Highly efficient removal of metal ion pollutants, such as toxic and nuclear waste-related metal ions, remains a serious task from the biological and environmental standpoint because of their harmful effects on human health and the environment. Recently, highly porous metal–organic frameworks (MOFs), with excellent chemical stability and abundant functional groups, have represented a new addition to the area of capturing various types of hazardous metal ion pollutants. This review focuses on recent progress in reported MOFs and MOF-based composites as superior adsorbents for the efficient removal of toxic and nuclear waste-related metal ions. Aspects related to the interaction mechanisms between metal ions and MOF-based materials are systematically summarized, including macroscopic batch experiments, microscopic spectroscopy analysis, and theoretical calculations. The adsorption properties of various MOF-based materials are assessed and compared with those of other widely used adsorbents. Finally, we propose our personal insights into future research opportunities and challenges in the hope of stimulating more researchers to engage in this new field of MOF-based materials for environmental pollution management.

Metal–organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions

More than 1000× uranium exists in the oceans than exists in terrestrial ores. With nuclear power generation expected to increase over the coming decades, access to this unconventional reserve is a matter of energy security. With origins in the mid-1950s, materials have been developed for the selective recovery of seawater uranium for more than six decades, with a renewed interest in particular since 2010. This review comprehensively surveys materials developed from 2000–2016 for recovery of seawater uranium, in particular including recent developments in inorganic materials; polymer adsorbents and related research pertaining to amidoxime; and nanostructured materials such as metal–organic frameworks, porous-organic polymers, and mesoporous carbons. Challenges of performing reliable and reproducible uranium adsorption studies are also discussed, as well as the standardization of parameters necessary to ensure valid comparisons between different adsorbents.

Materials for the Recovery of Uranium from Seawater

Metal–organic frameworks (MOFs) have recently been receiving increasing attention in various scientific fields, including nuclear industry, due to their unique properties. In this work, the acid-resistant chromium-based MOF, MIL-101, and its amino derivatives were prepared to explore their potential usage in separation, removal and/or recovery of radionuclides from aqueous solutions. The synthesized MIL-101-NH2, MIL-101-ED (ED = Ethanediamine), and MIL-101-DETA (DETA = Diethylenetriamine) were characterized by X-ray diffraction spectrometry (XRD), infrared spectrometry (IR), N2 adsorption–desorption measurements, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA), which confirm the successful modification of amino groups and the preservation of porous structures. The sorption performances of these materials toward U(VI) from an aqueous solution were investigated in detail. It was found that all the amine-grafted MOFs were highly efficient in capturing U(VI) compared to raw MIL-101. The sorption capacity of these MOFs for U(VI) sorption follows the order of MIL-101-DETA > MIL-101-ED > MIL-101-NH2 > MIL-101, in which MIL-101-DETA possesses the highest sorption capacity of 350 mg g−1 at pH ∼5.5. Moreover, the sorbed U(VI) can be easily desorbed by lowering the pH (pH ≤ 3.0), and the prepared materials also display a desirable selectivity toward U(VI) in a solution containing a range of competing ions. Based on the FTIR and EXAFS characterizations, the sorption mode of U(VI) onto MOFs is fully discussed. This work promises to provide a facile approach for developing acid-resistant MOFs toward a highly efficient and selective extraction of radionuclides from aqueous solutions.

Introduction of amino groups into acid-resistant MOFs for enhanced U(VI) sorption

Emerging natural and tailored materials for uranium-contaminated water treatment and environmental remediation

Efficient nuclear waste treatment and environmental management are important hurdles that need to be overcome if nuclear energy is to become more widely used. Herein, we demonstrate the first case of using two-dimensional (2D) multilayered V2CTx nanosheets prepared by HF etching of V2AlC to remove actinides from aqueous solutions. The V2CTx material is found to be a highly efficient uranium (U(VI)) sorbent, evidenced by a high uptake capacity of 174 mg g–1, fast sorption kinetics, and desirable selectivity. Fitting of the sorption isotherm indicated that the sorption followed a heterogeneous adsorption model, most probably due to the presence of heterogeneous adsorption sites. Density functional theory calculations, in combination with X-ray absorption fine structure characterizations, suggest that the uranyl ions prefer to coordinate with hydroxyl groups bonded to the V-sites of the nanosheets via forming bidentate inner-sphere complexes.

