Enhanced Glyphosate Removal by Montmorillonite in the Presence

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Enhanced Glyphosate Removal by Montmorillonite in the Presence of Fe(III) Zhong Ren,†,‡ Yuanhua Dong,*,† and Yun Liu*,† †

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, P. R. China ‡ Graduate University of Chinese Academy of Sciences, 100049 Beijing, P. R. China S Supporting Information *

ABSTRACT: N-(Phosphonomethyl)glycine (glyphosate, PMG) is an effective broad-spectrum organic phosphoric herbicide. The mass production and wide use of PMG presents a potential environmental hazard. This study evaluated the feasibility of using montmorillonite (MT) as an adsorbent to remove PMG from wastewater in the presence of Fe(III) ions. The results showed that the PMG adsorption process is dependent on the Fe(III) concentration. Adsorption was fast, and the adsorption kinetics were best described by a pseudo-second-order model. Adsorption of PMG was most effective over the pH range from 1.9 to 4.1. Ionic strength was found to have little effect on PMG removal at pH < 4.1 but to enhance PMG removal at pH > 4.1. Competing ions reduced the PMG removal at low pH (pH < 5.9) but increased the PMG removal at pH > 5.9. The adsorption capacity was found to be greater than 210 mg/g. The mechanism of this removal process is discussed in detail in terms of XRD and species calculations.

1. INTRODUCTION The herbicidal properties of the organophosphate compound N-(phosphonomethyl) glycine (PMG) were discovered by the Monsanto Company in 1970, after which the herbicide was widely used throughout the world. PMG is a small molecule with three polar functional groups (NH, COOH, and PO3H2). It is a nonselective herbicide that can inhibit some enzymes in plants.1 Environmental problems relating to the extensive use of PMG have been noted during the manufacture, transportation, and application of the molecule.2−4 For herbicide manufacturers, effluents with high concentrations of PMG and NaCl are extremely hard to purify. Several methods have been used to treat PMG-containing wastewaters, including photocatalytic degradation,5 ferrioxalatephotoinduced degradation,6 chlorination,7 electrochemical oxidation,8 and UV radiation.9 These techniques are effective to varying degrees, but degradation products and byproducts need further treatment to avoid secondary pollution. Adsorption has been recognized as an effective, convenient, and recyclable method. It can be easily applied in practice using engineering equipment such as batch adsorption-settling ponds, fixed beds, or fluidized beds. An effective adsorbent is the key of this method. Adsorbents that can be used for PMG removal such as layered double hydroxides (LDHs),10 ferric-supported active carbon,11 and water industrial residual12 have been investigated systematically. Montmorillonite (MT) is a 2:1-type aluminosilicate [unit cell formula (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O] that is widely used in wastewater treatment.13,14 However, PMG, which is anionic in aqueous solution at nearneutral pH, has a low affinity for cationic clay. Therefore, most research relating to PMG and MT has focused on the fate of PMG in soil.15−20 In the first reports by Sprankle et al.,21 Fe3+and Al3+-saturated clays and organic matter were demonstrated to adsorb more glyphosate than Na+- or Ca2+-saturated clays © 2014 American Chemical Society

and organic matter. Glass discovered that the adsorption process begins with cation release from MT.20 Shoval and Yariv17,18 studied the interaction between PMG and MT by infrared spectroscopy and observed complexation between the metal ions and PMG in the interlayer space. The Afonso group investigated PMG adsorption on raw MT and proposed structures of the surface complexes based on X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) patterns and explored them using molecular modeling techniques.15,16 As an inexpensive and environmentally compatible natural mineral, it is surprising that there are no previous reports on the use of MT in PMG removal from aqueous solution with the help of Fe(III). Although metal cation enhancement effects have been observed for many years,20−22 the exact mechanism and extent of the effect of Fe(III) on the process of PMG removal by MT and elucidation of the critical factors remain unresolved. The primary objective of this study was to investigate the performance of PMG removal by MT in the presence of Fe(III). The Fe(III) dosage, contact time, temperature, initial pH, and presence of foreign ions were investigated.

