Montmorillonite Functionalized with Zwitterionic Surfactant as a Highly

Article pubs.acs.org/IECR

Montmorillonite Functionalized with Zwitterionic Surfactant as a Highly Efficient Adsorbent for Herbicides Zheng Gu, Manglai Gao,* Laifu Lu, Yuening Liu, and Senfeng Yang State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, P. R. China S Supporting Information *

ABSTRACT: A novel adsorbent (DHAPS-Mt) was prepared with Na montmorillonite (Na-Mt) and a zwitterionic surfactant (3-(N,N-dimethylhexadecylammonio)propanesulfonate, DHAPS). Then DHAPS-Mt was characterized by a series of characterization techniques and applied in herbicide adsorption. Two representative herbicides in agriculture, paraquat (PQ, 1,1′-dimethyl-4,4′-dipyridinium dichloride) and amitrole (AMT, 3-amino-1,2,4-triazole), were removed by DHAPS-Mt varying with different experimental conditions. The adsorption mechanisms, obtained from a comparison of adsorption capacities of DHAPS-Mt and two other kinds of organomontmorillonite (CTAB-Mt and SDS-Mt), suggested that the surface electrostatic adsorption and electrostatic attraction between herbicide cations and negatively charged group of DHAPS were the main interaction in PQ and AMT removal. The results of anionic dye (methyl orange) removal onto these three kinds of clay indicated that DHAPS-Mt might be applied in the treatment of mixed-ion wastewater. The kinetic study indicated that the adsorption of herbicides onto DHAPS-Mt followed the pseudo-second-order model, and the Langmuir isotherm model agreed well with the experimental data. Furthermore, thermodynamic parameters illustrated that PQ removal by DHAPS-Mt was more spontaneous at higher temperature, and the process was endothermic and randomness increasing in nature, while the AMT removal presented an opposite tendency. silicate.28 Montmorillionite is a kind of abundant and lowcost clay mineral, which has been always used to adsorb the organic waste because of its excellent properties such as the high surface area and cation exchange capacity.29−32 Compared with the natural montmorillonite, montmorillonite modified by organic molecule has also been employed much more widely for the separation and purification of wastewater over the past few decades.33,34 The modifiers are usually conventional quaternary ammonium salts with long chain or other kinds of surfactants, whose removal mechanism to organic contaminants is mainly due to the partition adsorption.35,36 As far as we know, there are few reports about the application of montmorillonite modified with zwitterionic surfactant for pesticide removal from aqueous solution. Therefore, it is essential to prepare organomontmorillonite modified by the new kinds of zwitterionic surfactant and to further study the relationship between its structure and adsorption performance. In this study, a new kind of organomontmorillonite (DHAPS-Mt), modified from zwitterionic surfactant (3-(N,Ndimethylhexadecylammonio)propanesulfonate (DHAPS)), was confirmed and used in herbicide (PQ and AMT) adsorption. The adsorption mechanisms and the effects of the experimental conditions (pH of the solution, temperature, and contact time) on the adsorption efficiency of DHAPS-Mt were thoroughly investigated, and the feasibility of different models of adsorption isotherms and kinetics to the adsorption system was also evaluated. In addition, a detailed thermodynamic

1. INTRODUCTION The pesticide effluents originating from many sources, such as manufacturing plants, surface runoff, accidental leaching, have seriously polluted the water in nature and our lives.1 Those kinds of pesticide residues are mainly toxic organic compounds, which have been detected in water system in recent years.2−4 The existence of those pesticides would markedly increase the number of some serious problems or risks to aquatic organism and human health. Paraquat (PQ, 1,1′-dimethyl-4,4′-dipyridinium dichloride) and amitrole (AMT, 3-amino-1,2,4-triazole), two of the most commonly applied herbicides because of their nonselective property in the cells of plant,5,6 are typical cationic and neutral herbicide molecules, respectively. However, their residues in water resource have caused many fatalities or side effects when deliberately or accidentally ingested.7−9 Tunç et al.10,11 have reported that some herbicides including diquat dibromide cause the changes in the structure of human serum albumin protein. Furthermore, AMT herbicide leads to liver cancer in animals and is resistant to degradation by abiotic hydrolysis and aqueous photolysis.11 Thus, it is very important to remove these harmful herbicides from the aquatic systems and purify the water resources. The traditional methods for the treatment of pesticide solution usually contain Fenton’s reagent,12 photo-oxidation,13 adsorption,14,15 and biological degradation.16,17 Among these numerous cleanup methods, adsorption is regarded as a very important way for treating contaminated water on an industrial scale.18 To date, various kinds of adsorbents have been used for herbicide removal, and parts of these adsorption materials are resins19,20 zeolite,21 biopolymeric membranes,22 activated carbon,23 silica,24 and rice husk.25 Recently, there has been an increase in the studies of herbicide adsorption onto clay11,26,27 or other kinds of © 2015 American Chemical Society

