Current limitations of in situ chemical oxidation (ISCO) remediation agents in treating contaminated groundwater include the difficulty of bringing reactants into contact with contaminants, particularly when the contaminants are located in low permeability matrices in which diffusion and mass transfer are minimal, and the non-beneficial reactions of oxidant sources with aquifer materials such as metal catalyzed decomposition or the oxidation of naturally occurring organic materials (NOM). The use of sodium persulfate for ISCO has the potential to overcome these limitations. In addition to reactivity with a broad spectrum of contaminants and its limited reactivity with NOM, the longevity of sodium persulfate results in increased transport through the subsurface. Furthermore, persulfate treatment may increase the permeability of some aquifer materials, resulting in enhanced contact with contaminants. This project focused on the potential for persulfate to diffuse into low permeability regions of the subsurface, promoting contact with contaminants to enhance in situ destruction.
The objectives of this project were to investigate (1) persulfate decomposition rates in the presence of metal oxides and subsurface solids with a wide range of physical and geochemical characteristics, (2) the reactivity of persulfate with contaminants under varying activation conditions, and (3) the effect of persulfate on soil microstructure, porosity, and permeability.
Mechanistic investigation of persulfate activation by naturally occurring iron oxides, manganese oxides, clay minerals, trace minerals, base, iron chelates, and organic compounds was conducted using reaction specific probe compounds, hydroxyl radical and sulfate radical scavengers, and electron spin resonance spectroscopy. In addition, the effect of activated persulfate formulations on the permeability and morphology of subsurface minerals and subsurface solids was investigated using falling head permeameters, x-ray computed tomography, x-ray diffraction, and surface area analysis. Diffusion and transport of different persulfate formulations into low permeability matrices was investigated using specially designed soil columns filled with kaolinite and a low permeability soil.
Results of the activation studies showed that most minerals do not activate persulfate, particularly in the concentrations commonly found in the subsurface. Iron chelate-activated persulfate and base-activated persulfate generate hydroxyl radical, sulfate radical, and reductants and provide the basis for widespread treatment of different classes of contaminants in the subsurface. Many organic compounds activate persulfate including phenoxides and ketones. Because soil organic matter contains phenolic and ketonic moieties, it is a potent activator of persulfate. Depending on the acidity of the soil organic matter, it may activate persulfate with minimal addition of base.
Persulfate formulations have varied effects on subsurface morphology. Persulfate has minimal effects on the mineralogy of the subsurface, with the exception of aging ferrihydrite. It can increase the permeability of some subsurface materials. Most importantly, activated persulfate formulations have the potential to diffuse into low permeability strata, such as clays, where they have the potential to treat contaminants of concern.
Activated persulfate was shown to be a highly reactive remediation system that has sufficient longevity and transport characteristics to treat contaminants in low permeability regions of the subsurface. The use of activated persulfate can expand ISCO capabilities at Department of Defense facilities beyond the treatment of chlorinated ethenes to include benzene, toluene, ethylbenzene, and xylene (BTEX), polychlorinated biphenyls (PCBs), and chlorinated alkanes.