RESEARCHERS have synthesised fibres grafted with a highly-adsorbing chemical group, that can be deployed in seawater to attract uranium dioxide (uranyl) to its surface.
A multinational and multidisciplinary team led by the Oak Ridge National Laboratory (ORNL) and the Pacific Northwest National Laboratory (PNNL), US used a chemical group called amidoxime, a chemical compound belonging to the imines, with the general formula R1R2C=NOH, that is selective towards uranium. The team used braids of polyethylene fibres as a base and grafted them with amidoxime, converted from acrylonitrile, to create a uranium-adsorbing material.
The fibre strands are deployed in seawater at laboratory scale where uranyl ions are attracted and form a complex with amidoxime. Once the process is complete, the fibres are collected and subjected to an acidic treatment that releases the uranyl ions, allowing the fibres to be reused. The recovered uranium can be processed and enriched to use as nuclear fuel.
In the team’s experiments, they were able to hold 5.2 g of uranium per kilogram of adsorbent material over a 49-day period of natural seawater exposure.
The team say this method would be a less damaging way to extract uranium than mining, as the oceans contain enough uranium dioxide to meet global energy needs for 10,000 years. They say there is limited risk to the ocean as the ions are attracted to the material naturally, and would not require expensive water pumping.
Gary Gill, deputy director of PNNL's Coastal Sciences Division said: 'We assessed how well the adsorbent attracted uranium versus other elements, adsorbent durability, whether build-up of marine organisms might impact adsorbent capacity, and we demonstrated that most of the adsorbent materials are not toxic.”
The team says challenges remain in that the material also attracts other metal ions such as vanadium, which is also useful, as well as copper and iron ions, which are undesirable to the team.
Also, there are challenges from the environment to be considered, for example uranyl adsorption is favoured at relatively high seawater temperatures, salinity and uranium concentrations, and the velocity of the seawater through the braided fibres must remain higher than a few cm/s.
Despite the challenges for this technology, “there are a lot of locations where the seawater current is sufficient,“ Costas Tsouris, a chemical engineer at ORNL’s Environmental Sciences Division, told The Chemical Engineer:
The next step for the team will be to further refine the adsorbing materials to reach higher capacities for uranium capture. Recent testing using modifications yielded 6 g of uranium per kilogram of adsorbent after 56 days in natural seawater. Costas said the work will continue at laboratory scale for the foreseeable future.
“There are no plans for large-scale demonstration in the near future,” he added.
Industrial & Engineering Chemistry Research, DOI: 10.1021/acs.iecr.6b01293
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