Risk analysis in alluvial soils contaminated by mercury from historical mining activities through metal speciation: a comparison between thermo-desorption and selective sequential extraction

Mercury (Hg) is recognised as a global pollutant occurring under various forms in the environment, each characterised by different mobility and bioavailability. Thus, determination of Hg speciation in environmental samples is essential for a proper evaluation of Hg pollution impact and potential associated health risk. Selective sequential extraction (SSE) techniques are usually used to assess Hg speciation in soils, but this approach is usually time-consuming and suffers of low reproducibility of results [Issaro et al., 2009]. An alternative option to SSE may be represented by thermo-desorption (TD), which allows a simple discrimination of Hg occurring as non-mobile red cinnabar (α-HgS) thanks to its specific release peak temperature. In this study, the application of TD for health risk assessment was tested in some sites within the Isonzo alluvial plain, impacted by past mining activity at upstream Idrija Hg mine [Acquavita et al., 2022]. Potential risk associated with Hg volatilisation, leaching and ingestion was modelled considering the average amount of non-cinnabar (non-α-Hgs) fraction determined on all available soil samples. Results were compared with those obtained through SSE approach [Bloom et al., 2003]. Due to contamination from mining, in all samples α-HgS was the predominant fraction. Overall, TD overestimated the risk associated with volatile and soluble fractions of Hg compared to SSE. Conversely, comparable and acceptable risk (hazard index <1) was obtained for ingestion pathway through both techniques. However, TD allows to analyse a higher number of samples than SSE in less time, resulting in an increased representativeness of real situation. Considering also the good reproducibility of the results, TD can thus represent a valuable tool for a first screening of potential health risk at Hg contaminated sites, possibly coupled with simple single-step extractions for the determination only of soluble Hg fraction.