Environmental Science and Pollution Research (2019) 26:28622–28632

https://doi.org/10.1007/s11356-019-04641-0

ALTERNATIVE ADSORBENT MATERIALS FOR APPLICATION IN INDUSTRIAL PROCESSES

Use of bentonite calcined clay as an adsorbent: equilibrium

and thermodynamic study of Rhodamine B adsorption
in aqueous solution
Fernanda Ribeiro dos Santos 1 & Heloísa Carolina de Oliveira Bruno 1 & Lisbeth Zelayaran Melgar 1

Received: 1 September 2018 / Accepted: 19 February 2019 / Published online: 6 March 2018
# Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract
The Rhodamine B adsorption was realized in batch using calcined bentonite clay. The effects of Rhodamine B initial concen-
tration, pH, and temperature were evaluated and the conditions where the adsorption was favored were in 500 mg L−1, pH 3, and
35 °C. The equilibrium isotherms studied were from Langmuir and Freundlich. The coefficients of determination (R2 > 0.99)
were found to confirm the best fitted to Langmuir isotherm, with a monolayer adsorption capacity (qmax) of 552.49 mg g−1. The
kinetic data agreed well with the pseudo-second order model (R2 > 0.99). The in natura and calcined clay were characterized by
the techniques of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 physisorption (BET), and
scanning electron microscopy (SEM). Thermodynamic parameters including Gibbs free energy (ΔG°), enthalpy change
(ΔH°), and entropy change (ΔS°) were calculated to estimate the nature of Rhodamine B adsorption in clay. The results
suggested that the adsorption was endothermic and spontaneous, with the enthalpy adsorption increasing with the increase of
temperature. Therefore, calcined bentonite can be used as an efficient adsorbent for discoloration of large volume of residual
water, presenting low-cost and high adsorptive capacity.

Keywords Rhodamine B . Adsorption . Isotherms . Kinetic and thermodynamic study . Bentonite clay . Characterization

Introduction et al. 2009); in addition, some may be bioaccumulative

(Robinson et al. 2001), carcinogenic (Banat et al. 1996;
In the course of the years, water quality is suffering losses. The Clarke and Anliker 1980; Gupta et al. 1990; Mishra and
main causes of hydric deterioration resources are associated Tripathy 1993; Rammel et al. 2001), and mutagenics
with anthropogenic activities, disordered population growth, (Yagub et al. 2014) and can cause kidney, reproductive
unplanned urbanization, fast industrialization, and unspecial- system, liver, brain, central nervous system (Kadirvelu
ized water use (De Gisi et al. 2016). The effluents that come et al. 2003), gastrointestinal tract, skin, and lung dysfunc-
from textile, leather, paper, and printing industries contain tion (Karthikeyan and Jothi Venkatachalam 2014). That is
toxic dyes that are stable to the processes of photo-degrada- the reason why it is essential to treat dye effluents before
tion, biodegradation, and oxidation (Khan et al. 2012). Dyes pouring them the recipient bodies.
present a high-colored charge and when discharged into the The methods used for dye removal were precipitation, floc-
receptor bodies, they prevent light penetration, reduce the culation, incineration, ion exchange, coagulation, reverse os-
photosynthetic activity, and inhibit the biota growth (Han mosis, electrochemistry, membrane filtration (Zheng et al.
2013), advanced oxidation processes, and biological methods
(Hayat et al. 2015; Punzi et al. 2015) which have many de-
Responsible editor: Tito Roberto Cadaval Jr grees of efficiency (Zhu et al. 2016) depending on the selected
compound (Salleh et al. 2011). Following this perspective,
* Fernanda Ribeiro dos Santos
these processes can have disadvantages such as high costs,
fernandaribeirods@hotmail.com
high energy demand and sludge production (Chuah et al.
2005), and by-products formation that can be toxic or difficult
1
Department of Chemical Engineering, Federal University of São to recover and discard (Singh et al. 2018), besides being used
João del-Rei, MG 443 (Kilometer 7), Ouro Branco, Minas
Gerais 36420-000, Brazil for a restricted group of compounds.
28623 Environ Sci Pollut Res (2019) 26:28622–28632

