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 S0reduction using an industrial mesophilic granular sludge as inoculum under thermoacidophilic conditions. Here, we examined the effect of pH on the S0reduction performance of the same inoculum, in a continuously fed gas-lift reactor run at 30 °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·L-1·d-1S2-·to 0.71 ± 0.07 g·L-1·d-1S2-· Microbial community analysis via 16S rRNA gene amplicon sequencing showed that at pH 6.9, the S0-reducing generaSulfurospirillum, Sulfurovum, Desulfurella, andDesulfovibriowere present at the highest relative abundance, while at pH 3.9Desulfurelladominated the sequenced reads. The detection of acetic acid and the relative abundance ofAcetobacteriumat pH 6.9 pointed towards acetogenesis, explaining the dominance of the heterotrophic genusSulfurospirillumin this H2and CO2–fed bioreactor.
