Environment International: Krishna Kumar Yadav, Neha Gupta, Vinit Kumar, Shakeel Ahmad Khan, Amit Kumar

Environment International 111 (2018) 80–108
Contents lists available at ScienceDirect
Environment International
journal homepage: www.elsevier.com/locate/envint
Review article
A review of emerging adsorbents and current demand for defluoridation of T

water: Bright future in water sustainability
⁎
Krishna Kumar Yadava, , Neha Guptaa, Vinit Kumara, Shakeel Ahmad Khanb, Amit Kumarb
a
Institute of Environment and Development Studies, Bundelkhand University, Jhansi 284128, India
b
Centre for Environment Science and Climate Resilient Agriculture, Indian Agricultural Research Institute, New Delhi 110012, India
A R T I C L E I N F O A B S T R A C T
Keywords: Fluoride contamination of groundwater is a serious problem in several countries of the world because of the
Adsorption intake of excessive fluoride caused by the drinking of the contaminated groundwater. Geological and anthro-
Column pogenic factors are responsible for the contamination of groundwater with fluoride. Excess amounts of fluoride
Defluoridation in potable water may cause irreversible demineralisation of bone and tooth tissues, a condition called fluorosis,
Fluoride
and long-term damage to the brain, liver, thyroid, and kidney. There has long been a need for fluoride removal
Potable water
from potable water to make it safe for human use. From among several defluoridation technologies, adsorption is
Regeneration
the technology most commonly used due to its cost-effectiveness, ease of operation, and simple physical process.
In this paper, the adsorption capacities and fluoride removal efficiencies of different types of adsorbents are
compiled from relevant published data available in the literature and represented graphically. The most pro-
mising adsorbents tested so far from each category of adsorbents are also highlighted. There is still a need to
discover the actual feasibility of usage of adsorbents in the field on a commercial scale and to define the reu-
sability of adsorbents to reduce cost and the waste produced from the adsorption process. The present paper
reviews the currently available methods and emerging approaches for defluoridation of water.
1. Introduction caries by decreasing the rate of demineralisation of dental enamel or

reverses the progression of existing decay by promoting the rate of
Fluoride is a widely distributed monoatomic anion of fluorine remineralisation (Margolis and Moreno, 1990; Martinez-Mier, 2012).
characterised by a small radius (0.133 nm). It has a marked tendency to The process of demineralisation occurs during dental plaque metabo-
behave as a ligand and also to form a great number of different organic lism in which acids (produced from the reaction of bacteria, saliva, and
and inorganic compounds in air, soil, rock, and water. The sources of food) interact with surface dental enamel and remove minerals from it
fluorine in water and soil are mostly geogenic and include several rock (Thylstrup and Fejerskov, 1994). Fluoride is a beneficial constituent,
forming minerals (García and Borgnino, 2015). Among these, cryolite but this is only the case when its concentration in potable water is
may contain about 54 wt% F and fluorite, topaz, and fluorapatite may within the permissible limit (Jiménez-Reyes and Solache-Ríos, 2010).
contain about 48 wt%, 11.5 wt%, and 3.8 wt% F, respectively. Some There are different international standards for fluoride in drinking
other minerals, such as biotite and muscovite, may contain about 1 wt% water. According to the EU Council (1998), WHO (2011), and BIS
F (Limaleite et al., 2015). Some of these minerals, including cryolite, (2012), the maximum acceptable limit of fluoride in drinking water is
fluorite, and fluorapatite, are highly soluble in water and release 1.5 mg/L, but this limit is 4 mg/L according to the USEPA (2009). Ex-
fluoride ions into it. Fluoride usually competes with other anions such cess intake of fluoride can cause various diseases, such as osteoporosis,
as sulphate, chloride, carbonate, and phosphate for surface sites (García brittle bones, arthritis, cancer, infertility, thyroid disorder, and Alz-
and Borgnino, 2015). In addition, various industries such as pesticides, heimer's syndrome (Wambu et al., 2013; Vinati et al., 2015; Tiwari
ceramics, refrigerants, aerosol propellants, Teflon™ cookware, and et al., 2017a). Skeletal deformities occur over long-time consumption of
glassware industries increase the load of fluoride in water. Fertiliser, drinking water with > 8 mg/L fluoride during adolescence. Fluoride
iron, and aluminium manufacturing industries release fluoride as an can cause weakening of bones, leading to an increase in hip and wrist
unwanted byproduct (Peckham and Awofeso, 2014). Fluoride is con- fractures. Some reports have mentioned that chronic fluoride toxicity
sidered to be a micronutrient for humans because it prevents dental occurs in the form of osteo-dental fluorosis in both children and adults.
⁎
Corresponding author.
E-mail addresses: envirokrishna@gmail.com (K.K. Yadav), nhgupta83@gmail.com (N. Gupta).
https://doi.org/10.1016/j.envint.2017.11.014
Received 25 July 2017; Received in revised form 17 November 2017; Accepted 18 November 2017
0160-4120/ © 2017 Elsevier Ltd. All rights reserved.
K.K. Yadav et al. Environment International 111 (2018) 80–108
Individuals with kidney disease are at higher risk of fluorosis at even are used to remove fluoride from aqueous solution. Each technique has
normal permissible limits due to their decreased ability to excrete its own advantages, limitations, and influencing factors and works ef-
fluoride in urine (Ghosh et al., 2013). Fluoride toxicity does not only ficiently under ideal conditions.
have adverse effects on human health; it also affects animals and plants.
Excessive intake of fluoride by animals can have toxic effects in re- 2.1. Precipitation/coagulation
production, growth, thyroid hormones, learning and memory abilities,
blood, and feeding efficiency (Dolottseva, 2013). The toxic action of Alum and lime are the most utilised coagulants for defluoridation by
fluoride in aquatic organisms is as an enzymatic poison, inhibiting the precipitation method (Waghmare and Arfin, 2015). The Nalgonda
enzyme activity and ultimately interrupting metabolic processes such as technique is the best example of a coagulation/precipitation method. It
glycolysis and synthesis of proteins (Ghosh et al., 2013). Plants uptake involves the addition of aluminium salts, lime, and bleaching powder to
fluoride from contaminated soil because it is highly soluble in acidic fluoride-contaminated water followed by rapid mixing, flocculation,
soils. The absorbed fluoride is translocated to shoots, causing physio- sedimentation, filtration, and disinfection (Renuka and Pushpanjali,
logical, biochemical, and structural damage and even cell death, de- 2013). With the addition of lime and alum, the disinfection process
pending on the concentration in the cell sap (Gupta and Mondal, 2015). takes place in the following steps: (a) insoluble aluminium hydroxide
Fluoride contamination of drinking water has been recognised as a flocs form, (b) sediment sinks to the bottom, and (c) bleaching powder
major public health hazard in many parts of the world (Lavecchia et al., and fluoride co-precipitate (Bhatnagar et al., 2011). Although this
2012), such as China (up to 21.5 mg/L) (Ayoob et al., 2008), India method is effective for defluoridation, it may not be able to lower the
(0.12–24.17 mg/L) (Jha et al., 2013), Pakistan (1.13–7.85 mg/L) fluoride concentration to a desirable limit (1.5 mg/L) (Ayoob et al.,
(Rafique et al., 2009), and Thailand (0.01–14.12 mg/L) (Chuah et al., 2008). The precipitation technique is rarely used because of its high
2016). Fluoride enters the human body primarily through the con- chemical costs, formation of sludge with a high content of toxic alu-
sumption of fluoride contaminated drinking water (Sujana et al., 2009), minium fluoride complex, unpleasant water taste, and high residual
and once absorbed in the blood, rapidly distributes throughout the aluminium concentration.
body. The greatest proportion of the fluoride (almost 60% in adults and
80–90% in infants) is retained in calcium-rich areas such as bones and 2.2. Membrane process
teeth because fluoride has an affinity for calcium phosphate. The rest of
the fluoride is excreted via urine (Barbier et al., 2010). The tea plant A semi-permeable membrane is used in membrane processes be-
(Camellia sinensis L.) is a known accumulator of fluorine compounds, tween the adjacent phases (Velazquez-Jimenez et al., 2015) to serve as
which are released upon forming infusions such as the common bev- a barrier for suspended solids, pesticides, organic pollutants, inorganic
erage, and it can be considered a potential vehicle for fluoride dosing pollutants, and microorganisms (Suneetha et al., 2015). Reverse os-
(Chan et al., 2013). High concentrations in tea can be caused by high mosis, nanofiltration, dialysis, and electrodialysis are examples of this
natural concentrations in tea plants or by the use of additives during technique.
growth or fermentation (Ghosh et al., 2013). The fertilisers used to
promote the growth of green tea trees inevitably cause significant 2.2.1. Reverse osmosis
fluoride accumulation in tea leaves. Thus, tea drinking populations are Reverse osmosis is a physical phenomenon in which hydraulic
at increased risk of dental and skeletal fluorosis (Chan et al., pressure beyond the osmotic pressure applied to the higher con-
2013).Therefore, its remediation is very important (Singh et al., 2014), centration side of a semi-permeable membrane results in a flow of the
and there is an urgent need to seek out an efficient and emphatic de- solvent toward the less concentrated side (Wimalawansa, 2013). The
fluoridation technology to prevent the negative effects on human selection of the membrane to be used for water purification depends on
health. Among the fluoride removal technologies, the adsorption pro- the recovery, cost, salt rejection, temperature, pressure, and char-
cess is a significant method for removing excess fluoride from potable acteristics of the water to be treated (Velazquez-Jimenez et al., 2015).
water. This process has been used extensively by many researchers and Several researchers have studied reverse osmosis technology for the
has shown remarkable results. This paper presents a brief overview of purification of water (Sara et al., 2013; Pontie et al., 2013; Bejaoui
the technical applicability of various adsorbents for the removal of et al., 2014).
fluoride from potable water.
Although excellent review articles have been published, these arti- 2.2.2. Nanofiltration
cles provide a somewhat scattered treatment of the topic because some Nanofiltration is a process that has properties between the reverse
discuss the importance of adsorbents for fluoride removal, others de- osmosis and ultrafiltration. The required pressures for nanofiltration are
scribe the adsorption capacity and pH (Tomar and Kumar, 2013; lower than those for reverse osmosis, which reduces the energy costs.
Velazquez-Jimenez et al., 2015), and yet others summarise the con- The permeability of nanofiltration membranes is also superior to those
centration range (Tomar and Kumar, 2013; Jadhav et al., 2015). Many of reverse osmosis. Nanofiltration is suitable for reducing the hardness
studies have also focused on applicable isotherms and kinetics of water because the membranes have high retention capacity for
(Mohapatra et al., 2009; Bhatnagar et al., 2011; Habuda-Stanic et al., charged particles, especially bivalent ions. This technique appears to be
2014; Vinati et al., 2015). The present review is a compilation of all the the best method of all membrane processes for fluoride removal due to
aspects of defluoridation, including fluoride adsorption capacity, re- the high and specific membrane selectivity (Tahaikt et al., 2007).
moval efficiency, optimum pH, adsorbent dose, contact time, initial Diawara et al. (2011) compared the efficiency of nanofiltration and
fluoride concentration, temperature, applicable isotherms and kinetics low pressure reverse osmosis (LPRO) membranes for removal of
model, presence of co-anions, and reusability of adsorbent, including fluoride and salinity from brackish drinking water. They observed that
the detailed mechanisms of the influencing factors that affect the nanofiltration membranes are more efficient than LPRO membranes if
whole/partial process of adsorption. Additionally, based on the pub- the drinking water to be treated has fluoride and salinity concentrations
lished data, this review highlights the least and most promising ad- slightly above the WHO permissible limits. In the opposite case, LPRO
sorbents according to their efficiency from each respective category. membranes are more effective than nanofiltration membranes. Hoinkis
et al. (2011) studied the performance of two commercial nanofiltration
2. Technologies for fluoride removal membranes, i.e. NF 90 and NF 270, for the removal of fluoride from
surface and groundwater. The results demonstrated that the NF 270
Various techniques such as coagulation/precipitation methods, membrane was able to reduce fluoride to 1.5 mg/L from an initial
membrane processes, ion-exchange processes, and adsorption processes concentration of 10 mg/L and that the NF 90 membrane was efficient
81
for reducing the fluoride in drinking water to 0.5 mg/L from an initial treatment improved the surface morphology of the dense membrane.
concentration of 20 mg/L. The presence of HCO3– anions did not have a The flux values and recovery factors of the plasma-modified AFX
significant effect, but the removal of fluoride was decreased at low pH. membrane were higher than those of pristine membrane, which was
Bejaoui et al. (2014) compared the performance of reverse osmosis (RO- due to the change of wettability and morphology.
SG) and nanofiltration (NF 90) membranes for removal of fluoride ions The ion exchange process has great potential (up to 95%) for re-
from model water and metal packaging industrial effluent. The effects moving fluoride from aqueous solutions. The resins are expensive and
of concentration, ionic strength, feed pressure, nature of cation asso- make the treatment economically unviable; however, resins can be re-
ciated with fluoride, and pH on the retention of fluoride ions were generated easily. Unfortunately, the regeneration process produces
studied. Fluoride retention was noted to be increased at higher pH due large amounts of fluoride-loaded waste and disposal needs for such
to an increase in negative charge of the membrane. waste are a disadvantage of this process (Jadhav et al., 2015).
Membrane fouling, insufficient separation and rejection, chemical
resistance, and limited lifetime of membranes are some of the draw- 3. Adsorption process
backs of the nanofiltration technique that require improvement (Van
der Bruggen et al., 2008). Defluoridation by adsorption has great potential for fluoride re-
moval because of its cost effectiveness, ease of operation, high removal
2.2.3. Electrodialysis capacities, and ability to reuse the adsorbent (regeneration). These
Electrodialysis is a process of removing ionic components from characteristics encourage researchers to continue to explore adsorbents
aqueous solution using an ion-exchange membrane under the influence for additional potential. Adsorption is a three-step process: (a) external
of an electric field. The field acts as the driving force responsible for the mass transfer, i.e. the transfer of fluoride ions from aqueous solution to
separation of contaminants (Suneetha et al., 2015). When an electric the external surface of the adsorbent, which is called molecular diffu-
current is applied between two electrodes, the cations move to the sion or film diffusion; (b) adsorption of fluoride ions onto particle
cathode through the negatively charged cation exchange membrane surfaces; and (c) intraparticle diffusion, i.e. the transfer of adsorbed
and the anions move to the anode through the positively charged anion fluoride ions to the internal surfaces of the adsorbent particles
exchange membrane (Jadhav et al., 2015). The ion-exchange mem- (Mohapatra et al., 2009; Habuda-Stanic et al., 2014). It is not ne-
brane allows the ions, but not water, to pass through it. cessarily the case that an adsorbent that has maximum adsorption ca-
Kabay et al. (2008) removed fluoride from aqueous solution by pacity would also have maximum fluoride removal efficiency because
electrodialysis with varying parameters, such as applied voltage, feed fluoride removal is affected by various factors, which are discussed in
flow rate, and fluoride concentration in the solution. The results the next section. Adsorption capacity may be defined as the amount of
showed that the separation performance improved as the fluoride fluoride adsorbed per unit mass of adsorbent, whereas removal effi-
concentration in the feed solution was increased. It was also reported ciency represents the maximum removal of fluoride as a percentage at
that the separation of fluoride was influenced by chloride ions but not optimal conditions, such as optimal pH, adsorbent dose, contact time,
by sulphate ions because divalent ions were retained in the membrane initial fluoride condition, and temperature. In this section, important
and transported very slowly to the solution. Gmar et al. (2015) reported adsorbents along with their adsorption capacity, fluoride removal ef-
a fluoride removal of 92% by electrodialysis. Many other researchers ficiency under optimal experimental conditions, and regeneration ca-
have also studied the electrodialysis method for the removal of fluoride pacity are summarised.
from aqueous solution (Lahnid et al., 2008; Arda et al., 2009;
Majewska-Nowak et al., 2015). 3.1. Alumina and aluminium-based adsorbents
A very high removal capacity (up to 98%) and no usage of chemicals
are some advantages of membrane processes. These are one-step pur- Alumina has been studied extensively and is considered a most ef-
ification and disinfection processes. Despite these advantages, the fective adsorbent for defluoridation of water (Mondal and George,
membrane process cannot be adapted completely because it removes all 2015). To be an effective adsorbent, it must be activated. Activated
ions from water. Because some minerals and ions are essential and must alumina (AA) is prepared by either slow or rapid pyrolysis of gibbsite or
be present in potable water, these processes require a re-mineralisation materials containing gibbsite (flash calcinations) (Rozic et al., 2006;
process to recover them. Moreover, the disposal of sludge, high initial Mohapatra et al., 2009). The adsorption capacity of activated alumina
membrane cost, re-mineralisation process, and operating costs make is dependent on the pH of the water and its crystalline form. Goswami
these processes economically inappropriate and environmentally un- and Purkait (2012) developed acidic alumina to defluoridate water and
sustainable (Jadhav et al., 2015). reported maximum fluoride removal at pH 4.4. The adsorption process
followed the Langmuir isotherm with an adsorbent capacity of 8.4 mg/
2.3. Ion exchange process g, and the kinetics followed a pseudo-second-order equation.
Activated alumina has been modified by many researchers, either
Ion exchange is a process in which water flows through a bed of ion thermally or chemically, to increase its adsorption efficiency. Tripathy
exchange material (ion exchanger) to remove the undesirable ions. Ion et al. (2006) synthesised alum-impregnated activated alumina (AIAA)
exchangers are of two types: cation exchangers, which exchange posi- and found it more efficient for defluoridation of water. The surface area
tively charged ions (cations), and anion exchangers, which exchange of activated alumina was found to be increased from 113 m2/g to
negatively charged ions (anions). Zeolite and greensand are commonly 176 m2/g by impregnation with alum. AIAA was able to remove 99% of
used cation exchangers, and inorganic metallic oxides are used as anion fluoride at pH 6.5, adsorbent dose of 8 g/L, contact time of 3 hours (h),
exchangers. and initial fluoride concentration of 20 mg/L in 50 mL of water. The
Samadi et al. (2014) reported a maximum sorption capacity of resin isotherm and variation of the adsorbent dose data were correlated to
of 13.7 mg/g from the Langmuir isotherm. In another study (Ho et al., the Bradley equation, which shows that the adsorption capacity de-
2004), zirconia was introduced into mesoporous titanium oxohydroxide creases as pH increases. Energy-dispersive analysis of X-rays (EDAX)
(TiOx(OH)y) using dodecylamine as a template to improve the ion ex- showed that the fluoride removal occurred not because of adsorption
change capacity. They found that mesoporous Ti oxohydroxide con- but rather due to simple surface precipitation.
taining zirconia exhibits the highest fluoride ion exchange capacity due Manganese-oxide-coated alumina (MOCA) was investigated by
to its smallest particle size and high uniformity. Alkan et al. (2008) Maliyekkal et al. (2006). They reported that the fluoride adsorption rate
studied fluoride removal using a plasma-modified AFX anion-exchange and adsorption capacity of MOCA are much higher than those of acti-
membrane by the Donnan dialysis method. They observed that plasma vated alumina. Optimal fluoride removal was observed in the pH range
82
K.K. Yadav et al.
Table 1
Optimum experimental conditions of various aluminium-based adsorbents with their maximum adsorption capacities and fluoride removal efficiency.
S. no. Adsorbent AC Removal AD pH CT IFC Temp SA Interfering co-anions Regeneration Isotherm Kinetic References
(mg/g) (%) (g/L) (mg/L) (m2/g)
1. Acidic alumina 8.4 94 4.5 4.4 90 min 15 25 °C 144.27 SO42 −, CO3−, HCO3− 85% Langmuir Psuedo-2nd order Goswami and Purkait, 2012
2. AIAA 40.65 99 8 6.5 3h 20 25 °C 176 – 98% Bradley – Tripathy et al., 2006
3. MOCA 2.85 91 5 4–7 3h – 30 °C 170.39 No effect 94.6% Langmuir 2nd order Maliyekkal et al., 2006
4. HMOCA 7.09 95 – 5.2 – 10–70 25 °C 315.5 HCO3−, SO42 −, PO43 − 85% Langmuir Psuedo-2nd order Teng et al., 2009
5. MCAA 0.16 98 8 7 3h 10 25 °C 203 SO42 −, PO43 −, Cl−, NO3− – Langmuir 2nd order Tripathy and Raichur, 2008
6. La-AA 6.7 – 2 4–8 2h 10 – – – – Langmuir Psuedo-2nd order Cheng et al., 2014
7. MIAA 0.76 95 10 7 1h 12 20 °C – – 85% Langmuir & Freundlich – Rafique et al., 2013
8. HMAA 14.4 – 7 8 8h 9 – – – – Langmuir & Freundlich – Tomar et al., 2015
9. MAAA 10.12 95 4 7 3h 10 30 °C 193.5 SO42 −, HCO3− 90% Sips Psuedo-2nd order Maliyekkal et al., 2008
10. MA450 8.26 78 4 3–9 24 h 5 30 °C 413.65 HCO3−, SO42-, NO3− – Langmuir Psuedo-2nd order Jagtap et al., 2011
11. COLMA 137 90 3 6.8 8h 10 30 °C 93 HCO3−, SO42-, NO3− – Langmuir Psuedo-2nd order Dayananda et al., 2014
12. ϒ-Al2O3 with 5.6 85 0.5 6.3–7.3 140 min 5 35 °C 139 PO43 −, HCO3−, NO3−, SO42 −, – Langmuir Psuedo-2nd order Nazari and Halladj, 2014
83
MgO Cl−
nanocrystals
13. LAA 16.9 77.2 1 7.6–8.6 – 10 25 °C 192 – 98% Langmuir – Shi et al., 2013
14. COCA 7.22 98 0.4 4–9 24 h 10 30 °C 189.25 CO3−, HCO3− 97% Langmuir Psuedo-2nd order Bansiwal et al., 2010
AC = adsorption capacity; AD = adsorbent dose; CT = contact time; IFC = initial fluoride concentration; SA = surface area; AIAA = alum-impregnated activated alumina; MOCA = manganese oxide coated alumina; HMOCA = hydrous
manganese oxide coated alumina; MCAA = manganese dioxide coated alumina; La-AA = lanthanum impregnated activated alumina; MIAA = modified immobilized activated alumina; HMAA = hydroxyapatite modified activated alumina;
MAAA = magnesia-amended activated alumina; MA450 = chitosan based mesoporous alumina; COLMA = calcium oxide loaded mesoporous alumina; ϒ-Al2O3 with MgO nanocrystals = gamma alumina with magnesium oxide nanocrystals;
LAA = lanthanum impregnated activated alumina; COCA = copper oxide coated alumina.
Fig. 1. Fluoride adsorption performance of various aluminium-