The adsorption is an efficient physicochemical separation bentonite, it was possible to study the adsorption mechanism,
method of mixtures for the residual water decontamination, to obtain parameters and characteristics of the adsorbent, and
non-destructive, simple, without interaction with toxic sub- to evaluate the efficiency of the use of this system in the
stances and low-cost operation (Bentahar et al. 2018). The treatment of water effluents.
adsorption process depends on the nature and adsorbent type,
with the possible use of organic and inorganic materials
(Vakili et al. 2014). The most widely used adsorbents are as Methods and materials
follows: activated carbon (Guibal et al. 2003), alumina (Iida
et al. 2004), zeolites (Alver and Metin 2012; Han et al. 2007), Materials
silica gel (Gaikwad and Misal 2010), industrial by-products
(Bhatnagar and Jain 2005), agricultural solid waste (Wang The adsorbate studied was the Rhodamine B (RB) cationic
et al. 2008b), clays (Lian et al. 2009; Öztürk and Malkoc dye with 90% purity grade (m/m), provided by the company
2014), turf (Fernandes et al. 2007), bacterial biomass Contemporary Chemical Dynamics LTDA. Table 1 shows us
(Ratnamala and Brajesh 2013), and polysaccharides (Saha the RB physicochemical properties. The clay used was the
et al. 2011). However, many of these adsorbents present slow Brazilian natural sodium bentonite, provided by the indus-
adsorption or inconvenient separation and regeneration rates, try Latin Alliance Commerce LTDA from the city of
which prevent them from being applied in large scale in the Uruguaiana/RS.
water treatment. Moreover, the adsorbent with characteristics
such as fast rate adsorption and easy separation is extremely
desirable (Zeng et al. 2014). Experimental procedures
The bentonite clay is an abundant and low-cost natural
material; it has high cation exchange capacity, large specific Calcined clay preparation The in natura clay granulometric
area, possible interlamellar expansion, and a large volume of analysis was realized by sieving in a Bertel sieve shaker, with
intercalation molecules (Pereira et al. 2017). Bentonites ad- a set of ABNT (Brazilian Association of Technical Standards)
sorptive property is attributed to the lamellar structure formed sieves with opening dimensions of 32, 48, 60, 100, and 200
by octahedron aluminum layers between two others of silicon mesh, for 30 min and 8 rpm. The in natura clay, classified and
tetrahedron. The substitution of Al3+ by Mg2+ or Fe2+ in the selected in a 200 mesh sieve, was subjected to the calcination
octahedral leaves results in negative charges that are process in the Mufla SP Labor furnace, SP-1200 model, at
counterbalanced by exchangeable cations, such as Na+, 500 °C for a period of 24 h.
Ca2+, K+, and Mg2+, located in the interlamellar space
(Kausar et al. 2018). In the adsorption process, these cations Adsorbent characterization The in natura and calcined clay
can be replaced by other molecules or neutralize the active were classified and selected with a particle size of 0.074 mm
hydroxyl groups. Looking at this perspective, the cationic (200 mesh). The X-ray diffraction patterns were obtained
molecules have good interaction with clays (Kittinaovarat using a diffractometer (Rigaku Rotaflex, model RU200B),
et al. 2010; Wang et al. 2008a) and Rhodamine B adsorption equipped with Cu anode, operating at 60 mA and 40 kV.
may be viable. The standards were recorded from 5° to 40° (2θ), range where
This mineral capacity to absorb water and expand prevents the peaks are observed concerning Montmorillonite interest
it from being applied in fixed beds for dye removal on an (Bertagnolli et al. 2011; Nones et al. 2015; Vieira et al.
industrial scale (Bertagnolli et al. 2011). In order to modify 2010) and speed of 2° min−1. The specific surface area and
the bentonites physicochemical properties, they can be sub-
jected to treatments with temperature, acids, functionalization, Table 1 Physicochemical Rhodamine B dye properties
pillarization, and composition with other materials (Huang
Parameter Character/value
et al. 2016); the method choice depends on the final purpose.
In the thermal treatment, non-crystalline phase transforma- Class Triphenylmethanea
tions occur that aim to increase the adsorption and improve CAS number 81-88-9
the mechanical properties in the material, increasing its usabil- Molecular formula C28H31CIN2O3b
ity beyond the natural form (Auta and Hameed 2014). Molecular weight 479.02 g mol−1b
In this paper, the bentonite clay calcination and character- Maximum wavelength (λmax) 554 nma
ization were performed with the main objective of obtaining a pKa 3.7c
material with a high capacity and a simple removal rate, prac-
a
tical, and low-cost. The pH effects, temperature, and dye ini- Baddouh et al. (2018)
b
tial concentration were investigated. Based on the kinetic and Khamparia and Jaspal (2017)
c
thermodynamic study of Rhodamine B adsorption in Wang et al. (2014)
Environ Sci Pollut Res (2019) 26:28622–28632 28624

