Article pubs.acs.org/est
Formation of Crystalline Zn−Al Layered Double Hydroxide Precipitates on γ‑Alumina: The Role of Mineral Dissolution Wei Li,*,† Kenneth J. T. Livi,‡ Wenqian Xu,§ Matthew G. Siebecker,† Yujun Wang,†,∥ Brian L. Phillips,⊥ and Donald L. Sparks† †
Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717, United States ‡ The High-Resolution Analytical Electron Microbeam Facility, Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, United States § Department of Chemistry, Brookhaven National Lab, Upton, New York 11973, United States ∥ Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China ⊥ Department of Geosciences and Center for Environmental Molecular Science, Stony Brook University, Stony Brook, New York, 11794, United States S Supporting Information *
ABSTRACT: To better understand the sequestration of toxic metals such as nickel (Ni), zinc (Zn), and cobalt (Co) as layered double hydroxide (LDH) phases in soils, we systematically examined the presence of Al and the role of mineral dissolution during Zn sorption/precipitation on γ-Al2O3 (γ-alumina) at pH 7.5 using extended X-ray absorption fine structure spectroscopy (EXAFS), high-resolution transmission electron microscopy (HR-TEM), synchrotron-radiation powder X-ray diffraction (SR-XRD), and 27Al solid-state NMR. The EXAFS analysis indicates the formation of Zn−Al LDH precipitates at Zn concentration ≥0.4 mM, and both HR-TEM and SR-XRD reveal that these precipitates are crystalline. These precipitates yield a small shoulder at δAl‑27 = +12.5 ppm in the 27Al solid-state NMR spectra, consistent with the mixed octahedral Al/Zn chemical environment in typical Zn−Al LDHs. The NMR analysis provides direct evidence for the existence of Al in the precipitates and the migration from the dissolution of γ-alumina substrate. To further address this issue, we compared the Zn sorption mechanism on a series of Al (hydr)oxides with similar chemical composition but differing dissolubility using EXAFS and TEM. These results suggest that, under the same experimental conditions, Zn−Al LDH precipitates formed on γ-alumina and corundum but not on less soluble minerals such as bayerite, boehmite, and gibbsite, which point outs that substrate mineral surface dissolution plays an important role in the formation of Zn−Al LDH precipitates.
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INTRODUCTION Mobility and bioavailability of heavy metals in soil and aquatic environments are heavily influenced by metal−mineral interactions. It is well documented that transition metals such as nickel (Ni), zinc (Zn), and cobalt (Co) can form mixed metal−Al hydroxide surface precipitates on aluminum oxides and Al-rich soil clays.1−7 Upon aging, these precipitates become less soluble, which is an important mechanism for toxic metal sequestration in natural environments. These precipitates are identified as hydrotalcite-like surface phases, because the shortrange structural order of these precipitates is similar to hydrotalcite minerals (e.g., takovite). The hydrotalcite minerals are also named layered double hydroxides (LDHs), which have a chemical formula commonly represented as [Mez+1−xAl3+x (OH)2]q+(anionn−)q/n·yH2O, where Me2+ may be Co2+, Ni2+, or Zn2+ and the anion can be nitrate, carbonate, or silicate.8−10 Even though it is clear that Me−Al LDHs can form in certain environments, there is a missing link between the mineralogy of © 2012 American Chemical Society
