1. Introduction

The isolation of radioactive waste from the environment has been a critical concern for public health and safety. Since the first research on a new concept for vitreous or crystalline materials for the immobilization of radioactive waste was conducted in the 1950s, various glass waste forms have been studied for high-level radioactive wastes [1-3]. Amorphous borosilicate glass has been selected as a solid waste form in the past because, due to the unstructured molecular arrangement of glass, it can incorporate a wide range of chemical elements (cɑ. 20-30 elements). Other necessary properties such as waste loading, chemical durability, and processability for amorphous borosilicate glass are all reasonably acceptable for radioactive waste immobilization applications [4]. Thus, borosilicate glass has been used as a reference solid waste form for comparison of the performance characteristics. However, borosilicate glass is thermodynamically unstable and less resistant to leaching and weathering [5] than more stable crystalline ceramic waste forms currently in use. Various titanate-based (zirconolite, hollandite, perovskite, etc.), alumina-based (magnetoplumbite, nepheline, etc.), and silicate-based (pollucite, etc.) ceramic waste forms are advantageous over glass waste form equivalents [6,7]. These minerals exhibit various crystalline structures, such as fluorite, ABO₃, and ABO₄. A-site cations and B-site cations can be exchanged with fission product elements such as Cs, Rb, Sr, Ba, Th, and Pu [7-11].

Processing temperature and chemical durability are noteworthy properties when screening for a potential solid waste form. Processing temperature is an important property in terms of cost-effectiveness and secondary off-gas treatment for volatile radionuclides [6], while chemical durability is one of the most important performance characteristics of solid waste. There are several standard leach test protocols developed by the International Atomic Energy Agency (IAEA), the International Organization for Standardization (ISO), the American Society of Testing and Materials (ASTM), and American Nuclear Society (ANS). However, there is no single standard leach test method that can evaluate the performance of chemical durability under various field conditions of disposal facilities.

In this study, a chemical durability test was conducted based on the protocol described by ASTM C1220-17 [12] with some minor modifications of the experimental conditions. We identified the Ti⁴⁺-Te⁴⁺-oxide system for the immobilization of radioactive waste. TiTe₃O₈ is a potential Ti⁴⁺-Te⁴⁺-oxide because we anticipate that the replacement of cations in the crystal framework at the Ti⁴⁺ site with dopant cations can produce stable Ti_1-xM_xTe₃O₈ (M = Er³⁺, Ce⁴⁺, and Sn⁴⁺) compounds [13-15]. In addition, a high quality solid solution between stoichiometric amounts of TiO₂ and TeO₂ is expected to be easily prepared through a simple solid-state reaction at 600-700℃.

2. Experimental

2.1 Solid-state synthesis of TiTe₃O₈

A polycrystalline sample of TiTe₃O₈ was prepared by the solid-state reaction of TiO₂ and TeO₂ powder. TiO₂ (Showa, 99.0%) and TeO₂ (Aldrich, 99.0%) were used as purchased without further purification. Stoichiometric amounts of TiO₂ and TeO₂ were thoroughly ground together in an agate mortar and pestle. After grinding, the reaction mixtures were pressed into pellets and transferred to an alumina crucible. The samples placed in the alumina crucible were heated to 650℃ at a rate of 5℃·min^-1, held at this temperature for 18 h for sintering, and cooled to room temperature at a rate of 5℃·min^-1.

2.2 Material characterization

Powder X-ray diffraction (XRD) analyses were conducted to examine the purity of the polycrystalline TiTe₃O₈ phase with a Bruker D8-Advance diffractometer using Cu Kα radiation at an operating voltage of 40 kV and a current of 40 mA. The powder samples, which were prepared by grinding the pellet samples, were mounted on the sample holder, and measured in the 2θ range of 10-80° with a step size of 0.02° and a step time of 0.1 s. To investigate the thermal effects, the powder samples were heated at a rate of 10℃·min^-1 up to the three different target temperatures of 300, 600, and 900℃. Each target temperature was maintained for 10 min and subsequently cooled to room temperature to measure the powder XRD patterns. The weight loss and melting temperature of the TiTe₃O₈ powder samples were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively, using a SCINCO STA N-1500 TGA/DSC analyzer at a heating rate of 5℃·min^-1 to 1,000℃ under a flow of argon gas.

