Forms of Mercury: Beyond the Silver Liquid

By: Noelle Selin

It seems a bit strange to hear delegates at an intergovernmental negotiation on mercury discussing how to define “mercury.” Doesn’t the periodic table define it? Not only is mercury an element, but it’s also the reason why we’re all here in Geneva to negotiate an agreement. But defining exactly what is being addressed by the treaty is a critical issue – especially since mercury exists in many different forms in the environment.

Mercury in its liquid form is most  familiar.

Mercury in its liquid form is most familiar.

The chair’s draft treaty text defines mercury as “elemental mercury”. Elemental mercury is the liquid substance that many people recall when they think of mercury. In the atmosphere, most mercury is in elemental form, but it is a gas rather than a liquid. Elemental mercury is often abbreviated as Hg(0).

Another definition in the convention is “mercury compounds,” which addresses forms of mercury other than elemental mercury. What other forms of mercury are there?

Methylmercury is of particular concern, because it is the toxic form of mercury found in fish. Mercury is converted to methylmercury in aquatic systems by sulfate- and iron-reducing bacteria. For more on the health effects of methylmercury, see our earlier post.

In addition to elemental mercury, atmospheric mercury also exists as divalent mercury. Divalent mercury, also referred to as Hg(II), is formed when elemental mercury has undergone a chemical reaction of oxidation, losing electrons. In the atmosphere, Hg(II) can bind with other elements, but scientists don’t yet know exactly what these forms are. The chemical form of Hg(II) in the atmosphere could be HgCl2, HgBr2, Hg(OH)2, or HgO. The leading candidate is HgCl2, [give the name for this?], but this is a topic of current research. When Hg(II) is measured in the atmosphere, it is referred to as reactive gaseous mercury. Forms of mercury found in the ocean include both Hg(0) and Hg(II).

Emissions from different sources release different forms of mercury. Emissions from the surface ocean and land are in the form of elemental mercury. Anthropogenic sources, such as coal power plants, can release both Hg(0) and Hg(II). This is important because the two forms of mercury have different environmental behavior.

Hg(0) lasts for a long time in the atmosphere (6 months to a year), meaning that it circulates around the globe and can travel long distances. Hg(II) can easily rain or settle out after only a few days in the atmosphere, which means it is more likely to enter the environment nearby its source. Thus, reducing Hg(II) emissions will have important local benefits, compared with reducing Hg(0), which has important global benefits.

The behavior of mercury in the environment, however, is complex. Thus, we need to use computer models [pdf] to determine how mercury changes form and travels after it is emitted. These models use the chemical and physical properties of mercury in its various forms to estimate where mercury will travel over time. Mercury deposited to the environment as Hg(II) can return to the atmosphere as Hg(0). Additionally, Hg(0) can react (oxidize) to form Hg(II) in the atmosphere, and Hg(II) can then reduce back to Hg(0). In other words, mercury can change its form. This can occur anywhere in the atmosphere, even when it is being released from power plant plumes [pdf]. Ultimately, all mercury released continues to cycle through the environment for centuries, contributing to the global mercury legacy.

Many of these reactions are not well understood by scientists, so the transport and fate of mercury in the environment is a topic of significant ongoing research.

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