A corn stalk-derived porous carbonaceous adsorbent for adsorption of ionic liquids from aqueous solution

Abstract

Corn stalks were used to prepare a porous carbonaceous material with a high surface area of 2442 m² g⁻¹ by the hydrothermal carbonization of corn stalks followed by chemical activation. The prepared corn stalk-derived carbonaceous material (CSCM) was used for the adsorption of ionic liquids (ILs), and showed super performance for the adsorption of ILs from aqueous solution and adsorption capacities of 0.52 and 2.41 mmol g⁻¹ were obtained for [Bmim]Cl and [Bmim][NTf₂], respectively. Adsorption was fast that the equilibrium could be reached within about 1 min. The influence of the chemical structure of different ILs on the adsorption onto the CSCM adsorbent was examined where it was revealed that the adsorption capacities of the CSCM for a series of ILs increased with increasing the length of the alkyl chain of the imidazolium cation and the hydrophobicity of the IL anions, followed the sequence of Cl⁻ < [TFA]⁻ < [BF₄]⁻ < [OTf]⁻ < [PF₆]⁻. The results of this work provide a facile method for the preparation of a highly efficient adsorbent from renewable agricultural waste that has potential applications in the environmental and chemical fields.

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1. Introduction

Ionic liquids (ILs) have attracted more and more attention in academic and industry fields during the last few years, mainly due to their unique properties such as non-flammability, low vapor pressure, high thermal and chemical stability, wide liquid range, and tunable solvent capacity for different substances.¹ Presently, ionic liquids have been broadly applied in organic synthesis,² catalysis,³ electrochemical application⁴ and material science.⁴ However, recent research has indicated that ILs present a certain toxicity on bacterial, algae, duckweed, daphnia, and zebrafish.⁵ Thus, more attentions have to be pained on the removal or recovery of ionic liquids from aqueous streams before they enter the environment system. Various treatment methods such as chemical oxidation,^6,7 biodegradation,^8,9 flocculation¹⁰ and adsorption^11–13 have been applied to remove ionic liquids from aqueous solutions. Among these methods, adsorption has been widely adopted since it has advantages such as cost-effective, eco-friendly, and non-destructive.

In the last years, various adsorbents such as ion-exchange resin,^14,15 montmorillonite¹¹ and activated carbon¹⁶ have been tried for the adsorption of ILs from aqueous solutions. As the most commonly used adsorbent in wastewater treatment, activated carbon has been shown to be effective for the removal of ILs from aqueous streams, due to its high surface area (ca. 1000 m² g⁻¹). Palomar et al. investigated the adsorption of a series of imidazolium-based ILs from aqueous streams with a commercial activated carbon, and adsorption capacities of 0.14–1.42 mmol g⁻¹ were achieved for these imidazolium-based ILs.¹⁷ The effect of the influence factors such as pH, temperature, initial concentration, and contact time on the adsorption of various ionic liquids has been examined, to explore the adsorption efficiency, kinetic and thermodynamic.^18–21 However, most of the applied efficient adsorbents were commercial activated carbons or those being subjected to H₂O₂ and HNO₃ modification.^12,17 Generally, activated carbons were prepared from the precursors such as wood or coal by pyrolysis and physical or chemical activation, which requires high temperature carbonization step and leads to odorous waste gases emission. From the view point of practical application, development of carbonaceous materials with mild preparation conditions from sustainable resources is required.

