Uranium minerals uranyl phosphates

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

Complex uranium ores are often associated with phosphate-bearing minerals, and the presence of soluble phosphate has been found to adversely affect the recovery of the uranium. Studies such as those outlined above have established that, although the iron(III)-phosphonato complexes are more reactive, the decreased leaching rate in the presence of phosphate is due to the formation of insoluble, non-conducting layers of uranyl phosphate on the surface of the mineral. 

To summarize uranium complexing in hydrothermal solutions, the predominant species will depend on the concentration of complexing anions, which is, in turn, dependent on temperature and pH. The activity of fluoride in many uranium mineralizing systems appears to be significant, as is indicated by the abundance of fluorite and other fluoride-containing gangue minerals. In these systems uranyl fluoride complexes would predominate in acid to neutral solutions. At low temperatures carbonate complexes predominate in alkaline solutions, but, as temperature increases, carbonate complexes become less important. Phosphate complexes may be important in nearneutral solutions in which as little as 0.1 ppm phosphate is present. As temperature increases, hydroxide complexes become more important. At temperatures of 300°C and above hydroxide complexes may be the only soluble uranium species. 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

The uranyl phosphates and arsenates comprise the largest group of uranium minerals and, except for uranophane, the most abundant mineral group. Table 10 lists the known minerals in this group. The autunite and meta-autunite families... 

The author would like to thank the several students and colleagues who have contributed to this study of uranium minerals. Frances V. Stohl, Christine A. Anderson and Barry E. Scheetz have contributed greatly to the study of the uranyl silicates. Michael E. Zolensky is still engaged in the studies of the uranyl phosphates. He has also helped review this manuscript. Steve A. Markgraf assisted with the literature search and compiling of data used in the tables. 

As was mentioned before, Arey et al. [19] conducted batch equilibration experiments to evaluate the ability of hydroxyapatite to remove uranium from contaminated sediments at the Savannah River Site of DOE and showed that removal of U was due to secondary phosphate minerals that had solubility even lower than autunite (Ca(U02)2(P04)2- IOH2O). The authors suggest formation of Al/Fe secondary phosphate. A similar conclusion was reached by Fuller et al. [20], who showed that uranyl ions can be removed by using hydroxyapatite. 

Simple thermodynamic calculations based on literature data (5-12) support the choice of phosphates as the optimum mineral phases for actinide immobilization. The calculations considered every relevant species reported (5-72) that contained protons, hydroxide, or the ligand in question for each metal ion. Where necessary, equilibrium constants were corrected to 0.1 M ionic strength using the Davies equation. As an example, the calculated solubility of europium, thorium, and uranium in various media at p[H] 7.0 (p[H] = - log of the hydrogen ion concentration), 0.001 M total ligand concentration, 0.1 M ionic strength, and 25 °C are shown in Table I. Within the constraints of the calculation, the solubility of thorium is limited by Th(OH)4, but the lowest europium and uranyl solubilities are observed for phosphates. 

The hexavalent-uranium phosphate minerals (Table 14) are important and widespread uranyl-oxysalt minerals. Their structures and behavior are dominated by the crystal chemistry of the (U 02) uranyl group they have been described in detail by Burns (1999) and will not be considered any further here. 

Uranium in its highest valence state forms a large number of colourful minerals that may deposit in the oxidized zone associated with the primary deposit or the uranium may go into solution and be transported a considerable distance from its source area before reprecipitation. Minerals that form at the source may mimic the original phases by direct replacement, but more often they form a nondescript mass that destroys any original structure. These minerals are usually hydrated uranyl oxides, silicates or phosphates. Further from the source the minerals usually form as one or more of the many hydrated uranyl oxysalts. 

The uranyl vanadates form mineral groups distinct from the phosphates and arsenates because of the markedly different chemistry of the vanadium ion. Like uranium, vanadium shows several valence states in nature, and its detailed mineralogy is very complex. The crystal chemistry of vanadium was reviewed by Evans.In its lower valence states it forms distinct vanadium minerals, but in its higher valence state 5-1- it com-... 

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