Resonant Anomalous X-ray Reflectivity Research Articles

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.

Direct Experimental Observations of Ion Distributions during Overcharging at the Muscovite-Water Interface by Adsorption of Rb+ and Halides (Cl- , Br- , I- ) at High Salinity.

Classical electric double layer (EDL) models have been widely used to describe ion distributions at charged solid-water interfaces in dilute electrolytes. However, the chemistry of EDLs remains poorly constrained at high ionic strength where ion-ion correlations control non-classical behavior such as overcharging, i. e., the accumulation of counter-ions in amounts exceeding the substrate's surface charge. Here, we provide direct experimental observations of correlated cation and anion distributions adsorbed at the muscovite (001)-aqueous electrolyte interface as a function of dissolved RbBr concentration ([RbBr]=0.01-5.8 M) using resonant anomalous X-ray reflectivity. Our results show alternating cation-anion layers in the EDL when [RbBr]≳100 mM, whose spatial extension (i. e., ~20 Å from the surface) far exceeds the dimension of the classical Stern layer. Comparison to RbCl and RbI electrolytes indicates that these behaviors are sensitive to the choice of co-ion. This new in-depth molecular-scale understanding of the EDL structure during transition from classical to non-classical regimes supports the development of realistic EDL models for technologies operating at high salinity such as water purification applications or modern electrochemical storage.

Inhibition of Strontium Adsorption and Desorption by Ethylenediaminetetraacetic Acid at the Barite (001)–Water Interface

Understanding the sorption behavior of ions at the mineral–water interface is important to determine the fate of elements in the environment. Here, we used barite to understand metal and organic molecule interactions with ionic crystals as organic molecules are used to remove scale that can form during oil recovery in sulfate-rich settings. The sorption behavior of strontium (Sr) on the (001) surface in the presence of ethylenediaminetetraacetic acid (EDTA) was measured using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). Based on the XR results, the presence of EDTA disrupts the barite surface structure, particularly that of the top two monolayers. A maximum of 0.7 (±0.2) EDTA molecules per surface unit cell are adsorbed, though the exact amount cannot be distinguished from that of other adsorbed species, such as Ba2+ and H2O. Based on the RAXR results, the presence of EDTA inhibits Sr incorporation into the barite surface and binds with Sr in solution to limit the amount of Sr2+ in solution. The Sr-EDTA2– complex then binds with barium ions at the barite surface, presumably in a bi-nuclear, bi-dentate state. Desorption of the Sr was also measured in the presence of EDTA. The desorption experiments show that ∼75% of the sorbed strontium was removed following extended reactions with EDTA and deionized water. There were minimal changes in the surface structure following desorption measurements implying that changes following Sr sorption in the presence of EDTA may be irreversible.

Publisher Correction: Resonant anomalous X-ray reflectivity for revealing ion distributions near electrodes

Publisher Correction: Resonant anomalous X-ray reflectivity for revealing ion distributions near electrodes

Resonant anomalous X-ray reflectivity for revealing ion distributions near electrodes

Resonant anomalous X-ray reflectivity for revealing ion distributions near electrodes

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ∼240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Simultaneous Adsorption and Incorporation of Sr2+ at the Barite (001)–Water Interface

Ionically bonded minerals are ubiquitous and play a determinative role in controlling the mobility of toxic metals in natural environments. However, little is known about the mechanism of ion uptake by these mineral surfaces. Here, the sorption of strontium ions (Sr2+) to the barite (001)–water interface was studied using a combination of synchrotron X-ray scattering and three types of computational simulations (density functional theory, classical molecular dynamics (CMD), and CMD-metadynamics). In situ resonant anomalous X-ray reflectivity (RAXR) revealed that Sr2+ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with CMD simulations confirmed the thermodynamic stability of these species, demonstrating almost equal magnitudes in the free energy of sorption between these species. Metadynamics simulations showed a more detailed feature in the free energy landscape for metal adsorption where a...

