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The Use of Zeolites for the Treatment of Radioactive Waste

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Abstract

The uptake of 137-Cs and 90-Sr on to synthetic zeolite A has been studied. The use of a combined barium exchange followed by a relatively mild heat treatment has been shown to fix the isotopes within a collapsed zeolite structure. Leaching experiments using deionized and simulated sea and pond waters were carried out on zeolite compacts and zeolite/cement composites. They demonstrated the effectiveness of the calcination and cement containment. Some comments on the inter- pretation of leach test data are made.
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... The porous titanosilicates proved to be a versatile class of compounds to remove selectively 137 Cs/ 90 Sr from nuclear waste because of its high sorption capacity and fast kinetics, and also it can be transformed to durable waste forms to prevent release of 137 Cs/ 90 Sr to the environment due to its resistance to high irradiation dose, chemical, mechanical, and structural stability comparable to that of borosilicates or aluminosilicates [1][2][3][4]. The radioactive 137 Cs decays to 137 Ba and 90 Sr to 90 Y and finally to 90 Zr. ...
... TheΔ f G o (Sr 2 TiSi 2 O 8 ,s,T) was obtained from the experimental Δ r G o (T) and 2Δ f G o (T) of TiO 2 (s), SiO 2 (s), SrTiO 3 (s) and NiO(s) [37] (given in Table 3). The corresponding expression is: Table 2 The coefficient of fitted polynomial equation of lattice parameters (a and c) and unit cell volume (V) as function of temperature along with average linear lattice thermal expansion coefficients α a(ave) and α c(ave) ) and volume thermal expansion coefficient (α V(aver) ) of Sr 2 Table 5. It is 160.5 kJ mol −1 more negative than that measured by Park et al. [26]. ...
Article
The titanosilicates: Ba2TiSi2O8(s)(BTS) and Sr2TiSi2O8(s)(STS) were synthesized by citrate nitrate gel combustion and complex polymerisation method, respectively and characterized using X-ray diffraction (XRD) technique. The heat capacity, thermal diffusivity, average thermal expansion coefficient, the standard molar Gibbs energy of formation of BTS and STS were measured with a heat flux-type differential scanning calorimeter, laser flash technique, high temperature XRD and solid oxide Gavanic cell, respectively. The heat capacities of BTS and STS were found to be comparable upto 600 K and afterwards, the increase is higher in BTS. Thermal conductivity of BTS was found to be higher than that of STS. The average volume thermal expansion coefficient of BTS was found to be lower than that of STS. The enthalpy of formation of BTS and STS were derived from their respective Gibbs energy data and their thermodynamic functions were calculated.
... For instance, zeolites loaded with Cs radioactive isotopes can be incorporated into borosilicate glass [9], cement [10] and glass [11]. Also, the pores of zeolites can be blocked with barium [12,13]. However, despite the presence of these matrices, the Cs will slowly diffuse through the matrix and eventually contaminate groundwater. ...
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A promising method for removal of Cs ions from water and their incorporation into stable crystal structure ready for safe and permanent disposal was described. Cs-exchanged X zeolite was hot-pressed at temperature ranging from 800 to 950 °C to fabricate dense pollucite ceramics. It was found that the application of external pressure reduced the pollucite formation temperature. The effect of sintering temperature on density, phase composition and mechanical properties was investigated. The highest density of 92.5 %TD and the highest compressive strength of 79 MPa were measured in pollucite hot-pressed at 950 °C for 3 h. Heterogeneity of samples obtained at 950 °C was determined using scanning electron microscopy. The pollucite hot-pressed at 950 °C had low linear thermal expansion coefficient of ∼4.67 × 10⁻⁶ K⁻¹ in the temperature range from 100 to 1000 °C.
... It is mined commercially in at least 16 countries, so ensuring its supply. Further, as an aluminosilicate, it is compatible with encapsulation for long-term storage and subsequent disposal [7]. It can also be used in particle sizes appropriate for columns, and most clinoptilolites are stable under presence of Group II cations reduces the effective CEC, because Ca 2+ , Mg 2+ and Sr 2+ are not exchanged as readily as Na + . ...
