Improving Permanganate Oxidations: Speciation, Pathways, Rates, and Products

Johns Hopkins University, Baltimore MD

Investigators

Abstract

1067075 Stone Permanganate is widely employed in water supply plants to remove iron and manganese, taste and odor compounds, endocrine disruptors, and cyanobacterial toxins. It is employed in aquaculture as a fungicide. Permanganate reportedly dehalogenates some organic substrates, but this capability is not yet utilized in practice. Permanganate oxidations can involve six manganese oxidation states: +II, +III, +IV, +V, +VI, +VII. Manganese speciation and reactivity in response to ambient chemistry govern rates of substrate consumption and the identities of oxidation products. Many permanganate oxidations are autocatalytic; an intermediate or product catalyzes forward reaction. Better understanding of the effects of ambient chemistry on the speciation, pathways, rates, and products of permanganate oxidations would enable us to (i) accelerate permanganate oxidations, thereby improving effectiveness, and (ii) anticipate and avoid situations where oxidation products might be toxic or otherwise problematic. The objective of this research to provide the scientific/technical basis for improving the performance of permanganate oxidations. Emerging contaminants that possess both classical sites of permanganate oxidation (e.g. alkene groups) and nonclassical sites (oxygen and nitrogen heterocycles) will be included as representative substrates. Small changes in functional group identity and molecular structure will enable us to probe reaction mechanisms and develop structure-reactivity relationships. Capillary electrophoresis with UV detection, and HPLC with both UV and ESI-MS/MS detection, will be used to identify and quantify reaction intermediates and products. Autocatalysis may arise from (i) heightened substrate reactivity upon adsorption to hydrous manganese oxides (HMOs), (ii) disproportionation reactions between permanganate and lower Mn oxidation states (+II, +III, +IV) that boost Mn(V) and Mn(VI) concentrations, (iii) permanganate-substrate reactions that generate novel oxidants on surfaces or in solution. Reagent probes, as well as exploration of pH and major ion composition effects will help resolve among possible mechanisms. High resolution transmission electron microscopy (HRTEM), combined with selective area electron diffraction (SAED) and electron energy loss spectroscopy (EELS), will be used to characterize manganese (hydr)oxide solids generated by permanganate oxidations. They will explore the intermediate oxidation state manganese species identified and quantified using capillary electrophoresis. The unique combination of expertise in the PI's lab, i.e. inorganic reaction mechanisms, organic reaction mechanisms, and heterogeneous speciation determination, is needed to address the scientific/technical issues posed. By improving the efficiency of permanganate oxidations, and by shedding light on permanganate oxidation product identity and yields, the proposed work will make a significant contribution to the field of water quality. The PI's laboratory has developed several key concepts and tools in the area of metal-organic interactions in water. Nine women and five men have completed their Ph.D. training in the PI's laboratory, including eight currently holding tenure track appointments. The PI has helped organize outreach activities at the Rawlings Conservatory in Baltimore, and leads monthly "Science Demonstrations" at the Conservatory.