montmorillonite composites and its application for diclofenac sodium removal

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Preparation of ionic liquids/montmorillonite composites and its application for diclofenac sodium removal ⁎

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Limei Wu , Qing Wang, Ning Tang , Lili Gao School of Materials Science and Engineering, Shenyang Jianzhu University, Shenyang 110168, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Montmorillonite Ionic liquid Water treatment Contaminants

Ionic liquid (IL) is an environment friendly organic solvent, which has a relatively low vapor pressure. This work focuses on adsorption of montmorillonite (Mt) to IL as well as removal of diclofenac sodium (DS), an anionic contaminant in water, by IL-modified Mt. The experiment shows absorption of DS increased by increasing IL dosage in modifying Mt. As a result, to modified Mt. with a concentration of IL of 200% cationic exchange capacity (CEC), its static absorption of modified Mt. to DS is 310 mmol/kg, with a rapid rate (reaching balance in 5 min). In dynamic column experiment, absorption of DS reaches balance after 24 h, which absorption amount is 2490 mmol/kg. It can be inferred that modification of IL change surface charge of Mt. and renders intercalation of DS into Mt. interlayers, thus increasing adsorption capacity to DS. These features could further expand the application of ILs and enable IL-modified Mt. to be used as inexpensive sorbents for the removal of chromate and other oxyanions from water.

1. Introduction Many researches have been carried out in the field of adsorption of contaminants by clay minerals. Moreover, there was report in absorption of organic modified Mt. to anionic organic contaminant, which achieved a wide application in pollution treatment (Li et al., 2014; Donga and Feng, 2005). Montmorillonite (Mt), one of the most important clay minerals, is layered silica tetrahedral and alumina octahedral sheets with negatively charged layers compensated by cations such as Na+, Ca2+. The cations can be exchanged by another inorganic cations or organic cations. IL is an environment friendly organic modifier, with special features such as minimal vapor pressure. Some ionic liquid has a strong adsorption or intercalation to clay minerals with negative layer charge, which can be used as modifier to clay minerals (montmorillonite) to enhance removal of anionic contaminants in water. The removal capacity increases with increasing addition of IL (Stepnorski et al., 2007). Therefore, modifying Mt. with long chain-length IL can enhance basal spacing of clay due to interaction force of organic molecules, and reaches adsorption equilibrium (Srivastava et al., 2009). Intercalating IL molecules into Mt. through cationic ion exchange, which process changes surface charge of clay, to prepare composite material applying in adsorb anionic contaminants. The more IL entered interlayer of Mt., the higher order degree of interlayer molecule arrangement (Ding et al.,

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2006). Also, desorption rate of IL modified Mt. is relatively low, indicating good stability of composite materials prepared by IL intercalation (Matzke et al., 2009). As an effective anti-inflammatory drug, DS has a short half-life in blood of about 1–2 h, while as an oral medicine, its concentration in blood increases and drop down rapidly, which may also cause many gastrointestinal adverse reactions. In addition, much of DS is released in excrement and urine to nature environment after metabolism, waste through this way would accumulate in soil and ground water (Boxall et al., 2004; Jjemba, 2006; Ahmed et al., 2015). DS cannot be completely decomposed or removed even by urban wastewater treatment plant and pollutes soil through irrigation. Therefore, an effective and efficient way of removing DS is an urgent need in modern agriculture and waste treatment. Among all current absorption methods, mineral adsorbent is an environment friendly and promising research topic in reducing diclofenac sodium contamination. Innovative point of our research is applying IL into environment field and focus on anionic contaminant removal, especially modification of soil material. The research purpose is investigating optimal modification degree of IL modified Mt., and using DS to test removal capacity and mechanism of IL-Mt to anionic contaminants. Through computer simulation program, the research configurates state and conformation of IL in Mt. interlayer, as well as its interaction style with DS. Furthermore, with overall consideration of static and dynamic

Corresponding authors. E-mail address: [email protected] (L. Wu).

https://doi.org/10.1016/j.jconhyd.2018.11.006 Received 15 September 2018; Received in revised form 12 November 2018; Accepted 14 November 2018 0169-7722/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Wu, L., Journal of Contaminant Hydrology, https://doi.org/10.1016/j.jconhyd.2018.11.006

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3.3 cm and top 3.3 cm was quartz sand, while the top 3.3–6.7 cm with IL-Mt. A peristaltic pump was used for the delivery of simulated groundwater containing 5 mmol/L DS in an upward direction. We prepared groundwater for the experiment; consist of Mg2+, Na+, K+ and Cl− with total concentration of 0.1 g/L. The flow rate was 4.15 mL/ min, resulting in a linear velocity of 7 m/d and a resident time of 10 h. The samples were taken total of 43 h. The DS concentrations were determined spectrophotometrically.

