Electrosorption of uranium on carbon fibers as a means of environmental remediation

Uranium-containing aqueous wastes have been treated by electrosorption on a carbon electrode composed of vapor-grown fibers in a continuous flow-through cell. Effective uranium (VI) removal is accomplished when a negative potential in the range of -0.45 to -0.9 V (vs. Ag/AgCl) is applied to the carbon electrode. For a feed concentration of 100 mg/l, the concentration of U(VI) in the cell effluent is reduced to less than 100 mug/l. The adsorbed uranium is stripped from the carbon fiber by passing a 0.1 M KNO3 solution through the cell and applying a positive potential on the electrode. Almost all of the stripped uranium is removed as a suspended precipitate and recovered in solid form by filtration. A sorption capacity over 1.20 g(uranium)/g(carbon) is reached. The electro-adsorbed uranium is mainly in the form of uranyl hydroxide (UO3 . H2O), indicating very limited reduction of U(VI) to U(IV) and precipitation of U(IV). It is proposed that ion exchange and double layer charging are the dominant mechanisms for electrosorption of uranium at potentials less negative than -0.3 V, whereas surface-induced precipitation of uranyl hydroxide (UO3 . H2O) occurs at more negative potentials, thereby greatly enhancing the sorption capacity. (C) 2000 Elsevier Science B.V. All rights reserved.