No abstract

Neptunium / Plutonium / Sulfate compounds / Crystallographic structure / Infra-red spectra / Optical spectra Summary Several techniques, X-ray diffraction, thermogravimetry, IR and optical spectroscopy were applied to study the formation of solid Np(VI) and Pu(VI) compounds with S0 4 2-oxoanions. In case of solutions containing equimolar amounts of Np0 2 2+ and cesium sulfate only one compound Cs 2 (Np0 2 ) 2 (S0 4 ), was obtained in crystalline form. But in solutions containing Np0 2 2+ (Pu0 2 2+ ) sulfates with a molar ratio [Cs]: [Np](Pu) higher than 2:1, Cs 2 Np0 2 (S0 4 ) 2 as well as Cs 2 Pu0 2 (S0 4 ) 2 were precipitated. The crystal structure of Cs 2 Np0 2 (S0 4 ) 2 is of layered type and is built up by anionic layers [Np0 2 (S0 4 ) 2 ]" 2 "~ linked together by Cs + ions. Each Np atom in the anionic layer is linked via the sulfateions to three other atoms by such a manner that the bonds form an hexagonal net. Attempts to synthesize hydrated sulfate compounds of Np(VI) and Pu(VI) with cesium via the slow solution evaporation procedure were unsuccessful.

No abstract

No abstract

The behavior of Np in higher oxidation states in alkaline solution containing silicate and aluminate ions was studied. In formation of a crystalline aluminosilicate in a solution, Np(V), (VI), and (VII) are not incorporated into its crystal structure but hamper formation of the solid phase. The possibility of sorption of Np on various aluminosilicates is primarily governed by its oxidation state. Np(V) and Np(VII) are not sorbed from strong alkali. Np(VI) is retained by aluminosilicate materials to various extents depending on the surface characteristics and surface area of these materials. On heating, the degree of Np(VI) sorption decreases, which suggests the physical nature of the process.We studied previously [1] the Pu(VI) sorption on aluminosilicates of the zeolite type, whose spontaneous formation is possible in prolonged storage of alkaline radioactive wastes containing silicate and aluminate ions at elevated temperature. It was shown that Pu(VI) existing in strong alkalis in the form of anionic hydroxo complexes hampers crystallization of aluminosilicate, but in the case of formation of a solid phase it is partially sorbed on its surface. The sorption capacity of crystalline aluminosilicates is low (approximately 6 mg per gram of the sorbent), which apparently reflects the ratio of neutral hydroxo complexes PuO 2 (OH) 2 (H 2 O) 2 and negatively charged complexes [PuO 2 (OH) n (H 2 O) 4 3n ] 2 3n existing in alkaline solution, among which PuO 2 (OH) 4 23 predominates at [OH 3 ] > 1 M. Highly charged ionic species are less sorbable because of electrostatic repulsion from the negatively charged aluminosilicate surface [2]. Almost quantitative sorption of Pu(VI) with natural montmorillonite is caused by reduction of Pu(VI) to Pu(IV), which is well retained by many materials.No less interesting is the behavior of long-lived radionuclide 237 Np, which can be present in alkaline wastes in oxidation state +5 and, under certain conditions, +6, since Np(IV) under these conditions is unstable and is oxidized to Np(V) even with atmospheric oxygen. The hydrolytic behavior of Np(V) and Np(VI) is different. Np(VI) can be expected to be similar to Pu(VI) in the hydrolytic behavior, although there are no published data on the speciation of Np(VI) in strong alkalis. Pentavalent neptunium is subject to hydrolysis to a lesser extent than Np(VI); in addition, it shows a weaker tendency to form polymeric species [3,4]. It was assumed [5,6] that, at relatively high concentration of hydroxide ions in a solution, the major ionic Np(V) species are [NpO 2 (OH) 2 ] 3 , [NpO 2 (OH) 3 ] 23 , and [NpO 2 (OH) 3 (H 2 O)] 23 . In this study we examined the reaction of Np(V) and Np(VI) with various aluminosilicates at [OH 3 ] of approximately 2 M. Under the same conditions, we also studied the behavior of Np(VII), which, as known, exists in alkaline solutions as a highly charged anion NpO 4 (OH) 2 33 [7, 8] and is highly soluble in alkalis. In addition, attention should be given to the possibility of disproportionation of Np(VI) in alkali...