# Adsorption of an Anionic Azo Dye UsingMoringa oleifera Seed Protein-Montmorillonite Composite

Authors: Xiao Mi; Zhenya Shang; Chunbao Du; Guoting Li; Ting Su; Xinying Chang; Ruiming Li; Zhihong Zheng; Jingxi Tie
Journal: Journal of Chemistry
 (2019)

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## Abstract

In this study,Moringa oleifera seed protein-montmorillonite (MOSP-MMT) composite was synthetized using the impregnation method. The MOSP-MMT adsorbent was characterized using scanning electron microscope, X-ray diffraction, infrared spectroscopy, surface area analysis, and thermogravimetric analysis. The removal of water-soluble reactive red 2 (RR-2) from artificial wastewater by the MOSP-MMT composite was carried out in a batch system. The results indicated that RR-2 adsorption increased with contact time, and the pseudo-second-order equation was best to describe the adoption process among the three models. The RR-2 adsorption decreased from 7.60 to 5.92 mg/g as pH increased from 3.2 to 9.1 and increased from 15.2 to 17.1 mg/g as NaCl concentration increased from 0 to 30 g/L. The Freundlich isotherm model provided the better fit of the experimental data than the Langmuir model. The result showed that the MOSP-MMT composite could be a potential adsorbent for the treatment of wastewater containing RR-2.


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## Body

## 1. Introduction

Currently, most of the commercial dyes used in many industries including textile, paper, plastic, leather, and rubber have a synthetic origin. Dyes have physicochemical, thermal, and optical stability for the sake of their aromatic molecular structures, which in turn make them resistant to degrade with time, sunlight, and biological and chemical treatments [1, 2]. Furthermore, some dyes or their degradation intermediates are cancerogenic, teratogenic, and mutagenic [3, 4]. Hence, dyes must be removed before the dye-containing wastewater is discharged into water body. A lot of methods including chemical, physicochemical, and biological methods have been developed to treat dye-containing wastewater [5–8]. Among these techniques, adsorption is believed to be most promising because of its easy operation, low cost, and high performance without formation of harmful by-products. Seeking for the effective and efficient adsorbent is critical to ensure the success operation of adsorptive removal of dyes from their water solutions.As a natural clay mineral, montmorillonite (MMT) has been used successfully for pollutants removal from contaminated water due to its advantages such as abundant reserve, low price, large specific surface area, good cation exchange capacity, excellent chemical and physical stability, versatility, and easy modification. [9, 10]. Unfortunately, the natural MMT has poor adsorption of anionic dyes due to the negative charge and hydrophilic characteristic of its surface [11]. Thus, many techniques have been used to modify MMT for improving its dye adsorption capacity [1, 12, 13], and organic modification of introducing active ligands to create hybrid composites has been proved to be an important and successful strategy to increase the dye adsorption capacity of many materials [14, 15]. For instance, the adsorption capacity of direct blue 74 on poly(dopamine) grafted biosilica composite was 67.8 mg/g, and the capacity increased to 386.4 mg/g after tetraethylenepentamine ligands were loaded on the surface of the composite [16].As a natural coagulant,Moringa oleifera seed protein (MOSP) has been proved to be very efficient for the removal of many negatively charged pollutants including anionic dyes since the protein is positively charged in the pH range below its isoelectric points between pH 10 and 11 [17–21].In this study, the commercial MMT was modified with MOSP to create a new selective organ-montmorillonite for anionic dye adsorption. Hence, the main objective of this paper is to test the anionic dye adsorption of theMoringa oleifera seed protein-montmorillonite (MOSP-MMT) composite. Moreover, the parameters affecting the adsorption including contact time, pH, inorganic salt, and initial dye concentration were investigated.


## 2. Materials and Methods

### 2.1. Synthesis of MOSP-MMT Composite

The purified MMT was mixed with MOSP solution (8.14 g/L and pH = 6.8) at the approximate ratio of 1 : 3.2 (W : V) determined in our previous study, and the mixture was reacted in a shaker at 30°C for 24 h. The sediment collected after the centrifugation of the mixture at 8000 rpm for 10 min was washed with deionized water for 5 times and dried in a vacuum drying oven at 40°C in sequence. Finally, the dried composite was smashed and sieved for the experiment.


