Work Description

Dataset title: Imaging the reduction of uranyl on Fe-bearing minerals surfaces using in situ electrochemical AFM
Dataset Creators: YoungJae Kim, Maria C. Marcano, Sooyeon Kim and Udo Becker
Dataset Contact: Udo Becker (ubecker@umich.edu)
Funding: DE-FG02-06ER15783 (DOE)

Key Points:
- This experimental data reveal how natural iron minerals mediate redox reactions of redox sensitive elements.
- We measure electrochemical responses of dissolved uranyl ions (UO22+) to potentials in the presence of organic molecules.
- The atomic force microscopic (AFM) images show nucleation of reduced uranyl on ilmenite (FeTiO3) as a function of potential.

Research Overview:
The main objective of this research is to integrate electrochemical and microscopic approaches to understand reaction mechanisms and pathways of the uranyl reduction and nucleation mediated by magnetite and ilmenite.

Reduction of mobile aqueous uranyl complexes can be hindered by the strength of the uranyl-ligand complex and the resistivity of the complexes in transferring electrons. Semiconducting solids, such as magnetite (Fe3O4) and ilmenite (FeTiO3), can facilitate otherwise-slow electron transfer between the reductant and oxidant, especially if these species are adsorbed on their surfaces as inner-sphere complexes. Then electrons can be shuttled through the surface or the structure. This process can be facilitated by either finding a new electron transfer path or by weakening or destroying the ligand-uranyl complex.

Using in situ electrochemical atomic force microscopy (EC-AFM), we imaged reduction of uranyl in the absence or presence of organic ligands such as ethylenediaminetetraacetate (EDTA) and oxalate. In situ images of uranyl reduction show gradual nucleation and growth of reduced uranium with decreasing reduction potentials. X-ray photoelectron spectroscopy (XPS) measurements indicate pentavalent uranium being the most dominant uranium species resulting from uranyl reduction mediated by magnetite and ilmenite. The spectroscopic and electrochemical data suggest that the catalysis of Fe-bearing minerals facilitates the reduction of uranyl-organic complexes. U(VI)-oxalate complexes are found to be redox-active and participate directly in the reduction mediated by magnetite. In the presence of EDTA, reduction of uranyl may proceed in concert with ligand exchange between metals (i.e., U and Fe) on the mineral surface. The proposed mechanism is the breaking of uranyl-organic bonds upon interaction with the mineral surface followed by the replacement by Fe-organic bonds, thereby allowing hexavalent uranyl to be reduced to pentavalent one.  

Methodology:
1. Mineral electrodes and reagents: The magnetite samples used in this study are octahedral–dodecahedral crystals collected from Mineville, New York, and the Itabira District, Minas Gerais, Brazil (Ward's Science, USA). Minor amounts of titanium and aluminum were measured using energy-dispersive x-ray spectroscopy (EDS). The ilmenite specimens used in this study were sourced from St. Urbain, Canada (Ward’s Science) and XRD and SEM analyses (not shown here) reveal intergrowth of ilmenite with hematite as reported from a previous study by Gillson (1932). The crystals were cut using a water-saw and polished with silicon carbide paper (grit sizes 320, 800, 1500, 3000, and 4000). The samples were then sonicated for 10 min and rinsed with Milli-Q® water (resistivity ≥ 18.2 MΩ·cm) to remove any contaminants introduced during polishing. The exposed areas of the final samples were approximately 1.0 to 2.0 cm2. Stock solutions of uranyl and organic compounds were prepared using Milli-Q® water and uranyl nitrate hexahydrate (UO2(NO3)2∙6H2O, International Bio-analytical Industries), disodium EDTA dihydrate (C10H14N2Na2O8∙2H2O, Fisher Chemical), oxalic acid dihydrate (C2H2O4·2H2O, Baker analyze), and ferrous sulfate heptahydrate (FeSO4∙7H2O, Fisher Chemical).

2. Solution chemistry: For all experiments, the concentration of uranyl was 0.3 or 0.5 mM, and the solution was adjusted to a pH of 3.0 by adding concentrated HCl or NaOH. The same concentrations of organic compounds and uranyl were added, unless stated otherwise, which corresponds to a uranyl/organic-compound molar ratio of 1:1. Acidic conditions were necessary to prevent wet precipitation of uranyl solids. The uranyl/organic ratio was chosen to maximize visualization of uranyl nucleation in the AFM measurement. Specifically, upon AFM imaging, uranium nucleation was found to be less pronounced with higher concentrations of organic compounds. Prior to electrochemical and AFM experiments, 10 ml of the solution was purged with N2 for 10 min to minimize the presence of oxygen and carbon dioxide, and an N2 atmosphere was maintained throughout each experiment. For our experimental conditions, aqueous U(VI) species and saturation index with respect to various solid phases were calculated by Visual MINTEQ ver. 3.0 with the Thermo.vdb database (See Table 1 of associated manuscript 2020). Under our experimental conditions (pH 3.0; [UO22+], [org] = 0.3 mM), 89 % and 93 % of the U speciation is accounted for by U-organic complexes, including [(UO2)2+HEDTA]– and [(UO2)2+oxalate]0, as the main species in solution containing EDTA and oxalate, respectively (See Table 1 of associated manuscript 2020).

