Long-term stability and risk assessment of lead in mill waste treated by soluble phosphate

In an effort to address public concerns of the long-term stability and ecological risk reduction of phosphate (P)-stabilized lead (Pb) in mine wastes, mill tailings located at the Jasper County Superfund Site of southwest Missouri, containing ~4000 mg Pb kg(-1), were treated in situ by phosphoric acid at three rates: 0; 7.5; and 10.0 g P kg(-1) soil. Field experiment consisted of 2- by 4-m plots arranged in a randomized complete block design with four replicates of each P level. Soil and plant samples were collected at a 3-month interval during five to six (5-6) years post treatments and analyzed for Pb bioaccessibility and leachability, microbial toxicity, Pb chemical fraction, and elemental composition of Pb solids, and Pb concentration in plant tissue. Results indicated that the P treatments significantly reduced bioaccessible and leachable Pb in the mill waste, and the reductions were maintained during the sampling period. Lead concentration in plant tissue was positively related to the Pb bioaccessibility. There was no significant toxicological effect of the treatments on soil microbial community. The treatment using 10 g P kg(-1) appeared to be most effective for overall risk reduction. The Pb stabilization and risk reduction by the P treatments were accomplished by the induced transformation of labile Pb species to relatively insoluble forms, probably pyromorphite-like minerals. This study illustrated that in situ Pb stabilization by soluble phosphate would be long-term and ecologically-safe, which could safeguard human health and ecosystem from Pb contamination in mining areas.