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

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.