Mercury speciation in contaminated soils by thermal release analysis

Thermal release analysis of mercury species in contaminated soils was performed by temperature controlled continuous heating of the samples in a furnace coupled to an Atomic Absorption Spectrophotometer (AAS). It was shown that this method allows the identification of different redox states of Hg-species through their characteristic releasing temperature ranges. The method was applied to Hg-contaminated samples from an inactive chlor-alkali production plant in former East Germany (GER), and from a gold mining area in Pocone, Mate Grosso, Brazil (BRA), as well as synthetic soil samples obtained by spiking pre-heated soil matrices (GER and BRA) with the following mercury species: Hg-0, Hg2Cl2, HgCl2, HgO and HgS. The samples GER, in general, frequently showed the presence of Hg2+ probably bound to humic substances, in the case of samples with higher total carbon content. Only in highly contaminated samples (>3000 ppm of mercury) was HgO the predominant species. The samples BRA more frequently showed the presence of mercury species in the lower oxidation states, i.e. Hg1+ in combination with Hg-0. The method allows observing changes in Hg-speciation in the samples with time, mainly changes among the oxidation states Hg-0, Hg1+ and Hg2+. The treated GER matrix showed a stronger tendency to oxidise Hg-species than the BRA treated matrix, in which only added HgO is partially oxidised to Hg1+ and Hg2+. In contrast, the BRA matrix showed a pronounced tendency to reduce spiked Hg2+ to Hg1+. This may be the reason for the presence of Hg1+ in the majority of original BRA samples. The method appears to be very useful to study speciation of mercury and its dynamics. It can be used as a tool for monitoring mercury oxidation states and/or reactions of mercury in soils.