Competitive Adsorption of ZrO2 Nanoparticle and Alkali Cations (Li+–Cs+) on Muscovite (001)

We studied the adsorption behavior of ZrO2 nanoparticles on a muscovite (001) surface in the presence of cations from the alkali series (Li+, Na+, K+, Rb+, and Cs+). The results of X-ray reflectivity, i.e., specular crystal truncation rod and resonant anomalous X-ray reflectivity in combination with AFM images, show that the sorption of ZrO2 nanoparticles is significantly affected by the binding mode of alkali ions on the muscovite (001) surface. From solutions containing alkali ions binding as outer sphere surface complexes (i.e., Li+ and Na+), higher uptake of Zr4+ is observed corresponding to the binding of larger nanoparticles, which relatively easily replace the loosely bound alkali ions. However, Zr4+ uptake in solutions containing alkali ions binding as inner sphere surface complexes (i.e., K+, Rb+, and Cs+) is significantly lower, and smaller nanoparticles are found at the interface. In addition, the uptake of Zr4+ in the presence of inner sphere bound cations displays a strong linear relationship with the hydration energy of the coexisting alkali ion. The linear trend can be interpreted as competitive adsorption between ZrO2 nanoparticles and inner sphere bound alkali cations, which are replaced on the surface and undergo rehydration after release to the solution. The rehydration of alkali ions gives rise to a large energy gain, which dominates the reaction energy of the competitive adsorption process. The competitive adsorption mechanism of ZrO2 nanoparticles and alkali ions is discussed comprehensively to highlight the potential relationship between the hydration effect of alkali ions and the effect of charge density of the nanoparticles.

Formation and Aggregation of ZrO2 Nanoparticles on Muscovite (001)

The aggregation of nanoparticles is a key step in the formation of solid phases and a controlling factor for the behavior of suspended nanoparticles in solution. Using a charged mineral surface [muscovite (001)], we apply the surface X-ray diffraction techniques crystal truncation rod (CTR) measurements and resonant anomalous X-ray reflectivity (RAXR) to investigate the aggregation process of Zr nanoparticles at the sub-nanometer scale. The aggregation process was studied as a function of ionic strength (0, 1, 10, and 100 mM NaCl), and the interfacial particles were characterized by CTR/RAXR and AFM. The observations are consistent with an aggregation process that follows a multistep mechanism, which starts with the 3D aggregation of primary building units to form nanosheets. These sheets continue to grow through addition of building units to their reactive edges at higher ionic strength. Once the size and concentration of aggregates are sufficient, “face-to-face” stacking of nanosheets becomes the preferred aggregation mechanism as this minimizes the electrostatic repulsion of the charge that accumulates along nanosheet edges.

Zinc Adsorption and Hydration Structures at Yttria-Stabilized Zirconia Surfaces

Zinc adsorption and interfacial hydration on yttria-stabilized zirconia (YSZ) surfaces in contact with aqueous zinc solutions at room temperature and neutral pH have been probed with combined specular high-resolution X-ray reflectivity and element-specific (Zn) resonant anomalous X-ray reflectivity techniques. The total and partial zinc-specific electron density profiles in the surface normal direction show the detailed interfacial hydration structures with zinc adsorption. Strongly depending on its crystallographic orientations, the YSZ (110) surface adsorbs zinc species only within adsorbed water layers above the terminal plane, while on (111) surface, zinc further penetrates the substrate (below the terminal plane). Considering that both surfaces are enriched with oxygen vacancies and metal-depleted sites, on which chemisorbed water species are expected, the observed contrast indicates that specific zinc adsorption is controlled strongly by the intrinsic surface chemistry that results from orientation-...

Real-time observation of cation exchange kinetics and dynamics at the muscovite-water interface

Ion exchange at charged solid–liquid interfaces is central to a broad range of chemical and transport phenomena. Real-time observations of adsorption/desorption at the molecular-scale elucidate exchange reaction pathways. Here we report temporal variation in the distribution of Rb+ species at the muscovite (001)–water interface during exchange with Na+. Time-resolved resonant anomalous X-ray reflectivity measurements at 25 °C reveal that Rb+ desorption occurs over several tens of seconds during which thermodynamically stable inner-sphere Rb+ slowly transforms to a less stable outer-sphere Rb+. In contrast, Rb+ adsorption is about twice as fast, proceeding from Rb+ in the bulk solution to the stable inner-sphere species. The Arrhenius plot of the adsorption/desorption rate constants measured from 9 to 55 °C shows that the pre-exponential factor for desorption is significantly smaller than that for adsorption, indicating that this reduced attempt frequency of cation detachment largely explains the slow cation exchange processes at the interface.