After the leaching experiments, scanning electron microscopy (SEM) analyses were conducted on the pelletized samples using a JEOL JSM-6610LV scanning electron microscope to investigate any harmful effects on the crystalline structures and surface morphologies. XRD analysis was also performed for the pelletized samples after leaching under the same operating conditions as those for the powder samples.

2.3 Chemical durability test

The static leaching test method described by ASTM C1220-17 using deionized water was used to test the chemical durability of pelletized TiTe₃O₈ samples. High-purity deionized water is one of the most frequently employed aqueous solvents for leaching, as the appropriate pH and salt compositions of the aqueous leaching solution can be manipulated to simulate the specific groundwater conditions at the disposal sites. The 15-mm-diameter and 1.4-mm-thick pellets were immersed in 10 mL of deionized water. The acidity of the leaching solution spanned from an acidic solution (0.01 M HCl, pH = 2) to an alkaline solution (0.01 M NaOH, pH = 12). A 17 mL Teflon-lined, stainless steel autoclave containing the leaching solution and the pelletized sample was placed in a convection oven at 90℃ for 7 and 14 d. After 14 d, the concentration of the released Ti was measured by inductively coupled plasma-mass spectrometry (ICP-MS) using a iCAP TQ ICP-MS (Thermo Fisher Scientific) with the limit of detection (LOD) of 0.005 mg·L^-1.

3. Results and discussion

3.1 Crystalline Structures

The incorporation of radionuclides into the crystalline framework of ceramic waste forms is key to the immobilization of radioactive wastes. Fundamental information on crystalline structures is helpful for understanding waste form stability and incongruent dissolution. The reported values of crystallographic information, bond distances, and bond angles for crystalline TiTe₃O₈ are given in Tables 1-3 [16]. Fig. 1 shows that the powder X-ray diffraction pattern for TiTe₃O₈ powder prepared through solidstate sintering matched the polycrystalline TiTe₃O₈ (PDF#: 01-070-2439 in Fig. 1). No other impurities such as TiO₂ and TeO₂ phases were found in the sintered powders. As previous studies have shown [17], the crystal structure of TiTe₃O₈ showed the centrosymmetric cubic space group, Iɑ-3 (No. 206). It reveals 3D frameworks consisting of TiO₆ octahedra and TeO₄ polyhedra (see Fig. 2). The unique Ti⁴⁺ cation is connected to six oxygen atoms in an octahedral coordination environment. The asymmetric unit indicated the presence of unique Te⁴⁺ cations, showing an unsymmetrical coordination moiety resulting from the stereoactive lone pairs. The three distorted TeO₄ polyhedra share their corners through one O atom and form Te₃O₈ trimers. The Ti⁴⁺ cations in the center of the oxide octahedron are shared by two Te₃O₈ trimers in an asymmetric coordination environment. Each Te₃O₈ trimer is further connected by a TiO₆ octahedron along the [100] direction, thereby resulting in the formation of a 3D framework.

Table 1

Crystallographic data of TiTe₃O₈ [16]

Table 2

Atomic positions, site occupancies, and equivalent isotropic displacement parameters for TiTe₃O₈ [16]

Table 3

Selected interatomic bond lengths and bond angles for TiTe₃O₈ [16]

Fig. 1

Powder X-ray diffraction patterns of the solid-state synthesized TiTe₃O₈ powders (black line), and the reference structure of pure TiTe₃O₈ (red line, PDF#: 01-070-2439).

Fig. 2

Ball-and-stick representation of 3D structure of TiTe₃O₈ in ab-plane, TiO₆ octahedron and Te₃O₈ trimers by corner sharing of TeO₄ seesaw groups. The purple octahedrons represent TiO₆, the blue balls Te atoms, and the red balls O atoms.