As an abundant agricultural waste, corn stalk mainly composes of organic cellulose, hemicellulose, lignin and inorganic silicon dioxide, and it has great potential to be a cheap precursor for production of carbonaceous adsorbents for a variety of purposes for the adsorption of various pollutants.^22–24 For example, Song et al.²² synthesized an amine-functionalized magnetic corn stalk composite adsorbents having a BET surface area of about 24 m² g⁻¹, and the adsorbent exhibited a high adsorption capacity of 231 mg g⁻¹ for Cr(VI). Char from fast pyrolysis of corn stalk was activated with CO₂ or H₃PO₄, and activated carbons that had a BET surface area of 600–880 m² g⁻¹ were prepared.²⁴ The direct use or functionalization of corn stalk for adsorption has only limited capacity due to its low surface area. The preparation of activated carbon from corn stalk normally requires high temperature that tends to cause serious air pollution. Hydrothermal carbonization of biomass is an emerging process to produce carbonaceous materials that has advantages such as no waste gases emission and mild preparation conditions. In this work, corn stalk as feedstock was used to prepare an adsorbent that had a large surface area of 2442 m² g⁻¹, by hydrothermal carbonization of corn stalk following with chemical activation using KOH as activation agent. Nine ionic liquids with different chemical structures were selected to study the adsorption performances of the prepared corn stalk-derived adsorbent for the ionic liquids.

2. Materials and methods

2.1. Materials

Ionic liquids (purity 99%) (1-ethyl-3-methylimidazolium chloride ([Emim]Cl), 1-hexyl-3-methylimidazolium chloride ([Hmim]Cl), 1-octyl-3-methylimidazolium chloride ([Omim]Cl), 1-butyl-3-methylimidazolium trifluoroacetate ([Bmim][TFA]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), 1-butyl-3-methylimidazolium trifluoromethansulfonate ([Bmim][OTf]), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) and 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim][NTf₂])) used in this work were received from Lanzhou Institute of Chemical Physics, Chinese Academic of Sciences (Lanzhou). 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) was purchased from Lihua pharmaceutical Co., Ltd. (Henan, China). Potassium hydroxide and ethanol were obtained from Guangfu fine chemicals research institute (Tianjin). Potassium phosphate and methanol (HPLC grade) were provided by Fisher Scientific Inc. (Geel, Belgium). Corn stalk was collected from a local farm (Tianjin, China) that was ground to pass 180 mesh, and oven-dried at 105 °C for one day before use.

2.2. Preparation and characterization of corn stalk-derived carbonaceous adsorbent

The corn stalk-derived carbonaceous adsorbent was prepared by hydrothermal carbonization of corn stalk followed by chemical activation with KOH. Typically, 12 g corn stalk and 60 mL of ultrapure water was mixed and placed in a 100 mL Teflon lined stainless steel autoclave. The autoclave was heated to 250 °C within 30 min and maintained for 10 h at 250 °C at the autogenous pressure. After the reaction was completed, the autoclave was cooled with cold water, and the resulting solid material (donated as HC) was separated by centrifugation. Then the solid material was washed with ultrapure water and ethanol for five times, and dried at 80 °C overnight.

The above prepared hydrothermal solid material was chemically activated with KOH in a horizontal tubular furnace (OTF-1200X, Hefei Kejing Materials Technology Co. LTD) by thoroughly mixing the HC with KOH at a given weight ratio (KOH/HC = 2). The mixture was then heat-treated from room temperature to 800 °C at a heating rate of 5 °C min⁻¹. The sample was maintained at 800 °C for 2 h under a flowing N₂ gas. The obtained solid sample was thoroughly washed with 1 mol L⁻¹ HCl aqueous solution and ultrapure water until the leachate became neutral. The resulting solid material was obtained (denoted as CSCM800-2-2 and simplified as CSCM) by oven-drying at 80 °C for 12 h.

Scanning electron microscopy (SEM) images were obtained on a HITACHI SU8020 instrument to observe the surface morphological properties of the prepared adsorbents. The porous texture properties of the samples were measured on the basis of nitrogen adsorption–desorption isotherms at −196 °C using a 3FLEX apparatus (Micromeritics Instrument Corp., USA). Elemental analysis was performed using Elementar Vario EL cube (Germany).

