Surface adsorption and precipitation of inositol hexakisphosphate on calcite: A comparison with orthophosphate

Chemical Geology 421 (2016) 103–111

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Surface adsorption and precipitation of inositol hexakisphosphate on calcite: A comparison with orthophosphate Biao Wan, Yupeng Yan, Fan Liu, Wenfeng Tan, Xiuhua Chen, Xionghan Feng ⁎ 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

a r t i c l e

i n f o

Article history: Received 24 July 2015 Received in revised form 4 December 2015 Accepted 11 December 2015 Available online 12 December 2015 Keywords: Myo-inositol hexakisphosphate Orthophosphate Calcite Sorption Surface precipitation

a b s t r a c t Unraveling the processes of adsorption–precipitation of organic phosphates (OPs) on calcite is important for understanding the transformation, mobility and bioavailability of phosphorus (P) in calcareous soils. To elucidate these processes, the interaction between myo-inositol hexakisphosphate (IHP) and calcite was studied using macroscopic sorption experiments and a variety of analytical approaches. The experiments were also conducted with orthophosphate (Pi) for comparison. Calcite presented a similar sorption capability for IHP and Pi through the rapid formation of surface precipitates based on sorption experiments and the results of Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and solidstate 31P nuclear magnetic resonance (NMR) spectroscopy. After interacting with IHP/Pi, two different types of precipitates on a calcite surface could be directly observed by SEM images and analyzed by energy dispersive spectroscopy (EDS): a sphere-shaped precipitate considered typical for the poorly crystallized calcium phytate and a plate-shaped precipitate considered typical for the crystalline hydroxylapatite. This study suggests that active calcite strongly influences the species and behavior of IHP via a rapid surface precipitation reaction in a natural environment and advances our knowledge for predicting the fate of dissolved OP species in a variety of calcareous soils and geological settings enriched with calcite. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Phosphorus (P) has long been recognized as an essential nutrient element for biological growth, as a major factor contributing to eutrophication, and as ‘the disappearing nutrient’ (Gilbert, 2009). Considerable research has been devoted to defining the adsorption and precipitation of P on the surfaces of soils and soil mineral components, which control the mobility, transformation, and availability of P to plants (Arai and Sparks, 2007; Giaveno et al., 2010; Neupane et al., 2014; Wang et al., 2013; Yan et al., 2014b). Calcareous soils are always considered to have a great phosphateimmobilization capacity (Wang et al., 2012). Calcite is widely distributed in calcareous soils and a variety of geological settings. P could adsorb on and desorb from calcite surfaces (Suzuki et al., 1986), thereby influencing the fate of P in alkaline soils. To interpret these chemical processes that occur on the surface of calcareous soils, it is necessary to distinguish between the different forms of this nutrient partition on the calcite surfaces, particularly including P in its surface-adsorbed form and P as mineral-precipitated phases (Tunesi et al., 1999). The influence of soluble Ca2 + in inducing P precipitation has usually been compared with P adsorption on calcite surfaces of calcareous soils ⁎ Corresponding author. E-mail address: [email protected] (X. Feng).

http://dx.doi.org/10.1016/j.chemgeo.2015.12.004 0009-2541/© 2015 Elsevier B.V. All rights reserved.

(Tunesi et al., 1999). Thus, the interaction of phosphate (Pi) on calcite surfaces involved adsorption at low concentrations of Pi and precipitation of calcium phosphate at high concentrations of Pi (Cole et al., 1953; Freeman and Rowell, 1981; Griffin and Jurinak, 1973). In addition, the interactions involving Pi on calcite have been studied extensively, even at the near-molecular level through in situ atomic force microscopy (AFM) (Klasa et al., 2013; Wang et al., 2012). However, little attention has been paid to the behavior of organic phosphate (OPs) on the surface of the calcite, which commonly occurs in calcareous soils (Celi et al., 2000; Kan et al., 2005). Myo-inositol hexakisphosphate (IHP), one of the most abundant OPs (IHP can make up 50% of the OPs), is an important P source in the environment and often appears in lake sediments (Jørgensen et al., 2011), pasture soils (Turner et al., 2002, 2012), and riparian soils (Young et al., 2013). The strong interaction of IHP with the surface of metal oxides in acidic soils and the surface of calcium carbonate in alkaline soils might be an important cause of its accumulation in nature (Celi et al., 2000; Yan et al., 2014a,b, 2015). IHP is generally bound to iron and aluminum (oxyhydr)oxides via the formation of outer- and inner-sphere complexes (Celi et al., 2001; Guan et al., 2006; Ruyter-Hooley et al., 2015; Yan et al., 2014a,b, 2015), which are affected by electrolytes, the solution pH (Celi et al., 2001), and the type of aluminum (oxyhydr)oxide (Yan et al., 2014b). Our previous studies have also indicated that the IHP surface complexes on amorphous aluminum