Received: Revised: Accepted: Published: 4947

September 19, 2014 March 30, 2015 April 20, 2015 April 20, 2015 DOI: 10.1021/acs.iecr.5b00438 Ind. Eng. Chem. Res. 2015, 54, 4947−4955

Article

Industrial & Engineering Chemistry Research

Brunauer−Emmett−Teller (BET) specific surface area were measured and calculated using an accelerated surface area and porosimetry system (Micromeritics, ASAP 2020), respectively. Thermal characterization of the samples was carried out by METTLER-TOLEDO thermogravimetric analysis (TG) with a TGA/DSC1 STARe system (operating conditions: air atmosphere, 25−800 °C, heat rate of 10 °C/min). The ζ potential measurements were performed using a BIC, Zetaplus model. 2.4. Adsorption Experiments. An amount of 25 mL of herbicide solution (PQ or AMT) with different initial concentrations was mixed with 0.08 g of the adsorbent. After the pH adjustment with dilute HCl or NaOH solution, the dispersion was placed in a THZ-98A thermostatic mechanical shaker and shaken with a stirring speed of 120 rpm at the desired temperature. After the adsorption equilibrium was reached, the suspension was separated by a centrifuge for 1 min under the speed of 4000 rpm. Then the pH of solution was adjusted to a certain value (pH of 2 for PQ, pH of 7 for AMT) and the residual concentration of herbicide solution was measured by Shimadzu UV2550 ultraviolet spectrophotometer at the maximum absorption wavelength of 227 nm for PQ and 202 nm for AMT, respectively. The removal amount of herbicide onto the adsorbent was determined by the change of concentration before and after the adsorption. The adsorption amount of herbicide onto organoclay (qe, mg g−1) can be calculated through a mass balance as follows:

evaluation of the adsorption process was performed. The obtained results of this work will provide a basis for the further preparation of the new kinds of adsorption materials in the area of environmental protection.

2. MATERIALS AND METHODS 2.1. Materials. Na montmorillonite (Na-Mt), obtained from Zhejiang Institute of Geology and Mineral Resources in China, was composed of SiO2 (67.16%), Al2O3 (20.60%), MgO (4.11%), CaO (3.21%), Fe2O3 (2.20%), Na2O (1.47%), K2O (0.92%), TiO2 (0.17%), and others (0.16%), whose cation exchange capacity (CEC) was 112 mequiv 100 g−1. 3-(N,Ndimethylhexadecylammonio)propanesulfonate (DHAPS) was provided by Nangjing Robiot Co., Ltd., China. Paraquat (PQ), amitrole (AMT), and methyl orange (MO) were of analytical grade and supplied by TCI, Shanghai, China. The structures and characteristics37,38 of DHAPS, PQ, and AMT are listed in Figure 1 and Table 1, respectively. Cetyltrimethylammonium

qe =

Table 1. Characteristics of PQ and AMT PQ37

AMT38

CAS no. molecular weight (g mol−1) solubility in water at 25 °C (g L−1) log Kow at 25 °C

1910-42-5 257.2 620 −4.5

61-82-5 84.08 280 −0.97

(1)

where C0 and Ce are the initial and equilibrium concentration of herbicide (mg L−1), respectively, m (g) is the mass of organomontmorillonite used in this study, and V (L) is the solution volume. The initial results showed that adsorption amounts of Na-Mt to PQ and AMT were much less than that of DHAPS-Mt under the same experimental conditions. The adsorption amount of organoclay DHAPS-Mt was evaluated under different reactant conditions, and the adsorption experiments of PQ and AMT on DHAPS-Mt were undertaken under the various pH of solutions (3, 5, 7, 9, 11) and temperatures (298, 308, 318 K). Each experiment was duplicated, and the results were given as average. The standard deviation values were all found to be below 3%.

Figure 1. Chemical structures of DHAPS (a), PQ (b), and AMT (c).

herbicide

(C0 − Ce)V m

bromide (CTAB) and sodium dodecyl sulfonate (SDS) were of analytical grade purchased from Beijing Dali Fine Chemical Factory, China. The other chemicals used in this study were also of analytical grade. 2.2. Preparation of Organomontmorillonite (DHAPSMt). DHAPS (the amount of DHAPS was equivalent to 1.0 times the CEC of Na-Mt) was added slowly to the slurry of NaMt (5 g) and deionized water (250 mL) at 60 ± 1 °C. Then the mixture was stirred for 3 h until the modification was accomplished. The wet product was centrifuged and washed with deionized water five times. The final DHAPS-Mt was obtained by the processes of desiccation and pulverization. 2.3. Characterization of Na-Mt and DHAPS-Mt. The Xray diffraction (XRD) patterns of Na-Mt and DHAPS-Mt were determined using a Shimadzu XRD-6000 with Cu Kα radiation at 40 kV and 40 mA and collected between 1.5° and 10° (2θ) at a scanning speed of 1°/min. The Fourier transform infrared (FT-IR) spectra of the samples were recorded in the spectral range of 4000−400 cm−1 on a Nicolet Magna 560 E.S.P. FT-IR spectrometer. The surface morphology was observed by Quanta 200F field emission scanning electron microscope (SEM). Nitrogen gas adsorption−desorption isotherms and

