Fractionation of lead in soil by isotopic dilution and sequential extraction
The chemical reactivity of lead in soil is difficult to assess and depends on both soil conditions and the origins of the lead. This paper tests the combined application of lead isotopic techniques and chemical extraction to our understanding of lead fractionation in soils. Possibly against expectation, it appears that the 'reactivity' of lead can be high and yet there is tentative evidence that the original source of the metal affects its fractionation in soil, even after long contact times.
... ng contact times. Abstract 'Reactivity' or 'lability' of lead is difficult to measure using traditional methods. We investigated the use of isotopic dilution with 204 Pb to determine metal reactivity in four soils historically contaminated with contrasting sources of Pb, including (i) petrol-derived Pb, (ii) Pb/Zn mine-spoil, (iii) long-term sewage sludge application and (iv) C19 th urban waste disposal; total soil Pb concentrations ranged from 217 -13600 mg kg -1 . A post-spike equilibration period of three days and 5.0 x 10 -4 M EDTA electrolyte provided reasonably robust conditions for measuring isotopically-exchangeable Pb although in acidic organic soils a dilute Ca(NO 3 ) 2 electrolyte may be better to avoid mobilisation of 'non-labile' Pb. Results showed that the reactive pool of soil Pb is often larger than may be intuitively expected but varies with the original Pb source. A comparison of isotopic exchangeability with the results of a sequential extraction procedure showed that (isotopically) 'non-labile' Pb may be broadly equated with 'residual' Pb in organic soils. However, in mineral soils the 'calcareous' and 'oxide-bound' Pb fractions included non-labile forms of Pb. De-coupling the isotopic signature of labile and nonlabile Pb pools suggested that, despite prolonged contact with soil, differences between the lability of the original contaminant and the native soil Pb may remain.