Hydrogen as an Indicator to Assess Biological Activity During Trace-Metal Bioremediation

Abstract

Trace-metal and/or radionuclide bioremediation schemes require that specific redox conditions be achieved at given zones of an aquifer. Tools are therefore needed to identify the terminal electron acceptor processes (TEAPs) that are being achieved during bioremediation in an aquifer. Dissolved hydrogen (H2) concentrations have been shown to correlate with specific TEAPs during bioremediation in an aquifer. Theoretical analysis has shown that these steady-state H2 levels are solely dependent upon the physiological parameters of the hydrogen-consuming microorganisms, with H2 concentrations increasing as each successive TEAP yields less energy for bacterial growth. The objective of this research was to determine if H2 can still be used as an indicator of TEAPs during a uranium bioremediation scheme where an organic substrate is injected into the subsurface and organisms may consume H2 and carbon simultaneously. In addition, the effect of iron bioavailability on H2 concentrations during iron reduction was observed. The first phase of research determined the effect of a competing electron donor (acetate) on the kinetics of H2 utilization by Geobacter sulfurreducens in batch cultures under iron reducing conditions. The results indicate that, though the Monod kinetic coefficients describing the rate of H2 utilization under iron-reducing conditions correlate energetically with the coefficients found in previous experiments under methanogenic and sulfate-reducing conditions, conventionally measured growth kinetics do not predict the steady state H2 levels typical for each TEAP. In addition, with acetate and H2 as simultaneous electron donors, there is slight inhibition between the two electron donors for G. sulfurreducens, and this can be modeled through competitive inhibition terms in the classic Monod formulation, resulting in slightly higher H2 concentrations under steady state conditions in the presence of acetate. This dual-donor model indicates that the steady state H2 concentration in the presence of an organic as electron donor is not only dependent on the biokinetic coefficients of the TEAP, but also the concentration of the organic substrate, and that the H2 concentration does not start to change very dramatically as long as the organic substrate concentration remains below the half saturation constant. The results for this phase of research are provided in Section 1. The second phase of research measured steady-state H2 concentrations under iron reducing conditions using NABIR Field Research Center background soil in a simulated bioremediation scenario involving acetate injection to stimulate indigenous microbial activity in a flow-through column. Steady-state H2 concentrations measured during this long-term (500 day) column experiment were higher than observed for iron-reducing conditions in the field even though evidence suggests that iron reduction was the dominant TEAP in the column. Additional column experiments were performed to determine the effect of iron bioavailability on steady-state H2 concentrations using the humics analogue, AQDS (9,10-anthraquinone-2,6-disulfonic acid). The iron reduction rate in the column with AQDS was double the rate in a parallel column without AQDS and lower steady state H2 levels were observed in the presence of AQDS, indicating that even though iron reduction does occur, a decreased bioavailability of iron may inhibit iron reduction such that H2 concentrations increase to levels that are more typical for less energetically favorable reactions (sulfate-reduction, methanogenigesis). The results for this phase of research are in Section 2. A final phase of research measured the effect of carbon concentration and iron bioavailability on surface bound iron reduction kinetics and steady-state H2 levels using synthetic iron oxide coated sand (IOCS). Results show a significant decrease in the microbial iron reduction and acetate oxidation rates for systems with surface bound Fe(III) (IOCS) compared to soluble Fe(III) (ferric citrate). The addition of AQDS did not affect the rate of iron reduction for soluble Fe(III) but did increase the rate of surface bound Fe(III) reduction to values similar to soluble Fe(III). IOCS column experiments will be performed using acetate concentration ranges above and below the half saturation constant calculated from the batch experiments to verify if steady-state H2 concentrations vary with change in carbon concentration as predicted from the dual-donor modeling in phase 1. These results will determine if, and at what concentration, a carbon source added as an electron donor during a biostimulation scenario will compete with H2 as a co-electron donor and how this interaction will influence the use of H2 as an indicator of TEAP during bioremediation in an aquifer. The results for this phase of research are in Section 3.