The potential of phosphate mine tailings in the remediation of acidic Pb-contaminated soil

We investigated the potential of tailings from phosphate mining, consisting of phlogopite, carbonate minerals calcite and dolomite, and residues of apatite, to serve as a stabilizing agent in the remediation of Pb-contaminated soil in situ or on-site. In a 2.5-year field trial, test plots located in a former shooting range area were surface-treated with the tailings and analyzed for tailings-induced changes in Pb solubility and thus potential mobility within the soil profile. The factors and mechanisms controlling tailings-induced changes in Pb solubility in various soil types, and the susceptibility of Pb to leaching down the soil profile following the treatment, were investigated in supplementary laboratory-scale experiments carried out with horizon-specific soil samples collected from the field site. In the tailings-treated soil, the dissolution of the carbonate fraction of the tailings and the subsequent increase in soil pH contributed to the displacement of shot-derived Pb²⁺ ions by the carbonate-derived calcium ions (Ca²⁺) and the adsorption of Pb²⁺ by soil organic matter and Al, Fe, and Mn (hydr)oxide surfaces. Moreover, the apatite fraction of the tailings formed poorly-soluble compounds with Pb, particularly in soils high in exchangeable Pb²⁺ with respect to their cation exchange capacity. Consequently, the Pb solubility in tailings-treated soils substantially decreased. The reduction in Pb solubility was most evident in the organic topsoil high in Pb. Despite the liming effect of the tailings, and the susceptibility of Pb to form organic complexes conducive to solubilization upon an increase in pH, we found no evidence of tailings-induced leaching of Pb down the soil profile.