pore distribution were determined by BET, N2 quantification determination. The limiting or slower step is the one which
technique to form a monolayer adsorbed on a solid surface of is responsible to control the process, and may be the mass
an adsorbent. We used the model 2020 ASAP, Micromeritics adsorbate transfer from the fluid phase to the adsorbent outer
equipment temperature of − 196.5 °C. The morphology of the surface, adsorbate diffusion into the material pores, and the
surface of bentonite was investigated in a scanning electron physical interaction or chemical reaction between adsorbent/
microscope (SEM), JEOL JSM-6360LV with secondary elec- adsorbate (Fogler 2009). The kinetic parameters were cal-
tron detector for high vacuum, electron backscattered detector culated from the adsorbed quantity qe (mg g−1) data, dye
for high and low vacuum, EDS detector (energy dispersive X- initial concentration Co (mg L−1), equilibrium concentra-
ray spectrometer), and EBSD detector (electron backscatter tion in solution Ce (mg L−1), solution volume V (L), and
diffraction), running the tests with 15 and 30 kV. For this, adsorbent mass m (g) that can be related by Eq. 1 (Auta
samples of in natura and calcined bentonite clay were previ- and Hameed 2014).
ously atomized with gold, making the electron conductive
ðC 0 −C e ÞV
surface. The structural bentonite composition was analyzed qe ¼ ð1Þ
by infrared absorption test (FTIR). Initially, we measured the m
cesium iodide (CsI) spectra, which showed a low water quan- The kinetic adsorption behavior was obtained by experi-
tity, remembering that this control is important, since CsI is mental data fit to the linearized pseudo-first order (Eq. 2) and
very hygroscopic. Thus, the existing water is a component of pseudo-second order (Eq. 3) models (Nascimento et al. 2014).
the sample. The spectra were measured using Nicolet 6700
model equipment. The test conditions were 64 SCANS, lnðqe −qt Þ ¼ lnðqe Þ−k 1 t ð2Þ
4 cm−1 resolution and spectral range 400 to 4000 cm−1. This t 1 t
analysis was performed with a single particle size fraction ¼ þ ð3Þ
qt k 2 q2e qe
(0.074 mm), by evaluating the chemical bonds of the groups
present in the clay, which are not altered by changes in the size where k1 is adsorption constant rate of pseudo-first order
of the particles (Bertagnolli et al. 2011). (min−1); qe and qt are amounts adsorbed per gram of ad-
sorbent at equilibrium and at time t (min) in mg g−1; k2 is the
Adsorption experiments The studies here performed to eval- pseudo-second order adsorption rate constant (g mg−1 min−1).
uate adsorption were by adding (1 g L−1) thermally modified The values of k1 and k2 were found from linear regression of
clay in 50 mL of RB dye solution, both maintained at constant ln(qe − qt) against t and t qt−1 against t, respectively.
stirring and temperature of 200 rpm and 25 °C, respectively.
The system was monitored using the Lucadema Shaker incu-
Isotherm models and thermodynamic study
bator, Luca-223 model, controlling time contact of 120 min
that was settled. The pH (3–10) was controlled using solutions
Starting from the isotherms, it is possible to obtain the maxi-
of HCl (1.0 and 0.1 mol L−1) and NaOH (1.0 and 0.1 mol L−1)
mum amount adsorbed by the adsorbent mass and to calculate
with the aid of a benchtop pH meter (Hanna PH21 model),
the parameters related to the thermodynamic adsorption prop-
keeping the RB initial concentration constant, 400 mg L−1.
erties. The isothermal models studied were Langmuir (Eq. 4)
Then varied the dye initial concentration (300–500 mg L−1)
and Freundlich (Eq. 5) (Konggidinata et al. 2017) that allowed
and remained fixed in pH 3. In order to analyze equilibrium
inferring in the adsorption mechanism and in the adsorbent
concentrations, absorbances were measured using the
characteristics.
Micronal UV-VIS spectrophotometer, AJX-1600 model spec-
trophotometer, 554 nm wavelength, using a 1 cm optical path C eq 1 C eq
¼ þ ð4Þ
quartz cuvette. The assays to obtain the isotherms adsorption qe qmax K L qmax
equilibrium were performed at pH 3; it was kept doing the 1
necessary solution applications HCl (1.0 and 0.1 mol L−1) lnqe ¼ lnK F þ lnC eq ð5Þ
n
and NaOH (1.0 and 0.1 mol L−1) with the aid of a benchtop
pH meter for the its control. This way, RB initial concentration where Ceq (mg L−1) is the equilibrium concentration, qe
varied (350–600 mg L−1) and temperature (15–35 °C). (mg g−1) is the amount adsorbed by adsorbent mass, qmax
Calcined clay (50 mg) was added to 50 mL RB solution and (mg g−1) is the maximum adsorbed per adsorbent mass, and
the system was kept under stirring (200 rpm) for 60 min. K L (L mg −1 ) is the Langmuir adsorption constant; K F
(mg−(1/n) g−1L(1/n)) and n are Freundlich isotherm constants
Kinetic adsorption models for a given system at a specific temperature. qmax and KL were
calculated from the slope and intercept of plot of Ceq qe−1
The kinetic study is performed in order to know the rate law, against Ceq, while n and KF were calculated from the slope
mechanisms that govern the adsorption and limiting step and intercept of plot of ln qe against ln Ceq, respectively.
28625 Environ Sci Pollut Res (2019) 26:28622–28632

