Uranyl Peroxide Nanocluster (U60) Persistence and Sorption in the

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Article 60

Uranyl Peroxide Nanocluster (U ) Persistence and Sorption in the Presence of Hematite Luke R. Sadergaski, Wynn Stoxen, and Amy E. Hixon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06510 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Environmental Science & Technology

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Uranyl Peroxide Nanocluster (U60) Persistence and

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Sorption in the Presence of Hematite

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Luke R. Sadergaski, Wynn Stoxen, and Amy E. Hixon*

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Department of Civil and Environmental Engineering and Earth Sciences, University of Notre

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Dame, Notre Dame, Indiana 46556 USA

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Keywords: uranyl peroxide nanoclusters, hematite, sorption, aqueous speciation

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ABSTRACT

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The presence of uranium-based nanomaterials in environmental systems may significantly

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impact our current understanding of the fate and transport of U(VI). Sorption of the uranyl

11

peroxide nanocluster [(UO2)(O2)(OH)]6060- (U60), to hematite (α-Fe2O3) was studied using batch

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sorption experiments with varying U60, hematite, and alkali electrolyte (i.e., NaCl, KCl, and

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CsCl) concentrations. Data from electrospray ionization mass spectrometry and centrifugal

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microfiltration revealed that U60 persisted in the presence of hematite and the background

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electrolyte for at least 120 days. K+ ions were removed from solution with uranium whereas Li+

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ions remained in solution, indicating that the U60 cluster behaved like an anion and that the Li+

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ions did not play a significant role in the sorption mechanism. Analysis of the reacted mineral

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surface using X-ray photoelectron and Raman spectroscopies confirmed the presence of U(VI)

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and uranyl species with bridged peroxo groups associated with the mineral surface. These results

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indicate that uranyl peroxide nanoclusters may persist in the aqueous phase under

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environmentally-relevant conditions for reasonably long periods of time, as compared to that of

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the uranyl cation.

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INTRODUCTION

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Freshwater sources have become contaminated with uranium from naturally-occurring uranium

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minerals and anthropogenic activities such as mining and ore processing, high-level radioactive

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waste storage, and the use of fertilizers.1 The fate and transport of uranium is important for

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environmental and public health due to its toxicity and long half-life (e.g., 4.47 x 109 years for

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238

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readily dissolves and forms the uranyl ion (UO2)2+, which is soluble and stable in solution at pH

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< 5. At higher pH values, uranyl hydroxide (e.g., (UO2)3(OH)5+, UO2(OH)2(aq)), uranyl hydroxy

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carbonate (e.g., (UO2)2CO3(OH)3-), and uranyl carbonate (e.g., Ca2UO2(CO3)3) species readily

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form.1

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The fate and transport of uranium has been addressed in part by sorption experiments of (UO2)2+

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to various minerals in dynamic geochemical systems.2 Sorption studies of the uranyl ion to

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Fe(III) minerals, such as hematite (α-Fe2O3), have been carried out in great detail due to their

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abundance and high surface reactivity.3-7 However, there is an absence of research regarding the

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behavior of U-based nanoclusters in the environment.8

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Actinide peroxide nanoclusters are a large class of materials that rapidly self-assemble and

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persist in aqueous solution with more than 60 variations reported in the literature.9-11 Uranyl

U). Mobility is linked to oxidation state. Under oxidizing conditions hexavalent uranium


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peroxide nanoclusters form when U(VI) is combined with hydrogen peroxide under alkaline

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conditions. Although the uranyl peroxide clusters have not been observed in nature, they have

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the potential to form at locations such as the Hanford Site (Washington), Fukushima-Daiichi

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(Japan), the Savannah River Site (South Carolina), used nuclear fuel cooling pools, and future

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geologic waste repositories.12-13 These clusters may significantly impact the current

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understanding of the fate and transport of U(VI) in the environment if their behavior is different

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than that of the uranyl cation. Therefore, it is important to understand the solution properties and

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sorption characteristics of U-based nanoclusters from an environmental perspective.

