Interface gypsum deposition in flow-electrode CDI treating brackish water: Impacts and mechanisms

Flow-electrode capacitive deionization (FCDI) is a promising electrically driven technology for brackish water desalination, but it suffers from scaling issues in the concentrate chamber when treating brackish water with high levels of SO₄^2- and Ca²⁺. In addition, how the key components (e.g., flow electrodes, spacer and ion exchange membranes) induce scaling in the concentrate chamber remains poorly understood. Therefore, this study systematically investigated the roles of the FCDI's components playing in the scaling process. Results showed substantial pressure loss in the concentrate chamber, which increased by 108% due to the scaling. The characterization results revealed that the scale attached to the surface of the spacer and membranes was gypsum. Gypsum crystallization experiments highlighted the crucial role of the cation exchange membrane and spacer in the heterogeneous nucleation process, which significantly shortened the induction time compared to the homogeneous nucleation process. The surface properties, such as the surface energy and surface charge, were found closely related to gypsum nucleation. In summary, the results of this work pave the way for understanding the gypsum nucleation process in FCDI continuously desalinating brackish hard water, potentially aiding in scaling removal and system optimization for broader environmental applications.