Abstract
Perovskites, such as tausonite, are crystalline metal oxides with excellent optical and photocatalytic properties and have also been used successfully in the retention of metals, simulating the isotopes of uranium and plutonium. In this work, different pseudo-order and thermodynamic models were studied to achieve the prediction of the sorption of Eu3+ (chemical analogous for actinides) in tausonite. The effects of gamma irradiation and temperature on the structural characteristics of the material were determined, as an additional step in the evaluation of material as an engineering barrier in the disposal of radioactive waste. The results obtained show that the tausonite is resistant to the gamma irradiation and thermal energy. Likewise, it was possible to determine that europium sorption occurs through an exothermic and spontaneous reaction, as well as through the formation of surface complexes, where Eu3+ ions bind to sites on the tausonite by dipole-dipole interaction. Furthermore, it was shown that the sorption mechanism is influenced by diffusive phenomena, which participate in the formation of surface complexes. Additionally, a new sorption model with respect to pH was proposed, which allowed determining the physical parameter π. The evidence obtained suggests that π is a physical parameter that relates pH to an optimal value and could explain the equilibrium between the surface complexes that tausonite forms with europium. Likewise, the evidence suggests that 50 kg of tausonite would have the capacity to retain at least 26.59 g of alpha-emitting radionuclides, equivalent to a waste package (900 kg) with a maximum activity of 4000 Bq/g.
Highlights
Perovskites are crystalline metal oxides with the general formula ABO3, in which A can be rare earth or an alkali or alkaline earth metal and B is a transition metal [1, 2]. ese oxides are excellent light absorbers and their properties include tunable band gaps, low exciton binding energy, high carrier mobility, long diffusion length, and so on [3], and, they have a narrow band gap, for which they could be considered as good candidates for thermoelectric generators [4, 5]
Perovskites, such as tausonite, have been used successfully in the retention of U6+ and Eu3+, simulating the different isotopes of uranium and actinides (Ac3+), which are produced in the burning and reprocessing of nuclear fuels [12, 13]. ese studies have shown that perovskites are insoluble in water and have high chemical stability under acidic and alkaline pH conditions and that retention of U and Eu is carried out by the formation of inner-sphere surface complexes at pH values low and high, respectively [14, 15]. ese results reveal the potential of perovskites as materials that could be used as engineering barriers in the disposal of radioactive waste
E IAEA defines the term “disposal” as the emplacement of radioactive waste into a facility or location with no intention of recovering it, when the objective of the radioactive waste disposal is to protect the environment and people from the potential radiological consequences due to the release and dispersal of radionuclides [16]. erefore, disposal involves (a) isolating waste from the biosphere and people, reducing the risk of human intrusion, and (b) containing the wastes, inhibiting, reducing, and delaying the migration of radionuclides into the biosphere [16, 17]. e physical and chemical properties of engineering barriers provide containment of radionuclides. is containment consists of impermeability to water; limited corrosion, dissolution, leaching rate, and solubility; retention of radionuclides; and delayed migration of radionuclides [16]
Summary
Perovskites are crystalline metal oxides with the general formula ABO3, in which A can be rare earth (mainly lanthanides) or an alkali or alkaline earth metal and B is a transition metal [1, 2]. ese oxides are excellent light absorbers and their properties include tunable band gaps, low exciton binding energy, high carrier mobility, long diffusion length, and so on [3], and, they have a narrow band gap (semiconductor), for which they could be considered as good candidates for thermoelectric generators [4, 5]. The evaluation of materials used as engineering barriers consists of determining their physical and chemical properties, including retention and transport of radionuclides, as well as determining the resistance to degradation of these materials with sources of thermal energy or gamma radiation [16, 17]. In the study of the retention capacities of materials used in engineering barriers, it is recommended to use a combination of macroscopic and microscopic models, since they allow estimating important constants and parameters, such as kinetics and sorption capacity, concentration of surface sites, dependence of the surface sites with respect to pH, the formation constants of the surface site, the identification of the surface complexes formed, and the respective complex formation constants, among others. Erefore, this work seeks to contribute to the environmental and global challenge that radioactive waste management represents, evaluating materials that can be used successfully in the isolation and containment of radioactive waste
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