Enhanced Dissolution and Transformation of ZnO Nanoparticles

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Enhanced Dissolution and Transformation of ZnO Nanoparticles: The Role of Inositol Hexakisphosphate Xionghan Feng, Yupeng Yan, Biao Wan, Wei Li, Deb P. Jaisi, Lirong Zheng, Jing Zhang, and Fan Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00268 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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Environmental Science & Technology

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Enhanced Dissolution and Transformation of ZnO

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Nanoparticles: The Role of Inositol Hexakisphosphate

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Xionghan Feng,*,† Yupeng Yan,† Biao Wan,† Wei Li,‡ Deb P. Jaisi,§ Lirong Zheng,ǁ

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Jing Zhang,ǁ and Fan Liu,†

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†

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Yangtze River), Ministry of Agriculture, College of Resources and Environment,

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Huazhong Agricultural University, Wuhan 430070, People’s Republic of China

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of

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‡

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Sciences and Engineering, Nanjing University, Nanjing 210093, People’s Republic of

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China

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§

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USA

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ǁ

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Academy of Sciences, Beijing 100039, China

Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth

Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716,

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese

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*Corresponding author:

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Xionghan Feng, Tel: +86 27 87280271; fax: +86 27 87288618; e-mail:

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[email protected]

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ACS Paragon Plus Environment


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ABSTRACT:

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The toxicity, reactivity and behavior of zinc oxide (ZnO) nanoparticles (NPs) released

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in the environment are highly dependent on environmental conditions. Myo-inositol

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hexakisphosphate (IHP), a common organic phosphate, may interact with NPs and

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generate new transformation products. In this study, the role of IHP in mediating the

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dissolution and transformation of ZnO NPs was investigated in the laboratory kinetic

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experiments using powder X-ray diffraction, attenuated total reflectance Fourier

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transform infrared spectroscopy,

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high-resolution transmission electronic microscopy, and synchrotron-based extended

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X-ray absorption fine structure spectroscopy. The results indicate that IHP shows a

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dissolution-precipitation effect, which is different from citrate and EDTA that only

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enhances Zn dissolution. The enhanced dissolution and transformation of ZnO NPs by

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IHP (< 0.5 h) is found to be strikingly faster than that induced by inorganic phosphate

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(Pi, > 3.0 h) at pH 7.0, and the reaction rate increases with decreasing pH and

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increasing IHP concentration. Multi-technique analyses reveal that interaction of ZnO

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NPs with IHP induces rapid transformation of ZnO NPs into zinc phytate complexes

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initially and poorly crystalline zinc phytate-like (Zn-IHP) phase finally. Additionally,

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ZnO NPs preferentially react with IHP and transform to Zn-IHP when Pi and IHP

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concurrently coexist in a system. Overall, results from this study contribute to an

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improved understanding of the role of organic phosphates (e.g., IHP) in the speciation

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and structural transformation of ZnO NPs, which can be leveraged for remediation of

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ZnO-polluted water and soils.

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P nuclear magnetic resonance spectroscopy,

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INTRODUCTION

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The large-scale industrial production and widespread usage of manufactured

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nanoparticles (MNPs) inevitably translates into their increasing presence in the

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biosphere and this trend will continue to increase in future.1–6 Risk assessment of

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these MNPs to human health and the environment, therefore, requires an

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understanding of their persistence, mobility, reactivity, and ecotoxicity.2, 7–10

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ZnO NPs are a common type of manufactured nanomaterials that have long been

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produced and used in semiconductors, pharmaceuticals, sunscreen products, textiles,

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paints, industrial coatings, and antibacterial agents.1, 11–13 The global production of

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ZnO NPs is 31500−34000 metric tons per year, and their average discharge into the

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water resources has been estimated to be at 8−20%.3 The concentrations of ZnO in

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surface waters, wastewater treatment plant (WWTP) effluent, biosolids from WWTP

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effluent, sediments and soils have been estimated at 10–2 µg L–1, 0.5−1.5 µg L–1,

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10−80 mg kg–1, 100 µg kg–1 and 16 µg kg–1, respectively.3, 14 ZnO NPs have shown

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toxicity toward ecological receptors across diverse taxa, including bacteria, algae and

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plants, as well as aquatic and terrestrial invertebrates and vertebrates.15–19 These

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negative environmental impacts have led to the increasing concern about the fate of

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ZnO NPs in the environment, which depends largely on the solution chemistry of the

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media (soil or water). For example, several studies have reported the role of solution

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conditions (e.g., ionic strength, pH and the presence of organic matter) and physical

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properties of ZnO itself in the stability, dissolution, and transformation of ZnO.20–24

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The presence of citric acid significantly enhanced the extent of ZnO dissolution.23



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ZnO NPs were found to be rapidly transformed into ZnS and Zn3(PO4)2 during

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anaerobic digestion of wastewater and post-treatment processing of sewage sludge.25,

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26

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found to induce the transformation of ZnO into zinc phosphate,27, 28 and this reaction

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can even occur inside plants, such as maize.29

Furthermore, the reaction between ZnO NPs and inorganic phosphate (Pi) has been

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Myo-inositol hexakisphosphate (IHP), one of the most common organic

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phosphate (OP) compounds in the environment,30 is commonly present in soils,31, 32

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sediments33, 34 and waters.34 Concentrations of inositol hexakisphosphate (IP6) were

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reported to be in the range of 0-460 mg P kg–1 in surface soil from various parts of the

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world and myo-IP6 represented up to 90% of the total IP6.30, 35 IHP is present as a

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major phosphorus species in seeds, grains, as well as in other plant tissues.36 It can

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interact with metal cations (e.g., Cd2+, Cu2+, Zn2+, and Cr3+) to form phytate

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complexes.37–41 In addition, IHP can be strongly sorbed on the surfaces of iron and

