Adsorption Process and Properties Analyses of a Pure Magadiite and a Modified Magadiite on Rhodamine-B from an Aqueous Solution

The result of an adsorption experiment indicated that the pure magadiite (MAG) and the modified MAG via cetyltrimethylammonium-bromide (CTAB-MAG) possessed pronounced affinity to the Rhodamine-B (Rh-B) dye molecules. CTAB-MAG was synthesized with an ion-exchange method between MAG and cetyltrimethylammonium-bromide (CTAB) in an aqueous solution. The adsorption capacities of CTAB-MAG and MAG on Rh-B were 67.19 mg/g and 48.13 mg/g, respectively; while the pH and the time were 7 and 60 min, respectively; however, the initial concentration of Rh-B was 100 mg/L, and adsorbent dosage was 1 g/L. Whereas, the adsorption capacity of CTAB-MAG was increased by 40% over MAG which indicated that CTAB-MAG can be used as an efficient low-cost adsorbent. Adsorption kinetics were consistent with the pseudo-second-order kinetic equation; the adsorption processes were dominated by film diffusion process which belonged to monomolecular layer adsorption.


Introduction
Many dyes are toxic for human health, and many dyes are widely used in numerous industries; however, a synthetic pink dye called Rhodamine-B (Rh-B) has been widely used as a pigment for textiles, food production, and biological staining (in biomedical research laboratories). But it is difficult to degrade because of stable chemical structure, as can be seen from Figure 1. There are many kinds of methods used to treat wastewater containing Rh-B, such as electrochemical oxidation [1], catalytic Magadiite (MAG), which was discovered by Eugster in Kenya's saline lake [7], is a natural layered silicate mineral; its plates present a rose petal shape; it presents as a white powder under normal circumstances. There are so many hydrated sodium ions between the plates that the cationexchange capacity (CEC) is higher than other silicates, such as montmorillonite and can reach up to 2.22 meq/g [8], which was determined from the ideal formulation of MAG (Na2Si14O29). So far, the cation-exchange properties of MAG investigated [9][10][11][12][13], indicate that MAG can be used as an adsorbent, based on cation exchange [14,15]. According to the current study, MAG can be synthesized [16][17][18][19][20][21]; the laminate of MAG was composed of SiO4 and has no other impurities. Therefore, the structure of the MAG is very stable and has good chemical stability [22][23][24]. The Figure 2 shows the lamellar structure of the MAG; the lines of squares, hexagons and octagons, were expressed as Si-O-Si bond [25]. So far, there are many ways to modify MAG based on ion exchange. Cetyltrimethylammonium (CTMA) [26,27], heterocyclic ammine [28], and octyl triethoxysilane (OTES) [29] have been used to modify MAG, based on cation exchange, which could effectively expand layer spacing and elevate Magadiite (MAG), which was discovered by Eugster in Kenya's saline lake [7], is a natural layered silicate mineral; its plates present a rose petal shape; it presents as a white powder under normal circumstances. There are so many hydrated sodium ions between the plates that the cation-exchange capacity (CEC) is higher than other silicates, such as montmorillonite and can reach up to 2.22 meq/g [8], which was determined from the ideal formulation of MAG (Na 2 Si 14 O 29 ). So far, the cation-exchange properties of MAG investigated [9][10][11][12][13], indicate that MAG can be used as an adsorbent, based on cation exchange [14,15]. According to the current study, MAG can be synthesized [16][17][18][19][20][21]; the laminate of MAG was composed of SiO 4 and has no other impurities. Therefore, the structure of the MAG is very stable and has good chemical stability [22][23][24]. The Figure 2 shows the lamellar structure of the MAG; the lines of squares, hexagons and octagons, were expressed as Si-O-Si bond [25].
Processes 2018, 6, x FOR PEER REVIEW 2 of 12 difficult to degrade because of stable chemical structure, as can be seen from Figure 1. There are many kinds of methods used to treat wastewater containing Rh-B, such as electrochemical oxidation [1], catalytic degradation [2], photocatalytic degradation [3], photoelectrocatalytic degradation [4], heterogeneous photo-Fenton degradation [5], and the adsorption method [6] which is the most common way because it is simple to operate, with wide range of options. Magadiite (MAG), which was discovered by Eugster in Kenya's saline lake [7], is a natural layered silicate mineral; its plates present a rose petal shape; it presents as a white powder under normal circumstances. There are so many hydrated sodium ions between the plates that the cationexchange capacity (CEC) is higher than other silicates, such as montmorillonite and can reach up to 2.22 meq/g [8], which was determined from the ideal formulation of MAG (Na2Si14O29). So far, the cation-exchange properties of MAG investigated [9][10][11][12][13], indicate that MAG can be used as an adsorbent, based on cation exchange [14,15]. According to the current study, MAG can be synthesized [16][17][18][19][20][21]; the laminate of MAG was composed of SiO4 and has no other impurities. Therefore, the structure of the MAG is very stable and has good chemical stability [22][23][24]. The Figure 2 shows the lamellar structure of the MAG; the lines of squares, hexagons and octagons, were expressed as Si-O-Si bond [25]. So far, there are many ways to modify MAG based on ion exchange. Cetyltrimethylammonium (CTMA) [26,27], heterocyclic ammine [28], and octyl triethoxysilane (OTES) [29] have been used to modify MAG, based on cation exchange, which could effectively expand layer spacing and elevate So far, there are many ways to modify MAG based on ion exchange. Cetyltrimethylammonium (CTMA) [26,27], heterocyclic ammine [28], and octyl triethoxysilane (OTES) [29] have been used to modify MAG, based on cation exchange, which could effectively expand layer spacing and elevate its adsorption performance. In this experiment, we prepared CTAB-MAG by using Processes 2019, 7, 565 3 of 12 cetyltrimethylammonium-bromide (CTAB) to modify MAG, which can effectively increase the layer spacing of MAG from 1.52 nm to 3.166 nm, thereby enhancing its adsorption capacity, then we used MAG and CTAB-MAG adsorption Rh-B from an aqueous solution to discuss the adsorption mechanism of CTAB-MAG compared with MAG.

