Article Cite This: Langmuir 2019, 35, 7995−8006
pubs.acs.org/Langmuir
Structural Approach to Understanding the Solubility of Metal Hydroxides Taishi Kobayashi,*,† Shogo Nakajima,† Ryuhei Motokawa,*,‡ Daiju Matsumura,‡ Takumi Saito,§ and Takayuki Sasaki† †
Department of Nuclear Engineering, Kyoto University, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8540, Japan Materials Sciences Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan § Nuclear Professional School, School of Engineering, The University of Tokyo, 2-22 Shirakata Shirane, Tokai-mura, Ibaraki 319-1188, Japan Downloaded via NOTTINGHAM TRENT UNIV on August 13, 2019 at 10:33:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We report the hierarchical structure of zirconium hydroxide after aging at different temperatures to elucidate the factors governing zirconium solubility in aqueous solutions. Zirconium hydroxide solid phases after aging at 25, 40, 60, and 90 °C under acidic to alkaline conditions were investigated using extended X-ray absorption fine structure (EXAFS), wide- and small-angle X-ray scattering (WAXS and SAXS), and transmission electron microscopy (TEM) techniques to reveal the bulk and surface structures of the solid phases from the nanoscale to sub-microscale. After aging at 25 °C, the fundamental building unit of the solid phase was considered to be tetrameric and dimeric hydroxide species. These polynuclear species formed amorphous primary particles that are approximately 3 nm in size, which in turn formed aggregates that are hundreds of nanometers in size. This hierarchical structure was found to be stable up to 60 °C under acidic and neutral conditions and up to 40 °C under alkaline conditions. After aging at 90 °C under acidic conditions and at 60 and 90 °C under alkaline conditions, the WAXS and EXAFS measurements suggested the crystallization of the solid phase. The SAXS profiles and TEM observations supported the existence of crystallized large particles about 60 nm in size, and the appearance of the Guinier region in the SAXS profiles indicated that the crystallization of the amorphous primary particles leads to the reduction of the size of the large aggregates. The transformation of the solid-phase structure by temperature was discussed in relation to the solubility product to understand the solubility-limiting solid phase. The solubility of zirconium hydroxide after aging at different temperatures was governed not only by the size of the amorphous primary particles or crystallized large particles but also by their surface configuration. temperature depending on the disposal concepts.5−7 It is interesting to clarify the effect of elevated temperature on the solubility limits of M(OH)4(am) because M(OH)4(am) is often subject to be transformed to a more thermodynamically stable solid phase by temperature. As a tetravalent ion, zirconium has been considered as a chemical analogue of tetravalent actinides, despite its different chemical characteristics.8−12 Moreover, because Zr has a high yield among the uranium fission products and Zr metal is used as a fuel cladding in light water reactors, it is a relevant element in the safety assessment of geological disposal of nuclear waste. In the previous studies, the solubilities of tetravalent zirconium were examined after aging sample solutions at elevated temperatures up to 90 °C, which showed a decrease by several orders of the magnitude with the increasing aging temperature.13,14
1. INTRODUCTION Among actinide elements and long half-life fission products, 237 Np, 233U, 229Th, and 93Zr behave as tetravalent metal ions (M(IV)) and 242Pu and 239Pu behave as trivalent and tetravalent metal ions (M(III/IV)) under reducing conditions of deep underground environments and are hydrolyzed to form sparingly soluble amorphous hydroxide solid phases (M(OH)4(am)). The migration behavior of radionuclides in the tetravalent oxidation state is primarily governed by the solubility of M(OH)4(am) under the geochemical conditions; thus, it is important to predict the solubilities of radionuclides reliably for the performance assessment of radioactive waste disposal. In international collaboration projects, much work has been dedicated to establishing robust databases for thermodynamic constants at standard conditions of 25 °C and 1 atm.1−4 However, in repository systems for highly radioactive waste or spent nuclear fuels, decay heat emissions from the waste are transferred to groundwater aquifers through engineered and geological barrier systems, increasing the © 2019 American Chemical Society
Received: April 17, 2019 Published: May 22, 2019 7995
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
Article
Langmuir Table 1. Summary of Samples Investigated in This Study sample no.
aging temperature (°C)a
target pHc
1-a 2-a 3-a 1-b 2-b 3-b
25
2.7 7.7 12.2 2.7 7.7 12.2
40
measured pHcb
sample no.
aging temperature (°C)a
target pHc
± ± ± ± ± ±
1-c 2-c 3-c 1-d 2-d 3-d
60
2.7 7.7 12.2 2.7 7.7 12.2
2.79 7.70 12.19 2.79 7.70 12.22
0.29 0.14 0.07 0.20 0.19 0.13
90
measured pHcb 2.68 8.11 12.21 2.61 7.58 12.20
± ± ± ± ± ±
0.14 0.17 0.09 0.11 0.23 0.07
a Temperature during aging in the incubator. bpHc values of sample solutions after aging at elevated temperatures and measured at 25 °C. The values are average pHc values of the sample solutions for WAXS/SAXS, EXAFS, and TEM measurements.
Moreover, SAXS and transmission electron microscopy (TEM) were used to reveal the size of the primary particles and their aggregates, together with their surface configuration. These informations are discussed to elucidate the solid-phase transformation caused by temperature and to clarify their effects on the solubilities of Zr hydrolysis products.
