Separation of Radiolytic Species at the Boehmite–Water Interface

May 31, 2019 - Uncovering the mechanisms that control oxide radiolytic activity has proven to be ... Previous work has ruled out the electronic struct...
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Cite This: J. Phys. Chem. C 2019, 123, 15534−15539

Separation of Radiolytic Species at the Boehmite−Water Interface Sebastien N. Kerisit,*,† Zhizhang Shen,†,§ Micah P. Prange,† and Eugene S. Ilton† †

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Department of Chemical and Biological Engineering, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States

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S Supporting Information *

ABSTRACT: The effects of radiation fields present in nuclear waste tanks on the structure and reactivity of Al-bearing phases, a major component of the solid waste inventory, and their implications for nuclear waste processing remain poorly understood. While H2 release from irradiated Al-(oxy)hydroxide phases can be measured experimentally, the mechanisms that lead to its formation and the fate of other radiolytic species are not known. Density functional theory calculations were therefore performed to determine the energetics of radiolytic species (O− and H0) across the interface between water and both the (010) and (101) facets of boehmite (γAlOOH), the two surfaces that dominate the morphology of boehmite particles at alkaline pH values relevant to tank waste. The DFT calculations employed semilocal and hybrid exchangecorrelation functionals and consisted of a combination of energy minimizations and ab initio molecular dynamics simulations. The calculations showed that the release of H0 radicals from boehmite into the liquid phase was highly exothermic, whereas the O− radicals remained trapped at the surface of boehmite particles. The solid−water interface is therefore the locus of separation of radiolytic species, and the accumulation of O− radicals under irradiation could lead to significant changes in particle reactivity.

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INTRODUCTION Uncovering the mechanisms that control oxide radiolytic activity has proven to be particularly challenging due to the sheer complexity of the cascade of physicochemical processes resulting from high-energy radiation particles impinging on oxide materials. The chemistry of irradiated oxides is critical to many topical issues such as the development of radiationtolerant materials and the performance of radiation detection materials for medicine and national security. Of particular interest is the radiolytic activity of oxide materials in the context of the treatment and disposal of legacy nuclear waste. Fifty-six million gallons of nuclear waste is stored in underground tanks at the U.S. Department of Energy Hanford Site, Washington.1 Aluminum is a major component of the waste, and Al-bearing phases such as boehmite (γ-AlOOH) and gibbsite (γ-Al(OH)3) are a critical part of the high-level radioactive waste (HLW) fraction. Pretreatment of the HLW fraction prior to immobilization in a borosilicate glass matrix will include caustic dissolution of Al-bearing phases to reduce the high Al content, which is detrimental to glass formation and quality. Tank waste boehmite dissolves much more slowly than synthesized boehmite on the basis of surface normalized rates2 for reasons that remain unknown. An important question is the effect of radiation fields present in waste tanks on the structure and reactivity of Al-bearing solid phases during HLW processing. In particular, unexplained differences between the behaviors of boehmite and gibbsite under irradiation have highlighted a general lack of understanding of radiolytic transformation of solid tank waste. Westbrook et al.3 demonstrated an enhancement of radiolytic © 2019 American Chemical Society

production of dihydrogen gas for dry powders at boehmite surfaces but not at gibbsite surfaces, and Kaddissy and coworkers4 reported a higher radiolytic activity of boehmite compared to bayerite (a close polymorph of gibbsite) for dry powders. Furthermore, the liquid-cell transmission electron microscopy experiments of Conroy et al.5 showed that nanosheets of boehmite exfoliated and dissolved rapidly during exposure to the electron beam, whereas gibbsite was stable under the same conditions. In sum, the mechanisms that lead to the production of H2 from irradiated Al-(oxy)hydroxides are not understood, and the development of engineering solutions for the treatment of nuclear waste would benefit from a deeper understanding of this phenomenon,1 particularly considering the flammable nature of H2. The hazards and cost associated with handling radioactive materials are a major challenge facing experimental investigations into this phenomenon. In contrast, first-principles simulations can bypass these obstacles and provide valuable insights into the radiolytic production of H2 in solid tank waste. Previous work has ruled out the electronic structure of boehmite and gibbsite as a root cause for the observed differences in radiolytic activity6 and instead pointed to the crystal structure, through its effect on the mechanisms of diffusion of radiolytic species, as a controlling factor.7 Specifically, hydrogen radicals can diffuse across structural Received: March 6, 2019 Revised: May 22, 2019 Published: May 31, 2019 15534

