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Simultaneous Removal of Cationic and Anionic Dyes from Environmental Water Using Montmorillonite-Pillared Graphene Oxide Lu Liu,† Bin Zhang,† Yaru Zhang,† Yanjin He,† Langhuan Huang,† Shaozao Tan,*,† and Xiang Cai*,‡ †

Department of Chemistry, Jinan University, Guangzhou 510632, P. R. China Department of Light Chemical Engineering, Guangdong Polytechnic, Foshan 528041, P. R. China

J. Chem. Eng. Data 2015.60:1270-1278. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/23/19. For personal use only.

‡

ABSTRACT: We investigate the removal of methyl orange (MO) and methylene blue (MB) from aqueous solution by montmorillonite-pillared graphene oxide (MGO). Experimental conditions were used that evaluate the potential of MGO in removing anionic and cationic dyes in single and binary systems, and we investigated the uptake capacity of MGO toward organic dye as a function of different pH, adsorbent dosage, temperature, and adsorption time. In the single system, the Langmuir and Freundlich adsorption models were used to describe the equilibrium isotherm and calculate the isotherm constants. Moreover, the pseudo-first-order and pseudo-second-order kinetic models were applied to study the mechanism of MGO adsorbing dyes. Thermodynamic studies demonstrated that the adsorption of MO and MB onto MGO was feasible and spontaneous. In the binary system, the adsorption capacities of MO and MB by MGO were dramatically higher than those in a single system. Therefore, through the recorded adsorption results under different conditions, we could illustrate that the MGO was absolutely used as an adsorbent to be capable of simultaneous removals of MO and MB. human daily life.6 Thus, it is necessary to remove these dyes from aqueous solution. There are many methods such as chemical, physical, and biological approaches to investigate the process of removing dyes from contaminated water.7,8 Among these water treatment techniques, adsorbents for removing dyes have been extensively used for their merits of simplicity, high efficiency, and economic feasibility.9−11 Therefore, adsorbents with a high removal capacity play an important role in the performance of adsorbing dyes. For example, activated carbon is a popular adsorbent for removing various organic or/and inorganic pollutants in wastewater because of its high adsorption capacity. However, these adsorbents are not only very expensive but also difficult to regenerate after use, therefore, there is a tendency to use commercially available and cut-price materials for the adsorption of dyes, such as peat,12 saw dust,13 sepiolite,14,15 clay and modified clay,16−19 and diatomite.20−24 Graphene oxide (GO) and graphene nanosheets have attracted great attention all over the world in the past few

1. INTRODUCTION In recent years, industrial developments have left an impression on the environmental society. Organic dyes with intense color and high toxicity are widely used in various industries, including the manufacture of dyestuff, leather, ceramics, and textiles.1 Among the various pollutants of wastewater, organic dye effluents are the identified as a significant contaminant. The presence of these dyes in water even at very low concentrations is highly observable. Furthermore, allergic dermatitis or skin irritation is easily caused by the trace dyes in water, and they may be carcinogenic and mutagenic to humans and aquatic organisms.2,3 Dyes contain three kinds of anionic, cationic, and nonionic dyes. Anionic dyes are divided into direct, acid, and reactive dyes while the cationic dyes are basic dyes.3 Methylene blue (MB) is a well-known cationic dye for wood, paper, leather, and silk.4 Methyl orange (MO) is a typically anionic dying material which has been extensively used in the printing, food, textile, scientific research, and pharmaceutical industries.5 They are both toxic colorants and can easily cause harmful effects such as gene mutations, allergic dermatitis, and cancer. If these dyes are discarded into effluents, they will cause serious damage to © 2015 American Chemical Society

Received: October 9, 2014 Accepted: April 21, 2015 Published: April 28, 2015 1270

