1,3-Dialkylimidazolium modified clay sorbents for perchlorate removal from water

Abstract

Sodium montmorillonite clays modified using 1-alkyl-3-methylimidazolium based ionic liquids with varying alkyl chain length viz. C4, C6, C8, C10 and C16 were used for perchlorate adsorption from water. Pristine MMT showed negligible adsorption whereas ionic liquid modified clays showed an increase in adsorption with increase in chain length of the exchanged cation. 1-Hexadecyl-3-methylimidazolium modified clay (C16-clay) with a d-spacing of 18.55 Å showed a maximum adsorption of 0.16 mmol g⁻¹ of clay. The d-spacing of the C16-clay decreased on adsorption of perchlorate to 13.70 Å without a change in the composition of the modified clay, as confirmed by CHN analysis. Raman spectroscopic studies substantiated the conformational change from gauche to trans for the imidazolium cations on perchlorate adsorption. The adsorption followed the Freundlich isotherm with pseudo second order kinetics. The modified clays were thermally stable (<200 °C) and regenerated by heating to 175 ± 5 °C in air and 95% regenerability was observed.

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1. Introduction

Perchlorate ions detected in soil, water and food originate mainly from different salts used in solid propellants for rockets and oxidizer components in various military and industrial processes.^1–10 Perchlorates are highly soluble in water and enter the human body through drinking water or the food chain and cause hypothyroidism by interfering with the ability of the thyroid gland to process iodine.^11–14 The persistent and toxic nature of perchlorates with their unusual physical and chemical properties make them difficult to remove. Different methods like adsorption, ion-exchange, membrane methods, biological treatment, chemical reduction, electrochemical reduction and so on have been studied and a reliable, repeatable, recyclable system is yet to be established for perchlorate removal from water.^15–28

Among alternatives for perchlorate removal, ion-exchange is apparently the most efficient method. However, it is costly and not efficient to deal with small concentration of perchlorate in water. Adsorption methods are promising and widely studied method for perchlorate removal. The first choice for adsorption, virgin activated carbon was not effective for perchlorate adsorption, however surface modification made it comparative to ion exchange.¹⁶ Considering the availability, low cost and thermal stability, clays were studied for perchlorate removal but no improvements were observed. The use of organoclay (modified clays) to ‘selective’ uptake of perchlorate from water marked the changing phase in terms of perchlorate removal systems.^{7,20,22–25,28} Alkyl ammonium and alkyl pyridinium modified clays were extensively used for perchlorate removal and no regenerable system was reported.^15–24

Luo et al.¹⁸ used benzyloctadecyldimethylammonium modified montmorillonite clay to achieve perchlorate removal efficiency of 0.90 mmol g⁻¹. Kim et al.²⁰ used octadecyltrimethylammonium, dodecyltrimethylammonium and hexadecyltrimethylammonium modified clays with perchlorate uptake of 0.07, 0.02 and 0.04 mmol g⁻¹ respectively. Seliem et al.²⁴ used commercially available clays, Cloisite 10A (benzyldimethyldodecylammonium modified), Cloisite 15A (dimethyldioctadecylammonium, 125 meq./100 g of clay) and Cloisite 20A (dimethyldioctadecylammonium, 95 meq./100 g of clay) for perchlorate removal with 0.26, 0.16 and 0.09 mmol g⁻¹ perchlorate uptake efficiency respectively. Bagherifam et al.²⁹ used hexadecylpyridinium modified montmorillonite clay and achieved perchlorate removal of 1.11 mmol g⁻¹. Al-Pillared montmorillonite by Komarneni et al.³⁰ showed perchlorate removal capacity of 0.01 mmol g⁻¹.

In this work, a new class of modified clays using thermally stable 1,3-dialkylimidazolium based ionic liquids (ILs) with varying chain lengths were explored for perchlorate removal from water. The adsorption mechanism, isotherms, kinetics and regeneration studies are reported. To the best our knowledge, this is the first report on perchlorate adsorption by imidazolium modified clay and also the first report on regeneration of perchlorate adsorbed clay.

