DYNAMICS OF HYDROGEN SUPPLY AND DEMAND FOR IN SITU DEHALORESPIRATION

摘要

The diffusive release of hydrogen (H2) was evaluated as part of a multitechnology pilot program to evaluate the feasibility of various electron donors and delivery methods for reducing chlorinated hydrocarbons in groundwater at a former industrial facility in California. The evaluation was performed as part of a one-year pilot program using separate in-situ test cells designed to intercept treatment-zone groundwater. Groundwater flowing into the two test cells used for this study contained a mixture of chlorinated hydrocarbons, where trichloroethene (TCE) was the most abundant with initial average concentrations exceeding 50 milligrams per liter (mg/L). Sulfate was an important competing electron acceptor, where average pre-test concentrations were approximately 500 mg/L. Waterloo EmittersTM were installed in the test cells to deliver molecular hydrogen (H2) and sulfur hexafluoride (SF6) tracer gas to treatment zone groundwater by a passive, diffusive mechanism. Stoichiometric calculations revealed that the amount of H2 required for reducing TCE and sulfate greatly exceeded its aqueous solubility. Methods to overcome H2 solubility by increasing the H2 utilization rate included adding nutrients and implementing mechanical mixing of groundwater in both test cells. Hydrogen transfer rates were controlled by varying the partial pressure inside the emitters. In both test cells, results were similar, where a 4-fold reduction in TCE, and similar increase in cisdichloroethene (cDCE) concentration was observed over the evaluation period. Hydrogen sulfide increased and sulfate concentrations also decreased. Dissolved H2 concentrations inside both test cells remained below 60 nanomolar for most of the study, although numerical simulations and co-released SF6 tracer concentrations confirmed that the emitters were functioning as designed, and that sufficient H2 was being added to the system to explain the observed changes in TCE and sulfate concentrations. These results suggest that H2 solubility limitations were avoided because of rapid utilization by sulfatereducing and dechlorinating bacteria. More importantly, this work suggests that amendment utilization and supply rates are important considerations when designing insitu systems, and that stoichiometric considerations based on concentration data alone can be misleading when considering in-situ treatment options.