Biological S0 reduction at neutral (pH 6.9) and acidic (pH 3.8) conditions: Performance and microbial community shifts in a H2/CO2-fed bioreactor

Sulfidogenesis is a promising technology for the selective recovery of chalcophile bulk metals (e.g. Cu, Zn, and Co) from metal-contaminated waters such as acid mine drainage (AMD) and metallurgy waste streams. The use of elemental sulfur (S0) instead of sulfate (SO42-) as electron acceptor reduces electron donor requirements four-fold, lowering process costs, and expands the range of operating conditions to more acidic pH. We previously reported autotrophic S0 reduction using an industrial mesophilic granular sludge as inoculum under thermoacidophilic conditions. Here, we examined the effect of pH on the S0 reduction performance of the same inoculum, in a continuously fed gas-lift reactor run at 30 {degrees}C under neutral (pH 6.9) and acidic (pH 3.8) conditions. Steady-state volumetric sulfide production rates (VSPR) dropped 2.3-fold upon transition to acidic pH, from 1.79 {+/-} 0.18 g{middle dot}L-1{middle dot}d-1 S2-{middle dot}to 0.71 {+/-} 0.07 g{middle dot}L-1{middle dot}d-1 S2-{middle dot} Microbial community analysis via 16S rRNA gene amplicon sequencing showed that at pH 6.9, the S0-reducing genera Sulfurospirillum, Sulfurovum, Desulfurella, and Desulfovibrio were present at the highest relative abundance, while at pH 3.9 Desulfurella dominated the sequenced reads. The detection of acetic acid and the relative abundance of Acetobacterium at pH 6.9 pointed towards acetogenesis, explaining the dominance of the heterotrophic genus Sulfurospirillum in this H2 and CO2-fed bioreactor.