140 100
Adsorption Capacity (mg/g)
based adsorbents along with removal efficiency
Fluoride Removal (%)

120 90
80
100 70
80 60
50
60 40
40 30
20
20 10
0 0
Adsorbents
Adsorption Capacity (mg/g) Fluoride Removal Efficiency (%)
of 4–7. The maximum fluoride uptake capacity for MOCA and AA was adsorption data followed pseudo-second-order kinetics and fit the
found to be 2.85 and 1.08 mg/g, respectively. Adsorption onto MOCA Langmuir isotherm well. Samarghandi et al. (2016) used activated
followed the Langmuir isotherm and second-order kinetics. MOCA alumina for defluoridation in the presence of natural organic matter
could be regenerated using 2.5% NaOH eluent. A hydrous manganese- using a response surface methodology. They reported that the presence
oxide-coated alumina (HMOCA) has been prepared with a redox pro- of natural organic matter and a higher pH enhanced fluoride adsorption
cess (Teng et al., 2009). Consistent with the Langmuir model, the on activated alumina. The experimental data fit the second-order
maximum adsorption capacity of HMOCA was found to be 7.09 mg/g. polynomial model well.
Optimal removal of fluoride was achieved in the pH range of 4–6. The Lanas et al. (2016) conducted a study to evaluate the feasibility of
adsorption with HMOCA followed a pseudo-second-order equation. mesoporous hierarchical alumina microspheres (HAM) for the removal
Tripathy and Raichur (2008) used manganese-dioxide-coated activated of fluoride. Their fluoride adsorption studies were carried out by means
alumina (MCAA) for fluoride removal, and the newly developed ad- of potentiometry and isothermal titration calorimetry (ITC). The max-
sorbent was able to reduce fluoride from 10 mg/L to 0.2 mg/L. Ad- imum sorption capacity of HAM was found to be 26 mmol/g. The ad-
sorption on MCAA followed the Langmuir isotherm and second-order sorption was well explained using the Langmuir isotherm model.
kinetics. The Dubinin–Raduskevich isotherm, zeta potential, and EDAX The defluoridation performances of aluminium-based adsorbents
analysis confirmed that fluoride uptake by MCAA was due to physical are appreciable, but they can be dangerous to human health because
adsorption as well as intra-particle diffusion. aluminium ions get solubilised from the surface of alumina in the
Cheng et al. (2014) prepared a lanthanum-modified activated alu- presence of high concentrations of fluoride ions in water and variation
mina (La-AA) by mixing 2 g of adsorbent with 40 mL of lanthanum of pH causes dissolution of more aluminium ions into the treated water
nitrate solution for 2 days. They reported that the adsorption rate and (Mondal and George, 2015). Fluoride ions in the presence of trace
capacity of La-AA were distinctly superior to those of AA. Maximum amounts of aluminium form aluminofluoride compounds. These me-
removal of fluoride was achieved in the pH range of 4–8. The adsorp- tallofluoride compounds may mimic or potentiate the action of nu-
tion process on La-AA and AA followed the Langmuir isotherm and merous extracellular signals and significantly affect many cellular re-
pseudo-second-order kinetics. Rafique et al. (2013) synthesised mod- sponses (Strunecká and Patocka, 1999).
ified immobilised activated alumina (MIAA) using alum with the sol-gel A comparison among various aluminium-based adsorbents under
method. The fluoride adsorption rate of MIAA was 1.35 times higher optimal experimental conditions for fluoride removal is presented in
than that of normal immobilised activated alumina. Fluoride removal Table 1. The efficiency of various adsorbents can differ depending on
reached a maximum at a 10 g/L adsorbent dose, 1 h contact time, the physical properties that affect the complex chemistry of fluoride in
150 rpm stirring rate, and 12 mg/L initial fluoride concentration. The water. The adsorption capacity and fluoride removal efficiency of var-
adsorption on MIAA conformed well with both the Langmuir and ious aluminium-based adsorbents are shown in Fig. 1. It is evident from
Freundlich isotherms. MIAA can be regenerated easily by both thermal a survey of the literature that calcium oxide-loaded mesoporous alu-
and chemical processes. Tomar et al. (2015) prepared the new ad- mina exhibits the highest fluoride adsorption capacity of 137 mg/g
sorbent hydroxyapatite-modified activated alumina (HMAA) by dis- (Dayananda et al., 2014), because calcium may react with fluoride ions
persing nanoparticles of hydroxyapatite within activated alumina to form calcium fluoride (CaF2) precipitates and because of the high
granules. HMAA was five times more efficient than AA at removing point of zero charge (pHPZC) value of COLMA. pHpzc determines how
fluoride from water with an adsorption capacity of 14.4 mg/g. The easily a substrate can adsorb the ions present in solution. It is worth
adsorption capacity was found to be highest around a pH of 8. HMAA noting that modified/impregnated/coated alumina has more potential
can be regenerated using 6 bed volumes of a solution containing in- for fluoride adsorption than unmodified alumina. This is the case be-
nocuous chemicals. cause modification of adsorbents with multivalent cations can drama-
Maliyekkal et al. (2008) developed magnesia-amended activated tically change the surface properties of the adsorption materials and
alumina (MAAA) for defluoridation of water. > 95% removal of their affinity for fluoride (Cheng et al., 2014).
fluoride (10 mg/L) was achieved with MAAA within 3 h of contact time
at pH 7. The BET surface area of MAAA was found to be 193.5 m2/g. 3.2. Carbon-based adsorbents
The adsorption data trim well with pseudo-second-order kinetics.
Higher concentrations of bicarbonate and sulphate reduced the fluoride Carbon-based adsorbents have also been studied extensively for
sorption capacity. Jagtap et al. (2011) used chitosan-based mesoporous fluoride removal as carbon has a high affinity for fluoride anions.
alumina (MA450) and reported that its maximum adsorption capacity is Karthikeyan and Elango (2008) used various grades of graphite as ad-
8.26 mg/g at an initial fluoride concentration of 5 mg/L. The sorbents to remove fluoride ions from aqueous solutions. Maximum
84
K.K. Yadav et al.
Table 2
Optimum experimental conditions of various carbon-based adsorbents with their maximum adsorption capacities and fluoride removal efficiency.
(mg/g) (%) (g/L) (mg/L) (m2/g)
1. ZILSSAC 3.25 – 2 3 2.5 h 10 25 °C – – – Langmuir – Joshi et al., 2012

2. WCS 4.86 – 1 – – 15 25–28 °C 629 – 92% Langmuir 2nd order Gupta et al., 2007
3. ZICFC 1.95 93 20 4 6h 20 25 °C 163.2 – 100% Langmuir Pseudo-2nd Sai Sathish et al., 2008
order
4. GAC-La0.05 9.98 92 0.1 7 – 20 25 °C 954 No effect 87% Langmuir Pseudo-2nd Vences-Alvarez et al., 2015
order
5. NC 5.9 82 3 4 1h 5 24 ± 2 °C 227.58 – – Langmuir – Regassa et al., 2016
6. PAC 8.36 86 3 2 1h 5 24 ± 2 °C 312 – – Langmuir – Regassa et al., 2016
7. CAC 11.35 90 3 2 1.5 h 5 24 ± 2 °C 711 – – Langmuir – Regassa et al., 2016
8. RS2/KMnO4 23.3 100 1.5 2 3h 20 25–55 °C 122.9 SO42 −, Br− – Langmuir & Freundlich Psuedo-2nd Daifullah et al., 2007
order
9. ACBG 4.702 83.77 12.5 5–5.5 105 min 3 25 °C – SO42 −, HCO3− 67.4% Redlich-Peterson & Langmuir – Alagumuthu et al., 2010
10. ZICSC 0.99 91 1 4 6h 10 25 °C 2.82 – 100% Langmuir Pseudo-2nd Sai Sathish et al., 2007
order
11. ZrOx-AC 7.40 94 – 7 – 40 25 °C 926.83 Cl−, SO42 −, PO43 −, NO3−, CO3− – Langmuir & Freundlich – Halla et al., 2014
12. GC (FeSO4·7H2O) 2.16 94 20 4 48 h 10 30 °C 80.94 – – Langmuir & Freundlich Pseudo-2nd Chen et al., 2011a
order
85
13. ZIGNSC 2.32 94 2 3 3h 3 25 °C 2.12 HCO3− – Freundlich Pseudo-2nd Alagumuthu and Rajan, 2010a
order
14. ZIWSC 6.38 94 15 3 3h 3 25 °C – – 96.2% Freundlich Pseudo-2nd Rajan and Alagumuthu, 2013
order
15. ZICNSC 1.83 80.33 0.015 7.6 3h 3 25 °C – – 96.2% Langmuir Pseudo 2nd Alagumuthu and Rajan, 2010b
order
16. CDC @ 500 °C 15 99 2 7.53 40 min 2 25 °C 19.2 HCO3−, NO3− NO2−, SO42 −, Cl− 100 Freundlich Psuedo-2nd Rajkumar et al., 2015
order
−
17. AC750NMW5 18.95 97.2 2 4 20 min – 30 °C 695 Cl – Langmuir Psuedo-2nd Dutta et al., 2012
order
18. ACBS 2.4 52 2 6 1h 8 30 °C 2 HCO3−, SO42 −, Cl− 8% Redlich–Peterson, Langmuir Psuedo-2nd Singh et al., 2017
order
19. ACBFR 1.65 88 4 7 1h 5 30 °C 375.03 PO43 −, HCO3−, SO42 −, NO3−, – Langmuir Psuedo-1st order Ravulapalli and Kunta, 2017
Cl−
20. ABC 1.15 57.6 4 6 1h 2.5 28 ± 1 °C – – – Freundlich Psuedo-2nd Yadav et al., 2013
order
ZILSSAC = zirconyl-impregnated lapsi seed stone activated carbon; WCS = waste carbon slurry; ZICFC = zirconium ion-impregnated coconut fiber carbon; GAC-La = lanthanum oxyhydroxide-impregnated granulated activated carbon;
NC = natural coal; PAC = physically activated coal; CAC = chemically activated coal; RS2/KMnO4 = KMnO4 modified activated carbon derived from steam pyrolysis of rice straw; ACBG = activated carbon from Bermuda grass;
ZICSC = zirconium-impregnated coconut shell carbon; ZrOx-AC = zirconium-impregnated activated carbon mixed with oxalic acid; GC (FeSO4·7H2O) = iron-impregnated granular ceramics; ZIGNSC = zirconium-impregnated groundnut shell
carbon; ZIWSC = zirconium-impregnated walnut shell carbon; ZICNSC = zircomium-impregnated cashew nut shell carbon; CDC = cow dung carbon; AC750NMW5 = microwave assisted activated carbon derived from Acacia auriculiformis scrap
wood; ACBS = activated carbon prepared from Bael shell (Aegle marmelos); ACBFR = activated carbon derived from bark of Ficus racemosa plant; ABC = activated bagasse carbon.
Fig. 2. Fluoride adsorption performance of various carbon-based

