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Effects of myo-inositol hexakisphosphate on Zn(II) sorption on #-alumina: A mechanism study Yupeng Yan, Biao Wan, Deb P. Jaisi, Hui Yin, Zhen Hu, Xiaoming Wang, Chunmei Chen, Fan Liu, WenFeng Tan, and Xionghan Feng ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00051 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Earth and Space Chemistry

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Effects of myo-inositol hexakisphosphate on Zn(II) sorption

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on γ-alumina: A mechanism study

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Yupeng Yan,† Biao Wan,† Deb P. Jaisi,‡ Hui Yin,† Zhen Hu,† Xiaoming Wang,†

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Chunmei Chen,§ Fan Liu,† Wenfeng Tan,† Xionghan Feng,†, *

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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

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‡

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

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

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19716, United States

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§

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30602, United States

Department of Crop and Soil Sciences, The University of Georgia, Athens, Georgia

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*Corresponding author: Xionghan Feng

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Tel: +86 27 87280271;

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Fax: +86 27 87282163;

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E-mail: [email protected]

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 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

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ABSTRACT: Myo-inositol hexakisphosphate (IHP), a most common organic

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phosphorus in many soils, can strongly interact with aluminum (Al) oxides and

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influence the fate of metal ions. In this study, the effects of presorbed IHP on γ-Al2O3

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(γ-alumina) surfaces on Zn(II) sorption were investigated in batch experiments using

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a combination of powder X-ray diffraction (XRD), in situ attenuated total reflectance

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Fourier transform infrared spectroscopy (ATR-FTIR), 31P and 27Al solid-state nuclear

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magnetic resonance spectroscopies (NMR), and Zn K-edge extended X-ray absorption

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fine structure spectroscopy (EXAFS). The results of the batch experiments show that

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the pre-sorption of IHP increases the sorption density of Zn(II) on γ-Al2O3 surfaces.

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The XRD data indicate that the pre-sorption of IHP hinders the formation of Zn−Al

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layered double hydroxide (LDH) precipitates by raising the critical concentration of

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Zn(II) required to precipitate the complex. Solid-state NMR spectra further suggest

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that the chemical environment and speciation of IHP presorbed change, i.e., from

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inner-sphere surface complexes to ternary surface complexes and to zinc phytate

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precipitates (Zn-IHP) with the increase in Zn(II) concentration or pH. Linear

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combination fittings (LCFs) of the EXAFS spectra indicate that the proportion of

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Zn(II) in binary or ternary surface complexes decreases and that in Zn-Al LDH

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increases with increasing concentration of Zn(II) at pH 7. Furthermore, the order at

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which IHP and Zn are added in the reaction can influence the co-sorption mechanism.

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At pH 7, more binary or ternary Zn surface complexes and Zn-IHP form, and less

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Zn−Al LDH precipitates form if Zn is added first. These results demonstrate that both

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the timing and concentration of IHP and divalent metals have sweeping influences on


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their solubility and speciation and these intricacies are needed to take into

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consideration towards predicting their fates in the environment.

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KEYWORDS: Myo-inositol hexakisphosphate, Zn(II), sorption, aluminum oxide,

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layered double hydroxide, zinc phytate

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INTRODUCTION

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Solubility, mobility, and toxicity of heavy metals in soils and aquatic

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environments are heavily affected by metal−mineral interactions1. Metals can interact

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with minerals or soil colloids via surface adsorption and precipitation reactions. It has

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been widely proposed that metals such as zinc (Zn), nickel (Ni), and cobalt (Co) could

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form metal−Al hydroxide surface precipitates on aluminum oxides and Al-rich

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clays2-4. It is further reported that under certain conditions, Zn- and Ni-layered double

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hydroxides (LDH) can be formed in soils5-8. Since these precipitates are less soluble

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and their formation provides an important template for sequestration of toxic metals,

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further research in these products and reaction mechanisms has long been an area of

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intensive scientific interest.

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Environmental conditions are generally far more complex than binary reactions.

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For example, the presence of anionic ligands may interfere the formation of LDH in

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soils9-11. Similarly, citrate and salicylate suppress the formation of Ni−Al LDH on

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gibbsite, and α-Ni hydroxide formed instead due to the complexation of Al by these

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two organic ligands9. In addition, the presence of glyphosate (GPS) inhibited the

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precipitation of Zn on aluminum oxide10. Recently, the role of extracellular polymeric

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substances has been found to enhance Zn sorption on γ-Al2O3 at pH 5.5, but to

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suppress Zn sorption at pH 7.5 and to form γ-Al2O3-EPS-Zn ternary complexes at

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both pHs12.

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Inorganic phosphate (IP) and organic phosphate (OP) are omnipresent in the

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environment. The presence of phosphate inevitably affects the interaction of metals


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with mineral surfaces. A number of studies have suggested that phosphate can alter

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sorption mechanisms of cadmium, copper, zinc, and lead on iron and aluminum

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oxides13-18. For instance, based on the analyses of EXAFS spectra and the CD-MUSIC

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modeling, Tiberg et al.

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ferrihydrite in the presence of phosphate was most satisfactorily explained by the

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formation of ternary metal-phosphate complexes in which the metal interacted

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directly with the surface. Furthermore, Elzinga and Kretzschmar16 showed that the

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addition of aqueous Cd(II) enhanced the amount of phosphate adsorbed across the pH

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ranges but in overall increased with increasing pH, and was interpreted to be due to

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the formation of two structurally distinct ternary Cd(II)-phosphate surface complexes.

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In addition, Ren et al.18 studied the mutual effects of phosphate and Zn(II) on γ-Al2O3

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and showed that multiple mechanisms were involved in Zn(II) retention in the

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presence of phosphate including electrostatic interaction, binary and ternary surface

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complexation, and the formation of Zn(II)-phosphate polynuclear complexes. In

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summary, the sorption mechanisms in the ternary system are dependent on the type of

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mineral, metal ions, and anions.

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proposed that the enhanced sorption of Cu(II) and Pb(II) to

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Organic P compounds, which constitute 20–80% of total P in soils and

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sediments19, involve in a series of biogeochemical reactions and transformations.

