S0-dependent bio-reduction for antimonate detoxification from wastewater by an autotrophic bioreactor with internal recirculation

Elemental sulfur (S⁰) autotrophic reduction is a promising approach for antimonate [Sb(V)] removal from water; however, it is hard to achieve effective removal of total antimony (TSb). This study established internal recirculation in an S⁰ autotrophic bioreactor (SABIR) to enhance TSb removal from Sb(V)-contaminated water. Complete Sb(V) reduction (10 mg/L) with bare residual Sb(III) (< 0.26 mg/L) was achieved at hydraulic retention time (HRT) = 8 h. Shortening HRT adversely affected the removal efficiencies of Sb(V) and TSb; meanwhile, an increased reflux ratio was conducive to Sb(V) and TSb removal at the same HRT. Sulfur disproportionation occurred in the SABIR and was the primary source for SO₄²⁻ generation and alkalinity consumption. The alkalinity consumption decreased with the shortening HRT and increased with an increased reflux ratio at the same HRT. The generated SO₄²⁻ was significantly higher (50–100 times) than the theoretical value for Sb(V) reduction. Coefficient of variation (CV), first-order kinetic models, and osmolality analyses showed that internal recirculation did not significantly affect the stability of SABIR but contributed to enhancing TSb removal by increasing mass transfer and reflowing generated sulfide back to the SABIR. SEM–EDS, Raman spectroscopy, XRD and XPS analyses identified that the precipitates in the SABIR were Sb₂S₃ and Sb-S compounds. In addition, high-throughput sequencing analysis revealed the microbial community structure's temporal and spatial distribution in the SABIR. Dominant genera, including unclassified-Proteobacteria (18.72–38.99%), Thiomonas (0.94–4.87%) and Desulfitobacterium (1.18–2.75%) might be responsible for Sb(V) bio-reduction and removal. This study provides a strategy to remove Sb from water effectively and supports the theoretical basis for the practical application of the SABIR in Sb(V)-contaminated wastewater.