Concentrations of atmospheric reactive mercury in Australia and emissions of reactive mercury from contaminated mine materials using cation exchange membranes
thesisposted on 28.03.2022, 15:51 by Matthieu B. Miller
Mercury (Hg) is a pervasive and persistent environmental contaminant with a significant atmospheric cycle that results in distribution to all spheres of the environment. In the atmosphere Hg is found primarily as gaseous elemental Hg0 (GEM) but under certain scenarios reactive Hg2+(RM) compounds may form a significant proportion of total atmospheric Hg. Physiochemical transformation processes play a leading role in the air-surface exchange of Hg and are a major determinant of Hg mobility and longevity within the environment, including bioavailability and biological uptake into ecosystem food webs, which constitutes the primary exposure pathway to the human population. Practical differentiation of gas and particle phase RM has proven to be a profound and lasting measurement challenge, and even the broadest distinction between GEM and RM is easily confounded. In addition to the intrinsic difficulties of measurement, data on atmospheric Hg in the Southern Hemisphere (SH) suffers the further limitation of being comparatively rare, and almost non-existent for RM. Lack of data inhibits international scientific and regulatory efforts to monitor, model, and mitigate the effects of Hg exposure to human populations. In an effort to help close some of the information gap, this study deployed a recently developed filter-based RM sampling system at two locations in Australia with existing GEM monitoring capabilities, and on board the Australian Research Vessel Investigator (RVI) during a research voyage to the East Antarctic coast. The ground-based field sites included a remote, temperate coastal site at the Cape Grim Baseline Air Pollution Station (CGBAPS) on the northwestern point of Tasmania, and a temperate urban site at Macquarie University Automatic Weather Station (MQAWS) in Sydney, New South Wales. Measurements were undertaken between November 2015 and May 2017, constituting the most extensive dataset on RM concentrations in Australia to date. As has been shown for GEM in the SH, concentrations of RM were relatively uniform over time and between sites, despite very different environments. The overall mean RMconcentration was 15.9 ± 6.7 pg m-3 at CGBAPS and 17.8 ± 6.6 pg m-3 at MQAWS. No seasonal trend in RM concentration was apparent at CGBAPS, in contrast to a small but statistically distinct difference in winter versus summer RM concentration apparent at MQAWS. The concentration of RM measured on RVI over the Southern Ocean averaged 23.7 ± 7.0 pg m-3 during four deployments in austral summer (January 10 – March 4, 2017). GEM was measured concurrently with the RM filter sampling, with an overall mean concentration of 0.65 ± 0.24 ng m-3 MQAWS and 0.90 ± 0.35 ng m-3 at CGBAPS, and 0.53 ± 0.10 ng m-3 on the RVI voyage. Additionally, tandem measurements of GEM were performed for one year at CGBAPS using both a Tekran® 2537B and a 2537X Automatic Ambient Air Analyzer. Inter-comparison of the two instruments indicated that the 2537X unit measured systematically higher GEM concentrations versus the 2537B, with an average difference of 8.6%. Complimentarily to the larger goal of quantifying ambient RM, the CEM material was deployed in a novel technique to measure RM air-surface exchange, in conjunction with an established method based on Teflon dynamic flux chambers (DFC) and a Tekran® 2537 mercury analyzer. Initial results using this combined CEM/DFC methodology indicate that RM flux can be detected at a minimum difference of ~ 13.5 pg m-3 between DFC inlet and outlet concentrations, based on the analytical detection limit of blank CEM material. Distinct differences in RM flux were observed between material types with a range of Hg concentration, and between wet and dry materials. Mercury contaminated mining materials such as tailings may have relatively large RM fluxes of several 1000 pg m-2 h-1. In addition to the ambient field site deployments, extensive method validation tests were undertaken in controlled laboratory conditions. As part of these tests, CEM material was exposed to high concentrations of GEM and a representative Hg2+ compound (HgBr2), using a custombuilt permeation system. The CEM material was found to take up a consistently negligible amount of GEM (0.004 ± 0.001% of total exposure) in clean laboratory air at varying levels of humidity. GEM concentrations during the permeation tests were 1.43x106 to 1.85x106 pg m-3,resulting in total GEM exposures ranging from 2.7x106 to 7.3x106 pg. The low rate of GEM uptake at these high concentrations indicates that, at typically much lower ambient atmospheric concentrations, GEM uptake would be concomitantly small and essentially unobservable, an important prerequisite for the CEM material to be successfully employed in ambient air sampling. The CEM material also exhibited very little breakthrough (< 0.5%) of total permeated HgBr2 at concentrations up to ~5000 pg m-3, indicating a high collection efficiency for this particular gaseous oxidized Hg compound under laboratory conditions.