### 2.2. Preparation of Artificial Wastewater

A typical anionic dye, reactive red 2 (RR-2), was selected as the target pollutant. The RR-2 purchased from an online shop without further purification was dissolved into distilled water to synthesize the RR-2 solution. The pH value of the solution was adjusted using 0.1 M·NaOH and 0.1 M·HCl solutions and measured using a pH meter (pHS-3C, Leici Ltd., China). All the reagents used in this study were of analytical grade. The chemical structure of RR-2 is shown in Figure1.Figure 1
Chemical structure of RR-2 (chemical formula: C19H10Cl2N6Na2O7S2 and molecular weight: 615.33).


### 2.3. Adsorption Experiment

Static adsorption experiments were carried out to test the uptake of RR-2 by MMT-MOSP as functions of reaction time, pH, inorganic salt, and initial RR-2 concentration. The effect of reaction time on the adsorption of RR-2 was carried out at an initial RR-2 concentration of 51.9 and 100.3 mg/L at 30°C and pH 7. The influence of pH was studied at an initial RR-2 concentration of 52.6 mg/L in the pH range of 3.2–9.1 at 30°C with a reaction time of 10 h. The presence of salt on the uptake of RR-2 was studied at an initial RR-2 concentration of 42.5 mg/L at 30°C and pH 7 with reaction time of 10 h. The effect of initial RR-2 concentration on the removal effectiveness was conducted in the RR-2 concentration range of 47.0–380.9 mg/L at 20°C and 30°C, pH 7, and with a reaction time of 10 h. For every test, 0.05 g MOSP-MM and 10 mL RR-2 solution were used, and the shaking speed was 100 rpm. The pH values of the solutions were adjusted in the same way mentioned above. The mixture were centrifuged at 8000 rpm for 8 min to get the supernatants when the reaction was over, and the residual RR-2 concentrations in supernatants were determined using a UV-Vis spectrophotometer (UV-5100, Yuanxi Instruments, Shanghai, China) at its longest adsorption wavelength ofλ = 538 nm [22].The RR-2 uptake by MOSP-MMT was calculated using the following equation:(1)q=C0−CeVm,where q is the RR-2 uptake by per unit MOSP-MMT (mg/g); C0 and Ce (mg/L) are the initial and final RR-2 concentrations, respectively; V (L) is the RR-2 solution volume; and m is the amount of MOSP-MMT (g). The mean values of three independent experiments were used in the study.


### 2.4. Sample Characterization

The morphological information of the raw MMT and MOSP-MMT was characterized using SEM (JSM- 6460LV, Japan Electronic Co., Ltd.). MMT and MOSP-MMT were degassed at 90°C for 1 h, and then the temperature was increased to 300°C at the speed of 10°C per minute and kept at the temperature for 6 h before analysis. The surface areas were measured using the Brunauer–Emmett–Teller (BET) method at at −196°C (Autosorb-I, Quantachrome, USA). XRD patterns of the two samples were acquired with an X-ray diffractometer (Smartlab 3kw, Rigaku Ltd., Japan) over the scanning range of 2θ = 2°–20° to study the changes in their structural properties. Bragg’s law 2d sinθ=nλ was used to calculate the d001 of the two samples. FTIR spectra of MOSP, MMT, and MOSP-MMT were obtained using a FTIR spectrum (Nicolet 5700, Thermo Nicolet Ltd., USA) to observe the surface functional groups of the samples. The thermogravimetric analysis was carried out using the simultaneous thermal analyzer (STA409PC, NETZSCH group, Germany).


## 2.1. Synthesis of MOSP-MMT Composite

The purified MMT was mixed with MOSP solution (8.14 g/L and pH = 6.8) at the approximate ratio of 1 : 3.2 (W : V) determined in our previous study, and the mixture was reacted in a shaker at 30°C for 24 h. The sediment collected after the centrifugation of the mixture at 8000 rpm for 10 min was washed with deionized water for 5 times and dried in a vacuum drying oven at 40°C in sequence. Finally, the dried composite was smashed and sieved for the experiment.