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Myo-inositol hexakisphosphate (IHP) strongly interacts with metal oxides such as

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aluminum oxides20-23, iron oxides24,

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ZnO)26-28. The presence of IHP significantly increases the sorption of Cd(II) to

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gibbsite below pH 8.0 especially at high concentrations of Cd(II) and IHP29. The 31P

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and metal nanoparticles (TiO2, CeO2, and



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NMR spectra combined with surface complexation modeling have been used to

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confirm the presence of two outer-sphere ternary complexes29. Wan et al.30 studied the

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co-adsorption of IHP and Cd(II) on hematite by ATR-FTIR spectroscopy and

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proposed that two structurally distinct ternary surface complexes were formed in the

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pH range of 4.0 to 9.0. Series of reactions mentioned above result in the precipitation

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of IHP and hence the accumulation in the environment19. It means the interaction

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between IHP and minerals will inevitably affect the reaction between metals and

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minerals. To the best of our knowledge, only a handful of studies have explored the

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interaction between Zn and IHP at the mineral/water interface29-31, but few studies

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simultaneously applied multiple spectroscopic tools to investigate the co-sorption

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mechanisms of IHP and metals on mineral surfaces. The sequence and concentrations

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at which metals and IHP are added and their influence on the composition and

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co-existence of metal-IHP complex, however, remain elusive.

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γ-Al2O3, an analog to naturally occurring aluminum hydroxides and Al-rich clays,

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is widely used as a proxy and a representative aluminum oxide in the studies of P, Zn

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and other metals adsorption due to its large specific surface area and high interface

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activity3, 10-12, 17, 22, 32. The adsorption mechanisms of IHP or Zn on γ-Al2O3 were

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systematically studied before3, 22, but the co-sorption mechanisms of IHP and Zn at

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the interface of γ-Al2O3 are not fully understood. Accordingly, the objectives of the

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present study are to investigate the co-sorption mechanisms of IHP and Zn(II) on the

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surfaces of γ-Al2O3 in controlled laboratory batch experiments by employing powder

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X-ray diffraction (XRD), in situ attenuated total reflectance Fourier transform infrared


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spectroscopy (ATR-FTIR), solid-state nuclear magnetic resonance spectroscopy

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(NMR), and Zn K-edge EXAFS spectroscopy. The results contribute to understanding

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the co-sorption interfacial processes of inositol phosphate and metals (such as Zn) on

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Al-rich minerals in the highly weathered and Zn polluted soil with the presence of

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IHP.

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MATERIALS AND METHODS

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Materials and Reagents. Analytical reagent grade chemicals and double

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distilled deionized water were used to prepare all of the solutions and suspensions

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used in this study. Dipotassium myo-inositol hexakisphosphate (myo-IP6, IHP, purity

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greater than 95%) salt was obtained from Sigma-Aldrich (P5681-5G). Stock solutions

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of IHP were prepared in double distilled deionized water and refrigerated. The

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hydrolysis of IHP in stock solutions was monitored by measuring orthophosphate

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level in solution before each use. The γ-Al2O3 mineral (SkySpring Nanomaterials Inc.,

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product number-1328QI) used in this study is pure with a specific surface area of 325

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m2 g-1 and with an isoelectric point (IEP) of 9.3.

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Synthesis of zinc phytate. Zinc phytate was prepared from K-phytate (K2H10IP6)

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and zinc chloride according to the established methods28, 33. Briefly, K2H10IP6 was

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mixed with 0.2 M HCl to make a final concentration of 15 mM. A sufficient quantity

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of Zn chloride was added to the solution so that a Zn-to-P ratio of 1:1 was obtained in

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the reaction mixtures. The solutions were adjusted to a pH of 6.0 by slowly adding 1

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M NaOH dropwise in an l-h time interval while keeping the solution stirred. The



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white precipitates (zinc phytate) then formed were filtered and washed with three

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20-mL portions of double distilled deionized water to remove residual reagents and

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any possible adsorbed or occluded ions. The washed precipitate was air-dried at the

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room temperature until the constant weight was achieved.

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Co-sorption of IHP and Zn(II) on γ-Al2O3. To avoid the possible precipitation

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of zinc phytate, Zn(II) and IHP were added to γ-Al2O3 suspensions successively

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instead of simultaneously. In addition, the mineral concentration of γ-Al2O3 was set as

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0.75 g L−1 to collect enough products for the ATR-FTIR, XRD, and EXAFS analyses.

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IHP with a maximum concentration of 240 µM and Zn(II) with a maximum

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concentration of 3.0 mM were used to achieve a good signal-to-noise ratio for the 31P

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NMR analysis and Zn K-edge EXAFS analysis of the co-sorption products.

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Before the start of adsorption experiment, 100-mL 1.5 g L-1 γ-Al2O3 suspensions

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in 0.1 M KCl were first equilibrated under nitrogen and stirred at pH 7.0 for 24 h; the

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pH was regularly adjusted with 0.1 M HCl or KOH using an automatic titrator. Then,

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100 mL of 480 µM IHP solution equilibrated at pH 7.0 and in 0.1 M KCl buffer was

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added to the 100-mL γ-Al2O3 suspension. During the equilibration, the pH of each

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batch sample was checked (after 6, 24, and 48 h) and adjusted, if needed, to pH 7.0 ±

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0.05 with 0.1 M HCl (or KOH). After 48 h of reaction, the suspensions were

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centrifuged (at 9500 g for 15 min); the supernatants were decanted and filtered

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through a 0.22-µm Millipore filter membrane and analyzed for IHP to obtain the

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equilibrium IHP concentration and to be able to calculate the adsorbed amount of IHP

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from the difference in concentration before and after IHP adsorption.


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After centrifugation and decantation, the obtained wet solid was washed to

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remove the residual IHP (i.e., free IHP in residual solution) using 5 mL of 0.1 M KCl

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at pH 7.0. The Zn(II) sorption experiments were performed by adding 0.24, 0.48, 0.72,

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1.0, 1.50, 1.75, 2.25, and 3.0 mM Zn(II) to the wet solid (i.e., γ-Al2O3 sorbed with

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IHP) with final volume of 200 mL. The wet solid was re-dispersed by shaking and

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ultrasonic treatment. All experiments were performed at pH 7.0 in 0.1 M KCl salt

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solution. During equilibration, the pH of each reactor measured and adjusted to pH

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7.0 (± 0.05) with 0.1 M HCl or KOH. After 24 h of reaction, the suspensions were

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centrifuged; the supernatants were decanted and filtered through a 0.22-µm Millipore

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membrane filter and analyzed for Zn to obtain the equilibrium Zn concentration. The

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adsorbed amount of Zn was calculated from the difference in concentration of Zn

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before and after the reaction.

