Effective Removal of Hexavalent Chromium from Aqueous Solutions Using Ionic Liquid Modified Graphene Oxide Sorbent

Ionic liquid modified reduced graphene oxide (IL-rGO) was prepared and examined for chromate removal. The sorbent was characterized by N2 adsorption-desorption measurement (BET), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis. The sorption behavior of chromate on the ionic liquid modified reduced graphene oxide sorbent from an aqueous medium was studied by varying the parameters such as contact time, initial chromate concentration, pH, and agitation speed. The results showed that sorption kinetics of chromate by IL-rGO follows the pseudo second order, which indicates that the sorption mechanism is both chemical and physical interaction. The sorption isotherm studies revealed that Langmuir model provided the best fit to all the experimental data with an adsorption capacity of 232.55 mg g–1 for IL-rGO. Thermodynamic parameters, such as Gibbs free energy (–2.85 kJ mol–1 at 298 K), enthalpy (55.41 kJ mol–1), and entropy (11.64 J mol–1 K–1) of sorption of the chromate on ionic liquid modified reduced graphene oxide was evaluated, and it was found that the reaction was spontaneous and endothermic in nature.


Introduction
Chromium is one of the heavy metals present in toxic effluents released by the aerospace, electroplating, leather, mining, dyeing, fertilizer, and photography industries 1,2 .The Cr(VI) ion generally exists in the form of extremely soluble and highly toxic chromate ions (HCrO 4 − or Cr 2 O 7 2− ), which can transfer freely to the biotic organisms prevailing in the aquatic ecosystem and food chain 3 .Therefore, the presence of chromium in the environment has received considerable attention.It has been found that chromium has a severe carcinogenic effect on human health, and causes cancer in the digestive system and lungs, and it may lead to other severe health hazards, such as dermatitis, bronchitis, perforation of the nasal septum 4,5 .Hence, the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA) have set a guideline for the maximum acceptable concentrations of 50 and 100 µg L -1 for total chromium in drinking water, respectively 6 .However, Cr(VI) concentrations in Cr(VI)-bearing wastewater and Cr(VI)-contaminated natural water resources are far above the permitted threshold level.Therefore, it is imperative to develop new materials and effective methods for Cr(VI) remediation.
Graphene oxide, a 2D carbon material, has unique physical and chemical properties which make it an ideal candidate for extended applications such as: nanoelectronics 56 , supercapacitors 57 , sensors 58 , hydrogen storage 59 , and drug delivery 60 .Graphene has become a sparkling rising star on the horizon of materials science due to its extraordinary electrical, thermal, and mechanical properties 59 .Graphene has a large theoretical specific surface area of about 2.6 •10 3 m 2 g -1 , high thermal conductivity (3 • 10 3 W m -1 K -1 ), and Young's modulus (1.06 •10 3 GPa) 57 .Recent research has indicated that graphene oxides and chemically modified graphene oxides have proved to be promising materials for the adsorption of metal ions due to their extraordinary mechanical strength and relatively large specific area 61 .
Ionic liquids (ILs) have been recently gaining widespread recognition from both research and industrial applications, such as electrochemistry, extraction, organic synthesis and catalysis for clean technology and polymerization processes [62][63][64] .This is due to their unusual properties and great potential as ''green'' solvents for industrial processes.Moreover, they are nonvolatile, nonflammable, thermally stable, and recyclable.However, the advantageous liquid state in most cases hinders the development of ILs in applications requiring solid shape.One way to circumvent this problem is loading ionic liquids on the surface of nanomaterials 63 .
In this study, reduced graphene oxide has been synthesized and modified with ionic liquid.Investigated and compared were the mechanism, isotherms, thermodynamics and kinetics of sorption of chromate by graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide.The sorption capacity and kinetics were studied using the batch method.The variables affecting the sorption process, including the agitation speed, contact time, initial concentration of chromate, solution pH, ionic strength, and temperature were optimized.

