Mineral–Water Interface Reactions of Actinides

Elemental and particle size fractionation during the transport of Eu(III)-silicate colloids in water-saturated porous media

Actinides (An)-bearing colloids could facilitate An migration in the environment. However, little is known about the transport behavior of An(III)-silicate colloids, which are readily formed by the reaction of An3+ with silicic acid under environmental conditions. Column experiments were conducted to investigate the transport of Eu(III)-silicate colloids (chemical analog of An(III)-silicate colloids) in water-saturated porous media as a function of pH, ionic strength (IS) and the presence of fulvic acid (FA). The results showed that colloid transport was more favorable at relatively low IS (≤ 50 mM) and high pH levels (pH ≥ 7). The presence of FA (5-10 mg/L) significantly enhanced the colloid transport. Under high IS (≥ 100 mM), the transport feature of colloids was turned from blocking to ripening due to the on-going aggregation of colloids. Additionally, an interesting elemental fractionation, i.e., a discrepancy in the breakthrough curves (BTCs) with respect to the C/C0 values of Si and Eu, was observed in the IS of 100-500 mM. A detailed investigation indicated that the elemental fractionation could be attributed to the partial Si dissolution of the colloids, the heterogeneity of the colloid size and element composition, and particle size fractionation during colloid transport. Extended Derjaguin-Landau-Verwey-Overbeek interaction energy calculations and convective-dispersive equation modeling were performed to illustrate variations in the colloid transport profiles. These findings illustrate the importance of Si dissolution in the migration of metal-silicate colloids and highlight the significant influence of the heterogeneity of colloid size and composition on the transport/migration behavior of colloids in the environment.

Machine learning surrogates for surface complexation model of uranium sorption to oxides

The safety assessments of the geological storage of spent nuclear fuel require understanding the underground radionuclide mobility in case of a leakage from multi-barrier canisters. Uranium, the most common radionuclide in non-reprocessed spent nuclear fuels, is immobile in reduced form (U(IV) and highly mobile in an oxidized state (U(VI)). The latter form is considered one of the most dangerous environmental threats in the safety assessments of spent nuclear fuel repositories. The sorption of uranium to mineral surfaces surrounding the repository limits their mobility. We quantify uranium sorption using surface complexation models (SCMs). Unfortunately, numerical SCM solvers often encounter convergence problems due to the complex nature of convoluted equations and correlations between model parameters. This study explored two machine learning surrogates for the 2-pK Triple Layer Model of uranium retention by oxide surfaces if released as U(IV) in the oxidizing conditions: random forest regressor and deep neural networks. Our surrogate models, particularly DNN, accurately reproduce SCM model predictions at a fraction of the computational cost without any convergence issues. The safety assessment of spent fuel repositories, specifically the migration of leaked radioactive waste, will benefit from having ultrafast AI/ML surrogates for the computationally expensive sorption models that can be easily incorporated into larger-scale contaminant migration models. One such model is presented here.

Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.

Structures of multinuclear U(VI) species on the hydroxylated α-SiO2(001) surface: insights from DFT calculations

Multinuclear U(VI) species may be dominant in aqueous solutions under environmental conditions, while the structures of the multinuclear U(VI) species on mineral surfaces remain unclear. This work reports the structural and bonding properties of the possible surface complexes of three aqueous multinuclear U(VI) species, i.e., (UO2)2(OH)3+, (UO2)2(OH)22+ and (UO2)3(O)(OH)3+, on the hydroxylated α-SiO2(001) surface based on density functional theory (DFT) calculations. The results show that (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ tend to form end-on structures at SiO(H)SiO(H) sites, whereas (UO2)2(OH)3+ prefers a side-on structure at SiO(H)O(H)-SiO(H)O(H) sites. The main driving forces for the formation of the multinuclear U(VI) surface complexes are electrostatic interactions and partially covalent chemical bonds. The Os-2p orbital hybridizes strongly with U-5f and U-6d orbitals, with a decreasing binding strength in the sequence of (UO2)2(OH)3+ > (UO2)2(OH)22+ > (UO2)3(O)(OH)3+ for the adsorption at the same type of surface sites. For the adsorption of the same multinuclear U(VI) species, the binding energy increases with the deprotonation extent of the identical sites. In addition, hydrogen bonds between surface hydroxyls and coordination waters as well as the acyl oxygen of uranyl moieties contribute to the formation of the multinuclear U(VI) surface complexes. The U-5f electron delocalization of far-side U atoms in the end-on structures of (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ surface complexes also contributes slightly to the overall binding energy. Overall, this study provides insights into the adsorption behavior of multinuclear U(VI) on silica.

Phyllite/bentonite mixture-an alternative effective buffer material for a geological disposal of radioactive waste

The use of phyllite (Phy) instead of quartz in mixtures with bentonite (B) is recommended as a buffer material for engineering barriers in a geological repository of nuclear waste. The recommendation is based on experimentally determined sorption properties of various Phy/B mixtures. The adsorption capacity of Phy/B mixtures (Phy/B: 75/25, 50/50, and 25/75), the removal efficacy of Eu(III) ions (an analog for fissiongenic lanthanides and actinides), and the rate of their binding reaction were studied using the batch adsorption equilibrium and kinetic experiments at different Eu(III) initial concentrations, solution pH, and solution to adsorbent (L/S) ratio. The adsorption capacity of the Phy/B mixtures increased with the increased bentonite content in the mixture depending on the L/S ratio and solution pH. The highest increase in the adsorption capacity of the Phy/B mixtures compared to phyllite was observed for the Phy/B proportions of 25/75 and 50/50. The rate of the Eu(III) adsorption was the best fitted by the pseudo-second-order kinetic model indicating that the adsorption rate was controlled by chemisorption. The Sips model provided the best correlation of the adsorption experimental data, indicative of more than one adsorption site. The results of this study show the advantage of the Phy/B mixtures in immobilizing Eu and certain fission products by combining adsorption properties of the materials.

