Dissolution and Transformation of ZnO Nano- and Microparticles in

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the Dissolution and Transformation of ZnO nanoand micro-particles in soil mineral suspensions Ping Wu, Peixin Cui, Huan Du, Marcelo Alves, Cun Liu, Dong-Mei Zhou, and Yujun Wang ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00165 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Earth and Space Chemistry

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The Dissolution and Transformation of ZnO nano- and micro-particles in Soil Mineral Suspensions (sp-2018-00165v.R1) Ping Wu 1,2,#, Peixin Cui 1,#, Huan Du 1, Marcelo E. Alves 3, Cun Liu 1, Dongmei Zhou 1, Yujun Wang 1,* 1

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil

Science, Chinese Academy of Sciences, Nanjing 210008, P.R. China; 2

University of Chinese Academy of Sciences, Beijing 100049, P.R. China;

3

Departamento de Ciencias Exatas, Escola Superior de Agricultura "Luiz de Queiroz"

13418-900 Piracicaba SP Brasil;

#

These authors contributed equally to this work.

* Corresponding author. Tel.: 86-25-86881182; fax: 86-25-86881000 E-mail address: [email protected] (Yujun Wang).

ACS Paragon Plus Environment

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2 Abstract The increasing use of ZnO nanoparticles (NPs) has generated serious concern about their fate, transportation, and toxicity in the environment. The present study focuses on the key geochemical processes controlling the environmental fate of NPs, that is, dissolution and transformation of ZnO NPs in a large time scale with two common soil minerals (γ-Al2O3 and goethite) suspensions in batch reactors for up to 360 days using synchrotron-radiation powder X-ray diffraction (SR-XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), and high-resolution transmission electron microscopy (HR-TEM). The aqueous dissolution experiments showed that smaller size ZnO NPs dissolved faster at lower pH. The combined spectroscopic analyses revealed that the interaction between ZnO NPs and γ-Al2O3 promoted dissolution of ZnO NPs by transforming into the Zn-Al layered double hydroxide (LDH) precipitate at pH 7.5 within 0.5 h, and the fraction of Zn-Al LDH precipitate increased with the incubation time. At pH 5.5, Zn-Al LDH precipitate was also observed, which was attributed to the locally elevated pH and Zn concentration near ZnO NPs surface during the dissolution. In the presence of goethite, ZnO NPs dissolved less, and Zn mainly existed as ZnCO3 at pH 5.5, but at pH 7.5, ZnO NPs barely dissolved and transformed even with prolonged incubation time. The findings of this study will facilitate a better understanding of the fate of ZnO NPs in soil mineral suspensions, which can be leveraged for remediation of ZnO NPs-polluted soils. Keywords: ZnO; nanoparticles; soil minerals; Zn-Al LDH; EXAFS


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4 1. Introduction Nanoparticles (NPs) currently receive much attention as they have been developed and applied extensively in many fields, including electronic, catalysis, and sensor applications.1-3 Manufactured NPs (MNPs) in particular are produced at a high volume, and they are widely used in personal care and industrial products, inevitably entering the environment.3-6 The increasing amount of researches have illustrated the biogeochemical transformations of NPs and the effects of their nanotoxicity on human and ecosystem health 7-11. However, most of those researches were using short-term experiments, but long-term studies are necessary for the better understanding of NPs’ stability, mobility, and reactivity in large time scales. ZnO NPs are one of the most MNPs,12 and they can serve as representative model NPs for the important class of metal oxide NPs.13 The global production of ZnO NPs reacted 31500-34000 t per year, and the growing quantities of ZnO NPs are expected to find their way into the ecological systems, through wastewater treatment, landfill deposition, or other processes 5. Increasing investigations have highlighted potential toxicities of ZnO NPs themselves to bacteria, plants, nematodes, and earthworms.8, 14-16

