Article pubs.acs.org/est
Mechanism of Myo-inositol Hexakisphosphate Sorption on Amorphous Aluminum Hydroxide: Spectroscopic Evidence for Rapid Surface Precipitation Yupeng Yan,† Wei Li,‡,∥ Jun Yang,§ Anmin Zheng,§ Fan Liu,† Xionghan Feng,*,†,‡ and Donald L. Sparks‡ †
Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China ‡ Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19716, United States § State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China ∥ Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: Inositol hexakisphosphates are the most abundant organic phosphates (OPs) in most soils and sediments. Adsorption, desorption, and precipitation reactions at environmental interfaces govern the reactivity, speciation, mobility, and bioavailability of inositol hexakisphosphates in terrestrial and aquatic environments. However, surface complexation and precipitation reactions of inositol hexakisphosphates on soil minerals have not been well understood. Here we investigate the surface complexation−precipitation process and mechanism of myo-inositol hexakisphosphate (IHP, phytate) on amorphous aluminum hydroxide (AAH) using macroscopic sorption experiments and multiple spectroscopic tools. The AAH (16.01 μmol m−2) exhibits much higher sorption density than boehmite (0.73 μmol m−2) and α-Al2O3 (1.13 μmol m−2). Kinetics of IHP sorption and accompanying OH− release, as well as zeta potential measurements, indicate that IHP is initially adsorbed on AAH through inner-sphere complexation via ligand exchange, followed by AAH dissolution and ternary complex formation; last, the ternary complexes rapidly transform to surface precipitates and bulk phase analogous to aluminum phytate (Al-IHP). The pH level, reaction time, and initial IHP loading evidently affect the interaction of IHP on AAH. In situ ATR-FTIR and solid-state NMR spectra further demonstrate that IHP sorbs on AAH and transforms to surface precipitates analogous to Al-IHP, consistent with the results of XRD analysis. This study indicates that active metal oxides such as AAH strongly mediate the speciation and behavior of IHP via rapid surface complexation−precipitation reactions, thus controlling the mobility and bioavailability of inositol phosphates in the environment.
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INTRODUCTION Phosphorus (P) is an essential nutrient element required for biological growth as well as a major contributor to eutrophication and nonpoint source pollution. Adsorption, desorption, and precipitation of P reactions on the surfaces of metal oxides and clays govern the mobility, transformation, and availability of this element in the environment, greatly affecting biomass production in ecosystems.1−5 In addition to inorganic phosphate (IP), myo-inositol hexakisphosphate (IHP), the most abundant organic phosphate (OP) in the environments,6 is an important P pool that is widely present in lake sediments,7 pasture soils,8,9 riparian soils,10 and poultry litter.11−13 A clear © 2014 American Chemical Society
understanding of the fate of IHP in the environment is of critical importance to effective nutrient management. IHP strongly interacts with iron (Fe) and aluminum (Al) oxides, leading to the accumulation of inositol hexakisphosphate in nature.6 Some researchers have suggested that sorption of IHP on Fe oxides (e.g., goethite) occurs via innersphere complexation, during which four phosphate groups in Received: Revised: Accepted: Published: 6735
February 27, 2014 May 20, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es500996p | Environ. Sci. Technol. 2014, 48, 6735−6742
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necessary. The suspensions were oscillated in the dark at 298 K for 24 h, centrifuged at 16 000g for 15 min, and then filtered through a 0.22-μm membrane filter. Sorption kinetics experiments were conducted with an automatic titrator (907 Titrando, Metrohm, Switzerland). The reaction temperature was maintained at 25.0 ± 0.2 °C by circulating water through the jacket with a water circulator. A 3 g L−1 AAH suspension was equilibrated in 0.1 M KCl at pH 5 under an atmosphere of N2 for 24 h. The kinetics experiments were initiated by adding 100 mL of a 1044 μM IHP solution to an equal volume of AAH suspension in 0.1 M KCl at pH 5. Upon IHP sorption, OH− ions released into solution were followed in time using a pH-STAT system (907 Titrando, Metrohm). The reaction was kept at pH 5 by adding a known amount of 0.5 M HCl. The molar amount of OH− released was equal to the amount of HCl added. In another parallel experiment, a 5 mL suspension was withdrawn at each reaction time and immediately filtered through a 0.22-μm membrane filter. IHP was hydrolyzed to IP by digestion with concentrated sulfuric and perchloric acids,33 then determined using the phosphomolybdate blue colorimetric method.34 Each run was completed in triplicate. AAH samples sorbed with IHP at various IHP loadings (174, 522, 1 044, and 1 305 μM), pHs (3.5, 4, 5, and 7), and equilibration times (0.5, 1 h, 3 h, 6 h, 8 h, 12 h, 1 d, and 2 d) were selectively prepared for in situ ATR-FTIR (wet), powder XRD (air-dried), and solid-state NMR (air-dried) analyses. Additionally, an AAH sample sorbed with IHP (2 610 μM) in 0.1 M KCl at pH 5 for 4 d was prepared for solid-state NMR (air-dried) analysis. AAH Characterization. The ζ potential variations of AAH over time after sorption with IHP were determined using a Malvern Zetasizer ZEN 3600 zeta potential analyzer (Malvern Instruments Ltd., Malvern, U.K.). The ATR-FTIR spectra of samples were recorded on a Bruker Vertex 70 FTIR spectrometer equipped with a deuterated triglycine sulfate detector (Bruker Optics Inc., Ettlingen, Germany). Powder XRD patterns were recorded on a Bruker D8 Advance diffractometer (Bruker AXS Gmbh, Karlsruhe, Germany). Solid-state 31P and 27Al single-pulse magic angle spinning (SP/MAS) NMR spectra of AAH-IHP sorption solids and standard samples were collected on a 500 MHz Bruker Ascend NMR spectrometer (11.7 T). Detailed operational conditions and analytical procedures are given in the Supporting Information (SI S1). Statistical Analysis. Data are presented as the means of triplicate measurements ± standard error of the mean (SEM). When not indicated, the SEMs were within the dimensions of the symbols shown in the graphs. The values of parameters for all sorption isotherms were fitted using Origin 8.0 (OriginLab Corporation, Northampton, MA, U.S.A.) at the 95% confidence level.
