Adsorption of switchable surfactant mixed with common nonionic surfactant on montmorillonite: Mechanisms and arrangement models

Applied Clay Science 146 (2017) 140–146

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Research paper

Adsorption of switchable surfactant mixed with common nonionic surfactant on montmorillonite: Mechanisms and arrangement models

MARK

Xiaojun Hua, Senlin Tiana,⁎, Shujiao Zhana, Jianxi Zhub a

Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650500, China Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Switchable surfactant (11-Ferrocenylundecyl) trimethylammonium bromide Montmorillonite Reversible adsorption

To improve understanding of adsorption behavior of mixed surfactants on typical clay components in soil for potential applications of surfactant-enhanced remediation (SER) technologies during remediation of hydrophobic organics-contaminated soil, the adsorption mechanisms and arrangement models of an electrochemical switchable cationic surfactant (11-Ferrocenylundecyl) trimethylammonium bromide (FTMA), mixed with the nonionic surfactant Tween 80 on montmorillonite were investigated. The mixed system in the presence of Tween 80 remains electrochemically reversible. The adsorption isotherms of the pure FTMA and FTMA-Tween 80 mixed system on montmorillonite were Langmuir type. The major mechanism of pure FTMA adsorption is via cation exchange as the total capacity is close to the cation exchange capacity (CEC) of montmorillonite. With the addition of Tween 80, the cation exchange of FTMA would be weakened by the function of hydrogen bonding. As the added concentration of FTMA and Tween 80 are 2874 mg/L and 50 times of critical micelles concentration of Tween 80, adsorption capacity of FTMA and mixed surfactants decrease from 280.6 to 235.2 mg/g and 400 to 298 mg/g. Moreover, the adsorption capacity of Tween 80 linearly decreases from 118.7 to 62.7 mg/g. Meanwhile, as the amount of FTMA increases from 0.4 to 1.0 times CEC of montmorillonite, the arrangement model of FTMA in the interlayer of montmorillonite changes from a flat monolayer, lateral bilayer to pseudotrilayer. When mixed with Tween 80, the interlayer space of montmorillonite increased significantly, and it showed a gradual increasing trend of interlayer space as the concentration of Tween 80 increased. The results of the present study show that the combined use of cationic and nonionic surfactants can reduce the adsorption loss of surfactants during remediation of polluted soil and then be conducive to the application of SER technologies.

1. Introduction Soil pollution by hydrophobic organic compounds (HOCs) is an environmental problem worldwide because of their long-term persistence in soil and adverse effects on human health (Paria, 2008; Whang et al., 2008; Zhou and Zhu, 2005). HOCs tend to adsorb to soil particles in an immobile manner because of their low water solubility and strong association with the organic matter in soils (Frankki et al., 2006). Surfactant-enhanced remediation (SER) technologies have shown great efficiency for HOCs removal via desorption of HOCs from soil because the surfactants can enhance the solubility of contaminants in the soil (Edwards et al., 1991; Khan et al., 2004; Mulligan et al., 2001; Torres et al., 2005). However, surfactants must be separated from the solubilization system for reuse, leading to negative effects on the environment caused by their direct emissions and higher costs. Therefore, reversible surfactant-enhanced remediation surfactant (RSER) technology based

⁎

Corresponding author. E-mail address: [email protected] (S. Tian).

http://dx.doi.org/10.1016/j.clay.2017.05.025 Received 11 December 2016; Received in revised form 21 May 2017; Accepted 23 May 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.

on switchable surfactants is proposed (Long et al., 2014). These kinds of surfactants can be reversibly controlled via the formation and disruption of micelles by external stimuli including pH, light, and potential (Feng et al., 2013; Liu et al., 2006). Contaminants are solubilized and released from soil via the formation and disintegration of the micelles. (11-Ferrocenylundecyl) trimethylammoniunm bromide (FTMA) is an electrochemical switchable surfactant that can be triggered by its redox reactions (Tustin et al., 2007). Our previous study revealed that the adsorption behavior of (11-Ferrocenylundecyl) trimethylammoniunm bromide (FTMA) is consistent with the conventional surfactant CTAB. Moreover, FTMA has a great advantage in soil remediation because less is adsorbed onto soil when compared to CTAB (Zhan et al., 2014). Montmorillonite, which is one of the most common clays in soils, exhibits a net negative surface charge relative to other primary components such as illite and kaolinite (Stumm and Morgan, 1970). This charge imbalance is offset by exchangeable cations (typically Na+

