Mercury Isotope Fractionation by Environmental Transport and Transformation Processes

Mercury is a toxic metal with well known health risks, but uncertainties regarding its environmental fate remain. Analytical tools capable of distinguishing small variations in mercury isotope composition have recently become available and there is considerable interest in applying these to help improve understanding of mercury's complex biogeochemical cycle, and to identify specific sources to remediate. In this dissertation, mercury isotope fractionation by three environmental transport and transformation processes - mercury diffusion through a polymer, thermal decomposition of HgS(s), and mercury diffusion through air - are investigated. Clear understanding of processes that affect mercury isotopes, such as these, is needed to ensure field scale isotopic data are interpreted correctly./nA new analytical method for measuring mercury isotopes with high precision was developed to pursue the work described here. In this method, both mercury and thallium (for instrumental mass bias corrections) are introduced to a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) as a liquid aerosol. The addition of cysteine to liquid samples effectively controlled mercury memory effects. A purge and trap sample preparation technique, using KMnO4 or HOCl as mercury oxidants, was used in this work to prepare mercury in a common matrix. The long-term reproducibility of the method was approximately 0.3 / for δ202Hg, which is similar to other contemporary methods./nMercury diffusion through a polymer was found to have a very large isotope effect. This effect was determined by measuring Hg0 that permeated PVC tubing and matching this with models of the rate and isotopic composition of this gas. The isotope fractionation factor for this process, α202 = 1.00288±0.00040, is the largest factor yet determined for mercury near ambient conditions. This fractionation factor represents the relative diffusion coefficients of 198Hg and 202Hg in the polymer./nThere have been recent observations of mercury isotope variations at mercury mines that were speculated to have resulted from heating of mercury ores. In experiments described here, thermal decomposition of HgS(s) did not result in bulk isotope fractionation of the remaining HgS(s). This was evaluated by heating HgS(s) particles in an argon gas flow for different periods of time and measuring the mass and isotopic composition of remaining HgS(s). A model of congruent evaporation from a solid explained this lack of bulk isotope fractionation well. This model indicates that, while changes in the isotopic composition of a thin surface layer are possible, isotopic changes of the bulk material are very small./nMercury diffusion in air was found to have a large isotope effect that can be predicted by kinetic gas theory using only the molecular masses of mercury isotopes and air. This effect was determined by observing mercury remaining in a well mixed reservoir that was depleted by diffusion through a set of hypodermic needles. The ratio of 198Hg to 202Hg diffusion coefficients in air was determined to be 1.00125±0.00011. Kinetic theory predicts this ratio to be 1.00126. The fractionation factor of this fundamental and common environmental process is similar to the larger isotope fractionation factors documented previously./nThe determination of mercury isotopic variations with new analytical tools offers a promising approach for examining mercury in the environment. Interpretations of field measurements will need to be guided by mechanistic understanding developed under controlled conditions. The work described in this dissertation enables better understanding of mercury isotope fractionation by environmental transport and transformation processes that lead to isotopic variations throughout the environment. These isotopic differences suggest not only a means of interpreting environmental transport and transformation processes but also determining the dominant sources of mercury where there have been multiple releases.

Advisor: Hunt, James R.