These facts indicated the transformation of solubilitylimiting solid phase from initial Zr(OH)4(am). The bulk structures of zirconium solid phases after aging at the elevated temperatures was examined using the X-ray diffraction (XRD).13,14 After aging at 90 °C, the peaks in the XRD patterns became sharper, suggesting the formation of crystalline particles in the bulk solid phase due to a heatinduced dehydration transformation of Zr(OH)4(am) to ZrO2(cr), respectively. The trend in the solubility product, Ksp, as a function of the crystalline particle size of the solid phase after aging at 90 °C was explained based on a particle size effect, where log Ksp decreases in inverse proportion to the particle size of the solid phase.13 On the other hand, the XRD patterns of the zirconium hydroxide solid phase were not changed after aging at 25−60 °C, indicating that the crystalline structure was absent, although Zr solubility was still decreased with aging temperature.14 These results suggest that the prediction of Zr solubility under moderate temperature conditions of 40 and 60 °C based on only its crystalline particle size derived from the investigation of bulk solid phase is unreliable. Therefore, to understand Zr solubility behavior under various elevated temperature conditions, we suppose that it is necessary to explore the solid-phase structure in greater detail over a wide length scale from approximately 0.1 to 100 nm together with the bulk and surface configurations of the solid phase. The microscopic structure of Zr solid phases has been investigated in a number of studies.15−18 Fryer et al.18 prepared the hydrolysis products by reflux of 0.055 mol/dm3 (M) zirconyl chloride solution and proposed that a tetramer, [Zr4(OH)8·16H2O]8+, was a fundamental building unit of a Zr solid phase based on high-resolution electron microscopy observations. Several extended X-ray absorption fine structure (EXAFS) and small-angle X-ray scattering (SAXS) studies have also supported the existence of a tetramer.19−23 Although these previous studies using different techniques have hinted that the Zr tetramer is involved in the structures of Zr(OH)4(am), the factor governing Zr solubility at 25 °C in relation to the bulk and surface configurations of the solid phase is still not fully understood. Studies of the transformation of the zirconium hydroxide solid phase at elevated temperatures have mainly focused on the transformation of the crystal structure at temperatures well above 100 °C,24−28 and only few studies investigated aqueous suspensions of Zr(OH)4(am) under the temperature conditions less than 100 °C.29,30 On the basis of these backgrounds, we focus on the bulk and surface configurations of zirconium hydroxide solid phases after aging at temperatures of 25, 40, 60, and 90 °C from the nanoscale to sub-microscale. Wide-angle X-ray scattering (WAXS) and EXAFS were used to reveal the crystal structure and coordination structure around the Zr ions, respectively.
2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. All chemicals used were of reagent grade. ZrCl4, ZrO2(cr), HClO4, NaOH, NaCl, and NaClO4 were purchased from Wako Pure Chemical Corporation. All solutions were prepared with deionized purified water (Milli-Q, Millipore). All sample preparation and subsequent aging were performed under the atmospheric condition. An effect of the dissolved carbon on the Zr solid phase was considered to be limited under the atmospheric pressure according to the thermodynamic calculation in the Zr-carbonate system.31 A concentrated aqueous Zr solution was prepared by dissolving ZrCl4 in purified water and NaOH was added quickly to precipitate the amorphous hydroxide, Zr(OH)4(am). The Zr(OH)4(am) precipitate was washed with purified water and dissolved in concentrated HClO4 to obtain a stock solution of Zr perchlorate ([Zr] = 0.3 M). Sample solutions were prepared by oversaturation at 25 °C. The initial solution of 0.01 M Zr with pHc 1.0 was prepared by diluting the Zr stock solution, and HClO4/NaOH was added to adjust the pH to approximately 2.7, 7.7, and 12.2. The pH values for the samples are shown in Table 1. The ionic strength (I) was kept at 0.5 M by adding an appropriate amount of NaClO4. The sample solutions were kept in incubators at 25, 40, 60, and 90 °C for 1 month. A combination glass electrode (9615-10D, HORIBA Ltd.) was used to measure hydrogen ion concentrations (pHc). The internal solution in the electrode consisted of 3.6 M NaCl and 0.4 M NaClO4 and prevented precipitation of KClO4 at the junction between the electrode and solution. The electrode was calibrated against standard HCl and NaOH solutions (Wako Pure Chemical Corporation) at the appropriate ionic strength to correct the measured pH values to those of the pHc values. After aging the sample solutions at elevated temperatures, the sample solutions were cooled to 25 °C to measure the pHc values, and the wet precipitate was separated by centrifugation. The samples are summarized in Table 1. During the aging at each temperature, the activity of OH− ion changes, accompanied by the temperature dependence of the ion product of water (pKw). At I = 0.5 M, pKw equals 13.74, 13.26, 12.69, and 11.94 at 25, 40, 60, and 90 °C, respectively,32 indicating that the activity of OH− ion decreases with increasing temperatures. For WAXS and SAXS measurements, the wet precipitate was loaded into a boron silicate capillary with an inner diameter of 1.5 mm (WJM-Glas Müller GmbH). For EXAFS measurements, the wet precipitate was loaded into a 2 mm thick quartz cell. For all measurements, ZrO2(cr) powder purchased was used as a reference. For TEM observations, a few microliter droplet of solid/liquid suspension was put on a TEM grid (NS-C15, Ohken Shoji) covered by a holey carbon film and was dried at room temperature under air. 2.2. Solid-Phase Characterization. The zirconium hydroxide solid phases after aging at different aging temperatures were 7996