DOI: 10.1021/je5009312 J. Chem. Eng. Data 2015, 60, 1270−1278

Journal of Chemical & Engineering Data

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2.3. Adsorption Experiment. Adsorption tests of cationic and anionic dyes onto the MGO were conducted at 30 °C in a series of experiments. In a single system solution, a range of concentrations in 100 mL of solution were added to a flask containing MGO at different dosages (0.05 to 0.5 g) and MB or MO solution at various initial concentrations were added to each flask. The conical flasks were agitated in a water-bath− vibrator shaker at 160 rpm and made to react under a certain time interval. To separate solid and liquid phases, the suspension solution was centrigued at 4000 rpm for 10 min, and then the supernatant liquid was analyzed for MB and MO. MB and MO were determined by spectrophotometer by measuring their absorbance at λmax = 664 nm and λmax = 464 nm (Spectronic 20 Genesys spectrophotometer), respectively. For binary system solutions, different dosages (0.05 to 0.5 g) of MGO were added to the 100 mL adsorbate solution. Then the follow-up experiment steps were in accord with the above. When the reaction was kept at equilibrium, the removal amount of cationic or anionic dye per unit mass of adsorbent (qe, mg/g) was determined according to the following mass balance:

years. There are many researchers using graphene oxide-based material as an adsorbent to remove contaminants in aqueous solution.25−27 Because it has a high cation-exchange amount, is very available, inexpensive, and easy to recycle, montmorillonite (MMT) is also used as a good adsorbent for the removal of cationic dye. However, most of these findings only deal with single-component aqueous solutions, and relatively few works discuss and compare the removal capacity of anionic and cationic dyes onto adsorbent. Additionally, the utilization of GO and MMT composite for adsorbing anionic and cationic dyes is only present in a limited amount of literature. The present work was undertaken mainly to report the adsorption of a cationic dye (MB) and an anionic dye (MO) over montmorillonite-pillared graphene oxide (MGO), However, simultaneously removing the cationic and anionic dyes has not been widely achieved,28 and the adsorption processes of the MGO are more easily understood through a contrast between the MB and MO.29 The adsorption behavior of cationic and anionic dyes on MGO was studied by kinetic and thermodynamic analyses.

2. MATERIALS AND METHODS 2.1. Materials. Graphite powder (spectral pure), KMnO4, P2O5, concentrated H2SO4, H2O2, MB, and MO were bought from Lingfeng Chemical Reagent Company (Table 1). All

qe =

graphite powder KMnO4 P2O5 H2SO4 H2O2 MB MO MMT a

source

initial mole fraction purity

purification method

final mole fraction purityb

analysis methodb

CRa

0.99

none

NA

NA

CRa CRa CRa CRa CRa CRa CRa

0.96 0.96 0.98 0.30 0.98 0.98 0.9

none none none none none none none

NA NA NA NA NA NA NA

NA NA NA NA NA NA NA

(1)

where C0 was the adsorbate initial concentration (mg/L), Ce was the adsorbate equilibrium concentration (mg/L), V was the liquid volume (L), and W was the dosage of the solid adsorbent (g), respectively. The linear form of the Langmuir isotherm model was given34 by

Table 1. Sample Purity chemical name

(C0 − Ce)V W

Ce C 1 = + e qe qmax KL qmax

(2)

where KL was the Langmuir constant consistent with the rate of adsorption (L/mg), and qmax was the Langmuir maximum removal capacity of MB or MO per unit mass of adsorbent (mg/g). The logarithmic form of the Freundlich model was given35 by

Chemical reagent. Co., Ltd. bNA = not applicable.

ln qe = ln KF +

materials were dissolved in the deionized water to prepare different concentrations in this research. The MMT was obtained from Fuchen Clay Co., Ltd. (Tainjin, China). All reagents were used in the experiments without any further purification. 2.2. Preparation of MGO. We used an improved Hummers method to prepared GO.30−33 Briefly, 2.5 g of natural graphite and 2.5 g of P2O5 were added to a flask. Then, 58 mL of concentrated H2SO4 was placed with stirring in an ice bath, and 12.5 g of KMnO4 was added slowly and uniformly for about 70 min. The mixture was stirred for 30 min in an ice bath. Afterward, the temperature was raised to 35 °C and the solution was stirred for 2 h at 35 °C. Next, the temperature was raised to 80 °C. When the mixture turned into a red brown viscous liquid, 150 mL of H2O and 6 mL of 30 % H2O2 was added. Finally, the mixture was washed with deionized water and centrifuged many times. After the solids were vacuum dried, GO appeared as flakes. Finally, 0.3 g of GO and 1 g of MMT were dissolved in 80 mL of H2O, and the mixture underwent ultrasound for 8 h, then the mixtures were dried for reserve. The resulting MGO was obtained.

