Immobilization of uranium from aqueous solutions by precipitation of lead apatite – pyromorphite (Pb5(PO4)3Cl)
- 1Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland (sordyl@agh.edu.pl)
- 2Department of Earth Sciences, Uppsala University, Villavӓgen 16, SE-752 36 Uppsala, Sweden
- 3Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN, 46556, USA
- 4Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN, 46556, USA
Apatite supergroup minerals are tolerant to various chemical substitutions. Data on the presence of uranium in natural lead apatite - pyromorphite (Pb5(PO4)3Cl) indicate that the content of U(VI) reaches up to 0.5 wt%. This indicates that significant amounts of U(VI) may be accommodated in the pyromorphite structure, which may affect the ultimate development of Pb-apatite nuclear waste forms. However, the U content of natural pyromorphite represents the concentration of U in source solutions rather than in the mineral structure. The structural constraints on the upper limit of U incorporated into pyromorphite at low temperature are unknown. This is relevant to U and Pb-apatite applications in radioactive waste remediation.
In the present study, eight compounds were synthesized from aqueous solutions in a still water column under ambient conditions. A solution containing UO2(NO3)2∙6H2O and Pb(NO3)2 in varied molar proportions was added slowly by dripping through a glass funnel into the solution containing dissolved NaH2PO4·6H2O and NaCl. In each synthesis, the molar ratio of UO2:Pb was varied as follows: 1:1; 1:10; 1:20; 1:30; 1:40; 1:50; 1:100; 1:200, aiming at the final composition of Pb5-x(UO2)x(PO4)3Cl. The overall goal was to reach the upper limit of U incorporation into pyromorphite upon precipitation at room temperature. The final solutions were analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES) for Pb and U concentrations, while solids were filtered, dried, and analyzed with powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and Raman spectroscopy.
In all experiments precipitation was observed. U was removed from the solution at levels ranging from 87.2% (σ = 1.9) to 94.1% (σ = 2.5), and Pb was removed at levels ranging from 95.7% (σ = 2.6) to 98.4% (σ = 1.9). PXRD patterns revealed that five of the eight synthesis products were the synthetic analogs of pyromorphite containing (UO2)2+ partially substituting Pb2+. The observed Raman bands at the regions: 1050 – 918 cm-1, 586 – 541 cm-1, and 439 – 392 cm-1 were attributed to the vibrations of the (PO4)3+ units, while those at 830 – 800 cm-1 were assigned to the (UO2)2+ units. As the U content of the initial solution increased, the intensity of the (UO2)2+ band increased relative to the highest band of (PO4)3+. When the initial concentration of U was the highest, coprecipitation of a second phase, the not-yet-described Pb-analog of meta-autunite (Ca(UO2)2(PO4)2∙6H2O), was observed.
This experimental study showed that precipitation of pyromorphite can effectively remove uranium from aqueous solutions although substitution in pyromorphite cannot exceed 1 wt% U(VI) when precipitated under ambient conditions. The coprecipitation of the potentially new lead uranium phosphate is further investigated.
This research was funded by the Polish NCN grant no. 2019/35/B/ST10/03379.
How to cite: Sordyl, J., Chamberlain, C., Sweet, T., Burns, P. C., and Manecki, M.: Immobilization of uranium from aqueous solutions by precipitation of lead apatite – pyromorphite (Pb5(PO4)3Cl), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3189, https://doi.org/10.5194/egusphere-egu23-3189, 2023.