Instrumentation
The structures of the synthesized graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide were analyzed by powder X-ray diffraction (XRD), Philips 1830 diffractometer, using graphite monochromated Cu Kα radiation.X-ray photoemission spectroscopy (XPS) spectra were obtained with a Scienta ESCA 200 analyzer (Gammadata, Sweden) using monochromatized Cu Kα X-ray source.Adsorption-desorption isotherms of the synthesized samples were measured at 77 K on micromeritics model ASAP 2010 sorptometer to determine an average pore diameter.Pore-size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method, while surface area of the sample was measured by the Brunauer-Emmet-Teller (BET) method.Transmission electron microscope (TEM) analysis was carried out with a JEOL JEM 2100 transmission electron microscope at 200 kV.The Cr(VI) concentration in the water was monitored by measuring the absorbance using an UV-vis spectrophotometer (CE 2501, CECIL instruments, Cambridge, UK) with 1.0 cm glass cuvettes at 517 nm.The pH of solutions was determined with a Metrohm 780 pH meter equipped with a combined glass calomel electrode (Metrohm Co., Herisau, Switzerland).

Synthesis of IL-rGO
Ionic liquid modified reduced graphene oxide was obtained by the method as described by Wang et.al. 65 In a typical synthesis, 0.5 g of the ionic liquid ([Bmim][Cys]) was put into 10 mL GO aqueous suspension of 0.5 mg mL -1 .The mixture was kept in a tightly sealed glass bottle, and stirred for 15 h at room temperature.Firstly, the black product was isolated by centrifugation at 8000 rpm, and then the obtained black slurry was washed with deionized water.One part of the as-prepared products was then dispersed in deionized water to prepare the suspension of IL-rGO, and the other part was used to produce the powder of IL-rGO by drying at 50 °C for 24 h under vacuum.

Batch sorption procedure
The sorption equilibrium studies of graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide for chromate were conducted with 50 mL of a Cr(VI) solution (200 mg L -1 ) and 0.5 g L -1 of the sorbent at 298 K, unless otherwise stated.The sorbent and sorbate was mixed on a shaking bath at 250 rpm until equilibrium was attained (typically in 5 hours).After the sorption was complete, the mixture of sorbent and sorbate was centrifuged at 3000 rpm for 3 minutes, and the amount of Cr(VI) remaining in the solution was determined by measuring its absorbance at 517 nm with a UV-Vis spectrophotometer.The amount of Cr(VI) sorbed on the sorbent was determined as follows: (1) where q e is the amount of Cr(VI) sorbed by the sorbent (mg of Cr(VI)/g of the sorbent), C 0 and C e are the Cr(VI) concentrations (mg L -1 ) at time 0 and equilibrium, respectively, V is the volume of the solution (L), and W is the mass of the sorbent (g).Each experiment was conducted three times, and the average and standard deviation of the results were determined.
The kinetic studies for all the five sorbents were carried out with 250 mg L -1 of Cr(VI) at pH 4.0.For such studies, a series of 50 mL of the Cr(VI) solution was added to 25 mg of the sorbent.The samples were shaken at 298 K and 250 rpm.The experimental data were analyzed by the most commonly used models of pseudo first-order and pseudo second-order.

Textural characterization
The nitrogen adsorption−desorption isotherms of reduced graphene oxide and ionic liquid modified reduced graphene oxide samples are shown in Figure 1.The nitrogen adsorption-desorption isotherms of graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide show a type IV isotherm according to IUPAC classification, corresponding to mesoporous solids.The specific surface area (S BET ) and specific volume of the rGO are 2302 m 2 g -1 and 0.88 cm 3 g -1 , respectively.The specific surface area and specific volume of IL-rGO are reduced to 1454 m 2 g -1 and 0.62 cm 3 g -1 , respectively.Therefore, it may be assumed that ionic liquids are located inside the pores or near the pore opening of reduced graphene oxide.
The XRD patterns of graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide are presented in Figure 2. Comparison of XRD pattern of IL-rGO and rGO with GO, suggested that the basal spacing between the (002) planes increased from 0.35 nm to about 0.8 nm, which was signified by a new broad peak at 2θ = 11.8ºreplacing the peak at 2θ = 25.4º.This revealed that IL-rGO, rGO, and GO were well modified and had the representative peaks of IL-rGO, rGO, and GO.
The chemical state of the carbon of the GO, rGO, and IL-rGO was examined by the XPS technique (Figure 3).The C1s XPS spectrum of GO shows that, beside the band of aromatic C-C bonds at BE = 284.6 eV, there is a very broad peak ranging from 286 to 290 eV, indicating a considerable degree of oxidation and the presence of different oxygen-containing groups.However, C1s XPS spectra of IL-rGO show almost identical features with that of rGO, which demonstrate the aromatic nature of graphene-like material.The broadened peak shape of reduced graphene oxide and ionic liquid modified reduced graphene oxide indicates that these materials have a much more disordered structure than graphite.
The structure of the IL-rGO was further studied by transmission electron microscopy (TEM) (Figure 4).The TEM image proves that IL-rGO hybrid material is formed and ionic liquids are well dispersed on the surface of reduced graphene oxide.

