Chemistry of Mercury to Methylmercury
Dr. Gary Gill
Texas A&M University - Galveston

Biogeochemical Controls on Monomethyl Mercury Production in Aquatic systems
It is now well recognized that the chemical form or chemical speciation of an element in aquatic systems dictates the elements transport, fate, bioavailability, and toxicity in aquatic systems. The chemical and phase speciation of mercury in aquatic systems is especially complex. Mercury can be found in the environment in two oxidation states, Hg (0) and Hg (II), it has an affinity to interact with natural organic material, it is very particle reactive, and it can be converted to methylmercury, which bioconcentrates in aquatic food webs, potentially leading to concentration levels in top fish which exceed safe consumption advisory levels.

From a thermodynamic viewpoint, the inorganic chemical speciation of mercury is fairly well understood. Interactions of mercury with organic material in aquatic systems has long been recognized as potentially important, but currently is poorly understood. Recent investigations in our laboratory have suggested that a substantial portion of what is often considered "dissolved" mercury is actually mercury associated with macromolecular colloidal organic matter. In addition, using competitive ligand equilibration techniques, we have found that a major portion of the mercury present in Galveston Bay is complexed by ~10 pM of a natural organic ligand(s), with a conditional stability constant > 10²⁹. These findings suggest that >99% of the solution forms of mercury in oxic estuarine systems exist as mercury-organic complexes. How broadly representative these findings hold true remains to be investigated.

It is now fairly well accepted that the main pathway for the introduction of methylated mercury forms into aquatic systems is via in situ production, mediated by sulfate-reducing bacteria. There is often a temptation in assessing methyl mercury concentrations and production in aquatic systems to focus predominately on loading or abundance of inorganic Hg as the dominant controlling factor. While this is indeed an important factor, it is by no means the only important factor, nor necessarily the controlling factor. A number of parameters have been identified as important in influencing the production and abundance of methyl mercury in aquatic systems including: mercury loading, the chemical form of mercury (chemical speciation), temperature, the availability of organic substrate for sulfate-reducing bacteria (i.e. a food source), mercury de-methylation activity (by bacteria), in situ reduction-oxidation conditions and in some cases photo-demethylation. To complicate the issue even more, many of these parameters vary temporally and spatially in aquatic systems. Any of these parameters can potentially limit the abundance of methylmercury in an aquatic system.

Benoit, Gilmour, Mason and colleagues have recently proposed that sulfide levels in aquatic systems can be very important in controlling methylmercury production by sulfate-reducing bacteria. This influence arises from the strong interaction between inorganic mercury and sulfide to form mercury-sulfide complexes and the bioavailability of these complexes to sulfate-reducing bacteria. They hypothesize that only neutrally charged mercury complexes (e.g. Hg⁰, HgS⁰ or HgCl₂⁰) are capable of readily passing bacterial membranes for intra-cellular mercury methylation. Hence, the in situ chemical speciation of mercury is very important in controlling Hg methylation. In anoxic systems, the inorganic speciation of mercury is dominated by sulfide complexes. At low sulfide levels (< 10 µM), neutrally charged HgS⁰ dominates the Hg-sulfide speciation. Above this concentration level, polysulfide (charged) complexes of Hg dominate.

The role of oil and gas platform operations and practices in directly or indirectly promoting the formation of methylmercury and its incorporation into the food webs around the platforms is currently of great interest. Platforms are frequently characterized as "Oases for Marine Life in the Gulf" or "Islands or Life", due to the proliferation of marine life which concentrates around the platforms. It is possible to speculate that this phenomenon might be promoting environmental conditions in sediments around the platforms which enhance the production of methylmercury. If the deposition of organic debris from the marine life around the platforms is locally elevated, this might provide the "fuel" needed to drive down oxygen levels, promote sulfate-reduction, and enhance methylmercury formation. This condition would exist even without the current concern of the possible augmentation of mercury in sediments from barite drilling mud. Whether such a localized phenomena is important on a Gulf-wide basis remains to be determined.

Complete Abstract List (suitable for printing)