U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions - Spectroscopic investigations of retention mechanisms

Unravelling the nature differences of halide anions affecting the sorption of U(VI) by hydrous titanium dioxide supported waste polyacrylonitrile fibers in the presence of carbonates

The co-occurrence of halides and carbonates with uranium in the natural water poses a challenge to the uranium recovery for nuclear power due to the potential complexation. Hydrated titanium dioxide (HTD) contains a lot of surface hydroxyl (-OH) groups and waste polyacrylonitrile fiber (WPANF) has the advantages of both weather and chemical resistances. Herein, the nature differences of halide anions affecting the sorption of U(VI) by WPANF/HTD was investigated in the presence of carbonates. The sorption capacity (qe) decreased with the increases of initial pH, total carbonates, and halides but increased at high temperature and initial U(VI) concentration. The U(VI) sorption was a spontaneous chemisorption, which mainly involved surface sorption rather than intra-particle diffusion. The order of inhibitory ability on U(VI) sorption for the four halides was F > I ∼ Br > Cl. The aqueous F- was shown to be the most strongly inhibited with the lowest qe value of 17.2 mg·g-1, due to the formation of U(VI)-F complex anions. The characteristic peaks with weakened relative intensity after U(VI) sorption for the surface -OH groups on HTD (HTD-OH), together with the results from DFT calculations, demonstrated a key role of HTD-OH in U(VI) sorption by WPANF/HTD via the coordination with U(VI) complex anions. This work unravels the nature differences of halide anions affecting U(VI) sorption in the presence of carbonates and provides a valuable reference for the U(VI) extraction toward halogen-rich natural uranium-containing water.

Study adsorbents based on bent-Al13-CS-CTA and its application to the removal of CR from wastewater

For rapid and efficient removal of Congo red (CR) from aqueous solutions, a composite of bent-Al13-CS-CTA was prepared from bentonite (bent), chitosan (CS), citric acid (CTA) and Al13 compounds. To comprehend the adsorption process, adsorption variables were changed, including initial pH of the solution, contact time, temperature, initial CR concentration, and adsorption dose. Bent intercalated with X-ray diffraction (XRD), specific surface area (BET), scanning electron microscopy (SEM) and Fourier transform infrared spectrophotometry (FTIR) were used to analyze the material. Physicochemical and structural analysis proven the incorporation of Al13, CS, and CTA into the bent matrix. The pseudo-second-order model aligns with the adsorption kinetics. The adsorption isotherm conformed to the Langmuir adsorption isotherm, with a maximum adsorption capacity of 476.8 mg g-1 at pH 9, a dosage of 2 g L-1, and a temperature of 25 °C. Upon examining the thermodynamic properties of ΔS, ΔH, and ΔG, it was found that the reaction is a spontaneous endothermic process that could potentially be utilized to eliminate CR.

Efficient uranium(VI) adsorbing bioinspired nano-sized hydroxyapatite composites: synthesis, tuning, and adsorption mechanism

The production of large amounts of uranium-containing wastewater and its potential hazards has stimulated green and efficient material removal of uranium (VI). Inspired by the natural mineralization of bone, a facile and eco-friendly biomimetic synthesis of nano-hydroxyapatite (HAP) was carried out using chitosan (CS) as a template. It was found that the reaction temperature and the amount of precursors influence the particle size, crystallinity and specific surface area of the CS/HAP nanorods, and consequently their U(VI) adsorption efficiency. Moreover, the synthesized CS/HAP-40 with smaller particle size, lower crystallinity, and larger specific surface area show a more efficient U(VI) removal compared with CS/HAP-55 and CS/HAP-55-AT. It has a maximum adsorption capacity of 294.12 mg·g^-1 of the CS/HAP-40. Interestingly, the U(VI) removal mechanism of CS/HAP-40 in acidic (pH = 3) and alkaline (pH = 8) aqueous solutions was found to be different. As one of the main results, the U(VI) adsorption mechanisms at pH 8 could be surface complexation and ion exchange. On the contrary, three different mechanisms could be observed at pH 3: dissolution-precipitation to form chernikovite, surface complexation, and ion exchange.

