Scaling behavior of Si in capacitive deionization (CDI) systems for brackish groundwater desalination

Abstract

Capacitive deionization (CDI) is a potentially cost-effective method of desalinating brackish groundwaters though the technology has not yet been widely utilised for this particular application. Silicon (Si) is a common element in groundwaters which may cause problems due to its tendency to induce severe fouling of equipment used for water treatment. In this study we focus on the impacts of the presence of Si on the performance of CDI. A systematic evaluation procedure is proposed, which is used to distinguish the "dynamic" and residual Si scaling and the impacts of these types of fouling on desalination performance. Results show that Si has minimal influence on CDI performance within low concentration ranges (<0.5 mM). Sorption of deprotonated silicic acid species is largely avoided in CDI, which contributes to sustaining the removal of major ions and stable operation of the system. However, at high concentrations of Si (>2.0 mM), scaling was observed to form on the electrode surface, leading to noticeable deterioration in CDI electrode performance. The presence of ion exchange membrane in CDI (i.e., membrane CDI or MCDI) can significantly alleviate Si scaling of the desalination system. Results of this study clearly demonstrate the scaling behavior of CDI and its derivatives when dealing with Si-laden water matrices, providing insights into the design and maintenance of CDI systems in practical applications.