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aluminum oxides, forming surface complexes or surface precipitates.42–46 IHP may

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also interact strongly with NPs (e.g., ZnO, TiO2 and CeO2), leaving the speciation of

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both IHP and NPs changed, which will affect the properties, mobility, toxicity and fate

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of NPs in the environment. Our recent results show that IHP adsorbs at the surface of

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insoluble TiO2 and CeO2 NPs via inner-sphere complexes, and plays an important role

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in their stability in solution media by regulating their surface charge.47, 48 However,

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roles of IHP towards stabilization, dissolution, and transformation may vary with

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nanomaterial types particularly the properties of cations in NPs. To the best of our

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knowledge, the interaction between IHP and soluble ZnO NPs has not been addressed


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so far. Therefore, characterizing the nature of IHP–ZnO NPs interaction is

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fundamental to understanding the mechanism of IHP and ZnO NPs transformation

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and an essential step for assessing the fate of ZnO NPs in the environment.

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The objective of this study is to explore the role of IHP on dissolution and

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transformation of ZnO NPs with and without Pi and differentiate from ligands

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induced dissolution reactions. The speciation of IHP in the reaction media was

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determined by solution

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Furthermore, the speciation and phase transformation of ZnO NPs were characterized

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by a combination of attenuated total reflectance Fourier transform infrared

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(ATR-FTIR) spectroscopy, powder X-ray diffraction (XRD) analysis, high-resolution

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transmission electronic microscopy (HRTEM), and synchrotron-based extended X-ray

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absorption fine structure (EXAFS) spectroscopy. Multi-method approached used and

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consistency of results among different approaches could provide in-depth insight into

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the stability and environmental fate of ZnO NPs in the presence inositol phosphate,

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one of the most dominant OP compounds in the environment.

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P nuclear magnetic resonance (NMR) spectroscopy.

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EXPERIMENTAL SECTION Materials and reagent. Dipotassium myo-inositol hexakisphosphate (P5681-5G)

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was obtained from Sigma-Aldrich, St. Louis, MO, USA. The ZnO NPs (99.9% purity)

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were purchased from Nanjing Emperor Nano Material Co., Ltd., Jiangsu, China. Both

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reagents were used as received. Powder XRD shows that the ZnO NPs display peaks

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for this mineral (JCPDS No. 01-089-1397) and the absence of extraneous peaks



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confirms its purity (vide infra). To readily compare the results in this study with

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previously similar researches,24, 27, 28 rhombohedral ZnO NPs with a particle size of 30

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± 10 nm were selected, as indicated by TEM (vide infra). The specific surface area

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(SSA) of ZnO NPs is 37 ± 2 m2 g−1, which is close to the published values (38 and 27

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m2 g−1 for 28 ± 11 and 40 ± 11 nm ZnO NPs, respectively).24, 27 The SSA is also close

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to the calculated geometric surface area of 36 ± 12 m2 g−1 which is based on the

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assumption of spherical geometry, and the density of the nanoparticles is the same as

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bulk ZnO.23

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Reaction of ZnO NPs with IHP and Pi. To better elucidate the reactivity of

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ZnO at the presence of IHP, Pi and other ligands, effect of ZnO NPs aggregation on

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their dissolution and transformation reaction was examined. Aggregation and

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sedimentation of ZnO NPs were determined using time-resolved optical absorbance at

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378 nm using a UV−vis spectrophotometer (UV-6300PC, Shanghai, China).20,

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Meanwhile, the hydrodynamic diameter (z-average diameter) of the ZnO NPs (81,

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162, 405 and 810 µg L−1) and reaction products of ZnO NPs (810 µg L-1) in the

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presence of IHP (IHP-P/ZnO=1) at pH 7.0 in 0.01 M KCl were directly measured by

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dynamic light scattering (DLS) (Malvern Nano ZEN 3600). Furthermore, dissolution

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of ZnO NPs at various initial concentrations (0.203, 0.405 and 0.810 g L−1) at pH 7.0

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in 0.01 M KCl, and the reaction of ZnO NPs with IHP at initial ZnO concentration of

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0.81 g L−1 under stirring conditions alone (without sonication treatment) were

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performed to evaluate the aggregation effect on kinetics of ZnO NPs dissolution and

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transformation.


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Triplicate batch experiments were conducted to understand the interaction

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between ZnO and IHP (or Pi, or both IHP and Pi) at different pH values and P/ZnO

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molar ratios. In order to collect enough intermediate products for the ATR-FTIR,

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XRD and EXAFS analyses, ZnO NPs of 0.81 g L−1 (containing 10 mM Zn) were

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chosen; to achieve a good signal-to-noise ratio for the

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intermediate speciation of P in solution, IHP-P with a maximum concentration of 10

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mM was chosen. ZnO NPs (0.081 g) were mixed with 100 mL 0.01 M KCl solution at

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varying concentrations of IHP, to obtain IHP-P/ZnO molar ratios (i.e., total P/ZnO

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molar ratios) of 0, 0.1, 0.2, 0.3, 0.5, and 1.0. The mixture was immediately sonicated

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for ~1 min to homogenously disperse ZnO NPs. Then the pH was maintained at 7.0 in

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all experiments by addition of 0.2 M HCl using a 907 Titrando automatic titrator

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(Metrohm, Herisau, Switzerland) under stirring. OH− released during the reaction was

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quantified by measuring the amount of HCl consumed. After 1 h of reaction, the

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suspension was centrifuged (at 16000 × g for 15 min) and separated (particle size > 8

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nm) for ATR-FTIR (wet) and XRD (air dried) analyses. In another set of experiments,

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a 4-mL suspension was sampled at different intervals (1, 2, 3, 5, 10, 15, 20, 30 and 60

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min) and immediately filtered through a 0.22-µm membrane filter to separate the

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reactants and to stop the reaction. The effect of pH on dissolution and transformation

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of ZnO NPs was examined in acidic (pH 5.0) and alkaline (pH 9.0) conditions at the

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constant IHP-P/ZnO molar ratio of 0.5. To compare the effect of IHP with other

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ligands, the interaction between ZnO and Pi, citrate and ethylenediaminetetraacetic

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acid (EDTA) was individually performed at a constant pH of 7.0 with Pi-P/ZnO,

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P NMR analysis of the


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citrate/ZnO and EDTA/ZnO molar ratios of 0.5 and/or 1.0. Finally, to identify the

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effect of Pi on the interaction between ZnO and IHP, additional experiments were

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performed at pH 7.0 with the IHP-P: Pi-P: Zn molar ratio of 1:1:1.