every IHP molecule are possibly bound with mineral surfaces while the other ones remain free.14−16 Differently, Johnson et al.17 have proposed that adsorption occurs via outer-sphere complexation and that hydrogen bonding plays an important role in binding IHP to goethite. Additionally, amorphous Al hydroxide (AAH) is the most active Al (oxyhydr)oxide in soils and sediments, showing significant interaction with IHP.18,19 During IHP sorption, only three of the six phosphate groups in every IHP molecule were thought to bind to the AAH surface, while the others remain free.20 While many studies are focused on the surface complexation mechanism for IHP on metal oxides, the possible precipitation reaction of IHP on minerals has received much less attention. Evidences for surface precipitation of phosphate have been observed on a number of Fe/Al oxides, including goethite,21 AAH,22,23 γ-Al2O3,24−27 gibbsite,23,28 and α-Al2O3.29,30 As IHP is a strong ligand that may promote dissolution of soil minerals, thus a surface precipitation process may occur during IHP sorption under certain conditions. Our recent study has shown that IHP sorption on AAH is much higher than that of other OPs (e.g., adenosine triphosphate, ATP; 10-fold difference) and phosphate (1.7-fold difference), which suggested a possible transformation of IHP surface complexes to surface precipitates.31 The objectives of the present study were to investigate IHP interaction with AAH and the underlying mechanism in acidic to neutral pH media (pH 3.5−7) using macroscopic sorption experiments and multiple spectroscopic approaches. Evolution of IHP in terms of speciation and phase transformation of AAH as a function of reaction time, pH, and IHP concentration were monitored by in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and powder Xray diffraction (XRD). Further, solid-state nuclear magnetic resonance spectroscopy (NMR) was applied to reveal the alteration of chemical environment of P and Al during the reaction. The results contribute to understanding OPassociated interfacial processes and biogeochemical cycles, and provide reference for management of OP-induced environmental issues.
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EXPERIMENTAL SECTION Materials and Reagents. Myo-inositol hexakisphosphate [C6H18O24P6, IHP], boehmite (γ-AlOOH) and α-alumina (αAl2O3) were obtained from Sigma-Aldrich (St. Louis, MO, U.S.A.). AAH was prepared following a modified procedure described by Shang et al.18 and Guan et al. (SI S1).20 The purity of Al (oxyhydr)oxides was demonstrated by powder XRD measurements. AAH, boehmite, and α-Al2O3 had specific surface areas of 73.0, 114.6, and 9.3 m2 g−1, respectively, and their points of zero charge were 9.3, 9.2, and 8.9, respectively. An aluminum phytate reference was synthesized according to the method of He et al. (SI S1).32 IHP Sorption Isotherms and Kinetics Experiments. Before sorption, Al (oxyhydr)oxides were dispersed in 0.1 M KCl for 24 h to hydrate the minerals, acidified with 0.2 M hydrochloric acid (HCl) to reach a final pH of 5, and purged overnight with N2 (g) to remove CO2. To keep a similar surface area of the Al (oxyhydr)oxides, each 10 mL of Al (oxyhydr)oxide suspension (corresponding to 30 mg AAH, 20 mg boehmite, and 250 mg α-Al2O3) was then added to an equal volume of IHP-containing solutions in 0.1 M KCl at pH 5. After equilibration at 25 °C for 6 and 24 h, the pH of each batch sample was measured and then adjusted to pH 5 if
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RESULTS AND DISCUSSION Sorption Isotherm and Kinetics. Figure 1a provides IHP sorption isotherms for AAH, boehmite, and α-Al2O3 over a wide range of IHP concentration (15−2200 μM). There is increasing IHP uptake for all Al (hydr)oxide sorbents (especially AAH) as IHP concentration is increased. The sorption isotherms can be fitted using a Langmuir equation, Q = QmKC/(1 + KC), where Q is the sorption density of IHP (μmol·m−2), C is the equilibrium IHP concentration (μM), Qm is the maximum sorption density of IHP, and K (L·μmol−1) is 6736
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sorption followed by a prolonged sorption process. Complete sorption equilibrium is reached after approximately 6 h of reaction. However, the OH− release is initially slow, increasing dramatically over time after 3 h of reaction and even after complete sorption of IHP, then stabilizing after 12 h of reaction (Figure 1b). These observations indicate that two reaction processes may exist for the sorption of IHP on AAH. First, IHP is sorbed onto AAH mainly through forming inner-sphere complexes; these complexes are formed via ligand exchange of aquo and hydroxo groups with a certain number of phosphate groups among the six groups in an IHP molecule.19,20 Second, the sorbed IHP molecules undergo continuous reactions, e.g., further complexation or coprecipitation with dissolved Al3+ ions at the AAH surface, through the rest of the phosphate groups; thus, there is a significant OH− release after complete IHP sorption at 6 h (Figure 1b). Compared with the control treatment in the absence of IHP, the concentration of aqueous Al3+ in the AAH suspension sharply increases after 6 h of reaction and reaches a maximum at 12 h, then decreases to zero at 24 h (SI Figure S1). These results confirm that the species of IHP sorbed on the AAH surface may change with further reaction with AAH or dissolved Al3+. We speculate that the sorbed IHP associates with Al3+ on the AAH surface and in the solution via chelation, thereby inducing AAH dissolution and Al3+/OH− release into solution (SI Figure S1 and Figure 1b). The sorbed IHP acts as a sorption site for dissolved Al3+ to form a ternary complex, which further transforms to surface precipitates. Due to the formation of surface precipitates, Al3+ ions are gradually depleted in the reaction solution at 12−24 h (SI Figure S1). Zeta Potential Measurements. The ζ potential dynamics of AAH at various pH levels (4−7) over time are provided in Figure 2. From acidic to neutral pH levels, both the initial ζ
Figure 1. Sorption isotherms of myo-inositol hexakisphosphate (IHP) on amorphous aluminum hydroxide (AAH, 1.5 g L−1), boehmite, and α-Al2O3 in 0.1 M KCl at pH 5 after 24 h of reaction (a), and sorption kinetics of IHP (522 μM) on AAH (1.5 g L−1) and associated OH− release in 0.1 M KCl at pH 5 (b).