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or Ca2 +) at the clay mineral surface. Long-chain surfactant compounds can substantially modify the surface properties of raw montmorillonite via cationic exchange and van der Waals interactions. Moreover, the amount of surfactants adsorbed by cation exchange is much larger than the amount adsorbed onto the surface. However, modified montmorillonite is hydrophobic and has a greater ability to retain contaminants in soil than its unmodified form. However, the amount of cationic surfactants has a decrease in soil remediation due to cation exchange. Therefore, the application of FTMA for RSER technology may significantly be limited due to ion exchange with montmorillonite (Toh et al., 2010; Wan et al., 2008). Cationic–nonionic mixed surfactants systems have been found to be a good choice for reduction of ion exchange between cationic surfactants on soil because of their ability to strongly enhance the solubility of nonionic surfactants (Zhang et al., 2012). Previous studies have shown that nonionic surfactants can reduce the adsorption of cationic surfactants onto soil via competitive effects (Pagac et al., 1998; Torn et al., 2003). Additionally, the solubilization and release of polycyclic aromatic hydrocarbons (PAHs) by FTMA-Tween 80 mixed surfactants illustrates that adding nonionic surfactants could enhance the solubility of PAHs when FTMA at lower concentration, enabling use of an overall lower amount of surfactants (Long et al., 2015). Based on these findings, the loss of the cationic surfactants for soil remediation can be effectively reduced by being mixed with nonionic surfactants. However, the adsorption mechanisms of switchable cationic surfactants mixed with nonionic surfactants systems have not yet been reported. The adsorption of surfactants on montmorillonite can lead to the structural change. The surface properties of surfactant modified montmorillonite and adsorption behavior are affected by the species and arrangement of surfactants in the interlayer of montmorillonites. (Paiva et al., 2008; Wang et al., 2004). The interlayer characteristics and elements loading of surfactant modified montmorillonite have been studied widely using X-ray diffraction (XRD) (Zhu et al., 2003). However, extensive structural analyses of montmorillonite adsorbing switchable cationic surfactant with and without nonionic surfactants have not yet been conducted. Accordingly, examination of the structural changes of modified montmorillonite is necessary to further elucidate surfactant adsorption mechanisms. Therefore, this study was conducted to explore the adsorption mechanisms and arrangement models of switchable cationic-nonionic surfactants onto montmorillonite. To enable their application into RSER technologies, reversible characteristics of mixed surfactants were investigated. Specifically, adsorption experiments were conducted to evaluate co-adsorption mechanisms of mixed surfactants on montmorillonite. Additionally, structural characterizations of the surfactants adsorbed montmorillonite were analyzed and investigated by X-ray diffraction (XRD) for further illustration of the co-adsorption mechanisms involved.

Fig. 1. The reduction-oxidation system for mixed surfactants. Electrochemical experiments were carried out in a three-electrode cell with FTMA-Tween 80 mixed surfactants in cell 1 and Li2SO4 in cell 2 while the solution was bubbled with N2 and stirred.

2.2. Electrochemical behavior of mixed surfactants Electrochemical experiments (Fig. 1) employing Li2SO4 (0.2 M) as the supporting electrolyte were conducted using LK2005A (LANLIKE Electrochemical High Technology Co. Ltd., Tianjin, China). Cyclic voltammetry (CV) and the constant potential method were performed with a three-electrode cell at 25 °C employing a platinum plate (8.64 cm2) as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Electrolytic oxidation of FTMA mixed with Tween 80 solutions was conducted at + 0.5 V (vs. SCE), a potential much more positive than the equilibrium potential of the mixed surfactants (Saji et al., 1991a). Oxidation was induced using an electrochemical working station for 21 h while the solution was bubbled with N2 and stirred. The reaction on the counter electrode is the reduction of water. In contrast, the reduction reaction was conducted by the constant potential method at a potential of + 0.0 V (vs. SCE) for 21 h at 25 °C under N2.

