Immobilization of Lead, Tungsten, and Phosphate by Steel Slag Fines: Metals Thresholding and Rate Studies

Abstract

This study presents an evaluation of 9.5-mm minus steel slag fines (SSF) to immobilize lead (Pb), tungsten (W), and phosphate (P as PO4PO4) at total contaminant dosing concentrations of 10,000 mg/kg10,000 mg/kg (W, PO4PO4) and 100,000 mg/kg100,000 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 PO4PO4. Batch rate studies showed that >95%>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/L500 mg/L (W, PO4PO4) and 5,000 mg/L5,000 mg/L (Pb), the TCLP-Pb concentrations for all multielement suites were <0.3 mg/L<0.3 mg/L, which is much less than the TCLP-Pb criterion of 5.0 mg/L5.0 mg/L. For the P-Pb-W suite (all three contaminants present simultaneously), the SPLP-W concentrations (<0.35 mg/L<0.35 mg/L) were lower than the TCLP-W concentrations (<1.1 mg/L<1.1 mg/L) regardless of PO4PO4 dose. Leached phosphate concentrations from the P-Pb-W suite were below or hovered at the detection limit (0.5 mg/L0.5 mg/L) under SPLP and TCLP conditions, respectively. A mineralogical evaluation revealed that lead pyromorphite [Pb5(PO4)3OHPb5(PO4)3OH] and scheelite [Ca(WO4)Ca(WO4)] were the key PO4PO4-containing, Pb-containing, and W-containing phases.