2. Methodology

2.1 Materials

Sodium montmorillonite, MMT-Na⁺, (CAS no. 1318-93-0) from Southern Clay Products, Inc., USA, 1-butyl-3-methylimidazolium chloride (C4MImCl), 1-methyl-3-octylimidazolium chloride (C8MImCl) and 1-decyl-3-methylimidazolium chloride (C10MImCl) from M/s Otto Chemie Pvt. Ltd, Mumbai, India. 1-Methylimidazole, 1-chlorohexane and 1-chloro hexadecane (Sigma Aldrich) were used to synthesize 1-hexyl-3-methylimidazolium chloride (C6MImCl) and 1-hexadecyl-3-methylimidazolium chloride (C16MImCl). Hydrochloric acid (purity > 36–38%) from Merck and ammonium perchlorate AR (99.9%) prepared in house were used.

2.2 Synthesis of ionic liquids

Scheme 1 shows the synthesis route for 1-hexyl-3-methylimidazolium chloride and 1-hexadecyl-3-methylimidazolium chloride.

Scheme 1 Synthesis of 1-alkyl-3-methylimidazolium chloride.

The reaction was monitored using ¹³C NMR analysis as long alkyl chains were involved. Fig. 1 shows the representative ¹³C NMR spectrum of reactants; 1-chlorohexane, 1-methylimidazole and product, 1-hexyl-3-methylimidazolium chloride. The CH₂–Cl peak in 1-chlorohexane at 45 ppm disappeared on completion of the reaction and a new quaternary N–CH₂ group appeared at 52.5 ppm as reaction progressed. C2, C4 and C5 peaks in the aromatic imidazolium ring also shifted. The product yield was 95%. Similar features were observed for C16MImCl (ESI†).

Fig. 1 Overlaid ¹³C NMR spectra of 1-chlorohexane, 1-methylimidazole and 1-hexyl-3-methylimidazolium chloride.

2.3 Modification of clay

Modification of sodium montmorillonite clay (MMT-Na⁺) with ILs were performed as per reported procedure.³¹ Clay (2%) in water was sonicated for 10 minutes using Hielscher-UIP1000hd probe sonicator and IL diluted in methanol was added followed by sonication for 15 minutes. It was allowed to settle and filtered. The residue was washed with distilled water (5–8 times) and dried at room temperature for 4 h and then at 100 °C for 2 h under vacuum. Table 1 shows the list of modified clays. Characterisation of modified clay is included in the ESI.†

Table 1 Modifier and abbreviations for modified clays

Modifier/cation	Modified clay
1-Butyl-3-methylimidazolium	C4-clay
1-Hexyl-3-methylimidazolium	C6-clay
1-Methyl-3-octylimidazolium	C8-clay
1-Decyl-3-methylimidazolium	C10-clay
1-Hexadecyl-3-methylimidazolium	C16-clay

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2.4 Perchlorate adsorption studies

0.5 g of modified clay was dispersed in 25 mL each of 50, 100, 250, 500 and 1000 mg L⁻¹ perchlorate solution and the mixture was equilibrated at 30 °C for 1, 2, 5, 10, 15, 30, 60, 120 and 180 min using a shaker. After equilibration, the suspension was centrifuged and the solution was analysed for perchlorate content using Ion Chromatograph (IC). The experiment was repeated with different pH from 2 to 8. The amount of sorbed perchlorate per gram of clay was calculated using eqn (1),where, C_i is the initial perchlorate solution concentration, V is the volume of perchlorate solution, m is the weight of adsorbent in g and C_e is the concentration of perchlorate after adsorption calculated from IC analysis.

2.5 Kinetic studies

The experiments were conducted at pH = 2 and various time intervals, viz. 1, 2, 5, 10, 15, 30 and 60 min. The experimental data were fitted with pseudo-first order (eqn (2)), pseudo-second order (eqn (3)) and intra-particle diffusion process (eqn (4)),

q_t = k_pt^1/2 + C (4)

where q_e and q_t are the amount of perchlorate (mg g⁻¹) adsorbed at equilibrium and time t (min). The first order rate constant, k₁ is obtained from the slope of the log(q_e − q_t) versus t plot. k₂ is the second order rate constant obtained from the plot of t/q_t versus t. k_p is the rate constant of intra-particle diffusion determined from the slope of q_t versus t^1/2 plot. The value of C relates to the thickness of the boundary layer. Larger the intercept, greater is the boundary layer effect.³² If the plot passes through the origin (C = 0), the intra-particle diffusion is regarded as the only rate controlling step.³³

