Determination of Hg binding forms in contaminated soils andsediments: state of the art and a case study approachingabandoned mercury mines from Mt. Amiata (Siena, Italy)

This paper, after an introduction about the state of the art in the methods for the evaluation of Hg chemical speciation,focuses on Hg polluted soils and sediments from Siele mining area (Piancastagnaio, Siena, Italy). In particular, the mineralogicalcomposition was evaluated via X-ray powder diffraction, total Hg was determined via inductively coupled plasma optical emissionspectroscopy on acid digested samples, and Hg binding forms were investigated both via sequential selective extraction andpyrolytic extraction. Solutions obtained via sequential selective extraction were checked for Hg content using inductively coupledplasma optical emission spectroscopy, whereas to carry out pyrolytic extraction a thermogravimetric analyser coupled with anevolved gas mass spectrometer was used. To our knowledge, this last method was never used for this purpose on natural samples.Experimental results enhance that: i) Hg detection via thermal analyses can qualitatively distinguish among different Hg compounds.ii) The temperature of Hg thermal release is affected by both the sample matrix and the Hg bond type. iii) Thermal analysesfail in the attempt of a quantitative determination of the evolved species; this goal is better achieved via the selective chemical leaching.iv) Most of the Hg is related to HgS-like bonds, thus pointing to the presence of very stable Hg compounds. v) Only in somesamples we found also organo-complexed Hg-like compounds and, in a limited amount, hydrosoluble Hg and/or Hg species solublein weakly acid environment. vi) No correlation between sampling point and both total Hg and/or Hg speciation was observed.