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hydroxide and nano-γ-Al2O3 might transform rapidly into surface precipitates under acidic and neutral conditions based on spectroscopic evidence (Yan et al., 2014a, 2015). The transformation of surface complexation to surface precipitation will obviously decrease the solubility and mobility of IHP, which has an inevitable impact on the availability of phytase (Giaveno et al., 2010; Giles et al., 2012; Tang et al., 2006) and the activity of the phytate-mineralizing bacteria (Lim et al., 2007). The adsorption and precipitation of IHP on Fe/Al (oxyhydr)oxides and clay minerals has also been widely researched, but the sorption mechanism for IHP on alkaline soils relative to several important environmental issues has hardly been addressed, mainly because the macroscopic sorption of IHP on minerals provides limited information for understanding the possible precipitation in the interfacial reactions between IHP and minerals (Celi et al., 2000). Thus, the objectives of this study were to investigate and compare the sorption mechanism of IHP and Pi on calcite surfaces by macroscopic sorption experiments, Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. The reaction time and IHP/Pi loading were considered key factors for discussing the transformation of IHP and Pi on calcite surfaces and for understanding the immobilization and sequestration of organic and inorganic phosphates in the environment. 2. Materials and methods 2.1. Materials and reagents Dipotassium myo-inositol hexaphosphate (K2H16(CPO4)6 or IHP, purity greater than 95%) was obtained from Sigma-Aldrich (P5681-5G). Calcite (BET surface area of 0.32 ± 0.03 m2 g− 1) was obtained from Sinopharm Chemical Reagent Co., Ltd., and contained no impurity phases as detected by powder X-ray diffraction (XRD) analysis (Fig. S1A). Ultrapure water (ρ N 18 MΩ·cm) was used to prepare all of the solutions and suspensions used in this study. Calcium phytate and hydroxylapatite were synthesized according to the method of He et al. (2006). To prepare calcium phytate and hydroxylapatite, 50 mL of 0.01 mol L− 1 CaCl2 (pH 2.0) was added to an equal volume of 0.0017 mol L − 1 K2H16(CPO4)6 (pH 2.0) or 0.01 mol L−1 KH2PO4 (pH 2.0) and adjusted to the final pH of 8.5 using 0.5 mol L−1 KOH. 2.2. IHP sorption kinetics Sorption kinetics experiments were conducted with an automatic titrator (Metrohm 907 Titrando, Switzerland). The reaction temperature was maintained at 25 ± 0.2 °C by circulating water through the jacket with a water circulator. Mineral suspensions were equilibrated at pH 8.5 in 0.1 mol L−1 KCl under an atmosphere of N2 for 24 h. The particle concentration of calcite was 1 g L−1 to keep the particle concentration similar to the highest possible total P concentration in the solution. The kinetics experiments were started by adding 100 ml of IHP/Pi solution at pH 8.5 in 0.1 mol L− 1 KCl to the 100 mL calcite suspension (pH 8.5, 2.0 g L−1). The reaction pH was stabilized at 8.5 by adding a known amount of 0.5 mol L−1 HCl and 0.5 mol L−1 KOH. At each reaction time, a 5-mL suspension was filtered through a 0.22-μm Millipore membrane to analyze the equilibrium IHP/Pi and Ca2+ concentration to calculate the sorbed amount of IHP/Pi and, subsequently, a concentration of Ca2+ dissolved from calcite. Each experiment of IHP/Pi sorption kinetics was performed in duplicate. IHP was hydrolyzed to Pi by digestion with concentrated sulfuric and perchloric acids (Martin et al., 1999) and was measured by the phosphate molybdate blue colorimetric method (Murphy and Riley, 1962). The dissolved Ca2+ concentration in the supernatant was quantified using atomic absorption spectrometry (Varian, 240FS). In addition, calcite samples (1.0 g L−1 or 10 mmol L−1) reacted with IHP for various IHP loadings and Pi for the same total P concentration as