2.6 Isotherm studies

It is essential to establish the most suitable adsorption equilibrium correlation for reliable prediction of adsorption parameters and quantitative comparison. Langmuir model (eqn (5)) (based on the assumption that monolayer adsorption on a homogeneous adsorbent takes place with no interacting forces between adsorbed molecules) and heterogeneous models, viz. Freundlich (eqn (6)) and Tempkin (eqn (7)) were used to fit the adsorption of perchlorate onto modified clay,

Tempkin model: q_e = B ln A + B ln C_e (7)

where C_e (mg L⁻¹) is the concentration of perchlorate solution at equilibrium, q_e (mg g⁻¹) is the amount of perchlorate adsorbed per unit mass of the adsorbent at equilibrium and Q₀ (mg g⁻¹) is the amount of adsorbate at complete coverage, which gives the maximum adsorption capacity and b (L mg⁻¹) is the Langmuir constant reflecting the energy of adsorption. k_F is the Freundlich constant, n is a constant related to energy of adsorption and its magnitude is an indication of the favourability of adsorption. A and B are the Tempkin constants.

2.7 Computational method

Geometries of gaseous ion pairs were optimized at B3LYP³⁴ level of density functional theory using 6-311+G(d,p) basis set as implemented in Gaussian09.³⁵ Basis set superposition error (BSSE) corrected binding energy for the ion-pairs ([RMIm]⁺[X]⁻) was calculated using eqn (8).

[RMIm]⁺ + [X]⁻ ⇌ [RMIm]⁺[X]⁻ (8)

2.8 Regeneration

The perchlorate adsorbed clays were regenerated by heating at different temperatures; 170, 175, 180 and 190 °C. The % regenerability was analysed by comparing the perchlorate adsorption efficiency of regenerated clay and parent modified clay.

2.9 Instrumental

Perchlorate was estimated using a Dionex model ICS 2000 Ion Chromatograph (IC) equipped with AS16 column, AG16 Guard column, ASRS 300 Suppressor column and a conductivity detector using 35.0 mM NaOH as eluent with a flow rate of 1 mL min⁻¹. Chromeleon chromatographic software was used for the data analysis. XRD analysis was done using Bruker D8 Discover diffractometer. CHN analysis was done using Perkin Elmer 2400 CHNS analyzer with thermal conductivity detector. Thermal analysis was carried out using TA instruments SDT Q600 TGA from room temperature to 600 °C at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere for modified clay and in air for regeneration studies. Conformational studies were performed using Witec alpha 300R confocal Raman microscope.

3. Results and discussion

3.1 Effect of solution pH on adsorption

Fig. 2 shows the variation of adsorption capacity of modified clays with pH. pH was limited to 8 due to the enhanced solubility of clay in alkaline medium.³⁶ Higher rate of adsorption was observed at lower pH and pH = 2 was selected for experiment. At low pH levels H⁺ ions dominate the surface of the clay. Perchlorate ions with negative charge at low pH result in electrostatic attraction with the positive charge on the clay, resulting in an increased adsorption at the surface.

Fig. 2 Effect of pH on perchlorate adsorption using C16-clay. Error bars are shown for the selected chart series with 5% value.

When pH is increased, there will be excess of OH⁻ ions present in the solution, lead to competition of adsorption sites with perchlorate ions which results in lower adsorption. If there is still significant amount adsorbed onto modified clay surface at higher pH values, it can be attributed to chemisorption.³⁷ From Fig. 2, at pH = 8, higher perchlorate adsorption capacity is retained (11.3 mg g⁻¹) by C16-clay and expected to show chemisorption.

3.2 Effect of contact time

The effect of contact time on adsorption at room temperature (303 K) was studied at various time intervals from 1 to 180 min. Fig. 3 shows the perchlorate adsorption by C16-clay. Maximum adsorption was observed at 15 min and equilibrium was reached. Among reported modified clay systems, benzyloctadecyldimethyl ammonium modified montmorillonite clay,¹⁸ hexadecyl pyridinium modified montmorillonite clay²⁹ and benzyldimethyldodecyl ammonium modified montmorillonite clay²⁴ taken 120 min, 240 min, and 1440 min respectively for attaining the equilibrium. Lower equilibration time is advantageous in its practical applications.

Fig. 3 Effect of contact time on perchlorate adsorption using C16-clay. Error bars are shown for the selected chart series with 5% value.

