New Fiber Materials with Sorption Capacity at 5.0 g-U/kg Adsorbent under Marine Testing Conditions Report uri icon

Overview

abstract

  • The Fuel Resources program of the Fuel Cycle Research and Development program of the Office of Nuclear Energy (NE) has focused on assuring that nuclear fuel resources are available in the United States for a long term. An immense source of uranium is seawater, which contains an estimated amount of 4.5 billion tonnes of dissolved uranium. Extraction of the uranium resource in seawater can provide a price cap and ensure centuries of uranium supply for future nuclear energy production. NE initiated a multidisciplinary program with participants from national laboratories, universities, and research institutes to enable technical breakthroughs related to uranium recovery from seawater. The goal is to develop advanced adsorbents to make the seawater uranium recovery technology a cost competitive, viable technology. Under this program, Oak Ridge National Laboratory (ORNL) has developed several novel adsorbents, which enhanced the uranium capacity 4-5 times from the state-of-the art Japanese adsorbents. Uranium exists uniformly at a concentration of ~3.3 ppb in seawater. Because of the vast volume of the oceans, the total estimated amount of uranium in seawater is approximately 1000 times larger than its amount in terrestrial resources. However, due to the low concentration, a significant challenge remains for making the extraction of uranium from seawater a commercially viable alternative technology. The biggest challenge for this technology to overcome to efficiently reduce the extraction cost is to develop adsorbents with increased uranium adsorption capacity. Two major approaches were investigated for synthesizing novel adsorbents with enhanced uranium adsorption capacity. One method utilized conventional radiation induced graft polymerization (RIGP) to synthesize adsorbents on high-surface area trunk fibers and the other method utilized a chemical grafting technique, atom-transfer radical polymerization (ATRP). Both approaches have shown promising uranium extraction capacities: RIGP adsorbent achieved 5.00 ± 0.15 g U/kg-ads., while ATRP adsorbent achieved 6.56 ± 0.33 g U/kg-ads., after 56 days of seawater exposure. These achieved values are the highest adsorption capacities ever reported for uranium extraction from seawater. Novel fiber adsorbents (AF1) comprised of acrylonitrile (AN) and itaconic acid prepared by RIGP onto high-surface-area, hollow-gear polyethylene fibers. The AF1 adsorbent was subsequently tested in natural seawater at the Pacific Northwest National Laboratory (PNNL). This report describes the preparation, characterization and testing of this novel adsorbent including results on simulated seawater testing and field adsorption testing using natural seawater. Investigation of the optimum reaction parameters for conversion of grafted cyano groups into amidoxime groups was conducted by reaction with hydroxylamine at different temperatures and time periods in a variety of water based and organic solvents. The 13C CP/MAS spectra of AF1 adsorbent fibers amidoximated in 50/50 (w/w) water-methanol and in dimethyl sulfoxide (DMSO) clearly revealed that both the open-chain amidoxime and cyclic imide dioxime were formed during the course of the reaction. Formation of imide dioxime from amidoxime was found to occur slowly and gradually with increasing reaction time. Higher diffusivity of DMSO as compared to water-methanol, in the grafted trunk PE fiber resulted in faster kinetics of the amidoximation reaction and a larger amount of cyclic imide dioxime throughout the adsorbent. The uranium adsorption capacity of the amidoximated AF1 samples was determined after: (i) 24 h contact with sodium based brine spiked with uranium and (ii) 56 days exposure in natural seawater (Sequim Bay, WA) in flowthrough-columns. The performance of the adsorbents after exposure in natural seawater was consistent with the laboratory screening results and the AF1 samples amidoximated in DMSO at 70 °C for 3 h resulted in the highest uranium adsorption capacity (5.00 ± 0.15 g U/kg-ads) with much faster adsorption kinetics compared to AF1 adsorbent amidoximated in water-methanol solution. The other novel fiber adsorbents were prepared by ATRP of 2-hydroxyethyl acrylate and acrylonitrile. The composition of grafted chain, i.e., varied comonomer ratio in simultaneous grafting with AN, had a significant impact on the uranium adsorption capacity, indicating the optimum reaction conditions to prepare high-performance fiber adsorbents. The optimized ATRP adsorbent achieved 6.56 ± 0.33 g U/kg-ads., after 56 days of seawater exposure. With this capacity, the estimated uranium production cost ranges from $420-860/kg or $370-760/kg depending on the assumptions. These values are so far the best cost estimates reported. In the case of a constant 5% adsorbent degradation rate, the optimal soaking campaign is around 50 days, while the worst case degradation optimizes closer to 10 days of exposure. The cost decrease, roughly 10%, is predominantly a result of the improvement in adsorbent capacity, as this has been previously identified as one of the most significant cost drivers. The study successfully demonstrated new fiber materials with sorption capacity at 5.0 g-U/kg adsorbent under marine testing conditions. Further optimization, investigation of other new materials as well as deepening our understanding will develop adsorbents that have even higher uranium adsorption capacity, increased selectivity, and faster kinetics.

publication date

  • 2016

Identity

report identifier

  • ORNL/LTR-2016/128