Speciation of Mercury (II) and Methylmercury in Cloud and Fog Water

D.R Bittrich,A.P Rutter,D.E Armstrong,H Manolopoulos,J Collett,P Herckes,C.L Babiarz,J.J Schauer,S.P Chadwick

doi:10.4209/aaqr.2010.08.0067

Abstract

The fate of oxidized mercury (Hg) in clouds and fogs is affected by the complexation of oxidized Hg(Ⅱ) with other chemical species present in cloud and fog water. Metal complexation often influences the types of reactions available to a metal in an aqueous system. The influence of pH, major inorganic ions, and organic acids on the complexation of Hg(Ⅱ) and methylmercury (MeHg) was examined for a range of cloud and fog water compositions. Fog water was collected in the San Joaquin Valley, CA and rain water was collected at Devil's Lake State Park, WI to provide additional measurements of the chemical conditions of atmospheric media. A thermodynamic model was used to determine the speciation of Hg(Ⅱ) and methylmercury (MeHg) over a range of atmospherically-relevant cloud and fog compositions. The speciation of Hg(Ⅱ) in cloud and fog water was highly dependent on pH. For conditions found in most clouds and fogs, the chloride ion was the most important major ion controlling Hg(Ⅱ) complexation, even under conditions of relatively low chloride content. However, Hg(OH)2(subscript (aq)), and HgClOH(subscript (aq)) were found to dominate over HgCl2 in locations with high pH due emissions of agricultural ammonia; i.e. San Joaquin and Sacramento Valleys, CA. At concentrations relevant to typical cloud and fog waters, carboxylic acids (e.g. formate and acetate) did not play a significant role in Hg(Ⅱ) speciation. Methyl mercury was speciated as MeHgCl in most locations, except for the locations with high pH, where MeHgOH dominated. These results provide constraints over potential reaction pathways that may transform oxidized Hg(Ⅱ) in clouds and fogs.

Highlights

Mercury (Hg) in the environment is a concern to human health due to its conversion to methylmercury (MeHg), a neurotoxin, and the subsequent bioaccumulation of MeHg in fish consumed by humans (Bloom, 1992; Westcott and Kalff, 1996; NRC 2000)
Speciation of Hg(II) Complexes in Fog and Cloud Water Fig. 2 illustrates the modeled speciation of Hg(II) for four of the regions that typify the spectrum of regional cloud and fog water chemical composition
Similar Hg(II) speciation was observed in the San Joaquin Valley of California (Supplemental Fig. 2)

Summary

Introduction

Mercury (Hg) in the environment is a concern to human health due to its conversion to methylmercury (MeHg), a neurotoxin, and the subsequent bioaccumulation of MeHg in fish consumed by humans (Bloom, 1992; Westcott and Kalff, 1996; NRC 2000). Elemental mercury (Hg0) can exist in the vapor phase and undergo long-range transport (Lindqvist and Rodhe, 1985; Lin and Pehkonen, 1999a). Hg(II) can enter the aquatic ecosystem either by wet or dry Understanding the chemistry of cloud and fog water is critical in determining the fate and transport of mercury in the atmosphere; since metal complexation can determine what reactions a metal may undergo (Stumm and Morgan, 1996; Benjamin, 2002). Other reactions of Hg(II) may include reduction back to Hg0 or for the mercury to become associated with particulate matter (Amyot et al, 1994; Lamborg et al, 1999). MeHg has been measured in atmospheric deposition, but questions remain regarding whether it is formed in-situ, or transferred from other compartments

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Conclusion