3.3 Effect of perchlorate concentration

Adsorption studies were done using different concentrations of perchlorate solutions from 50 mg L⁻¹ to 1000 mg L⁻¹ and the adsorption increased linearly (R² = 0.98) with increasing concentrations of perchlorate solutions (Fig. 4).

Fig. 4 Effect of initial concentration on adsorption using C16-clay. Error bars are shown for the selected chart series with 5% value.

3.4 Perchlorate adsorption by modified clays

Adsorption studies were carried out using MMT-Na⁺, C4-clay, C6-clay, C8-clay, C10-clay and C16-clay with 1000 mg L⁻¹ perchlorate solution at pH = 2 and contact time of 15 min. The adsorption capacity (q_e) and d-spacing of clays before and after adsorption are tabulated in Table 2.

Table 2 d-Spacing of clay and its adsorption capacity (q_e)

Clay	d-Spacing (Å)	qe (mg g^−1)	d-Spacing after ClO4^− adsorption (Å)
MMT-Na^+	12.09	0.1	12.10
C4-clay	13.64	0.7	13.08
C6-clay	13.97	1.1	13.43
C8-clay	14.20	3.6	14.11
C10-clay	14.38	12.4	14.20
C16-clay	18.55	15.6	13.70

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The value of q_e showed the same trend as d-spacing of modified clays; MMT-Na⁺ < C4-clay < C6-clay < C8-clay < C10-clay < C16-clay (Table 2). C16-Clay showed maximum perchlorate adsorption of 15.6 mg g⁻¹ (0.16 mmol g⁻¹) of clay. d-Spacing of C16-clay decreased from 18.55 Å to 13.70 Å on adsorption of perchlorate (Fig. 5) with composition unchanged while all other modified clays showed negligible change (Table 2). The increased adsorption of perchlorate with increasing d-spacing is attributed to the enhanced access of perchlorate ions on to the clay surface.

Fig. 5 XRD spectra of C16-clay and perchlorate adsorbed C16-clay (C16-clay–ClO₄).

Raman spectroscopy was used for conformational analysis to account for the decreased d-spacing on adsorption by measuring the intensity of symmetric (2847 cm⁻¹) and asymmetric (2878 cm⁻¹) stretching vibrations of methylene groups present.³⁸ 1-Alkyl-3-methylimidazolium ring with trans conformation shows higher intensity for asymmetric stretching than symmetric stretching and in gauche conformer symmetric stretching is reported to possess higher intensity.³⁸

Fig. 6 shows the representative Raman spectra of C16-clay, C16MImCl and C16-clay–ClO₄ with intensity of methylene stretching vibrations in comparison with C4 analogues. Modified clay showed an increase in intensity ratio of asymmetric to symmetric stretching vibrations, suggesting more of trans conformation. The perchlorate adsorbed clays showed a decrease in intensity ratio of asymmetric to symmetric C–H stretching vibrations compared to modified clays, an indication of change in conformation from trans to gauche. The change was significant in C16-clay and accounts for the large decrease (4.85 Å) in d-spacing observed for C16-clay on perchlorate adsorption. The analysis reveals the possibility of formation of a new ion pair within the clay layer between imidazolium cation and perchlorate anion. From the Fig. 6, it is clear that the CH₂ stretching pattern in free ionic liquid C16MImCl and newly formed ion pair within the clay layers are comparable.

Fig. 6 Raman spectra of C16-clay, C16MImCl and C16-clay–ClO₄. The deconvoluted spectrum is shown in inset. The table shows the intensity of C–H str. vibrations of selected ILs, modified clay and perchlorate adsorbed clay.

The ion-pair formation possibility was studied using B3LYP/6-311+G(d,p) method. Fig. 7a shows the model for MMT-Na⁺, a 2 : 1 phyllosilicate with alumina octahedral sandwiched between two silica tetrahedra forming one layer of clay with some sites of Al³⁺ substituted by Mg²⁺ ions. The resulting negative charge on Mg²⁺ substitution in the clay was neutralized by Na⁺ ion present in between the clay layers. In modified clays such as C4-clay these Na⁺ ions are replaced with 1-butyl-3-methylimidazolium cation (C4MIm⁺) as shown in Fig. 7b. The most electrophilic C2H site in the imidazolium cation ring is responsible for major interaction with anionic silicate layer in the modified clay. Subsequently a simplest model of C4MIm⁺–Si(OH)₃O⁻ is used for computational studies as depicted in Fig. 7c where the C4MIm⁺ represent the intercalated imidazolium cation and Si(OH)₃O⁻ represent the anionic clay layer.