Experimental Reagents
The Rh-B (chemical pure) and CTAB (chemical pure) were obtained from Tianjin Fuchen Chemical Reagent Factory, (Tianjin, China). Other necessary chemicals (chemical pure) were obtained from Guangzhou Qianhui Company (Guangzhou, China).

Measuring Instruments
The X-ray diffraction (XRD) analyses were characterized using an AXS D8 ADVANCE X-ray diffractometer (Bruker AXS, Karlsruhe, Germany). Using the range of 500-4000 cm −1 at room temperature, the Fourier transform infrared spectroscopy (FTIR) analyses were characterized by a NEXUS 670 type FTIR in a KBr pellet (Nicolet, Waltham, MA, USA). The microscopic surface morphology was observed by SEM analyses by Nova Nano type SEM 430 (Merlin, CA, USA).

Preparation of Sorbents.
The specific preparation method of MAG was made in our laboratory [21]; the synthesis method of CTAB-MAG was an ion exchange method. The interlayer Na + of MAG exchanges with CTA + of CTAB to form CTAB-MAG; therefore, the chemical composition of CTAB-MAG was that of the skeleton was MAG, but the interlayer cation was CTA + . The method of preparation for CTAB-MAG was as follows. First, 5 g MAG was weighed and added to deionized water, 50mL, ultrasonically dispersed for 10 min, and magnetically stirred for 1 h. Then we weighed 2.5 g CTAB and added it to MAG disperse solution. We magnetically stirred the solution with MAG and CTAB at 60 • C for 7 h. We washed the CTAB-MAG solution with deionized water until no foam was visible, and filtered it by suction filtration, then put the CTAB-MAG into a vacuum drying oven and dried at 60 • C for 24 h to obtain CTAB-MAG composite powder.

Adsorption Performance Experiment
Batch adsorption experimentations were completed to explore the possessions factors of adsorption process in order to investigate the adsorptive performance of MAG and CTAB-MAG on Rh-B, such as initial concentration of Rh-B, contact time, solution pH, and adsorbent dose. After adsorption, the MAG and CTAB-MAG were separated from the Rh-B solution by centrifuge at 6000 rpm/min for 10 min; then the concentration of Rh-B were measured by ultraviolet spectrophotometer [30,31]. The adsorption capacity (q e ) and the removal of Rh-B by the adsorbent is shown in Equations (1) and (2).
where C 0 is the initial concentration of Rh-B (mg/L), C e is the equilibrium concentration of Rh-B (mg/L), V is adsorption solution volume (mL), and M is adsorbent mass (mg).

Effect of Adsorption Time
At room temperature, with the initial concentration of 100 mg/L, 40 mL of Rh-B solution was added to seven beakers, then 40 mg of adsorbent was added to each beaker. The adsorption times were set in the seven beakers as 5, 10, 20, 30, 60, 90, and 120 min, respectively.

