Phosphate effects on the transformation of Sb(V)-bearing ferrihydrite under microbial iron- and sulfate-reducing conditions

Antimony (Sb) is a toxic metalloid that poses environmental risks in terrestrial and aquatic systems. Immobilization of the Sb(V) oxyanion, Sb(OH)6-, by ferric oxides can be influenced by competitive adsorption from other anions such as phosphate (PO43-). This study investigated the unknown effect of PO43- on Sb(V) behavior under Fe(III) and sulfate-reducing conditions. Anoxic reactors that contained Sb(V)-bearing ferrihydrite (Fh), varying PO43- concentrations (0, 0.2, and 2 mM), and sulfate, were inoculated with a microbial community sourced from Sb-contaminated soil. Additionally, abiotic reactors with either Sb(V)-adsorbed to or Sb(V)-coprecipitated with Fh and different PO­43- loadings (0–100 mM) were created to investigate the competitive adsorption mechanisms in the absence of microbial activity. Results from the abiotic reactors suggest that Sb(V) is likely incorporated into the Fh structure, with only minor amounts remaining surface-bound and extractable by PO43-. In the biotic reactors, Fe(III) and sulfate reduction were more extensive in the presence of PO43-. At 0.2 mM PO43-, microbial activity transformed Fh into siderite and led to the complete reduction of Sb(V) to Sb(III) as stibnite (Sb2S3). At 2 mM PO43, the greater coverage by PO­43- stabilized Fh and decreased the extent of both Fe(III) and Sb(V) reduction, and shifted the reduced products to mackinawite and Sb(III) adsorbed onto Fh, in addition to stibnite formation. This study demonstrates that while PO­43- may not directly compete with Sb(V) for sorption sites, it can influence Sb mobility in Fe(III)- and sulfate-reducing environments by altering the mineralization pathways.