Enhancing pyromorphite formation in lead-contaminated soils by improving soil physical parameters using hydroxyapatite treatment.

During chemical immobilization in soil, enhancement of insoluble phases is required to prevent toxic metal from leaching into the surrounding environment. Understanding the effects of physicochemical parameters of soil on the reaction between lead and hydroxyapatite is important to enhance the formation of the insoluble pyromorphite-lead phase. However, the combined effect of soil physical parameters on pyromorphite formation and compressive strength has not been reported. This study aimed to investigate the relationship between soil texture and pyromorphite formation, as well as unconfined compressive strength in lead-contaminated soils treated with hydroxyapatite under different compaction states and moisture conditions. Our findings showed that in compacted soil, >20% of lead was formed as pyromorphite compared with 10% of lead in uncompacted soil. In particular, low porosity and a high saturation ratio of soil under the unsaturated moisture condition were favorable for pyromorphite formation. Under the saturated moisture condition, the addition of hydroxyapatite enhanced pyromorphite formation compared with that under the unsaturated moisture condition. In addition, the leaching of soluble lead into the surrounding environment could be suppressed to <0.05% of lead in soil. The addition of hydroxyapatite also increased compressive strength of the compacted soil with increased curing period despite the soil texture. Our results suggest that treatments such as compacting and seepage control with hydroxyapatite may simultaneously increase pyromorphite formation and compressive strength. Furthermore, when performing soil recycling with hydroxyapatite at sites in the groundwater zone, the soluble lead in the soil is prevented from leaching to the surrounding environment. Hydroxyapatite could be used to enable the reuse of lead-contaminated soil for lead immobilization and to increase compressive strength.