PNNL

Abstract

The reductive dissolution of hematite (?-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone- 2,6 disulfonate (AQDS), which is often used as an electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow-rate, pH, Fe(II) and phosphate concentrations were varied to investigate the redox reaction kinetics. The effluent AH2DS, AQDS, and Fe(II) concentrations changed significantly within the first half hour of AH2DS reaction with hematite and then gradually evolved toward steady-state. The steady-state 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 AQDS sorption on hematite surfaces were independently investigated for interpreting hematite reductive dissolution kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to ?-Al2O3, a redox stable analog of ?-Fe2O3. Decreasing Fe(II) and increasing AH2DS sorption by controlling flow residence time, influent pH, 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, as shown by the effect of increasing the influent concentration of Fe(II).