Simultaneous removal and recovery of ammonium and phosphate ions using flow electrode capacitive deionization through the struvite generation mechanism

Removing and recovering nitrogen (N) and phosphorus (P) from water is essential to prevent eutrophication and support sustainable development. Flow electrode capacitive deionization (FCDI) is a promising electrosorption technology often used to remove and recover charged ions from wastewater. This study presents an FCDI system using Mg(OH)2/BC as a flow electrode in a short-circuit closed-cycle (SCC) mode, enhancing the simultaneous removal and recovery of NH4+ and phosphate ions through struvite formation. Results show improved removal of NH4+ and phosphate ions at an optimal Mg(OH)2 amount (M- BC- 0.336) and an initial influent pH of 8. The NH4+ and phosphate ions adsorbed into the flow electrode solution can react with Mg2+ ions released from the electrode material, resulting in the formation of struvite nanowires on the surface of the electrode material. This process reduces the back-diffusion effect of NH4+ and phosphate ions, thereby enhancing the performance of FCDI in removing N and P. In actual domestic sewage treatment, the system achieves high removal efficiency for N (64.6 %) and P (87.3 %) while consuming low energy (0.46 kWh/kg for NH4+-N and 4.1 kWh/kg for PO4 3--P). Our study demonstrates an efficient method for removing and recovering N and P from wastewater while producing struvite-rich biochar.