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Mud Hills clinoptilolite has been used in an effluent treatment plant (SIXEP) at the Sellafield nuclear reprocessing site. This material has been used to remove 134/137Cs and ⁹⁰Sr successfully from effluents for 3 decades. Samples of the zeolite have been tested in column experiments to determine their ability to remove radioactive Cs⁺ and Sr²⁺ ions under increasing concentrations of competing ions, Ca²⁺, Mg²⁺, Na⁺ and K⁺. These ions caused increased elution of Cs⁺ and Sr²⁺. Ca²⁺, Mg²⁺ and K⁺ were more effective competitors than Na⁺. For Na⁺, it was found that if concentration was reduced, then column performance recovered rapidly. Electronic supplementary material The online version of this article (10.1007/s10967-018-6329-8) contains supplementary material, which is available to authorized users.
... Three big NPP incidents in the near past in NPP Three Mile Island in 1979, NPP Chernobyl in 1986, and NPP Fukushima Daiichi in 2011 have caused damages over a wide range. The consequences have been confined by decontamination of accident radioactive wastewaters using zeolitic ion-exchangers for uptake radioactive caesium ( 137 Cs), strontium ( 90 Sr) and other radionuclides holding them in three-dimensional crystal framework [2,3]. Zeolites react readily with cement and glass systems thus allowing the radioactive waste to be entrapped and contained safely. ...
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The relationship of pore solution composition to observed leach behavior for cesium-doped, cement-based waste form has been demonstrated. A leaching model is proposed, based on early diffusion and homogenization of the pore solution and leachant followed by re-equilibration of the waste form with the greater pore solution (pore solution plus leachant) over the long term.
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The thermal, mechanical, and volumetric behavior of silicalite-1, an all-silica MFI zeolite, is elucidated by atomistic simulations. A flexible force field was selected and validated from a set of force fields to capture the intramolecular interactions of the crystal lattice. This force field accounts for realistic bond, angle, and torsional interactions among atoms of the framework alongside with conventional Lennard-Jones and Coulomb interactions. By monitoring the behavior of silicalite-1 as a function of pressure and temperature, a fully reversible monoclinic to orthorhombic phase transition (polymorphism) was revealed in accordance with experimental data. Thermodynamic considerations dictate that this is a second order phase transition in the Ehrenfest classification. Additionally, reversible pressure-induced amorphization was captured by our model and was associated with the formation of linear zones of increased distortion running parallel to the straight and sinusoidal channels of this zeolite. Remarkably high isothermal compressibility (small bulk modulus) was calculated for orthorhombic silicalite-1, in excellent agreement with experimental data, rendering silicalite-1 the most compressible zeolite known to date. The rigid unit mode (RUM) model was identified as the dominant structural mechanism for negative thermal expansion (NTE), typically observed over a wide temperature range in MFI zeolites. Better understanding of the monoclinic to orthorhombic phase transition, and molecular mechanisms associated with energy dissipation and NTE in zeolites provides control over the framework microstructure, allowing for enhanced molecular sieving, tunable selectivity in separation processes, mechanical stability, and substantially amplified catalytic efficiency in petrochemical applications.
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Natural zeolite is considered to be an effective and economical crystalline aluminosilicate adsorbent and catalyst. In the treatment of coal seam gas (CSG) co-produced water, it has the potential to become an economic alternative filtration medium for the reduction of excessive Na content. A variety of methods for the modification of natural zeolite have been used to improve its surface properties. This study reports the results of X-ray photoelectron spectroscopy (XPS) analysis on the near-surface of natural zeolite (clinoptilolite and quartz) particles before and after modification by sulphuric acid. The atomic % and binding energies of the chemical elements comprising zeolite are changed significantly following acid treatment. With increasing sulphuric acid concentration, the Si/Al atomic ratio increases from 2.99 at 0 M to 4.92 at 5 M. The binding energy (BE) shifts show a similar trend with increasing acid concentration. High-resolution and valence band spectra show that the BE shifts are influenced by cation removal from the zeolite structure. These detailed XPS results are useful for understanding changes in the sodium adsorption capacity of zeolite by acid modification to enhance its suitability for use in treatment of CSG co-produced water.