## 2.2. Preparation of Artificial Wastewater

A typical anionic dye, reactive red 2 (RR-2), was selected as the target pollutant. The RR-2 purchased from an online shop without further purification was dissolved into distilled water to synthesize the RR-2 solution. The pH value of the solution was adjusted using 0.1 M·NaOH and 0.1 M·HCl solutions and measured using a pH meter (pHS-3C, Leici Ltd., China). All the reagents used in this study were of analytical grade. The chemical structure of RR-2 is shown in Figure1.Figure 1
Chemical structure of RR-2 (chemical formula: C19H10Cl2N6Na2O7S2 and molecular weight: 615.33).


## 2.3. Adsorption Experiment

Static adsorption experiments were carried out to test the uptake of RR-2 by MMT-MOSP as functions of reaction time, pH, inorganic salt, and initial RR-2 concentration. The effect of reaction time on the adsorption of RR-2 was carried out at an initial RR-2 concentration of 51.9 and 100.3 mg/L at 30°C and pH 7. The influence of pH was studied at an initial RR-2 concentration of 52.6 mg/L in the pH range of 3.2–9.1 at 30°C with a reaction time of 10 h. The presence of salt on the uptake of RR-2 was studied at an initial RR-2 concentration of 42.5 mg/L at 30°C and pH 7 with reaction time of 10 h. The effect of initial RR-2 concentration on the removal effectiveness was conducted in the RR-2 concentration range of 47.0–380.9 mg/L at 20°C and 30°C, pH 7, and with a reaction time of 10 h. For every test, 0.05 g MOSP-MM and 10 mL RR-2 solution were used, and the shaking speed was 100 rpm. The pH values of the solutions were adjusted in the same way mentioned above. The mixture were centrifuged at 8000 rpm for 8 min to get the supernatants when the reaction was over, and the residual RR-2 concentrations in supernatants were determined using a UV-Vis spectrophotometer (UV-5100, Yuanxi Instruments, Shanghai, China) at its longest adsorption wavelength ofλ = 538 nm [22].The RR-2 uptake by MOSP-MMT was calculated using the following equation:(1)q=C0−CeVm,where q is the RR-2 uptake by per unit MOSP-MMT (mg/g); C0 and Ce (mg/L) are the initial and final RR-2 concentrations, respectively; V (L) is the RR-2 solution volume; and m is the amount of MOSP-MMT (g). The mean values of three independent experiments were used in the study.


## 2.4. Sample Characterization

The morphological information of the raw MMT and MOSP-MMT was characterized using SEM (JSM- 6460LV, Japan Electronic Co., Ltd.). MMT and MOSP-MMT were degassed at 90°C for 1 h, and then the temperature was increased to 300°C at the speed of 10°C per minute and kept at the temperature for 6 h before analysis. The surface areas were measured using the Brunauer–Emmett–Teller (BET) method at at −196°C (Autosorb-I, Quantachrome, USA). XRD patterns of the two samples were acquired with an X-ray diffractometer (Smartlab 3kw, Rigaku Ltd., Japan) over the scanning range of 2θ = 2°–20° to study the changes in their structural properties. Bragg’s law 2d sinθ=nλ was used to calculate the d001 of the two samples. FTIR spectra of MOSP, MMT, and MOSP-MMT were obtained using a FTIR spectrum (Nicolet 5700, Thermo Nicolet Ltd., USA) to observe the surface functional groups of the samples. The thermogravimetric analysis was carried out using the simultaneous thermal analyzer (STA409PC, NETZSCH group, Germany).