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The sorption of Zn(II) on γ-Al2O3 without presorbed IHP were also conducted at

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the same pH and concentration of Zn(II) as above. In addition, the sorption of Zn(II)

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(3.0 mM) on γ-Al2O3 with/without presorbed IHP were also conducted at three pHs:

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4.0, 5.0, and 6.0 all under 0.1 M KCl. Finally, the sorption of IHP (240 µM) on

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γ-Al2O3 with Zn(II) (3.0 mM) presorbed onto it was performed at pH 7.0. After

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washing with 0.1 M KCl at the corresponding pH to remove the residual Zn2+ and IHP,

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the sorption products prepared for further analyses. Each adsorption experiment was

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performed in duplicate at room temperature (25 ºC). All products generated from

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various treatments above were analyzed by using ATR-FTIR. Furthermore, an aliquot

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of each was air-dried for XRD, NMR, and Zn K-edge EXAFS analyses.



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IHP and Zn2+ Analysis. Before the colorimetric determination of phosphate by

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the phosphomolybdate blue method34, IHP was hydrolyzed to inorganic phosphate by

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digestion with concentrated sulfuric and perchloric acids35. The dissolved

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concentration of Zn2+ in the solution was analyzed by using atomic absorption

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spectrometry (AAS, Varian AAS 240FS).

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Powder X-ray Diffraction (XRD) Measurements. The powder XRD patterns

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were recorded on a Bruker D8 Advance diffractometer (Bruker AXS Gmbh, Karlsruhe,

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Germany) equipped with a LynxEye detector by using Ni-filtered Cu Kα radiation (λ=

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0.15418 nm). The diffractometer was operated at a tube voltage of 40 kV and a

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current of 40 mA with a scanning rate of 4°/min and at a step size of 0.02°.

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In situ ATR-FTIR Spectroscopy. The ATR-FTIR spectra were recorded on a

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Bruker Vertex 70 FTIR spectrometer equipped with a deuterated triglycine sulfate

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(DTGS) detector. A single-reflection diamond ATR accessory (Pike Technologies, Inc.)

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was used to acquire spectra of wet samples. Spectra were collected in the spectral

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range extending from 900 to 1500 cm-1 as the average of 512 scans at an instrument

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resolution of 4 cm-1. The wet sample paste was directly and uniformly applied to the

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diamond ATR crystal. The sample-holding region was covered with a glass lid to

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prevent water evaporation during measurements, and ATR-FTIR spectra were then

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recorded immediately. The supernatant spectra were used to check for possible

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contributions to the paste spectra from ligands remaining in solution. Also, to isolate

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the spectra of the ligands at the water-solid interface, the paste spectra were subtracted

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from the supernatant spectra to remove the strong contributions from the water bands.


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Solid-state NMR Spectroscopy. Solid-state

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Al and

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P single-pulse MAS

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(SP/MAS) NMR spectra of some samples were collected on an Infinity Plus 300 MHz

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spectrometer (7.05 T) (Oxford). The

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collected at the operating frequencies of 121.42 and 78.15 MHz, respectively, using a

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4 mm (o.d.) double resonance ZrO2 rotors at a spinning rate of 10 kHz. The

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chemical shifts (δP) are reported relative to an external (NH4)2HPO4 standard. The 31P

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SP/MAS spectra were obtained with an excitation 90° pulse of 2.7 µs, with a 30-s

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relaxation delay. The 27Al chemical shifts (δAl) are reported relative to an external 1 M

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Al(NO3)3 solution set to δAl = 0 ppm. The pulse delay was optimized at 1 s, and

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approximately 200 scans were collected for each spectrum to obtain an acceptable

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signal-to-noise ratio.

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P and

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Al SP/MAS NMR spectra were

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P

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The solid-state SP/MAS NMR spectra of other sorption samples were collected

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on a 400 MHz NMR spectrometer (9.4 T) (Burker Advance III 400 M, Switzerland).

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The

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MHz using a 5 mm PABBO BB-1 H/D Z-GRD probe with samples contained in 4

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mm (o.d.) ZrO2 rotors at a spinning rate of 12 kHz. High-power decoupling was

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employed during the acquisition. The 31P chemical shifts (δP) were reported relative to

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an external 85% H3PO4 solution. The

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excitation pulse of 2 µs, at a 2-s relaxation delay.

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P SP/MAS NMR spectra were collected at the operating frequencies of 161.8

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P SP/MAS spectra were obtained with a 90°

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Zn K-edge EXAFS spectroscopy. Air-died powder samples were ground and

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homogeneously distributed onto Kapton or cellophane tape for X-ray absorption

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spectroscopy (XAS). Zn K-edge (9659 eV) EXAFS spectra were collected at the



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1W1B beamline of the Beijing Synchrotron Radiation Facility (Beijing, China). One

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Al and Zn co-sorbed sample [Al2O3-IHP-Zn(0.24), pH 7.0], zinc phytate precipitate

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(Zn-IHP), and Zn−Al layered double hydroxide precipitate (LDH) synthesized by a

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previous study36 were used as reference compounds. The EXAFS spectra were

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collected in fluorescence or transmission mode within the energy range of ~150−1000

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eV from the K-edge of Zn (9659 eV). A Zn-foil was regularly used to perform energy

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calibration of the spectra.

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The EXAFS spectra were processed and analyzed using the Athena software

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package. After normalization, linear combination ﬁts (LCFs) were performed using

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the special co-sorption samples, Zn-IHP, and LDH as standards. It was assumed that

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the co-sorbed Al2O3-IHP-Zn complex with the lowest concentration of sorbed Zn(II)

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represented the speciation of Zn(II) in the binary and ternary surface complexes.

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In addition, the χ(k) function was Fourier transformed using k3 weighting, and all

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shell-by-shell fitting was performed in R-space. A single threshold energy value (∆E0)

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was allowed to vary during fitting. The amplitude reduction factor, S02, was 0.90.

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More details on the data analysis are provided in the Supporting Information.

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RESULTS

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Sorption isotherm and sorption pH edge of Zn(II) on γ-Al2O3. The sorption

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density of IHP on γ-Al2O3 decreases with increasing pH with 0.89 µmol m-2 at pH 7.0

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(Figure 1a), consistent with our previous study22. Sorption isotherms of Zn(II) on

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pristine γ-Al2O3 and γ-Al2O3 presorbed with IHP at pH 7.0 are showed in Figure 1b.


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The isotherms are fitted using a Langmuir equation, Q = QmKC/(1 + KC), where Q is

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the sorption density of Zn(II) (µmol·m-2), C is the equilibrium Zn(II) concentration

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(µM), Qm is the maximal sorption density, and K (L·µmol-1) is the equilibrium

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constant for sorption reaction. The maximal sorption density of Zn(II) on pristine

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γ-Al2O3 is found to be 8.24 µmol m-2 (Table S1). Compared with the control, the

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sorption density of Zn(II) on γ-Al2O3 presorbed with IHP increases with a maximal

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sorption density of 12.04 µmol m-2 (Table S1). Our previous study showed that the

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adsorption of IHP greatly decreased the Zeta potential of γ-Al2O3 at various pHs22.

