Research Article: Coconut-Shell Derived Carbon/carbon Nanotube Composite or Fluoride Adsorption From Aqueous Solution
Research Article: Coconut-Shell Derived Carbon/carbon Nanotube Composite or Fluoride Adsorption From Aqueous Solution
Research Article: Coconut-Shell Derived Carbon/carbon Nanotube Composite or Fluoride Adsorption From Aqueous Solution
Research Article
Coconut-shell derived carbon/carbon nanotube composite for fluoride adsorption from aqueous solution†
Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory, Department of Chemical
Engineering, Indian Institute of Technology Hyderabad, Kandi, Telangana, India
Correspondence: Dr. R. Araga, Creative & Advanced Research Based on Nanomaterials (CARBON) Laboratory,
Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Kandi-502285, Telangana, India
E-mail: ch13p0003@iith.ac.in
†This article has been accepted for publication and undergone full peer review but has not been through the
copyediting, typesetting, pagination and proofreading process, which may lead to differences between this
version and the Version of Record. Please cite this article as an ‘Accepted Article’, doi:
[10.1002/clen.201800286]
Abstract
Devastating effect of fluoride in drinking water on human health is a great concern and defluoridation is essential to
make groundwater suitable for drinking. The aim of this study was to evaluate the dissolved fluoride removal
efficiency of a novel and low-cost carbon/carbon nanotube (CNT) composite under batch conditions. CNTs were
coated on the coconut-shell charcoal surface at 450 °C by using plasma enhanced chemical vapor deposition.
Thereafter, processed charcoal samples were ball milled and used for the fluoride removal from aqueous medium. The
amount of fluoride removal was found to be approximately 65% of the initial concentration of 4.4 mg L--1 in 3 h
contact time at the adsorbent dose of 10 g L--1. The linear forms of three isotherm models (Langmuir, Freundlich, and
Temkin) and two kinetic models (pseudo-first order and pseudo-second order) were applied to the adsorption data to
determine the best fit for equilibrium expression. Isotherm data fit the Langmuir model while the adsorption kinetics
was represented by the pseudo-second-order kinetic model. The fluoride adsorption process onto prepared
carbon/CNT composite occurred spontaneously (ΔG° = --1.656 kJ mol--1) in an endothermic nature (ΔH° = 11.07 kJ
mol--1) with increased randomness (ΔS° = 41.69 J mol--1 K--1). To validate the performance further, the as-prepared
adsorbent was successfully used to treat groundwater samples with excess fluoride concentration collected from
Nalgonda district, Telangana, India.
Abbreviations: BET, Brunauer-Emmett-Teller; CNT, carbon nanotube; FESEM, field emission scanning electron
microscopy; SS, stainless steel; WHO, World Health Organization
1 Introduction
Since groundwater is one of the primary sources of drinking water, contamination of groundwater due to natural causes
and human activities is a worldwide problem. Among the numerous aquatic pollutants, fluoride is one of the most
potent groundwater pollutants. According to the guidelines of World Health Organization (WHO), the maximum
permissible limit of fluoride in drinking water is 1.5 mg L-1 [1]. High fluoride concentrations i.e., above 1.5 mg L-1
causes adverse effects on human health and the most common problems associated with fluoride are dental and skeletal
fluorosis. Besides fluorosis, excessive intake of fluoride may also lead to neurological disorders, reduced immunity,
gastrointestinal problems, urinary tract malfunctioning, etc. [2, 3]. Therefore, globally there is great need to treat fluoride
contaminated water. Several defluoridation techniques have been developed such as coagulation/precipitation,
membrane separation, ion exchange, and adsorption. Among these techniques, adsorption is found to be a widely
accepted method to treat fluoridated water because of its ease of operation, cost-effectiveness, and the possibility to
regenerate the adsorbent [3]. In the past several decades, many adsorbent materials have been developed for the removal
of fluoride from contaminated water. However, some of them are not effective at low fluoride concentrations. While
for some other adsorbents, the preparation method is much complicated hence, the defluoridation process is not
economically feasible for large-scale applications. Fluoride removal capacity was found to be low when
bioremediation was implemented for the defluoridation purpose [4, 5]. Considering all these drawbacks, development
of cost-effective and efficient adsorbents is highly essential for the treatment of fluoride-contaminated water,
especially for low fluoride concentrations.
Carbon nanotubes (CNTs) have received much attention over the past two decades and are being used for a wide
choice of promising applications including wastewater treatment because of its significant pollution removal capacity
[6, 7]
. Unlike many adsorbents, CNTs possess different features such as high aspect ratio, large surface area and
encouraging adsorption behavior which are beneficial to improve the water treatment performance [8, 9]. However, the
strong intermolecular interactions between the nanotubes may lead to the development of aggregates. This aggregation
decreases the accessible surface area for physicochemical interactions; hence, the use of CNTs as the adsorbent is
limited [10]. In order to solve this problem, CNT-based composites were synthesized and used as effective adsorbents
for the treatment of several aquatic pollutants including arsenate, cadmium, chromium, and dyes (such as methylene
blue, neutral red, cresyl blue, methyl orange and rhodamine B) [11-17].
