Redox surface reactions occurring at plattnerite, pyrolusite, and hydrous manganese oxide particle-water interfaces relevant to water supply systems and biochemical systems.

摘要

Several water supply systems have experienced significant increases in Pb at the tap following a switch from free chlorine to chloramine disinfectant. Loss of the PbIVO2(s) protective layer through the reduction of PbIV to PbII likely contributes to these concentration increases. In this study, PbIVO 2(s, plattnerite) nanoparticle aggregrates in aqueous suspension are readily reduced by Mn2+(aq), Fe2+(aq), and natural organic matter (NOM), yielding Pb2+(aq) and adsorbed Pb II. Fe2+(aq) oxidation generates FeIII (hydr)oxides that impede PbIV reduction, especially at pH ≥ 5. Mn2+(aq) oxidation generates mixed MnIII/Mn IV (hydr)oxides that are less of an impediment to PbIV reduction. Adding both Fe2+(aq) and Mn2+(aq) can set into motion a Mn redox cycle that catalyzes PbO2(s) reduction by Fe2+(aq).;NOM is a diverse collection of molecules, each possessing its own reductant, complexant, and adsorption properties. Granular activated carbon (GAC) column treatment is more efficient at removing the reductant NOM fraction; coagulation/filtration, with either FeCl3 or Al2(SO4)3 coagulant, is most efficient at removing the inhibitory NOM fraction. Inhibition may arise from (i) NOM adsorption at the mineral/water interface, which blocks approach of reductant molecules and (ii) a micelle-like aggregate nature, which provides hydrophobic pockets that capture reductant molecules, again keeping them away from the mineral/water interface.;Alkene-containing natural products with complex structures, abietic acid and retinoic acid, taste and odor compounds, precursors of the carcinogen Mutagen X (MX), and other chlorinated disinfection by-products (DBPs), are investigated for their redox reactivities toward hydrous manganese oxide (HMO) and permanganate. These two oxidants are selective in oxidizing various functional groups, with distinctive oxidation pathways. Autocatalytic kinetics and synergistic effects are observed in some oxidation reactions.;Representative biochemical constituents present in blood, sweat, tears, mucus, and human gastrointestinal tract fluids were investigated for pathways and rates of redox reactions with MnO2(s, pyrolusite) and PbO 2(s, plattnerite) nanoparticles. Water soluble antioxidants, such as ascorbic acid, uric acid, cysteine, glutathione, and ergothioneine, are highly reactive with either metal oxide, although the ranking of reactivity is different with MnO2(s) than with PbO2(s), indicating distinctive oxidation pathways. Other biochemical constituents, such as N-acetyl-neuraminic acid, urocanic acid, trehalose, and caffeine are much less reactive.