Uranyl phosphate formation

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. 

Suzuki and Banfield (1999) classify methods of microbial uranium accumulation as either metabolism dependent or metabolism independent. The former consists of precipitation or complexa-tion with metabohcally produced ligands, processes induced by active cellular pumping of metals, or enzyme-mediated changes in redox state. Examples include precipitation of uranyl phosphates due the activity of enzymes such as phosphatases, formation of chelating agents in response to metal stress, and precipitation of uraninite through enzymatic uranium reduction. 

A mixed bacterial culture obtained by enrichment with tributyl phosphate as sole source of carbon and phosphorus was immobilized in a polyurethane foam matrix (Thomas and Macaskie 1996). These nongrowing cells produced phosphate by hydrolysis of the substrate, and this could be coupled to the formation of uranyl phosphate that was washed out of the column and then precipitated. 

N. Boukis and B. Kanellakopoulos. Extraction phase distribution of uranyl nitrate with tri-n-butyl phosphate Part II - The formation of a third phase in the system U02(N03)2-TBP-HN03. Technical report, Kernforschungszentrum Karlsruhe (KfK-3352), 1983. 

Hahn, H.T., Vander Wall, E.M. 1964. Complex formation in the dilute uranyl nitrate-nitric acid-dibutyl phosphoric acid-tributyl phosphate-amsco system. J. Inorg. Nucl. Chem. 26 191-192. 

Uranium(VI) dioxydichloride, 5 148 Uranium (VI) hydrogen dioxyortho-phosphate 4-hydrate, 6 150 analysis of, 5 151 Uranium(IV) oxalate, 3 166 Uranium (IV) oxide, formation of, by uranyl chloride, 6 149 Uranium (IV) (VI) oxide, U3Oa, formation of, by uranyl chloride, 5 149... 

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. 

Indirect determinations of several types can be carried out. Sulfate has been determined by adding an excess of standard barium(II) solution and back-titrating the excess. By titrating the cations in moderately soluble precipitates, other ions can be determined indirectly. Thus sodium has been determined by titration of zinc in sodium zinc uranyl acetate, and phosphate by determination of magnesium in magnesium ammonium phosphate. Quantitative formation of tetracyano nickel-ate(II) has been used for the indirect determination of cyanide. ... 

We have shown that phytic acid readily hydrolyzes to produce phosphate with a projected lifetime of 100-150 years in the absence of microbiological effects, that actinide-phytate compounds are insoluble, and that europium and uranyl phytates are converted to phosphates within a month at 85 °C. Thorium solubility, on the other hand, is controlled by hydroxide or oxide species. Furthermore, the solubilities of radiotracer europium and uranyl are reduced by phosphate dosing of a simulated groundwater solution, even in the presence of citric acid. In the same systems, neptunium(V) solubility is only affected by 0.01 M phosphate at pH greater than 7. The results of these tracer-scale immobilization experiments indicate that phosphate mineral formation from representative deposits is under thermodynamic control. 

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