25 120
adsorbents along with removal efficiency.
100

20
80
15
60
10
40
5 20
0 0
Adsorbents
fluoride adsorption was achieved at a low range of pH and high tem- fluoride adsorption capacities of natural coal (NC), physically activated
perature. The adsorption data fit the Langmuir and Freundlich iso- coal (PAC), and chemically activated coal (CAC). The adsorption pro-
therms well and followed first-order kinetics. The thermodynamics in- cess was found to be pH dependent. The optimum pH was observed to
dicated that the adsorption is an endothermic process. Joshi et al. be 2 for PAC and CAC and 4 for NC. The adsorption data were described
(2012) developed zirconyl-impregnated activated carbon. The activated well by the Langmuir isotherm with adsorption capacities of 5.9 mg/g,
carbon was derived from Lapsi (Choerospondias axillaries) seed stone. 8.36 mg/g, and 11.35 mg/g for NC, PAC, and CAC, respectively.
Maximum defluoridation was found to occur in the pH range of 3–4 Daifullah et al. (2007) prepared activated carbon from rice straw
within a contact time of 3 h. The Langmuir isotherm model fit the ad- modified by KMnO4 for defluoridation of water. The adsorbent was
sorption isotherm data well. Said and Machunda (2014) studied the found to be very efficient with almost 100% fluoride removal and a
defluoridation of water using coconut shell activated carbon. Batch 23.3 mg/g adsorption capacity. The experimental data fit the Langmuir
experiments were carried out to evaluate the effects of various ad- and Freundlich isotherms well and followed a pseudo-second-order
sorbents on the adsorption efficiency. Adsorption was favoured in the equation. In another study (Alagumuthu et al., 2010), activated carbon
acidic range, and maximum adsorption (58.4%) was recorded at was derived from Bermuda grass (Cynodon dactylon). The batch ad-
pH 2.The Langmuir and Freundlich isotherms were a good fit to explain sorption experiments were carried out at neutral pH as a function of
the adsorption of fluoride onto coconut shell activated carbon. adsorbent dose, temperature, contact time, initial fluoride concentra-
Gupta et al. (2007) used waste carbon slurry for fluoride removal. tion, and effect of coexisting anions. The adsorbent was characterised
The adsorbent was developed from a fuel oil-based generator in the by X-ray diffraction (XRD), scanning electron microscopy (SEM), and
fertiliser industry. The maximum adsorption capacity (4.861 mg/g) was Fourier transform infrared spectroscopy (FTIR). The adsorption process
observed at 15 mg/L initial fluoride concentration using a 1 g/L ad- was endothermic and spontaneous in nature. The adsorbent could be
sorbent dose. Moreover, the experimental data followed the Langmuir regenerated easily up to 67.4% using 2% NaOH. A summary of the
isotherm and second-order kinetics. Desorption from fluoride-loaded various experimental conditions for various carbon-based adsorbents is
carbon slurry could be achieved under alkaline conditions. Sai Sathish given in Table 2. Zirconium has been used extensively by many re-
et al. (2008) synthesised zirconium ion-impregnated coconut fibre searchers to enhance the fluoride potential of carbon-based adsorbents
(ZICFC) for fluoride removal. The surface area calculated from BET (Sai Sathish et al., 2007; Sai Sathish et al., 2008; Alagumuthu and
plots was 163.2 m2/g. ZICFC was able to adsorb 93% of fluoride at pH 4 Rajan, 2010a; Alagumuthu and Rajan, 2010b; Rajan and Alagumuthu,
and 6 h of agitation time. The adsorption data fit the Langmuir isotherm 2013; Joshi et al., 2012; Halla et al., 2014). The adsorption capacity
well and followed pseudo-second-order kinetics. The adsorption of and fluoride removal efficiency of various adsorbents are shown in
fluoride onto ZICFC was due to chemisorption and physisorption pro- Fig. 2. KMnO4-modified activated carbon derived from steam pyrolysis
cesses with intra-particle diffusion. of rice straw showed a marked fluoride adsorption capacity and re-
Poudyal and Babel (2015) compared the efficiencies of granular moval efficiency of 23.3 mg/g and 100%, respectively (Daifullah et al.,
activated carbon (GAC) and domestic sewage sludge for fluoride re- 2007). The high adsorption capacity of this material is probably due to
moval. The maximum fluoride adsorption efficiency was 88% and 78% the sorption mechanism, which includes both ion exchange and com-
for sewage sludge and GAC, respectively. The experimental adsorption plexation.
data were described better by the Freundlich isotherm than the Lang-
muir isotherm. Vences-Alvarez et al. (2015) developed granular acti-
vated carbon by impregnation of lanthanum oxyhydroxides (GAC-La). 3.3. Calcium-based adsorbents
The adsorption capacity of the GAC-La was 5 times higher than that of
the unmodified GAC. This difference was due to a higher number of Some researchers have explored calcium-based adsorbents for
eOH active sites in the adsorbent. The maximum adsorption capacity of fluoride removal from aqueous solution because of their excellent af-
GAC-La was reported to be 9.98 mg/g, and the adsorption best fit the finity for fluoride and biocompatibility with the human body. Gandhi
Langmuir isotherm. It was also reported that the presence of coexisting et al. (2013) studied chalk powder as an adsorbent for fluoride removal.
anions did not affect the adsorption capacity of the adsorbent material Chalk powder was chosen as an adsorbent because of its high porosity.
at concentrations between 5 and 30 mg/L. However, concentrations of Batch adsorption studies were carried out, and it was found that the
coexisting anions > 30 mg/L had a negative effect on the fluoride ad- adsorption of fluoride onto chalk powder was directly proportional to
sorption capacity of GAC-La. Regassa et al. (2016) compared the the fluoride concentration, adsorbent dose, and contact time. The ki-
netic model revealed the pseudo first-order and second-order rate, and
86
the adsorption data fit the Langmuir and Freundlich isotherms best. The Jiménez-Reyes and Solache-Ríos (2010) used hydroxyapatite for
adsorption reaction was exothermic and spontaneous in nature. He and fluoride retention from aqueous solutions. Maximum removal was
Cao (1996) used different combinations of tricalcium phosphate (TCP), achieved in the pH range 5.0–7.3 with 16 h of contact time using 0.1 g
bone char (BC), hydroxyapatite (HAP), and related substances to re- of hydroxyapatite for 25 mL of solution. The adsorption data followed
move fluoride from drinking water. The batch experiment showed that the Freundlich isotherm and pseudo-second-order kinetics. The fluoride
the defluoridation efficiencies of TCP, HAP, and BC were 87%, 68%, uptake by hydroxyapatite was due to chemical adsorption. Garg and
and 66.4%, respectively, under routine conditions. The bone char Chaudhari (2012) developed a magnesium-substituted hydroxyapatite
monocalcium phosphate (BC-MCP) combination was found to be best (Mg-HAp) adsorbent for defluoridation of potable water. The char-
for removal of fluoride. The BC-MCP combination was efficient for re- acteristics of the Mg-HAp were analysed using an X-ray diffractometer
ducing fluoride from 10.4 mg/L to 0.6 mg/L in 24 h at the pH range of (XRD). Batch adsorption experiments were conducted to study the ef-
6.5–8.5. fects of various parameters, such as pH, contact time, initial fluoride
Nath and Dutta (2010) used crushed limestone for defluoridation of concentration, and coexisting ions, on the fluoride adsorption capacity
water and found it very efficient for removal of fluoride. Gogoi and of Mg-HAp. The authors observed that an increase in pH reduced the
Dutta (2016) modified limestone powder hydrothermally using phos- adsorption capacity. The presence of coexisting anions did not sig-
phoric acid. Hydroxyapatite was formed during the hydrothermal nificantly affect the fluoride adsorption. The experimental data fol-
modification, as demonstrated by FTIR and XRD analyses. The ad- lowed the Freundlich isotherm and pseudo-second-order kinetics. Sani
sorption capacity of the hydrothermally modified adsorbent was et al. (2016) carried out a comparative study of nano-hydrotalcite/
6.45 mg/g. The isotherm model showed that the process was controlled hydroxyapatite (n-HT/HAp) and calcined hydrotalcite with Mg-Al (cHT
by physical adsorption with an exchange of hydroxide ions of HAP with Mg-Al) at a ratio of 4:1. The results showed that a higher adsorption of
fluoride ions. The adsorption data followed second-order kinetics. A 98% compared to the cHT Mg-Al sample (97%) was achieved with n-
thermodynamics study confirmed that the adsorption reaction was HT/HAp at 10 g/L of adsorbent dose for a 20 mg/L fluoride solution. A
endothermic, spontaneous, and irreversible. A new calcium aluminate list of the various experimental conditions for various calcium-based
(CA) adsorbent was synthesised by Sakhare et al. (2012) using the adsorbents is presented in Table 3. Fig. 3 shows the maximum ad-
combustion method. The newly developed adsorbent was able to re- sorption capacity and fluoride removal efficiency of each of those ad-
move up to 85% of the fluoride from an initial fluoride concentration of sorbents. Fig. 3 reveals that Ce-Zr oxide nanospheres encapsulating
8.9 mg/L using a 3 g/L adsorbent dose. The kinetics of fluoride ad- calcium alginate beads have the highest adsorption capacity of
sorption on CA could be described well by the pseudo-second-order rate 137.6 mg/g (Chen et al., 2016). This might be due to the exchange
law, and the Langmuir isotherm fit the experimental data well. In ad- between surface hydroxyl ions and fluoride ions. On the other hand, Ce
dition, the thermodynamic parameters indicated that the fluoride ad- (IV) was reduced to Ce (III) during adsorption, which indicates that the
sorption onto CA was dominated by chemisorption and that the process hydroxyl groups bonded to Ce are also involved in the removal of
was spontaneous and endothermic. Regeneration could be achieved by fluoride. Contrary to this result, NCA is highly dependent on pH and
treating the fluoride-loaded material with NaOH and H2SO4. performs best at pH 6. Fluoride removal was decreased by either in-
Basu et al. (2013) synthesised alumina-impregnated calcium algi- creasing or decreasing pH from this level (Jayarathne et al., 2015).
nate beads (Cal-Alg-Alu) for fluoride removal. The nature and mor-
phology of the Cal-Alg-Alu were analysed by scanning electron micro- 3.4. Oxides/hydroxides and layered double hydroxides
scopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and
attenuated total reflectance Fourier transform infrared spectroscopy Several researchers have reported that two or more metal oxides/
(ATR-FTIR). The batch experimental data demonstrated that adsorption hydroxides have great potential for fluoride adsorption. Kumar et al.
was > 99.9% at the pH range of 3.5–9.0 and in concentration range of (2009) used granular ferric hydroxide (GFH) for the removal of fluoride
1–100 mg/L. The Langmuir isotherm fit the experimental data well and from aqueous solutions. The maximum removal of fluoride was 7 mg/g
followed pseudo-second-order kinetics. The maximum calculated at 25 °C and pH range of 4–8. The isotherm model confirmed the
fluoride adsorption was 17 mg/L. Chen et al. (2016) determined the Langmuir model, and the kinetic study revealed pseudo-first-order ki-
fluoride adsorption efficiency of hierarchically porous Ce-Zr oxide na- netics controlled by pore diffusion. The adsorption of fluoride onto GFH
nospheres encapsulating calcium alginate beads (CZ-CABs) by the sol- was influenced most strongly by the presence of phosphate ions, fol-
gel technique. The experimental data fit the Langmuir and Freundlich lowed by carbonate and sulphate ions. Ce-Fe bimetal oxides with
isotherms best with an adsorption capacity of 137.6 mg/g under neutral hierarchical pore structure having high efficiency for fluoride removal
conditions. The kinetic data were well described by the pseudo-second- were synthesised and characterised by Tang and Zhang (2016). The
order equation. characteristics of the adsorbent were determined by XRD, XPS, and
Jayarathne et al. (2015) used natural crystalline apatite for fluoride HRTEM. The highest defluoridation capacity was noted to be 60.97 mg/
adsorption from aqueous solution. The adsorption process was heavily g. The experimental data were described well by the Langmuir isotherm
dependent on pH, and the optimal pH for maximum removal was ob- model, and the kinetic study revealed the pseudo-second-order equa-
served to be 6. It is also illustrative that the adsorption increased with tion. The adsorption was influenced most by the presence of carbonate
adsorbent dose because of the increase in adsorbent sites. The highest ions, followed by bicarbonate, sulphate, nitrate, and chloride ions.
adsorption was achieved at pH 6 for a solution containing 15 mg/L Hussain et al. (2015) studied the fluoride adsorption efficiency of a
fluoride within 10 min of contact time. The adsorption data showed a Mg (III) – Al (III) – La (III) triple-metal hydrous oxide developed by the
monolayer adsorption capacity of 0.212 mg/g and were best fit by the co-precipitation method. The adsorbent efficiently removed 98.28% of
Langmuir isotherm. The experimental data obeyed pseudo-second- fluoride in the pH range of 2–12 from a 20.66 mg/L fluoride solution.
order kinetics. Mondal et al. (2016) assessed the defluoridation po- The adsorption process followed the Langmuir isotherm model, and the
tential of magnesium-incorporated hydroxyapatite (M-i-HAP) having a reaction was endothermic. Kinetically, adsorption took place according
surface area of 46.62 m2/g. The highest removal of fluoride (94.5%) by to the pseudo-second-order model. The adsorbent could be regenerated
M-i-HAP was achieved under the optimum conditions of 303 K, pH 7, easily up to 95.71% recovery with a mixture of methanol and HCl.
3 h of contact time, and 10 g/L adsorbent dose for a 10 mg/L fluoride Biswas et al. (2010) assessed the feasibility of hydrated Fe (III) eAl (III)
solution. The adsorption process followed the Langmuir isotherm, with eCr (III) ternary mixed oxide (HIACMO) for the removal of fluoride
an adsorption capacity of 1.16 mg/g, and a pseudo-second-order kinetic from aqueous solution. Optimal fluoride removal was observed in the
model. The desorption study showed that 91% of the adsorbent could pH range of 4–7 within 1.5 h of contact time. The equilibrium data fit
be regenerated using 0.1 M NaOH solution. the Langmuir isotherm model reasonably well, and the adsorption data
87
HAP = hydroxyapatite; CA = calcium aluminate material; Cal-Alg-Alu = alumina impregnated calcium alginate beads; CZ-CABs = Ce-Zr oxide nanospheres encapsulating calcium alginate beads; NCA = natural crystalline apatite; M-i-
HAP = magnesium incorporated hydroxyapatite; d-Hap = Ca-deficient hydroxyapatite; GLS = gypsiferous limestone; QL = quick lime; G-HAP = glass derived hydroxyapatite; CBSBB = charred beef shoulder blade bones; Al-HAP = aluminium-
were described well by the pseudo-second-order kinetic model. The
thermodynamic parameters indicated that the fluoride adsorption was
Jiménez-Reyes and Solache-Ríos,

endothermic and spontaneous. Equilibrium solution pH analyses sug-
gested that the fluoride adsorption onto HIACMO was due to the ion/
ligand exchange mechanism. The HIACMO could be regenerated with
Jayarathne et al., 2015
Islam and Patel, 2007
Gourouza et al., 2014

Mourabet et al., 2015
Sakhare et al., 2012
up to 90% recovery with 0.5 M NaOH. Kanrar et al. (2016) prepared a
Mondal et al., 2016

Gandhi et al., 2013
Liang et al., 2011

Chen et al., 2016
Basu et al., 2013
new adsorbent by incorporating graphene oxide into hydrous Fe (III)
Nie et al., 2012

Li et al., 2010
Melidis, 2015
eAl (III) mixed oxide (HIAGO) composite for the defluoridation of
Fufa, 2014
References
water. The nature and morphology of the new adsorbent were char-
acterised by XRD, SEM, transmission electron microcopy (TEM), FTIR,
2010
thermogravimetric analysis/differential thermal analysis (TGA/DTA),

and Raman spectroscopy. The adsorption of fluoride by HIAGO was
Pseudo-1st & 2nd order
highest at pH ~ 7 owing to electrostatic interaction between the

Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-2nd order
Psuedo-1st order
fluoride ions and the positive surface charge of the adsorbent. The
2nd order equilibrium data were described best by the Langmuir adsorption iso-
1st order
therm with a monolayer adsorption capacity of 27.8 mg/g at 318 K. The

Kinetic
adsorption reaction followed pseudo-second-order kinetics and was

endothermic in nature.
–
Alemu et al. (2014) synthesised an aluminium oxide – manganese

Langmuir & Freundlich

oxide (AOMO) composite material. The AOMO was prepared from

manganese (II) chloride and aluminium hydroxide. An adsorption ca-
pacity of 4.8 mg/g was achieved at an adsorbent dose of 4 g/L and
Freundlich
Freundlich
Freundlich
Langmuir
Langmuir
Langmuir
Langmuir
Langmuir
Langmuir
Langmuir
Langmuir
within the pH range of 5–7. The experimental data of fluoride ad-

Isotherm
sorption onto AOMO were described well by the Freundlich isotherm

model, and kinetic study revealed a non-linear pseudo-second-order
reaction model. Hydrous zirconium oxide modified with β-cyclodextrin
Regeneration
Optimum experimental conditions of various calcium-based adsorbents with their maximum adsorption capacities and fluoride removal efficiency.
(CY-HZO) was prepared by Saha et al. (2015) for efficient removal of

fluoride from aqueous solution. The characteristics of CY-HZO were
50%
84%
63%
91%
86%
6%
determined by XRD, SEM, TEM, atomic force microscopy (AFM), and

–
–
–
–
–
–
–
Brunauer–Emmett–Teller (BET) surface analysis. The adsorption data

Cl−, SO42 −, PO43 −, NO3−
HCO3−, PO43 −, SO42 −
followed the Langmuir isotherm model with an adsorption capacity of

PO43 −, SO42 −, NO3−
Interfering co-anions
31.45 mg/g. The reaction had pseudo-second-order kinetics and was

endothermic and spontaneous in nature. Approximately 95% re-
PO43 −, CO3−
generation from fluoride-loaded adsorbent could be achieved using 3 M

No effect
No effect
NaOH solution.
HCO3−
Patankar et al. (2013) used mixed alumina-magnesia hydroxide

adsorbent (PURAL® MG-20) for defluoridation of water. The maximum
–
–
–
–
–
–
–
absorption capacity achieved was 5.62 mg/g within 24 h of contact

(m2/g)
16.27
46.62
11.75
11.32
258.6
time at pH 7 for an initial fluoride concentration of 5.13 mg/L. The

SA
22
–
–
–
–
–
–
–
–
adsorption data were described well by the Langmuir isotherm and

24.5–26.5 °C
followed pseudo-second-order kinetics. Lanthanum hydroxide was

25 ± 2 °C
25 ± 2 °C
27 ± 1 °C
22–35 °C
30–50 °C
20–40 °C
synthesised and characterised for defluoridation by Na and Park

Temp
25 °C
25 °C
35 °C
25 °C
(2010). Batch tests were conducted to evaluate the influence of dif-

–
ferent parameters, such as pH, contact time, competing anions, ad-

(mg/L)
sorbent dose, temperature, and initial fluoride concentration, on the

1–100
11.28
100
IFC
8.9
4.6
10
15
10
20
50
10
10
adsorption process of the lanthanum hydroxide. The fluoride removal

2
–
efficiency of the lanthanum hydroxide was optimal at pH ≤ 7.5. The

20 min
10 min
75 min
presence of competing anions did not affect the adsorption process. The
1.5
4h
CT
16
24
12
12
5
3
–
Langmuir isotherm equation described the data well with a monolayer

7.4–8.2
adsorption capacity of 242.2 mg/g. The pseudo-second-order equation

12.71
5–7.3
3–11
4.16
12.5
6.72
4–7
3–5
pH
described all the kinetic data very well. Thermodynamic parameters

5
7
6
7
5
–
indicated that the adsorption reaction of fluoride was endothermic and

(g/L)
0.28
spontaneous. It was suggested that the adsorption of fluoride was due to

0.6
0.1
0.5
0.5
AD
10
15
3
2
5
5
8
–
the chemical process. Regeneration of fluoride-adsorbed material was

Removal
possible by washing with NaOH solution.