2.3. Electrochemical AFM experiments: The experimental setup within the AFM fluid cell consists of a three-electrode configuration in which magnetite and ilmenite serve as the working electrode, with a platinum wire as the counter electrode and a silver wire as the quasi-reference electrode (+250 mV vs. the standard hydrogen electrode, SHE). In this electrochemical cell (see Fig. A1 in Walker et al., 2016; Bruker Corporation, Santa Barbara, California),the mineral samples were affixed flat on an etched Teflon® disk using Torr Seal® low vapor pressure epoxy. The Teflon disk was then mounted to the base of the AFM fluid cell, and electrical contact was established with a spring through a small opening in the disk. The redox potential was controlled using a CHI 760D potentiostat, and EC-AFM experiments were performed using an in-house Dimension Icon atomic force microscope (Bruker).

The Eh investigated ranged from +1.15 to –0.65 V (+0.9 to –0.9 V vs. Ag/AgCl electrode). Cyclic voltammetry was performed using the mineral electrode, and collected data were analyzed to identify specific redox species responsible for electrical signals at given pH and Eh values. Peak Force Tapping (PFT) mode experiments were performed in fluid using silicon nitride tips (DNP, nom. Frew. 23 kHz, nom. Spring constant 0.12 N/m). In PFT, developed by Bruker, the tip makes intermittent contact with the sample surface. Scratches and grooves produced when polishing the surface can serve as reference features during the experiment.

4. Material characterization: XPS analysis was performed using a Kratos Axis Ultra spectrometer with monochromatic and focused Al K α radiation (1486.6 eV). The x-ray emission anode voltage and current applied during spectra acquisition were 14 keV and 8 mA, respectively. Pass energies of 160 and 20 eV were used on survey and core scans, respectively. All measurements were performed with a hybrid lens and slot aperture (700 × 300 μm2) under ultra-high vacuum conditions (< 10–8 Torr). Spectra were analyzed using the CASA XPS software (version 2.3.16,  www.casaxps.com), and binding energies were calibrated to the C 1s peak (284.9 eV). Caution should be taken on XPS analysis for uranium because the X-ray beam is likely to induce reduction of U(VI) during the measurement (Ilton et al., 2007). For each specimen, sequential analyses were performed on the same spot in order to examine the possibility of beam-induced reduction, as previously reported by Ulrich et al. (2009). Specimens obtained from reaction with EDTA and oxalate were stable over the first several spectra. All XPS spectra presented in this study were collected from the first two scans in each measurement, such that the contribution of the beam-induced reduction to the XPS spectra was minimal. 5. Molecular structure and energy calculation The Gaussian 09 package was used to model the atomic structures of hydrated uranyl with and without organic ligands based on molecular orbital calculation. All calculations were performed using the DFT-HF hybrid B3LYP density functional (Lee et al., 1988). Relativistic effects for U were included in the calculation using pseudopotentials and basis set information (Stuttgart RSC 1997) collected from the EMSL Basis Set library. Energy-consistent pseudopotentials and basis sets for the C, O, and N atoms (ECP2MWB) were adopted from the Stuttgart/Cologne Group library (Bergner et al., 1993;  www.tc.uni-koeln.de). The 6-31++G** basis is used to treat the H atoms in the calculations. Cluster models of uranyl species (as UO22+ and UO2+) and their complexes with organic ligands (oxalate and EDTA) were constructed and serve as the basis for investigation of electron exchange reactions between these species. All reactions are assumed to occur in the water solvent such that hydration is an essential parameter in this reaction system. All models include explicit water molecules that comprise the first and second hydration spheres. Solvation effects are further included using the conductor-like polarizable continuum model (CPCM) as implemented in Gaussian 09. This model accounts for the contribution of higher-order spheres of hydration within a homogeneous dielectric fluid around the first and/or second coordination environment(s).  

Instrument and/or Software specifications:
The redox potential was controlled using a CHI 760D potentiostat, and electrochemical atomic force microscopy (EC-AFM) experiments were performed using an in-house Dimension Icon atomic force microscope (Bruker). The Eh investigated ranged from +1.15 to ñ0.65 V (+0.9 to ñ0.9 V vs. Ag/AgCl electrode). Cyclic voltammetry was performed using the mineral electrode and collected data were analyzed to identify specific redox species responsible for electrical signals at given pH and Eh.