Effect of Ca(II) on U(VI) and Np(VI) retention on Ca-bentonite and clay minerals at hyperalkaline conditions - New insights from batch sorption experiments and luminescence spectroscopy

In deep geological repositories for radioactive waste, interactions of radionuclides with mineral surfaces occur under complex geochemical conditions involving complex solution compositions and high pH resulting from degradation of cementitious geo-engineered barriers. Ca²⁺ cations have been hypothesized to play an important role as mediators for the retention of U(VI) on Ca-bentonite at (hyper)alkaline conditions, despite the anionic character of both the mineral surface and the aqueous uranyl species. To gain deeper insight into this sorption process, the effect of Ca²⁺ on U(VI) and Np(VI) retention on alumosilicate minerals has been comprehensively evaluated, using batch sorption experiments and time-resolved laser-induced luminescence spectroscopy (TRLFS). Sorption experiments with Ca²⁺ or Sr²⁺ and zeta potential measurements showed that the alkaline earth metals sorb strongly onto Ca-bentonite at pH 8-13, leading to a partial compensation of the negative surface charge, thereby generating potential sorption sites for anionic actinyl species. U(VI) and Np(VI) sorption experiments in the absence and presence of Ca²⁺ or Sr²⁺ confirmed that these cations strongly enhance radionuclide retention on kaolinite and muscovite at pH ≥ 10. Concerning the underlying retention mechanisms, site-selective TRLFS provided spectroscopic proof for two dominating U(VI) species at the alumosilicate surfaces: (i) A ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡ Ca - OH - U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates under hyperalkaline conditions. Consequently, the present study confirms that alkaline earth elements, which are ubiquitous in geologic systems, enable strong retention of hexavalent actinides on clay minerals under hyperalkaline repository conditions.

Synthesis and characterization of waste commercially available polyacrylonitrile fiber-based new composites for efficient removal of uranyl from U(VI)-CO3 solutions

The existing species of uranium determines the design of novel sorbents towards uranium extraction from the natural waters. Herein, three composites based on waste commercially available polyacrylonitrile fiber (WPANF), namely WPANF/TiO₂·xH₂O, WPANF/CTAB-bentonite, and WPANF/NZVI, were first prepared and employed for the removal of U(VI) from the carbonate coexisted aqueous solutions. Among them, the WPANF/TiO₂·xH₂O exhibited the optimum sorption capacity of ~40.6 mg·g^-1 (pH 8.0, C₀ = 50 mg·L^-1, and [CO₃]_Total = 2 mmol·L^-1), which is significantly greater than the WPANF/CTAB-bentonite (~12.6 mg·g^-1) and WPANF/NZVI (~10.3 mg·g^-1). All sorption capacities decreased with the increases of initial pH, [NaCl], and [CO₃]_Total, due to the species transformation from UO₂(CO₃)₂^2- and (UO₂)₂CO₃(OH)₃^- to UO₂(CO₃)₃^4- that enhanced the electrostatic repulsion and the competitive sorption. The XPS analysis and DFT calculations indicated that in the composites, WPANF was a role in strengthening the mechanical properties of composites rather than the main sorption sites for uranyl carbonates. The sorption mechanisms were mainly involved in -OH group coordination, Br^- anions exchanges, and redox reactions. Desorption, reusability and U(VI) sorption test in the simulated seawater demonstrated that WPANF/TiO₂·xH₂O could be an alternative candidate for acquiring uranium resource. This work has screened the potential composites for U(VI) extraction from the natural waters, especially based on the practical U(VI) speciation, and provides a novel research approach for the removal of U(VI) towards U(VI)-CO₃ systems.