Loading Actinides in Multilayered Structures for Nuclear Waste Treatment: The First Case Study of Uranium Capture with Vanadium Carbide MXene

Uranium is one of the most hazardous heavy metal due to its long half-life radioactivity, high toxicity and mobility as aqueous uranyl ion (UO22+) under ordinary environmental conditions. Herein, amino functionalized SBA-15 (APSS) was developed as a rapid and efficient sorbent for removal of U(VI) from the environment. The APSS sample was synthesized by grafting method and was characterized by SEM, NMR, SAXS, and N-2 sorption/desorption isothermal experiments. The sorption of U(VI) by APSS was investigated under different conditions of pH, contact time, initial U(VI) concentration, ionic strength and solid-liquid ratio. The results show that the sorption of U(VI) by APSS is strongly dependent on pH but independent of ionic strength and solid-liquid ratios (m/V). The sorption is ultrafast with an equilibrium time of less than 30 min, and the sorption capacity is as large as 409 mg/g at pH 5.3 +/- 0.1. Besides, the U(VI) sorption by APSS from extremely diluted solution and the desorption of U(VI) from APSS were also studied. It is found that 100 mg of APSS can almost completely remove the U(VI) ions from 4 L aqueous solution with the U(VI) concentration as low as 4.2 ppb and the sorbed U(VI) can be completely desorbed by 0.1 mol/L nitric acid. The results strongly reveal the high performance of the APSS material in the removal and preconcentration of U(VI) from the aqueous solution.

A high efficient sorption of U(VI) from aqueous solution using amino-functionalized SBA-15

Mesoporous silicas have a very attractive ability of sorption and enrichment of metal ions due to their huge surface area and facile functionalization by organic ligands. In this work, phosphonate-amino bifunctionalized mesoporous silica SBA-15 (PA-SBA-15) as U(VI) sorbent was fabricated through post-grafting method. The obtained mesoporous silica was characterized by SEM, XRD, NMR and nitrogen sorption/desorption experiments, which revealed the existence of ordered mesoporous structure with uniform pore diameter and large surface area. The adsorptivity of PA-SBA-15 for U(VI) from aqueous solution was investigated using batch sorption technique under different experimental conditions. The preliminary results show that the U(VI) sorption by PA-SBA-15 is very quick with equilibrium time of less than 1 h, and the U(VI) uptake is as large as 373 mg/g at pH 5.5 under 95 °C. The sorption isotherm has been successfully modeled by the Langmuir isotherm, suggesting a monolayer homogeneous sorption of U(VI) in PA-SBA-15. The sorption is pH-dependent due to the pH-dependent charge of sorbent in the aqueous solution. The thermodynamics research shows that the sorption is a feasible and endothermic process. Based on these results, PA-SBA-15 could be a promising solid phase sorbent for highly-efficient removal of U(VI) ions from waste water and enrichment of U(VI) from a solution at a very low level.

Mesoporous silica SBA-15 functionalized with phosphonate and amino groups for uranium uptake

Electroseparation of thorium from ThO2 and La2O3 by forming Th-Al alloys in LiCl-KCl eutectic

Diffusion Coefficient of Ho3+at Liquid zinc Electrode and Co-reduction Behaviors of Ho3+ and Zn2+ on W Electrode in the LiCl-KCl Eutectic

This work presents a comparative study of electrochemical properties of Gd3+ on W and Zn electrodes in LiCl-KCl eutectic. It was found that Gd3+ reduction on a tungsten cathode occurs in a single step pattern. Relevant kinetic properties of the Gd3+/Gd redox reaction were investigated by cyclic voltammetry (CV). The reduction of Gd3+ at a liquid zinc electrode in LiCl-KCl-GdCl3 melt and a film zinc electrode in LiCl-KCl-GdCl3-ZnCl2 eutectic were also studied at 773 K. The diffusion coefficient of Gd3+ at the liquid zinc electrode was calculated to be 5.44 x10(-5) cm(2)s(-1) in LiCl-KCl-GdCl3 melt. The standard rate constants for the charge transfer of Gd3+ at the liquid zinc electrode or on the tungsten electrode was calculated to be about 1 x10(-2) cms(-1) at 773 K by Nicholson method, revealing the redox reaction of Gd3+ to Gd is quasi-reversible based on the Matsuda-Ayabe criteria for practical concept of electrochemical reversibility. Electrodeposition of Gd3+ was carried out on both the film zinc electrode and the liquid zinc electrode. X-ray diffraction (XRD) results showed that intermetallic compounds GdZn12, Gd2Zn17, Gd13Zn58 and GdZn were obtained on the film zinc electrode, whilst intermetallic compound GdZn12 was gained at the liquid zinc electrode. The morphological properties of all the intermetallic compounds were characterized by scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). (c) 2015 The Electrochemical Society. All rights reserved.

Yi-Xiao Feng

Papers

A high efficient sorption of U(VI) from aqueous solution using amino-functionalized SBA-15

Mesoporous silica SBA-15 functionalized with phosphonate and amino groups for uranium uptake

Electroseparation of thorium from ThO2 and La2O3 by forming Th-Al alloys in LiCl-KCl eutectic

Diffusion Coefficient of Ho3+at Liquid zinc Electrode and Co-reduction Behaviors of Ho3+ and Zn2+ on W Electrode in the LiCl-KCl Eutectic

Electroreduction of Gd3+ on W and Zn Electrodes in LiCl-KCl Eutectic: A Comparison Study