the equilibrium constant for the IHP sorption reaction. The fitted Qm values follow the order of AAH (16.01 μmol·m−2) ≫ α-Al2O3 (1.13 μmol·m−2) > boehmite (0.73 μmol·m−2). The mass-normalized sorption order of the three Al (hydr)oxides is AAH (1168.99 μmol·g−1) ≫ boehmite (83.77 μmol·g−1) > αAl2O3 (10.53 μmol·g−1). The substantially high sorption density of IHP on AAH is unusual for surface sorption, which suggests a possible process involving AAH dissolution and Al-IHP precipitation, rather than a two-dimensional surface adsorption process as proposed by Ler and Stanforth21 and Jia et al.35,36 Figure 1b presents the sorption kinetics of IHP on AAH and the accompanying OH− release. To reveal different reaction processes during IHP sorption, the molar ratio of IHP/AAH (0.03) is controlled under sorption saturation. The sorption process of IHP is significantly different from the accompanying OH− release (Figure 1b), in contrast to the same kinetic trends in phosphate sorption and OH− release reported previously.1,5 Sorption kinetics of IHP on AAH involves a first step of rapid
Figure 2. Zeta potential variations of amorphous aluminum hydroxide (AAH, 1.5 g L−1) over time as a function of pH in the presence of 87 μM myo-inositol hexakisphosphate (IHP) in 0.1 M KCl.
potential of AAH and its growth rate exhibit declining trends. The results demonstrate that the reaction between IHP and AAH is not a simple process of surface complexation. Because of the strong chelation ability of IHP, the dissolved Al3+ is further bound with the sorbed IHP, thus notably increasing the ζ potential of AAH in the initial stage of IHP sorption. An acidic pH level is favorable for the reaction of IHP on AAH, mainly because of rapid AAH dissolution and Al3+ release at a low pH level. The ζ potential of AAH in the presence of IHP at different initial concentrations (43.5−522 μM, pH 5) over time is 6737
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respectively. With increasing initial IHP loading, the original absorbance band at 958 cm−1 gradually disappears while the relative intensity of the new absorbance bands continuously increase at 1138 cm−1 and decrease at 1042 cm−1 (Figure 3b). These results indicate that the sorption product is more analogous to the Al-IHP reference material. The ATR-FTIR spectrum of the AAH sample prepared at the initial IHP loading of 1305 μM exhibits a similar pattern to that of the AlIHP reference, indicating that the higher IHP loading facilitates the faster reaction and the completion of precipitate formation. XRD Patterns. The XRD pattern of nonreacted AAH exhibits three broad peaks centered at 0.44 (20° 2θ), 0.22 (42° 2θ), and 0.15 nm (64° 2θ), respectively (Figure 4), consistent
presented in SI Figure S2. With increasing concentration of IHP, the ζ potential growth rate of AAH obviously declines. In the presence of 43.5 μM IHP, the ζ potential of AAH initially occurs at 9.4 mV and rapidly approaches the original level at 6 h (35.7 mV). At the IHP concentrations of 174 and 522 μM, the ζ potentials of AAH are respectively, 34.4 and 24.7 mV at 24 h (SI Figure S2). These results indicate that, when the initial concentration of IHP is higher, a longer time is needed to dissolve sufficient AAH and release more Al3+ to offset the surface negative charge caused by higher adsorption of IHP. Additionally, excessive free IHP in the solution can compete with the dissolved Al3+ through chelation and thus retard its coadsorption on the surface of AAH. ATR-FTIR Spectra. In situ ATR-FTIR spectra (1300−800 cm−1 region) of AAH-IHP sorption solids at different reaction times (0.5 h−2 d) are presented in Figure 3a. The ATR-FTIR
Figure 4. XRD patterns of amorphous aluminum hydroxide (AAH) sorbed with myo-inositol hexakisphosphate (IHP, 1305 μM) in 0.1 M KCl at pH 5 as a function of time. AAH and aluminum phytate (AlIHP) are used as reference materials. Asterisk denotes the characteristic peak (0.49 nm) of minor gibbsite impurity in AAH (JCPDS No. 00-002-0173).