Impact of chromium (VI) as a co-contaminant on the sorption and co-precipitation of uranium (VI) in sediments under mildly alkaline oxic conditions

Uranium (U) waste, generated at a variety of mines and nuclear production sites, migrates in the subsurface, posing a serious threat to contaminate groundwater systems. In this study, batch equilibrium and kinetic experiments, geochemical modeling and solid phase characterization were conducted to investigate the impact of Cr(VI), a common co-contaminant, on the adsorption of U(VI) to quartz, plagioclase feldspar, and carbonate-dominated sediment (≤2 mm). Batch experiments were performed under slightly alkaline conditions (7.80 ± 0.18) and in the presence of major groundwater components (Ca2+, Mg2+, Na+, K+, carbonate, chloride, and sulfate) at different U(VI):Cr(VI) molar ratios 10:1, 1:1, and 1:10 at lower U(VI) concentration (10.5 μM [2.5 mg/L]) and at U(VI):Cr(VI) molar ratios of 168:1.05, 168:10.5, 168:105, 1:1, and 1:10 at higher U(VI) concentration (168 μM [40 mg/L]). At the low U(VI) concentration (10.5 μM [2.5 mg/L]), the distribution coefficients (Kd) were unchanged for the 10:1 and 1:1 U:Cr molar ratios indicating an excess of available U(VI) adsorption sites on the sediment. However, the U(VI) Kd values in the presence of Cr(VI) at the 1:10 M ratio decreased suggesting competition between U(VI) and Cr(VI) for adsorption sites. At the higher U(VI) concentration (168 μM [40 mg/L]), the Kd values were unaffected by U(VI):Cr(VI) molar ratios of 168:1.05, 168:10.5, 168:105, again indicating an excess of available sediment adsorption sites. At U:Cr molar ratios 1:1 and 1:10, there was a slight increase in Kd for U(VI) possibly reflecting an increase in inner-sphere binding of uranyl carbonate complexes. The adsorption process followed pseudo second-order kinetic parameters. These results indicate the sediment affinity for U(VI) adsorption appears to be largely independent of Cr(VI) under our experimental conditions. The intraparticle diffusion modeling of U(VI) adsorption to the sediment suggests a multi-step process, including film and intraparticle diffusion. The intraparticle diffusion stage remained constant in the presence of Cr(VI) suggesting the rate determining factors for overall equilibrium adsorption process for U(VI) was independent of Cr(VI). Understanding the mobility of U(VI) under natural conditions as simulated in our study is critical in developing effective remediation strategies and effective monitored natural attenuation (MNA) following the remediation of contaminated sites.

An electrochemical technique for sensing uranium adsorption and desorption

Uranium is a toxic, heavy metal that can pose elevated health risks if leached into the environment. Uranium mobility is dependent on many factors, including speciation, solution pH, ligands in solution, and presence of surfaces. Surface adsorption is one phenomenon that inhibits uranium mobility in the environment and is studied as a naturally occurring phenomenon as well an intentional tool for environmental remediation. This work presents and validates a potentiometric, electrochemical technique for sensing uranium adsorption on and desorption from an electrochemically active surface solely through changes of the electrode potential. This novel electrochemical technique presents a new lens to study adsorption that complements external techniques (e.g., spectroscopy). Indication of adsorption and desorption via the electrochemical technique are gravimetrically validated using an electrochemical quartz crystal microbalance. This work contributes a unique, complementary technique that corroborates the adsorption of uranium on an electrode surface.

Impact of a Biological Chelator, Lanmodulin, on Minor Actinide Aqueous Speciation and Transport in the Environment

Minor actinides are major contributors to the long-term radiotoxicity of nuclear fuels and other radioactive wastes. In this context, understanding their interactions with natural chelators and minerals is key to evaluating their transport behavior in the environment. The lanmodulin family of metalloproteins is produced by ubiquitous bacteria and Methylorubrum extorquens lanmodulin (LanM) was recently identified as one of nature's most selective chelators for trivalent f-elements. Herein, we investigated the behavior of neptunium, americium, and curium in the presence of LanM, carbonate ions, and common minerals (calcite, montmorillonite, quartz, and kaolinite). We show that LanM's aqueous complexes with Am(III) and Cm(III) remain stable in carbonate-bicarbonate solutions. Furthermore, the sorption of Am(III) to these minerals is strongly impacted by LanM, while Np(V) sorption is not. With calcite, even a submicromolar concentration of LanM leads to a significant reduction in the Am(III) distribution coefficient (Kd, from >104 to ∼102 mL/g at pH 8.5), rendering it even more mobile than Np(V). Thus, LanM-type chelators can potentially increase the mobility of trivalent actinides and lanthanide fission products under environmentally relevant conditions. Monitoring biological chelators, including metalloproteins, and their biogenerators should therefore be considered during the evaluation of radioactive waste repository sites and the risk assessment of contaminated sites.

Adsorption and diffusion of actinyls on the basal gibbsite (001) surface: a theoretical perspective

Actinides are an important component of nuclear fuel for nuclear power and affect human health, and a key process in the transport of radionuclides in the environment is adsorption on mineral surfaces. In this work, we have used density functional theory (DFT) to investigate the microscopic adsorption and diffusion mechanisms of actinyls, U(V), U(VI), Np(V), Np(VI) Pu(V), and Pu(VI), on the gibbsite (001) surface. Actinyls(VI) are attached to the gibbsite surface through two An-Os bonds, which results in a bidentate inner sphere mode, while actinyls(V) favor a monodentate inner sphere adsorption mode with the gibbsite (001) surface. The solvent effects were considered through an explicit water cluster model. All the actinyls studied can be efficiently adsorbed on the gibbsite (001) surface with binding energies ranging from -113.9 kJ mol-1 to -341.2 kJ mol-1. Electronic structure analyses indicate that the cooperation of the An-Os bonds and hydrogen bonds leads to high adsorption stability of the actinyls with the gibbsite surface. The diffusion barriers of the actinyls on the gibbsite surface were determined, and the high energy barriers indicate that this type of gas-phase diffusion process is not likely to take place.

Facet-dependent U(VI) removal of hematite with confined ferrous ions

The presence of ferrous minerals has been demonstrated to have a significant impact on the destiny, migration, and availability of uranyl (U(VI)) in natural surroundings. The iron oxide/Fe(II) system is a multifaceted iron reduction system anchored to surfaces, encompassing various forms of iron and ferrous ions. Several studies have investigated the effectiveness of adsorbed ferrous iron on iron-based minerals to facilitate the reduction of heavy metal ions and radioactive nuclides. A range of techniques for characterization, including X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy, were employed to explore the process of U(VI) adsorption and deposition, focusing on the limited region containing ferrous iron on the exposed crystalline surface of hematite. In this specific investigation, two kinds of hematite nanocrystals primarily exposing {001} and {012} crystal facets, referred to as HNPs and HNCs, were synthesized. Their ability to remove U(VI) was examined. Ferrous ions (Fe(II)) adsorbed onto the surface of hematite nanocrystals significantly enhanced the efficiency of U(VI) remediation. Furthermore, the HNCs/Fe(II) system showed better U(VI) reduction ability than the HNPs/Fe(II) system. Remarkably, HNCs produced and consumed more electrons and hydroxyl radicals, indicating a more intense response. This finding serves to highlight the significance of their role in interfacial effects and in predicting the spatial distribution of U(VI) in aqueous systems.

