Factors Influencing the Effectiveness and Stability of Aqueous Lead Immobilization by Hydroxyapatite

Abstract

The effects of Ca, K, Na, calcite, and EDTA on Pb immobilization by hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ; HA] were investigated. The stability of hydroxypyromorphite [Pb 10 (PO 4 ) 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 ( 85%) of the Pb from the same mixture. Hydroxyapatite effectively immobilized >71% of the aqueous Pb in four PbHAsO 4 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.