1 ln Ce n

(3)

where n and KF were defined as the Freundlich constants, indicating the favorableness of the adsorption process and the removal capacity of the adsorbent, respectively. KF was the distribution or adsorption coefficient, and represented the adsorbing capacities of the MGO for dyes per unit equilibrium concentration. The slope 1/n, ranging from 0 to 1, was used to measure the surface heterogeneity or adsorption intensity, and reflected greater heterogeneity when its value got closer to zero. The thermodynamic model of MB and MO adsorption on MGO was evaluated by the thermodynamic parameters including the changes in entropy (ΔS), free energy (ΔG), and enthalpy (ΔH). All of the thermodynamic parameters were calculated according to the equations ln(Kd) =

ΔS ΔH − R RT

ΔG = −RT ln(Kd) 1271

(4) (5) DOI: 10.1021/je5009312 J. Chem. Eng. Data 2015, 60, 1270−1278


Kd =

(C0 − Ce) V · Ce W

Article

(6)

Where Kd was the distribution coefficient of the adsorption process, R was the universal gas constant [8.314 J/(mol·K)], and T was the temperature (K). The pseudo-first-order kinetic model was used for low concentration of solute. It was written by36 ln(qe − qt ) = ln qe − k1t

(7)

where qt was the MB or MO amount adsorbed at time t (mg/ g); qe was that adsorbed at the equilibrium (mg/g); k1 was the rate constant of the pseudo-first-order equation. The pseudo-second-order equation was more suitable for the amount of the solute adsorbed on the surface of adsorbent and the dosage adsorbed at equilibrium,36 which is given by the following equation:37 t 1 t = + qt qe k 2qe2

Figure 1. Pore size distributions of the GO, MMT, and MGO.

(8)

where k2 was the rate constant of pseudo-second-order equation, and t, qe, and qt had the same meanings as those in the pseudo-first-order equation. The intraparticle diffusion model was also used to study the kinetic adsorption. The intraparticle diffusion parameters were calculated using the following form:38 qt = kp(t

1/2

)

Figure 2. FTIR spectra of the MMT (a), GO (b), and MGO (c).

(9)

frequency), 1385 cm−1 (CC bending), and 1052 cm−1 (C−O−O bending). From Figure 2c, it could be concluded that MGO not only had MMT characteristic bands, but also had GO characteristic bands. Because of these changes in the FTIR spectra, it was thought that the MMT modification of GO had been achieved. 3.1.3. SEM Analysis. Figure 3 panels a, b, and c illustrate the SEM images of MMT, GO, and MGO, respectively. The SEM

where kp was the intraparticle diffusion rate constant. It was generally found that the plot of qt against t1/2 might present a multilinearity, which indicated that two or more steps occurred in the adsorption processes. The first sharper portion was external surface adsorption or instantaneous adsorption stage. The second portion was the gradual adsorption stage, where the intraparticle diffusion was rate-controlled. The third portion was the final equilibrium stage, where the intraparticle diffusion started to slow down due to the extremely low solute concentration in solution.39