LDHs and the formation mechanism of LDH surface precipitates. Such knowledge is important to improve surface sorption/precipitation models and to better predict the environmental fate of toxic transition metals. To understand the LDH formation mechanism, a crucial step is the characterization of the surface precipitates. Extended Xray absorption fine structure spectroscopy (EXAFS) has been extensively used to identify LDH phase in either labortory1−7,11−16 or natural environments.17−23 However, the information obtained from EXAFS alone is not adequate to fully characterize the LDH surface precipitates. Technically, EXAFS only provides average structural information over a short-range order (usually less than 5 Å); therefore, EXAFS Received: Revised: Accepted: Published: 11670
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and solid-state NMR characterization. Replicate samples of the wet paste at initial Zn concentrations of 0.2, 0.4, 0.8, and 3 mM were prepared for EXAFS analysis. Synchrotron Radiation Powder X-ray Diffraction. SRXRD patterns were recorded in transmission geometry with a Perkin−Elmer amorphous silicon detector at an incident X-ray energy of 38794 eV (λ = 0.3196 Å) at beamline X7B at the National Synchrotron Light Source (NSLS), Brookhaven National Lab (Upton, NY). Dry powders were placed in kapton capillary tubes. Two-dimensional XRD patterns were calibrated with lanthanum hexaboride (LaB6, NIST 660a) and integrated to XRD profiles of intensity versus 2θ with Fit2d.26 X-ray Absorption Fine Structure Spectroscopy. EXAFS data were collected on the sorption samples at beamline X11A at the NSLS. All samples were mounted in a thin plastic sample holder covered with Kapton tape and placed at 45° to the incident beam. Data were collected in both fluorescence and transmission modes using a Lytle detector positioned 90° to the beam. Zn foil was used for energy calibration. A pair of Si(111) crystals for the monochromator were employed, which were detuned by 30% to suppress high order harmonic X-rays. Data processing was performed with the EXAFS data analysis program SIXPACK.27 Spectra were averaged after careful energy calibration using Zn foil (E0 = 9659 eV). The χ(k) function was Fourier transformed using k3 weighting, and all shell-by-shell fitting was done in R-space. A single threshold energy value (ΔE0) was allowed to vary during fitting. The amplitude reduction factor, S02, was estimated based on the fitting of the spectra of a Zn solution and the Zn−Al LDH model compound and then applied to sorption samples. High-Resolution Transmission Electron Microscopy. Small quantities of powdered reactants were dispersed in deionized water and ultrasonicated for three minutes. A 200 mesh Cu grid with a lacey-carbon support film was dipped into the suspension and dried. TEM analyses were made using a Philips CM300 FEG microscope equipped with an Oxford light element energy dispersive X-ray spectroscopy (EDS) detector and a Gatan GIF 200 CCD imaging system. The point-to-point resolution of the TEM is better than 0.2 nm, and the line resolution is 0.09 nm. Images were analyzed and processed using the Gatan Digital Micrograph 1.8 software. The software package ES Vision4 was used to acquire and process the EDS spectra. Solid-State NMR. Solid-state 27Al single-pulse MAS (SP/ MAS) NMR spectra of Zn sorption samples were collected on a 400 MHz Varian Inova spectrometer (9.4 T), with Larmor frequencies of 130.2 MHz. Spectra were collected using a Varian/Chemagnetics T3-type probe with samples contained in 3.2 mm ZrO2 rotors and a spinning rate of 20 kHz. A 6 μs 90° pulse was calibrated using the 1 M Al(NO3)3 solution standard, but only 1 μs rf (i.e., radio frequency) pulse length was chosen for measuring the solid samples. The pulse delay was optimized as 5 s, and approximately 400 scans were collected for each spectrum to obtain an acceptable signal-to-noise ratio. The 27Al chemical shifts (δAl) are reported relative to an external 1 M Al(NO3)3 solution set to δAl = 0 ppm.
