Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide

Design and Application of Materials for Sequestration and Immobilization of 99Tc

⁹⁹Technetium (⁹⁹Tc) is a hazardous radionuclide that poses a serious environmental threat. The wide variation and complex chemistries of liquid nuclear waste streams containing ⁹⁹Tc often create unique, site specific challenges when sequestering and immobilizing the waste in a matrix suitable for long-term storage and disposal. Therefore, an effective management plan for ⁹⁹Tc containing liquid radioactive wastes (such as storage (tanks) and decommissioned wastes) will likely require a variety of suitable materials/matrixes capable of adapting to and addressing these challenges. In this review, we discuss and highlight the key developments for effective removal and immobilization of ⁹⁹Tc liquid waste in inorganic waste forms. Specifically, we review the synthesis, characterization, and application of materials for the targeted removal of ⁹⁹Tc from (simulated) waste solutions under various experimental conditions. These materials include (i) layered double hydroxides (LDHs), (ii) metal-organic frameworks (MOFs), (iii) ion-exchange resins (IERs) as well as cationic organic polymers (COPs), (iv) surface modified natural clay materials (SMCMs), and (v) graphene-based materials (GBMs). Second, we discuss some of the major and recent developments toward ⁹⁹Tc immobilization in (i) glass, (ii) cement, and (iii) iron mineral waste forms. Finally, we present future challenges that need to be addressed for the design, synthesis, and selection of suitable matrixes for the efficient sequestration and immobilization of ⁹⁹Tc from targeted wastes. The purpose of this review is to inspire research on the design and application of various suitable materials/matrixes for selective removal of ⁹⁹Tc present globally in different radioactive wastes and its immobilization in stable/durable waste forms.

Vanadium Oxidation States and Structural Role in Aluminoborosilicate Glasses: An Integrated Experimental and Molecular Dynamics Simulation Study

Vanadium-containing glasses have aroused interest in several fields such as electrodes for energy storage, semiconducting glasses, and nuclear waste disposal. The addition of V₂O₅, even in small amounts, can greatly alter the physical properties and chemical durability of glasses; however, the structural role of vanadium in these multicomponent glasses and the structural origins of these property changes are still poorly understood. We present a comprehensive study that integrates advanced characterizations and atomistic simulations to understand the composition-structure-property relationships of a series of vanadium-containing aluminoborosilicate glasses. UV-vis spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure (XANES) have been used to investigate the complex distribution of vanadium oxidation states as a function of composition in a series of six-component aluminoborosilicate glasses. High-energy X-ray diffraction and molecular dynamics simulations were performed to extract the detailed short- and medium-range atomistic structural information such as bond distance, coordination number, bond angle, and network connectivity, based on recently developed vanadium potential parameters. It was found that vanadium mainly exists in two oxidation states: V⁵⁺ and V⁴⁺, with the former being dominant (∼80% from XANES) in most compositions. V⁵⁺ ions were found to exist in 4-, 5-, and 6-fold coordination, while V⁴⁺ ions were mainly in 4-fold coordination. The percentage of 4-fold-coordinated boron and network connectivity initially increased with increasing V₂O₅ up to around 5 mol % but then decreased with higher V₂O₅ contents. The structural role of vanadium and the effect on glass structure and properties are discussed, providing insights into future studies of sophisticated structural descriptors to predict glass properties from composition and/or structure and aiding the formulation of borosilicate glasses for nuclear waste disposal and other applications.

Development of a Geochemical Speciation Model for Use in Evaluating Leaching from a Cementitious Radioactive Waste Form

Cast Stone has been developed to immobilize a fraction of radioactive waste at the Hanford Site; however, constituents of potential concern (COPCs) can be released when in contact with water during disposal. Herein, a representative mineral and parameter set for geochemical speciation modeling was developed for Cast Stone aged in inert and oxic environments, to simulate leaching concentrations of major and trace constituents. The geochemical speciation model was verified using a monolithic diffusion model in conjunction with independent monolithic diffusion test results. Eskolaite (Cr₂O₃) was confirmed as the dominant mineral retaining Cr in Cast Stone doped with 0.1 or 0.2 wt % Cr. The immobilization of Tc as a primary COPC in Cast Stone was evaluated, and the redox states of porewater within monolithic Cast Stone indicated by Cr are insufficient for the reduction of Tc. However, redox states provided by blast furnace slag (BFS) within the interior of Cast Stone are capable of reducing Tc for immobilization, with the immobilization reaction rate postulated to be controlled by the diffusive migration of soluble Tc in porewater to the surface of reducing BFS particles. Aging in oxic conditions increased the flux of Cr and Tc from monolithic Cast Stone.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO₄^-) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26(H₂O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO₄^-. Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, and diffraction methods, support successful incorporation of TcO₄^- into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO₄^- and one OH^- during substitution, where Ca²⁺-coordinated water near the substitution site is deprotonated to form OH^- for charge compensation upon TcO₄^- substitution. Furthermore, AIMD calculations support favorable TcO₄^- substitution at the SO₄^2- site in ettringite rather than gypsum (CaSO₄·2H₂O, formed as a secondary mineral phase) by at least 0.76 eV at 298 K. These results are the first of their kind to suggest that ettringite may contribute to TcO₄^- immobilization and the overall lifetime performance of cementitious waste forms.