Formation of a lead-insoluble phase, pyromorphite, by hydroxyapatite during lead migration through the water-unsaturated soils of different lead mobilities

This study combined the original unsaturated-column-percolation test with X-ray diffraction (XRD) analysis to understand how lead is transformed into lead-insoluble phase and immobilized by hydroxyapatite during lead migration in the water-unsaturated soil of different lead mobilities. The amounts of lead migrated from the soils without hydroxyapatite ranged from 4 to 46%, depending on the lead mobilities of soils. On the other hand, those of soils with hydroxyapatite were greatly suppressed by > 95% as compared with those without hydroxyapatite. The XRD analysis showed that the amounts of lead transformed into pyromorphite were compatible with those of lead migrated from the soil irrespective of the different lead mobilities. To the best of our knowledge, this study provides the first experimental evidence that lead migration can induce lead to transform into pyromorphite in the water-unsaturated soil. In addition, this study quantitatively demonstrates that the amount of lead migrated is almost equal to that of lead formed into pyromorphite. Thus, it was found that even if soluble lead remains after the application of immobilization material, it would be immobilized by the material during the lead migration as long as adequate material is applied to the soil.