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Characterization of ZnO NPs and Reaction Products. The pristine ZnO NPs

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and the dissolution and transformation products after reacting ZnO with IHP and/or Pi

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were characterized using a series of complementary techniques. HRTEM analysis was

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carried out using an electronic microscope (JEM-2100F STEM/EDS; JEOL, Japan) at

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200 kV, and the samples were prepared by spreading 10-µL aliquots of ZnO NPs or

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reaction samples onto carbon-coated copper grids. The ζ potentials of ZnO NPs, zinc

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phytate and the reaction products of ZnO NPs in the presence of IHP (IHP-P/ZnO=1)

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were measured at different pH values (6.5−10) in 0.01 M KCl solution with a Malvern

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Zetasizer ZEN 3600 zeta potential analyzer (Malvern Instruments Ltd., Malvern, UK).

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The ζ potentials of reaction products of ZnO NPs in the presence of IHP (IHP-P/ZnO

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ratio=0.1, 0.2, 0.3, 0.5 and 1.0) were also measured. The speciation and phase

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transformation of ZnO NPs were characterized using in situ ATR-FTIR spectroscopy,

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X-ray diffraction (XRD), and EXAFS spectroscopy. Additional details on

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experimental conditions and analytical procedures are included in the Supporting

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Information (SI S1).

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Solution

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P NMR Spectroscopy. Solution

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P single-pulse magic angle

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spinning NMR spectra of standard samples as well as residual IHP at different time

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points were collected on a 600-MHz Varian UNITY INOVA spectrometer (Santa

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Clara, CA, USA), at an operating frequency of 242.8 MHz for


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P. The NMR

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parameters were 5.02 µs pulse width, 1.60 s acquisition time, 1.5 s pulse delay, 25 °C,

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and more than 2000 scans. The

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external 85% H3PO4 solution. Standard samples included 3.00 mM IHP, and two zinc

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phytate complexes (with 1.67 and 0.83 mM IHP, respectively) with an IHP-P/Zn2+

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ratio of 6:1 and 6:2 (referred to as IHP-P/Zn2+ = 6:1 and IHP-P/Zn2+ = 6:2),

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respectively, all at pH 7.0. According to Crea et al.,39 the predominant species of IHP

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(> 95%) at pH 7.0 are Zn2H2L6− and Zn2HL7− (L: C6H6(PO4)6) for the two zinc

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phytate complexes.

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P chemical shifts (δP) are reported relative to an

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Quantitative Analysis of Pi, IHP and Zn2+. We measured the changes in

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solution chemistry at different time points during the reactions. The concentration of

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Pi was measured using the phosphomolybdate blue colorimetric method.49 It was

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found that the presence of IHP did not affect the determination of Pi. Before the

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colorimetric determination, IHP was hydrolyzed to Pi by digestion with concentrated

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sulfuric and perchloric acids.50 The dissolved Zn concentration in the supernatants

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was quantified using a 240FS atomic absorption spectrometer (Varian, Palo Alto,

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USA).

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RESULTS AND DISCUSSION

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Dissolution and Transformation of ZnO NPs in IHP Solutions. The

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dissolution and stability of ZnO NPs are affected by the particle aggregation. The

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ZnO NPs are poorly dispersed and readily aggregated within 2 h in 0.01 M KCl

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solution at pH 7.0 (SI Figure S2). Both ultrasonication and stirring treatment could



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improve the stability of ZnO NPs. The kinetics of ZnO NPs dissolution with various

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ZnO loadings at pH 7.0 shows that the dissolved Zn concentration increases rapidly

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within the initial 1 hour and approaches equilibrium at 8 hours (SI Figure S3A). The

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dissolved Zn concentration at 8 hours for the highest ZnO loading (0.810 g L−1) is

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much lower than those for the others (0.405 and 0.203 g L−1), 195.53 ± 6.36 µM

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versus 812.85 ± 4.13 and 594.5 ± 32.73 µM. While the dissolved Zn concentration

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decreases to some extent when ZnO loading reduces from 0.405 to 0.203 g L−1. The

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possible explanation could be higher dissolution percentage for the lower ZnO

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loading likely causing greater particle aggregation and thus impeding dissolution in

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the other way. In addition, it is indicated that the mass normalized ZnO dissolution

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rate constant within the initial 20 min obtained by a pseudo-first-order model decrease

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with increasing ZnO concentration, i.e., 0.1150, 0.0968 and 0.0251 L g−1 min−1 for the

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0.203, 0.405 and 0.810 g L−1 loadings, respectively (SI Figure S3C and S3D). Further

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DLS data show that the the z-average (intensity weighted) diameters of ZnO NPs are

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423 ± 98, 586 ± 81 and 902 ± 38 nm for 81, 162 and 405 µg L-1 ZnO NPs,

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respectively, directly suggesting that the aggregation is enhanced with increasing ZnO

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loadings. Given that ZnO loading is the only variable parameters in above ZnO

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dissolution experiments, the lower dissolved Zn concentration and initial dissolution

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rate for the 0.810 g L−1 loading than for the others can be ascribed to an effect of

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aggregation. The decreasing dissolved Zn concentration and dissolution rate of the

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aggregated NPs may be due to reduction of surface energy and surface area and/or

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decreased interaction of the surface of an individual NP with H+.51 Therefore, to


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minimize differences in influence of aggregation on ZnO NPs dissolution and

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transformation, ZnO concentration, and ultrasonication and stirring treatment were

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kept identical in the following experiments.

