Effects of ferrihydrite nanoparticle incorporation in cementitious materials on radioactive waste immobilization

Role of Al substitution in the reduction of ferrihydrite by Shewanella oneidensis MR-1

Substitution of aluminum under natural environmental conditions has been proven to inhibit the transformation of weakly crystalline iron (oxyhydr)-oxides towards well crystalline iron oxides, thereby enhancing their long-term stability. However, exploration on the role of aluminum substitution in bacteria-mediated iron oxides transformation is relatively lacking, especially in the anaerobic underground condition where iron (oxyhydr)-oxides are easy to reduced. In this study, we selected four different levels of substitution aluminum prevalent in iron oxides under natural conditions, which are 0 mol%, 10 mol%, 20 mol%, and 30 mol% (mol Al/mol (Al + Fe)) respectively. With the presence of Shewanella oneidensis MR-1, we conducted a 15-day anaerobic microcosm experiment in simulated groundwater conditions. The experiment data suggested that aluminum substitution result in a decrease in bio-reduction rate constants of ferrihydrite from 0.24 in 0 mol% Al to 0.17 in 30 mol% Al. Besides, when containing substituted aluminum, secondary minerals produced by biological reduction of ferrihydrite changed from magnetite to akaganeite. These results were attributed to the surface coverage of Al during the reduction process, which affects the contact between S. oneidensis MR-1 and the unexposed Fe(III), thus inhibiting the further reduction of ferrihydrite. Since iron (oxyhydr)-oxides exhibit a strong affinity on multiple kinds of pollutants, results in this study may contribute to predicting the migration and preservation of contaminants in groundwater systems.

Low carbon binder modified by calcined quarry dust for cemented paste backfill and the associated environmental assessments

Cemented paste backfill (CPB) favors the sustainable development of mine industry. However, as the primary cementitious binders in CPB, the high cost of ordinary Portland cement (OPC) discourages CPB utilization. In the present work, low-carbon and low-cost binders activated by Na₂CO₃ supplemented by calcined quarry dust were used in CPB. The binder was prepared using a 'one-part' method. It was found that binders prepared using 8% Na₂CO₃ and 5% CQD show the best performance. The superior properties of the binders were attributed to the promoted binder hydration and special phase assembles of the hydration products. Cost and carbon emission analysis showed that Na₂CO₃ activated binder was cheaper and greener. The cost and CO₂ emission of binder B8Q5 were lower than OPC by around 34.16% and 87.76%, respectively. Besides, leaching tests showed that all the toxic metals were stabilized, which posed no environmental risk.

Toward Sustainable Cementitious Radioactive Waste Forms: Immobilization of Problematic Operational Wastes

Developing effective radioactive waste management practices is essential for ensuring the sustainability of the nuclear industry. The immobilization of radioactive wastes is one of the main activities conducted during the management of these wastes; it aims to produce a durable waste form that has sustainable performance over long periods of time. In this work, the challenges that face the design of durable cementitious waste forms are addressed for problematic operational wastes. In this respect, the problematic characteristics of evaporator concentrates, spent ion exchangers, and organic liquid wastes are overviewed, and the factors that affect the durability of their cementitious waste forms are identified. A summary of potential conventional and innovative cementitious matrices is presented by reviewing the cementation practices in national programs and recent research devoted to developing durable matrices. Finally, a guide to optimize the mix design of these waste forms was proposed that includes the selection of the testing procedure, factors that affect the waste form performance, and the optimization technique. This guide was presented with special focus on leaching tests, which are a means to test the stabilization performance of nuclear waste forms.

Encapsulation of cesium with a solid waste-derived sulfoaluminate matrix: A circular economy approach of treating nuclear wastes with solid wastes

It is of great importance to safely dispose nuclear wastes with the development of nuclear industries. Past approaches to this problem have included immobilizing radioactive cesium in Portland cement-based matrices; however, the leaching rates of cesium are relatively high, especially as the leaching temperature increases. This paper explores a high-efficiency and cost-effective approach for encapsulating cesium using a sulfoaluminate cement (SAC) matrix, which was prepared via synergetic use of industrial solid wastes. Leaching results showed that, the apparent diffusion coefficient values of cesium were only ~1.4 × 10^-15 cm²/s and ~5 × 10^-18 cm²/s at 25 ℃ and 90 ℃ leaching conditions, respectively. These values were several orders of magnitude lower when compared with previously reported values, indicating the excellent encapsulation performance of the solid-waste-based SAC for cesium. Moreover, the heavy metals contained in the industrial solid waste were also effectively immobilized. A mechanistic analysis revealed that cesium was encapsulated in the SAC matrices stably by a physical effect. Finally, a life cycle assessment and economic analysis indicated that this approach was environmental-friendly, cost-effective, and energy-saving. This work provides a promising strategy for effective encapsulation of cesium and synergetic treatment of industrial solid wastes.