Hydrotalcite Colloidal Stability and Interactions with Uranium(VI) at Neutral to Alkaline pH

In the United Kingdom, decommissioning of legacy spent fuel storage facilities involves the retrieval of radioactive sludges that have formed as a result of corrosion of Magnox nuclear fuel. Retrieval of sludges may re-suspend a colloidal fraction of the sludge, thereby potentially enhancing the mobility of radionuclides including uranium. The colloidal properties of the layered double hydroxide (LDH) phase hydrotalcite, a key product of Magnox fuel corrosion, and its interactions with U(VI) are of interest. This is because colloidal hydrotalcite is a potential transport vector for U(VI) under the neutral-to-alkaline conditions characteristic of the legacy storage facilities and other nuclear decommissioning scenarios. Here, a multi-technique approach was used to investigate the colloidal stability of hydrotalcite and the U(VI) sorption mechanism(s) across pH 7-11.5 and with variable U(VI) surface loadings (0.01-1 wt %). Overall, hydrotalcite was found to form stable colloidal suspensions between pH 7 and 11.5, with some evidence for Mg²⁺ leaching from hydrotalcite colloids at pH ≤ 9. For systems with U present, >98% of U(VI) was removed from the solution in the presence of hydrotalcite, regardless of pH and U loading, although the sorption mode was affected by both pH and U concentrations. Under alkaline conditions, U(VI) surface precipitates formed on the colloidal hydrotalcite nanoparticle surface. Under more circumneutral conditions, Mg²⁺ leaching from hydrotalcite and more facile exchange of interlayer carbonate with the surrounding solution led to the formation of uranyl carbonate species (e.g., Mg(UO₂(CO₃)₃)^2-_(aq)). Both X-ray absorption spectroscopy (XAS) and luminescence analysis confirmed that these negatively charged species sorbed as both outer- and inner-sphere tertiary complexes on the hydrotalcite surface. These results demonstrate that hydrotalcite can form pseudo-colloids with U(VI) under a wide range of pH conditions and have clear implications for understanding the uranium behavior in environments where hydrotalcite and other LDHs may be present.

New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength

The influence of ionic strength up to 3 mol kg^-1 (background electrolytes NaCl or CaCl₂) on U(VI) sorption onto montmorillonite was investigated as function of pH_c in absence and presence of CO₂. A multi-method approach combined batch sorption experiments with spectroscopic methods (time-resolved laser-induced fluorescence spectroscopy (TRLFS) and in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR)). In the absence of atmospheric carbonate, U(VI) sorption was nearly 99% above pH_c 6 in both NaCl and CaCl₂ and no significant effect of ionic strength was found. At lower pH, cation exchange was strongly reduced with increasing ionic strength. In the presence of carbonate, U(VI) sorption was reduced above pH_c 7.5 in NaCl and pH_c 6 in CaCl₂ system due to formation of aqueous UO₂(CO₃)_x^(2-2x) and Ca₂UO₂(CO₃)₃ complexes, respectively, as verified by TRLFS. A significant ionic strength effect was observed due to the formation of Ca₂UO₂(CO₃)_3(aq), which strongly decreases U(VI) sorption with increasing ionic strength. The joint analysis of determined sorption data together with literature data (giving a total of 213 experimental data points) allowed to derive a consistent set of surface complexation reactions and constants based on the 2SPNE SC/CE approach, yielding log K°_{≡S^SOUO2⁺} = 2.42 ± 0.04, log K°_≡S^SOUO2OH = -4.49 ± 0.7, and log K°_{≡S^SOUO2(OH)3^2-} = -20.5 ± 0.4. Ternary uranyl carbonate surface complexes were not required to describe the data. With this reduced set of surface complexes, an improved robust sorption model was obtained covering a broad variety of geochemical settings over wide ranges of ionic strengths and groundwater compositions, which subsequently was validated by an independent original dataset. This model improves the understanding of U(VI) retention by clay minerals and enables now predictive modeling of U(VI) sorption processes in complex clay rich natural environments.