Time-resolved laser-induced fluorescence spectroscopy and chemometrics for fast identification of U(VI)-bearing minerals in a mining context

We evaluated the potential of time-resolved laser-induced fluorescence spectroscopy (TRLFS) combined with chemometric methods for fast identification of U(VI)-bearing minerals in a mining context. We analyzed a sample set which was representative of several environmental conditions. The set consisted of 80 uranium-bearing samples related to mining operations, including natural minerals, minerals with uranium sorbed on the surface, and synthetic phases prepared and characterized specifically for this study. The TRLF spectra were processed using the Ward algorithm and the K-nearest neighbors (KNN) method to reveal similarities between samples and to rapidly identify the uranium-bearing phase and the associated mineralogical family. The predictive models were validated on an independent dataset, and then applied to test samples mostly taken from U mill tailings. Identification results were found to be in accordance with the available characterization data from X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). This work shows that TRLFS can be an effective decision-making tool for environmental investigations or geological prospection, considering the large diversity of uranium-bearing mineral phases and their low concentration in environmental samples.

Np(V) Retention at the Illite du Puy Surface

In this study, Np(V) retention on Illite du Puy (IdP) was investigated since it is essential for understanding the migration behavior of Np in argillaceous environments. The presence of structural Fe(III) and Fe(II) in IdP was confirmed by Fe K-edge X-ray absorption near-edge structure (XANES) and ⁵⁷Fe Mössbauer spectroscopy. In batch sorption experiments, a higher Np sorption affinity to IdP was found than to Wyoming smectite or iron-free synthetic montmorillonite. An increase of the relative Np(IV) ratio sorbed onto IdP with decreasing pH was observed by solvent extraction (up to (24 ± 2)% at pH 5, c₀(Np) = 10^-6 mol/L). Furthermore, up to (33 ± 5)% Np(IV) could be detected in IdP diffusion samples at pH 5. Respective Np M₅-edge high-energy resolution (HR-) XANES spectra suggested the presence of Np(IV/V) mixtures and weakened axial bond covalency of the NpO₂⁺ species sorbed onto IdP. Np L₃-edge extended X-ray absorption fine structure (EXAFS) analysis showed that significant fractions of Np were coordinated to Fe─O entities at pH 9. This highlights the potential role of Fe(II/III) clay edge sites as a strong Np(V) surface complex partner and points to the partial reduction of sorbed Np(V) to Np(IV) via structural Fe(II).

Sorption study of long-lived 94Nb on laterite: Effects of physicochemical parameters on sorption

Zr-Nb alloy is used as the pressure tube in pressurized heavy water reactors (PHWR). Prolonged neutron irradiation of the pressure tubes leads to the formation of a long-lived radioisotope ⁹⁴Nb. Thus, the discharged pressure tubes possess huge ⁹⁴Nb activity which persists for a prolonged period.If these discharged pressure tubes come in contact with ground water, ⁹⁴Nb isotope may leach and migrate and this can lead to a long-term radiological impact in the environment.In the present study, we have explored the capability of laterite as a filler material for the containment and retarding the migration of ⁹⁴Nb. In this regard, detailed characterization of the laterite soil was carried out using energy dispersive X-ray fluorescence (EDXRF), X-ray diffraction (XRD), fourier transform infra-red spectrometry (FTIR), total cation exchange capacity determination, zeta potential measurement and thermogravimetric analysis (TGA). The sorption study of ⁹⁴Nb on laterite was carried and the effects of different physico-chemical parameters like pH, ionic strength, temperature and equilibration time were evaluated. Ionic strength, temperature and time dependent sorption studies assist to explore the probable sorption mechanism of ⁹⁴Nb on laterite, which helps in understanding the migration behaviour of ⁹⁴Nb in natural aquatic environment. This study suggests that laterite is a promising material in containment of ⁹⁴Nb isotope owing to its good cation exchange behaviour in the acidic medium and ability to form surface complex in the neutral medium.

Theoretical insights into the coordination structures, stabilities and electronic spectra of Cm3+ species at the gibbsite-water interface: A computational study combing ab initio molecular dynamics and wave function theory

The information of structure and stability of actinide species is key to understand the sorption mechanism of actinides at mineral-water interface. Such information is approximately derived from experimental spectroscopic measurements and needs to be accurately obtained by a direct atomic-scale modelling. Herein, systematic first-principles calculations and ab initio molecular dynamics (AIMD) simulations are carried out to study the coordination structures and absorption energies of Cm(III) surface complexes at gibbsite-water interface. Eleven representative complexing sites are investigated. The most stable Cm³⁺ sorption species are predicted to be a tridentate surface complex in weakly acidic/neutral solution condition and a bidentate one in the alkaline solution condition. Moreover, luminescence spectra of the Cm³⁺ aqua ion and the two surface complexes are predicted based on the high-accuracy ab initio wave function theory (WFT). The results give a gradually decreasing emission energy in good agreement with experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. This work is a comprehensive computational study involving AIMD and ab initio WFT methods to gain the coordination structures, stabilities, and electronic spectra of actinide sorption species at the mineral-water interface, thus providing important theoretical support for geological disposal of actinide waste.

Y(III) Sorption at the Orthoclase (001) Surface Measured by X-ray Reflectivity

Interactions of heavy metals with charged mineral surfaces control their mobility in the environment. Here, we investigate the adsorption of Y(III) onto the orthoclase (001) basal plane, the former as a representative of rare earth elements and an analogue of trivalent actinides and the latter as a representative of naturally abundant K-feldspar minerals. We apply in situ high-resolution X-ray reflectivity to determine the sorption capacity and molecular distribution of adsorbed Y species as a function of the Y³⁺ concentration, [Y³⁺], at pH 7 and 5. With [Y³⁺] ≥ 1 mM at pH 7, we observe an inner-sphere (IS) sorption complex at a distance of ∼1.5 Å from the surface and an outer-sphere (OS) complex at 3-4 Å. Based on the adsorption height of the IS complex, a bidentate, binuclear binding mode, in which Y³⁺ binds to two terminal oxygens, is proposed. In contrast, mostly OS sorption is observed at pH 5. The observed maximum Y coverage is ∼1.3 Y³⁺/A_UC (A_UC: area of the unit cell = 111.4 Ų) for all the investigated pH values and Y concentrations, which is in the expected range based on the estimated surface charge of orthoclase (001).