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The enhancement effect by IHP indicates the presence of favorable electrostatic

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and/or chemical interactions (e.g., the formation of ternary surface complexes or

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surface precipitates) between Zn(II) and IHP ions at the γ-Al2O3 surface. The sorption

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of Zn(II) on pristine γ-Al2O3 increases with the increase of pH (Figure 1c). In all pH

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values tested, the sorption of Zn(II) on γ-Al2O3 with presorbed IHP is higher than that

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in pristine but is highest at pH 7.0 (Figure 1c). The ratio of sorbed Zn(II) to presorbed

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IHP on γ-Al2O3 increases with increasing pH (Figure 1d), suggesting that high pH is

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more favorable for sorption of Zn(II). The sorption mechanism on molecular-scale is

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helpful for us to understand the sequestration of Zn(II) on minerals. Previous EXAFS

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analysis revealed that Zn adsorbed on the aluminum oxide surface mainly as bidentate

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mononuclear surface complexes at acidic pH (5.5), whereas Zn−Al layered double

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hydroxide (LDH) precipitates formed at alkaline pH (8.0)10.

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ATR-FTIR spectroscopy. The ATR-FTIR spectra of γ-Al2O3 with presorbed

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IHP and then with Zn(II) sorbed at pH 7.0 are showed in Figure 2. Compared with the



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spectrum of pristine γ-Al2O3, the spectrum of γ-Al2O3 sorbed with IHP alone shows

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peaks at 1124, 1080, and 1003 cm−1, which could be assigned to the P-O stretching

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vibration20, 28. When the amount of sorbed Zn(II) further increases (≥ 0.24 mM Zn(II)),

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the spectrum exhibits a distinct increase in the relative intensity of the peak at 1080

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cm−1 implying that the changes in speciation of sorbed IHP on the surfaces. When the

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concentration of Zn(II) is above 1.5 mM, the relative intensity of the peak at 1080

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cm−1 becomes maximum, and the spectrum of the product tend to be comparable to

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that of zinc phytate.

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Interestingly, IR peak at 1355 cm−1 was unique and appears only above certain

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Zn(II) concentration. Generally, the IR intensity of this peak was most obvious when

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the Zn(II) concentration was 3.0 mM. This IR band is assigned to the carbonate group,

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which suggests the anion in the interlayers of Zn−Al LDH is CO32−. It is because

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carbonate can replace chloride anion to form more stable carbonate−LDH3, 37. The

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source of carbonate, however, is unclear but most likely originated from the diffusion

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of atmospheric CO2 in the water during the reaction.

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The ATR-FTIR spectra of γ-Al2O3 with presorbed IHP and then with Zn(II)

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sorbed at various pH levels are presented in Figure S1. The intensity of the peak at

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1080 cm-1 increases with the rise of the pH and becomes the main peak at pH 7.0,

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consistent with that of zinc phytate. This result indicates that with increasing pH, the

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speciation on IHP-sorbed mineral surface changes and differs from that at pHs 4.0,

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5.0, and 6.0.

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Figure S2 compares the ATR-FTIR spectrum of γ-Al2O3 with Zn(II) sorbed first


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and then with IHP sorbed at pH 7.0 with that of zinc phytate. The spectrum of the

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Zn-IHP co-sorbed standard is comparable to that of zinc phytate. In addition, a weak

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shoulder occurs at 1156 cm-1, similar to that of aluminum phytate. These results

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suggest that the initially formed Zn-Al LDH is dissolved and gradually transforms to

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zinc phytate and aluminum phytate precipitates. The IR peak at 1355 cm−1 in Figure 2

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is no longer observed (Figure S2), which also supports this suggestion.

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XRD. The XRD pattern of γ-Al2O3 sorbed with low concentration Zn(II) (0.24

299

mM), which is similar to the pristine γ-Al2O3 is shown in Figure 3. When the

300

concentration of Zn(II) is above 0.72 mM, new peaks appear at 0.79 nm (11°, 2θ) and

301

0.39 nm (23°, 2θ), indicating the formation of Zn-Al LDH, consistent with the

302

literature38. After pre-sorption of IHP on γ-Al2O3, when the concentration of Zn(II)

303

added is lower than 0.72 mM, the sorption product does not present the characteristic

304

peaks of Zn−Al LDH precipitate. When 1.5 mM Zn(II) is added, the sorption product

305

shows characteristic peaks of LDH indicating that a few Zn−Al LDH precipitates are

306

formed. In addition, obvious characteristic peaks of LDH show up when the

307

concentration of Zn(II) is above 2.25 mM. These results indicate that the pre-sorption

308

of IHP inhibits the formation of Zn−Al LDH precipitate and raises the critical

309

concentration of Zn(II) to form Zn−Al LDH precipitate.

310

The effect of presorbed IHP on the γ-Al2O3-Zn(II) sorption products at various

311

pH values is presented in Figure S3. At low pHs (pH 4.0 and 5.0), the pre-sorption of

312

IHP presents a limited impact on XRD patterns of sorption product and no LDH

313

diffraction peak is observed (Figure S3). The formation of LDH phase is



314

thermodynamically unfavorable upon Zn(II) uptake onto γ-Al2O3 at those low pHs (Li

315

et al., 2012)3. At pH 6, it shows the formation of minor amount of LDH in the

316

γ-Al2O3-Zn(II) binary system. However, when IHP-sorbed γ-Al2O3 is used, no LDH is

317

formed at the surface of γ-Al2O3, suggesting that the presence of IHP impedes the

318

formation of LDH at pH 6.0.

319

Figure S4 compares the effect of the addition sequence of sorption reagents.

320

When IHP reacts with γ-Al2O3 for 48 h before the addition of Zn(II) (3 mM), Zn−Al

321

LDH precipitate forms. When Zn(II) is reacted with γ-Al2O3 for 24 h before the

322

addition of IHP, Zn−Al LDH does not form implying that IHP prevents the formation

323

of LDH via dissolution and re-precipitation, but poorly crystalline zinc phytate and

324

aluminum phytate may eventually form as reaction products39. It is also possible that

325

the well crystalline LDH is transformed into a poorly crystalline one, which could not

326

be well identified by XRD. These results suggest that zinc and aluminum phytate are

327

more kinetically stable than Zn−Al LDH precipitate. This effect of IHP is different

328

from that of glyphosate10, i.e., if Zn was added prior to glyphosate, Zn−Al LDH

329

precipitates still formed at pH 8.010.

