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Electron Transfer Pathways Facilitating U(VI) Reduction by Fe(II) on Al- vs. Fe-Oxides Sandra D. Taylor, Udo Becker, and Kevin M. Rosso J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06893 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Electron Transfer Pathways Facilitating U(VI) Reduction by Fe(II) on Al- vs. Fe-Oxides S.D. Taylor1*, U. Becker2, and K.M. Rosso1* 1Pacific

Northwest National Laboratory, Physical Sciences Division, PO Box 999, Richland, WA 99352, U.S.A.

2The

University of Michigan, Department of Earth and Environmental Sciences, 2534 C. C. Little Building, 1100 North University Ave., Ann Arbor, MI 48109–1005, U.S.A. *To whom correspondence should be addressed: Sandra Taylor, [email protected], (509) 371-6374 Kevin Rosso, [email protected], (509) 371-6357 Revised and prepared for acceptance to Journal of Physical Chemistry C (section C2)

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Abstract This study continues mechanistic development of heterogeneous electron transfer (ET) pathways at mineral surfaces in aquatic environments that enable the reduction U(VI) by surface-associated Fe(II). Using computational molecular simulation within the framework of Marcus Theory, our findings highlight the importance of the configurations and interaction of the electron donor and acceptor species with the substrate, with respect to influencing its electronic structure and thereby the ability of semiconducting minerals to facilitate ET. U(VI) reduction by surface-associated Fe(II) (adsorbed or structurally incorporated into the lattice) on an insulating, corundum (001) surface (α-Al2O3) occurs when proximal inner-sphere (IS) surface complexes are formed, such that ET occurs through a combination of direct exchange (i.e., Fe d- and U f-orbitals overlap through space) and superexchange via intervening surface oxygen atoms. U(VI) reduction by coadsorbed Fe(II) on the isostructural semiconducting hematite (α-Fe2O3) basal surface requires either their direct electronic interaction (e.g., IS complexation) or mediation of this interaction indirectly through the surface via an intrasurface pathway. Conceptually possible longer-range ET by charge-hopping through surface Fe atoms was investigated to determine whether this indirect pathway is competitive with direct ET. The calculations show that energy barriers are large for this conduction-based pathway; interfacial ET into the hematite surface is endothermic (+80.1 kJ/mol) and comprises the rate-limiting step (10–6 s–1). The presence of the IS adsorbates appears to weaken the electronic coupling between underlying Fe ions within the surface, resulting in slower intra-surface ET (10–5 s–1) than expected in the bulk basal plane. Our findings lay out first insights into donor-acceptor communication via a charge-hopping pathway through the surface for heterogeneous reduction of U(VI) by Fe(II) and help provide a basis for experimental interrogation of this important process at mineral-water interfaces.


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Introduction In aquatic environments, the reduction of soluble U(VI) to U(IV) and subsequent precipitation to insoluble U(IV)O2(s) is an important process limiting the mobility of uranium in soils and sediments. Coupled sorption-redox reactions occurring on mineral surfaces are dominant processes in these settings and as surface-associated reductants like sorbed Fe(II) facilitate uranyl reduction.1, 2, refs therein This heterogeneous redox process is particularly important as U(VI) reduction by Fe(II) in homogeneous aqueous solution is predicted to be kinetically inhibited in anoxic, near-neutral pH solutions.3-4 A detailed understanding of surface-mediated redox reaction mechanisms is currently still lacking, despite being needed to accurately predict the transport and sequestration of U in the environment. Filling this knowledge gap could also have broader implications for understanding surface-catalyzed redox processes driven by sorbed Fe(II) in general, such as reductive dissolution and oxidative growth underlying the recrystallization of Fe(III)-(oxyhydr)oxides.5-6 Certain aspects of heterogeneous electron transfer (ET) kinetics have been developed previously. For example, electrically insulating minerals, such as corundum (α-Al2O3), can catalyze redox reactions by acting as coordinating surfaces.5 The surface strips hydration spheres around ions during adsorption and aids in transitioning the donor/acceptor ions from outer-sphere (OS) to more strongly interacting inner-sphere (IS) complexes.1-2, 4, 7-11 IS complexation enables atomic orbitals between the acceptor and donor to overlap through-space and through bridging oxygen ligands so that ET occurs via direct- and superexchange interactions, respectively.12 Uranyl reduction occurs by Fe(II) adsorbed onto surfaces3, 13-17 and/or structurally-incorporated in the lattice18-20, 21, refs therein, though it is not well understood how the local structural environment of the surface-bound reductant influences its reactivity. Electron transfer pathways (ETPs) possible on semiconducting surfaces, however, such as those of hematite (α-Fe2O3) or goethite (α-FeOOH), are nominally more complex than those observed for insulating surfaces because of their propensity to also facilitate charge redistribution. That is, potentially numerous inter- and intrasurface ETPs that can connect the donor and acceptor exist in this case. Solidstate charge carrier mobility can couple redox reactions between remote surface sites, first examined theoretically as the proximity effect at the nanoscale9, 22 and later experimentally shown to extend into macroscopic distances.23 Evidence that the role of electron conduction through minerals is a prominent ET mechanism includes isotope-labelling studies monitoring the 57Fe(II)-accelerated recrystallization of Fe (oxyhydr)oxides (naturally abundant in 56Fe), which show facile exchange of 57Fe atoms in the aqueous phase with 56Fe from the solid. The invoked mechanism entails solid-state charge transport coupling oxidative Fe(II) adsorption to spatially distinct Fe(II) reductive release.6, 24-28