2. MATERIALS AND METHODS 2.1. Materials. PMG was purchased from Sigma-Aldrich (St. Louis, MO) (97% purity). MT was purchased from R&L Chemical Inner Mongolia Co., Ltd. (Neimenggu Province, China). The particle diameter was 0.99, and the equilibrium rate constant (k2) of the pseudo-second-order model was 7.977 × 10−3. Furthermore, the calculated qe value of 170.94 mg/g was very close to the experimental value. For the Elovich model, the R2 value of 0.88 was not good and indicated a poor fit. Fe(III) adsorption onto MT with and without PMG was carried out and is shown in Figure 2. Rapid adsorption occurred, but the quantity of Fe(III) adsorbed was much lower than that in the presence of PMG. Thus, PMG removal by MT aided by Fe(III) is a coadsorptive process. 3.2. Effect of Fe(III) Concentration. Metal-cationenhanced adsorption effects were documented previously for Fe(III);21 however, the direct addition of Fe(III) to the reaction between PMG and MT in aqueous solution has not been studied. The effects of the added Fe(III) concentration on the PMG removal rate and equilibrium concentration of Fe(III) in aqueous solution are shown in Figure 3. In the absence of Fe(III), PMG removal by MT was very low at an initial concentration of 350 mg/L, which was ascribed to electrostatic repulsion between the dissolved PMG and MT layers. The PMG removal rate increased linearly with increasing Fe(III) concentration to a value 0.11 mmol. Meanwhile, the Fe(III) concentration in the aqueous solution remained at a low level. This indicates that Fe(III) completely reacted with the MT and PMG. The PMG adsorption maximum reached 0.10 mmol at

Figure 3. Effect of Fe(III) dose on the removal of PMG by MT.

an Fe(III) concentration of 0.11 mmol. In this case, the Fe(III) concentration was equal to the concentration of PMG. Figure S1 (Supporting Information) shows the relationship between the quantity of PMG removed and the Fe(III) consumed over a concentration range of 0 to 0.11 mmol. The molar ratio of PMG removed to Fe(III) consumed obtained from a linear fit was close to 1:1 (0.93:1). The complexation ratio of PMG and Fe(III) was reported by Barja and Afonso to be 1:1.28 After the equivalence point, Fe(III) consumption varied slightly, and the dissolved Fe(III) concentration increased linearly with the initial concentration. This indicates that the excess Fe(III) did not participate in the complexation but adsorbed to MT. It can thus be inferred that Fe(III) and PMG formed a stable 1:1 complex that was strongly adsorbed to MT in this system. 3.3. Effects of pH and Ionic Strength. It is important to understand how pH affects PMG removal, as it has implications for engineering. Figure 4 shows the relationship between equilibrium pH and initial pH. In comparison with the initial pH, the equilibrium pH decreased significantly. At an initial pH of 12.0, the equilibrium pH changed little. This initial equilibrium pH curve resulted from the addition of FeCl3· 6H2O as OH− was consumed during the hydrolysis of Fe(III), decreasing the pH of the solution. Figure 5 indicates that the initial PMG adsorption was favored over the range of 2.0−10.5, which corresponds to an equilibrium pH between 1.9 and 4.1 (Figure 5). It was also noted that the PMG removal rate declined rapidly when the initial pH was increased above 10.5. 14487

dx.doi.org/10.1021/ie502773j | Ind. Eng. Chem. Res. 2014, 53, 14485−14492

Industrial & Engineering Chemistry Research

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by MT. At pH > 4.1, Fe(III) formed anionic or neutral complexes with PMG that could no longer be immobilized on MT as a cation, preventing precipitation. 3.4. Effects of Competing Ions. In a real wastewater system, many ions such as Cl−, SO42−, and CO32− are often present. PO43− often coexists with PMG in manufacturing wastewater. The effects of anions such as SO42−, CO32−, and PO43− on PMG adsorption by MT in the presence of Fe(III) are shown in Figure 6. At pH < 5.9, PO43− and SO42− were

Figure 4. Equilibrium pH as a function of initial pH.