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The uranyl peroxide nanocluster U60, for which the crystalline composition is

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Li44K16[(UO2)(O2)(OH)]60·255H2O,14 has been selected for this study. Containing sixty

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compositionally identical uranyl peroxide hydroxide polyhedra, U60 has the same topology as the

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Buckminsterfullerene C60.15 The exterior and interior of the uranyl peroxide cage is truncated by

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the relatively unreactive axial oxygen atoms of the uranyl ions. U60 behaves as an aqueous

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species when dissolved in aqueous solution16 and is stable in water for more than one year.12

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Aqueous solutions containing an excess of U60 crystals can reach concentrations up to 177,000

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ppm uranium at steady-state, which is considerably higher than what would be expected if

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simple U(VI) aqueous species prevailed in solution.17

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In solution U60 has the composition [(UO2)(O2)(OH)]6060- and the effective negative charge is

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partially balanced by Li+ and K+ counter-cations.18 Some of the counter-cations are located

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inside the cage while others are found within the electric double layer surrounding the cage.17 Li+

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ions have a higher propensity to dissociate from the uranyl peroxide cage due to its larger

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hydrated size, while K+ ions are more closely associated and are typically found in the

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pentagonal windows of the cage.15,18 The cage of U60 maintains an effective negative charge in


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solution that changes as a function of U60 concentration.16,19 One unexplored aspect of uranyl

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peroxide nanocluster chemistry is the behavior of these clusters at the mineral-water interface. It

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is not apparent if the nanoclusters will have a primarily cationic or anionic character when

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interacting with mineral surfaces.

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In typical sorption experiments, the sorption of discrete cations onto mineral surfaces increases

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with increasing pH. We hypothesized that U60 will exhibit the opposite trend: as the pH

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decreases, the fraction of U60 removed from solution will increase. U60 sorption will be

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dominated by electrostatic interactions between the negatively charged uranyl peroxide cage and

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a positively- charged mineral surface. To test this hypothesis, we have probed the removal of

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U60 from solution in the presence of hematite as a function of U60, hematite, and alkali salt

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concentrations.

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MATERIALS AND METHODS

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All ACS grade chemicals were commercially obtained and used as received unless otherwise

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stated.

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U60 Preparation and Characterization.

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Crystals containing U60 nanoclusters were synthesized according to published procedures.10

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Briefly, solutions of 0.5 M uranyl nitrate hexahydrate (UO2(NO3)2·6H2O), 0.4 M potassium

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chloride (KCl), and 30% hydrogen peroxide (H2O2) were combined in glass vials. At pH 9,

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achieved by the addition of 2.4 M lithium hydroxide hydrate (LiOH·H2O), crystals of U60 form

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within 7 to 10 days. The crystals were collected with a Buchner funnel, rinsed with Milli-Q water

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(18.2 MΩ·cm at 25°C), and examined under a microscope to confirm homogeneity and purity. A

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70 mg/mL stock solution of U60 was made by dissolving recovered crystals in Milli-Q water.


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Single crystal X-ray diffraction (SC-XRD) was used to definitively identify the nanocluster by

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matching the unit cell with published data from Olds et al.14 SC-XRD data were collected at 100

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K using a Bruker APEXII single-crystal diffractometer with monochromated Mo Kα X-ray

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radiation. Electrospray ionization mass spectrometry (ESI-MS) has been used successfully as a

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method for “fingerprinting” nanoclusters in solution.20 In this study, ESI-MS was used to

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confirm that the U60 stock solution was monodisperse and that U60 remained stable in solution

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throughout the experiments. U60 spectra were collected in negative ion mode using a Bruker

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microTOF-Q II high resolution quadrupole time-of-flight spectrometer. Data was averaged over

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180 seconds for the range of 1000 – 5000 m/z. Samples were introduced by direct infusion at

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rates between 300 - 600 µL/min.

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Hematite Synthesis and Characterization.