IHP at pH 8.5, and equilibration time (6 h, 1 d, 3 d and 5 d) were selectively prepared for FTIR, powder XRD and SEM analysis. The total concentration of P was 1.67, 3.33, 5.0, and 10.0 mmol L−1, where for IHP, the total P concentration of 1.67, 3.33, 5.0, and 10.0 mmol L− 1 refers to the IHP concentration of 0.278, 0.555, 0.833, and 1.667 mmol L−1 because there are six P molecules per IHP molecule. Prior to FTIR, XRD, SEM, and NMR analyses, the selected calcite suspensions mentioned above were centrifuged (at 16,000 g for 15 min), washed twice using P-free 0.1 mol L− 1 KCl at corresponding pH to remove the residual IHP/Pi and dried at room temperature to prepare the solid samples. These air-dried samples were directly and uniformly used for FTIR, XRD, and SEM analyses because the surface speciation likely changes under other rigorous conditions (e.g., drying using an oven at high temperature or freeze-drying). The calcite-only sample sorbed with IHP at pH 8.5 at the different times (6 h, 1 d, 3 d and 5 d) was prepared for solid-state 31P single-pulse magic-angle spinning (SP/MAS) NMR (air-dried) analysis. 2.3. Characterization of calcite and reaction products The FTIR spectra were recorded on a Bruker Vertex 70 FTIR spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector (Bruker Optics, Inc., Ettlingen, Germany). Pellets were prepared by mixing 0.5 mg of the sample with 200 mg of KBr (spectrometry grade). Spectra were collected in the spectral range extending from 800 to 1600 cm−1 for an average of 512 scans at an instrument resolution of 4 cm−1. Bands between 800 and 1600 cm−1 originated mainly from the stretching vibrations of the phosphate group and carbonate group. Powder XRD patterns were obtained on a Bruker D8 Advance diffractometer (Bruker AXS Gmbh, Karlsruhe, Germany) equipped with a LynxEye detector using Ni-filtered Cu Kα radiation (λ = 0.15418 nm). The diffractometer was operated at a tube voltage of 40 kV and a current of 40 mA with a scanning rate of 4° min−1 and at a step size of 0.02°. The morphology and surface element composition analyses of reaction products were conducted using an electron microscope, operating at an accelerating voltage of 15 kV for photomicrographs with an Energy Dispersion X-ray (EDS) (Japanese Netherlands FESEM Quanta SU8010). The reaction products were initially placed in a vacuum chamber for coating with a thin layer (a few nanometers) of gold (Au). Solid-state 31P SP/MAS NMR spectra of calcite-IHP sorption products and standard samples (air-dried solids) were collected on a 400 MHz NMR spectrometer (9.4 T) (Bruker Advance III 400 M, Switzerland). The 31P SP/MAS NMR spectra were collected at the operating frequencies of 161.8 MHz using a 5-mm PABBO BB-1 H/D Z-GRD probe at a spinning rate of 12 kHz. High-power decoupling was employed during acquisition. The 31P chemical shifts (δP) are reported relative to an external 85% H3PO4 solution. The 31P SP/MAS spectra were obtained with a 90° excitation pulse of 2 μs, with a 2-s relaxation delay. The pulse delay was optimized at 5 s to obtain an optimal signal-to-noise ratio. 3. Results 3.1. Sorption kinetics The sorption kinetics of IHP/Pi onto the calcite surface are shown in Fig. 1, and dissolution kinetics of calcite in the absence and presence of the IHP/Pi system are presented in Fig. S2. These figures were used to illustrate the influence of IHP/Pi on the dissolution and transformation of calcite at pH 8.5. Fig. 1A and B shows that the total P concentration of IHP/Pi in the calcite suspension decreased gradually and reached equilibrium within 5 d. In the presence of IHP/Pi at a Ca/P molar ratio of 1, the total P concentration of the reaction equilibrium was approximately 6300 μmol L−1 (1050 μmol L−1 of IHP) for IHP and 6000 μmol L−1 for Pi sorbed onto calcite after sorption for 5 d. Meanwhile, the percentage of

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Fig. 1. Concentration of sorbed P for IHP (A) and for Pi (B) as a function of time with different Ca/P molar ratios (IHP and Pi, total concentration of P was 1.67–10 mmol L−1) for 5 d. Error bars are standard deviations.

total P sorbed for IHP and for Pi as a function of time with different Ca/P molar ratios for 5 d was shown in Fig. S5. The concentration of Ca2+ released from the dissolution of calcite decreased gradually and stopped at 75 μmol L−1 for IHP and 35 μmol L−1 for Pi, which were obviously lower (by approximately 410 μmol L−1) than the concentration of calcite without adding IHP/Pi (Fig. S1 A and B). It is suspected that the decreased concentration of both P and Ca2+ resulted from the occurrence of Ca2+ precipitation with IHP/Pi. In addition, the concentration of dissolved Ca2+ decreased with the increasing concentration of Pi in solution over 5 d of reaction time, whereas the dissolved Ca2+ increased with the increasing IHP loading. The possible reason for this difference was that Pi decreased the Ca2 + concentration by the formation of precipitates with Ca2 +, whereas IHP could form complexes with Ca 2 + that were soluble before the insoluble precipitates were formed (Yan et al., 2014a, 2015). However, another difference was encountered in the presence of IHP when the Ca/P molar ratio was 6:1. At this molar ratio, the concentration of dissolved Ca2 + gradually increased over time after 12 h, possibly due to the percentage of total P (for IHP) sorbed on calcite being close to 100% when the Ca/P molar ratio was 6:1 (Fig. S5). The calcite, which remained, could have continuously dissolved to increase the concentration of Ca2 + in solution after IHP was completely sequestrated and precipitated, considering that dissolution of pure calcite at pH 8.5 potentially releases approximately 400 μmol/L Ca2 +.

