Selective removal of chloride ions by bismuth electrode in capacitive deionization

Common methods in capacitive deionization (CDI) for selective removal are derived from membrane separation mechanism or size-based intercalation mechanism. In this study, based on the electrochemical activity of bismuth (Bi) material to chloride ions (Cl − ) and inertness to sulfate ions (SO 4 2− ), Bi electrode is used as anode in CDI to selectively remove Cl − ions from mixed NaCl and Na 2 SO 4 solution. Through the reaction with Bi, Cl − ions are stored in bismuth oxychloride (BiOCl). Results show that Bi electrode exhibits selectivity for Cl − over SO 4 2− with a selectivity coefficient of more than 1.5 in solutions with mole ratio of Cl − to SO 4 2− larger than 1. The selectivity coefficient is greatly dependent on the mole ratio of Cl − to SO 4 2− and increases when the ratio rises. In solution with a fixed Cl − /SO 4 2− mole ratio, higher applied voltage and prolonged time result in higher Cl − ion removal capacity, but not necessarily better selectivity. The highest selectivity coefficient of 4.5 is achieved in 8.5 mM NaCl and 1.1 mM Na 2 SO 4 mixed solution at voltage of 1.6 and 2.0 V after charging for 1 h. These results demonstrate that Bi electrode can selectively remove Cl − ions from mixed solution with a relatively lower concentration of SO 4 2− ions, which provides new insights into ion separation and selective removal by CDI.