Electrochem. Energy Technol. 2016; 2:17–23 Review Open Access Mahendra S. Gaikwad* and Chandrajit Balomajumder Current Progress of Capacitive Deionization for Removal of Pollutant Ions DOI 10.1515/eetech-2016-0004 nology was used for desalination. CDI has better perfor- Received Apr 09, 2016; accepted Jun 29, 2016 mance in removal of divalent ions [3] and in purification of insulin [4]. Microbial fuel cells act as a power source in Abstract: A mini review of a recently developing water CDI for feeds of low salinity [5, 6]. Recently, researchers are purification technology capacitive deionization (CDI) ap- much more interested in CDI technique for removal of toxic plied for removal of pollutant ions is provided. The cur- ions. In this minireview, we present a general overview rent progress of CDI for removal of different pollutant ions about removal of pollutant ions by capacitive deioniza- such as arsenic, fluoride, boron, phosphate, lithium, cop- tion. per, cadmium, ferric, and nitrate ions is presented. This paper aims at motivating new research opportunities in ca- pacitive deionization technology for removal of pollutant ions from polluted water. 2 Capacitive deionization and Keywords: Capacitive deionization; membrane capacitive membrane capacitive deionization; pollutant ions deionization (MCDI) CDI works on the principle of electrostatic adsorption us- 1 Introduction ing porous electrodes. In a CDI transport of ion is due to electrostatic adsorption, not due to oxidation and reduc- Global availability of fresh water has decreased due to pop- tion reactions [7]. CDI has benefits of being eco-friendly, ulation growth, global warming and huge increases in pol- having less energy consumption and working costs than lution, such as a widespread release of various organic and other desalination technologies, simplicity in regenera- inorganic pollutants into the environment [1]. In such con- tion and maintenance compared with other conventional ditions developing or applying new technologies for de- techniques of desalination [8, 9]. In capacitive deioniza- livering pure water to society has been a growing chal- tion techniques one or several pairs of oppositely charge lenge. Nowadays, worldwide demand of fresh water has electrodes are used as a capacitor for deionization of water. rapidly increased. Based on current developments in re- Electrodes used in CDI have a positive and negative pole search activity various techniques are available for treat- under the application of external voltage. The salt solution ment of polluted water such as sedimentation, membrane contains positively and negatively charged ions, namely separation, adsorption, coagulation–flocculation, precip- cations and anions, respectively. These ions are captured itation, chemical reaction, flotation, ion exchange pro- by oppositely charged electrodes due to electrostatic force cesses and biological processes with varying levels of suc- under application of electric potential. Purification and re- cess [2]. Globally, during the last few years, researchers are generation are the two important operating steps used in much attracted to the newly developing technique capac- CDI technology. In purification mode ions are captured or itive deionization. Generally capacitive deionization tech- deposited on oppositely charged electrode under applica- tion of potential difference and pure water stream comes out from the CDI cell. When the electrodes getting satu- rated due to sorption of ions there is a necessity of des- *Corresponding Author: Mahendra S. Gaikwad: Depart- orption of ions from that electrode. In such conditions ei- ment of Chemical Engineering, Indian Institute of Technology ther reversed potential difference is applied or zero volt- Roorkee, Roorkee-247667, Uttarakhand, India; Email: mahen- [email protected] age is applied to the electrodes for removal of ions from Chandrajit Balomajumder: Department of Chemical Engineering, electrodes and the outlet of CDI cell contains a highly con- Indian Institute of Technology Roorkee, Roorkee-247667, Uttarak- centrated solution. Schematics of purification mode and hand, India © 2016 M. S. Gaikwad and C. Balomajumder, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Unauthenticated Download Date | 1/9/17 6:02 PM 18 Ë M. S. Gaikwad and C. Balomajumder (a) (b) (c) Figure 1: (a) CDI purification mode, (b) CDI regeneration mode, (c) membrane capacitive deionization. regeneration mode are presented in Fig. 1. Recently, ion ex- 3 Electrode materials in CDI change membranes are used in CDI systems that are called membrane capacitive deionization (MCDI) systems. Work- Removal efficiency has been enhanced by application of ing mechanism is shown in Fig. 1(c). In MCDI ion exchange novel electrodes synthesized from single materials and membranes are placed in front of porous electrode oper- composites [12]. Electrodes used in CDI have a high surface ated by applying a potential difference [10, 11]. Cation and area and wettability, better electrical conductivity, and anion exchange membranes are placed in