Factors influencing the effectiveness and stability of aqueous lead immobilization by hydroxyapatite

The effects of Ca, K, Na, calcite, and EDTA on Pb immobilization by hydroxyapatite [Ca-10((PO4)(6)(OH)(2); HA] were investigated. The stability of hydroxypyromorphite [Pb-10(PO4)(6)(OH)(2); HP] (a reaction product of HA with Pb) and the effectiveness of using HA to immobilize Pb in Pb-contaminated soils also were examined. Neither Na nor K affected Pb immobilization. Aqueous Ca and calcite slightly inhibited Pb immobilization, but HA still reduced aqueous Pb > 97%, in the presence of these species; however, EDTA significantly reduced the effectiveness of HA to immobilize Pb from Pb-EDTA solutions, with only 24 to 31% resultant reduction in aqueous Pb. Hydroxypyromorphite was stable in the presence of HA up to 8 wk. Both higher Ca and lower P concentrations (following P removal by anion exchange resin) dissolved HP from a mixture of HA and HP, but the final aqueous Pb concentrations remained low (<168 nmol L(-1)). High EDTA concentrations dissolved most (>85%) of the Pt,from the same mixture. Hydroxyapatite effectively immobilized >71% of the aqueous Pb in four PbHAsO4 contaminated soils. The effectiveness of HA to immobilize aqueous Pb in the presence of Ca, K, Na, and calcite, and to immobilize aqueous Pb from Pb-contaminated soils, together with the stability of HP in the presence of high Ca and low P concentrations support our previous conclusion that HA has the potential to treat Pb contaminated water, soils, and wastes and that such a treatment can result in long-term in situ Pb stabilization; however, HA did not effectively immobilize Pb, nor was HP, stable in the presence of high EDTA concentrations. Thus, HA treatment will be ineffective in EDTA rich environments.