Uranyl phosphate thermodynamics

Fowle et al. (2000) have measured the sorption by a soil bacterium (B. subtilis) of uranyl in 0.1 M NaC104 at 25°C as a function of pH, time, and solid solute ratio, using a batch technique. The stoichiometiy and thermodynamic stability of the important uranyl-surface complexes indicated that uranyl formed two different surface complexes, one involving neutral phosphate functional groups, and another with deprotonated carboxyl functional groups, on the bacterial cell wall ... 

Probing Ca++—Phosphate Binding. Although this question has been at the center of the current investigations, little experimentation was reported on direct evidence which could be furnished by IR spectroscopy. In a brief mention of IR absorption of DPL-uranyl nitrate in nujol paste, it was concluded that indeed Ca++ interacts with the lipid phosphate group (7). Subsequently, after thermodynamic analysis of calorimetric studies with DPL dispersions in aqueous electrolyte, the same laboratory suggested the absence of direct Ca++-phosphate interaction (8). Strangely, the authors of the second work (8) failed to provide explanations for the discrepancy between this and a previous report (7). The particular predicament implies that either one of the two experiments had to be... 

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. 

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. 

W. Davis, Jr., J. Mrochek, and R. R. Judkins. 1970. Thermodynamics of the two-phase water-uranyl nitrate-tributyl phosphate-Amsco 125-82 system. J. Inorg. Nucl. Chem. 32 1689. 

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