Fig. 7 (a) Model structure of MMT-Na⁺, (b) C4-clay, (c–f) optimized structures of species involved in perchlorate adsorption mechanism.

C4-Clay in acidic medium (pH = 2) weakens the cation clay layer interaction due to the presence of excess H⁺ ions, which is supported by the increase in C2H⋯O bond distance from 1.471 Å to 2.078 Å in the optimized model structures given in Fig. 7c and d, respectively. The free imidazolium cations thus generated on acidification, can also interact with perchlorate anions to form 1-butyl-3-methylimidazolium perchlorate, C4MIm⁺ClO₄⁻ (Fig. 7e) as substantiated by Raman spectroscopic analysis. The BSSE corrected binding energy for C4MIm⁺Si(OH)₃O⁻ (412.5 kJ mol⁻¹) and C4MIm⁺ClO₄⁻ (337.3 kJ mol⁻¹) suggest imidazolium cation–silicate interaction is much stronger than the imidazolium cation–perchlorate interaction. Hence, on thermal activation of the perchlorate adsorbed system, reversal of cation interaction from perchlorate to silicate is feasible with the elimination of HClO₄ as shown in Fig. 7f.

3.5 Adsorption kinetics

Pseudo-second order and intra-particle diffusion models were best fitted for perchlorate adsorption with R² = 0.99. The pseudo-second order linear plot suggests that the rate limiting step is chemisorption including valence force through sharing and exchange of electrons between ClO₄⁻ and modified MMT.³⁹ The q_e values calculated with pseudo-second order model (16.6 mg g⁻¹) was in good agreement with the experimental results. In Fig. 8b, the intercept not passing through the origin (C ≠ 0) suggests that intra-particle diffusion is not the rate controlling step.³³ The conformational change and thereby fluidic property observed may be the reason for higher correlation of intra-particle diffusion kinetics. Pseudo-first order plot (R² = 0.87) did not fit well for perchlorate adsorption by modified clay (ESI†).

Fig. 8 (a) Pseudo-second order model and (b) intra-particle diffusion model for perchlorate uptake by C16-clay. Error bars are shown for the selected chart series with 5% value.

3.6 Adsorption isotherm study

Adsorption isotherm indicates the distribution of perchlorate between the solution phase and the solid phase (modified clay) at the equilibrium state. The adsorption behaviour for the solid–liquid adsorption system can be characterized in terms of the type of adsorption isotherm and to know this, the experimental data were fitted in eqn (5)–(7) to obtain the adsorption isotherms. Freundlich adsorption describes the most favourable adsorption process with R² = 0.98 (Fig. 9). The magnitude of n from Freundlich plot, n > 1 indicates a favourable adsorption process. In this study, n = 1.85 and 1/n = 0.54, a value below unity implies chemisorption.⁴⁰ Langmuir plot (R² = 0.85) and Tempkin adsorption isotherms (R² = 0.87) were not fitting well with the experimental data (ESI†).

Fig. 9 Freundlich plot for perchlorate uptake by C16-clay. Error bars are shown for the selected chart series with 5% value.

3.7 Thermal stability of modified clays

Thermal stability of modified clays were evaluated using TG analysis. Among studied clays, the order of thermal stability was C16-clay < C10-clay < C8-clay < C6-clay < C4-clay, thermal stability depends on alkyl chain length as side chain degradations were observed for long alkyl chains. Fig. 10 shows the TG/DTG curves of C4-clay and C16-clay with initial decomposition temperatures of 260 °C and 200 °C and maximum decomposition temperature of 462 °C and 291 °C, respectively. 1-Alkyl-3-methylimidazolium modified montmorillonite clay showed higher thermal stability than commercially available alkyl ammonium modified clays, montmorillonite clay modified with benzyldimethyldodecyl ammonium showed initial decomposition at 150 °C and maximum decomposition at 250 °C (ESI†) and that with dimethyldioctadecyl ammonium showed initial decomposition at 135 °C and maximum decomposition at 227 °C (ESI†).

Fig. 10 TG/DTG curve of (a) C4-clay and (b) C16-clay.

