Membrane capacitive deionization (MCDI) is an effective way for selective removal of divalent cations (e.g. Ca2+) over monovalent cations (e.g. Na+) and can thus be used for water softening. However, the dependence of selectivity on operating conditions and feed solution composition has not been systematically investigated and is the focus of the current study. In this study, we used both experiments and numerical simulations to investigate selective removal of Ca2+over Na+in MCDI. Specifically, the impacts of current density, hydraulic retention time, and feed composition on the selectivity of Ca2+over Na+were investigated. Our study reveals that (1) regardless of operating conditions and feed composition, the ratio between the Ca2+and Na+ molar fluxes is always proportional to the ratio between the Ca2+and Na+ concentrations in the spacer channel; and (2) there exists a universal tradeoff between selectivity and the ion removal rate.

Water softening using membrane capacitive deionization: What affects selectivity?

2019 Spring National Meeting

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