Leon Maya

A Raman spectroscopy study of hydroxo and carbonato species of the uranyl (VI) ion

Abstract The Raman spectra of a number of uranyl species in carbonate and alkaline solutions have been obtained as well as those of solids obtained from carbonate media. The frequency of the UO2−ν1 symmetric stretching vibration of these species can be used to complement evidence derived by other means in their identification. Thus, the existence of uranyl hemicarbonate, (UO2)2CO3(OH)3−, has been demonstrated as well as that of anionic hydroxo species, such as UO2(OH)42−. The conversion of UO2(CO)32− into UO2(CO3)34− by addition of HCO3− was followed by Raman spectroscopy and 13C NMR. Solid solutions with compositions within that of UO2CO3 and sodium uranate have been detected. A correlation was observed between the ν1 frequency and the number and nature of the ligands surrounding the uranyl ion.

Hydrolysis and carbonate complexation of dioxoneptunium(V) in 1. 0 M NaClO/sub 4/ at 25/sup 0/C

The purpose of experiment was to obtain data on the hydrolysis and carbonate complexation of NpO/sub 2//sup +/ which might then be used to establish speciation schemes of this ion under conditions similar to those found in nature. Chosen experimental approach was based on determination of Np solubility of a well-defined solid phase as a function of carbonate concentration in a medium of constant ionic strength. The carbonate complexation constants and the hydrolytic constants measured were used to calculate a species distribution scheme for Np(V) in the pH range 7-9 under a partial CO/sub 2/ pressure if 10/sup -2/ atm. Analysis of data shows that the dominant species to be found in nature would be NpO/sub 2/CO/sub 3//sup -/ and/or NpO/sub 2/(CO/sub 3/)/sub 2//sup 3 -/. Formation of NpO/sub 2/(CO/sub 3/)/sup 3 -/ would require an unusually high degree of oversaturation by the common carbonate minerals. Np(V) shows a tendency to form relatively stable carbonate complexes, leading to the prediction that it would be found in groundwaters in the form of a complex. 18 references, 2 figures, 1 table.

Ruthenium(IV) in nitric acid media

Abstract Nitrosyl ruthenium nitrates undergo photolytic dissociation to yield nitric oxide and a Ru(III) intermediate which is spontaneously oxidized in nitric acid to a Ru(IV) species. The Ru(III)-Ru(IV) conversion is acid dependent and proceeds faster at high acid concentrations. The photodissociation reaction can be reversed by sparging the nitric acid solution containing Ru(IV) with NO. The Ru(IV) species obtained by photodissociation in nitric acid has been identified as Ru 4 (OH) 12 4+ by the commonality of its chemical and electrochemical behavior in perchloric acid. The Ru(IV) species can be reduced in two one-electron processes to species with average oxidation numbers of 3.75 and 3.5. The Ru(IV) species can be oxidized to Ru(VIII); in addition, a species with an intermediate oxidation state with an average value of 4.38 has been observed.

Chemistry of extractable nitrosyl ruthenium species in the system nitric acid-tributyl phosphate-dodecane

Abstract Fission product ruthenium is present in solutions resulting from the dissolution of spent nuclear fuel in the form of nitro and nitrato complexes of nitrosyl ruthenium. A fraction of the ruthenium, mostly higher nitrato complexes, is extracted into the organic solvent phase in the Purex process. Stripping tests reveal a component in the solvent phase that cannot be readily transferred to the aqueous phase. Experiments were performed to determine whether the retained fraction is a highly extractable species originally present in the aqueous phase or if it is the product of a reaction between the extracted species and the solvent. A chromatographic procedure was developed to separate the species in the aqueous phase. This revealed a species more extractable than the tetranitrato complex; however, since the separation process involved TBP, the ambiguity regarding the origin of this species could not be resolved. On the other hand, in a separate series of experiments evidence was found showing that the retained species result from a reversible reaction in the solvent phase. The equilibrium concentration of this species is dependent on the temperature and the tributyl phosphate activity. This complex apparently results from the substitution of aquo ligands in the extracted species by tributyl phosphate.

Extraction of zirconium from nitric acid media by butyl lauryl phosphoric acid in dodecane

Abstract Butyl lauryl phosphoric acid (HBLP) was chosen as a model compound of secondary degradation products of Purex process solvent. The extent of zirconium complexation by this compound is similar to that of dibutyl phosphoric acid or bis(2 ethyl hexyl) phosphoric acid. The mechanism of zirconium extraction by HBLP was studied. The nature of the extracted species could not be unambiguously established. Tributyl phosphate has an antagonistic effect on the zirconium extraction by HBLP. Removal of HBLP from the solvent cannot be accomplished by alkaline washes representative of solvent clean-up procedures used in the Purex process. This could lead to a long term build-up of this type of compound under Purex processing conditions. The threshold HBLP concentration for an enhancement of zirconium extraction is 2 × 10 −4 M.

