Photochemistry of dissolved natural organic matter and its interactions with methylmercury and inorganic mercury

This thesis presents results of studies on the photochemistry and binding of Hg2+ and CH3Hg+ with natural organic matter (NOM). NOM was isolated from surface water samples of Minnesota wetlands (bog and fen) and lakes (Scrapper, Blandin and Spring). Bog and fen NOM were isolated with DEAE-cellulose and XAD-8 and were compared with CP-MAS 13C NMR spectroscopy. Binding studies were conducted for Hg 2+ with International Humic Substance Society (IHSS) peat humic acid. A competitive ligand-exchange technique with dl-penicillamine was used to determine binding constants. Experiments were conducted to study the effect of pH, Hg/Sre mole ratios and competitive ligand concentrations. Results showed that the distribution coefficients varied with pH from 1022.6 (pH 1.89) to 1032.8 (pH 5.8). Binding studies were conducted for CH3Hg+ (MeHg) with IHSS SRFA using a dialysis membrane competitive ligand-exchange technique with 100 molecular weight cut off (MWCO) membranes and Br - as a competitive ligand. Experiments were conducted to study the effect of MeHg/Sre mole ratios, ligand concentrations and pH. Distribution coefficients (∼108) were nearly constant over a range of ligand concentrations (0.03--0.001 M), MeHg/S re mole ratios (1/1220--1/12240) and pH (2.98--7.62). Methylmercury photolysis experiments were conducted to determine the effect of humic substances on photolysis rate constants and to determine the products, rates and mechanisms for direct and nitrate-induced hydroxyl radical mediated indirect photolysis. Results for direct photolysis showed that MeHg forms Hg(0) and Hg(II) and photolysis proceeds with a rate of 0.003 min-1 under a UV lamp. Experiments with nitrate-induced hydroxyl radicals showed that the second-order rate constant between McHg and ·OH is 6.81 x 109 M-1 s-1 and the primary product of McHg photodegradation is Hg(II). Experiments in the presence of NOM isolates of Minnesota surface waters showed that pseudo first-order rate constants of methylmercury photolysis initially increased with increasing DOC concentrations and then decreased with further increase in DOC concentrations.