Abstract
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.
Highlights
Mercury (Hg) is recognized as a priority hazardous substance that, once released locally, can travel long distances and becomes a toxic global pollutant.[1]
The risk associated with a Hg-contaminated site in terms of Hg toxicity and mobility strongly depends on the particular Hg species present
While it is difficult to provide a clear explanation for why the Hg concentration maximum in core K2 occurred at this particular depth within a relatively homogeneous silty loess layer, one can conclude that downward transport of Hg from the soil surface must have taken place
Summary
Mercury (Hg) is recognized as a priority hazardous substance that, once released locally, can travel long distances and becomes a toxic global pollutant.[1]. The main research questions of this study were (i) the extent to which Hg isotope signatures are variable and reflect the process transformation history in the subsurface of the site and (ii) the implications of the findings for source and process tracing at contaminated sites using Hg isotope signatures To this end, Hg isotope fractionation was studied from three viewpoints: (1) between two soil cores taken at different locations, (2) between bulk soil samples from different depths, and (3) between operationally defined subpools of Hg within individual samples. For PTD, sample aliquots were continuously heated from room temperature to 700 °C at a rate of 0.5 °C s−1 under a N2 gas flow (300 mL min−1) and the Hg release curves were measured using atomic absorption spectrometry (AAS) as a function of temperature They were compared to characteristic release curves of Hg reference compounds (see Figure S1).[36] Mercury concentration analysis of the extracts was carried out by a cold vapor flow injection atomic absorption spectrometer (CV-AAS) (FIMS 100, PerkinElmer, Waltham, MA, USA). No Δ200Hg and Δ204Hg anomalies were observed in the samples examined in this study
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