Crystal Structure of Mixed Np(V)-Ammonium Carbonate

This work presents details of the synthesis, properties and structure of a novel neptunium carbonate (NH4)[NpO2CO3], a member of the M[AnO2CO3] (M = K, (NH4), Rb, Cs) class of compounds. Carbonates play an important role in the migration of actinides in the environment, and thus are relevant for handling and disposal of radioactive wastes, including spent nuclear fuel and vitrified raffinates. Knowledge of the crystallographic structure of these compounds is important for models of the environmental migration behavior based on thermodynamic descriptions of such chemical processes. (NH4)[NpO2CO3] crystals were obtained during long-term hydrothermal treatment of Np(VI) in aqueous ammonia at 250 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show that a single-phase sample containing only Np(V) was obtained. Structural features of (NH4)[NpO2CO3] were elucidated from single crystal X-ray diffraction and confirmed by vibrational spectroscopy. The results obtained are of interest both for fundamental radiochemistry and for applied problems of the nuclear fuel cycle.

Mechanistic understanding of Curium(III) sorption on natural K-feldspar surfaces

To assess a reliable safety case for future deep underground repositories for highly active nuclear waste the retention of radionuclides by the surrounding host rock must be understood comprehensively. Retention is influenced by several parameters such as mineral heterogeneity and surface roughness, as well as pore water chemistry (e.g., pH). However, the interplay between those parameters is not yet well understood. Therefore, we present a correlative spectromicroscopic approach to investigate sorption of the actinide Cm(III) on: 1) bulk K-feldspar crystals to determine the effect of surface roughness and pH (5.5 and 6.9) and 2) a large feldspar grain as part of a complex crystalline rock system to observe how sorption is influenced by the surrounding heterogeneous mineralogy. Our findings show that rougher K-feldspar surfaces exhibit increased Cm(III) uptake and stronger complexation. Similarly, increasing pH leads to higher surface loading and stronger Cm(III) binding to the surface. Within a heterogeneous mineralogical system sorption is further affected by neighboring mineral dissolution and competitive sorption between mineral phases such as mica and feldspar. The obtained results express a need for investigating relevant processes on multiple scales of dimension and complexity to better understand trivalent radionuclide retention by a potential repository host rock.

The mechanism of Fe induced bond stability of uranyl(v)

The stabilization of uranyl(v) (UO₂ ^{1 +} ) by Fe(ii) in natural systems remains an open question in uranium chemistry. Stabilization of U^VO₂ ¹⁺ by Fe(ii) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(ii) induced stability and the change of the bonding properties have not been elucidated up to date. We demonstrate that U(v) - oaxial bond covalency decreases upon binding to Fe(ii) inducing redirection of electron density from the U(v) - oaxial bond towards the U(v) - equatorial bonds thereby increasing bond covalency. Our results indicate that such increased covalent interaction of U(v) with the equatorial ligands resulting from iron binding lead to higher stability of uranyl(v). For the first time a combination of U M_4,5 high energy resolution X-ray absorption near edge structure (HR-XANES) and valence band resonant inelastic X-ray scattering (VB-RIXS) and ab initio multireference CASSCF and DFT based computations were applied to establish the electronic structure of iron-bound uranyl(v).

Stability of Eu(III)-silicate colloids: Effect of Eu content, pH, electrolyte and fulvic acid

Dissolved silicic acid in the environment has strong affinity for actinides (An), but An(III)-silicate colloids have been scarcely investigated. In this study, Eu(III)-silicate colloids, an analogue to An(III)-silicate, were prepared and the aggregation kinetics of the colloids was investigated as a function of Eu content (Si/Eu molar ratio), pH, background electrolyte (NaCl, NaNO₃, NaClO₄, KCl and CsCl) and fulvic acid (FA). Results indicated that the colloids with higher Si/Eu molar ratio exhibited higher stability under the same conditions. The stability of the colloids increased with increasing aqueous pH (7.1-9.4) and decreasing ionic strength, and the inhibition effect of monovalent electrolytes on the colloid stability followed the order of Na⁺ < K⁺ < Cs⁺ and Cl^- < NO₃^- < ClO₄^-. In addition, the presence of FA significantly increased the stability of the colloids. The dependence of the stability on the chemical conditions in all cases could be illustrated by DLVO theory. Disaggregation kinetics showed that the aggregation process of the colloids was not fully reversible, because a time-dependent size memory effect led to a bigger mean size of disaggregated colloids as compared to the initial ones. The present work provides detailed insight in the formation and stability of An(III)-silicate colloids under the alkaline conditions relevant to geological disposal of radioactive waste, which is critical for understanding the behavior of this type of colloids in the environment.

Development of Heavy Element Chemistry at Interfaces: Observing Actinide Complexes at the Oil/Water Interface in Solvent Extraction by Nonlinear Vibrational Spectroscopy

Understanding the chemistry of elements at the bottom of the periodic table is a challenging goal in chemistry. Observing actinide species at interfaces by using interface-selective second-order nonlinear optical spectroscopy, such as vibrational sum frequency generation (VSFG) spectroscopy, is a promising route for developing heavy element chemistry; however, such attempts are scarce. Here, we investigated the phase transfer mechanism of uranyl ions (UO₂²⁺) in solvent extraction using the di(2-ethylhexyl)phosphoric acid (HDEHP) extractant dissolved in the dodecane organic phase by probing the oil/water liquid-liquid interface using VSFG spectroscopy. The POO^- symmetric stretch vibrational signals of the HDEHP ligands clearly demonstrated that uranyl ions form interfacial complexes with HDEHP at the oil/water interface. The interfacial uranyl-HDEHP complexes were formed with uranyl ions coming from both the aqueous and oil phases, strongly suggesting that the interfacial complex is an intermediate to cross the oil/water interface. Density functional theory calculations proposed the molecular structure of the interfacial uranyl-HDEHP complex.

Fate of Neptunium in nuclear fuel cycle streams: state-of-the art on separation strategies

Neptunium, with a half life of 2.14 million years is one of the most notorious activation products in the nuclear fuel cycle. It has been more than 5 decades in the reprocessing of nuclear fuels by the well documented PUREX process, but the fate of Np in the PUREX cycle is still not well controlled. Although Np being stable in its pentavalent state in low acid media, its starts to undergo disproportionation at higher acidities. This disproportionation along with the oxidizing conditions of the HNO3 medium makes Np to exits as Np(IV), Np(V) and Np(VI) in the dissolver solution. The overall extractability of Np in the co-decontamination step of the PUREX cycle is dependent on its oxidation state in the medium as Np(VI) and Np(IV) being extractable while Np(V) being least extractable. The present review article discusses about the speciation of Np in HNO3 and its disproportionation. The variety of redox reagents are discussed for their effectiveness towards controlling Np redox behavior in the HNO3 media. The extraction of Np with the different class of extractant has also been discussed and the results are compared for better understanding. Solid phase extraction of Np using both commercially available resin and lab based synthesized resins were discussed. The anion exchange resins with the different cationic centers were shown to behave differently towards the uptake of Np form the acidic medium. The present review also highlight the chemical conditions required for controlling or minimizing the fate of Np in different process streams of the nuclear fuel cycle.