X-ray Analyses of Lead Adsorption on the (001), (110), and (012) Hematite Surfaces.

Predicting the environmental fate of lead relies on a detailed understanding of its coordination to mineral surfaces, which in turn reflects the innate reactivity of the mineral surface. In this research, we investigated fundamental dependencies in lead adsorption to hematite by coupling extended X-ray absorption fine structure (EXAFS) spectroscopy on hematite particles (10 and 50 nm) with resonant anomalous X-ray reflectivity (RAXR) to single crystals expressing the (001), (012), or (110) crystallographic face. The EXAFS showed that lead adsorbed in a bidentate inner-sphere manner in both edge and corner sharing arrangements on the FeO6 octahedra for both particle sizes. The RAXR measurements confirmed these inner-sphere adsorption modes for all three hematite surfaces and additionally revealed outer-sphere adsorption modes not seen in the EXAFS. Lead uptake was larger and pH dependence was greater for the (012) and (110) surfaces, than the (001) surface, due to their expressing singly- and triply coordinated oxygen atoms the (001) surface lacks. In coupling these two techniques we provide a more detailed and nuanced picture of the coordination of lead to hematite while also providing fundamental insight into the reactivity of hematite.

A Comparison of Adsorption, Reduction, and Polymerization of the Plutonyl(VI) and Uranyl(VI) Ions from Solution onto the Muscovite Basal Plane.

X-ray scattering techniques [in situ resonant anomalous X-ray reflectivity (RAXR) and specular crystal truncation rods (CTR)] were used to compare muscovite (001) surfaces in contact with solutions containing either 0.1 mM plutonyl(VI) or 1 mM uranyl(VI) at pH = 3.2 ± 0.2, I(NaCl) = 0.1 M, as well as in situ grazing-incidence X-ray absorption near-edge structure (GI XANES) spectroscopy and ex situ alpha spectrometry. Details of the surface coverage are found to be very different. In the case of Pu, alpha spectrometry finds a surface coverage of 8.3 Pu/AUC (AUC = 46.72 Å2, the unit cell area), far in excess of the 0.5 Pu/AUC expected for ionic adsorption of PuO22+. GI XANES results show that Pu is predominantly tetravalent on the surface, and the CTR/RAXR results show that the adsorbed Pu is broadly distributed. Taken together with previous findings, the results are consistent with adsorption of Pu in the form of Pu(IV)-oxo-nanoparticles. In contrast, uranyl shows only negligible, if any, adsorption according to all methods applied. These results are discussed and compared within the context of known Pu and U redox chemistry.

Hydration Structures and Water Chemistry at Zirconia-Water Interface

The life cycle of zirconium cladding is a key safety factor and design parameter for higher fuel burnups in pressurized water reactors. The degradation of the metal alloy is primarily attributed to interactions of native oxide with water. Here we probe the detailed interfacial hydration structures at the 8 mol% yttria-stabilized cubic zirconia (YSZ) surfaces as a model system using synchrotron-based high-resolution X-ray reflectivity technique. Interfacial hydration structures in the major crystallographic orientations, (100), (110), and (111), were determined with sub-angstrom resolution and compared with each other to identify common features as well as different surface chemistry effects on the interfacial processes. All three surfaces terminate with significant number of point defects originating from the metal depletion and the intrinsic oxygen vacancies. Water molecules fill those vacancies, forming an ordered, layered structure on the top surface. Above the termination layer on each surface, two additional adsorbed layers form characteristic interfacial hydration structures. The first adsorbed layers likely include metal species based on the observed local densities, whereas the second layers consist of pure water. The current results provide valuable references for future studies of water chemistry effect on the interfacial hydration structure and the high temperature stability test of the zirconia in pressurized water conditions simulating realistic environment in pressurized water reactors of nuclear power plants. We also studied the effect of zinc adsorption on the interfacial hydration structure in zinc acetate solutions. Zinc injection into the reactor coolant system has been known to be effective in reducing the radioactive wastes as well as stabilizing crud oxide layers of the cladding materials. Our X-ray reflectivity data qualitatively reveal that there are obvious hydration structure changes at (100) and (111) surfaces while (110) surface show little changes. The interfacial electron density profiles were derived from the structure factor analysis of the measured reflectivity data. The structure factor analysis indicated an inner-sphere adsorption of Zn2+ layer at YSZ (111)-water interface, while there appeared to be an outer-sphere adsorption of Zn2+ layer at YSZ (100)-water interface. We further confirmed the detailed element specific adsorption profile of Zn2+ ions on YSZ (110) and (111)-water interfaces with resonant anomalous X-ray reflectivity measurements.