## 3. Results and Discussion

### 3.1. Sample Characterization

Figure2(a) shows the SEM image of MMT and MOSP-MMT. There is no obvious difference between MMT and MOSP-MMT.Figure 2
SEM of MMT and MOSP-MMT (a); XRD of MMT and MOSP-MMT (b); FTIR spectra of MMT, MOSP, and MOSP-MMT (c); TG of MMT and MOSP-MMT (d).Figure2(b) shows the XRD patterns of MMT and MOSP-MMT. The d001 reflection at 2θ for MMT and MOSP-MMT is 6.29 and 5.82°, and their corresponding interlayer spacings are 14.2 and 15.2 Å, respectively. Considering the large spatial structure of MOSP, it was impossible for the MOSP to enter the interlayer of MMT, and the increase of the interlayer was probably caused by the existence of the small molecule such as H2O with the bond length of about 1.0 Å. Thus, it could be confirmed that MOSP was only adsorbed on the surface of MMT.Figure2(c) shows the FTIR results of MOSP, MMT, and MOSP-MMT. The characteristic bands at 1659, 1543, and 1237 cm−1 in the MOSP spectrum were assigned to C=O stretching (amide I), CN stretching, and NH bonding (amide II) and CN stretching (amide III), respectively. A new band at 1540 cm−1 appeared in the MOSP-MMT, indicating the reaction of MOSP with MMT.Figure2(d) shows the thermogravimetric results of MMT and MOSP-MMT. It can be seen that both MMT and MOSP-MMT lost absorbed water on its surface and interlayer water below 200°C. MMT lost residual interlayer water and hydroxyl in the temperature range of 590–700°C. MOSP in MOSP-MMT decomposed and carbonized in the temperature range of 270–700°C. MOSP-MMT was thermostable below 63°C. The mass proportion of MOSP in MOSP-MMT was about 13.9%.Table1 summarizes the BET parameters of MMT and MOSP-MMT. MMT showed the specific surface area of 21.91 m2/g and total pore volume of 0.090 cm3/g with the average pore size of 16.38 nm. After the modification by MOSP, the specific surface area and total pore volume of MOSP-MMT decreased to 4.90 m2/g and 0.025 cm3/g, respectively, with the average pore size of 20.94 nm, which might result from the fact that the pores in MMT were occupied by MOSP.Table 1
BET parameters of MMT and MOSP-MMT.
Samples
Specific surface area (m2/g)
Total pore volume (cm3/g)
Average pore size (nm)
MMT
21.91
0.090
16.38
MOSP-MMT
4.90
0.025
20.94


### 3.2. Effect of Reaction Time and Kinetics Study

Figure3 shows the effect of reaction time on RR-2 uptake at two initial RR-2 concentrations. It can be seen that the adsorption of RR-2 on MOSP-MMT can be divided into a fast and a slow stage for both of the two concentrations. The fast stage lasted from the beginning to the 15th min when MOSP-MMT offered more free active sites in this stage, indicating that chemical adsorption might be the main mechanism for RR-2 uptake in this stage. There were less and less active sites left for the adsorption in the following slow stage to slow down the adsorption. Additionally, the repulsion between the absorbed and the free dye molecules in solution further weakened the reaction.Figure 3
Effect of reaction time on RR-2 uptake (T = 30°C and pH = 7.0).To further understand the adsorption, three classic kinetic equations including pseudo-first-order, pseudo-second-order, and Elovich equations are used to describe the adsorption.The linearized form of the pseudo-first-order equation is shown as follows:(2)logqe−qt=logqe−k12.303t,where qt and qe (mg/g) are the RR-2 uptake at time t and at equilibrium, respectively; k1 (min−1) is the equilibrium rate constant; and k1 and predicted qe could be obtained from the slope and intercept of the plot of log (qe − qt) vs t.The linearized form of the pseudo-second-order equation is shown as follows:(3)tqt=tqe+1k2qe2,where k2 (g/(mg·min)) is the equilibrium rate constant and k2 and qe could be determined from the slope and intercept of the plot of t/qt vs t.The linearized form of the Elovich equation is shown as follows:(4)qt=1βlnαβ+1βlnt,where α (mg/(g min)) and β (g/mg) are the initial adsorption rate and the constant associated with the fraction of surface coverage and activation energy for chemisorption, respectively. The two parameters could be determined from the intercept and slope of the plot of qt vs. lnt.The kinetics parameters obtained from the three linearized plots are shown in Table2. The correlation coefficients of the pseudo-second-order kinetics are highest among the three models. The calculating values of qe,cal got from the equations are in good accordance with those values of qe,exp obtained from the experiments, indicating that the adsorption followed the pseudo-second-order kinetics. Thus, chemical adsorption or chemisorption might be the rate-determining step for the adsorption of RR-2 on MOSP-MMT, and the valency force between the RR-2 molecular and MOSP-MMT might be involved in the adsorption as well.Table 2
Parameters of the three kinetic equations for RR-2 adsorption on MOSP-MMT.
Models
Parameters
Values
C
0 = 51.9 mg/L
C
0 = 100.3 mg/L
Pseudo-first-order equation
q
e,exp
6.3
10.5
q
e,cal
6.51
3.12
k
1
0.0039
0.0053
R
2
0.964
0.9
Pseudo-second-order equation
q
e,cal
6.60
10.81
k
2
0.0050
0.0021
R
2
0.9959
0.9918
Elovich equation
α
5.71
1.58
β
1.35
0.63
R
2
0.9357
0.9796


