V. V. Milyutin

Sorption of cesium on finely dispersed composite ferrocyanide sorbents

Sorption of microamounts of cesium on finely dispersed composite ferrocyanide sorbents was studied. The sorbents were prepared by precipitation of nickel potassium ferrocyanide in the presence of various mineral supports (chalk, wollastonite, bentonite, clinoptilolite, diatomite, biosilica). The distribution coefficient (Kd) of 137Cs on composite ferrocyanide sorbents is considerably higher than on nickel potassium ferrocyanide without support. The Kd values on the composite sorbents increase by a factor of 25–110 when separating the solid phase with a paper filter and by a factor of 4.3–8.2 when using a microfiltration membrane. Considerable increase in Kd of 137Cs on composite sorbents is attributed to the formation of a nickel potassium ferrocyanide phase firmly fixed on the support surface and resistant to peptization. Composite ferrocyanide sorbents were tested for the 137Cs recovery from a simulated NPP bottom residue in the pH range 8.5–12.0. On the composite sorbents, Kd of 137Cs is 2.5–3.0 times higher than on the nickel ferrocyanide precipitate throughout the examined pH range. The composite sorbent based on biosilica and nickel potassium ferrocyanide was tested for treatment of real liquid radioactive waste with a total salt content of 22.0 g dm−3 to remove 137Cs. The decontamination factor as high as 5190 was attained owing to simultaneous use of the finely dispersed composite ferrocyanide sorbent and an ultrafiltration ceramic membrane.

Sorption of cesium on ferrocyanide sorbents from highly saline solutions

Sorption of microamounts of Cs from highly saline solutions (bottom residues from nuclear power plants) on various types of ferrocyanide sorbents was studied. The dependences of the distribution coefficient (Kd) of 137Cs on pH of solution were determined under static conditions. They strongly depend on the type of the sorbent used. FS-2 sorbent prepared by joint precipitation of silicic acid and copper ferrocyanide exhibits the best sorption characteristics toward Cs. On this sorbent, Kd of 137Cs in a model solution with pH < 11 is (5–6) × 105 cm3 g−1. In alkaline solutions at pH > 11, the Cs sorption drastically decreases because of dissolution of the ferrocyanide component of the sorbent. Data on Cs sorption under dynamic conditions on various types of ferrocyanide sorbents from the bottom residue with pH 8–11 are presented. The volume of the solution passed up to 1% 137Cs breakthrough was determined. FS-2 sorbent exhibits the best dynamic characteristics. Its use allows decontamination of 1000–4500 column volumes of the bottom residue depending on pH.

Coprecipitation of Microamounts of Cesium with Precipitates of Transition Metal Ferrocyanides in Alkaline Solutions

Experiments on coprecipitation of microamounts of Cs with precipitates of copper, nickel, and cobalt ferrocyanides in alkaline solutions showed that the dependences of the distribution coefficients (Kd) of 137Cs on pH of the liquid phase have a pronounced maximum at pH 10.0 ± 0.2 (Kd ≈ 8 × 105), 10.5 ± 0.2 (Kd ≈ 2 × 106), and 10.6 ± 0.3 (Kd ≈ 5 × 105), respectively. This phenomenon is associated with the formation of mixed precipitates of transition metal ferrocyanides and hydroxides, surpassing in the sorption characteristics the phases of the corresponding ferrocyanides. The mechanism of coprecipitation of microamounts of cesium with precipitates of transition metal ferrocyanides in alkaline solutions and the conditions of maximum recovery of cesium from solutions of various compositions were determined.

Sorption of actinide ions onto mesoporous phosphorus-containing silicas

Equilibrium and kinetic characteristics of template mesoporous silicas containing phosphonic acid residues in sorption of various actinide ions were studied. The sorption equilibrium involving these sorbents is attained within 20 min after introducing the sorbent into the solution. The calculated values of the internal diffusion coefficient

Optimal Conditions for Coprecipitation of Cesium Radionuclides with Nickel Ferrocyanide

(\bar D)