2.3. Adsorption experiments

Batch adsorption experiments were conducted in a thermostated shaker bath (model DSHZ-300A, Jiangsu) at a shaking rate of 120 rpm in 50 mL flasks containing 0.05 g adsorbent and 20 mL ionic liquids aqueous solution for 2 h. Initial ionic liquid concentrations of 0 to 6 mmol L⁻¹ were used. Adsorption kinetics experiments revealed that the adsorption was so quick that equilibrium could reach within several minutes.

The concentration of ionic liquid was quantified by Ultra Performance Liquid Chromatography (UPLC, Waters) equipped with a UV detector and a column ACQUITY UPLC®BEH C18 (1.7 μm 2.1 × 100 mm). The mobile phase consisted of 15% v/v methanol and 85% v/v 10 mM potassium phosphate buffer, and the flow rate was 0.2 mL min⁻¹. The injection volume of the sample was 2 μL and each sample was detected twice. The absorption wavelength of the UV detector was set at 211 nm for imidazolium-based ionic liquids. Adsorption amount of the ionic liquids was calculated on the basis of the difference in concentrations of ionic liquids before and after adsorption. All adsorption experiments were conducted for three times and average values of triplicate experiments were adopted.

3. Results and discussion

3.1. Characterization of corn stalk-derived carbonaceous adsorbent

Nitrogen sorption isotherms were measured to characterize the BET surface area and pore characteristic of the obtained carbonaceous adsorbent. The N₂ adsorption–desorption isotherms of the obtained sample CSCM are shown in Fig. 1a. It shows that a sharp knee appeared at P/P₀ = 0.05, indicating the occurrence of large amount of micropores. The nitrogen adsorption isotherm of corn stalk-based activated carbon is type I and typical microporous material according to the IUPAC (International Union of Pure and Applied Chemistry). The pore size distributions (PSD) of CSCM (Fig. 2b) is defined as heterogeneity degree of porous material and is directly related to the adsorption equilibrium and kinetic properties of prepared activated carbon. A narrow pore size distribution could be observed in the region below 2 nm, which is in agreement with N₂ adsorption–desorption isotherms where a horizontal plateau could be observed for higher relative pressures. Previous study proved that the pores within the size range up to 8 nm are playing a main role in ionic liquids adsorption from aqueous solution.¹²

Fig. 1 Nitrogen adsorption/desorption isotherms (a) and pore size distribution (b) of the prepared CSCM adsorbent.

Fig. 2 SEM images of (a) raw corn stalk, (b) carbon material obtained from the hydrothermal carbonization of corn stalk at 250 °C for 10 h, (c and d) activated carbon obtained from the chemical activation of HC materials by KOH.

The effect of activation temperature and KOH/HC weight ratio was examined (Table S1†). It can be seen that both of the specific surface area and pore volume increased with increasing activation temperature from 600–800 °C, mainly contributed by the increasing developed porosity by KOH at high temperatures. Then they had a little decline at 900 °C. The effect of KOH/HC weight ratio exhibited a similar tendency. The decrease in the specific surface area and pore volume at 900 °C or KOH/HC weight ratio of 2 should be resulted from the collapse of the formed pores in the carbonaceous materials. Since the carbonaceous material that prepared under the conditions of 800 °C, KOH/HC weight ratio of 2 and activation time 2 h (denoted as CSCM), exhibited the best structural characteristics, it was used to study the adsorption behavior of ionic liquids.

The SEM images of the raw corn stalk, HC and CSCM are given in Fig. 2. It shows that the crude corn stalk was mainly composed of rod-shaped fibers and some large size particles, and some carbonaceous microspheres were found from the HC that obtained from the hydrothermal carbonization of corn stalk. When the HC sample was further chemically treated with KOH, the morphology of the HC was completely destroyed, and the obtained carbonaceous material (CSCM) is now consisted of macrometer-sized monolithic fragments with sharp edges.

During KOH activation, the HC material reacts with KOH as following reaction.

