Cometabolic biotransformation of 1,4-dioxane in mixtures with hexavalent chromium using attached and planktonic bacteria

Abstract

Biological treatment of 1,4-dioxane, a probable human carcinogen and a recalcitrant contaminant of concern, is often complicated by the presence of inhibitory co-contaminants. Due to its use as a solvent, wetting agent, and stabilizer for chlorinated solvents employed in metal vapor degreasing, 1,4-dioxane has often been found to occur with a variety of co-contaminants, including heavy metals such as hexavalent chromium [Cr(VI)]. Cr(VI) also occurs naturally in groundwater due to geological formations, but also has sources that can coincide with 1,4-dioxane from anthropogenic activities such as metal vapor degreasing. Biodegradation of 1,4-dioxane can be accomplished by microbes that use it as a source of carbon or energy as well as those that cometabolize it after growth on other organic substrates. A propanotroph, Mycobacterium austroafricanum JOB5, was grown in planktonic pure cultures and biofilms to determine its ability to cometabolize 1,4-dioxane in the presence of varying concentrations of Cr(VI). 1,4-Dioxane cometabolism by JOB5 planktonic cells was uninhibited by Cr(VI) at levels up to 10 mg/L, while biofilms were only mildly inhibited at 10 mg/L. As an important part of the biofilms commonly found in subsurface aquifers and engineered systems, extracellular polymeric substances (EPS) were found to play an important role in preventing Cr(VI) exposure to cells. We observed that soluble EPS were able to bind to Cr(VI) and theorize that biofilm-associated EPS additionally served to impede penetration of the biofilm structure by Cr(VI), thus mitigating exposure and toxicity. These findings suggest that bioremediation would be a viable treatment strategy for 1,4-dioxane-contaminated waters that contain elevated levels of Cr(VI) in natural and built environments.