Phenol Red Adsorption from Aqueous Solution on the Modified Bentonite

In the present work, the modified bentonites were prepared by the modification of bentonite with cetyltrimethylammonium bromide (CTAB), both cetyltrimethylammonium bromide and hydroxy-Fe cations and both cetyltrimethylammonium bromide and hydroxy-Al cations. X-ray diffraction (XRD), thermal analysis (TG-DTA), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and nitrogen adsorption/desorption isotherms were utilized to characterize the resultant modified bentonites. ,e modified bentonites were employed for the removal of phenol red dye from aqueous solution. Phenol red adsorption agreed well with the pseudo-second-order kinetic model.,e equilibrium data were analyzed on the basis of various adsorption isotherm models, namely, Langmuir, Freundlich, and Dubinin‒Radushkevich models. ,e highest monolayer adsorption capacity of phenol red at 30°C derived from the Langmuir equation was 166.7mg·g, 125.0mg·g, and 100.0mg·g for CTAB‒bentonite, Al‒CTAB‒bentonite, and Fe‒CTAB‒bentonite, respectively. Different thermodynamic parameters were calculated, and it was concluded that the adsorption was spontaneous (∆G°< 0) and endothermic (∆H°> 0), with increased entropy (∆S°> 0) in all the investigated temperature ranges.


Introduction
In recent years, dyes are used widely in many industries such as textiles, paper, printing, and cosmetics. Besides the economic benefits, these industries release a large amount of highly toxic dyes in wastewater every year, most of which have negative effects on human and animal health. Various techniques such as adsorption [1][2][3][4], catalysis [5,6], electrochemical oxidation [7], ozonation [8], membrane filtration [9], biological treatment [10], and enzymes [11] have been used to remove the dye components from polluted wastewater. Phenol red is a fabric dye and if we are exposed to the dye, it will affect the respiratory system and human skin [12]. erefore, many research studies focus on the preparation of adsorbents or redox agents for the removal of phenol red. Actually, oxidation and adsorption are the two main methods for the removal of phenol red. In recent reports on the first method, advanced oxidation processes have received the most attention [14][15][16]. For the second one, Abdullah et al. [13] prepared composite silica-filled ENR/PVC beads for phenol red adsorption. e results showed that the maximal adsorption capacity is 2.67 mg·g −1 . Iqbal et al. [14] found that the phenol red adsorption process on activated carbon materials was spontaneous with increasing entropy. e kinetic and thermodynamic studies of the phenol red absorption on bottom ash and deoiled soya were investigated very specifically by Mittal et al. [12]. e highest monolayer adsorption capacity has been obtained for the phenol red-bottom ash system (2.6 × 10 −5 ·mol·g −1 ) at 50°C. Recently, many researchers have been using natural materials like bentonite and modified bentonite for the removal of phenol and other dyes [15][16][17][18][19][20][21][22][23][24][25][26]. Organobentonites have been formed by introducing organic cations into bentonite layers. e purpose of organic bentonite synthesis is to create hydrophobic materials from hydrophilic form, so that they can be used as sorbents for hydrophobic organic compounds. Meanwhile, the inorganic bentonites with hydrophilic surfaces are suitable for adsorbing heavy metals. us, simultaneously inserting both metal ions and organic substances into bentonite, a new material is created named inorganic-organic bentonite. Because of the novel properties of the inorganic-organic bentonite, their structural characteristics have drawn great interest in recent years. Intercalation methods, intercalation agents, and bentonite/organic compound/inorganic compound ratios will affect the structures of inorganic-organic bentonite. To date, few studies have clarified the adsorption of the dyes onto organic bentonite and inorganic-organic bentonite.
In the work, the synthesis of CTAB-bentonite, Al-CTAB-bentonite, and Fe-CTAB-bentonite and phenol red adsorption are demonstrated. e kinetics, isotherms, thermodynamics, and the mechanism of phenol red adsorption process on modified bentonite materials are addressed.  3 .9H 2 O, Guangzhou, 99%), and silver nitrate (AgNO 3 , Guangzhou) were used in this study.

