Synergistic mechanisms of iron-carbon micro-electrolysis in promoting arsenic reduction and remediation in aquatic sediment

Combination of biochar and iron oxide enhanced As(III) oxidation in aquatic sediment. • Increased fulvic and humic acid DOM favored for iron cycle and bioavailable carbon source. • Iron and biochar amendment improved sucrase, urease and catalase activity. • Iron-biochar enriched enzymes abundance that related to As cycling, glycolysis and TCA. Arsenic (As) is ubiquitous in the aquatic environment, among which arsenite (As(III)) possessing extreme toxicity and environmental risk. Adding biochar is an extensive strategy for As remediation in sediment via immobilization, while the redox transformation of As is often ignored. Considering the effect of iron‑carbon micro-electrolysis on interspecies electron transfer, the present study intends to explore the role of biochar and iron oxide in the behavior of As in aquatic sediments, and elucidate the underlying mechanisms among iron‑carbon-As. Results reflected that the synergistic effect of biochar and magnetite reduced 40.7% of As(III) contents in sediment, related to the As(III) transformation rate of 27.4%, which were 14.6%, 26.6%, 29.7% higher than that in magnetite, biochar, and control groups, respectively. Correspondingly, the co-existence of biochar improved the generation of Fe(II) by 15.6%, indicating the increased inorganic electrons and iron dependent oxidation of As(III). It was attributed to the degradation of soluble organic carbon and increased proportions of fulvic acid-like and humic acid-like materials, which could also provide carbon source for microbial As metabolism. The iron and biochar up-regulated the abundance of catalase, and microbes that related to As metabolism like Bacteroidota , Acidobacteriota , and Chloroflexota , which reduced the bioavailability of As and alleviated their toxic pressure to sediment. They also increased the expression of arsR and arsC that related to As remediation, and enzymes in glycolysis (like K00134, K00927) and TCA (like K00024, K00031), which contributed to higher metabolic efficiency. This study provided comparative findings on the synergistic mechanisms of biochar and magnetite in enhancing As(III) oxidation, and provided an effective strategy for sediment remediation.