3.2. FTIR spectra Normalized FTIR spectral analysis of the solids of IHP/Pi-sorption calcite at different reaction times (6 h–5 d) with a Ca/P molar ratio of 1 and the reacted samples with various Ca/P (IHP) and Ca/P (Pi) molar ratios (6:6, 6:3, 6:2, and 6:1) are presented in Figs. 2 and 3, respectively, which compare the 1600–800 cm− 1 regions of the sample spectra with the spectra of the initial calcite and standard references of calcium phytate and hydroxylapatite. The FTIR spectrum of calcite showed a broad absorbance band at 1422 cm−1 (v3) and a sharp absorbance band at 875 cm−1 (v2) due to the C–O stretching vibrations of CO2− (Figs. 2 and 3) (Celi et al., 2000; 3 Gunasekaran et al., 2006). The IR bands of the calcite samples reacted with various amounts of IHP at 5 d and at different reaction times with a Ca/P molar ratio of 1 showed bands at 1126, 993, 901 and 847 cm− 1, consistent with the spectrum of calcium phytate (Fig. 2) (Ganesan and Epple, 2008; He et al., 2006). A broad band at approximately 1126 cm−1 could be assigned to the P–O stretching vibration of the P–O–Ca coordination on calcium phytate precipitates (Ganesan and Epple, 2008; He et al., 2006; Yan et al., 2014a). The bands at 993, 901, and 847 cm−1 were assigned to the C–O–P vibrations from calcium phytate (Ganesan and Epple, 2008; He et al., 2006). The FTIR spectrum of calcite-IHP at 6 h showed two slightly new absorbance bands at approximately 1126 and 993 cm−1. The intensity

Fig. 2. Normalized FTIR spectra of calcite (A) in the presence of IHP (total concentration of P was 10 mmol L−1) as a function of time and (B) in the presence of IHP with different Ca/P molar ratios (total concentration of P was 1.67–10 mmol L−1) after 5 d of sorption in 0.1 mol L−1 KCl at pH 8.5. Calcium phytate was used as a reference material.

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Fig. 3. Normalized FTIR spectra (A) of calcite in the presence of Pi (total concentration of P was 10 mmol L−1) as a function of time and (B) in the presence of Pi with different Ca/P molar ratios (total concentration of P was 1.67–10 mmol L−1) after 5 d of sorption in 0.1 mol L−1 KCl at pH 8.5. Hydroxylapatite was used as a reference material.

of the absorbance band at 1126 and 993 cm−1 was enhanced, and the appearance of the new absorbance bands at 901 and 847 cm− 1 was gradually obvious over time. The spectrum of calcite reacted with IHP at 5 d ultimately approached that of the calcium phytate reference (Fig. 2). However, bands at 1422 cm−1 and 875 cm−1 were still very obvious due to the v3(C–O) and v2(C–O) stretching vibrations of the carbonate group, indicating that there are still some calcites remaining in the reaction products. In addition, the FTIR spectra of the reacted products with different initial IHP loadings (total concentration of P was 1.67–10 mmol L−1) are presented in Fig. 2B. With an increasing initial IHP loading, the original absorbance bands at 1422 and 875 cm− 1 remained unchanged while the relative intensity of the new absorbance bands at 1126, 993, 901 and 847 cm−1 continuously increased (Fig. 2B). This finding suggests that the calcium phytate-like surface precipitate was formed on all calcite samples and that the higher concentration of added IHP facilitated the formation of a calcium phytate-like precipitate. Furthermore, the FTIR bands, observed in the calcite-Pi samples at different reaction times with a Ca/P molar ratio of 1 and reacted with various amounts of Pi at 5 d, showed bands at 1112, 1026, and 961 cm−1, consistent with the spectrum of hydroxylapatite (Fig. 3) (He et al., 2006). The FTIR spectrum of the hydroxylapatite reference shows characteristic bands at approximately 961 cm−1 (v1), and 1026 and 1112 cm−1 (v3) due to the P–O stretching vibrations of the phosphate group (Ślósarczyk et al., 2005). The intensity of the absorbance bands at 1112, 1026 and 961 cm− 1 were generally enhanced over time, and the spectrum of calcite-Pi at 1 d became very close to the spectrum of the hydroxylapatite reference (Fig. 3A). With increasing initial Pi loading (total concentration of P at 1.67–10 mmol L−1) and reaction time, the original absorbance bands at 1442 and 875 cm−1 gradually disappeared while the relative intensity of the new absorbance bands at 1112, 1026 and 961 cm−1 constantly increased (Fig. 3B), indicating that the sorption products were similar to the hydroxylapatite reference and that calcite was completely transformed into hydroxylapatite.