330

Solid-state NMR spectroscopy. The

31

P solid-state NMR spectrum of IHP

331

sorbed on γ-Al2O3 at pH 7.0 exhibits two peaks at δP-31 =−1.8 and −5.4 ppm (Figure

332

4a), which is different from the chemical shift of IHP (δP-31 = −0.5 ppm)20. Previous

333

NMR studies have reported inner-sphere phosphate surface complexes at δP = 0 and

334

−6 ppm on boehmite40, 41, at δP = −2.8 ppm on α-Al2O342, at δP = −3 ppm on γ-Al2O343,

335

at δP = −6 ppm on amorphous Al(OH)344, and at δP = −4.5 ppm on gibbsite45. From


Page 16 of 41

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336

these results, it can be suggested that NMR peaks in the range of δP = 0 to −6 ppm

337

appear to be typical for inner-sphere P complexes on aluminum (hydr)oxide surfaces40,

338

41

339

complexes on γ-Al2O3. When IHP is reacted with γ-Al2O3 for 48 h before the addition

340

of 0.24 mM Zn(II), the spectrum of the sorption product is similar to IHP sorbed on

341

γ-Al2O3 alone, suggesting that probably the IHP binds to γ-Al2O3 via phosphate

342

groups, and Zn associates with free phosphate groups in IHP molecules to form

343

γ-Al2O3-IHP-Zn ternary surface complexes (see details in the section of Zn K-edge

344

EXAFS spectroscopy). It was proposed that bivalent metal ions coordinated with two

345

phosphate groups of IHP molecules46. When 1.5 mM Zn(II) is added, the chemical

346

shift of the sample NMR spectrum at δP-31 = −5.4 ppm decreases in intensity, while

347

the intensity of the chemical shift at δP-31 =1.92 ppm has some decreases, implying

348

some changes in the co-sorption mechanism. When 3.0 mM Zn(II) is added, the

349

intensity of the chemical shift at δP-31 =2.48 ppm further increases and the spectrum

350

becomes similar to that of zinc phytate33. These results collectively indicate that with

351

increasing Zn(II) concentration, the chemical environment of IHP and Zn(II) changes,

352

which is consistent with ATR-FTIR and XRD results.

. Therefore, the result of this study suggests the formation of inner-sphere IHP

353

The 27Al solid-state NMR spectra of γ-Al2O3 and several other sorption products

354

exhibit two peaks at δAl-27 = 8.1 ppm and 65 ppm (Figure 4b). The peak at ~10 ppm is

355

assigned to AlO6 octahedral coordination and that at δAl-27 = 65 ppm is assigned to

356

AlO4 tetragonal coordination3, 47. With the increase of Zn(II) concentration, a signal

357

intensity reduction of the peak at δAl-27= +65 ppm is observed (Figure 4b), indicating



358

the transformation of some tetrahedral Al from the bulk γ-Al2O3 to octahedral Al in

359

the Zn−Al LDH precipitates. As compared with the

360

for the Zn reacted sample at low loading (0.24 mM and 1.5 mM), that for the Zn

361

reacted samples at high loading (3.0 mM) shows a small shoulder at δAl= +12.5 ppm

362

(Figure 4c). The NMR signal at +12.5 ppm indicates the formation of Zn−Al LDH

363

with a mixed Zn and Al octahedral structure3.

364

27

Al solid-state NMR spectrum

The effect of presorbed IHP on the γ-Al2O3-Zn(II) sorption products at various 31

365

pH values is presented in Figure 5. At pH 4.0, the

366

product exhibits the main peak at δP-31 = −6 ppm with a shoulder at 0 ppm. Another

367

shoulder at δP-31 = ~ −11 ppm can be assigned to Al-IHP precipitates20. With the

368

increase of pH, the intensity of the shoulder at −11 ppm gradually decreases,

369

indicating the decrease of the relative amount of Al-IHP precipitates. With increasing

370

pH (4.0 to 6.0), the relative intensity of the shoulder around δP-31 = 1.92 ppm

371

gradually increases. When the pH further increases to 7.0, the peak around δP-31 = 1.92

372

ppm becomes the main peak, which is consistent with the spectrum of Zn-IHP

373

precipitates, with a small shoulder peak at δP-31= −6.0 ppm. These results indicate that

374

with the increase of pH, the chemical environment of P in the sorption products is

375

gradually altered. In addition, the solid-state

376

product under ternary system shows minor difference from that under binary system

377

at pH 5.0 (Figure S5). This result implies that the presence of Zn(II) would cause little

378

change in the relative intensities or positions of the 31P NMR spectra for the adsorbed

379

species compared with similar species bound in the absence of Zn(II). In this complex,

P NMR spectrum of sorption

31

P NMR spectrum of the sorption


Page 18 of 41

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380

the Zn2+ is most probably bound to free phosphate groups of the chemisorbed IHP

381

species and, as a result, is unlikely to significantly affect the chemical environment of

382

the P atoms.

383

The effect of addition sequence of Zn(II) and IHP on sorption products is showed

384

in Figure S6. When Zn(II) added is prior to IHP, the characteristic peak of Zn-IHP

385

precipitates at δP-31 = 1.92 ppm occurs in the 31P NMR spectrum of sorption product.

386

In addition, a shoulder peak occurs at δP-31 = −3.15 ppm, which could be attributed to

387

a signal of inner-sphere IHP complexes or possible ternary surface complexes.

388

Meanwhile, the right shoulder is very wide, similar to the signal of Al-IHP

389

precipitates which exhibits the main peak at δP-31 = −11.2 ppm20. It was also reported

390

that the chemical shifts at δP-31= −13 and −21 ppm were assigned to Al–IHP surface

391

precipitates29. The result in this study indicates that some Zn-Al LDH initially formed

392

is dissolved and then transfers to form IHP surface complexes, Al-IHP precipitates

393

and Zn-IHP precipitates due to the addition of IHP, which is consistent with the XRD

394

results (Figure S4).

395

Zn K-edge EXAFS spectroscopy. Linear combination fittings (LCFs) of the k

396

space of Zn K edge EXAFS spectra are performed to quantify different speciation of

397

Zn(II) in co-sorption products. Our previous study showed that two zinc phytate

398

complexes (with 1.67 and 0.83 mM IHP, respectively) with an IHP/Zn2+ ratio of 1:1

399

and 1:2, respectively were stable enough at pH 7.028. At pH 7.0, the sorption density

400

of IHP and Zn(II) is both approximately 0.9 µmol m-2 (Figures 1 and S1), close to a

401

1:1 in stoichiometry, for the special co-sorption sample [Al2O3-IHP-Zn(0.24), pH 7.0].



