OR I G I N A L A R T I C L E

Adsorption and precipitation of myo-inositol hexakisphosphateonto kaolinite

Zhen Hu1 | Deb P. Jaisi2 | Yupeng Yan1 | Hongfeng Chen3 | Xiaoming Wang1 |Biao Wan4 | Fan Liu1 | Wenfeng Tan1 | Qiaoyun Huang1 | Xionghan Feng1

1Key Laboratory of Arable LandConservation (Middle and Lower Reaches ofYangtze River), Ministry of Agriculture andRural Affairs, College of Resources andEnvironment, Huazhong AgriculturalUniversity, Wuhan, People's Republic ofChina2Department of Plant and Soil Sciences,University of Delaware, Newark, Delaware3College of Hydraulic and EnvironmentalEngineering, China Three GorgesUniversity, Yichang, People's Republic ofChina4School of Earth and Atmospheric Sciences,Georgia Institute of Technology, Atlanta,Georgia

CorrespondenceXionghan Feng, Key Laboratory of ArableLand Conservation (Middle and LowerReaches of Yangtze River), Ministry ofAgriculture and Rural Affairs, College ofResources and Environment, HuazhongAgricultural University, Wuhan 430070,People's Republic of China.Email: fxh73@mail.hzau.edu.cn

Funding informationFundamental Research Founds for theCentral Universities, Grant/Award Number:2662017PY070; National Natural ScienceFoundation of China, Grant/AwardNumbers: 41471194, 41603100; U.S.National Science Foundation, Grant/AwardNumber: 1709724; National ScienceFoundation; National Key Research andDevelopment Program of China, Grant/Award Number: 2017YFD0200201

AbstractSorption of myo-inositol hexakisphosphate (IHP), a common type of organic phos-

phorus in soils, largely controls its mobility and bioavailability. Research on the

interaction between IHP and phyllosilicate minerals such as kaolinite, which is com-

monly present in highly weathered soils, has often been neglected, probably due to

the common assumption that negatively charged phyllosilicate minerals have low

sorption capacity and binding affinity to IHP and thus do not play any significant role

in its fate. Here, the interaction between IHP and poorly crystallized kaolinite (KGa-

2) was investigated in batch experiments using Zeta (ζ) potential measurement and31P nuclear magnetic resonance (NMR) spectroscopy. The results showed that dis-

solved Al(III) concentration at the adsorption initiation stage increased with increas-

ing IHP concentration at pH 4.0. From pH 2.5 to 9.0, IHP presented a maximum

sorption capacity (50 μmol g−1) at pH 4.0 at 24 hr. With IHP sorption, the ζ potential

of kaolinite first decreased sharply to a negative value, then gradually increased with

resorption of Al(III) released from kaolinite dissolution at acidic pH, and finally

approached the original value of the pure kaolinite. 31P NMR spectroscopy and ζ

potential analyses revealed that IHP formed inner-sphere surface complexes and alu-

minium phytate precipitated on kaolinite at low pH (2.5 and 4.0), whereas the forma-

tion of inner-sphere surface complexes was the dominant sorption mechanism at

pH ≥ 5.5. This study implies that various mechanisms, depending on ambient pH

condition, can dominate the IHP sorption onto kaolinite, which impacts the mobility

and bioavailability of phosphorus in highly weathered soils.

Highlights

• IHP promotes the dissolution of kaolinite mainly through the formation of alu-minium phytate complex.

• IHP sorption presents a sharp maximum at pH 4.0.• IHP forms inner-sphere complexes at the surface of kaolinite.• Formation of aluminium phytate surface precipitates is favourable at relatively

low pH.

KEYWORD S31P NMR, clay minerals, dissolved Al(III), inner-sphere complexes, interfacial reaction, organic

phosphorus, Phytate (IHP)

Received: 26 May 2018 Revised: 4 June 2019 Accepted: 11 June 2019

DOI: 10.1111/ejss.12849

Eur J Soil Sci. 2019;1–10. wileyonlinelibrary.com/journal/ejss © 2019 British Society of Soil Science 1

