Direct sol-gel, novel template, and additional high-temperature reduction procedures for preparing iron oxides and their composites, showing promise for selective sorption of dissolved U(VI) from aqueous media of various acidities, are described. The sorption activity of the materials was studied, the kinetic curves of the sorption were obtained, and the efficiency of the selective recovery of U(VI) from aqueous solutions with different pH values using the new sorbents was compared. The probable mechanism of the U(VI) sorption onto the sorbents studied was suggested on the basis of SEM, XPS, emf, and BET data. The quantitative sorption of U(VI) is determined to a greater extent by the composition of the sorbent solid phase, rather then by the specific surface area of the sorbents, which ranges from 0.1 to 47.3 m2 g−1 depending on the synthesis procedure. The crystalline Fe0 phase in the sorbents prepared using additional high-temperature reduction plays the key role in the U(VI) sorption by the reducing deposition mechanism. The saturation magnetization for this type of sorbents can reach 133–140 emu g−1, which is an additional advantage allowing magnetic separation of the spent sorbents from the treated solutions.

The possibility of using spark plasma sintering (SPS) for preparing high-density ceramic matrices suitable for firm long-term immobilization of Cs radionuclides was examined. The kinetic features of sintering and phase formation of natural zeolite from the Far Eastern deposit, loaded with the adsorbed Cs ions (surrogate of radiocesium), under nonequilibrium SPS conditions were analyzed. The optimum SPS conditions were determined, and high-quality glass-ceramic matrices based on zeolites from various deposits, characterized by high density (98.5–99.8% of theoretical density), high compression strength (470–490 MPa), Cs content of up to 20.8 wt%, and low Cs leach rates (<10−5–10−6 g cm−2 day−1), were prepared. The SPS technology shows promise for radioactive waste management (in particular, for solidification of spent radioactive sorbents) and radioisotope industry (in particular, for production of special-purpose radionuclide sources).

The work presents a novel method for fabrication of the high-quality ionizing radiation source (IRS), which is promising to replace unsafe commercial products based on (CsCl)-Cs-137 prohibited by IAEA. Spark plasma sintering (SPS) technique has been applied to produce dense ceramic and glass-ceramic matrixes from Cs-containing ((similar to)13.5 wt.%) zeolite yielding in non-dispersible cores sealed in the container of radiation-resistant steel (J93503, US standard). One-stage SPS regimes to provide high-quality product have been optimized: sintering temperature < 1000 C, heating and holging duration 13 and 5 min, respectively, pressure 24.5 MPa. XRD, SEM, EDX, BET, XFS and solid-state MAS NMR 133Cs methods prove exceptional physico-chemical and mechanical characteristics of the obtained materials, namely: density 99.8% from theoretical, compressive strength (similar to)477 MPa, leaching rate 10(-4)-10(-6)g cm(-2) day(-1). Results of the investigation can be promising for fabrication of the IRS cores on a large scale as done for similar Russian products RSL, IGI-C, M37C, GID-C.

High selectivity and ion-exchange character of birnessite-type sorbents make these materials promising for Sr-90 removal from radioactive wastes containing seawater. However, it is still debatable how their functional properties are related to the nature of active phases and structure and, therefore, routes to improve birnessite performance are virtually unknown. Here we study the evolution of structure, surface morphology, and redox and sorption properties of birnessite-type sorbents in sorption/regeneration cycles and after reductive treatment with hydrazine and hydrogen to enhance birnessite uptake of Sr-90 from highly mineralized solutions. The influence of various treatment procedures on physico-chemical characteristics of birnessite has been evaluated using XRD, SEM, low temperature nitrogen sorption, and temperature-programmed reduction. Temperature programmed reduction was implemented to reveal changes in birnessite's active sites distribution and morphology, which play a crucial role in sorption behavior of the material. The sorption activity towards Sr-90 has been evaluated for the birnessites under static and dynamic conditions. The original approach used for characterization of birnessite sorption materials has helped to reveal how these sorbents change under operating conditions, and provided routes to the targeted modification of these manganese-oxide materials.

Temperature-programmed reduction (TPR) is a widely used method for characterization of oxide-based catalysts, sorbents, and functional materials, but its results lack quantitative assessment. Here, we present a novel approach to kinetic analysis of the TPR that can be applied to a large variety of systems involving multiple limiting stages. Implementation of cubic splines to approximate rate constant vs. conversion dependencies obtained from several TPR curves recorded at different heating rates yields in a set of kinetic parameters (activation energy and preexponential factors) for all reduction stages. Relationship between preexponential factor of the first reduction stage and the specific surface area of the sample has been shown. Reduction of hematite has been studied to prove the performance of the developed kinetic analysis technique.