402

A previous study indicated that the critical sorption density for surface adsorption and

403

surface precipitation is 1.5 µmol m-2, i.e., when the sorption density of Zn(II) was

404

below 1.5 µmol m-2, Zn(II) was sorbed by surface adsorption on γ-Al2O33. The XRD

405

data also show that no LDH form at this condition (240 µM Zn) (Figure 3). Therefore,

406

we assume that the co-sorption sample [Al2O3-IHP-Zn(0.24), pH 7.0], a sample with

407

the lowest concentration of Zn(II) (i.e., 240 µM) sorbed, represents the speciation of

408

Zn(II) in “binary or ternary surface complexes”.

409

The LCF results (Table 1) reveal that the proportion of binary or ternary surface

410

complexes decreases from 100% to 24% with Zn(II) concentration increasing from

411

0.24 mM to 3.0 mM. As the concentration of Zn(II) increases from 1.50 mM to 3.0

412

mM, the proportion of Zn-Al LDH formed increases from 19% to 49%. While the

413

proportion of Zn(II) in the speciation of Zn-IHP is in the range of 38% to 27% as the

414

concentration of Zn(II) increases from 0.72 mM to 3.0 mM. These results suggest that

415

Zn(II) in the co-sorption products is likely composed of two or three kinds of species,

416

which is dependent on the concentration of Zn(II) (Table 1).

417

For the experiment with 3.0 mM Zn(II) sorbed first, the speciation of Zn(II) in

418

binary or ternary surface complexes, Zn-IHP and LDH is 29%, 29%, and 42%,

419

respectively. The ratio of LDH decreases by 7% when compared with the treatment

420

with IHP sorption prior to 3.0 mM Zn(II). This result also supports that the well

421

crystalline LDH formed is eventually transformed to poorly crystalline one, which

422

could not be well identified by XRD but by Zn K-edge EXAFS.

423

Shell-by-shell fitting results show that the coordination number of Zn-P is in the


Page 20 of 41

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424

range of 0.52-1.84 (Table S2 and Figure S7). Specially, as low concentration of Zn(II)

425

is added, the coordination number of Zn-P for the co-sorption sample is low (i.e.,

426

0.52), suggesting that part of Zn also coordinates with the surface of mineral. When

427

the concentration of added Zn(II) increases, the coordination number of Zn-P

428

increases, indicating that the proportion of Zn coordinating with phosphate groups of

429

IHP molecules increases. While the concentration of added Zn(II) further increases,

430

more LDH forms, resulting in the decreasing Zn-P coordination number and

431

meanwhile the increasing Zn-Al/Zn coordination number. With increasing

432

concentration of Zn(II) added for the co-sorption samples, the coordination number of

433

Zn-O increases, likely due to more tetrahedral coordination of Zn at low Zn(II)

434

loading. The fitting results suggest that the Zn−Zn and Zn-Al distances for the

435

co-sorption samples are in the range of 3.06 to 3.09 Å, consistent with a previous

436

study3.

437 438

DISCUSSION

439

Al oxides and Al-rich soil minerals are abundant in the environment and exert

440

impacts on the mobility, speciation, and bioavailability of both organic P compounds

441

and heavy metals. In the natural environment, organic phosphorus (e.g., IHP) and

442

heavy metals (e.g., Zn(II)) can co-exist with Al oxides and undergo ternary reaction.

443

In this study, the results of sorption isotherm and sorption pH edges indicate that the

444

presorbed IHP on γ-Al2O3 promotes the sorption of Zn(II) due to lowering of the

445

surface positive charge of γ-Al2O322. A portion of presorbed IHP can provide new



446

adsorption sites for Zn(II). On the other hand, the pre-sorption of the IHP raises the

447

critical concentration of Zn(II) needed to form Zn−Al LDH precipitate as indicated by

448

XRD. Previous study showed that the critical concentration of Zn(II) to form Zn−Al

449

LDH precipitate was 0.4 mM, i.e., when the concentration of Zn(II) was below 0.4

450

mM, Zn(II) was sorbed by surface adsorption; while the concentration of Zn(II) was

451

above 0.4 mM, Zn(II) was sorbed by surface precipitation3.

452

The order at which reagents are added or the composition of reagents have

453

significant difference on the product formed. For example, it is shown that at pH 8.0,

454

Zn−Al LDH precipitates are formed if Zn is added first, and no precipitates formed if

455

glyphosate is added first or simultaneously with Zn; while, at pH 5.5, only

456

γ-Al2O3-GPS-Zn ternary surface complexes form regardless of whether GPS or Zn is

457

added first or both were added simultaneously10. Yamaguchi et al.9 showed that the

458

presence of either citrate or salicylate can suppress the formation of Ni−Al LDH on

459

pyrophyllite and gibbsite. Further Tan et al.11 revealed that the surface coating of

460

silicate on γ-Al2O3 reduced Al release and finally resulted in a high Ni : Al ratio due

461

to a lower extent of Al substitution into the precipitates. The presence of silicate has

462

been found to prevent the growth of the precipitates, which leads to the formation of

463

less stable Ni−Al LDH11. From these results, it could be concluded that the ligands

464

may play three different roles in these systems: (i) complexing or precipitating metals

465

to prevent further reaction of metal ions with the mineral surface; (ii) preventing the

466

formation of LDH via dissolution and re-precipitation, and (iii) covering the mineral

467

surface and thus reducing the possibility of dissolving γ-Al2O3 and the interaction


Page 22 of 41

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468

between metal ions and mineral surface Al11, 39.

