Immobilization of radioactive corrosion products by cold sintering of pure hydroxyapatite

Bi0-Reduced Graphene Oxide Composites for the Enhanced Capture and Cold Immobilization of Off-Gas Radioactive Iodine

Radioactive waste management is critical for maintaining the sustainability of nuclear fuel cycles. In this study, we propose a novel bismuth-based reduced graphene oxide (Bi0-rGO) composite for the immobilization of off-gas radioactive iodine. This material synthesized via a solvothermal route exhibited a low surface area (2.96 m2/g) combined with a maximum iodine sorption capacity of 1228 ± 25 mg/g at 200 °C. The iodine sorbent was mixed with Bi2O3 powder and distilled water to fabricate waste matrices, which were cold-sintered at 300 °C under a uniaxial pressure of 500 MPa for 20 min to achieve a relative density of ∼98% and Vickers hardness of 1.3 ± 0.1 GPa. The utilized methodology reduced the iodine leaching rate by approximately 3 orders of magnitude through the formation of a chemically durable iodine-bearing waste form (BiOI). This study demonstrates the high potential of Bi0-rGO as an innovative solution for the immobilization of radioactive waste at relatively low temperatures.

Incorporation of lead into pyromorphite: Effect of anion replacement on lead stabilization

Previous studies demonstrate that the leaching of heavy metals in unreliable waste forms causes serious environmental pollution and health concerns. Thus, research is focused on identifying an effective, safe strategy for disposing of metal-laden solid waste such as lead (Pb). This study evaluated the effect of anion replacement in the structure of pyromorphite (Pb₁₀(PO₄)₆Cl₂, a common mineral phase for Pb sequestering) on Pb stabilization. Phosphate (PO₄^3-) at the tetrahedral pyromorphite site was simultaneously replaced by silicate (SiO₄^4-) and sulphate (SO₄^2-) in a controlled thermal treatment. The lattice expanded with the incorporation of additional SiO₄^4- and SO₄^2-. Furthermore, the unit cell parameters of the solid solutions evolved linearly with an increase in the substitution degree (x in Pb₁₀(SiO₄)_x(SO₄)_x(PO₄)_(6-2x)Cl₂). This research also demonstrated that Pb distributed into amorphous in a PO₄^3--deficient matrix, while asisite (Pb₇SiO₈Cl₂) was formed when the matrix was dominated by SiO₄^4- and SO₄^2-. The leaching results showed the isomorphous substitution in the target system rendered the products less durable towards acidic attack. Moreover, the fully isomorphous-substituted product (x = 3) showed more than two orders of magnitude lower leaching resistance than the PO₄^3--rich phase (x = 0). The lattice expansion, resulting from the isomorphous substitution, suggested that a lower dissolution energy was required in a PO₄^3--deficient matrix. The leaching kinetics pointed to a product with a lower apparent activation energy in the leaching process. The findings of this study provide unique insight into the design and optimization of waste forms for the immobilization of heavy metals.

Application of response surface methodology for uranium(VI) adsorption using hydroxyapatite prepared from eggshells waste material: study of influencing factors and mechanism

Hydroxyapatite (HAp) was synthesized from biowaste hen eggshells by wet precipitation method in which calcium hydroxide and phosphoric acid were used as precursors. The effectiveness of uranium(VI) adsorption onto HAp was investigated by batch adsorption experiments from aqueous solutions. The obtained HAp powder was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectrometry, point of zero charge and Scanning electron microscope. The factors and levels used during the experiments were pH (2-5), adsorbent mass (0.01-0.05 g), and initial U(VI) concentration (100-310 mg L^-1). A Box-Behnken design combined with analysis of variance was used to interpret the main effect influencing the adsorption. The results showed that pH was the most significant parameter affecting U(VI). The kinetic data correlates well with the pseudo-second-order model. The adsorption isotherms fitted the Langmuir-1 type model with the q_max = 175.22 mg g^-1 at 25 °C. The calculated value of the mean free energy indicates the chemisorption process. Under optimal conditions, the uranium effluent derived from the precipitation of ammonium uranyl carbonate removal performance of 98% was achieved. This study proved that HAp prepared from eggshell was an ecofriendly and low-cost adsorbent and was very effective for the adsorption of U(VI) from aqueous solutions.

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.

Non-volatile immobilization of iodine by the cold-sintering of iodosodalite

A cold-sintering process at a very low temperature (300 °C) achieved stable immobilization of simulated radioiodine waste incorporated in sodalite (iodosodalite). The reported sintering temperature was much lower than conventional ceramic waste form processing temperatures (600-1100 °C) and had no effect on the stability of the loaded iodine waste. Excellent iodine retention (>93%) with relative sintered density 91% were achieved by the cold-sintering at 300 °C, respectively. The sintered body exhibited a micro-hardness value of 3.9 ± 0.1 GPa and compressive strength of 198 ± 11 MPa. The seven-day product consistency test found iodine leaching rates on order of the magnitude 10^-4 g/m²⋅d. These results are the first example of the low temperature consolidation of iodine-bearing sodalite without using any additional material (e.g. glass, cement, etc.). High retention of the loaded simulated radioiodine without volatilization warrants the cold-sintering process for the environmental-friendly disposal of radioiodine.

Efficient immobilization of ionic corrosion products by a silica-hydroxyapatite composite via a cold sintering route

We have successfully demonstrated a new method of radioactive waste immobilization by hosting a waste-bearing form in another waste matrix. A cold sintering route was used to consolidate a silica-incorporated hydroxyapatite (Si-HAp) composite at 200 °C by applying a uniaxial pressure of 500 MPa for a short holding time of 10 min. The higher relative sintered density of up to 98.0 ± 1.3% was achieved by 25 wt% Si loaded HAp. Results from high resolution X-ray diffraction, micro-hardness, and high resolution scanning electron microscopy confirmed the densification with good mechanical strength (micro-hardness = 2.9 ± 0.3 GPa). For practical applications, two kinds of wastes (25 wt% ionic corrosion product-sorbed EDTA functionalized mesoporous silica and 75 wt% ionic corrosion product-sorbed HAp) were mixed, consolidated and tested. The chemical stability of the solidified composite matrix was positively assessed for low leaching rates of 5.9 × 10^-9 to 1.2 × 10^-5 g per m² per day using a standard product consistency test. The consolidated composite can bear compressive stress up to 358 MPa, which is orders of magnitude higher than the waste acceptance criteria of 3.5 MPa. The low process temperature can make this sintering process very powerful for the immobilization of radionuclides with volatility and low boiling point. Such a low temperature solidified matrix hosting various wastes may be a promising path for waste management because of its simplicity, reliability, scalability, cost effectiveness and environmental friendliness.