86.34
60.27
99.9
94.5
94.5
80.6
96.6
(%)
Layered double hydroxides (LDHs) have also been studied for their
90
96
85
78
65
85
89
75
fluoride adsorption potential. The efficiency of Li-Al layered double

(mg/g)
137.6
0.212
26.11
16.67
17.34
32.57
hydroxide was determined by Zhang et al. (2012), who reported a

3.65
4.37
1.16
3.12
12.4
1.07
3.15
4.7
AC
17
maximum removal of 97.36%. Koliraj and Kannan (2013) tested the

modified hydroxyapatite.
potential of nitrate-containing ZnCr layered double hydroxide (ZnCr3-

Chalk powder
Cal-Alg-Alu
Quick lime
NO3-LDH) and found that the maximum fluoride uptake capacity was
Adsorbent
CZ-CABs
M-i-HAP
Al-HAP
G-HAP
CBSBB
31 mg/g. The adsorbent was prepared by the co-precipitation method.

d-HAP
NCA
HAP
HAP
HAP
GLS
The authors reported that fluoride uptake by ZnCr3-NO3-LDH was

CA
constant within the pH range of 3–10 because of the buffering nature of

Table 3
S. no.
LDH. Mg-Cr-Cl layered double hydroxide was synthesised by Mandal

10.
11.
12.
13.
14.
15.
1.
2.
3.
4.
5.
6.
7.
8.
9.
et al. (2013) for fluoride adsorption from aqueous solutions. The
88
Fig. 3. Fluoride adsorption performance of various calcium-

140 100
based adsorbents along with removal efficiency.
90
120

80
100 70
80 60
50
60 40
40 30
20
20
10
0 0
Adsorbents
adsorbent was able to remove up to 88.5% of the fluoride at pH 7 with adsorbent dose, and 100 mg/L initial fluoride concentration. The ex-
an adsorbent dose of 0.6 g/100 mL solution and an initial fluoride perimental data best fit the Freundlich isotherm model and the pseudo-
concentration of 10 mg/L. The desorption study suggested that re- first-order kinetic equation. The thermodynamic parameters showed
generation of the adsorbent is not possible. Wang and He (2014) ex- that the reaction was endothermic and spontaneous. The probable
amined the adsorption performance of Ni-Al layered double hydroxide reason for fluoride uptake is an ion-exchange mechanism forming CaF2
prepared by the solvothermal method and found that it removed a by replacing hydroxide ions with fluoride ions from CaO nanoparticles.
considerable amount of fluoride. The structure of LDHs consists of po- A maximum 95% desorption can be achieved by adding 0.1 M NaOH or
sitively charged hydroxide sheets. When fluoride is removed from 0.1 M HCl to adjust the pH range to between 2 and 12. Devi et al.
aqueous solution by LDHs, the adsorption probably occurs primarily in (2012) investigated nano-sized magnesium oxide (nano-MgO) for
response to Coulomb attractions between the anionic adsorbent and the abatement of fluoride from water. The fluoride adsorption efficiency of
positively charged external and interlayer surfaces of LDHs via the nano-MgO was found to be 90% when using a 0.6 g/L adsorbent dose.
memory effect. Kameda et al. (2015) intercalated the Mg-Al layered Adsorption of fluoride by nano-MgO was influenced by the presence of
double hydroxide with nitrate (NO3·Mg-Al LDH) and chloride (Cl·Mg-Al OH– ions. The presence of other ions and variations in pH reduced the
LDH). The maximum adsorption was found to be 3.3 mmol/g and fluoride adsorption negligibly. The equilibrium data followed the
3.2 mmol/g for NO3·Mg-Al LDH and Cl·Mg-Al LDH, respectively. The Freundlich isotherm model more closely than the Langmuir isotherm
removal of fluoride was described by the Langmuir isotherm and model, which suggested multilayer adsorption of the adsorbent fol-
pseudo-second-order kinetics. In another study, Elhalil et al. (2016), lowing pseudo-second-order kinetics. A regeneration study showed that
calcined Mg/Al layered double hydroxide (CLDH) was developed for 1 M HCl was the best eluent for fluoride removal with 95% desorption
defluoridation of water. The adsorbent was synthesised by the co-pre- capacity, followed by 2 M NaOH with 25% regeneration of the ad-
cipitation method, and an optimal pH of 6.85 was noted for maximum sorbent.
removal of fluoride. Adsorption by CLDH was strongly favoured by The fluoride removal efficiency of synthesised nano-alumina was
higher temperature. Cai et al. (2016) developed a new sorbent, LALDH- examined by Kumar et al. (2011) using batch tests as a function of pH,
201, by impregnating nanocrystalline Li/Al LDHs inside the commercial temperature, contact time, initial fluoride concentration, and co-ex-
polystyrene anion exchanger D201. The maximum capacity of fluoride isting anions. The structural characteristics of the nano-alumina were
adsorption onto LALDH-201 was 62.5 mg/g as calculated from the Sips analysed by SEM, XRD, EDX, and FTIR. The results showed that 14 mg/
model. The fluoride-loaded adsorbent could be regenerated easily using g of fluoride was removed at 25 °C and pH of 6.15. The adsorption
a mixture of 0.01 M NaOH and 1 M NaCl. A list of various oxide/hy- experiment data fit the Langmuir isotherm well and showed pseudo-
droxide and layered double hydroxide adsorbents from the literature second-order kinetics. The adsorption capacity of nano-alumina was
and their adsorption capacities and removal efficiencies under various found to be influenced by the presence of phosphate, sulphate, and
experimental conditions is provided in Table 4. The list shows that bicarbonate ions. Raul et al. (2012) prepared iron oxide-hydroxide na-
metal and tri-metal oxide/hydroxides and layered double hydroxides noparticles for the removal of fluoride from water. The maximum ad-
have better fluoride removal potential than mono-oxide/hydroxides. sorption capacity of the adsorbent was found to be 16.70 mg/L. The
Fe–Mg–La tri-metal composite performed best, having an adsorption equilibrium adsorption data were described well by the Freundlich
capacity of 270 mg/g (Fig. 4), due to ion-exchange between sulphate isotherm model. The adsorption capacity of the prepared nanoparticles
anions in the adsorbent and fluoride anions in water. FTIR study con- was reduced due to the presence of hydroxide ions; however, the pre-
firmed the abundant existence of sulphate and hydroxide anions in the sence of other co-anions did not have a significant effect. It was re-
adsorbent. ported that the fluoride uptake by iron oxide-hydroxide nanoparticles is
due to physical adsorption and ion exchange mechanisms. The nano-
3.5. Nanoparticles particles could be regenerated easily (up to 76%) using 0.2 M and 0.5 M
sodium hydroxide as eluent.
The emergence of nanotechnology in recent years has prompted all- Wen et al. (2015) developed a three-element adsorbent, nano-
encompassing research. Nanotechnology involves the synthesis, devel- magnetite graphite-La (γ-Fe2O3-graphite-La, MGLNP), using magnetic
opment, and characterisation of nano-sized particles (1–100 nm) and graphite nanoparticles. The maximum adsorption capacity of MGLNP
has become one of the most active avenues of research for purifying was observed to be 77.12 mg/g. As the pH decreased, adsorption by
contaminated water. Patel et al. (2009) synthesised CaO nanoparticles MGLNP increased. The adsorption onto MGLNP was described well by
with the sol-gel method. The maximum observed adsorption of fluoride the Langmuir isotherm, and kinetic study revealed that the adsorption
by the CaO nanoparticles was 92% at 30 min of contact time, 0.6 g/L followed pseudo-first-order kinetics. Magnesium oxide-coated
89
Table 4
Optimum experimental conditions of various oxide/hydroxide/layered double hydroxide adsorbents with their maximum adsorption capacities and fluoride removal efficiency
K.K. Yadav et al.
(mg/g) (%) (g/L) (mg/L) (m2/g)
1. GFH 7 95 10 4–8 24 h 20 25 °C – CO3− – Langmuir Psuedo-1st order Kumar et al., 2009

2. Ce/Fe MO 60.97 90 0.5 2.9–10.1 40 min – 20 °C 164.9 HCO3−, CO3−, SO42 −, – Langmuir Psuedo-2nd order Tang and Zhang, 2016
NO3−
3. MALMHO 1.37 98.28 2 7 40 min 10 25 °C 169 CO3−, HCO3−, HPO42 −, 95.71% Langmuir Psuedo-2nd order Hussain et al., 2015
Cl−,
NO3−, SO42 −
4. HIACMO 31.89 95.7 4 5.6 90 min 10 30 ± 1 °C – – 91% Langmuir Psuedo-2nd order Biswas et al., 2010
5. AOMO 18.6 96.4 4 5–7 2h – 23 ± 2 °C 30.7 – – Freundlich Psuedo-2nd order Alemu et al., 2014
6. CY-HZO 31.45 94 0.5 5.5 2h 10 30 °C 0.53 PO43 −, SO42 − 95% Langmuir Psuedo-2nd order Saha et al., 2015
7. PURAL® MG-20 5.62 90 6 7 24 h 5.13 32 ± 2 °C 260 HCO3−, CO3−, SO42 − 98% Langmuir Psuedo-2nd order Patankar et al., 2013
8. Li-Al LDH 47.24 97.36 2 7 1h 20 25 °C 37.24 PO43 −, HPO42, CO32 −, – Freundlich Psuedo-2nd order Zhang et al., 2012
HCO3−
9. ZnCr3-NO3 LDH 31 95 1 3–10 3h 10 – 12 CO3−, SO42 −, OH−, 80% Langmuir Psuedo-2nd order Koliraj and Kannan,
2013
10. Mg-Cr-Cl LDH 13.15 88.5 6 6 40 min 10 25 ± 2 °C – PO43 −, SO42 −, NO3− 2% Langmuir 1st order Mandal et al., 2013
11. Calcined LDH 79.37 99.2 0.5 6–7 1h 10 25 °C – Cl−, SO42 − – Langmuir – Sadik et al., 2014
12. Ni-Al LDH 93.41 37.12 2 3–7 30 min 60 30 °C 39.87 NO2−, HPO42 −, CO3−, – Psuedo-2nd order Lu et al., 2014
SO42 −
13. Mg-Al LDH 55.22 93.92 4 7 2.5 h 10 25 °C – – 86% Freundlich Psuedo-2nd order Bo et al., 2016
14. Ca-Al-La 29.30 96.65 1 6.8 3h 10 25 °C – PO43 −, SO42 −, HCO3− – Langmuir Psuedo-2nd order Xiang et al., 2014
15. AH 7 90 1.6 – 1h 20 23 ± 2 °C – – – Freundlich Psuedo-2nd order Shimelis et al., 2006
90
16. Fe-Al-Ce TMO 178 90 0.15 7 – 84.5 25 °C 114 NO3−, PO43 −, AsO3− 97% Langmuir – Wu et al., 2007
17. TMO 2.19 78 1 7 70 min 10 30 ± 2 °C – – – Langmuir Psuedo-2nd order Rafique et al., 2014
18. HBO 1.93 65 50 4–12 3h 10 25 ± 2 °C 76.04 HCO3−, SO42 −, Cl− – Langmuir Psuedo-2nd order Srivastav et al., 2013
19. CTDO 15.35 75 1 7–8 3h 5 25 °C 56 HCO3− 79% Langmuir Psuedo-2nd order Babaeivelni and
Khodadoust, 2013
20. HITMO 10.47 95.36 0.8 6.4 2h 25 55 °C 127 HCO3− 75% Langmuir Psuedo-2nd order Biswas et al., 2009
21. α-FeOOH@rGO 24.67 87.4 – 10.90 1h 50 25 °C – F−, SO42 −, Cl−, NO3−, 95% Langmuir Psuedo-2nd order Fan et al., 2016
SiO32 −,
PO43 −, HCO3−
22. MMA (Fe3O4@Fe-Ti) 41.8 92.4 1 3 2 4 – 99.2 Cl−, NO3−, SO42 −, 96.8% Langmuir Quasi-2nd order Zhang et al., 2016a
SiO32 −,HCO3−, PO43 −,
Ca2 +, Mg2 +
23. Fe–Mg–La TMC 270 85 0.1 4 5h 10 25 °C – SO42 −, HCO3− 96.7% Langmuir – Yu et al., 2015a
GFH = granular ferric hydroxide; Ce/Fe MO = Ce-Fe bimetal oxide; MALMHO = magnesium-aluminum-lanthanum triple metal hydrous oxide; HIACMO = hydrated iron-aluminum-chromium ternary mixed oxide; AOMO = aluminium oxide-
manganese oxide; CY-HZO = β-Cyclodextrin modified hydrous zirconium oxide; PURAL® MG-20 = alumina-magnesia mixed hydroxide; LDH = layered double hydroxide; Ni-Al LDH = non-thermal plasma modified Ni-Al layered double hy-
droxide; Mg-Al LDH = calcined Mg-Al layered double hydroxide; Ca-Al-La = calcium-aluminum-lanthanum composite; AH = aluminium hydroxide; Fe-Al-Ce = iron-aluminium-cerium trimetal oxide; TMO = trimetal oxide; HBO = hydrous
bismuth oxide; CTDO = crystalline titanium dioxide; HITMO = hydrous iron-tin mixed oxide; α-FeOOH@rGO = goethite anchoring regenerated graphene oxide; MMA = micron-sized magnetic adsorbent; Fe-Mg-La TMC = Fe-Mg-La trimetal
composite.
Fig. 4. Fluoride adsorption performance of various oxide/hy-

300 100
droxide-based and layered double hydroxide adsorbents along
90 with removal efficiency.
250
Fluoride removal (%)

