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(IP31) Complexation and Redox Transformations of Inorganic Arsenic by Natural Organic Matter (NOM).
Redman, Aaron*,1, Macalady, Don2, Ahmann, Dianne1, 1 Division of Environmental Science and Engineering, Colorado School of Mines, Golden, Co2 Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Co
ABSTRACT- Inorganic arsenic forms stable complexes with NOM, possibly through the formation of ternary complexes with NOM-bound metals. Arsenate (As(V)) and arsenite (As(III)) were incubated individually with either raw filtered river water or ground walnut shells that had been pretreated with various concentrations of Al(NO3)3 at pH 5. The extent of As complexation with dissolved NOM was then determined by HPLC-ICP-MS. Free As(III) eluted first from the strong anion exchange HPLC material followed by free As(V) and then an unknown As species. The concentration of the unknown As species increased with increasing Al(NO3)3 concentration in NOM solutions. The broad character of the unknown As peak indicates heterogeneity of the species as expected for a mixture of NOM-As complexes, while its elution after the other As species suggests a strong anionic character which is consistent with the organic acid nature of the NOM molecules. The structure of the NOM-Al(III)-As complex(es) is still unresolved and could include a combination of As that is bound to NOM-coated colloidal aluminum oxides, As that has formed esters with NOM or As that has formed ternary NOM-Al(III)-As complexes. Concomitant with the observed complexation was the ability of dissolved NOM to reduce As(V) to As(III) under certain conditions. However, the presence of light and EDTA tended to inhibit the reduction of As(V), indicating that cationic metals and photoredox chemistry are important processes in these systems.
Key words: arsenic, speciation, natural organic matter, redox
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