Lead bioavailability in soil and soil components

The bioavailability of Pb in polluted soil as well as in Pb-spiked individual soil components (carbonates, Fe-oxides, clays, organic matter, and quartz) was determined using a genetically engineered bioluminescent bacterial reporter strain. Calculated bioavailability was compared to total concentrations of the metal in the samples and to its fraction leached into the aqueous assay medium. Our data show that adsorbed Pb is unavailable to the bacterial reporter, which responds only to dissolved Pb. We also found that leaching with aqueous solutions do not desorb any measurable Pb (<0.1 ppm) from contaminated soils, but does so from most of the freshly spiked soil components, at different efficiencies (10%, 25-60%, 25-55%, 0%, and 0-3% for carbonate, quartz, clay, Fe-oxide, and organic matter, respectively). This tendency to retain or release Pb, and hence determine its bioavailability, corresponds to the affinity of the various soil fractions to Pb as reported in the literature. For example, Fe-oxides which have the highest affinity for Pb retain all its Pb, making it unavailable to microorganisms in the soil. Nonetheless, even in the soil fractions with the lowest affinity for Pb (quartz), only 55% of the desorbed Pb is bioavailable.