Rapid thermoscanning technique for direct analysis of mercury species in contaminated sediments: From pure compounds to real sample application

Mercury (Hg) in aquatic environments accumulates in sediments in several chemical forms, both inorganic and organic, which are often determined through time-consuming selective and sequential extraction procedures. Thermal desorption technique (pyrolysis) coupled with continuous determination by atomic absorption spectrometry (AAS) may be an easy-to-use alternative technique for the rapid identification and quantification of Hg species in the solid matrix. This technique is based on the gradual heating of a sample that releases Hg at different temperature intervals depending on its chemical form. Thus, a single Hg species that desorbs at a specific temperature may be identified via a thermogram of the sample. In this work, several commercial pure Hg compounds, natural Hg mineral species (red cinnabar, α-HgS) and one compound synthesised in the lab (α-FeOOH–––Hg) were mixed with synthetic calcium carbonate (CaCO3), silica (SiO2) and natural matrices (silicate and carbonate marine sediments) which were then desorbed in order to determine the desorption peak temperatures corresponding to each Hg species. Moreover, possible interference caused by the matrix was also considered. The results obtained from 52 desorbed MIX samples displayed different desorption temperatures for the same Hg species depending on the matrix used. Indeed, Hg species mixed with synthetic SiO2 desorbed at a temperature lower than the same species mixed with synthetic CaCO3 with a difference of approximately 100 ◦C. The analytical approach was applied to selected coastal sediments from the Gulf of Trieste (Northern Adriatic Sea, Italy), contaminated by Hg from the five centuries of cinnabar (α-HgS) mining activity from Idrija (Slovenia), in order to identify the unknown Hg species. The results revealed the presence of two peaks with a distinct temperature of desorption (~250 and 350 ◦C). The highest temperature corresponds to the mostly refractory red cinnabar (α-HgS) compound, whereas the lowest temperature is related to other species (e.g. β-HgS) that may also include Hg associated with more mobile and potentially bio-accessible species (e.g. α-FeOOH–––Hg) if compared to α-HgS. This methodological approach is a rapid and cost-effective technique useful to preliminarily quantify more stable Hg species (mainly α-HgS), underlining the relevance in considering the chemical form of the element rather than merely the total concentration for simplifying the environmental management of Hg-contaminated sediments.