Recovery of ionic liquids from dilute aqueous solutions by electrodialysis

doi:10.1016/j.desal.2011.10.003

Desalination

Volume 285, 31 January 2012, Pages 205-212

https://doi.org/10.1016/j.desal.2011.10.003 Get rights and content

Abstract

Ionic liquid/s (IL/ILs) are ideal solvents for many separation processes, such as cellulose dissolution, extraction of heavy metal ions and coal liquefaction residues, etc. During the above processes, ILs would inevitably remain in effluents. Due to their high costs and potential detrimental impacts on environment, it is extremely important to recycle ILs from dilute aqueous solutions. Electrodialysis (ED) was used for primary separation and recycling ILs. In order to evaluate the performance of ED process, the effects of initial concentration, applied voltage and initial volume of the dilute solutions on the overall current efficiency (η), recovery ratio (R) and concentration ratio (ω) were investigated. Among these experimental results, the highest recovery ratio could reach 85.2% and the highest overall current efficiency could reach 80.9%. The ED energy consumption is also discussed and the highest specific energy consumption (R_e) could reach about 1350 g/kW h. The effects of IL cation and anion sizes on concentrating process are also evaluated. The experimental results indicate that ED is an effective method to concentrate IL aqueous solutions.

Highlights

► Electrodialysis was used to concentrate ionic liquid aqueous solutions. ► Increasing initial concentration will decrease R, η, ω and R_e. ► Increasing voltage will increase R, η, ω and R_e in a certain range. ► ω increases almost linearly as initial volume of dilute solution increases. ► Cation and anion sizes of ILs may have some influences on ion transport.

Introduction

ILs, as potential room temperature melted salts, are essentially composed of ions. At present, interesting air-stable and water-stable ILs are increasingly employed to replace organic solvent in a variety of chemical processes. As a novel category of solvents, ILs show unique physicochemical properties, such as good non-volatility and non-flammability, high electric conductivity, wide electrochemical windows, excellent catalytic activities, good phase separation performance, etc. [1]. By virtue of these peculiar properties, ILs are widely used in a number of fields, such as catalysis [2], electrochemistry [3], as well as cellulose dissolution [4], gas sorption and other separation processes.

For instance, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) is a highly efficient direct solvent for the dissolution and regeneration of cellulose. The regenerated cellulose could be precipitated from the IL solutions by the addition of water [5], [6]. Thus, in this separation processes, large volumes of dilute aqueous solutions of ILs are produced. The disposal of aqueous ILs solutions may cause severe environment issues due to their degradation and toxicity. The high values of ILs also call for efficient recycling of them from these aqueous solutions [7].

Yuefeng Deng and co-workers reported that [Amim]Cl could be recycled from ILs aqueous solutions using the Aqueous Biphasic Systems (ABS) [8]. But, in the ABS separation process, salts (such as K₂HPO₄, K₂CO₃) may remain in concentrated solutions as impurities; the separation process also lacks efficiency. In this case this work adopts ED with high separation efficiency to separate ILs aqueous solutions. In addition ED barely brings impurities into concentrated solutions.

Compared with simple distillation method, ED can reduce energy consumption [9], for it is a membrane separation process based on ionic selective migration through ion-exchange membranes with the aid of an electrical driving force.

ED is widely applied in effluent treatment, brackish water desalination, lactic acid concentration and some other separation processes [[10], [11], [12], [13], [14]]. However, the performance of ED process in treating IL aqueous solutions has not been investigated to the best of our knowledge. The paper presents experiments we conducted to investigate the performance of an ED process in concentrating IL aqueous solutions. [Bmim]Cl is used as a model IL. The effects of initial concentration, applied voltage and initial volume on the concentrating process were investigated. We also evaluated effects of IL cation and anion sizes on the concentrating process.

Section snippets

ED equipment and reagents

ED equipment was produced and modified by the Zhejiang Qianqiu Group Co., Ltd. As is shown in Fig. 1, the ED is equipped with a membrane stack, a circulation system and a DC power supply unit. The membrane stack has 20 pairs of anion-exchange membranes (AEM) and cation-exchange membranes (CEM) with an effective area of 200 × 120 mm. The AEM and CEM were provided by the Zhejiang Qianqiu Group Co., Ltd too. The DFG-210 AEM and PEG-001 CEM are both homogeneous ion-exchange membranes; the

Limiting current density

In ED process, polarization appears when current density approaches to the maximum value defined as the limiting current density (LCD). Polarization is a phenomenon of loading current through H⁺ ions and OH⁻ ions, which are dissociated from water molecules between the ion-exchange membrane and the boundary layer. Polarization usually causes sedimentation and contamination on the membrane surface of diluate compartment. The polarization has some effects on the overall current efficiency,

Conclusion

In this research, ED was used to concentrate the IL aqueous solutions. In order to evaluate the performance of ED, the effects of initial concentration, applied voltage and initial volume of dilute solutions were investigated. The experiment results show that ED is an effective and significant method to concentrate ILs aqueous solutions.

With an increase of initial concentration, the overall current efficiency η and the recovery ratio R decrease slightly; the specific energy consumption R_e

Acknowledgments

This work is financially supported by National Basic Research Program of China (973 Program, No. 2009CB219901), Key Program of National Natural Science Foundation of China (No. 21036007) and Knowledge Innovation Program of the Chinese Academy of Sciences (No. KGCX2-YW-321). The authors also gratefully acknowledge the support from National Institute of Clean and Low Carbon Energy (NICE), Beijing, China, and the National Natural Science Foundation of China Project (No. 21076113).

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Recovery of ionic liquids from dilute aqueous solutions by electrodialysis

Abstract

Highlights

Introduction

Section snippets

ED equipment and reagents

Limiting current density

Conclusion

Acknowledgments

Solid State Ionics

Catal. Today

Anal. Chim. Acta

J. Biotechnol.

Carbohydr. Polym.

Trends Anal. Chem.

Sep. Purif. Technol.

Desalination