3. RESULTS AND DISCUSSION 3.1. Characterization of Clay Samples. XRD technique was usually used to investigate the arrangement of organic modifier between the layers of clay.39−41 The XRD patterns of the original montmorillonite (Na-Mt) and organomontmorillonite (DHAPS-Mt) are shown in Figure 2. The result showed that the insertion of DHAPS led to the increase of d(001) spacing (from 1.22 nm of Na-Mt to 3.92 and 2.14 nm of DHAPS-Mt), which might be much easier for herbicide adsorption. In addition, the interlayer spacing (d(001) spacing minus 0.96 nm) of DHAPS-Mt was 2.96 and 1.18 nm, which illustrated that DHAPS might exist in the form of paraffin bilayer and pseudotrilayer between the layers of montmorillonite.42 Figure 3 depicted the FT-IR spectra of Na-Mt, DHAPS-Mt, and DHAPS-Mt loaded with PQ or AMT to compare the differences among three kinds of montmorillonites. It can be seen that both Na-Mt and DHAPS-Mt had the same characteristic peaks of montmorillonite. The two absorption 4948

DOI: 10.1021/acs.iecr.5b00438 Ind. Eng. Chem. Res. 2015, 54, 4947−4955

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Figure 4 showed the nitrogen adsorption−desorption isotherms of Na-Mt and DHAPS-Mt. The two samples showed

Figure 2. XRD patterns of Na-Mt and DHAPS-Mt samples.

Figure 4. BET analysis of Na-Mt and DHAPS-Mt.

type IV adsorption isotherms based on BDDT classification with close hysteresis loops.43 The values of BET specific surface area, total pore volume, and average pore diameter for Na-Mt and DHAPS-Mt were also listed in Figure 4. As seen from the values, DHAPS-Mt had a lower specific surface area (14.02 m2 g−1) than Na-Mt (20.07 m2 g−1), which indicated that nitrogen gas was only adsorbed on the external surface of DHAPS-Mt but not the internal surface.44 In other words, DHAPS molecules, those inserted between the layers of clay, blocked a part of holes in montmorillonite, which prevented some nitrogen molecules from entering the inner surface of clay; thus, the BET specific surface area of organomontmorillonite decreased. Similar phenomenon was also found in other research.45 The SEM pictures about Na-Mt and DHAPS-Mt are given in Figure 5a and Figure 5b, respectively. It is seen that both NaMt and DHAPS-Mt possessed the layer structure, but the organomontmorillonite was much more ordered. Additionally, DHAPS-Mt had a much larger interlayer spacing than that of original montmorillonite, which proved the complete insertion of DHAPS molecules between the layers of clay. The analysis of thermogravimetric (TG) and derivative thermogravimetric (DTG) about the two clay samples are shown in Figure 6. DHAPS-Mt exhibited a relatively low mass loss compared with Na-Mt from 25 to 200 °C, which suggested that DHAPS-Mt had less free water than Na-Mt, and it also indicated the intercalary or adsorbed surfactants decreased the surface energy of montmorillonite and led to the appearance of a hydrophobic clay surface.46 The mass loss at 650.74 and 628.67 °C were all from the dehydroxylation of the two samples.47 As indicated from the DTG curves, the peaks at 73.62 and 86.62 °C corresponded to the loss of physically adsorbed water for Na-Mt and DHAPS-Mt, respectively, while the peak at 140.59 °C resulted from the bonding water of NaMt. Additionally, the two obvious peaks of DHAPS-Mt in the temperature range 250−550 °C were due to the thermal degradation of modifier. It is known that the binding forms between modifier and clay are classified into three kinds, i.e., surface absorption, interlayer-adsorbed, and intercalated, which meant that the modifier would degrade at different temperatures.48 Thus, the peaks in DTG curve of DHAPS-Mt at

Figure 3. FT-IR spectra of Na-Mt (a), DHAPS-Mt (b), and DHAPSMt loaded with PQ or AMT (c).

bands around 3430 and 3626 cm−1 were −OH stretching vibrations, while the peaks around 1639 and 1036 cm−1 were related to the bending vibration of −OH and stretching vibration of Si−O. Figure 3b also gave the adsorption bands at 2924 and 2853 cm−1, which were classified as the asymmetric and symmetric stretching vibrations of H−C−H in the methylene group, respectively. Furthermore, the decreasing intensity of the adsorption bands at 3624 and 1641 cm−1 might have resulted from the increase of hydrophobic character of DHAPS-Mt compared with hydrophilic Na-Mt. In combination with the analysis of XRD, it indicated that DHAPS molecules have inserted into the silicate layers of montmorillonite. Additionally, as seen from Figure 3c, it is important to note that the intensities of C−H stretching vibrations band decreased in the FT-IR spectrum of DHAPS-Mt loaded with PQ or AMT, which is because the functional groups of the DHAPS-Mt surface have been occupied with PQ or AMT. Meanwhile, the FT-IR spectra of DHAPS-Mt after PQ and AMT adsorption showed the new characteristic PQ and AMT bands. Adsorption band at 3053 cm−1 was attributed to C−H stretching vibration of nitrogen heterocyclic ring, while bands at 1637 and 1632 cm−1 might be due to N−H bending vibration or CN stretching vibration, which were overlapped with the bending vibrations of −OH. 4949


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Figure 5. SEM analysis of Na-Mt (a) and DHAPS-Mt (b).

