Mercury isotopes in contaminated soil-aquifer systems

Mercury (element symbol Hg) is a naturally occurring toxic heavy metal. The danger of mercury for humans and the environment strongly depends on its chemical form. Mercury occurs naturally mostly in ore deposits in the mineral cinnabar (HgS). In this form, mercury is strongly bound to sulfur and exhibits a very low solubility and toxicity. Cinnabar was mined for many centuries by humans. The contained mercury was transformed into more dangerous forms and used e.g., in industrial processes or in gold mining. For example, metallic mercury, which is liquid at room temperature and was used in old thermometers, evaporates easily into the gaseous phase and the inhalation of the fumes can cause severe neurological damage. Another very toxic form is mercury(II)-chloride, which was used until the 1960s in the impregnation of wood. This Hg form prevents the growth of fungi and renders the treated wood more resistant against decay. However, it possesses a high solubility in water and can thus be transported easily in soils and groundwaters. Although the use of mercury by humans has now been strongly restricted, there are still many contaminated sites, e.g. in the vicinity of former industrial facilities. The investigation of the behavior of mercury at such industrial legacy sites stands in the center of this research project. There are big knowledge gap concerning the occurrence, transformations, and transport of different mercury forms in soil and groundwater, and new research approaches are needed. Mercury consists of seven stable isotopes, i.e. Hg atoms with a slightly different mass due to a different number of neutrons in its atomic core. The relative abundance of the Hg isotopes can change as a result of physico-chemical processes (e.g., volatilization, redox reactions). Therefore, the isotopic fingerprint of an environmental sample may be used to derive information on different sources and past transformation processes. In this research project, we will combine, for the first time, different modern analysis methods to determine the concentration, the speciation (Hg forms), and the stable isotope ratios of mercury. We will test the hypothesis that the transformation of different Hg forms in contaminated soil and groundwater leads to characteristic changes of the isotopic fingerprint. In addition to sampling soil and groundwater at two contaminated sites in SW-Germany, we will perform laboratory experiments and modeling. We expect that our results will contribute significantly to a better understanding of the behavior of mercury in the environment and for the prediction of the risk potential at contaminated legacy sites.

Contaminated sites represent local hotspots of Hg which pose a risk to humans and the environment because of their toxicity and mobility. In the present thesis, Hg transformation processes at such hotspots were examined at two sites where soil and groundwater had been contaminated by the release of Hg during wood preservation using highly soluble mercury chloride. The findings demonstrate how the interpretation of Hg geochemistry can be improved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid and liquid phases. A particular focus lies in testing the applicability of Hg stable isotopes for tracing transformation processes changing the physico-chemical properties of Hg. Volatilization of Hg as gaseous elemental mercury (GEM) from one of the facilities was investigated using Hg accumulated in growth rings of trees. Tree ring samples indicated significantly higher Hg concentrations in samples dating back to the active industrial period compared to samples from after the closing. Using Hg isotope ratios of Hg archived in tree rings and bark samples from trees surrounding the site internal tree Hg cycling was investigated. Using analyses of Hg binding forms and Hg stable isotope ratios in-stream Hg transformation processes after exfiltration of contaminated groundwater were investigated. A large shift in Hg isotope ratios towards negative 202Hg values was observed downstream of the contaminated site (change of 2 ) along with a minor offset in mass-independent fractionation. Considering both mass-dependent and mass-independent Hg isotope fractionation a conceptual model explaining the observed isotope signatures as a result of kinetic isotope effects during sorption, redistribution of Hg within the sediment, and the preferential transport of the isotopically fractionated redistributed fraction preventing re-equilibration was proposed. The presence of Hg(0) at both field sites raised the question about the dominant reduction pathway. To be able to distinguish between different Hg(II) reduction pathways using Hg stable isotopes the knowledge of isotopic enrichment factors is required but not available for many processes. The Hg isotope fractionation during the reduction of Hg(II) by dissolved, surface-bound, and structural Fe(II) was investigated using batch experiments and analyzing both reactants and products, and isotope enrichment factors were determined. Notably, these experiments provide the first experimental observation of a small but consistent mass-independent fractionation of even mass isotopes (200Hg, 204Hg). The findings of this thesis provide insights into the applicability and limitations of Hg isotope as process tracers and the benefits of combining multiple analytical approaches to assess the geochemical behavior of Hg at contaminated sites. The results of this work not only contribute to the understanding of transformation processes at contaminated sites but have implications for the general interpretation of Hg isotope ratios in natural samples.