Reduction of Hg(II) to Hg(0) by Dicarboxylic Acids in the atmosphere: Kinetic, Product and Mechanism Studies

Physics

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0317 Chemical Kinetic And Photochemical Properties

Scientific paper

Mercury is an environmental contaminant of global concern. The recent global circulation studies clearly indicate that there are main reduction mechanisms that are yet to be identified in cycling of mercury in the Earth's environment. The reduction of oxidized mercury species by organic matter to the elemental mercury (Hg0) is of vital importance for understanding the cycling of elemental Hg between the atmospheric and aquatic systems. We targeted at the reduction of HgII by small, semi-volatile dicarboxylic acids. The objectives of this research were to study the reaction kinetics using cold vapor atomic fluorescence spectroscopy (CVAFS) under different environmental-related conditions and to analyze the intermediates and products of the reaction using ultraviolet-visible (UV-VIS), direct mass spectrometry (MS) and Matrix-Assisted Laser Desorption Ionization Time of Flight Mass spectrometry (MALDI-TOF-MS). The reaction rate constants for three different dicarboxylic acids under various experimental conditions were obtained. The presence of the intermediate Hg- organic complex was confirmed by using MALDI-TOF-MS and NMR, suggesting that even smaller organic molecules can undergo Hg-organic complexation and further reduction. Our results also indicated that the presence of chloride ion significantly reduces the reduction rate by competing the complexation of HgII with dicarboxylic acids, while dissolved oxygen retards the production of Hg0 by providing the oxidant for the transformation of Hg0 to HgII after photolysis. Based on our experimental results, a tentative mechanism is proposed and the potential atmospheric implication is discussed.

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