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C14H11Cl2NO2 C14H10Cl2NO2

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2.4. Materials characterization 20

The equilibrium DS concentrations were analyzed by a UV–Vis spectrophotometer (Model T6 New Century 1650, made by General Instrument, Inc. LLT, Beijing China) at the wavelength of 276 nm, corresponding to its maximal absorbance. Calibrations were made following standards of 10, 20, 30, 40, 50, and 60 mg/L with a regression coefficient of 0.9998. The amount of DS adsorbed was calculated as the difference between the initial and final concentrations. Powder XRD analyses were performed on a Rigaku D/max-III adiffractometer (Tokyo, Japan) with a Ni-filtered CuKα radiation at 30 kV and 20 mA. Orientated samples were scanned from 3° to 70° with a scanning step of 0.01°. FTIR spectra of samples were collected on a Nicolet-560 spectrometer (Thermal Nicolet Co., USA) from 400 to 4000 cm−1 with a nominal resolution of 4 cm−1. Molecular simulation was performed under the module ‘CASTEP’ of Materials Studio 6.1 software to investigate the sorption sites of IL and DS on Mt. The resulting primitive unit cell was characterized by the parameters a = 15.540 Å, b = 17.940 Å, c = 23.36 Å, and α = γ = 90°, β = 99°. Based on the primitive unit cell, a series of (3 × 2 × 1) supercells were built with the spacing of layers set to16 and 20 Å, respectively.

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pH Fig. 1. Speciation of DS (C14H11Cl2NO2 and C14H10Cl2NO2-).

experiment, the research studies on feasibility of IL-Mt application as PRB materials. 2. Experiments and methods 2.1. Materials The montmorillonite (Mt) used was obtained from the Clay Mineral Repositories in Purdue University (West Lafayette, IN) without further purification. The basic information of the Mt. was detailed described in our previous work (Wu et al., 2014). The 1-hexadecy1–3-methylimidazolium chloride monohydrate (C16mimCl, CAS#: 404001-62-3) was obtained from Shanghai Darui Fine Chemical Co. Ltd. (Shanghai, China). The Diclofenac sodium (DS) (15307-79-6) was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China). DS has a pKa values of 4.4 ± 0.1 (Fig. 1). The pKa was calculated on ACD online services website.

3. Results and discussions 3.1. IL uptake on Mt. and DS removal by IL-modified Mt IL-Mt is prepared by C16minCl intercalated Mt., and 50% CEC IL-Mt, 100% CEC IL-Mt and 200% CEC IL-Mt are prepared by modifying Mt. with different concentrations of C16minCl (50%CEC, 100%CEC and 200%CEC, respectively). It is shown in Fig. 2 that Mt. adsorption capacity for DS has been increased a great deal after modification. The adsorption capacity of unmodified Mt. to DS is 1.5 mmol/kg, while C16minCl modified Mt. to DS is higher and increasing continuously as modifying agent, C16mimCl, concentration increased. 200% CEC IL-Mt has the highest adsorption capacity and reaches balance at equilibrium concentration of 2.5 mmol/L, which adsorption capacity is 310 mmol/

2.2. Batch tests The initial IL concentrations varied from 8.5 (50% CEC), 17 (100% CEC) and 34 (200% CEC) mmol/L for the intercalation isotherm study. The mass of Mt. used was 1.0 g, while the volume of solution used was 50 mL for all studies. The solid and solution were combined in each 50 mL centrifuge tube and shaken for 5 h at 150 rpm and room temperature for all studies. The mixtures were centrifuged at 10000 rpm for 20 min, then dried at 60 °C and ground before characterizations. The initial DS concentrations varied from 0.5 to 5.0 mmol/L for the intercalation isotherm study, and fixed at 5 mmol/L for the kinetic study. The mass of IL-Mt used was 0.2 g, while the volume of solution used was 20 mL for all studies except the kinetic study, for which 20 mL of solution was used. The solid and solution were combined in each 50 mL centrifuge tube and shaken for 120 min at 150 rpm and room temperature for all studies except the kinetic study, in which the shaking time was 1, 3, 5, 10, 30, 60 and 120 min. After the mixtures were centrifuged at 10000 rpm for 20 min, the supernatants were filtered through 0.22 μm syringe filters before being analyzed for equilibrium DS concentrations.

