Mechanistic insights into Fe 3 O 4 -mediated inhibition of H 2 S gas production in sludge anaerobic digestion.

The addition of iron-based conductive materials has been extensively validated as a highly effective approach to augment methane generation from anaerobic digestion (AD) process. In this work, it was additionally discovered that Fe 3 O 4 notably suppressed the production of hazardous H 2 S gas during sludge AD. As the addition of Fe 3 O 4 increased from 0 to 20 g/L, the accumulative H 2 S yields decreased by 89.2 % while the content of element sulfur and acid volatile sulfide (AVS) respectively increased by 55.0 % and 30.4 %. Mechanism analyses showed that the added Fe 3 O 4 facilitated sludge conductive capacity, and boosted the efficiency of extracellular electron transfer, which accelerated the bioprocess of sulfide oxidation. Although Fe 3 O 4 can chemically oxidize sulfide to elemental sulfur, microbial oxidation plays a major role in reducing H 2 S accumulation. Moreover, the released iron ions reacted with soluble sulfide, which promoted the chemical equilibrium of sulfide species from H 2 S to metal sulfide. Microbial analysis showed that some SRBs (i.e., Desulfomicrobium and Defluviicoccus) and SOB (i.e., Sulfuritalea) changed into keystone taxa (i.e., connectors and module hubs) in the reactor with Fe 3 O 4 addition, showing that the functions of sulfate reduction and sulfur oxidation may play important roles in Fe 3 O 4 -present system. Fe 3 O 4 presence also increased the content of functional genes encoding sulfide quinone reductase and flavocytochrome c sulfidedehydrogenase (e.g., Sqr and Fcc) that could oxidize sulfide to sulfur. The impact of other iron-based conductive material (i.e., zero-valent iron) was also verified, and the results showed that it could also significantly reduce H 2 S production. These findings provide new insights into the effect of iron-based conductive materials on anaerobic process, especially sulfur conversion.