1 | INTRODUCTION

Phosphorus (P) is an essential element for life and a majorlimiting nutrient for biological productivity in many ecosys-tems. Organic phosphorus (OPs) often accounts for as muchas 50–80% of total soil P (Turner, Papházy, Haygarth, &McKelvie, 2002) and myo-inositol hexakisphosphate (IHPor phytate) can represent up to 45% of OPs (Gerke, 2015).The preferential accumulation of IHP in soils is attributed toits strong interaction with soil minerals, especially with iron/aluminium (Fe/Al) oxides (Celi, De Luca, & Barberis, 2003;Celi, Presta, Ajmore-Marsan, & Barberis, 2001; Guan,Shang, Zhu, & Chen, 2006; Jørgensen, Turner, & Reitzel,2015; Yan et al., 2014; Yan et al., 2015), which limits IHPbiodegradation and bioavailability. However, the classicalassumption on the longer residence of IHP in soils than otherP forms has been questioned recently (Doolette, Smernik, &Dougherty, 2010; Stout, Nguyen, & Jaisi, 2016). In addition,the transformation of surface complexes into surface precipi-tates further results in the decrease in the mobility and bio-availability of IHP in soils (Gerke, 2015; Giaveno, Celi,Richardson, Simpson, & Barberis, 2010; Tang, Leung,Leung, & Lim, 2006). In terms of kinetics, the initial fastsorption reaction is followed by a diffusive penetration ofthe intrapores in particles/aggregates. That means that fastsorption characterizes the equilibrium of phosphate betweenthe solution and particle surface, and slow sorption describesthe transformation of phosphate, and this reaction causes thisvital nutrient to become unavailable for plant growth(Barrow, 1974; Barrow & Shaw, 1975). Myo-inositolhexakisphosphate generally adsorbs onto Fe and Al (hydr)oxides via phosphate groups to form outer- or inner-spherecomplexes (Celi et al., 2001; Guan et al., 2006; Ruyter-Hooley, Larsson, Johnson, Antzutkin, & Angove, 2015).Moreover, it is suggested that the possible precipitation reac-tion of IHP on mineral surfaces could occur with long-terminteraction. This means that with increasing reaction timemore IHP is adsorbed onto the mineral surface resulting inthe mineral dissolution and the formation of surface precipi-tates. For example, IHP is initially adsorbed on amorphousAl hydroxides through inner-sphere complexation via ligandexchange, followed by amorphous Al hydroxides dissolu-tion, and then the ternary complex formation, and finally,the rapid transformation from ternary complexes to surfaceprecipitates (Yan et al., 2014). With increasing reaction time,poorly crystalline calcium phytate is formed on the calcitesurface (Wan et al., 2016). A recent study applied X-rayscattering and spectroscopic techniques to monitor the pro-gressive change in the surface reaction from complexation toprecipitation during the interaction between IHP andferrihydrite (Wang et al., 2017). It was found that the pro-portion of precipitates on the surface increased with

increasing reaction time, and the overall rate of precipitationwas slower than that of surface adsorption (Wang et al.,2017). In addition, atomic force microscopy (AFM) wasused to directly assess the temporal changes in the adsorp-tion and precipitation process of IHP on brucite (Wanget al., 2016). In the initial stage, the adsorption on mineralsis very fast and is dominated by surface adsorption, and thenit gradually transitions into a slow sorption process(i.e. surface precipitation).

As mentioned above, several researchers have studiedsurface adsorption, precipitation and other mechanisms ofIHP interaction on metal (hydr)oxides (Celi et al., 2001;Guan et al., 2006; Ruyter-Hooley et al., 2015; Wan et al.,2016; Wang et al., 2016; Wang et al., 2017; Yan, Li, et al.,2014). However, IHP interaction with phyllosilicate min-erals, including reaction mechanisms, has received muchless attention. It is well known that IHP has low sorptioncapacity and binding affinity on negatively chargedphyllosilicate minerals. Particularly, 1:1 phyllosilicates havea greater adsorption capacity for IHP than 2:1 phyllosilicates(Celi, Lamacchia, Marsan, & Barberis, 1999). With soil evo-lution, the amount of 1:1 phyllosilicates, such as kaolinite,increases and could be much larger than that of Fe/Al (hydr)oxides (Anda, Shamshuddin, & Fauziah, 2015; Dixon,1989). Given that kaolinite derived from highly weatheredsoils usually has a high defect structure and poorly crystal-line morphology (Hughes, Gilkes, & Hart, 2009; Khawmee,Suddhiprakarn, Kheoruenromne, Bibi, & Singh, 2013), itcan exhibit overall high surface reactivity. Under low pHconditions in highly weathered soil, kaolinite is less nega-tively charged or could even be positively charged, whichmakes it more reactive towards P sorption. It was alsoreported that the contribution of kaolinite to soil P sorptioncould approach up to 85% in clay-rich horizons (Gérard,2016). The nature of phosphate sorbed onto kaolinite wasfound to be dominated by the presence of both amorphousand crystalline precipitate phases at low pH and high phos-phate concentrations (Van Emmerik, Sandstrom, Antzutkin,Angove, & Johnson, 2007). By comparing the sorbedamount of IHP and inorganic phosphate (Pi), the IHP sorp-tion onto the kaolinite surface was interpreted through twophosphate groups in inositol (Celi et al., 1999). Furthermore,surface complexation models for the sorption of IHP ontokaolinite indicated that the inner-sphere surface complexwas the major adsorbed species below pH 6, whereas theouter-sphere surface complex was the dominant speciesabove pH 7 (Ruyter-Hooley, Johnson, Morton, & Angove,2017). However, the effect of reaction time at various IHPconcentrations and pH ranges and the potential transforma-tion from surface adsorption of IHP to surface precipitationon phyllosilicate minerals and the underlying mechanism ofthis transformation are not well understood yet.