469

The NMR data show that the chemical shift of δP-31 changes gradually with the

470

increase in Zn(II) concentration, indicating that the chemical environment of IHP, i.e.,

471

the speciation of IHP sorbed on γ-Al2O3, changed. There must be an IHP species that

472

is different from “regular” IHP surface complexes in the γ-Al2O3-IHP binary system

473

as well as zinc phytate precipitates. Similarly, previous study reported that Cd(II) and

474

phosphate were adsorbed on the surface of hematite by forming two structurally

475

distinct ternary Cd(II)–phosphate surface complexes, with the relative proportions of

476

the various complexes varying with pH, based on the observation, distinct differences

477

between the IR spectra of the ternary difference spectra and phosphate solutions,

478

binary spectra, and Zn(II)-phosphate precipitates16. Previous studies have concluded

479

that multiple mechanisms are involved in Zn(II)/Cu(II) sorption on γ-Al2O3 in the

480

presence of phosphate17, 18. Based on three different references, the relative proportion

481

of different Zn(II) speciation in the co-sorption products was further quantified by the

482

LCFs of Zn K edge EXAFS spectra. The fitting results (Table 1) suggest that

483

generally three species of Zn(II) co-exist in the IHP and Zn(II) co-sorption system,

484

which is dependent on the solution chemistry (such as the concentration of aqueous

485

Zn(II), pH and so on). Based on the NMR, XRD, ATR-FTIR and EXAFS analyses,

486

we speculate that: (i) at low concentration of Zn(II), IHP binds to γ-Al2O3 via 2-3

487

phosphate groups, and Zn2+ is bound to free phosphate groups of the chemisorbed IHP

488

species; (ii) when the concentration of Zn(II) increases, the ternary complexes

489

transforms to zinc phytate precipitates; and (iii) when the concentration of Zn(II)



490

further increases, Zn(II) interacts with γ-Al2O3 to form Zn−Al LDH precipitate.

491 492

CONCLUSIONS AND IMPLICATIONS

493

Our studies reveal that the sorption mechanisms in the ternary system

494

(γ-Al2O3/IHP/Zn(II)) are quite different from that in the binary system (γ-Al2O3/IHP,

495

or γ-Al2O3/Zn(II)). The speciation and stability of reaction products are determined by

496

the pH values, the molar ratio of the reactants, and sequence at which reactants are

497

added in the system. This study shows that the pre-sorption of IHP increases the

498

sorption of Zn(II) on γ-Al2O3 than that on pristine γ-Al2O3. The pre-sorption of IHP

499

impedes the formation of Zn−Al LDH precipitate by apparently raising the critical

500

concentration of Zn(II) required to precipitate Zn−Al LDH. The chemical

501

environment and speciation of IHP presorbed onto γ-Al2O3 change with the gradual

502

increase in the concentration of Zn(II) spiked afterward or pH. The proportion of

503

Zn(II) speciation in “binary or ternary surface complexes” decreases and that in Zn-Al

504

LDH increases with increasing concentration of Zn(II) at pH 7.0 as indicated by LCFs

505

of the EXAFS spectra. More importantly, the sequence at which IHP and Zn are

506

added are found to affect the sorption mechanism. Therefore, the results of this study

507

provide implications for deep understanding the complicate geochemical systems

508

where inositol phosphate are concurrently present with dissolved heavy metals. The

509

confounding interactions between Zn(II) and IHP with Al oxides should be taken into

510

consideration in prediction of the mobility and bioavailability of heavy metals and

511

organic phosphorus with comparable properties in natural environments.

512 ACS Paragon Plus Environment

Page 24 of 41

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513

ASSOCIATED CONTENT

514

Supporting Information

515

The Supporting Information is available free of charge on the ACS Publications

516

website at http://pubs.acs.org.

517

(1) EXAFS data analysis and fitting results, (2) Langmuir parameters of sorption

518

isotherm of Zn(II) on γ-Al2O3, (3) ATR-FTIR spectra of zinc phytate reference and

519

γ-Al2O3 sorbed with IHP and Zn(II) at various pH values, (4) XRD patterns of

520

γ-Al2O3 with Zn(II) and/or IHP sorbed at various pH values, and (5) Solid-state

521

SP/MAS NMR spectrum of zinc phytate reference and γ-Al2O3 with Zn(II) and/or

522

IHP sorbed (PDF).

31

P

523 524

AUTHOR INFORMATION

525

Corresponding Author

526

*Telephone: +86 27 87280271. E-mail: [email protected]

527

ORCID

528

Xionghan Feng: 0000-0001-5499-7174

529 530

Yupeng Yan: 0000-0001-7965-6173 Deb P. Jaisi: 0000-0001-8934-3832

531

Note

532

The authors declare no competing financial interest.

533 534

ACKNOWLEDGMENTS

535

This research is supported by the National Key Research and Development Program

536

of China (No. 2017YFD0200201), the National Natural Science Foundation of China

537

(Grant Nos. 41603100 and 41471194), and the Fundamental Research Funds for the

538

Central Universities (No. 2662017PY070).

539 ACS Paragon Plus Environment


540


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541

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545

hydroxide precipitates at the mineral/water interface: A multiple-scattering XAFS

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(3) Li, W.; Livi, K. J. T.; Xu, W.; Siebecker, M. G.; Wang, Y.; Phillips, B. L.; Sparks,

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γ-alumina: The role of mineral dissolution. Environ. Sci. Technol. 2012, 46,

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(12) Li, C.; Wang, Y.; Du, H.; Cai, P.; Peijnenburg, W. J. G. M.; Zhou, D. Influence of

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nano-γ-Al2O3. J. Colloid Interface. Sci. 2015, 451, 85–92.

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Surface complexation modeling of inositol hexaphosphate sorption onto gibbsite.

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electrolyte on inositol hexaphosphate interaction with goethite. Soil Sci. Soc. Am.

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of inositol hexaphosphate to goethite. J. Colloid Interface. Sci. 2012, 367,

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hexakisphosphate

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nanoparticles: Effects of pH and surface coverage. Environ. Chem. 2016, 13,

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myo-inositol hexakisphosphate adsorbed on TiO2 nanoparticles and its impact on

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their colloidal stability in aqueous. Sci. Total Environ. 2016, 544, 134–142.

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inositol hexakisphosphate. Environ. Sci. Technol. 2016, 50, 5651–5660.

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The effect of inositol hexaphosphate on cadmium sorption to gibbsite. J. Colloid

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Interface. Sci. 2016, 474, 159–170.

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Quantitative and spectroscopic investigations of the co-sorption of myo-inositol

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hexakisphosphate and cadmium(II) on to haematite. Eur. J. Soil Sci. 2017, 68,

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of myo-inositol hexaphosphate onto kaolinite and its effect on cadmium retention.

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structure of arsenate adsorbed on nano-crystalline γ-alumina. Environ. Sci.

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Zhang, T. Q. Solid-state Fourier transform infrared and

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resonance spectral features of phosphate compounds. Soil Sci. 2007, 172,

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delamination of highly crystalline transition-metal-bearing layered double

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hydroxides. Langmuir 2007, 23, 861−867.

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(37) Yang, Y.; Zhao, X.; Zhu, Y.; Zhang, F. Transformation mechanism of magnesium

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and aluminum precursor solution into crystallites of layered double hydroxide.