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The dissolution kinetics of ZnO NPs with different IHP-P/ZnO molar ratios in

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0.01 M KCl solution was examined at a constant pH of 7.0 to evaluate the reactivity

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between ZnO NPs and IHP. In the absence of IHP, dissolution of ZnO NPs is

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insignificant at pH 7.0, with the dissolved Zn concentration limited to < 100 µM

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(0.8% dissolution of total ZnO) (Figure 1A). In the presence of IHP at IHP-P/ZnO

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molar ratios of 0.5 and 1.0, the concentration of dissolved Zn rapidly increases at first,

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with the maximum concentration reaching 829 and 976 µM at 3 min (8.29%

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dissolution of total ZnO) and 10 min (9.76% dissolution of total ZnO), respectively.

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Thereafter, the concentration of dissolved Zn decreases sharply, and approaches to

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zero at 30 min (Figure 1A). The concentrations of total IHP-P in the experiments at

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two different IHP-P/ZnO molar ratios follow a similar pattern as a function of time.

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Total P in the solution decreases dramatically and becomes exhausted within 10–20

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min of reaction (Figure 1B). This result suggests a rapid reaction between dissolved

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Zn and IHP after ~10 min, resulting in the precipitation and removal of both reactants

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from the solution. Interestingly, the role of IHP is quite different from that of other

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common ligands, citrate and EDTA, which enhance the rate of dissolution (within 20

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min) and form soluble Zn(II)-citrate or Zn(II)-EDTA complexes (Figure 1C). The

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dissolved Zn is relatively low in the IHP-ZnO system owing to its fast precipitation

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(see spectroscopic analyses below). To compare the dissolution rate constant of ZnO



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in the presence of different ligands, the experimental data are fitted with the

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pseudo-first-order kinetics model: Ct = Ce (1 − e−kt), where Ce (µM) is the Zn(II)

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concentration at equilibrium, Ct (µM) is the Zn(II) concentration at a given time t

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(min), and k (min−1) is the apparent rate constant (Figure 1C). The k of ZnO

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dissolution in the presence of IHP (1.30 min−1) is much higher than that in the

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presence of citrate (0.37 min−1) and that of the control (0.09 min−1), but much lower

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than that in the presence of EDTA (2.24 min−1) (SI Table S1).

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P solution NMR spectroscopy was employed to analyze the species of IHP in

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the reaction involving ZnO and IHP with an IHP-P/ZnO molar ratio of 1:1 at a pH of

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7.0. Three standard solutions were prepared and showed remarkable differences in

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peak positions (Figure 2). The NMR spectrum of the free IHP solution at pH 7.0

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yields three peaks at the chemical shift of 1.37, 0.39, and 0.07 ppm, respectively,

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whereas the spectrum of the IHP and Zn2+ mixed solution with an IHP-P/Zn2+ ratio of

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6:1 (IHP-P/Zn2+ = 6:1) shows four peaks at the chemical shift of 1.10, 0.39, −0.17,

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and −0.51 ppm, respectively. Similarly, the spectrum of the IHP and Zn2+ mixed

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solution with an IHP-P/Zn2+ ratio of 6:2 (IHP-P/Zn2+ = 6:2) shows four peaks at the

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chemical shift of 1.05, 0.52, 0.22, and −0.47 ppm, respectively. The spectra of IHP

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and Zn2+ mixed solution are different from that of the IHP alone, implying the

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formation of complexes, i.e. zinc phytate complexes, between IHP and Zn2+. The

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chemical shifts of zinc phytate complexes are different from that of IHP from other

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studies31, 52 because of the difference in pH values.

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After reaction with ZnO NPs, the evolution of IHP species in solution is clearly


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evident from progressive change in NMR spectra run at 1, 2, 3, and 5 min of reaction.

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Because the concentration of IHP in the solution after 5 min is too low to be measured,

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the NMR spectra of solution after 5 min were not collected. The NMR spectrum of

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IHP in solution at 1 min of the reaction yields peaks at the chemical shift of 1.39, 0.43,

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and 0.11 ppm, highly similar to that of free IHP (Figure 2). This observation suggests

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that IHP remains mostly as a free species in solution at the beginning of the reaction.

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The 31P NMR spectrum of the sample at 2 min exhibits peaks at the chemical shifts of

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1.11, 0.41, −0.11, and −0.48 ppm, similar to that of the IHP-P/Zn2+ = 6:1 reference.

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This indicates that IHP starts to react with Zn2+ to form zinc phytate complex at an

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IHP-P/Zn2+ ratio of 6:1. With increasing reaction time, their spectra remain

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unchanged. They are still similar to the IHP-P/Zn2+ = 6:1 reference but differ from the

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IHP-P/Zn2+ = 6:2 reference. These observations reveal that IHP promotes the

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dissolution of ZnO NPs mainly through the formation of zinc phytate complex with

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an IHP-P/Zn2+ ratio close to 6:1 in the solution. With increasing Zn2+ concentration

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(due to ZnO dissolution) less soluble zinc phytate complex with an IHP-P/Zn2+ ratio

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close to 6:2 in solution may form, but rapidly transform into zinc phytate precipitates

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when the IHP-P/Zn2+ ratio is greater than 6:2.

