Immobilization of uranium from aqueous solutions by precipitation of lead apatite – pyromorphite (Pb5(PO4)3Cl)
Authors: Sordyl Julia , Chamberlain Carmen , Sweet Teagan , Burns Peter C. , Manecki Maciej 
Published: February 2023

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&#8729;6H2O and Pb(NO3)2 in varied molar proportions was added slowly by dripping through a glass funnel into the solution containing dissolved NaH2PO4&#183;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% (&#963; = 1.9) to 94.1% (&#963; = 2.5), and Pb was removed at levels ranging from 95.7% (&#963; = 2.6) to 98.4% (&#963; = 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 &#8211; 918 cm-1, 586 &#8211; 541 cm-1, and 439 &#8211; 392 cm-1 were attributed to the vibrations of the (PO4)3+ units, while those at 830 &#8211; 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&#8729;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.&#160;&#160;