Enhancing Elemental Sulfur Selectivity in Sequential Membrane Biofilm Reactors via Decoupled pH and ORP Control

A sequential membrane biofilm reactor (MBfR) system─comprising a hydrogen-based reducing module (H2-MBfR) for sulfate (SO42–) reduction and an oxygen-based oxidizing module (O2-MBfR) for elemental sulfur (S0) recovery─was operated for 269 days to evaluate decoupled oxidation-reduction potential (ORP) and pH control strategies for treating mining-influenced waters. ORP control (−360 to −400 mV) at mildly alkaline pH (∼8) achieved moderate S0 selectivity (67.7 ± 11.4%) but promoted significant thiosulfate (S2O32–) accumulation (−5.00 ± 1.58 g S m–2 d–1), revealing the limitations of redox-only strategies. Shifting to mildly acidic conditions (pH ∼ 6) suppressed S2O32– formation by 92%, attributable to the pH-mediated reduction of reactive bisulfide (HS–) concentration─which governs abiotic oxidation kinetics─and the enhanced direct uptake of undissociated H2S by sulfur-oxidizing bacteria (SOB). This suppression remained robust under high-rate stress testing (average SO42– surface loading of 25.8 g S m–2 d–1) despite severe hydraulic transients. The system achieved a peak SO42– removal flux of 27.81 g S m–2 d–1, volumetric S0 production of 4.16 g S L–1 d–1, and a peak S0 recovery efficiency of 94.1%. These results demonstrate that decoupled pH-ORP control effectively minimizes undesirable byproducts, widening the operational window for selective biological sulfur recovery in MBfR systems.