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The dissolution of ZnO NPs at different pHs in the presence of IHP at an

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IHP-P/ZnO molar ratio of 0.5 is illustrated in Figure 3. At pH 5.0, in the absence of

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IHP, the proton-promoted dissolution of ZnO NPs is fast, being completed within

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approximately 10 min (SI Figure S4). In the presence of IHP, the concentration of

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total Zn2+ persistently increases over time, from 928 µM at 5 min to 1227 µM at 60



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min (Figure 3A). However, the concentration of IHP rapidly decreases and becomes

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close to zero at 5 min (Figure 3B), which is consistent with the formation of zinc

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phytate (Zn-IHP) precipitates as observed by XRD and ATR-FTIR analyses described

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in the following sections. The dissolved Zn increases even after the complete

291

consumption of IHP, which suggests further that the proton-promoted dissolution of

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ZnO occurs even at the absence of IHP. However, at pH 7.0 and 9.0, in the absence of

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IHP, the dissolution of ZnO NPs is limited (SI Figure S4); in the presence of IHP, the

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concentrations of dissolved Zn sharply increase in first 10 min, then decrease

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gradually, and then become close to zero at 60 min (Figure 3A). On the other hand,

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the concentrations of IHP decrease uniformly over time (Figure 3B), albeit at different

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rates at different pH values. Based on the Zn and P-IHP concentrations, our results

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suggest that an acidic pH level is more favorable for the dissolution and

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transformation of ZnO NPs in the presence of IHP. These results are consistent with

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the Pi-induced pH-dependent transformation of ZnO NPs reported in the literature.28

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Surface Charge of the Reaction Product of ZnO. The zeta potential of ZnO

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NPs was measured during the reaction to examine the evolution of surface charge and

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to identify transformation process. It decreases from 28.1 mV to −17.4 mV for the

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pristine ZnO NPs with the increase of pH from 6.5 to 11.0, with a point of zero charge

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(PZC) of 9.4 (SI Figure S5A). The zeta potential of reaction products of ZnO NPs in

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the presence of IHP at pH 7.0 decreases and becomes asymptotic at about −32 mV

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with the increase of IHP-P/ZnO ratio. This indicates that as IHP concentration

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increases, surface charge becomes increasingly negative at pH 7.0 (SI Figure S5B).


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The reaction between IHP and ZnO (IHP-P/ZnO=1.0) causes a significant reduction

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of zeta potentials that becomes negative (about −30 mV) at various pH (SI Figure

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S5A). The zeta potentials of the reaction products are close to these of zinc phytate

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reference at the corresponding pH levels (SI Figure S5A). In addition, surface

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adsorption of IHP could also result in decreasing of zeta potentials of NPs, such as

314

TiO2 and CeO2.47,

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precipitate or/and adsorption of IHP at the surface of the products.

48

Thus, this implied the transformation of ZnO to Zn-IHP

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XRD and ATR-FTIR Analysis. XRD and ATR-FTIR were used to characterize

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the evolution of ZnO in the reaction. XRD patterns of the reaction products with the

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IHP-P/ZnO molar ratios of 0.1, 0.2, 0.3, and 0.5 display ZnO reflection peaks at the

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same positions, but the peak intensities decrease with increasing IHP-P/ZnO molar

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ratio (Figure 4A). These results indicate that the content of residual ZnO in the

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reacted samples decreases rapidly with increasing IHP concentration. The spectrum of

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the product from the reaction with an IHP-P/ZnO molar ratio of 1.0 shows only one

323

broad peak at around 0.30 nm, which is very similar to that of the zinc phytate

324

standard.53 Thus, the above results indicate that the reaction between IHP and ZnO

325

produces a poorly crystalline zinc phytate-like phase.

326

The results of the ATR-FTIR analysis of the reacted samples with various

327

IHP-P/ZnO molar ratios (0.1, 0.2, 0.3, 0.5, and 1.0) for 1 h are presented in Figure 4B,

328

which compares the 1250−850 cm−1 regions of the sample spectra to those of the

329

unreacted ZnO NPs, free IHP solution and the Zn-IHP standard. In the experiments

330

where ZnO is reacted with various amounts of IHP, the IR bands appear at 1124, 1080,



331

and 1003 cm-1, which could be assigned to the P–O stretching vibration of the

332

P–O–Zn coordination. The IR spectra of the reaction products show some differences

333

from those of the free IHP (1175, 1128, 1167 and 983 cm-1) and some similarity with

334

that of Zn-IHP, as indicated in previous studies.53 Similarly, P–O–Al in aluminum

335

phytate and aluminum phosphate results in absorbance bands at 1138 and 1042 cm-1,

336

and at 1137 cm-1, respectively.45, 55 These results indicate that ZnO rapidly reacts with

337

IHP and transforms into a zinc phytate-like phase, which is consistent with the XRD

338

results (Figure 4A).

339

EXAFS Analysis. Quantitative analysis of the component proportion in the

340

reaction samples is also performed by a linear combination fitting (LCF) of the k

341

space of Zn K edge EXAFS spectra (SI Figure S6A) using ZnO and Zn-IHP as

342

reference compounds. The LCF results are presented in SI Figure S6B. The

343

proportion of Zn-IHP in the samples increases from 17.6% to 93.6% with the

344

IHP-P/ZnO molar ratio increasing from 0.1 to 1.0. The Zn K-edge XAS spectra of the

345

reaction products formed at different times from the reaction of ZnO NPs with IHP at

346

an IHP-P/ZnO molar ratio of 1.0 are also analyzed. Zn K-edge k space EXAFS

347

spectra and LCF results of the kinetic reaction products (SI Figure S6C and D) reveal

348

that the proportion of Zn-IHP precipitate increases from 42.7% to 90.6% with reaction

349

times increasing from 1 min to 20 min, respectively. This means that over 40% and

350

over 90% of ZnO NPs are transformed into Zn-IHP precipitates within the first 1 min

351

and 20 min, respectively.