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The Journal of Physical Chemistry C layers in gibbsite (three-dimensional (3D) diffusion) but not in boehmite (two-dimensional (2D) diffusion). In this work, the energetics of O− and H0 radicals across the boehmite−water interface were determined for the (010) and (101) surfaces, the two facets that dominate the morphology of boehmite particles at pH values relevant to tank waste.8−10 The calculations demonstrated that a thermodynamic driving force exists for release of hydrogen radicals into the aqueous phase but not for oxygen radicals, which were predicted to remain at the surface of boehmite particles. The solid−water interface is therefore the locus of separation of radiolytic species. The overall emerging picture is one where H2 formation from combination of hydrogen radicals occurs in the vicinity of the interface. In this picture, H2 production is more efficient for boehmite than for gibbsite due to the concentration of H radicals to side surfaces through 2D diffusion in boehmite and is also more efficient at low water coverage. Oxygen radicals are predicted to accumulate at the surface under irradiation, which could lead to significant changes in reactivity.

− − n > OH(s) + mH 2O(l) + H(g)0 → [n > OH(s) ··· H 0

··· mH 2O(l)]

(1)

where n and m are the number of boehmite hydroxyl groups (>OH−) and water molecules involved in bonding with H0, respectively, and − − >OH(s) → >O(s) + H(g)0

(2a)

H 2O(l) → HO(l)0 + H(g)0

(2b)

or

depending on whether the hydrogen atom is removed from a hydroxyl group (as in the bulk solid, subsurface, and surface environments discussed in the Results and Discussion section) or a water molecule (interface, water slab, and bulk liquid environments). In all cases, the energy of reaction was calculated by subtracting the sum of the total energies of the simulations representing the reactants from the sum of the total energies of the simulations representing the products. The two reactions are treated independently; that is, ΔE1 and ΔE2 are not compared to each other. Instead, the changes in either energy of reaction as a function of the location of the corresponding radical are evaluated to determine the fate of the isolated O and H radicals. As expected, the results of the calculations presented below show that the isolated O and H radicals are metastable with respect to an intact O−H bond. The energy necessary to cleave an O−H bond is provided through interactions with primary or secondary radiation particles. On the basis of experimental evidence for the radiation-induced formation of the radicals of interest inside boehmite particles,4 this work does not simulate the radical production stage; instead, it uses the end of that stage as a starting point to examine the fate of the radicals as they reach a particle’s surface. While one O radical and one H radical may recombine as they diffuse through a particle, H2 formation involves the reaction of two H radicals that avoided recombination and also causes an imbalance between the O and H radical populations that leads to isolated O radicals. Therefore, this work considers these two radicals in isolation. Moreover, considering the energetics of one radical in the presence of the other would make for a practically intractable problem as it would require exploring all possible positions of the spectator radical for each site that the radical of interest can occupy across the boehmite−water interface. The O− and H0 radicals are considered in this work as they were the dominant species detected in EPR measurements of irradiated boehmite particles. Two signals in these EPR measurements were assigned to polyatomic O radicals (O2− and O3−), but these species were not considered as their atomic and electronic structures are not known and O− was the dominant oxygen radical species. Potential pH effects on the surface charge were not considered to provide reference results for a neutral surface and because of the difficulties associated with determining and simulating relevant surface charges. Moreover, the O and H radicals are neutral defects, and surface charge is therefore not expected to have a pronounced effect on the results of this work. For the case of bulk boehmite, the dependence of the energy of reactions 1 and 2a on the plane-wave cutoff energy, k-point mesh, and supercell size was evaluated, as detailed in Tables S1 to S3 of the Supporting Information, and a 2 × 1 × 2 supercell