Spark plasma sintering (SPS) is a highly prospective technology providing rapid consolidation of nano-disperse compositions into structured ceramics with complex structural organization. Characterizing pore structure of such systems remains a challenge due to different types, size and shape of the pores as well as structural defects occurring in the material, thus requiring special methods to quantify certain porosity type. The paper presents a complex approach to studying structure of SPS hematite-based structured ceramics. The method involves a number of gas, intrusion and electron microscopy probing techniques to characterize open and closed pores in the ceramics bulk. Low temperature nitrogen sorption has been implemented to estimate open micro- and mesoporosity in the size range 0.7-50 nm using BET, BJH and t-plot methods to process experimental data. Additionally, structural peculiarities of consolidated structured ceramics have been studied using temperature-programmed reduction (TPR) with the kinetic analysis of the results being carried out. Volume of macropores sized above 50 nm has been measured with mercury intrusion. Closed pores and structural defects have been quantified with SEM and FIB/SEM tomography. By comparing experimental and estimated results we have shown that addressing all porosity types provides a highly precise structural characterization of the material.

Sorption of micro- and microamounts of Sr from seawater has been studied using granulated Na-birnessite. Distribution coefficients of 90Sr in the natural seawater are 0.8–1.2 × 103 ml g−1, in the model seawater they are 1.6–1.8 × 103 ml g−1. Application of Na-birnessite was shown to be prospective in sorption–desorption–regeneration regime. In dynamic sorption conditions, over 150 bed volumes of seawater can be purified till 5% breakthrough occurs at feed rate 10 BV h−1. Na-birnessite can be used for 90Sr radionuclide removal from liquid radioactive wastes containing seawater.

Hydrothermal oxidation, a promising method for removal of cobalt radionuclides from EDTA-containing liquid radioactive waste streams, is in the focus of the present study. It has been demonstrated that Co(III)-EDTA complexes, which are very stable under normal conditions, undergo oxidation as a result of the electron transfer from the EDTA carboxyl group to Co(III) ions under elevated temperature. The electron transfer reaction follows the first-order rate law with an activation energy of 91.8 kJ/mol at pH 12 and time of Coale ions half-conversion of 0.2 s at 200 degrees C. The rate of EDTA oxidation is proportional to the concentration of Co(III) ions and solution pH. Based on quantum chemistry simulations, possible intermediate structures formed upon the electron transfer from EDTA to Co(III) ions have been suggested. It has been shown that the introduction of hydrogen peroxide provides a continuous generation of Co(III) ions and a sequential decarboxylation of the EDTA until complete degradation of the chelate structure. The pathways of cobalt immobilization have been clarified.

The article investigates spark plasma sintering (SPS) of commercial uranium dioxide (UO2) powder of ceramic origin into highly dense fuel pellets using non-standard die instead of usual graphite die. An alternative and formerly unknown method has been suggested to fabricate UO2 fuel pellets by SPS for excluding of typical problems related to undesirable carbon diffusion. Influence of SPS parameters on chemical composition and quality of UO2 pellets has been studied. Also main advantages and drawbacks have been revealed for SPS consolidation of UO2 in non-standard molybdenum die. The method is very promising due to high quality of the final product (density 97.5-98.4% from theoretical, absence of carbon traces, mean grain size below 3 μm) and mild sintering conditions (temperature 1100 ºC, pressure 141.5 MPa, sintering time 25 min). The results are interesting for development and probable application of SPS in large-scale production of nuclear ceramic fuel.

The morphology, specific surface area, and sorption properties toward Sr-90 radionuclides in seawater of a sorption material based on manganese oxide have been investigated. The material was fabricated through interaction of KMnO4 and H2O2 with subsequent annealing at 500 °C. The sorbent is characterized with the mechanical strength sufficient for using under dynamic sorption conditions: here, the efficiency of Sr-90 removal at feeding of 150 bed volumes exceeds 95 %. The values of Sr-90 distribution coefficients are equal to 0.8–1.2×103 ml/g in the real seawater and to 1.6-1.8 ×103 ml/g in the simulated one.

