V. A. Avramenko

Colloid stable sorbents for cesium removal: Preparation and application of latex particles functionalized with transition metals ferrocyanides

In this paper we suggest a principally new approach to preparation of colloid stable selective sorbents for cesium uptake using immobilization of transition metals (cobalt, nickel, and copper) ferrocyanides in nanosized carboxylic latex emulsions. The effects of ferrocyanide composition, pH, and media salinity on the sorption properties of the colloid stable sorbents toward cesium ions were studied in solutions containing up to 200 g/L of sodium nitrate or potassium chloride. The sorption capacities of the colloid sorbents based on mixed potassium/transition metals ferrocyanides were in the range 1.3-1.5 mol Cs/mol ferrocyanide with the highest value found for the copper ferrocyanide. It was shown that the obtained colloid-stable sorbents were capable to penetrate through bulk materials without filtration that made them applicable for decontamination of solids, e.g. soils, zeolites, spent ion-exchange resins contaminated with cesium radionuclides. After decontamination of liquid or solid radioactive wastes the colloid-stable sorbents can be easily separated from solutions by precipitation with cationic flocculants providing localization of radionuclides in a small volume of the precipitates formed.

HfB2-SiC (45 vol %) ceramic material: Manufacture and behavior under long-term exposure to dissociated air jet flow

Ultra-high-temperature composite materials HfB2-SiC containing 45 vol % SiC were prepared by spark plasma sintering. The behavior of a sample of this composition under exposure to a subsonic jet of dissociated air of a high-frequency induction plasmatron was studied; the total time was more than 30 min. Under certain test conditions, some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. So, most of the surface area of the sample experienced exposure to temperatures up to 2500–2700°C for more than 15–18 min, while the rest of the surface had a temperature of 1700–1800°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), X-ray powder diffraction, and X-ray computed microtomography enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Flocculation of humic substances and their derivatives with chitosan

The efficiency of the flocculation of humic compounds and their derivatives with chitosan was studied as a function of pH, concentrations of chitosan and dissolved humic compounds, and also the presence of inorganic coagulants. Samples of humic compounds were studied by the methods of potentiometric titration and gel permeation chromatography. The molecular weight distributions and the spectra of ionization constants of their functional groups were calculated. The flocculation efficiency of chitosan with respect to humic compounds is much higher than that of polyacrylamide.

Dynamics of sorption of cesium radionuclides on selective ferrocyanide sorbents. Distribution of the 137Cs activity in the stationary phase

A method was suggested for studying the dynamic sorption properties of ferrocyanide sorbents toward Cs radionuclides by analyzing the radiocesium distribution in the solid phase of the sorbent. The dynamic sorption characteristics are determined by measuring the activity of successive sorbent layers after passing definite volumes of the solution. The method is efficient in analysis of the stability of various sorbents in liquid media. The sorption characteristics of the following ferrocyanide sorbents were examined: Termoksid-35 (nickel-potassium ferrocyanide on zirconium hydroxide), FNS-2 (nickel-potassium ferrocyanide on silica), and FS-2 (copper-potassium ferrocyanide on silica). In alkaline solutions, Termoksid-35 is the most stable. The effect of oxalic acid on the activity distribution in a ferrocyanide sorbent bed was studied. In alkaline solutions containing oxalic acid, copper ferrocyanide (FS-2) is the least stable. At the same time, at low pH values and in the absence of oxalic acid, it is preferable to use sorbents based on copper ferrocyanides.

HfB2-SiC (10–20 vol %) ceramic materials: Manufacture and behavior under long-term exposure to dissociated air streams

Ultra-high-temperature composite materials HfB2-SiC containing 10, 15, and 20 vol % SiC were prepared by spark plasma sintering. The behavior of the samples prepared under long-term exposure to subsonic dissociated airstreams of a high-frequency induction plasmatron was studied. The total test time per sample was 35–42 min. Under certain exposure conditions (which were dependent on the composition of a sample), some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. These regions enlarged over time, so that most of the surface area of the sample experienced exposure to temperatures of up to 2500–2700°C for 19–38 min, while the rest of the surface had a temperature of up to 1800–1900°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), and X-ray powder diffraction enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Behavior of a sample of the ceramic material HfB2–SiC (45 vol %) in the flow of dissociated air and the analysis of the emission spectrum of the boundary layer above its surface

