Immobilization of Co and Ni in Mining-Impacted Soils Using Phosphate Amendments

Synthetic hydroxyapatite (HA) and natural phosphate rock (PR) were applied to heavy metal-contaminated soils from sulfide mine areas in Sardinia and Tuscany (Italy). The application of phosphate amendments to the polluted mine waste soils reduced water-soluble concentrations of Co and Ni by about 99 %. In general, phosphate treatment was slightly more effective in reducing water solubility of Co and Ni in the Sardinian soils than in the Tuscan ones. This result suggests that the mineralogical composition of the mine waste soils may impact the effectiveness of metal immobilization. The formation of complexes of the heavy metals on the surface of phosphate grains and partial dissolution of the amendments and precipitation of heavy metal-containing phosphates are the dominant immobilization mechanisms. Between the phosphate amendments, PR was slightly less effective than HA in immobilizing Co and Ni. This result could be attributed to PR inability to provide soluble phosphate. Although with lower effectiveness, the use of PR to immobilize heavy metals from contaminated soils may reduce the risk of phosphate-induced eutrophication due to the application of amendments with highly soluble phosphate.