Using molybdenum carbiding to induce digestion of carbon in H2O2: A sustainable approach to eliminate radioactivity for hazardous graphite waste inherited from nuclear enterprise

To properly manage nuclear wastes is critical to sustainable utilization of nuclear power and environment health. Here, we show an innovative carbiding strategy for sustainable management of radioactive graphite through digestion of carbon in H₂O₂. The combined action of intermolecular oxidation of graphite by MoO₃ and molybdenum carbiding demonstrates success in gasifying graphite and sequestrating uranium for a simulated uranium-contaminated graphite waste. The carbiding process plays a triple role: (1) converting graphite into atomic carbon digestible in H₂O₂, (2) generating oxalic ligands in the presence of H₂O₂ to favor U-precipitation, and (3) delivering oxalic ligands to coordinate to Mo^VI-oxo anionic species to improve sample batching capacity. We demonstrate > 99% of uranium to be sequestrated for the simulated waste with graphite matrix completely gasifying while no detectable U-migration occurred during operation. This method has further been extended to removal of surface carbon layers for graphite monolith and thus can be used to decontaminate monolithic graphite waste with emission of a minimal amount of secondary waste. We believe this work not only provides a sustainable approach to tackle the managing issue of heavily metal contaminated graphite waste, but also indicates a promising methodology toward surface decontamination for irradiated graphite in general.

Elucidation of the sorbent role in sorption thermodynamics of uranium(VI) on goethite

The sorption process of radionuclides, often conducted at ambient temperature, shows significant sensitivity to the surrounding temperature. Prediction of fate and transport in the environment, therefore, requires accurate thermodynamic data of their species defining sorption-desorption onto solid surfaces. Herein, we examined the thermodynamics of uranium(VI), U(VI), sorption onto goethite with particular emphasis on directly calculating the enthalpy of U(VI) surface species formed under slightly acidic pH conditions. To achieve this aim, a sorption study of U(VI) was carried out on goethite in the pH range 3-10 and modelled using a 2-pK single-site diffuse layer surface complexation model. A binuclear bidentate species of U(VI), (FeO)₂UO₂, reproduces the sorption profile at pH 3-5 while the sorption was under-estimated in the pH >5 region. Precipitation of schoeptite at pH 5-8 was attributed to the underestimation of the predicted sorption behaviour. The species complexation constant was employed in the analysis of heat consumed, measured using an isothermal titration calorimeter, in the titration of the goethite suspension with U(VI) at pH 4.5 ± 0.1. Enthalpy for the U(VI) species was found to be 41 ± 7 kJ mol^-1, suggesting that sorption is an entropically driven process. Comparing thermodynamic data with that of similar U(VI)-iron oxide systems, binding energy of U(VI) surface species, surface hydration and hydrogen binding are suggested as main factors in the sorbent role towards the thermodynamics of the sorption process.

Insights into sorption speciation of uranium on phlogopite: Evidence from TRLFS and DFT calculation

Knowledge of the sorption speciation of uranium at mineral/water interface is essential to construct reliable retention and migration models. In this work, the sorption speciation of U(VI) at the phlogopite/water interface was studied at trace concentrations by combining batch sorption, time-resolved luminescence spectroscopy, and theoretical calculation. Batch experiments showed that the sorption of U(VI) on phlogopite was strongly dependent on pH but weakly affected by ionic strength, implying that the inner-sphere surface complexation was mainly responsible for U(VI) sorption on phlogopite. The diverse luminescence spectral characteristics indicated the formation of multiple inner-sphere surface species at the phlogopite/water interface, whose abundances varied as a function of pH. A portion of U(VI) precipitated as uranyl oxyhydroxides such as metaschoepite and becquerelite at high pH. Density functional theory calculation revealed that the bidentate complex at the edge of phlogopite (≡AlO-MgO-UO₂(H₂O)₃) was the most favorable sorption configuration for U(VI) at acidic condition. The increasing temperature enhanced the sorption of U(VI) on phlogopite without altering the sorption species, and such enhancement in U(VI) sorption was withdrawn once the temperature decreased. These findings are essential for understanding the immobilization mechanism of U(VI) in mica-rich granitic terrains at a molecular scale and building a reliable retention model.

The strong interaction of actinyl ions with fullerenol driven by multiple hydrogen bonds

The removal of actinides from radioactive wastewater is an important subject with the continuous application of nuclear energy. All-electron density functional theory (DFT) calculations were carried out to understand the adsorption behaviors of actinyl ions on C₆₀(OH)₂₄ fullerenol in this work. The outer-sphere (OS) bonding mode is more stable than the inner-sphere one because of the formation of multiple hydrogen bonds in the OS mode. The actinyl(VI) ions can be more efficiently absorbed by fullerenol than actinyl(V) ones. The bonding nature of actinyl ions with C₆₀(OH)₂₄ was revealed by various electron density topological analyses. Multiple hydrogen bonds formed in the OS complexes show moderate bond strength with partial covalent nature and are responsible for their high stability. IR spectra were fingerprinted to distinguish the interaction modes of actinyl ions with C₆₀(OH)₂₄.

Community Data Mining Approach for Surface Complexation Database Development

This paper presents a comprehensive data-to-model workflow, including a findable, accessible, interoperable, reusable (FAIR) community sorption database (newly developed LLNL Surface Complexation/Ion Exchange (L-SCIE) database) along with a data fitting workflow to efficiently optimize surface complexation reaction constants with multiple surface complexation model (SCM) constructs. This workflow serves as a universal framework to mine, compile, and analyze large numbers of published sorption data as well as to estimate reaction constants for parameterizing reactive transport models. The framework includes (1) data digitization from published papers, (2) data unification including unit conversions, and (3) data-model integration and reaction constant estimation using geochemical software PHREEQC coupled with the universal parameter estimation code PEST. We demonstrate our approach using an analysis of U(VI) sorption to quartz based on a first L-SCIE implementation, concluding that a multisite SCM construct with carbonate surface species yielded the best fit to community data. Surface complexation reaction constants extracted from this approach captured all available sorption data available in the literature and provided insight into previously published reaction constants and surface complexation model constructs. The L-SCIE sorption database presented herein allows for automating this approach across a wide range of metals and minerals and implementing novel machine learning approaches to reactive transport in the future.