6KOH + 2C → 2K + 3H₂ + 2K₂CO₃

K₂CO₃ → K₂O + CO₂

CO₂ + C → 2CO

K₂CO₃ + 2C → 2K + 3CO

C + K₂O → 2K + CO

The activation mechanism of the hydrothermal carbon material HC should be ascribed to the redox reaction between potassium species and carbon, gasification of carbon with CO₂ and the expansion of carbon matrix that results from the as-prepared metallic K.²⁵ During this process, the specific surface area was continuously increased along with micropores appeared.

The chemical composition of the raw corn stalk, HC and CSCM were analyzed and the results are listed in Table 1. The C content in the raw corn stalk is 43.4%, which increased to 70.3% after being subjected to hydrothermal carbonization. The increase of carbon content in the HC material was mainly resulted from the deoxygenating reactions such as dehydration and decarboxylation occurred during the hydrothermal carbonization of the raw corn stalk.²⁶ After the HC material was chemically activated with KOH, the C content in the prepared CSCM material increased to 90.7%, which should be contributed from the pyrolysis effect promoting the removal of oxygenated groups in carbonaceous material at high temperature of 800 °C.

Table 1 Physical and chemical characterization of corn stalk, the prepared HC and CSCM materials

Sample	C (wt%)	H (wt%)	N (wt%)	ABET (m^2 g^−1)	Vtotal (cm^3 g^−1)	Vmicro (cm^3 g^−1)
Corn stalk	43.4	6.22	0.48	—	—	—
HC	70.3	6.20	1.72	0.5	0.006	0.006
CSCM	90.7	2.00	0.84	2442	1.560	0.860

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3.2. Adsorption kinetics

In this work, two common ionic liquids, [Bmim]Cl and [Bmim][NTf₂] were selected as representative hydrophilic and hydrophobic ionic liquids, respectively, to investigate the adsorption behavior of the prepared corn stalk-based carbonaceous material for the ionic liquids. Several empirical and phenomenological kinetic models, including Elovich model, intraparticle diffusion model, pseudo-first-order and pseudo-second-order models have been applied to describe the adsorption of ionic liquids onto various adsorbents,^19,21 in which pseudo-first-order and pseudo-second-order models are the most frequently-used kinetic models that can well describe the adsorption of ionic liquids onto activated carbons.^13,25 Therefore, herein pseudo-first-order and pseudo-second-order models were used to fit the adsorption data to investigate the adsorption kinetics of the ionic liquids onto the CSCM adsorbent.

Pseudo-first-order kinetic model is derived from mass balance equation that is usually used to describe external mass transfer process and calculate equilibrium adsorption capacity well. The pseudo-first-order equation is:

q_t = q_e(1 − e^{−k₁t/2.303}) (1)

where k₁ is the adsorption rate constant of pseudo-first order equation (min⁻¹), q_t and q_e (mmol g⁻¹) are the amount of ionic liquids adsorbed on the adsorbent at time t (min) and equilibrium.

Pseudo-second-order kinetic model is based on adsorption isotherm of Langmuir and it focuses on describing chemisorption process at active sites. The pseudo-second order has the following form:

t/q_t = 1/k₂q²_e + t/q_e (2)

where k₂ is the adsorption rate constant (g mg⁻¹ min⁻¹), q_e and q_t (mmol g⁻¹) are the amount of ionic liquids adsorbed on the adsorbent at equilibrium and time t (min), respectively.

Fig. 3 shows the adsorption kinetics of [Bmim]Cl and [Bmim][NTf₂] on the CSCM adsorbent, and very good agreements between the simulated curves and the experimental data was obtained from the fact that the correlation coefficient for both models are higher than 0.999 (Table 2), indicating that both pseudo-first-order and pseudo-second-order models could satisfactorily fit the experimental adsorption data of [Bmim]Cl and [Bmim][NTf₂]. It can be observed that the adsorption of two ionic liquids was so fast that the equilibrium could be reached within about 1 min, which is much faster than the adsorption of octylpyridinium ionic liquids onto activated carbons, and [Bmim]Cl onto functional carbon microspheres that 2–24 h was required to reach equilibrium.^19,21 The concentration of [Bmim]Cl and [Bmim][NTf₂] in aqueous solution decreased rapidly from 0.57 mmol L⁻¹ to 0.24 mmol L⁻¹ and 0.01 mmol L⁻¹, respectively, and remained constant afterwards. This can be explained by the nature of adsorbent and its large available amount of adsorption sites which are responsible for the short time to reach the equilibrium. The slope of the tangent at any point on ionic liquids uptake curve reveals the adsorption rate at corresponding test time, so it showed that the fast adsorption happened in initial adsorption stage. Therefore, it can be deduced from the analysis above that the adsorption of ionic liquids at inner active sites is the rate-controlling step.²⁷

