Uranium hydrolysis products

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

As indicated earlier Kraus (9) has recognised three types of hydrolysis products of tetravalent thorium, uranium, plutonium and americium. These products are ... 

Several uranium compounds tested on the eyes of animals caused severe eye damage as well as systemic poisoning. The anion and its hydrolysis products determine the degree of injury. A hot nitric acid solution of uranyl nitrate spilled on the skin caused skin burns, nephritis, and heavy metal encephalopathy. Prolonged skin contact with uranium com-... 

In another study of workers exposed to UF, the review of two years of follow-up medical data on 31 workers who had been exposed to uranium (VT) fluoride and its hydrolysis products following the accidental rupture of a 14-t shipping cylinder in early 1986 indicated that none of the 31 workers sustained any observable health effects from exposure to U even though an exposure limit of 9.6 mg was exceeded by eight of the workers (244). 

Hexavalent. As with most reactions, the hydrolysis of U02 + is the best studied of the hexavalent actinides. The hydrolysis of U02 + begins at pH 3, while the onset for the hydrolysis of Np02 + and Pu02 + each occur at a higher pH. The monomeric hydrolysis products of the uranyl ion, U02(0H) n = 1, 2) can be studied in solutions with uranimn concentrations less than 10 M. For solutions with higher uranium concentrations, multinuclear cationic species dominate the speciation, for example, (U02)2(0H)22+, (U02)3(0H)42+, and (U02)3(OH)s+. These cations have been crystallized from solutions with the formulas (U02)2(at2-OH)2(OH2)6 + and (U02)3(M3-0)(/x2-0H)3(0H2)6+ (21). For Np and Pu, the dimer of the first hydrolysis product, (An02)2(OH)2 + (22), has also been identified but not fully stracturally characterized. 

Leach LJ, Gelein RM, Panner BJ, et al. 1984. The acute toxicity of the hydrolysis products of uranium hexafluoride (UE6) when inhaled by the rat and guinea pig. Pinal report. ISS K/SUB-81-9039-3. NTIS... 

Morrow PE, Leach LJ, Smith FA, et al. 1982b. Metabolic fate and evaluation of injury in rats and dogs following exposure to the hydrolysis products of uranium hexafluoride. Implications for a bioassay program related to potential releases of uranium hexafluoride. Govt Rep Announce Ind, Issue 11. NTIS/NUREG/CR-2268. 

Hydroxides. The hydrolysis of Np has been studied more than that of any other pentavalent actinide because it is the most stable oxidation state for Np and it is an actinide ion of significant concern for environmental migration. Pentavalent uranium disproportionates in aqueous solution at pH values where hydrolysis would occur. Hydrolysis products for Pa, Pu, and Am are very similar to, but much less stable than those of Np, so only Np hydroxides will be described in detail. Neptunyl hydrolyzes at about pH 9, to form the stepwise products, Np02(0H) and Np02(0H)2 ", which have been identified by optical absorbance and Raman spectroscopy. " In addition to the hydroxide these complexes likely have two or three inner-sphere waters in the equatorial plane and pentagonal bipyramidal coordination geometry. 

Hydrolysis of TBP. Hydrolysis of TBP occurs stepwise via dibutyl and monobutyl phosphoric acid ([C4H9O] jPOjH and C4H9OPO3H2) and leads eventually to phosphoric acid. Dibutyl phosphoric acid is the most abundant degradation product. Its rate of formation is influenced by temperature, the nitric acid concentration, the uranium content, and the presence of a diluent, beside the radiation dose. The acidic nature of these hydrolysis products allows in principle cleanup by an alkaline wash. 

Morrow PE, Leach LJ, Smith FA, Gelein RM, Scott JB, Beiter HD, Amato FJ, Picano JJ, Yuile cl and Consler TG (1982) Metabolic Fate and Evaluation of Injury in Rats and Dogs Following Exposure to the Hydrolysis Products of Uranium Hexafluoride, NUREG/CR-2268, New York. 

Cassidy et al. [48] used this reagent for the determination of lanthanides in thorium dioxide/uranium dioxide fuel rods. Arsenazo I and PAR are not suitable for this analysis, because both have to be used under strongly alkaline conditions. However, thorium hydrolyzes at alkaline pH, so that thorium-containing solutions cannot be injected directly under these conditions. In addition, the hydrolysis product is not soluble in water. Maximum sensitivity is obtained at pH 2.5, but the sensitivity obtained with a purely aqueous solution with the optimum concentration of c = 0.15 mol/L (pH 4.6) is sufficient. The measuring wavelength is 653 nm. A sample chromatogram was shown in Fig. 4-58 in Section 4.5.3. 

Fodor, M., Z. Poko, and J. Mink, 1966. Investigation of hydrolysis products from uranium trioxide and uranyl salts by derivatography and infrared spectroscopy. Mikrochim. Acta 865. 

The last reaction cited above as shown is very effectively catalyzed by bacterial action but is very slow chemically by recycling the spent ferrous liquors and regenerating ferric iron bacterially, the amount of iron which must be derived from pyrite oxidation is limited to that needed to make up losses from the system, principally in the uranium product stream. This is important if the slow step in the overall process is the oxidation of pyrite. The situation is different in the case of bacterial leaching of copper sulfides where all the sulfide must be attacked to obtain copper with a high efficiency. A fourth reaction which may occur is the hydrolysis of ferric sulfate in solution, thus regenerating more sulfuric acid the ferrous-ferric oxidation consumes acid. 

A different type of bridging occurs in hydrolysis complexes of tho-rium(IV) (219) and uranium(IV) (130). Here a distinct peak at 3.94(2) A in the hydrolyzed solutions can be ascribed to the metal-metal distances in the hydrolysis complexes. Discrete dinuclear complexes with a very similar metal-metal distance, 3.988(2) A, in which the metal atoms are joined by double hydroxo bridges have been found in crystals ofTh2(OH2)(N03)6(H20)8 (229). The same type of bridging, therefore, must occur in solution. When hydrolysis is increased, however, the number of metal-metal distances per metal atom increases beyond a value of 0.5, valid for a dinuclear complex, and larger hydrolysis complexes are obviously formed. These structures are unknown but an extensive X-ray investigation of highly hydrolyzed thorium(IV) solutions has shown that there is probably no close relation between the structures of the hydrolysis complexes in solution and the structure of thorium dioxide, which is the ultimate product of the hydrolysis process (230). 

Wet proces.ses for the production of UO2 precipitation of poorly soluble uranium compounds from the hydrolysis of UF(, ... 

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