Assessment of POFA -Cementitious material as backfill barrier in DSRS borehole disposal: 226Ra confinement

Disused Sealed Radioactive Sources (DSRS) borehole disposal is an innovative concept recommended by international atomic energy agency (IAEA) to improve the safety and security of the management end point for these sources. A green application of Palm Oil Fuel Ash (POFA) as a supplementary material for cementitious backfill barrier in DSRS borehole disposal facility is proposed. Samples with up to 50% POFA replacement complied with the mechanical and hydraulic performance requirements for backfill barriers in retrievable radioactive waste disposal facilities. The structures of one year old OPC and optimum OPC-POFA cement backfills were evaluated using FESEM, XRD, EDXRF, BET, and TGA and their ²²⁶ Ra confinement performances were assessed. 30% POFA replacement improved the geochemical conditions by reducing competitive Ca²⁺ release into the disposal environment. It enhanced ²²⁶Ra confinement performance independently on the amount of water intrusion or releases below 2% of 1 Ci source. The improved performance is attributed to the higher fraction of active sites of OPC-POFA backfill compared to that of OPC backfill. ²²⁶Ra sorption onto C-S-H is irreversible, spontaneous, endothermic, and independent on the degree of the surface filling. The provided experimental data and theoretical analysis proved the feasibility of this green use of POFA in reducing the radiological hazard of ²²⁶Ra.

Hydroxyapatite nanoparticle functionalized activated carbon particle electrode that removes strontium from spiked soils in a unipolar three-dimensional electrokinetic system

Biohazard performance of Sr radionuclide can be significantly magnified by its release from the contaminated sedimentation. In this study, hydroxyapatite nanoparticle-functionalized activated carbon electrode (AC-HAP) was synthesized and stacked to the cathode compartment of the electrokinetic (EK) system to develop a unipolar three-dimensional (3D) electrochemical process for Sr²⁺ removal from spiked soils. Sr²⁺ adsorption by AC-HAP can be fitted by the pseudo-first-order and pseudo-second-order kinetic models and the Langmuir, Freundlich, and Temkin isotherm models. The largest monolayer adsorption capacity of AC-HAP of 69.49 mg g^-1 was evaluated in the pH range of 10-12 and at 40 °C. 3D EK further intensified the adsorption process of AC-HAP and the corresponding Sr²⁺ removal from aqueous environments. Voltage gradients and proposing time had a significant effect on the migration and transmission of Sr²⁺ in the electrolyzer. The influence of competitive ions on Sr²⁺ removal in the stock solutions followed Al³⁺ < Mg²⁺ < K⁺ < Na⁺ < Ca²⁺ while followed Al³⁺ < Na⁺ < K⁺ < Mg²⁺ < Ca²⁺ in 3D EK. The first three cycles for AC-HAP had taken roughly 50% of the reusability percentage. Sr²⁺ removal from spiked samples in 3D EK was achieved by acid dissolution, electromigration, and selective uptake on particle electrode.

Ultrafast removal of radioactive strontium ions from contaminated water by nanostructured layered sodium vanadosilicate with high adsorption capacity and selectivity

Layered sodium vanadosilicates was synthesized for removing soluble strontium ion (Sr²⁺) from simulated radioactive wastewater. SEM, EDS and TEM images indicated that layered sodium vanadosilicates had a nanostructure with flake-like shape providing an extremely large surface area. XRD, FTIR and XPS analyses further revealed a framework structure of adsorbent consisting of corner-sharing SiO₄ tetrahedra and VO₆ octahedra, with sodium ion being the major cation in the synthesized nanostructured layered sodium vanadosilicate. This study for the first time showed ultrafast adsorption of Sr²⁺ by nanostructured layered sodium vanadosilicates in a time frame of seconds with a high adsorption capacity of 174.3 mg/g estimated from Langmuir isotherm. It was found that about 99.0% of Sr²⁺ at an initial concentration of 5.00 mg/L and adsorbent dosage of 0.5 g/L could be removed within several seconds. The kinetic analysis further revealed that pseudo-second-order instead of pseudo-first-order kinetics could satisfactorily describe the observed ultrafast removal of Sr²⁺. In particular, the nanostructure layered sodium vanadosilicates exhibited an excellent affinity to Sr²⁺ over a wide pH range of 3-11. It was also demonstrated that the working mechanisms of nanostructured layered sodium vanadosilicates for Sr²⁺ removal mainly included surface electrostatic interaction and ion-exchange with sodium ion. Furthermore, nanostructure layered sodium vanadosilicates had significant advantages for Sr²⁺ removal compared with other adsorbents. Consequently, it is reasonable to expect that nanostructured layered sodium vanadosilicates synthesized in this study could be considered as a promising adsorbent for ultrafast and high-efficiency removal of radioactive Sr²⁺ from radioactive wastewaters.

