Hydroxyl-regulated ZnFe2O4/UV/PDS synergy for green conversion of NH4+-N: Insights from experiment and theory

ZnFe₂O₄ catalysts synthesized via hydrothermal (ZFO-H) and sol-gel (ZFO-S) methods were employed to activate peroxodisulfate (PDS) under UV light for the efficient conversion of low-concentration NH₄⁺-N into environmentally benign N₂, offering a promising solution for advanced wastewater treatment. Characterization revealed that the sol-gel-derived ZFO-S exhibited uniform agglomeration, smooth surfaces, and an 80% reduction in surface hydroxyl density (as indicated by the FTIR peak area at 3320 cm^-1), outperforming both hydrothermally prepared and commercial ZnFe₂O₄ (ZFO-H and ZFO-M, respectively). This study addresses the critical challenge of ammonia nitrogen removal in wastewater, where conventional methods often struggle with low efficiency or secondary pollution. Mechanistic studies demonstrated that ZnFe₂O₄ facilitated electron transfer at Zn sites to activate PDS, generating reactive species dominated by singlet oxygen (¹O₂), which drove NH₄⁺-N oxidation. Density functional theory (DFT) calculations further elucidated the reaction pathway, highlighting preferential adsorption of NH₃ at oxygen sites on the ZnFe₂O₄ (311) surface, followed by sequential dehydrogenation and coupling to form N₂. Under optimal conditions (pH >9.25, 10 mM PDS, 25°C, 3 h), 97.40% of initial 50 mg/L NH₄⁺-N was converted to N₂, with negligible NO₃⁻ byproduct formation (< 6%), demonstrating high feasibility for practical wastewater remediation. The reduced surface hydroxyl groups and lower work function of ZFO-S enhanced electron transfer, underscoring its superior catalytic activity. This work provides a novel strategy for designing hydroxyl-regulated photocatalysts tailored for wastewater treatment and advances the mechanistic understanding of persulfate-based ammonia nitrogen remediation in engineered water systems.