fails to determine if the LDH-like precipitates are crystalline or amorphous, which is important in understanding the stabilization of LDH precipitates. Crystalline LDHs have periodic interlayers between the mixed metal−Al hydroxide layers, with interlayer distances being ∼7.5−8 Å depending on the interlayer anions. The layered structure allows LDH precipitates to be further reactive toward anions. Another limitation of EXAFS is the challenge of analyzing light elements (e.g., Al), because light elements usually have weak backscattering properties and thus do not contribute significantly to the amplitudes in the background subtracted chi(k) EXAFS data. This weakness makes it difficult to distinguish between LDH species and metal hydroxides with distorted structures while using EXAFS. This issue has been addressed by Scheidegger and co-workers,4,5 who showed that adding an additional Ni−Al scattering path (besides the Ni−Ni path) in the second atomic shell would lead to better fitting. However, this only provided indirect evidence for the presence of Al; until recently, direct measurement of Al in the precipitates is still lacking. Additionally, EXAFS is mainly used for structural analysis and can hardly provide information about the chemical composition, crystallinity, and morphology, all of which is of equal importance in understanding the LDH formation. For these reasons, we employed several state-of-the-art techniques including synchrotron-radiation X-ray diffraction (SR-XRD), high-resolution transmission electron microscopy (HRTEM), and 27Al solid-state nuclear magnetic resonance (NMR) and traditional EXAFS to investigate zinc sorption/ precipitation mechanisms on γ-alumina (γ-Al2O3). The objective of this study is to thoroughly characterize the Zn− Al LDH surface precipitates and elucidate the role of Al in its formation. One important finding is the identification of crystalline LDH precipitates on γ-alumina at a low Zn concentration (∼26 ppm) using SR-XRD and HRTEM. In addition, the role of surface Al dissolution from the substrate as an important mechanism in the formation of LDH precipitates is discussed.
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EXPERIMENTAL SECTION Sorption Experiments. Aluminum oxide (aluminum oxide C, Degussa), identified by powder X-ray diffraction as the γphase Al2O3 (γ-alumina), was used as the adsorbent. The γalumina powder exhibited a Brunauer−Emmett−Teller (BET) specific surface area of 136 m2 g−1 and a pHPZC of 9.1.24 Sorption of Zn on γ-alumina was conducted at ambient conditions using a batch technique. A 0.10 g aliquot of dry γalumina powder was added to 40 mL of 0.01 M NaNO3 background electrolyte at pH 7.5. The pH was maintained by addition of a HEPES buffer (pKa = 7.5 at 298 K) at a final concentration of 10 mM. Previous studies showed that HEPES does not interfere with transition metal sorption to mineral surfaces.6,25 Small amounts of 50 mM Zn(NO3)2 solution were added into each tube to reach the desired initial Zn concentration. A speciation diagram of aqueous Zn is provided in the Supporting Information (Figure S1), suggesting at the experimental pH that the dominant dissolved Zn species is Zn2+ (aq). A reaction time of 24 h was chosen based on the sorption kinetics (Supporting Information, Figure S2). After the reaction, the samples were centrifuged to separate the solid and solution. The supernatant was filtered with a 0.2 μm filter and then analyzed for Zn by inductively coupled plasma− atomic emission spectroscopy (ICP-AES), while the solid samples were then freeze-dried for further HRTEM, SR-XRD,
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RESULTS Sorption Isotherm. A sorption isotherm was carried out over a Zn concentration range of 0.1−3 mM (Figure 1). Over this range, the surface coverage increased from 0.44 to 11.3 μmol m−2. The isotherm was fitted well using a Langmuir model (R2 = 0.97) and yielded a maximum sorption capacity of 11671
