Fractionation of lead in soil by isotopic dilution and sequential extraction

Abstract

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. 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.

Lability, solubility and speciation of Cd, Pb and Zn in alluvial soils of the River Trent catchment UK

“…The large ionic radius of Pb prevents it from being readily incorporated into the metal oxide structure during re-crystallisation and ageing processes, thus remaining primarily 10 surface associated 72 . This may help explain the higher than intuitively expected lability of Pb found in this and other studies 24,26 where previously it was assumed that Pb sorbs strongly both to poorly crystalline oxides and organic matter.…”

“…However, using 0.0005M EDTA as suggested by Atkinson et al 24 , allowed more analytically robust Pb isotope measurements to be obtained for all soils (E-values with CV <0.4-7%), as previously reported by Izquierdo et al 2 . Thus, the final dataset for Pb modelling 15 ( Table S2) contains only E-values obtained using 0.0005M EDTA as the background electrolyte.…”

“…Lead displayed a lower reactivity and the lability ranged between 9-56%, which was on the lower range for %E Pb values in the literature i.e. 31-78% 17,24,26,[54][55] . Zinc was the least geochemically responsive metal, with 65 only 13-37 % labile across the dataset.…”

“…Previous experience of undertaking stable metal ID assays has demonstrated that the selection of the background electrolyte is of great importance. It must be able to solubilise 65 sufficient labile-metal for robust analytical analysis whilst not dissolving non-labile metal 24 . Soils with large clay contents, relatively large organic C and circum-neutral pH values such as many of these alluvial soils have the potential for insufficient metal to be solubilised for robust isotopic measurement.…”

“…The measurement of the labile pool using isotopic dilution (ID) assays is generally accepted as the most robust and mechanistically sound estimate 40 of reactive metals 23 . Methods for Cd, Zn and Pb have used both stable and radio isotopes [24][25][26][27][28][29][30][31] . However, as a result of the ID assay being analytically challenging through the use of either stable isotopes or potentially hazardous using radio-isotopes, analogues of the labile pool have been suggested and these 45 include 0.43M HNO 3 and the EU standard 0.05M EDTA extractions [32][33][34] .…”