Figure 6. Effects of competing ions at different pH values.

found to inhibit PMG removal, with PO43− showing greater inhibition. The removal-rate inhibition by PO43− decreased from 98.05% to 74.80%. This agrees with previous reports31−33 in which phosphate was found to compete with Fe(III) binding to suppress the formation of PMG−Fe(III) complexes. SO42− exhibited a smaller influence over this pH range, with a reduction in the maximum removal rate from 98.05% to 85.87%. For CO32−, the pH cannot fall below 5.6 because of the H2CO3−CO2 equilibrium. At pH > 5.9, PO43−, CO32−, and SO42− were found to promote PMG removal. All three competing ions can defer the decline of the removal rate following an increase in pH, as described above for the NaCl/ ionic strength mechanism mentioned previously. 3.5. Adsorption Isotherms. Figure 7 shows the isotherms of PMG adsorption on MT with Fe(III) at three temperatures.

Figure 5. Effect of initial pH on PMG removal.

At an ionic strength of 0.1 M NaCl, the PMG removal rate was the same as that for the blank when the initial pH was 10.5 was not as steep as for the NaCl-free system. Thus, increased ionic strength promotes PMG removal at high pH. The effect of NaCl concentration on PMG removal in the presence of Fe(III) is shown in Figure S2 (Supporting Information). The removal rate did not change as the ionic strength was increased at an initial pH of 3.0. Similar behavior was observed by Damonte et al.16 and McBride,29 with the latter suggesting that inner-sphere surface complexes can show small adsorption increases with increasing solution ionic strength, due to the increased activity of the available counterions to compensate the surface charges generated by specific ion adsorption. The variation of the Fe(III) concentration with solution pH was found to be similar to that of PMG. It is important to note that, instead of precipitating, the concentration of Fe(III) dissolved in solution increased with increasing pH. The same phenomenon was also observed for Cu adsorption on MT, and PMG can reduce Cu adsorption under certain conditions.30 The relationship between Fe(III) and PMG in aqueous solution over the equilibrium pH range from 4.1 to 7.1 is illustrated in Figure S3 (Supporting Information). The linear relationship indicates that Fe(III) coordinates to PMG over this pH range [otherwise, the Fe(III) would precipitate instead of remaining dissolved] and that the mole ratio is maintained at 1:1. At equilibrium pH 1.9−4.1, the complex could be adsorbed

Figure 7. PMG adsorption isotherms. 14488



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The three curves are quite close to one another, indicating that the adsorption behavior was barely affected over the temperature range studied. The Langmuir and Freundlich adsorption isotherms were employed in this study. For the Langmuir equation, the following assumptions were made: adsorption is limited to monolayer coverage; all surface sites are alike, and each site can accommodate only one adsorbed molecule; and the ability of a molecule to be adsorbed on a given site is independent of the occupancy of neighboring sites (i.e., nonlateral surface interactions between adsorbates are ignoredy). By applying these assumptions and the kinetic principle that the rates of adsorption and desorption from the surface are equal, the Langmuir model relates the coverage of molecules on a solid surface to the concentration of the medium above the solid surface at a fixed temperature.34 It can be represented by the equation

qe =

Table 3. Comparison of Glyphosate Adsorption Capacity and Equilibrium Time with Other Adsorbents adsorbent water industrial residual MgAl LDH NiAl LDH activated carbon MT resin zeolite ZSM-5 goethite Fe(III)−MT system

