Mechanistic insights into FeO-mediated inhibition of HS 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 FeO notably suppressed the production of hazardous HS gas during sludge AD. As the addition of FeO increased from 0 to 20 g/L, the accumulative HS 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 FeO facilitated sludge conductive capacity, and boosted the efficiency of extracellular electron transfer, which accelerated the bioprocess of sulfide oxidation. Although FeO can chemically oxidize sulfide to elemental sulfur, microbial oxidation plays a major role in reducing HS accumulation. Moreover, the released iron ions reacted with soluble sulfide, which promoted the chemical equilibrium of sulfide species from HS 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 FeO addition, showing that the functions of sulfate reduction and sulfur oxidation may play important roles in FeO-present system. FeO 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 HS production. These findings provide new insights into the effect of iron-based conductive materials on anaerobic process, especially sulfur conversion.