Uranyl Adsorption Research Articles

The collaboratively selective uranyl adsorption of marine fungal modification biosorbent linked by the open-chain polyether terminal with amidoxime

Abstract To further improve the uranyl adsorption capacity and the selectivity adsorption ability of marine fungus ZZF51 from Zhanjiang sea area in China, its two new modification biosorbents (ZTBA/ZTDA) linked by the open-chain polyether terminal with two/one amidoxime unit(s) on mycelium were designed according to the synthesis process of etherification, sulfonylation, substitution, and amidoximation. By the reasonable characterization of Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TG), and scanning electron microscopy (SEM), it was confirmed that the above target materials were successfully prepared. The relevant experiments showed that both of ZTBA and ZTDA had not only the excellent uranium (VI) adsorption performance with the maximum adsorption capacity of 525.7 mg g−1 and 465.7 mg g−1, respectively, but also the better uranyl adsorption selectivity when in the simulated wastewater containing the various ions of UO2 2+, Th4+, Ba2+, Pb2+, Fe3+, Cu2+, and Ca2+. In addition, the selectivity analysis explored the longer polyether in the middle bridge and the more number of terminal amidoxime unit could synergistically improve their uranyl adsorption capacity and selectivity performance. Surely, the adsorption isotherm/kinetics models, the Gibbs free energy analysis, and the favourable reusability of the target materials were also discussed in this study in detail.

Surface-Activated Graphite Oxide Nanosheets with Promoted U(VI) Adsorption Performance

Adsorption is widely regarded as the most promising method for uranium extraction. Among the various materials that have been studied, graphene oxide (GO) has attracted intensive interest because of its large specific surface area and abundant oxygen-containing functional groups. However, the layers tend to aggregate owing to pronounced Van der Waals forces, which reduce the surface area and diminish the likelihood of contact between uranyl ions and adsorption sites. Graphite oxide is an intermediate product of GO, with a simple preparation process and low cost. In this study, graphite oxide nanosheets (GONs) were synthesized using graphite oxide powder as the raw material and the NaOH activation method. GONs possessed a larger specific surface area and more carboxyl groups, which resulted in an excellent uranium adsorption capacity. The maximum adsorption capacity was found to be 578.0 mg·g−1, and the adsorption rate was 90.8% within 30 min. The adsorption process closely resembled the pseudo-second-order model and the Langmuir model. The mechanism of uranium adsorption by GONs was the synergistic coordination of -COOH and -OH with U(VI). This research suggests that the novel uranium adsorbent GONs can be applied to efficiently capture U(VI) from radioactive wastewater.

Polyamidoxime‐Coated Coconut Haustorium Derived Magnetic Biochar Adsorbent with Photothermal Conversion for Highly Efficient Uranium Recovery from Nuclear Wastewater

AbstractEfficient removal of uranium from uranium‐containing nuclear wastewater is of great significance for utilizing uranium resources and protecting the environment from uranium hazards. In this paper, a polyamidoxime‐coated coconut haustorium derived magnetic biochar (CHMBC‐PAO) composite adsorbent with excellent adsorption performance, magnetism, recyclability, and photothermal conversion properties in complex environments is prepared through a simple soak‐gelation coating method for efficient removal of uranium from nuclear wastewater. The coconut haustorium biochar, carbonized by a sponge‐like organ inside the sprouted coconut, can provide rich functional groups and electronegative surface, which are conducive to the loading of functional materials and the adsorption of uranyl, respectively. The CHMBC‐PAO presents 2.4 emu g−1 saturation magnetization, making it can be efficiently recovered through magnetic recovery. It can convert absorbed light into thermal energy through molecular thermal vibration and non‐radiative relaxation process of excitons. Enhanced by the photothermal effect, CHMBC‐PAO shows an extra high uranium adsorption capacity of 310.30 mg g−1 in acidic nuclear wastewater within a short period of time, surpassing the adsorption capacity of most reported uranium removal materials. Meanwhile, the CHMBC‐PAO also presented excellent ion selectivity and reusability. Therefore, CHMBC‐PAO composite adsorbent can be considered as a potential candidate for treating radioactive nuclear wastewater.

