Unveiling synergistic enhancement mechanism of nitrogen removal in surface flow constructed wetlands: Utilizing iron scraps and elemental sulfur as integrated electron donors

Sulfur-iron interactions forming activated FexSy pool in-situ to synergistically improve nitrogen removal in denitrification system

Sulfur-iron coupling has received increasing attention for improving nitrogen removal. However, the boosting mechanisms of denitrification in sulfur-iron coupling biological system are still ambiguous, and no reasonable explanation has been given for the mismatch between the amount of S0 loss and the amount of SO42- produced in the coupling system. Therefore, this study established sulfur-iron coupling denitrification systems, and investigated the nitrogen removal performances and coupling mechanisms of the systems. The research results showed that the TN removal efficiencies of the sulfur-iron coupling systems were 122.73-149.27 % higher than those of the single electron donor systems. In the process of nitrogen removal, about 26.03-35.32 % of the more leached S0 and Fe0 in the coupling systems co-precipitated to form activated FexSy pool in-situ, contributing about 25.41 % of the nitrogen removal and allowing the systems to remove 76.32-100 % of TN without external electron donors; moreover, the oxidation process of S2- provided electrons for the reduction of Fe (Ⅲ) to Fe (Ⅱ), generating more electron donors. Metagenomic sequencing results showed significant increases in the richness and diversity of functional microorganisms associated with sulfur and iron autotrophic denitrification in the coupling systems, and their contributions to the key genes in the denitrification, sulfur transformation and iron cycle processes increased substantially. In general, this study offered deeper understanding for assessing the nitrogen removal potential of the sulfur-iron coupling system, as well as investigating the interactions between S0 and Fe0 and elucidating nitrogen removal pathways within the system.