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Hematite was synthesized according to the method of Schwertmann and Cornell.21 A complete

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description of synthetic procedure is provided in Supporting Information. Hematite was

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characterized by powder X-ray diffraction (5-60° 2-theta, Cu-Kα radiation) and Raman

99

spectroscopy, and no impurities were detected. The specific surface area was determined by N2

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adsorption-desorption at 77 K with a Micromeritics ASAP 2020 accelerated surface area and

101

porosimetry system. The sample was degassed at 100°C for 24 hours before Brunauer-Emmett-

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Teller (BET) surface area analysis. The specific surface area of hematite used in this study was

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determined to be 36.4 m2/g, which is typical of iron oxide minerals.21

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ICP-OES Analysis

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Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the

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concentration of each element in the reactor solutions. The elemental analyses were evaluated


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using a PerkinElmer Optima 8000 DV ICP-OES instrument with 165 – 800 nm coverage and a

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resolution of approximately 0.01 nm for multi-elemental analysis. Aliquots from the reactors

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were dissolved in 10 mL of 5% nitric acid. External calibration was used to determine the

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unknown elemental concentrations of U, K, Li, Fe and Na. Each standard, blank, and sample

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contained 1 ppm Y as the internal standard to monitor for instrument drift.

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Raman Spectroscopy

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Raman spectroscopy was used to determine the presence of vibrational frequencies associated

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with hematite as well as uranyl and peroxide stretches at the beginning and end of the batch

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sorption experiments.20,22-23 Raman spectra were collected using a Bruker Sentinel system with

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fiber optics and a video-assisted Raman probe equipped with a 785 nm laser source and a high-

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sensitivity, TE-cooled, 1024 x 255 CCD array. Spectra were typically collected using a 200 mW

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light source and six, 45-second scans over the range 80 - 3200 cm-1. Reacted hematite was

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collected and rinsed with Milli-Q water before analysis by Raman spectroscopy. The solid phase

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formed by adding ~0.1 mL of 1 M NaCl to 1 mL of 70 g/L U60 was also analyzed by Raman

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spectroscopy.

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X-ray Photoelectron Spectroscopy

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X-ray photoelectron spectroscopy (XPS) was used to examine the valence states of uranium and

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iron by measuring their respective photoelectron spectra using a PHI VersaProbe II X-ray

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photoelectron spectrometer. Spectra were collected at high resolution with monochromatic Al-K

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radiation using a pass energy of 93.9 eV and a 100-micron spot size. Reacted U60-hematite

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samples were rinsed twice with Milli-Q water and placed on carbon tape before analysis. Surface

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charge neutralization was performed automatically and spectra were collected for U 4f, Fe 2p,


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and C 1s peaks. Measured binding energies were referenced by fixing the position of

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adventitious C 1s to 285.0 eV. Shirley background and asymmetric peak shape profile

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parameters were used to model the fitted bands.24 Satellite peak positions were used to determine

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oxidation state of U and Fe by comparison with published data.25,26

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Batch Sorption Experiments

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Batch sorption experiments were performed in duplicate by spiking the appropriate amount of

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the U60 stock solution into suspensions containing 34 m2/L – 200 m2/L hematite. For some

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experiments, hematite suspensions were allowed to pre-equilibrate for 48 hours with 0.023 – 2.3

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mM NaCl, KCl, or CsCl to allow the pH of solution to stabilize before the addition of U60.

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Justification for these alkali salt concentrations is provided in the Supporting Information.

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Reactors were sampled at various time points within a four month time frame. At each time

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point, a 150 µL aliquot was centrifuged for 10 min at 4,600 g and then diluted for ICP-OES

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analysis. The centrifugation speed was chosen so that if U60 aggregates formed in the aqueous

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phase, they would not be artificially removed from solution. In some cases, an additional aliquot

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was passed through a 0.2 µm PTFE filter and then passed through 30K, 50K, or 100K molecular

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weight cut-off (MWCO) Amicon Ultra-0.5 centrifugal filter devices (microfilters). The

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microfilters were spun at 14,000 g for 20 minutes and the filtrate was collected and analyzed

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with ICP-OES. The filtrate and concentrate from the 50K and 100K filters were also analyzed

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using ESI-MS. In certain cases, the removal of uranium by centrifugation was compared to

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removal by the 0.2 µm PTFE filter. In order to avoid adding any salt to the reaction mixture, the

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solution pH was measured at each time point by taking a 600 µL aliquot from the main reactor.