Fig. 3 Adsorption kinetics of [Bmim]Cl (■) and [Bmim][NTf₂] (●) on CSCM (conditions: 30 °C, pH: 6.0, initial ILs concentration: 0.57 mmol L⁻¹, adsorbent dosage: 2.5 g L⁻¹). The solid and dashed lines are the simulated adsorption kinetics using pseudo-first-order model and pseudo-second-order model, respectively.

Table 2 Kinetic parameters for the pseudo-first-order and pseudo-second-order models for the adsorption of [Bmim]Cl and [Bmim][NTf₂] onto the prepared CSCM adsorbent

ILs	Pseudo-first-order model qt = qe(1 − e^−k1t/2.303)			Pseudo-second-order model t/qt = 1/k2q^2e + t/qe
	qe (mmol g^−1)	k1 (min^−1)	R^2	qe (mmol g^−1)	k2 (g mmol^−1 min^−1)	R^2
[Bmim]Cl	0.093	5.14	0.9999	0.093	4984	0.9999
[Bmim][NTf2]	0.258	4.82	0.9999	0.258	1928	0.9999

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3.3. Adsorption isotherms and thermodynamic analysis

To better understand the adsorption behavior of the ILs onto the CSCM adsorbent from aqueous solution, Freundlich and Langmuir models as given below, were used to simulate the adsorption experimental data of the ILs.

Langmuir model:

q_e = bq_mC_e/(1 + bC_e) (3)

Freundlich model:

q_e = KC^1/n_e (4)

where q_e is the equilibrium adsorption amount (mmol g⁻¹) of ionic liquid on the CSCM adsorbent, C_e is the equilibrium concentration (mmol L⁻¹) of ionic liquid in aqueous solution, b is the Langmuir adsorption equilibrium constant (L mmol⁻¹), q_m represents the maximum adsorption capacity of ionic liquid on the CSCM adsorbent (mmol g⁻¹), K is a constant that represents adsorption capacity (L mmol⁻¹), and n is an exponent depicting the sorption intensity and n⁻¹ = 1 indicates linear isotherm.

The adsorption isotherms of the ionic liquids [Bmim]Cl and [Bmim][NTf₂] on the CSCM adsorbent at different temperatures were investigated (Fig. 4). It can be seen that for the hydrophilic ionic liquid [Bmim]Cl, both of the two equations could fit the experimental data well. However, the adsorption of the hydrophobic [Bmim][NTf₂] was better described with the Langmuir model than the Freundlich model. Obviously, it cannot be concluded that which model described all sorption isotherms better according to the obtained correlation coefficients. Langmuir equation is derived from the assumption of monolayer coverage, and the better fitting results of the adsorption of [Bmim][NTf₂] on the CSCM adsorbent hint that the monolayer sorption might occurred. The Freundlich model is an empirical isotherm model being used in heterogeneous surface energy systems. All of the isotherms are nonlinear according to the value of n⁻¹ (ranging from 0.24 to 0.57) in Freundlich fitting parameters. Many causes should be responsible for nonlinearity such as sorbate–sorbate interaction. As both models can fit the adsorption data of [Bmim]Cl well, it can be concluded that sorbate–sorbate interaction, van der Waals interaction and hydrogen bonding interaction might exist during the adsorption process. The adsorption capacities of both [Bmim]Cl and [Bmim][NTf₂] on the prepared adsorbent decreased with increasing temperature.

