Rocking-chair capacitive deionization for phosphate recovery via rejection mode using ion-exchange membranes

Abstract

Rocking-chair capacitive deionization (CDI), in combination with commercially available ion-exchange membranes, was evaluated for the feasibility of recovering phosphate. Here, contrary to the typical ion-selective adsorption methods in CDI, we aim to isolate and recover phosphate via a rejection mode. We have successfully removed 80 % of chloride from a binary solution containing equimolar concentrations of chloride and monovalent phosphate, while retaining virtually 100 % of phosphate. Further studies have revealed that ≥90 % of chloride can be removed while sacrificing as little as 15–20 % of the original phosphate concentration. To optimize the electrochemical ion separation process, we have studied several different working potentials, flowrates, and cycle times in these experiments. Moreover, we have observed similar selectivity results for both carbon-based and intercalation electrodes (Prussian blue analogues, PBAs) in a direct comparison. Within the constraints of the testing parameters, both systems exhibit no pronounced differences, with PBAs marginally outperforming carbon-based electrodes in terms of total chloride removed (94 % vs. 92 %). However, during the early stages of a desalination run, the carbon-based system revealed a minor edge on the ion separation performance compared to the PBA system.