Peak Force Tapping (PFT) mode experiments were performed in fluid using silicon nitride tips (DNP, nom. Frew. 23 kHz, nom. Spring constant 0.12 N/m). In PFT, developed by Bruker, the tip makes intermittent contact with the sample surface. Scratches and grooves produced when polishing the surface can serve as reference features during the experiment.

References:
-Gillson J. L. (1932) Genesis of the ilmenite deposits of Saint Urbain, County Charlevoix, Quebec. Econ. Geol. 27, 554-577
-Walker S. M., Marcano M. C., Bender W. M. and Becker U. (2016) Imaging the reduction of chromium (VI) on magnetite surfaces using in situ electrochemical AFM. Chem. Geol. 429, 60-74.
-Ilton E. S., Boily J.-F. and Bagus P. S. (2007) Beam induced reduction of U (VI) during X-ray photoelectron spectroscopy: the utility of the U4f satellite structure for identifying uranium oxidation states in mixed valence uranium oxides. Surf. Sci. 601, 908-916
-Ulrich K.-U., Ilton E. S., Veeramani H., Sharp J. O., Bernier-Latmani R., Schofield E. J., Bargar J. R. and Giammar D. E. (2009) Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate. Geochim. Cosmochim. Acta 73, 6065-6083.
-Lee C., Yang W. and Parr R. G. (1988) Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 37, 785.
-Bergner A., Dolg M., Küchle W., Stoll H. and Preuß H. (1993) Ab initio energy-adjusted pseudopotentials for elements of groups 13–17. Mol. Phys. 80, 1431-1441

Files contained here:
We have two data sets: Fig. 1 and 3
Fig. 1. Cyclic voltammetry (CV) measured using the magnetite working electrode. Comparison between the uranium-containing and blank solutions with and without organic molecules. ([UO22+] = 0.5 mM; [Org.] = 0.5 mM). ). All the solutions contained 0.1 M NaCl as a background electrolyte.
- Fig_1A_NaC_vs_U+NaCl.csv is comparison between the blank (without organic molecules) and the uranyl containing solutions.
- Fig_1B_oxalate_vs_U_oxalate.csv is comparison between the blank (with oxalate) and the uranyl containing solutions.
- Fig_1C_EDTA_vs_U_EDTA.csv is comparison between the blank (with EDTA) and the uranyl containing solutions.

Fig. 3. EC-AFM peak force error (PFE) images (A to F) and height images (G and H) as a function of electric potentials applied. The electrode was a substrate of ilmenite immersed in a solution with 0.5 mM uranyl at pH 3.0. Cumulative times elapsed during the potential sequence are shown on the images.
- Fig. 3A. A._IL_dry_PF.tif is PFE image taken on the ilmenite surface before immersed in the solution.
- Fig. 3B. B._IL_wet_PF.tif is PFE image taken on the ilmenite surface after immersed in the solution.
- Fig. 3C. C._IL_M250_PF.tif is PFE image taken on the ilmenite surface in the solution after -250 mV is applied for 5min.
- Fig. 3D. D._IL_M450_1st__PF.tif is PFE image taken on the ilmenite surface in the solution after -450 mV is applied for 5 min (cumulative time is 10 min).
- Fig. 3E. E._IL_M450_2nd__PF.tif is PFE image taken on the ilmenite surface in the solution after -450 mV is applied for 5 min (cumulative time is 15 min).
- Fig. 3F. F._IL_M650_PF.tif is PFE image taken on the ilmenite surface in the solution after -450 mV is applied for 5 min (cumulative time is 20 min).
- Fig. 3G. G._IL_M250_H.tif is height image taken on the ilmenite surface in the solution after -250 mV is applied for 5min.
- Fig. 3H. H._IL_M650_H.tif is height image taken on the ilmenite surface in the solution after -450 mV is applied for 5 min (cumulative time is 20 min).

Related publication(s):
Walker S. M., Marcano M. C., Bender W. M. and Becker U. (2016) Imaging the reduction of chromium (VI) on magnetite surfaces using in situ electrochemical AFM. Chemical Geology 429, 60-74.

Kim Y., Marcano M.C., Sooyeon Kim, and Becker U. (under review) Imaging the reduction of uranyl as mediated by ilmenite and magnetite using in situ electrochemical AFM. Geochimica et Cosmochimica Acta.

Use and Access: This data set is made available under an Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License: http://creativecommons.org/licenses/by-nc/4.0/

To Cite Data:
Kim Y., Marcano M.C., Sooyeon Kim, and Becker U. (under review) Imaging the reduction of uranyl as mediated by ilmenite and magnetite using in situ electrochemical AFM. Geochimica et Cosmochimica Acta.