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A simple one-step method with direct thermal conversion at lower temperatures for preparing a stabile Cs-aluminsilicate phase, known as pollucite, is presented. Cs-exchanged form of Na, Ca-LTA type of zeolite (Cs-LTA) was pressurless sintered and hot pressed at certain temperatures in order to obtain pollucite. XRD and FTIR analysis were used to study structural changes of Cs-LTA before and after thermal treatments. Pressurless sintered sample recrystallized into pollucite phase after heat treatment at 1000 0C (3h) (PLS1000) and hot pressed sample at 750 0C (3h) using pressure of 35 MPa (HP750), indicating reduced temperature of 250 degrees. SEM micrographs confirmed that HP750 has higher density than PLS1000 which leads to higher value of compressive strength. The HP750 showed better resistance to Cs leaching than the PLS1000. Base on these results one can conclude that hot pressing is the promising method for the permanent disposal of Cs radionuclides.
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Amorphous zirconium phosphates (ZrP) loaded with137Cs and90Sr/90Y isotopes are contained in cements and their leaching behaviour with simulated pond, ground, sea and deionized waters, evaluated. Acceptable compositions of the waste mixes in terms of the proportions of various components and their solid to liquid ratios, are also described.
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The saturation of negative charges of zeolites by specific cations to modify their physicochemical and catalytic properties has broadened the applications of zeolites. The adsorption behavior of H+ to Li+, Na+, K+, Rb+ and Cs+-saturated Linde-type A, Na-P1, mordenite, X type and Y type zeolites was evaluated at different pH-pM, where pH-pM is equal to log {(M+)/(H+)} and M+ represents either Li+, Na+, K+, Rb+, or Cs+. In all cases, with decreasing pH-pM, the amounts of alkali metal retention decreased due to the adsorption of H+ via cation exchange reaction. The adsorption selectivity of H+ into the zeolites had a negative correlation with the Si/Al ratio of the zeolites. In each zeolite species, Cs+-saturated zeolite showed the lowest H+ selectivity, and this suggested that Cs+ had the strongest adsorption energy in the alkali metal cations. The adsorption of H+ was strongly affected by diameter and hydration energy of the alkali metal cations, and was also affected by the framework type and Si/Al ratio of the zeolites. The adsorption of H+ into zeolites decreases the amount of cation retention other than with H+ and may cause the elution of Si and Al into aqueous solutions.
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Radiochemical investigations have been carried out to determine the ease of replacement of Na+ ions from X and Y zeolites by the divalent calcium, strontium and barium cations. Full replacement often can be achieved only at elevated temperatures and fully-exchanged M2+ zeolites prepared in this way have a proportion of their cations which are difficult to exchange at room temperature. The significance of these results is discussed in relationship to the population of the various cation sites available.
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The reduction of cesium leachability by addition of mineral zeolites to enhance the safety of solidification of wastes with cement has been examined. Leaching tests were carried out by the method recommended by IAEA. The naturally occurring mineral zeolites containing mordenite and clinoptilolite (Zeolite I), which have high atom ratios of silicon and aluminum, possess an extremely high effectiveness for immobilizing 137Cs in a cement matrix. By addition of zeolite I, the leachability is reduced by a factor of about 100. However, since increasing the amount of zeolites decreases the leachability and workability concurrently, the content of zeolites should be less than 0.25 for good workability. On the basis of the results obtained, estimates were made on the amount of 137Cs leached from a cement composite (200 l. drum size), with or without Zeolite I, that is exposed to a water environment for extended periods. (C)1978Health Physics Society
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Self-diffusion parameters of zinc into zeolites X and Y, of Si:Al atios of 1·26, 1·87 and 2·62 respectively, have been determined at low and high temperatures. Derived values of EA and ΔS‡ have been rationalized in terms of mechanisms in which water played an important part, and a possible explanation for anomalous results obtained with X at low temperatures has been put forward.
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The compressive strength of concrete is shown to be increased by the presence of sodium chloride or ocean salts in the mixing water. The strength was found to increase with increasing salinity even up to 7% by weight of water.RésuméNous avons remarqué que la résistance a la compression du béton augmentait si l'on accroissait la tenair en chlorure de sodium ou en sels marins mélanges dans l'eau. De telles augmentations ont été trouvées pour des teneurs en sels aussi élevées que 7% du poids de l'eau.
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