Preparation of CTAB-Modified Bentonite and Fe-CTAB-Modified and Al-CTAB-Modified Bentonites.
e CTAB bentonite was synthesized as follows: 1.0 g of the purified bentonite (B) in 100 mL distilled water was stirred for 1 h. e 100 mL solution containing 0.8 g of CTAB was dissolved completely by vigorous stirring for 1 h and then was dropped slowly into the bentonite dispersion. e mixture was stirred with a magnetic stirrer for 4 h at 50°C. e obtained mixture was kept at 60°C for 20 h. e modified bentonite was washed many times with deionized water, dried at 100°C for 5 h, and then ground into the powder. e obtained CTABbentonite was denoted as CB.
e synthesis of Fe-CTAB-bentonite was carried by the following steps. Firstly, the Fe pillaring solution was prepared from the mixture of 100 mL 0.1 M Fe (NO 3 ) 3 .9H 2 O solution, and 30 mL 0.1 M NaOH (OH − /Fe 3+ molar ratio � 0.3). en, it was stirred for 2 h and kept at room temperature for 24 h. Secondly, the 1% CB suspension was prepared by dispersing 1 g of CB in 1 L of deionized water. en, the Fe pillaring solution prepared above was dropped slowly into the CB suspension. e mixture was stirred for 24 h at room temperature. e resultant solid was separated by centrifugation and dried at 100°C for 10 h to obtain the Fe-CTAB-bentonite (denoted as FeCB). e Al-CTAB-bentonite was synthesized in the same manner, except for the pillaring solution which was prepared by adding drop by drop 200 mL of 0.1 M NaOH solution to 100 mL 0.1 M AlCl 3 solution. is resultant pillaring bentonite suspension was stirred for 7 h at 70°C and then kept at room temperature for 24 h. e Al-CTAB-bentonite solid was separated by centrifugation, dried at 100°C, and denoted as AlCB.

Characterization.
e purified and modified bentonites were studied by X-ray diffraction (XRD, D8-Advance Brucker, Germany with λ CuKα � 1.5406Å). e thermal curves of the products were obtained in a DTG-60H (Shimadzu). Fourier transform infrared spectra (FT-IR) using KBr pressed disk technique were gained on SHI-MADZU FT-IR 8010M. Nitrogen adsorption-desorption isotherms were measured on Tri Star 3000. e samples were outgassed for 5 h at 250°C in the degas port of the instrument. e specific surface areas were calculated using the BET model. SEM ima4ges were taken by scanning electron microscopy (Hitachi, S-4500). e point of zero charge values (pH PZC ) of the samples were calculated by the pH drift method.

Effect of pH.
e phenol red adsorption of modified bentonites was carried out in a batch system. An exact amount (0.05 g) of modified bentonite was placed into the 100 mL flask containing 50 mL of 100 mg·L −1 phenol red solution.
e pH solution was adjusted from 2 to 10 by adding amounts of 0.01 M NaOH or 0.01 M HCl, and the flasks were stirred by a magnetic stirrer for 6 h at room temperature (30°C) to reach equilibrium. After equilibrium, the supernatant was collected, and the concentration of phenol red was analyzed by UV-Vis method at λ max � 435 nm. e adsorption uptake of phenol red by the adsorbent was calculated by using where q t is the adsorption uptake of phenol red on the adsorbent at time t, C o and C t (mg·L −1 ) are the phenol red concentration at the initial time and a certain time, t, respectively, V (L) is the volume of phenol red solution, and m (g) is the mass of the adsorbent.

Adsorption Kinetics and ermodynamics Studies.
A measured amount (0.2 grams) of the adsorbent (B, CB, AlCB, or FeCB) was added to 250 mL of 100 mg·L −1 phenol red at pH 3.0. e mixture was stirred by a magnetic stirrer at different temperatures (20, 30, 40, and 50°C). is suspension was then centrifuged after a particular time interval, and the phenol red concentration in the supernatant was measured by UV-Vis spectroscopy.
Kinetic data were applied to pseudo-first/second-order kinetic models [27]. e pseudo-first-order model is given by where q e and q t are the phenol red uptake of adsorbent at equilibrium, and at time t (mg·g −1 ), k 1 is the first-order rate constant (min −1 ). e pseudo-second-order kinetic model is given by where k 2 is the rate constant of the pseudo-second-order kinetic model (g·mol −1 ·min −1 ).