Figure 3. ATR-FTIR spectra of amorphous aluminum hydroxide (AAH) in the presence of myo-inositol hexakisphosphate (IHP, 1305 μM) in 0.1 M KCl at pH 5 as a function of time (a), and ATR-FTIR spectra of AAH in the presence of IHP at different loadings (174− 1305 μM) in 0.1 M KCl at pH 5 after 2 d of sorption (b). AAH and aluminum phytate (Al-IHP) are used as reference materials.
with an amorphous phase. In addition, there exists a weak peak at 0.49 nm, indicating the presence of a minor gibbsite impurity (JCPDS No. 00-002-0173). Except for the newly emerging weak peak at 0.32 nm (28° 2θ), the XRD patterns of IHPreacted samples at 3 and 6 h are highly similar to that of the bulk AAH (Figure 4), suggestive of low-degree AAH dissolution and dominant inner-sphere surface complexation at these times. The three broad peaks centered at 0.44 (20° 2θ), 0.22 (42° 2θ), and 0.15 nm (64° 2θ) gradually weaken over time, while the intensity of the peak centered at 0.32 nm (28° 2θ) increases. For AAH-IHP at 2 d, the original peaks become less resolved and the spectrum is more analogous to the Al-IHP reference, demonstrating that IHP sorbed on AAH finally converts to a poorly crystalline Al-IHP-like phase. The XRD spectra at various IHP loadings (174−1305 μM) and pH levels (3.5−7) indicate that the transformation processes of IHP on AAH become more pronounced with a decrease in pH and an increase in IHP loading (SI Figure S3), consistent with the results of the ATR-FTIR analysis (Figure 3). 31 P and 27Al NMR Spectra. The 31P NMR spectra of IHP sorbed on AAH as a function of time (3 h−4 d, pH 5) are shown in Figure 5a. The nonreacted IHP shows a NMR signal at δP‑31 = −0.5 ppm, whereas the poorly crystalline Al-IHP reference yields a broad peak, including two main peaks (δP‑31 = −11.2 and −6.4 ppm, respectively) with a small shoulder at
spectrum of nonreacted AAH shows only one broad absorbance band at 958 cm−1; by comparison, the absorbance band of AAH-IHP at 3 h shifts to a slightly higher wavenumber (Figure 3a). These results suggest that in the initial stage of the surface reaction, IHP is sorbed on AAH predominantly via surface complexation while AAH dissolution is not pronounced. The ATR-FTIR spectrum of AAH-IHP at 6 h shows two new absorbance bands at 1138 and 1032 cm−1, respectively (Figure 3a), which can be assigned to the PO stretching vibration of the POAl coordination of an AlIHP precipitate on AAH.29,37 The intensity of the absorbance band at 1138 cm−1 is generally enhanced over time, and the spectrum of AAH-IHP at 2 d becomes very close to that of the poorly crystalline Al-IHP reference (Figure 3a). Together, these observations demonstrate that an Al-IHP-like surface precipitate is formed on AAH following the initial surface complexation of IHP and the gradual dissolution of AAH. The ATR-FTIR spectra of reacted solids with differently initial IHP loadings (174, 522, 1044, and 1305 μM) are presented in Figure 3b. Compared with data for nonreacted AAH, the ATR-FTIR spectrum of the AAH sample prepared at the initial IHP loading of 174 μM exhibits minor changes and shows two weak absorbance bands at 1138 and 1042 cm−1, 6738
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Figure 5. Solid-state 31P (a) and 27Al (b) SP/MAS NMR spectra of myo-inositol hexakisphosphate (IHP) sorbed on amorphous aluminum hydroxide (AAH) in 0.1 M KCl at pH 5 as a function of time; and 31P (c) and 27Al (d) SP/MAS NMR spectra of AAH sorbed with IHP for 2 d as a function of pH. IHP, AAH, and aluminum phytate (Al-IHP) are used as references. For SP/MAS experiments, the spinning rate is 10 kHz; pulse delays are 30 s for 31P NMR and 5 s for 27Al NMR. Asterisks denote spinning side bands.
−0.5 ppm. Similar to that of phosphate,30,38,39 the peak at δP‑31 = −11.2 ppm can be assigned to an Al-IHP precipitate, the peak at δP‑31 = −6.4 and −0.5 ppm can be assigned to surface-sorbed IHP, and there is a small contribution from physical sorption ascribed to the absorbance band at δP‑31 = −0.5 ppm. The 31P NMR spectrum of AAH-IHP at 3 h (with a sorption amount of 196.37 μmol g−1) exhibits a main peak at δP‑31 = −6.5 ppm with a shoulder at −11.8 ppm. The intensity of the shoulder at −11.8 ppm barely changes over time, suggesting that rapid surface precipitation occurs while inner-sphere surface complexes and surface precipitates coexist in the products. The effect of pH on the 31P NMR spectra of IHP sorbed to AAH is shown in Figure 5c. The 31P NMR spectrum of AAHIHP at pH 7 and 2 d exhibits a main peak at δP‑31 = −6.5 ppm, with two shoulders around −1.0 and −11.0 ppm, respectively. This result suggests that formation of inner-sphere surface complexes is the dominant mechanism, while minimal surface precipitation is developed at this time. With decreasing pH (5 to 3.5), the relative intensity of the peak around δP‑31 = −11.0 ppm gradually increases. The 31P NMR spectrum of AAH-IHP at pH 3.5 is highly similar to that of the Al-IHP reference, demonstrating that low pH is favorable for IHP transformation from surface complexes to surface precipitates. The 27Al NMR spectra of AAH in the absence and presence of IHP further corroborate the above-mentioned transformation process of IHP. The 27Al NMR spectrum of AAH shows a main peak at δAl‑27 = 7.2 ppm and two small peaks at δAl‑27 = 35 and 63 ppm, respectively (Figure 5b). The chemical shift of 27Al NMR around 10 ppm is assigned to AlO6
