PYROMORPHITE FORMATION FROM GOETHITE ADSORBED LEAD

Conversion of labile soil Ph species into stable Pb minerals is a cruc ial process in the development of an in situ soil Pb remediation strat egy. A complete understanding of the reaction with specific soil Pb sp ecies is therefore of fundamental importance. A synthetic goethite, al pha-Fe(OH)(3), with surfaces saturated with Pb2+ was used to represent adsorbed Pb, one of the primary sail Pb species. Hydroxyapatite (Ca-5 (PO4)(3)OH) and NaH2PO4 were the source of solid and soluble phosphate , respectively. When stoichiometric concentrations of soluble phosphat e were added directly to the suspension of Ph-adsorbed goethite, the t hermodynamically stable lead phosphate mineral chloropyromorphite (Pb- 5(PO4)(3)Cl) was rapidly precipitated. By contrast, when the same goet hite suspension was reacted in a dialysis system in the presence of hy droxyapatite, the formation of chloropyromorphite is slow and appears to be controlled by the rate of dissolution of the hydroxyapatite. Chl oropyromorphite minerals produced in these experiments vary in morphol ogy in accordance with the reaction conditions in which the mineral fo rmed. However, the extractability of the goethite-adsorbed Pb by MgCl2 was dramatically reduced because of the reaction with added phosphate s. This supports the hypothesis that the bioavailability of adsorbed s oil Pb could potentially be reduced to insignificant levels in Pb-cont aminated soil, under normal soil pH conditions, by amendment with a ph osphate source.