Effect of pH
At the normal temperature with an initial concentration of 100 mg/L, 40 mL of Rh-B solution was added to six beakers; then 40 mg of adsorbent was added to each beaker; however, the pH of Rh-B solution in the six beakers was adjusted by hydrochloric acid and sodium hydroxide solution to be 4, 6, 7, 8, 10, and 12, respectively, and the adsorption time was set for 60 min.

Effect of the Absorbent Dosage
At the normal temperature with the initial concentration of 100 mg/L, 40 mL of Rh-B solution was added to six beakers, then the dosage of adsorbent was introduced in the six beakers as 10, 20, 30, 40, 50, and 60 mg, respectively, and the adsorption time was set for 60 min.

XRD Analyses
It can be seen from Figure 3a that using CTAB modified MAG could effectively expanded its layer spacing, from original 1.52 nm to 3.166 nm, because CTAB can be inserted into the inter-layer of the MAG; meanwhile, the reflection at 5.809° was still visible, indicating that a small portion of MAG was still not intercalated by CTAB. However, the diffraction peak at 2.788° was higher than the diffraction peak at 5.809°, indicating that the intercalation rate was high, which met the needs of this experiment. The enlargement of the layer spacing means that there will be more space between the layers, which can absorb more pollutants.    Figure 3b shows that the absorption reflection bands at 3430 cm −1 and 1630 cm −1 belong to the stretching and bending vibration of the O-H bond; the absorption reflection band at 1089 cm −1 belongs to the symmetric stretching vibration of the [SiO 4 ] tetrahedron; the absorption reflection bands at 785 cm −1 and 619 cm −1 belong to the double rings vibrations. However, CTAB-MAG has three more absorption peaks (at bands 2920 cm −1 , 2852 cm −1 , and 1484 cm −1 ) than the MAG spectrum; the symmetric vibration of C-H functional groups belong to the absorption reflection band at 2920 cm −1 ; the asymmetric vibration of C-H functional groups belong to the absorption reflection band at 2852 cm-1; however, the bending vibration of C-H functional groups belong to the absorption reflection band at 1484 cm −1 , thus those results can be proof that MAG and CTAB were presented in the CTAB-MAG sample. Therefore, the addition of CTAB does not destroy the structure of MAG. Combined with XRD analysis, we can prove that CTAB was inserted into the MAG interlayer, thereby increasing the layer spacing of CTAB-MAG. Figure 4 shows the SEM images of MAG and CTAB-MAG. The particles of MAG were rose petal-like and the particle size was nanometer grade in the z direction; however, the particle size was micrometer grade in two other directions (shown in Figure 4a) while part of the laminate of CTAB-MAG was stripped because of the change of layer spacing (shown in Figure 4b).

SEM Analysis
Processes 2018, 6, x FOR PEER REVIEW 5 of 12 symmetric vibration of C-H functional groups belong to the absorption reflection band at 2920 cm −1 ; the asymmetric vibration of C-H functional groups belong to the absorption reflection band at 2852 cm-1; however, the bending vibration of C-H functional groups belong to the absorption reflection band at 1484 cm −1 , thus those results can be proof that MAG and CTAB were presented in the CTAB-MAG sample. Therefore, the addition of CTAB does not destroy the structure of MAG. Combined with XRD analysis, we can prove that CTAB was inserted into the MAG interlayer, thereby increasing the layer spacing of CTAB-MAG. Figure 4 shows the SEM images of MAG and CTAB-MAG. The particles of MAG were rose petallike and the particle size was nanometer grade in the z direction; however, the particle size was micrometer grade in two other directions (shown in Figure 4a) while part of the laminate of CTAB-MAG was stripped because of the change of layer spacing (shown in Figure 4b).