Facile and scalable synthesis of functionalized hierarchical porous polymers for efficient uranium adsorption

Efficient uranium capture from wastewater holds great importance for the environmental remediation and sustainable development of nuclear energy, but it is a tremendous challenge. Herein, a facile and scalable approach is reported to fabricate functionalized hierarchical porous polymers (PPN-3) decorated with high density of phosphate groups for uranium adsorption. The as-constructed hierarchical porous structure could allow rapid diffusion of uranyl ions, while abundant phosphate groups that serve as adsorption sites could provide the high affinity for uranyl. Consequently, PPN-3 shows a high uranium adsorption uptake of 923.06 mg g−1 and reaches adsorption equilibrium within simply 10 min in uranium-spiked aqueous solution. Moreover, PPN-3 affords selective adsorption of uranyl over multiple metal ions and possesses a rapid and high removal rate of U(VI) in real water systems. Furthermore, this study offers direct polymerization strategy for the cost-effective fabrication of phosphate-functionalized porous organic polymers, which may provide promising application potential for uranium extraction.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012).

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

Novel chitosan-based derivative material loaded with the diethylenetriamine pentamethylene phosphonic acid for efficient adsorption of uranyl ions from weakly acidic wastewater

An ideal adsorption material capable of efficiently and selectively capture uranyl ions from radioactive wastewater remains a serious challenge. In this work, diethylenetriamine pentamethylene phosphonic acid (DTPMP), one of the common metal ion extractants with five negatively charged phosphate groups, was innovatively used to modify chitosan chains for better uranyl adsorption capacity. Two chitosan derivatives were designed, DTPMP protonated chitosan crosslinked with potassium tripolyphosphate (DTPP) and DTPMP reinforced potassium tripolyphosphate crosslinked chitosan (DCTPP), in which DCTPP possessed an excellent maximum adsorption capacity of up to 1316.82 mg/g for uranyl ions at 298 K. Compared to the 60 min taken-for 90 % by DTPP, the time taken to reach same percentage by DCTPP was only about 5 min, indicating its superior uranyl removal efficiency. Outstanding affinity and capture performance could be attributable to the remarkable mesoporous surfaces and abundant P-containing active sites. Influence of initial solution pH on the removal efficiency for uranyl was significantly attenuated due to the C-P bond. The fitting results of kinetic and isotherm models revealed that the uranyl adsorption mechanisms by as-prepared composites were physicochemical processes, mainly including surface complexation/chelation, chemisorption, and intra-particle diffusion. Theory calculations predicted potential binding modes and electrons transfer between the organophosphonic acid molecule and uranyl unit, validating the strong interaction and reduction of phosphorus-oxygen groups on uranyl ions. Furthermore, DCTPP showed great potentials to separate uranyl from simulated uranium-contaminated wastewater. Design and development of novel chitosan derivatives will provide theoretical support and technical reference for an effective treatment of weakly acidic radioactive wastewater.

Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at −0.34 V (vs. standard hydrogen electrode), indicative of U60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at −0.71 V is assigned to the subsequent U60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e- (per step) reduction of U60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and −0.05 V (U(V)/U(IV)). For uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum–mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (EH-pH) diagrams for adsorbed and structural species.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Adsorption of uranium (VI) from aqueous solutions using boron nitride/polyindole composite adsorbent

AbstractTurbostratic boron nitride (tBN) surface is modified with polyindole (PIn) by a facile polymerization technique and the uranyl adsorption efficiency of this mesoporous hybrid is investigated. The successful surface modification is confirmed by FT‐IR, Raman, XRD, TEM, SEM, EDX, EDS mapping XPS, BET, and zeta potential techniques. The batch experiments are performed in various temperatures (T), contact times (t), pH, and initial solution concentrations (C0) to evaluate its adsorption performance. The optimum adsorption performance is achieved at pH = 5.0–5.5, T = 307 K, t = 10 min, C0 = 18 mg L−1. These experimental results are evaluated using Freundlich, Redlich–Peterson, and Langmuir isotherm models, which presents equivalent regression coefficients. Maximum adsorption capacity (qm) of the nanoadsorbent (tBN/PIn), determined by the Langmuir isotherm, is 315.29 mg g−1. The adsorption kinetics of uranyl ions on tBN/PIn are in harmony with the pseudo‐second order model. tBN/PIn nanoadsorbent provides high adsorption efficiency even at exceptionally low UO22+ concentration range (4–40 mg L−1) and low adsorbent mass (0.005 g). XPS analysis results show that 0.05% of uranium is adsorbed on tBN/PIn via mainly U‐O coordination. The results of present study demonstrate that tBN/PIn can a potential adsorbent for removing uranium from aqueous solutions.

Self-Aggregated Nanoscale Metal-Organic Framework for Targeted Pulmonary Decorporation of Uranium.