Table 2 Granulometric analysis

of in natura clay Mesh Upper diameter (mm) Bottom diameter (mm) Mass retained (g) Fraction retained (x%)

− 32 + 48 0.500 0.295 0.040 0.040

− 48 + 60 0.295 0.250 0.060 0.060
− 60 + 100 0.250 0.149 0.220 0.220
− 100 + 200 0.149 0.074 18.520 18.520
− 200 0.074 0.000 81.160 81.160

Three thermodynamic parameters were studied: Gibbs free 1 nm in width, and is considered a nanoparticulate material.
energy (ΔG°), enthalpy change (ΔH°), and entropy change Clay classification on sieves with smaller openings guarantees
(ΔS°). The values of ΔH° and ΔS° were calculated from the the attainment of free clay impurities, such as quartz and ka-
Eq. 7 (Ratnamala and Brajesh 2013), whereas Gibbs free en- olinite, which are naturally found on this type. Besides that, a
ergy was obtained by Eq. 8 (Konggidinata et al. 2017). granulometry with great fines presence provides the capacity
for cation exchange and a specific area increasing (Ferreira
K ads ¼ ρw K L ð6Þ 2009). Therefore, the sieve fraction 0.074 mm (200 mesh),
ΔS ° ΔH ° corresponding to 81% of the sample, it was selected for the
lnK ads ¼ − ð7Þ kinetic and thermodynamic tests in order to standardize the
R RT
adsorbent granulometry effect in the adsorption process.
ΔG° ¼ ΔH ° −TΔS ° ð8Þ

where Kads is the equilibrium constant, ρw is the water density Characterizations

(mg L−1), R the ideal gas constant (8.314 J/mol K), T the
absolute temperature (K), ΔS° (KJ mol−1 K−1), ΔG°, and Figure 1 shows the X-ray in natura and calcined clay
ΔH° (KJ mol−1) (Frantz et al. 2017). The ΔH° and ΔS° diffractogram. It can be observed that the in natura clay peak
values were calculated from the slope and intercept, respec- 2θ = 6.52° is observed in 2θ = 8.24° in the calcined clay.
tively of plot of ln Kads against T−1, by the Van’t Hoff’s These peaks represent the basal interplanar distance of the
method. montmorillonite which could be calculated by Bragg’s Law.
Following this perspective, it was found that the interplanar
distance was 13.53 Å for the in natura clay and reduced to
Results and discussion 10.72 Å after the thermal clay treatment. This result was
shown to be consistent since it indicates plane interlayer dis-
Granulometric classification tances (d001) reductions due to the hydration water elimination
from interlamellar cations (Rossetto et al. 2009). It happens
The data of granulometric analysis are shown in Table 2. because at 500 °C, the mineral clay dehydration and dehy-
The bentonite is composed of submicrometric lamellar par- droxylation causes structural alteration and mass loss
ticles with sizes in the order of 100 to 200 nm in length and