Additionally, dissolved Zn2+ released from ZnO NPs may generate adverse

biological effects in organisms.9, 17, 18 It is becoming clear that the dissolution and transformation of ZnO NPs contribute significantly to their toxicities and potential risks on human health and the environment. Dissolution rates mostly depend on the properties of ZnO NPs, especially the primary particle size.13, 19, 20 Smaller ZnO NPs are found to be more active and show


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5 greater solubility due to their larger specific surface area and higher surface energy, which accelerate the ions to break away from the lattice structure 13, 19, 20. More importantly, the dissolution and transformation of ZnO NPs are strongly governed by the environmental conditions, including pH, inorganic and organic ligands 19, 21-25. Under acidic or alkaline conditions, protons or hydroxide ions are expected to react with ZnO particles, thus promoting the dissolution of ZnO particles.21, 22 The presence of inorganic phosphate and organic phosphate enhances the dissolution of ZnO NPs and transformed ZnO NPs into Zn3(PO4)2 25 and Zn-organic phosphate precipitates.23 ZnO particles were also reported to react rapidly with EDTA and dissolve completely after 3h upon addition of 1 mM EDTA. 21Organic matter has also been found to be effective in promoting the dissolution of ZnO NPs.24 Soil minerals are ubiquitous in soil, and play a crucial role in controlling the fate of NPs released to soils.17, 26 It was evidenced that ZnO NPs cannot be detected after incubation into soil for 1 h and it was primarily caused by sorption and precipitation on soil minerals.17 ZnO NPs rapidly dissolve within 1 day on kaolinite due to their sorption to the negative charge sites of the kaolinite and the formation of Zn2+ inner-sphere complexes.26 Fe/Al-bearing minerals especially, Fe/Al (oxyhydr)oxides are relatively abundant in soil and constitute one of the most important geosorbents for many transition metals, including Zn.27-29 Previous studies have demonstrated that Zn could form inner-sphere and polynuclear complexes on the surface of ferrihydrite.27, 29 It has been well documented that dissolved Zn2+ can form mixed Zn−Al layered double hydroxide (LDH) precipitates

on the surface of aluminum



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6 oxides, which are more stable than single Zn hydroxide .30-32 Hence, the interactions between ZnO NPs and soil minerals may contribute to decreasing the potential long-term toxicity and risk of ZnO NPs. However, the detailed mechanisms of the effects of soil minerals on the dissolution and transformation of ZnO NPs under different conditions, especially over long time scales have not been addressed so far. Thus, this study focuses on the transformation of ZnO NPs and microparticles (MPs) on the long time scale up to 360 days as a function of solution pH and type of Fe/Al (oxyhydro)oxide minerals present in the suspension. The speciation and phase transformation of ZnO NPs and MPs were characterized by a combination of synchrotron-radiation X-ray diffraction (SR-XRD), synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (HR-TEM) fitted with a Bruker X-ray energy dispersive spectrometry (EDS). The results will help characterize the long-term interactions between Fe/Al (oxyhydro)oxide minerals and ZnO NPs which will contribute for better understanding and assessment of the fate and transformation of ZnO NPs in the environment.

2. Materials and Methods 2.1. Materials and Reagent The ZnO MPs and NPs were obtained from XFNANO Ltd. (China) with particle sizes of 3 µm and 30 nm, respectively. γ-Al2O3 was purchased from Alfa Aesar Chemical Co., Ltd. with an average particle size of 20 nm and a surface area of 164


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7 m2/g. The point of zero charge (PZC) measured by the zeta potentiometer (Malvern) was 8.6. NaNO3, MOPS, MES, HNO3, and NaOH were obtained from Sinopharm Chemical Reagent Co., Ltd; goethite (98.5% purity) was purchased from Sigma-Aldrich (USA). All reagents were used without further purification.

