Microcrystalline Hexagonal Tungsten Bronze. 1. Basis of Ion Exchange Selectivity for Cesium and Strontium

Mechanism and Kinetics of the Phase Formation and Dissolution of NaxWO3 on a Pt Electrode in a Na2WO4-WO3 Melt

A comprehensive study concerning the phase formation mechanism and growth/dissolution kinetics of sodium tungsten bronze crystals during the electrolysis of a 0.8Na2WO4-0.2WO3 melt was carried out. The regularities of deposit formation on a Pt(111) working electrode were investigated experimentally using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and X-ray diffraction analysis. Models have been developed to calculate the current response during the formation, growth and dissolution of a two-phase deposit consisting of NaxWO3 and metallic tungsten or two oxide tungsten bronzes with different sodium content. These models consider mass transfer to the electrode and nuclei; chemical and electrochemical reactions with the participation of polytungstate ions, Na+, Na0, and O2-; as well as the ohmic drop effect. The approach was proposed to describe the dissolution of an NaxWO3 crystal with a nonuniform sodium distribution. The fitting of cyclic voltammograms was performed using the Levenberg-Marquardt algorithm. The NaxWO3 formation/growth/dissolution mechanism was determined. Concentration profiles and diffusion coefficients of [WnO3n]-, reaction rate constants, number density of nuclei, and time dependencies of crystal size were calculated. The proposed approaches and models can be used in other systems for the cyclic voltammogram analysis and study of the mechanism and kinetics of electrode processes complicated by phase formation; parallel and sequential electrochemical and chemical reactions; as well as the formation of a deposit characterized by a nonuniform phase and/or chemical composition.

Efficient Co-Adsorption and Highly Selective Separation of Cs+ and Sr2+ with a K+ -Activated Niobium Germanate by the pH Control

¹³⁷ Cs and ⁹⁰ Sr are hazardous to ecological environment and human health due to their strong radioactivity, long half-life, and high mobility. However, effective adsorption and separation of Cs⁺ and Sr²⁺ from acidic radioactive wastewater is challenging due to stability issues of material and the strong competition of protons. Herein, a K⁺ -activated niobium germanate (K-NGH-1) presents efficient Cs⁺ /Sr²⁺ coadsorption and highly selective Cs⁺ /Sr²⁺ separation, respectively, under different acidity conditions. In neutral solution, K-NGH-1 exhibits ultrafast adsorption kinetics and high adsorption capacity for both Cs⁺ and Sr²⁺ (q_m ^Cs  = 182.91 mg g^-1 ; q_m ^Sr  = 41.62 mg g^-1 ). In 1 M HNO₃ solution, K-NGH-1 still possesses q_m ^Cs of 91.40 mg g^-1 for Cs⁺ but almost no adsorption for Sr²⁺ . Moreover, K-NGH-1 can effectively separate Cs⁺ from 1 M HNO₃ solutions with excess competing Sr²⁺ and M^{n +} (M^{n +}  = Na⁺ , Ca²⁺ , Mg²⁺ ) ions. Thus, efficient separation of Cs⁺ and Sr²⁺ is realized under acidic conditions. Besides, K-NGH-1 shows excellent acid and radiation resistance and recyclability. All the merits above endow K-NGH-1 with the first example of niobium germanates for radionuclides remediation. This work highlights the facile pH control approach towards bifunctional ion exchangers for efficient Cs⁺ /Sr²⁺ coadsorption and selective separation.

Ultrafast removal of radioactive strontium ions from contaminated water by nanostructured layered sodium vanadosilicate with high adsorption capacity and selectivity

Layered sodium vanadosilicates was synthesized for removing soluble strontium ion (Sr²⁺) from simulated radioactive wastewater. SEM, EDS and TEM images indicated that layered sodium vanadosilicates had a nanostructure with flake-like shape providing an extremely large surface area. XRD, FTIR and XPS analyses further revealed a framework structure of adsorbent consisting of corner-sharing SiO₄ tetrahedra and VO₆ octahedra, with sodium ion being the major cation in the synthesized nanostructured layered sodium vanadosilicate. This study for the first time showed ultrafast adsorption of Sr²⁺ by nanostructured layered sodium vanadosilicates in a time frame of seconds with a high adsorption capacity of 174.3 mg/g estimated from Langmuir isotherm. It was found that about 99.0% of Sr²⁺ at an initial concentration of 5.00 mg/L and adsorbent dosage of 0.5 g/L could be removed within several seconds. The kinetic analysis further revealed that pseudo-second-order instead of pseudo-first-order kinetics could satisfactorily describe the observed ultrafast removal of Sr²⁺. In particular, the nanostructure layered sodium vanadosilicates exhibited an excellent affinity to Sr²⁺ over a wide pH range of 3-11. It was also demonstrated that the working mechanisms of nanostructured layered sodium vanadosilicates for Sr²⁺ removal mainly included surface electrostatic interaction and ion-exchange with sodium ion. Furthermore, nanostructure layered sodium vanadosilicates had significant advantages for Sr²⁺ removal compared with other adsorbents. Consequently, it is reasonable to expect that nanostructured layered sodium vanadosilicates synthesized in this study could be considered as a promising adsorbent for ultrafast and high-efficiency removal of radioactive Sr²⁺ from radioactive wastewaters.