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Glass columns (2 cm in diameter and 10 cm in length) were used for the column experiments. Two column settings were assembled. The amount of quartz sand and Mt. used was at 2:1 (v:v) ratio for all two settings. In column 1, the bottom 3.3 cm and top 3.3 cm was quartz sand, while the top 3.3–6.7 cm with Mt. For column 2, the bottom

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Equilibrium DSconcentrations (mmol/L) Fig. 2. Uptake of DS on Raw Mt. and IL-Mt. 2

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kg. In the experiment, adsorption capacity of raweMt, 50% CEC IL-Mt, 100% CEC IL-Mt and 200% CEC IL-Mt are 1.5 mmol/kg, 136 mmol/kg, 173 mmol/kg and 310 mmol/kg, respectively. In sum, modification of C16mimCl to highly increased its adsorption to DS, where 200% CEC ILMt has a remarkable 200 times of adsorption capacity of unmodified raw Mt. In summary, C16mimCl modification has equipped Mt. with adsorption to anionic organic contaminants. Adsorption of Surfactant modified zeolite (SMZ) to chromate is mainly due to anion exchange of chromate and Br- or Cl- absorbed on SMZ surface (Li et al., 2014; Li and Bowman, 1997). Therefore, IL-Mt adsorption to DS has similar mechanism, part of DS intercalates into Mt. interlayer and interacts with IL, and part of DS is absorbed on Mt. surface. Based on the results of 200% CEC IL-Mt adsorbing DS, the adsorption rate of composite materials has been studied (Fig. 3). With an initial concentration of 3 mmol/L, DS has been adsorbed by 200% CEC ILMt very rapidly, which adsorption amount reaches 174 mmol/kg in 5 min, and reaches equilibrium at 20 min. Dynamic experiment exhibits IL-Mt adsorbing DS is an instantaneous process. It can be inferred that physical adsorption is dominant in process of IL modified Mt. adsorbing DS (Wu et al., 2014).

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Wavenumber (nm) Fig. 4. X-ray diffraction patterns of Mt., 200% CEC IL-Mt and DS- 200% CEC ILMt (d/ Å) (a), FTIR analyses of Mt. and 200% CEC IL-Mt.

intercalated into Mt. interlayer. Absorption peaks at 2850–3000 cm−1 are attributed to CH2 stretching (υs (CH2)) and ant symmetric (υas (CH2)) modes (Frost et al., 2008; Venkataraman and Vasudevan, 2001; Ha and Char, 2005; Suga and Rusling, 1993). No further new absorption peak appears in FTIR pattern, which demonstrates C16mimCl has only intercalated or adsorbed on surface without form new chemical bond, maintaining composite material a similar property as original Mt. (Ha and Char, 2005; Li and Ishida, 2003; Xi et al., 2005; He et al., 2004; Vaia et al., 1994). C16mim+ exists as cation in solution and interacts with Mt. Organic cations have higher affinities with surface and interlayer of Mt. than inorganic cations (Liu et al., 2008). The instantaneous adsorption of DS supported surface adsorption and interlayer adsorption. Computer simulation was used to calculate the arrangement C16mimCl and DS in the interlayer of Mt., and interaction between Mt. sheets and interlayer organic. Molecule dynamics simulation plays an important role in study of molecular interaction mechanism. Silicon (Si) is used to represent Mt. sheet and N (positive charged group in C16mimCl and feature atom in DS) to represent C16mim+ and DS position in Mt. interlayer. The results of simulation are shown in Fig. 5. Mt. sheet (Si) located in position of 0–6.6 Å, and interlayer C16mimCl and DS in 100% CEC IL-Mt and 200% CEC IL-Mt in 9.5–13.8 Å and 9–18 Å. It can be observed that, with increasing amount of interlayer, distance of C16mimCl and DS to silicon‑oxygen tetrahedron in Mt. gradually increased. It proves amount of intercalated organic molecule in Mt. interlayer affects basal spacing of Mt. (Wu et al., 2017). Through exchange of cation, C16mimCl modified Mt. and entered