2.2. Time-dependent Dissolution of ZnO NPs and MPs in Water First, 810 milligrams of ZnO NPs or MPs were added into 500 mL polytetrafluoretyhylene bottles containing 400 mL of 10 mM NaNO3 background electrolytes at pH 5.5 or 7.5. The pH was maintained by adding 62.5 mM MES or MOPS buffer adjusted by HNO3 or NaOH. This experiment was performed in duplicate. Samples were collected periodically for characterization. The detailed sampling process is provided in Text S1 of the supporting information.

2.3. Time-dependent Dissolution of ZnO NPs and MPs in Different Mineral Suspensions This experiment was conducted in a similar manner to the experiment discussed in Section 2.2. 810 milligrams of ZnO NPs or MPs with 8.0 g γ-Al2O3 or 8.0 g goethite were added into 500 mL polytetrafluoretyhylene bottles containing 400 mL of 10 mM NaNO3 background electrolytes at pH 5.5 or 7.5. Other experimental conditions were kept identical to those presented in Section 2.2. The wet solids isolated from the suspensions were washed twice by Milli-Q water and freeze-dried for SR-XRD, EXAFS, and HR-TEM measurements.



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2.4. Synchrotron Radiation Power X-ray Diffraction The milled mixtures were structurally characterized by SR-XRD. The data were collected from the references and the samples using a CCD detector at beamline BL15U1 in Shanghai synchrotron radiation facility (SSRF), Shanghai, China. Dry powder was placed in kapton capillary tubes. Two-dimensional XRD patterns were calibrated using CeO2 as the standard material. The resulting 2D images were integrated to 1D diffractograms using the Fit2D program.33

2.5. Extended X-ray Absorption Fine Structure Spectroscopy EXAFS data were carried out on solid samples collected from dissolution experiment at beamline BL14W1 in SSRF. All dry samples were mounted on a thin plastic sample holder covered with Kapton tape and positioned at 45° to the incident beam. Data were collected in a fluorescence mode with a Lytle detector, which was positioned at 90° to the beam. A Si (111) double crystal was employed as the monochromator, and the energy was calibrated by a Zn foil (K-edge at 9659 eV). The raw EXAFS data were processed using the software packages Demeter.34 Linear Combination Fitting (LCF) analysis was used for the identification and quantification of Zn species. The κ3-weighted χ(k) function was Fourier transformed, and all shell-by-shell fittings were done in R-space. Details of data processing are shown in Text S2 of the supporting information.


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9 2.6. High-resolution Transmission Electron Microscopy HRTEM images and energy-dispersive spectrometry (EDS) spectra were obtained through HRTEM (Tecnai 20U-Twin, Philips, Netherlands) at an accelerating voltage of 200 kV. Small quantities of powered samples were spread with ethanol for three minutes and put onto carbon-coated copper grids.

3. Results and Discussion 3.1. Dissolution of ZnO NPs and MPs in Water: The Effects of size and pH The time-dependent dissolution and transformation of ZnO NPs and MPs in an aqueous solution are important processes controlling their stability and bioavailability. The kinetic measurements of the released Zn2+ concentration showed greater extent of dissolution for both ZnO NPs and MPs at pH 5.5, with the Zn2+ concentration reaching 1240 mg/L (76.3% dissolution of total ZnO) and 420 mg/L (25.8% dissolution of total ZnO) for ZnO NPs and MPs after 360 days, respectively (Figure 1). At pH 7.5, the dissolution of ZnO NPs and MPs was insignificant, with dissolved Zn concentrations of 16 mg/L (0.98% dissolution of total ZnO) and 1.7 mg/L (0.10% dissolution of total ZnO) for ZnO NPs and MPs after 360 days, respectively

(Figure

1). These results suggested that the dissolution of ZnO particles was pH- and size-dependent. In this study, the ZnO NPs and MPs were more readily to dissolve at pH 5.5, + + forming solvated ionic species of Zn2(aq) and Zn(OH)(aq) , which is consistent with

previous studies that ZnO particles were expected to be dissolved quickly under acidic



