To
understand the transformations of mercury (Hg) species in the subsurface
of a HgCl2-contaminated former industrial site in southwest
Germany, Hg isotope analysis was combined with an investigation of
Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic
thermodesorption. Data from two soil cores revealed that the initial
HgCl2 was partly reduced to metallic Hg(0) and that Hg
forms of different mobility and oxidation state coexist in the subsurface.
The most contaminated sample (K2-8, 802 mg kg–1 Hg)
had a bulk δ202Hg value of around −0.43 ±
0.06‰ (2SD), similar to published average values for industrial
Hg sources. Other sample signatures varied significantly with depth
and between SEP pools. The most Hg-rich samples contained mixtures
of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool
exhibited heavy δ202Hg values of up to +0.18‰.
Sequential water extracts revealed slow dissolution kinetics of mobile
Hg pools, continuously releasing isotopically heavy Hg into solution.
This was further corroborated by heavy δ202Hg values
of groundwater samples. Our results demonstrate that the Hg isotope
signature of an industrial contamination source can be significantly
altered during the transformations of Hg species in the subsurface,
which complicates source tracing applications but offers the possibility
of using Hg isotopes as process tracers in contaminated subsurface
systems.
In addition to analytical speciation, reliable Hg species modeling is crucial for predicting the mobility and toxicity of Hg, but geochemical speciation codes have not yet been tested for their prediction accuracy. Our study compares analyses of Hg species in highly Hg-contaminated groundwater (Hgtot: 0.02-4 μmol·L(-1)) at three sites with predictions of Hg speciation obtained from three geochemical codes (WHAM, Visual MINTEQ, PHREEQC) with and without implementation of Hg complexation by dissolved organic matter (DOM). Samples were analyzed for chemical composition, elemental, inorganic, and DOM-bound Hg (Hg(0), Hginorg, HgDOM). Hg-DOM complexation was modeled using three approaches: binding to humic/fulvic acids, binding to thiol-groups, or a combination of both. Results of Hg(0) modeling were poor in all scenarios. Prediction accuracy for Hginorg and HgDOM strongly depended on the assumed DOM composition. Best results were achieved when weaker binding sites, simulated by WHAMs DOM submodel, were combined with strongly binding thiol groups. Indications were found that thiol-DOM ratios in groundwater are likely to be lower than in surface water, highlighting the need for analytical thiol quantification in groundwater DOM. This study shows that DOM quality is a crucial parameter for prediction of Hg speciation in groundwater by means of geochemical modeling.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.