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

doi:10.1016/S0378-3820(00)00114-4

Fuel Processing Technology

Volume 68, Issue 3, December 2000, Pages 189-208

https://doi.org/10.1016/S0378-3820(00)00114-4 Get rights and content

Abstract

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 μg/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.

Introduction

A variety of physical, chemical and biological methods have been proposed for the removal and recovery of uranium (VI) from contaminated water and waste streams. Methods, such as microbial reduction/precipitation, coagulation, lime-softening, ion exchange, adsorption on activated alumina or activated carbon, have been successful in efficient U(VI) removal [17]. However, many of these techniques have limited application because of their limited capacity, especially when the concentration of U(VI) in the waste is relatively high [11].

Electrodeposition of metals using porous carbon electrodes has been extensively investigated. Electrosorption may play an important role in treatment of metal-bearing wastes because dissolved and hazardous metals, once deposited, can be considered as a resource to be recovered quantitatively and potentially marketed [5]. Earlier results showed that the deposition of copper, lead, cadmium and nickel on a variety of carbon materials, including carbon nanofiber monoliths, coal-derived carbon foams, etc., was very effective [1]. For some metals like uranium and strontium, however, electrodeposition is not a practical approach because of the high reduction potential of these particular cations.

An alternative to electrodeposition is electrosorption, that is, adsorption of the metal cations onto a negatively charged carbon surface. The negative potential exerted on the carbon surface is not enough to reduce the metal to the elemental state, but it can attract and adsorb cations much more efficiently than the naturally formed negative surface charge caused by dissociation of the oxygen-containing surface groups. Thus, the adsorption capacity is greatly enhanced for the charged surface. Electrosorption takes advantage of a combination of the high adsorption capacity and the electrical conductivity of graphitic carbons. It has been suggested as a minimally polluting, energy-efficient, and potentially cost-effective alternative to ion exchange, reverse osmosis, electrodialysis, and evaporation [2].

Electrosorption of metal ions on carbon electrodes has been studied mainly for the purpose of water demineralization [8]. No parallel investigations on U(VI) removal from aqueous wastes by electrosorption have been found in the literature. An approach similar to electrosorption is biological adsorption and reduction of U(VI). A number of metal-reducing bacteria have been used to adsorb and reduce dissolved uranium to insoluble U(IV) and remove it from aqueous solution [4], [12], [13], [16]. Biological reduction of U(VI) to U(IV) was carried out under anoxic conditions and in the presence of appropriate electron donors (organic acids or H₂) [13].

In this study, the electrosorption technique was examined for the treatment of U(VI) in aqueous solution by using a continuous flow-through cell packed with a novel form of carbon material, vapor-grown carbon fiber.

Section snippets

Carbon fibers

A series of vapor-grown carbon fibers was provided by Applied Sciences Inc. (ASI), Cedarville, OH. These fibers are produced by a catalytic vapor-deposition process that allows for control of the fiber dimensions, i.e., filament length and diameter. Lengths are available from 1 mm to tens of centimeters and diameters can vary from under 0.2 to over 100 μm. These carbon fibers are extremely light. Their apparent density is around 0.0009 mg/cm³ if no subsequent treatment is made and can be raised

Electrosorption at various potentials

Electrosorption of U(VI) onto the ASI (oxidized) carbon fiber was carried out at various potentials. The uranium concentration in the effluent samples from these tests is shown in Fig. 3. It can be seen that U(VI) is effectively removed from a 100 mg/l uranium feed solution by a single pass through the electrochemical cell. At a potential of −0.6 V, for instance, the U(VI) concentration in the effluent sample is reduced to 150 μg/l in 2 h. This corresponds to a 99.85% removal of uranium from

Conclusions

From this study, the following conclusions can be drawn.

(1) Uranium (VI) in aqueous wastes is effectively removed by electrosorption onto an electrode made of vapor-grown carbon fibers. Under suitable potentials (e.g., −0.45 to −0.9 V), up to 99.9% of uranium is removed from a 100 mg/l feed solution by a single pass through the electrochemical remediation cell. A sorption capacity of over 1.20 g_uranium/g_carbon is observed.

(2) The adsorbed uranium is easy to strip and recover in solid form by

Acknowledgements

This work was supported by a grant from Morgantown Energy Technology Center (under the WVU-DOE Cooperative Agreement, Project MC-17, Contract No. DE-FC21-92MC292467). We thank Dr. Robert Alig of ASI for supplying the carbon fiber samples.

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

Abstract

Introduction

Section snippets

Carbon fibers

Electrosorption at various potentials

Conclusions

Acknowledgements

Colloids Surf.

Water Res.

Energ. Fuels

Energ. Fuels

J. Radioanal. Chem.

Environ. Technol.

J. Appl. Electrochem.

Radiochim. Acta

Cation Adsorption by Hydrous Metal Oxides and Clay in Adsorption of Inorganics at the Solid–Liquid Interfaces