Fig. 1 X-ray in natura and calcined clay diffractogram Fig. 2 Nitrogen adsorption and desorption isotherms at − 196.5 °C
Environ Sci Pollut Res (2019) 26:28622–28632 28626

mesoporous those comprising between 2 and 50 nm di-

ameter and finally macropores, those greater than 50 nm.
As shown in Fig. 3, it can be noticed the predominance
of mesoporous in natura and calcined clays. The
mesopority bentonite clay result was also found by
Bertagnolli et al. (2011).
The values obtained for the surface area by the BET meth-
od and pore volume for samples of in natura and calcined clay
are shown in Table 3. From the N2 adsorption isotherms, the
micropore volume was obtained by reading the volume
adsorbed at p p0−1 = 0.10 and the mesopore volume given by
the value found in p p0−1 = 0.95 subtracted the value in
p p0−1 = 0.10 (Gañán-Gómez et al. 2006). The calcination
contributed to the surface area reduction and volume of mi-
Fig. 3 Increase in intrusion (cm3 g−1) versus pore diameter (nm) on cropore and mesopore, which also occurs at the pore collapse
bentonite clay
structure (Anadão et al. 2014; Auta and Hameed 2012).
The electronic microscopes have the function of observing
(Bojemueller et al. 2001). In addition, the system can be ap- the morphological aspects of the materials surface, with a
plied in fixed bed, as the basal spacing reduction minimizes large increase (in the order of 2 to 5 nm) and good spatial
the clay expansion in water, preventing the column filling and resolution. In this way, they provide pictures with very easy
effluent flow passage. interpretation. Thus, analyzing the micrographs, Fig. 4, it was
Through this work, by BET analysis, it can be inferred that possible to note a typical lamellar morphology of the benton-
the N2 adsorption isotherms in natura and calcined bentonite ite, both in natura and calcined clay. Structural collapse after
clay would from type II, as shown in Fig. 2, according the calcination was visible, as it can be viewed in Fig. 4a and b, in
updated classification of isotherms of physisorption was pre- which the clay presented an Bexpanded^ appearance when
sented by Thommes et al. (2015) on his IUPAC (International compared to Fig. 4d and e, which showed to be more
Union of Pure and Applied Chemistry) technical report. The compacted. This aspect prove the interllamelar spacing reduc-
type II isotherm presents itself in the form of S or sigmoidal, tion presented in the analysis of the XRD and it is in line with
being found in non-porous solids or with larger pores than the BET result, since the structure collapse favors the material
micropores (Vieira et al. 2010). Another characteristic of this surface area reduction. It was also possible to notice a slight
model is in unrestricted monolayer adsorption to high values increase in surface roughness after thermal treatment, when
of p p0−1 and the thickness of multilayered sound increases observing Fig. 4c and f (Anadão et al. 2014; Nones et al. 2015;
without limits when p p0−1 = 1. The point B, which marks the Rossetto et al. 2009).
beginning of the section approximately linear, represents the The FTIR spectra were used to evaluate the group chemical
end of monolayer coverage. A more curvature, at point B, bonds in bentonite samples, before and after calcination, as
means that there is a large overlap monolayer coverage and illustrated in Fig. 5. Tracks from bands between 470 and
the beginning of multilayer adsorption. The hysteresis loop 1120 cm−1 correspond to the filosilicate structure, which is
shape that was found is very similar to H4 type, suggest- associated with the elongation and angular deformation of
ing that the adsorption is more pronounced at low values Si–O–Si and Si–O–Al. Such vibrations occur within the crys-
of p p0−1, being associated with the micropore completion. talline structures and are not affected by other cations present
In addition, the H4 type is found in crystals of some zeolites, in all cuts (Li et al. 2008). In this track, we observed three
some mesoporosos, and micro-mesoporosos (Thommes et al. relevant spectra: stretching of the Si–O bond in the 1120 and
2015). 1040 cm−1, circular deformation of Si–O around 470 cm−1,
The IUPAC (1985) classifies in your reporting the octahedral layers represent in the range between 790 and
pore diameter as those smaller than 2 nm micropores, 800 cm−1 (Bertagnolli et al. 2011). The range of 1625–

Table 3 Structural parameters of

natural bentonite and calcined Clay Surface area R2 Volume of micropore Volume of mesopore
−1
2
(m g ) (cm3 g−1) (cm3 g−1)

