Mercury (Hg) Speciation in the Soil–Plant System of Formerly Polluted Soils

The distribution of mercury (Hg) species in soils, as affected by (i) the physicochemical and biological properties of the soils, (ii) the lithogenic vs. anthropogenic sources of soil Hg pollution, and (iii) soil–plant interactions, was investigated in a model pot experiment in which Sinapis alba (Brassicaceae) was planted. The pseudototal (aqua regia-soluble) Hg contents in soils originating from the vicinity of the former cinnabar mine varied between 16.6 and 44.7 mg/kg, whereas in soils from sites where the amalgamation technique had been used for the extraction of gold from ore-bearing materials, the pseudototal Hg values varied between 1.63 and 10.1 mg/kg. However, Hg accessibility was low, with mobilizable Hg(II) accounting for 3–11% of total soil Hg and mobilizable methylmercury (MeHg) remaining below 1%, indicating a limited bioavailable pool under the studied conditions. Mobilizable Hg(II) showed significant negative relationships with total soil carbon and cation exchange capacity (CEC), reflecting its strong association with charged functional groups of the soil sorption complex. The low Hg accessibility in the soil resulted in low Hg contents in plants, not exceeding the feed safety thresholds, with a significant proportion of Hg taken up by the plants being retained in the roots. The results of the determination of gaseous elemental mercury (GEM) indicated its relevance in soil mercury cycling, where further research on the role of plants in GEM emissions is necessary. In this context, the GEM concentrations increased in plants found in soils collected close to the former cinnabar mine. These aspects should be investigated further in future studies.