Abstract
Effective and safe management of wastes are primary requirements for the nuclear industry. These wastes whether solid, liquid or gaseous arise from every stage of the nuclear fuel cycle and are treated to ensure they comply with stringent regulatory standards before final disposal into the environment. In the early period of nuclear energy the emphasis was largely on the treatment of operational wastes but over the past two decades the need to accommodate wastes arising from the decommissioning of nuclear installations has gradually increased and in the next two decades will become the prime focus. Significant components of decommissioning are post operation clean out (POCO) and the decontamination of plants/equipment to minimize dose uptake to operatives. It is crucial now and in the future that an integrated approach to waste management has been/is developed to ensure the overall decommissioning process produces an end product that can be disposed of safely with the minimal impact on the environment and is acceptable to all stakeholders. It is imperative that effective treatments of liquors arising from POCO, decontamination and even operational activities are developed and employed.
This work is to develop novel nanostructured hybrid materials that can be employed in-situ that have high capacities for a variety of radionuclides in aqueous solution. Especially, hybrid composites are synthesized with a focus on how to separate the used composite materials from treated solution as follows: magnetic nanoparticles that can be easily retrieved from an aqueous solution by a magnet, hydrogel beads that are packed in fixed-bed column reactors, three-dimensional aerogels can be removed from an aqueous solution by a net. In addition, functionalization techniques are used to synthesize specific radionuclide targeted ligands and extraction experiments are performed to selectively remove radionuclides from aqueous solution.
First of all, magnetic Prussian blue (PB) nanocomposites synthesized by binding PB to a core of magnetite (Fe3O4) nanoparticles for highly efficient and rapid separation of cesium (Cs+) from aqueous solution. The average particle size of the magnetic PB nanocomposites was 13.6 nm, and they had a high surface area (322.19 m2/g), leading to efficient Cs+ adsorption capability. The nanocomposites showed a maximum sorption capacity of 280.82 mg/g at an initial Cs+ concentration of 50 mM, pH 7, and 10°C, which is much higher than those of previously reported PB-based adsorbents for removing Cs+. Furthermore, magnetite nanoparticles supported on organically modified montmorillonite were successfully synthesized by a facile coprecipitation method and were used as an adsorbent to remove iodide from aqueous solutions. The individual and combined effects of key process parameters (pH, temperature, and initial iodide concentration) were studied using a response surface methodology. The maximum iodide removal efficiency of 93.81% was obtained under the optimal conditions of pH 3.9, a temperature of 41.3°C, and an initial iodide concentration of 113.8 mg/L.
Secondly, a continuous fixed-bed column study was performed using PVA-alginate encapsulated Prussian blue graphene oxide (PB-GO) hydrogel beads as a novel adsorbent for the removal of cesium from aqueous solutions. The effects of different operating parameters, such as initial cesium concentration, pH, bed height, flow rate, and bead size, were investigated. The maximum adsorption capacity of the PB-GO hydrogel beads was 164.5 mg/g at an initial cesium concentration of 5 mM, bed height of 20 cm, and flow rate of 0.83 mL/min at pH 7. The Thomas, Adams?Bohart, and Yoon?Nelson models were applied to the experimental data to predict the breakthrough curves using non-linear regression. Although both the Thomas and Yoon?Nelson models showed good agreement with the experimental data, the Yoon?Nelson model was found to provide the best representation for cesium adsorption on the adsorbent, based on the χ2 analysis.
Thirdly, a separation process using porous aerogels as separation medium was performed using three-dimensional barium-sulfate-impregnated reduced graphene oxide (BaSO4-rGO) aerogel. The BaSO4-rGO aerogel was successfully synthesized by a facile one-step hydrothermal method and was used as an adsorbent to remove strontium from aqueous solutions. The characterization of BaSO4-rGO aerogel confirmed that barium sulfate particles were firmly embedded in the surface of the rGO sheets and exhibited a porous 3D structure with a high surface area of 129.37 m2/g. The mass ratio of BaSO4 in the BaSO4-rGO aerogels substantially affected strontium adsorption, and the optimal BaSO4/rGO ratio was found to be 1:1. The synthesized BaSO4-rGO aerogels not only reached adsorption equilibrium faster (within 1 h), but also showed much higher adsorption capacity than an rGO aerogel. The experimental data were well fitted to a pseudo-second-order kinetic model and the adsorption behavior followed the Langmuir isotherm. The adsorption capacity of strontium on BaSO4-rGO aerogels remained relatively high even under ionic competition in simulated seawater.
In the final study, amino-functionalized multi-walled carbon nanotubes (MWCNTs) were synthesized by a simple, cost-effective method using 3-aminopropyltriethoxysilane and were evaluated for cesium ion removal in aqueous solution. Experimental results showed that the maximum cesium adsorption capacity of amino-functionalized MWCNTs was 136.3 mg/g at an initial cesium concentration of 50 mM, pH 7, and 35°C, reaching 95% of the ultimate adsorption capacity within 30 min. The adsorption capacity of amino-functionalized MWCNTs at pH 7 was not significantly affected by the presence of competing ions (Ca2+, Mg2+, Na+, and K+). Equilibrium adsorption data showed that the Langmuir model was more suitable than the Freundlich and Tempkin models. The thermodynamic parameters indicated the spontaneous and endothermic nature of cesium adsorption on the amino-functionalized MWCNTs. The large surface area and amino functional groups on the modified MWCNT surface contributed to the high cesium ion adsorption capacity. In addition, 5-bromo-2,9-bis(5,6-diphenyl-1,2,4-triazin-3-yl)-1,10-phenanthrolin (5-bromo-Ph4-BTPhen) ligands are performed to selectively remove radionuclides from aqueous solution. The 5-bromo-Ph4- BTPhen ligands were characterized by H, C NMR spectra, FT-IR, and mass spectra (m/z). Experimental results showed high capacity of over then 95% and high selectivity for strontium and cobalt at an initial concentration of 1 mg/L, pH 7, and 25°C. The capacity of 5-bromo-Ph4-BTPhen was not significantly affected by the presence of co-existing ions. Furthermore, the bromination at the 5-position of neocuproine can substitute 4-hydroxyphenol linking group through Suzuki coupling, which enables immobilization to the magnetic nanoparticles. The ligand immobilized with the magnetic nanoparticles can be easily retrieved from aqueous solution after decontamination of radionuclides.