3.2. Mechanism of IL uptake on Mt. and DS removal by IL-modified Mt With the ion-exchange of the sodium ion for the C16mim+, expansion of the NaeMt layers occurred. This expansion was readily measured by XRD. The interlayer spacing of the modified Mt. by the C16mimCl was expanded as compared with that of the raw Mt.; the basal and interlayer spacing were observed for the different clays as shown in Fig. 4a. The trend for the basal spacing (d0 0 1) is the same as that in the interlayer spacing. The differences between the different organic-Mt are due to the different incorporation and arrangement of the surfactants (Heinz et al., 2003; Heinz et al., 2007; Wu et al., 2014). C16mimCl exchanges Na+ ion in NaeMt interlayers, leading to obvious increase in d001 of Mt. Basal spacing values of Mt. intercalated by 200% CEC C16mimCl are 20.01 Å. Whereas after 200% CEC C16mim-Mt adsorbing DS, basal spacing value has been extended to 23.36 Å. The result is not only related to the C16mimCl intercalation capacity, but also to the alkyl chain length of C16mimCl, in that both amount and forming pattern of C16mimCl and DS between Mt. layers determine the increase of the basal spacing (Li et al., 2014). FTIR pattern of C16mimCl intercalated Mt. indicates formation and break of chemical bond before and after intercalation (Fig. 4b). Scanning the range of 450–4000 cm−1 to identify difference in FTIR absorption peaks, comparing with original Mt., only peaks at 2850–3000 cm−1 show changes among all peaks of 200% CEC C16mimMt, where represents CH2 vibration peak and indicates C16mimCl has 3

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Mt. interlayer. The study uses molecule dynamics simulation program to calculate arrangement and interaction of layer sheet of C16mim+ and DS in Mt. space (Fig. 5), with experiment results from 100% CEC IL-Mt and 200% CEC IL-Mt adsorbing DS. The simulation shows there are two C16min+ ions and one DS molecule in interlayer space of 100% CEC ILMt, where three molecules overlap and form a stable layer paralleling to Mt. sheet. The two kind of molecular interlaced with each other and to form stable monolayer paralleled in the Mt. interlayer (Fig. 5a). DS is more than 98% anionic-type organic, which cannot enter Mt. interlayer. Nevertheless, after C16minCl intercalation into Mt., the interaction force of intercalated molecule with layer has been changed and makes it possible for DS to enter Mt. space in a stable way. From Fig. 5b it can be seen that four C16min+ ions and two DS molecules in the space of Mt. These four C16mim+ ions are configured by strong interaction force into two groups in Mt. interlayer, while DS form a stable molecular layer with interaction from C16min+. It can be clearly seen that 200% CEC IL-Mt space has two more C16mim+ and one more DS molecule, which yield different configuration of ion/molecule in the interlayer. C16mim+ shows stronger modification than 200% CEC IL-Mt, rendering 137 mmol/kg more adsorption amount of DS than 100% CEC IL-Mt.

