Distribution of natural and anthropogenic thallium in the soils in an industrial pyrite slag disposing area

The total concentrations combined with the chemical speciation of thallium (Tl) were examined in order to track the distribution of natural and anthropogenic Tl in the soils in an industrial pyrite slag disposing area. Their geochemical behaviors in the soils were further discussed. Soil samples were collected from three soil profiles adjacent to a large open-disposed pile of industrial Tl-rich pyrite slag, and from one soil profile in the background area. The results show that the soil contamination with Tl derived from slag (slag-Tl) is generally limited; slag-Tl was mainly accumulated in the upper part (< 16.5 cm) of the vicinal soils of the slag pile and shows large variation in concentration among different sampling sites. Basically, the soils surrounding the slag pile within 5 m are more Tl-contaminated than those under the slag pile and those far away from the slag pile. In respect of the concentrations of total Tl, the deeper soils of the studied profiles seem to be uncontaminated. However, the percentages of Tl in the easily reducible fraction indicate that these soils have been actually contaminated by slag-Tl. Natural Tl and anthropogenic Tl are distributed differently among the soil components in the studied soils. Natural Tl in the background soils is predominantly hosted in the residual fraction ( approximately 98%), while anthropogenic Tl was significantly incorporated into the more labile fractions of the soil (up to approximately 80%), especially in the acid-extractable fractions and easily reducible fraction (up to approximately 30% and approximately 45%, respectively). Detailed analysis of speciation data of Tl suggests that despite being predominantly controlled by the degree of Tl pollution, the distribution of slag-Tl in the soils can be further affected by the general differences in soil properties. In this study, the order for preferential immobilization of anthropogenic Tl among major soil components can be roughly summarized as: Tl(III) carbonates and hydroxides > Mn oxide-hydroxides > Fe oxide-hydroxides > adsorption sites on the surface of soil, while the order can be significantly mediated by the pH conditions in the soils. The correlations between the fractions of Tl in the slag and in the soils indicate that the anthropogenic Tl in the soils in the studied slag disposing area should be mainly derived from the dissolved slag-Tl that was leached by rainwater rather than from the washed-out particles of slag.