X-ray Absorption Near Edge Structure Study of Lead Sorption on Phosphate-Treated Kaolinite

Presorbed phosphate significantly increases Pb sorption on the phyllosilicate kaolinite in the pH range from 4 to 8. The sorbed Pb-to-P molar ratios over this pH range stray little from the molar ratio found in the mineral pyromorphite, suggesting sorbed phosphate reacts with soluble Pb to form a surface precipitate similar to pyromorphite. X-ray absorption near edge structure ONES studies at the Pb L-3-edge support this interpretation. In particular, the fine structure of first-derivative Pb L-3-edge XANES spectra of Pb species sorbed to phosphate-treated kaolinite samples covering a pH range extending from 4 to 10 match the fine structure of the spectrum of pyromorphite. Although the product is structurally and compositionally similar to pyromorphite, the ion activity product in the pH range 4-6 was undersaturated with respect to the solubility product of pyromorphite.