Optimization of sulfate removal from brackish water by membrane capacitive deionization (MCDI).

Removal of sulfate from water is an environmental challenge faced by many industrial sectors as most existing options are inefficient, costly or unsustainable. The situation is further complicated by the typical coexistence of other ions. In this work, the feasibility of sulfate removal from brackish water by single-pass constant-current membrane capacitive deionization (MCDI) under reverse-current desorption was investigated. Results revealed that sulfate is preferentially removed from the aqueous solution by MCDI compared to chloride. Equivalent circuits of the MCDI system during adsorption and desorption were proposed and the dynamic variation of cell voltage and charging voltage at different adsorption currents was satisfactorily elucidated. Optimization studies were conducted with attention given to discussing the effects of four operating parameters, i.e., adsorption current, pump flow rate, ending cell voltage and desorption current, on three performance indicators (i.e., water recovery, energy consumption and sorption ratio of sulfate to chloride) on the premise of maintaining the effluent sulfate concentration below the specified threshold of 300 mg L-1. Water recovery-energy consumption mapping and sorption ratio of sulfate to chloride-energy consumption mapping indicated that the combination of a lower adsorption current and a lower matching pump flow rate which reduced the effluent sulfate concentration to 300 mg L-1 was more favorable in practical applications. An appropriately small ending cell voltage was advantageous while a trade-off between water recovery and energy cost was required in optimizing the desorption current.