Uptake of thallium from artificially contaminated soils by kale (Brassica oleracea L. var. acephala)

A pot experiment focused on the study of factors influencing thallium transfer from contaminated soils into kale (green cabbage, Brassica oleracea L. var. acephala, variety Winterbor F1) was evaluated. Three different types of topsoils with naturally low content of thallium (heavy, medium and medium-light soil) were used for pot experiments. The soils were contaminated with thallium sulfate to achieve five levels of contamination (0, 0.52, 2.10, 4.20 and 5.88 mg/kg). There were six replicates for each combination (90 pots in the experiment). The first part of the experiment started in the year of contamination (2001) and continued in 2003. The soil samples and the samples of kale (leaves and stalks were sampled separately) were collected and analysed. Kale was found to be able to accumulate Tl without any influence on yield. The highest thallium concentration was found in the leaves of kale in the first year of the experiment and reached 326 mg/kg dry matter. Bioaccumulation factor (Biological Absorption Coefficient – BAC) was found to be over 80 during the first year of the experiment. In the third year the BAC was around 3 for the soil with the highest pH and the highest organic matter content but as high as 15 for an acid soil with the lowest content of organic matter and the lowest Cation Exchange Capacity (CEC) of soils. The content of thallium in the leaves of kale was found to be 7 to 10 times higher than in the stalks in the third year. In the first year this ratio was up to 18. From these findings it can be concluded that the ability of some plants of Brassicacea family, that are planted as common vegetables, to accumulate thallium is very high and can be a serious danger for food chains. Neutral soils high in CEC and organic matter are able to bind thallium more effectively than poor acid soils and the transfer of Tl into plants from these soils is substantially lower. The uptake of Tl from contaminated soils into kale can be very high and without any negative effect on the plant growth. The transfer of Tl into kale decreases with the time necessary to reach the equilibrium between the added Tl and the soil (ageing of a sample).