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The pH of the reactors was not adjusted to avoid introducing species to solution that could affect

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the stability of the nanoclusters. Controls containing only U60 and the various salts (no hematite)


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at respective concentrations were monitored in a similar fashion throughout the four month time

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period. U60 sorption curves were calculated as the % U removed according to equation 1 since

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U60 breaks down to (UO2)2+ in 5% HNO3. The % filtrate recovery was calculated by equation 2.

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% =

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% =

∗ 100

∗ ∗

(1)

∗ 100

(2)

158 159

In equations 1 – 2, C0 is the initial uranium concentration (ppm), Cf is the concentration of

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uranium in the filtrate (ppm), M0 is the mass (g) of material added to the filter, and Mf is the mass

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(g) of the filtrate.

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RESULTS AND DISCUSSION

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U60 Sorption as a Function of Time, U60 Concentration, and Hematite Concentration

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The percentage of uranium removed from solution decreases with increasing U60 concentration

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(see Figure 1A). As the uranium concentration increases at constant mineral concentration, the

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sorbent sites become saturated and a smaller fraction of the sorbate is removed from solution. In

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general, the pH of the reactors containing U60 and hematite drops (without adjustment) by

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approximately one pH unit over the course of the batch sorption experiments (see Figure 1B).

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Controls containing U60 but no hematite also exhibited a pH change, dropping approximately two

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pH units over the same time period (see Supporting Information) due to gradual equilibration

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with the ambient atmosphere. In either case the pH remains in the range of U60 stability (i.e., pH


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7.5–11).10 As shown previously, as the concentration of U60 increases the counter-cations (K+

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and Li+) become more closely associated with the negatively-charged uranyl peroxide cage;

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however, the overall effective charge of the cluster remains negative.16,19 The point of zero

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charge (pzc) for hematite reported in Cornell and Schertmann21 gives a large range for measured

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values (8.5–9.5); reported values in the range 8.5 – 9 are more common.3,27-28 In the present

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study we assume that the hematite surface has a net positive charge near the final pH of 8 in the 1

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g/L and 2.5 g/L U60 systems. Thus, U60 interactions in these systems may be dominated by

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electrostatic attraction between the negatively-charged U60 and the positively-charge hematite

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surface.

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U60 removal from solution is a two-step process. There is an initially fast removal of U60 from

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solution that occurs within 2 – 3 days. For solutions containing 1 g/L U60, this rapid removal is

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followed by a slower removal over at least 120 days; steady-state is not achieved within the time

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frame studied. After approximately 20 days, the pH of the 1 g/L U60 system is below the pzc of

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hematite. From 20 to 120 days, only minor changes in the pH of the system are observed yet

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there is a significant increase in the fraction of uranium that is removed from solution. This is

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similar to systems containing discrete U(VI), wherein the sorption edge shifts to lower pH values

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with increasing time. Thus, U60 interactions with hematite not only depend upon the pH of the

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system, but exhibit a kinetically-slow reaction with the hematite surface at pH values below the

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pzc. At higher U60 concentrations, the removal of U60 from solution is at steady-state until ~80

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days after which slow removal of U60 from solution is observed. The steady-state condition may

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be due to the closer association of cations with the uranyl peroxide cage in the more concentrated

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systems and the associated formation of a cation-bridged surface complex at high pH values

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where the mineral surface is negatively charged. When the pH drops below the pzc in the system


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containing 2.5 g/L U60, increased U60 removal from solution may be due to electrostatic

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interactions with the positively charged mineral surface.