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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Figure 1. WAXS spectra of zirconium hydroxide solid phases aged at 25, 40, 60, and 90 °C at pHc (a) 2.7, (b) 7.7, and (c) 12.2, together with monoclinic ZrO2(cr) as a reference. Down arrows in (a,c) represent the peak position of tetragonal ZrO2(cr). intensity, Q, which is proportional to the mean square scattering length density fluctuation36,37
investigated by a combined method of EXAFS, WAXS, SAXS, and TEM. The wet precipitates of zirconium hydroxides and ZrO2(cr) powder were used as references with no treatment and were characterized by Zr EXAFS measurements at beamline BL14B1, SPring-8, Hyogo, Japan. A pair of Si(311) crystals were used in the double-crystal monochromator. The monochromatic radiation (∼1010 photons/s at the Zr K-edge) was focused to a spot of size ∼3 × 1 mm (horizontal × vertical) at the sample position. Higher harmonics radiation in the incident beam was suppressed by two Rh-coated mirrors. Spectra were recorded in transmission mode using argon- and nitrogen-filled (1:1) ionization chambers. The EXAFS data analysis was based on standard least-square fit techniques using the Athena and the Artemis program packages with FEFF6.0.33−35 E0, the origin used to calculate the EXAFS χ(k) function, was fixed at the first-derivative maximum in the individual spectra at ∼18.009 keV. The k-range up to ∼12 Å−1 was investigated. EXAFS parameters (neighboring atomic distances Ri within the resolution of ΔRi ≈ π/2Δk, EXAFS Debye−Waller factors σi2, and coordination numbers Ni for coordination shell i) were determined using the IFEFFIT code. Backscattering amplitude and phase-shift functions for single scattering paths were obtained from FEFF6.0. Prior to analysis, the k3-weighted Zr K-EXAFS spectra were Fouriertransformed over a k-space range of 2.0−12 Å−1, using symmetric square windows with Δk = 1.0 Å−1. All fit operations were performed in Ri-space over the radial distance range of 1−3 Å. The amplitude reduction factor, S02, was fixed at 0.823. WAXS and SAXS measurements were performed at BL8S3 of Aichi Synchrotron Radiation Center, Seto, Japan. The wavelength of the incident X-ray beam, λ, was 0.92 Å, and the X-ray beam was focused to a spot size of about 1.0 × 0.5 mm (horizontal × vertical, full width at half maximum) at the sample position. The scattered X-rays from the zirconium hydroxide samples aged at 25, 40, 60, and 90 °C and the ZrO2(cr) reference suspended in purified water were detected by an imaging plate (R-axis IV++, Rigaku) with 3000 × 3000 pixels, which covered a q range of 15 nm−1 < q < 45 nm−1 at a sample-to-detector distance of 0.2 m (q region of WAXS) and q range of 0.06 nm−1 < q < 10 nm−1 at sample-to-detector distances of 1.0 and 4.0 m (q region of SAXS). Note that q, which is equal to (4π/λ)sin θ, is the magnitude of the scattering vector, where 2θ is the scattering angle. The scattering data recorded on the imaging plate were circularly averaged and corrected by the counting efficiency and instrumental background on a pixel-to-pixel basis. The cell scattering and solvent scattering were subtracted from the corrected scattered intensity, I(q), by considering the transmission. Because the zirconium hydroxide solid-phase samples probably did not disperse homogeneously in the glass capillary cell during the SAXS measurements owing to their specific gravity, the scattering intensity is not reflected on a relative scale. Therefore, I(q) was normalized by the second moment of the
Q=
∫0
∞
q2I(q) dq ∝ 2π 2ϕsample(1 − ϕsample)Δρ2
(1)
where ϕsample is the volume fraction of the zirconium hydroxide samples and Δρ is the scattering contrast between the sample and solvent, Δρ = (ρsample − ρwater), where ρsample and ρwater are the scattering length densities of the sample and of water, respectively. Using Inor(q) (=I(q)/Q) canceled the effects of the scattering contrasts and volume fraction.38,39 The TEM investigation was supported by National Institute of Materials Science (NIMS) microstructural characterization platform. The high-angle annular dark-field scanning TEM (HAADF−STEM) and high-resolution TEM (HRTEM) images were obtained by using JEM-ARM200F (JEOL). The size distributions of the observed particles were obtained from the HAADF−STEM images of the solid phases after aging at from 25 to 90 °C.
3. RESULTS 3.1. WAXS Spectra. Figure 1 shows the WAXS spectra of the zirconium hydroxide solid phases after aging at pHc 2.7, 7.7, and 12.2 at temperatures of 25, 40, 60, and 90 °C, together with that of monoclinic ZrO2(cr). At pHc 2.7, peaks at q = 19.5, 22.0, 34.4, 37.6, and 40.4 nm−1, corresponding to those of monoclinic ZrO2(cr) (JCPDS no. 371484),40 were visible in the solid phase aged at 90 °C, whereas these peaks were not observed in the solid phases aged at less than 60 °C. Note that a small peak at 21 nm−1, marked by a down arrow in Figure 1, after aging at 90 °C corresponds to tetragonal ZrO2(cr) (JCPDS no. 421164).41 The observed WAXS spectra are comparable with the XRD patterns obtained in the acidic pH range in the previous reports.13,14 At pHc 7.7, no indication of a crystalline structure was observed after aging at 25−60 °C, suggesting that the solid phase remained amorphous after aging from 25 to 60 °C. After aging at 90 °C at pHc 7.7, only very small peaks at the positions of monoclinic ZrO2(cr) were observed. In contrast, sharp, intense peaks corresponding to monoclinic ZrO2(cr), together with a small shoulder peak at 21 nm−1 corresponding to tetragonal ZrO2(cr) were visible after aging at pHc 12.2 and 60 and 90 °C. The XRD patterns indicated that the crystallization of bulk zirconium hydroxide solid phases depended on both temperature and pH of the aqueous solution. Note that the pHc values were measured at 25 °C after aging at elevated temperatures. 7997
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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Figure 2. (a) k3-weighted χ(k)-functions and (b) corresponding FT magnitudes of the solid phases aged at pHc 2.7, 7.7, and 12.2 at 25 to 90 °C.
