pyromorphite and lead-sulfide

The aqueous concentration of lead [Pb(II)] in geochemical environments is controlled by the solubility of Pb-bearing minerals and their weathering products. In contaminated soils, a common method for in situ stabilization of Pb(II) is the addition of phosphate to convert more redox sensitive sulfide minerals into sparingly soluble pyromorphite [Pb

Topics: Biological Availability; Bradyrhizobiaceae; Chemoautotrophic Growth; Lead; Minerals; Oxidation-Reduction; Phosphates; Sulfides; Sulfur

Determining the effectiveness of in situ immobilization for P-amended, Pb-contaminated soils has typically relied on non-spectroscopic methods. However in recent years, these methods have come under scrutiny due to technical and unforeseen error issues. In this study, we analyzed 18 soil samples via X-ray diffraction (XRD), selective sequential extraction (SSE), and a physiologically based extraction test (PBET). The data were compared against each other and to previous data collected for the soil samples employing X-ray absorption fine structure spectroscopy coupled with linear combination fitting (XAFS-LCF), which spectroscopically speciates and quantifies the major Pb species in the samples. It was observed that XRD was incapable of detecting pyromorphite, the hopeful endpoint of the immobilization strategy for reduced Pb bioavailability in our studies. Further, the SSE and PBET extraction methods demonstrated an increase of recalcitrant Pb forms in comparison to the XAFS-LCF results suggesting that SSE and PBET methods induced the precipitation of pyromorphite during the extraction procedures. The theme of this paper illustrates the experimental concerns of several commonly employed methods to investigate immobilization strategies of amended, metal-contaminated systems which may not be in true equilibrium. We conclude that appropriate application of spectroscopic methods provides more conclusive and accurate results in environmental systems (i.e., Pb, Zn, Cd, etc.) examining P-induced immobilization.

Topics: Environmental Monitoring; Iron; Lead; Minerals; Phosphates; Soil Pollutants; Spectrum Analysis; Sulfides; X-Ray Diffraction

Electrochemical Scanning Tunneling Microscopy (EC-STM) and electron microscopies have been used to follow the nucleation and growth of approximately 10-15 nm pyromorphite (Pb5(PO4)3Cl,OH) particles on a galena (PbS) substrate under oxidative conditions. The particle sizes and crystal morphologies are found to be strongly affected by solution and oxidation potential, and in the earliest stages the particles are generally sufficiently small to be mobilized in a soil. It is clear that the particles grow epitaxially under these conditions, based on observations of the particles' adherence to the surface during imaging, their preferred crystallographic orientation, their growth along surface features on the galena, and commensurate atomic structures. Through cyclic voltammetry, we show that the presence of phosphate also partially passivates the surface of the galena to oxidation. We propose two possibilities for the mechanism of passivation, one is that pyromorphite nucleation inhibits the retreat of steps, and the second is that adsorbed phosphate stabilizes a lead-terminated surface structure by coordinating lead and slowing its dissolution.

Topics: Electrochemistry; Lead; Microscopy, Electron, Scanning Transmission; Minerals; Oxidation-Reduction; Particle Size; Phosphates; Soil; Sulfides; Surface Properties