Effect of iron bioavailability on dissolved hydrogen concentrations during microbial iron reduction

Dissolved hydrogen (H ₂) concentrations have been shown to correlate with specific terminal electron accepting processes (TEAPs) in aquifers. The research presented herein examined the effect of iron bioavailability on H ₂ concentrations during iron reduction in flow-through column experiments filled with soil obtained from the uncontaminated background area of the Field Research Center (FRC), Oak Ridge, TN and amended with acetate as the electron donor. The first column experiment measured H ₂ concentrations over 500 days of column operation that fluctuated within a substantial range around an average of 3.9 nM. Iron reduction was determined to be the dominant electron accepting process. AQDS (9,10-anthraquinone-2,6-disulfonic acid) was then used to determine if H ₂ concentrations during iron reduction were related to iron bioavailability. For this purpose, a 100-day flow-through column experiment was conducted that compared the effect of AQDS on iron reduction and subsequent H ₂ concentrations using two columns in parallel. Both columns were packed with FRC soil and inoculated with Geobacter sulfurreducens but only one was supplied with AQDS. The addition of AQDS increased the rate of iron reduction in the flow-through column and slightly decreased the steady-state H ₂ concentrations from an average of 4.0 nM for the column without AQDS to 2.0 nM for the column with AQDS. The results of this study therefore show that H ₂ can be used as an indicator to monitor rate and bioavailability changes during microbial iron reduction.