Comparative toxicity of hydrogen peroxide, hydroxyl radicals, and superoxide anion to Escherichia coli

Abstract

The toxicity of hydrogen peroxide, hydroxyl radical, and superoxide radical anion to Escherichia coli was investigated as a basis for understanding the effects of hydrogen peroxide when it is injected into the subsurface for in situ bioremediation or in situ chemical oxidation. Hydrogen peroxide toxicity was evaluated by maintaining its steady state concentration at a series of concentrations ranging from 0.7 to 3.0 mM in the presence of E. coli. Hydroxyl radical toxicity was studied by conducting parallel reactions of equal steady state concentrations of hydrogen peroxide, but using an iron (III)–nitrilotriacetic acid complex to decompose hydrogen peroxide to hydroxyl radicals. Superoxide was also generated from equal steady state concentrations of hydrogen peroxide, but with the addition of pyrolusite (manganese oxide) to catalyze its decomposition to superoxide radical. Hydrogen peroxide was toxic to E. coli at all concentrations investigated. The generation of hydroxyl radicals in hydrogen peroxide solutions showed no increase in toxicity relative to hydrogen peroxide toxicity, indicating minimal additional toxicity of the hydroxyl radicals. Hydrogen peroxide solutions of equal concentrations in which superoxide was generated showed less toxicity relative to hydrogen peroxide systems. The results indicate that toxicity of hydrogen peroxide to microorganisms may be lower when it is injected to subsurface systems containing high manganese oxide contents.