Inhibition of Pb redistribution by two complexing agents (cryptand and NTA) during a sequential extraction of soil models

We previously reported a severe redistribution of Pb in both model soils and spiked real soil systems during two different sequential extraction schemes, and concluded that a manganese oxide and humic acid were the two main solid phases responsible for scavenging Pb liberated at the first extraction step. As an extension to that study, we have been examining a range of modifications to the first extraction step (0.11 M acetic acid, 16 h) of the three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) for a possible means of counteracting the Pb redistribution problem. In this study, the potential usefulness of two structurally-distinct soluble complexing ligands was investigated; cryptand 2.2.2 (a macrocyclic crown-like ligand) and nitrilotriacetic acid (NTA). The addition of each ligand to the first extraction solution of the BCR procedure generally improved the percentages of Pb recovered from our multi-phase model soils, with Pb originally spiked on calcite. However, the effectiveness of the cryptand and NTA at inhibiting redistribution (at concentrations of 9x10(-3) and 1x10(-4) M, respectively) was dependent on the percentages by weight of hausmannite and humic acid in each model system. In the presence of no more than 2% hausmannite, both cryptand and NTA, at the concentration used, could satisfactorily recover about 60% of the Pb, provided that the percentages of humic acid in the systems were not higher than 10% and 5%, respectively. The important preliminary criteria for selecting a potential useful soluble complexing agent suggested by the results of this study are that the ligand possess (a) a high stability-constant for the ion involved, and (b) a semi-encapsulated metal binding site (which appears to inhibit direct surface extraction from occurring). For Pb, we also found it desirable to decrease the extraction time of the first step in the BCR scheme as an aid to minimising opportunity for redistribution.