Surface Charge of the Calcite (104) Terrace Measured by Rb+ Adsorption in Aqueous Solutions Using Resonant Anomalous X-ray Reflectivity

Adsorption of Rb+ on the (104) plane of single crystal calcite was investigated to estimate the charge of the ionic crystal calcite–water interface. The adsorbed Rb+ coverage was quantified as a function of Rb concentration (1–100 mM) in calcite-saturated solutions at pH 8.3 by using in situ resonant anomalous X-ray reflectivity in transmission cell geometry. The Rb+ coverages for all solution conditions were small with the maximum ion coverage (Γmax) of 0.12(4) Rb+/nm2 estimated by the best-fit Langmuir isotherm model. This result provides an estimate of the upper limit to the effective surface charge density of about −0.02 C/m2 considering that any Rb+ adsorption to the terrace plane is likely induced by electrostatic attraction. This charge density is significantly lower (by a factor of ≥10) than those estimated from macroscopic measurements, implying that any excess charge likely originates from surface defects.

Effects of the background electrolyte on Th(IV) sorption to muscovite mica

The adsorption of tetravalent thorium on the muscovite mica (001) basal plane was studied by X-ray crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR) measurements and alpha spectrometry in the presence of perchlorate background electrolytes LiClO4, NaClO4, and KClO4 ([Th(IV)]=0.1mM, I=0.1M or 0.01M, pH=3.3±0.3). RAXR data directly reveal a strong influence of the background electrolyte on the actinide sorption. No significant Th adsorption was observed in 0.1M NaClO4, i.e., the Th coverage θ(Th), the number of Th per unit cell area of the muscovite surface (AUC=46.72Å2), was ⩽0.01 Th/AUC, whereas limited uptake (θ(Th)∼0.04Th/AUC) was detected at a lower ionic strength (I=0.01M). These results are in stark contrast to the behavior of Th in 0.1M NaCl which showed a coverage of 0.4Th/AUC (Schmidt et al., 2012a). Th uptake was also influenced by the electrolyte cation. Weak adsorption was observed in 0.1M KClO4 (θ(Th)∼0.07Th/AUC) similar to the results in NaClO4 at lower ionic strength. In contrast, strong adsorption was found in 0.1M LiClO4, with θ(Th)=4.9Th/AUC, a ∼10-fold increase compared with that previously reported in NaCl. These differences are confirmed independently by ex situ alpha spectrometry, which shows no measurable Th coverage in 0.1M NaClO4 background in contrast to a large coverage of 1.6Th/AUC in 0.1M LiClO4. Alpha spectrometry cannot be obtained in situ, but sample preparation requires several washing steps that may affect Th(IV) sorption, RAXR, however, is considered to reflect the in situ sorption structure. The CTR/RAXR analyses of Th-LiClO4 show the sorption structure consisting of Th species that are broadly distributed, centered at heights of 4.1Å and 29Å distance from the interface. Neither the very large distribution height of the second species nor the high coverage can be explained with (hydrated) ionic adsorption, suggesting that the enhanced uptake is presumably due to the formation and sorption of Th nanoparticles.

Rb+ Adsorption at the Quartz(101)–Aqueous Interface: Comparison of Resonant Anomalous X-ray Reflectivity with ab Initio Calculations

