Charge-Enhanced Zirconium-Biochar Nanocomposites for Highly Efficient Removal of Phosphate from Water

Eutrophication of water bodies caused by excessive phosphate discharge remains a critical environmental issue worldwide. Existing biochar-based adsorbents often suffer from limited efficiency, poor selectivity, and low stability under realistic conditions. To address these challenges, we carefully designed a positively charged biochar (N-BC) to encapsulate hydrated zirconium oxide (HZO) nanoparticles. The resulting novel nanocomposite, HZO-N-BC, exhibited high adsorption capacity (up to ∼27.65 mg of P/g) and great adsorption rate (equilibrium time around 100 min) toward phosphate. Moreover, HNO-N-BC had high adsorption preference toward phosphate in copresence of high levels of competing anions and humic acid, with adsorption capacity decline of only 29.7∼67.5%. In fixed-bed column tests, when 0.1 mg P/L was set as the breakthrough point, HZO-N-BC treated approximately 2.3 times the volume of synthetic phosphate-containing wastewater of 1 mg P/L compared to D201, a commercial phosphate adsorbent. Additionally, HZO-N-BC can treat real municipal wastewater with effective treatable volume of ∼720 BV. More importantly, the performance of HZO-N-BC showed little degradation after multiple adsorption-regeneration processes, demonstrating its excellent stability. The cooperative adsorption mechanism was revealed through structural, spectroscopic, and computational analyses. The positively charged quaternary ammonium groups on the surface facilitated the pre-enrichment and diffusion of phosphate, while HZO achieved preferable phosphate capture through specific inner-sphere complexation. This study provides a robust and sustainable strategy for developing high-performance adsorbents to mitigate eutrophication and advance water purification technologies.