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It is postulated that the rapid reduction of U(VI) by Fe(II) in the presence of semiconducting Fe (oxyhydr)oxides, observed experimentally,3, 28 may be linked to these inter- and intrasurface ETPs. For instance, in a scenario where Fe(II) and U(VI) may not interact with one another (e.g., at low Fe(II)/U(VI) sorption densities), an electron from Fe(II) adsorbed onto a hematite surface could be injected into and sequentially transferred through the semiconducting lattice to reduce a U(VI) ion at a distant site. However, details of these ETPs on semiconducting surfaces/bulk remain largely unknown. Due to the complexity of the redox environment in heterogeneous systems (e.g., differences in surface reactivity, sorption site density, adsorbate speciation, etc.),29 it was and continues to be difficult to link enhanced rates for U(VI) reduction by surface-bound Fe(II) to intrasurface ETPs. The extent to which such mechanisms occur in semiconducting mineral systems is a research frontier with broad prospective implications in geochemistry and biogeochemistry, including understanding of processes controlling the mobility of metals and contaminants in the environment.30-31 Quantum mechanical calculations are a useful platform for addressing mechanistic aspects of ETPs at (semiconducting) surfaces. In a recent study32 planewave-based density functional theory (DFT) calculations were used to compare sorption/redox reactions and ET mechanisms between Fe(II) and U(VI) coadsorbed on isostructural, periodic (001) surfaces of the insulator corundum (α-Al2O3) vs. the semiconductor hematite (α-Fe2O3). Coadsorbed Fe(II) and U(VI) ions were spatially separated from one another on the surfaces (⩾5.9 Å) to determine whether electronic-coupling through the semiconducting hematite surface facilitates ET between the adsorbates. The calculations highlighted how the different chemical and electronic properties between the isostructural corundum and hematite surfaces led to considerably different ET mechanisms. ET on the insulating corundum (001) surface was inhibited by the large distance of separation between adsorbates and lack of an intrasurface pathway. In contrast, electronic coupling through the hematite surface linked the spatially separated adsorbates and enabled ET. The extent of ET depended on overlap of orbitals of the Fe and U adsorbates with those of neighboring O and Fe ions at the hematite surface. While this prior work established that intra-surface ETP’s exist, because of the methods used those studies could not examine ET reaction (ETR) rates. That is, to determine whether or not these indirect ETP’s are competitive with direct through-space ET between proximal co-adsorbed Fe(II) and U(VI), methods that can predict ET kinetics for both the direct and indirect ETP’s are needed. This requires a treatment that specifically examines all one-ET steps involved, in sequence. Computational simulations combined with Marcus Theory (MT), which describes the rate of ET, can provide detailed descriptions of ET energetics and kinetics. This approach has been used as a framework for