352

Morphology of the transformation products of ZnO. Pristine ZnO NPs and


Page 16 of 39

Page 17 of 39


353

1-h reaction samples with IHP-P/ZnO molar ratios of 0.5 and 1.0 are analyzed by

354

HRTEM to obtain more detailed information on the morphological evolution of ZnO

355

NPs affected by IHP. The TEM image shows that the pristine ZnO NPs are

356

rhombohedral (SI Figure S7A) with a particle size ranging from 20 to 40 nm.

357

However, the z-average (intensity weighted) diameters of ZnO NPs (810 µg L-1) and

358

the transformation product of ZnO NPs (IHP-P/ZnO=1.0) as measured by dynamic

359

light scattering (DLS) are 916 ± 133 nm and 569 ± 46 nm at pH 7.0 in 0.01 M KCl,

360

respectively, indicative of strong aggregates for both initial ZnO NPs and the product.

361

The aggregation may affect the dissolution and transformation rate of ZnO NPs as

362

indicated by Jiang et al.24 However, it would exert limited impact on the

363

transformation products, considering that the ATR-FTIR spectrum and XRD pattern

364

of the product formed under stirring conditions alone presents no obvious difference

365

from that formed under stirring conditions combined with ultrasonication (SI Figure

366

S8). The high resolution TEM (HRTEM) image of a single ZnO NP (SI Figure S7D)

367

further displays a clear crystal lattice structure, indicative of the high crystallinity. The

368

HRTEM image of the reaction sample with an IHP-P/ZnO molar ratio of 0.5 shows

369

the amphibolous particle boundaries and the presence of larger aggregates (SI Figure

370

S7B), indicative of dissolution features on the surface of ZnO NPs. The HRTEM

371

image of the reacted sample (SI Figure S7E) further displays fuzzy, poorly crystalline

372

structures in some areas while a clear crystal lattice is still present. When the

373

IHP-P/ZnO molar ratio increases to 1.0, rhombohedral NPs almost disappear and

374

larger amorphous (poorly crystalline and non-uniform) particles dominate the product



375

Page 18 of 39

(SI Figure S7C and F).

376

IHP-induced Transformation of ZnO. Results from several complementary

377

research techniques described above (XRD, NMR and ATR-FTIR) consistently

378

indicate the dramatic release of Zn2+ from ZnO NPs followed by the precipitation of

379

Zn-IHP, suggesting rapid transformation of ZnO NPs. During the dissolution of ZnO,

380

a significant amount of OH– is released (SI Figure S9). Based on these results, the

381

chemical reactions during dissolution and transformation of ZnO in the presence of

382

IHP in the aqueous solution are proposed as follows:

383

ZnO (s) + H2O (l) ↔ Zn2+ (aq) + 2OH− (aq)

(1)

384

Zn2+ (aq) + H5L7− (aq) + OH− (aq) ↔ (ZnH4L)6− (aq) + H2O (l)

(2)

385

Zn2+ (aq) + (ZnH4L)6− (aq) + 2OH− (aq) ↔ (Zn2H2L)6− (aq) + 2H2O (l)

(3)

386

(Zn2H2L)6− (aq) + 3Zn2+ (aq) ↔ Zn5H2L (s)

(4)

387

As indicated in these reactions, dissolved Zn is complexed with aqueous IHP to

388

form soluble zinc phytate complexes,39 followed by the precipitation of a poorly

389

crystalline zinc phytate phase. Based on the NMR results (Figure 3), the complexes

390

with an IHP-P/Zn2+ ratio of 6:1, denoted as (Zn2H2L)6− or (Zn2HL)7−, dominate the

391

soluble zinc phytate species in the solution, while the soluble complexes with an

392

IHP-P/Zn2+ ratio of 6:2 may react quickly with more aqueous Zn(II) and convert into

393

Zn-IHP precipitates. These reactions result in the decrease of free Zn2+ ions released

394

from ZnO, and thus allow subsequent transformation of ZnO NPs to zinc phytate

395

complexes and Zn-IHP precipitates.

396

The formation of Zn-IHP precipitate is also supported by chemical equilibrium


Page 19 of 39


397

thermodynamic prediction. According to the solubility constant of ZnO (Ksp = ~2.5 ×

398

10−17),23 the total concentration of aqueous Zn through ZnO dissolution at pH 7.0 is

399

estimated to be 2.5 ×10−3 M. The main IHP species at pH 7.0 are H5L7−.44 To be brief,

400

the molar ratio of [Zn(H5L)]5−/H5L7− is calculated on the basis of the equilibrium

401

equation (Zn2+ + H5L7− ↔ (ZnH5L)5−, SI Table S2),56 i.e., 8.67×105. Results from this

402

thermodynamic calculation are indicative of complete transformation of free IHP into

403

zinc phytate complex when the reaction is close to equilibrium. This calculation, in

404

fact, agrees well with the IHP species identified by

405

(Figure 3). The complexation reaction between IHP and aqueous Zn2+ released from

406

ZnO proceeds continuously until the formation of Zn-IHP precipitate consumes

407

almost all IHP and Zn2+ in the solution. Further quantitative thermodynamic analysis,

408

however, is limited due to the unavailability in the literature of the solubility product

409

data for Zn-IHP precipitate.