The limited availability of effective agents for removing actinides from the lungs significantly restricts the effectiveness of medical treatments for nuclear emergencies. Inhalation is the primary route of internal contamination in 44.3% of actinide-related accidents, leading to the accumulation of radionuclides in the lungs and resulting in infections and potential tumor formation (tumorigenesis). This study focuses on the synthesis of a nanometal-organic framework (nMOF) material called ZIF-71-COOH, which is achieved by post-synthetic carboxyl functionalization of ZIF-71. The material demonstrates high and selective adsorption of uranyl, while also exhibiting increased particle size (≈2100nm) when it aggregates in the blood, enabling passive targeting of the lungs through mechanical filtration. This unique property facilitates the rapid enrichment and selective recognition of uranyl, making nano ZIF-71-COOH highly effective in removing uranylfrom the lungs. The findings of this study highlight the potential of self-aggregated nMOFs as a promising drug delivery system for targeted uranium decorporation in the lungs.

Uranyl adsorption: recent progress of covalent organic framework materials

Uranyl adsorption: recent progress of covalent organic framework materials

Analysis of Uranium Sorption in a Laboratory Column Experiment Using a Reactive Transport and Surface Complexation Model

The transport and retardation of radioactive elements in hyper-alkaline conditions of radioactive waste repositories is a challenging field that is still poorly understood. In this study, the transport and attenuation of uranium in a column experiment was modelled by considering kinetic reactions, advection–dispersion and chemical/physical retardation processes. The modelling was first performed for three alluvium samples from Yucca Mountain in circumneutral pH to moderately alkaline conditions. Sorption of uranyl ({mathrm{UO}}_{2}^{2+} ({U}_{mathrm{VI}})) was found to strongly depend on the surface complexation model assumed, with no significant removal of {U}_{mathrm{VI}} by precipitation or ion exchange process. The surface/edge site reaction of Al-hydroxyl group in kaolinite was shown to have a high affinity for uranyl adsorption, while the hydrous ferric oxide edge on hematite adsorbed most of the uranyl ions. The model was then used to interpret uranium transport in a laboratory column filled with Hollington sandstone under hyper-alkaline (pH 13) conditions. The simulation results show that uranium adsorption on the Al-hydroxyl edge of kaolinite exceeds adsorption by the calcium silicate hydrate phase. This result may reflect the lack of surface complexation parameters for calcium silicate hydrate minerals. Hence, further studies are required in the field of surface complexation reactions for calcium silicate hydrate phases.Graphical

Modeling Uranyl Adsorption on MoS2/Mo2CTx Heterostructures Using DFT and BOMD Methods.

Uranium-bearing wastewaters exert a great threat to the ecological environment due to its high radiotoxicity level. The designing and fabrication of novel adsorption materials can be promoted for radionuclide elimination from wastewater. In this work, results from density functional theory and Born-Oppenheimer molecular dynamics simulations are reported for the uranyl adsorption behavior on the MoS2/Mo2CTx heterostructure in the gas phase and in an aqueous environment. Uranyl ions prefer to be adsorbed at deprotonated O sites on the Mo2COH surface and S sites on the MoS2 side of the heterojunctions, resulting in the formation of bidentate configurations. In addition to coordination interaction, H-bond and van der Waals interactions can also play an important role in binding configurations. More importantly, the oxidation state U(VI) can be reduced to U(V) and then to U(IV) caused by the strong reducibility of the Mo2COH surface at room temperature, whereas the uranyl complex can move freely on the MoS2 surface. However, the coordination number of U with respect to H2O in the first hydration shell on the Mo2COH surface remains unchanged and is found to be 3, which is similar to that on the MoS2 surface. This work provides novel nanosorbents for the removal of uranyl from wastewater. The present viewpoint provides valuable mechanistic interpretations for uranyl adsorption and will give a supplement to the experimental research.

Highly efficient sulfonated poly (ether ether ketone) (sPEEK) adsorbent for removal of uranium (VI) from aqueous solution

Sulfonated polyether ether ketone (sPEEK) polymer matrix was used as adsorbent for uranyl adsorption for first time. sPEEK showed high affinity against uranyl cations and the maximum adsorption capacity was found as 0.82 mmol/g. The maximum adsorption was achieved at pH 6. The adsorption kinetic was described by pseudo-second order model. Pore diffusion was involved in adsorption but not rate controlling step. The adsorption reaction is endothermic, and the activation energy is 85.8 kJ/mol. The desorption kinetics into 2 M HNO3, 0.1 M Na2CO3, 0.05 M EDTA and 5 % citric acid solutions were searched, and 99 % desorption was achieved when 0.05 M EDTA and 2 M HNO3 were used as leaching solution. The recovery efficiency decreased from 98 % to 88 % in 0.05 M EDTA solution and from 96 % to 86 % in 2 M HNO3 solution at the end of the fifth cycle. Uranium removal and recovery from liquid waste generated by mining and nuclear plants is of great importance. This study presents an opportunity for highly efficient removal using a low-cost and easily handled sorbent, as well as high recovery of uranyl ions with the aid of a common eluent, allowing for five cycles.