Surface Coverage- and Excitation Laser Wavelength-Dependent Luminescence Properties of U(VI) Species Adsorbed on Amorphous SiO2

Time-resolved luminescence spectroscopy is usefully used to identify U(VI) surface species adsorbed on SiO2. However, the cause of the inconsistent luminescence lifetimes and spectral shapes reported previously remains undetermined. In this study, the U(VI) surface coverage (Γ) and excitation laser wavelength (λex) were examined as the predominant factors governing the luminescence properties of U(VI) surface species. At neutral pH, the luminescence lifetimes of U(VI) surface species increased with decreasing Γ. In the low-Γ region, where a relatively large number of adjacent surface sites are involved in the formation of multidentate surface complexes, the displacement of more number of coordinated water molecules in the equatorial plane of U(VI) results in a longer lifetime. The pH-dependent luminescence lifetimes of U(VI) surface species at the same U(VI) to SiO2 concentration ratio in the pH range of 4.5–7.5 also explain the effect of the surface binding sites on the luminescence lifetime. The time-resolved luminescence properties of the U(VI) surface species were also investigated at different excitation wavelengths. Continued irradiation of the SiO2 surface with a UV laser beam at λex = 266 nm considerably reduced the luminescence intensities of the U(VI) surface species. The higher the laser pulse energy, the greater the decrease in luminescence intensity. Laser-induced thermal desorption (LITD) of U(VI) surface species is suggested to be the origin of the decrease in luminescence intensity. LITD effects were not observed at λex = 355 and 422 nm, even at high laser pulse energies.

Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

Relativistic Multiconfigurational Ab Initio Calculation of Uranyl 3d4f Resonant Inelastic X-ray Scattering

We applied relativistic multiconfigurational all-electron ab initio calculations including the spin-orbit interaction to calculate the 3d4f resonant inelastic X-ray scattering (RIXS) map (3d3/2 → 5f5/2 U M4 absorption edge and 4f5/2 → 3d3/2 U Mβ emission) of uranyl (UO22+). The calculated data are in excellent agreement with experimental results and allow a detailed understanding of the observed features and an unambiguous assignment of all involved intermediate and final states. The energies corresponding to the maxima of the resonant emission and the non-resonant (normal) emission were determined with high accuracy, and the corresponding X-ray absorption near edge structure spectra extracted at these two positions were simulated and agree well with the measured data. With the high quality of our theoretical data, we show that the cause of the splitting of the three main peaks in emission is due to the fine structure splitting of the 4f orbitals induced through the trans di-oxo bonds in uranyl and that we are able to obtain direct information about the energy differences between the 5f and 4f orbitals: Δ5f δ/ϕ - 4f δ/ϕ, Δ5f π* - 4f π, and Δ5f σ* - 4f σ from the 3d4f RIXS map. RIXS maps contain a wealth of information, and ab initio calculations facilitate an understanding of their complex structure in a clear and transparent way. With these calculations, we show that the multiconfigurational protocol, which is nowadays applied as a standard tool to study the X-ray spectra of transition metal complexes, can be extended to the calculation of RIXS maps of systems containing actinides.

Complexation of Np(V) with the Dicarboxylates, Malonate, and Succinate: Complex Stoichiometry, Thermodynamic Data, and Structural Information

The complexation of Np(V) with malonate and succinate is studied by different spectroscopic techniques, namely, attenuated total reflection Fourier transform infrared (ATR FT-IR) and extended X-ray absorption fine-structure (EXAFS) spectroscopy, as well as by quantum chemistry to determine the speciation, thermodynamic data, and structural information of the formed complexes. For complex stoichiometries and the thermodynamic functions (log β_n^°(Θ), Δ_rH_n^°, Δ_rS_n^°), near infrared absorption spectroscopy (vis/NIR) is applied. The complexation reactions are investigated as a function of the total concentration of malonate ([Mal^2-]_total) and succinate ([Succ^2-]_total), ionic strength [I_m = 0.5-4.0 mol kg^-1 Na⁺(Cl^-/ClO₄^-)], and temperature (Θ = 20-85 °C). Besides the solvated NpO₂⁺ ion, the formation of two Np(V) species with the stoichiometry NpO₂(L)_n^1-2n (n = 1, 2, L = Mal^2-, Succ^2-) is observed. With increasing temperature, the molar fractions of both complex species increase and the temperature-dependent conditional stability constants log β_n^'(Θ) at given ionic strengths are determined by the law of mass action. The log β_n^'(Θ) are extrapolated to IUPAC reference-state conditions (I_m = 0) according to the specific ion interaction theory (SIT), revealing thermodynamic log β_n^°(Θ) values. For all formed complexes, [NpO₂(Mal)^-: log β₁^°(25 °C) = 3.36 ± 0.11, NpO₂(Mal)₂^3-: log β₂^°(25 °C) = 3.95 ± 0.19, NpO₂(Succ)^-: log β₁^°(25 °C) = 2.05 ± 0.45, NpO₂(Succ)₂^3-: log β₂^°(25 °C) = 0.75 ± 1.22], an increase of the stability constants with increasing temperature was observed. This confirmed an endothermic complexation reaction. The temperature dependence of the log β_n^°(T) values is described by the integrated Van't Hoff equation, and the standard reaction enthalpies and entropies for the complexation reactions are determined. Furthermore, the sum of the specific binary ion-ion interaction coefficients Δεn^°(Θ) for the complexation reactions are obtained as a function of the t from the respective SIT modeling as a function of the temperature. In addition to the thermodynamic data, the structures of the complexes and the coordination modes of malonate and succinate are investigated using EXAFS spectroscopy, ATR-FT-IR spectroscopy, and quantum chemical calculations. The results show that in the case of malonate, six-membered chelate complexes are formed, whereas for succinate, seven-membered rings form. The latter ones are energetically unfavorable due to the limited space in the equatorial plane of the Np(V) ion (as NpO₂⁺ cation).

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.

Influences of geological factors on the distribution of uranium in drinking water limescale in the junction zone of the East European Platform and the Southern Urals

An assessment of uranium contents and distribution in drinking water limescale has been conducted in the Republic of Bashkortostan (RB), Russia. A total of 515 limescale samples from 262 settlements of the RB were analyzed. The spread of U concentration values in limescale samples ranged from 0.01 to 61.0 μg/g. Elevated U concentrations in the West of the RB corresponded with the horsts of the granite-gneiss crystalline basement of the South-Tatar Dome and their Eastern slopes, the areas with the Lower Permian red beds and the oil and gas fields. The U migration from the granite-gneiss basement is attributed to the tectonic factor and hydrocarbons movement. Elevated concentrations of U within the South of the RB are associated primarily with the deposits of the Southern Ural brown coal basin. The Bashkir Trans-Urals anomalies are mainly associated with Lower Paleozoic eclogite complex, Devonian and Carboniferous volcanic-sedimentary, carbonate, intrusive formations, as well as the Jurassic cover of terrigenous marine sediments. The negative anomalies of the spatial distribution of U are located in the area of the Ufimian Plateau mainly composed of limestone.