3. RESULTS AND DISCUSSION 3.1. Material Characterization. 3.1.1. Specific Surface Area. Surface areas were investigated using BET analysis. The surface area values of the GO, MMT, and MGO were 611, 243, and 972 m2·g−1, respectively. The lower surface area of GO was probably due to the part of graphite not stripping completely and aggregation in the process of oxidation. The higher surface area of MGO was due to the pillared MMT and enhanced adsorption of N2 in the wider ranges of micropores and mesopores40 (Figure 1). Since MGO had larger specific surface area than MMT or GO, it provided more adsorption sites to adsorb cationic and anionic dyes. 3.1.2. FTIR Analysis. The FTIR spectra of MMT, GO, and MGO are shown in Figure 2. The MMT (Figure 2a) was characterized by the bands: the broad bands in the 3626 cm−1 and 3452 cm−1 region were ascribed to the −OH stretch of the lattice hydroxyl and the −OH stretch from free H2O, respectively, while the O−H bending vibration band occurred at 1636 cm−1 and the Si−O stretching vibration band was observed at 1035 cm−1.41,42 In the FTIR spectrum of GO (Figure 2b), the characteristic bands appeared at 3399 cm−1 (which stood for the −OH stretching band from water), 1730 cm−1 (CO bending), 1622 cm−1 (C−OH vibration

Figure 3. SEM micrograph of MMT (a), GO (b), and MGO (c).

micrograph showed that GO was sheet-shaped with a little agglomeration, which might be due to the aggregation during the reduction process. The SEM micrograph of MGO showed the formation of a hybrid composite. MMT exhibited stratified structure-like morphology, and MGO contained irregular sheets 1272



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increasing from 2 to 6. When the pH was up to 9, the percentage removal decreased. This occurred because the negative charge appeared on the adsorbent surface and the positive charge decreased on the solution interface at high pH solution.44 As a consequence, the anionic dye adsorption showed a decrease and the cationic dye adsorption increased. The zero-point-charge of MGO appeared at pH = 6.2; when the pH was less than 6.2, the adsorbent surface appeared positively charged, while when the pH was greater than 6.2, it was negatively charged. In comparison, at a low pH solution, the adsorbent surface appeared positively charged and the positive charge at the solution interface would increase, which resulted in an increase in anionic dye adsorption and showed a decrease in cationic dye adsorption. 3.2.2. Effect of Adsorbent Dosage on Dye Adsorption. The effect of adsorbent dosage on removal was studied by adding various amounts of the MGO (0.05−0.5 g) into the flask containing 100 mL of 150 mg/L (MB) and 50 mg/L (MO) solution at pH = 6.0. The results were presented in Figure 5b. The removal percentages of MB and MO by MGO increased from 93.9 % to 97.7 % and from 39.4 % to 71.7 %, respectively, as the sorbent dosage increased from 0.05 g to 0.5 g and then remained almost constant. The increase in percent dye removal with increasing adsorbent dosage in the first stage could be attributed to the greater availability of the exchangeable sites of the adsorbent. Therefore, 0.05 g of MGO for MB and 0.4 g of MGO for MO were selected for further experiments. 3.2.3. The Comparison of Adsorption on MB and MO by MGO, MMT, or GO. To verify MGO could provide more adsorption sites based on BET surface area. We added MGO, MMT, and GO into a series of adsorption tests of different concentrations of MB (C0 = 0 mg/L to 400 mg/L) and MO (C0 = 0 mg/L to 3000 mg/L) solutions. According to Figure 6, when the concentration of MB and MO were increasing, the adsorbing capacities of MB and MO both increased at the beginning, but when the concentration of MB and MO continued to increase, the adsorbing capacities started to decrease, This may have occurred because when the concentration of MB and MO increased to a certain degree, the adsorbing capacities had reached maximum, and when the concentration of MB and MO continued to increase, there would be competition for the adsorption sites which would cause the decrease of the adsorbing capacities. Therefore, from Figure 6, we could clearly see that MGO owned the best adsorption ability among the MMT and GO.

in the interlayer and a lot of fold on its surface, which was ascribed to GO. The results indicated that the MMT was pillared in each layer of the GO. 3.1.4. TEM Analysis. The TEM image exhibited that the diameter of the MMT was generally homogeneous in size at about 0.3 μm (Figure 4a), The TEM image clearly showed the

Figure 4. TEM images of MMT (a), GO (b), and MGO (c).