3.8 Regeneration studies

The chemisorbed species are not easy to remove as it forms chemical bonds and the possibility of thermal decomposition of adsorbed species has to be analysed due to higher thermal stability of imidazolium modified clays. Computational studies predict the formation of perchloric acid as decomposition product (Fig. 7f) on heating. The maximum decomposition temperature of perchloric acid was 173 °C from TG/DTG analysis (Fig. 11).

Fig. 11 TG/DTG curve of perchloric acid in air.

The regeneration studies of perchlorate adsorbed clays were done at different temperatures from 170 to 190 °C (Table 3). The initial C16-clay perchlorate adsorption of 15.6 mg g⁻¹ is taken as 100% (cycle-I) and it is compared with perchlorate adsorption after regeneration at different temperatures (cycle-II). Maximum regenerability of 95% was achieved at 175 °C. This agreed well with the proposed mechanism of perchloric acid removal. Above 175 °C regeneration capacity decreased due to initiation of alkyl chain degradation of intercalated imidazolium cation.

Table 3 Regeneration of modified clays at different temperatures

Temp. (°C)	Regeneration (adsorption capacity in %)
	Cycle-I	Cycle-II
170	100	87
175	100	95
180	100	82
190	100	72

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C16MIm–ClO₄ was synthesized using reported procedure by Wang et al.⁴¹ for further confirmation. The characteristic peak of perchlorate anion at 931 cm⁻¹ is seen in Raman spectra of C16MImClO₄ (ESI†). Clay shows a characteristic vibration of Si–O_b–Si at 705 cm⁻¹ (O_b represents bridging oxygen atoms that connects the SiO₄ tetrahedra in clay). Raman spectra of perchlorate adsorbed C16-clay, shows the presence of characteristic peaks due to Si–O_b–Si and ClO₄⁻ with a peak shift. i.e. the peak due to Si–O_b–Si at 705 cm⁻¹ shifted to 701 cm⁻¹ indicating the change in inter gallery spacing and the peak due to ClO₄⁻ at 931 cm⁻¹ is downshifted to 929 cm⁻¹. This further confirms the formation of C16MIm–ClO₄ in the clay gallery. TG/DTG analysis of C16MIm–ClO₄ showed maximum decomposition at 287 °C (ESI†). The activation energy for the decomposition calculated using Kissinger method, a multiple heating rate TG analysis method was 232 kJ mol⁻¹ (ESI†). The high activation barrier was lowered by the presence of H⁺ ions entrapped in the clay during the perchlorate adsorption at pH = 2, leading to elimination of HClO₄ at a lower temperature than the actual decomposition temperature of C16MIm–ClO₄. This confirms the regeneration mechanism proposed using computational studies.

4. Conclusions

1-Alkyl-3-methylimidazolium modified clays were used for perchlorate adsorption from water. The clay with maximum d-spacing showed maximum adsorption and the conformational changes associated on adsorption were studied using Raman spectroscopy. Experimental data fitted with different adsorption isotherms and kinetic models revealed Freundlich adsorption and pseudo second order kinetics for the adsorption process. On thermal activation 95% regeneration of clay was observed at 175 °C and regeneration mechanism involving perchloric acid removal was proposed based on computational studies. The organoclay prepared represents a potential adsorbent for perchlorate with advantage of very low contact time and regenerability of the system.

Abbreviations

IL	Ionic liquid
C4MImCl	1-Butyl-3-methylimidazolium chloride
C6MImCl	1-Hexyl-3-methylimidazolium chloride
C8MImCl	1-Methyl-3-octylimidazolium chloride
C10MImCl	1-Decyl-3-methylimidazolium chloride
C16MImCl	1-Hexadecyl-3-methylimidazolium chloride
C4MIm	1-Butyl-3-methylimidazolium cation
ClO4^−	Perchlorate anion
BSSE	Basis set superposition error
IC	Ion chromatograph
qe	Adsorption capacity
C16-clay–ClO4	Perchlorate adsorbed C16-clay
HClO4	Perchloric acid

Acknowledgements

The authors thank Director, Vikram Sarabhai Space Centre, Thiruvananthapuram, for granting permission to publish this work. One of the authors (ET) thanks Indian Space Research Organisation for providing research fellowship. We greatly acknowledge the analytical support from Analytical and Spectroscopy Division.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18107d

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