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+ .
Chen et al. (Wed,) studied this question.