### 3.3. Effect of pH

Solution pH can impact both the surface charges of adsorbent and the existing forms of adsorbate [23, 24], which in turn affects the adsorption. Figure 4 shows the effect of pH on RR-2 uptake. RR-2 uptake decreased from 7.6 to 6.5 mg/g as the pH increased from 3.2 to 4.1, and the RR-2 uptake decreased to 5.9 gradually as the pH rose to 9.1. Overall, rising pH weakened the RR-2 uptake. MMT could hardly adsorb RR-2 (data not shown) due to its hydrophilicity and net negative charge. However, the MOSP-MMT could adsorb RR-2 since the positively charged amino groups of MOSP in the MOSP-MMT could react with anionic groups of the RR-2 in the pH range of 3–9 set for the experiment, which improved the RR-2 adsorption. The probable mechanism for RR-2 adsorption on MOSP-MMT could be described by the following equations. P is used to represent the coagulating protein since its structure is still unknown so far:(5)where P, M, and R denote MOSP, MMT, and RR-2, respectively.Figure 4
Effect of pH on RR-2 uptake (C0: 52.6 mg/L, T: 30°C, and time: 10 h).The weakened adsorption with the increasing pH could be explained by the fact that the negative charge of RR-2 and positive charge on MOSP-MMT surface decreased with the rising pH. Additionally, the competition between RR-2 molecules and increasing OH− groups for the active sites weakened the RR-2 uptake as well.


### 3.4. Effect of Inorganic Salt

NaCl is an inorganic salt used widely to prompt dyeing efficiency because it can drive the dye molecules out of solution onto fibers [25]. Thus, there are a large amount of NaCl in dye-containing wastewater. Hence, the effect of NaCl on RR-2 removal was investigated. Figure 5 shows that RR-2 uptake increased from 15.2 to 17.1 mg g−1 as NaCl increased from 0 to 30 g/L, indicating that NaCl can improve the removal of RR-2. In our previous study, we found NaCl could increase the adsorption of reactive green on polyaniline/bentonite as well [25]. Ip et al. reported that the adsorption capacities of reactive dyes on four adsorbents increased with rising NaCl concentration as well [26]. The rise of the dye uptake caused by NaCl addition was attributed to the aggregation of reactive dye molecules resulted from the increased intermolecular forces, including van der Waals forces, ion-dipole forces, and dipole-dipole forces [27]. Additionally, the salts interacted with the adsorbate and screened the repulsive force between the dye molecules and the carbon surface between the adsorbed species [26], which could be another explanation for the increase of RR-2 removal.Figure 5
Effect of inorganic salt (C0: 142.5 mg/L, T: 30°C, reaction time: 10 h, and pH: 7).


