Orchestrating the Reduction-Capture-Precipitation Cascade: A Mg–nZVI Functionalized Platform for Simultaneous Deep Nitrogen and Phosphorus Removal

To achieve simultaneous deep removal of total nitrogen (TN) and total phosphorus (TP) in constructed wetlands (CWs), a novel composite (Mg–nZVI@ZASC) was engineered by modifying a zeolite–alum sludge-based composite (ZASC) with nanoscale zerovalent iron (nZVI) and magnesium (Mg). This study evaluates the synergistic “reduction–capture–precipitation” pathway within the Mg–nZVI@ZASC substrate CW system. Results show that nZVI serves as an electron donor to chemically reduce nitrate (NO3––N) into ammonium (NH4+–N), which is adsorbed in situ by the ZASC substrate and subsequently coprecipitates with phosphate (PO43––P) as struvite upon activation by the Mg coating. Compared to bare ZASC, the Mg–nZVI@ZASC averagely increased removal rates of NO3––N, NH4+–N, TN, TP and chemical oxygen demand (COD) by 2.90%, 22.18%, 16.32%, 14.02% and 8.67%, respectively. Layer-wise analysis identified the Mg–nZVI@ZASC layer as the primary reactive zone, dominating the removal of NH4+–N (42.03%), TP (45.72%), and COD (35.56%). Although the metallic layer selectively suppressed microbial diversity, the enhanced abiotic chemical synergy effectively compensated for biological limitations. This study provides a robust, sustainable strategy for intensifying simultaneous nitrogen and phosphorus removal in CWs through multifunctional substrate engineering.