Development of Methods for Determining Dry Deposition of Mercury Using an Ion-exchange membrane: Relative Rates of Mercury Dry Deposition at Sardis, Enid, and Grenada Lakes

摘要

This research focuses on studies developing methods to measure dry deposition of mercury (Hg) using an ion-exchange (IX) membrane to capture gaseous mercury species in the air. Mercury is a toxic heavy metal that is spread globally through the atmosphere. Atmospheric Hg deposits to terrestrial and aquatic ecosystems through wet and dry deposition. While routine methods have been developed to study wet deposition of Hg, measuring dry deposition of Hg is more problematic and often overlooked. In this study, we developed an inexpensive means to deploy a polyethersulfone cation exchange membrane in the field by dangling it within a polycarbonate bottle containing holes in the bottom to allow gas exchange. We tested several different analytical methods to measure the Hg on the membranes including atomic absorption spectrometry, atomic fluorescence spectrometry, and mass spectrometry. After demonstrating that the field method is capable of capturing and retaining airborne Hg on the membrane, we deployed the bottles containing the membranes at Sardis, Enid, and Grenada Lakes, located in north Mississippi. The purpose was to estimate the relative rates of dry deposition of Hg in order to explore differences in the levels of Hg found in fish from these lakes. We hypothesized that point sources near Grenada Lake, including a coal-fired power plant, may result in higher Hg deposition rates, which may be the reason for the higher Hg levels observed in fish from Grenada Lake compared to the other lakes. However, results show that Sardis Lake had the highest dry deposition rates followed by Enid and Grenada Lakes. Thus, the higher levels of Hg in fish from Grenada Lake remain unexplained.