octahedral coordination,40,41 which indicates that AAH is primarily made up of octahedral Al. In addition, the signals at δAl‑27 = 35 and 65 ppm are respectively attributed to AlO5 pentahedral coordination and AlO4 tetragonal coordination.40,41 Compared to that of the pristine AAH, the 27Al NMR spectrum of AAH-IHP at 3 h exhibits no substantial changes. For AAH-IHP at 6 h, a small shoulder occurs at δAl‑27 = −4.6 ppm, indicating that part of the Al in the bulk AAH transforms into a new species. Taking into consideration the chemical shift of 27Al for AlPO4 surface precipitates on AAH,22,23 and the results of 31P NMR analysis (Figure 5a), we infer that Al transforms from the bulk mineral to surface complexes and precipitates after IHP sorption at 6 h. For AAH-IHP at 2 d, the intensity of the 27Al NMR spectrum at −4.6 ppm increases further and becomes the main peak (Figure 5b), suggesting that substantial Al in AAH transforms into a complexation−precipitation species. For AAH-IHP at 4 d, the chemical shift of the main peak drifts to δAl‑27 = −5.6 ppm, closer to that of the Al-IHP reference (δAl‑27 = −8.8 ppm) (Figure 5b). At a reaction time of 4 d, the original peak of AAH at δAl‑27 = 7.2 ppm becomes very weak, suggesting that surface precipitates transform into an Al-IHP bulk phase. With decreasing pH, the peak of the 27Al NMR spectrum at δAl‑27 = −4.6 ppm for AAH-IHP at 2 d becomes more discernible (Figure 5d). This result reveals that the conversion process is facilitated at low pH, consistent with the 31 P NMR data (Figure 5c). 6739
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according to the equilibrium equation (Al3+ + H6L6− ↔ [Al(H6L)]3−, SI Table S2),43 i.e., 1.3 × 102.48, which is indicative of complete transformation of the free IHP into the complex form when the reaction approaches equilibrium. The multiple phosphate groups and strong chelating ability of IHP would make the further complexation reactions between complexed IHP and aqueous Al3+ released from AAH proceed continuously until the final precipitate is formed. Unfortunately, to the best of our knowledge, the solubility product data are unavailable for aluminum phytate precipitates in the literature. Therefore, an attempt to provide a further quantitative thermodynamic analysis is hindered. Roles of IHP in Surface Complexation−Precipitation. Boehmite and α-Al2O3 sorbed with IHP yield identical 27Al NMR spectra to the pristine boehmite and α-Al2O3 (SI Figure S4). These results imply that inner-sphere surface complexes are dominantly formed on boehmite and α-Al2O3 surfaces, while the amount of surface precipitates is negligible within the sorption period (24 h). The solubility of boehmite and α-Al2O3 is significantly lower than that of AAH, leading to great difficulty in providing sufficient active Al3+ ions to form IHP surface precipitates over a short time. This finding indicates that the crystallinity of Al (oxyhydr)oxides affects surface adsorption−precipitation of IHP. Under certain long-term reactions, AlPO4 surface precipitates can form on AAH,22,23,37 γ-Al2O3,24−27 gibbsite,23,28 and α-Al2O3.29,30 However, no surface precipitates are observed on boehmite under longterm reaction (22 d) with phosphate.39 The possible surface complexation and precipitation of IHP on crystalline Al and Fe (oxyhydr)oxides in longer time and at higher temperature require further study. Owing to the six phosphate groups in every IHP molecule, IHP has great chelating ability. Thus, IHP surface complexes can more easily bind with the incoming cations and transform to surface precipitates than phosphate and other OPs (e.g., adenosine triphosphate, β-glycerophosphate, and glucose-6phosphate).31 It was reported that Al phosphate surface precipitate was formed on AAH after reacting with 33 mM phosphate for 10 days at 60 °C.22 In contrast, we observed that Al-IHP precipitate was developed with 2.2 mM IHP for 4 days at ambient temperature. Therefore, it is reasonable to surmise that these surface precipitation processes of IHP may occur in soil environments where AAH and IHP coexist. Environmental Implications. Al (oxyhydr)oxides are widespread in the environment and can greatly affect the fate, mobility, and bioavailability of IHP and other OPs. It is reported that there exists a significant positive correlation between IHP and total Al levels in soils and sediments.7,8,44 The observed sorption density of IHP on AAH (16.01 μmol m−2) is 10 to 20 times greater than those on boehmite (0.73 μmol m−2) and α-Al2O3 (1.13 μmol m−2), implying that besides surface adsorption, an additional process is also operating for the AAH system, and the amorphous Al oxide is probably an important sink for IHP in soils. Results of our multitechnique analyses confirm that IHP surface complexes are initially formed on AAH surface, then rapidly transform to Al-IHP-like surface precipitates and bulk phases under acidic and circumneutral conditions. Substantial reaction of IHP with active Al may also decrease Al toxicity to crops in acid soils.45 More importantly, the transformation process will decrease the solubility and mobility of IHP, inevitably affecting the availability of phytase3,46,47 and phytate-mineralizing bacteria.48 Phosphate/arsenate irreversibly bound into the framework of