the peak intensities decreased gradually with increasing the Ca/P (IHP) molar ratio (Fig. 4B). The XRD pattern of the calcium phytate reference showed a characteristic hump of amorphous phase appearing at approximately 2θ = 28° (Fig. 4C), and no discernible peaks of crystalline materials were observed. Additionally, literature data does not exist for crystalline calcium phytate, and only X-ray amorphous phases were reported (Cheang et al., 2010; Ganesan and Epple, 2008). Thus, it is suggested that the reaction sample of calcite consisted of calcite and the poorly crystallized precipitates of calcium phytate, and that the XRD peak intensities decreased mainly due to the increasing components of calcium phytate in the reaction samples. In addition, considering the small amount of information provided by powder XRD patterns, SEM, EDS, and solid-state NMR techniques were used to further study the existence of poorly crystallized calcium phytate shown in Figs. 6 and 7. X-ray powder diffraction of calcite-sorbed Pi displayed the four peak positions of hydroxylapatite (JCPDS number: 09-0432): (002) (26°, 2θ), (102, 210) (28°, 2θ), (211, 112, 300) (31°, 2θ), and (202) (34°, 2θ) (Figs. S1B and 5C). XRD spectra of the reaction samples with a Ca/P (Pi) molar ratio of 1 displayed the same peak positions as calcite, but the peak intensities gradually weakened over 1 d (Fig. 5A). After a single day of reaction time, seven sharp diffraction peaks of the initial calcite, well-defined in the XRD pattern, completely disappeared instead of appearance of hydroxylapatite diffraction peaks (Fig. 5), demonstrating that Pi-sorbed calcite was finally converted into a crystalline hydroxylapatite-like phase. This result was consistent with the result of the FTIR spectra, where the carbonate group absorbance bands at 1442 and 875 cm−1 gradually disappeared over time (Fig. 3A). The XRD spectra with Ca/P (IHP) molar ratios of 6, 3, 2 and 1 at pH 8.5 indicated that the transformation of calcite into hydroxylapatite became more pronounced with the increasing concentration of Pi (Fig. 5B), consistent with the results of the FTIR spectrum (Fig. 3B). 3.4. SEM morphology

3.3. XRD patterns The XRD pattern of calcite reacted with water exhibited seven sharp diffraction peaks at (012) (23°, 2θ), (104) (29°, 2θ), (110) (36°, 2θ), (113) (36°, 2θ), (202) (44°, 2θ), (024) (48°, 2θ) and (116) (49°, 2θ) (Fig. S1B), consistent with all characteristic peaks as defined by the standard JCPDS number 47-1743, and presented no impurity phases. XRD patterns of calcite sorbed with IHP (total concentration of P at 10 mmol L−1) as a function of time displayed the same peak positions as calcite reacted with water, but the peak intensities decreased with increasing reaction time (Fig. 4A). Similarly, the XRD spectra of the reaction samples with a Ca/P (IHP) molar ratio of 6, 3, 2, and 1 showed that

Because FTIR spectra and XRD patterns suggested the presence of some different type of IHP/Pi precipitate on the calcite surface, SEM and EDS analyses were used to examine the morphology and elemental distribution of selected samples. Fig. 6 (the high magnification images) and Figs. S3/S4 (the low magnification images) show the SEM images of calcite reacted with water for 5 d (Fig. 6A), sorbed with IHP for 1 d and 5 d (Fig. 6B and C), and sorbed with Pi for 1 d and 5 d (Fig. 6D and E) in 0.1 mol L− 1 KCl solution at pH 8.5, and EDS analysis of the selected regions (Fig. 6F). As shown in Figs. 6A and S3A, the morphology of calcite reacted with water was characteristic as rhombohedral calcite with a smoothly cleaved surface (Wang et al., 2012). Two different

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Fig. 4. XRD patterns of calcite (A) reacted with IHP (total concentration of P was 10 mmol L−1) as a function of time and (B) in the presence of IHP with different Ca/P molar ratios (total concentration of P was 1.67–10 mmol L−1) after 5 d of sorption in 0.1 mol L−1 KCl at pH 8.5; XRD patterns (C) of calcium phytate. Calcite and calcium phytate were used as reference materials.

types of precipitate illustrated in the low magnification image (Figs. S2/ S3) were observed: sphere-shaped precipitate (Fig. S3B and C), which was previously reported as typical for the poorly crystalline calcium phytate (Ganesan and Epple, 2008), and plate-shaped precipitate (Fig. S3D and E), which was viewed as typical for the crystalline hydroxylapatite (Yoshimura et al., 2004). However, the SEM image shown in Fig. S3 was not a representative image because these precipitates were small. The high magnification SEM images (Fig. 6B, C, D and E) and EDS spectra of the selected regions (Fig. 6F) revealed that the particles in regions I and II with regular shapes were composed of C, O, P and Ca (Fig. 6F), and thus, two types of precipitate, rich in P, supported the morphological