Fig. 4 Adsorption isotherms of (a) [Bmim]Cl and (b) [Bmim][NTf₂] on the CSCM adsorbent at different temperatures. The solid lines are simulated by Freundlich model and the dashed lines are simulated by Langmuir model (conditions: dosage of adsorbent: 2.5 g L⁻¹, pH = 6, contact time: 2 h).

Thermodynamic analysis was performed according to the adsorption isotherms at different temperatures presented above, to get some important information on the spontaneity of the adsorption process and the stability of the adsorbed phase. The variations in the standard Gibbs energy (ΔG⁰), entropy (ΔS⁰) and enthalpy (ΔH⁰) in the adsorption process were calculated from the experimental data under different temperatures. The ΔG⁰ values was calculated by using the equation ΔG = −RT ln b, where R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is absolute temperature (K), and b represents the equilibrium constant calculated from the Langmuir model. Then, the ΔH⁰ and ΔS⁰ values were obtained from the slope and intercept of a plot of ΔG⁰ versus T on the basis of the equation ΔG⁰ = ΔH⁰ − TΔS⁰, as listed in Table 3.

Table 3 Standard thermodynamic values (ΔG⁰, ΔH⁰, ΔS⁰) for the adsorption of [Bmim]Cl and [Bmim][NTf₂] on CSCM at different temperatures (conditions: dosage of adsorbent: 2.5 g L⁻¹, pH 6.0, 2 h)

ILs	T (°C)	b (L mol^−1)	ΔG^0 (kJ mol^−1)	ΔH^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)	R^2
[Bmim]Cl	20	1193	−17.3	−7.06	35	0.9932
	30	1144	−17.7
	40	975	−18.0
[Bmim][NTf2]	20	8854	−22.1	−17.7	15	0.9643
	30	7153	−22.3
	40	5489	−22.4

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The calculated ΔG⁰ values for both ionic liquids were negative, implying that the adsorption of [Bmim]Cl and [Bmim][NTf₂] onto the CSCM adsorbent were spontaneous. The more negative ΔG⁰ values for [Bmim][NTf₂] with respect to [Bmim]Cl demonstrated that the adsorption of the former was more spontaneous.¹⁸ The ΔH⁰ values of the adsorption of [Bmim]Cl and [Bmim][NTf₂] were −7.06 and −17.7 kJ mol⁻¹, respectively, which means that the adsorption of both of the hydrophilic [Bmim]Cl and hydrophobic [Bmim][NTf₂] was exothermic and lower temperatures was favorable for stronger adsorption. In addition, the values of enthalpy of [Bmim][NTf₂] was more negative than that of [Bmim]Cl, indicating stronger interactions of the [Bmim][NTf₂] molecule with the CSCM surface, possibly with unsaturated bond electrons (π–π interactions).¹⁸ The positive ΔS⁰ values for [Bmim]Cl (35 J mol⁻¹ K⁻¹) and [Bmim][NTf₂] (15 J mol⁻¹ K⁻¹) indicate an increase in the disorder at the CSCM-solution interface during adsorption process, which may result from the desorption of water molecules or OH⁻ from the positively charged carbon surface while the ionic liquids became adsorbed in the interlayer space.^11,21 Specifically, the absolute value of ΔH⁰ of [Bmim]Cl (−7.06 kJ mol⁻¹) was lower than the value of [Bmim][NTf₂] (−17.7 kJ mol⁻¹), implying the stronger interaction between the [Bmim][NTf₂] species and the surface of the CSCM adsorbent.