Agricultural waste is abundantly available, renewable, non-toxic and widely used as precursors for synthesizing the
[18]
carbonaceous materials . These agro-waste derived carbons are found to have prominent effects as adsorbents in
the eco-friendly treatment of wastewater contaminated with a variety of pollutants [19-23]. However, the major problem
associated with these adsorbents is the difficulty of choosing an appropriate precursor for effective adsorption of a
particular pollutant as they possess a variety of physicochemical properties based on the precursor material.
Among the several carbonaceous adsorbents, coconut-shell derived carbon has unique properties like low ash content,
optimized porous structure, and large internal surface area [24]. Furthermore, coconut shell is also reported as one of
the potential feedstocks for different minerals, which can be used for CNT growth [25]. Adsorption ability of coconut-
shell derived carbon/activated carbon have been tested for fluoride, however, there is no report in the literature about
fluoride adsorption using coconut-shell derived carbon/CNT composite. With this perspective, in the present work
CNTs have been coated on coconut-shell derived charcoal which can further increase the effective surface area and
active sites of the adsorbent by preventing the CNT aggregates. The resulting carbon/CNT composite was evaluated
for its potentiality towards the removal of fluoride ions from aqueous solution under batch adsorption conditions.
The major objectives of this study were to (a) evaluate the effect of various influential parameters (i.e., contact time,
adsorbent dosage, initial fluoride concentration, solution pH and temperature) on adsorption process; (b) determine
the applicability of various isotherm and kinetic models; (c) determine the nature of adsorption process with respect
to thermodynamic parameters; (d) find out the applicability of synthesized adsorbent material with groundwater
samples collected from different places which are adversely affected with fluorosis problem.
2.2 Characterization
Morphology of the as-prepared carbon/CNT composite was characterized by field emission scanning electron
microscopy (FESEM) (Zeiss). The surface area of the as-prepared adsorbent was evaluated using nitrogen adsorption-
-desorption isotherms which were recorded using a Brunauer-Emmett-Teller (BET) surface analyzer (Autosorb-iQ;
Quantachrome Instruments) at 77 K by evacuating the samples at 573 K for 2 h under vacuum.
chromatography (Basic IC plus; Metrohm). The amount of fluoride ions adsorbed per unit mass of adsorbent at
equilibrium (qe, mg g--1) and percentage adsorption (%) was calculated using the following equations:
𝑉(𝐶i −𝐶e )
𝑞e = (1)
𝑊
(𝐶i −𝐶e )
Percentage adsorption (%) = × 100 (2)
𝐶i
where Ci and Ce are the initial and equilibrium concentrations of fluoride in solution (mg L--1), V is the volume of
solution (L) and W is the weight of adsorbent (g).
3.2.4 Effect of pH
The effect of pH on fluoride ion uptake by carbon/CNT composite was examined at different initial pH values ranging
from 2 to 10 and the results are presented in Fig. 5. The solution pH was adjusted by using 0.1 M HCl or NaOH and
the adsorption process was observed to be strongly dependent upon pH. Utmost fluoride removal was observed at pH
2 and the percentage adsorption was found to be decreased with an increase in pH. Utmost removal of fluoride ions
in the acidic medium could be the result of attraction of fluoride ions towards the positive surface charges of the
adsorbent and least removal in the alkaline medium caused by the repulsion of fluoride ions by the negatively charged
adsorbent surface [27].
The obtained rate constants of pseudo-first-order and pseudo-second-order kinetic models for fluoride adsorption onto
coconut-shell derived carbon/CNT composite are summarized in Table 1. Regression coefficient (R2) of the pseudo-
first-order kinetic model is much lower compared with the pseudo-second-order kinetic model. The attained
adsorption capacity (qe,cal) and R2-values revealed that the adsorption of fluoride ions onto the prepared adsorbent was
better described by the pseudo-second-order kinetic model than the pseudo-first-order model, which reveals that the
fluoride removal might be a chemisorption process [24]. The experimental data has also been tested with the Elovich
kinetic model, however, the low R2-value (0.849) indicates that the model does not apply to the present adsorption
system (in Supporting Information Table S1).
S2).
The separation factor (RL) is a dimensionless constant and essential characteristics of Langmuir isotherm, described
by the following equation [31]:
1
𝑅L = (8)
1+𝐾L 𝐶i
where Ci (mg L--1) is the initial concentration of adsorbate in the aqueous phase and KL (L mg--1) is the Langmuir
constant described above. The value of RL reveals the nature of adsorption process as follows:
RL = 0 for irreversible adsorption
0 ˂ RL ˂ 1 for favorable adsorption
RL = 1 for linear adsorption
RL > 1 for unfavorable adsorption
For the present study, the obtained RL-values were in the range of 0.069 to 0.147, which indicates that the fluoride ion
adsorption was favorable onto prepared carbon/CNT composite under the studied circumstances. Furthermore, the
lower RL-values represent strong interaction of fluoride ions with the prepared adsorbent material [32].