80
Adsorption Capacity (mg/g) 70
200
60
150 Fluoride Removal Efficiency (%) 50
40
100
30
20
50
10
0 0
GFH
Ce/Fe MO
MALMHO
HIACMO
AOMO
CY-HZO
PURAL® MG-20
Li-Al LDH
ZnCr3-NO3 LDH
Mg-Cr-Cl LDH
Calcined LDH
Ni-Al LDH
Mg-Al LDH
Ca-Al-La
AH
Fe-Al-Ce TMO
TMO
HBO
CTDO
HITMO
MMA
Fe-Mg-La TMC
Adsorbents
magnetite nanoparticles were developed by Minju et al. (2013) using was spontaneous in nature. Chinnakoti et al. (2016a) also developed a
the sol-gel method. The adsorbent efficiently removed up to 98.5% of nano-sized gamma alumina using the surfactant-assisted combustion
fluoride at pH 6 within 2 h using an adsorbent dose of 2 g/L for an method. Nano gamma alumina was efficient at reducing fluoride levels
initial fluoride concentration of 13.6 mg/L. The equilibrium data were by up to 96% (from 10 mg/L to 0.3 mg/L) within 1 h using 1 g/L ad-
described well by the Langmuir isotherm model. The kinetic data best sorbent dose at pH 4. The experimental data followed the Langmuir
fit pseudo-second-order kinetics. Rout et al. (2015) compared the isotherm and pseudo-second-order kinetics. It was reported that ad-
fluoride adsorption capacities of iron oxide-based nanocomposite sorption of fluoride onto the nano-sized gamma alumina was due to
(IBNC), titanium-based nanocomposite, and micro-carbon filter. The surface adsorption and intra-particle diffusion. The regeneration study
maximum adsorption capacity was attained by IBNC (97%), followed suggested that nano gamma alumina could be regenerated using 0.1 M
by titanium-based nanocomposite (92%) and micro-carbon fibre (88%). sodium hydroxide as eluent.
The same trend was apparent for both the adsorption capacity and the Carbon nanotubes have been used extensively in recent years due to
adsorption intensity. The adsorption data best fit the Freundlich iso- their controlled pore size distribution, high surface active sites, small
therm model. Wang et al. (2009) examined fluoride adsorption onto size, and hollow and layered structure. These are one of the allotropes
nanoscale aluminium oxide hydroxide (nano-AlOOH). A batch experi- of carbon and are composed of a cylindrical shape rolled up in a tube-
ment was performed to study the effects of various parameters, such as like structure (Santhosh et al., 2016). Balarak et al. (2016) investigated
pH, contact time, adsorbent dose, initial fluoride concentration, and the potential of single-walled carbon nanotubes (SWCNTs) for fluoride
competing anions, on the adsorption process. The maximum adsorption adsorption from water. The adsorbent was efficient at removing
by nano-AlOOH was 3259 mg/kg at pH 7. The presence of sulphate and fluoride in the range of 87–100% with an adsorption capacity of
phosphate ions was found to considerably reduce fluoride adsorption. 50–150 mg/g. The adsorption process was endothermic and sponta-
The adsorption data followed pseudo-second-order kinetics and the neous. A maximum adsorption capacity of 4.42 mg/g was reported for
Langmuir isotherm model. A desorption study revealed that fluoride zirconia/multi-walled carbon nanotubes (Ramamurthy et al., 2011).
could be desorbed easily at pH 13. Various experimental conditions such as adsorbent dose, pH, con-
Bazrafshan et al. (2016) synthesised cupricoxide (CuO) nano- tact time, and initial fluoride concentration for various nano-adsorbents
particles to analyse their fluoride scavenging potential. The maximum have been compiled from the available literature and are presented in
removal of fluoride was estimated to be 3152 mg/g for an initial Table 5. Adsorption capacities and fluoride removal efficiencies of the
fluoride concentration of 5 mg/L at pH 6, 0.005 g/L adsorbent dose, studied nano-adsorbents are presented as Fig. 5. It is evident from lit-
and 40 min of contact time. The Freundlich model was found to suitably erature survey that some nano-adsorbents, such as CaO nanoparticles
describe the adsorption equilibrium of fluoride onto CuO nanoparticles. (Patel et al., 2009), CuO nanoparticles (Bazrafshan et al., 2016), and Fe-
The kinetic study revealed that the adsorption data fitted well with Ce-Ni nanoparticles (Dhillon and Kumar, 2015), have good potential
second-order kinetics. Thermodynamic analysis suggested that the ad- compared to other adsorbents. Fe-Ce-Ni nanoparticles are considered
sorption process was endothermic and spontaneous. Nanoparticles of best, having the maximum adsorption efficiency of 285.7 mg/g due to
gamma alumina (NPGA) were investigated by Singh et al. (2016) with the high surface area, high porosity, and high adsorbing efficiency at
the sol-gel process. The average particle size seemed to be 30 nm, as normal water pH of 7.
determined from the scale bar. The surface area determined from BET
analysis was 137 m2/g. The maximum adsorption capacity of NPGA for 3.6. Natural materials
fluoride removal was calculated to be around 14 mg/g. The adsorption
isotherm study showed that the adsorption data were more compatible A number of natural materials, including clay, soil, chitosan, and
with the Langmuir isotherm model. A new adsorbent, nano-sized hy- zeolite, have been used as adsorbents for removal of fluoride from
droxyapatite onto graphene oxide sheets (GOs-nHAp), was prepared by water. Natural clay minerals have been proven to be effective ad-
Prabhu et al. (2016) using the in situ co-precipitation fabrication sorbents due to their high surface area, chemical and mechanical sta-
method. The maximum monolayer adsorption capacity was calculated bility, molecular sieve structure, and variety of surfaces and structural
to be 44.068 mg/g. The adsorption data fit the Freundlich isotherm properties (Srinivasan, 2011; Vinati et al., 2015). The potential of using
well. The kinetics were described well by the pseudo-second-order rate clay and clay minerals for fluoride removal has been well documented
law. The thermodynamic parameters showed that the fluoride adsorp- by many researchers. Peter (2009) reported that soil rich in bauxite has
tion was due to the physical force of attraction and that the adsorption more adsorption capacity than soil rich in kaolinite. It was also reported
91
Table 5
Optimum experimental conditions of various nano-adsorbents with their maximum adsorption capacities and fluoride removal efficiency.
K.K. Yadav et al.
S. no. Adsorbent AC Removal AD pH CT IFC Temp SA Interfering Regeneration Isotherm Kinetic References
(mg/g) (%) (g/L) (mg/L) (m2/g) co-anions
1. CaO NPs 163.3 98 0.6 – 30 min – 25 ± 1 °C – SO42 −, NO3−, PO43 − 50% Freundlich Pseudo-1st order Patel et al., 2009
2. Nano-MgO 14 90 0.6 3–11 2h 10 25 ± 1 °C 92.46 OH−, SO42 −, HCO3−, 95% Freundlich Pseudo-2nd order Devi et al., 2012
Cl−
3. Nano-Al2O3 14 85 1 6.15 24 h 20 25 ± 2 °C 151.7 PO43 −, CO3−, SO42 − – Langmuir Pseudo-2nd order Kumar et al., 2011
4. Fe-OOH NPs 16.7 80 1 7.28 3h 10 25 °C 6.57 OH− 76% Freundlich – Raul et al., 2012
5. MgO-Fe2O3 NPs 10.96 98.6 2 6 2.5 h 13.6 28 ± 1 °C – – – Langmuir Pseudo-2nd order Minju et al., 2013
6. nano-AlOOH 3.25 90 5 5.2 6h 13 55 °C 240.38 SO42 −, PO43 − 97% Langmuir Pseudo-2nd order Wang et al., 2009
7. CuO NPs 357 89 0.05 5 80 min 20 22 ± 1 °C – – – Freundlich Pseudo-2nd order Bazrafshan et al., 2016
8. NPGA 14 85 0.1 7 1.5 h – 30 ± 2 °C 137 – – Langmuir Pseudo-2nd order Singh et al., 2016
9. GOs-nHAp 44.06 96.3 0.5 3–7 25 min 50 50 °C – Cl−, SO42 −, NO3−, 95% Freundlich Pseudo-2nd order Prabhu et al., 2016
PO43 −
10. Nano-ϒ-Al2O3 32 96 1 4 1h 10 30 ± 2 °C 221 – 98% Freundlich Pseudo-2nd order Chinnakoti et al., 2016a
11. NZVI 18.91 84 0.6 4 35 min – 25 ± 2 °C – – – Freundlich – Jahin, 2014
12. Fe-Ce-Ni NP 285.7 98.7 0.4 7 30 min 10 30 °C 436.8 PO43 − 95% Freundlich & Dubinin- Pseudo-2nd order Dhillon and Kumar,
Radushkevich 2015
13. n-Hap 2.30 91 4 5 100 min 10 25 °C – – – Langmuir & Freundlich Pseudo-2nd order Gao et al., 2009
14. MgO@Al2O3 37.35 90 3 6.8 8h 30 30 ± 2 °C 105 HCO3−, SO42 −, NO3−, – Langmuir & Freundlich Pseudo-2nd order Dayananda et al., 2015
Cl−
15. Nano-AlOOH 11.88 95 1.6 7 1h 20 22 ± 2 °C – – – Langmuir Pseudo-2nd order Adeno et al., 2014
16. α-FeOOH 59 49 1 – 2h 30 25 °C – Cl−, SO4− – Freundlich Psuedo-2nd order Mohapatra et al., 2010
17. Fe-Ti oxide 47 98.7 1 – 12 h 50 25 °C – O2− – Langmuir – Chen et al., 2012a,
nano-adsorbent 2012b
92
18. MWCNTs 3.5 94 – 5 18 min – – – Cl−, SO42 −, NO3−, Freundlich – Ansari et al., 2011
CO3−
19. SWCNTs 2.4 58 0.5 5 30 min 1 24 °C 700 – – Freundlich Psuedo-2nd order Dehghani et al., 2016
20. MWCNTs 2.83 55 0.5 5 30 min 1 24 °C 270 – – Freundlich Psuedo-2nd order Dehghani et al., 2016
21. SWCNTs 16.63 100 0.6 6 25 min 1 30 °C 712.9 OH− – Langmuir Psuedo-2nd order Balarak et al., 2016
22. ZrO2/MWCNTs 4.94 99.67 2 3 5h 10 25 °C 168.5 Cl−, SO42 −, NO3−, Small Regeneration Langmuir – Ramamurthy et al., 2011
HCO3−
23. SWCNTs 3.0 58 0.5 5 70 min 1 25 °C 700 – – – – Haghighat et al., 2012
24. TNTs 58 95 1 2 2h 10 80 °C 282 OH− – Langmuir Psuedo-2nd order Chinnakoti et al., 2016b
CaO NPs = calcium oxide nanoparticles; nano-MgO = nanomagnesium oxide; Nano-Al2O3 = nano-alumina; Fe-OOH NPs = iron oxide - hydroxide nanoparticles; MgO-Fe2O3 NPs = magnesium oxide coated magnetite nanoparticles; Nano-
AlOOH = nano-scale aluminium oxide hydroxide; CuO NPs = cuprioxide nanoparticles; NPGA = nanoparticles of gamma alumina; GOs-nHAp = nano-sized hydroxyapatite onto graphene oxide sheets; Nano-ϒ-Al2O3 = nano gamma alumina;
NZVI = nanoscale zero-valent iron; Fe-Ce-Ni NP = hybrid Fe-Ce-Ni nanoporous adsorbent; n-HAp = nano-hydroxyapatite; MgO@Al2O3 = MgO nanoparticles loaded with mesoporous alumina; α-FeOOH = nano-sized goethite; MWCNTs = multi-
walled carbon nanotubes; SWCNTs = single-walled carbon nanotubes; ZrO2/MWCNTs = zirconia/multi-walled carbon nanotubes; TNTs = tritinate nanotubes.
400 100 Fig. 5. Fluoride adsorption performance of various nano-ad-

