Stabilization of As and Sb in contaminated acidic shooting range soil with apatite mine tailings: Challenge of co-contamination

Differences in the behaviors of shot-derived metal Pb and metalloids Sb and As render the remediation of metal(loid)-contaminated shooting range soils challenging. Treatment methods that generally reduce Pb solubility may simultaneously increase Sb and As solubility due to pH changes and ion competition. We investigated the potential of tailings from phosphate mining, previously used to immobilize Pb, to stabilize acidic shooting range soil without incurring the risk of enhanced Sb and As solubility. In a 2.5-year field trial, the soil of a former shooting range, surface-treated with tailings consisting of phlogopite, carbonate minerals calcite and dolomite, and residues of apatite, displayed no evidence of increased Sb or As solubility. Results from a parallel laboratory-scale pot experiment, carried out with test soils from the field site, supported the findings. Under acidic conditions, dissolution of the carbonate fraction of the tailings, and the subsequent decrease in soil acidity, contributed to the release of Sb and As from organic associations and/or Al/ Fe (hydr)oxide surfaces. We concluded that the abundant Ca²⁺ ions liberated upon carbonate dissolution probably reacted with the anionic species of Sb and As to form sparingly soluble Ca-antimonates and Ca-arsenates. Moreover, the solubility of intrinsic and apatite-derived P in the test soils, initially hypothesized to compete for adsorption with Sb and As and thereby increase their solubility, also decreased after tailings treatment. In conclusion, Pb-contaminated shooting range soil was successfully stabilized with the tailings without increasing Sb or As solubility.