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a Zn−Zn scattering path. The fitting results suggest the Zn−Zn distance is similar to the Zn−Al distance at ∼3.10 Å, which is in good agreement with the fitting results of the Zn−Al LDH model compound. The XANES (Supporting Information, Figure S3) and EXAFS (Figure 2) spectra of the 0.2 mM Zn sorption sample is different from those of the Zn−Al LDH model compound and aqueous Zn solution. Thus, formation of either surface precipitates or outer-sphere surface complexes was excluded. The 0.2 mM Zn EXAFS spectrum (Figure 2) can best be fitted with a first Zn−O shell with coordination number (CN) of 4 at a distance of 1.97 Å and a second Zn−Al shell (CN = 1) at 2.99bÅ, which corresponds to an inner-sphere bidentate mononuclear complex.7 Our EXAFS results are in good agreement with the previous research of Trainor et al.7 that Zn−Al LDH precipitates formed at high-sorption-density samples (≥1.5 μmol m−2) and inner-sphere mononuclear sorption complexes formed at low-sorption-density samples (0.7 nm basal reflection is very close to the central beam and can easily overwhelm faint reflections. In the 3 mM Zn/γ-alumina reacted sample (Figure 5), a higher concentration of the dark and poorly crystalline precipitates were observed (Figure 5B). Again, the SAED profiles contained no extra peaks other than those that could be identified for γ-alumina. However, the presence of an LDH phase is clearly indicated by the SR-XRD data, and the EDS data show the LDH phase to be Zn rich. Analysis II indicates that the poorly crystalline material unambiguously contains Al. The presence of both crystalline and poorly crystalline Znrich phases is in contrast with the observation that only amorphous Ni−Al LDH phases were found on pyrophyllite.31,32 The low concentrations of Zn on γ-alumina crystals neither preclude sorption of Zn onto their surfaces nor does it eliminate the possibility that Zn surface precipitates have
reflection and slightly shift to a lower diffraction angle as the Zn concentration increased from 0.4 to 3 mM. This change is consistent with the increasing coordination number of the Zn− Zn/Al shell in the EXAFS spectra (Supporting Information, Table S1). For the 3 mM Zn reacted sample, we found that the peaks indicative of LDH (Supporting Information, Figure S4) are as broad as those for bulk γ-alumina, which suggests the precipitates are crystalline and of nanometer size. High-Resolution Transmission Electron Microscopy. Since SR-XRD suggests the presence of some type of crystalline precipitate, TEM analysis was undertaken to examine the morphology, elemental distribution, and crystallinity of selected samples. Figure S5A, B of the Supporting Information shows a low magnification and an HRTEM image of pristine γ-alumina, which demonstrates that the γ-alumina is crystalline with a particle size around 10−20 nm. Figure S6 of the Supporting Information contains a low magnification TEM image of the 0.2 mM Zn sample and a selected area electron diffraction (SAED) pattern with a rotationally averaged intensity profile. The SAED profile compares well with the SR-XRD for pristine γ-alumina. SAED, EDS, and TEM data all indicate that very little Zn is present and that no separate precipitates have formed. After reaction with 0.8 mM Zn (Figure 4A), a separate apparently needle-shaped precipitate (150 nm × 25 nm) was observed with a particle size larger than pristine γ-alumina. Upon tilting the sample by 40°, the precipitate was identified to have a flake-shaped morphology. The flake morphology is consistent with the layered structure of LDH materials. EDS analysis of I (Figure 4B, C) showed the precipitate to be rich in Zn. The presence of Al cannot be determined due to the overlap of γ-alumina. Neighboring γ-alumina did not contain significant Zn (analysis II). In addition to the dark needle/flake precipitate, there were regions of Zn-rich poorly crystalline material (region and analysis III in Figure 4A, C). The poorly 11674