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THEORETICAL METHODS All of the plane-wave density functional theory calculations were performed with VASP (Vienna ab initio simulation package)11−14 using the projector augmented-wave (PAW) approach.15,16 The revised Perdew, Burke, and Ernzerhof17,18 (revPBE) generalized gradient approximation (GGA) exchange-correlation (XC) functional of Zhang and Yang19 with the Grimme dispersion corrections (D3)20 was used for the majority of the energy minimizations and all the ab initio molecular dynamics (AIMD) simulations. The hybrid revPBE0 XC functional (75% revPBE exchange functional + 25% exact exchange + 100% revPBE correlation functional, following the PBE0 XC functional of Adamo and Barone21 but with the revPBE XC functional) was also used (without Grimme corrections) for some of the energy minimizations. This approach allowed a comparison of the two XC functionals. As the hybrid functional is expected to be superior to revPBE-D3 but its use in the AIMD simulations would be computationally prohibitive, the results obtained with revPBE-D3 were validated using energy minimizations in order to increase confidence in the findings derived from the revPBE-D3 AIMD simulations. The comparison of the two XC functionals was therefore restricted to the cases where a single representative configuration was adequate (e.g., radicals in the bulk solid). The PAW potentials were obtained from the VASP database for aluminum (10), oxygen (2), and hydrogen (0), with the number of core electrons shown in parentheses. Constant-pressure energy minimizations were first performed using a plane-wave cutoff energy of 750 eV and a 12 × 3 × 12 (revPBE-D3) or 4 × 1 × 4 (revPBE0) k-point mesh8 to determine the optimized crystal structures at the revPBE-D3 and revPBE0 levels of approximation. The optimized boehmite unit cells (revPBE-D3: a = 3.733 Å (+1.1%), b = 11.901 Å (−2.6%), and c = 2.871 Å (+0.2%); revPBE0: a = 3.709 Å (+0.4%), b = 12.239 Å (+0.1%), and c = 2.877 Å (+0.4%)) agreed well with the crystal structure obtained from X-ray diffraction (a = 3.693 Å, b = 12.221 Å, and c = 2.865 Å).22 The A21am space group is used throughout this work. The energetics of H and O radicals across the boehmite− water interface, ΔE1 and ΔE2, respectively, were calculated using the following reactions for the H and O radicals 15535

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Figure 1. Atomic density profiles projected on the normal to the (010) (left) and (101) (right) boehmite slabs. A snapshot of the AIMD simulation is also shown for both interfaces in which Al, O, and H atoms are shown in blue, red, and white, respectively.

Figure 2. Energies of reactions 1 and 2a/2b for the formation of H0 and O− radicals in bulk boehmite (leftmost bars), bulk water (rightmost bars), and at sites across the (010)−water interface (middle bars) as obtained from energy minimizations with the revPBE-D3 (black bars) or revPBE0 (red bars) XC functionals or from AIMD simulations with revPBE-D3 (green bars). Tabulated values can be found in Table S5 of the Supporting Information. The O− radical did not reside long enough within the water slab to determine ΔE2 accurately, and this case is therefore shown as “n/ a”. Snapshots of the calculations with isocontour surfaces of the net spin density are also shown to illustrate the various positions (left to right: subsurface, surface, and interface) considered for the H0 (top row) and O− (bottom row) radicals.

with a 450 eV cutoff energy and 2 × 1 × 2 k-point mesh was used in the AIMD simulations of bulk boehmite on the basis of this evaluation. A 450 eV plane-wave cutoff energy was used in the remainder of the calculations reported in this work. The AIMD simulations of bulk water and the two boehmite−water interfaces were performed at the Γ point (1 × 1 × 1 k-point mesh). The constant-volume energy minimizations used 5 × 7 × 1 (3 × 5 × 1) and 3 × 3 × 1 (3 × 3 × 1) k-point meshes for the (010) and (101) surfaces, respectively, with the revPBE-D3 (revPBE0) functional. In the energy minimizations, convergence was reached when the force on any atom was less than 0.01 eV/Å. All of the AIMD simulations were performed at 323.15 K in the NVT (constant number of particles, constant volume, and constant temperature) ensemble with a 0.5 fs integration time step. As is typical for AIMD simulations of liquid water,23 a temperature of 323.15 K was used instead of 298.15 K to compensate for

the increased intermolecular water structure predicted by GGA XC functionals. The velocities were scaled to the target temperature every 12.5 fs for the first picosecond of simulation. The first 2 ps of each simulation were omitted in the analyses. The AIMD simulations of bulk water used a cell containing 64 water molecules, and those of the (010)((101))−water interface used a boehmite slab 1(2) unit cell thick and with a 2 × 3(1 × 2) surface area (42.9(112.1) Å2, 24(16) AlOOH units). A gap of approximately 18 Å between the two faces of the boehmite slab was created and filled with 36(76) water molecules. Table S4 lists all the AIMD simulations performed in this work and the corresponding simulated times.