Evaluation of humic acid embedded Chitosan/PVA composite performance in the removal of uranyl ions

In this study, the retention of uranyl ions on a novel, natural and cost-effective adsorbent, humic acid (HA) embedded chitosan/polyvinyl alcohol (Ch/PVA-HA) bio composite was investigated. The highest adsorption capacity reported in literature was achieved by Ch/PVA-HA in a column system in this study. Specific surface area of Ch/PVA-HA was found as 1.308 m2g-1 which is rather high comparing that of Ch/PVA (0.254 m2g-1). The maximum adsorption on Ch/PVA and Ch/PVA-HA were achieved in the range of 1 < pH < 5, and 1 < pH < 4, respectively. The batch tests showed that adsorption isotherm of uranyl ions was changed from Freundlich to Langmuir when the surface of Ch/PVA copolymer was covered with HA. The maximum uranyl adsorption capacity of Ch/PVA-HA was found as 37.8 mg/g while the column capacity was 1909 mg/g. The presence of HA enhanced the distribution ratio of uranyl ions between adsorbent phase and aqueous solution. Considering the results of the batch and column tests, it can be said that the Ch/PVA-HA may have high potential to use as alternative low-cost adsorbent for removal of uranyl ions. Also, this study will lead up synthesis of new adsorbents to provide higher and recoverable uranyl adsorption.

Precisely modulating the branching functional groups of MIL-53(Al) for highly efficient sequestration of uranium

To fully utilize the customizable characteristics of metal-organic framework (MOF) materials in environmental applications, it is important to precisely modulate the branching groups of MOFs for improved performance. In this work, MIL-53(Al) was modulated by different numbers of nitrogen atoms and carboxyl groups/phosphite groups (MOF-N2-(COOH)3, MOF-N3-(COOH)4, and MOF-N2-(PO(OH)2)3) via a post-synthesis modification method, and the influence of functional groups and chain length on adsorption and the mechanism of modified groups on the adsorption of uranium (U) are demonstrated. Batch experiments suggest the removal of U(VI) reaches 37%, 27%, and 90% for MOF-N2-(COOH)3, MOF-N3-(COOH)4 and MOF-N2-(PO(OH)2)3, respectively (pH = 4.0, T = 25 ℃, I = 0.01 M NaNO3). The surface complexation between UO22+ and the surface of functional groups on the modulated MOF dominates the adsorption. Nevertheless, the steric hindrance in MOF porous structure can explain the inferiority of U(VI) adsorption on MOF-N3-(COOH)4 (27%) to MOF-N2-(COOH)3 (37%). MOF-N2-(PO(OH)2)3 is more conducive to uranyl adsorption than MOF-N2-(COOH)3 or MOF-N3-(COOH)4, which is due to better affinity of U(VI) to phosphate group than the carboxyl group by density functional theory (DFT) calculation. Moreover, the excellent regeneration performance and U(VI) adsorption of MOF-N2-(PO(OH)2)3 in natural water or wastewater further confirmed the practical application of modulated MOFs in practical application.

Fabrication of phosphate-containing mesoporous carbon for fast and efficient uranium (VI) extraction

Uranium has been recognized as one of the most desired radionuclides that has long been used in reactor. The efficient extraction of uranyl (VI) ions from radioactive effluents and seawater is an ideal way to collect uranium, however it is always a challenge to develop high-efficiency adsorbents for effective uranium uptake. In this paper, a series of phosphate-containing mesoporous carbon (PC) are fabricated by the carbonization of sucrose and phytic acid upon using nanosized silica as hard template. The obtained PC have high-content phosphate group, large specific surface area and tunable mesoporous structure. The PC can efficiently adsorb uranium at a wide pH conditions. The maximum uptake capacity of PC for uranium reaches 928 mg g −1, ranking as one of the most highly effective adsorbents. The mesopores of PC enables them to quickly reach the adsorption equilibrium within 60 s and meantime PC show excellent adsorption selectivity. Investigations of adsorption mechanism reveals that uranyl (VI) ions tend to bind with the surface P-OH groups to form P-O-U complexes, leading to the effective uranium extraction. Moreover, PC displays good recyclability and can be further used for dynamic column adsorption of uranyl (VI) ion from uranium-containing solution. The excellent adsorption performances of PC highlight their great potential in uranium extraction.