2 HU ET AL.

The objective of the present study is to investigate thekinetics and mechanisms of IHP adsorption and precipitationon poorly crystalline kaolinite mineral. A series of analyticaltechniques, including zeta (ζ) potential, solid-state nuclearmagnetic resonance (NMR) spectroscopy and Fourier trans-form infrared spectroscopy (FTIR), were applied to identifythe sorption products to reveal the corresponding reactionmechanisms. In this study, the kinetics of sorption, includingIHP sorption, Al(III) release and ζ potential evolution, areinvestigated to unveil the rates and corresponding pathwaysof the reaction. Furthermore, the effects of various IHP con-centrations and pH ranges are carefully analyzed becausethey are the key factors determining the sorption and specia-tion of IHP on kaolinite.

2 | MATERIALS AND METHODS

2.1 | Reagents and materials

Dipotassium myo-inositol hexakisphosphate (phytic aciddipotassium salt, K2H16(CPO4)6 or IHP) was obtained fromSigma-Aldrich (St. Louis, MO, USA). The IHP stock solu-tion (5.8 mM of IHP) was prepared in deionized water andrefrigerated. Kaolinite (KGa-2) used in this study was pur-chased from the Clay Minerals Society Source Clays Repos-itory (University of Colorado, USA) with a purity of95–100%. The cation exchange capacity (CEC) quoted bythe supplier was 3.3 cmol(+) kg−1. The organic matter in theclay was oxidized with hydrogen peroxide (H2O2) and the<2-μm size fraction was separated by wet sedimentation.The size-fractioned kaolinite was freeze-dried and then gro-und to pass through a 100-mesh sieve and stored until fur-ther analysis. The N2 adsorption–desorption (total 86 dots)experiments were conducted with Quantachrome Autosorb-1(Boynton Beach Florida, USA) after degassing at 110�C for3 hr under vacuum to measure the specific surface area. TheBET specific surface area of the freeze-dried kaolinite was19.7 m2 g−1. Aluminium phytate was prepared following theprocedures described by He, Honeycutt, Zhang, and Bertsch(2006). In brief, 30 mL of 0.3 M aluminium chloride (AlCl3)(pH 2.3) was added dropwise to 30 mL of 0.05 M dip-otassium phytate, and the final pH of the reaction mixturewas adjusted to 2.4 with 0.5 M KOH.

2.2 | Sorption kinetics of IHP at differentinitial concentrations

Duplicate batch experiments were conducted to understandthe interaction between IHP and kaolinite at different IHPconcentrations. Before starting the sorption experiments,kaolinite (0.1 g) was dispersed in 25 mL deionized water(DIW) at 25�C and pH 4.0 for 24 hr to sufficiently hydrate

the mineral. The pH was maintained at 4.0 by adding 0.1 M

nitric acid (HNO3) or sodium hydroxide (NaOH) solution,and the suspensions were purged overnight with nitrogen(N2) to remove dissolved carbon dioxide (CO2).

All sorption experiments were performed at 2 g L−1 kao-linite and at three different IHP concentrations (50, 100 and150 μM), with a final volume of 50 mL at constant pH for96 hr under stirring (400 rpm). The control experiment wasthe same but also without IHP at pH 4.0. In each kineticexperiment, 3 mL suspension was taken out at the differenttime intervals (1, 10, 30 min, 1, 2, 6, 12, 24, 48, 72 and96 hr) from a single flask. A 2-mL aliquot of the collectedsample was immediately filtered through a 0.22-μm mem-brane syringe filter to measure the dissolved concentrationsof IHP and Al. The remaining 1 mL aliquot suspension wasused to measure the ζ potential.

