Implications from concentrations and isotopic data for Pb partitioning processes in soils

Abstract

Lead concentrations and stable isotopic measurements were examined in the different chemical fractions of Czech forest soils to investigate the mechanisms of Pb partitioning. A method of selective sequential dissolution (SSD) was employed that distinguished between five different fractions: exchangeable, surface bound, organic matter, Fe-oxides, and silicates (non-labile). From an analysis of the concentrations and isotopic compositions associated with the different fractions, it is apparent that Pb in the deep Czech mineral soils is of predominantly natural origin and is primarily associated with silicates (69–81%) and Fe-oxides (11–19%). Natural Pb associated with surface bound and organic matter fractions in mineral soils accounts for only 7 to 15%. Anthropogenic Pb in the Czech soils is concentrated primarily in the organic horizons and is strongly associated with the surface-bound and organic matter fractions in which the proportion of total Pb is 33 to 50% and 23 to 47%, respectively. At high and low levels of contamination, Pb isotopic signatures within the labile fractions of the same soil samples are generally homogenous, although a degree of heterogeneity among these fractions is noted in samples of intermediate degrees of contamination. Such heterogeneity probably reflects different levels of natural and anthropogenic Pb mixing. Determination of the mass-normalized affinity of Pb to the primary components using solid-solution distribution coefficients suggests that in Czech forest soils, the order of affinity may be summarized as Fe-oxides > organic matter > silicates. A similar treatment of the data reported for semiarid Mediterranean soils indicates the prevailing order to be Fe-oxides > carbonates > organic matter > silicates. The general similarity of the behaviour of Pb with respect to the different soil components in both temperate and semiarid soils suggests that these orders of affinity may have wider significance for a variety of other soil types.