### 3.5. Effect of Initial RR-2 Concentration

Figure6 shows the effect of initial RR-2 concentration on its adsorption on MOSP-MMT. Effect of initial RR-2 concentration on its adsorption was carried out ranging from 47.0 to 380.9 mg/L. Figure 6 clearly reveals that the extent of adsorption of RR-2 increased linearly with the rising RR-2 concentration. Meanwhile, at every RR-2 concentration, RR-2 adsorption at 30°C was better than that at 20°C.Figure 6
Effect of initial RR-2 concentration (reaction time: 10 h and pH: 7).The adsorption isotherm gives an idea of how the adsorbate molecules distribute between the liquid-solid interface at equilibrium [28, 29], which helps us to further understand the nature of adsorption. Thus, the Langmuir and Freundlich models are adopted to analyze the adsorption data at equilibrium, as shown in Figure 7.Figure 7
Adsorption isotherm of RR-2 on MOSP-MMT at two temperatures (initial RR-2 concentration: 47.0 mg/L−380.9 mg/L, reaction time: 10 h, and pH: 7).The linearized form of the Langmuir equation is shown as follows:(6)ceqe=1qmb+ceqm,where ce (mg/L) and qe (mg/g) are the concentration and adsorption capacity at equilibrium, respectively, and b (L/mg) and qm (mg/g) are the Langmuir constant related to binding energy and the maximum adsorption capacity, respectively, which could be obtained from the slope and intercept of the plots of 1/ce versus ce/qe, respectively.The linearized form of the Freundlich equation is shown as follows:(7)logqe=logkf+1nlogCe,where qe (mg/g) and ce (mg/L) have same meaning as that mentioned above and kf and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively, which could be obtained from the slope and intercept of the plots of ln ce versus ln qe, respectively.Parameters of Langmuir and Freundlich isotherms are shown in Table3. It can be seen that the R2 values of isotherm at both of the two temperatures are better than those of Langmuir isotherm, indicating that the Freundlich equation is more suitable to describe the adsorption of RR-2 on MOSP-MMT, which was a multilayer adsorption of RR-2 on the heterogeneous surface MOSP-MMT [30].Table 3
Parameters of Langmuir and Freundlich isotherms.
20°C
30°C
Langmuir
q
m
—
93.45
b
−2991.27
0.0052
R
2
0.0408
0.9138
Freundlich
n
0.9834
0.8270
k
f
0.1246
0.0666
R
2
0.9754
0.9972


## 3.1. Sample Characterization

Figure2(a) shows the SEM image of MMT and MOSP-MMT. There is no obvious difference between MMT and MOSP-MMT.Figure 2
SEM of MMT and MOSP-MMT (a); XRD of MMT and MOSP-MMT (b); FTIR spectra of MMT, MOSP, and MOSP-MMT (c); TG of MMT and MOSP-MMT (d).Figure2(b) shows the XRD patterns of MMT and MOSP-MMT. The d001 reflection at 2θ for MMT and MOSP-MMT is 6.29 and 5.82°, and their corresponding interlayer spacings are 14.2 and 15.2 Å, respectively. Considering the large spatial structure of MOSP, it was impossible for the MOSP to enter the interlayer of MMT, and the increase of the interlayer was probably caused by the existence of the small molecule such as H2O with the bond length of about 1.0 Å. Thus, it could be confirmed that MOSP was only adsorbed on the surface of MMT.Figure2(c) shows the FTIR results of MOSP, MMT, and MOSP-MMT. The characteristic bands at 1659, 1543, and 1237 cm−1 in the MOSP spectrum were assigned to C=O stretching (amide I), CN stretching, and NH bonding (amide II) and CN stretching (amide III), respectively. A new band at 1540 cm−1 appeared in the MOSP-MMT, indicating the reaction of MOSP with MMT.Figure2(d) shows the thermogravimetric results of MMT and MOSP-MMT. It can be seen that both MMT and MOSP-MMT lost absorbed water on its surface and interlayer water below 200°C. MMT lost residual interlayer water and hydroxyl in the temperature range of 590–700°C. MOSP in MOSP-MMT decomposed and carbonized in the temperature range of 270–700°C. MOSP-MMT was thermostable below 63°C. The mass proportion of MOSP in MOSP-MMT was about 13.9%.Table1 summarizes the BET parameters of MMT and MOSP-MMT. MMT showed the specific surface area of 21.91 m2/g and total pore volume of 0.090 cm3/g with the average pore size of 16.38 nm. After the modification by MOSP, the specific surface area and total pore volume of MOSP-MMT decreased to 4.90 m2/g and 0.025 cm3/g, respectively, with the average pore size of 20.94 nm, which might result from the fact that the pores in MMT were occupied by MOSP.Table 1
BET parameters of MMT and MOSP-MMT.
Samples
Specific surface area (m2/g)
Total pore volume (cm3/g)
Average pore size (nm)
MMT
21.91
0.090
16.38
MOSP-MMT
4.90
0.025
20.94