2.3 | Sorption kinetics of IHP at different pHs

The experimental set-up for sorption kinetic reactions withIHP at different pHs was the same as that for pH 4.0(explained above). The reaction pHs were chosen to be 2.5,5.5, 7.0 and 9.0 at IHP concentrations of 100 μM. The pHwas adjusted at regular time intervals and maintained con-stant during the experiment. For comparative purposes, theexperiment was carried out also without IHP.

2.4 | Zeta (ζ) potential measurements duringkinetics

The changes in the ζ potential of the kaolinite suspensionduring the kinetic sorption reaction at different IHP concen-trations and pHs were determined by using a MalvernZetasizer ZEN 3600 (Malvern Instruments Ltd, Malvern,U.K.). At each sampling point, 1 mL suspension was takenout and immediately used to measure the ζ potential. Eachsample was measured three times with 12–30 runs in everymeasurement.

2.5 | Solid-state NMR and FTIR spectroscopicmeasurements

A series of 100-mL kaolinite suspensions (2 g L−1 with100 μM IHP) was prepared in different flasks under identicalconditions with the experiment above for spectroscopic analy-sis. The suspensions were centrifuged, washed twice usingdeionized water at corresponding pH to remove the residualIHP, and then freeze-dried for analyses. The IHP sorbed onkaolinite (KGa-2-IHP) collected at various reaction timepoints(2, 6, 24, 48 and 96 hr) at pH 4.0 and at single timepoint(96 hr) at four other pHs (2.5, 5.5, 7.0 and 9.0) was chosenfor FTIR and solid-state NMR analyses.

HU ET AL. 3

2.5.1 | Solid-state NMR spectroscopy

Solid-state 31P single-pulse magic-angle spinning (MAS)(SP/MAS) NMR spectra of KGa-2-IHP sorption samples, stan-dards and control treatments were collected on a 500 MHzBruker AscendTM (11.7 T). The NMR operating frequencywas 202.6 MHz using a PH MASDVT 500WB BL 4 X/Y/F/Hprobe. The samples were kept in 4-mm (outer diameter) ZrO2

rotors spinning at a rate of 10 kHz. The 31P chemical shifts(δP) were reported relative to an external 85% H3PO4 standard.The other measurement parameters were: excitation 30� pulseof 5.5 μs and 30 s relaxation delay. The pulse delay was opti-mized at 5 s, and more than 300 scans were collected for eachspectrum to obtain an acceptable signal-to-noise ratio.

2.5.2 | FTIR spectroscopy

The FTIR spectra were recorded on a Bruker Vertex 70 FTIRequipped with a deuterated triglycine sulphate (DTGS)detector (Bruker Optics, Inc., Ettlingen, Germany). Pelletswere prepared by mixing 0.5 mg of the freeze-dried samplewith 200 mg of KBr (spectrometry grade). Spectra were col-lected in the spectral range from 400 to 4,000 cm−1, with anaverage of 256 scans at an instrument resolution of 4 cm−1.

2.6 | Quantitative analyses of IHP and Al(III)concentrations

The concentrations of IHP and Al(III) in the solution weremeasured at different timepoints during the reaction. The IHPwas hydrolyzed to form inorganic orthophosphate (Pi) usingthe digestion method (Martin, Celi, & Barberis, 1999) withminor modification. Briefly, a drop of saturated MgCl2 solutionwas added to 1 mL of IHP solution in a 50-mL digestion tube,and then the solution was dried at 290�C. The residue wasreacted with 1 mL concentrated H2SO4 (18 M) and 0.5 mL ofconcentrated HClO4 (12 M) and digested at 320�C for 30 minuntil the yellow colour disappeared. The digestion reaction wasstopped after the evaporation of HClO4 was complete. Aftercooling, the solution was adjusted to neutral pH with NaOH.Then the concentration of Pi was measured by using the phos-phomolybdate blue method (Murphy & Riley, 1962). The per-centage recovery of IHP in control samples (100 μM) withoutkaolinite at different pH values ranged from 98% to 102% witha coefficient of variation less than 5%. Once the IHP concentra-tion was quantified, the amount of sorbed IHP (Qa) (μmol g−1)was calculated by using the following equation:

Qa = Co−Ceð Þ×V=m,

where Co is the initial concentration (μmol L−1), Ce is theresidual concentration (μmol L−1), V is the solution volume(L) and m is the mass of sorbent (g).

