Pyromorphite Formation from Goethite Adsorbed Lead

Abstract

Conversion of labile soil Pb species into stable Pb minerals is a crucial process in the development of an in situ soil Pb remediation strategy. A complete understanding of the reaction with specific soil Pb species is therefore of fundamental importance. A synthetic goethite, α-Fe(OH)3, with surfaces saturated with Pb2+ was used to represent adsorbed Pb, one of the primary soil Pb species. Hydroxyapatite (Ca5(PO4)3OH) and NaH2PO4 were the source of solid and soluble phosphate, respectively. When stoichiometric concentrations of soluble phosphate were added directly to the suspension of Pb-adsorbed goethite, the thermodynamically stable lead phosphate mineral chloropyromorphite (Pb5(PO4)3Cl) was rapidly precipitated. By contrast, when the same goethite suspension was reacted in a dialysis system in the presence of hydroxyapatite, the formation of chloropyromorphite is slow and appears to be controlled by the rate of dissolution of the hydroxyapatite. Chloropyromorphite minerals produced in these experiments vary in morphology in accordance with the reaction conditions in which the mineral formed. However, the extractability of the goethite-adsorbed Pb by MgCl2 was dramatically reduced because of the reaction with added phosphates. This supports the hypothesis that the bioavailability of adsorbed soil Pb could potentially be reduced to insignificant levels in Pb-contaminated soil, under normal soil pH conditions, by amendment with a phosphate source.