similarly considered to be kinetically hindered by the lower Zr solubility in the neutral pH region. The higher crystallization under alkaline pH condition has been discussed in literature studies.25−27,29 Teseng et al. explained the difference in transformation of solid phases with the difference in surface energy and internal strain during the transformation, based on the observation of decreasing powder density of the solid phases with increasing pH.25 The difference in the solid-phase transformation was also explained with different extents of the loss of structural water of the initial solid phases.26,27 On the other hand, Š tefanić et al. suggested that the dissolution and precipitation mechanisms are also responsible for the solid phase transformation under alkaline pH region.29 An understanding of the crystallization mechanism at high pHc, therefore, needs further investigation. In the present study, we focused on the solid phase only after aging for 1 month and not discussed in detail over the crystallization process. The dominant phase was found to be amorphous hydroxide and/or monoclinic ZrO2(cr). 3.2. EXAFS Spectra. For the zirconium hydroxide solid phases aged at pHc 2.7, 7.7, and 12.2 and 25 to 90 °C, the k3weighted EXAFS-functions, χ(k), are shown in Figure 2a, and the corresponding Fourier transform (FT) magnitudes are shown in Figure 2b. All spectra exhibit an intense FT peak at ∼1.7 Å, corresponding to a phase-corrected distance value of about 2.1 Å. This peak represents the oxygen atoms comprising the first coordination sphere of the central absorbing Zr. All FT spectra also show a second-shell contribution at around ∼3.0 Å, corresponding to a phasecorrected distance of 3.4 Å. Characteristic parameters determined in the fits of the EXAFS equation to the experimental data are summarized in Table 2. A typical fit to
Based on the phase diagram, although monoclinic ZrO2(cr) is the stable phase up to around 1100 °C,42 tetragonal and cubic ZrO2(cr) have been often reported in literature studies after the hydrothermal treatment of amorphous hydroxides in practice.24−29 The crystalline phase significantly depends on the solution pHc, temperature, and precipitation rate; however, it has been widely observed that the prolonged treatment at elevated temperature leads to the formation of thermodynamically stable monoclinic ZrO2(cr). Although the sample preparation method with rapid precipitation used in this study might cause the formation of tetragonal ZrO2(cr), the observed contribution of tetragonal ZrO2(cr) was found to be limited after aging for 1 month. The WAXS spectra of the solid phases aged at acidic and neutral pHc indicated that the crystallization under neutral pHc hardly proceeded comparing to that under acidic pHc as reported in the literature studies.25−29 The solubility of zirconium hydroxides and oxides at neutral pHc is several orders of magnitude lower than that at acidic pHc,43−46 and the kinetics of crystallization has been often interpreted by the dependence of zirconium solubility on pHc.25−29 For example, Denkewicz et al. prepared 0.2 M Zr solution, adjusted to pH 5 with 5 M KOH and treated hydrothermally at 200 °C for 2 h. The resulting component remained tetragonal ZrO2(cr), whereas that adjusted at pH 3.6 resulted in monoclinic ZrO2(cr). The authors suggested that the formation of monoclinic ZrO2(cr) was caused by a dissolution/precipitation process and the transformation from tetragonal to monoclinic ZrO2(cr) did not occur because of the low solubility of tetragonal ZrO2(cr) at pH 5.27 In the present study, the transformation of the amorphous solid phase at pHc 7.7 was 7998
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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Table 2. Characteristic Parameters Determined by Least-Squares Fitting Analysis of the EXAFS Spectra Shown in Figure 3 sample no.
aging temperature and pHca
shell
1-a
25 °C, 3.08
1-b
40 °C, 2.99
1-c
60 °C, 2.82
1-d
90 °C, 2.72
2-a
25 °C, 7.84
2-b
40 °C, 7.89
2-c
60 °C, 7.78
2-d
90 °C, 7.81
3-a
25 °C, 12.26
3-b
40 °C, 12.35
3-c
60 °C, 12.30
3-d
90 °C, 12.27
O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr O Zr
ZrO2 (monoclinic)
Ri (Å) 2.16 3.46 2.16 3.46 2.16 3.46 2.15 3.47 2.16 3.47 2.16 3.47 2.16 3.46 2.16 3.45 2.16 3.47 2.16 3.46 2.14 3.47 2.14 3.47 2.18 3.41
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.01 0.01 0.02 0.02 0.02
CN 8.0 ± 1.5 ± 8.0 ± 1.7 ± 8.0 ± 1.9 ± 7.8 ± 2.4 ± 7.8 ± 1.7 ± 7.8 ± 1.7 ± 7.7 ± 1.7 ± 7.6 ± 1.7 ± 7.8 ± 1.7 ± 7.8 ± 1.8 ± 7.9 ± 4.2 ± 7.0 ± 4.1 ± 7.0c 6.0c
0.3 0.6 0.3 0.6 0.3 0.7 0.4 0.8 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.5 0.8 0.5 0.9
ΔE0 (eV)
σ2 (Å2)b
R-factor
−1.6 ± 0.8
0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.007 0.009 0.008 0.003 ± 0.003 0.003 ± 0.003
0.015
−1.6 ± 0.7 −1.6 ± 0.8 −2.2 ± 0.9 −3.7 ± 0.8 −3.6 ± 0.8 −3.6 ± 0.9 −3.7 ± 0.9 −3.5 ± 0.8 −3.6 ± 0.8 −5.0 ± 1.2 −5.5 ± 1.3 −5.0 ± 1.6 −4.8 ± 1.6
0.014 0.017 0.022 0.017 0.016 0.017 0.017 0.016 0.017 0.037 0.042 0.039
The pHc values were those measured at 25 °C after aging at elevated temperatures. bFixed value was used except the case of ZrO2 (monoclinic). Fixed value.
a c
Figure 3. R-space fitting results for samples after aging at pHc 2.7 and 25 °C. (a) Fourier-filtered data (open circles) and back-transformed fit (solid red line). (b) FT magnitude of EXAFS data (open circles), fit magnitude (solid red line), imaginary part of the fit (open green squares), and real part of the fit (open blue triangles).
strongly acidic solution (pH ≈ 0) and crystallized the solid phase from the solution. The EXAFS spectra were analyzed by fixing the structure of the Zr tetramer, [Zr4(OH)8(H2O)16]8+, with a coordination number of 2 for neighboring Zr, resulting in similar distances of 3.590 and 3.588 Å for Zr−Zr in the aqueous and solid state of the Zr hydrolysis products, respectively.20 In this study, the distances and coordination numbers are obtained for the paths 8.3 ± 0.3 O at 2.16 ± 0.01 Å and 1.4 ± 0.6 Zr at 3.46 ± 0.03 Å for the solid phase after aging at pHc 2.7 and 25 °C. The distance and coordination number of neighboring O atoms agree within the errors from those in the previous EXAFS measurements and theoretical calculations by density functional theory (DFT) for Zr aqueous species.19−22,47 On the other hand, the distance of neighboring Zr atoms of 3.46 Å obtained here is slightly shorter than 3.53,
the FT spectra and corresponding back-transformed χ(k) function are shown in Figure 3 for the solid phase aged at pHc 2.7 and 25 °C. The overall goodness of the fit evaluated by fefffit is given as “R-factor” in Table 2. Rf = 0.02 signifies that the theory and data agree within 2%. Walther et al.22 measured the EXAFS spectra of the polymeric hydrolysis species in the acidic pH range of pHc 0.1−1.8 at Zr concentrations of 0.01 and 0.0015 M. A radial distance of 3.65 ± 0.03 Å and coordination number of 1.3 ± 0.4 for the neighboring Zr atoms were obtained for the sample at pHc 0.1 and [Zr] = 0.01 M. These results agreed with the assumption that the hydroxo-bridged Zr tetramer was the main aqueous hydrolysis species. The deviation from the expected number of 2 for Zr−Zr pairs in the tetramer was attributed to the effect of the remaining monomers at pHc 0.1.22 Hagfeldt et al. prepared aqueous Zr hydrolysis species ([Zr] = 0.3 M) in 7999
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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Figure 4. Double-logarithmic plots of the SAXS profiles of the solid phases aged at pHc (a) 2.7, (b) 7.7, and (c) 12.2 and temperatures of 25, 40, 60, and 90 °C. The SAXS profiles in (a−c) are vertically shifted by offset constants of 1.5, 0.5, −1, and −2.5, respectively, to avoid overlap. The solid black lines are the theoretical SAXS profiles obtained by using eq 2 together with the characteristic parameters listed in Table 3. Dashed lines in the figures indicate the power law with α.