flake-like shape of GO (Figure 4b), and the size of GO was 3 μm to 4 μm (length) × 2 μm to 3 μm (width). The MGO pattern (Figure 4c) presented particles as mixed shapes of sheet and blocks; the MMT with a high density was distributed over the surface of GO, and the fold of GO had disappeared, which indicated that MMT was pillared in each layer of GO. 3.2. Adsorption Study of Cationic and Anionic Dyes in Single Solution. 3.2.1. Effect of Solution Initial pH on Dye Uptake. The adsorption efficiency of dyes on adsorbent was highly affected by solution pH. Therefore, the optimized pH was critical for further investigation.43 The effect of pH in the range of 2.0 to 11.0 with a stirring time of 50 min on the removal of dyes was studied using 0.1 mol/L HCl or NaOH solution for pH adjustment, with the initial MB and MO concentration fixed at 150 mg/L and 50 mg/L, respectively. Figure 5a showed that the percentage removal of MB on MGO (with 0.4 g mass loading) increased with an increase in solution pH, while the percentage removal of MO on MGO (with 0.4 g mass loading) remained almost unchanged with the pH

Figure 5. (a) Effect of solution pH on adsorption amounts of the MB and MO. (b) Effect of amount of MGO on MB and MO dye solution adsorption. 1273



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Figure 6. Comparison of the adsorptive capacities of MB (a) and MO (b) by MGO, MMT, or GO.

Figure 7. Freundlich (a) and Langmuir (b) plots for the adsorption of MB and MO by MGO.

Table 2. Langmuir and Freundlich Isotherms Constants for the Adsorptions of the MB and MO on MGO Langmuir

MB MO

Freundlich

qe(exp)

KL

Qmax

mg/g

L/mg

mg/g

R2

S2

mg/g

n

R2

S2

345.0 131.8

0.207 0.005

250.0 144.9

0.9824 0.9623

0.1412 0.1453

224.1 12.9

18.9 3.34

0.5762 0.8418

0.6185 0.7348

KF

Figure 8. (a) Plot of ln (Kd) versus 1/T for MO and MB adsorption by MGO; (b) effect of adsorption time on the removal amount of MB and MO by MGO.

correlation coefficients (R2) are given in Table 2. The correlation coefficients obtained from the Langmuir isotherm were higher than the value from the Freundlich isotherm; therefore, the Langmuir isotherm was a better fit for the adsorption of MB and MO on the MGO. The results

3.2.4. Adsorption Isotherms. The Langmuir and Freundlich equations (eqs 2 and 3) are necessary to evaluate the adsorption properties of the adsorbent. Figure 7 displays the adsorption isotherms for MB and MO. The calculated parameters for Langmuir and Freundlich isotherms and the 1274



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3.2.6. Adsorption Kinetics. We also studied the effects of contact time on the adsorption. The data indicated that the adsorbent was highly effective for the adsorption of MB and MO, and the adsorption balance was reached in a short time (ca. 50 min) (Figure 8b). At the initial stage the adsorption went very fast, which might be due to the remaining active sites on the adsorbent and the availability of the uncovered surface area. The experimental data of the kinetic model for MB and MO were calculated by the pseudo-first-order, pseudo-secondorder, and intraparticle diffusion model, according to eqs 7 to 9, respectively. The calculated results are listed in Table 4. According to the correlation coefficients, the experimental data fit to the pseudo-second-order model (R2 > 0.99) better than to the pseudo-first-order model and intraparticle diffusion model (Figure 9), which suggested that the rate-limiting step might be the chemical adsorption.46−48 3.3. Adsorption Study of Cationic−Anionic Dye Binary Mixture. 3.3.1. Effect of Simultaneous Adsorption of Cationic and Anionic Dyes. In this work, we studied the coadsorption of a mixture of anionic and cationic dyes, because it owned greater practical relevance. The experiments of simultaneous adsorption of MB and MO included the effect on MO removal with the presence of MB and the effect on MB adsorption with the presence of MO. In two binary systems, the initial concentration of MO was fixed to 50 mg/L, whereas the concentration of MB was varied from (0 to 3000) mg/L. As shown in Figure 10a, the presence of MO with the MB adsorption was much more pronounced than that in the absence of MO by MGO, and the adsorption capacities qe (MB) increased from 350 mg/g to 715 mg/g. In another binary system, the initial concentration of MB was constant at 150 mg/L, and the concentration of MO was varied from (0 to 3000) mg/L. For MGO, the adsorbing capacity of MO in the presence of MB was also better than that in the absence of MB (Figure 10b). It was seen that MB and MO adsorption both increased in the binary system, which suggested a synergistic adsorption occurring. 3.3.2. Effect of Adsorbent Dosage and pH on Binary Adsorption. To investigate the effect of varying the MGO dosage in the adsorption medium on the percent of the MB− MO binary mixture removed, the dosage of MGO used as adsorbent was varied in the range of 0.05 g to 0.4 g and the adsorption process was carried out using the batch technique in the adsorbate solution of fixed concentration (CMB = 150 mg/ L, CMO = 20 mg/L) and at the same temperature (T = 30 °C). The results of the percent dye removal using different adsorbent doses were presented in Figure 11a. The removal percent of MB and MO using the MGO dosage of 0.05 g was about 100 % and 97 %, respectively. Then, the removal percent of MB and MO remained almost constant by increasing the MGO dosage in the adsorbate solution, indicating that an adsorbent dose of 0.05 g was sufficient for the optimum removal of the MB and MO.