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10 and alkaline conditions.19, 21, 35 Under acidic conditions (pH < 6), the ZnO surface was more likely to be directly attacked by protons, resulting in a greater propensity for dissolution.19, 21 When pH was greater than 10, ZnO particles could also dissolve by forming soluble hydroxyl complexes, such as Zn(OH)2(aq), Zn(OH)3-(aq), and Zn(OH)42-(aq). Between pH 6 and pH 10, the solid Zn(OH)2 precipitate was formed, lowering the solubility of ZnO particles.19, 21 Thus, under the weak alkaline condition (pH = 7.5), the dissolution of ZnO particles was limited with the relativelylow concentrations of proton and hydroxyl ions in the solution. The measurements of released Zn2+ concentrations also showed the greater extent of dissolution for ZnO NPs than ZnO MPs (Figure 1), which was mostly due to the larger specific surface area of ZnO NPs.1, 20 Additionally, ZnO NPs had a much larger fraction of atoms at the edges and corners than ZnO MPs, which made it easier for surface ions and small clusters to break away from the lattice structure, resulting in enhanced solubility.19

3.2. Dissolution and Transformation of ZnO NPs and MPs in Different Mineral Suspensions Understanding the effects of different soil minerals on the dissolution and transformation of ZnO NPs and MPs is important for revealing the fate of ZnO particles in soil. The dissolved Zn2+ concentrations of ZnO NPs and MPs in γ-Al2O3 and goethite suspensions as a function of time are depicted in Figure 2. The measured Zn2+ concentration decreased over time after addition of γ-Al2O3 and goethite – irrespective of the pH being 5.5 or 7.5 (Figure 2). This may be resulted from the


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11 sorption of dissolved Zn2+ on γ-Al2O3 and goethite. Substantial studies have shown a strong uptake of Zn2+ on γ-Al2O3 and goethite via adsorption and(or) precipitation. 28,30, 32, 36

In addition, the released Zn2+ concentration in goethite suspensions was

higher than that in γ-Al2O3 suspensions. It could be preliminarily

speculated that the

sorption capacity of γ-Al2O3 for Zn2+ was higher than that of goethite, which may be attributed to the formation of Zn-Al LDH precipitate on the surface of γ-Al2O3.30, 32

3.3. SR-XRD Analysis SR-XRDs were used to characterize the changes in crystalline structure of ZnO particles in the reaction systems. Previous studies had found that Zn could form (co)precipitates on some minerals at high Zn sorption densities.30, 32, 37 Thus, the samples of ZnO NPs with higher dissolution rates collected from the dissolution kinetic experiments were subject to SR-XRD analysis. The SR-XRD patterns of the ZnO NPs in γ-Al2O3 suspensions showed that the characteristic diffraction peaks of ZnO disappeared at pH 5.5 and pH 7.5 after 1 h (Figure 3A, B), which suggested that the crystalline structures of ZnO NPs were completely dissolved in γ-Al2O3 suspensions within 1 h. An additional diffraction peak at about 3.59°(2θ) was observed on samples in the presence of γ-Al2O3 within 1 day at pH 7.5 (Figure 3B). The d-spacing of the peak was 0.732 nm, which can be indexed as the (006) reflection of the typical Zn−Al LDH.38 More additional peaks (2θ at 3.59°, 7.15°, and 10.69°) were evolved on samples in the presence of γ-Al2O3 at pH 7.5, and the peak intensities increased with reaction time prolonged (Figure 3B), which were in line



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12 with the characteristic peaks of Zn-Al LDH. Similar findings were also observed at pH 5.5, but the reflection peaks shifted slightly to the higher diffraction angles (Figure 3A). These changes were attributed to lower coordination numbers of the Zn−Zn/Al shell (discussed in the following EXAFS section). The results confirmed that ZnO NPs dissolved rapidly and were transformed into Zn-Al LDH precipitate on the surface of γ-Al2O3, and then the fraction of Zn-Al LDH precipitate increased with the reaction time prolonged. In goethite suspension, the characteristic reflection peak of ZnO disappeared and no new reflection peaks appeared at pH 5.5 after 1 day (Figure 3C). In contrast, the characteristic reflection peak of ZnO at about 14.07° (2θ) remained at pH 7.5 after 1 day, and the peak intensity decreased insignificantly over time (Figure 3D), suggesting that ZnO NPs dissolved faster at pH 5.5 than at pH 7.5.