2.15 ± 0.01 Å and those for neighboring Zr were 2.4 ± 0.8 Zr at 3.47 ± 0.02 Å, respectively. In the previous study, diffraction peaks corresponding to monoclinic ZrO2(cr) appeared in the XRD pattern of the solid phase after aging at 90 °C,13 and the WAXS spectra shown in Figure 1 also confirmed the existence of the crystalline structure. Despite the uncertainties in the coordination number of neighboring Zr, the result of EXAFS analysis suggested the progress of crystallization of the solid phase with increasing aging temperature. At pHc 7.7, the distance and coordination number for O and Zr did not change after aging at 25 to 90 °C (Table 2). This indicates that crystallization hardly occurred under neutral pH conditions up to 90 °C. In the alkaline pH region at pHc 12.2, the coordination number for Zr started increasing after aging at 60 °C and it was 4.1 ± 0.9 after aging at 90 °C, which is close to the value for ZrO2(cr) (Table 2). The appearance of diffraction peaks in the WAXS spectra after aging at pHc 12.2 and 60 and 90 °C also indicated that crystallization toward monoclinic ZrO2(cr) occurred. 3.3. SAXS Profiles. Figure 4 shows the SAXS profiles, log Inor(q), for the solid phases aged at pHc 2.7, 7.7, and 12.2 and temperatures from 25 to 90 °C. The q dependence of all the SAXS profiles was divided into the two characteristic q regions: the lower-q region (q < 4.0 nm−1; region I) and the higher-q region (q > 4.0 nm−1; region II). The small-angle scattering intensity distributions varied with power-law scattering, Inor(q) ≈ q−α, where the exponent, α, in region II was close to 4.0. In region I, the α value was around 2.4 for the solid phases aged at pHc 2.7 at 25−60 °C, at pHc 7.7 at 25−90 °C, and at pHc 12.2 at 25−40 °C. These asymptotic behaviors support the formation of amorphous primary particles several nanometers in size, and the aggregation of the primary particles without a homogeneous dispersion. Here, note that, for pHc 7.7 at 90 °C, very small peaks corresponding to those for monoclinic ZrO2(cr) were observed in the WAXS spectra (Figure 1), and the EXAFS spectra in Figure 2 showed no significant difference between those for the samples aged at pHc 7.7 at 25−90 °C. Therefore, it was considered that the crystallization of the solid phase partially occurred in a narrow region of the sample under the neutral pH condition. The constant values of α in region I for pHc 2.7 at 25−60 °C, pHc 7.7 at 25−90 °C,
3.59, and 3.65 Å19,20,22 reported as the distance for [Zr4(OH)8(H2O)16]8+ prepared at pHc < 1.8 and close to 3.43 Å for the species prepared by adding NaOH to 0.001 M Zr solution up to pHc 2.8.21 In the present study, the solid phases were prepared from the initial solution with [Zr] = 0.01 M at pHc 1. Based on the simulation of speciation diagram using a set of hydrolysis constants for polynuclear species up to 12 mers,48 tetramer species were found to be the dominant species in the solution of [Zr] = 0.01 M at pHc 1. The sample preparation method by rapid precipitation from the initial solution might lead an existence of the tetramer species as a fundamental building unit of the solid phases. On the other hand, Cho et al. interpreted the shorter distance of Zr−Zr atoms observed in the aqueous species at pHc 2.8, indicating that the species were built up of primarily ZrO8 building blocks and that the associated Zr sublattice was highly disordered and showed a great variety of geometric variations in its primary coordination sphere.21 Considering that the Zr−Zr distance became shorter, 2.98 Å in dimer species of [Zr2(OH)4(H2O)8]4+, compared to 3.77 Å in the tetramer species of [Zr4(OH)8(H2O)16]8+ according to on the DFT calculation,47 the fundamental building unit of the solid phase should contain not only the tetramers but also the dimeric species. Figure S3 in the Supporting Information shows the structures of the tetramer of [Zr4(OH)8(H2O)16]8+ and dimer of [Zr2(OH)4(H2O)8]4+ optimized by quantum calculations using DFT, as reported previously.47 After aging at pHc 2.7 and 40 and 60 °C, the distances and coordination numbers for O and Zr were similar to those after aging at 25 °C. This suggests that the fundamental building unit of the Zr solid phase is stable at temperatures up to 60 °C. Previously, no clear peak corresponding to monoclinic ZrO2(cr) appeared in the XRD patterns of the solid phases after aging below 60 °C.14 The WAXS spectra in Figure 1 also confirmed this after aging at 25 to 60 °C. The EXAFS spectra also indicated that the fundamental Zr units, that is, tetramer/ dimer mixture, were not connected to each other after aging at pHc 2.7 and 25, 40, and 60 °C, and they aggregated loosely with a random orientation. After aging at pHc 2.7 and 90 °C, the distances and coordination numbers for neighboring O were 7.8 ± 0.4 O at 8000
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Langmuir and pHc 12.2 at 25−40 °C indicate that the size of large aggregates was larger than 105 nm. This is because the low q limit in our SAXS experiment was q = 0.06 nm−1 (2π/0.06 = 105 nm). After aging the solid phases at pHc 2.7 at 90 °C and pHc 12.2 at 60 and 90 °C, the α value changed to be from 4.0 to approximately 3.2 at the boundary of region I and II and changed from 3.2 to 0 at qc = 0.1−0.2 nm−1 with decreasing q in region I (qc; see thick arrows in Figure 4). The large decreases in α from 3.2 to 0 at q < qc for these solid phases may correspond to the Guinier region in small-angle scattering because of the dispersion of the crystallized large particles which are tens of nanometer in size,49 where the Guinier region corresponds to the q region of q < 1/Rg (Rg is the size of the crystallized large particles). The appearance of the Guinier region in SAXS profiles with the diffraction peaks in WAXS profiles obtained for pHc 2.7 at 90 °C and pHc 12.2 at 60 and 90 °C indicate that the size of the large aggregates becomes smaller during the crystallization of the amorphous solid phase. It is worthy to note that the change of α from 4 to 3.2 at around q = 4.0 nm−1 is unlikely to be recognized for pHc 12.2 at 60 and 90 °C. This fact indicates that the contribution of amorphous primary particles to SAXS profiles decreases because of their crystallization to form the dispersed large particles. We defined the radius of gyration of the crystalized large particles, Rg1, and the small amorphous primary particle, Rg2. To analyze Rg1 and Rg2 and the asymptotic q-behavior in Inor(q) quantitatively, we carried out a numerical analysis of the SAXS profiles using a combined profile with the unified Guinier and power-law approach50
Figure 5. Contribution of the first to fourth terms in eq 2 ((1)−(4)) shown in the double-logarithmic plots of the SAXS profiles of the solid phases aged at pHc 2.7 and 90 °C.
