Thallium contamination of desert soil in Namibia: Chemical, mineralogical and isotopic insights

摘要

We studied arid desert soils from Namibia (Rosh Pinah) that were contaminated with up to 7 mg kg(-1) of thallium (Tl) via dust emitted from a local flotation tailing dam. Chemical extractions of waste and soil materials indicated that most of the Tl is strongly bound, in accordance with X-ray diffraction and X-ray absorption spectroscopy data that point to the predominant association of Tl with metal sulfides and phyllosilicates. The isotope fractionation factor epsilon Tl-205 of the soil samples (from -0.4 to +3.8) shows a positive linear relationship (R-2 = 0.62) with 1/Tl, indicative for the mixing of two major Tl pools, presumably anthropogenic Tl and geogenic Tl. The epsilon Tl-205 value for the topmost soil samples (similar to+3) closely matches the epsilon Tl-205 value for post -flotation waste particles with a diameter of 0.05 mm, whereas the bulk flotation waste exhibits a significantly larger epsilon Tl-205 value (similar to+6). These variations are in accordance with predominant atmospheric transfer of Tl from the tailings to the adjacent soils via fine (dust) particles. The identified minimal Tl alteration in soils indicates that only a small part of the Tl could be potentially released and passively enter the vegetation, local population and/or food chain in the long term. From this viewpoint, Tl does not represent such an important environmental concern as other (abundant) contaminants at the locality. Furthermore, there could be a relevance for other alkaline desert soils, including those where Tl pollution plays a major role. (C) 2018 Elsevier Ltd. All rights reserved.