3.4. Extended X-ray Absorption Fine Structure Spectroscopy The k3-weighted χ(k)-spectra of Zn samples that reacted with γ-Al2O3 are presented in Figure 4. The spectral signature of ZnO associated with γ-Al2O3 was invisible after 0.5 h at pH 7.5 and 5.5 in the EXAFS spectra (Figure S1). These observations suggested a considerably fast dissolution of ZnO NPs in γ-Al2O3 suspensions. A remarkable characteristic of Zn-Al LDH precipitate (the distinctive double pattern between 6.0 and 8.0 Å-1) was observed inγ-Al2O3 suspensions at pH 7.5 after 1 h, and the double pattern brought out gradually with interaction time increasing (Figure 4B). This suggested that ZnO NPs quickly dissolved and transformed into Zn-Al LDH precipitate on the surface of γ-Al2O3. The Zn-Al LDH


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13 precipitate was also detected at pH 5.5, but the double pattern was less conspicuous compared with the amplitude at pH 7.5 (Figure 4A). The spectral trends suggested that the Zn-Al LDH precipitate formation proceeded more favorably at pH 7.5 than 5.5. To quantify the content of residual ZnO in γ-Al2O3 suspensions and to obtain information on the Zn species formed upon ZnO NPs’ dissolution, the EXAFS spectra were analyzed by linear combination fitting (LCF). The fitting results indicated that ZnO NPs were completely dissolved within 0.5 h at both pH 7.5 and pH 5.5 (Table 1). At pH 7.5, Zn-Al LDH precipitate was the dominant species and became more dominant over time (the fraction increased from 27.1% to 67.6%) (Table 1). Therefore, the dissolved Zn2+ from ZnO NPs could be potentially sequestered on the surface of γ-Al2O3 by the formation of relatively stable Zn-Al LDH precipitate. The results were slightly different from the results of SR-XRD that Zn-Al LDH was formed after 1 day at pH 7.5. The reason for this was probably that either the amount of the Zn-Al LDH precipitate was too low or the initial precipitate was in an amorphous state at the time of first contact, which was difficult to be detected by SR-XRD. Zn-Al LDH precipitate was also observed at pH 5.5, which was inconsistent with previous study that Zn formed inner-sphere complexes on the surface of γ-Al2O3 at pH 5.5.36 That may be caused by the locally elevated pH and Zn concentration near the dissolving surface of ZnO NPs.31 The fraction of Zn-Al LDH at pH 7.5 was higher than that at pH 5.5, and this was attributed to the higher pH, which favored the formation of Zn-Al LDH.39



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14 A considerable fraction of ZnO (22%) was still present after 2 days at pH 5.5 in goethite suspensions (Table S5), while they were completely dissolved within 0.5 h in γ-Al2O3 suspensions as discussed above. The results indicated that γ-Al2O3 promoted faster dissolution of ZnO NPs compared to goethite, which may be caused by the formation of Zn-Al LDH precipitates on the surface of γ-Al2O3. ZnO NPs were not detected after reaction time of 360 days, implying that ZnO NPs were completely dissolved after 360 days (Table S5). Comparing with previous studies regarding the tightly sorbed Zn2+ on Fe minerals through inner-sphere complexation 40, the released Zn from ZnO NPs was mainly present as ZnCO3, and only a small fraction of Zn was sorbed on goethite (Table S5). A possible reason for this was that ZnCO3 was a precipitation form, which was more stable than inner-sphere Zn complexes on goethite surface. At pH 7.5, ZnO NPs were still unequivocally detected in goethite suspensions (80%) after 2 days, and only a small fraction of ZnO NPs was subsequently dissolved as the reaction time proceeded to 360 days (reduced to 64%) (Table S5). The results further confirmed the fact that ZnO NPs dissolved faster under acidic conditions than weakly alkaline conditions. Shell by shell fitting analysis was also conducted in this study. Zn-C bond was observed in ZnO NPs reacted with γ-Al2O3 samples at pH 5.5 (Table S6), which implied the presence of ZnCO3. Whereas, Zn-C bond was not observed after 1 day at pH 7.5 (Table S7). The results suggested that CO32- played a more effective role in Zn complexation at a relatively low pH. No obvious changes were observed in the first shell Zn-O coordination number at pH 5.5 (Table S6), while the coordination number


