Technetium-99 (99Tc) is one of the most-problematic radioisotopes in used nuclear fuel owing to its intrinsic features of a high fission yield, long half-life, high environmental mobility, volatile nature during waste vitrification, and its redox interface capability with actinides during used fuel repossessing. The selective separation of pertechnetate (TcO4 –) from legacy nuclear waste and contaminated natural water is therefore highly desirable but still a significant challenge, because the conditions of a strong radiation field, high ionic strength, high acidity/alkalinity, and large amounts of competing anions are often involved in these systems. Until now, there are a handful of functional materials that can efficiently remove TcO4 – from nuclear waste solutions with high uptake capacities, fast kinetics, and good selectivity, but room still remains to further improve our capabilities for controlling the contamination/separation of TcO4 –. In this Perspective, we discuss the current state of the art TcO4 – separation materials including precipitation agents, reducing materials, ion-exchange resins, inorganic cationic frameworks, cationic metal–organic frameworks (MOFs), and cationic polymeric networks (CPNs) materials. The intriguing separation mechanisms of these materials for TcO4 – are also disclosed, which may hopefully shed light on further development in this field.

The nano-MOF+ technique was employed by assembling nanoporous metal-organic framework (MOF) UiO-66 with nanoscale zero-valent iron (ZVI) particles to remove uranyl ions from aqueous solution under anoxic condition for the first time. The synthesized composite of Fe0@UiO-66-COOH exhibits a synergic effect between uranyl sorption by MOF host of UiO-66-COOH and chemical reduction by ZVI, reaching much elevated removal capacity and rate in comparison to those of the pristine UiO-66-COOH. The combined complexation and reduction mechanisms are further elucidated by the synchrotron radiation X-ray absorption near-edge structure analysis. This work highlights the bright future of the nano-MOF+ technique in the application of uranium removal, especially for the remediation of the uranium-contaminated subsurface environment.

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The silica-based CeO 2 adsorbent (CeO 2 /SiO 2) was prepared for removing fluoride from the aqueous solution. The synthesized adsorbent was characterized by scanning electron microscope, energy dispersive spectrum, X-ray diffractometer, Fourier transform infrared spectrometer, and zeta potential measurement analyses. The adsorption batch experiments in the various experimental conditions including solution pH, contact time, initial fluoride concentration, and adsorption temperature were performed and investigated. The maximum adsorption capacity of fluoride into CeO 2 /SiO 2 was 2.441 mmol/g at pH 3 and 298 K. The adsorption kinetics and isotherms were well described by the pseudo-second-order model and the Langmuir model, respectively. The fluoride adsorption reached the equilibrium in 15 min from the aqueous solution with the initial fluoride concentration of 400 mg/l at 298 K. In the temperature range of 298-338 K, the maximum adsorption capacity of fluoride decreased from 2.441 mmol/g to 2.109 mmol/l at pH 3. The adsorption thermodynamics study revealed that this process was a spontaneous, exothermic, and entropy-driving adsorption. Furthermore, the mechanism of adsorption was identified as the anion exchange and the electrostatic interaction. The desorption efficiency of fluoride-loaded CeO 2 /SiO 2 adsorbent could reach about 95% by 0.1 mol/l NaOH.

For the adsorption and recovery of rare earth elements from aqueous nitric acid, solid-phase extraction resins were prepared by impregnating and immobilizing of bis(2-ethylhexyl)phosphoric acid extractant into the macroporous silica-based polymeric (SiO 2-P) particles. It was found that bis(2-ethylhexyl)phosphoric acid/SiO 2-P had higher adsorption distribution coefficient for heavy rare earth elements than for light rare earth elements. The adsorption capacity of Gd(III) was observed to be 0.315 mmol g À1 by bis(2-ethylhexyl)phosphoric acid/SiO 2-P in 0.1 M HNO 3 at 298 K, which increased slightly when increasing temperature from 298 to 323 K. The adsorption isotherms of Gd(III) matched well with the Langmuir and Freundlich models. The obtained thermodynamic parameters (ÁH o and ÁG o) showed that the adsorption of Gd(III) by bis(2-ethylhexyl)phosphoric acid/SiO 2-P was a spontaneous and exothermic process. This study also evaluated the chemical stability of bis(2-ethylhexyl)phosphoric acid/SiO 2-P treated with nitric acid at different temperatures and demonstrated that bis(2-ethylhexyl)phosphoric acid/SiO 2-P had considerable stability against nitric acid and heat.

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