The dissolved Al(III) concentration in the supernatantswas quantified using a NexION 350X ICP-MS (PerkinElmer,Shelton, CT).

3 | RESULTS

3.1 | Sorption kinetics of IHP at differentinitial IHP concentrations

The sorption kinetics of IHP on kaolinite at pH 4.0 with dif-ferent initial concentrations (50, 100 and 150 μM IHP) arepresented in Figure 1a. During the course of the reaction, IHPwas almost completely sorbed by kaolinite at 50 and 100 μMIHP, and the time for sorption equilibria extended withincreasing initial IHP concentration. When the IHP increasedto 150 μM, IHP persistently sorbed throughout the experiment.This trend suggested that it might involve kaolinite dissolutionand Al-IHP precipitation, rather than pure surface adsorption(Celi et al., 1999; Celi et al., 2003; Ruyter-Hooley et al.,2017; Ruyter-Hooley, Morton, Johnson, & Angove, 2016).

Figure 1b presents Al(III) dissolution kinetics in theabsence and presence of IHP. In the absence of IHP, the dis-solved Al(III) gradually increased over time, with the concen-tration reaching 236.1 μM at 96 hr. With increasingconcentration of IHP, the trend of Al(III) dissolution was dif-ferent. For example, in the presence of 50 μM IHP, the dis-solved Al(III) first decreased to zero before 10 min, thengradually increased over time but its amount was smaller thanthat in pure kaolinite. At the IHP concentration of 100 and150 μM, the dissolved Al(III) concentration was higher in thepresence of IHP than in the absence of IHP at the initial reac-tion stage (within 0.5–1 hr) (Figure S1 in File S1). At the IHPconcentration of 100 μM, the concentration of dissolved Al(III)in the supernatant first increased before 10 min, then decreasedrapidly and approached zero during the first 24 hr, and after-wards it steadily increased until the end of the experiment. Incontrast, at the IHP concentration of 150 μM, the concentrationof dissolved Al(III) in the supernatant first increased before30 min, then gradually decreased to near zero.

3.2 | Surface charge of the sorption kinetics ofkaolinite

The ζ potential dynamics of kaolinite at various initial IHPconcentrations (0, 50, 100 and 150 μM) at pH 4.0 are pro-vided in Figure 2. In the absence of IHP, the ζ potentialincreased slightly from +32.2 to +45.9 mV from 1 min to96 hr of the experiment at pH 4.0 after the 24 hr pre-equilib-rium. With the sorption of IHP, the ζ potentials sharplydecreased to a negative value at the beginning, and thenincreased to positive values during the remaining time attwo IHP concentrations (50 and 100 μM IHP), and finally

4 HU ET AL.

became close to the ζ potential of pure kaolinite suspension.The trend of the change in the ζ potential of kaolinite(increasing from −45.9 mV to −38.9 mV) at 150 μM IHPwas smaller than that at 50 and 100 μM IHP during 96 hrreaction.

3.3 | 31P NMR spectroscopy of experimentalproducts at pH 4.0

To examine the effect of time on IHP sorption, 31P SP/MASspectra of IHP-sorbed kaolinite (Figure 3) were collected atpH 4.0 at different time intervals (2–96 hr). The reference for

NMR analysis included phytic acid dipotassium salt and poorlycrystalline aluminium phytate. The NMR spectrum demon-strated that IHP showed a chemical shift at δP–31 = −0.5 ppm,and that the poorly crystalline aluminium phytate yielded a

FIGURE 2 Zeta potential variations of kaolinite (2 g L−1) overtime as a function of initial myo-inositol hexakisphosphate (IHP)concentrations (0, 50, 100 and 150 μM) at pH 4.0. Each data pointrepresents the mean of two replicate experiments, with standarddeviation shown by error bars

FIGURE 3 Solid-state 31P single-pulse (SP/MAS) nuclearmagnetic resonance (NMR) spectra of myo-inositol hexakisphosphate(IHP) sorbed on kaolinite at pH 4.0 at selected timepoints. IHP andaluminium phytate (Al-IHP) were used as references. In SP/magic-angle spinning (MAS) experiments, the spinning rate was kept at10 kHz with pulse delay of 30 s

FIGURE 1 (a) Sorption kinetics of myo-inositol hexakisphosphate (IHP) on kaolinite (2 g L−1) and (b) change in concentrations of dissolvedAl(III) over time at pH 4.0 at initial IHP concentrations of 0, 50, 100 and 150 μM. Each data point represents the mean of two replicate experiments,with standard deviation shown by error bars