Figure 6. TG and DTG curves for Na-Mt and DHAPS-Mt.

343.61 and 403.30 °C resulted from the thermal decomposition of interlayer-adsorbed and intercalated modifier molecules, respectively.44 The TG-DTG analysis of the two samples finally confirmed the insertion of DHAPS into the montmorillonite layers. 3.2. Effects of Different Factors on Herbicide Removal. 3.2.1. Effect of pH on Herbicide Adsorption. The acidity or alkalinity of solution is always closely related to the removal efficiency of pollutants, so it is very important to study the influence of pH on contaminant adsorption. Figure 7 displayed the effects of the initial pH on the uptake of PQ and AMT. As can be seen in Figure 7a, the equilibrium adsorption capacity (qe, mg g−1) of PQ on DHAPS-Mt increased when the pH changed from 3 to 7, which might be explained by the ζ potential (mV) of DHAPS-Mt surface (Figure 7c). The ζ was more negative when the pH reached 7 in contrast with the solution with pH of 3 and 5, which meant that there were more OH− in the solution and it would increase the electrostatic interaction between PQ2+ and negatively charged surface of DHAPS-Mt, so the qe value of PQ increased. With the further increase of pH (7 to 11), the adsorption system gradually reached the equilibrium and there was no change in the uptake. Distinctly different from the effect of pH on PQ removal, Figure 7b showed that AMT adsorption onto DHAPS-Mt took place more readily at lower pH and strongly depended on pH value. The equilibrium adsorption capacity decreased rapidly when the pH rose from 3 to 9 and did not change from pH 9 to

Figure 7. Effect of pH on the adsorption of PQ (a) and AMT (b) onto DHAPS-Mt and the ζ potentials of DHAPS-Mt at different pH values (c): 0.08 g/25 mL adsorbent; C0 = 300 mg L−1 for PQ, 40 mg L−1 for AMT; t = 120 min; T = 298 K.

11. This phenomenon was largely related to the existing form of AMT at different pH of solution. As Fontecha-Cámara et al. indicated, AMT existed as a neutral molecule between pH 6 and 8, as cation (AMTH+) when pH < 6, and as anion (AMT−) at pH 8−13.38 Therefore, the electrostatic interaction between AMTH+ and negatively charged surface of DHAPS-Mt at lower acidic solution (pH 3) increased the adsorption capacity. Conversely, the uptake of AMT would decrease in alkaline solution because of the electrostatic repulsion between AMT− and negatively charged surface of DHAPS-Mt. 3.2.2. Effect of Contact Time on Herbicide Adsorption. Figure 8 listed the impacts of contact time on herbicide adsorption at 298, 308, and 318 K. The uptake of PQ and AMT increased rapidly during the initial period, and then the adsorption equilibrium was obtained at 60 and 20 min, respectively. As depicted in Figure 8, the removal amounts of PQ onto DHAPS-Mt increased with increasing temperature, while the adsorption efficiency of AMT changed toward the opposite direction. This phenomenon indicated that the adsorption processes of PQ and AMT by DHAPS-Mt were endothermic and exothermic, respectively. 4950


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and SDS, which had a cationic group (CH3(CH2)15N+(CH3)2−) on one side, and an ionic group (−SO3−) on the other side. Accordingly, it is necessary to investigate the herbicide adsorption onto these three kinds of organomontmorillonite individually. The ζ potential confirmed the negatively charged surfaces of DHAPS-Mt and SDS-Mt in the range of pH 3−11, while CTAB-Mt exhibited a positively charged surface. Therefore, the surface electrostatic adsorption only existed between the two kinds of organomontmorillonite with negatively charged surface (i.e., DHAPS-Mt and SDS-Mt) and herbicide cations (PQ2+ and AMTH+) in the experimental conditions (pH of 7 and 3 for PQ and AMT, respectively). But for CTAB-Mt, there was just partition adsorption between the long carbon chain of CTAB and herbicides due to the similar intermiscibility of the modifier and adsorbate. Additionally, the lower adsorption capacities of CTAB-Mt to herbicides (6.4483 mg g−1 for PQ and 1.0961 mg g−1for AMT) verified the inefficiency of partition mechanism. On the basis of the above analysis and the higher adsorption capacities of DHAPS-Mt and SDS-Mt, we can safely conclude that the partition adsorption could be neglected in the removal processes of herbicides onto DHAPS-Mt and SDS-Mt. As for DHAPS-Mt and SDS-Mt, there was also the electrostatic attraction between the negative charged group (−SO3−) and herbicide cations (PQ2+ and ATMH+) apart from the surface electrostatic adsorption, which was another reason for the stronger adsorption capacities of DHAPS-Mt and SDSMt compared with CTAB-Mt. The data in Table 2 showed that equilibrium adsorption capacities (qe) of SDS-Mt to PQ and AMT were slightly higher than those of DHAPS-Mt, which might arise from the different structures of modifiers. As displayed in Figure 1, the sulfonate goup (−SO3−) in SDS has a negative charge, while DHAPS molecular carries both a negative charge (−SO 3 − ) and a positive charge (CH3(CH2)15N+(CH3)2−). Hence, there was only electrostatic