Equilibrium Adsorption
where C e is the equilibrium concentration of adsorbate in the solution (mg·g −1 ), q m is the monolayer maximum adsorption uptake of the adsorbent (mg·g −1 ), and K L is the Langmuir constant (L·mg −1 ) which is related to the energy of adsorption, respectively. e other parameters were described above. q m and K L can be calculated from the intercept and slope of the linear plot of C e /q e versus C e .
Additionally, the essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant called separation factor (R L , also called equilibrium parameter) which is defined by the following expression: where C o (mg·L −1 ) is the initial dye concentration and K L (L·mg −1 ) is the Langmuir constant. Freundlich equation [27] is given as where K F and n are the Freundlich constants. ese constants can be calculated from the slope and intercept of the linear plot of ln q e versus ln C e . e linear form of the Dubinin-Radushkevich isotherm [28] is given by where the constant β is related to the mean free energy E (kJ·mol −1 ) of adsorption per molecule of the adsorbate when it is moved to the surface of the solid from the solution (mol 2 ·J −2 ) and ε is the Polanyi potential which can be calculated by e mean free energy can be computed by using the following relation: e thermodynamic parameters including the standard free-energy changes of Gibbs, ∆G°, standard enthalpy changes, ∆H°, and standard entropy changes, ∆S°, were also studied to better understand the adsorption mechanism. e ∆G°was calculated based on e values of ∆H°and ∆S°can be calculated from the slope and intercept of the linear plot of ln K L versus 1/T.