31

P solution NMR spectroscopy

410

Based on the batch experimental results and quantitative analyses of different

411

reaction samples by Zn K edge EXAFS spectra, speciation of Zn(II) can be calculated

412

(Figure 5). As shown, the proportion of Zn(II) in solution (aq), Zn(II) in ZnO NPs (s)

413

and Zn(II) in Zn-IHP precipitate (s) changes as a function of time. At an IHP-P/ZnO

414

molar ratio of 1.0 and pH 7.0 in 0.01 M KCl, the proportion of Zn(II) in Zn-IHP

415

precipitate increases over time, and that of Zn(II) in ZnO NPs decreases quickly, with

416

a minor amount (less than 10%) of Zn(II) present in solution. This result indicates that

417

in the presence of IHP, Zn(II) in the speciation of ZnO is quickly (within 1 h)

418

transformed to Zn(II) in Zn-IHP precipitate, another speciation with greater stability



419

and less bioavailability.

420

Comparison of Pi and IHP on the stability of ZnO NPs. In the presence of

421

phosphate alone, with Pi-P/ZnO molar ratios of 0.5 and 1.0 at pH 7.0, the

422

concentration of dissolved Zn greatly decreases in comparison with that of the control

423

(SI Figure S10A). Our results are consistent with the previous studies that observed

424

dissolved Zn concentration decreasing in the presence of Pi.27, 28 The concentrations

425

of Pi constantly decrease over time in all the experiments (SI Figure S10B). At a

426

Pi-P/ZnO molar ratio of 0.5, the reaction lasts for 3 h for Pi, which is much longer

427

than that for IHP (< 0.5 h). At a Pi-P/ZnO molar ratio of 1.0, the reaction reaches

428

equilibrium at 6 h and the final concentration of Pi (3328 ±18 µM) is ~ 33% of its

429

initial concentration. The XRD and in situ ATR-FTIR spectra of the reaction products

430

from ZnO and Pi are in good agreement with those of the zinc phosphate hydrate

431

reference, thereby confirming the complete dissolution of ZnO and formation of zinc

432

phosphate hydrate (SI Figure S11). In addition, the molar ratio (2:3) of Pi and ZnO

433

consumed in the above reaction also supports the stoichiometry of zinc phosphate

434

hydrate [Zn3(PO4)2·4H2O].

435

In the ZnO-(IHP-Pi) ternary system with an IHP-P:Pi-P:ZnO molar ratio of 1:1:1

436

at pH 7.0, IHP reacts with ZnO NPs more quickly than in the ZnO-IHP binary system

437

(Figure 6A and B). Compared with the ZnO NPs-IHP binary system, in the initial

438

stage, more Zn2+ is released from ZnO and a greater amount of IHP is consumed in

439

the ternary system. Similarly, the interaction between Pi and ZnO NPs is faster in the

440

ZnO-(IHP-Pi) ternary system than that in the ZnO-Pi binary system. The Pi


Page 20 of 39

Page 21 of 39


441

consumption rate is greatly accelerated initially, and then the reaction almost ends in

442

20 min in the co-presence of IHP due to the depletion of ZnO (Figure 6C). In situ

443

ATR-FTIR spectra of the reaction products are consistent with that of the Zn-IHP

444

reference, indicating that mainly Zn-IHP-like precipitates are produced in the reaction

445

(SI Figure S12). XRD data further confirm that the reaction products are

446

predominantly the Zn-IHP-like phase (SI Figure S13). The sequestration of Pi and the

447

absence of crystalline peaks in the XRD pattern imply that a minor amount of poorly

448

crystalline Zn3(PO4)2·4H2O is formed in the ternary system. These results

449

demonstrate that the reaction of P with ZnO has a synergistic rather than a competitive

450

effect, and that ZnO preferentially reacts with IHP when Pi and IHP coexist.

451

Environmental Implications. Results from our multi-technique analyses

452

suggest that the combination of dissolution kinetics, conventional solid phase

453

identification and solution speciation studied by XRD, in situ ATR-FTIR and NMR

454

provide most valuable information to evaluate the reactivity of ZnO with IHP and the

455

underlying reaction pathway. It is showed that IHP greatly enhances the dissolution of

456

ZnO NPs and release of Zn2+, a micronutrient at low concentrations and a

457

contaminant at high concentrations,15,

458

transformation product. Such ability of IHP to immobilize Zn2+ by dissolution and

459

transformation is more effective with respect to other common ions and ligands (e.g.,

460

Pi, citrate, and EDTA). Our results also indicate that ZnO NPs preferentially react

461

with IHP and transform to Zn-IHP precipitate when IHP and Pi are concurrently

462

present. Therefore, on the one hand, IHP-mediated complexation reaction could

57, 58

but rapidly sequesters Zn2+ into solid



463

provide a novel remediation strategy for sequestering dispersed ZnO NPs and relevant

464

metal-based NPs in the environment. On the other hand, given that IHP is

465

omnipresent in many environments,30 role of IHP needs to be accounted when

466

evaluating the mobility and toxicity of ZnO NPs and potentially other metallic NPs

467

entered into the environment. In addition, the effects of particle size and other

468

competing or inhibiting ions and compounds (e.g., anions, organic matter, and low

469

molecular weight organic acids) should also be considered for predicting their more

470

realistic fate and behavior, as well as toxicity.

471 472

Supporting Information

473

Additional data are provided. This material is available free of charge via the Internet

474

at http://pubs.acs.org.

475 476

Acknowledgments

477

The authors gratefully acknowledge five anonymous reviewers and the associate

478

editor Dr. Daniel Giammar for their constructive comments on how to improve the

479

manuscript. We are grateful to the National Natural Science Foundation of China (Nos.

480

41171197 & 41471194), and the Strategic Priority Research Program of the Chinese

481

Academy of Sciences (No. XDB15020402) for the financial support.

482 483


Page 22 of 39

Page 23 of 39


484

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(52) Wu, J.; Paudel, P.; Sun, M. J.; Joshi, S. R.; Stout, L. M.; Greiner, R.; Jaisi, D. P.