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15 of Zn-O increased with reaction time increasing at pH 7.5 (Table S7). The results indicated that Zn-O coordination transformed to high coordination. Previous studies indicated that inner-sphere sorption Zn complexes and Zn(OH)2 were in four-fold coordination with the first neighboring oxygen while Zn-Al LDH and ZnCO3 were in six-fold coordination with oxygen.32, 41 Considering that ZnCO3 disappeared at pH 7.5 (no Zn-C bond after 1 day), we could conclude that the inner-sphere sorption Zn complexes and Zn(OH)2 were transformed to a more stable precipitate of Zn-Al LDH over time. Moreover, the second shell of Zn-Al, Zn-Zn distance inγ-Al2O3 suspensions at pH 7.5 after 12 h were similar to those of synthesized Zn-Al LDH precipitate (RZn-Al = 3.06-3.10 Å, RZn-Zn = 3.08-3.10 Å).32, 36, 42 Combining the results of XRD, ZnO NPs were mostly transformed to Zn-Al LDH in the presence of γ-Al2O3 at pH 7.5. In goethite suspensions, the Zn-C scattering path was confirmed at pH 5.5, which was diagnostic of the presence of ZnCO3 (Table S8). After reaction for 360 days, the coordination numbers of Zn-O and Zn-C were 5.3and 5.1, respectively (Table S8), which were close to the crystalline structures of ZnCO3 (ICSD #100679). Combining the results of XRD, we could conclude that ZnO NPs dissolved with a long reaction time and mainly transformed to ZnCO3 at pH 5.5. At pH 7.5, the coordination number of Zn-Zn decreased from 9.5 to 4.7 when the reaction time increased from 2 days to 360 days. Crystalline structures showed that the Zn-Zn coordination numbers of Zn-Zn for ZnO (ICSD #26170) and ZnCO3 were 12 and 6, respectively. The reduction of Zn-Zn coordination numbers at pH 7.5 and the higher Zn-Zn coordination numbers



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16 at pH 7.5 than pH 5.5 suggested that most ZnO NPs remained unchanged in goethite suspensions but the minority transformed into ZnCO3 over time.

3.5. High-Resolution Transmission Electron Microscopy Since SR-XRD and EXAFS suggested that ZnO NPs formed some different types of crystalline precipitates on γ-Al2O3, HR-TEM equipped with EDX analysis was undertaken to further examine the changes in morphology, elemental distribution of ZnO NPs in γ-Al2O3 suspensions. After the reaction of ZnO NPs with γ-Al2O3 for 360 days at pH 7.5, the TEM images combined with EDS analysis of such solid samples are shown in Figure S4 A. Graininess and flake morphology are presented in Figure S4A, and EDS analysis of all regions in TEM images showed that the main metals were Zn and Al with the ratio of approximately 2 : 5. The flake morphology observed in region 1 was Zn rich, which was proven to be the layered structure of Zn-Al LDH materials.30 The graininess material in region 2 only contained metal of Al, and its morphology was consistent with γ-Al2O3 particles (Figure S3 of Supporting Information). At pH 5.5, the crystalline nature of γ-Al2O3 weakened compared to pH 7.5 (Figure S4B), indicating that γ-Al2O3 dissolved faster at pH 5.5 than at pH 7.5. Needle-shaped precipitate and flake morphology were also observed, suggesting that Zn-Al LDH was formed at pH 5.5 (Figure S4B). Liu et al. measured the facile and large-scale production of Zn-Al LDH and found that at acidic conditions, Zn-Al LDH precipitate was depressed due to insufficient Al(OH)4- and Zn(OH)42- in the


