Selective Ammonium Removal from Synthetic Wastewater by Flow-Electrode Capacitive Deionization Using a Novel K2Ti2O5-Activated Carbon Mixture Electrode.

Ammonium (NH4+) in wastewater is both a major pollutant and a valuable resource. Flow-electrode capacitive deionization (FCDI) is a promising technology for chemical-free and environmentally friendly NH4+ removal and recovery from wastewater. However, the coexisting sodium (Na+) in wastewater, with a similar hydrated radius to NH4+, competes for the adsorption sites, resulting in low NH4+ removal efficiency. Here, potassium dititanate (K2Ti2O5 or KTO) particles prepared by the electrospray method followed by calcination were mixed with activated carbon (AC) powder to form a novel KTO-AC flow-electrode for selective NH4+ removal over Na+. The mixed KTO-AC electrode exhibits a much higher specific gravimetric capacitance in NH4Cl solution than in NaCl solution. Compared with the pure AC electrode in the FCDI tests on NH4+ removal from synthetic wastewater, 25 wt % KTO addition in the electrode mixture increases the adsorption selectivity from 2.3 to 31 toward NH4+ over Na+, improves the NH4+ removal from 28.5% to 64.8% and increases the NH4+ desorption efficiency from 35.6% to over 80%, achieving selective NH4+ recovery and effective electrode regeneration. Based on DFT calculations, NH4+ adsorption on the K2Ti2O5 (0 0 1) surface is more thermodynamically favorable than that of Na+, which contributes to the high NH4+ adsorption selectivity observed.