Nuclear field shift effect in isotope fractionation of mercury during abiotic reduction in the absence of light.

We investigated the abiotic reduction of inorganic Hg(II) by dissolved organic matter (DOM) and stannous(II) chloride (SnCl(2)) in the absence of light and quantified fractionation of Hg isotopes during these processes. The kinetics of reduction by DOM was characterized using multiple parallel pseudo-first-order reactions, implying different reactive Hg(II) species resulting from Hg-DOM complexation. Significant mass independent isotopic anomalies were observed in reduction by both reducing reagents. Isotopes with odd atomic masses ((199)Hg and (201)Hg) showed less enrichment in reactants Hg(II) than expected for a mass dependent fractionation process. The fractionation factors (alpha) showed an odd-even staggering pattern that resembles the variation of nuclear charge radii. We demonstrated that these isotopic anomalies originated from nuclear field shift effect (NFS). The contribution of NFS to the measured fractionation factors was estimated and found to be as significant as the mass dependent effect. The observed Delta(199)Hg/Delta(201)Hg slope was explained by NFS and determined to be between 1.5 and 1.6 in abiotic nonphotochemical reduction, which is distinguishable from slopes determined for photochemical reduction. Therefore, we first demonstrated experimentally the significance of the nuclear field shift effect during reduction of Hg(II) and showed the application of isotope fractionation to distinguish between different reduction pathways.