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environmental ET calculations for over fifteen years,19, 22, 33-42 and has provided significant understanding of redox reaction rates observed in macroscopic experiments. We adopt this approach for this study, combining ab initio molecular orbital simulations with MT, to examine the heterogeneous reduction of U(VI) by surface-bound Fe(II). Similar to what was done in Taylor et al. 32, we compare coupled sorption/redox reactions occurring on insulating corundum (α-Al2O3) and semiconducting hematite (α-Fe2O3) clusters, representing the isostructural (001) surfaces. U(VI) and Fe(II) were coadsorbed onto the surface within close proximity to one another, but spatially separated from one another (dU-Fe ~ 4.6 Å). The energetics and kinetics for the one-electron reduction step of U(VI) to U(V) by Fe(II) via three different ETPs were evaluated (Figure 1). For pathway 1 ET occurs between coadsorbed Fe and U ions as a through-space interaction. For pathway 2 ET occurs between structurally incorporated Fe(II) and adsorbed U(VI). This ETP tests whether the electron donor reactivities of adsorbed Fe(II) and structural Fe(II) are different. Pathway 3 investigates indirect ET between coadsorbed Fe(II) and U(VI) via charge hopping though the underlying surface, for hematite only. In this case, three sequential ET steps are modelled: interfacial ET between the adsorbed Fe(II) and an underlying Fe(III) in the surface, Fe(II)-Fe(III) valence interchange transport in the surface basal plane, and interfacial charge transfer from a hematite surface Fe(II) to the overlying adsorbed U(VI). This is the first time donor-acceptor communication via a charge-hopping pathway into and through the semiconducting hematite surface has been modelled as a sequential series of ETRs. Comparison of the kinetics of these three pathways enables some initial conclusions regarding the relevance of indirect versus direct ET.

Methods Molecular models Small molecular cluster models were chosen to represent corundum and hematite (001) surfaces to enable the computation of MT quantities, such as the electronic coupling matrix element, using established methods based on molecular orbital theory. This approach emphasizes that the basis of electrical semiconduction in hematite derives from localized charge carriers and associated mobility through polaronic electron hopping, rather than band conduction which would instead require a scattering formalism via an accurate band structure approach. For both materials the cluster was constructed to be two cation-bilayers thick and approximately three octahedra wide (Fig. 2). The surfaces and edges of the cluster were protonated in a symmetry-preserving fashion along the cut M – O bond vectors, forming a charge-neutral cluster with the stoichiometry M8O30H36 (where M is either Fe3+ or Al3+). Protonation of the (001) surface was consistent with the charge neutral (doubly coordinated) hydroxyl termination. The 5 ACS Paragon Plus Environment


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small size of the cluster limited the distance between available adsorption sites but was found to be sufficient to reproduce experimentally known adsorption geometries, as discussed in the Results section, and short intra-surface ETPs. Pre-adsorbed metal complexes were modeled as charge neutral, mononuclear UO2(OH)2(H2O)3 and Fe(OH)2(H2O)4 species (see Eqns. 1-3). The uranyl complex entailed pentagonal bipyramid coordination43 while the ferrous complex was octahedral (with the OH ligands in trans-isomerization positions). Here forward, ions are distinguished according to the reactant they belong to; i.e., O- and Alor Fe-ions belonging to the corundum or hematite surfaces are referred to as OCor and AlCor or OHem and FeHem, respectively, while the U and Fe adsorbates are referred to as UAds and FeAds, respectively. Previous ab initio models of Fe and/or U (co)adsorbed on periodic, hydrated (001) corundum and hematite slab models32 guided the guess configurations of U and/or Fe (co)adsorbed on respective cluster models here. The uranyl molecule was partially dehydrated (i.e. two waters were removed) and adsorbed to the edge of a M-octahedron on the (001) surface of the substrate as a bidentate, mononuclear complex sharing two hydroxyl ligands. The uranyl’s fivefold coordination environment is maintained by the water ligands in the first coordination sphere. Furthermore, the inclusion of water in the first solvation shell is necessary for ET calculations as previous studies have shown that water directly bonded to the redox active ions significantly influences ET energetics and kinetics.36 This structural configuration of UAds is consistent with experimental results from X-ray absorption spectroscopy for mononuclear, bidentate uranyl complexes adsorbed on Al- and Fe-octahedral sites.11, 44-47 Two protons were removed from the resulting model to maintain a charge neutral system, yielding a stoichiometry of [UO2(H2O)3 – M8O30H34]0. During the initial geometry optimization (with all atoms relaxed) proton-transfer occurred between an equatorial water ligand of UAds and a nearby, non-bridging hydroxyl ligand associated with the surface, yielding the sorption complex 2·SOH ≡ UO2(OH)(H2O)2 and a cluster stoichiometry of [UO2(OH)(H2O)2 – M8O30H35]0. This surface complex is consistent with the uranyl sorption configuration also found by Glezakou and deJong 48. Models for adsorbed Fe(II) were guided by the previous computational results of Kerisit et al. 39 and the experimental results of Tanwar et al. 49. In general, adsorbed Fe(II) is hypothesized to maintain an octahedral coordination50 and to adopt positions where it acts as a continuation of the bulk before relaxation.39, 49, 51-52 Molecular-dynamics simulations confirmed that adsorption of Fe(II) onto hematite (001) occurs preferentially above a vacant octahedral site, adopting a tridentate configuration bound to three hydroxyl ligands at the corners of three octahedra.39 Because of the limited adsorption sites on our small cluster models, the Fe(II) molecule was partially dehydrated (i.e. three waters were removed) and


