Plant availability of thallium in the rhizosphere of hyperaccumulator plants: a key factor for assessment of phytoextraction

The dynamics of thallium (Tl) fractions in the rhizosphere of two Tl hyperaccumulator plants, kale (Brassica oleracea acephala L. cv. Winterbor) and candytuft (Iberis intermedia Guers.), were examined to evaluate the efficiency of their possible use in phytoextraction. Plants were grown in a rhizobox system with a soil contaminated by Tl deposits from a cement plant in Leimen, Germany (1300 g Tl kg^–1 soil (aqua regia extraction) and 106 g Tl kg^–1 soil NH₄NO₃-extractable Tl). After 6 and 8 weeks growth of kale and candytuft, respectively, Tl fractions were sequentially extracted and compared with Tl uptake by plants. The uptake from `plant-available' Tl (fraction 1 – 4) in the rhizosphere (0–2 mm distance from root compartment) of both hyperaccumulator plants kale and candytuft accounted for 18 and 21% of the Tl accumulated in their shoots, respectively. The uptake from the `non-plant available' Tl (fraction 5 – 7) accounted for 50 and 40% of the mass of Tl accumulated by kale and candytuft, respectively. The high uptake capacity for Tl and the subsequent marked depletion in the rhizosphere soil might have resulted in a rapid shift in the equilibrium between the various Tl fractions. In addition, the high depletion in the rhizosphere indicates that the transport of Tl to roots is mainly diffusion driven. In conclusion, the easy access of the so called `non-plant available' Tl fraction in the rhizosphere soil by both hyperaccumulator plants indicates a high efficacy of possible phytoremediation of Tl contaminated soils such as the soil at the site in Leimen.