(7)

where qe (mg/g) is the amount of PMG adsorbed per unit mass of MT particles at equilibrium, Ce (mg/L) is the equilibrium liquid-phase concentration of PMG, b is the equilibrium constant (L/mg), and qm is the amount of adsorbate required to form a monolayer (mg/g). The Langmuir constants qm and b at temperatures from 298 to 318 K are reported in Table 2. Table 2. Freundlich and Langmuir Isotherm Constants for the Adsorption of PMG onto MT with Fe(III) Freundlich constants

Langmuir constants

T (K)

Kf

n

R2

qm

b

R2

298 308 318

129.693 133.510 132.237

9.706 10.456 10.652

0.922 0.986 0.985

202.992 198.003 196.238

0.983 2.362 2.088

0.908 0.833 0.852

ref

85.9−113.6

60

12

27.4−184.5 172.4 58.4 ∼60.7 362.2 98.5 ∼31 203.0

45−60 30 100 − 90 − − 5

10 36 37 16 35 38 39 this study

Table 4. Hydrolysis Reactions for Fe(III) at 25 °C Fe3+ + H2O = FeOH2+ + H+ Fe3+ + 2H2O = Fe(OH)2+ + 2H+ Fe3+ + 3H2O = Fe(OH)0 + 3H+ Fe3+ + 4H2O = Fe(OH)4+ + 4H+ 2Fe3+ + 2H2O = Fe2(OH)24+ + 2H+ Fe(HO)3(s) + 3H+ = Fe3+ + 3H2O

The Freundlich isotherm is an empirical equation. This equation is among the most widely used isotherms for the description of adsorption equilibria and is described by qe = K f Ce1/ n

equilibrium time (min)

not economical and are not as conveniently available as MT, as they have to be artificially synthesized or processed. In addition, our reaction rates are the highest reported to date. The fact that efficiency is maintained under high-salinity conditions makes the Fe(III)−MT system very promising for PMG removal. 3.6. Removal Process and Mechanism. Speciation of PMG was calculated using the deprotonation constants pKa1 = 2.22 (carboxylate), pKa2 = 5.44 (second phosphonic), and pKa3 = 10.13 (amine) (Table S2, Supporting Information),40 as shown in Figure S4 (Supporting Information). Over the pH range from 2.0 to 4.0, the species H3L and H2L− (where L represents fully deprotonated PMG) both coexist in solution. Hydrolysis of Fe(III) was enhanced by increasing the pH. The corresponding reactions are listed in Table 4,41 and the

qmbCe 1 + bCe

Langmuir or maximum qm (mg/g)

(8)

speciation of Fe(III) hydrolysis simulated by the program Visual MINTEQ 3.1 is shown in Figure S4 (Supporting Information). It is known that Fe(III) exists as FeOH2+ over the pH range of 2.0−4.0. Fe(III) acts as a “bridge” between PMG and MT, and the valence of Fe species has to be more than 1+ to effectively bridge PMG and MT. Four complexes can be formed between PMG and Fe(III):42,43 FeL, FeHL+, FeLOH−, and FeL23−. Of these, FeHL+ is the only positively charged complex, which can be adsorbed onto MT. By combined analyses of the speciation of PMG and Fe(III), the removal mechanism was deduced. Over the pH range of 2.0−4.0, there are two potential binding modes. One pathway involves the initial formation of the complex, as described in eqs 9 and 10, followed by adsorption of the FeHL+ complex onto MT (eq 11). FeOH2+ is consumed in the formation of other species such as Fe(OH)2+, which will equilibrate to form FeOH2+. The other pathway involves Fe(III) adsorption directly to MT (eqs 12), followed by PMG binding to Fe(III)-saturated MT (eqs 13). The first pathway (complexation−adsorption) might dominate, as the homogeneous reaction (collision complex formation) is faster than the heterogeneous step (complex adsorption).