Influencing Factors of the Adsorption Capacity
As can be seen from Figure 5a, the adsorption capacity of MAG and CTAB-MAG increased from 21.79 mg/g to 57.87 mg/g, and 27.16 mg/g to 77.68 mg/g with the increasing of the initial concentration of Rh-B from 30 mg/L to 150 mg/L. This was because with the increasing of the initial concentration of Rh-B, the mass transfer power to the adsorbent increases, resulting in an adsorption capacity increase. As can be seen from Figure 5b, the adsorption capacity of MAG and CTAB-MAG increased quickly from 31.85 mg/g to 46.56 mg/g, and from 45.36 mg/g to 65.34 mg/g with the increase of the adsorption time from 5 min to 40 min; however, the adsorption capacity of MAG and CTAB-MAG increased slowly from 46.56 mg/g to 49.07 mg/g, and 65.34 mg/g to 68.82 mg/g with the increase of adsorption time from 40 min to 120 min, respectively. The reason is that the adsorption capacity was increased rapidly first and then increased slowly. The active site of adsorbent was decreased gradually with the adsorption process; on the other hand, the concentration of Rh-B in the solution was gradually decreased; therefore, the rate of particle diffusion was promoted by the concentration difference decreases, resulting in the decrease of the adsorption rate. Figure 5c shows that the adsorption capacity of MAG and CTAB-MAG decreased quickly from 52.39 mg/g to 34.90 mg/g, and from 84.12 mg/g to 40.52 mg/g with the increasing of the pH from 4 to 12, respectively. This decrease could be attributed to competition between the Rh-B dye molecules and the hydroxyl ions present at these pH values [32]. Figure 5d shows that the adsorption capacity of MAG and CTAB-MAG decreased quickly from 128.52 mg/g to 35.54 mg/g, and 149.24 mg/g to 52.43 mg/g with the increasing of the dosage of MAG and CTAB-MAG from 0.25 g/L to 1.5 g/L, respectively.

Influencing Factors of the Adsorption Capacity
As can be seen from Figure 5a, the adsorption capacity of MAG and CTAB-MAG increased from 21.79 mg/g to 57.87 mg/g, and 27.16 mg/g to 77.68 mg/g with the increasing of the initial concentration of Rh-B from 30 mg/L to 150 mg/L. This was because with the increasing of the initial concentration of Rh-B, the mass transfer power to the adsorbent increases, resulting in an adsorption capacity increase. As can be seen from Figure 5b, the adsorption capacity of MAG and CTAB-MAG increased quickly from 31.85 mg/g to 46.56 mg/g, and from 45.36 mg/g to 65.34 mg/g with the increase of the adsorption time from 5 min to 40 min; however, the adsorption capacity of MAG and CTAB-MAG increased slowly from 46.56 mg/g to 49.07 mg/g, and 65.34 mg/g to 68.82 mg/g with the increase of adsorption time from 40 min to 120 min, respectively. The reason is that the adsorption capacity was increased rapidly first and then increased slowly. The active site of adsorbent was decreased gradually with the adsorption process; on the other hand, the concentration of Rh-B in the solution was gradually decreased; therefore, the rate of particle diffusion was promoted by the concentration difference decreases, resulting in the decrease of the adsorption rate. Figure 5c shows that the adsorption capacity of MAG and CTAB-MAG decreased quickly from 52.39 mg/g to 34.90 mg/g, and from 84.12 mg/g to 40.52 mg/g with the increasing of the pH from 4 to 12, respectively. This decrease could be attributed to competition between the Rh-B dye molecules and the hydroxyl ions present at these pH values [32]. Figure 5d shows that the adsorption capacity of MAG and CTAB-MAG decreased quickly from 128.52 mg/g to 35.54 mg/g, and 149.24 mg/g to 52.43 mg/g with the increasing of the dosage of MAG and CTAB-MAG from 0.25 g/L to 1.5 g/L, respectively.

Isothermal Adsorption Experiment
The adsorption capacity was fitting by the Langmuir model equation and the Freundlich model equation [33,34], as shown in Equations (3) and (4).

= 1 +
where Ce is the concentration at equilibrium (mg·L −1 ), qe is the adsorption capacity when the adsorption balance (mg·g −1 ), and KL is Langmuir equilibrium constant (L·mg −1 ). = (4) where KF and n are the Freundlich equilibrium constant and the characteristic constant, respectively. Figure 6 shows that the adsorption capacity was fitted by Langmuir model equation and Freundlich model equation; meanwhile, Table 1 shows that the related parameters had been well presented. By using the Langmuir model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found-0.99 and 0.993, respectively; however, by using the Freundlich model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found to be 0.984 and 0.987, respectively, thus indicting that the Langmuir model and Freundlich model can simulate the adsorption process together. However, the Freundlich model constants (1/n) for MAG and CTAB-MAG were found to be 0.40486 and 0.32086, respectively. They were less than 1, indicating an adsorption process consist with monolayer adsorption; meanwhile, this conclusion is also consisted with the assumptions of the Langmuir model.