Measuring multiple 17O–13C J-couplings in naphthalaldehydic acid: a combined solid state NMR and density functional theory approach

A combined multinuclear solid-state NMR and a density functional theory computational approach, with SIMPSON simulations, is evaluated to determine the four heteronuclear ¹J(¹³C,¹⁷O) couplings in naphthalaldehydic acid.

Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review

Liquid wastes containing non-ferrous heavy metal ions and some radionuclides, ¹³⁷Cs and ⁹⁰Sr in particular, represent one of the most dangerous sources of environmental contamination. The remediation of wastewater containing such pollutants continue to be among the biggest challenges of Sustainable Development and Environmental Safety. Sorption-based technologies have proven their efficiency also in reducing the radionuclide content in aqueous streams to low-level residual activity, with the concomitant decrease in the amount of ultimate solid waste generated. Although sorption of cesium and strontium by resins, clays, and zeolites has been investigated intensively and even used in real applications, there is still considerable scope for improvement in terms of retention capacity and selectivity. Recent progress in design and preparation of nanostructured inorganic materials has attracted growing interest based on the potential for improving the retention performance when coupling such functionalities as ion exchange capacity, structural flexibility that may result in steric retention effects, as well as the propensity to interact specifically with the target metal cations. Titanate, vanadate, and tungsten based materials, manganese oxides, hexacyanoferrates, metal sulfides, ammonium molybdophosphates, or hydroxyapatite, characterized by various structures and morphologies, are reviewed with the emphasis being put on synthesis and shaping of such materials, their structure in relationship with the capacity and selectivity of trapping cesium and strontium from either single or multi-component aqueous solutions, as well as the possible retention mechanism. The potential candidates for remediation uses are selected with regard to their sorption capacity and distribution coefficient towards target cations, and also the pH window for an optimum cation capture.

Direct solid state NMR observation of the 105Pd nucleus in inorganic compounds and palladium metal systems

Although ¹⁰⁵Pd is a very challenging nucleus for solid state NMR, these initial observations demonstrate its potential for characterising catalytically relevant Pd metal systems.

The novel composite mechanism of ammonium molybdophosphate loaded on silica matrix and its ion exchange breakthrough curves for cesium

Long-lived ¹³⁷Cs (over 30 years), a byproduct of the spent fuel fission processes, comprises the majority of high-level and prolific heat-generating waste in downstream processing. This study reports a novel sequential annealing mechanism with cross-linked network of polyvinyl alcohol, fabricating the composite of ammonium molybdophosphate loaded on silica matrix (SM-AMP20, 20wt% AMP) as an excellent granular ion exchanger for removal Cs⁺. When the matrix is remarkably sequential annealed, well-dispersed SM-AMP20 particles are formed by firmly anchoring themselves on controlling the porous characteristics and stable structure. The material crystallizes in the complex cubic space group Pn-3m with cell parameters of crystalline AMP formation. The breakthrough curve of Cs⁺ by SM-AMP20 follows the Thomas model with a high removal rate of 88.23% (∼10mg/L of Cs⁺) and breakthrough time as high as 26h (flow rate Q≈2.5mL/min and bed height Z≈11cm) at neutral pH. We also report on sorbents that could efficiently remove Cs⁺ ions from complex solutions containing different competitive cations (Na⁺, Al³⁺, Fe³⁺, and Ni²⁺, respectively) in large excess. Furthermore, this study shows that there is a probability for SM-AMP20 to recycle cesium using an eluent of 2-3mol/L NH₄NO₃ solution.

Enhancing the adsorption capacity of Sr2+ and Cs+ onto hexagonal tungsten oxide by doped niobium

The adsorption capacity of Sr²⁺ and Cs⁺ onto hexagonal tungsten oxide was enhanced by the incorporation of Nb.

Guanidine sulfate-assisted synthesis of hexagonal WO3 nanoparticles with enhanced adsorption properties

The hexagonal WO₃ nanowires prepared with the assistance of C₂H₁₀N·H₂SO₄ possess a large specific surface area and numerous adsorption functional groups, consequently improving the Sr²⁺ adsorption capacity considerably.

Strontium adsorption on tantalum-doped hexagonal tungsten oxide

Hexagonal tungsten oxide (hex-WO3) has the potential to separate (137)Cs and (90)Sr from nuclear power plant or fission (99)Mo production waste. This study aims to increase the capacity of hex-WO3 to adsorb Sr(2+). Ta-doped hex-WO3 was synthesized by the hydrothermal treatment of sodium tungstate dihydrate and tantalum chloride in concentrated HCl, in the presence of ammonium sulfate. Incorporating Ta into the WO3 framework caused the interlayer spacing to expand, and the band gap to shift to higher energy. The Sr(2+) adsorption capacity of Ta-doped hex-WO3 was significantly higher than that of hex-WO3. Sr(2+) adsorption reached equilibrium within 2h in acidic solution. Maximum Sr(2+) removal occurred at pH 4. Sr(2+) uptake by hex-WO3 was described better by the Freundlich model than by the Langmuir model. Sr(2+) adsorption on hex-WO3 was spontaneous under the studied conditions.

On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3

The synthesis route followed to prepare h-WO₃ type oxides results in the production of nanostructured crystals by the spontaneous formation of self-assembled and regular nano-cavities on their surface.