The nickel phytoextraction potential of some ultramafic soils as determined by sequential extraction

Sequential extractions at three pH values were used to model the effect of successive harvests of Ni hyperaccumulator plants on the Ni availability of several ultramafic soils. All soils, with one exception, contained about 2 000 mu g/g Ni and all behaved in a similar manner. It was found that betwee n 13 and 80% of the total Ni in the soil was potentially available to plant s. The remainder could not be removed even after a large number of theoreti cal hyperaccumulator harvests. This necessarily limits the soils' potential for phytoremediation and phytomining. The concentration of residual solubl e Ni after several extractions could be predicted mathematically, As it has been shown that the Ni concentration in hyperaccumulator plants is related to the soluble Ni fraction in the soils, an estimate was made on how many economic harvests of Ni could be obtained from each soil. It was found that between three (Dun Mountain, New Zealand) and 18 (Kouaoua, New Caledonia) economic Ni extracts could be obtained before the soil would need to be mod ified. Similar sequential extractions may be used to determine the bioavail ability of other heavy metals in soils that are to be phytoremediated or ph ytomined. (C) 1999 Elsevier Science B.V. All rights reserved.