sorbents along with removal efficiency.
350 90

80
300
70
250 60
200 50
150 40
30
100
20
50 10
0 0
Adsorbents
that activated soil has higher adsorption capacity than non-activated endothermic and spontaneous. MB could be regenerated up to 97%
soil in the case of both bauxite-rich and kaolinite-rich soil. Soil rich in using 1 M NaOH. Vhahangwele et al. (2014) prepared Al3 + modified
bauxite was found to be more efficient at low fluoride concentration bentonite clay (Alum-bent) by ion exchange of base cations. The ad-
than at high fluoride concentration. In another study (Gogoi and sorption capacity of the alum-bent was found to be 5.7 mg/g at room
Baruah, 2008), acid-activated kaolinite clay was found to be more ef- temperature. The adsorption data fit the Langmuir and Freundlich
fective than raw kaolinite clay due to its high adsorption. The kaolinite isotherms well, confirming monolayer and multilayer adsorption. In
clay was activated by concentrated sulphuric acid. The potential of another study, Gitari et al. (2015) developed Fe3 + modified bentonite
laterite soil for fluoride removal was assessed by Sarkar et al. (2006), clay capable of removing almost 100% of fluoride from groundwater.
who recorded 78.2% removal from 10 mg/dm fluoride solution. The The Fe3 + modified bentonite clay was found to be effective for high-
thermodynamic study revealed that the process was exothermic and fluoride groundwater samples.
spontaneous. Patnaik et al. (2016) determined the fluoride adsorption efficiency
Pyrophyllite clay, composed of Si (74.03%) and Al (21.20%), was of a chitosan iron complex in batch experiments. The highest removal
used by Kim et al. (2013), who found that the maximum adsorption rate of fluoride was found to be 2.34 mg/g using a 10 g/L adsorbent
capacity was 0.737 mg/g. The adsorption reaction was endothermic in dose for a 50 mg/L fluoride solution. The experimental data followed
nature. The presence of coexisting anions such as sulphate, carbonate, the Langmuir–Freundlich and D–R isotherm models. The adsorption
and phosphate affected the adsorption rate of pyrophyllite clay sig- data best fit pseudo-second-order kinetics. The thermodynamic study
nificantly; however, the effects of chloride and nitrate were negligible. revealed that the adsorption process is exothermic and spontaneous.
Ngulube (2016) used silica-rich reddish-black Mukondeni clay soil and The presence of sulphate ions followed by bicarbonate and nitrate ions
observed that the adsorption efficiency of Mukondeni clay soil was influenced the adsorption rate adversely. Bentonite/chitosan beads
optimal at 25 °C, pH 2, 1.5 g of adsorbent dose, 9 mg/L initial fluoride showed an adsorption efficiency of 0.895 mg/g (Zhang et al., 2013).
concentration, and 1 h of contact time. The activation energy was Adsorption onto the bentonite/chitosan beads followed the Freundlich
shown to be 58.86 kJ/mol by the Arrhenius equation, indicating that isotherm and pseudo-second-order kinetics. The adsorbent could be
the adsorption process was due to chemisorption. Montmorillonite clay regenerated using NaOH.
has also been used for fluoride removal (Karthikeyan et al., 2005; Tor, Zeolite and its modifications have also been used extensively by
2006). many researchers in recent years. Zeolites are crystalline alumino-sili-
Modification of clay using various metals to maximise adsorption cates consisting of three-dimensional frameworks of SiO4 and AlO4
efficiency has been tested by several researchers. Atasoy and Sahin tetrahedra linked through oxygen bridges (Dessalegne et al., 2016). A
(2014) reported that magnesium-enriched clay has a high adsorption chitosan-modified zeolite (Ch-Z) was synthesised by Peng et al. (2013)
efficiency compared to raw clay due to increased and stabilised positive and achieved a performance three times higher than the performance of
sites. The clay was modified by calcination. In another study (Zhang unmodified zeolite. Ch-Z had an adsorption capacity of 4.16 mg/g for
et al., 2016b), natural clay that was modified by lanthanum and alu- an initial fluoride concentration of 40 mg/L. The Freundlich and Red-
minium had an adsorption capacity of 1.30 mg/g. The modified clay lich–Peterson isotherm models fit best with the adsorption data. The
also demonstrated a good regeneration capacity of > 80% using KAl adsorption onto Ch-Z was due to intra-particle diffusion and followed
(SO4)2.12H2O. The adsorption data followed the Langmuir isotherm pseudo-second-order kinetics. Waghmare et al. (2015) modified zeolite
and pseudo-second-order kinetics. The highest recorded adsorption with calcium sulphate and aluminium sulphate for better adsorption of
capacity was 18.40 mg/g for a 1.25 g/L adsorbent dose using clay fluoride from water. The maximum adsorption by calcium-aluminium-
mixed with hydroxyapatite (C-HAp) as the adsorbent (Dang-i et al., modified zeolite (CAZ) was recorded to be 8.03 mg/g according to the
2015). The data obtained from adsorption onto C-HAp were described Langmuir isotherm, and the adsorption data fit the Freundlich isotherm
well by the Freundlich isotherm and pseudo-first and second-order ki- well. The adsorption process was endothermic and spontaneous. The
netic models. presence of competing anions did not affect the adsorption process.
Chemical modification of bentonite clay has also been reported by Teutli-Sequeira et al. (2015) used aluminium-modified zeolitic tuff and
some researchers. Thakre et al. (2010) used magnesium chloride to hematite in a column experiment at different bed depths. The highest
modify bentonite clay. They found a maximum fluoride removal ca- adsorption obtained was 3.24 mg/g and 2.37 mg/g for modified zeolitic
pacity of 2.26 mg/g, which was much better than the result for un- tuff and hematite, respectively, at 4 cm bed depth, 1 mL/min flow rate,
modified bentonite clay. The thermodynamic study revealed that the and 10 mg/L initial fluoride concentration. The thermodynamic study
adsorption onto the magnesium-incorporated bentonite clay (MB) was revealed that adsorption by both adsorbents was endothermic and
93
spontaneous and was controlled by physical adsorption. Zhou et al. multilayer sorption. Ranjeeta (2015) tested the fluoride scavenging
(2014) reported that zirconium-modified zeolite with a maximum ad- potential of fly ash generated from Chula. The maximum removal of
sorption rate of 97.62% was a more efficient adsorbent than raw zeolite fluoride occurred at an adsorbent dose of 100 g/L within a contact time
with a rate of 32.94% and acid-modified zeolite with a rate of 57.05%. of 2 h. The fly ash was found to be effective for high concentration
Salifu et al. (2016) used granular aluminium-coated bauxite (GACB) fluoride solution. This might be due to the presence of unburnt carbon
and achieved better adsorption of fluoride. The fluoride adsorption particles in the fly ash, which are known to be very efficient adsorbents.
capacity of GACB was reported to be 0.426 mg/g with thermal activa- Ramesh et al. (2012) investigated the adsorption potential of bottom
tion at 500 °C. The probable reason for fluoride uptake by GACB as ash for fluoride-contaminated water. The highest adsorption capacity
suggested by kinetic and isotherm analysis, thermodynamic calcula- was found to be 16.26 mg/g with 83.2% fluoride removal at pH 6. The
tions, FTIR, and Raman analysis was physical adsorption and chemi- adsorption data were described well by the Langmuir isotherm.
sorption processes. Chen et al. (2010) performed a batch experiment to Various industries produce large amounts of solid wastes as by-
study the adsorption capacity of ceramic adsorbent. The ceramic ad- products. These wastes have been used by several researchers as ad-
sorbent was prepared using Kanuma mud and zeolite. The ceramic sorbents for fluoride removal from aqueous solution. The fluoride re-
adsorbent was able to remove the fluoride at capacities up to 2.16 mg/ moval ability of sludge or waste residue produced from the alum
g. The adsorption data followed the Langmuir and Freundlich models manufacturing process was investigated by Nigussie et al. (2007). A
isothermally and the pseudo-second-order model kinetically. The ad- batch study was carried out to assess the influence of pH, contact time,
sorption was adversely affected by the presence of phosphate, followed adsorbent dose, initial fluoride concentration, and effect of co-anions on
by carbonate and sulphate. The adsorption capacity of fluoride by Ka- sorption rate of alum sludge. The maximum adsorption capacity was
numa mud was studied by Chen et al. (2011b) in batch and fixed-bed achieved at the pH range of 3–8, with an adsorbent dose of 16 g/L and
column experiments. The batch test results showed the highest ad- an initial fluoride solution with a concentration of 10 mg/L. The Du-
sorption capacity of Kanuma mud to be 3.06 mg/g in the pH range of binin–Radushkevick (D–R) isotherm model and second-order kinetics
5–7 and with 2 h of contact time. The adsorption data were described were found to fit the adsorption data. Higher concentration of bi-
well by the Freundlich isotherm and pseudo-second-order kinetics. The carbonate anions retarded the fluoride removal significantly, but others
fixed-bed column experiment showed that the breakthrough time and showed no effect. Biswas et al. (2016) compared the performance of
exhaustion time decreased with increasing flow rate, decreasing bed heat-activated Mahabir colliery shale (HAMBS550) with that of heat-
depth, and increasing influent fluoride concentration. The bed depth activate Sonepur Bazari colliery shale (HASBS550) for fluoride removal
service time model and the Thomas model best fit the experimental from potablewater. The collected adsorbents were activated at the high
data. An extensive list of adsorbents with the adsorption capacity, temperature of 550 °C to increase the adsorption efficiency prior to use
fluoride removal efficiency, applicable isotherm model, kinetic model, for remediation. The maximum fluoride removal was found to be 88.3%
and optimum experimental conditions of each is given in Table 6. The and 88.5% for HAMBS550 and HASBS550, respectively, at an adsorbent
highest potential for fluoride was reported to be almost 100% with dose of 70 g/L, initial fluoride concentration of 10 mg/L, and pH 3.
Fe3 + modified bentonite clay (Gitari et al., 2015), siliceous mineral FTIR analysis confirmed the elimination of hydroxyl ions on the
(Wambu et al., 2013), and alum-bent (Vhahangwele et al., 2014), as fluoride-loaded shale samples. Kinetic study revealed that the adsorp-
represented in Fig. 6. Alum-bent and Fe3 + modified bentonite clay tion data followed the pseudo-second-order model, which indicated
performed best due to their high cation exchange capacity and high chemisorptive binding of fluoride ion on the adsorbents. Hence, the
surface area. The absorbents were modified by introducing high ca- fluoride-loaded adsorbent could be used in road construction or any
tionic density species such as Fe3 + and Al3 + in exchange of low ca- other civil work.
tionic density species such as Na+, Mg2 +, and Ca2 +. Due to the high Lv et al. (2013) evaluated the potential of a zirconium hydroxide-
density of electronegative oxygen atoms associated with it, the siliceous modified red-mud porous material (Zr-RMPM) for fluoride removal
mineral was assumed to have negative surface charge, which favoured from aqueous solutions. The red mud was an industrial by-product. The
the acid activation of the adsorbent for anionic adsorption resulting in adsorption capacity of Zr-RMPM was calculated to be 0.6 mg/g at pH 3
increased surface positive charge and fluoride adsorption potential. and 60 min contact time. The sorption data were described well by the
Langmuir isotherm and the pseudo-second-order kinetic model. Deso-
3.7. Building material and industrial waste adsorbents rption of fluoride-loaded adsorbent with better than 90% recovery was
possible at pH 12 using NaOH solution. In another study, Islam and
Many building materials such as cement, concrete, brick powder, Patel (2011) tested the fluoride scavenging ability of thermally acti-
sand, and fly ash have been used by several researchers to examine their vated basic oxygen furnace (BOS) slag. The optimum conditions for
fluoride removal potential with or without treatment and modification. maximum removal of fluoride were contact time (35 min), pH (6–10),
Bibi et al. (2015) evaluated the ability of hydrated cement to remove adsorbent dose (5 g/L), and initial fluoride concentration (10 mg/L).
fluoride from potable water. The adsorbent was found to have a good The BOS potential for fluoride removal was noted to be 93%. The ad-
fluoride removal efficiency of 80% at pH 7 with 60 min contact time sorption process was spontaneous and endothermic, as revealed by
and 30 g/L adsorbent dose. The maximum fluoride uptake was calcu- thermodynamic study. The presence of phosphate ions, followed by
lated as 1.72 mg/g. Regeneration was not required because the ad- sulphate and nitrate ions, significantly retarded the fluoride adsorption
sorbent is very inexpensive. In another study, Kagne et al. (2008) in- on BOS. Aside from the detailed adsorbents, much research on using
vestigated the potential of hydrated cement at various time intervals for building materials and industrial waste materials for fluoride removal
the removal of excess fluoride from aqueous solution by using batch has been reported in the literature in the past few years, and some of
adsorption studies. They found that the hydrated cement showed sig- these results including the maximum adsorption capacity and fluoride
nificant fluoride removal over a wide range of pH (3 − 10). The ex- removal, optimum conditions, and applicable isotherm and kinetic
perimental data generated from batch adsorption experiments fit the model are listed in Table 7. Fig. 7 reveals that calcinated electro-
linearly transformed Freundlich and Langmuir isotherms well. coagulation sludge (Yilmaz et al., 2015) is the most-promising ad-
Togarepi et al. (2012) performed an experiment with sand for sorbent among all the studied adsorbents based on building materials
fluoride removal. The sand was treated thermally and chemically prior and industrial wastes. CES showed an excellent adsorption capacity of
to use as an adsorbent. Batch experiments were conducted to examine 44.25 mg/g and a potential of fluoride removal of 99.99% because the
the influence of various parameters, such as adsorbent dose, pH, and adsorption phenomenon is an ion-exchange mechanism on CES. The
initial fluoride concentration, on adsorption. The equilibrium data of adsorption was increased by acidic pH, smaller particle size, high initial
the activated sand were found to fit the Freundlich isotherm with fluoride concentration, and high adsorbent dose.
94
K.K. Yadav et al.
Table 6
Optimum experimental conditions of various natural materials with their maximum adsorption capacities and fluoride removal efficiency.
(mg/g) (%) (g/L) (mg/L) (m2/g)
1. Laterite soil 0.8461 80.4 1 6.8 − 20 303 K 12.97 cm2 − 80.4% Langmuir Psuedo-1st order Sarkar et al., 2006
2. Montmorillonite clay 1.836 97 70 2 50 min 3 298 K 750 HCO3− − Langmuir Intra-particle diffusion Karthikeyan et al., 2005
model
3. C-Hap 18.409 96.5 1.25 6 30 min 5 − − − − Freundlich Psuedo-2nd order Dang-i et al., 2015
4. MB 2.26 95.47 3 3–10 12 h 5 25 ± 2 °C − HCO3− 97% Langmuir Psuedo-1st order Thakre et al., 2010
5. Alum-bent 5.7 99.97 10 2–12 30 min 60 26 ± 2 °C 44.3 − − Langmuir & Freundlich Psuedo-2nd order Vhahangwele et al., 2014
6. Fe3 + − bent 2.91 100 20 2–10 30 min 10 26 °C 49.95 − − Langmuir − Gitari et al., 2015
7. CIC 2.34 90 10 3–10 1.5 h 50 − − NO3−, HCO3−, SO42 − 84.3% L–F & D–R Psuedo-2nd order Patnaik et al., 2016
8. CAZ 8.03 96 1.5 4–8 6h 10 25 °C 65.69 CO3−, HCO3−, PO43 − 90% Freundlich Psuedo-2nd order Waghmare et al., 2015
9. Zr-Zeolite 28.57 97.62 10 7 1h 40 Room 36.49 Cl−, SO42 −, HCO3− − Freundlich Psuedo-2nd order Zhou et al., 2014
10. GACB 0.426 94.1 10 2 176 h 5 20 °C 174 SO42 −, HCO3− − Langmuir Psuedo-2nd order Salifu et al., 2016
11. Ceramic adsorbent 2.16 94.4 20 4–11 48 h 10 30 °C 80.94 SO42 −, CO3−, PO43 − − Langmuir & Freundlich Psuedo-2nd order Chen et al., 2010
12. Kanuma mud 3.06 90 20 5–7 2h 10 30 ± 1 °C 144.01 − 85% Freundlich Psuedo-2nd order Chen et al., 2011b
13. Pyrophyllite 2.2 85 4 4.9 2 10 24 ± 2 °C 424 − − Langmuir Psuedo-2nd order Goswami and Purkait, 2011
14. AC-Na+ 1.013 88 4 4 30 min 6 − − − − Langmuir − Ramdani et al., 2010
15. ANC-Na+ 1.324 88 3 4 30 min 6 − 110 − − Langmuir − Ramdani et al., 2010
16. Calcined meixnerite 16.1 97.3 − − 50 h 75 20 ± 2 °C − − 61% Langmuir & Freundlich 1st order Guo and Reardon, 2012
17. ATRL 39.10 96 0.5 5 24 h 10 305 K 371–382 HCO3−, PO43 − 96% Langmuir & Freundlich Shrinking core model Maiti et al., 2011
95
18. Brushite 36.26 90.83 1.5 5.36 2h 49.06 31.96 °C − − − Langmuir Psuedo-2nd order Mourabet et al., 2017
19. Pumice 0.31 85.75 20 7 3h 7 25 °C − − − Freundlich Psuedo-2nd order Malakootian et al., 2011
20. HMP 11.765 70.8 6 6 210 min 10 293–333 K 53.11 SO42 −, NO3− 98% Freundlich Psuedo-2nd order Sepehr et al., 2013
21. ATDE 51.1 92 500 3.4 10 min 400 298 ± 0.5 K − Cl− − Langmuir − Wambu et al., 2011
22. Mg/Ce/Mn OMDE 12.633 97.1 6 6.52 1h 40 24 5.9056 CO3−, Cl−, NO3−, SO42 − 60.8% Langmuir Psuedo-2nd order Gitari et al., 2017
23. Raw bauxite 0.275 96.1 120 5.5 90 min 6.17 283–308 K − − − − 1st order Kayira et al., 2014
24. LIB 18.18 99 2 8.5 120 20 25 14 Cl−, SO42 −, PO43 −, 95% Langmuir Psuedo-2nd order Vivek Vardhan and
HCO3−, NO3− Srimurali, 2016
25. Iron ore 1.72 89 5 6 2h 14.22 22 ± 2 °C − PO43 −, CO3−, HCO3− − Freundlich Psuedo-1st order Kebede et al., 2014
26. Siliceous mineral 12.4 100 500 3.4 20 min 200 293 K − Cl−, SO42 −, NO3− Langmuir − Wambu et al., 2013
27. Montmorillonite 0.263 65 8 6 3h 4 25 °C 18.5 − 93% Freundlich Non-linear Tor, 2006
28. Clay soil 93.45 85 – 3 24 h − − 35.4 − − Langmuir − Hamdi and Srasra, 2009
29. Vermiculite 2.36 51 2 4 70 min 4 25 °C − − − Freundlich Psuedo-2nd order Ologundudu et al., 2016
30. Bauxite 33.6 93.8 12.5 4 2h 8 − − − − Langmuir Psuedo-1st order Sajidu et al., 2008
(Gibbsite and Kaolinite)
C-Hap = Clay mixed hydroxyapatite; MB = Magnesium incorporated bentonite clay; Alum-bent = Al3 + modified bentonite clay; Fe3 +-bent = Fe3 + modified bentonite clay; CIC = Chitosan iron complex; CAZ = Calcium aluminium modified
zeolite; Zr-Zeolite = Zirconium modified zeolite; GACB = Granular aluminium coated bauxite; AC-Na+ = Sodium activated Algerian montmorillonite clay having calcium; ANC-Na+ = Sodium activated Algerian non-calcium montmorillonite
clay; ATRL = Acid treated raw laterite; HMP = Hydrogen peroxide modified pumice; ATDE = Acid treated diatomaceous earth; Mg/Ce/Mn OMDE = Mg/Ce/Mn oxide-modified diatomaceous earth; LIB = Lanthanum-impregnated bauxite.
Fig. 6. Fluoride adsorption performance of various natural ma-

100 100
terials-based adsorbents along with removal efficiency.

90 90

80 80
70 70
60 60
50 50
40 40
30 30
20 20
10 10
0 0
Adsorbents
3.8. Agricultural and biomass-based adsorbents irreversible. The authors suggested that PDPC can be used for removal
of fluoride from contaminated water or wastewater having low pH.
Agricultural and biomass wastes have been used tremendously by Pandey et al. (2012) prepared a biomass from Tinospora cordifolia for
researchers in recent years for removal of fluoride from aqueous solu- defluoridation of potablewater. The optimum conditions were found to
tion. These are used very much in practise due to their abundant be pH 7, adsorbent dose of 7 g/50 mL, and 120 min of contact time. The
availability, economic feasibility, and biodegradability in nature. Many experimental data were described well by the Langmuir and Freundlich
researchers have modified the plant-based adsorbents with a suitable isotherms. FT-IR spectrum analysis showed that the fluoride binding
chemical to enhance fluoride removal efficiency. Singh et al. (2015) was in different frequency ranges. Paudyal et al. (2011) used orange
carried out a batch adsorption experiment to study the fluoride ad- waste to explore a new biosorbent called Al3 + loaded saponified orange
sorption capacity of sugarcane bagasse. The effects of various para- juice residue (Al-SOJR). The adsorbent was prepared by chemical
meters such as pH, contact time, adsorbent dose, temperature, and in- modification, saponification, and metal loading. Al-SOJR was found to
itial fluoride concentration were studied. The maximum uptake of be a promising adsorbent for removal of fluoride at pH 6 and contact
fluoride was found to be 4.12 mg/g. The Redlich–Peterson isotherm time of 4 h. The fluoride scavenging potential of Al-SOJR was 1.03 mg/
model and the pseudo-second-order equation described the adsorption g. The isotherm model suggested that the Langmuir isotherm with
rate of fluoride well. The adsorption process was found to be en- monolayer sorption was the best fit with the equilibrium data. The
dothermic, as revealed by thermodynamic study. Dwivedi et al. (2014b) presence of sulphate and bicarbonate ions retarded the fluoride removal
used mosambi peel powder (Citrus limetta) as an adsorbent for fluoride capacity to some extent. Complete elution of adsorbed fluoride was
removal from water. The maximum uptake of fluoride was achieved at achieved using NaOH solution.
neutral pH with a 10 g/L adsorbent dose within a contact time of Dwivedi et al. (2014a) focused on peepal leaf powder (Ficus re-
30 min. The Langmuir isotherm was the best fit with the equilibrium ligiosa) as an adsorbent for sorption of fluoride from aqueous solution.
data, and kinetic study showed that the pseudo-second-order equation The maximum adsorption capacity of peepal leaf powder was 2.24 mg/
was the best fit for the adsorption data. Sanchez-Sanchez et al. (2013) g at neutral pH and 45 min of contact time. The adsorption data were
prepared mechanically modified guava seeds (MGS) for removal of described well by the Freundlich isotherm. In another study, Yu et al.
fluoride ions from aqueous solution. The maximum removal of fluoride (2015b), an innovative lanthanum-modified carbon (LMC) adsorbent
using MGS was noted to be 85% with an adsorption capacity of rooted in Sargassum sp. was developed for removal of fluoride. A
15.6 mg/g. Particle size was a remarkable parameter as the adsorption maximum fluoride removal efficiency of 90% was achieved at neutral
was found to increase with decreasing particle size. The authors re- pH, 0.4 g adsorbent dose, and 4 h of contact time. Significant inter-
commended this adsorbent for domestic systems because of its low cost ference of co-anions, such as SiO32 −, SO42 −, HCO3−, PO43 −, and
and high efficiency of fluoride removal. AsO3−, was noticed on fluoride uptake of LMC. The study suggested
A plant material, Cuminum cyminum, was used to synthesise a new that LMC has marked potential in industrial applications. Kumari et al.
adsorbent called restructured lignite (RSL) (Msagati et al., 2014). The (2015) explored the fluoride removal efficiency of sal (Shorea robusta)
adsorbent was characterised by BET, SEM, and XRD. The BET surface leaf powder by conducting a batch study. The sal leaf powder was
area of RSL was found to be 3.12 times greater than that of lignite. The characterised by SEM, FTIR, and EDX to obtain a good understanding of
fluoride scavenging ability of RSL (15.8 mg/g) was also greater than the adsorption mechanism. An excellent fluoride potential of 98.6%
that of lignite (13.8 mg/g). RSL was efficient for removing up to 60% was observed at pH 7.5 and 60 g/L adsorbent dose. The isotherm study
fluoride, even after the fifth cycle of regeneration. Desorption experi- suggested that the Freundlich isotherm with multilayer sorption fit best
ments were conducted using 0.01 M NaOH. Ajisha and Rajagopal with the experimental data.
(2015) developed a new material, pyrolyzed Delonix regia pod carbon Many more adsorbents have been studied for their fluoride removal
(PDPC), for sequestration of fluoride from water. The defluoridation uptake and efficiency at optimum conditions, and these are given in
potential of PDPC was found to be 97% at pH 2 with a 1.5 g/100 mL Table 8. The up-to-date list was prepared from data available in the
adsorbent dose and 300 min of contact time. The Freundlich isotherm scientific literature, and these data were compared to find the adsorbent
model best fit the experimental data, which indicated multilayer sorp- having the maximum defluoridation potential. Fig. 8 shows that lan-
tion of fluoride onto PDPC. Thermodynamic parameters showed that thanum-modified carbon from Sargassum sp. (Yu et al., 2015b) has the
the adsorption process is spontaneous, endothermic, stable, and maximum adsorption capacity of 94.34 mg/g among the studied
96
hydroxide modified red mud porous material; BOF slag = basic oxygen furnace slag; CES = calcinated electrocoagulation sludge; BCC = broken concrete cubes; SPUF = surface-functionalized polyurethane foam; CFA = calcium hydroxide treated
HC = hydrated cement; Alum-sludge = sludge produced from manufacturing of aluminium sulphate (alum); HAMBS550 = heat-activated Mahabir colliery shale; HASBS550-heat-activated Sonepur Bazari colliery shale; Zr-RMPM = zirconium
adsorbents. This result is justified by the fact that an exchange process
Bandyopadhyay et al., 2009

might have occurred between lanthanum ions and hydrogen ions
Geethamani et al., 2014

Krupadam et al., 2010
Islam and Patel, 2011
during the lanthanum solution modification process, and the removal of
Togarepi et al., 2012
Nigussie et al., 2007

Ramesh et al., 2012
Kumari et al., 2013

Yilmaz et al., 2015
Biswas et al., 2016
Biswas et al., 2016
Bibi et al., 2015 fluoride was increased due to the presence of lanthanum ions through
Lv et al., 2013
electrostatic attraction and Lewis acid-base interaction. The highest
fluoride removal efficiency of 98.6% was achieved by sal leaf powder
References
due to its increased surface area for adsorption (Kumari et al., 2015).
4. Factors affecting the process of defluoridation

order
order
order
order
order
order
order
The adsorption of fluoride ions depends on various parameters such
Pseudo-2nd
Pseudo-2nd
Pseudo-2nd
Pseudo-2nd
Pseudo-2nd
Pseudo-2nd
Pseudo-2nd
as solution pH, initial fluoride concentration, surface area of adsorbent,
2nd order
Kinetic
adsorbent dose, contact time, and temperature. The adsorption process

is assumed to be complete when equilibrium is achieved between
–
–
–
–
–
fluoride ions and adsorbent (Kumar et al., 2016; Sivarajasekar et al.,

Dubinin – Radushkevick
Freundlich & Langmuir
2017).
4.1. Temperature
Freundlich
Freundlich
Freundlich
Freundlich
Langmuir
Langmuir
Langmuir
Langmuir
Langmuir
Isotherm
Temperature plays a double role in the fluoride adsorption process.