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The two-step removal might also be attributed to sorption and partial diffusion of U60 into

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micropores on the hematite surface. BET analysis shows that approximately 20% of the reported

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surface area can be attributed to micropores, most of which are 100 Å (10 nm) or less in

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diameter. Assuming a site density of 2.3 sites per nm2,29 the concentration of sites within the

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micropores is approximately four times the concentration of U60 at 1 g/L, 1.5 times the

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concentration of U60 at 2.5 g/L, and 25% less than the concentration of U60 at 5 g/L. These

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calculations agree nicely with the trends displayed in Figure 1A but assume that U60 is associated

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with hematite through only one sorption site. As shown in Figure 2, this is not a valid assumption

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due to the size of U60, which is approximately 2.7 nm in diameter. Even though the full diameter

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of U60 would not be interacting directly with the mineral surface, it would effectively block

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access of other U60 clusters to available sites. The spacing between U60 clusters in Figure 2 is

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identical to the packing that is observed in the crystal structure and represents the closest we

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would expect the clusters to be when associated with the hematite surface.

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The percentage of U60 removed from solution increases with increasing hematite concentration

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(see Figure 3). This is similar to results typical for discrete ions; as more surface sites become

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available more solute is sorbed. Note that the systems in Figure 3 contain 0.23 mM KCl. The

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additional cations in solution do not appear to significantly impact the rate of uranium uptake

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from solution in comparison to the system shown in Figure 1A, which contained no added

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potassium chloride. This suggests that the sorption of U60 and stability of the clusters in solution

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is not significantly impacted by small changes in K+ ion concentration.


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ICP-OES was used to measure the concentrations of K+ and Li+ ions in the aqueous phase as a

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function of time. Li+ cations are present from the original U60 crystals that were dissolved; the

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concentration of K+ cations results from both the U60 crystal dissolution and the addition of KCl

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to the system. The concentration of lithium in solution does not change significantly throughout

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the duration of the experiments, while the concentration of potassium decreases at a rate similar

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to that of uranium (see Figure 4). This suggests that lithium dissociates from U60, which

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increases the effective negative charge of the cluster,16-17,20 and does not participate in U60

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interactions with the mineral surface. Note that the U/K and U/Li ratios remain constant for the 1

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g/L U60 controls for each sorption experiment (see Supporting Information).

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Effect of Added Alkali Salts

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Figure 5 shows the results of sorption experiments as a function of alkali salt (NaCl, KCl, and

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CsCl) and alkali salt concentration (0.023 – 2.3 mM). In systems containing (UO2)2+, a decrease

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in fraction sorbed with increasing ionic strength is an indirect indication of outer-sphere sorption

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due to changes in the electric double layer at the mineral-water interface.30 In systems containing

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U60, the addition of alkali salts has the added potential effect of inducing aggregation and

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affecting the long-term aqueous stability of the clusters.19,31-32 Based on the results of Gao et

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al.,19 we expect systems with 2.3 mM alkali salt to contain some U60 aggregates; lower alkali salt

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concentrations should not induce aggregation.

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Increasing the concentration of NaCl over two orders of magnitude had an insignificant effect on

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the removal of U60 from solution, although in general the amount of U60 removed from solution

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increases with increasing NaCl concentration (see Figure 5A). Regardless of NaCl concentration,


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99% of uranium was removed from solution after contact time of 120 days. We attribute the

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slight difference in the 2.3 mM NaCl system to pH (see Supporting Information).

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There is no difference in uranium sorption between systems containing 0.023 mM and 0.23 mM

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KCl (see Figure 5B). However, increasing the KCl concentration to 2.3 mM results in a

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significant increase in the percent of uranium removed from solution, especially at early time

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points. After 120 days, approximately 99% of uranium is removed from solution regardless of

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KCl concentration.

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The removal of uranium from solution in the presence of hematite and 2.3 mM CsCl is

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significantly different than any of the other systems we studied (see Figure 5C). An apparent

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multi-step reaction occurs. The first step reaches steady-state within 45 days whereas the second

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step does not reach steady-state within the time frame of our experiments. We attribute this

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multi-step reaction to the formation of blackberries or other aggregates (as discussed below) and

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the subsequent association of these larger aqueous species with the hematite surface.

