Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Acid sulfate systems commonly contain the metastable ferric oxyhydroxysulfate mineral schwertmannite, as well as phosphate (PO₄³⁻) - a nutrient that causes eutrophication when present in excess. However, acid sulfate systems often experience reducing conditions that destabilize schwertmannite. Under such conditions, the longterm fate of both schwertmannite and PO₄³⁻ may be influenced by interactions during microbially-mediated Fe (III) and SO₄²⁻ reduction. This study investigates the influence of PO₄³⁻ on Fe(III) and SO₄²⁻ reduction and the subsequentmineralogical transformation(s) in schwertmannite-rich systems exposed to reducing conditions. To accomplish this, varied PO₄³⁻ loadings were established in microbially-inoculated schwertmannite suspensions that were incubated under anoxic conditions for 82 days. Increased PO₄³⁻ attenuated the onset of microbial Fe (III) reduction. This delayed consequent pH increases, which in turn had cascading effects on the initiation of SO₄²⁻ reduction and subsequent mineral species formed. Under zero PO₄³⁻ loading, goethite (αFeOOH) formed first, followed by mackinawite (FeS) and siderite (FeCO₃). In contrast, in higher PO₄³⁻ treatments, vivianite (Fe₃(PO₄)₂) and/or sulfate green rust (Fe^II₄Fe^III₂(OH)₁₂SO₄) became increasingly important over time at the expense of goethite and mackinawite compared to PO₄³⁻-free conditions. The findings imply that PO₄³⁻ loading alters the rates and onset of microbial Fe(III)- and SO₄²⁻- reduction and the subsequent formation of secondary Febearing phases. In addition, schwertmannite reduction and the associated mineralogical evolution under anoxic conditions appears to sequester large quantities of PO₄³⁻ in the form of green rusts and vivianite.