Antimony coprecipitation suppresses microbial reduction of goethite and hematite and limits Sb release across particle sizes

Antimony (Sb) is a toxic environmental contaminant, whose environmental behaviour is tightly linked to sorption or co-precipitation reactions with iron(III) (oxyhydr)oxides. These nanoparticulate minerals are susceptible to reductive dissolution via the activity of dissimilatory Fe(III)-reducing microorganisms (DIRB), thereby potentially mobilizing associated Sb. However, the impacts of Sb (generally in the form of Sb(V)) associated with iron(III) (oxyhydr)oxides of different particle sizes on microbial Fe(III) reduction, as well as on subsequent Sb speciation and distribution, remain poorly understood. In this study, we synthesized Sb(V)-containing goethite and hematite of varying particle sizes and characterized them using micro X-ray diffraction (µ-XRD), scanning/transmission electron microscopy (STEM), 57 Fe Mössbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and wet chemical extractions. We then examined their reductive dissolution by Shewanella oneidensis MR-1. Our findings reveal that (i) Sb(V) was incorporated into nanoscale iron(III) (oxyhydr)oxides to varying extents (Sb: Fe ratios ranging from 2:100 to 8:100), with no clear correlation between the extent of coprecipitation and mineral type or particle size. (ii) The presence of Sb(V) significantly inhibited both the extent and kinetics of microbial Fe(III) reduction across all samples of varying particle sizes, with suppression extent ranging from 25 to 80%. (iii) Sb release was limited (<10% of total Sb for goethite and <4% of total Sb for hematite), exhibited incongruence with the reductive dissolution of Fe, and showed only negligible Sb(V) reduction to Sb(III) across all sample conditions. These collective results indicate that the incorporation of Sb(V) into goethite and hematite may enhance the stability of these iron(III) (oxyhydr)oxides and serve as an effective sink for Sb immobilization. This stabilization persists even under reducing conditions, highlighting the potential for long-term Sb sequestration in Fe-rich environments.