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formed. However, if they have formed, they must be in low concentrations relative to the separate precipitates. What is known is that separate crystals of Zn-rich LDH along with a more poorly crystallized or amorphous, and possible precursor, phase have formed. Solid-State NMR. The observation of separate crystalline LDH precipitates with large particle size implies a large amount of Al mass transfer from γ-alumina to the LDH precipitates, which could be an important mechanism for LDH formation. Because NMR spectroscopy is quantitatively sensitive to the changes of Al coordination environment (i.e., 4-, 5- and 6coordinated Al), 27Al solid-state NMR analysis was carried out on several samples to examine the changes in the Al environment. The crystal structure of γ-alumina contains ∼30% tetrahedral and 70% octahedral Al,33,34 which is consistent with the intensities of NMR peaks occurring at δAl = +8 ppm and δAl = +68 ppm, respectively (Figure 6A). If dissolution occurs, substantial tetrahedral Al in the bulk structure will dissolve in the solution and convert to octahedral Al in either the solution (e.g., aqueous Al(H2O)63+) or solid state (e.g., Al(OH)3). Also, formation of large amounts of Zn− Al LDH would be reflected in the NMR spectrum, as the chemical shifts for the Al in Zn−Al LDH and the octahedral Al in γ-alumina differ by about 4−5 ppm. As compared to the bulk γ-alumina, the 27Al solid-state NMR spectrum (Figure 6C) for the 3 mM Zn reacted sample showed a small shoulder at δAl = +12.5 ppm. The NMR signal at +12.5 ppm indicates a mixed Zn and Al octahedral structure that is consistent with a Zn−Al LDH model compound (Figure 6C), whereas the signal at δAl‑27 = +8 ppm is attributed to a spinel-like octahedral Al environment of γ-alumina. To further confirm the assignment of the small shoulder to Zn−Al LDH precipitates, a sorption sample was prepared at a higher Zn concentration of 10 mM and a higher temperature of 333 K, enhancing precipitation of LDH phases. As predicted, a much more significant NMR peak at ∼δAl = +12.5 ppm was observed. In addition, a signal intensity reduction of the peak at δAl = +68 ppm was observed (Figure 6B), indicating the transformation of some tetrahedral Al from the bulk γ-alumina to octahedral Al in the Zn−Al LDH precipitates. However, the differences are very small, and more information cannot be deduced because the precipitates only amount to a small fraction of the whole sample with respect to the majority substrate, γ-alumina. In considering the 3 mM Zn reacted sample (0.1 g), for example, roughly 125 μmol of Zn was sequestrated in this sample. By assuming that all of the Zn is in the form of the Zn−Al LDH and a Zn/Al ratio of 2, the precipitates contain 62.5 μmol of Al (1.688 mg), that is, 3.2% of the total Al (53 mg) of the whole sample. Since the fraction of tetrahedral Al in γ-alumina is ∼30%, only a 1% difference would be predicted in the tetrahedral Al before and after reaction, which agrees with our 27Al NMR analysis. Zn Sorption Mechanism on Different Al (Hydr)oxides. To examine the role of mineral surface dissolution on Zn sorption/precipitation, a series of Al oxides and hydroxides were selected as adsorbents. For comparison purposes, sorption samples were prepared under the same experimental conditions, that is, a Zn concentration of 0.8 mM buffered at pH 7.5 using 10 mM HEPES and a reaction time of 15 min. This short reaction time was chosen because our preliminary results suggested that formation of Zn−Al LDH precipitates on γ-alumina was rapid, within 15 min (Supporting Information, Figure S8). Figure 7 shows the background subtracted k3-
Figure 7. EXAFS analysis of Zn adsorption/precipitation on different Al (hydr)oxides. (A) k3 weighted χ functions; (B) the corresponding Fourier transform (phase shift not corrected). Experimental and fitted data are presented as black solid lines and red dashed lines, respectively.