2.3. Adsorption experiments of mixed surfactants on montmorillonite Adsorption of FTMA and Tween 80 on montmorillonite was conducted in 40-ml Corex centrifuge tubes with Teflon-lined screw caps. The samples were equilibrated on a reciprocating shaker for 24 h at 25 °C, after which they were centrifuged at 3000 rpm for 1 h to separate the undissolved solute. In addition, all experiments had two duplications and a blank sample including ultrapure water. The supernatant was collected and analyzed by UV–vis spectrophotometry and the adsorption capacity (Q, unit: mg/g) of single or mixed surfactants on montmorillonite were calculated by the following equation:

2. Materials and method 2.1. Materials

Q= The redox-active surfactant (11-Ferrocenylundecyl) trimethyl ammonium bromide (Fc (CH2)11N+ (CH3)3) Br−; Fc is ferrocene) with a critical micelle concentrations (CMC) of about 0.6 mmol/L was synthesized according to the procedures described by (Saji et al., 1991b). The nonionic surfactant Tween 80 with a CMC value of about 0.02 mmol/L, Li2SO4, Fe2SO4·7H2O and HCl were purchased from Aladdin (Jingchun Chemical Co., Shanghai, China), and all reagents were used as received. N2 (volume fraction = 99.99%) was obtained from Kunming Messer gas company. Ultrapure water was used throughout the experiment.

(c0 − c1) × V m

(1)

where, c0 (mg/L) and c1 (mg/L) are the concentration of surfactant in the initial and final solution, respectively, V (mL) is the volume of solution and m (g) is the mass of montmorillonite. FTMA and Tween 80 (cobalt nitrate–ammonium thiocyanate colorimetric method) were analyzed using a Shimadzu UV–vis spectrophotometer (Kyoto, Japan) at λmax = 435 and 620 nm, respectively. The added amounts of FTMA were equal to 0–1.0 times montmorillonite's CEC, while the amount of Tween 80 added in the mixed surfactants was 10, 30, or 50 times the CMC.

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2.4. Preparation and characterization of modified montmorillonite The surfactants modified montmorillonites were prepared according to the following process: raw Ca2 +-montmorillonite was collected from Wenshan, Yunnan Province, China, and then ground to be able to pass through a 100-mesh sieve after drying at 105 °C. The cation exchange capacity (CEC) of the montmorillonite was determined to be 65.8 cmol (+)/kg clay by the cation exchange method (JC/T593-1995). The prepared montmorillonite (10 g) was mixed with 100 mL ultrapure water, and then stirred until they were thoroughly dispersed. Desired amounts of FTMA and Tween 80 were mixed with 100 mL ultrapure water and then added into the montmorillonite dispersion. The mixtures were subsequently stirred for 24 h at 120 rpm, after which they were filtered and washed with ultrapure water several times until no traces of Br− could be detected. The modified montmorillonites were then air-dried in a desiccator at 60 °C until constant weight, after which they were roughly ground and sieved between 100 and 200 meshes. The resulting organic montmorillonites were denoted as xFTMA/ yTween 80-Mt, where x and y represent the amounts of FTMA and Tween 80, respectively. For example, 0.2FTMA/10Tween 80-Mt was used to denote the sample with FTMA equal to 0.2 times the CEC of montmorillonite and with Tween 80 equal to 10 times the CMC. The symbol FTMA/Tween 80-Mt was used to represent all of the FTMA and Tween 80 modified organic montmorillonite. XRD was conducted using a Phillips 2 kW model X-ray spectrophotometer with a nickel filtered copper X-ray diffraction (Cu Kɑ 1.5418 Å) to characterize the unmodified and modified montmorillonite samples. The range of 2θ from 1° to 20° was recorded at a scanning speed of 2°/min. The basal space values were calculated according to the 2θ values of the refection on the XRD patterns.

