Assessment of the extent of lead redistribution during sequential extraction by two different methods

Synthetic model soils were used to evaluate redistribution of lead (Ph) in both the three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) and the commonly applied procedure of Tessier et al. (1979). Redistribution of Pb in both schemes was found to be substantial, sufficient to render meaningless any attempts to quantify the original phase associations of Pb in cases where these are not known. In the model systems, the manganese oxide and humic acid phases were found to be responsible for most of this redistribution in both schemes. Despite the fact that both schemes allow severe Pb redistribution to occur, differences are apparent in the manner in which this occurs. Whereas Tessier et al.'s procedure in all cases yielded the highest recovery of Pb at the reducing step, the three-stage BCR procedure released the most Pb at the oxidizing step, both irrespective of where it originated. These differences are attributable to a combination of high temperature (96 degrees C) and the presence of acetic acid (25%, v/v) in Tessier et al.'s reducing step. These conditions were found to reduce the sorptive capacity of humic acid by more than 50%, in addition to retaining more of the Pb in solution by acetate ions. Extensive redistribution in the three-stage BCR procedure was also observed in systems where natural soils were mixed with Pb-spiked synthetic phases. In keeping with the results related to synthetic soil models, the organic matter and Fe/Mn oxide fractions of the natural soils appear to dominate the redistribution of Pb during the extraction.