Formation of Pyromorphite and Lead Mobilization in Contaminated Soils Amended with Hydroxyapatite in the Presence of Iron Oxyhydroxide and Water Percolation

Abstract

The application of hydroxyapatite (HAP) can transform lead into pyromorphite in the soil. However, it is not clear how the physicochemical properties of soil enhance or reduce the formation of pyromorphite. This study determined that the presence of ferrihydrite or soil moisture condition was a more important factor to enhance the formation of pyromorphite. We also evaluated lead sorption characteristics and stability in soil with HAP in the presence of ferrihydrite. The difference in the maximum lead removal capacity of soil with and without 5 wt% ferrihydrite corresponded to 10.4% of the difference in lead removal between soils with and without HAP. In artificially contaminated soil with a 50% water-holding capacity, the ratio of lead that formed into pyromorphite was compatible between soils with and without ferrihydrite at 22% and 28% of added lead, respectively. In a percolation test, almost all of the added lead was transformed into pyromorphite, despite the presence of ferrihydrite. In both water and a 0.1-M citric acid extraction, the differences in lead extracted from the contaminated soil with HAP with or without ferrihydrite were very small compared with water-soluble lead in soil without HAP. This study indicated that in soil with 5 wt% ferrihydrite, lead was removed and converted into pyromorphite by HAP with a little disturbance by ferrihydrite, and the immobilized lead would be stable. In addition, this study suggested that the soil moisture condition was a more important factor for the formation of pyromorphite than the presence of ferrihydrite.