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17 solutions.43 While in the present study, Zn-Al LDH precipitate was formed at pH 5.5, which could be explained by locally elevated pH and high Zn concentrations near dissolving surface of ZnO NPs.31

4. Conclusions To accurately predict the stability and toxicity of ZnO NPs and MPs in the soil environment, the long-term dissolution and transformation of ZnO particles in the presence of typical Fe/Al bearing minerals, i.e., γ-Al2O3 and goethite suspensions for up to 360 days were investigated in the current study. Dissolution of ZnO particles was found to be enhanced with smaller particle size, and ZnO particles underwent rapid dissolution in acidic solution (pH 5.5) compared to weak alkaline solution (pH 7.5). EXAFS and HR-TEM results indicated that in γ-Al2O3 suspensions, ZnO NPs were completely dissolved within 1 h and transformed into the Zn-Al LDH precipitate at pH 7.5, whose fraction increased with reaction time increasing. Zn-Al LDH precipitate was also detected at pH 5.5 with a smaller fraction, which was attributed to the locally elevated pH and Al/Zn concentration near the immediate surface of ZnO NPs during dissolution. While in goethite suspensions, most of the ZnO NPs were unequivocally unchanged at pH 7.5. The dissolution rate increased but was still lower than in γ-Al2O3 suspensions at pH 5.5, and the dissolved Zn2+ was mainly transformed into ZnCO3.



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18 Acknowledgments We acknowledge the support from the National Natural Science Foundation of China (Project No. 41430752, 41701359 and 41301559), and the Natural Science Foundation of Jiangsu Province, China (Project No. BK20130050). We are grateful to the staff of the beamline BL 14W1 and BL15U1 in the Shanghai Synchrotron Radiation Facility for their support in EXAFS and SR-XRD measurements.

Supplementary material Supplementary materials and methods, and results are available online:


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19

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24 Zn(II) with Hematite Nanoparticles and Microparticles. Part 1. EXAFS Study of Zn(II) Adsorption and Precipitation. Langmuir 2009, 25 (10), 5574-5585. (38)Liu, Z. P.; Ma, R. Z.; Ebina, Y.; Iyi, N.; Takada, K.; Sasaki, T. General synthesis and delamination of highly crystalline transition-metal-bearing layered double hydroxides. Langmuir 2007, 23 (2), 861-867. (39) Jacquat, O.; Voegelin, A.; Kretzschmar, R. Soil properties controlling Zn speciation and fractionation in contaminated soils. Geochim. Cosmochim. Acta 2009, 73 (18), 5256-5272. (40)Juillot, F.; Marechal, C.; Ponthieu, M.; Cacaly, S.; Morin, G.; Benedetti, M.; Hazemann, J. L.; Proux, O.; Guyot, F. Zn isotopic fractionation caused by sorption on goethite and 2-Lines ferrihydrite. Geochim. Cosmochim. Acta 2008, 72 (19), 4886-4900. (41)Roberts, D. R.; Ford, R. G.; Sparks, D. L. Kinetics and mechanisms of Zn complexation on metal oxides using EXAFS spectroscopy. J. Colloid Interf. Sci. 2003, 263 (2), 364-376. (42) Li, W.; Wang, Y. J.; Zhu, M. Q.; Fan, T. T.; Zhou, D. M.; Phillips, B. L.; Sparks, D. L. Inhibition Mechanisms of Zn Precipitation on Aluminum Oxide by Glyphosate: A P-31 NMR and Zn EXAFS Study. Environ. Sci. Technol. 2013, 47 (9), 4211-4219. (43) Liu, J. P.; Huang, X. T.; Li, Y. Y.; Sulieman, K. M.; He, X.; Sun, F. L. Facile and large-scale production of ZnO/Zn-Al layered double hydroxide hierarchical heterostructures. J. Phys. Chem. B 2006, 110 (43), 21865-21872.