Natural 46.177 0.999 11.837 16.224

Calcined 30.299 0.999 7.684 14.943
28627 Environ Sci Pollut Res (2019) 26:28622–28632

Fig. 4 Micrographs clay samples: in natura with the resolution 2 μm (a), 1 μm (b), and 500 nm (c); calcined with resolution 2 μm (d), 1 μm (e), and
500 nm (f)

1635 cm−1 corresponds to the H–O–H angular deformation, pH effect

while the 3500 to 3645 cm−1 corresponds to O–H stretching
(Xu et al. 2000). The range intensity has been reduced by For pH (3 to 10) effect analysis, the experiment was per-
calcination. This suggests the water and silicon displacement formed in acid and alkaline medium, maintaining the initial
caused by thermal treatment. concentration of RB of 400 mg L−1. It can be seen in Fig. 6
that the pH reduction slightly favored the RB dye increased
removal. Below pH 3.5, Rhodamine B molecules are in the

Fig. 6 Kinetic study to evaluate the pH variation effect (3, 6, and 10) on
Fig. 5 FTIR spectrum in natura and calcined bentonite clay the RB adsorption (400 mg L−1) in calcined bentonite (1 g L−1) at 25 °C
Environ Sci Pollut Res (2019) 26:28622–28632 28628

Fig. 8 Adsorption isotherms of RB in calcined clay (1 g L−1), for a range

Fig. 7 Kinetic study to evaluate the initial concentration variation effect of dye concentration 350 to 600 mg L−1, at temperatures of 15, 25, and
(300, 400, and 500 mg L−1) on the RB adsorption in calcined bentonite 35 °C, at pH 3
(1 g L−1), pH 3 at 25 °C
The experimental data were fitted to the pseudo-first order
monomeric form, so that the molecule can easily go into ad- and pseudo-second order. The best-correlated model was the
sorbent pores. Above pH 3, Rhodamine B in water takes the pseudo-second order, as can be seen in Table 4. This model is
zwitterionic form, in which there is an aggregation of based on the adsorption capacity of the solid phase and con-
Rhodamine B molecules, resulting in larger molecules. siders that the driving force of the adsorption is the difference
In this way, the input of this molecule inside the pores between the solid phase concentration at any time and at equi-
is hampered, reducing the adsorption (Anandkumar and librium (Mesquita et al. 2017).
Mandal 2011; Ma et al. 2016).
Adsorption isotherms
Initial concentration effect
The adsorption isotherms were performed at the initial con-
The initial concentration effect study was performed by vary- centration range of 350–600 mg L−1 at pH 3, at 15, 25, and
ing the RB concentration of 300 to 500 mg L−1, maintaining 35 °C temperatures. The isotherms are shown in Fig. 8 and
pH 3 and 25 °C. Through Fig. 7 analysis, it is observed that the from these data, it was possible to determine the thermody-
initial concentration increasing favors a better dye adsorption. namic properties and the best correlated isotherm model.
It can be explained by the fact that the initial concentration It can be seen in Table 5 the Langmuir and Freundlich
enlargement, a driving force for the occurrence of adsorption correlations and the respective values of the calculated param-
increases (Goel et al. 2005), furthering the mass transfer of eters of each model. Thus, it can be observed that the exper-
dye from the solution to the surface of the adsorbent. imental data were better fitted by the Langmuir model,

Table 4 Parameters and fit values of pseudo-first and pseudo-second order to the experimental data obtained by the kinetic adsorption assays of RB in
calcined clay

Parameters Pseudo-first order Pseudo-second order

qe (mg g−1) k1 (min−1) R2 qe (mg g−1) k2 (g mg−1 min−1) R2

pH
3 65.171 0.028 0.568 400.000 0.002 0.999
6 128.161 0.016 0.732 377.358 0.001 0.998
10 112.739 0.023 0.830 381.679 0.002 0.999
Initial concentration
(mg L−1)
300 113.309 0.322 0.838 319.488 0.001 0.999
400 65.171 0.028 0.568 400.000 0.002 0.997
500 75.776 0.025 0.852 500.000 0.002 0.999
28629 Environ Sci Pollut Res (2019) 26:28622–28632

Table 5 Adsorption parameters for Langmuir and Freundlich models Table 7 Thermodynamic Rhodamine B adsorption in calcined
RB clay in batch fit bentonite parameters

