Comparison of mercury distribution and mobility in soils affected by anthropogenic pollution around chloralkali plants and ancient mining sites

Abstract

A comparative study of total mercury, elemental mercury and mercury fractionation has been performed on particle-size fractions of soils affected by anthropogenic sources of different origins, namely two former chloralkali plants (CAPs) and two historical cinnabar mine sites in Spain. A specifically developed sequential extraction procedure was applied in order to evaluate mercury fractionation, including Labile mercury (F1), mercury bound to humic and fulvic acids (F2), elemental mercury and mercury bound to crystalline iron oxides (F3), and mercury bound to sulfide and refractory species (F4). Markedly higher total mercury concentration were found in ancient mining soils (AMS), with concentration values ranging from 18.6 to 1509 mu g.g(-1), whereas soils from CAPs exhibited lower total Hg concentrations (1.7-61.6 mu g.g(-1)). Hg(0) was not detected in AMS samples and this indicates the absence of this species in the studied areas. In contrast, very low Hg(0) concentration were found in CAP soils as a consequence of the intense interaction of gaseous Hg deposited along with soil matrix compounds. Sequential extraction analyses revealed different fractionation patterns for both kinds of studied soils: AMS exhibited a clear predominance of Hg bound to sulfide in all particle-size subsamples whereas soils from CAPs mainly contained elemental Hg and Hg bound to crystalline iron oxides in the coarser subsamples. Conversely, Hg bound to sulfide was the major fraction in the finest soils. In summary, the results of the present study indicate a high level of Hg immobilization in CAP soils because of the interaction with soil phases and the prevalence of highly refractory cinnabar in AMS samples. Finally, it is necessary to highlight that, in some cases, compounds with higher reactivity can lead to a lower toxification risk if differences in concentrations are sufficiently marked, as detected in CAPs with respect to AMS.