The values of thermodynamic parameters at different initial fluoride concentrations (i.e. 2.68, 4.4 and 6.24 mg L--1)
are summarized in Table 4. ΔH° and ΔS° were determined from the slope and intercept of lnKo versus 1/T plot (Eq.
(9)) whereas ΔG° was directly calculated from Eq. (8). The positive values of ΔH° confirm that the adsorption of
fluoride ions onto the prepared adsorbent is endothermic. The endothermic process adsorbs energy in the form of heat
from its surroundings due to which higher percentage removal of fluoride was achieved at the high temperatures as
mentioned in Section 3.2.5. However, the enthalpy change value was found to be decreased with respect to the increase
in the fluoride ion concentration. Furthermore, positive ΔS° values reveal the increased randomness at the solid-
solution interface. The ΔG° values were observed to be negative in all cases except at 303 K with fluoride
concentrations of 2.68 and 6.24 mg L--1 which indicates that the adsorption process is spontaneous at higher
temperatures [33].
under the identical experimental conditions of the batch equilibration study. Fluoride concentrations of the samples
were analyzed before and after the adsorption treatment and the results are presented in Table 5. Achieving the
standard drinking quality for the three water samples confirms the suitability of as-prepared adsorbent to treat the
fluoride contaminated groundwater.
4 Conclusion
As-prepared carbon/CNT composite (ball-milled sample) was measured to be having the surface area of 358 m2 g--1
and found to be effective to treat the fluoride contaminated water due to its high removal efficiency towards fluoride
ions (approximately 71% of removal was attained at the concentration of 4.4 mg L--1). The results of this investigation
revealed that the adsorbent performance was better even at low fluoride concentrations (˂3 mg L--1). Langmuir
isotherm fits better with the equilibrium data, while the adsorption process was influenced by pseudo-second-order
kinetics. Thermodynamic parameters revealed that the adsorption process was spontaneous at high temperatures.
Finally, the practical utility of the prepared adsorbent was also tested with groundwater samples collected from one
of the most severely affected places in India. The results of this study clearly show the effective applicability of the
prepared adsorbent for reduction of elevated fluoride concentration from groundwater to the permissible limit.
Acknowledgment
The authors gratefully acknowledge Indian Institute of Technology Hyderabad for providing necessary research
facilities.
Contribution
CSS designed the work while RA and SK performed all the experiments. All authors analyzed the data and
prepared the manuscript.
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Figure 1. FESEM images of (a) hand-grinded and (b) ball-milled samples of PECVD treated coconut shell derived
charcoal
Figure 2. Effect of agitation time on the adsorbed amount of fluoride (initial dye concentration: 10 mg L--1; adsorbent
dosage: 1 g L--1; pH: 2 and temperature: 30 °C)
Figure 3. Effect of adsorbent dosage on percentage removal of fluoride (initial dye concentration: 10 mg L--1; pH: 2;
temperature: 30 °C and time: 3.5 h)
Figure 4. Variation of percentage adsorption onto carbon/CNT composite with initial fluoride concentration
(adsorbent dosage: 10 g L--1; pH: 2; temperature: 30 °C and time: 3.5 h)
Figure 5. Effect of pH on adsorption of fluoride onto carbon/CNT composite (initial solution concentration: 10 mg L-
-1
; adsorbent dosage: 10 g L--1; temperature: 30 °C and time: 3.5 h)
Figure 6. Effect of temperature on fluoride adsorption process onto prepared carbon/CNT composite at different initial
fluoride concentrations (adsorbent dosage: 10 g L--1; pH: 2 and time: 3.5 h)
Figure 7. (a) pseudo-first-order and (b) pseudo-second-order kinetic plots of fluoride adsorption from aqueous
solution by coconut shell derived carbon/CNT composite (initial fluoride concentration: 10 mg L--1; adsorbent dose:
1 g L--1; pH: 2 and temperature: 303 K)
Figure 8. Equilibrium isotherms of (a) Langmuir (b) Freundlich and (c) Temkin for adsorption of fluoride onto
coconut shell derived carbon/CNT composite at 303 K
Table 3: Isotherm parameters for adsorption of fluoride onto the prepared adsorbent
Temperature Langmuir constant Freundlich constant
--1 --1 2
(K) qm (mg g ) KL (L mg ) R KF (mg g ) (L mg--1)n
--1
n R2
303 0.36 2.158 0.9965 0.270 8.35 0.9076
Temperature Temkin constant
(K) A (L mg--1) b ((J mol--1) (mg g--1)--1) R2
303 1344.48 67.72 0.8977
Table 5: Fluoride ion concentration of groundwater samples (before and after adsorption)
Initial fluoride concentration Final fluoride concentration
Sample
--1
(mg L ) (mg L--1)
1 2.24 ± 0.41 1.12 ± 0.12
2 2.81 ± 0.29 1.18 ± 0.11
3 2.93 ± 0.27 1.4 ± 0.09
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8