Characterization of the Adsorbents.
Basal spacing is one of the most important factors reflecting the structural characteristics of the modified layered clays. XRD was employed to characterize the layer structures of the intercalated bentonite in this study. e XRD patterns of B, CB, AlCB, and FeCB are shown in Figure 1. e purified bentonite (B) exhibits the characteristic peaks of the montmorillonite according to JCPDS No. 00-003-0014. e changes of basal spacing of the modified bentonite can reflect the intercalations of CTAB, hydroxy-Al, and hydroxy-Fe cations into bentonite interlayer spaces. e bentonite (B) had a basal spacing (d 001 ) of 1.44 nm. For the CB, the (001) reflection was observed at a lower angle, corresponding to a larger basal spacing d 001 � 1.81 nm, which indicated that the CTAB intercalated into bentonite interlayer spaces. As reported by Hu et al. [29], the height of alkyl chain was about from 0.40 to 0.45 nm, and the value of 1.81 nm approximated to the sum of the height of the two alkyl chains and the bentonite thickness (0.96 nm), indicating that the alkyl chains took the arrangement of a bilayer in the interlayer. For AlCB, the peak in the pattern shifted to a smaller value of 2θ, and the corresponding basal spacing was 1.99 nm, which was larger than that of CB. is indicated that hydroxy-Al ([Al 13 O 4 (OH) 24 (H 2 O) 12 ] 7+ , Al 13 ) could be intercalated and expand the interlayers of CB even when the "channels and grooves" between bentonite layers were occupied by CTAB (or CTA + cations). For FeCB, there was also change in d 001 basal spacing (d 001 � 1.82 nm) indicating that polyhydroxy-Fe or polyoxo-Fe cations ([Fe 13 O 4 (OH) 24 (H 2 O) 12 ] 7+ ) intercalated into bentonite interlayer spaces [30], and there was an ion exchange between the intercalated surfactant cations and the polyhydroxy-Fe or polyoxo-Fe cations. It is notable that the (001) reflections of all modified bentonites were more intensive than that of purified bentonite (B), suggesting that these materials had better crystallinity than that of B sample. is might explain that when inserting CTA + cations into bentonite, CTAB acted as an ion exchanger with hydrated cation localized in the interlayer spaces of bentonite and promoted the rearrangement of the clay layers (Figure 2), so the material structure obtained was highly orderly, shown in sharp reflection (001) peaks. In other words, CTAB had "assembled" small bentonite particles into larger particles with an orderly structure due to the connection between CTAB particles with negatively charged edges of small bentonite particles.
FT-IR has been widely used for the characterization of the modified bentonite. FT-IR spectra between 4000 and 400 cm −1 for B, CB, AlCB, and FeCB are presented in Figure 3.
For B sample, the peak at 3414 cm −1 is related to the stretching vibration of the adsorbed water molecules [31], and the peak at 1638 cm −1 represented the deformation band (δ (O-H)). e peak at 964 cm −1 was due to Si-OH vibration [18], and the peak at 816 cm −1 corresponded to Fe-Fe-OH deformation vibration [32]. For organobentonite (CB), the new peaks at 2922 cm −1 and 2851 cm −1 were attributed to the antisymmetric and symmetric stretching vibrations of -CH 2 , respectively. e corresponding deformation modes of these groups were observed between 1470 and 1418 cm −1 [33]. e peak at 1488 cm −1 was assigned to the vibration of the quaternary amine functional group [33]. All of these vibration bands confirmed the modification of bentonite with CTAB. With the intercalation of CTAB, H 2 O content was reduced with the replacement of the hydrated cations by CTAB, and the surface of bentonite was changed from hydrophilic to hydrophobic. is could be explained by a decrease in the intensity of peak at the 1635 cm −1 of the CB sample. FT-IR spectra of organic-inorganic bentonite samples displayed a doublet at 2922 cm −1 and 2851 cm −1 with lower intensity compared with that of CB, indicating that some intercalated surfactants were exchanged by hydroxy-Al or hydroxy-Fe. For AlCB, a new peak appeared at 726 cm −1 with low intensity, and it corresponded to Al-O vibration in Al-OH, suggesting that Al 13 cations were intercalated into the layers of bentonite.
TG and DTA curves of the samples are shown in Figure 4. e TG curves of B and modified bentonites samples showed mass loss below 200°C and above 600°C due to the vaporization of adsorbed water or interlayer water in bentonite and the dehydroxylation of the silicate layers, respectively. A mass loss attributed to the decomposition of the organic compounds on the CB was observed between 200 and 600°C (11.9%). e decomposition took place in several steps, which could be seen in the DTA curve ( Figure 4(b)). e first strong DTA peak at 289.3°C corresponded to the decomposition of the organic compounds presented in CB, and the second small DTG peak at approximately 510°C was assigned to decompose the residuals of CTAB occluded inside capillaries of bentonite. e TG curves of AlCB and FeCB samples included the stages similar to those of CB. However, the two later mass losses of these samples were larger than those of CB (CB: ∆m 2 � 11.9%, ∆m 3 � 7.5%; AlCB: ∆m 2 � 29.9%, ∆m 3 � 9.3%; FeCB: ∆m 2 � 15.1%, ∆m 3 � 9.3%), suggesting that the stage in the range of the temperatures over 200°C could also be assigned to dehydration of hydroxy-Al or hydroxy-Fe cations.
e SEM micrographs of the materials are presented in Figure 5. e B sample consisted of small-sized clay particles and the particles presented face-face layer aggregations, which were classical arrangements for smectites. e morphology of CB consisted of clay sheets of a few micrometers in diameter, around which were smaller-sized particles, which were not observed in the original bentonite sample. e Fe-CTAB or Al-CTAB modified bentonite samples included sheets of several micrometers in size stacked on top of each other, showing that their layer structures were much clearer than those of the original B sample. In addition, the AlCB and FeCB samples showed less face to edge interactions, increasing the interactions face to face and gave consequently a more ordered morphology. is could be explained by the interaction between CTAB particles and hydroxyl-Al cations or between CTAB particles and hydroxyl-Fe cations inserted into bentonite, between these ions and bentonite layers, leading to the arrangement of the material mainly composed of bentonite layers (face-face). e nitrogen adsorption and desorption isotherms of the B, CB, AlCB, and FeCB materials are shown in Figure 6. e isotherms were all of type IV in accordance with IUPAC classification. Particularly for the CB sample, it was found that this material hardly adsorbed N 2 molecules, and the adsorption/desorption branches of N 2 sorption isotherm overlapped each other. Table 1 shows that the specific surface area was strongly decreased after intercalating organic cations or both inorganic and organic cations into interlayer spaces of bentonite, from 114.44 (B) to 0.25 m 2 ·g −1 (CB), 10.07 m 2 ·g −1 (AlCB), and 16.56 m 2 ·g −1 (FeCB). is could be explained that the large size of CTAB particles located in bentonite interlayer spaces or CTAB adsorbed at the outer surface of bentonite blocked the pathway of N 2 molecules. It could be argued that CTAB intercalated into the interlayers of bentonite (filling the micropores of about 5-6Å) and CTAB could also be located on the outer surface (in the mesopores created between primary bentonite particles), which made the specific surface significantly reduced and the mesopores system wider. erefore, the outside surface of bentonite would become hydrophobic and could not be able to receive large-sized hydrophobic molecules. Compared with B, the AlCB and FeCB samples had the larger specific surface area, suggesting that a part of the large organic cations was replaced by the inorganic cations of smaller size. In addition, it was found that the organic-inorganic bentonite samples contained no micropores or the windows of micropores were closed, so N 2 molecules could not be able to access these spaces.