Bradley
It can impact the physical binding processes of fluoride to adsorbent

–
and it may also alter the physical properties of an adsorbent if the

Optimum experimental conditions for various building materials and industrial wastes with their maximum adsorption capacities and fluoride removal efficiency
adsorbent is treated thermally prior to use (Sharma et al., 2017). Most

Regeneration
existing adsorption studies were conducted at room temperature in

laboratory settings. Adsorption was shown to be less favoured at high
90%
96%
0%
0%
4%
temperature, most likely due to increased deprotonation or hydro-

–
–
–
–
–
–
–
–
xylation of the surface, causing more negatively charged adsorbent

PO43 −, SO42 −, NO3−
surfaces (Sreenivasa et al., 2015). In addition, changing the tempera-

Interfering co-anions
ture changes the equilibrium capacity of the adsorbent for a particular

HCO3−, CO3−
adsorbate (Bakas et al., 2014; Elhalil et al., 2016).
4.2. pH
CO3−
CO3−
–
–
–
–
–
–
–
–
The pH is an important factor affecting defluoridation at water-

(m2/g)
adsorbent interfaces (Ma et al., 2009). Some adsorbents perform well in

11.02
8.39
144
acidic media, some perform best at neutral pH, and some adsorbents
SA
–
–
–
–
–
–
–
–
–
–
work well over a wide range of pH (3 −11). This can be explained by

25 ± 3 °C
22 ± 2 °C
the change of surface charge of an adsorbent. The surface of an ad-

30–50 °C
sorbent is highly protonated in highly acidic media, but it is neutralised

Room
293 K
Temp
25 °C
30 °C
30 °C
25 °C
13 K
and tends to have negative charge in alkaline media. Therefore, the

–
high efficiency in acidic media can be attributed to a gradual increase in

(mg/L)
attractive forces, and the low efficiency in alkaline media can be ex-
IFC
2.5
plained by the repulsion between the negatively charged surface and

10
10
10
10
20
10
25
20
10
5
8
–
fluoride (Rajan and Alagumuthu, 2013). The most plausible reaction for
105 min
fluoride removal is the following (Teng et al., 2009; Thakre et al., 2010;
35 min
24 h
24 h
24 h
Xiang et al., 2014; Yilmaz et al., 2015):

1h
1h
1h
1h
2h
2h
1h
2h
CT
MOH + H+ ⇔ MOH2+ (1)

6–10
3–8
6.3
6.0
7.0
6.7
pH
MOH2+ + F− ⇔ MF + H2 O (2)
7
3
3
3
7
3
(g/L)
Total equation can be written as follows:

240
0.7
3.5
AD
30
16
70
70
60
10
fly ash; HAS = heat activated shale (Coal mine waste).

5
4
3
–
MOH + H+ + F− ⇔ MF + H2 O (3)
Removal
MOH + F− ⇔ MF + OH− (4)

99.99
95.46
83.2
88.3
88.5
(%)
80
90
85
90
93
80
96
80
where M = metal ion.

Eq. (3) expresses that fluoride removal is enhanced at acidic pH
(mg/g)
16.26
0.358
2.154
44.25
10.86
0.358
because the formation of positively charged surface sites is maximal at

1.72
10.3
0.53
8.07
1.23
0.6
7.8
AC
acidic pH, which attracts more negatively charged fluoride ions by

electrostatic attraction. Contrary to this, fluoride adsorption decreases
Activated sand
at high solution pH because of strong competition between hydroxide

Alum-sludge
Bottom ash
HAMBS550
Adsorbent
HASBS550
Zr-RMPM
ions and fluoride ions for active adsorption sites. At alkaline pH,
BOF slag
fluoride is adsorbed by an ion-exchange mechanism (Eq. (4)). Tables 2

SPUF
HAS
BCC
CFA
CES
HC
and 7 show that most of the carbon-based adsorbents and industrial

waste materials, respectively, have maximum adsorption in acidic
Table 7
S. no.
media. The adsorbents that give optimum results at neutral pH are

10.
11.
12.
13.
1.
2.
3.
4.
5.
6.
7.
8.
9.
considered to be the best adsorbents for field studies.
97
Fig. 7. Fluoride adsorption performance of various building

50 100
material and industrial waste-based adsorbents along with re-
45 90

moval efficiency.
40 80
35 70
30 60
25 50
20 40
15 30
10 20
5 10
0 0
Adsorbents
4.3. Adsorbent dose stage and adsorption can take place rapidly, but the number of re-
maining active sites of the adsorbent are decreased after equilibrium
Fluoride removal increases with increased adsorbent dose due to and these sites are occupied with difficulty due to the repulsive forces
greater surface area and availability of more adsorption sites (Sakhare between the solute molecules on the solid and the bulk phase
et al., 2012; Mourabet et al., 2015; Mandal et al., 2013). However, no (Kalavathy and Miranda, 2010). Fluoride adsorption of magnesia-
significant adsorption occurs beyond the optimum dose of an ad- loaded mesoporous alumina increased from 12% to 76% as contact time
sorbent. This can be attributed to either of two reasons: (i) a decrease in was increased from 15 min to 480 min. The adsorption was very rapid
surface area and overlapping of active sites at higher doses (Joshi et al., during the first 120 min, but no appreciable removal occurred beyond
2012; Rajan and Alagumuthu, 2013; Mandal et al., 2013) or (ii) limited this time. The initial rapid adsorption occurred because of the exchange
availability of fluoride with respect to the adsorbent dose (Koliraj and of fluoride ions with surface hydroxyl ions of the adsorbent, and the
Kannan, 2013). Jagtap et al. (2011) noticed that fluoride removal in- slow adsorption in the later stage was due to the gradual uptake of
creased from 13% to 85% with an increase in adsorbent dose from 0.2 fluoride at the inner surface of adsorbent (Dayananda et al., 2015). The
to 6 g/L. However, only a slight improvement occurred after the dosage optimum contact times for various adsorbents, the times at which they
of 4 g/L. Sakhare et al. (2012) observed that the percentage of fluoride reach equilibrium and achieve maximum adsorption, are given in
removal increased from 61.34% to 88.16% with increase in CA dose Tables 1 to 8.
from 0.5 to 4 g/L. This was a result of an increase in active sites. Sai
Sathish et al. (2008) also reported that fluoride uptake increased with
4.6. Presence of co-anions
increased ZICFC dosage up to 20 g/L and that no significant removal
occurred beyond this optimum dose. Fluoride uptake was increased by
Fluoride-contaminated water is generally associated with other co-
increasing to an optimum ZILSSAC dosage of 2 g/L. Further addition of
anions such as chloride, sulphate, phosphate, carbonate, bicarbonate,
ZILSSAC dosage did not produce a considerable increase in de-
and others, which can compete with the uptake of fluoride ions during
fluoridation (Joshi et al., 2012).
the adsorption process. The defluoridation efficiency of adsorbents
exhibits a negligible effect due to the interference of chloride anions
4.4. Initial fluoride concentration because these are outer-spherically sorbing anions. The presence of
sulphate ions slightly affects fluoride removal as these ions are partially
The initial fluoride concentration provides an important driving inner-sphere complex forming species. However, the presence of car-
force to overcome the mass transfer resistance of fluoride ion between bonate and bicarbonate anions produces a negative trend in the re-
the aqueous and solid phases (Abas et al., 2013; Panchore et al., 2016). moval of fluoride (Kumar et al., 2011; Goswami and Purkait, 2012;
The percentage of fluoride removal decreases from 88.5% to 72.1% Patankar et al., 2013). This might be due to the competition of bi-
with variation of fluoride concentration from 10 mg/L to 100 mg/L. carbonate ions with fluoride for sorption sites. The defluoridation ef-
The higher adsorption value at the lower fluoride concentration is due ficiency of hydrous bismuth oxide (HBO) was retarded in the presence
to the availability of more active sites on the surface of the adsorbent, of increasing concentrations of bicarbonate, sulphate, and chloride
which decreases with increases in fluoride concentration (Mandal et al., ions, respectively (Srivastav et al., 2013). The presence of competing
2013). Bazrafshan et al. (2016) reported that removal efficiency of anions does not have any significant effect on the adsorption process
cupricoxide nanoparticles decreases from 75% to 50% with increase of (Maliyekkal et al., 2006; Jiménez-Reyes and Solache-Ríos, 2010; Basu
initial fluoride concentration from 5 mg/L to 100 mg/L. Mohammad et al., 2013; Ajisha and Rajagopal, 2015; Vences-Alvarez et al., 2015;
and Majumder (2014) also found a decrease in the fluoride removal Dobaradaran et al., 2016).
percentage of groundnut shell of 93% to 82% by increasing the fluoride
concentration from 5 mg/L to 30 mg/L.
4.7. Kinetics
4.5. Contact time A kinetics study is carried out to understand the mechanism of an
adsorption process (Aly et al., 2014), which is one of the important
Independent of other parameters, contact time plays a vital role in characteristics defining the efficiency of the sorption (Sai Sathish et al.,
the adsorption process of fluoride. The adsorption of fluoride increases 2008). Adsorption kinetics depend on the adsorbate–adsorbent inter-
with increasing contact time until equilibrium is achieved. Only very action (Ho, 2004). Pseudo-first-order, pseudo-second-order, in-
slight improvement occurs after equilibrium is achieved. This occurs traparticle diffusion, and pore diffusion models are the most popular
because the active sites of the adsorbent are vacant during the initial kinetic models. The pseudo-first-order kinetic model describes the
98
K.K. Yadav et al.
Table 8
Optimum experimental conditions for various agricultural and biomass-based adsorbents with their maximum adsorption capacities and fluoride removal efficiency.
(mg/g) (%) (g/L) (mg/L) (m2/g)
1. SB 4.12 45 1 5.4 1h 4 30 ± 1 °C – SO42 −, HCO3−, Cl− 3–4% Redlich-Peterson Psuedo-2nd order Singh et al., 2015
2. MPP 1.35 82.5 10 7 30 min 5 30 °C – – – Freundlich Psuedo-2nd order Dwivedi et al., 2014b
3. MGS 15.6 85 80 6.0 6h 100 25 °C – – – Langmuir & Freundlich Psuedo-2nd order Sanchez-Sanchez et al., 2013
4. RSL 15.8 95 8 7.93 – 3 – 5.52HCO3−, SO42 −, PO43 −, – Langmuir Psuedo-2nd order Msagati et al., 2014
NO3−, Cl−
5. PDPC 33.34 97 15 2 5h 2–10 28 ± 1 °C 54.80 No effect – Freundlich Psuedo-2nd order Ajisha and Rajagopal, 2015
6. TCPP 25 70 140 7 2h 5 – – Cl−, NO3−, SO42 −, – Langmuir & Freundlich Psuedo-2nd order Pandey et al., 2012
PO43 −
7. Al-SOJR 1.03 80 1 6 4h 0.52 mmol/L 30 °C – – – Langmuir – Paudyal et al., 2011
8. PLP 2.24 85.7 10 7 45 min 20 29 ± 1 °C – – – Freundlich – Dwivedi et al., 2014a
9. LMC 94.34 90 0.4 7 4h 20 – – HCO3−, AsO3− – Langmuir Psuedo-2nd order Yu et al., 2015b
10. SLP 1.28 98.6 60 7.5 25 min 5 30 °C – – – Freundlich – Kumari et al., 2015
11. ZCOP 5.605 97.2 0.7 6 50 min 10 25 ± 2 °C 20.56 HCO3−, PO43 − 91% Langmuir Psuedo-2nd order Jha et al., 2015
12. PLDC 0.81 81.60 3 7 40 min 3 30 ± 1 °C – – – Langmuir 1st order Emmanuel et al., 2015
99
13. CMPNS 2.3 84 8 7 – 20 30 °C 17 – – Langmuir – Hernandez-Montoya et al., 2012
14. NSC 1.27 94 5 2 3h 2 30 °C – – – Langmuir & Freundlich Psuedo-2nd order Chakrabarty and Sarma, 2012
0
15. ASEG 7 85 1.5 2–10 210 min 5 C 3.7 ± 0.12 SO42 −, NO3− Langmuir Intra-particle Manna et al., 2015
diffusion
16. Bark of babool 1.12 77.04 5 8 8h 5 30 °C – CO3−, HCO3−, SO42 −, – Langmuir Psuedo-2nd order Mamilwar et al., 2012
PO43 −, NO3−
17. CPGC 1.68 78.1 0.75 7 75 min 2–10 25 °C 6.42 HCO3−, SO42 − – Freundlich – Kanaujia et al., 2015
18. Banana peel 1.340 94.34 14 6 1h 20 30 ± 1 °C – – – Langmuir Psuedo-2nd order Abas et al., 2013
19. Padina sanctae 1.65 97 48 7 5 min 8 25 ± 1 °C – No effect – Freundlich Psuedo-2nd order Dobaradaran et al., 2016
crucis algae and pore diffusion
models
20. SDR 1.73 56.4 4 6 1h 2.5 28 ± 1 °C – – – Freundlich Psuedo-2ndorder Yadav et al., 2013
21. WSR 1.93 49.8 4 6 1h 2.5 28 ± 1 °C – – – Freundlich Psuedo-2nd order Yadav et al., 2013
SB = sugarcane bagasse; MPP = mosambi peel powder; MGS = mechanically modified guava seeds; RSL = restructured lignite; PDPC = pyrolyzed Delonix regia pod carbon; TCPP = Tinospora cordifolia plant powder; Al-SOJR = Al3 + loaded
saponified orange juice residue; PLP = peepal leaf powder; LMC = lanthanum modified carbon from Sargassum sp.; SLP = sal leaf powder; ZCOP = zirconium (IV) loaded carboxylated orange peel; PLDC = pithacelobiumdulce carbon;
CMPNS = carbons obtained from pecan nut shells modified with a calcium solution extracted from egg shells; NSC = activated neem stem charcoal; ASEG = alkali steam-treated elephant grass; CPGC = carbonised Punica granatum carbon;
SDR = Sawdust raw; WSR = Wheat straw raw.
100 100 Fig. 8. Fluoride adsorption performance of various agricultural

Adsroption Capacity (mg/g)
and biomass-based adsorbents along with removal efficiency.

