Development of high-concentration ammonium removal process via NH4+ ion exchange and Mg–P recycling using struvite

This study developed a selective leaching and crystallization process based on counter-ion exchange to recover magnesium (Mg) and phosphorus (P) from struvite and reuse them for ammonium removal. Struvite was partially leached using sulfuric acid to selectively release ammonium while retaining Mg and P for subsequent recovery. Acid leaching was conducted using 3 N H 2 SO 4 at a liquid-to-solid ratio of 2.5 mL/g, under which ammonium was released from struvite and remained in solution as ammonium sulfate. Subsequent pH adjustment to 5.0 enabled over 90% of Mg and P to precipitate as magnesium hydrogen phosphate (MgHPO 4 ·3H 2 O, MHP), with the recovered solid consisting of approximately 95.4% MHP. The recovered MHP was then reused as a Mg and P source for struvite re-crystallization in synthetic wastewater containing 1,000 mg NH 4 + –N/L, achieving more than 80% ammonium removal at P/N molar ratios above 1.4 within 30 min. In real electroplating wastewater, high Ca 2+ concentrations inhibited struvite formation due to competitive calcium phosphate precipitation, resulting in limited ammonium removal. After Ca 2+ reduction pretreatment, the NH 4 + –N removal efficiency reached 80.4%, demonstrating the applicability of the recovered MHP in real wastewater systems. Overall, the proposed process offers a practical and resource-efficient approach for treating ammonium-rich wastewater while promoting internal recycling of magnesium and phosphorus. • A process was developed to recover Mg and P from struvite for reuse. • Selective NH 4 + exchange was achieved during partial acid leaching of struvite. • Over 90% of Mg and P were recovered as magnesium hydrogen phosphate (MHP). • The recovered MHP was reused as Mg and P sources for sequential struvite formation. • Recovered MHP removed >80% NH 4 + –N within 30 min.