Environ. Sci. Technol. 1994, 28, 1589-1597
A Chemical Kinetic Mechanism for Atmospheric Inorganic Mercury Christian Seigneur,' Jacek Wrobel, and Elpida Constantinou
ENSR Consulting and Engineering, 1320 Harbor Bay Parkway, Alameda, California 94502
A chemical kinetic mechanism for the atmospheric chemistry of inorganic mercury has been developed. Computer simulations conducted with this mechanism suggest that the half-life of elemental mercury could be of the order of hours for reaction with Cl2 in a nocturnal marine layer and for reactions with oxidants such as H202 in the ambient atmosphere. However,large uncertainties exist in the gasphase reaction rates of Hg(0) with Cl2, 03,and H202, and the overall half-life of Hg(0) due to chemical reactions in the atmosphere cannot be assessed with certainty without further laboratory kinetic data. In the presence of an atmospheric liquid phase, oxidation and reduction reactions lead toward an equilibrium between Hg(0) and Hg(I1). This equilibrium is a strong function of pH, liquid water content, and concentrations of SO2 and HC1. Under most simulation conditions, Hg(0) concentrations exceed Hg(I1) concentrations by at least 1 order of magnitude, which is consistent with observations.
Introduction
Mercury is emitted into the atmosphere from various anthropogenic and natural sources and is removed through dry and wet deposition processes. Mercury that has deposited on surface soils and water bodies may then bioconcentrate in vegetation and fish. Consumption of produce and fish that have high concentrations of mercury may lead to adverse health effects in humans and some predator animals (e.g., Florida panther, mink, bald eagle). Deposition rates of mercury from the atmosphere to soil and surface water bodies depend on the chemical and physical forms of mercury. Mercury can be present in the air either as a vapor or as particulate-bound species. Vapors tend to have different deposition rates than particles. In addition, various gaseous forms of mercury, which include, for example, elemental mercury and mercury chloride, have different water solubilities,which lead to different wet and possibly dry deposition rates. It is, therefore, essential to understand the chemical and physical transformations that govern the atmospheric behavior of mercury in order to be able to assess its environmental fate. Mercury exists in various valence states. Elemental mercury, Hg(O), is primarily present as a gas in the atmosphere. The second oxidation state of mercury is monovalent, Hg(I),such as the mercurous cation, Hg+.An example of a mercurous salt is Hg2C12. The third oxidation state of mercury is divalent, Hg(II), and includes the mercuric cation Hg2+. Mercuric compounds can be present in the atmosphere either as gases [e.g., HgClz or Hg(OH)21, in the aqueous phase [e.g., HgC12, Hg(OH)2or sulfites] or in the solid particulate phase (e.g., HgO or HgS) (I, 2). In addition, mercury also combines with organic compounds to form organomercuric compounds, which are environmentally important because of their ability to
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@ 1994 American Chemical Society
Table 1. Typical Concentrations of Mercury Species in the Atmospheric Environment mercury species
typical gas-phase concn (ng/m3)
Hg(0) Hg(I1)
0.09-0.19
2-5
typical liquid-phase concn (ng/L) 6-27 X 10-8 3.5-13.3
ref 1 3
Estimated from gas-phaseair concentrations by means of Henry's law.
bioconcentrate. Methylmercury and phenylmercury are examples of organomercuric compounds. The objective of this work is to develop a foundation for our understanding of the atmospheric chemistry of mercury. We review first the atmospheric chemistry of mercury and present an atmospheric chemical kinetic mechanism that we developed based on present knowledge of mechanistic, kinetic, and thermodynamic data. This kinetic mechanism is then applied to the simulation of various atmospheric scenarios to investigate the conversion rate of mercury between its elemental and divalent forms and the distribution of mercury between the gas phase and the particulate phase/droplets. Through those simulations, we can identify the key components of the mercury chemical mechanism and assess whether any significant uncertainties or data gaps exist in our knowledge. This process allows us then to recommend specific laboratory experiments to reduce existing uncertainties or fill eventual data gaps. As new data become available, it will be possible to incorporate them into this model, thereby improving our capabilities for simulating the atmospheric behavior of mercury. Review of Atmospheric Chemistry
The chemistry of mercury in the atmospheric environment may take place in the gas phase and in the aqueous phase. Aqueous-phase chemistry is associated with particle and droplet chemistry. In addition, heterogeneous reactions may occur at the surface of atmosphericparticles. Reactions that take place in the atmosphere will depend on the concentration of mercury or its compound as well as on the concentration of the species that react with mercury. The chemistry of the organomercurials (e.g., methylmercury) is not included in this atmospheric chemistry for the following reasons. First, although organic mercury may be emitted from some sources, anthropogenic emissions of mercury are primarily inorganic forms of mercury, and organic forms of mercury are primarily produced through biogenic transformations. Second, although organic mercury species may be present in the atmosphere as they are directly emitted from some anthropogenic sources or as they volatilize from water bodies or surface soil, there is no information on the reaction products and kinetics of the atmospheric reactions that may transform these organic mercury species in the atmosphere. Typical atmospheric concentrations of mercury species reported in the literature are presented in Table 1. Environ. Sci. Technol., VoI. 28, No. 9, 1994
I589
Table 2. Gas-Phase Reactions of Mercury
reaction
(1)
-.Hg(II)(g) + Clz(g) - HgCMg) + Odg)
-
(2) (3) Hgo(g) + BrzW HgBrz(g) (4) Hgo(g)+ I&) HgIz(g) (5) Hgo(g) + HzOz(g) Hg(OH)z(g) (6) Hgo(g)+ 2NOz(g) Hg(NOz)z(s,g) (7) Hgo(g) + HCl(g) products (8)2Hgo(g) + Oz(g) 2HgO(s,g) (9) Hgo(g)+ SOz(g) products (10) Hgo(g) + H&g) products (11)Hgo(g) + NzO(g) -products (12) Hgo(g) + NH3(g) -products (13) Hgo(g) + 2HI(g) HgIz(g) + Hz(g) hu (14) HgCldg) products +
---
-5
(15) Hg(OH)z(g)
-
Hgo
Table 3. Aqueous-Phase Reactions of Mercury
equilibrium or rate parametera (cm3molecule-' 9-11
ref