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Fig. 5. XRD patterns of calcite (A) reacted with Pi (total concentration of P was 10 mmol L−1) as a function of time and (B) in the presence of Pi with different Ca/P molar ratios (total concentration of P was 1.67–10 mmol L−1) after 5 d of sorption in 0.1 mol L−1 KCl at pH 8.5; XRD patterns (C) of hydroxylapatite. Calcite (C) and hydroxylapatite (H) were used as reference materials.

identification of the two phases. In the presence of a Ca/P (IHP) molar ratio of 1, SEM images showed that the morphology of the calcium phytate precipitate was sphere-shaped at approximately 50 nm and that the size was very uniform on the calcite surface (Fig. 6C and D). After a single day's reaction time, a small number of sphere-shaped calcium phytate nucleates were found on the calcite surface, resulting in a rough surface instead of a smoothly cleaved surface (Fig. 6A and B). A large amount of sphere-shaped precipitate covered the calcite (Fig. 6C) and changed the rhombohedral morphology of calcite after 5 d of reaction (Fig. S3C). In the presence of a Ca/P (Pi) molar ratio of 1, partial conversion of calcite crystals to plate-like hydroxylapatite crystals on the calcite surface that still

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Fig. 6. SEM images of calcite reacted with water over 5 d (A), reacted with IHP (total concentration of P was 10 mmol L−1) for 1 d (B), and 5 d (C), and reacted with Pi (total concentration of P was also 10 mmol L−1) for 1 d (D) and 5 d (E) in 0.1 mol L−1 KCl at pH 8.5; EDS analysis (F) of the selected regions.

showed a cubic habit was observed for calcite sorbed Pi at 1 d (Figs. 6D and S3D), and calcite crystals sorbed with Pi for 5 d were completely converted to hydroxylapatite crystals (Figs. 6E and S3E), consistent with the phase identification by analyses of the XRD patterns and FTIR spectrum (Figs. 3 and 5). 3.5. 31P NMR spectra NMR spectra can provide molecular-level insights into the adsorption and precipitation of this environmentally important P source on calcite. Because Pi was widely investigated with regard to the extent of dissolution and precipitation on calcite surfaces through a variety of spectroscopic measurements (Mason et al., 2007), such as in situ AFM (Klasa et al., 2013; Wang et al., 2012), and surface complexation modeling (Sø et al., 2011), solid state 31P SP/MAS NMR spectra of only IHP sorbed on calcite, shown in Fig. 7, were collected as a function of time (6 h–5 d, pH 8.5) in 0.1 mol L−1 KCl solution to explain the mechanism of the OP reaction at the calcite surface. The non-reacted and solid state IHP displayed an NMR signal at δP-31 = −0.5 ppm, whereas the poorly

crystalline calcium phytate reference generated a broad peak at approximately 0.64 ppm assigned to the calcium phytate precipitate (He et al., 2007; Yan et al., 2014a). The 31P NMR spectrum of calcite sorbed with ~ 1500 μmol IHP yielded a broad peak (δP-31 = 0.33 ppm), which was different from the chemical shift of IHP at δP-31 = −0.5 ppm and was assigned to the precipitate formation of calcium phytate (He et al., 2007). This result indicated that the interaction of IHP with calcite within 6 h may involve the processes of rapid surface precipitate formation, while surface complexes and surface precipitates may coexist in the reaction products at the beginning of the reaction (Celi et al., 2000). In addition, the 31P NMR spectrum of calcite absorbing IHP after 1, 3 and 5 d presented a main peak at approximately 0.64 ppm with sites basically in line with the peak site of the calcium phytate reference, suggesting that the longer sorption periods might lead to a large amount of IHP precipitate to form calcium phytate (He et al., 2007). The change of NMR peaks suggested that rapid precipitation was the dominant mechanism for sorption of IHP on calcite and that calcium phytate precipitates were a main component of the final reaction products.

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Fig. 7. Solid state 31P SP/MAS NMR spectra (A) of IHP reacted with calcite (total concentration of P was 10 mmol L−1) as a function of time in 0.1 mol L−1 KCl at pH 8.5; (B) represents a portion of (A) with between the chemical shift range of +30 to −30 ppm; IHP and calcium phytate were used as reference materials. For SP/MAS experiments, the spinning rate is 12 kHz; pulse delays are 5 s for 31P NMR. The NMR data of non-reacted IHP was provided by Yan et al. (2014a).

