Remediation of lead (Pb)-contaminated sites with phosphate amendments is one of the best studied and cost-effective methods for in situ immobilization. In this treatment, a very stable mineral, pyromorphite Pb₅(PO₄)₃Cl, is formed. Several studies propose to improve this treatment method with the addition of phosphate-solubilizing bacteria (PSB). The effect of bacteria on solubilization of pyromorphite is unknown. In this study, the effect of the soil microorganisms on the stability of pyromorphite Pb₅(PO₄)₃Cl has been investigated in a set of batch solution experiments. The mineral was reacted with Pseudomonas putida, a common soil microorganism. Dissolution of pyromorphite was enhanced by the presence of P. putida, resulting in an elevated Pb concentration in the solution. This occurred even when the bacteria were provided with an additional source of phosphate in the solution. Pyromorphite has been shown to be a potential source of nutrient phosphorus for common soil bacteria. Thus, the use of PSB in remediation treatments of Pb contaminated sites may have adverse long-term impacts on Pb immobilization. Conscious phosphate management is suggested for long-term sustainability of the in situ Pb immobilization by pyromorphite formation.

A number of compounds of the pyromorphite-mimetite solid solution series were synthesized at room temperature and analyzed with infrared spectroscopy and powder X-ray diffraction. High-resolution high-quality powder diffraction data were obtained by means of the state-of-the-art instrument at the 11-BM beamline of the Advanced Photon Source at Argonne National Laboratory, Argonne, IL, USA. The unit-cell dimensions, atomic position and atomic displacement parameters, as well as site occupancy for analyzed phases, were refined by the Rietveld method and reported herein for the first time. The results of this study indicate that the pyromorphite-mimetite series is continuous in near-Earth-surface environments. Detailed structural changes caused by P(5+)-As(5+) substitutions in minerals from the pyromorphite-mimetite solid solution series are reported.

Lead-containing (Pb-B-X)-2Hapatites encompass a number of [A^F]₄[A^T]₆[(BO₄)₆]X²compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb²⁺on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb²⁺at both theA^FandA^Tresults in substantial adjustments to the Pb^FO₆metaprism twist angle, φ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2H(ideally Pb₁₀(VO₄)₆C_l2), pyromorphite-2H(Pb₁₀(PO₄)₆Cl₂), mimetite-2H/M(Pb₁₀(As⁵⁺O₄)₆Cl₂) and finnemanite-2H(Pb₁₀(As³⁺O₃)₆Cl₂). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s²lone-pair electrons on the Pb²⁺that occupy a volume similar to an O²⁻anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lonepair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the Pb^FO₆metaprism twist-angles (φ) as the temperature changes. Finally, crystalchemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.