where qe and Ce are as defined before, Kf is the Freundlich constant related to sorption capacity, and n is a constant related to energy or intensity of adsorption. The fit line is shown in Figure 7, and the parameters Kf and n are presented in Table 2. The Freundlich plot was fitted to the experimental data and was found to be a better fit than the Langmuir plot, as it showed higher regression coefficients at each temperature (Table 2). These results also indicate that temperature had a minor effect on PMG removal by MT with Fe(III). The practical maximum adsorption capability is greater than 210 mg/g. The adsorption capabilities were found to be in the range of 1.253−1.281 mmol/g, which corresponds to the CEC data (115−139 mmol/100 g). The factor determining PMG adsorption capacity is the CEC of MT in the presence of Fe(III). Materials used for PMG adsorption are briefly summarized in Table 3. The adsorption capacity of MT with Fe(III) is much higher than those of most mineral adsorbents investigated so far, except for the resin reported by Jia et al.35 MgAl LDH10 and NiAl LDH36 have capacities comparable to that of the Fe(III)− MT system. However, these LDHs and the Jia et al. resin are 14489



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Figure 8. XRD patterns of the solids.

FeOH2 + + H3L → FeHL+ + H+ + H 2O

indicating that the interlayer spacing reached 0.82 nm. The diameter of the PMG anion is about 0.45 nm;46 however, the increase in interlayer spacing between Fe(III)−MT and PMG− Fe(III)−MT was found to be only 0.15 nm. This strongly suggests that the PMG molecule adopts a horizontal orientation between the MT sheets. It has been documented that the Fe(III) plays a bridging role in trapping PMG. In the case of the XRD of solids desorbed by NaOH, the basal spacing of MT decreased to 1.50 nm (Figure 8). This result agrees with eq 15: The PMG−Fe(III) complex was deintercalated at high pH, leaving Na+ or a water molecule to fill the interlayer. This process led to shrinking of the basal spacing. This is similar to the behavior of Cu−PMG complexes described by Morillo et al.30 It is therefore likely that a ternary surface complexation reaction occurs. The Fe is in close proximity to the MT surface, and the ligand is bonded directly to the Fe ion, as described by Sheals et al.39 When the pH is increased, the complex changes to Fe(OH)2−PMG. This electrically neutral complex cannot be stabilized by a layer of MT.

(9)

FeOH2 + + H 2L− → FeHL+ + H 2O

(10)

SiO− + FeHL+ → SiOFeHL

(11)

SiO− + FeOH2 + → SiOFeOH+

(12)

SiOFeOH+ + H 2L− → SiOFeHL + H 2O (13)

On the other hand, for pH > 4.0 Fe(OH)2+ + H 2L− → FeL + 2H 2O

(14)

In this case, the coordination product FeL cannot adsorb onto MT, and the removal rate decreases rapidly. The desorption process occurs as described by the equation SiOFeL + OH− → SiO− + FeL

(15)

0

Species based on Fe(OH)3 form with an increase in pH, which is in equilibrium with colloidal Fe(OH)3. The colloidal Fe(OH)3 aggregates to form an Fe(OH)3 precipitate. However, during our experiments, we found that Fe(III) can exist at high concentrations at equilibrium pH levels between 4.1 and 7.1, which suggests that the formation of colloidal Fe(OH)3 can be suppressed by complexation with PMG. Liu and Millero reported that certain interactions can prohibit colloidal Fe(OH)3 from precipitating, and observation of this phenomenon is shown in Figure 5.44 The desorption results showed that 91.36% of the PMG was desorbed from the PMG−Fe− MT matrix in NaOH solution. Figure 8 shows the XRD patterns of a series of MT−Fe(III) solid samples. The original MT sample (wet) exhibits a diffraction peak at 2θ = 5.78°, which corresponds to a basal spacing of 1.53 nm. The interlayer spacing for MT is thus equal to 0.57 nm after taking into account the thickness (0.96 nm) of an MT clay layer.45 There are no apparent changes to MT upon reaction with PMG. However, the basal spacing increased from 1.53 to 1.63 nm after reaction with Fe(III), meaning the interlayer spacing increased from 0.57 to 0.67 nm as a result of the intercalation of Fe(III). The basal spacing was further increased to 1.78 nm after reaction with Fe(III) and PMG,