Structure and Thermodynamics of Eu(III) and Cm(III) Complexes with Glucuronic Acid

Complexation by small organic ligands controls the bioavailability of contaminants and influences their mobility in the geosphere. We have studied the interactions of Cm³⁺, as a representative of the trivalent actinides, and Eu³⁺, as an inactive homologue, with glucuronic acid (GlcA) a simple sugar acid. Time-resolved laser-induced luminescence spectroscopy (TRLFS) shows that complexation at pH 5.0 occurs only at high ligand to metal ratios in the form of 1:1 complexes with standard formation constants log β⁰ = 1.84 ± 0.22 for Eu³⁺ and log β⁰ = 2.39 ± 0.19 for Cm³⁺. A combination of NMR, QMMM, and TRLFS reveals the structure of the complex to be a half-sandwich structure wherein the ligand binds through its carboxylic group, the ring oxygen, and a hydroxyl group in addition to five to six water molecules. Surprisingly, Y³⁺, which was used as a diamagnetic reference in NMR, prefers a different coordination geometry with bonding through at least two hydroxyl groups on the opposite side of a distorted GlcA molecule. QMMM simulations indicate that the differences in stability among Cm, Eu, and Y are related to ring strain induced by smaller cations. At higher pH a stronger complex was detected, most likely due to deprotonation of a coordinating OH group.

Characterization of Americium and Curium Complexes with the Protein Lanmodulin: A Potential Macromolecular Mechanism for Actinide Mobility in the Environment

Anthropogenic radionuclides, including long-lived heavy actinides such as americium and curium, represent the primary long-term challenge for management of nuclear waste. The potential release of these wastes into the environment necessitates understanding their interactions with biogeochemical compounds present in nature. Here, we characterize the interactions between the heavy actinides, Am³⁺ and Cm³⁺, and the natural lanthanide-binding protein, lanmodulin (LanM). LanM is produced abundantly by methylotrophic bacteria, including Methylorubrum extorquens, that are widespread in the environment. We determine the first stability constant for an Am³⁺-protein complex (Am₃LanM) and confirm the results with Cm₃LanM, indicating a ∼5-fold higher affinity than that for lanthanides with most similar ionic radius, Nd³⁺ and Sm³⁺, and making LanM the strongest known heavy actinide-binding protein. The protein's high selectivity over ²⁴³Am's daughter nuclide ²³⁹Np enables lab-scale actinide-actinide separations as well as provides insight into potential protein-driven mobilization for these actinides in the environment. The luminescence properties of the Cm³⁺-LanM complex, and NMR studies of Gd³⁺-LanM, reveal that lanmodulin-bound f-elements possess two coordinated solvent molecules across a range of metal ionic radii. Finally, we show under a wide range of environmentally relevant conditions that lanmodulin effectively outcompetes desferrioxamine B, a hydroxamate siderophore previously proposed to be important in trivalent actinide mobility. These results suggest that natural lanthanide-binding proteins such as lanmodulin may play important roles in speciation and mobility of actinides in the environment; it also suggests that protein-based biotechnologies may provide a new frontier in actinide remediation, detection, and separations.

Impact of Updated OECD/NEA Thermodynamic Database on the Safety Assessment of Radioactive Waste Repository Studied Using RESRAD-OFFSITE Code

A RESRAD-OFFISTE computational code for the safety assessment model of a radioactive waste repository was utilized to evaluate the influence of the updated OECD/NEA thermodynamic database on the safety assessment model in terms of exposure dose. The solubility data as the input parameter for the RESRAD-OFFSITE code obtained with two different sets of chemical thermodynamic databases such as JAEA-TDB and amended JAEA-TDB reflecting the updates of the OECD/NEA thermodynamic database were calculated and compared with each other. As a result, almost identical exposure doses were obtained due to the remarkable similarity between the solubility data of various radionuclides for both chemical thermodynamic databases. In contrast, dramatic changes in exposure dose were observed with varying distribution coefficients. Thermodynamic calculations indicated that the aqueous species distribution can be significantly changed by the selection of a chemical thermodynamic database and thus the relevant distribution coefficient can also be influenced as a consequence. Accordingly, the result obtained in the present work indicated that (i) the impact of the updated chemical thermodynamic data was somewhat minor from the viewpoint of the solubility and (ii) the distribution coefficient, which can be sensitively influenced by the predominant chemical species, produced a remarkable change in the exposure dose. This work provided an insight into the precise exposure dose calculation in terms of the reliable estimation of the distribution coefficient by means of a surface complexation model, which can predict the distribution coefficient as a function of groundwater composition coupled with a chemical speciation calculation based on up to date chemical thermodynamic data.

Localised solution environments drive radionuclide fractionation in uraninite

We explore the role of various solution environments - chloride brines, acid mine drainage (sulfate) and groundwater (carbonate), as well as pore pressure in producing secular disequilibrium among the various radionuclides (RN) in the U-decay series upon leaching of uraninite - the most abundant U-ore and a widespread accessory mineral in U-rich rocks. We observed that the end products of the U-decay chain, ²⁰⁶Pb and ²⁰⁷Pb, exist primarily at the surface/edges of grains or within large pores in the uraninite. In contrast, the intermediate daughters ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, and ^234/230Th, exist primarily within the bulk of uraninite, requiring breakdown by leaching for subsequent mobility to occur. Overall, pore pressure had little effect on RN mobility, with solution environment being the primary factor in creating significant mobility and disequilibrium among the RN, as it drives the initial breakdown of uraninite and influences the subsequent differential solubility of individual RNs. This was particularly the case for carbonate-bearing fluids, leading to significant fractionation of the various daughter RN arising from variable complexation and sorption phenomena. Understanding the geochemical behaviour of the RN in the U-decay series is important for predicting and managing the risks associated with RN in both environmental (acid-mine drainage) and engineered (metallurgical extraction) processes. Effective modelling of long-term RN behaviour should incorporate this strong relative fractionation caused by contrasting geochemical behaviour of individual RN during and after their release into the water from uraninite and subsequent interaction with the surrounding aquifer host rocks.

Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a ⁶⁰Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50-100, 10-50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic -OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs⁺, Sr²⁺, and Eu³⁺ ions in the molecular-weight fractions of the metal-HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs⁺ ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr²⁺ and Eu³⁺ ions redistributed organic carbon toward the larger molecular-weight fractions.