HU ET AL. 5

broad peak with two main chemical shifts (δP–31 = −11.2 and− 6.4 ppm) and a small shoulder peak at −0.5 ppm (Figure 3).The chemical shifts at δP–31 = −0.5 ppm and δP–31 = −6.4 ppm were assigned to surface-sorbed IHP species asinner-sphere surface complexes. Also, there was a small contri-bution to free or physically adsorbed IHP at δP–31 = −0.5 ppm,and the chemical shift at δP–31 = −11.2 ppm was attributed toaluminium phytate precipitates (Yan, Li, et al., 2014). The 31PNMR spectrum of IHP-sorbed kaolinite at 2 hr at pH 4.0showed a broad chemical shift peak at δP–31 = −6.4 ppm,whereas that of the 6-hr product showed two obvious chemicalshifts at δP–31 = −6.4 ppm and − 11.2 ppm. The intensityratio of the chemical shift at δP–31 = −11.2 ppm to δP–31 = −6.4 ppm only slightly increased over the reaction timeafter 6 hr. This result suggested that surface adsorption and sur-face precipitation may occur simultaneously at the initial stageof the reaction, and the longer reaction time might lead toslightly increased aluminium phytate precipitation.

3.4 | Kinetics of IHP sorption at different pHs

The results of the IHP sorption kinetics on kaolinite with arange of pH (2.5–9.0) at an initial IHP concentration of100 μM showed that the fastest sorption reaction occurred atpH 4.0 with the sorption capacity of 50 μmol g−1 at 24 hr(Figure 4). At pH 2.5, IHP sorption steadily increased until72 hr. As a result, almost all IHP was removed from the solu-tion. Possible reasons for this result are discussed below.Within the intermediate pH range (from 5.5 to 9.0), the amountof IHP sorbed on kaolinite decreased with increasing pH.

The ζ potential of the control (kaolinite only) experimentafter the 24-hr pre-equilibrium was measured during thereaction to examine the change in surface charge(Figure S2a in File S1). At a specific and constant pH valuefrom 1 to 96 hr, the ζ potential increased from +31.7 mV to+45.9 mV at pH 4.0 and from +1.4 mV to +10.2 mV atpH 5.5, respectively. However, there was no significantchange of ζ potential of kaolinite at pH 2.5, 7.0 and 9.0within 96 hr. The comparison of the ζ potential of controlkaolinite (Figure S2a in File S1) and IHP-sorbed kaolinite(Figure S2b in File S1) at various pHs revealed that the sorp-tion of IHP led to a sharp decrease in surface charge(to negative) early in the reaction. At pH 4.0, the ζ potentialof IHP-sorbed kaolinite was stable around −40.0 mV duringthe first 12 hr, and afterwards sharply increased to+14.8 mV until 48 hr, and then slowly increased to+29.6 mV from 48 to 96 hr. The same trend was alsoobserved in the experiment at pH 2.5, but the increase in ζpotential was relatively slow, compared to that at pH 4.0. AtpH 5.5, the ζ potential of kaolinite gradually increased from−54.7 to −48.3 mV over time, whereas at pH 7.0 and 9.0,

the ζ potential fluctuated within a very narrow range (from−58 to −61 mV).

Figure S3 in File S1 shows the 31P NMR spectra of IHPsorption onto kaolinite at different pHs after 96 hr. AtpH 2.5, the NMR spectrum displayed two chemical shifts atδP–31 = −11.2 and − 6.4 ppm. At pH 5.5, the 31P NMRspectrum of IHP-sorbed kaolinite presented a major peakwith a chemical shift at δP–31 = −6.4 ppm and a small shoul-der at δP–31 = −11.2 ppm. At pH 7.0 or 9.0, the 31P NMRspectra of IHP-sorbed kaolinite showed only one major peakaround −6.4 or − 0.5 ppm, respectively.

3.5 | FTIR spectroscopic analysis

The FTIR spectra of the IHP sorbed on the kaolinite surfaceas a function of time and after 96 hr at various pHs areshown in Figure S4 in File S1. The peak positions observedin FTIR spectra and their corresponding functional groupsare listed in Table S1 in File S1. Because of the low amountof sorbed IHP on kaolinite, no adsorption bands of IHP weredetected and no obvious change in peak position wasobserved. This result was similar to that of FTIR spectra ofhumic acid sorbed onto kaolinite reported in a previousstudy (Chen, Koopal, Xiong, Avena, & Tan, 2017).