4. CONCLUSIONS In this study, the adsorption of PMG onto MT in the presence of Fe(III) in aqueous solutions was investigated. The adsorption process was found to follow pseudo-second-order kinetics, with a large adsorption quantity reached after 5 min. The high-speed adsorption can lead to a short period of batch adsorption−settling pond operation or a high flow velocity in a fixed-bed reactor. Fe(III) was found to be responsible for PMG adsorption at a 1:1 molar ratio. The maximum adsorption capacity was greater than 210 mg/g. Equilibrium adsorption data gave a better fit to the Freundlich isotherm than to the Langmuir isotherm. Complexation was found to depend on pH and be independent of ionic strength. PMG−Fe forms innersphere complexes when adsorbed on MT particles. The ionic strength can widen the effective pH range and can enhance the adsorption capacity at final pH values above 4.1, which is critical for applications in high-salinity wastewater systems. In the presence of competitively binding ions, the PMG removal rate was reduced by PO43−. SO42− can also reduce PMG adsorption by MT at pH < 5.9, but the PMG removal 14490



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(9) Manassero, A.; Passalia, C.; Negro, A. C.; Cassano, A. E.; Zalazar, C. S. Glyphosate Degradation in Water Employing the H2O2/UVC Process. Water Res. 2010, 44, 3875. (10) Li, F.; Wang, Y. F.; Yang, Q. Z.; Evans, D. G.; Forano, C.; Duan, X. Study on Adsorption of Glyphosate (N-Phosphonomethyl Glycine) Pesticide on MgAl-Layered Double Hydroxides in Aqueous Solution. J. Hazard. Mater. 2005, 125, 89. (11) Xie, M.; Xu, Y. Glyphosate Adsorption by Ferric Supported Active Carbon. China Environ. Sci. 2011, 31, 239. (12) Hu, Y. S.; Zhao, Y. Q.; Sorohan, B. Removal of Glyphosate from Aqueous Environment by Adsorption Using Water Industrial Residual. Desalination 2011, 271, 150. (13) Liu, Y.; Kang, Y.; Huang, D.; Wang, A. Cu2+ Removal from Aqueous Solution by Modified Chitosan Hydrogels. J. Chem. Technol. Biotechnol. 2012, 87, 1010. (14) Na, P.; Jia, X.; Yuan, B.; Li, Y.; Na, J.; Chen, Y.; Wang, L. Arsenic Adsorption on Ti-Pillared Montmorillonite. J. Chem. Technol. Biotechnol. 2010, 85, 708. (15) Khoury, G. A.; Gehris, T. C.; Tribe, L.; Sanchez, R.; Afonso, M. D. Glyphosate Adsorption on Montmorillonite: An Experimental and Theoretical Study of Surface Complexes. Appl. Clay Sci. 2010, 50, 167. (16) Damonte, M.; Sanchez, R.; Afonso, M. D. Some Aspects of the Glyphosate Adsorption on Montmorillonite and Its Calcined Form. Appl. Clay Sci. 2007, 36, 86. (17) Shoval, S.; Yariv, S. Interaction between Roundup (Glyphosate) and Montmorillonite. Part I. Infrared Study of the Sorption of Glyphosate by Montmorillonite. Clays Clay Miner. 1979, 27, 19. (18) Shoval, S.; Yariv, S. Interaction between Roundup (Glyphosate) and Montmorillonite. Part II. Ion Exchange and Sorption of IsoPropylammonium by Montmorillonite. Clays Clay Miner. 1979, 27, 29. (19) McConnell, J. S.; Hossner, L. R. pH-Dependent Adsorption Isotherms of Glyphosate. J. Agric. Food Chem. 1985, 33, 1075. (20) Glass, R. L. Adsorption of Glyphosate by Soils and Clay Minerals. J. Agric. Food Chem. 1987, 35, 497. (21) Sprankle, P.; Meggitt, W. F.; Penner, D. Adsorption, Mobility, and Microbial Degradation of Glyphosate in the Soil. Weed Sci. 1975, 23, 229. (22) Hensley, D. L.; Beuerman, D. S. N.; Carpenter, P. L. The Inactivation of Glyphosate by Various Soils and Metal Salts. Weed Res. 1978, 18, 287. (23) Ibanez, M.; Pozo, O. J.; Sancho, J. V.; Lopez, F. J.; Hernandez, F. Re-Evaluation of Glyphosate Determination in water by Liquid Chromatography Coupled to Electrospray Tandem Mass Spectrometry. J. Chromatogr. A 2006, 1134, 51. (24) Ibanez, M.; Pozo, O. J.; Sancho, J. V.; Lopez, F. J.; Hernandez, F. Residue Determination of Glyphosate, Glufosinate and Aminomethylphosphonic Acid in Water and Soil Samples by Liquid Chromatography Coupled to Electrospray Tandem Mass Spectrometry. J. Chromatogr. A 2005, 1081, 145. (25) de Llasera, M.; Gomez-Almaraz, L.; Vera-Avila, L. E.; PenaAlvarez, A. Matrix Solid-Phase Dispersion Extraction and Determination by High-Performance Liquid Chromatography with Fluorescence Detection of Residues of Glyphosate and Aminomethylphosphonic Acid in Tomato Fruit. J. Chromatogr. A 2005, 1093, 139. (26) Standard Methods for the Examination of Water and Wastewater; American Public Health Association (APHA): Washington, DC, 2005. (27) Ho, Y. S. Second-Order Kinetic Model for the Sorption of Cadmium onto Tree Fern: A Comparison of Linear and Non-Linear Methods. Water Res. 2006, 40, 119. (28) Barja, B. C.; Afonso, M. D. An ATR-FTIR Study of Glyphosate and Its Fe(III) Complex in Aqueous Solution. Environ. Sci. Technol. 1998, 32, 3331. (29) McBride, M. B. A Critique of Diffuse Double Layer Models Applied to Colloid and Surface Chemistry. Clays Clay Miner. 1997, 45, 598. (30) Morillo, E.; Undabeytia, T.; Maqueda, C. Adsorption of Glyphosate on the Clay Mineral Montmorillonite: Effect of Cu(II) in Solution and Adsorbed on the Mineral. Environ. Sci. Technol. 1997, 31, 3588.

