Using AFM atomic force microscopy to study the transformation of monazite (Ce,U,Th)PO4 into pyromorphite Pb5(PO4)3Cl. This research will be carried out at Weber State University in Utah (USA) in collaboration with Dr. Marek Matyjasik. It is part of a larger PRELUDIUM BIS research project: “Low-temperature transformation of monazite (Ce,U,Th)PO4 into pyromorphite Pb5(PO4)3Cl – basic research for future technologies ” carried out as part of the applicant’s PhD.
This project tests the hypothesis that in the presence of Pb2+ and Cl– ions, the dissolution of (Ce,U,Th)PO4 monazite proceeds somewhat differently than in the pure aqueous solutions studied so far: it is accompanied by the crystallization of the pyromorphite Pb5(PO4)3Cl, which accelerates the decomposition of monazite and may impair its properties as a material for use in radioactive waste repositories. Lead is widely used in all technologies related to radioactive materials, so the existence of solutions containing Pb2+ ions is very likely in connection with radioactive waste and related facilities. Pyromorphite has lower solubility than monazite by which its formation may be preferred in the environment. In preliminary studies carried out within the framework of this project, it was found that cerium-lead phosphates crystallize very easily from solutions in the presence of Pb (Staszel et al., 2023). Such reactions lead to complete removal of Ce and Pb from solutions (Sordyl et al., 2023). So far, it has been found that a thin layer of phosphate products forms on the surface of weathering monazite, which limits the access of fresh solution and slows down the reaction (Cetiner et al., 2005; Gausse et al., 2018). Never before has the formation of such a layer been studied using AFM atomic force microscopy, nor has it been studied in the presence of solutions containing Pb ions. There are both thermodynamic indications and literature on the formation of transitional (metastable) phosphates indicating that in the presence of Pb2+ and Cl– ions, the precipitation of pyromorphite Pb5(PO4)3Cl is possible and likely: the source of phosphate ions is dissolving monazite and the process is controlled by the availability of Pb2+ and Cl– ions. Atomic force microscopy is a unique device that allows direct observation of the formation of such layers and the evolution of changes on the surface of the reacting mineral directly in solution, in real time at very high, sub-atomic magnifications. There are many literature examples indicating that the use of this technique makes it possible to visualize and describe the reaction mechanism at the mineral-solution interface. The aim of the research is to identify the nano-structures of the precipitating phases and to understand the mechanism of their formation in the form of layers, coatings, pseudomorphoses and other forms that can hinder or accelerate the progress of this reaction. The “fluid-cell AFM” technique will be used to analyze etching pits, the mechanism of dissolution, and the formation of new phases on a surface immersed in solution and reacting in real time. This is a very effective approach, still not very often used in earth sciences. This methodology allows us to study the basic mechanisms of mineral dissolution and reactions with solutions by analyzing the surface with subatomic resolution in real time in solution (Bajda et al., 2019; Guren et al., 2020; Li et al., 2018; Ruiz -Agudo & Putnis, 2012). The oriented grains will be embedded in epoxy resin to expose specific crystal walls. This will allow consideration and quantification of possible dissolution anisotropy, if any exists in the case of monazite. The dissolution of monazite has never been studied in this way. Experiments will be performed both in the absence (control experiment) and in the presence of Pb2+ and Cl– ions in solution. Samples will be placed in an AFM flow cell containing the solution and imaged as the reaction progresses. Between imaging, the solution will be passed through the system at a rate of approx. 3 ml/min. The evolution of the surface morphology of the dissolved monazite (formation of etch figures, evolution of degree morphology and islands of groups of atoms and molecules, smoothing or increasing of surface roughness on the atomic scale) will allow direct identification of the dissolution mechanism: controlled by surface morphology vs. controlled by solution concentration (Maurice, 2009). More than a dozen experiments are planned using different crystallographic walls of monazite and solutions of different concentrations and acidity. The reacted monazite surface will also be analyzed ex-situ after the experiment to further characterize the observed reaction products.
The project supervisor Prof. Maciej Manecki, together with Weber State professors Dr. Marek Matyjasik and physics professor Dr. C. Ingelfield, developed a detailed methodology for the analysis of mineral transformations using the flow cell of an AFM atomic force microscope in 2001 as part of the grant “Dissolution of pyromorphite in lactic acid – an atomic force microscopy study.” Since then, collaborative research in the field of mineral-water interaction leading to joint publications and conference presentations.
References
Bajda, T., Manecki, M., & Matyjasik, M. (2019). The Early Stages of Mimetite Dissolution in EDTA Studied with Atomic Force Microscopy and Scanning Electron Microscopy. Microscopy and Microanalysis: The Official Journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada, 25(3), 810-816. https://doi.org/10.1017/S1431927619000217
Cetiner, Z. S., Wood, S. A., & Gammons, C. H. (2005). The aqueous geochemistry of the rare earth elements. Part XIV. The solubility of rare earth element phosphates from 23 to 150 °C. Chemical Geology, 217(1), 147-169. https://doi.org/10.1016/j.chemgeo.2005.01.001
Gausse, C., Szenknect, S., Mesbah, A., Clavier, N., Neumeier, S., & Dacheux, N. (2018). Dissolution kinetics of monazite LnPO4 (Ln = La to Gd): A multiparametric study. Applied Geochemistry, 93, 81-93. https://doi.org/10.1016/j.apgeochem.2018.04.005
Guren, M. G., Putnis, C. V., Montes-Hernandez, G., King, H. E., & Renard, F. (2020). Direct imaging of coupled dissolution-precipitation and growth processes on calcite exposed to chromium-rich fluids. Chemical Geology, 552, 119770. https://doi.org/10.1016/j.chemgeo.2020.119770
Li, M., Wang, L., & Putnis, C. V. (2018). Atomic force microscopy imaging of classical and nonclassical surface growth dynamics of calcium orthophosphates. CrystEngComm, 20(21), 2886-2896. https://doi.org/10.1039/C7CE02100C
Maurice, P. A. (2009). Environmental surfaces and interfaces from the nanoscale to the global scale. Wiley.
Ruiz -Agudo, E., & Putnis, C. V. (2012). Direct observations of mineral fluid reactions using atomic force microscopy: The specific example of calcite. Mineralogical Magazine, 76(1), 227-253. https://doi.org/10.1180/minmag.2012.076.1.227
Sordyl, J., Staszel, K., Leś, M., & Manecki, M. (2023). Removal of REE and Th from solution by co-precipitation with Pb-phosphates. Applied Geochemistry, 158, 105780. https://doi.org/10.1016/j.apgeochem.2023.105780
Staszel, K., Jędras, A., Skalny, M., Dziewiątka, K., Urbański, K., Sordyl, J., Rybka, K., & Manecki, M. (2023). New synthetic [LREE (LREE = La, Ce, Pr, Sm), Pb]-phosphate phases. Mineralogy, 54(1), 58-68. https://doi.org/10.2478/mipo-2023-0006