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 long-term fate of both schwertmannite and PO43 may be influenced by interactions during microbially-mediated Fe (III) and SO41- 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 (alpha FeOOH) formed first, followed by mackinawite (FeS) and siderite (FeCO3). In contrast, in higher PO43- treatments, vivianite (Fe-3(PO4)(2)) and/or sulfate green rust ((Fe4Fe2III)-Fe-II(OH)(12)SO4) 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 beating 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. (C) 2018 Elsevier B.V. All rights reserved.