Impact of ZVI on microbial selenite removal from long-term low pH conditions: Process performance and stability

Abstract

The low pH and carbon-deficient characteristics of selenite-containing mine wastewater inhibit microbial activity and reduce selenite removal rate. This study investigated the effect of zero-valent iron (ZVI) on microbial selenite removal under long-term acidic influent conditions (pH 4) using an anaerobic sequencing batch reactor operated over 473 days. ZVI enhanced selenite removal efficiency from 86.0 to 96.1% in the control to 95.9–97.8%, and increased the first-order kinetic rate constant by 0.95 to 6.32 times. ZVI promoted the reduction of selenite to elemental selenium and further to selenide. Enzyme activity assays showed that ZVI elevated the activities of sulfite reductase, glutathione reductase, fumarate reductase, and nitrite reductase by factors of 0.71–1.56, 0.57–1.71, 1.08–1.31, and 4.20–4.80, respectively. Microbial community analysis revealed that ZVI enriched unclassified SBR1031 and Rhodocyclaceae. This suggests that the increased abundance of these genera facilitates selenite reduction and enhances electron transfer through the cycling of ferric and ferrous iron. Although ZVI partially mitigated acidification, external buffering was still required to maintain long-term pH stability. Therefore, ZVI promotes microbial metabolic performance and enhances electron transfer, thus promoting the microbial reduction of selenite. These findings provide theoretical support for the biological treatment of acidic selenite-contaminated wastewater using ZVI-assisted systems.