Formation of Chloropyromorphite During the Physiologically Based Extraction Test: Experimental Artifact?

Abstract

We examined the potential formation of chloropyromorphite [Pb5(PO4)3Cl(s)] as an experimental artifact in the physiologically based extraction test (PBET), a common surrogate method used to assess the effectiveness of in situ phosphate-based treatment to reduce Pb bioavailability in soil. Although the conditions of the PBET are strongly thermodynamically favorable for the formation of Pb5(PO4)3Cl(s) due to the high concentrations of chloride (0.25 M) in the extraction solution, Pb precipitation was minimal (<5%) over the normal 1-h duration of the PBET. Lead concentrations in the PBET fluid did, however, decrease by 70%–85% over 5–7 days. Precipitation rates increased sharply as the glycine concentration was decreased from 0.4 M (standard PBET) to 0 M, approaching the rapid Pb5(PO4)3Cl(s) precipitation rates typically observed in aqueous solutions. Thus, we conclude that glycine substantially lowers the precipitation rate of Pb5(PO4)3Cl(s), and at the standard 0.4 M glycine concentration of the PBET, insignificant formation of Pb5(PO4)3Cl(s) will occur in the extraction fluid itself (i.e., there will be no experimental artifact) over the normal duration of the test. Further, the concentration of Pb stabilized in our solutions at an apparent solubility significantly exceeding what would be predicted thermodynamically (i.e., log Kso apparent of −79.5±0.9 vs. a reported value of −84.4). In sum, we conclude that significant formation of Pb5(PO4)3Cl(s) does not occur in the PBET, supporting its use in assessing the effectiveness of in situ phosphate immobilization. Our results also support the emerging body of evidence that the thermodynamic potential for Pb precipitation via the formation of Pb5(PO4)3Cl(s) may not be as great as previously believed.