Oxidative and reductive pathways in manganese-catalyzed Fenton's reactions

Soluble manganese (11) and amorphous and crystalline manganese (IV) oxides were investigated as catalysts for the Fenton-like decomposition of hydrogen peroxide into oxidants and reductants. 1-Hexanol was used as a hydroxyl radical probe and carbon tetrachloride (CT) was used as a reductant probe. Soluble manganese (II)-catalyzed reactions at acidic pH resulted in >99% degradation of 1-hexanol and no measurable transformation of CT, indicating that hydroxyl radicals were generated but reductants were not. However, when these reactions were conducted at near-neutral pH, an amorphous manganese oxide precipitate formed and 89% of the CT degraded in 60 min, while 1-hexanol degradation was negligible. Using an amorphous manganese oxide synthesized in a separate reactor, CT was rapidly degraded while 1-hexanol oxidation was undetectable. Reactions catalyzed by the crystalline manganese oxide pyrolusite (beta-MnO2) at near-neutral pH also resulted in significant CT degradation, indicating that reductants are generated by both the crystalline and amorphous manganese oxide-catalyzed decomposition of H2O2. The presence of manganese oxides in the subsurface and their ability to catalyze the generation of reductants in modified Fenton's reactions has important implications for hydrogen peroxide stability and contaminant transformation pathways during the in situ Fenton's treatment of contaminated soils and groundwater.