Synthesis of hierarchical mesoporous cerium titanate brannerite and uranyl adsorption properties at pH 3.8.

Cerium titanates possessing brannerite structure are produced by employing soft and hard templates via sol-gel processing. Powders synthesized with various hard template sizes and template to brannerite weight ratios are composed of nanoscale 'building blocks' with size ∼20-30 nm and characterized on macro-, nano- and atomic scales. These polycrystalline oxide powders exhibit specific surface area up to ∼100 m2 g-1, pore volume ∼0.4 cm3 g-1, and uranyl adsorption capacity ∼0.221 mmol (53 mg) U per gram powder. Remarkably, the materials possess significant proportion of mesopores with 5-50 nm pores representing 84-98% of total pore volume, which facilitate fast accessibility of the adsorbate to the internal surfaces of the adsorbent with adsorbed uranyl reaching over 70% of the full capacity within 15 min of contact. These mesoporous cerium titanate brannerites synthesized by the soft chemistry route are highly homogenous, stable at least in 2 mol L-1 acidic or basic solution, and may attract attention for other applications like catalysis at high temperature.

Atom-scale understanding the adsorption mechanism of uranyl in the interlayer of montmorillonite: Insight from DFT + U calculation

In this work, the adsorption mechanism of uranyl with varied water ligands (UO22+·xH2O; x = 3, 4 and 5) in the interlayer of montmorillonite (MMT) was studied theoretically based on the first-principles DFT + U calculation. The adsorption energy, adsorption geometry configuration and electronic properties (PDOS, electron density difference, Bader charge, ELF) of MMT-UO22+·xH2O systems were investigated systematically. It was found that UO22+·5H2O and UO22+·3H2O are more likely adsorbed above OM (the bridging oxygen on the six-membered ring of SiO4 tetrahedra sheet, signed as T site) and UO22+·4H2O is preferentially adsorbed between OM atoms (signed as B site). DFT + U calculations showed that the migration of uranyl in the interlayer of MMT could be inhibited by forming hydrogen bonds through OM and water ligands. In addition, with the decrease of water ligands, the positive charge of UO22+ increases and UO22+ is directly exposed to the internal surface of MMT. Resultantly, the U atom in UO22+ can form weak chemical bond with OM, thus blocking the migration of UO22+. In summary, this work can provide insights into the adsorption of uranyl in the interlayer of MMT at the microscopic atom-scale, which contributes to understanding the experimentally based internal surface complexation model.

Probing Adsorption of Uranium Species at Electrochemical Interfaces in Support of Environmental Radiochemistry

Special nuclear material, like enriched uranium and plutonium, can contaminate environmental soils and water because of weapons development and testing or improper waste storage. To remediate affected lands and design long-term waste storage solutions, it is crucial to understand uranium speciation in the environment which depends on its complexation, oxidation state, and associated adsorption mechanisms. These phenomena are complex and poorly understood because they depend on many variables including the environmental pH, water or soil composition, uranium concentration, and surface chemistry of the minerals or soils. In previous studies, the proposed mechanism for adsorption of the hexavalent uranyl species, UVIO2 2+, on metal-oxide surfaces follows these general steps: 1) adsorption of UVIO2 2+ on the mineral, followed by 2) reduction to UVIO2 2+ due to the coupled reaction of a redox active species within the mineral. To gain better mechanistic insight into coupled electron transfer and adsorption phenomena associated with uranyl species, we demonstrate here a novel technique to probe an electrochemical interface to detect adsorbed uranium species under reducing conditions. In this technique, we monitor the transient response of the electrode potential immediately after electrochemical reduction of UVIO2 2+ on a rotating disc electrode is halted. This potential relaxation rate, when analyzed, contains information on presence and coverage of surface adsorbed species. In combination with electrochemical quartz crystal microgravimetry (e-QCM), these experiments confirm the presence of an adsorbed layer of uranium ions during the reduction of UVIO2 2+. To simulate uranyl adsorption on minerals under environmental conditions, future work will aim to probe adsorption onto a metal-oxide film prepared on an e-QCM electrode under varying redox and pH conditions.