90 90

80 80
70 70
60 60
50 50
40 40
30 30
20 20
10 10
0 0
Adsorbents
Fig. 9. Applicable kinetic model on surveyed adsorbents

Pseudo-first-order
100
80
60
40
13
20
0
6
Pseudo-second-
Others 81
order
Applicable Kinetic Model (%)
Langmuir fluoride ions are transferred to the internal surfaces of adsorbent par-
60 53 ticles. Bangham's equation is used to describe the pore diffusion model
50 of the adsorption process (Aharoni and Ungarish, 1977). In this model,
the solute moves from the surface of particles to interior sites of par-
40
ticles by pore diffusion (Emmanuel et al., 2015). A graph was prepared
30 by plotting some of the adsorbents that were surveyed in the present
Others 20 Freundlich review to highlight the most applicable kinetic model. As is evident
27
10 from the survey result (Fig. 9), most of the adsorbents obey the pseudo-
3
0 second-order kinetic model, followed by the pseudo-first-order model.
2 Other kinetic models such as the intraparticle diffusion pore diffusion,
non-linear, and shrinking core models seldom fit adsorption data.
14
4.8. Equilibrium isotherms
Mixed Langmuir & Freundlich Adsorption isotherms indicate how molecules subjected to adsorp-
tion distribute between liquid and solid phases at the time of equili-
brium. They provide some insight into the adsorption mechanism as
well as the surface properties and affinities of the adsorbent (Aly et al.,
Applicable Isotherm Model (%) 2014). Analysis of isotherm data is an interesting way to predict the
Fig. 10. Applicable isotherm models on surveyed adsorbents
adsorption capacity of an adsorbent while designing an adsorption
system (Mondal and George, 2015). The most popular isotherm models
are the Langmuir model and the Freundlich model. Other isotherm
liquid–solid system based on solid capacity (Lagergren, 1898), whereas models, which adsorption data follow rarely, include the Dubinin–Ra-
the pseudo-second-order model (Ho and McKay, 1999) describes the dushkevich (D–R) model, Redlich–Peterson (R–P) model, Temkin
chemical adsorption involving valence forces through the sharing or model, Sips model, and Bradley equation. Fig. 10 clearly shows that
exchange of electrons between the adsorbent and adsorbate as covalent most of the adsorption data fit well with the Langmuir isotherm, which
forces and ion exchange (Ho, 2006). The intraparticle diffusion model indicates monolayer adsorption on a uniform homogeneous surface
(Weber and Morris, 1963) is another popular model in which adsorbed having identical sites, whereas some data fit best with the Freundlich
100
model, which indicates heterogeneity of the adsorbent surface and recycling of these adsorbents for further use is not possible. Hence,
considers multilayer adsorption (Dayananda et al., 2015). A very few further desorption studies are needed to develop a most practical ad-
adsorbents followed other isotherms, such as the Dubinin–Radushke- sorbent.
vich isotherm, which attempts to distinguish between physical and
chemical sorption mechanisms, the Temkin and Pyzhev equation,
5.2. Column studies
which reflects the heat of adsorption and the adsorbent–adsorbate in-
teraction on surfaces, and the Redlich–Peterson model, which is a
Batch adsorption experiments have shown the marked potential of
special case model that incorporated the features of the Langmuir and
various adsorbents for removing fluoride from water. However, a gap
Freundlich isotherms into a single equation and presents a general
between laboratory-scale studies and practical feasibility of these ad-
isotherm equation (Raul et al., 2012). The adsorption data of some
sorbents remains, and this gap needs to be bridged using column stu-
adsorbents fit well with more than one isotherm (Alagumuthu et al.,
dies. Very few researchers have studied adsorbents from this aspect
2010; Dhillon and Kumar, 2015; Patnaik et al., 2016).
(Maliyekkal et al., 2006; Gupta et al., 2007; Jagtap et al., 2011;
Goswami and Purkait, 2012; Nazari and Halladj, 2014; Samarghandi
et al., 2016). Column studies are used to test the practical applicability
5. Important parameters for practical feasibility of the adsorption
of adsorbents in the field and are necessary for designing industrial/
process
commercial-scale fixed-bed adsorbent systems. Column experiments are
carried out as a function of initial fluoride concentration, inlet flow
5.1. Desorption/regeneration studies
rate, bed depth, and adsorbent dose at various throughput volumes.
Gupta et al. (2007) tested the potential of carbon slurry by drawing
The reusability of an adsorbent is very important from the per-
breakthrough curves for fluoride adsorption and noted the break-
spectives of economy and waste management, but very few researchers
through capacity of 4.155 mg/L. Maliyekkal et al. (2006) also reported
have studied this aspect of adsorption. pH plays a vital role in the re-
that the breakthrough curve was steeper for MOCA at low contact time,
generation of an adsorbent. An adsorbent can be regenerated using
which indicated faster exhaustion of bed. Sivasankari et al. (2010)
acids, alkalis, or other chemicals (Lata et al., 2015). Hardly any fluoride
tested the practical utility of granular polymer-agglomerated alumina
is leached at acidic pH, but adsorbed fluoride begins to desorb when pH
(GPAA) for removal of fluoride from water. The maximum adsorption
moves toward the alkaline range, as shown in Fig. 11. This occurs be-
capacity of GPAA was found to be 1.948 mg/g at an optimum bed
cause hydroxide ions compete with the fluoride ions at alkaline pH. The
height of 31 cm in a 1.5 cm diameter column with a flow rate of 10 mL/
concentration of hydroxide ions (10− 4 mol/L) is higher than that of
min. GPAA could be regenerated using a sufficient volume of 0.25%
fluoride ions (0.789 × 10− 4 mol/L) at pH 10 (Darchen et al., 2016).
aluminium sulphate solution, and cyclic defluoridation and subsequent
Tripathy et al. (2006) reported that no desorption of AIAA took place
regeneration were possible for 8 cycles without appreciable loss of
up to pH 8 but that fluoride leached back into solution when pH was
adsorption capacity. Wambu et al. (2013) used siliceous mineral and
increased from 8 to 12 and reached a maximum of 98%. Thakre et al.
observed that removal was maximum at low flow rate and low initial
(2010) treated fluoride-loaded MB with 1 M NaOH for 1 h and achieved
fluoride concentration.
approximately 97% defluoridation. The regenerated adsorbent showed
a decrease in fluoride removal efficiency from 95.47% to 73% on reuse.
This was due to the treatment of MB at highly alkaline pH (12 −13). 6. Worldwide status of fluoride
There was very little regeneration of HMOCA at pH values lower than
10. A maximum of 85% desorption occurred at pH 12 (Teng et al., The problem of excessive fluoride in drinking water has engulfed
2009). Zhang et al. (2015) used ferric chloride solution for desorption many parts of the world. At the latest status, > 35 countries worldwide
of fluoride-loaded Fe-impregnated chitosan. Most of the adsorbents have reported high fluoride water. The most well-known countries with
based on building materials and agricultural wastes have not been fluoride on land are India, China, Ethiopia, Kenya, Ghana, Pakistan,
tested by desorption studies (refer to Tables 7 and 8) because these Iran, Germany, Sri Lanka, Nigeria, Tanzania, and South Africa. Some
adsorbents are waste materials and are available in abundance. How- other countries have also reported excess fluoride (refer to Table 9 and
ever, a very small number of researchers did study this aspect and found Fig. 12). In view of the fact that the problem persists mainly in devel-
either no regeneration or very little regeneration (Islam and Patel, oping countries, defluoridation technology with the attributes of ease of
2011; Singh et al., 2015; Biswas et al., 2016), which indicates that operation, simplicity of design, and low cost need to be adopted. There
Fig. 11. Desorption of fluoride from surface of some adsorbents

13
100 14
12.2
13
11.6
12
12
12
as a function of pH
12
12
12
12
11
10
80
Desorption (%)
10
60 8
7
pH
40 6
4
20
2
0 0
Adsorbents
Desorption (%) pH
101
Table 9
Range of fluoride concentration in groundwater of different countries.
Country Region/province/state Fluoride concentrationa (mg/L) References
Algeria Ouargla, Wilaya 0.4–4.32 Sekkoum et al., 2012

Argentina Almirante Brown, Chaco Province 0.8–4.2 Buchhamer et al., 2012
Brazil Southern Brazil 0.1–4.79 Mirlean and Roisenberg, 2007
Cameroon Mayo Tsanaga 0.19–15.2 Fantong et al., 2010
Canada – Up to 3.3 Saleh et al., 2001
China Yuanmou County 1.0–7.20 Chen et al., 2012a, 2012b
Yuncheng Basin 0.2–14.1 Li et al., 2015
Egypt – Up to 10.0 Saleh et al., 2001
Ethiopia – 0.0–75.0 Ayenew, 2008
– 0.4–21.4 Rango et al., 2010
Ziway-Shala Basin 1.1–68.0 Rango et al., 2012
Finland – 0.7–1.8 Sehn, 2008
Germany Muenster region Up to 8.8 Queste et al., 2001
Ghana Bongo, Upper East Region 1.5–4.0 Anku et al., 2009
India Birbhum District, West Bengal 0.01–18.0 Batabyal and Gupta, 2017
Iran Maku City, West Azarbaijan 0.23–5.0 Mesdaghinia et al., 2010
Italy Sillicy and Caltanissetta 0.023–3.28 D'Alessandro et al., 2008
Aosta Valley Region 0.03–1.14 Tiwari et al., 2017b
Japan Mizunami Up to 12.0 Abdelgawad et al., 2009
Kenya Gilgil, Nakuru County 0.026–21.5 Wambu and Muthakia, 2011
Malawi Lilongwe, Malawi 0.5–7.02 Msonda et al., 2007
Mexico Chihuahua 0.05–11.8 González-Horta et al., 2015
Morocco Khouribga 0.21–2.97 El Jaoudi et al., 2012
Nepal Kathmandu Valley 0.06–1.92 Pant, 2011
Niger Tibiri 4.8–6.6 Arji, 2001
Nigeria Makurdi, Benue State 0.03–6.7 Akpata et al., 2009
Norway Hordaland 0.05–9.04 Bardsen et al., 1999
Pakistan Khalanwala, East Punjab 0.11–22.8 Farooqi et al., 2007
Poland Malbork 0.05–2.45 Czarnowski et al., 1996
Saudi Arabia Riyadh Region Up to 2.5 Aldosari et al., 2002
Qaseem Region 0.0–6.2 Aldosari et al., 2002
Senegal Sine Saloum Region 0.4–2.85 Kane et al., 2012
South Korea Gimcheon 0.02–2.15 Kim et al., 2011
Spain La Guancha 2.88–6.20 Hardisson et al., 2001
Sri Lanka Nikawewa-Siyabalangamuwa Up to 5.30 Chandrajith et al., 2011
Giribawa, Nochchiyagama
0.01–4.34 Young et al., 2011
Sudan East of Blue Nile Communities 0.3–7.0 Abdellah et al., 2012
Sweden Kalmar County 0.1–15.0 Augustsson and Berger, 2014
Tanzania Northern Tanzania 10.5–46.0 Thole, 2013
Thailand Lamphun, Northern Thailand 0.01–14.12 Chuah et al., 2016
Tunisia Louza −2 Up to 3.39 Nasr et al., 2011
Turkey Anatolia 0.05–13.7 Oruc, 2008
Uganda Rift Valley Area 0.5–2.5 Rwenyonyi et al., 2000
USA Pennsylvanian Up to 4.0 Senior and Sloto, 2006
a
Values are for a particular region/province/locality.
is also a need to inspect and assess the issues of fluoride and fluorosis in humans.
developing countries. • The fluoride removal efficiencies of carbon and calcium-based ad-
sorbents were found to be satisfactory. However, the adsorbents
7. Conclusions and future recommendations have not been tested using actual water in the field. Furthermore,
carbon-based adsorbents work efficiently at acidic pH, which can be
This review described the adsorption efficiency of a wide range of a drawback for field studies.
adsorbents for the removal of fluoride from water. The literature ana- • Oxides/mixed oxides and layered double hydroxides were identified
lysis led to the following conclusions. as robust adsorbents for fluoride adsorption, but they were found to
be expensive and the presence of co-anions existing in water can
• The adsorption process appears to be the most attractive method affect the defluoridation process significantly.
among all the techniques due to its key features such as high re- • A wide range of nanomaterials has been investigated for fluoride
moval capacity, cost effectiveness, ease of operation, and simplicity adsorption. Carbon nanotubes have attracted the attention of re-
of design. However, it is worth noting that characteristics of the searchers for defluoridation and have been proven to be good ad-
adsorbent such as selectivity, regeneration potential, and physical sorbents in recent years. However, the production of a sufficient
and chemical stability are among the drawbacks of this technique, quantity of nanomaterials for industrial use might be difficult and
which require improvement. could be expensive. Additionally, the cumulative release of nano-
• Alumina and aluminium-based adsorbents have shown a remarkable materials into the environment through a long period of time might
potential for fluoride adsorption because of their high affinity for be a drawback. Hence, further investigations are required to over-
fluoride ions. However, the consumption of water treated with these come these issues.
adsorbents can be dangerous to human health because the alumi- • Various natural materials, building materials, and industrial wastes
nium from the adsorbents leaches into the water during the de- have also been explored for defluoridation of water. Modifications
fluoridation process and may cause neurodegenerative diseases in of these materials to enhance their fluoride adsorption efficiency
102
Fig. 12. World map showing fluoride affected countries.
have also been reported by various researchers. However, con- of inorganic/organic-based adsorbents for drinking water de-
tinuous flow experiments have been reported by very few re- fluoridation and their understanding of different models/processes
searchers and the regeneration efficiency of these materials is either is continuously increasing. This review paper is an up-to-date
negligible or very low. summary of the recent research progress in this field with respect to
• The literature survey revealed that the Langmuir isotherm and defluoridation. In parallel, a critical investigation of the applic-
pseudo-second-order kinetic models were the best fit models for the ability of developed adsorbents was attempted, considering the
experimental data in most studies. economic viability, environmental safety, and sustainability of the
• Fluoride removal efficiency depends on many factors, so actual proposed processes.
applications may differ from laboratory experiments. Hence, it is • Future needs include low-cost, highly advanced systems, with low
suggested that the practical utility of any adsorbent be tested using waste, minimum process wastage, and maximum utilisation of the
actual water prior to implementation in the field. available waste at the large-scale to continue defluoridation.
• It is worthwhile to note that regeneration of any exhausted ad-
sorbent is essential to overcome the issues of economy and waste Conflict of interest
management. Finally, it is suggested that research in this field re-
quires more investigations into better adsorption materials and The authors declare no conflict of interest.
more efficient and cost-saving methods of adsorption processes.
• Depending on the properties of each class of adsorption materials, Acknowledgements
research has focused mainly on their optimisation as adsorbents for
fluoride, but has not focused on their availability, cost reduction, The authors are very grateful to Lisa M. Reece, World Health
and large-size defluoridation methodology. As is routinely stated, Organization Collaborating Center for Vaccine Research, Evaluation
the efficiency of an adsorbent in a defluoridation methodology is and Training on Emerging Infectious Diseases, University of Texas
proportional to its specific area and the reactivity of its surface. Medical Branch, Galveston, TX 77555 and Richard Chadd, Senior
Therefore, one of the critical tasks in the design of low-cost en- Ecologist, Environment Agency of England, UK for their consistent
gineered adsorbents and their technical incorporation is to ensure support and guidance. The authors are also thankful to the editor and
product stability against aggregation and chemical transformations anonymous reviewers for their valuable suggestions to improve the
due to storage, safe-handling, and safe use. However, higher effi- manuscript.
ciency is one of the most desired parameters for validating the ef-
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Environment International: Krishna Kumar Yadav, Neha Gupta, Vinit Kumar, Shakeel Ahmad Khan, Amit Kumar

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Environment International 111 (2018) 80–108

Contents lists available at ScienceDirect

A review of emerging adsorbents and current demand for deﬂuoridation of T

1. Introduction caries by decreasing the rate of demineralisation of dental enamel or

Fig. 1. Fluoride adsorption performance of various aluminium-

based adsorbents along with removal eﬃciency

Fluoride Removal (%)

1. ZILSSAC 3.25 – 2 3 2.5 h 10 25 °C – – – Langmuir – Joshi et al., 2012

Fig. 2. Fluoride adsorption performance of various carbon-based

adsorbents along with removal eﬃciency.

Fluoride Removal (%)

Jiménez-Reyes and Solache-Ríos,

Jayarathne et al., 2015

Islam and Patel, 2007

Gourouza et al., 2014

Mondal et al., 2016

Liang et al., 2011

Nie et al., 2012

thermogravimetric analysis/diﬀerential thermal analysis (TGA/DTA),

highest at pH ~ 7 owing to electrostatic interaction between the

therm with a monolayer adsorption capacity of 27.8 mg/g at 318 K. The

adsorption reaction followed pseudo-second-order kinetics and was

Alemu et al. (2014) synthesised an aluminium oxide – manganese

Langmuir & Freundlich

Langmuir & Freundlich

oxide (AOMO) composite material. The AOMO was prepared from

within the pH range of 5–7. The experimental data of ﬂuoride ad-

sorption onto AOMO were described well by the Freundlich isotherm

(CY-HZO) was prepared by Saha et al. (2015) for eﬃcient removal of

determined by XRD, SEM, TEM, atomic force microscopy (AFM), and

Brunauer–Emmett–Teller (BET) surface analysis. The adsorption data

followed the Langmuir isotherm model with an adsorption capacity of

31.45 mg/g. The reaction had pseudo-second-order kinetics and was

generation from ﬂuoride-loaded adsorbent could be achieved using 3 M

Patankar et al. (2013) used mixed alumina-magnesia hydroxide

absorption capacity achieved was 5.62 mg/g within 24 h of contact

time at pH 7 for an initial ﬂuoride concentration of 5.13 mg/L. The

adsorption data were described well by the Langmuir isotherm and

followed pseudo-second-order kinetics. Lanthanum hydroxide was

synthesised and characterised for deﬂuoridation by Na and Park

(2010). Batch tests were conducted to evaluate the inﬂuence of dif-

ferent parameters, such as pH, contact time, competing anions, ad-

sorbent dose, temperature, and initial ﬂuoride concentration, on the

adsorption process of the lanthanum hydroxide. The ﬂuoride removal

eﬃciency of the lanthanum hydroxide was optimal at pH ≤ 7.5. The

Langmuir isotherm equation described the data well with a monolayer

adsorption capacity of 242.2 mg/g. The pseudo-second-order equation

described all the kinetic data very well. Thermodynamic parameters

indicated that the adsorption reaction of ﬂuoride was endothermic and

spontaneous. It was suggested that the adsorption of ﬂuoride was due to

the chemical process. Regeneration of ﬂuoride-adsorbed material was

possible by washing with NaOH solution.

ﬂuoride adsorption potential. The eﬃciency of Li-Al layered double

hydroxide was determined by Zhang et al. (2012), who reported a

maximum removal of 97.36%. Koliraj and Kannan (2013) tested the

potential of nitrate-containing ZnCr layered double hydroxide (ZnCr3-

31 mg/g. The adsorbent was prepared by the co-precipitation method.

The authors reported that ﬂuoride uptake by ZnCr3-NO3-LDH was

constant within the pH range of 3–10 because of the buﬀering nature of