weighted χ functions of the sorption samples and the corresponding Fourier transforms. Precipitates on γ-alumina and corundum (α-Al2O3) were identified, as suggested by the two features shown at about 7.2 and 9.5 Å−1 in the χ functions and the appearance of a significant second shell in the radial distribution function (Figure 7). Further shell-to-shell fitting confirms that the precipitates were Zn−Al LDH phases (Supporting Information, Table S2). In contrast, the EXAFS spectra for sorption samples of boehmite (γ-AlOOH), bayerite (β-Al(OH)3), and gibbsite (α-Al(OH)3) are similar to that of 0.2 mM Zn adsorbed γ-alumina, and the FT transforms do not show a significant second shell. Generally, these EXAFS spectra can be best fit with a first Zn−O shell with a CN of ∼4 at a distance of around 1.97 Å and a second Zn−Al shell (CN = 1− 1.5) at 3.01 Å, suggesting a predominant inner-sphere bidentate mononuclear surface complexes.7
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DISCUSSION Formation of Crystalline Precipitates. The combined SR-XRD and HRTEM analyses have identified the formation of crystalline Zn−Al LDH precipitates as the dominant Zn sorption mechanism on γ-alumina. This suggests a dissolution− reprecipitation process that happens at a high enough Zn concentration in the solution. The observation of crystalline LDH precipitates strongly supports all the previous EXAFS investigations and adds important complementary information. These observations are consistent with previous research by Paulhiac and Clause,1 who identified well-crystallized Zn−Al LDH precipitates by XRD when reacting γ-alumina with a high concentration Zn solution (10 mM) at neutral pH. It is worth noting that much lower Zn concentrations (0.4−3 mM) were used in this study. Besides the crystallinity, the TEM analysis clearly revealed that the precipitates were separate from bulk γalumina. This supports an early study by d’Espinose de la Caillerie et al.,2 who used a dialysis bag to isolate γ-alumina during the Ni (10−100 mM) impregnation experiment (pH 9 and 333 K) and identified crystallized Ni−Al LDH precipitates outside of the bag. In this study, our TEM images show large pieces of crystallites with particle size much larger than γalumina substrate. The large particle size of the LDH precipitates in this study is comparable to bulk LDH materials 11675
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synthesized by coprecipitation method.28,29 Furthermore, we employed FTIR to analyze the anions type in these samples (Supporting Information, Figure S9), where IR peaks at 1355 cm−1 were observed that showed increasing intensities as the Zn concentration increases. This IR band is assigned to the carbonate group,2,35 which suggests the anions in the interlayers of Zn−Al LDH are CO32−. The carbonate is orginated from the atmospherical CO2 during the reaction, since carbonate can compete nitrate to form more stable carbonate−LDH. The observation of crystalline Zn−Al LDH precipitates differs from previous TEM studies that amorphous Nicontaining precipitates were observed and associated/deposited on the pyrophyllite surface.31,32 This is probably due to the dissolved Si from pyrophyllite that prevents the crystallization of Ni−Al LDH. It has been reported that coprecipitation of Zn and Al in the presence of silicate would lead to a very poorly crystallized hydrotalcite-like phase.36 The present study was conducted with Si-free adsorbent (i.e., γ-alumina); therefore, crystalline LDH precipitates can be identified. Another interesting observation is that the HRTEM images also showed the presence of poorly crystalline precipitates that could not be detected by diffraction methods (Figure 5). Since the samples were properly washed and sonicated prior to TEM analysis, it is unlikely that the poorly crystalline Zn phase precipitated during drying of the TEM sample. It is unclear if this precipitate is an LDH-like (or a-LDH) phase and why it formed. During the synthesis of Mg−Al LDH, Yang et al.35 observed amorphous colloidal aluminum hydroxide and suggested this phase as a precursor for the LDH formation. In our study, the poorly crystalline precipitates might be a precursor of a crystalline phase as well. It is not possible with EXAFS to determine whether the amorphous phase is LDHlike, since it is in the presence of a crystalline Zn-LDH phase identified by SR-XRD. Further understanding of the role of this phase was hampered by the current technical limitations. Role of Al in Precipitation. The role of Al was shown to be important in the formation of secondary LDH precipitates. Scheinost et al.37 indicated Ni−Al LDH seems to be thermodynamically favored when Al is available. Using DRS, they observed the formation of α-Ni(OH)2 on Al-poor minerals (e.g., talc and amorphous silica) whereas Ni−Al LDH precipitates formed on Al-rich adsorbents (e.g., pyrophyllite