Use of ozone for dissolving high-level plutonium dioxide in nitric acid in the presence of Am(V,VI) ions

and half-exchange time (τ0.5) in sorption of uranium were ∼3.5 × 10−16 m2 s−1 and ∼390 s, respectively. Mesoporous phosphorus-containing silicas efficiently sorb from acid solutions uranyl ions, Th(IV), and Pu(IV). In sorption of uranium from sulfuric acid solutions, the capacity of the sorbents is 125–132 mg g−1, and in sorption from nitric acid solutions (0.5–3.0 M HNO3), 276–299 mg g−1. In sorption of Th(IV) from nitric acid solutions, the capacity of the sorbents is 60–66 mg g−1. In sorption of microamounts of 239Pu(IV), the distribution coefficient reaches 4500 cm3 g−1. Phosphorus-containing silicas in nitric acid solutions do not noticeably sorb 241Am, which allows using them for efficient separation of the Pu/Am pair with the separation factor of no less than 2 × 103.

Sorption of uranium from nitric acid solutions on various ion exchangers

Various factors influencing coprecipitation of cesium tracer with nickel ferrocyanide were studied. The precipitate was separated by filtration of a suspension through filtering materials of different pore size. The following optimal conditions for decontamination of solutions from cesium radionuclides were found: pore size of the filtering material no more than 0.2 μm; molar ratio Ni2+/Fe(CN)64− = 1.33–1.75; electrolyte (NaNO3) concentration no less than 0.05 M; and pH of the solution no higher than 10.

Sorption of cesium and strontium radionuclides onto crystalline alkali metal titanosilicates

Plutonium dioxide recovered in the course of reprocessing of SNF from WWER reactors (so-called high-level PuO2) was subjected to dissolution in 0.6–3.0 M HNO3 in the presence of Am(III) ions under ozonation with an ozone-oxygen mixture containing 30–180 mg l−1 O3. Measurements of the rate of the PuO2 dissolution in 3 M HNO3 in the temperature interval from 30 to 80°C showed that, with an increase in the ozone concentration in the ozone-oxygen mixture from 30 to 180 mg l−1, the dissolution rate increases by a factor of 4–5. The acceleration of the PuO2 dissolution is attributed to the formation of Am(V,VI) by homogeneous oxidation of Am(III) ions with ozone dissolved in HNO3. The Am dioxocations formed act as PuO2 oxidants and are continuously regenerated by the oxidation of Am(III) with ozone. This assumption is confirmed by an additional increase in the dissolution rate, observed on introducing Am(III) into the initial electrolyte for the PuO2 dissolution.

Removal of radionuclides and corrosion products from neutral and weakly alkaline solutions by microfiltration

Sorption of uranium from model nitric acid solutions on ion-exchange resins of various types was studied. Phosphoric acid cation exchanger KRF-20, an ampholyte with iminodiphosphonic groups (S-950), and a cation exchanger with phosphonic and sulfonic groups (S-957) exhibit the best sorption properties. The dependences of the static exchange capacity of the above resins for uranium on the HNO3 concentration, the sorption isotherms in 1 and 3 M solution of HNO3, and the capacity for uranium to breakthrough were determined. The possibility of uranium desorption with a hot solution o Na2CO3 was demonstrated.

Sorption of REE(III), Th(IV), and U(VI) ions from nitric acid solutions with sorbents based on tetraoctyldiglycolamide

Sorption of tracer amounts of 137Cs and 90Sr radionuclides from model solutions of various compositions onto synthetic titanosilicates, framework ivanyukite and layered SL3, both synthesized at the Center for Nanomaterials Science, Kola Scientific Center, Russian Academy of Sciences, was studied. Synthetic ivanyukite and titanosilicate SL3 well compete with Termoxid-25 ferrocyanide sorbent in the ability to take up cesium from neutral NaNO3 solutions and from a simulated solution of bottom residue from a nuclear power plant with RBMK reactors. The maximal sorption of 137Cs onto ivanyukite is observed at pH 6–7. The dependence of the 137Cs distribution coefficient (Kd) on ivanyukite on the concentration of sodium and potassium ions in the solution was studied. Potassium ions affect the cesium sorption more strongly than sodium ions do. In the ability to take up 90Sr, synthetic ivanyukite well competes with synthetic zeolite of type A and with the sorbent based on modified manganese dioxide. The dependences of Kd of 90Sr on the concentrations of the Na+ and Ca2+ ions in the solution were determined. Calcium ions affect the strontium sorption more strongly than sodium ions do. Ivanyukite and SL3 show promise as sorbents for removing cesium and strontium radionuclides from multicomponent salt solutions.