was enhanced by those ions at pH > 5.9. Finally, PMG in the PMG−Fe−MT solid matrix could be desorbed easily using NaOH solution, which makes a fixed-bed reactor more convenient to regenerate. The fact that the PMG−Fe(III) complex was intercalated into the MT was confirmed by XRD and the correspondence of the adsorption capacity to the CEC. The MT−Fe(III) system is considered to have great potential for the rapid removal of high PMG concentrations from highsalinity wastewaters.

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ASSOCIATED CONTENT

S Supporting Information *

Main chemical compositions of MT mentioned in section 2.1 (Table S1), deprotonation constants of PMG in Na(Cl) ionic medium (T = 25 °C) mentioned in section 3.6 (Table S2), linear fit of the amount PMG removed versus consumed Fe(III) mentioned in section 3.2 (Figure S1), effect of ionic strength on the removal of PMG by MT mentioned in section 3.3 (Figure S2), relationship between PMG and Fe(III) concentration mentioned in section 3.3 (Figure S3), and speciation of Fe(III) and PMG in aqueous solution mentioned in section 3.6 (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 25 86881317, +86 25 86881370. Fax: +86 25 86881000. E-mail: [email protected] (Y.D.). *E-mail: [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Foundation of Innovation Research program of the Chinese Academy of Sciences (Grant KSCX2-EW-B-6) and the Natural Science Foundation of Jiangsu Province (Grant BE2009698). The scientific editing company “International Science Editing” is also thanked for improving the quality of the manuscript.

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REFERENCES

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