3.4. Effect of the concentration of ionic liquids on the adsorption

The effect of initial concentration of ionic liquids on the removal efficiency was examined, and the results are illustrated in Fig. S1.† It shows that the effect of initial concentration is quite different for the two ionic liquids. For the hydrophilic [Bmim]Cl, the removal decreased sharply with the increase of the initial concentration, and only 32% of removal efficiency was retained for an initial ionic liquid concentration of 3.9 mmol L⁻¹. On the other hand, the removal efficiency of hydrophobic [Bmim][NTf₂] ionic liquids could be remained at 94.7% when the initial concentration was 4.8 mmol L⁻¹, and 85% was remained even at the initial concentration of 6 mmol L⁻¹. Therefore, the prepared CSCM adsorbent is a super adsorbent for the adsorption of [Bmim][NTf₂] at a wide range of initial concentrations up to 6 mmol L⁻¹.

3.5. Effect of pH on the adsorption of the ionic liquids onto the CSCM adsorbent

Generally, adsorption of an adsorbate onto a solid adsorbent is determined by van der Waals, polar, electrostatic and hydrogen-bonding interactions among the solute, adsorbent and solvent, in which electrostatic interaction is strongly dependent on the solution pH. Thus, the effect of solution pH on the adsorption of ionic liquids on the prepared corn stalk-derived carbonaceous material was investigated (Fig. 5). It can be seen that the adsorption capacity is dependent on solution pH, and the adsorption capacities of the CSCM for both ionic liquids increased with increasing solution pH. The adsorption capacity of [Bmim]Cl increased from 0.455 mmol g⁻¹ to 0.525 mmol g⁻¹ as the initial solution pH was varied from 3 to 10. The adsorbent surface is positively charged when the solution pH is low, resulting in electrostatic repulsion interactions between the adsorbent and the cation of the ionic liquids and lower adsorption capacity for the ionic liquids. When the solution pH was higher, the oxygen containing functional groups such as –OH and –COOH groups on the adsorbent surface became deprotonated, the adsorption of [Bmim] cation onto the CSCM adsorbent was promoted, leading to a higher adsorption capacity for the ionic liquids due to the increased ionic interaction, chemical interaction, and electrostatic attractions.^28–30

Fig. 5 Adsorption isotherms (Freundlich fits (solid lines) and the Langmuir fits (dashed lines)) of the CSCM adsorbent for (a) [Bmim]Cl and (b) Bmim[NTf₂] at different initial pH values (conditions: dosage of adsorbent: 2.5 g L⁻¹, 30 °C, contact time: 2 h).

3.6. Effect of ILs types on the adsorption onto the CSCM adsorbent

It has been stated that the carbonaceous adsorbents performance depends on the structure and the surface character of the adsorbent and the physical and chemical nature of adsorbates.³¹ Furthermore, the adsorption behaviour of imidazolium-based ionic liquids is significantly affected by the alkyl chain size of the cation and the hydrophobicity of the anion. Thus, the adsorption of imidazolium-based ionic liquids with various chemical structures on the prepared CSCM adsorbent was studied (Fig. 6).

Fig. 6 Adsorption isotherms (Freundlich fits (solid lines) and the Langmuir fits (dashed lines)) of the CSCM adsorbent for ILs with (a) different anions and (b) different lengths of the alkyl chains (conditions: dosage of adsorbent: 2.5 g L⁻¹, 30 °C, pH 6.0).

First, the effect of anion on the adsorption of ionic liquids with 1-butyl-3-methylimidazolium cation onto the CSCM adsorbent was studied. The isotherms in Fig. 6a show that the corn stalk-derived carbonaceous adsorbent performed as a good adsorbent for the 1-butyl-3-methylimidazolium based ionic liquids with different anions, showing adsorption capacities of 0.52, 0.65, 1.12, 1.14, 2.23 mmol g⁻¹ for [Bmim]Cl, [Bmim][TFA], [Bmim][BF₄], [Bmim][OTf] and [Bmim][PF₆], respectively, which are much higher than that of these ionic liquids on the activated carbon (AC-MkU) supplied by Merck (Table 4). It seems that the anion has large influence on the adsorption of ionic liquids on the CSCM adsorbent. The adsorption capacities were observed to increase with increasing hydrophobicity of the IL anions following the sequence of Cl⁻ < [TFA]⁻ < [BF₄]⁻ < [OTf]⁻ < [PF₆]⁻. Therefore, it can be concluded that imidazolium-based ILs containing hydrophobic anions can be more effectively removed and present high affinity towards the prepared CSCM adsorbent.

