Mass-independent fractionation of mercury isotopes during photochemical reduction in freshwater systems

Mercury is a globally distributed environmental toxin. Both inorganic and methylated species have severe detrimental effects on humans and animals, but it is methyl mercury (MeHg) that bioaccumulates in food webs and results in significant human exposure via fish consumption. Photochemical reduction of aqueous Hg species to dissolved gaseous Hg(0) can result in a net transfer of Hg from aquatic systems to the atmosphere, making it unavailable for methylation. In addition, photo-reduction of MeHg is an alternative fate to bioaccumulation for this powerful neurotoxin. Both mass-dependent isotope fractionation (MDF) and mass-independent fractionation (MIF) are observed in natural samples. MIF is the deviation in isotope ratios from those predicted by MDF based on 202Hg/198Hg. Bergquist and Blum 2007 showed that aqueous photo-reduction of Hg2+ and MeHg in the presence of dissolved natural organic matter results not only in Rayleigh-type MDF but also significant MIF, with the odd isotopes 199Hg and 201Hg being preferentially retained in the reactant (soluble) phase. Berquist and Blum 2007 also observed that the ratio of the MIF for the odd isotopes was different for each of the photo-reduction pathways (MeHg versus Hg2+) and suggested this ratio could be unique to certain pathways, which might allow identification of photo-reduction among other pathways in natural samples. They also suggested that the magnitude of MIF might relate quantitatively to the amount of photo-reduction Hg undergoes in aqueous systems. To better understand the causes of MIF and its capacity along with MDF as a tool for tracing photo-reduction of Hg, further experiments mimicking the freshwater photo-reduction of Hg2+ and MeHg were carried out. Each species was photo-reduced in the presence of Suwannee River Fulvic Acid with different portions of the electromagnetic spectrum blocked by filters. Bergquist and Blum 2007 suggested the magnetic isotope effect (MIE) as the cause of the MIF they observed. The MIE should be expressed during reactions involving excited radical pair intermediates and therefore should be affected by the wavelengths of light available for radical production. Experiments were performed (1) under full spectrum, (2) with over 90 percent of UVB rays blocked and (3) with over 95 percent of both UVA and UVB rays blocked. These experiments were run simultaneously under natural sunlight. The Hg2+ full spectrum experiments showed an 84 percent loss of Hg2+ to Hg vapor, which was larger than the 77 percent loss of the UVB blocked reactor and the 14 percent loss of the UVA and UVB blocked reactor. Significant differences in isotope ratios were also observed, and initial results suggest that MIF may be most sensitive to UVA. Since radical production is key to the MIE, this suggests that the UVA region may be most important to producing Hg radicals. In contrast, the losses in all three MeHg reservoirs were similar suggesting that the UVA and UVB portions of the spectrum are not critical in MeHg photo-reduction.