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

Abstract

SummaryTrace-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 (H 2 ) concentrations have been shown to correlate with specific TEAPs during bioremediation in an aquifer. Theoretical analysis has shown that these steady-state H 2 levels are solely dependent upon the physiological parameters of the hydrogen-consuming microorganisms, with H 2 concentrations increasing as each successive TEAP yields less energy for bacterial growth. The objective of this research was to determine if H 2 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 H 2 and carbon simultaneously. In addition, the effect of iron bioavailability on H 2 concentrations during iron reduction was observed.The first phase of research determined the effect of a competing electron donor (acetate) on the kinetics of H 2 utilization by Geobacter sulfurreducens in batch cultures under iron reducing conditions. The results indicate that, though the Monod kinetic coefficients describing the rate of H 2 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 H 2 levels typical for each TEAP. In addition, with acetate and H 2 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 H 2 concentrations under steady state conditions in the presence of acetate. This dual-donor model indicates that the steady state H 2 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 H 2 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 H 2 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 H 2 concentrations measured during this 1 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 stead...