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FORMATION OF PYROMORPHITE IN ANGLESITE-HYDROXYAPATITE SUSPENSIONS UNDER VARYING PH CONDITIONS
Citation:
Zhang**, P. AND J A. Ryan*. FORMATION OF PYROMORPHITE IN ANGLESITE-HYDROXYAPATITE SUSPENSIONS UNDER VARYING PH CONDITIONS. DOI: 10.1021/es98023, ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 32(21):3318-3324, (1998).
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Description:
Addition of phosphate to lead [Pb(II)] contaminated soil to immobilize soil Pb by formation of pyromorphite has been proposed as an alternative remediation technique. Lead sulfate (PbSO4, anglesite), a Pb-bearing form found in contaminated soils and wastes, was reacted with a synthetic phosphate mineral, hydroxyapatite [Ca5(PO4)3OH], under constant pH (pH 2 to 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 pyromorphite and the solubility of Pb. Under constant pH condition, complete transformation of anglesite to chloropyromorphite, [Pb5(PO4)3Cl], was obtained at pH 4 and 5. At pH 6 and 7, the newly formed chloropyromorphite precipitated on the surface of undissolved apatite. The coverage of the apatite surface may reduce apatite dissolution rate and the transformation rate of Pb from anglesite to chloropyromorphite. Increasing the P/Pb ratio increased the transformation rate, but anglesite was still present after a 120 min reaction time. In the dynamic pH system, the added apatite was rapidly dissolved at the initial low pH and complete transformation of anglesite to chloropyromorphite was obtained within 25 min. The soluble Pb level was controlled by the solubility of chloropyromorphite during the entire reaction process. These results demonstrate the effect of reaction kinetics on the formation rate of chloropyromorphite and the mechanisms controlling the solubilization of Pb in the anglesite-apatite system. Further, they illustrate that a complete transformation of ingested anglesite to chloropyromorphite can be achieved under gastrointestinal track pH conditions if sufficient phosphate is provided.
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FORMATION OF PYROMORPHITE IN ANGLESITE-HYDROXYAPATITE SUSPENSIONS UNDER VARYING PH CONDITIONS