The article is devoted to the evolution of structural, morphological, and sorption characteristics of layered manganese oxide (birnessite) under various conditions close to the real operating regime of the sorbents for radioactive waste processing. To identify the phase composition in the birnessites, we implemented XRD analysis, while SEM and temperature-programmed reduction (TPR) were used to study morphological and redox features of the materials, respectively. Structural changes after various kinds of treatment of birnessites were tracked using low temperature nitrogen sorption. Sorption characteristics were assessed under static and in dynamic conditions on the efficiency of Sr2+ removal from simulated seawater. TPR combined with kinetic analysis revealed the decrease of particle sizes in the birnessites after repeated use in sorption-regeneration cycle and reduction with hydrazine. Despite the fact that the porous structure of the materials remains preserved, the surface morphology of birnessite changes drastically depending on the reducing agent. Hydrazine treatment increases the sorption performance of the birnessite followed by degradation of mechanical properties, thus, preventing such sorbent from repeated use. Kinetic analysis of TPR allows quantifying differences in morphology and porous structure of manganese oxide materials. The specific surface area, amorphous surface structure, and accessibility of Mn+3 sites are the most important factors for birnessite sorption performance.

To date, kinetic computations have been carried out efficiently for a great variety of physico-chemical processes including crystallization, melting and solid-solid transitions. However, appropriate methods for the kinetic analysis of chemical reactions, especially multi-staged reactions, are currently lacking. Here we report on an alternative way of treating temperature-programmed reaction data using the reduction of iron(iii) oxide as an example. The main principle in the suggested approach is to take into account every stage of the studied process, resulting in a system of kinetic differential equations. Kinetic parameters (activation energy and preexponential factors) are optimized for each of the stages, and cubic splines are used to approximate the conversion functions that reflect changes in reaction-specific surface area throughout the process. The applicability of the suggested method has been tested on temperature-programmed reduction (TPR) data for iron(iii) oxide samples produced from the original Fe2O3 powder by annealing it at 600, 700 and 800 °C. Results of kinetic analysis obtained at different temperature regimes demonstrate the good stability and performance of the method. Peculiarities of iron(iii) oxide reduction have been revealed, depending on the stage and heating rate. The influence of material morphology on the reduction kinetics has been assessed by comparing preexponential factors corresponding to the first reduction stage. This approach allows a comparison of the structural characteristics of the materials based on the kinetic analysis of the TPR data. Using optimized conversion functions, the initial particle size distribution has been reproduced. Theoretically found particle size distribution was found to correlate well with the experimental distribution obtained via laser diffraction.

The possibilities of creating special-purpose ceramics by the spark plasma sintering (SPS) according to the principle of the low-voltage electro-pulse consolidation of powders under the conditions of an external mechanical load is investigated. Ceramics applicable in the nuclear, medical, and electrotechnical industries are fabricated. A methodological description and special features of the presented technique are provided for the first time in terms of fabricating nuclear ceramics in the form of a pellet product of UO 2 ; composite bioceramics with a theoretical density exceeding 97.6% (based on ZrO 2 doped by hydroxyapatite) characterized with controlled (meso-, macro-) porosity and a high compression strength of ~400 MPa; and magnetic ceramics resistant to reverse magnetization based on nanostructured iron oxides (Fe 3 O 4 /α-Fe 2 O 3 ) with a saturation magnetization of 50 emu/g.

A new method, which included the sol–gel synthesis of a HfB 2 –(SiO 2 –C) reactive composite powder and its subsequent consolidation by hot pressing (1700°C, 30 MPa, 15 min) with simultaneous carbothermic synthesis of nanocrystalline silicon carbide, was used to produce HfB 2 –SiC ultra-high-temperature ceramic material promising for using in an air atmosphere at temperatures above 2000°C. Its elemental and phase compositions, as well as its microstructure were investigated. The density and calculated porosity were 7.6 g/cm 3 and 13.5%, respectively. The behavior of a cylindrical sample of the material was studied on long-term (40 min) exposure to a subsonic dissociated air flow in a high-frequency induction plasmatron. The change in the temperature of the surface of the material was examined in the context of its relationship with the HfB 2 and SiC oxidation and the evaporation of the oxidation products. The phase composition and microstructure were determined in regions of the oxidized surface of a HfB 2 –SiC sample containing 30 vol % SiC that were heated on exposure to high-enthalpy flows to 2600–2700°C and in regions the temperature of which was 1850–1950°C. By scanning electron microscopy, the thickness, microstructure, and composition of the oxidized layer were found.