PARTITIONING OF LEAD ISOTOPES IN FOUR SOIL STANDARDS USING A SIX STEP SEQUENTIAL EXTRACTION SCHEME

It is generally accepted that knowledge of the concentration of heavy metals in soil is not enough to understand the behavior and impact of these metals on ground water and plants. Therefore, understanding the distribution of lead isotope ratios in different soil fractions would yield more information about the fate of heavy metals in soils. This was done by analyzing soils derived from geologically old rocks, because different soil phases may have different isotope ratios inherited from progenitor minerals with different U/Pb and Th/Pb ratios and this provided a means of detecting the partitioning of lead and lead isotopes in different soil fractions. Accordingly, three Canadian soil standards TILL-1, TILL-3, and SO-2, were studied and the antiquity of their source rocks confirmed by model Sm-Nd ages of the soils which were 1.5 Ga for SO-2, and TILL-1, and 2.5 Ga for TILL-3. The standard SRM 2709 from a Californian agricultural soil was also studied. Thermal ionization was used and Pb isotope ratios and concentrations were simultaneously measured with a ²⁰⁵Pb spike on a Finnigan MAT 261 mass spectrometer. Erel et al's six step sequential extraction scheme was used. The Pb contents of all reagents was <50 pg/mL. All extractions were done in triplicate and good agreement in both concentration and isotope ratio obtained. In all four soils, the amount of Pb increased in the order: exchangeable (0.005-0.016 ppm), Mn-Ox (0.42-1.1 ppm), organic (0.13-1.6 ppm), carbonate (0.92-3.9 ppm), and Fe-Ox (1.8-6.8 ppm). The residual was 10.6-14.2 ppm and amounted to between 50 and 80% of the Pb processed. The agricultural soil SRM 2079 was isotopically homogeneous as might be expected for a soil derived from young rocks, but large variations were found in the Canadian soils. The most interesting result is that if the ratios ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb are plotted for the residual, the whole soil, the Mn-Fe Oxides and the organic phases, good isochrons are obtained for SO-2 and TILL-3 with apparent ages of 2.0 and 2.93 Ga which reflect the model Sm-Nd ages. Each of the six soil fractions for the older Canadian soils as operationally defined by Erel et al, scheme had a unique isotopic signature. As Such using this same scheme to analyze soils on which food crops are grown would be essential in determining the behavior and phytoavaliability of soil Pb to plants.