Fig. 3. FTMA-Tween 80 mixed surfactants in reduced (a) and oxidized (b) states. During the electrochemical experiments switching cycles, the colors of the FTMA-Tween 80 system varied from yellow to green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. Electrochemical reversibility of FTMA-Tween 80 mixed solutions The typical CVs of the FTMA-Tween 80 mixed solutions of various molar ratios are presented in Fig. 2. The difference between the oxidation and reduction potential Δ Ep was 50.2 mV, and the ratio of Ipa/Ipc was 1.28. Therefore, it can be inferred that FTMA-Tween 80 mixed surfactants are also reversible, and the formation and disintegration of mixed micelles can be achieved through an electrochemical approach. Additionally, during the electrochemical experiments switching cycles,

Fig. 4. CVs of FTMA-Tween 80 mixed surfactants in reduced and oxidized states. Redox of mixed surfactants can be considered the transformation between Fe3 + and Fe2 + according to the oxidation peak current of mixed surfactants increased obviously relative to that of the reduction state. And the redox efficiency of the mixed surfactants was also obtained.

the colors of the FTMA-Tween 80 system varied from yellow to green (Fig. 3). The oxidation peak current of mixed surfactants increased obviously relative to that of the reduction state (Fig. 4), indicating that redox of mixed surfactants can be considered the transformation between Fe3 + and Fe2 +. The equation to determine the redox efficiency is expressed as follows (Yun et al., 2011):

i = 2.85n + 7.52

(2)

where, i is the redox efficiency and n is the value of the oxidation peak current. The n value of the mixed surfactant was obtained from Fig. 3. The redox efficiency of the mixed surfactants was determined to be 84%, indicating that FTMA-Tween 80 mixed surfactant has good reversibility. The high conversion efficiency of FTMA-Tween 80 provides a potential application for RSER technology. 3.2. Adsorption isotherm of FTMA and Tween 80 on montmorillonite The adsorption isotherms of FTMA in the complete range of concentrations used (0–2874 mg/L) under different Tween 80 concentrations are shown in Fig. 5. The adsorption isotherm of FTMA on montmorillonite corresponded with that of conventional surfactants including similar structure (CTAB). At the sub-CMC concentration, FTMA in the aqueous solution and at the solid-water interface existed in monomeric forms. In this region, the cationic surfactant is primarily

Fig. 2. Cyclic voltammogram curves of FTMA-Tween 80 mixed solutions. FTMA-Tween 80 mixed surfactants are electrochemical reversible for the difference between the oxidation and reduction potential ΔEp was 50.2 mV, and the ratio of Ipa/Ipc was 1.28.

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Table 1 Isotherm parameters for the adsorption of FTMA on montmorillonite. Tween 80 concentrations

Langmuir equation

0 10 CMC 20 CMC 50 CMC

lg Qe = lg Kf +

Qm (mg/ g)

B (L/g)

R2

Kf

1/n

R2

296.8 284.9 282.8 281.6

0.0142 0.0072 0.0043 0.0022

0.9838 0.9865 0.9935 0.9951

37.46 23.86 14.70 6.83

0.2916 0.3364 0.3921 0.4813

0.8689 0.8754 0.9332 0.9721

(4)

where, Qm (mg/g) is the maximum adsorption capacity, b (g/L) is the Langmuir constant, and Kf and n are the constants of the Freundlich adsorption isotherm. The values of all parameters for the adsorption of FTMA on montmorillonite at 25 °C are included in Table 1. As indicated by the R2 values (Table 1), the Langmuir model presents a better fit for single and mixed surfactants adsorption data than the Freundlich model. The values of both Qm and b decreased as Tween 80 increased, indicating that the adsorption is generally weak on montmorillonite and the adsorption capacity of FTMA on montmorillonite is lower. These findings are consistent with the experiment results. Additionally, as shown in Table 1, the value of 1/n was less than 1, and the value of Kf decreased with increasing concentrations of Tween 80, which also verifies that addition of Tween 80 into the system leads to weak adsorption strength of FTMA on montmorillonite. As shown in Fig. 6, the adsorbed capacity of Tween 80 decreased linearly with increasing concentrations of FTMA in solution, indicating that adsorption is dominated by a partition effect. Moreover, as the FTMA concentration increased, adsorption amount of Tween 80 decreased gradually, which indicated that Tween 80 was replaced by FTMA in the solid-liquid interface. It can be explained that as the FTMA concentration increases, more adsorption sites of montmorillonite are occupied by FTMA. Meantime, the existence of FTMA enhance the diffusion of Tween 80, which hinder the adsorption between Tween 80 and montmorillonite. As indicated by the slope in Fig. 6, the higher original Tween 80 concentration indicates a lower adsorbed capacity of FTMA. The higher Tween 80 concentration results in easier formation of FTMA micelles that do not adsorb onto montmorillonite (Hand, 1989). Therefore, as the amount of Tween 80 increased, more FTMA

