Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Abstract

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 long-term 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 subsequent mineralogical 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 (FeII4FeIII2(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 Fe-bearing 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.