Preparation of ion exchanger layered electrodes for advanced membrane capacitive deionization (MCDI)

Abstract

A noble electrode for capacitive deionization (CDI) was prepared by embedding ion exchanger onto the surface of a carbon electrode to practice membrane capacitive deionization (MCDI). Bromomethylated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) was sprayed on carbon cloth followed by sulfonation and amination to form cation exchange and anion exchange layers, respectively. The ion exchange layers were examined by Scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FT-IR). The SEM image showed that the woven carbon cloth was well coated and connected with BPPO. The FT-IR spectrum revealed that sulfonic and amine functional groups were attached on the cationexchange and anionexchange electrodes, respectively. The advantages of the developed carbon electrodes have been successively demonstrated in a batch and a continuous mode CDI operations without ion exchange membranes for salt removal using 100 mg/L NaCl solution.

Introduction

Recently, capacitive deionization (CDI) has been used as a water treatment technology due to the simple principle and low operating potential, without the need for chemicals (Ito et al., 2007). Therefore, it is considered as an environmentally-friendly and economical system (Foo and Hameed, 2009). A CDI system operation consists of adsorption and desorption periods for obtaining purified water and concentrated water, respectively (Lee et al., 2010). When an electric potential is applied to CDI cells, charged ions in contaminant water are adsorbed onto the surface of charged electrodes, and formed an electric double layer due to the charged electrode and adsorbed ions, producing purified water. After the adsorption of ions, the saturated electrode undergoes regeneration by desorption of the adsorbed ions under zero electrical potential or reversed electric field (Seo et al., 2010). Hereby utilization of the adsorption ability of an electrode is the key parameter for the CDI operation. In order to maintain acceptable operation efficiencies, the complete adsorption and desorption of charged ions should be accomplished within appropriate periods. Practically, however, when a potential is applied to a CDI cell, counter ions are attracted onto the electrode surface, simultaneously co-ions expelled from the counter electrode (Kim and Choi, 2010). It leads to a higher energy consumption and a lower operation efficiency due to mobility of unwanted ions.

To avoid this phenomenon, a membrane-CDI (MCDI) is employed with the help of ion selective membranes in the CDI cell. A MCDI has two types of ion exchange membranes, i.e. anion exchange and cation exchange membranes (AEM & CEM, respectively). The AEM and CEM are positioned in front of the positively and negatively charged electrodes, respectively (Lee et al., 2006). The ion exchange membrane has the ability to selectively permeate ions, i.e., a CEM permit the passage of cations only, while an AEM allow the passage of anions only. The selectivity of ion exchange membranes prevent reverse adsorption and prohibit the mobility of unwanted ions. However, a MCDI requires strong physical pressure for smooth contact between the membrane and electrode material. Also the diffusion layer on the membrane surface will become thick when the concentration of contaminants in the feed water is low (Dlugolecki et al., 2010). Accordingly, this phenomenon induces a decreased mobility of wanted ions and a high interfacial resistance of the membrane.

In this study to solve the problems of contact resistance and the diffusion layer, an advanced-MCDI (A-MCDI) was developed by adhering the ion exchanger onto the carbon electrode surface as a thin layer, which reduces the contact resistance between the ion exchanger and electrode of a MCDI. Therefore, an A-MCDI is expected to exhibit a high removal efficiency and a low current consumption compared to a conventional MCDI.