3.3. Dynamic column experiment Adsorption capacity of IL-Mt to DS is high in batch experiment conditions, especially 200% CEC IL-Mt reaching 320 mmol/kg in removal. Therefore, 200% CEC IL-Mt is used to investigate removal of ILMt to DS and its application outside laboratory. Mt. as well as quartz sand is put into glass column, where quartz sand is used to ensure fluidity of liquid. DS solution flows into glass column from top and out from bottom, going through Mt. and quartz sand and being adsorbed. In Fig. 6a, the unmodified Mt. reaches its adsorption equilibrium at 2 h, so unmodified Mt. could adsorb certain amount of DS for only 2 h. However, IL modified Mt. column (Fig. 6b) adsorbs DS and get equilibrated at 24 h. In sum of this comparison, modified Mt. increases adsorption capacity of column and elongated time to reach equilibrium (Lu et al., 2011; Lv et al., 2014). When the flow rate is set at 4.15 mL/min, the water comes out of the column reaches 2490 mL at 10 h, with a DS concentration below 2.5 mmol/L. As the time goes above 10 h, the concentration of DS increases gradually. After 24 h, corresponding to 5976 mL, the concentration of contaminant of the water from the outlet is almost the same as that from the inlet (5 mmol/L), which indicates that IL-Mt in the column has totally lost its ability for the removal of contaminants. This equilibrium goes on until distilled water is added into the system at 300 mL. Then the concentration of contaminant from the outlet starts to decrease and becomes lower 0.5 mmol/L after 35 h. It takes 43 h for the whole process. The volume of water passes the column during this time is 10,707 mL and the overall removal capacity is 2.49 mmol/g. In batch experiment, both the solvent and solid are static. In a batch experimental part, IL modified Mt. have saturated surface adsorption, which hinders DS enter into Mt. interlayer and maximum adsorption amount is only 320 mmol/Kg; while in column test part, surface of IL-Mt interacts with DS ions, with solution flow, DS enters IL-Mt interlayer and exchange anions with Mt. surface (Li et al., 2014), reaching highest adsorption amount. Various research of application of clay minerals has been carried out in environment field. Due to negative charge on outer surface of clay minerals, cationic surfactants are used to modify clay with bilayer surface coverage to enhance its adsorption of anionic contaminants (Li and Bowman, 1998; Krishna et al., 2000; Atia, 2008; Hu and Luo, 2010; Brum et al., 2010; Sarkar et al., 2010). Highlight of this research is that modifier in preparation of Mt. composite material is environmentally friendly solvent and supposed to eliminate environmental pollution of commonly used anionic organic.

Fig. 5. Molecular dynamic simulation of arrangement (a, c)and radial distributions (b, d) of DS-100% CEC IL-Mt (a, b) and DS-200% CEC IL-Mt (c, d).

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4. Conclusions In this study, IL modified Mt. (IL-Mt) has been prepared, which is considered as an environmentally friendly composite material. Through modification, IL changes layer charge of Mt. and significantly increases IL-Mt adsorption to DS. In this experiment, a batch adsorption amount of 200% CEC IL-Mt to DS is 320 mmol/kg and reach equilibrium in a remarkably short time of 5 min; in dynamic column experiment, its adsorption to DS saturates after 24 h, which amount is 2490 mmol/kg, greatly increased from unmodified material. The results show a possible application of IL-Mt in PRB materials in adsorbing anionic-type contaminants. This study extended the application of Mt. and explored new applications of ILs for the removal of anionic contaminants from water. Conflicts of interest There are no conflicts of interest to declare. Acknowledgments This research was jointly supported by National Natural Science Foundation of China (51604248), National Natural Science Foundation of China (51508344), China Postdoctoral Science Foundation funded project (2018 M631818) and the Doctoral Startup Foundation of Liaoning (20170520315). References Ahmed, A.A., Thiele-Bruhn, S., Aziz, S.G., Hilal, R.H., 2015. Interaction of polar and nonpolar organic pollutants with soil organic matter: sorption experiments and molecular dynamics simulation. Sci. Total Environ. 508, 276–287. Atia, A.A., 2008. Adsorption of chromate and molybdate by cetylpyridinium bentonite. Appl.Clay Sci. 41, 73–84. Boxall, A., Fogg, L., Blackwell, P., Kay, P., 2004. Veterinary medicines in the environment. Rev. Environ. Contam. Toxicol. 80, 1–91. Brum, M.C., Capitaneo, J.L., Oliveira, J.F., 2010. Removal of hexavalent chromium from water by adsorption onto surfactant modified montmorillonite. Miner. Eng. 23, 270–272. Ding, Y.S., Wang, S.S., Zha, M., Wang, Z.G., 2006. Physicochemical adsorption and aggregative structures of the organic cation [C18mim]+ in the interlayer of montmorillonite. Acta Physico-Chim. Sin. 22, 548–551. Donga, Y., Feng, S., 2005. Poly (D,L-lactide-co-glycolide) / montmorillonite nanoparticles for oral delivery of anticancer drugs. Biomaterials 26, 6068–6076. Frost, R.L., Zhou, Q., He, H., Xi, Y., 2008. An infrared study of adsorption of paranitrophenol on mono-, di-and tri-alkyl surfactant intercalated organoclays. Spectrochim. Acta A 69, 239–244. Ha, B., Char, K., 2005. Conformational behavior of dodecyldiamine inside the confined space of montmorillonites. Langmuir 21, 8471–8477.

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