Complexation−Precipitation Transformation of IHP on AAH. The sorption kinetics of IHP and accompanying OH− release (Figure 1b), as well as ζ potential dynamics (Figure 2), indicate that IHP sorption on AAH undergoes surface complexation and precipitation reactions: First, the incoming IHP forms a surface complex on the AAH surface in a mono- or multidentate complex via ligand exchange, accompanied by OH− release; second, AAH dissolution occurs with Al3+ release, while the sorbed IHP provides a sorption site for the dissolved Al3+, leading to the formation of ternary complexes and the depletion of Al3+ from solution; last, the ternary surface complexes convert to surface precipitates via polymerization or condensation, and the reaction continues, resulting in surface precipitates analogous to the poorly crystalline Al-IHP reference. The adsorption−precipitation transformation process of IHP on AAH is similar to that of arsenate35,36,42 and phosphate21 on Fe (oxyhydr)oxides. The ζ potential is a measure for surface charge of particles, which in turn reflects the proportion of positively, neutrally, and negatively charged groups on the surface.21 The decrease in surface negative charge on AAH with extended IHP sorption time (Figures 2) is attributed to the transformation of IHP surface complexes into surface precipitates.21,31 The combined XRD and ATR-FTIR data (Figures 3 and 4) confirm that poorly crystalline Al-IHP precipitates are finally formed in the IHP-AAH sorption systems under acidic to neutral conditions (pH 3.5−7). Solution pH, reaction time, and initial IHP loading are important factors that jointly control the rate and the degree of reactions between IHP and AAH. Additionally, the adsorption-precipitation mechanism of IHP on AAH can be corroborated with the solid-state NMR data (Figures 5). In a 31P NMR spectrum, peaks in the range of δP‑31 = 0 to −6 ppm appears to be typical for inner-sphere phosphate surface complexes on the surface of Al (oxyhydr)oxides, e.g., boehmite,38 α-Al2O3,30 γ-alumina,26 gibbsite,28 and AAH.22 A chemical shift for IHP surface precipitates on AAH may be similar to those of Al phosphate minerals, which are commonly more negative (δP‑31 = −10 to −30 ppm) due to chemical shielding by Al.30 In the 31P solid-state NMR spectrum of AAHIHP at pH 5 and 3 h, the coexisting peaks at δP‑31 = −6.5 and −11.8 ppm (Figure 5a) indicate that both inner-sphere surface complexes and surface precipitates are rapidly formed in the products. It should be noted that the broad peak in the 31P solid-state NMR spectrum of the Al-IHP reference also comprises the component at δP‑31 = −6.0 ppm. Therefore, the P species that generate the peak at δP‑31 = −6 ppm presumably include Al-IHP complexes or precipitates with low polymerization, that likely exist in the Al-IHP reference. Correspondingly, the 27Al NMR spectrum of AAH sorbed with IHP for 6 h at pH 5 shows a new peak at δAl‑27 = −4.6 ppm (Figure 5b), confirming that a new Al-IHP-like phase is formed. Surface complexation of IHP seems to be an important step in the precipitation process on AAH. Previous studies also show that phosphate adsorption and precipitation can be predicted via calculation of the solubility equilibrium.24,25,30 On the basis of the available equilibrium constants of relevant reactions, it is suggested that the complexation−precipitation transformation of IHP on AAH is thermodynamically favorable. Taking pH 5 for example, according to the solubility of AAH (Ksp = 1.3 × 10−33), the equilibrium aqueous Al3+ concentration with AAH is 1.3 × 10−6 M. The main IHP species at pH 5 is H6L6−.17 For the sake of simplicity, the molar ratio of [Al(H6L)]3−/H6L6− is calculated 6740
