Stable isotope fractionation of mercury(Hg) during its transformations and cycling the environment
Various combinations of all possible sources (global vs. local; anthropogenic vs. natural) and transformations (biological vs. abiotic) can lead to the buildup of different species of Hg (i.e. Hg(0), Hg(II), Methylmercury etc.). In order to design and implement effective remediation strategies, we need new and effective tools to track the actual causes of Hg bio-accumulation in a given ecosystem.
Stable isotope fractionation
Stable isotopes are non-radioactive isotopes which differ from each other in their atomic masses. The difference in atomic mass leads to a small quantitative difference in the physico-chemical properties (e.g. boiling point, rate of reaction or diffusion) of the molecules carrying different isotopes which leads to fractionation (i.e. change in the relative abundance of stable isotopes in a sample as compared to a standard). See the simple example of a lighter element!
The measurement of relative amounts of two isotopes (i.e. stable isotope ratio of heavier over lighter isotope) in typical sources and determination of stable isotope fractionation mechanisms has provided insight into the biogeochemistry of many elements spanning the periodic table from hydrogen to molybdenum. The advent of high precision mass spectrophotometers in the past decade makes it possible to measure very small differences in stable isotope ratios of elements which were until recently considered ‘too heavy’ to fractionate.
Hg stable isotopes
Hg has seven stable isotopes and has characteristics (redox sensitivity and high degree of covalent character of its compounds) which suggests that it could undergo fractionation. Indeed, in recent years, a number of groups have measured and reported significant Hg isotope ratio variation in natural samples (hydrothermal ores, sediment cores and fish tissues). Thus the isotopic ratios of Hg in a sample may suggest its sources and/or the kind of transformations it has undergone. But development of a new stable isotope proxy for addressing complex biogeochemical phenomena requires the determination of stable isotope ratios for representative sources as well as extent of fractionation by individual biotic and abiotic transformations that occur in the environment.
In our lab, we are determining if the isotopic composition of Hg is altered during its various transformations such as methylHg degradation and Hg(II) reduction by various biological and abiological processes and also if the environmental variable such as temperature change the extent of fractionation. This research is carried out by Kritee in collaboration with Prof. Joel Blum’s lab at the University of Michigan and is supported by a Biogeoscience award from the National Science Foundation.
Recent Publications and Posters:
Kritee K., Joel. D. Blum, Marcus. W. Johnson, Bridget. A. Bergquist and Tamar Barkay (2006), Mercury stable isotope fractionation during reduction of Hg(II) to Hg(0) by mercury resistant microorganisms. Environmental Science and Technology. Web released Feb 9th, 2007
Poster presented at the 8th International Conference on Mercury as a Global Pollutant held in Madison, WI from 6-11 August, 2006. (Outstanding student presentation award)