Adsorption of Rb+ to the quartz(101)–aqueous interface at room temperature was studied with specular X-ray reflectivity, resonant anomalous X-ray reflectivity, and density functional theory. The interfacial water structures observed in deionized water and 10 mM RbCl solution at pH 9.8 were similar, having a first water layer at height of 1.7 ± 0.1 Å above the quartz surface and a second layer at 4.8 ± 0.1 Å and 3.9 ± 0.8 Å for the water and RbCl solutions, respectively. The adsorbed Rb+ distribution is broad and consists of presumed inner-sphere (IS) and outer-sphere (OS) complexes at heights of 1.8 ± 0.1 and 6.4 ± 1.0 Å, respectively. Projector-augmented planewave density functional theory (DFT) calculations of potential configurations for neutral and negatively charged quartz(101) surfaces at pH 7 and 12, respectively, reveal a water structure in agreement with experimental results. These DFT calculations also show differences in adsorbed speciation of Rb+ between these two conditions. At pH 7, the lowest energy structure shows that Rb+ adsorbs dominantly as an IS complex, whereas at pH 12 IS and OS complexes have equivalent energies. The DFT results at pH 12 are generally consistent with the two site Rb distribution observed from the X-ray data at pH 9.8, albeit with some differences that are discussed. The surface charge estimated on the basis of the measured total Rb+ coverage was −0.11 C/m2, in good agreement with the range of the surface charge magnitudes reported in the literature.

Incorporation of Pb at the calcite (104)-water interface.

Lead (Pb) is a common environmental pollutant, and its transport in surface waters and groundwater is controlled in part by sorption and precipitation reactions at mineral surfaces. Using in situ specular and resonant anomalous X-ray reflectivity measurements, we investigated the interaction of the calcite (104) surface with a dilute Pb- and EDTA-bearing solution that is slightly undersaturated with respect to calcite. The X-ray results reveal Pb coherently substituting for Ca in the near-surface layers of strained calcite with Pb/(Pb + Ca) atom fractions as high as 0.28 in the outermost layer. The larger ionic radius of Pb(2+) relative to Ca(2+) is accommodated in calcite by vertical displacements of Pb relative to the Ca site. In situ atomic force microscopy images obtained during the reaction suggest that Pb incorporation below the surface occurs after initial dissolution followed by regrowth of a strained epitaxial Pb-rich calcite solid-solution at the calcite (104)-water interface. This process could produce a widespread host phase for Pb in groundwater aquifers and soil pore fluids.

Surface-Mediated Formation of Pu(IV) Nanoparticles at the Muscovite-Electrolyte Interface

The formation of Pu(IV)-oxo-nanoparticles from Pu(III) solutions by a surface-enhanced redox/polymerization reaction at the muscovite (001) basal plane is reported, with a continuous increase in plutonium coverage observed in situ over several hours. The sorbed Pu extends >70 Å from the surface with a maximum concentration at 10.5 Å and a total coverage of >9 Pu atoms per unit cell area of muscovite (0.77 μg Pu/cm(2)) (determined independently by in situ resonant anomalous X-ray reflectivity and by ex-situ alpha-spectrometry). The presence of discrete nanoparticles is confirmed by high resolution atomic force microscopy. We propose that the formation of these Pu(IV) nanoparticles from an otherwise stable Pu(III) solution can be explained by the combination of a highly concentrated interfacial Pu-ion species, the Pu(III)-Pu(IV) redox equilibrium, and the strong proclivity of tetravalent Pu to hydrolyze and form polymeric species. These results are the first direct observation of such behavior of plutonium on a naturally occurring mineral, providing insights into understanding the environmental transport of plutonium and other contaminants capable of similar redox/polymerization reactions.

Investigation of Structure, Adsorption Free Energy, and Overcharging Behavior of Trivalent Yttrium Adsorbed at the Muscovite (001)–Water Interface

The speciation, coverage, and thermodynamics of trivalent Y3+ adsorbed at a negatively charged muscovite (001)–water interface were measured to investigate electrostatic ion–ion correlations in a strong coupling regime. In situ specular crystal truncation rod and resonant anomalous X-ray reflectivity data show that Y3+ adsorbs as three distinct species (inner-sphere, adsorbed outer-sphere, and extended outer-sphere) among which the fractional coverage of the inner-sphere species (∼10%) is the smallest. Uptake measurements show that the maximum Y3+ coverage is higher by 50% than the amount needed to satisfy the muscovite surface charge in the absence of background electrolyte, but this overcharging is largely suppressed in the presence of 0.1 m NaCl. The measured intrinsic standard-state Gibbs free energy for Y3+ adsorption (−36.9 ± 0.9 kJ/mol) is smaller than that expected solely on the basis of its charge, and this discrepancy can be partly explained by the difference in adsorbed cation speciation at the charged interface.