Building the bridge between U(VI) and Ca-bentonite - Influence of concentration, ionic strength, pH, clay composition and competing ions

In the context of a high-level nuclear waste disposal, the retention of U(VI) on non-pre-treated Ca-bentonite as potential technical barrier is studied. The objective of this study is to reveal the retention behaviour of U(VI) under extreme geochemical conditions, such as hyperalkaline pH range as well as high salinity at the same time, and taking into account other relevant parameters. This should lead to a better understanding of necessary safety precautions for avoiding a release of U(VI) in the environment. Batch experiments were conducted to determine the influence of the initial U(VI) concentration, salinity, pH value, clay composition and the presence of other elements (Ca(II), I^-, Cs(I), Eu(III)). After the sorption experiments, the remaining U(VI) concentration in solution was determined via mass spectrometry with inductively coupled plasma. U(VI) can be immobilised from 10% to 100% under all investigated conditions. Precipitation plays a role in the U(VI) retention but only at higher concentrations (≥10^-5 mol L^-1). The retention is reversible especially with decreasing pH (<10.5) as the aquo complex Ca₂UO₂(CO₃)₃(aq) is formed. Ca(II) strongly enhances the U(VI) adsorption onto Ca-bentonite in the hyperalkaline pH range, probably due to the formation of Ca(II)-bridges. The best retention could be observed on natural bentonite compared to pure montmorillonite and altered bentonite. From a waste cocktail containing important elements of the repository inventory (Cs(I), Eu(III), U(VI) and iodide), only Eu(III) as homologous element to trivalent actinoids competes with U(VI) for binding sites, especially at low metal concentrations, but also facilitates the precipitation at higher concentrations.

Technetium immobilization by chukanovite and its oxidative transformation products: Neural network analysis of EXAFS spectra

The uptake of the fission product technetium (Tc) by chukanovite, an Fe^II hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards Tc^VII is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log R_d ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of Tc^VII to Tc^IV by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO₂-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, Tc^IV is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.

Development, characterization, and first application of a resonant laser secondary neutral mass spectrometry setup for the research of plutonium in the context of long-term nuclear waste storage

Plutonium is a major contributor to the radiotoxicity in a long-term nuclear waste repository; therefore, many studies have focused on interactions of plutonium with the technical, geotechnical, and geological barriers of a possible nuclear waste storage site. In order to gain new insights into the sorption on surfaces and diffusion of actinides through these complex heterogeneous materials, a highly sensitive method with spatial resolution is required. Resonant laser secondary neutral mass spectrometry (Laser-SNMS) uses the spatial resolution available in time-of-flight secondary ion mass spectrometry (TOF-SIMS) in combination with the high selectivity, sensitivity, and low background noise of resonance ionization mass spectrometry (RIMS) and is, therefore, a promising method for the study and analysis of the geochemical behavior of plutonium in long-term nuclear waste storage. The authors present an approach with a combined setup consisting of a commercial TOF-SIMS instrument and a Ti:sapphire (Ti:Sa) laser system, as well as its optimization, characterization, and improvements compared to the original proof of concept by Erdmann et al. (2009). As a first application, the spatial distributions of plutonium and other elements on the surface of a pyrite particle and a cement thin section were measured by Laser-SNMS and TOF-SIMS, respectively. These results exemplify the potential of these techniques for the surface analysis of heterogeneous materials in the context of nuclear safety research.

Uranium and neptunium retention mechanisms in Gallionella ferruginea/ferrihydrite systems for remediation purposes

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg-1 for uranium and 3.07 × 105 L kg-1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.

The determination of the thermodynamic constants of MgUO2(CO3)32- complex in NaClO4 and NaCl media by time-resolved luminescence spectroscopy, and applications in different geochemical contexts