At pHc 2.7, the P2 values gradually increase with the aging temperature. This represents that the surface configuration of the amorphous primary particle may be slightly changed as will be discussed in detail in Section 4.3 even without significant difference observed in the WAXS and EXAFS spectra. In contrast, in the neutral and alkaline pH conditions, the P2 values were almost constant, suggesting that the surface configuration of the amorphous primary particles was maintained during the aging process. 3.4. TEM Observations. Figure 6 shows HAADF−STEM images of the zirconium hydroxide solid phases after aging at pHc 7.7 and 12.2 at temperatures ranging from 25 to 90 °C. The HRTEM images of these solid phases are presented in Figure S1 in the Supporting Information. After aging at pHc 7.7, network structures consisting of small particles with the size of a few manometers were observed after aging at 25 to 90 °C and no large particle with a crystal structure was found within the investigated temperature range up to 90 °C. For pHc 7.7 at 90 °C, very small peaks corresponding to those for monoclinic ZrO2(cr) were observed in the WAXS spectra (Figure 1), and the EXAFS spectra in Figure 2 showed no significant difference between those for the samples aged at pHc 7.7 at 25−90 °C. Therefore, in the SAXS analysis, we considered that the crystallization of the solid phase partially occurred only in a narrow region of the sample under the neutral pH condition at 90 °C. The SAXS profile for pHc 7.7 at 90 °C was analyzed by assuming the network structure consisting of amorphous primary particles, similar to those at lower aging temperatures. In the TEM images for pHc 7.7 at 90 °C, no large crystalline particle was found. After aging at pHc 12.2, the network structures consisting of small particles were only found after aging at 25 and 40 °C. Because no evidence for the crystallization of these small particles was observed in the HRTEM images, these particles were considered to form the amorphous structures. These observations agree with the WAXS spectra and SAXS profiles of the solid phases after aging at pHc 12.2 and at 25 and 40 °C (Figures 1 and 4). The size distribution of the small particles after aging at pHc 12.2 at 25 °C exhibits a normal distribution with the average size of 6.0 nm (see Figure S2 in the Supporting Information). The observed average particle size by TEM is slightly larger than the size of amorphous primary particles (2Rg2 = 2.4 nm) determined in the analysis of SAXS profiles. In the TEM observations, the particle with a size
Inor(q) = G1 exp( −q2R g12/3) + B1 exp( −q2R g2 2/3){[erf(qR g1/ 6 )3 ] /q}P1 + G2 exp( −q2R g2 2/3) + B2 {[erf(qR g2/ 6 )3 ] /q}P2 (2)
where P1 and P2 correspond to the exponents of the power-law scattering in q regions I and II, respectively, and G1, B1, G2, and B2 are the proportionality constants describing the relative contributions of the four terms in eq 2 to the scattering intensity, Inor(q). In the analysis of the SAXS profiles for pHc 2.7 at 25−60 °C, pHc 7.7 at 25−90 °C, and pHc 12.2 at 25, 40 °C, the first term and [erf(qR g1/ 6 )3 ] in the second term, which functioned as a damping factor for the power-law scattering at q < 1/Rg1, in eq 2 were negligible because no crystalized large particles were formed under these conditions. Figure 5 shows the contribution of each term on the rhs of eq 2 to the best-fit theoretical scattering curve for the SAXS profile after aging at pHc 2.7 and 90 °C. The decomposed lines numbered (1)−(4) represent the contributions of the first to fourth terms of eq 2. The characteristic parameters Rg1, Rg2, P1, and P2 were treated as free parameters. It was found that the unified function of eq 2 and their parameter values listed in Table 3, indicated as solid black curves in Figure 4, wellreproduced all of the experimentally obtained SAXS profiles. The average value of Rg2 was determined to be Rg2 = 1.2 nm, regardless of the pHc. On the other hand, after aging at pHc 2.7 at 90 °C and pHc 12.2 at 60 and 90 °C, the crystallized large particles with an average size of Rg1 = 25 nm were observed in addition to the amorphous primary particles. 8001
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Langmuir Table 3. Metrical Parameters Extracted by Least-Squares Fitting Analysis of the SAXS Profiles Shown in Figure 4 sample no. 1-a 1-b 1-c 1-d 2-a 2-b 2-c 2-d 3-a 3-b 3-c 3-d
aging temperature and pHca 25 40 60 90 25 40 60 90 25 40 60 90
°C, °C, °C, °C, °C, °C, °C, °C, °C, °C, °C, °C,
2.70 2.69 2.72 2.50 7.63 8.28 7.61 7.46 12.15 12.17 12.15 12.17
Rg1 (nm)
26.3 ± 0.7
25. 2 ± 0.7 22.9 ± 0.1
P1 2.47 2.46 2.42 3.13 2.45 2.42 2.36 2.36 2.36 2.60 3.35 3.27
± ± ± ± ± ± ± ± ± ± ± ±
P2a
Rg2 (nm) 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01
1.07 1.12 1.22 1.36 1.12 1.16 1.34 1.66 1.18 1.15 1.09 0.68
± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.04 0.03
4.05 4.16 4.28 4.34 4.06 4.16 4.09 3.99 4.02 4.01 4.03 4.04
± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
The pHc values were those measured at 25 °C after aging at elevated temperatures.
