Mass balance on surface-bound, mineralogic, and total lead concentrations as related to industrial aggregate bioaccessibility

Previous investigations into lead bioavailability have focused an the solubility of the mineralogic fraction. However, the potential for surface-bound (sorbed) lead to contribute to the bioavailable fraction has not yet been determined. This study investigated the Pb distribution between the mineralogic and surface-bound pools in aggregate generated by thermal vitrification of a Pb-bearing organic and metalliferous waste feedstock. Electron microprobe and laser ion mass spectroscopy demonstrated that amorphous slag comprised >95% of the Vitreous material, with lead sulfosalts, oxides, and carbonates encapsulated in the refractory slag matrix representing the mineralogic Pb fraction. The maximum potentially bioavailable surface-bound fraction was 27% in the 467 mg/kg primary aggregate-impacted soil and 5% in the 6520 mg/kg slagged aggregate, averaging 14% of the Pb pool tar the six samples evaluated. The low concentration of Pb on the surface of aggregate particles, the refractory nature of the Pb-bearing slag, the low percentage of liberated Ph-bearing (non-slag) particles (1:6300 to 1:38 000 of the total grain population), and the large size of the slag particles explain the low bioaccessibility of Pb in modern aggregate (1.5%) as compared to urban Cincinnati soils (51%) and street dusts (77%).