Abstract
Sulfur autotrophic denitrification (SAD) is commonly utilized for nitrate (NO(3) (-)-N) removal from groundwater because of its efficiency, minimal sludge production, cost-effectiveness, and carbon source independence. However, elevated Fe(2+) and Mn(2+) concentrations in groundwater may influence its efficiency. The purpose of this work was to explore the effects of coexisting Fe(2+) and Mn(2+) at varying 5 mM ratios on SAD efficiency and its underlying mechanisms. The results showed that adding 5 mM Fe(2+) and Mn(2+) at different ratios inhibited NO(3) (-)-N removal, reducing efficiency from 92.73% (without Fe(2+)/Mn(2+)) to 60.96% (Fe(2+): Mn(2+) = 9:1) by Day 6. All the systems with coexisting Fe(2+) and Mn(2+) accumulated NO(2) (-)-N and N(2)O. The generation of SO(4) (2-) by the system gradually diminished, the Fe(2+) removal rate gradually decreased, and the Mn(2+) removal rate gradually increased as Fe(2+) and Mn(2+) concentrations increased and decreased, respectively. The coexistence of Fe(2+) and Mn(2+) reduced pH, decreased the relative abundance of Thiobacillus, and downregulated the expression of key denitrification (nirS, norB, nosZ) and sulfur oxidation (dsrA, soxB) genes, thereby compromising the denitrification efficiency of the SAD system. The rate-limiting reactions for system denitrogenation with Fe(2+) and Mn(2+) coexistence included NO reduction and N(2)O reduction. Furthermore, the key driving factors were the nosZ/narG, nosZ/nirK, norB/nirK, dsrA/16S rRNA, soxB/nirK, and soxB/nirK gene ratios. The findings of this study provide theoretical support for employing SAD technology to remove NO(3) (-)-N from water with elevated levels of coexisting Fe(2+) and Mn(2+).