Selective adsorption of phosphate by carboxyl-modified activated carbon electrodes for capacitive deionization

Capacitive deionization (CDI) has been considered as a promising technology for removing phosphate from water but suffer inferior selectivity and electrosorption performances for phosphate of current carbon electrodes in CDI. Herein, we achieved highly selective phosphate removal from a ternary effluent of Cl^-, PO₄^3-, and SO₄^2- by using nitric acid-treated activated carbon (AC) with various modification times and pure AC as the anode and cathode, a novel phosphate selective asymmetric CDI reactor. The results showed that carboxyl groups greatly grafted on the materials after modification (varying from 0.00084 to 0.0012 mol g^-1). The phosphate selectivity of the present research was higher than that of unmodified CDI, and it increased with the increase of carboxyl groups content. The highest phosphate selectivity (2.01) in modified materials is almost six times higher than that of pure AC. Moreover, the modified electrodes exhibited good regenerative ability with a phosphate desorption efficiency of around 72.12% during the adsorption/desorption process and great stability during the cycling experiment. These results demonstrated that the innovative application of nitric acid-modified AC can effectively selectively remove phosphate from mixed anion solution, opening a hopeful window to selective adsorption in water treatment by CDI.