Figure 8. Effect of contact time on the adsorption of PQ (a) and AMT (b) onto DHAPS-Mt: 0.08 g/25 mL adsorbent; C0 = 300 mg L−1 for PQ, 40 mg L−1 for AMT; pH = 7 for PQ, pH = 3 for AMT.

3.3. Adsorption Mechanism of Herbicide onto DHAPS-Mt. The modifier DHAPS, a zwitterionic surfactant, was known as the combination of quaternary ammonium cations with long carbon chain (CH3(CH2)15N+(CH3)2−) and ionic group (−SO3−). So the removal mechanisms of PQ and AMT onto DHAPS-Mt were supposed to be quite distinct from those of common surfactant modified clay, arising from the special structure of DHAPS. In order to illuminate the removal mechanism, cetyltrimethylammonium bromide modified montmorillonite (CTAB-Mt) and sodium dodecyl sulfonate modified montmorillonite (SDS-Mt), prepared under the same experimental conditions as DHAPS-Mt, were also used as adsorbents for comparison in this study. As indicated in Table 2, DHAPS could be considered as a complex of CTAB

Table 2. Equilibrium Adsorption Capacities (qe, mg g−1) and % Removal (R) of DHAPS-Mt, CTAB-Mt, and SDS-Mt to Organic Contaminants and the ζ Ootential of Organomontmorillonite Samplesa

R (%) = [(C0 − Ce)/C0] × 100, where C0 and Ce are the initial and equilibrium concentration of herbicide (mg L−1), respectively; C0 values for PQ and AMT are 300 and 40 mg L−1, respectively. Adsorbent concentration of 0.08 g/25 mL; T = 298 K; pH = 7 for PQ, pH = 3 for AMT; C0 for MO is 100 mg L−1; adsorbent concentration of 0.1 g/25 mL; pH = 7 for MO. a

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describing the removal processes of PQ and AMT onto DHAPS-Mt. The experimental data were also treated according to the pseudo-second-order equation:50,51 t 1 t = + 2 qt q2.cal k 2q2.cal (3)

attraction between SDS and herbicide cations, while for DHAPS, there was electrostatic repulsion in addition to the electrostatic attraction. As a result, the different structures of the modifiers gave rise to the higher adsorption efficiency of SDS-Mt than that of DHAPS-Mt. To sum up, the main adsorption mechanisms of PQ and AMT onto DHAPS-Mt consisted of the surface electrostatic interaction and the electrostatic attraction between the herbicide cations and the negatively charged group of DHAPS (−SO3−). On the basis of the above analysis, it seemed that it might be unnecessary for choosing DHAPS-Mt as an adsorbent for herbicide removal because of the strong adsorption capacity of SDS-Mt. However, the adsorption capacity of the zwitterionic surfactant modified montmorillonite (DHAPS-Mt) to cationic contaminants is one part of our study. The other purpose of the present research is investigating the comprehensive adsorption ability of DHAPS-Mt to different kinds of ionic pollutants. In other words, DHAPS-Mt was selected not only for the removal of the single kind of cationic contaminants from aqueous solution in future study but also for the adsorption of the mixed wastewater including both anionic and cationic pollutants or even neutral contaminants. For example, the adsorption data of methyl orange (MO), a commonly used anionic dye, onto those three kinds of organomontmorillonite were also displayed in Table 2. As seen from the data in the table, the % removal of DHAPS-Mt to MO (70.85%) was lower than that of CTAB-Mt (99.81%) but much higher than that of SDS-Mt (3.17%), which suggested that the cationic group (CH3(CH2)15N+(CH3)2−) in DHAPS was also conducive to the removal of anionic dye through electrostatic attraction, while SDS-Mt showed the poorest adsorption ability through the partition mechanism. The adsorption capacity of DHAPS-Mt to PQ, AMT, and MO was not the strongest among those three kinds of organomontmorillonite, but it possessed the moderate adsorption ability to both cationic and anionic contaminants. This phenomenon illustrated that DHAPS-Mt or other series of zwitterionic surfactant modified montmorillonite might be considered as suitable adsorption material for the treatment of the mixed-ion wastewater, and our team will continue to study the removal efficiency of DHAPS-Mt to the mixed solution of anionic and cationic pollutants and investigate the corresponding relationship between the structure and function in further research. 3.4. Adsorption Kinetics. Adsorption kinetics is generally considered as a key factor to understand the adsorption characteristic of adsorbent. Two commonly used kinetic models, pseudo-first-order and pseudo-second-order, were employed in this work. The linear form of pseudo-first-order equation or Lagergren’s kinetics equation is49 log(qe.exp − qt) = log q1.cal −

