Mercury speciation and reactivity in the coastal and estuarine waters of Long Island Sound

Date of Completion

January 2003


Biogeochemistry|Environmental Sciences




Mercury (Hg) is a toxic metal present in trace amounts in the environment. Its biogeochemistry is intricate and includes atmospheric transport, aquatic physical and chemical cycling and in situ transformation to the more toxic and bioavailable species monomethylmercury (MMHg). Once formed, MMHg bioaccumulates and biomagnifies in aquatic ecosystems, reaching concentrations in piscivorous fish that may pose health risks to humans and wildlife that consume fish. Human activity has significantly perturbed the cycling of Hg since the Industrial Revolution by increasing the amount of Hg in the environment worldwide by about 3-fold. There are indications that emissions continue to increase. As a result, there is an urgent need to understand the fate of Hg in the environment. ^ It is hypothesized that the chemical speciation of Hg in natural waters exerts a first order control on its fate by determining which biogeochemical pathway the metal is most likely to follow. Natural dissolved organic matter (DOM) is known to form complexes with Hg, and this chemical form is thought to have a very different biogeochemical reactivity than Hg bound to inorganic complexing agents (such as hydroxide and chloride). There is little quantitative information, however, on the strength and abundance of the DOM complexing agents in natural waters. This dissertation is focused on the development and application of a technique to determine these complexation properties from a variety of important waters in the Long Island Sound system. The technique developed is a wet chemical analog to electrochemical techniques used for other metals and is applied in a ligand titration. ^ Results indicate that the material that binds Hg ranges from <1–30 nN concentrations in Long Island Sound, the Connecticut, Pawcatuck and East Rivers, the Mid-Atlantic Bight, sediment porewaters and sewage effluent. The conditional equilibrium constants of this material are high (logK = 20–25). These properties imply that in freshwaters, Hg is >99.9% bound to organic matter. In estuarine and coastal waters, the amount of organic complexation can range widely and is dependent on season and location. ^ In an example of speciation control on biogeochemistry, experiments conducted under controlled conditions suggest that Hg must be bound to DOM in order to be effectively transformed to the elemental Hg form. This is an important reaction, as it results in gas exchange of Hg to the atmosphere and general detoxification of water bodies. ^