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was adsorbed similarly as a tridentate complex to three hydroxyl ligands at the edges of two surface octahedra. The octahedral coordination environment was maintained with directly bonding water ligands to the three remaining sites in the first coordination sphere. As was done for uranyl, two protons were removed from the cluster upon adsorption of Fe (which required exchange of two coordinating OH and a water ligand to form three bonds to surface hydroxyls) to maintain a charge neutral system, yielding a stoichiometry of [Fe(H2O)3 – M8O30H34]0. Separate U and Fe adsorption energies were calculated according to Eqns. 1 and 2, respectively: = (/ + 2 ) − ( + )

(1)

= (/ + 3 ) − ( + )

(2)

Coadsorbed U and Fe, each located at their respective individual adsorption sites, were separated by 4.6 Å – 5.2 Å depending on whether the surface model was corundum or hematite, and yielded a stoichiometry of [UO2(OH)(H2O)2– M8O30H33 – Fe(H2O)3]0. These coadsorption models are referred to as UFeCoadsCor or UFeCoadsHem, respectively. For these models, to prevent proton-transfer during geometry optimization for different charge distributions the O-H bond distances were fixed (further discussed below). The coadsorption energies were calculated two ways, one that can be compared to Eqns. 1 and 2 and the other that examines the proximity effect between FeAds and UAds, according to Eqns. 3a and 3b: () = (// + 5 ) − ( + + )

(3a)

() = (// + ) − (/ + / )

(3b)

As mentioned earlier, the different ETPs are highlighted in the simplified schematic of the coadsorbed models (Figure 1). Pathway 1 examines the energetics and kinetics of ET through space between the coadsorbed Fe(II) and U(VI) surface complexes using the UFeCoadsCor and UFeCoadsHem clusters (Figure 2a, b). This pathway entails direct ET from Fe(II)Ads to U(VI)Ads ~5 Å away, without the direct participation of the mineral substrate. When the electron is initially localized on FeAds this is referred to as the pre-ET structure while when it is transferred to and localized on UAds (i.e., U(VI) has been reduced to U(V)) this is referred to as the post-ET structure. These ETRs are denoted as cETFeAds-UAds and hETFeAds-UAds for the corundum and hematite systems, respectively. Because they are topologically analogous they enable comparison of the substrate effect on the energetics and kinetics of this ETP.