front of nega- suitable pore arrangement for removal of ions in sea water tive and positively charged electrodes, respectively, and have been reported [13, 14]. Worldwide carbon materials are separated by inserting a non-conductive spacer in be- are attractive for preparation of electrodes due their prop- tween them to avoid short circuit. Ion exchange membrane erties [15]. Normally activated carbon [16], mesoporous role is to keep away counter ions. Finally, pure water come carbon [15] and carbon aerogels [17, 36] are used as car- out at the outlet of MCDI cell. In MCDI co-ion effect, i.e. bon materials for electrode preparation. Recently applica- coadsorption of ions with opposite charge because of e.g. tion of nanomaterial-based electrode has yielded better re- specific interactions between ions, is restricted. sponse in desalination [18, 19]. Mostly graphene as high performance electrode material is suggested for capaci- tive deionization [20]. Merging metal oxides with nanos- Unauthenticated Download Date | 1/9/17 6:02 PM Progress of Capacitive Deionization in Removal of the pollutants Ë 19 tructured graphene improves capacitance [21–23]. Also rameters was reported by Huang et al. [30]. Maximum re- nanostructured electrodes [24] and polymer-coated elec- moval (at flow rate 4.8 L/min and 50 mg P/L initial con- trodes [25] were used for capacitive deionization. centration) of phosphate was found to be 86.5%. The re- ported energy consumption was 7.01 kWh/kg P (removed) (or 1.65 kWh/m3) [30]. Ryu et al. suggested a novel sys- 4 Current status of CDI in pollutant tem for recovery of lithium with the help of electrostatic field assistance (EFA) to improve the conventional adsorp- ion removal tion process. They prepared a lithium-selective electrode with a lithium selective adsorbent and the hydrophilic PVA The current state of CDI for removal of pollutant ions (ar- binder. A test set was fabricated by modifying the con- senic, fluoride, boron, phosphate, lithium, copper, cad- ventional MCDI technique. The adsorption performance mium, ferric ion, and nitrate) is comparatively and system- for lithium by EFA was superior to that found with plain atically collected in Table 1. physisorption in the range of the initially tested lithium concentrations. The suggested system also showed good reproducibility and durability during repeated adsorp- 5 Discussion of pollutant ion tion/desorption cycles [31]. A study of removal of Cd(II) ions from aqueous solution with a newly prepared man- removal by CDI ganese oxide/active carbon fiber (MO/ACF) electrode was reported by Chen et al. [32]. Results prove that the elec- A graphene nano-flake (GNFs) electrode was fabricated for trosorptive capacity of MO/ACF electrode was six times capacitive deionization because of better specific surface greater as compared to ACF. It has a greater adsorptive ca- area and good electronic conductivity and such electrode pacity compared to pure ACF because of more adsorptive was employed for ferric chloride electrosorption. An elec- sites and higher capacitance of MO/ACF [32]. Hu et al. re- trosorption capacity of 0.5 mg/g for an initial concentra- ported preparation of a manganese dioxide/carbon fiber tion of FeCl3 was reported. Electrosorption experiments (MnO2/CF) electrode by anodic eletrodeposition and ap- were carried out for different ions and GNFs electrode re- plied it for electrosoption of Cu2+ ions [33]. Uniform distri- 3+ 2+ 2+ + sults were found in the order Fe > Ca > Mg > Na bution of MnO2 nanoflowers with several “petals” was ob- [26]. Removal of boron present as boric acid form in water served on the surface of the carbon fibers. MnO2/CF elec- by CDI with activated carbon electrodes was reported by trode showed maximum adsorption capacity (172.88 mg/g) Avraham et al. [27]. It was removed in two steps. In the first which was two times that of simple MnO2 absorbent with- step dissociation of boric acid proceeded done on the neg- out an electric field imposed [33]. Work on removal of fluo- ative electrode due to local basic pH caused by polarizing ride and nitrate removal by batch-mode CDI from brack- the cell. Then electrosorption of borate ions was done at ish groundwaters was examined by Tang et al. [34]. Re- the positive electrode [27]. Zhang and coworkers reported moval experiments were carried out under different condi- that solar-powered capacitive deionization could be used tions such as feed concentration, flow rate and copresence for removal of arsenic from water. A Box–Behnken statis- of sodium chloride. At constant concentration of sodium tical experiment design was used to examine the effects of chloride, fluoride concentration was found to decline in major process parameters. Experimental values and pre- the euent with declining initial fluoride concentration. dicted values of arsenic removal were found good agree- At higher initial concentration of chloride a higher equi- ment. Removal of arsenate ions by CDI favors lower salin- librium dissolved concentration of fluoride was found be- ity conditions and high pH [28].