Biological S⁰ reduction at neutral and acidic 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 expanding the range of operating conditions to a 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 gas-lift reactor run at 30°C under neutral (pH 6.9) and acidic (pH 3.8) conditions, continuously fed with mineral media and H₂ and CO₂. Steady-state volumetric sulfide production rates (VSPR) dropped 2.5-fold upon transition to acidic pH, from 1.79 ± 0.18 g S²⁻·L⁻¹·d⁻¹ to 0.71 ± 0.07 g S²⁻·L⁻¹·d⁻¹. Microbial community composition was analyzed using 16S rRNA gene amplicon sequencing. At neutral pH (6.9), the high relative abundance of the S⁰-reducing genus Sulfurospirillum, previously known only for heterotrophic members, combined with the presence of Acetobacterium and detection of acetate, suggests an important role for heterotrophic S⁰ reduction facilitated by acetogenesis. Conversely, at acidic pH (3.9), S⁰ reduction appeared autotrophic, as indicated by the high relative abundance of Desulfurella.