The formation constants and specific ion interaction coefficients of MgUO₂(CO₃)₃^2- complex were determined in 0.1 to 1.0 mol kg_w^-1 NaCl and 0.10 to 2.21 mol kg_w^-1 NaClO₄ media in the framework of the specific ion interaction theory (SIT), by time-resolved laser-induced luminescence spectroscopy. The upper limits of ionic strength were chosen in order to limit luminescence quenching effects generated by high concentrations of Cl^- and ClO₄^- already observed during our earlier studies on Ca_nUO₂(CO₃)₃^(4-2n)- complexes (Shang & Reiller, Dalton Trans., 49, 466; Shang et al., Dalton Trans., 49, 15443). The cumulative formation constant determined is , and the specific ion interaction coefficients are ε(MgUO₂(CO₃)₃^2-, Na⁺) = 0.19 ± 0.11 kg_w mol^-1 in NaClO₄ and ε(MgUO₂(CO₃)₃^2-, Na⁺) = 0.09 ± 0.16 kg_w mol^-1 in NaCl. Two gratings of 300 and 1800 lines per mm were used to acquire MgUO₂(CO₃)₃^2- luminescence spectra, where the high-resolution 1800 lines per mm grating detected slight spectral shifts for the principal luminescent bands relative to Ca_nUO₂(CO₃)₃^(4-2n)-. The applications of the consistent set of thermodynamic constants and ε values for M_nUO₂(CO₃)₃^(4-2n)- (M = Mg and Ca) were examined in different geochemical contexts, where Mg over Ca concentration ratio varies to help defining the relative importance of these species.

Surrogate Model for Multi-Component Diffusion of Uranium through Opalinus Clay on the Host Rock Scale

Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick’s law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32−. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.

Modeling the risk of U(VI) migration through an engineered barrier system at a proposed Chinese high-level radioactive waste repository

The migration of U(VI) through the engineered barrier system (EBS) and into the natural environment in a geological repository for high-level radioactive waste depends on the chemical and physical environment of the repository. Modeling is widely used to understand the risk associated with migration of U(VI) for different barrier designs for repository sites. In this study, coupled thermal, hydrological, and chemical (THC) models were used to evaluate the risk of U(VI) migration at a proposed deep geological repository in northwestern China. The models incorporated two-site protolysis nonelectrostatic surface complexation, dissolution/precipitation of minerals and cation exchange as the major reactions controlling U(VI) migration. Modeling results showed that the main factors influencing U(VI) migration were pH, and the smectite content in the bentonite, as dissolution of the hydrous uranium oxide mineral schoepite is suppressed at higher pH values, and smectite is the most important adsorbent of dissolved U(VI). Therefore, an alkaline bentonite with a smectite volume fraction of >0.6 is suggested as the backfill material for this EBS. The THC model results also showed that in 100,000 years, U(VI) migration is constrained within EBS if the suggested bentonite is used as backfill in a repository that is hosted within Beishan granite. This study provides a feasible method for selecting a bentonite backfill and predicting the effect of environmental conditions on U(VI) migration.

Cm(III) retention by calcium silicate hydrate (C-S-H) gel and secondary alteration phases in carbonate solutions with high ionic strength: A site-selective TRLFS study

We studied the Cm(III) retention by calcium silicate hydrate (C-S-H), portlandite (Ca(OH)₂) and their alteration products calcite, vaterite, and aragonite in high ionic strength carbonate-containing solutions representing specific formation waters. For this, we synthesized C-S-H gels with calcium to silicon (C/S) ratios of 1.0 and 2.0 in the absence and presence of Cm(III), resulting in Cm(III)-free and Cm(III) doped C-S-H gel, respectively. For phase identification purposes we applied X-ray diffraction (XRD) while for the identification of the Cm(III)/C-S-H binding mode we applied site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS). The stability of Cm(III) doped phases under repository-relevant conditions was evaluated by studying the time-dependent release of Cm(III) from the Cm(III) doped C-S-H gel into leaching solutions containing 0.02 M NaHCO₃ or 2.5 M NaCl/0.02 M NaHCO₃ over 60 d. Speciation changes of Cm(III) due to leaching were followed with TRLFS while C-S-H structure alterations and secondary phase formation were monitored with XRD. From the results it could be concluded that Cm(III) is not mobilized by aqueous carbonate but either remains incorporated in the C-S-H structure and portlandite or becomes partially re-immobilized into secondary CaCO₃ phases. The presence of NaCl led to an accelerated conversion of metastable secondary CaCO₃ phases into calcite.