Nitric acid media

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

The depolymerization of the plutonium polymers is a very slow reaction at room temperatures and moderate acid concentrations. It is only upon heating or by the use of very concentrated acid that the depolymerization rate can be made appreciable. Kraus (16) has compared, in two nitric acid media, 6M and 10M, the depolymerization of already formed polymers—one at room temperature and the other at 95 °C. for one hour. The values for the half time of the reactions ranged from 29 minutes for the non-heated, 10M HN03 solution to 730 minutes for the heated, 6M HN03 solution. Lindenbaum and Westfall (22) found that at pH 7.8, a region of environmental interest, the rate of depolymerization was only about 0.04% per hour and at pH 4.0 increased to only 0.18% per hour. 

Musikas and co-workers studied extensively the extraction behavior of inorganic acids and U/Pu extraction chemistry with A/,A-dialkyl amides (202-205). Based on the extraction data, they proposed certain dialkyl amides suitable for the reprocessing of irradiated nuclear fuels in nitric acid media. Most of the work reported earlier on amides referred to either aromatic or substituted aliphatic hydrocarbons employed as diluents. However, these diluents are not suitable for commercial-scale reprocessing due to their poor radiation and chemical stability in the presence of nitric acid, as well as their tendency to form a three-phase system. 

Horwitz, E.P. Kalina, D.G. Kaplan, L. Mason, W.G. Diamond, H. Selected alkyl(phenyl)-V,V-dialkylcarbamoylmethylphosphine oxides as extractants for Am(III) from nitric acid media, Sep. Sci. Technol. 17 (1982) 1261-1279. 

Sriram, S. Mohapatra, P.K. Pandey, A.K. Manchanda, V.K. Badheka, L.P. Facilitated transport of americium(III) from nitric acid media using dimethyldibutyltetradecy 1-1,3 -malonamide, J. Memb. Sci. Ill (2000) 163-175. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

In the absence of any deliberately added reductant, Pu(IV) remains unchanged under UV irradiation in aerated or de-aerated aqueous carbonate solutions. However, in nitric acid media, both Pu(III) and Pu(IV) are transformed to Pu(VI), although both oxidations are almost certainly mediated by disproportionation of Pu(IV) to Pu(III) and Pu(VI) ... 

The experimental results in Figure 39.9 revealed that when the flow rate of the aqueous phase is higher than that of the organic phase in a hydrophobic membrane and with feed in 4 M HNO3 and 20% TBP as an extractant, the transport of uranium across the membrane is fast. Therefore, a HF contactor in strip dispersion mode with 20% TBP in nph as an extractant could be successfully used for the separation and concentration of U(VI) from nitric acid media. The feasibihty of this technique for the separation of U(VI) from dilute solutions using 0.01 M HNO3 solution as strippant even in the presence of fission products from waste streams makes it a viable alternative to conventional methods. 

Several studies have looked at the extraction of Th and ions by malonamides of varying structural character. Nigond et al. investigated the extraction of Pu from nitric acid media with the malonamide A, 7V -dimethyl-7V,7V -dibutyltetradecylmalonamide (DMDBTDMA)." UV-vis experiments indicate the presence of two extracted species that are formed according to the following equilibria shown in Equation (27) n=l or 2) ... 

The equilibrium for the extraction of U02 by TBP into various organic solvent from nitric acid media has been thoroughly characterized the extracted species is proposed to be U02(N03)2 2TBP. ... 

The extractant 1-hydroxy-6-A-octylcarboxamide-2(lH)-pyridinone (octyl-1,2-HOPO) has an appreciable affinity for Pu , though much less than for Pu under identical conditions. The equilibrium for the extraction from nitric acid media is given by Equation (58). Deprotonated HOPO generally coordinates the metal in a bidentate fashion, but in this case, m can range from 0 to 2. Extraction is greatest at low acid concentrations of around 0.001 M. Extractant dependency gives a slope of 1.3, which equates to a value of 2 for m in the above extraction stoichiometry ... 

The extraction of by a synergistic mixture of 2-thenoyltrifluoroacetone (HTTA) and tributylphosphate (TBP) from nitric acid media into benzene has been studied. Previous literature reports have described the synergistically extracted species as U02(N03)(TTA) TBP. However, Patil et have used extraction studies to show that the nitrate anion is not present in the extracted complex. Rather, the only species involved in the synergistic extraction is UO2-(TTA)2 TBP. ... 

The extraction of TCO4 from nitric acid media by TBP/OK has been reported to follow equation (160) in the absence of 002 " and equation (161) in the presence of U02 . The values of Dtc" between nitric acid and TBP/OK were found to increase with HNO3 concentration to a maximum value between 0.6 and 1.0 M HNO3 before falling off because of competition... 

The solubility of plutonium peroxide in nitric acid media ( .,is a function of the acid concentration as well as the concentrations of H2O2 and foreign ions. Typical solubilities range from 7 to 1190 mg plutonium per liter as the acid concentration varies from 1.2 to 5.2N (2). 

Although a variety of extractants have been studied and tested for their suitability in analytical separations of actinides, perhaps the most recent milestone in the process use of actinide extractants is the application of the bidentate carbamoylmethyl-phosphonates (17,18,19). The pioneering work on these compounds was done 15 years ago by Siddall (20,21). Their principal advantage is their ability to extract the trivalent, tetravalent, and hexavalent actinides from considerably more concentrated nitric acid media than other extractants. 

The recent photochemistry interests at Oak Ridge have been turned to an examination of these aspects for neptunium in perchloric (15) and nitric acid (16) solutions. A perchloric acid medium provides a much simpler starting point because the CIO ion is not subject to photolysis as is the NO ion when exposed to light of 250-360 nm wavelength. Nevertheless, nitric acid media were ultimately examined because of their obvious practical importance. 

Nitric Acid Media. The photo-redox reactions of neptunium in nitric acid are, as stated earlier, complicated by the NO3 photo-reduction which prevents the complete photo-oxidation of neptunium to the (VI) state. At first this might seem to pose a dilemma which ought to be avoided, but recent results have shown how this apparent complication is essential to a photo-separation process. Two photochemical reactions working together will be shown to be necessary in order to reach what we have accepted as a practical result. 

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