Table 4 Comparison of maximum adsorption capacity (q_m) of ionic liquids onto the CSCM (q_m-CSCM) and a commercial activated carbon (CAC) (q_m-CAC)

Ionic liquids	qm-CAC^a	qm-CSCM
a Values adapted from ref. 17.
[Bmim]Cl	0.17	0.52
[Hmim]Cl	0.45	0.67
[Omim]Cl	0.65	1.17
[Bmim][TFA]	0.24	0.65
[Bmim][BF4]	0.26	1.12
[Bmim][OTf]	0.64	1.14
[Bmim][PF6]	1.21	2.23
[Bmim][NTf2]	1.07	2.41

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A diversity of inter-molecular interactions are involved in the affinity of the adsorbate toward the adsorbent surface, including polar, π–π, van der Waals and hydrogen bonding interactions. Palomar et al. used the quantum-chemical COSMO-RS model to predict the adsorption behavior of ILs in aqueous solution and observed that the van der Walls interactions are dominated in the adsorption of hydrophobic ILs.¹⁷ On the contrary, hydrogen bonding interaction plays the main role in the adsorption of hydrophilic ILs and the influence of van der Waals interactions becomes lower. Because of the low content of oxygenated groups on the surface of CSCM, hydrogen bonding interactions between the hydrophilic ionic liquids and the adsorbent surface are much weaker than that with water molecules, resulting in the unfavorable adsorption of hydrophilic ionic liquids on the adsorbent. On the other hand, the variation in the anion with remarkably different polarities resulted in different uptakes, where ILs with hydrophobic anions present much higher adsorption capacity than hydrophilic ones, due to the weaker hydrogen bonding interactions between ILs and H₂O and the stronger attractive van der Waals interactions between ILs and the CSCM adsorbent, which is in good agreement with the increasing van der Waals forces described by COSMO-RS model.¹⁷

In addition, the head group and anion type of the ionic liquids was fixed to investigate the effect of the alkyl chain length of the cation of the ILs on their adsorption onto the CSCM adsorbent (Fig. 6b). It shows that the adsorption capacities were highly dependent on the length of alkyl chain of the imidazolium cation, and the adsorption capacity obviously increased with increasing the number of the carbon atoms in the alkyl chain of the imidazolium cation. This is consistent with results presented by Lemus et al. who proposed that improved adsorption capacities for ionic liquids that have longer alkyl chain in imidazolium cation can be ascribed to the high attractive interaction energies between the ionic liquids and the adsorbent surface.²⁰ Furthermore, the improvement of the hydrophobicity of the cation of ILs with the increase of the alkyl chain length could also contribute to stronger adsorption of the ILs with longer alkyl chains in the imidazolium cation due to hydrophobic IL-water interactions.

4. Conclusion

In this work, agricultural waste corn stalk was used to prepare a carbonaceous adsorbent by the hydrothermal carbonization followed by chemical activation. The prepared carbonaceous adsorbent CSCM had a high BET surface area of 2442 m² g⁻¹, and exhibited excellent adsorption capacities for both hydrophilic and hydrophobic ionic liquids, and high adsorption capacities of 0.52 and 2.41 mmol g⁻¹ could be achieved for [Bmim]Cl and [Bmim][NTf₂]. The adsorption capacities of ILs onto CSCM increased with increasing hydrophobicity of the anions and the length of alkyl chains of imidazolium cation.

Acknowledgements

The authors gratefully appreciate the financial supports from the National Natural Science Foundation of China (No. 21577073), The Science and Technology Innovation Program and Elite Youth program of Chinese Academy of Agricultural Sciences (to Dr Xinhua Qi).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06908h

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