4 | DISCUSSION

4.1 | IHP-induced dissolution of kaolinite andIHP sorption process at pH 4.0

Kaolinite, as a 1:1-type phyllosilicate mineral, possesses alayer structure with alternate silica tetrahedral and alumin-ium octahedral layers. Kaolinite partially dissolves in acidic

FIGURE 4 Sorption kinetics of myo-inositol hexakisphosphate(IHP, 100 μM) on kaolinite (2 g L−1) under various pH (2.5, 4.0, 5.5,7.0 and 9.0). Each data point represents the mean of two replicateexperiments, with standard deviation shown by error bars

6 HU ET AL.

aqueous solution and releases Al3+ (Khawmee et al., 2013).The dissolution of kaolinite in the presence of ligands can beinterpreted as surface complexation and exchange reactionsat mineral surfaces (Huertas, Chou, & Wollast, 1998; Wie-land & Stumm, 1992). Previous studies showed that theorganic ligands markedly increased the rate of kaolinite dis-solution (Cama & Ganor, 2006; Chin & Mills, 1991; Wang,Li, Hu, Zhang, & Zhou, 2005; Wieland & Stumm, 1992).Similarly, IHP has multiple phosphate groups, which havestrong chelation with Al3+ (Yan, Li, et al., 2014). The stabil-ity constants of Al3+ and IHP complexes are described inthe following reactions (Torres et al., 2005).

Al3+ +H2L10− Ð Al H2Lð Þð Þ7− log K=23:7 ð1Þ

Al3+ +H3L9− Ð Al H3Lð Þð Þ6− log K=20:1 ð2Þ

Al3+ +H4L8− Ð Al H4Lð Þð Þ5− log K=16:4 ð3Þ

Al3+ +H5L7− Ð Al H5Lð Þð Þ4− log K=12:2 ð4Þ

Al3+ +H6L6− Ð Al H6Lð Þð Þ3− log K=8:48 ð5Þ

The Al(III) concentration in solution of kaolinite at pH 4.0is around 5.0 × 10−5 M in the first minute (Figure 1b). Themajor species of IHP (H12L) at pH 4.0 is H6L

6− (Johnson,Quill, & Angove, 2012). According to the equilibriumEquation (5), the molar ratio of (Al(H6L))

3−/ H6L6− was

5.0 × 103.48, which means that such complexes are the pre-dominant forms of IHP in solution. The Al(III) concentrationincreased with increasing IHP concentration at the initialstage, and was higher than that in pure kaolinite solution whenthe initial IHP concentration was 100 or 150 μM (Figures 1band S1 in File S1), which might be attributed to the fact thatthe excess IHP in solution was combined with Al(III), pro-moting Al(III) dissolution from the kaolinite surface(Figures S1 and S5 in File S1). Similar outcomes of IHP-promoted dissolution of metal (hydr)oxides such as zinc oxideand amorphous aluminium hydroxides have been reported inprevious studies (Feng et al., 2016; Yan, Li, et al., 2014).

The ζ potential is the bulk measure of the sum of positiveand negative charges on the particle surface (Ler &Stanforth, 2003). With sorption of IHP ions at pH 4.0, itsvalue initially dropped from positive to negative (Figure 2)as a result of the accumulation of a negative charge withinthe shear plane, which changes the point of zero charge ofkaolinite and is characteristic of inner-sphere adsorption(Arai & Sparks, 2007). When the IHP concentration was50 μM the concentration of dissolved Al(III) first decreasedto zero and then increased, but it was lower than that in purekaolinite solution, and the ζ potential sharply increased

