Lead fractionation and bioaccessibility in contaminated soils with variable chemical properties

The hazard imposed by trace elements within soils is dependent on soil properties and the relative distribution of metal species. Hence, a greenhouse column study was conducted to investigate the geochemical speciation and bioaccessibility of lead (Pb) as a function of soil properties. Four different soil types (Immokalee, Belle Glade, Tobosa and Millhopper series) varying in physico-chemical properties were selected and amended with Pb as Pb(NO3) at 400, 800, and 1,200 mg kg(-1). A sequential extraction was employed to define the reactive metal pool, which was correlated with Pb bioaccessibility as determined by the physiologically based extraction test. Results show that Pb was mainly distributed in soluble + exchangeable phase in Immokallee (82%) and Millhopper (45%) series, and carbonate and Fe + Mn oxide fractions in Belle Glade (14-74%) and Tobosa (31-64%) series at time zero. With soil aging, Pb underwent chemical transformations in the soils and the majority of added Pb was associated with Fe + Mn oxide fraction (64-81%). Also, Pb bioaccessibility varied widely as a function of soil type and soil aging. Gastric phase (IVG-S) extracted 34-81% and 29-75% and the absorbed intestinal phase (IVG-Al) extracted 12-79% and 12-45% of amended Pb in all the soils at time zero and 6 months, respectively. Among soil types, Tobosa and Belle Glade showed reduced bioaccessibility relative to Immokalee and Millhopper. Statistical analysis revealed that the IVG-S Pb decreases as soil organic matter and cation exchange capacity (CEC) increases and total P decreases. While the Mehlich extractable P and Ca + Mg, total Fe + Al and organic matter predicted the Pb in an intestinal system.