demonstrated that the MB and MO in a severe environment were absorbed with high efficiency by MGO; especially the qmax values of the adsorbent of MB and MO were 250 and 145 mg/ g, respectively. Moreover, MGO was superior to other absorbents in terms of easy separation and little environmental pollution. 3.2.5. Adosrption Thermodynamics. The adsorption of the MB and MO on the MGO was investigated at different temperatures (293 K, 303 K, and 313 K) to decide the thermodynamic parameters. The changes in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) could be obtained for the transfer of a unit mole of solute from solution onto the solid− liquid interface. According to eq 5 and eq 6, the values of Kd and the ΔG parameters at each temperature were calculated. As shown in Figure 8a, the plot of ln (Kd) versus 1/T gave a straight line with a slope of ΔH (kJ/mol) and an intercept of ΔS (kJ/mol K). The thermodynamic parameter values (measured at different temperatures) are listed in Table 3. Table 3. Thermodynamic Adsorption Parameters of MB and MO by MGO at Different Temperature MB

MO

temp (K)

ΔG/(kJ/mol)

293 303 313 293 303 313

−3.08 −3.76 −3.97 −5.63 −2.01 −1.83

ΔH/(kJ/mol)

ΔS/(J/mol K)

10.09

45.17

−85.67

−274.20

On the one hand, the values of ΔH were positive for MB, suggesting the endothermic nature. The positive ΔS values indicated that the randomness at the solid/solution interface increased during the adsorption of MB onto MGO. On the other hand, the negative value of ΔH manifested the exothermic process of the removal of MO onto MGO. The negative ΔS value reflected that the randomness at the solid/ solution interface decreased during the adsorption of MO onto MGO. Furthermore, the negative free energy changes ΔG at different temperatures suggested that the adsorption of the MB and MO onto MGO was thermodynamically spontaneous and feasible. As shown in Table 3, the negative values of ΔG decreasing with increasing temperature suggested the favorable and spontaneous adsorption process of MB on the MGO. The physical adsorption energies were in the range of 0 kJ/mol to −20 kJ/mol and chemisorption energies in the range of −80 kJ/mol to −400 kJ/mol, which were reported by some researchers.45 When the intermolecular attractive forces between the adsorbate and adsorbent were greater than those among the adsorbate molecules themselves, it would cause the physical adsorption. However, chemisorption involved the formation of chemical bonds between the molecules of adsorbate and adsorbent. In this study, the adsorption of MB and MO on MGO supported a physical adsorption.

