Influence of Iron Reducing Bacteria on Phosphate and Arsenate Release and Sequestration Onto Iron Oxides

This study investigates the potential for iron oxides to capture phosphate and arsenate. While iron oxides clearly bind phosphate and arsenate, one should not assume that iron oxides would straightforwardly trap these contaminants. Instead, iron oxides in shallow groundwater sediments undergo chemical transformations that could lead to the release of sorbed contaminants. Most notably, scientists commonly observe that the biologically driven reduction of ferric iron (Fe[III]) to ferrous iron (Fe[II]) releases phosphate and/or arsenate under some conditions. Despite this observation, it is generally unclear whether iron reduction will lead to greater or lesser mobility of phosphate and arsenic. This is because sorbed phosphate and arsenate may dissolve into the aqueous phase under iron-reducing conditions, thereby becoming more mobile, while under the same reducing conditions, phosphate and arsenate may precipitate with ferrous iron solids, thus decreasing their mobility. Further complications arise when the system is exposed to cycles of oxic and anoxic conditions. Under oxic conditions, ferrous iron will oxidize and form new iron oxides, while existing solid ferrous oxides will transform to new ferric oxides. In shallow groundwater, flooding and drought lead to cycles of oxic and anoxic conditions that could produce cyclic patterns in iron redox states and in the mobility of arsenic and phosphate. To better predict the evolution of arsenate and phosphate in the field, this research employs a simplified laboratory model. This study tracks the fate of arsenate and phosphate sorbed onto goethite, a common sediment component, as geobacter sulfurreducens reduces iron from Fe[III] to Fe[II], and after re-oxidation of reduced iron. By studying that particular phenomenon, this research sheds light on how microbes and one soil constituent influence the mobility of arsenate and phosphate contamination. The results of this study show that freshly precipitated ferrous iron solids can sequester arsenate and phosphate as they remove these anions from the aqueous phase and incorporate them into the matrix of freshly precipitated ferrous iron solids.