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Presence and Size Distribution of U60 in Solution

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ESI-MS and microfilter data demonstrate that the U60 clusters remain intact and are, except for

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the 2.3 mM CsCl system, a relatively consistent size throughout the duration of these

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experiments. At U60 concentrations as low as 1 g/L, microfilters are the best tool we have for

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probing the size distribution of U60 in solution. These filters are rated according to a Nominal

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Molecular Weight Limit (NMWL). This means that a 30K filter is rated for a 30,000 NMWL

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cut-off and the membrane will exclude approximately 90% of organic molecular species, which

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are approximately linear, and have a molecular weight of 30,000 Daltons. Based on the

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crystallographic chemical formula, U60 has a molecular weight of 27,920 Daltons. Baseline


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studies showed that a majority of the clusters were rejected by a 30K filter (e.g., Table 1). U60

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does not pass through the filters with the reported efficiencies of linear organic molecules due to

262

the spherical nature of the clusters in solution. If the clusters break apart, we would expect to see

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less U60 rejected by the filter and measure higher concentrations of uranium in the filtrate. Using

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only the 30K filters will not tell us with confidence if U60 forms blackberries or other aggregates

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in solution. Blackberries refer to aggregated clusters and the reported hydrodynamic radius of

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U60 blackberries ranges from 11 – 58 nm depending upon experimental conditions.32 Therefore,

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we also used 50K and 100K filters to obtain information about larger species in solution. ESI-

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MS spectra of the filtrate and concentrate of the 50K and 100K MWCO filters show the

269

characteristic fingerprint for U60 (data not shown).

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Table 1 compares the filtration data of a U60 control to systems containing U60 + hematite and

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U60 + hematite + CsCl. Similar uranium recoveries are obtained from the U60 and U60 + hematite

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systems but when 2.3 mM CsCl is included in the system, the amount of uranium recovered from

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the filters decreases, suggesting the formation blackberries or other aggregates in solution. These

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trends are consistent throughout all experiments (see Supporting Information).

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The major driving force for blackberry formation is counter-ion-mediated attraction.33 These

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attractions are related to the hydrated radii of each alkali ion in solution, which decrease from Li+

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to Cs+ and correspond to the strength of interaction between clusters in solution. Table S3 in the

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Supporting Information shows the percent filtrate recovery for 1 g/L U60 in the presence of 200

279

m2/L hematite and 2.3 mM NaCl. Based on previously published data19 we would expect to see

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aggregates at this concentration of Na+. However, the microfilter data presented here indicates

281

that the U60 size distribution is similar to that of the U60 control. This suggests that the major


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aqueous species is still U60, although some small fraction of U60 may form blackberries or other

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aggregates.

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The impact of U60 blackberries or other aggregates is most apparent when 2.3 mM CsCl is added

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to the system (see Table 1 and the Supporting Information). It is clear that the percent filtrate

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recovery is much less in this system than in the U60 control or U60 in the presence of hematite

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and suggests the aggregation of U60 is significant. Our results are consistent with those of the

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macroion Mo72Fe30, where aggregation follows the sequence Cs+ > Rb+ > K+ > Na+, Li+.34 Even

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though significant aggregation occurs, Figure 5C shows that uranium is still removed from

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solution. This suggests that even blackberries may remain suspended in solution and behave like

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aqueous species.

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ESI-MS was also used to confirm the presence of U60 in systems containing alkali ions and

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hematite (see Supporting Information). The U60 fingerprint was observed in aliquots from all

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experimental systems except when 2.3 mM CsCl was present. These results speak to the

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significant stability of uranyl peroxide nanoclusters in the presence of a reactive mineral surface

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and significant quantities of light alkali cations for relatively long periods of time.

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U60 Associated with the Hematite Surface

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Raman spectra for U60 reacted with hematite after ~120 days and aggregated U60 solids are

299

presented in the Supporting Information. Raman signals at 797.4 cm-1 and 834.5 cm-1 are

300

assigned to symmetric stretching of U=O bonds in the uranyl groups and vibrations of O-O

301

bonds of the bridging peroxo groups, respectively, and confirm the presence of a uranyl species

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with bridged peroxo groups associated with the mineral surface. Both peaks are red shifted with

303

respect to those for U60 crystals, U60 bound to porous silica,8 and U60 precipitates, which suggests


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the elongation of the uranyl bond. Raman spectra collected at earlier time points do not show the

305

presence of a uranyl or peroxo groups. However, given the low fraction of U60 associated with

306

hematite at these early time points, any signal from the uranyl or peroxo groups would have been

307

below the detection limits of the instrument.