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NMR spectroscopy and spectral deconvolution. Soil Sci. 2003, 168, 469−478. (9) Turner, B. L.; Cheesman, A. W.; Godage, H. Y.; Riley, A. M.; Potter, B. V. L. Determination of neo- and D-chiro-inositol hexakisphosphate in soils by solution 31P NMR spectroscopy. Environ. Sci. Technol. 2012, No. 46, 4994−5002. (10) Young, E. O.; Ross, D. S.; Cade-Menun, B. J.; Liu, C. W. Phosphorus speciation in riparian soils: A phosphorus-31 nuclear magnetic resonance spectroscopy and enzyme hydrolysis study. Soil Sci. Soc. Am. J. 2013, 7, 1636−1647. (11) Turner, B. L.; Leytem, A. B. Phosphorus compounds in sequential extracts of animal manures: Chemical speciation and a novel fractionation procedure. Environ. Sci. Technol. 2004, 38, 6101−6108. (12) Giles, C. D.; Cade-Menun, B. J. Phytate in animal manure and soils: Abundance, cycling and bioavailability. In Applied Manure and Nutrient Chemistry for Sustainable Agriculture and Environment; He, Z.; Zhang, H., Eds.; Springer: Netherlands, 2014; pp 169−190. (13) Hashimoto, Y.; Takamoto, A.; Kikkawa, R.; Murakami, K.; Yamaguchi, N. Formations of hydroxyapatite and inositol hexakisphosphate in poultry litter during the composting period: Sequential fractionation, P K-edge XANES, and solution 31P-NMR investigations. Environ. Sci. Technol. 2014, 48 (10), 5486−5492. (14) Ognalaga, M.; Frossard, E.; Thomas, F. Glucose-1-phosphate and myo-inositol hexaphosphate adsorption mechanisms on goethite. Soil Sci. Soc. Am. J. 1994, 58, 332−337. (15) Celi, L.; Lamacchia, S.; Ajmone-Marsan, F.; Barberis, E. Interaction of inositol hexaphosphate on clays: Adsorption and charging phenomena. Soil Sci. 1999, 164, 574−585. (16) Celi, L.; Presta, M.; Ajmone-Marsan, F.; Barberis, E. Effects of pH and electrolyte on inositol hexaphosphate interaction with goethite. Soil Sci. Soc. Am. J. 2001, 65, 753−760. (17) Johnson, B. B.; Quill, E.; Angove, M. J. An investigation of the mode of sorption of inositol hexaphosphate to goethite. J. Colloid Interface Sci. 2012, 367, 436−442. (18) Shang, C.; Huang, P. M.; Stewart, J. W. B. Kinetics of adsorption of organic and inorganic phosphates by short-range ordered precipitate of aluminum. Can. J. Soil Sci. 1990, 70, 46l−470. (19) Shang, C.; Stewart, J. W. B.; Huang, P. M. pH effects on kinetics of adsorption of organic and inorganic phosphates by short-range ordered aluminum and iron precipitates. Geoderma 1992, 53, 1−14. (20) Guan, X. H.; Shang, C.; Zhu, J.; Chen, G. H. ATR-FTIR investigation on the complexation of myo-inositol hexaphosphate with aluminum hydroxide. J. Colloid Interface Sci. 2006, 293, 296−302. (21) Ler, A.; Stanforth, R. Evidence for surface precipitation of phosphate on goethite. Environ. Sci. Technol. 2003, 37, 2694−2700. (22) Lookman, R.; Grobet, P.; Merckx, R.; Vlassak, K. Phosphate sorption by synthetic amorphous aluminum hydroxides: A 27Al and 31P solid-state MAS NMR spectroscopy study. Eur. J. Soil Sci. 1994, 45, 37−44. (23) Lookman, R.; Grobet, P.; Merckx, R.; van Riemsdijk, W. H. Application of 31P and 27Al MAS NMR for phosphate speciation studies in soil and aluminium hydroxides: Promises and constraints. Geoderma 1997, 80, 369−388. (24) Laiti, E.; Persson, P.; Ö hman, L. Surface complexation and precipitation at the H+-orthophosphate-aged γ-Al2O3/water interface. Langmuir 1996, 12, 2969−2975. (25) Laiti, E.; Persson, P.; Ö hman, L. Balance between surface complexation and surface phase transformation at the alumina/water interface. Langmuir 1998, 14, 825−831. (26) Johnson, B. B.; Ivanov, A. V.; Antzutkin, O. N.; Forsling, W. 31P nuclear magnetic resonance study of the adsorption of phosphate and phenyl phosphates on γ-Al2O3. Langmuir 2002, 18, 1104−1111. (27) Kim, Y.; Kirkpatrick, R. An investigation of phosphate adsorbed on aluminium oxyhydroxide and oxide phases by nuclear magnetic resonance. Eur. J. Soil Sci. 2004, 55, 243−251. (28) van Emmerik, T. J.; Sandstrom, D. E.; Antzutkin, O. N.; Angove, M. J.; Johnson, B. B. 31P solid-state nuclear magnetic resonance study of the sorption of phosphate onto gibbsite and kaolinite. Langmuir 2007, 23, 3205−3213.
precipitates would make their desorption much lower than the corresponding adsorbed phase on minerals.4,49 It is suggested that the interaction between IHP and soil AAH would to some extent slow down the biogeochemical cycle of IHP and thus influence the dynamics, bioavailability, and ecological significance of inositol hexakisphosphate.
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ASSOCIATED CONTENT
S Supporting Information *
More detailed information about (1) the procedures and methods for characterization of AAH and reaction products; (2) stability constants of the complexes between IHP and Al cation; (3) Langmuir parameters for sorption isotherms of IHP on AAH; (4) the amount of Al3+ released; (5) Zeta potential variations of AAH sorbed with IHP over time; (6) XRD patterns of IHP-AAH sorption solids as a function of initial IHP loading and pH; and (7) solid-state 27Al SP/MAS NMR spectra for boehmite and α-Al2O3 sorbed with IHP. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 27 87280271; fax: +86 27 87288618; e-mail: fxh73@ mail.hzau.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 41171197, 40971142, and 30890132), and the Program for the Yangtze River Scholar and Innovative Research Team in University of China (IRT1247) for financial support of this research. We appreciate Dr. Zhengfeng Zhang, Xianfeng Yi, and Xinqi Tang (State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, China) for collecting the NMR spectra.