Influence of contact time and initial concentration on Cr(VI) removal
The influence of contact time on Cr(VI) sorption by graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide sorbents were studied (Figure 5).The results showed that the sorption capacity increased with time and reached a constant value after 5 h.Thus, an optimum contact time of 5 h was selected for further studies.The effect of chromate ion initial concentration on its removal by IL-rGO is shown in Figure 6.The initial concentration of Cr(VI) significantly influenced the extent of Cr(VI) sorption, i.e., an increase from 50 to 250 mg L -1 caused a significant increase in the sorbent capacity from 91.3 to 249.4 mg g -1 , respectively.Thus, an increase in Cr(VI) initial concentration enhances the interaction between the Cr(VI) molecules and the surface of the sorbents.Therefore, the selected optimum initial concentration of Cr(VI) was 250 mg L -1 for the rest of the experimental studies.
The higher sorption capacity of IL-rGO as compared to rGO and GO can be attributed to the fact that IL-rGO can interact more effectively than the other sorbent through complexation of chromate to ionic liquid.

Influence of agitation speed
The influence of agitation speed on the removal of Cr(VI) by the IL-rGO, rGO, and GO sorbents was examined by changing the agitation speed between 0 to 300 rpm, while keeping all other experimental conditions constant, as presented in Figure 7.It was found that, with a fixed stirring time, an increase in the agitation speed up to 250 rpm caused an increase in the sorbents capacity, which leveled off at a higher speed.Thus, the agitation speed of 250 rpm was chosen as the optimum agitation speed for all the sorption experiments.

The influence of pH
The pH is a key parameter controlling the Cr(VI) sorption process.The pH dependence of Cr(VI) sorption can largely be related to the type and ionic state of the functional groups of the sorbent surface, as well as to the Cr(VI) chemistry in the solution.The influence of pH on the sorption of Cr(VI) by IL-rGO was studied by changing the pH ˉ) compete for the same sorption site on the graphene oxide, resulting in a lower removal of Cr(VI).Thus, the sorption quantities of Cr(VI) at lower pH are larger than those at higher pH.

Kinetics of sorption
The dynamics of the sorption procedure from an aqueous solution and the process of sorption of Cr(VI) on IL-rGO sorbent were considered through the fitness of the experimental data with various kinetic models.The kinetics data were fitted with pseudo first order (eq.2) and pseudo second order (eq.3) kinetic model, described as follows: (2) 1 e t e log ( ) log 2.303 k t q q q − = − (3) 2 e e 1 t t q k q q = + where q e and q t are the sorption capacities at equilibrium and at time t, respectively, and k 1 is the pseudo first order rate constant (min -1 ).The pseudo first order rate constant (k 1 ), and the pseudo second order sorption (k 2 ).The k 1 , k 2 and R 2 were calculated, and are provided in Table 1 and Figure 9.
The results in Table 1 and Figure 9 clearly indicate that the pseudo second order model with high calculated linear regression correlation coefficient (R 2 ) can best describe the kinetics of Cr(VI) sorption for IL-rGO sorbent.Thus, these results further support the assumption that the sorption is chemisorption.The sorption on IL-rGO can also be fitted to pseudo second order model due to its high correlation coefficient.Thus, in the case of IL-rGO, sorption occurs by both chemical and physical interactions.This observation further proves the higher sorption capacity of IL-rGO sorbent as compared to the other sorbents.