Fig. 5. Adsorption isotherms of FTMA on montmorillonite with different Tween 80 concentrations. It shows the relationship of the adsorption capacity of FTMA on montmorillonite with the addition of different concentration of Tween 80. And with the addition of Tween 80, the adsorption of FTMA decreased.

adsorbed onto the solid through cation exchange and electrostatic interaction (Law and Kunze, 1966). After the adsorption sites on montmorillonite are occupied, the surfactant monomers self-aggregate into micelles in solution and are adsorbed onto the clay mineral more slowly (Beall and Goss, 2004). As the concentration is higher than CMC, FTMA exists completely in micelles, and the adsorption isotherm reaches the “plateau” region. With the adsorption capacity being the same as the CEC of the montmorillonite, the major mechanism of FTMA adsorption is via cation exchange (Elsherbiny, 2013; Law and Kunze, 1966). Our previous study (Zhan et al., 2014) has illustrated that the adsorption of FTMA on montmorillonite reaches to equilibrium after 8 h and the adsorption kinetic is pseudo-second-order kinetics. As mentioned above, the main mechanism of mixed surfactants adsorbed on montmorillonite is cation exchange. Therefore, the adsorption equilibrium of mixed surfactants on montmorillonite can be considered after about 8 h. The adsorption isotherm of FTMA with the addition of Tween 80 is also Langmuir-type. However, the adsorption capacity of FTMA on montmorillonite decreased with increasing addition of Tween 80, indicating decreased affinity for FTMA. This is because Tween 80 is adsorbs onto montmorillonite by hydrogen bonds, and partition and the polyoxyethylene chain will shield a portion of the adsorption sites via steric effects (Lei et al., 2006). The adsorption capacity of FTMA decreases as the concentration of Tween 80 increases, indicating a strong antagonistic effect between FTMA and Tween 80 (Portetkoltalo et al., 2001; Varade and Bahadur, 2005). However, the adsorption of montmorillonite with different Tween 80 concentrations differ a little when FTMA concentration is high. It is because that adsorption capacity on montmorillonite by cation exchange is larger than that of hydrogen bonding (Zhang et al., 2006). The adsorption capacity of FTMA decreased from 280, 250, 242 to 235 mg/g after the addition amount of Tween 80 changed from 0, 10, 20 and 50 times the CMC at an initial concentration of 2874 mg/L. Additionally, mixing with Tween 80 decreased the CMC value effectively and FTMA can form micelles at lower concentration; hence, adsorption capacity of FTMA by cation exchange decreases (Long et al., 2015). The FTMA adsorption isotherms with and without Tween 80 were fitted to the Langmuir and Freundlich isotherm models using Eq. (3) and (4), respectively:

Ce 1 C = + e Qe Qm b Qm

1 lg Qe n

Freundlich equation

Fig. 6. Adsorption capacity of Tween 80 on montmorillonite with different FTMA concentrations. The adsorbed capacity of Tween 80 decreased linearly with increasing concentrations of FTMA in solution, indicating that adsorption mechanism of Tween 80 is dominated by a partition effect.

(3) 143

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Fig. 7. Adsorption capacity of FTMA and FTMA-Tween 80. FTMA (with an initial concentration of 2874 mg/L) and Tween 80 (10, 30, and 50 CMC) were used in this experiment. The adsorption capacity of FTMA-Tween 80 was significantly lower than that of the sum of single surfactants, indicating more surfactants were used in enhanced remediation.

