Slow-release permanganate versus unactivated persulfate for long-term in situ chemical oxidation of 1,4-dioxane and chlorinated solvents

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The objective of this research was to evaluate slow-release permanganate and unactivated persulfate for in situ treatment of dioxane and associated chlorinated solvents. Laboratory batch studies with unactivated persulfate in deionized water or in soil and groundwater demonstrated dioxane removal with pseudo second-order rate constants ranging from 10−5 to 10−3 M−1 s−1. Flow-through column studies demonstrated over 99% dioxane removal with slow-release unactivated persulfate but not with slow-release permanganate. The slow-release permanganate cylinders became coated with a rind that limited oxidant mass transfer and dioxane oxidation. A field study was conducted with slow-release persulfate cylinders transverse to groundwater flow. Over 99% removal of dioxane and chlorinated solvents was observed 2.5 m downgradient of the cylinders. Density-driven flow associated with the released persulfate was observed and was attributed to a low horizontal hydraulic gradient. Thus, most of the contaminant and persulfate flux was thought to be isolated to a deep aquifer zone that was bound by an underlying silt aquitard. Contaminant reductions were also observed in shallow groundwater samples, albeit at a lesser extent. The longevity of the persulfate oxidant cylinders was estimated to be 6–12 months. Results of this study demonstrate that dioxane and co-mingled chlorinated solvents can be effectively treated using slow-release persulfate cylinders. Careful consideration to cylinder placement during the design phase is essential to prevent the contaminant plume from bypassing and not coming into contact with the released oxidant.