Sorption isotherm
Equilibrium sorption studies were performed to investigate the sorption capacities and mechanisms  where K f and n are Freundlich empirical constant, C e is the Cr(VI) equilibrium concentration in the solution (mg L -1 ), and q e is the equilibrium sorption capacity (mg g -1 ).
The Langmuir equation is presented as follows: (5) where C e is the equilibrium liquid phase concentration of Cr(VI) (mg L -1 ), q m (mg g -1 ) is the maximum sorption capacity, q e (mg g -1 ) is the equilibrium sorption capacity, and b is the Langmuir constant associated with the sorption energy.The Temkin equation is presented as follows: e T e = ln ln where A is the Temkin isotherm constant, R is the gas constant (8.314J mol -1 K -1 ), b is Temkin isotherm constant, T is the temperature (K), and K T is the equilibrium binding constant (mol -1 ) corresponding to the maximum binding energy.The coefficient of determination (R 2 ) of the linearized isotherm data of the three models at 298 K and pH of ~4.5 were calculated.The results are provided in Table 2.The comparison of the correlation coefficients values of the models suggested that the Langmuir model with a higher value of correlation was best fitted to the experimental results.Furthermore, the higher q m values for the sorption of Cr(VI) by IL-rGO (250.0 mg g -1 ) indicated that its mono layer sorption capacity was very high.

Thermodynamics of the sorption of Cr(VI) on sorbents
Thermodynamic parameters including Gibbs free energy change (ΔG 0 ), enthalpy (ΔH 0 ), and entropy (ΔS 0 ) were calculated for sorption of Cr(VI) on ionic liquid modified reduced graphene oxide sorbent by varying the temperature in the range of 298-328 K under other optimized conditions (Table 3).As it was observed, an increase in the temperature from 298 K to 328 K caused an increase in the Cr(VI) removal by all the sorbents.
The Gibbs free energy, as well as the enthalpy process were calculated from experimental results using the following equations:  where C e is the equilibrium concentration (mg L -1 ) of the Cr(VI) solution, q e is the equilibrium sorption capacity (mg g -1 ), R is the universal gas constant (8.314 •10 −3 kJ mol -1 K -1 ), T is the temperature in Kelvin, and K C is the equilibrium constant calculated as the surface and solution metal distribution ratio.The calculated thermodynamic parameters are listed in Table 3.As may be seen, the value of K C increased with an increase in the temperature from 298 to 328 K for all sorbents, favoring the sorption of analyte.The negative value of ΔG 0 and the positive value ΔH 0 at different temperatures indicated that the sorption of Cr(VI) by the sorbents was spontaneous and endothermic, respectively.The positive values of ΔS 0 suggested the increased randomness at the sorbent-solution interface during the sorption of Cr(VI) from the aqueous solution to the sorbents.

Performance evaluation
The maximum sorption capacity (q max ) of IL-rGO for Cr(VI) was calculated by the Langmuir isotherm model and compared to the other sorbents 51,[66][67][68][69] with high capacity used for this purpose (Table 4).It is clear that IL-rGO has the advantages of higher sorption capacity.

Conclusion
In this study, graphene oxide, reduced graphene oxide, and ionic liquid modified reduced graphene oxide were prepared.Their structural order and tex-tural properties were studied by N 2 adsorption-desorption measurement (BET), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analysis, and the potentials of these sorbents for removal of Cr(VI) were comparatively examined.The sorption kinetics of Cr(VI) by IL-rGO was found to fit the pseudo second order, indicating that the major sorption mechanism is physical.From an equilibrium study, it was found that the Langmuir model provided the best fit to the experimental data.Thus, the removal of Cr(VI) occurs through monolayer sorption.It was also found that, among the sorbents investigated for Cr(VI) removal, the IL-rGO had a higher capacity, which can be attributed to the higher sorption availability of the active sites.