Five lead-arsenate apatites (mimetites)-Pb5(AsO4)3X—where X denotes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and hydroxide (OH)—were synthesized via precipitation from aqueous solutions. The crystal structures were determined through Rietveld refinement of powder synchrotron X-ray data. All the compounds crystallized in the hexagonal class symmetry (space group P63/m). The Rietveld refinement indicated that mimetite-Cl, -Br, -I, and -OH had an anion deficiency at position X. Substitution of halogens in a mimetite structure brought about systematic changes in unit-cell parameters, interatomic distances, and metaprism twist angles φ, proportional to the substituted halogen’s ionic radius. Mimetite-OH did not follow the linear correlations determined within the series. Twist angle φ, a useful device for monitoring changes in apatite topology, ranged from 20.34° for mimetite-F to 11.42° for mimetite-I. The geometric method has been proposed for determining the diameter of hexagonal channels hosting halogens in apatites. A comparison of the results with halogenated pyromorphites showed similar systematic trends: the substitutions in mimetites have comparable effect on the interatomic distances as in their phosphorous analogues.

<p>Apatite supergroup minerals are tolerant to various chemical substitutions. Data on the presence of uranium in natural lead apatite - pyromorphite (Pb<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl) 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.</p><p>In the present study, eight compounds were synthesized from aqueous solutions in a still water column under ambient conditions. A solution containing UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>∙6H<sub>2</sub>O and Pb(NO<sub>3</sub>)<sub>2</sub> in varied molar proportions was added slowly by dripping through a glass funnel into the solution containing dissolved NaH<sub>2</sub>PO<sub>4</sub>·6H<sub>2</sub>O and NaCl. In each synthesis, the molar ratio of UO<sub>2</sub>: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 Pb<sub>5-x</sub>(UO<sub>2</sub>)<sub>x</sub>(PO<sub>4</sub>)<sub>3</sub>Cl. 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.</p><p>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 (UO<sub>2</sub>)<sup>2+</sup> partially substituting Pb<sup>2+</sup>. The observed Raman bands at the regions: 1050 – 918 cm<sup>-1</sup>, 586 – 541 cm<sup>-1</sup>, and 439 – 392 cm<sup>-1</sup> were attributed to the vibrations of the (PO<sub>4</sub>)<sup>3+</sup> units, while those at 830 – 800 cm<sup>-1</sup> were assigned to the (UO<sub>2</sub>)<sup>2+</sup> units. As the U content of the initial solution increased, the intensity of the (UO<sub>2</sub>)<sup>2+</sup> band increased relative to the highest band of (PO<sub>4</sub>)<sup>3+</sup>. When the initial concentration of U was the highest, coprecipitation of a second phase, the not-yet-described Pb-analog of meta-autunite (Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>∙6H<sub>2</sub>O), was observed.</p><p>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.</p><p>This research was funded by the Polish NCN grant no. 2019/35/B/ST10/03379.  </p>

<p>Synthetic REE-enriched Pb-apatites are potentially important materials in the industry. The size and morphology of the crystals can influence the physical properties, and therefore affect technological processes. The conditions of the synthesis can determine the size and morphology of the crystals. Lead apatite Pb<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl (analogue of mineral pyromorphite) was chosen for this study because the morphology of its crystals shows particular sensitivity to changes in synthesis conditions or solution composition. The addition of REE elements was used because there are reports that the morphology of synthetic Ca-apatite crystals depends on the presence of REE elements. Therefore, in the present study, synthesis by solution mixing at room temperature was carried out and the change in morphology of the precipitated pyromorphite crystals was observed as a function of solution chemistry (presence or absence of La or Sm) and concentration, pH, and mixing parameters. Powder X-ray diffraction (XRPD) was used to identify the phase composition of precipitates, scanning electron microscopy (SEM) to examine the morphology of the crystals, and energy-dispersive X-ray spectroscopy (EDS) for analysis of the elemental composition of analyzed crystals.</p><p>XRPD results showed that pyromorphite was identified in all samples. No changes in the crystalline structure were observed (hexagonal system, P6<sub>3</sub>/m space group, typical for apatites). Also, EDS analyses showed that the chemical composition remained unchanged despite the morphological differences and the studied REEs (La or Sm) were incorporated into the structure in similar amounts in all precipitates. SEM images indicated that both the size and morphology of the pyromorphite crystals were sensitive to small modifications of the synthesis conditions. The size ranged from 2 µm up to 500 µm. Stirring resulted in smaller crystals than precipitation in the still water column. Crystals appeared in the form of long hexagonal needles (both single and cross-twinned), or slightly rounded, elongated and spear-like rods, or flower-like forms and intergrowths. The presence of REE caused elongation parallel to crystallographic c axis and formation of long needles compare to stubby hexagonal rods in the control sample.</p><p>The variation in size and morphology of Pb-apatites synthesized by the precipitation in aqueous solutions in different conditions were reported for the first time. Further research is needed to explain the contributing factors.</p><p>Slight changes in the synthesis protocol strongly affect the size and shape of Pb-apatite crystals. Therefore, determining the optimal conditions for the synthesis of homogeneous and well-formed crystals could be of great importance in the potential future applications of these materials.</p><p>This research was funded by NCN research grant no. 2019/35/B/ST10/03379.</p>