a
smaller than a few nanometers is not a detectable because of an identification limit, which might cause the difference in the observed particle size by both methods. It should also be noted that the samples for TEM observations were dried under air, while those for SAXS measurements were the wet suspensions. The difference in the sample condition possibly resulted in the difference in the observed particle size. After aging at pHc 12.2 at 60 and 90 °C, HAADF−STEM images revealed that the particles with the size of more than 50 nm appeared. The small particles with a size similar to those observed at 25 and 40 °C were still observed between the large particles. The HRTEM images show lattice-fringe patterns for the large particles, indicating that they have a crystal structure. These observations supported the WAXS spectra and SAXS profiles of the samples after aging at 60 and 90 °C. After aging at pHc 2.7, we have obtained the HAADF− STEM and HRTEM images in the previous study under similar experimental conditions of pHc from 1.9 to 2.8.13 The solid phase aged at 25 °C has been found to form network structures consisting of small particles with the size of a few nanometers.13 After aging at 70 and 90 °C, on the other hand, large particles more than 50 nm in size with crystal structures have appeared.13 The WAXS and EXAFS spectra and SAXS profiles obtained in the present study (Figures 1, 2, and 4) showed similar patterns after aging at 20−60 °C and showed different patterns after aging at 90 °C. The transformation of the solid phase to form large crystalline particles started after 60 °C.
= 1.2 nm in size and their aggregates are larger than 105 nm. To investigate the primary particles further, the SAXS profiles of [Zr4(OH)8(H2O)16]8+ and [Zr2(OH)4(H2O)8]4+ were calculated from the position of all of the atoms obtained by DFT calculation in the single tetramer and dimer using the Debye function for scattering (see Supporting Information).47 The calculated SAXS profiles were compared with the experimentally obtained SAXS profiles to examine the relationship between [Zr4(OH)8(H2O)16]8+, [Zr2(OH)4(H2O)8]4+, and the amorphous primary particles. A typical SAXS profile obtained after aging at pHc 12.2 and 40 °C is shown with blue open circles in Figure 7. Additionally, the bold black line in Figure 7 indicates the contribution of small-angle scattering from the primary particles, which is given by the sum of the third and fourth terms of eq 2 using the best-fit parameters. The small-angle scattering intensity distributions shown by the tetramer (red line) and dimer (green line) are quite different from that of the primary particles (black line). This fact indicates that [Zr4(OH)8(H2O)16]8+, [Zr2(OH)4(H2O)8]4+, and the amorphous primary particles are different species. The SAXS profile of amorphous primary particle (black line) starts to decrease at q ≥ 1 nm−1, whereas that indicated by the red line starts to decrease at q ≥ 2 nm−1. This indicates that the primary particles are larger than [Zr 4 (OH) 8 (H 2 O) 16 ] 8+ and [Zr2(OH)4(H2O)8]4+. Moreover, the power-law scattering of SAXS profiles in q-region II provides insight into the difference between these species. The exponent is close to 4 for the black line, according to Porod’s law,51−57 implying the formation of colloidal particles. On the other hand, the exponent is 2 for the red and green lines, which is typically observed in planar scatterers, such as [Zr4(OH)8(H2O)16]8+ (see Figure S3). Therefore, we speculate that primary particles were formed from aggregation of several [Zr4(OH)8(H2O)16]8+ and [Zr2(OH)4(H2O)8]4+. Considering that only Zr and O atoms in [Zr4(OH)8(H2O)16]8+ and [Zr2(OH)4(H2O)8]4+ were observed in the EXAFS spectra, the stacking may be weak, via electrostatic interactions. 4.2. Solubility of Zirconium Hydroxide Solid Phases. The solubility products of zirconium hydroxides and oxides in different solid-phase states have been measured and discussed previously.13,14,43−46,58 Focusing on the molar surface free energy of the solid phase, the particle size effect of the solubility product has been introduced to systematize the various solubility products for the different solid phases.4 Figure 8 represents the reported solubility products of
4. DISCUSSION 4.1. Structure of Zirconium Hydroxide Solid Phases. From the analysis of the EXAFS spectra of the zirconium hydroxide solid phase aged at pHc 2.7 and 25 to 60 °C, the average coordination numbers and bond distances of neighboring O were 8.0 ± 0.3 O at 2.16 ± 0.01 Å and those of Zr were 1.7 ± 0.6 Zr at 3.46 ± 0.02 Å, respectively. These values suggested that the fundamental building unit of the solid phase was a polynuclear species, which can be considered as precipitated aqueous [Zr 4 (OH) 8 (H 2 O) 16 ] 8 + and/or [Zr2(OH)4(H2O)8]4+ species20−22,44,47,48 (see Figure S3). Similar parameters were obtained for the solid phases aged at pHc 7.7 and 25 to 90 °C and for those aged at pHc 12.2 and 25 and 40 °C, also indicating that the polynuclear species was the fundamental building unit of the solid phase. The SAXS profiles of these solid phases suggested that the solid phase consists of amorphous primary particles about Rg2 8002
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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Figure 7. Comparison of experimental and theoretical scattering curves. The experimental results (open blue circles) and contribution of small-angle scattering from the primary particles (bold black line) for the solid phase aged at pHc 12.2 at 40 °C are shown. The theoretical curves (red and green curves) were calculated from the known structures of [Zr4(OH)8(H2O)16]8+ and [Zr2(OH)4(H2O)8]4+, respectively, using the Debye function for scattering and vertically shifted by an offset constant.