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Pathway 2 investigates ET between Fe(II) structurally-incorporated into the corundum and hematite surfaces to U(VI)Ads. Because this pathway entails incorporation of Fe(II) in a lattice site nominally occupied by a M(III) cation, it is hypothesized that the local strain on the incorporated Fe(II) cation could decrease its reduction potential and thereby overcome ET energetic/kinetic barriers relative to Pathway 1. For corundum, such a situation could arise from dissolution of a surface Al(III) site as an Al(OH)3(H2O)3 monomer that is subsequently reoccupied by dehydration of an adsorbed Fe(II). These reactions on the corundum or hematite clusters are denoted as cETFeSub-UAds and hETFeSub-UAds, respectively. The reactions are again analogous to one another; comparison between their structures, energetics, and kinetics enables insight into how the difference between insulating and semiconducting substrates affects ET. To evaluate Pathway 2, clusters were constructed to model ET with U(VI) adsorbed over a Fe(II)substituted metal site on the cluster surface, where Fe(II) replaces an Al(III) or a Fe(III) cation in the surface site directly underneath UAds. The site on the cluster chosen to make this substitution yields a complete inner shell coordination environment; the incorporated Fe(II) shares an O/OH ligand with each of the M cations in the substrate clusters. Protons were added to create a charge neutral cluster with stoichiometry Fe2+M7O30H37. Uranyl was adsorbed to the surface in the same manner as was done for the coadsorbed model described above ([UO2(OH)(H2O)2 – Fe2+M7O30H35]0. These models with U adsorbed to the Fe(II)-substituted corundum and hematite clusters are referred to as UAdsFeSubCor and UAdsFeSubHem, respectively (Figure 2c, d). The substituted Fe ion is referred to as FeSub. The relative stabilities of the Fe(II)-substituted clusters with respect to the pure corundum and hematite cases were calculated according to Eqn. 4, and the adsorption energy of uranyl onto the Fe(II)substituted cluster was calculated according to Eqn. 5. = (

= (#

+ ! ) − (

+ ("") )

+ 2 ) − (

(4) + )

(5)

Pathway 3 uses the UFeCoadsHem model (Figure 2b) to investigate intra-surface ET through semiconducting surfaces. The treatment applied here is consistent with conduction by hopping of small polarons.22 This contrasts indirect electronic interaction between the donor/acceptor mediated by the electronic structure of the surface, or the so-called proximity effect.9, 53 The difference between the two types of intra-surface interactions for hematite derives from its strongly polarizable lattice sites, wide band gap and narrow conduction band, which tends to favor localization of charge carrier electronic states via self-trapping, and their transport by thermally-activated hopping. In smaller band gap wider 8 ACS Paragon Plus Environment

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conduction band minerals, delocalized charge carriers would tend to dominate and the relevant donor/acceptor electronic interaction would be via the continuous electronic states, i.e., band conduction. Thus, here we treat Pathway 3 as a multistep reaction of sequential one-ETRs. That is, in the first ET step (hETFeAds-FeHem1) an electron from Fe(II)Ads (pre-ET structure) is transferred to an underlying, edge-sharing Fe(III)em cation (FeHem1) within the hematite cluster surface (intermediate 1 structure). In the second ET step (hETFeHem1-FeHem2) the electron is transferred in the c-direction between edge-sharing FeHem cations in the basal plane, from FeHem1 to FeHem2 (intermediate 2 structure). In the final ET step (hETFeHem2-UAds) the electron is transferred from FeHem2 to the U(VI) adsorbed (UAds) on the cluster surface (post-ET structure) via a IS, edge-sharing complex.

Wavefunction Methods All the clusters and molecules described above were geometry optimized in NWChem54 using spin-unrestricted Hartree-Fock (UHF). The UHF approach is useful for ET calculations due to its ability to strongly localize electrons, although because of its lack of dynamic electron correlation treatment this tendency is stronger than it should be and ET energetics are typically overestimated. The accuracy of the calculations can be improved at the DFT-B3LYP level. However, this hybrid functional treatment could not be universally applied due to the opposite imperfection in DFT to under-bind and thereby delocalize electrons. The UHF approach was able to reproduce structures, energetics, and electronic properties generally comparable to that observed in previous studies (as described in the Results and Discussion). Initial guess wavefunctions for the reactant and product structures for each ETR were preconditioned with the desired charge and spin density distribution prior to initiation of the self-consistent field solution. For instance, when modelling ET between the Fe(II) and U(VI), the Fe2+ ion of the pre-ET structure was initially assigned a net charge and spin of +2 and +4, respectively, while the UO22+ ion was initially given a net charge and spin of +2 and 0, respectively. In the post-ET structure the oxidized donor (now Fe3+) was assigned a net charge and spin of +3 and +5, respectively, while the reduced acceptor (now U5+) has a charge and spin of +1 and -1, respectively. FeHem3+ cations were assigned net charges and spins of +3 and ±5, in a pattern consistent with its known antiferromagnetic structure below the Morin transition. The majority spins on Fe(II)Ads cations were ferromagnetically coupled to those of underlying FeHem cations to enable the forward ET of the minority spin without violating Hund’s rules.35 High-spin states were applied to the ferric and ferrous iron cations in hematite and FeAds based on results from Mossbauer spectra.26-27 The Fe(III)-U(V) complexes were modelled using a ferrimagnetic spin configuration (i.e., opposite majority spin directions for the Fe3+ and U5+ with a net residual moment). The minority spin is transferred for the Fe(II)-Fe(III) and Fe(II)-U(VI) ETRs in this study. The models were optimized in the gas-phase, treating only the first hydration shell around the donor/acceptor explicitly as 9 ACS Paragon Plus Environment