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U 4f and Fe 2p electrons were probed using XPS to determine oxidation state of each component

309

on a reacted hematite surface (see Supporting Information). U 4f core-level peaks show satellites

310

at approximately 4 and 10 eV for U(VI) and the spin-orbit interactions separates the U 4/ and

311

U 4 / peaks by around 10.9 eV. U 4/ and U 4 / peaks occur at 381.73 and 392.53 eV.

312

U(VI) satellites occur at 384.39, 396.09, and 402.3 eV. Although hematite contains iron

313

primarily in the +3 oxidation state, trace Fe(II) may facilitate U(VI) reduction to U(IV), which

314

would presumably break apart the cluster. XPS indicates that the iron in our hematite is almost

315

entirely Fe(III) and that the uranium species deposited on the surface is entirely U(VI) with no

316

contribution from U(IV) or U(V). This does not prove the existence of U60 nanoclusters on the

317

surface, but supports the possibility, and confirms that uranium is indeed associated with the

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mineral surface.

319

U60 Removal Mechanism

320

Our results indicate that as the pH of solution drops there is an increase in the amount of U60

321

sorbed onto hematite. We hypothesize that this is due to electrostatic interactions between the

322

negatively-charged U60 cluster and the positively-charged hematite surface when the pH of the

323

solution is below the point of zero charge for hematite. When the pH of the solution is above the

324

point of zero charge, we hypothesize that U60 interacts with the mineral surface through the

325

formation of a cation-bridged complex. By monitoring the concentration of potassium and


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326

lithium in solution, we determined that lithium remains in solution while potassium is removed

327

from solution at the same rate that uranium is removed from solution. This suggests that lithium

328

dissociates from U60 and if a cation-bridged surface complex is formed, it is facilitated by

329

potassium instead of lithium. Dissociation of lithium also eliminates surface-induced

330

recrystallization of U60 at the hematite surface as a potential sorption mechanism, since lithium is

331

required for U60 crystallization.

332

Due to the unreactive '-yl' oxygens that truncate the uranyl peroxide cage, we expect the primary

333

mechanism for U60 – hematite interactions to be outer-sphere sorption. Changing the ionic

334

strength of the system through the addition of NaCl, KCl, and CsCl does not necessarily support

335

this hypothesis. However, this also assumes that U60 will behave like a discrete ion and that the

336

minimal changes in alkali salt concentration (compared to what is present from U60 synthesis)

337

were sufficient to see a change in sorption behavior. Although changing the NaCl concentration

338

had a minor effect on U60 sorption, the addition of KCl and CsCl revealed significant changes in

339

U60 sorption behavior. The multi-step sorption observed in the presence of CsCl is due to the

340

formation of U60 blackberries or other aggregates in solution and the association of these larger

341

uranium-bearing species with the mineral surface.

342

ESI-MS and microfilter data attest to the stability of the clusters in the presence of hematite and

343

alkali ions during the sorption experiments and suggest that the removal of U60 from solution is

344

due to interactions with the mineral surface. XPS confirms the presence of U(VI) associated

345

with the hematite surface. This, in combination with Raman spectra showing peaks for the

346

symmetric stretching of U=O bonds in the uranyl groups and vibrations of O-O bonds of the

347

bridging peroxo groups, indicates the possible presence of U60 on the hematite surface. Future

348

studies will focus on identifying the U60 surface complex.


16

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349

Environmental Implications

350

Although uranyl peroxide nanoclusters have not been observed in nature, they have the potential

351

to form at legacy nuclear waste sites, used nuclear fuel cooling pools, and future geologic

352

repositories. If present in the environment, they represent a source term that is not considered by

353

current surface complexation models or reactive transport models. Our results show that the

354

behavior of U60 in the presence of hematite is different than that of the uranyl cation. U60 appears

355

to behave as an anion under the conditions of our experiments and persists in the aqueous phase

356

for longer than what is observed for the uranyl cation. Thus, these results have the potential to

357

challenge predictions concerning the fate and transport of uranium relative to the usual U(VI)

358

aqueous species.


