Tracing Anthropogenic Thallium in Soil Using Stable Isotope Compositions

Thallium stable isotope data are used in this study, for the first time, to apportion Tl contamination in soils. In the late 1970s, a cement plant near Lengerich, Germany, emitted cement kiln dust (CKD) with high Tl contents, due to cocombustion of Ti-enriched pyrite roasting waste. Locally contaminated soil profiles were obtained down to 1 m depth and the samples are in accord with a binary mixing relationship in a diagram of Ti isotope compositions (expressed as epsilon Tl-205, the deviation of the Tl-205/Tl-263 ratio of a sample from the NIST SRM 997 Ti isotope standard in parts per 104) versus 1/[Tl]. The inferred mixing endmembers are the geogenic background, as defined by isotopically light soils at depth (epsilon Tl-205 approximate to -4), and the Tl emissions, which produce Tl-enriched topsoils with epsilon Tl-205 as high as +/- 0. The latter interpretation is supported by analyses of the CKD, which is also characterized by epsilon Tl-205 approximate to +/- 0, and the same epsilon Tl-205 value was found for a pyrite from the deposit that produced the cocombusted pyrite roasting waste. Additional measurements for samples from a locality in China, with outcrops of Tl sulfide mineralization and associated high natural Ti backgrounds, reveal significant isotope fractionation between soils (epsilon Tl-205 approximate to +/- 0.4) and locally grown green cabbage (epsilon Tl-205 between -2.5 and -5.4). This demonstrates that biological isotope fractionation cannot explain the isotopically heavy Tl in the Lengerich topsoils and the latter are therefore clearly due to anthropogenic Tl emissions from cement processing. Our results thus establish that isotopic data can reinforce receptor modeling for the toxic trace metal Tl.