Effect of pH.
e pH is one of the most important factors affecting the adsorption capacity of phenol red on modified bentonite surface. e change of pH affects the dissociation of functional groups on the adsorbent and then affects its affinity to adsorbate. e pH effect on the adsorption of phenol red on the modified bentonites is shown in Figure 7.
As shown in Figure 7, the phenol red absorption on the CB sample was generally less affected by pH than that on the AlCB and FeCB samples. e amount of the adsorbed phenol red increased slightly in the range of pH from 2.0 to 3.0 and then decreased slightly from 3.0 to 9.0. For AlCB and FeCB samples, the red phenol absorption had a similar trend: a maximum absorption at pH 3.0 and a remarkable decrease of adsorption capacity were observed when increasing pH from 3.0 to 9.0. is could be explained by electrostatic interactions between phenol red and adsorbents. In aqueous solution, phenol red exists in different forms of H 2 + PS − , HPS − , and PS 2depending on the pH of the solution (Scheme 1). us, in any environment, the red phenol also contains a group of sulfate with a negative charge. e pH PZC of CB, AlCB, and FeCB samples determined by the pH drift method was found to be 7.1, 6.3, and 5.8, respectively, so at pH < pH PZC , the modified bentonite surface was positively charged which was favorable for the adsorption of phenol red by electrostatic interaction. However, at a pH > pH PZC , the modified clay surface was negatively charged, causing a repulsion force between bentonite surface and the phenol red as well as OH − anion (hydroxyl ions and phenol red could compete for adsorption at high pH), resulting in the decrease of the adsorption capacity when pH increased. In addition, due to the presence of CTAB in bentonite, phenol red absorption on modified bentonite was also conducted by ligand exchange, but this process did not involve a change  in pH (this was verified experimentally): CTA + -Br − + HPS − ⟶ CTA + -HPS − + Br − In short, the reduction of phenol red adsorption over the pH range could be related to two possible mechanisms: electrostatic interaction and chemical reaction between modified bentonites and phenol red. From this result, the optimum pH 3.0 for red phenol adsorption on modified bentonite samples was chosen for further studies.

Adsorption Kinetics.
e kinetics data obtained at different temperatures using the pseudo-first and pseudosecond kinetic models are presented in Figures 8-10.
Parameters of the pseudo-first-order and pseudo-second-order kinetic model are listed in Table 2.
Based on the value of R 2 and the difference of q e calculated by two models compared with the empirical q e , it was found that pseudo-second-order model showed better goodness of fit to the experimental data than the pseudofirst-order model. Similar results were reported for the phenol red on bottom ash and deoiled soya [12]. From Table 2, it can be seen that the k 2 value increased with the temperature for all three samples (CB: 0.778.10 −3 -1.524.10 −3 g·mg −1 ·min −1 , AlCB: 0.73.10 −3 -2.53.10 −3 g·mg −1 ·min −1 , FeCB: 4.1.10 −3 -6.53.10 −3 g·mg −1 ·min −1 ). Under the same conditions, the phenol red adsorption rate on FeCB sample was faster than that on other samples. e interesting result of the simultaneous insertion of organic and inorganic species into bentonite was that the adsorption capacity of phenol red was reduced, but the adsorption rate increased compared to those of bentonite intercalated by organic cations. e phenol red has a molecular size of about 15Å, so the phenol red absorption had to occur in the mesopores of materials (∼several hundredÅ) and could not occur between the smectite layers (∼5-10Å). e values of k 2 of all modified bentonite samples were the same order (10 −3 ), which suggested that the phenol red adsorbed mainly at the outer surface and in the capillaries created between the primary particles. e values of rate constants from the pseudo-secondorder model can be used to calculate the activation energy of the sorption process by the Arrhenius equation. Figure 11 depicts the Arrhenius plots of phenol red adsorption onto CB, AlCB, and FeCB.      Journal of Chemistry e values of the activation energy were found to be 17.82 kJ·mol −1 , 33.87 kJ·mol −1 , and 17.52 kJ·mol −1 for the phenol red adsorption onto CB, AlCB, and FeCB, respectively. e magnitude of activation energy can give information if the adsorption process is physical or chemical. e value of E a was not large, indicating that phenol red adsorption on the three investigated materials was mainly physical adsorption (due to electrostatic interaction) as analyzed above. Comparing the E a values of phenol red adsorption on the three materials, the E a value of AlCB material was the highest.
is suggested that phenol red adsorption on CB and FeCB materials was more advantageous in terms of energy than that on AlCB material.

