Uptake of thallium by vegetables: Its significance for human health, phytoremediation, and phytomining

Eleven common vegetables (green bean, beetroot, green cabbage, lettuce, onion, pea, radish, spinach, tomato, turnip, and watercress) as well as the thallium hyperaccumulator Iberis intermedia, were grown in pot trials containing 0.7 and 3.7 mg/kg thallium added to a silt loam soil. The aims of the experiments were threefold: to estimate risks to human health of vegetables grown in thallium-rich soils, to demonstrate the potential of crops of these plants to remove thallium from polluted soils (phytoremediation), and finally to establish the degree to which part of the costs of remediation could be recouped by selling the extracted thallium (phytomining). Maximum thallium levels ranged from nearly 400 mg/kg (d.m.) in Iberis down to just over 1 mg/kg in green bean. The four vegetables with the highest thallium levels (watercress, radish, turnip and green cabbage) were all Brassicaceous plants, followed by the Chenopods beet and spinach. At a thallium concentration of 0.7 mg/kg in the soil only green bean, tomato, onion, pea and lettuce would be safe for human consumption. At 3.7 mg/kg thallium, only green bean and tomato could be eaten. The Iberis had by far the best potential for phytoremediation of thallium-contaminated land and would need 5 sequential croppings to reduce 1 mg/kg thallium to 0.1 mg/kg in the soil. By contrast rape would take 9 years and green cabbage over 30 years. Some of the costs of phytoremediation might be recouped by selling the thallium which currently has a world price of $US300/kg. It was concluded that phytoremediation of thallium-contaminated soils containing > 1 mg/kg thallium will never be feasible by use of common vegetables. For soils containing 1 mg/kg thallium or less, use would have to be made of Iberis intermedia or Brassica napus (rape) rather than common vegetables.