Table 4. Adsorption Kinetic Parameters for the Adsorption of the MB and MO on MGO pseudo-first-order model

MB MO

pseudo-second-order model

intraparticle diffusion model

qe(exp)

K1

mg/g

min−1

R2

mg/g)

S2

g/mg/min

R2

mg/g

S2

Kp

R2

S2

345.0 131.8

0.0157 0.0092

0.6742 0.7390

63.7 10.9

0.7359 0.5101

2.69 1.37

0.9999 0.9999

285.7 11.8

0.0223 0.0279

2.5375 0.1438

0.7323 0.6836

0.5006 0.7126

qe(cal)

qe(cal)

K2

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Figure 9. Pseudo-first-order (a) and pseudo-second-order (b) plots and intraparticle diffusion model (c) for adsorption of MB and MO by MGO.

Figure 10. (a) Effect of MO (C0 = 50 mg/L) on the adsorption of MB by MGO. (b) Effect of MB (C0 = 150 mg/L) on the adsorption of MO by MGO.

the efficiency of the adsorbent in the dye removal process, different adsorbate solutions having various initial pH values were prepared. The concentrations of these solutions were the same (CMB = 200 mg/L, CMO = 40 mg/L) and the MGO dosage was fixed at 0.05 g, but the initial pH was changed in the range of 2 to 11, and the adsorption efficiency was determined in each case. The results of the dye removal efficiency were presented in Figure 11b, which showed the change in the percent of dye removal as a function of pH. It was clear that maximum removal percent of MB (99.9 %) and MO (97.9 %) was attained at pH 9 and 7, respectively. As the pH values got higher, the percent removal of MB and MO showed decrement in its value. 3.4. Adsorption Mechanism. Many factors control the dye adsorption mechanism, for example, the electrostatic forces between adsorbent and adsorbate, the mass-transfer process, chemical and/or physical properties, and so on.49,50 MB and MO are both noncovalent functionalization molecules that would likely adsorb on MGO through π−π bond interactions

Figure 11. (a) Effect of amount of MGO on MB−MO binary mixture adsorption. (b) Effect of pH on MB−MO binary mixture adsorption.

The pH of the adsorbate solution affected to great extent the adsorption process as a whole, as pH played an important role in determining the ionization degree of the adsorbate, in addition to affecting the surface charge of the adsorbent particles. To study the effect of the initial pH of adsorbate on 1276



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between the columnar structure of MGO and the aromatic ring of the dyes. For example, we could see that the adsorption capacity for MO under acidic conditions was found to be lower than that under basic conditions (Figure 5a), which suggested that π−π stacking interactions play an important role in dye adsorption on MGO. While the zeta potential of MGO is −30.4 mV and the zero- point-charge of MGO appeared at pH = 6.2, the MGO surface solution is negatively charged. The dissolved MB solution is positively charged which would attract when approaching the MGO. Therefore, the electrostatic attraction force between MB and MGO might partially explain the adsorption mechanism.

4. CONCLUSION MGO could efficiently remove anionic dye (MO) and cationic dye (MB) from a sewage solution under severe conditions. The experimental data of adsorption kinetics showed that the pseudo-second-order model was better than the pseudo-firstorder model to describe the adsorption of MO and MB on MGO, and the adsorption behavior was fitted to the Langmuir better than the Freundlich isotherm, with maximum adsorption capacities of 144.9 mg/g for MO and 250.0 mg/g for MB. In a single system, the adsorption capacity of MB increased while that of MO decreased when the solution pH increased from 2.0 to 11.0, and higher adsorbent results in higher adsorption in both MO and MB. In the binary system, MGO exhibited better adsorption of MO and MB than those in single system, the adsorption capacities qe (MO and MB) increasing from 131 mg/g to 466 mg/g and 350 mg/g to 715 mg/g, respectively. In view of the above, MGO could be used as an effective and economical adsorbent for simple or simultaneous removals of the anionic and cationic dyes from environmental sewage.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

The authors acknowledge financial support from the National Natural Science Foundation of China (21271087, 51172099 and 21476052), the Foundation of Enterprise-UniversityResearch Institute Cooperation from Guangdong Province and the Ministry of Education of China (2013B090600148), and The Science and Technology Innovation Platform Project of Foshan City (2014AG100171). Notes

The authors declare no competing financial interest.

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