The recovery of uranium (U) from seawater has been investigated for over six decades in efforts to secure uranium sources for future energy production. The majority of the research activities have focused on inorganic materials, chelating polymers, and nanomaterials. Previous studies of uranium adsorption from aqueous solutions, mainly seawater, are reviewed here with a focus on various adsorbent materials, adsorption parameters, adsorption characterization, and marine studies. Continuous progress has been made over several decades, with adsorbent loadings approaching 3.2 mg U/g adsorbent in equilibrium with seawater. Further research is needed to improve first, the viability including improved capacity, selectivity, and kinetics, and second, the sorbent regeneration for multicycle use. An overview of the status of the uranium adsorption technology is provided and future research needs to make this technology commercially competitive are discussed.

Recovery of Uranium from Seawater: A Review of Current Status and Future Research Needs

Removal of chromium from electroplating industry effluents by ion exchange resins

Resources and the environment are two eternal themes of social development. Nuclear energy, a green source with high energy density, can greatly alleviate the pressure of the energy crisis in today's society. To guarantee the long-term sufficient supply of nuclear fuel, mining seawater uranium is imperative. Meanwhile, the great threats of uranium to ecological security and human health make the removal of uranium from the environment urgent. To achieve these ends, a large number of materials with specific functions have been born as a result. Among them, amidoxime-based materials serve as one of the most promising candidates and are the main tool used for uranium extraction from aqueous systems owing to their special affinity for uranium. However, there is still huge room for improvement in amidoxime-based materials in terms of their economic efficiency and performance. In this paper, we provide a comprehensive review of amidoxime-based materials for uranium recovery and removal, including synthesis strategies, characterization and types of amidoxime-based materials, the factors that influence uranium extraction, and the binding mechanisms between amidoxime ligands and uranyl ions, as well as the cost drivers in applications. Meanwhile, the shortcomings of current research as well as future development directions and research hotspots are also pointed out. Based on the in-depth analysis of the currently available literature, a demand-oriented strategy for fabricating a new generation of amidoxime-based adsorbents was proposed, and means to enhance the adsorbent performance were discussed with regards to four aspects, including adsorption capacity, selectivity, kinetics and regenerability. This paper aims to provide guidance for the purposeful design of novel amidoxime-based materials, and to provide advice on circumventing unfavorable factors and solving the technical problems relating to uranium recovery and removal.

Amidoxime-based materials for uranium recovery and removal

The adsorption of uranium (VI), cesium and strontium ions from aqueous solutions onto a commercial activated carbon obtained by physical activation of coconut shell has been studied in batch systems. In particular the adsorption of uranium, studied as a function of contact time and metal ion concentration, followed pseudo-first-order kinetics. Equilibrium adsorption data were fitted by Langmuir and Freundlich isotherm models and the maximum adsorption capacity of the activated carbon resulted to be 55.32 mg/g. The study showed that the considered activated carbon could be successfully used for uranium adsorption from aqueous solutions. Feasibility of cesium and strontium adsorption onto the same activated carbon has been also investigated. Results showed that no affinities with both of these ions exist.

Adsorption of uranium, cesium and strontium onto coconut shell activated carbon

Integration of ion exchange and electrodeionization as a new approach for the continuous treatment of hexavalent chromium wastewater

A number of lightly cross-linked poly(acrylonitrile-co-divinylbenzene) beads (RN-5) have been synthesized by suspension polymerization. The use of solvating diluents such as chloroform, dichloroethane, and tetrachloroethane resulted in copolymer beads having highly porous structures. The chelating resins containing amidoxime as a functional group (RNH-5) have been prepared by the reaction of copolymer beads with 3% hydroxylamine in methanol. A detailed analysis is made of the pore structure of lightly cross-linked copolymers of acrylonitrile–divinylbenzene and their amidoxime derivatives in the anhydrous state including pore-size distribution, specific surface area, and pore structure in the aqueous media by means of gel permeation chromatography (GPC). A set of experiments have been performed to ascertain the potential of the resins for the adsorption of uranium from seawater. Because of their modified pore structures, the chelating resins exhibited a marked adsorption rate for uranium in seawater as high as 23 μg of U/cm3 of resin/day without alkaline treatment.

