Kinetics of reductive dissolution of hematite by bioreduced anthraquinone-2,6-disulfonate

The reductive dissolution of hematite (alpha-Fe2O3) was investigated in a flow-through system using AH(2)DS, a reduced form of anthraquinone-2,6-disulfonate (AQDS), which is often used as a model electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow rate, pH, and Fe(II) and phosphate concentrations were varied to investigate the redox kinetics in a flow-through reactor. The hematite reduction rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and AGDS sorption on hematite surfaces were independently investigated for interpreting hematite reduction kinetics. AH(2)DS sorption to hematite was inferred from the parallel measurements of AGDS and AH(2)DS sorption to alpha-Al2O3, a redox stable analog of alpha-Fe2O3. Decreasing Fe(II) and increasing AH(2)DS sorption by controlling flow rate, influent pH, and Fe(II) and phosphate concentrations increased the rates of reductive dissolution.The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium. This study demonstrated the importance of the geochemical variables for the reductive dissolution kinetics of iron oxides.