Review on Selection and Experiment Method of Commonly Studied Simulated Radionuclides in Researches of Nuclear Waste Solidification

Although many types of simulated radionuclides have been widely used as a substitute for actual nuclear waste in the studies of nuclear waste solidification, the understanding of the applicability and validity of simulated radionuclides is still insufficient. In particular, the selection and use of simulated radionuclides, which can play a decisive role in the accuracy of the experimental results, still lack unified or integrated references. This paper provides a critical review on the selection, experimental methods, and applicability of the most commonly studied simulated radionuclides, followed by a careful discussion and recommendation of simulated radionuclides suitable for different solidified bodies. The main factors (e.g., temperature, pH, and atmosphere) affecting the choice of simulated radionuclides were analyzed in detail. This work helps to integrate the selection and use of simulated radionuclides, and it will be beneficial for improving the effectiveness of nuclide solidification research.

Nanostructured chitosan/molecular sieve-4A an emergent material for the synergistic adsorption of radioactive major pollutants cesium and strontium

A fresh adsorbent nanostructured chitosan/molecular sieve 4A hybrid (NSC@MS-4A) was fabricated for the rapid adsorption of strontium (Sr²⁺) and cesium (Cs⁺) ions from aqueous solutions. The as-obtained NSC@MS-4A were thoroughly characterized by XRD, FE-SEM, EDS, BET, XPS and FT-IR. The physio-chemical properties and structural aspects revealed that NSC@MS-4A acquires fine surface area (72 m²/g), porous structure as well as compatible functional groups (-P-O-P and -C-O-C) for the admission of Cs⁺ and Sr²⁺ ions. The batch adsorption studies concluded that prepared adsorbent displayed a maximum adsorption of 92-94 % within 40 min. Fast adsorption of Cs⁺ and Sr²⁺ was achieved at neutral pH (6-7), ambient temperature (25-30 °C) and slow agitation speed (50-60 rpm) which could propose vast benefits such as little power utilization and uncomplicated operation. Among six types of adsorption isotherms, Freundlich isotherm showed the best fit with R²>0.997. Pseudo-second order made a better agreement as compare to other kinetic models. The thermodynamic coefficients suggested the passage of Cs⁺ and Sr²⁺ ions through the liquid solid boundary is exothermic and spontaneous. The NSC@MS-4A displayed excellent regenerability properties over five repetitive adsorption/desorption cycles, which specified that as-obtained NSC@MS-4A is a sustainable as well as efficient adsorbent for practical decontamination of radioactive liquid waste.

Natural Organic Matter (NOM) Imparts Molecular-Weight-Dependent Steric Stabilization or Electrostatic Destabilization to Ferrihydrite Nanoparticles

Ferrihydrite nanoparticles (Fh NPs) are ubiquitous in natural environments. However, their colloidal stability, and fate and transport behavior are difficult to predict in the presence of heterogeneous natural organic matter (NOM) mixtures. Here, we investigated the adsorption and aggregation behavior of Fh NPs exposed to NOM fractions with different molecular weights (MW). The NOM fraction with MW < 3 kDa destabilized the NPs, resulting in accelerated aggregation even at high C/Fe mass ratios, whereas higher MW NOM fractions imparted better colloidal stability with increasing MW and C/Fe ratio. Despite differences in the functional group composition of the bulk (dissolved) NOM fractions, all NOM fractions produced similar adsorbed layer compositions on the NPs, suggesting minimal contribution of chemical properties to the distinctive aggregation behavior. Rather, the higher adsorbed mass and larger size of the higher MW fractions were key factors in stabilizing the NPs through steric repulsion, whereas the lowest MW fraction had low adsorbed mass and was unable to counter electrostatic patch-charge attraction when the NPs are positively charged. This mechanistic understanding helps us predict the transport and fate of Fh NPs and the associated contaminants in natural environments with varying NOM compositions.