Enhancing Uranium Uptake by Amidoxime Adsorbent in Seawater: An Investigation for Optimum Alkaline Conditioning Parameters

Abstract

A high-surface-area polyethylene-fiber adsorbent (AF160-2) has been developed at the Oak Ridge National Laboratory by radiation-induced graft polymerization of acrylonitrile and itaconic acid. The grafted nitriles were converted to amidoxime groups by treating with hydroxylamine. The amidoximated adsorbents were then conditioned with potassium hydroxide (KOH) by varying different reaction parameters such as KOH concentration (0.2, 0.44, and 0.6 M), duration (1, 2, and 3 h), and temperature (60, 70, and 80°C). Adsorbent screening was then performed with simulated seawater solutions containing sodium chloride and sodium bicarbonate, at concentrations found in seawater, and uranium nitrate at a uranium concentration of ∼7−8 ppm and pH 8. Fourier transform infrared spectroscopy and solid-state NMR analyses indicated that a fraction of amidoxime groups was hydrolyzed to carboxylate during KOH conditioning. The uranium adsorption capacity in the simulated seawater screening solution gradually increased with conditioning time and temperature for all KOH concentrations. It was also observed that the adsorption capacity increased with an increase in concentration of KOH for all the conditioning times and temperatures. AF160-2 adsorbent samples were also tested with natural seawater using flow-through experiments to determine uranium adsorption capacity with varying KOH conditioning time and temperature. Based on uranium loading capacity values of several AF160-2 samples, it was observed that changing KOH conditioning time from 3 to 1 h at 60, 70, and 80°C resulted in an increase of the uranium loading capacity in seawater, which did not follow the trend found in laboratory screening with stimulated solutions. Longer KOH conditioning times lead to significantly higher uptake of divalent metal ions, such as calcium and magnesium, which is a result of amidoxime conversion into less selective carboxylate. Scanning electron microscopy showed that long conditioning times may also lead to adsorbent degradation. ■ INTRODUCTIONCapturing uranium from seawater is a challenging task that requires consideration of chemical, transport, and process design aspects. The concentration of uranium in seawater is on the order of 1.4 × 10 −8 mol L −1 (3.3 ppb) and is the anionic triscarbonato-uranate (VI) [UO 2 (CO 3 ) 3 ] 4− species under the prevailing conditions. 1 The major problems pertaining to the development of a suitable uranium recovery system from seawater are due to the low concentration, the stability of triscarbonato uranate (VI), and the large excess of competing ions. 1−3 Triscarbonato uranate (VI) has a high stability constant that limits the choice of functional groups for uranium recovery from seawater. Poly(acrylamidoxime) (pAO) has been found to be chemically suitable for uranium recovery from seawater. 1−20 However, the major challenge for making uranium recovery economically viable is developing an adsorbent that has a high uranium sorption rate and a high uptake capacity.For this current study, pAO adsorbents have been ...

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