Uranium extraction from seawater

Abstract

To support the use of nuclear power as a sustainable electric energy generating technology, long-term supply of uranium is very important. The objective of this research is to investigate the use of new adsorbent material combined with electrosorption technique to extract uranium from seawater. For that purposes, an activated carbon-based adsorbent material was developed and tested through an electrosorption technique in this research. This research was divided into two major task. The first task was intentionally dedicated to fabricate the adsorbent materials based on the activated carbon, characterizations, and to investigate the performance of activated carbon electrode for electrosorption-based separation of uranium from seawater. The second task were performed in order to investigate the electrosorption isotherms and kinetics of the uranyl tricarbonate complex $[UO_2(CO_3)_3]^{4-}$ (one of the dominant uranium ion species in seawater) by an activated carbon electrode through batch-mode electrosorption experiments. The activated carbon electrode were fabricated by slurry coating on stainless steel gauze, drying, and a thermal cross-linking process. The activated carbon powder was mixed with polyvinylidene fluoride (acting as polymer binder) and N,N-dimethylacetamide (acting as organic solvent) to form a carbon slurry. The mixture was vigorously stirred for homogenization and casted onto a piece of stainless steel gauze followed by drying process. The physical structure, pore and specific surface area, strength and durability, molecular interaction, and electrochemical properties of the fabricated activated carbon electrode were characterized using various instruments and techniques. The characterizations has revealed that the fabricated activated carbon electrode has a continuous randomly structure porous material with the BET surface area and total pore volume were $50 m^2/g and 0.1 cm^3/g$ , respectively. The analysis on the strength and durability, cyclic voltammetry and molecular interaction indicated that the fabricated activated carbon electrode has enough strength and been optimize for electrosorption application. Subsequently, the performance of the fabricated activated carbon electrode to adsorb uranium from seawater was investigated through electrosorption experiments up to 300 minutes by changing voltage potentials from +0.8V to -0.8V (vs Ag/AgCl). Results indicate that the uranium adsorption by the activated carbon electrode developed in this research reached up to 3.4 g-U/kg-Ads, which is comparable with the performance of amidoxime-based adsorbent materials. Electrosorption of uranium ions from seawater was found to be most favorable at +0.4V (vs Ag/AgCl). The cost of chemicals and materials in the present research was also compared with that of the amidoxime-based approach as part of the engineering feasibility examination. The finding shows that the material and the chemical cost was significantly be reduced compared to amidoxime-based adsorbent material. However, when includes with the electricity cost, the estimated extraction cost was much higher than the amidoxime approach. Besides that, the energy consumption to extract uranium from seawater was also estimated to be higher than the energy harvested based on 30% of energy efficiency. The absorption performance of the fabricated activated carbon electrode toward $[UO_2(CO_3)_3]^{4-}$ from aqueous solutions was investigated through batch-mode electrosorption experiments with respect to bias potential, contact time, and solution concentration. The finding shows that the maximum electrosorption capacity improved with increase in applied voltage potential

the maximum value, 7.18 mg-Uranium/g-Ads, was obtained at +0.8 V. The separation factor ( $R_L$ ) revealed that the electrosorption of $[UO_2(CO_3)_3]^{4-}$ onto the activated carbon electrode improved with the increase in initial $[UO_2(CO_3)_3]^{4-}$ concentration. Electrosorption by the activated carbon electrode material was also modeled using Langmuir and Freundlich isotherms. The analysis has revealed that the electrosorption of $[UO_2(CO_3)_3]^{4-}$ onto the activated carbon electrode followed pseudo-second-order (rather than pseudo-first-order) kinetics. In addition of the two main tasks, electro-desorption of uranium on the activated carbon electrode was also performed. The electro-desorption test showed that more than 78% of the adsorbed uranium can be recovered in 120 minutes. The desorption efficiency was influenced by the applied negative potential. The FE-TEM and EDX analysis also verified presence of uranium ions adsorbed onto the fabricated activated carbon electrode due to electrosorption process. The purification test was also indicated that $[UO_2(CO_3)_3]^{4-}$ can be transformed into a solid form of $UO_2SO_4$ after chemical and drying process treatment. To the best of my knowledge, these works were expected to be the first attempt to study the activated carbon electrode along with electrosorption technique to extract uranium from seawater. This research has demonstrated that application of activated carbon electrode combined with electrosorption may provide a new way for uranium separation from seawater. Therefore, it is believe that the findings from this research may provide useful information for further development in this research areas.