molecules were replaced. The calculated adsorption capacities of FTMA (with an initial concentration of 2874 mg/L) and Tween 80 (10, 20, and 50 CMC) were compared with the adsorption capacity of mixed surfactants under the same initial concentration. As shown in Fig. 7, the adsorption capacity of FTMA-Tween 80 was significantly lower than that of the sum of single surfactants. At initial concentrations of 2874 mg/L FTMA and 50 CMC Tween 80, the adsorption capacities of FTMA, Tween 80 (50 CMC) and FTMA-Tween 80 were 280.6 mg/g, 118.7 mg/g, and 298 mg/g, respectively. These results can be ascribed to the competition between cationic and nonionic surfactants, which reinforced their adsorption on clay minerals (Sehgal et al., 2009; Zhang et al., 2012). Based on the above discussion, we propose the following adsorption mechanism of FTMA-Tween 80 mixed surfactants on montmorillonite. Below the CMC of FTMA-Tween 80, both FTMA and Tween 80 are adsorbed onto montmorillonite through hydrogen bonding and electrostatic effects, with more cationic surfactant adsorbed than nonionic surfactant. At intermediate concentrations, FTMA generally aggregates into micelles, and more Tween 80 is adsorbed onto montmorillonite by hydrogen bonding (Zhao, 2008). When the concentrations exceed the CMC, mixed surfactants form into mixed micelles, and the adsorption process becomes saturated. These results imply that the addition of Tween 80 can efficiently decrease the loss of surfactants adsorbed onto montmorillonite, thereby promoting the solubilization of pollutants in soil. 3.3. Arrangement of modified montmorillonite models XRD was conducted to investigate the structures of FTMA and Tween 80 on montmorillonite. Previous studies reported that the interlayer structure of organic montmorillonite can be inferred by the basal refection (Sun et al., 1995; Xi et al., 2005). Based on Bragg's law, the d-value was calculated by the following equation:

2d sin θ = nλ

(5)

where, θ and λ are the diffraction maximum and wavelength. The XRD patterns of raw montmorillnoie and FTMA modified montmorllonite are presented in Fig. 8(a). As calculated, the d-value increases from 1.51, 1.56, 1.64, 1.78, 1.90 to 2.11 nm with the addition of FTMA changing from 0, 0.2, 0.4, 0.6, 0.8 to 1.0 CEC, indicating that FTMA was inserted into the interlayer of montmorillonite. Previous studies (Klapyta et al., 2001; Lagaly, 1981; Vaia et al.,

(caption on next page)

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X. Hu et al. Fig. 8. XRD pattern of FTMA (a) and FTMA-Tween 80 (b) modified montmorillonite It shows the diffraction peak of unmodified and modified montmorillonite. With the concentration of FTMA increased, the diffraction peak of FTMA modified montmorillonite was moving to small degree. And with the addition of Tween 80, the diffraction peak of modified montmorillonite was in smaller degree.

Table 2 d-values of FTMA-Tween 80 modified montmorillonite.

d (nm)

1994) reported that the arrangement of alkylammonium ions in the interlayer of montmorillonite depends on the interlayer cation density and the alkyl chain length, including the lateral-monolayer (LM), lateral-bilayer (LB), pseudotrilayer and paraffin-type monolayer (PM) arrangement models. Zhu et al. (2003) provided a method to determine the interlayer arrangement of cations on montmorillonite by employing the refection value of XRD less the thickness of the phyllosilicate tetrahedral-octahedral-tetrahedral (TOT) layer (0.96 nm) and comparing the thickness of the organic phase with the height of cations in different arragement models. The steric configuration, size and shape of organic molecules or cations could then be calculated according to the van der Waals radius, covalent bond radius and bond angle (Kadish et al., 2000; Müller, 2007). In this study, we adopted the arrangement model theory proposed by Lagaly and took the molecular structure as rod-like. Therefore, when the FTMA is lying flat, the length and height of the alkyl-chain are about 2.25 and 0.4 nm, and the height of the ferrocenyl head and three methyl end are about 0.43 or 0.67 nm in the macroaxis and 0.33 or 0.51 nm in the brachyaxis. As shown in Fig. 9, when the concentrations of FTMA were 0.2 and 0.4 CEC, the d-value of XRD was 1.51 and 1.64 nm; thus, the thicknesses of the d-values were calculated to be 0.6 and 0.68 nm, respectively. These thicknesses were comparable to the interlayer space of LM (0.67 nm). Therefore, the cation surfactant is lying flat between the TOT in the LM model. When the concentration of FTMA increases to 0.6 CEC, the main refection value of XRD increases to 1.78 nm, which holds an interlayer space of 0.82 nm, and it approaches the height of LB (0.81 nm). In addition, the height of the organic phase is 0.94 nm at a concentration of 0.8 CEC FTMA, which is lower than a bilayer-mirrorimage arrangement (1.09 nm). Based on these findings, it can be inferred that the organic ion arrangement can also be considered LB. According to Zhu et al. (2003), the arrangement model of LB is formed by the protrudent methyl inserting into the cavity between organic cations or into the hexagonal hole of the basal oxygen plane. Consequently, the alkyl chains may be close together and arrange in LB model. Accordingly, we conclude that the height of the LB model depends on the height of the double layers of alkyl chains rather than that of the cation end of FTMA. As the concentration of FTMA increases to 1.0 CEC, a basal