Isotherms Parameter 15 °C 25 °C 35 °C Parameter 15 °C 25 °C 35 °C

Langmuir Kads × 10 6.30 9.60 18.30 ΔH° (KJ mol−1) 19.33 39.82 58.97
(L g−1) ΔS° (KJ mol−1 K−1) 0.17 0.24 0.31
qmax 467.29 483.09 552.49
ΔG° (KJ mol−1) − 31.98 − 34.12 − 36.92
(mg g−1)
R2 0.99 0.99 0.99
Freundlich KF 329.60 378.50 403.90
(mg−(1/n) g−1 L(1/n)) favored the disorder increase and randomness at the solid so-
n 13.62 20.17 11.26 lution interface. The ΔG° was negative and increased in mod-
R2 0.93 0.85 0.88 ulus with increasing temperature, indicating that the adsorp-
tion process is spontaneous and more favorable at higher tem-
peratures (Nascimento et al. 2014).
according to the coefficient of determination found (R2). The
Langmuir model admits that adsorption is homogeneous and
monolayer, occurring only in finite numbers of identical and
equivalent sites without interaction between adsorbed mole- Conclusion
cules (Patil et al. 2016). In addition, it was observed that the
maximum amount adsorbed and the Langmuir adsorption The study showed that the clay can be used as Rhodamine B
constant were favored with increasing temperature. This can adsorbent in aqueous solutions, being efficient in dye removal
be explained by the fact that the increase in temperature can and low-cost compared to other adsorbents and retention
lead to the increase of the kinetic energy and the mobility of methods. The bentonite clay was submitted to calcination
the adsorbate species, increasing the rate of intraparticle dif- which is a simple treatment for modifying the structure of
fusion of adsorbate (Jimenez et al. 2004), thus furthering the the material and which results in the improvement of mechan-
adsorption. This can also be observed in Fig. 8. ical properties, minimization of expansion in water, allowing
Table 6 shows the adsorption capacity of other adsorbents it to be used in fixed bed columns. Besides loss of hydration
widely used in RB retention. Making a comparison of the water, which caused the collapse of the structure and, conse-
maximum adsorbed amount per gram of adsorbent found by quently, reducing the surface area and pore volume, but without
the Langmuir isotherm (552.49 mg g−1), it can be seen that the changing the morphology and chemical composition of the
calcined bentonite is an efficient adsorbent in dye removal, clay in a significant way. At this paper, it was possible to con-
besides having low-cost, it also has desirable characteristics clude that the Rhodamine B adsorption was favored at acid pH
for treatment of large effluent volume. and the increase of the initial dye concentration, increasing the
The thermodynamic parameters, enthalpy, entropy, and adsorption rate. The analysis made of kinetics adsorption re-
Gibbs energy were studied to understand the effect of temper- vealed that the data were better correlated to the pseudo-second
ature on adsorption. The data obtained are shown in Table 7. order model (R2 > 0.99). The Langmuir isotherm obtained a
As it can be observed, positive values of ΔH° were found for better fitted (R2 > 0.99), showing that the adsorption occurs in
different RB concentrations indicating the endothermic nature monolayer and homogeneously, in addition to obtaining a max-
of the interactions between adsorbent and adsorbate. This imum adsorption capacity of 552.49 mg g−1. Going through
trend is confirmed by examining Fig. 8 and Table 5 which thermodynamic parameters, it can be inferred that the adsorp-
show that with increasing temperature, there was an adsorp- tion was endothermic (ΔH° from 19.33 to 58.97 KJ mol−1) and
tive increasing capacity. Medium positive values of ΔS° spontaneous (ΔG° from − 31.98 to − 36.92 KJ mol−1).

Table 6 Langmuir adsorptive

comparison capacities of isotherm Adsorbent pH qmax (mg g−1) References
for RB in other adsorbents
Activated carbon – 478.5 Ding et al. (2014)
Fe-montmorillonite 4 258.76 Ma et al. (2016)
Fe-bentonite 5 98.62 Hou et al. (2011)
Sodium montmorillonite 7 42.19 Selvam et al. (2008)
Acid-treated montmorillonite 6.9 188.67 Bhattacharyya et al. (2014)
Acid-treated kaolinite 6.9 23.70 Bhattacharyya et al. (2014)
Environ Sci Pollut Res (2019) 26:28622–28632 28630

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