Isothermal Adsorption Experiment
The adsorption capacity was fitting by the Langmuir model equation and the Freundlich model equation [33,34], as shown in Equations (3) and (4).
where C e is the concentration at equilibrium (mg·L −1 ), q e is the adsorption capacity when the adsorption balance (mg·g −1 ), and K L is Langmuir equilibrium constant (L·mg −1 ).
where K F and n are the Freundlich equilibrium constant and the characteristic constant, respectively. Figure 6 shows that the adsorption capacity was fitted by Langmuir model equation and Freundlich model equation; meanwhile, Table 1 shows that the related parameters had been well presented. By using the Langmuir model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found-0.99 and 0.993, respectively; however, by using the Freundlich model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found to be 0.984 and 0.987, respectively, thus indicting that the Langmuir model and Freundlich model can simulate the adsorption process together. However, the Freundlich model constants (1/n) for MAG and CTAB-MAG were found to be 0.40486 and 0.32086, respectively. They were less than 1, indicating an adsorption process consist with monolayer adsorption; meanwhile, this conclusion is also consisted with the assumptions of the Langmuir model.

Adsorption Kinetics Model
The adsorption capacity was fitted by the pseudo first order dynamic equation and the pseudo second order dynamic equation [35][36][37], as shown in Equations (5) and (6). Equation (5) was the pseudo first order dynamic equation expression.
where K1 was pseudo first order rate constants(min −1 ), qeq was the adsorption capacity when the adsorption balance(mg·g −1 ), and qt was the adsorption capacity when the time was t (mg·g −1 ). Equation (6) was the pseudo second order dynamic equation expression.
where K2 is the pseudo second order rate constants (g·mg −1 ·min −1 ), qeq is the adsorption capacity when the adsorption reached at equilibrium (mg·g −1 ), and qt is the adsorption capacity at time t (mg·g −1 ). Figure 7 shows that the adsorption capacity was fitted by of the pseudo first order kinetic model and the pseudo second order kinetic model; meanwhile, Table 2 shows that the related parameters had been well presented. By using of the pseudo second order kinetic model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found to be 0.999 and 0.999, respectively; however, by using the pseudo first order kinetic model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were 0.988 and 0.979, respectively, thus the correlation coefficients (R 2 ) of the pseudo second order kinetic model were larger than the correlation coefficients (R 2 ) of the pseudo first order kinetic model for MAG and CTAB-MAG. Therefore, it indicates that the pseudo second order kinetic model was more appropriate for describing the adsorption process.

Adsorption Kinetics Model
The adsorption capacity was fitted by the pseudo first order dynamic equation and the pseudo second order dynamic equation [35][36][37], as shown in Equations (5) and (6). Equation (5) was the pseudo first order dynamic equation expression.
where K 1 was pseudo first order rate constants(min −1 ), q eq was the adsorption capacity when the adsorption balance(mg·g −1 ), and q t was the adsorption capacity when the time was t (mg·g −1 ). Equation (6) was the pseudo second order dynamic equation expression.
where K 2 is the pseudo second order rate constants (g·mg −1 ·min −1 ), q eq is the adsorption capacity when the adsorption reached at equilibrium (mg·g −1 ), and q t is the adsorption capacity at time t (mg·g −1 ). Figure 7 shows that the adsorption capacity was fitted by of the pseudo first order kinetic model and the pseudo second order kinetic model; meanwhile, Table 2 shows that the related parameters had been well presented. By using of the pseudo second order kinetic model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were found to be 0.999 and 0.999, respectively; however, by using the pseudo first order kinetic model, the correlation coefficients (R 2 ) for MAG and CTAB-MAG were 0.988 and 0.979, respectively, thus the correlation coefficients (R 2 ) of the pseudo second order kinetic model were larger than the correlation coefficients (R 2 ) of the pseudo first order kinetic model for MAG and CTAB-MAG. Therefore, it indicates that the pseudo second order kinetic model was more appropriate for describing the adsorption process.  The Table 2 shows that the experimental results (qeqe) of MAG and CTAB-MAG were 49.07 and 68.82, respectively. The calculated results (qeqc) for the pseudo-second-order dynamic equation of MAG and CTAB-MAG were 50.994 and 71.327, respectively; however, the calculated results (qeqc) for the pseudo first order dynamical equation of MAG and CTAB-MAG were 22.81 and 27.99, respectively. Thus, the calculated results (qeqc) of the pseudo-second-order dynamic equation approached the investigational results (qeqe) indicting that the pseudo-second-order kinetic model was more appropriate for relating the adsorption process.

