Том 58, № 6 (2016)

Two new chromates of hexavalent uranium, [C₃H₁₀N][(UO₂)(CrO₄)(NO₃)] (1) and [C₂H₈N]₂[(UO₂)₂· (CrO₄)₂(Cr₂O₇)](H₂O)₂ (2), were prepared by a combination of hydrothermal synthesis and isothermal evaporation. Compounds 1 and 2 crystallize in the triclinic system, space group \(P\overline 1 \); a = 7.245(3), b = 7.329(3), c = 11.359(4) Å; α = 85.549(6)°, β = 82.547(6)°, γ = 80.174(6)° for 1; a = 7.2063(4), b = 11.5107(7), c = 16.0980(11) Å; α = 70.736(4)°, β = 80.246(4)°, γ = 71.759(4)° for 2. The structures were solved by the direct methods and refined to R₁ = 0.064 [for 1495 reflections with Fo > 4σ(Fo)] and 0.047 [for 4529 reflections with Fo > 4σ(Fo)] for 1 and 2, respectively. The crystal structure of 1 is based on [(UO₂)(CrO₄)(NO₃)]^– chains between which the isopropylamine molecules are arranged. In the structure of 2, the amine and H₂O molecules are localized between the [(UO₂)₂(CrO₄)₂(Cr₂O₇)]^2– layers. The uranyl chromate complexes described are derived from [(UO₂)(TO₄)(H₂O)_n]⁰ chains (T = Cr⁶⁺, S⁶⁺, Se6+, n = 0–2). A brief review of uranyl compounds with tetrahedral TO₄^2– anions (T = Cr⁶⁺, S⁶⁺, Se⁶⁺) and similar structural organization is given.

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5–74 mM sodium 1,2-cyclohexanediaminetetraacetate (Na₂CHDTA) or in a 96–97 mM Na₂CHDTA solution at 25–45°С was studied. The reaction rate at Na₂CHDTA concentrations in the range 5–60 mM and pH 3.5–5.9 is described by the equation V = k[Np(V)]^1.4[CHDTA], and at Na₂CHDTA concentrations in the range 70–100 mM and pH 4.1–5.2, by the equation V = k_A[Np(V)]^1.4. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)}₂ forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V).

Gas-phase conversion of metallic U, Zr, and Al into water-soluble compounds in the NO_x–H₂O (vapor)–air and HNO₃ (vapor)–air atmospheres was studied. Monolithic U_met and powdered Zr_met undergo gasphase conversion to form water-soluble compounds (nitrates, hydroxynitrates), whereas monolithic Zr_met remains unchanged. The degree of conversion of Al_met into water-soluble compounds in the examined nitrating media at 25–150°С does not exceed 10%. The principal possibility of separating U from Al and Zr by gasphase conversion of monolithic samples in a nitrating atmosphere was demonstrated.

The extraction of ²⁴¹Am and ¹⁵²Eu from alkaline carbonate solutions with solutions of functionalized thiacalix[4]arenes in m-nitrobenzotrifluoride was studied. The dependence of the radionuclide distribution ratios and separation factors on pH of the aqueous phase in the interval from 10 to 13.5, kind of the organic diluent, and position and electronic characteristics of functional groups in the thiacalixarene platform was examined. The composition of the extractable solvates of Am and Eu with functionalized thiacalix[4]arenes was determined by slope analysis. The hydrolytic stability of tert-butylthiacalix[4]arene in alkaline and acid solutions was evaluated. Thiacalix[4]arenes efficiently extract Am from alkaline solutions. The bromine-substituted thiacalix[4]arene exhibits the maximal selectivity, with the Am/Eu separation factor in extraction with this agent exceeding 30.

The kinetics of ⁹⁹Мо sorption onto T-5 sorbent in the batch mode at different values of the stirring rate, temperature, and sorbent granule size was studied. All the experimental kinetic curves are described by a sum of two exponents. In the first time interval of phase contact, from 0 to 5–10 min, the sorption is controlled by external diffusion, and in the interval from 10 to 40 min the step of transformation of unsorbable Mo species into sorbable hydroxo species also becomes a rate-determining step (external kinetic control). In the final stage (time interval from 40 min to 2–3 h), the sorption occurs at the lowest rate, and the Mo diffusion in micropores and overcoming of electrostatic hindrance on the surface of titanium hydroxide pores become the most probable limiting steps. In sulfate solutions, the Mo sorption rate considerably decreases already in the first stage, and the external kinetic control becomes prevalent because of hindered redistribution of unsorbable sulfate complexes into sorbable hydroxo complexes. The controlling factor in the sulfate and nitrate solutions in the second stage is presumably the same.

Removal of Mg²⁺, Сa²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Fe³⁺, and Pr³⁺ ions, tracer amounts of ¹³⁷Cs, ⁸⁵Sr, and ⁶⁰Co radionuclides, and oxalate and ethylenediaminetetraacetate ions from solutions using a polymeric nanofiltration (NF) membrane produced by RM Nanotekh (Russia) was studied. The selectivity (S) of the NF membrane to multicharged metal cations is considerably higher than to single-charged cations. The NF membrane exhibits the highest selectivity (S = 94–97%) to triple-charged Fe³⁺ and Pr³⁺ ions and double-charged Cu²⁺ ions. The selectivity to double-charged Mg²⁺ and Zn²⁺ ions is somewhat lower (90–94% on the average). Among doublecharged cations, the NF membrane exhibits the lowest selectivity (80–88%) to Ni²⁺ and Cа²⁺ ions. The NF membrane shows high performance in retention of tracer amounts of Sr (S = 95%) and Co (S > 99%) radionuclides, and also of oxalate (S = 94.5%) and ethylenediaminetetraacetate (S = 97.5%) ions. Examples of using the nanofiltration method for removing ⁶⁰Co from a model solution simulating NPP bottom residue and for removing transition metal impurities from a LiCl solution are presented.

The solubility of UO₃ in sodium almino(ferro)phosphate (SAFP) glasses prepared by quenching of melts reaches almost 50 wt %. The structure of the anionic motif of the network of such glasses is formed by polyanions and consists of alumino(ferro)phosphate and uranium–oxygen constituents. Irradiation with electrons of up to 8 MeV energy to an absorbed dose of 10⁶ Gy does not lead to appreciable changes in the glass structure, except insignificant increase in the fraction of octahedrally coordinated aluminum. After annealing, samples of sodium aluminophosphate glasses with low content of uranium oxides (up to 10 wt %) partially crystallize with the formation of phosphotridymite, whereas high-uranium glasses (up to ~50 wt % UO₃) remain X-ray amorphous. Samples of SAFP glasses at low concentrations of uranium oxides remain amorphous and at high concentrations undergo phase segregation with the formation of a SAFP glass phase enriched in uranium oxides and crystalline phase of sodium aluminum iron orthophosphate Na₃(Al,Fe)₂(PO₄)₃ containing impurity amounts of uranium ions. The incorporation of uranium ions is most probably due to the occurrence of redox processes between uranyl and iron ions.