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(38) Zhang, X. Q.; Li, S. P. Mechanochemical approach for synthesis of layered double hydroxides. Appl. Surf. Sci. 2013, 274, 158–163. (39) Nachtegaal, M.; Sparks, D. L. Nickel sequestration in a kaolinite-humic acid complex. Environ. Sci. Technol. 2003, 37, 529−534.

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phosphate on boehmite (γ-AlOOH) determined from NMR spectroscopy.

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Langmuir 2010, 26, 4753−4761.

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(41) Li, W.; Feng, X.; Yan, Y.; Sparks, D. L.; Phillips, B. L. Solid-state NMR

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(hydr)oxides. Environ. Sci. Technol. 2013, 47, 8308–8315.

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L. Molecular level investigations of phosphate sorption on corundum (α-Al2O3)

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by 31P solid state NMR, ATR-FTIR and quantum chemical calculation. Geochim.

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Cosmochim. Acta 2013, 107, 252–266.

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673

resonance study of the adsorption of phosphate and phenyl phosphates on

674

γ-Al2O3. Langmuir 2002, 18, 1104−1111.

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(44) Lookman, R.; Grobet, P.; Merckx, R.; Vlassak, K. Phosphate sorption by

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synthetic amorphous aluminum hydroxides: A 27Al and 31P solid-state MAS NMR

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spectroscopy study. Eur. J. Soil Sci. 1994, 45, 37−44.

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(45) van Emmerik, T. J.; Sandstrom, D. E.; Antzutkin, O. N.; Angove, M. J.; Johnson, 31

679

B. B.

680

phosphate onto gibbsite and kaolinite. Langmuir 2007, 23, 3205−3213.

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681

(46) He, Z.; Honeycutt, C. W.; Zhang, T.; Bertsch, P. M. Preparation and FT-IR

682

characterization of metal phytate compounds. J. Environ. Qual. 2006, 35,

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1319−1328.

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(47) Isobe, T.; Watanabe, T.; Caillerie, J. B.; Legrand, A. P.; Massiot, D. Solid-state 1H

685

and 27Al NMR studies of amorphous aluminum hydroxides. J. Colloid Interface.

686

Sci. 2003, 261, 320−324.

687 688 689 690 691 692 693 694 695 ACS Paragon Plus Environment


696

Figures and Tables

697

698 699

Figure 1. Sorption pH edge of myo-inositol hexakisphosphate (IHP) on γ-Al2O3 at

700

various pH values (pH 4.0, 5.0, 6.0 and 7.0) (a), sorption isotherms of Zn(II) on

701

γ-Al2O3 without/with IHP pre-sorbed at pH 7.0 (b), sorption pH edge of Zn(II) on

702

γ-Al2O3 without/with IHP pre-sorbed at various pH values (c), and the ratio of sorbed

703

Zn(II)/IHP at various pH values (d). γ-Al2O3-Zn and γ-Al2O3-IHP-Zn in the legend (b

704

and c) denote the sorption Zn(II) on γ-Al2O3 without and with IHP pre-sorbed,

705

respectively.

706


Page 34 of 41

Page 35 of 41 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


707 708

709 710

Figure 2. ATR-FTIR spectra of γ-Al2O3 with myo-inositol hexakisphosphate (IHP)

711

pre-sorbed and then with Zn(II) sorbed at pH 7.0.

712



713 714

715 716

Figure 3. XRD patterns of γ-Al2O3 with/without myo-inositol hexakisphosphate (IHP)

717

pre-sorbed and with various concentration of Zn(II) sorbed at pH 7.0. In some

718

samples, a small peak (see the asterisks) appeared at ~18°, 2θ, indicating the presence

719

of minor level of impurity from the magnetic seed due to stirring.

720 721


Page 36 of 41

Page 37 of 41 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


722 723

724 31

27

725

Figure 4. Solid-state

726

myo-inositol hexakisphosphate (IHP) pre-sorbed and then with various concentration

727

of Zn(II) sorbed at pH 7.0, and 27Al SP/MAS NMR spectra of the samples presented

728

in (b) in the chemical shift range of 25 to 0 ppm that represents the octahedral Al (c).

729

Zinc phyate (Zn-IHP) in (a) is a reference. The texts above each spectrum in (a)

730

denote the corresponding concentration of Zn(II). Al2O3-Zn (3.00 mM) in the legend

731

(b) denotes the sample of γ-Al2O3 with 3 mM Zn(II) sorbed.

P (a) and

Al (b) SP/MAS NMR spectra of γ-Al2O3 with

732



733 734

735 736

Figure 5. Solid-state 31P SP/MAS NMR spectra of γ-Al2O3 with 240 µM myo-inositol

737

hexakisphosphate (IHP) pre-sorbed and then with Zn(II) (3.0 mM) sorbed at various

738

pH levels. Zinc phyate (Zn-IHP) is used as a reference.

739 740


Page 38 of 41

Page 39 of 41 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


741 742

743 744

Figure 6. Fourier transforms in R space of Zn K-edge EXAFS spectra for

745

Zn/myo-inositol hexakisphosphate (IHP) co-sorption samples and model compounds

746

(phase shift not corrected). The special co-sorption sample [Al2O3-IHP-Zn(0.24), pH

747

7.0], zinc phytate precipitate (Zn-IHP), and Zn−Al layered double hydroxide (LDH)

748

are used as standards in LCF fitting.

749 750



Page 40 of 41

751 752 753

Table 1. Linear combination fits (LCFs) of the EXAFS spectra. LCFs were performed

754

using one special co-sorption sample [Al2O3-IHP-Zn(0.24), pH 7.0], zinc phytate

755

precipitate (Zn-IHP), and Zn−Al layered double hydroxide precipitate (LDH) as

756

standards. It was assumed that the special co-sorption sample [Al2O3-IHP-Zn(0.24),

757

pH 7.0], a sample with the lowest concentration Zn(II) sorbed, represented the

758

speciation of Zn(II) in binary and ternary surface complexes.

759 Samples

binary and ternary Zn

Zn-IHP

LDH

surface complexes Al2O3-IHP-Zn(0.24), pH 7.0

1.000

0.000

0.000

Al2O3-IHP-Zn(0.72), pH 7.0

0.619

0.381

0.000

Al2O3-IHP-Zn(1.50), pH 7.0

0.510

0.299

0.191

Al2O3-IHP-Zn(2.25), pH 7.0

0.257

0.343

0.400

Al2O3-IHP-Zn(3.00), pH 7.0

0.243

0.270

0.488

Al2O3-Zn(3.00)-IHP, pH 7.0

0.285

0.292

0.423

760 761 762


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763 764

TOC Figure

765 766

767 768 769 770


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