k1t 2.303

where q2.cal (mg g−1) is the calculated capacity and qt (mg g−1) is the removal capacity of DHAPS-Mt at any time t (min), respectively. k2 (g mg−1 min−1) is the equilibrium rate constant. In addition, the initial sorption rate h (mg (g min)−1) is equal to k2q2.cal2. Table S1 (in Supporting Information) displayed the main parameters of pseudo-second-order model. q2.cal and k2 were evaluated by the slopes and intercepts of the plots of t/qt against t (Figure 9). As seen from the data in the table, all the

Figure 9. Pseudo-second-order kinetic plots for the adsorption of PQ (a) and AMT (b) onto DHAPS-Mt at various temperatures: 0.08 g/25 mL adsorbent; C0 = 300 mg L−1 for PQ, 40 mg L−1 for AMT; pH = 7 for PQ, pH = 3 for AMT.

R2 values were higher than 0.999 for the tests and the values of q2.cal and qe.exp agreed well with each other. Hence, the analysis result demonstrated that the adsorption processes of PQ and AMT onto DHAPS-Mt could be well described by the pseudosecond-order equation. According to the equation of initial sorption rate h, it revealed that the initial sorption rate was both relevant with the equilibrium rate constant and the adsorption capacity. The calculating data showed that the removal amounts of PQ were far more than that of AMT at three temperatures, indicating PQ was more prone to be adsorbed on DHAPS-Mt than AMT. Furthermore, the parameters shown in Table S1 suggested that both initial sorption rate values of PQ and AMT decreased with the increase of temperature, demonstrating that the higher temperature could cause an adverse effect on initial sorption rate. 3.5. Adsorption Isotherms. Adsorption isotherm is usually used to illuminate the relationship between the adsorption capacity of adsorbent (qe) and the residual concentration of adsorbate (Ce).52 Figure 10 showed the adsorption isotherms of PQ and AMT at 298, 308, and 318 K. From the figure, qe increased with the increasing concentration of herbicides, and the adsorption equilibrium for PQ was reached when the initial concentration of herbicide solution reached 700 mg L−1, but for AMT adsorption was 80 mg L−1. In this study, the experimental data were analyzed with two

(2)

−1

where qt (mg g ) is the removal capacity of adsorbent at any time t (min), q1.cal and qe.exp are the adsorption amounts of calculation and experiment at equilibrium (mg g−1), respectively. k1 is the equilibrium rate constant (min−1) and can be obtained by a plot of log(qe.exp − qt) versus t. The Supporting Information (Table S1) listed the data of q1.cal, k1, correlation coefficient (R2), and residual sum of squares (RSS). The result suggested that there was a significant difference in adsorption amount between q1.cal and qe.exp, and the low values of R2 also illustrated the inadaptability of pseudo-first-order model in 4952


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The Freundlich isotherm is a typical empirical equation employed in the adsorption from dilute solutions, and its linearized expression is given as follows:55 1 log qe = log KF + log Ce (6) n where KF (mg/g (L/mg)1/n) and n are the Freundlich constants indicating the capacity and intensity of the adsorption, respectively. The two constants can be determined from the intercept and slope of the above equation. From Table S2 (in Supporting Information), the R2 values of PQ and AMT were found to be lower than 0.9, suggesting the infeasibility of the Freundlich model to the adsorption data of the both herbicides. 3.6. Thermodynamic Studies. In general, experimental temperature will always affect the reaction results to some extent, so the analysis about the effect of temperature is necessary for understanding the reaction characteristic. In this research, three thermodynamic parameters, i.e., the change of Gibbs free energy (ΔG°, kJ mol−1), standard enthalpy (ΔH°, kJ mol−1), and standard entropy (ΔS°, J mol−1 K−1), were used to reveal the influence of temperature on herbicide removal. The relevant parameters were calculated using the following equations:

Figure 10. Adsorption isotherms of PQ (a) and AMT (b) onto DHAPS-Mt at various temperatures: 0.08 g/25 mL adsorbent; t = 120 min; pH = 7 for PQ, 3 for AMT.

classical isotherm models, i.e., Langmuir and Freundlich isotherm models. The basic assumption of Langmuir isotherm is that the surface of adsorbent is homogeneous and smooth, and this theory is applied successfully in a lot of monolayer adsorption processes.53 The linearized expression of the Langmuir model is represented by the following equation:

Ce C 1 = e + qe qmax KLqmax

ΔG° = −RT ln K

ln K = K=

ΔS° ΔH ° − R RT

(7) (8)

qe Ce

(9)

where T and R are the temperature in kelvin and the gas constant (8.314 J mol−1 K−1), respectively. K is the distribution coefficient and equal to the ratio of qe (mg g−1) to Ce (mg L−1).56,57 Figure 11 showed the plot of ln K against 1/T, which

(4)

where qe is the adsorption capacity of DHAPS-Mt at equilibrium (mg g−1), Ce is the equilibrium concentration of herbicide in solution (mg L−1), and qmax and KL are the maximum concentration retained by DHAPS-Mt (mg g−1) and the Langmuir equilibrium constant related to the capacity of saturated monolayer (L mg−1), respectively. These two parameters could be calculated by the slope and intercept of the plots of Ce/qe versus Ce. The corresponding data values were given in Supporting Information (Table S2). The high R2 (>0.998) of the two adsorption processes confirmed a good agreement of the adsorption data with the Langmuir model, and the qmax values of DHAPS-Mt to those contaminants at three temperatures were 86.3558, 87.7963, 88.5740 mg g−1 (for PQ) and 3.6967, 3.2245, 2.8516 mg g−1 (for AMT). Additionally, it can be seen that the higher the temperature was, the bigger qmax of PQ would appear, which determined an endothermic nature. But for AMT, the qmax changed in the opposite direction and the removal process was exothermic. The separation factor or equilibrium parameter (RL), a dimensionless constant, is an essential characteristic of the Langmuir isotherm, which could be expressed by the following equation:54 1 RL = (1 + KLC0) (5)

Figure 11. Plot of ln K vs 1/T for estimation of thermodynamic parameters for PQ and AMT onto DHAPS-Mt: 0.08 g/25 mL adsorbent; C0 = 300 mg L−1 for PQ, 40 mg L−1 for AMT; t = 120 min; pH = 7 for PQ, pH = 3 for AMT.

can be used to evaluate the values of ΔH° and ΔS°. The Supporting Information (Table S3) displayed these values of thermodynamic parameters. As shown in Table S3, the values of ΔG° (0.0898, −0.1143, and −0.2961 kJ mol−1) of PQ removal decreased with an increase of temperature (298, 308, and 318 K), which suggested that the adsorption of PQ onto DHAPS-Mt was relatively

where C0 is the highest initial concentration of adsorbate. All of the RL values listed in Table S2 were within the range of 0−1, showing the favorable adsorption processes of PQ and AMT onto DHAPS-Mt. 4953


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(2) López-Ramón, M. V.; Fontecha-Cámara, M. A.; Á lvarez-Merino, M. A.; Moreno-Castilla, C. Removal of diuron and amitrole from water under static and dynamic conditions using activated carbons in form of fibers, cloth, and grains. Water Res. 2007, 41, 2856−2870. (3) Fernandez, M.; Ibanez, M.; Pico, Y.; Manes, J. Spatial and temporal trends of paraquat, diquat, and diffenzoquat contamination in water from marsh areas of the Valencian community (Spain). Arch. Environ. Contam. Toxicol. 1998, 35, 377−384. (4) Ritter, L.; Solomon, K.; Sibley, P.; Hall, K.; Keen, P.; Mattu, G.; Linton, B. Sources, pathways, and relative risks of contaminants in surface water and groundwater: a perspective prepared for the walkerton inquiry. J. Toxicol. Environ. Health, Part A 2002, 65, 1−142. (5) Khan, S. U. Pesticides in the Soil Environment; Elsevier: Amsterdam, 1980. (6) Da Pozzo, A.; Merli, C.; Sirés, I.; Garrido, J. A.; Rodríguez, R. M.; Brillas, E. Removal of the herbicide amitrole from water by anodic oxidation and electro-Fenton. Environ. Chem. Lett. 2005, 3, 7−11. (7) World Health Organization (WHO). Paraquat and Diquat; WHO: Geneva, 1984. (8) Chen, C. M.; Lua, A. C. Lung toxicity of paraquat in the rat. J. Toxicol. Environ. Health, Part A 2000, 60, 477−487. (9) Gülen, J.; Turak, F.; Ozgur, M. U. Removal of amitrole by activated clay. Int. J. Mod. Chem. 2012, 2, 47−56. (10) Tunç, S.; Duman, O.; Soylu, I.̇ ; Bozoğlan, B. K. Study on the bindings of dichlorprop and diquat dibromide herbicides to human serum albumin by spectroscopic methods. J. Hazard. Mater. 2014, 273, 36−43. (11) Tunç, S.; Duman, O.; Soylu, I.̇ ; Bozoğlan, B. K. Spectroscopic investigation of the interactions of carbofuran and amitrol herbicides with human serum albumin. J. Lumin. 2014, 151, 22−28. (12) Santos, M. S. F.; Alves, A.; Madeira, L. M. Paraquat removal from water by oxidation with Fenton’s reagent. Chem. Eng. J. 2011, 175, 279−290. (13) Lee, J. C.; Kim, M. S.; Kim, C. K.; Chung, C. H.; Cho, S. M.; Han, G. Y.; Yoon, K. J.; Kim, B. W. Removal of paraquat in aqueous suspension of TiO2 in an immersed UV photoreactor. Korean J. Chem. Eng. 2003, 20, 862−868. (14) Tsai, W. T.; Hsien, K. J.; Chang, Y. M.; Lo, C. C. Removal of herbicide paraquat from an aqueous solution by adsorption onto spent and treated diatomaceous earth. Bioresour. Technol. 2005, 96, 657− 663. (15) Fontecha-Cámara, M. A.; López-Ramón, M. V.; PastranaMartínez, L. M.; Moreno-Castilla, C. Kinetics of diuron and amitrole adsorption from aqueous solution on activated carbons. J. Hazard. Mater. 2008, 156, 472−477. (16) Jiang, J. D.; Zhang, R. F.; Li, R.; Gu, J. D.; Li, S. P. Simultaneous biodegradation of methyl parathion and carbofuran by a genetically engineered microorganism constructed by mini-Tn5 transposon. Biodegradation 2007, 18, 403−412. (17) Singh, D. K. Biodegradation and bioremediation of pesticide in soil: concept, method and recent developments. Indian J. Microbiol. 2008, 48, 35−40. (18) Al Duri, B. Introduction to adsorption. In Use of Adsorbents for the Removal of Pollutants from Wastewaters; McKay, G., Ed.; CRC Press: Boca Raton, FL, 1996; Chapter 1. (19) Tanada, S.; Nakamura, T.; Miyoshi, T.; Nakamura, M.; Yamada, Y.; Takahashi, H.; Terada, H. Kinetics of in vitro paraquat removal by cation-exchange resin. Bull. Environ. Contam. Toxicol. 1988, 41, 12−16. (20) Leite, M. P.; dos Reis, L. G. T.; Robaina, N. F.; Pacheco, W. F.; Cassella, R. J. Adsorption of paraquat from aqueous medium by Amberlite XAD-2 and XAD-4 resins using dodecylsulfate as counter ion. Chem. Eng. J. 2013, 215−216, 691−698. (21) Walcarius, A.; Mouchotte, R. Efficient in vitro paraquat removal via irreversible immobilization into zeolite particles. Arch. Environ. Contam. Toxicol. 2004, 46, 135−140. (22) Cocenza, D. S.; de Moraes, M. A.; Beppu, M. M.; Fraceto, L. F. Use of biopolymeric membranes for adsorption of paraquat herbicide from water. Water, Air, Soil Pollut. 2012, 223, 3093−3104.