Adsorption
Isotherms. e mechanism of adsorption reactions could be determined by evaluating the equilibrium data obtained from the experiments. e experimental data plotted with the Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models are shown in Figures 12-14. e estimated adsorption constants with the correlation coefficient obtained from the isotherms are listed in Table 3.   In terms of the correlation coefficient R 2 values, the experimental data could be well fitted to the Langmuir isotherm model rather than the Freundlich and D-R models at all investigated temperatures, indicating that the adsorption process occurred on the surface of modified bentonites. e calculated values of R L were found to be between 0 and 1, indicating that the adsorption of phenol red onto CB, AlCB, and FeCB samples was favorable. At the same conditions, the value of q max calculated from the Langmuir equation of all samples decreases in the order of CB, AlCB, and FeCB in the temperature range 20-40°C. e maximum phenol red adsorption capacities (q max ) at 30°C (room temperature) are 166.7 mg·g −1 for CB, 125.0 mg·g −1 for AlCB, and 100 mg·g −1 for FeCB. It is known that the specific surface area of CB was smaller but its adsorption capacity is higher than that of other two materials. is suggested that the presence of hydroxy-Al and hydroxy-Fe cations in CTAB-bentonite reduced the phenol red adsorption capacity of this material. In addition, the K L value at the same temperature on three investigated materials was different illustrating that active sites on these three materials were different. In comparison with previous studies [25,26], the phenol adsorption capacity of organobentonite was higher than phenol red adsorption capacity of CTAB-bentonite in this study; this could be explained by the fact that the size of phenol was smaller than that of phenol red, so it was more easily diffused into the capillary than phenol red. However, it is notable that the present adsorbents exhibit much higher adsorption capacity compared with those of adsorbents for phenol red reported previously, suggesting that modified      bentonite materials are attractive candidates for dye removal. Table 4 presents the absorption capacities based on the Langmuir model of some adsorbents.

ermodynamics Studies.
e plot of lnK L versus 1/T for the determination of thermodynamic parameters is depicted in Figure 15 and the thermodynamic parameters are listed in Table 5.
e negative values of ∆G°at different temperatures indicated the feasibility of the process and the spontaneous nature of the adsorption. e positive ∆H°values indicated that the adsorption of phenol red onto CTAB-bentonite and Fe-or Al-CTAB bentonites was endothermic. Furthermore, a higher positive ∆S°of phenol red adsorption process clearly stated that the randomness increased the modified bentonite-solution interface during adsorption.
is result suggested that the phenol red adsorption onto three investigated materials was promoted by entropy than by enthalpy.

Conclusions
e synthesis of CTAB-bentonite, Al-CTAB-bentonite, and Fe-CTAB-bentonite by ion exchange was demonstrated. It was found that the CTAB, hydroxy-Al, and hydroxy-Fe cations were intercalated into the interlayer spaces of bentonite leading to an increase in the basal spacing d 001 . Among the investigated materials, CTABbentonite exhibited the highest monolayer adsorption capacity at the temperature of 30°C (166.7 mg·g −1 ), followed by Al-CTAB-bentonite (125.0 mg·g −1 ) and Fe-CTAB-bentonite (100 mg·g −1 ). Introduction of hydroxy-Al and hydroxy-Fe cations into CTAB-bentonite clearly reduced the phenol red adsorption capacity of this material. Phenol red adsorption agreed well with the pseudo-second-order kinetic model in all the investigated temperature ranges. e results of activation energy calculation showed that phenol red adsorption on CTAB-bentonite and Fe-CTAB-bentonite materials was more advantageous in terms of energy than that on Al-CTAB-bentonite. e negative values of ∆G°indicated that the adsorption was spontaneous whereas the positive values of ∆H°and ∆S°indicated the endothermic nature and increase in randomness of the adsorption, respectively. From these studies, organic and organic-inorganic bentonite materials could be used as an effective adsorbent for the removal of dyes from aqueous solution.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.