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RESULTS AND DISCUSSION The (010) basal surface of boehmite is terminated by doubly coordinated hydroxyls (μ2-OH). A single adsorbed water layer was evident at the (010)−water interface (Figure 1). The 15536

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The Journal of Physical Chemistry C water molecules adsorbed at the (010) surface accepted a hydrogen bond from hydroxyl groups in one surface row and donated a hydrogen bond to hydroxyl groups in a neighboring surface row. Layering effects did not extend beyond this first adsorbed water layer. No significant effect of the surface area on the interfacial water structure was found (Figure S1). Energy minimizations were performed with the revPBE-D3 and revPBE0 XC functionals (Figure 2) for a (010) boehmite slab with a water monolayer adsorbed and a H0 introduced in the interlayer region (subsurface), in between rows of surface hydroxyls (surface), or within the water monolayer (interface). AIMD simulations were also performed with the revPBE-D3 XC functional with the (010)−water interface and H0 introduced at the same positions as well as within the water slab. Separate calculations were also performed with H0 in the bulk solid and bulk liquid. H0 resides in a single site in the boehmite crystal structure.7 This combination of calculations allowed determination of the energetics of H0 across the interface and the effect of the XC functional (Figure 2). The calculations indicated a clear thermodynamic driving force for H0 to be released into the bulk liquid with general agreement between the energy minimizations and AIMD simulations and between the XC functionals. In the AIMD simulation where H0 was initially positioned in a surface site, H0 quickly moved to an interfacial site, and an energy of reaction for this site was therefore not calculated. This result was consistent with the energy minimizations, which showed that the transfer from a surface site to an interface site was exothermic. A different picture emerged for the O− radical whereby ΔE2 was lower in the bulk boehmite than in the bulk liquid. In these calculations (Figure 2), O− radicals were created by removing a hydrogen atom from a hydroxide ion in the interlayer (subsurface), a surface hydroxyl (surface), a water molecule in the adsorbed water layer (interface), or a water molecule in the water slab (water slab). As for the H0 radical, separate calculations were performed for the equivalent bulk systems. The calculations predicted a slight decrease of ΔE2 when the O− radical was located at the boehmite surface or at the interface, with the surface site as a shallow minimum in the revPBE-D3 calculations but not in the revPBE0 calculations. When the O− radical was initially placed in the water slab, it quickly moved, through a series of hydrogen exchanges between water molecules, to the interface where it was then exchanged between surface and interfacial sites multiple times. In sum, the calculations showed that unlike for H0, a thermodynamic driving force to release the O− radical in the liquid phase was lacking, and the interfacial region acted as a shallow trap for O− radicals. For the H0 radical, the energetics of transfer across the interface can be understood from a steric argument, as illustrated by the radial distribution functions (RDF) computed from the bulk AIMD simulations (Figure 3). When the H0 radical is in bulk water, the first peak in the H0−O(H2O) RDF (“HrOw” trace in Figure 3) is located at approximately 2.9 Å compared to 1.8 Å for the first intermolecular peak in the H(H 2 O)−O(H 2 O) RDF (“HwOw”). The first peak in the H0−H(H2O) RDF (“HrHw”) is also shifted to longer distances compared to the first intermolecular H(H2O)−H(H2O) (“HwHw”) peak. Water is therefore able to form a cavity for the H0 radical, and given the weak interaction between H0 and water molecules, ΔE1 was only slightly negative as a result.

Figure 3. Oxygen−oxygen (top left), hydrogen−oxygen (top right), oxygen−hydrogen (bottom left), and hydrogen−hydrogen (bottom right) radial distribution functions from AIMD simulations of bulk boehmite and bulk water. In the key, “w”, “b”, and “r” stand for water, boehmite, and radical, respectively.

In contrast, the first peak in the H0−O(AlOOH) RDF (“HrOb”) is located at approximately 2.3 Å, and the first peak in the H0−H(AlOOH) RDF (“HrHb”) is also shifted to shorter distances compared to that in the H0−H(H2O) RDF by as much as 0.8 Å. The crystal structure of boehmite therefore offers a much smaller cavity for H0 to reside in than bulk water, leading to a significant increase of ΔE1 with respect to that for bulk water. The interactions of the O− radical within boehmite and bulk water were much different from those of the H0 radical. Because the O− radical results from the removal of a hydrogen atom, a neutral species, the formal −1 charge, is retained at this site in bulk boehmite (reaction 2a). Furthermore, the O− radical accepts one hydrogen bond from a hydroxide ion in boehmite as does OH−, but of course, it cannot donate a hydrogen bond like OH−. In bulk water, the situation is similar whereby the OH0 radical accepts two hydrogen bonds as does H2O but donates only one hydrogen bond. The RDFs also showed the presence of one hemibonded water molecule, which accounted for the additional peaks at approximately 2.3 and 2.6 Å in the O−−O(H2O) (“OrOw”) and O−−H(H2O) (“OrHw”) RDFs, respectively, as reported previously.24 Consequently, the values of ΔE2 in the bulk solid and bulk liquid were much closer to each other than those for ΔE1, and the calculations predicted the absence of a driving force for O− to be released in the liquid phase. The (101) surface is the dominant side surface at alkaline pH.8−10 Unlike the (010) surface, which has a single type of functional group (μ2-OH), the (101) surface exposes three different functional groups (μ1-OH, μ1-H2O, and μ2-OH) (Figure 1). The corrugated atomic structure of the (101) surface led to a split-adsorbed water layer (Figure 1), but similar to the (010)−water interface, any layering beyond this first adsorbed layer was lower than the noise in the data. The same approach as described above was followed to determine the values of ΔE1 and ΔE2 across the (101)−water interface (Figure 4). The general conclusions from these calculations in terms of the behavior of the H0 and O− radicals at the interface 15537