17


359

Page 18 of 29

FIGURES

360 361

Figure 1. (A) Removal of U from solution as a function of time and U60 concentration in systems

362

containing 200 m2/L hematite. (B) Corresponding pH for the systems in panel A. Legend applies

363

to both panels. The error bars in panel A represent propagation of error based on the uncertainty

364

of ICP-OES measurements. Error bars in panel B are smaller than the data points and reflect an

365

accuracy of ±0.01 pH units.


18

Page 19 of 29


366 367

Figure 2. Overlay of U60 clusters with the [001] plane of hematite. The relative size of the

368

clusters to the hematite plan is to scale, where the diameter of U60 is approximately 2.7 nm. The

369

spacing of U60 clusters represents the closest packing that would be expected based on the crystal

370

structure of U60.

371

372 373

Figure 3. Removal of uranium from solution as a function of time and hematite concentration in

374

systems containing 1 g/L U60 and 0.23 mM KCl. The 'no salt' data set corresponds to a system

375

containing 1 g/L U60 and 200 m2/L hematite. Error bars represent propagation of error based on

376

the uncertainty of ICP-OES measurements.


19


Page 20 of 29

377 378

Figure 4. Percent uranium, potassium, and lithium removed from solution versus time in a

379

system containing 1 g/L U60, 0.023 mM KCl, and 200 m2/L hematite. Error bars represent

380

propagation of error based on the uncertainty of ICP-OES measurements.

381


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382 383

Figure 5. Percent uranium removed as a function of time and (A) NaCl concentration, (B) KCl

384

concentration, and (C) CsCl concentration in systems containing 1 g/L U60 and 200m2/L


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385

hematite. Error bars represent propagation of error based on the uncertainty of ICP-OES

386

measurements.

387 388

TABLES

389

Table 1. Fraction of uranium passing through 30K, 50K, and 100K MWCO microfilters. Filter

% Filtrate Recovery

% Filtrate Recovery

% Filtrate Recovery

Size

(U60)

(U60 + hematite)

(U60 + hematite + salt)

30K

3.1 ± 0.1

6.0 ± 0.2

1.1 ± 0.5

50K

51± 2

51 ± 2

1.0 ± 0.5

100K

63 ± 2

61 ± 2

0.9 ± 0.5

*Experimental conditions: 1 g/L U60, 200 m2/L hematite, and 2.3 mM CsCl. All measurements made at 43 days. 390 391

ASSOCIATED CONTENT

392

Supporting Information. Detailed description of hematite synthesis, justification for alkali salt

393

concentrations, additional ESI-MS, Raman, and XPS spectra.

394

The following files are available free of charge.

395

Supporting Information (MS Word)

396

AUTHOR INFORMATION

397

Corresponding Author

398

*E-mail: [email protected]


22

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399

Author Contributions

400

The manuscript was written through contributions of all authors. All authors have given approval

401

to the final version of the manuscript.

402

Funding Sources

403

This material is based on work supported as part of the Material Science of Actinides, an Energy

404

Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of

405

Basic Energy Sciences under award number DE-SC0001089.

406

ACKNOWLEDGMENT

407

The authors would like to thank Dr. Travis Olds for assistance in collecting and interpreting

408

results from Raman spectroscopy and XPS and Jennifer E.S. Szymanowski for assistance in

409

collecting and interpreting ESI-MS results. The following centers and facilities at the University

410

of Notre Dame provided access to instrumentation used in this research study: the Center for

411

Environmental Science and Technology (BET, ICP-OES), the Mass Spectrometry and

412

Proteomics Facility (ESI-MS), and the Center for Sustainable Energy’s Materials

413

Characterization Facility (powder X-ray diffraction, Raman, XPS).

414

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TOC Graphic 75x47mm (150 x 150 DPI)


Uranyl Peroxide Nanocluster (U60) Persistence and Sorption in the

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