Adsorption Ratio Model
In order to investigate the adsorption rate, we research the dynamic boundary models, such as the film diffusion model, particle diffusion model, and chemical reaction model. We defined qt/qeq as F, where qt was the adsorption capacity at time t (mg·g −1 ), and qeq was the adsorption capacity when the adsorption reached equilibrium (mg·g −1 ), in the three equations that follow [38,39].
Film diffusion model: Particle diffusion model: Chemical reaction model: Figure 8 shows the three moving boundary models. Figure 8a is the film diffusion; 8b is the particle diffusion; 8c is the chemical reaction. The results of the moving boundary models have been presented in Table 3. As shown in Table 3, the correlation coefficients R 2 of the film diffusion model for MAG (0.988) and CTAB-MAG (0.979) were larger than in the particle diffusion model for MAG (0.909) and CTAB-MAG (0.887), as well as chemical reaction model for MAG (0.934) and CTAB-MAG (0.911), indicating that the film diffusion was more suitable for describing the adsorption.  The Table 2 shows that the experimental results (q eqe ) of MAG and CTAB-MAG were 49.07 and 68.82, respectively. The calculated results (q eqc ) for the pseudo-second-order dynamic equation of MAG and CTAB-MAG were 50.994 and 71.327, respectively; however, the calculated results (q eqc ) for the pseudo first order dynamical equation of MAG and CTAB-MAG were 22.81 and 27.99, respectively. Thus, the calculated results (q eqc ) of the pseudo-second-order dynamic equation approached the investigational results (q eqe ) indicting that the pseudo-second-order kinetic model was more appropriate for relating the adsorption process.

Adsorption Ratio Model
In order to investigate the adsorption rate, we research the dynamic boundary models, such as the film diffusion model, particle diffusion model, and chemical reaction model. We defined q t /q eq as F, where q t was the adsorption capacity at time t (mg·g −1 ), and q eq was the adsorption capacity when the adsorption reached equilibrium (mg·g −1 ), in the three equations that follow [38,39].
Film diffusion model: Particle diffusion model: Chemical reaction model: Figure 8 shows the three moving boundary models. Figure 8a is the film diffusion; 8b is the particle diffusion; 8c is the chemical reaction. The results of the moving boundary models have been presented in Table 3. As shown in Table 3, the correlation coefficients R 2 of the film diffusion model for MAG (0.988) and CTAB-MAG (0.979) were larger than in the particle diffusion model for MAG (0.909) and CTAB-MAG (0.887), as well as chemical reaction model for MAG (0.934) and CTAB-MAG (0.911), indicating that the film diffusion was more suitable for describing the adsorption.  The adsorption capacities of MAG and CTAB-MAG were 48.13 mg/g, 67.19 mg/g, respectively. When pH was 7, adsorption time was 60 min, the initial concentration of Rh-B was 100 mg/L, and the adsorbent dosage was 1 g/L. The Table 4 shows that the adsorption capacity of MAG (48.13 mg/g) and CTAB-MAG (67.19 mg/g) were both higher than kaolinite (46.08 mg/g) [40], sodium montmorillonite (42.19 mg/g) [41], and duolite C-20 resin (28.57 mg/g) [42]. Those results indicate in  The adsorption capacities of MAG and CTAB-MAG were 48.13 mg/g, 67.19 mg/g, respectively. When pH was 7, adsorption time was 60 min, the initial concentration of Rh-B was 100 mg/L, and the adsorbent dosage was 1 g/L. The Table 4 shows that the adsorption capacity of MAG (48.13 mg/g) and CTAB-MAG (67.19 mg/g) were both higher than kaolinite (46.08 mg/g) [40], sodium montmorillonite (42.19 mg/g) [41], and duolite C-20 resin (28.57 mg/g) [42]. Those results indicate in this process that CTAB-MAG can be used as an efficient low-cost adsorbent for removing Rh-B from an aqueous solution.

Conclusions
In this work, we prepared CTAB-MAG by using CTAB to modify MAG, based on ion exchange. Compared with MAG, CTAB-MAG can effectively increase the layer spacing of MAG from 1.52 nm to 3.166 nm, thereby enhancing its adsorption capacity. Meanwhile, the adsorption results shown the pronounced affinity of the CTAB-MAG to the Rh-B dye molecules. The adsorption capacities of MAG and CTAB-MAG were 48.13 mg/g and 67.19 mg/g. The adsorption capacity of CTAB-MAG was increased by 40% over MAG, indicating that CTAB-MAG can be used as an efficient, low-cost adsorbent. The pseudo-second-order kinetic equation was more suitable for describing the adsorption; the adsorption process was dominated by a film diffusion process. The adsorption process belongs to monomolecular layer adsorption processes.