FORMATION OF PYROMORPHITE IN ANGLESITE HYDROXYAPATITE SUSPENSIONS UNDER VARYING PH CONDITIONS

Addition of phosphate to lead [Pb(II)]-contaminated soil to immobilize soil Pb by formation of pyromorphite has been proposed as an alternat ive remediation technique. Lead sulfate (PbSO4, anglesite), a Pb-heari ng form found in contaminated soils and wastes, was reacted with a syn thetic phosphate mineral, hydroxyapatite [Ca-5(PO4)(3)-OH], under cons tant pH (pH 2-7) and simulated gastric pH conditions (pH varied from 2 to 7 within 30 min) to assess the effects of reaction kinetics on the formation rate of chloropyromorphite and the solubility of Pb. Under constant pH condition, complete transformation of anglesite to chlorop yromorphite, [Pb-5(PO4)(3)Cl], was obtained at pH 4 and pH 5. At pH 6 and pH 7, the newly formed chloropyromorphite precipitated on the surf ace of undissolved apatite. The coverage of the apatite surface may re duce apatite dissolution rate and the transformation rate of Pb from a nglesite to chloropyromorphite. Increasing the P/Pb ratio increased th e transformation rate, but anglesite was still present after a 120-min reaction time. In the dynamic pH system, the added apatite was rapidl y dissolved at the initial low pH, and complete transformation of angl esite to chloropyromorphite was obtained within 25 min. The soluble Pb level was controlled by the solubility of chloropyromorphite during t he entire reaction process. These results demonstrate the effect of re action kinetics on the formation rate of chloropyromorphite and the me chanisms controlling the solubilization of Pb in the anglesite-apatite system. Furthermore, they illustrate that a complete transformation o f ingested anglesite to chloropyromorphite can be achieved under gastr ointestinal tract pH conditions if sufficient phosphate is provided.