favorable and spontaneous at higher temperatures. But the positive values of ΔG° for AMT adsorption at different temperatures confirm its nonspontaneous nature of adsorption onto DHAPS-Mt and the system would gain energy from surroundings.58,59 The diametrically opposite values of ΔH° (PQ > 0, AMT < 0) revealed that the removal of PQ and AMT onto DHAPS-Mt was endothermic and exothermic, respectively. Besides, the positive value of ΔS° for PQ adsorption indicated an increase in the randomness, while the negative ΔS° for AMT removal corresponded to a randomness decreasing process.

4. CONCLUSIONS In this research, the adsorption capacities of zwitterionic surfactant modified clay (DHAPS-Mt) to herbicides and its removal mechanisms were fully investigated. The results of the influences of solution pH, contact time, and adsorption temperature on herbicide adsorption illustrated that PQ and AMT adsorption onto DHAPS-Mt could achieve equilibrium in a short time, and the herbicide removal highly depended on the pH value and temperature. Additionally, a cationic surfactant modified montmorillonite (CTAB-Mt) and an ionic surfactant modified montmorillonite (SDS-Mt) were employed as adsorbents for studying the removal mechanism due to the similarity of their structure with DHAPS. The results indicated that the partition adsorption of long carbon chain had little efficiency in PQ and AMT removal, but the electrostatic interaction (including the surface electrostatic adsorption and the electrostatic attraction between herbicide cation and negatively charged group of DHAPS) enhanced the adsorption ability. Furthermore, the high adsorption amounts of anionic dye (methyl orange) and cationic herbicides (PQ and AMT) onto DHAPS-Mt indicated that the wide application range of zwitterionic surfactant modified clay in the treatment of the mixed wastewater containing anionic and cationic contaminants. The studies of adsorption isotherms and kinetics suggested the applicability of experimental data to Langmuir isotherm model and the pseudo-second-order equation, respectively. And the thermodynamic study illustrated that the thermodynamic characteristic of PQ removal was completely contrary with that of AMT. In conclusion, DHAPS-Mt or its similar kinds of organoclay might be a lowcost and efficient adsorbent for treating organic pollutants from aqueous solution.

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ASSOCIATED CONTENT

* Supporting Information S

Parameters of kinetic, isotherm, and thermodynamics of the adsorption of PQ and AMT onto DHAPS-Mt. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00438.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 89733680. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Montmorillonite Functionalized with Zwitterionic Surfactant as a Highly

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