4. Discussion A large number of studies have shown that the processes of Pi adsorption and precipitation on calcite could be described via the dissolution equilibrium calculation (Klasa et al., 2013; Liu et al., 2012; Wang et al., 2012). The IHP fixation on calcite can be thus controlled by the speciation of IHP in the solution as well as the dissolution of calcite. When IHP (Phy) or calcite exists in water, the chemical species can be described through the dissolution-precipitation equilibrium reactions occurring in the system (Supplementary Information SI) (Crea et al., 2008; Liu et al., 2012). Complex formation constants for the different IHP–Ca2 + complexes via interactions between IHP and Ca2 + are described in the following reactions (Crea et al., 2006). Ca2þ þ 3Hþ þ Phy

12−

7−

↔CaH3 Phy

log K ¼ 33:09

ð1Þ

Ca2þ þ 4Hþ þ Phy12− ↔CaH 4 Phy6− log K ¼ 40:1

ð2Þ

Ca2þ þ 5Hþ þ Phy12− ↔CaH 5 Phy5− log K ¼ 46:77

ð3Þ

2Ca2þ þ 3H þ þ Phy12− ↔Ca2 H 3 Phy5− log K ¼ 36:14

ð4Þ

2Ca2þ þ 4H þ þ Phy12− ↔Ca2 H 4 Phy4− log K ¼ 43:28

ð5Þ

109

2Ca2þ þ 5Hþ þ Phy12− ↔Ca2 H 5 Phy3− log K ¼ 48:75

ð6Þ

3Ca2þ þ 2Hþ þ Phy12− ↔Ca3 H 2 Phy4− log K ¼ 30:97

ð7Þ

3Ca2þ þ 3Hþ þ Phy12− ↔Ca3 H 3 Phy3− log K ¼ 39:08

ð8Þ

3Ca2þ þ 4Hþ þ Phy12− ↔Ca3 H 4 Phy2− log K ¼ 45:39

ð9Þ

3Ca2þ þ 5Hþ þ Phy12− ↔Ca3 H 5 Phy− log K ¼ 50:97:

ð10Þ

K presents formation constants of IHP-Ca2+ complexes in aqueous KCl (0.25 mol L−1) media, at t = 25 °C (Crea et al., 2006). Ion pair formation constants of IHP species in solution are illustrated in SI via reactions (11)–(17). In the presence of excess phytate, formation of soluble complexes were predominant; however, when the metal ion was the component in excess, sparingly soluble species tended to form (Crea et al., 2008). According to the equilibrium constants in SI equations (11)–(17), Ca2+ and CO2− were the predominant forms in the calcite 3 system at pH 8.5. The main species of IHP in solution were H2Phy10−, H3Phy9− and H4Phy8− at this pH (De Stefano et al., 2003; Johnson et al., 2012). As a result of the initial excess IHP concentration, the dissolved and existing Ca2+ formed the soluble Ca–IHP complexes on the surface of the calcite. With the continuous dissolution of calcite in the presence of IHP solutions providing a reliable source of Ca2+ ions, the Ca–IHP complexes further coordinated with aqueous Ca2+. This resulted in nucleation and growth of the calcium phytate precipitate phases on the dissolving calcite surface due to the multiple phosphate groups and strong chelating ability of IHP. Despite the assumption of Martin and Evans (1986) that precipitation occurred only if one phosphate group was transformed into the oxo-dianion form, some authors have also reported the formation of the precipitate with formula Ca5H2Phy (Graf, 1983). The precipitation of Ca salts at the concentration of IHP (0.8 mol L−1, expressed as P mol) and Ca2+ (3.3 mmol L−1) started at approximately pH 5.0 to 6.0 and accounted for the complete disappearance of IHP from the solutions at pH N 6.0 (Graf, 1983). Moreover, based on the report of Crea et al. (2004), the sparingly soluble species formed as a function of both the pH and CCa/CPhy molar ratios were Ca6Phy, Ca5.5HPhy, Ca5.25H1.5Phy, Ca5H2Phy, and Ca4.75H2.5Phy, and the formation constant and enthalpy for Ca6Phy were reported as log K = 58.3 ± 0.4 and ΔH = 7.6 kJ mol−1, respectively (Crea et al., 2004). Thus, for the sake of simplicity, this study assumed that the formation constant of the calcium phytate precipitates might be in the range of 50.97 to 58.3. On the basis of the available equilibrium constants (1)–(10), the transformation of IHP from complexation into precipitation on calcite could be thermodynamically favorable as a result of Ca2+ and IHP concentrations high enough to reach oversaturation of the corresponding precipitate (Table S1) during the initial reaction stage. It is possible that the complexation–precipitation reaction between IHP and aqueous Ca2+, released from the calcite, proceeded continuously until the formation of calcium phytate precipitate consumed large amounts of IHP and Ca2+ in the solution. Meanwhile, multiple processes, such as adsorption, surface precipitation and precipitation through oversaturation of dissolved Ca and P (Table S1), should be considered together to account for removal of IHP from the solution even though the contribution of each process cannot be quantitatively addressed with the current data. Thus, the further investigations of organic P sorption on calcite are needed in the future to elucidate the dynamic processes of their interaction and the operating mechanism with in situ techniques, such as atomic force microscopy (AFM) and quick-scanning X-ray absorption spectroscopy (Q-XAS) (Wang et al., 2012; Siebecker et al., 2014). In addition, the molar ratio (3:5) of Pi and calcite consumed in the above reaction supports the stoichiometry of hydroxylapatite [Ca10(PO4)6(OH)2], also in agreement with the results of XRD and FTIR analyses. The formation of hydroxylapatite in this study was based on a dissolution–precipitation mechanism where Ca2 + was dissolved

