Effect of illite and birnessite on thallium retention and bioavailability in contaminated soils

The influence of illite and birnessite (δ-MnO(2)) amendments on the retention and bioavailability of Tl in contaminated soils was investigated. The efficiency of both phases was evaluated using Tl uptake by white mustard (Sinapis alba L.), sequential extraction and sorption experiments. The obtained data demonstrate that the application of birnessite can effectively transform Tl from the labile (easily mobilizable) fraction to its reducible form, thus lowering Tl bioavailability in soil and subsequent accumulation by plants. The Mn oxide added to the soils reduced substantially Tl uptake; Tl levels in the plants decreased by up to 50%, compared to the non-amended soil. The effect of illite on the immobilization and uptake of Tl was less pronounced, and in the carbonate-rich Leptosol has not been proved at all, suggesting the importance of bulk soil mineralogy and nature of the soil sorption complex on the behavior of this amendment. Therefore, the general applicability of illite for Tl stabilization in soils seems to be limited and strongly dependent on soil composition. In contrast, the use of birnessite like soil additive might be an efficient and environment-friendly solution for soil systems contaminated with Tl.