Although vanadinite (Pb5(VO4)3Cl) occurs in abundance in various terrestrial geochemical systems of natural and anthropogenic origin and is seriously considered as a potential nuclear waste sequestering agent, its actual application is severely limited by a lack of understanding of its basic thermodynamic parameters. In this regard, the greatest challenge is posed by its incongruent dissolution, which is a pivotal hurdle for effective geochemical modeling. Our paper presents an universal approach for geochemical computing of systems undergoing incongruent dissolution which, along with unique, long-term experiments on vanadinites’ stability, allowed us to determine the mineral solubility constant. The dissolution experiments were carried out at pH = 3.5 for 12 years. Vanadinite has dissolved incongruently, continuously re-precipitating into chervetite (Pb2V2O7) with the two minerals remaining in mutual equilibrium until termination of the experiments. The empirically derived solubility constant KspV,298 = 10–91.89 ± 0.05 of vanadinite was determined for the first time. The proposed modeling method is versatile and can be adopted to other mineral systems undergoing incongruent dissolution.

<p>   Supply of technologically important rare earth elements (REEs) is of concern in Europe. Important European sources are associated with apatites and phosphate rocks. Due to high production, this is a potential resource, but technical and cost challenges hinder the commercial recovery of REEs. A new extraction approach exploring selective co-precipitation of REEs and Pb in the form of phosphates offers cheap and effective technology which can be included in the existing flow of ore processing.</p> <p>   Most hydrometallurgical REE enrichment processes vary in efficiency for heavy and light REE. The aim of the present study was to verify whether this novel method of removing REEs from solution by co-precipitation with Pb-phosphates also has this drawback. Solution containing ca. 100 mg/L of Sc, Y, Th, and lanthanides (except Pm) was mixed with a solution containing Pb<sup>2+</sup>, PO<sub>4</sub><sup>3-</sup>, and Cl<sup>-</sup> to induce precipitation (pH between 2 and 4, ambient conditions). The initial and final solutions were analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES) for Pb and REE concentrations, while solids were filtered, dried, and analyzed with powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM).</p> <p>   In all experiments, the formation of a precipitate composed mainly of crystalline pyromorphite (Pb,REE)<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl was found, accompanied in smaller amounts by a second, less crystalline phase. In the SEM images, pyromorphite is apparent as hexagonal rods and needles (micrometers in size) while the second phase forms Cl-free, loose aggregates of globular grains, tens of nanometers in size. The extent of REE substitution for Pb in the pyromorphite structure, determined in a separate study for La, is at the order of ca. 1 wt. % La<sub>2</sub>O. At the experiment conditions, the charge difference between Pb<sup>2+</sup> and REE<sup>3+</sup> is compensated by Na<sup>+</sup>. Significant amounts of REEs are also precipitated in the form of Cl-free Pb-REE-phosphate, which constitutes an accompanying phase or a mixture of phases. At this stage of research, the structure and chemical composition of these phases could not be identified conclusively: the XRD pattern is obscured by pyromorphite, and the precipitate is too fine for regular microprobe analysis.</p> <p>   The concentrations of metals in question were reduced in the solution very significantly. For initial concentrations in the 1-10 ppm range, they were completely removed from solution to concentrations below detection limits of 0.002 ppm (concentrations of Y and La dropped down to below 0.01 ppm). For initial concentrations of 80 ppm, only Sc and Th were removed completely while concentrations of Ce, Pr, Nd, Sm, Eu, Gd, and Tb were reduced by over 60%, these of Dy, Ho, Er, Tm, Yb, and Lu by ca. 50 % and concentrations of Y and La dropped down only by ca. 40%. This may indicate that although the fractionation of REEs is not systematic and not very significant, heavy REEs are removed from the solution slightly less effectively. However, the crystalline and heavy precipitate allows easy separation of the solution.</p> <p>This research was partially funded by NCN research grants no. 2019/35/B/ST10/03379 and 2021/43/O/ST10/01282. </p>