3.2 Thermal properties

TGA data showed that no significant weight loss of the synthesized TiTe₃O₈ powders was observed until the temperature reached the melting point of TiTe₃O₈, as shown in Fig. 3(a). The weight loss started at approximately 840℃, where the decomposition of TiTe₃O₈ took place. At 1,000℃, almost 30% of the total weight was evaporated from the TiTe₃O₈ powder. In the DSC analysis, an endothermic peak was observed at 820℃, which indicates the melting of TiTe₃O₈. To investigate the chemical behavior introduced by the TeO₂ volatilization on the TiTe₃O₈ melt, XRD patterns measured at room temperature were compared after heat treatments at different temperatures up to the volatilization temperature. Fig. 3(b) reveals that polycrystalline TiTe₃O₈ maintains crystallinity up to 600℃. However, it is evident from the XRD pattern of the sample heat-treated at 900℃ that an amorphous residue was formed after the selective evaporation of TeO₂ from the TiTe₃O₈ melt. Small XRD peaks also appeared, corresponding to the polycrystalline TiO₂ phase (PDF#: 21- 1276) as well as polycrystalline TeO₂.

Fig. 3

Thermal behavior of TiTe₃O₈ powders. (a) Thermogravimetric analysis (black line), differential scanning calorimetry analysis (red line) data, and (b) X-ray diffraction patterns of TiTe₃O₈ powder treated at different temperatures for 10 min.

3.3 Chemical durability

Crystalline samples of pelletized TiTe₃O₈ were subjected to a static leach test. The leaching rate of TiTe₃O₈ was calculated using Eq. (2) from the normalized mass loss in Eq. (1) [12, 19].

where NL_Ti is the normalized mass loss of Ti (g·m^-2), LR_Ti is the leaching rate of Ti (g·m^-2·d^-1), C_Ti is the concentration of elemental Ti in the leaching solution (g·L^-1), V is the volume of the leaching solution (L), f_Ti is the mass fraction of element Ti in the sample, S is the surface area of the sample (m²), and t is the soaking time (d). All Ti concentrations in the leaching solutions of the 0.01 M HCl aqueous solution, pure deionized water, and 0.01 M NaOH aqueous solution were below the limit of detection (LOD). Based on the LOD values, LR_Ti under various leaching conditions in this study was less than the order of 10^-4 g·m^-2·d^-1. The leaching rates of other titanate- based solid waste forms are shown in Table 4. The chemical durability of TiTe₃O₈ is comparable to that of known titanate-based solid waste forms, and is more stable than borosilicate glass, while the melting temperatures of titanate-based materials are higher than those of TiTe₃O₈. Because the high processing temperature can cause volatilization of radionuclides in waste, a low temperature process is advantageous in field practices.

Table 4

Leaching rate of constituent Ti and Si element in various waste forms

The leaching rate of pelletized TiTe₃O₈ was low. Therefore, no changes were expected in the surface and crystal structure. The XRD results in Fig. 4 confirm that the crystal structures of TiTe₃O₈ remained unchanged after 7 and 14 d of leaching with highly acidic and alkaline aqueous solutions, as well as pure deionized water. SEM images also demonstrated that notable surface cracks or damages were not observed after leaching.

Fig. 4

XRD patterns of TiTe₃O₈ after leaching in (a) 0.01 M HCl aqueous solution, (b) pure deionized water, and (c) 0.01 M NaOH aqueous solution over t = 7 d (red line), 14 d (blue line), and XRD pattern of the sample before leaching (black line) as a reference.

4. Conclusions

Pure crystalline TiTe₃O₈ powder was synthesized through the solid-state sintering reaction of stoichiometric amounts of TiO₂ and TeO₂ powder. The two most important and fundamental properties required for the nuclear waste immobilization, thermal stability, and chemical durability of TiTe₃O₈ were tested to examine the potential as a promising solid waste form. Although the TiTe₃O₈ melt became unstable at around 840℃, a thermally stable TiO₂ residue was formed after the volatilization of TeO₂ from TiTe₃O₈. The low melting temperature of the TiTe₃O₈ is advantageous in terms of cost, processability, and stability of wastes. The leaching rates of Ti in pelletized TiTe₃O₈ crystals were comparable to the well-known solid waste forms even under severe acidic and basic conditions. Further studies are required to investigate other important performance characteristics such as the loading capacity of fission products, mechanical strength, and radiation resistance.