## 3.2. Effect of Reaction Time and Kinetics Study

Figure3 shows the effect of reaction time on RR-2 uptake at two initial RR-2 concentrations. It can be seen that the adsorption of RR-2 on MOSP-MMT can be divided into a fast and a slow stage for both of the two concentrations. The fast stage lasted from the beginning to the 15th min when MOSP-MMT offered more free active sites in this stage, indicating that chemical adsorption might be the main mechanism for RR-2 uptake in this stage. There were less and less active sites left for the adsorption in the following slow stage to slow down the adsorption. Additionally, the repulsion between the absorbed and the free dye molecules in solution further weakened the reaction.Figure 3
Effect of reaction time on RR-2 uptake (T = 30°C and pH = 7.0).To further understand the adsorption, three classic kinetic equations including pseudo-first-order, pseudo-second-order, and Elovich equations are used to describe the adsorption.The linearized form of the pseudo-first-order equation is shown as follows:(2)logqe−qt=logqe−k12.303t,where qt and qe (mg/g) are the RR-2 uptake at time t and at equilibrium, respectively; k1 (min−1) is the equilibrium rate constant; and k1 and predicted qe could be obtained from the slope and intercept of the plot of log (qe − qt) vs t.The linearized form of the pseudo-second-order equation is shown as follows:(3)tqt=tqe+1k2qe2,where k2 (g/(mg·min)) is the equilibrium rate constant and k2 and qe could be determined from the slope and intercept of the plot of t/qt vs t.The linearized form of the Elovich equation is shown as follows:(4)qt=1βlnαβ+1βlnt,where α (mg/(g min)) and β (g/mg) are the initial adsorption rate and the constant associated with the fraction of surface coverage and activation energy for chemisorption, respectively. The two parameters could be determined from the intercept and slope of the plot of qt vs. lnt.The kinetics parameters obtained from the three linearized plots are shown in Table2. The correlation coefficients of the pseudo-second-order kinetics are highest among the three models. The calculating values of qe,cal got from the equations are in good accordance with those values of qe,exp obtained from the experiments, indicating that the adsorption followed the pseudo-second-order kinetics. Thus, chemical adsorption or chemisorption might be the rate-determining step for the adsorption of RR-2 on MOSP-MMT, and the valency force between the RR-2 molecular and MOSP-MMT might be involved in the adsorption as well.Table 2
Parameters of the three kinetic equations for RR-2 adsorption on MOSP-MMT.
Models
Parameters
Values
C
0 = 51.9 mg/L
C
0 = 100.3 mg/L
Pseudo-first-order equation
q
e,exp
6.3
10.5
q
e,cal
6.51
3.12
k
1
0.0039
0.0053
R
2
0.964
0.9
Pseudo-second-order equation
q
e,cal
6.60
10.81
k
2
0.0050
0.0021
R
2
0.9959
0.9918
Elovich equation
α
5.71
1.58
β
1.35
0.63
R
2
0.9357
0.9796


## 3.3. Effect of pH

Solution pH can impact both the surface charges of adsorbent and the existing forms of adsorbate [23, 24], which in turn affects the adsorption. Figure 4 shows the effect of pH on RR-2 uptake. RR-2 uptake decreased from 7.6 to 6.5 mg/g as the pH increased from 3.2 to 4.1, and the RR-2 uptake decreased to 5.9 gradually as the pH rose to 9.1. Overall, rising pH weakened the RR-2 uptake. MMT could hardly adsorb RR-2 (data not shown) due to its hydrophilicity and net negative charge. However, the MOSP-MMT could adsorb RR-2 since the positively charged amino groups of MOSP in the MOSP-MMT could react with anionic groups of the RR-2 in the pH range of 3–9 set for the experiment, which improved the RR-2 adsorption. The probable mechanism for RR-2 adsorption on MOSP-MMT could be described by the following equations. P is used to represent the coagulating protein since its structure is still unknown so far:(5)where P, M, and R denote MOSP, MMT, and RR-2, respectively.Figure 4
Effect of pH on RR-2 uptake (C0: 52.6 mg/L, T: 30°C, and time: 10 h).The weakened adsorption with the increasing pH could be explained by the fact that the negative charge of RR-2 and positive charge on MOSP-MMT surface decreased with the rising pH. Additionally, the competition between RR-2 molecules and increasing OH− groups for the active sites weakened the RR-2 uptake as well.


