Incomplete transformations of Pb to pyromorphite by phosphate-induced immobilization investigated by X-ray absorption fine structure (XAFS) spectroscopy

For an accurate assessment of immobilization technologies, it is necessary to illustrate the transformation of target metal species into their final products. The present study employed extended X-ray absorption fine structure (EXAFS) spectroscopy combined with linear combination fitting (LCF) to determine Pb species and their proportions in contaminated soils treated with phosphate amendments. Lead contaminated soils collected from a shooting range were separately treated with calcium phosphate (CP), hydroxyapatite synthesized from ceramic waste (CHA), and incinerated poultry litter (PW). Soils were incubated at 32% water content for 7 and 380 d. The EXAFS–LCF analysis illustrated that Pb speciation in the control soil included organically-complexed phases (Pborg, 32%), PbO (22%), PbCO3 (28%), and Pb3(CO3)2(OH)2 (8%). As the incubation period increased, the proportion of chloropyromorphite [Pb5(PO4)3Cl] increased from 20% to 27% in CHA and from 19% to 31% in CP soils. The spectra of PW-amended soils were reproduced adequately with a combination of Pborg, PbO, and chloropyromorphite in the proportion of about 20%, 45%, and 23%, respectively. The effectiveness of amendments on Pb immobilization as indicated by the chloropyromorphite proportion was in the order of CP (31%) > CHA (27%) > PW (23%) after 380 d of incubation. Our study indicates that about 70% of Pb species was not immobilized as a form of chloropyromorphite, and the additional supply of phosphate amendment scarcely promoted chloropyromorphite formation. The EXAFS–LCF approach illustrated that organically-complexed Pb was persistent in all amended soils, suggesting that an enriched soil organic carbon may be an inhibitory factor for pyromorphite transformations.