The use of in situ chemical oxidation (ISCO) has dramatically increased at Department of Defense (DoD), Department of Energy (DOE), and defense contractor sites as responsible parties attempt to aggressively destroy chlorinated solvent and dense non-aqueous phase liquid (DNAPL) source areas with the goal of reducing the duration and cost of site remediation. Unfortunately, ISCO performance has varied significantly due to limited understanding of the chemical reaction mechanisms and kinetics and the interactions between the oxidant, contaminants, and aquifer matrix. This research has improved understanding of ISCO applicability and optimized deployment conditions.
Research objectives were addressed through two separate bench-scale efforts. Objective I was a comprehensive perspective of the kinetics of oxidation of groundwater contaminants by ISCO oxidants (mainly permanganate anion, hydroxyl radical, and sulfate radical), while Objective II was an assessment of how aquifer matrix properties (e.g., soil mineralogy, natural carbon content) affect subsurface oxidant mobility and stability leading to the development of a standardized natural oxidant demand (NOD) measurement protocol. A third field-based objective to determine the effects of ISCO on long-term groundwater quality was discontinued based on timing and logistical constraints.
The objectives of this project are: (1) develop a comprehensive perspective on the kinetics of oxidation of common groundwater contaminants by the most commonly used oxidants (permanganate [MnO4-] and Fenton’s reagent [H2O2/Fe2+]); (2) evaluate the effect of the aquifer matrix on oxidant mobility and stability using standardized oxidant demand measurement protocols; and (3) identify significant secondary impacts of ISCO on groundwater geochemistry and microbial activity at the field-scale.
A comprehensive literature review was conducted and subsequent data gaps were filled using a novel experimental method for measuring new kinetic data. New kinetic data and previously published data were subjected to correlation analysis, enabling more accurate kinetic predictions. Aquifer materials from nine sites were characterized and evaluated with respect to their physiochemical properties and total theoretical and experimental reductive capacities. Batch and column experiments conducted with permanganate, Fenton’s reagent, and persulfate evaluated fundamental chemical properties affecting oxidant consumption, maximum NOD of aquifer materials, kinetic behavior, and oxidant transport.
The addition of ferrous iron generated the largest hydrogen peroxide decomposition rate coefficients, while chelating agents generated the lowest hydrogen peroxide decomposition rate coefficients and offer promise as green in situ hydrogen peroxide applications. The transport of iron and manganese was observed associated with the hydrogen peroxide application. Since iron and manganese can promote hydrogen peroxide decomposition, such transport is expected to affect the subsurface behavior of hydrogen peroxide. Iron and manganese may also be involved in catalytic activation of persulfate. Activated persulfate produces sulfate radical, hydroxyl radicals, and other reactive intermediates. A method was developed for measuring rate constants for contaminant oxidation by sulfate radical and applied to a variety of contaminants, including benzene, toluene, ethylbenzene and xylenes (BTEX); chlorinated methanes; 1,4-dioxane; and methyl tert-butyl ether (MTBE). Decomposition tests indicated persulfate will have moderately high stability in most aquifer systems. The oxidation of explosives with heat-activated persulfate resulted in fairly rapid degradation of trinitrotoluene (TNT); 1,3,5-trinitroperhydro-1,3,5-triazine (RDX); octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX); and 4-nitrophenol.
This project has provided valuable information towards more effective ISCO deployment. For example, relative to permanganate consumption, total organic carbon determines the maximum NOD value while amorphous iron along with the cation exchange capacity (CEC) determines the permanganate consumption rate, thereby suggesting a means to optimize control of unproductive permanganate consumption by aquifer materials through multiple oxidant injection episodes. Further, a proposed permanganate-COD (chemical oxidant demand) test method was deemed superior to the current dichromate COD test and can also estimate the maximum NOD for site screening and initial design purposes.
While chlorinated ethenes have been successfully remediated with both ISCO and in situ thermal remediation (ISTR), there is evidence that a combined treatment approach may result in synergistic advantages. Sodium persulfate is an ideal oxidant for ISTR combination because persulfate exposure to high temperatures leads to the formation of highly reactive sulfate radicals in addition to the higher reaction rates typically induced by higher temperatures. For tetrachloroethene (PCE), permanganate oxidation may be more favorable at lower temperatures.
Finally, project results indicate that the use of batch test data for design is questionable since column experiments can provide more realistic aquifer material contact and therefore are believed to better mimic in situ conditions.