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REFERENCES
(1) Rajan, S. S. S. Adsorption of divalent phosphate on hydrous aluminum-oxide. Nature 1975, 253, 434−436. (2) Arai, Y.; Sparks, D. L. Phosphate reaction dynamics in soils and soil minerals: A multiscale approach. Adv. Agron. 2007, 94, 135−179. (3) Giaveno, C.; Celi, L.; Richardson, A. E.; Simpson, R. J.; Barberis, E. Interaction of phytases with minerals and availability of substrate affect the hydrolysis of inositol phosphates. Soil Biol. Biochem. 2010, 42, 491−498. (4) Ruttenberg, K. C.; Sulak, D. J. Sorption and desorption of dissolved organic phosphorus onto iron (oxyhydr)oxides in seawater. Geochim. Cosmochim. Acta 2011, 75, 4095−4112. (5) Wang, X.; Li, W.; Harrington, R.; Liu, F.; Parise, J. B.; Feng, X.; Sparks, D. L. Effect of ferrihydrite crystallite size on phosphate adsorption reactivity. Environ. Sci. Technol. 2013, 47, 10322−10331. (6) Turner, B. L.; Papházy, M. J.; Haygarth, P. M.; McKelvie, I. D. Inositol phosphates in the environment. Philos. Trans. R. Soc. London B 2002, 357, 449−469. (7) Jørgensen, C.; Jensen, H. S.; Andersen, F. Ø.; Egemose, S.; Reitzel, K. Occurrence of orthophosphate monoesters in lake sediments: Significance of myo- and scyllo-inositol hexakisphosphate. J. Environ. Monit. 2011, 13, 2328−2334. (8) Turner, B. L.; Mahieu, N.; Condron, L. M. Quantification of myo-inositol hexakisphosphate in alkaline soil extracts by solution 31P 6741
dx.doi.org/10.1021/es500996p | Environ. Sci. Technol. 2014, 48, 6735−6742
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(29) del Nero, M.; Galindo, C.; Barillon, R.; Halter, E.; Madé, B. Surface reactivity of α-Al2O3 and mechanisms of phosphate sorption: In situ ATR-FTIR spectroscopy and ζ potential studies. J. Colloid Interface Sci. 2010, 342, 437−444. (30) Li, W.; Pierre-Louis, A.; Kwon, K. D.; Kubicki, J. D.; Strongin, D. R.; Phillips, B. L. Molecular level investigations of phosphate sorption on corundum (α-Al2O3) by 31P solid state NMR, ATR-FTIR and quantum chemical calculation. Geochim. Cosmochim. Acta 2013, 107, 252−266. (31) Yan, Y. P.; Liu, F., Jr.; Li, W.; Liu, F.; Feng, X. H.; Sparks, D. L. Sorption and desorption characteristics of organic phosphates of different structures on aluminum (oxyhydr)oxides. Eur. J. Soil Sci. 2014, 65, 308−317. (32) He, Z.; Honeycutt, C. W.; Zhang, T.; Bertsch, P. M. Preparation and FT-IR characterization of metal phytate compounds. J. Environ. Qual. 2006, 35, 1319−1328. (33) Martin, M.; Celi, L.; Barberis, E. Determination of low concentrations of organic phosphorus in soil solution. Commun. Soil Sci. Plant Anal. 1999, 30, 1909−1917. (34) Murphy, J.; Riley, J. P. A modified single solution method for the determination of phosphates in natural water. Anal. Chim. Acta 1962, 27, 31−36. (35) Jia, Y.; Xu, L.; Fang, Z.; Demopoulos, G. P. Observation of surface precipitation of arsenate on ferrihydrite. Environ. Sci. Technol. 2006, 40, 3248−3253. (36) Jia, Y.; Xu, L.; Wang, X.; Demopoulos, G. P. Infrared spectroscopic and X-ray diffraction characterization of the nature of adsorbed arsenate on ferrihydrite. Geochim. Cosmochim. Acta 2007, 71, 1643−1654. (37) Nanzyo, M. Diffuse reflectance infrared spectra of phosphate sorbed on alumina gel. J. Soil Sci. 1984, 35, 63−69. (38) Li, W.; Feng, J.; Kwon, K. D.; Kubicki, J. D.; Phillips, B. L. Surface speciation of phosphate on boehmite (γ-AlOOH) determined from NMR spectroscopy. Langmuir 2010, 26, 4753−4761. (39) Li, W.; Feng, X.; Yan, Y.; Sparks, D. L.; Phillips, B. L. Solid-state NMR spectroscopic study of phosphate sorption mechanisms on aluminum (hydr)oxides. Environ. Sci. Technol. 2013, 47, 8308−8315. (40) Isobe, T.; Watanabe, T.; Caillerie, J. B.; Legrand, A. P.; Massiot, D. Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides. J. Colloid Interface Sci. 2003, 261, 320−324. (41) Li, W.; Livi, K. J. T.; Xu, W. Q.; Siebecker, M. G.; Wang, Y. J.; Phillips, B. L.; Sparks, D. L. Formation of crystalline Zn−Al layered double hydroxide precipitates on γ-alumina: The role of mineral dissolution. Environ. Sci. Technol. 2012, 46, 11670−11677. (42) Carlson, L.; Bigham, J. M.; Schwertmann, U.; Kyek, A.; Wagner, F. Scavenging of As from mine drainage by schwertmannite and ferrihydrite: A comparison with synthetic analogues. Environ. Sci. Technol. 2002, 36, 1712−1719. (43) Torres, J.; Dominguez, S.; Cerda, M. F.; Obal, G.; Mederos, A.; Irvine, R. F.; Diaz, A.; Kremer, C. Solution behaviour of myo-inositol hexakisphosphate in the presence of multivalent cations. Prediction of a neutral pentamagnesium species under cytosolic/nuclear conditions. J. Inorg. Biochem. 2005, 99, 828−840. (44) Turner, B. L.; Condron, L. M.; Richardson, S. J.; Peltzer, D. A.; Allison, V. J. Soil organic phosphorus transformations during pedogenesis. Ecosystems 2007, 10, 1166−1181. (45) Zheng, S. J. Crop production on acidic soils: Overcoming aluminium toxicity and phosphorus deficiency. Ann. Bot. 2010, 106, 183−184. (46) Tang, J.; Leung, A.; Leung, C.; Lim, B. L. Hydrolysis of precipitated phytate by three distinct families of phytases. Soil Biol. Biochem. 2006, 38, 1316−1324. (47) Giles, C. D.; Richardson, A. E.; Druschel, G. K.; Hill, J. E. Organic anion-driven solubilization of precipitated and sorbed phytate improves hydrolysis by phytases and bioavailability to Nicotiana tabacum. Soil Sci. 2012, 177, 591−598. (48) Lim, B. L.; Yeung, P.; Cheng, C. W.; Hill, J. E. Distribution and diversity of phytate-mineralizing bacteria. ISME J. 2007, 1, 321−330.
(49) Violante, A.; Gaudio, S. D.; Pigna, M.; Ricciardella, M.; Banerjee, D. Coprecipitation of arsenate with metal oxides. 2. Nature, mineralogy, and reactivity of iron(III) precipitates. Environ. Sci. Technol. 2007, 41, 8275−8280.
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