Dioxo uranium(VI) carbonate complexation in uranium recovery by reactive ion-exchange

Abstract The reaction between uranyl(VI) ions and a strongly basic anion exchange resin in the bicarbonate form is found to lead to complete uranium absorption into the resin and evolution of carbon dioxide. The uranium species absorbed by the resin is the tricarbonato complex. The use of this reaction for uranium separations and recovery was explored. Dowex 2-X8 has a marked selectivity for the dioxo uranium tricarbonato complex in the presence of competing anions. Increasingly higher urnaium distribution coefficients are observed as the competing anion concentration decreases but this trend is limited by competition with uranium hydrolysis.

Zirconium behavior in the system HNO3-30% tributyl phosphate-dodecane in the presence of monobutyl phosphoric acid

Abstract Dibutyl and monobutyl phosphoric acids result from the radiolytic degradation of tributyl phosphate in the solvent extraction processing of spent nuclear fuel. Information is needed about the interaction between zirconium and monobutyl phosphoric acid to assess the relative importance of this degradation product in terms of zirconium extraction, retention and solids formation. Monobutylphosphoric acid (H2MBP) extracts zirconium from nitric acid to a similar extent as dibutyl phosphoric acid (HDBP). Zirconium extracts containing H2MBP aged at low acidity, ≤0.1 M H+, undergo a reaction to produce a species that is retained by the solvent. There is no retention in the presence of HDBP alone. Gelatinous solids are formed in zirconium solutions in nitric acid by the addition of H2MBP. Analyses of these solids correspond to the empirical formula Zr(MBP)2·H2O. In addition to this compound, an intermediate solid with a MBP Zr molar ratio of one is formed in 0.2 M HNO3. The addition of H2MBP to zirconium extracts in 1.09 M TBP in dodecane containing HDBP leads to a solid having a formula corresponding to Zr(OH)x(NO3)y (DBP)-(MBP) where x + y = 1. Addition of H2MBP to a zirconium extract in the absence of HDBP leads to a solid with a MBP Zr molar ratio of two. Solubility estimates of these solids show that the MBP compounds are more insoluble, by at least two orders of magnitude, than the corresponding DBP complexes. Extraction of zirconium in the system is mostly determined by HDBP while H2MBP determines zirconium retention and precipitation.

The chloride-catalyzed decomposition of ammonium nitrate in nitric acid media at 100°C

Abstract The rate of disappearance of ammonium ion in chloride-catalyzed nitric acid mixtures at 100°C can be expressed by the rate equation d(NH 4 + )/d t = − k [H + ] n [NO 3 − ] p [NH 4 + ], where k =(2.430±0.186)×10 −7 , (sec −1 ), n = 2.426 ± 0.034, o = 3.096 ± 0.039, and p = 1.295 ± 0.035. The proposed reaction mechanism is: Fluoride and bromide had no catalytic activity. This is in agreement with the proposed reaction sequence since they do not form stable analogs of hypochlorous acid or chloramine.

Niobium(V) extraction by butyl lauryl phosphoric acid-dodecane from nitric acid☆

Abstract Extraction of niobium by tributyl phosphate (TBP) in the Purex process is inefficient; and in spite of this, niobium is found in solvent streams. Alternate mechanisms for niobium transfer into the solvent were sought. Butyl lauryl phosphoric acid (HBLP), a model compound of secondary degradation products of the solvent used in the process, was found to enhance niobium extraction above a threshold concentration of 5 × 10−4 M. A study of the niobium distribution coefficient dependences in the system HNO3-HBLP-dodecane on the HBLP, hydrogen ion and nitrate ion concentration suggests an extraction mechanism involving neutralization of an hydroxyl group bound to the niobium and no incorporation of nitrate into the extracted species. These results are supported by chemical analysis of the extracted species which show an empirical formula corresponding to Nb(OH)4BLP. Silica and zirconium monobutyl phosphate, under certain conditions, disperse adsorbed niobium in the solvent phase in the form of emulsions or suspensions. Centrifugation of these give no net enhancement of niobium transfer.

Fluoroboric acid and its hydroxy derivatives—solubility and spectroscopy

Abstract The composition of fluoroboric acid and its hydroxy derivatives are described within the H 3 OBF 4 -HBO 2 -H 2 O system. Solubility isotherms at 25 and 65°C were determined. Boric acid is the equilibrium solid phase at high water concentrations while metabolic acid is the solid phase at low water concentrations. Dihydroxyfluoroboric acid is a metastable species at 25° that disproportionates into H 3 OBF 3 OH and HBO 2 . Trihydroxyfluoroboric acid does not exist in the pure state. NMR, IR and Raman spectra for dihydroxyfluoroboric acid, boron trifluoride dihydrate and intermediate compositions were obtained. The fundamental vibrations in the Raman spectrum of BF 3 ·2H 2 O (H 3 OBF 3 OH) are almost identical to those of an aqueous solution of NaBF 3 OH. Dihydroxyfluoroboric acid appears to be a tetrahedral molecule. Fluorine as well as boron exchange processes take place at ambient temperature in mixtures of fluoroboric acid and its hydroxy derivatives.