Uranium , photochemical

Following biological degradation, the extract is exposed to photochemical degradation, which removes uranium from solution as polyuranate. The metals and uranium are captured in separate treatment steps, allowing for the separation of wastes into radioactive and nonradioactive waste streams. This treatment process does not create additional hazardous wastes and allows for the reuse of the contaminated soil. The technology has been the subject of bench-scale tests and is not currently commercially available. 

Two major instances of photochemical reactions that have reached deeply into modern civilization are the photosensitive silver and uranium salts and dyes which are the basis of photography and the manufacture of Vitamin D by the ultraviolet irradiation of ergosterol,... 

Ial >340 nm, 450 W medium pressure Hg-lamp, Pyrex, uranium-glass filter. Scheme 5.13. Photochemically induced formation of a tetrasubstituted furan. 

Uranium (IV) can also be reduced to U(III) photochemically. Irradiation of a solution of UCI4 in methanol at 248 nm gave up to 85% reduction with a quantum yield of 0.17. The U(III) produced could be stabilized as its (18-crown-6) complex (24a). 

Borohydride compounds of trivalent actinides are limited to those of uranium. The initial reports of U(BH4)4 indicated it was prepared from thermal or photochemical decomposition of U(BH4)4 as in Equation (6) ... 

The first attention given to actinide photochemistry was for the purpose of identifying any photochemical activity which might alter the efficiency of the extraction or exchange processes. Subsequently, the identification of photochemically active species of uranium and plutonium gave some indication that the photoreactions could be turned to a useful end and, perhaps, offer a cleaner way to separate actinides from each other and from the other elements accompanying them in nuclear fuel elements. 

Goldstein, M Barker, J. J. Gangwer, T. A Photochemical Technique for Reduction of Uranium and Subsequently Plutonium in the Purex Process , BNL-22443 (1976). 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

Although the redox potentials for aqueous solution indicate that uranium(IV) should reduce plutonium(IV), anions and other complexing agents can change the potentials sufficiently that uranium(IV) and plutonium(IV) can coexist in solution (25). Since one of the products of photochemical reduction of uranyl by TBP is dibutyl phosphate (DBP), which complexes plutonium(IV) strongly, experiments were done to test the photochemically produced urani-um(IV) solutions as plutonium(IV) reductants (26). Bench-scale stationary tests showed these solutions to be equivalent to hydroxylamine nitrate solutions stabilized with hydrazine (27). 

The photochemical reduction of a solution containing both uranium(VI) and plutonium(IV) is also of interest for reprocessing applications. Early experiments (12a) showed a significant reduction of plutonium(IV) by light in Purex-type process solutions. Since the quantum yield for plutonium redox reactions is about one-tenth that for uranyl reduction (7b,c) the most likely path of plutonium(IV) reduction in these experiments appears to have been by uranium(IV) or uranium(V) generated by photochemical reduction of uranyl by other components of the solutions. Further experiments in this area would be useful. 

DePoorter, G. L., Rofer-DePoorter, C. K., and Hayter, S. W., "Photochemically Produced Uranium (IV) and Application in LWR Fuel Reprocessing," Back End of the LWR Fuel Cycle Conf. held in Savannah, GA, 1978 (CONF-780304), p. V-15. 

While stable binary actinide carbonyls are still unknown, research in this area focused mainly on the detection and theoretical investigation of unstable molecules such as the monocarbonyl complexes of thorium and uranium. The possible molecular structures U-GO, U-OG, and GUO of carbon monoxide interacting on a uranium metal surface have been studied by density functional theory (DFT).14 GUO has been produced experimentally by reaction of laser-ablated U atoms with CO in excess argon and trapped in a triplet state in solid argon at 7 K.15 Studies of the reaction of thorium atoms with CO have been carried out. The reaction of laser-ablated thorium atoms with carbon monoxide in excess neon gave the first thorium carbonyl complex, Th-GO, which rearranges photochemically to CThO (Scheme l).16... 

A solution of 5.4b (150 mg, 0.54 mmol) in a mixture of dichloromethane (150 mL) and hexane (200 mL) was placed into an oven-dried photochemical reaction vessel, equipped with a pyrex immersion-well and a uranium glass filter. To this solution was added... 

In the following procedures, /3-uranium pentafluoride is conveniently prepared by the photochemical reduction of uranium hexafluoride, in a manner similar to an earlier smaller-scale preparation.9 Pentaethoxyuranium is prepared directly from 0-UFs and sodium ethoxide in ethanol. The preparation of hexafluoro-uranium salts from /3-UFs in nonaqueous solvents is described in a procedure that avoids the use of hydrofluoric acid common to previous methods.10,11... 

Our examination of the photochemical literature of uranium clearly shows that extensive attention has been given to UFg, while other compounds, until recently, have been almost ignored. The attention given to UFg, of course, relates back to the great interest in achieving a low cost laser induced isotope separation process for uranium isotopes. The economics of isotope separation, which have been briefly discussed by Letokhov and Moore (61), have consequently dictated the direction of much of the applied photochemical research on uranium compounds. Nonetheless, from the existing spectroscopic and photochemical data outlined here it would be expected that coordination and... 

Systematics attempts should be made to systematize photochemical reactions of uranium compounds. 

Urey, H. C. "Investigations of the Photochemical Method for Uranium Isotope Separation," Project SAM, Columbia University, July 1943 (A-750). 

Freed, S. Sancier, K. M. "Photochemical Activity of Salts of Uranium in Solutions at the Temperature of Liquid Nitrogen," J. Chem. Phys., 1954, 22, 928. 

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