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25 Table and Figure Captions Table 1 EXAFS-based ZnO speciation in γ-Al2O3 suspensions at pH 5.5 and pH 7.5. Figure 1. Changes in concentrations of dissolved Zn released from ZnO NPs and MPs in solutions as a function of time at pH 5.5 and pH 7.5. Figure 2. Changes in concentrations of dissolved Zn released from ZnO NPs and MPs in γ-Al2O3 and goethite suspensions as a function of time at pH 5.5 (A) and pH 7.5 (B), respectively. Figure 3. SR-XRD analysis ofγ-Al2O3, ZnO and ZnO reacted with γ-Al2O3 at different time points at pH 5.5 (A) and pH 7.5 (B); nonreacted goethite, ZnO and ZnO reacted goethite at pH 5.5 (C) and pH 7.5 (D). Figure 4. Linear combination fitting results for bulk-EXAFS spectra of ZnO in γ-Al2O3 suspensions at pH 5.5 (A) and pH 7.5 (B). Solid lines represent the k3-weighted χ(k)-spectra and the red dotted line represent the best fits obtained with linear combination fitting.



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26 Tables and figures Table 1 EXAFS-based ZnO speciation in γ-Al2O3 suspensions at pH 5.5 and pH 7.5. Zn speciation (LCF) (%) Samples

ZnO

Outer-sphere

Zn-Al LDH

ZnCO3

Inner-sphere

sorption

sorption

complexes Zn

complexes Zn

Zn(OH)2

pH=5.5 0.5h 1h 4h 12h 1d 7d 14d 150d 360d

0 0 0 0 0 0 0 0 0

46 49 48 45 52 44 42 35 45

19 26 30 32 35 31 25 43 36

13 13 9 7 1 6 10 9 8

22 12 13 16 12 19 23 13 11

0 0 0 0 0 0 0 0 0

11 9 16 7 0 0 0 0 0

14 3 2 0 0 0 0 0 0

12 0 0 0 0 0 0 0 0

pH=7.5 0.5h 1h 4h 12h 1d 7d 14d 150d 360d

0 0 0 0 0 0 0 0 0

36 32 30 31 30 29 31 30 33

27 56 52 62 70 71 69 70 67


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27

1200 800

Zn concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

16 pH=5.5 ZnO NPs pH=5.5 ZnO MPs pH=7.5 ZnO NPs pH=7.5 ZnO MPs

12 8 4 0 0

50

100

150

200

250

300

350

Time (day) Figure 1. Changes in concentrations of dissolved Zn released from ZnO NPs and MPs in solutions as a function of time at pH 5.5 and pH 7.5.



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28

Figure 2. Changes in concentrations of dissolved Zn released from ZnO NPs and MPs in γ-Al2O3 and goethite suspensions as a function of time at pH 5.5 (A) and pH 7.5 (B), respectively.


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29

Figure 3. SR-XRD analysis ofγ-Al2O3, ZnO and ZnO reacted with γ-Al2O3 at different time points at pH 5.5 (A) and pH 7.5 (B); nonreacted goethite, ZnO and ZnO reacted goethite at pH 5.5 (C) and pH 7.5 (D).



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30

Figure 4. Linear combination fitting results for bulk-EXAFS spectra of ZnO in γ-Al2O3 suspensions at pH 5.5 (A) and pH 7.5 (B). Solid lines represent the k3-weighted χ(k)-spectra and the red dotted line represent the best fits obtained with linear combination fitting.


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Dissolution and Transformation of ZnO Nano- and Microparticles in

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