Recovery of uranium from seawater. XII. Preparation and characterization of lightly crosslinked highly porous chelating resins containing amidoxime groups

Large-scale adsorption/elution cycles were performed to investigate the long-term stability of the chelating resins employed The adsorbed metal ions were rapidly and quantitatively eluted from the resins with acid eluants The shrinkage of the resins with successive adsorption/elution cycles influenced the adsorption capacity The uranium recovery was maintained at a nearly constant value by the employment of bicarbonate eluants In particular, 2 mol dm[sup [minus]3] NH[sub 4]HCO[sub 3] yielded an efficient stripping for uranium However, it was clarified that the elution with 025 mol dm[sup [minus]3] H[sub 2]SO[sub 4], which gave a high efficiency, was better than the bicarbonate eluants

Recovery of uranium from seawater. 13. Long-term stability tests for high-performance chelating resins containing amidoxime groups and evaluation of elution process

In order to design the amidoxime resins (RNH) suitable for circulating fluidized bed adsorbers, RNH were prepared from precursory acrylonitrile-divinylbenzene copolymer beads of different particle sizes, and chemical and physical properties of the resulting RNH were evaluated. Specific surface areas, pore structures, swelling ratios, and anion and cation-exchange capacities of RNH are little affected by the particle size, while their sedimentation velocities in water increase with an increase in particle size as expected from fluid dynamics. Although the uptake of uncomplexed uranyl ion from a uranyl nitrate solution (0.01 M) was not influenced by the particle size, the uranium uptake from seawater decreases with an increase in the particle size, indicating that the particle diffusion of the bulky complexed species UO[sub 2](CO[sub 3])[sub 3][sup 4[minus]] essentially controls the overall adsorption rate in the recovery of uranium from seawater.

Recovery of uranium from seawater; 15: Development of amidoxime resins with high sedimentation velocity for passively driver fluidized bed adsorbers

The poly(styrene-co-divinylbenzene) beads were synthesized by suspension polymerization using benzoylperoxide as an initiator and i -octane as a diluent. The copolymer beads with a particle size range of 32-60 mesh were selected. Chloromethylation was performed with chloromethylether at 0°C using tetrachloroethane as swelling agent and AlCl 3 as catalyst. The phosphorylation of copolymer beads (RS) and its chloromethylated derivative (RCS) was carried out using a mixture of PCl 3 and AlCl 3 under the reflux. The phosphorylated resins (RSP) were further oxidized using concentrated HNO 3 to obtain the oxidized form of RSP resins (RSPO). The removal of Cr(III) by the resins RSP, RSPO, and RCSP containing phosphinic, phosphonic, and methylenephosphonic groups, respectively, was investigated by batch and column methods. The results were compared with that of Diaion CRP-200 resin having methylenephosphonic functional groups. These resins exhibited a high affinity for Cr(III). The percent removal of Cr(III) reac...

CR(III) removal by macroreticular chelating ion exchange resins

In order to evaluate performances of lightly cross-linked highly porous amidoxime resins in uranium-adsorption systems utilizing natural seawater motions, uranium uptake by the resins from seawater was studied by different approaches, such as simulated sea current exposure tests, towing trials, and/or mooring trials. In general, the efficiency of uranium uptake became higher with a decrease in the thickness of packing layers, indicating important roles of fluidization of the resin particles. On the basis of these fundamental data, mooring tests in the natural sea current were designed and conducted. By mooring flat adsorption beds (base area 260 cm[sup 2], height 3.0 cm) packed with 780 ml of the resin for 40 h, promising uranium uptake as high as 44 mg/kg of resin (9.9 mg/l of resin) was achieved under sea conditions in which the velocity of sea currents and the vertical velocity of waves were 5.5-49.7 cm/s and 3.4-27 cm/s, respectively.

Recovery of uranium from seawater. 14. System arrangements for the recovery of uranium from seawater by spherical amidoxime chelating resins utilizing natural seawater motions

Taketomi Shuto

Papers

Recovery of uranium from seawater. XII. Preparation and characterization of lightly crosslinked highly porous chelating resins containing amidoxime groups

Recovery of uranium from seawater. 13. Long-term stability tests for high-performance chelating resins containing amidoxime groups and evaluation of elution process

Recovery of uranium from seawater; 15: Development of amidoxime resins with high sedimentation velocity for passively driver fluidized bed adsorbers

CR(III) removal by macroreticular chelating ion exchange resins

Recovery of uranium from seawater. 14. System arrangements for the recovery of uranium from seawater by spherical amidoxime chelating resins utilizing natural seawater motions