A slow-release oxygen composite based on sulfur/CaO₂ for sustained in-situ ammonia degradation form farmland drainage

The in-situ nitrification process continuously requires a stable supply of oxygen. However, the application of conventional oxygen-releasing materials is limited by its high alkalinity and rapid oxygen release rates. In this study, a novel sulfur-based slow-release oxygen material (SOSM) was designed to address these challenges. SOSM releases oxygen through the decomposition of CaO₂ and maintains pH balance with sulfur (S⁰). An 88-day continuous flow experiment for microbial degradation of ammonia nitrogen (NH₄⁺-N) was conducted with SOSM as a carrier. The results showed that the dissolved oxygen (DO) remained above 8 mg/L during 15 d, with oxygen being released following Fickian diffusion. S⁰ is oxidized by sulfur bacteria, forming a CaSO₄ precipitate within the material, while hydrogen ions (H⁺) are generated to counteract the alkalinity caused by CaO₂. The continuous flow experiments indicated that nitrification with SOSM occurred in three distinct phases with the following NH₄⁺-N removal efficiency: the domestication phase (1-21 d, 86.2%), the stabilization phase (22-76 d, 93.4%), and the deterioration phase (77-88 d, 60.5%). The enrichment of Proteobacteria and Actinobacterota promoted NH₄⁺-N removal when oxygen was abundant, while the enrichment of Acidobacteriota facilitated active sulfur cycling. The system was dominated by nitrification and supplemented by the complete autotrophic nitrifying anaerobic ammonia oxidation (CANON) process. The SOSM developed in this study can effectively address the critical issues of in-situ agricultural drainage remediation and expand the application scope of nitrification technology. It offers a novel approach to biological nitrogen removal treatment technology, especially nitrification technology to remove NH₄⁺.