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from the calcite crystal surface, reacted with the phosphate ions from solution, then precipitated as Ca–P nuclei and grew as hydroxylapatite crystals on active surface sites of calcite crystals (Wang et al., 2012; Yoshimura et al., 2004). Clearly, the rate of IHP precipitation on calcite has been shown to be very similar to the rate of Pi precipitation (Fig. 1). However, when the Ca/P molar ratio was 1, the normalized intensity of P–O stretching vibrations generated by the calcium phytate was clearly lower than that of hydroxylapatite after 6 h of reaction (Figs. 2A and 3A). And the amount of the sphere-shaped calcium phytate phase is also smaller than the amount of plate-like hydroxylapatite crystals on the calcite surface after 1 day of reaction (Fig. 6B and E). These results suggested that the transformation mechanism of IHP at the surface of calcite was different from that of Pi, and specifically, before the formation of calcium phytate precipitate, IHP tends to experience the process of complexing with dissolved Ca2+ ions, forming a ternary complex at the surface of calcite (Yan et al., 2014a). Additionally, the P/Ca ratios in the EDS of the selected region (Fig. 6F) were estimated to be 1:1.4 and 1.77:1 for Pi and IHP, respectively, indicating that the P/Ca ratio of the calcium phytate phase was much higher than the P/Ca ratio of the hydroxylapatite phase (Li et al., 2012). This result suggests that calcite has a greater ability to fix IHP than to fix Pi, considering the fact that each phosphate group of Pi molecules has more oxo-dianion than the IHP molecule. Finally, considering the strong chelating ability of the six phosphate groups in each IHP molecule, the IHP surface complexes can easily transform into surface precipitates such as Pi and other OPs (e.g., aminoalkylphosphonates) (Kan et al., 2005; Wang et al., 2012). The aluminum phosphate surface precipitate could be formed on amorphous aluminum hydroxide after 10 d of reaction with 33 mmol L− 1 Pi at 60 °C under acidic conditions (Lookman et al., 1994). Additionally, the aluminum phytate precipitate might be formed on amorphous aluminum hydroxide with 2.2 mmol L− 1 IHP for 4 d at pH 5.0 (Yan et al., 2014a). However, our results from the experimental work showed that calcite could act as a quickly responding source-to-sink for IHP under alkaline conditions. Interaction between IHP/Pi and calcite might decrease the release of the aqueous P from the calcite surface, and sorption periods relatively longer than 6 h may lead to the precipitation of calcium phytate and hydroxylapatite minerals or to incorporation of P in the calcite lattice during recrystallization. Thus, the great P-fixing capacity of calcite, which might affect its accumulation in the irreversible precipitate forms, would make desorption of P much lower than the corresponding sorbed phase on metal oxides and clay minerals (Yan et al., 2014a). The interaction between P (OPs and Pi) and calcareous soils might cause the soil biogeochemical processes of the P cycle to decline through the nucleation and growth of Ca–P, despite the possible influence of other sorbing components, and has a significant impact on the bioavailability and ecological significance of P resources.

5. Conclusions Calcite showed a great capacity to fix IHP and Pi. In terms of the same P concentration, this study showed that a similar amount was sorbed by calcite in either the organic form or the inorganic form through surface P precipitates at pH 8.5. The interaction of IHP/Pi with calcite involved dissolution of calcite and re-precipitation of dissolved Ca2 + when IHP/Pi was added. The FTIR, XRD, SEM and NMR results indicated that two different types of precipitates could rapidly be formed on the smooth cleavage surface of rhombohedral calcite after interacting with IHP/Pi: a sphere-shaped precipitate for the poorly crystalline calcium phytate and a plate-shaped precipitate for the crystalline hydroxylapatite. These results may have implications for understanding the interaction between dissolved OP species and calcite and predicting the resulting products in a variety of environments associated with calcite and other carbonate minerals.

Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 41171197, 41271474), and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB15020402) for financial support of this research. We gratefully acknowledge the Editor-in-Chief Dr. Jeremy Fein and the anonymous reviewers for their constructive comments on the manuscript.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.chemgeo.2015.12.004.

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