Ceramic HfB2–SiC samples with a silicon carbide content of 45 vol % have been obtained by spark-plasma sintering, their density and calculated porosity determined, and certain studies performed using IR spectroscopy, XRD, and other methods. The behavior of the HfB2–SiC (45 vol %) material has been studied using the VGU-4 high-frequency induction plasmatron with heating by a subsonic flow of dissociated air. It has been shown that the average temperature of the surface of the samples in the process of heating rises up to 2680–2690°C, which is connected with the formation on the surface, which has the temperature of ∼1700–1900°C, of local regions with the temperature of 2600–2700°C with gradually expanding areas. The total time during which the average surface temperature is higher than 2000°C is ∼30 min. The change in the composition of the gas phase above the sample surface in the course of the experiment was investigated using optical emission spectroscopy. Assumptions, which explain the changes in the concentration of boron and silicon under the action of a high-enthalpy flow, have been advanced. The elemental composition and the phase composition have been determined, and the microstructure of the surface and of various cuts of the samples have been studied.

Sol–gel synthesis of porous inorganic materials using “core–shell” latex particles as templates

Here we report on the sol–gel synthesis of porous inorganic materials based on manganese, molybdenum, and tungsten compounds using the “core–shell” siloxane-acrylate latex as a template. The chemical composition and structural characteristics of the materials obtained have been investigated. It was shown that temperature conditions and gaseous media composition during the template destruction controlled the composition and structure of porous materials. To obtain porous inorganic materials for catalytic applications, the “core–shell” latex template was preliminarily functionalized by gold and palladium nanoparticles obtained by thermal reduction of noble metal ions-precursors in a polycarboxylic “shell”. Upon the template removal, noble metals nanoparticles of a size of dozens of nanometers were homogeneously distributed in the material porous structure. The evaluation of the catalytic activity of macroporous manganese, tungsten, and molybdenum oxides under the conditions of liquid phase catalytic oxidation of organic dyes has been performed. The prospects of employing macroporous oxide systems with immobilized nanoparticles of noble metals in the processes of hydrothermal oxidation of radionuclide organic complexes in radioactive waste decontamination have been demonstrated.

Humic acids in brown coals from the southern Russian Far East: General characteristics and interactions with precious metals

Brown coals with high Au and PGE concentrations from six deposits in the southern Russian Far East were analyzed for elemental composition, acid-base properties, and the molecular-size distribution of humic acids (HA). The ash contents of the coals were determined to be negatively correlated with their Au concentrations, and the content of “organic Au” (which is chemically bound to humic substances, HS) reaches 95%. The most probable mode of Au occurrence in the brown coals is submicrometer-sized particles of elemental gold stabilized by HA. Quantum-mechanical calculations of interactions between Au(0) clusters with model HS fragments confirm that HS could be originally strongly chemically adsorbed on the surface of elemental gold particles. Different stability of colloids during centrifuging of alkali extracts of the gold-bearing brown coals was proved to be likely responsible for the selective separation of free HA and those bound with gold particles, and this can be used to develop a technology for gold recovery from coals without decomposing their organic matrix.

Sorption Recovery of Strontium from Seawater

Sorption recovery of strontium from seawater and prospects of using selective sorbents for treatment of seawater or mixtures of liquid radioactive wastes (LRW) with seawater to remove 90Sr are examined. A comparative analysis is made of characteristics of various sorbents. The most of the sorbents studied demonstrate low distribution coefficients and selectivity with respect to strontium, being unable to provide efficient removal of 90Sr from seawater. The exceptions are new sorption reagents developed by the authors, which show promise for treatment of LRW to remove strontium. The possible mechanism of strontium sorption with these sorbents is suggested.

Behavior of HfB2-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB2–SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550–2675°C during heating, due to the generation of surface localities having temperatures of 2600–2700°C (the initial surface temperature was ~1700–1900°C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000°C, was about 31–32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (~40-min) exposure to high-enthalpy air flows.