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described earlier. First the clusters were geometry optimized using a small basis set (SBS calculations): 321G for O, H,55, Al56, and Fe57, and CRENBL ECP for U.58 Fully unconstrained geometry optimizations of these clusters (i.e., all atoms relaxed) in different electronic configurations (such as in the pre- and post-ET state) would sometimes lead to bond breakage associated with the change in the charge density distribution, e.g., proton transfer. In simple ETRs no chemical bonds are broken or formed; changes in bond distances associated with changes in the redox state are the most prevalent structural alteration. The transfer of a proton as a result of a new charge distribution between donor/acceptor, while normal, does not necessarily mean that the ET rate is dependent on the accompanying proton transfer rate. Such is the case in so-called proton-coupled ET.4, 33 Models whose structures have been altered considerably from the pre- to post-ET state, such as via a proton transfer reaction, can yield energetic and kinetics values for ET that are wholly inaccurate, particularly because a realistic proton transfer via second-shell waters of solvation is not available in our models. Such reaction energetics and kinetics would not be representative of the ET step itself. Thus, after optimizing the initial pre-ET structures (as described earlier), we isolated just the ET processes on the UFeCoads- and UAdsFeSub- models by applying partially constrained geometry optimizations using Z-matrices, where the only limitation applied to the SBS calculations was that OH interatomic distances were held fixed (to prevent proton transfer from occurring). This approach produced topologically intact models before and after ET in all cases, while allowing all interatomic angles and all other interatomic distances to relax without constraint. Self-consistent field cycles converged with an energy difference kBT) ET occurs adiabatically, where kB is Boltzmann’s constant (8.3 × 10–3 kJ mol–1 K–1), and T is the temperature (298.2 K, assuming room temperature); if smaller (VAB < kBT), then ET occurs nonadiabatically.19, 36 The strength of the donor/acceptor electronic interaction, contained in VAB, was also qualitatively corroborated by examining isosurface plots of the beta (i.e., minority spin) highest occupied molecular orbitals (β-HOMO) at the TS geometry. That is, the β-HOMO for the pre-ET state shows the electron localized on Fe while the β-HOMO for the post-ET state shows it localized on U. Adding the β-HOMOs for the pre- and post-ET electronic states at the TS yields a visual illustration of the extent of orbital overlap and coupling between the acceptor and donor states and their bridging (hydr)oxo ligands, with larger overlap equating to larger values of VAB. These isosurfaces enable clear observations of ET occurring as direct and/or superexchange interactions, and also enable visualization of physical pathways enabling ET between electron and acceptor species. Given VAB, the ET probability κ is given by (Eqn. 6): $ =

%&

(6)

' %&

P12 is the probability of the reactants to be converted to the products per single passage through the TS, and was calculated as (Eqn. 7): ( = 1 − exp [.−

/01 67 4 589 :] 23 1

(7)

where h is Planck’s constant (3.9 × 10–13 kJ·s/mol) and v is the typical frequency for nuclear motion (1013 s–1)33. κ can also be correlated to the reaction adiabaticity; if κ ≈ 1 then the probability of ET occurring is 100% and the ETR proceeds adiabatically, for example. In the adiabatic regime the ET rate, kET, is expressed as (Eqn. 8): ?@ ∗ ) 1:

exp( 9

(8)

where ∆G* is the free energy barrier and the energy required to (thermally) excite the system to the TS configuration (Eqn. 9):


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∆C ∗ =

(8' ?@ D ) E8

− F#G

(9)

In the nonadiabatic regime kET is calculated as (Eqn. 10):

Reduction by Fe(II) - American Chemical Society

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