FTMA (0.6CEC)

FTMA-Tween 80 (10CMC)

FTMA-Tween 80 (20CMC)

FTMA-Tween 80 (50CMC)

1.78

1.76

1.84

2.02

reflection peak at the d-value of 2.11 nm occurs corresponding to an interlayer spacing of 1.15 nm in height, which is consistent with the height of the two layer of FTMA in a bilayer-mirror-image arrangement (1.10 nm) calculated theoretically. According to previous studies (Bonczek et al., 2002; Lagaly et al., 1976; Zhu et al., 2003), the cation interlayer arrangement was presumed to be pseudotrilayer when the dvalue ranging from 1.78 to 2.15 nm. Thus, we deduce that pseudotrilayer arrangement may explain well the basal reflection peak at the d-value of 2.11 nm. XRD patterns of FTMA-Tween 80 mixed modified montmorllonite are presented in Fig. 8 (b) and the d-values are listed in Table 2. As the concentration of Tween 80 increased from 10 to 50 CMC containing 0.6 CEC FTMA, the d-value increased gradually from 1.76 to 2.02 nm. These results confirm that Tween 80 expands the interlayer space at lower concentrations of FTMA, resulting in diminishing adsorption capacity on montmorillonite. However, when the Tween 80 is 10 CMC in the mixed surfactants, the d-value is lower than that of FTMA, implying that low concentrations of nonionic surfactant do not increase the basal space of the composite montmorllnoite. This is because low levels of nonionic surfactant mainly affect the interface between the soil-water system (Guégan, 2010). The XRD results for mixed surfactants were consistent with the change in adsorption capacity of mixed surfactants on montmorllnoite, further indicating that Tween 80 can reduce the adsorption capacity of FTMA on the montmorillonite. 4. Conclusion In summary, the switchable cationic and nonionic surfactants mixed surfactants have electrochemical reversibility and can easily undergo oxidation-reduction. The adsorption iosthem of FTMA is a Langmiur model and the main mechanism is cation exhange. With the addition of nonionic surfactants, the function of hydrogen bonding has an negative influence on cation exchange. The presence of nonionic surfactants can significantly reduce the amount of FTMA and the total adsorbed capacity of mixed surfactants on montmorillonite, which is attributed to competive adsorption on active sites and the cation exchange of FTMA. Moreover, as FTMA increased, the adsorption capacity of Tween 80 decreased linearly due to the partition effect on montmorillonite. These findings indicate that the mixed system can reduce the adsorption loss on montmorillonite. As the concentration of switchable cationic surfactants increases, the arrangement model changes from a flat monolayer, lateral bilayer to pseudotrilayer. Additionally, when mixed with nonionic surfactant, similar interlayer space can be explored at lower concentrations of cationic surfactants. The addition of nonionic surfactant provides a convenient means of realizing reversible surfactantenhanced remediation to promote the remediation efficiency. It reflects the relationships of d-value, loading amount of FTMA and arrangement models in the interlayers of montmorillonite. As the amount of FTMA increases from 0.4 to 1.0 times CEC of montmorillonite, the arrangement model of FTMA in the interlayer of montmorillonite changes from a flat monolayer, lateral bilayer to a pseudotrilayer. Acknowledgments This work was supported by the open Fund of Guangdong Provincial Key Laboratory of Mineral Physics and Materials (GLMPM-015); the National Natural Science Foundation of China (No. 21077048); the

Fig. 9. Influence of FTMA loading amount on d-values of FTMA-Mt. and arrangement models in montmorillonite.

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