Mass-Independent Fractionation of Mercury Isotopes during Photoreduction of Soot Particle Bound Hg(II)

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (epsilon Hg-202) of 1.48 +/- 0.02 parts per thousand(+/- 2 standard deviation) to 1.75 +/- 0.05 parts per thousand for aerosol-phase reactions (RH 28-68%) and from 1.26 +/- 0.11 to 1.50 +/- 0.04 parts per thousand for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Delta Hg-199 values for reactant Hg(II) as high as 5.29 parts per thousand, but negative odd-MIF occurred in aerosol-phase reactions, in which Delta Hg-199 values of reactant Hg(II) varied from -1.02 to 0 parts per thousand. The average ratio of Delta Hg-199/Delta Hg-201 (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.