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Mechanisms and pathways of phytate degradation: Evidence from oxygen isotope

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ratios of phosphate, HPLC, and phosphorus-31 NMR spectroscopy. Soil Sci. Soc.

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Am. J. 2015, 79, 1615–1628.

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(53) Pierce, A. G. Structure studies of phytate-zinc ion complexes: X-Ray diffraction and thermal analysis. Inorg. Chim. Acta 1985, 106, L9–L12. (54) He, Z.; Honeycutt, C. W.; Xing, B.; McDowell, R. W.; Pellechia, P. J.; Zhang, T. 31

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Solid-state Fourier transform infrared and

P nuclear magnetic resonance

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spectral features of phosphate compounds. Soil Sci. 2007, 172, 501–515.

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(55) del Nero, M.; Galindo, C.; Barillon, R.; Halter, E.; Madé, B. Surface reactivity of

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α-Al2O3 and mechanisms of phosphate sorption: In situ ATR-FTIR spectroscopy

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and ζ potential studies. J. Colloid Interface. Sci. 2010, 342, 437–444.

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(56) Torres, J.; Dominguez, S.; Cerda, M. F.; Obal, G.; Mederos, A.; Irvine, R. F.; Diaz,

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A.; Kremer, C. Solution behaviour of myo-inositol hexakisphosphate in the

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presence of multivalent cations. Prediction of a neutral pentamagnesium species

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under cytosolic/nuclear conditions. J. Inorg. Biochem. 2005, 99, 828–840.

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(57) Franklin, N. M.; Rogers, N. J.; Apte, S. C.; Batley, G. E.; Gadd, G. E.; Casey, P. S.

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Comparative toxicity of nanoparticulate ZnO, bulk ZnO, and ZnCl2 to a

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freshwater microalga (Pseudokirchneriella subcapitata): The importance of

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particle solubility. Environ. Sci. Technol. 2007, 41, 8484–8490.


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(58) Li, M.; Lin, D. H.; Zhu, L. Z. Effects of water chemistry on the dissolution of

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ZnO nanoparticles and their toxicity to Escherichia coli. Environ. Pollut. 2013,

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173, 97–102.

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666 667

Figures

668

669

670 671 672

Figure 1. Changes in concentrations of dissolved Zn (A) and myo-inositol

673

hexakisphosphate-P (IHP-P) (B) in solution as a function of time with three different

674

IHP-P/ZnO molar ratios in 0.01 M KCl at pH 7.0, and changes in concentrations of

675

dissolved Zn in solutions as a function of time with IHP-P/ZnO, citrate/ZnO and

676

EDTA/ZnO molar ratios of 1.0 at constant pH 7.0 in 0.01 M KCl (C). The dissolution

677

kinetic curves of ZnO are fitted by pseudo-first-order kinetics model. For ZnO

678

dissolution in the presence of IHP, the data are only fitted within 10 min due to the


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679

decrease of Zn2+ concentration after that. The reaction was performed with 0.81 g L−1

680

ZnO NPs. Each value represents the mean of three replicates with standard deviation

681

shown by error bars.

682 683



684

685 686

Figure 2. 31P solution NMR spectra of starting myo-inositol hexakisphosphate (IHP),

687

aqueous zinc phytate complexes with IHP-P:Zn2+ ratios of 6:1 (IHP-P/Zn2+ = 6:1) and

688

6:2 (IHP-P/Zn2+ = 6:2), and zinc phytate complexes in solution as a function of time

689

at IHP-P/ZnO molar ratio of 1:1. Initial concentration of ZnO NPs was 0.81 g L−1 in

690

the experiment performed in 0.01 M KCl at pH 7.0.

691 692


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693

694 695

Figure 3. Concentration of dissolved Zn (A) and IHP-P (B) in solution as a function

696

of time under various pH (5.0, 7.0 and 9.0) in 60 min (IHP-P/ZnO = 0.5). The

697

reactions were performed with 0.81 g L−1 ZnO NPs. Each value represents the mean

698

of three replicates with standard deviation shown by error bars.

699 700



701

702 703

Figure 4. XRD patterns (dry) (A), and normalized in situ ATR-FTIR spectra (wet) (B)

704

of ZnO NPs and zinc phytate (Zn-IHP) precipitate in experimental runs with various

705

IHP-P/ZnO ratios (R = 0.1, 0.2, 0.3, 0.5 and 1.0) in 0.01 M KCl at pH 7.0 after 1.0 h

706

of reaction. Initial ZnO NPs concentration was 0.81 g L−1. The pattern of ZnO NPs is

707

matched with the reference pattern of ZnO (JCPDS No. 01-089-1397), shown at the

708

bottom. The ATR-FTIR spectrum of Zn-free IHP at pH 7.0 is also included for

709

comparison.

710 711


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712

713 714

Figure 5. The proportion of Zn(II) (aq) in solution, Zn(II) (s) in ZnO NPs and Zn(II) (s)

715

in zinc phytate (Zn-IHP) precipitate as a function of time at IHP-P/ZnO molar ratio of 1.0

716

at pH 7.0 in 0.01 M KCl. The proportion calculation is based on linear combination ﬁt of

717

the EXAFS spectra and wet chemistry analysis.

718 719



720

721 722

Figure 6. Changes in concentrations of dissolved Zn (A), IHP-P (B), and dissolved Pi-P

723

(C) as a function of time. Experiments were performed with IHP and/or Pi

724

(IHP-P:Pi-P:ZnO = 1:1:1) and 0.81 g L−1 ZnO nanoparticles in 0.01 M KCl at pH 7.0.

725

Each data point represents the mean of three replicates with standard deviation shown by

726

error bars.

727 728


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729 730

TOC Figure

731

732 733 734


Enhanced Dissolution and Transformation of ZnO Nanoparticles

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