Stop-flow discharge operation aggravates spacer scaling in CDI treating brackish hard water

Brackish water desalination has become a major opportunity to fulfill the rising clean water needs globally. Capacitive deionization (CDI) is an attractive electrochemical separation technology to treat water salinity, and the innovative discharge operation, termed stop-flow (SF) discharge, has demonstrated high efficiency to minimize the brine volume. This work systematically compared the desalination performance of CDI when single-pass and SF discharge operation modes were respectively used. Results showed that while the SF discharge operation could achieve remarkably higher water recovery (>80 %), this inevitably resulted in the deterioration of CDI performance when the feed contained higher concentrations (5 mM) of Caz} and Sat. Compared to the electrodes, the spacer and the desalination chamber could be more vulnerable to scaling in brackish water desalination. With the formation of glauberite and gypsum in SF discharge operation, the carbon materials from the electrodes constantly existed on the scaled spacer, which contributed to the blocking of the flow path and a significant increase in pressure loss. This study provides insight into the scaling mechanism for SF discharge in multi-cycle operation, which is crucial for the design of CDI materials and optimization of operating protocols for brackish water desalination.