Immobilization of lead, tungsten, and phosphate by steel slag fines: Metals thresholding and rate studies

This study presents an evaluation of 9.5-mm minus steel slag fines (SSF) to immobilize lead (Pb), tungsten (W), and phosphate (P as PO₄) at total contaminant dosing concentrations of 10,000 mg/kg (W, PO₄) and 100,000 mg/kg (Pb), as a potential new construction material for firing-range backstop berms (or treating firing-range soils). Direct contaminant uptake and kinetic rate, rerelease, and mineralogical studies were undertaken for metal loadings totaling up to six combinations of Pb, W, and PO₄. Batch rate studies showed that > 95% of Pb and W were removed from aqueous solution within 2 h in the presence of the SSF media. For equivalent aqueous doses of 500 mg/L (W, PO₄) and 5,000 mg/L (Pb), the TCLP-Pb concentrations for all multielement suites were < 0.3 mg/L, which is much less than the TCLP-Pb criterion of 5.0 mg/L. For the P-Pb-W suite (all three contaminants present simultaneously), the SPLP-W concentrations ( < 0.35 mg/L) were lower than the TCLP-W concentrations ( < 1.1 mg/L) regardless of PO₄ dose. Leached phosphate concentrations from the P-Pb-W suite were below or hovered at the detection limit (0.5 mg/L) under SPLP and TCLP conditions, respectively. A mineralogical evaluation revealed that lead pyromorphite [Pb₅(PO₄)₃OH] and scheelite [Ca(WO₄)] were the key PO₄-containing, Pb-containing, and W-containing phases.