## 3.4. Effect of Inorganic Salt

NaCl is an inorganic salt used widely to prompt dyeing efficiency because it can drive the dye molecules out of solution onto fibers [25]. Thus, there are a large amount of NaCl in dye-containing wastewater. Hence, the effect of NaCl on RR-2 removal was investigated. Figure 5 shows that RR-2 uptake increased from 15.2 to 17.1 mg g−1 as NaCl increased from 0 to 30 g/L, indicating that NaCl can improve the removal of RR-2. In our previous study, we found NaCl could increase the adsorption of reactive green on polyaniline/bentonite as well [25]. Ip et al. reported that the adsorption capacities of reactive dyes on four adsorbents increased with rising NaCl concentration as well [26]. The rise of the dye uptake caused by NaCl addition was attributed to the aggregation of reactive dye molecules resulted from the increased intermolecular forces, including van der Waals forces, ion-dipole forces, and dipole-dipole forces [27]. Additionally, the salts interacted with the adsorbate and screened the repulsive force between the dye molecules and the carbon surface between the adsorbed species [26], which could be another explanation for the increase of RR-2 removal.Figure 5
Effect of inorganic salt (C0: 142.5 mg/L, T: 30°C, reaction time: 10 h, and pH: 7).


## 3.5. Effect of Initial RR-2 Concentration

Figure6 shows the effect of initial RR-2 concentration on its adsorption on MOSP-MMT. Effect of initial RR-2 concentration on its adsorption was carried out ranging from 47.0 to 380.9 mg/L. Figure 6 clearly reveals that the extent of adsorption of RR-2 increased linearly with the rising RR-2 concentration. Meanwhile, at every RR-2 concentration, RR-2 adsorption at 30°C was better than that at 20°C.Figure 6
Effect of initial RR-2 concentration (reaction time: 10 h and pH: 7).The adsorption isotherm gives an idea of how the adsorbate molecules distribute between the liquid-solid interface at equilibrium [28, 29], which helps us to further understand the nature of adsorption. Thus, the Langmuir and Freundlich models are adopted to analyze the adsorption data at equilibrium, as shown in Figure 7.Figure 7
Adsorption isotherm of RR-2 on MOSP-MMT at two temperatures (initial RR-2 concentration: 47.0 mg/L−380.9 mg/L, reaction time: 10 h, and pH: 7).The linearized form of the Langmuir equation is shown as follows:(6)ceqe=1qmb+ceqm,where ce (mg/L) and qe (mg/g) are the concentration and adsorption capacity at equilibrium, respectively, and b (L/mg) and qm (mg/g) are the Langmuir constant related to binding energy and the maximum adsorption capacity, respectively, which could be obtained from the slope and intercept of the plots of 1/ce versus ce/qe, respectively.The linearized form of the Freundlich equation is shown as follows:(7)logqe=logkf+1nlogCe,where qe (mg/g) and ce (mg/L) have same meaning as that mentioned above and kf and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively, which could be obtained from the slope and intercept of the plots of ln ce versus ln qe, respectively.Parameters of Langmuir and Freundlich isotherms are shown in Table3. It can be seen that the R2 values of isotherm at both of the two temperatures are better than those of Langmuir isotherm, indicating that the Freundlich equation is more suitable to describe the adsorption of RR-2 on MOSP-MMT, which was a multilayer adsorption of RR-2 on the heterogeneous surface MOSP-MMT [30].Table 3
Parameters of Langmuir and Freundlich isotherms.
20°C
30°C
Langmuir
q
m
—
93.45
b
−2991.27
0.0052
R
2
0.0408
0.9138
Freundlich
n
0.9834
0.8270
k
f
0.1246
0.0666
R
2
0.9754
0.9972


## 4. Conclusion

In the present study, MOSP-MMT composite was synthesized and characterized, and the adsorption properties of the composite were investigated with a typical reactive dye, namely, RR-2. The adsorption process was found to be dependent on contact time, pH, inorganic salt, and initial dye concentration and the adsorption process followed the pseudo-second-order kinetics. The data obtained from adsorption isotherms at two temperatures fitted to the Freundlich model better than the Langmuir model. The composite has considerable potential for the removal of RR-2 from water solution.



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*Source: 8464815-2019-06-03.xml*