and gibbsite). Gao et al.30 observed single crystal Zn−Al hydrotalcite while reacting Zn solution with an Al-bearing glass substrate. They also found that the precipitates grew on the Al surface but not on the glass surface. Both Scheidegger et al.3 and Towle et al.4 suggested that adding an additional Me−Al scattering path (Me could be Ni or Co) would lead to a better fitting of EXAFS data, suggesting the presence of Al in the LDH phase. All of these studies suggested Al was an essential component in the precipitates. However, direct characterization of the presence of Al in the previous studies was hampered due to experimental limitations. In our study, SR-XRD and NMR analyses provided more direct evidence. X-ray diffraction patterns reflect the long-range structural order of the entire crystal lattice whereas EXAFS only provides average local structural information with a short-range order at most of 5 Å. The 27Al solid-state NMR analysis provided additional direct evidence for the presence of octahedral Al in the LDH precipitates and some evidence for the dissolution of bulk γalumina. Since γ-alumina is the only source of Al in our research system, migration of Al from γ-alumina to the precipitates is
critical for the formation of Zn−Al LDH phase. Dissolution of the mineral surface has been hypothesized to be the driving force for metal−Al LDH formation, but experimental evidence is sparse. In this research, we show a clear link between mineral surface dissolution and surface precipitation. Thus, we compared Zn sorption/precipitation on a series of aluminum (hydro)xides (Figure 7). These minerals have similar chemical composition but have different surface dissolution constants (Supporting Information, Table S3). Their well-known surface chemistry shows the following dissolution trend:7 γ-alumina (γAl2O3) > corundum (γ-Al2O3) > bayerite (β-Al(OH)3) > boehmite (γ-AlOOH) > gibbsite (α-Al(OH)3). Under the same experimental conditions (i.e., Zn concentration of 0.8 mM, solid/solution ratio of 2.5 g/L, and reaction time of 15 min), Zn−Al LDH surface precipitates were detected on γ-alumina and corundum using EXAFS. In contrast, on bayerite, boehmite, and gibbsite, no precipitates were observed but mainly adsorption complexes were identified. Additional timedependent EXAFS studies were performed on γ-alumina and boehmite. As shown in Figure S8 and Table S4 of the Supporting Information, the results demonstrate that Zn−Al LDH precipitates formed on γ-alumina after reaction of 15 min and remained up to 7 days. In contrast, for Zn sorption on boehmite, only Zn inner-sphere mononuclear sorption complexes formed during the initial 15 min and later converted to Zn−Al LDH precipitates after 24 h. These results clearly indicate that formation of precipitates is highly related to mineral surface dissolution. A similar relationship has been suggested for Ni precipitation on pyrophyllite, montmorilollite, and gibbsite.12 Ni−Al LDH precipitates formed only 15 min after reaction with pyrophyllite but required 2 days on montmorillonite. Given that corundum has the lowest specific surface area and the lowest surface loading, we conclude that a crucial step for surface precipitation is mineral dissolution. However, more detailed investigations will be needed to establish a quantitative relationship between the kinetics of surface precipitation and mineral surface reactivity.
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ASSOCIATED CONTENT
S Supporting Information *
Zn speciation diagram, Zn sorption kinetics, XANES spectra of Zn reacted γ-alumina, SR-XRD patterns for precipitates, additional TEM images, EXAFS of Zn sorption samples, FTIR spectra, and EXAFS fitting parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (302) 831-3219; fax: (302) 831-6505; e-mail: weili@ udel.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We sincerely appreciate the helpful comments from three anonymous reviewers and from the editor, Dr. Scherer. This research was funded by the National Science Foundation (NSF) through the Delaware EPSCoR program (grant no. EPS0814251). We thank Cathy Olsen at the University of Delaware for assistance with the ICP-OES analyses and Dr. Kaumudi Pandya for help with XAS data collection at beamline X11A of the NSLS. Y.J.W. is grateful to the Shanghai 11676
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Environmental Science & Technology
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Synchrotron Radiation Facility (SSRF) for use of the synchrotron radiation facilities for analyzing the model compounds. Drs. Mengqiang Zhu (LBNL) and Paul Northrup (BNL) are acknowledged for assistance with EXAFS data analysis. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-98CH10886.
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dx.doi.org/10.1021/es3018094 | Environ. Sci. Technol. 2012, 46, 11670−11677