Figure 8. Solubility products of zirconium hydroxides and oxides at 25 °C as a function of particle size. The solubility products were determined in previous studies. The solid line represents the calculated solubility product based on the Schindler equation, assuming ZrO2(cr) solid-phase particles. Figure 6. HAADF−STEM images of zirconium hydroxide solid phases after aging at pHc 7.7 at (a) 25, (b) 40, (c) 60, (d) 90 °C, and at pHc 12.2 at (e) 25, (f) 40, (g) 60, (h) 90 °C.
where γ is the mean free surface energy per unit surface area and Ksp(A → 0) is the solubility product of large particles with a much smaller molar surface area. M is the molecular weight, ρs is the density of the solid, ri is the ionic radii of the ions, β is the geometry factor, R is the universal gas constant, T is the temperature (T = 298 K), and NA is the Avogadro number. In Figure 8, the calculated solubility product of ZrO2(cr) is plotted based on the Schindler equation with the known data M = 123.218 g/mol, ρs = 5.68 g/cm3, rZr = 0.85 Å, and rO = 1.4 Å. β was taken as 6, assuming spherical particles. The calculated curve suggested that Zr(OH)4(am) after aging at 25 °C with the solubility product of log Ksp = −56.1959 had a particle size of 3.3 nm, which is close to the size of the primary particles of 2.4 nm determined from the SAXS profile at pHc 2.7, provided that d = 2Rg2. This particle size was slightly smaller than that of the small particles observed in the TEM images. We concluded that the solubility of Zr(OH)4(am)
zirconium hydroxides and oxides as a function of the particle size. Zr crystalline oxides, ZrO2(cr), with large particles showed low solubility products compared with the value determined for Zr(OH)4(am). Based on this particle size effect, the solubility product, Ksp(A), with molar surface area A can be described as a function of particle size d using the Schindler equation4,59 1 2 ln K sp(A) = × γA + ln K sp(A → 0) (3) RT 3 A=
Mβ , ρs d
γ=−
3RT ln K sp(A → 0) 2NA ∑ 4πri 2
(4) 8003
DOI: 10.1021/acs.langmuir.9b01132 Langmuir 2019, 35, 7995−8006
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aged at 90 °C. Decreasing the slope corresponds to the steeper power-law scattering of Inor(q), α > 4. This fact indicates the finite size of the interfacial thickness in principle. As the WAXS and EXAFS spectra did not confirm any transformation of the average microscopic structure of solid phases and their particle sizes aged at from 25 to 60 °C at pHc 2.7, the change in the slope of the Porod plots by temperature suggested that the solid phase transformation is likely limited to near the surface. In the previous study, we examined the temperature dependence of Ksp for the solid phases aged at from 25 to 60 °C.14 The obtained values for enthalpy of reaction suggested the endothermic dehydration as the possible transformation of initial Zr(OH)4(am) to the solid phase aged at 60 °C. The growth of the interface with increasing the aging temperature observed in the Porod plot may be interpreted as the progress of such a dehydration reaction near the surface of the solid phase and to be responsible for the decrease of apparent Ksp, however, these surface configurations should be examined in future work.
after aging at 25 °C is governed by the amorphous primary particles. The solubility product of the solid phase after aging at 90 °C determined in the previous study9 was plotted against the size of the crystallized large particle in Figure 8. As discussed in ref 13, the solubility product after aging at 90 °C was well explained by the proposed Schindler equation based on the size of the crystallized large particle,4,59 which indicates that the solubility-limiting solid phases depended on temperature. In contrast, after aging at pHc 2.7 and 40 and 60 °C, the WAXS and EXAFS spectra did not change greatly from those after aging at 25 °C. This indicates that both the fundamental building unit and primary particles with Rg2 = 1.2 nm remained up to 60 °C at pHc 2.7, whereas the observed solubility consecutively decreased with the aging temperatures.14 The possible effect of the surface configuration on the solubility is further discussed in Section 4.3. Similar to the observation at pHc 2.7, the solubility may be governed by the size of the primary particles and of the aggregates with the crystalline structure depending on the temperature at neutral and alkaline pH. However, the Zr solubility in these pH ranges is extremely low, even at 25 °C, and it has been difficult to confirm the solubility decrease after aging at elevated temperatures.13,14 4.3. Surface Configuration of Zirconium Hydroxide Solid Phases. We discuss the reason for the decrease in Ksp from 25 to 60 °C at pHc 2.7 with same size of the primary particles. The power-law scattering of Inor(q) in region II should reflect the interfacial structure of the primary particles. Thus, we examined the scattering profiles in detail at q < 5.0
5. CONCLUSIONS Zirconium hydroxide solid phases after aging at 25, 40, 60, and 90 °C under acidic to alkaline pH conditions were investigated using WAXS, EXAFS, SAXS, and TEM to reveal the bulk and surface structures of the zirconium hydroxides on the nanoscale to sub-microscale. After aging at 25 °C, the fundamental building unit of the solid phase was a zirconium hydroxide polynuclear species of tetramer and dimer, and these species formed primary particles with 2Rg2 = 2.4 nm in size, which also formed aggregates larger than hundreds of nanometers. This hierarchical structure was stable up to 60 °C under acidic and neutral conditions and up to 40 °C under alkaline conditions. After aging at 90 °C under acidic conditions and at 60 and 90 °C under alkaline conditions, the WAXS and EXAFS spectra showed the formation of a crystalline structure. The SAXS profiles and TEM images indicated the formation of the crystallized large particles with 2Rg1 = 50 nm after aging at 90 °C under acidic conditions and at 60 and 90 °C under alkaline conditions. The transformation of the solid-phase structure by temperature was discussed in relation to the solubility product based on the proposed particle size effect. It was concluded that the Zr solubility at 25 °C was governed by the amorphous primary particles. At moderate elevated temperatures of
