Isotope fractionation of mercury during its biogeochemical transformations: Volatilization and reduction.

摘要

This study focuses on the isotope fractionation of Hg during volatilization of Hg(0) and reduction of Hg(II) in aquatic systems for the purpose of understanding and tracking the biogeochemical cycle of Hg. Volatilization of Hg(0) across the water-air interface led to predominantly mass dependent isotope fractionation (MDF) with an average fractionation factor 10 3lnalpha202 = -0.46 +/- 0.02 (2SE). Lighter isotopes were preferentially volatilized, following a Rayleigh fractionation model. Photochemical reductions of Hg(II) by natural DOM and various low-molecular-weight organic compounds (LMWOC) yielded significant mass independent isotope fractionation (MIF) dominated by the magnetic isotope effects (MIE), which specifically enrich ((+)MIE) or deplete ((-)MIE) magnetic isotopes (99Hg and 201Hg) in reactants Hg(II). The direction of MIE depends on the initial spin multiplicity of the radical pairs generated as intermediates of primary photochemical procedures. In contrast, the MIF during abiotic non-photochemical reduction of Hg(II) is characterized by a distinct nuclear field shift effect (NFS), originating from the size and shape of nuclei instead of mass. The mechanisms of MIF ((+)MIE, (-)MIE and NFS) are intimately related to reaction mechanisms, and can consequently serve as a powerful tool in discerning different reduction pathways of Hg(II).;KEYWORDS: mercury, isotope fractionation, mass independent, magnetic isotope effect, nuclear field shift effect, volatilization, photochemical reduction, abiotic reduction, dissolved organic matter, low-molecular-weight organic compounds;The reactivity of Hg(II) in natural water is controlled by its organic ligands. Both kinetic and isotopic studies suggested the presence of multiple pools of Hg(II) with different reactivity caused by the binding of Hg(II) to different functional groups of natural DOM. Generally, Hg(II) bound to O/N donor groups was more reducible than those bound to reduced sulfur groups. (-)MIE was consistently induced by various sulfur-containing LMWOC during photochemical reduction of Hg(II), while most non-sulfur LMWOC led to (+)MIE. Therefore, the source of Hg(0) and the binding , environment of Hg(II) in water can be discerned using the direction of MIE.