In situ transformation of labile lead compounds to pyromorphites

Abstract

Phosphate-induced lead (Pb) immobilization that transforms labile species to pyromorphites is being tested as a cost-effective remedial alternative for reducing the health and ecological risks associated with Pb-contaminated soil. Several studies have revealed that the phosphate-induced transformation of soil Pb was incomplete and in vitro Pb bioavailability over-estimated the reduction of in vivo bioavailability of the treated soils. A laboratory study was conducted to synthesize pyromorphites from labile Pb compounds under simulated soil surface moisture conditions, and to examine the reactivity of the labile compounds and the mechanism of Pb transformation. Two highly labile compounds: lead carbonate (PbCO3) and lead oxide (PbO), were selected to react with phosphoric acid (H3PO4) at a Pb:P molar ratio of 1:5 and incubated under the conditions of naturally wet-dry cycles. The lead mineralogy of the mixtures was identified by x-ray diffraction (XRD), and the elemental composition characterized by electron beam microspectroscopy (SEM-WDS). Results indicated that the dissolution of labile Pb compounds and the initial formation of pyromorphites were kinetically rapid. Nevertheless, only 25-30% of the mixtures were identified as pyromorphites, and the remaining portion was accounted for by lead phosphates after reaction. This study suggests that pyromorphites form through intermediate lead phosphates, rather than precipitation from Pb ions as was previously described. Formation of such intermediate lead phosphates during the transformation to pyromorphites could result in limited pyromorphite formation, which may account for the incomplete Pb immobilization in H3PO4-treated contaminated soil, and the discrepancy between the in vivo and in vitro bioavailability of soil Pb. © 2007 EPP Publications Ltd.