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Experimental study on modeling of Pu sorption onto quartz

Plutonium(IV) sorption onto quartz in carbonate solutions was systematically investigated under anaerobic conditions to analyze the sorption behaviors of Pu(IV) with a non-electrostatic model (NEM). Pu(IV) sorption data was obtained from batch sorption experiments as a function of pH and carbonate concentration. The Pu(IV) sorption onto quartz showed similar tendencies to Th(IV), which is considered to be chemically analogous as a tetravalent actinoid. The distribution coefficient, K d , of Pu(IV) onto quartz showed inverse proportionality to the square of the total carbonate concentration under the investigated pH conditions of 8–11. The modeling study, however, revealed a Th(IV) sorption model, which is ≡SOTh(OH)4 − and ≡SOThOH(CO3)2 2−, could not be applied to simulate the Pu(IV) sorption onto quartz. It was inferred that the electrostatic repulsion between negatively charged ligands limited the formation of ≡SOM(OH)4 − and ≡SOMOH(CO3)2 2− for Pu(IV) with smaller ionic radii than Th(IV). The Pu(IV) sorption model was developed as ≡SOPu(OH)3 and ≡SOPu(OH)4 −. In addition, data of Pu(IV) sorption onto muscovite was obtained in order to be compared with data for quartz.

Selective radionuclide co-sorption onto natural minerals in environmental and anthropogenic conditions

Anthropogenic activities can redistribute the constituents of naturally occurring radioactive materials (NORM), posing potential hazards to populations and ecosystems. In the present study, the co-sorption of several RN from the U decay chain- ²³⁸U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, onto common minerals associated with mining activities (chalcopyrite, bornite, pyrite and barite) was investigated, in order to identify the various factors that control long-term NORM mobility and retentivity in environmental acid-mine drainage systems and hydrometallurgical processing. The results show selective RN co-sorption to the various natural minerals, suggesting that mineral-specific mechanisms govern the variability in NORM mobility and retentivity. Both ²²⁶Ra and ²¹⁰Po underwent significant sorption onto the natural minerals investigated in this study. The order of co-sorption in sulfate media for chalcopyrite and bornite was ²¹⁰Po>²²⁶Ra>²⁰⁶Pb>²¹⁰Pb>²³⁸U/^230Th. Conversely, both pyrite and barite showed increased affinity for ²²⁶Ra; the order of co-sorption in sulfate media was ²²⁶Ra>²¹⁰Po>²⁰⁶Pb/²¹⁰Pb>²³⁸U/²³⁰Th for pyrite and ²²⁶Ra>²⁰⁶Pb/²¹⁰Pb>²³⁰Th/²³⁸U/²¹⁰Po for barite. Similar orders of co-sorption were observed in the nitrate media: for chalcopyrite and bornite the order was ²¹⁰Po>²²⁶Ra/²⁰⁶Pb/²¹⁰Pb/²³⁸U/²³⁰Th compared to ²²⁶Ra>²¹⁰Po/²⁰⁶Pb/²¹⁰Pb/²³⁸U/^230Th for pyrite and barite. The behavior of ²¹⁰Po was found to the anomalous: in both sulfate and nitrate solutions, ²¹⁰Po had little affinity for barite compared to the sulfides. Thermodynamic modeling indicated the formation of a reduced PoS(s) phase at the surface of sulfide minerals, leading to the suggestion that ²¹⁰Po likely undergoes reductive precipitation on the surface of sulfide minerals. The high sorption of both ²⁰⁶Pb and ²¹⁰Pb observed in the sulfate systems were likely as a result of co-precipitation as insoluble anglesite compared to nitrate where they mainly remained in solution. Overall, barite showed the highest affinity for ²²⁶Ra, given its propensity to sorb ²²⁶Ra (similar ionic size). Both ²³⁸U and ²³⁰Th were highly mobile in acidic sulfate and nitrate solutions. The results highlighted here identify the various constraints on the natural variability and fractionation of NORM in the environment, as well as the mineral-specific mechanisms that control co-sorption of RN. This information provides a framework for predicting RN transport within soils and ground waters with variable geochemical conditions and in metallurgical extraction processes, in order to develop effective strategies towards NORM mitigation.

Molecular-Level Speciation of Eu(III) Adsorbed on a Migmatized Gneiss As Determined Using μTRLFS

The interaction of Eu(III) with thin sections of migmatized gneiss from the Bukov Underground Research Facility (URF), Czech Republic, was characterized by microfocus time-resolved laser-induced luminescence spectroscopy (μTRLFS) with a spatial resolution of ∼20 μm, well below typical grain sizes of the material. By this approach, sorption processes can be characterized on the molecular level while maintaining the relationship of the speciation with mineralogy and topography. The sample mineralogy was characterized by powder X-ray diffraction and Raman microscopy, and the sorption was independently quantified by autoradiography using ¹⁵²Eu. Representative μTRLFS studies over large areas of multiple mm² reveal that sorption on the heterogeneous material is not dominated by any of the typical major constituent minerals (quartz, feldspar, and mica). Instead, minor phases such as chlorite and prehnite control the Eu(III) distribution, despite their low contribution to the overall composition of the material, as well as common but less studied phases like Mg-hornblende. In particular, prehnite shows high a sorption uptake as well as strong binding of Eu to the mineral surface. Sorption on prehnite and hornblende happens at the expense of feldspar, which showed the highest sorption uptake in a previous spatially resolved study on granitic rock. Similarly, sorption on quartz is reduced, even though only low quantities of strongly bound Eu(III) were found here previously. Our results illustrate how competition of mineral surfaces for adsorbing cations drives the metal distribution in heterogeneous systems.

A new effective way to degrade methylene blue by introducing negative ions powder into Fe3O4/H2O2 system to accelerate Fe(III)/Fe(II) transformation

Negative ions powders (NIP) have been widely applied in many fields because of their natural electric field and far infrared radiation, especially in wastewater treatment. In this study, the NIP was first introduced into Fe₃O₄/H₂O₂ system to degrade methylene blue (MB). The MB removal was completely achieved at 5 h via a non-photochemical pathway and the degradation rate constant of this system is about 0.565 h^-1, which is about 16 times higher than in Fe₃O₄/H₂O₂ Fenton-like system (0.035 h^-1). In addition, the results of quenching experiments indicate that the electron (e^-) and negative oxygen ion (•O₂^-) are the main reactive species. It was determined that Fe₃O₄@NIP is the effective component that leads to the activation of H₂O₂ to produce •OH, which derive from the pathway: NIP acts as an electron donor to reduce Fe(III) into Fe(II). Moreover, NIP can produce negative ions, which is also conductive to degradation. This study suggests a promising direction for the practical application of NIP based catalysis by integrating it with the Fe(III)/Fe(II) transformation process.