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Figure 4. Energies of reactions 1 and 2a/2b for the formation of H0 and O− radicals in bulk boehmite (leftmost bars), bulk water (rightmost bars), and at sites across the (101)−water interface (middle bars) as obtained from energy minimizations with the revPBE-D3 (black bars) or revPBE0 (red bars) XC functional or from AIMD simulations with revPBE-D3 (green bars). Tabulated values can be found in Table S6 of the Supporting Information. Snapshots of the calculations with isocontour surfaces of the net spin density are also shown to illustrate the various positions considered for the H0 (top, left to right: subsurface, surface, and interface) and O− (bottom, clockwise: subsurface, μ1-OH, μ1-H2O, and μ2-OH) radicals.

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were the same as for the (010)−water interface. For H0, ΔE1 decreased significantly as H0 moved from the subsurface to the surface−interface. In the AIMD simulations with the revPBED3 XC functional, the unpaired electron transferred between multiple sites several times. As a result, the O− radical did not reside long enough at the subsurface and μ1-OH sites to determine ΔE2 accurately. As for the (010) surface, the DFT calculations predicted that functional groups at the (101) surface could act as shallow traps for the O− radical although the nature of this site varied between the two XC functionals. The lowest ΔE2 was obtained for the μ1-H2O group with revPBE-D3, whereas revPBE0 predicted the μ1-OH group to have the lowest ΔE2 with the μ2-OH group having only a slightly more positive ΔE2.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02144.

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Evaluation of the dependence of ΔE1 and ΔE2 on the plane-wave cutoff energy, size of the k-point mesh, and size of the boehmite supercell, list of all the AIMD simulations performed in this work, atomic density profiles at the (010)−water interface for two boehmite slab sizes, and tabulated values of the energy of reactions presented in Figures 2 and 4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (509) 371-6382.

CONCLUSIONS In combination with previous theoretical work7 that showed rapid 2D diffusion of H0 within the boehmite interlayer region, the results of this work indicated that H0 radicals formed upon irradiation of boehmite particles should escape boehmite particles rapidly from the side (101) surfaces to the interfacial region and into the liquid phase where they can combine to form H2 molecules. The concentration of H0 radicals at side surfaces could explain the higher H2 yields measured for boehmite compared to gibbsite3,4 whose crystal structure allows for 3D diffusion. Moreover, a thin water film, as opposed to bulk water, is thus expected to concentrate H0 radicals at the interface and facilitate H2 formation. The H2 yield should therefore be negatively correlated to the water coverage, a conclusion that is consistent with experimental observations for Al-(oxy)hydroxides.3,25 O− radicals diffuse at a much slower rate,7 and the radicals that eventually reach the surface may get trapped at surface sites, which could affect the reactivity of boehmite particles if their concentration becomes sufficiently high. This contrasting behavior of the two types of radicals, as revealed by the present calculations, offers an atomic-scale mechanism for the potential long-lasting impact of irradiation on boehmite particles present in tank waste.

ORCID

Sebastien N. Kerisit: 0000-0002-7470-9181 Zhizhang Shen: 0000-0002-8837-5573 Eugene S. Ilton: 0000-0003-4931-5217 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Laboratory Directed Research and Development (LDRD), Nuclear Process Science Initiative (NPSI) at the Pacific Northwest National Laboratory (PNNL). The computational work was performed using PNNL Institutional Computing. PNNL is a multiprogram national laboratory operated for the U.S. Department of Energy by the Battelle Memorial Institute under contract no. DE-AC05-76RL01830.

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