within the first 2 hr (Figure 2 and Figure S1 in File S1). Thisindicated that the phosphate groups of the sorbed IHP gradu-ally bound with the dissolved Al(III) cations resulting in neu-tralization of negative charges from the surface (Ler &Stanforth, 2003). At the IHP concentration of 100 μM, theconcentration of dissolved Al(III) in the supernatant firstincreased due to complexation of Al(III) with the remainedIHP in solution, then decreased rapidly due to the sorption ofdissolved aluminium phytate complexes. The further uptakeof aluminium phytate complexes onto kaolinite finally led tothe formation of aluminium phytate precipitates, which wassupported by the chemical shift at δP–31 = −11.2 ppm in theNMR spectra (Figure 3). After a 12-hr reaction, IHP wascompletely sorbed with a very small amount of dissolvedAl(III) remaining in the solution (Figures 1b and S2). Subse-quently, the ζ potential increased sharply, implying that sur-face complexes were rapidly transformed into surfaceprecipitates after further binding with dissolved Al(III). Atthe IHP concentration of 150 μM, the concentration of dis-solved Al(III) in the supernatant was higher at first comparedto that of the experiment with IHP concentration of 100 μM,then continuously decreased due to the sorption of dissolvedaluminium forming phytate complexes. These results collec-tively indicated that a fraction of IHP was sorbed on the sur-face of kaolinite via inner-sphere surface complexes at theinitial stage of the reaction, while sorbed IHP on kaoliniteand free IHP in the solution could complex with Al(III),which in turn promoted Al(III) dissolution from the kaolinitesurface. After the complete sorption of IHP (for initial IHPconcentration of 50 and 100 μM), the sorbed IHP on the kao-linite surface continued to complex with Al(III) ions causingthe ζ potential of kaolinite to increase sharply, and graduallysurface complexes transformed into surface precipitates. Thetrend of change of kaolinite ζ potential under different initialconcentrations of IHP was similar to that of goethite at vari-ous Pi concentrations (Ler & Stanforth, 2003).

4.2 | Sorption mechanisms of IHP on kaoliniteat different pHs

The maximum amount of sorption of IHP onto kaolinite wasobserved at pH 4.0 at 24 hr, but the sorption occurred withinthe range of pH from 2.5 to 9.0 (Figure 4). At low pH such as2.5 or 4.0, the IHP interface behaviour with kaolinite was pre-dominantly impacted by the sorption of dissolved Al(III) andthe formation of aluminium phytate precipitates (Figure S3 inFile S1). The major species of IHP (H12L) was H6L

6− atpH 4.0, whereas it was H7L

5− at pH 2.5 (Johnson et al.,2012). The highly deprotonated H6L

6− species was found tohave a higher affinity to the metal cation and oxide surfacethan H7L

5−. Thus, at relatively low pH (pH ≤4.0), Al3+

released from kaolinite (Figure 5a,b) formed aluminium

HU ET AL. 7

phytate precipitates, causing higher sorption of IHP at pH 4.0than at pH 2.5. At relatively high pHs (pH > 4.0), IHP sorp-tion became less favourable because IHP was highlydeprotonated. At pH 5.5, the formation of inner-sphere sur-face complexes was the dominant mechanism with minimalsurface precipitate (Figure S3 in File S1). When pH increasedto 7.0 or 9.0, the 31P NMR spectra of IHP-sorbed kaoliniteshowed the major chemical shift was a gradual drift towardsthe left (i.e. to the low-field region). A similar trend was alsoobserved in the sorption of phosphate on aluminium (hydr)oxides at various pHs (Li, Feng, Yan, Sparks, & Phillips,2013). Our data on a chemical shift suggested the formationof inner-sphere complexes at pH 7.0 and 9.0. The gradualchange in the chemical shift with pH increasing from 5.5 to9.0 might be attributed to the decreased number of P-O-Allinkages and the decreased phosphate protonation as pHincreased (Kim & Kirkpatrick, 2004; Li et al., 2013).

Although limited Al of kaolinite still dissolves at pH 9.0(Figure 5c) without IHP, dissolved Al(III) mainly existed asAl(OH)4

− species (Lützenkirchen et al., 2014). At pH 9.0,IHP was sorbed onto the kaolinite surface primarily via theformation of inner-sphere complexes, which was supportedby the results of NMR (Figure S3 in File S1) and ζ potential(Figure S2b in File S1). Because of the low sorption capacityat high pH, the concentration of IHP in solution increased.Meanwhile, the highly deprotonated IHP (such as H2L

10−) isthe main IHP species in solution and is readily complexedwith Al3+, AlOH2+ or Al(OH)2

+, which can promote therelease of additional Al(III) from kaolinite in the presence ofIHP (Figure 5c). However, the main species of Al(III) atpH 9.0 was Al(OH)4

−, and a much lower amount of Al(III)dissolved due to the lower solubility of kaolinite at pH 9.0(Figure 5c), which did not facilitate the formation of alumin-ium phytate.

FIGURE 5 Kinetics of kaolinite (2 g L−1) reaction with or without myo-inositol hexakisphosphate (IHP) (100 μM) at pH 2.5 (a), pH 4.0(b) and pH 9.0 (c) as a function of time. The sorbed IHP concentration is shown in black squares (left axis) and aluminium dissolution concentrationis shown in blue circles and triangles (right axis). Each value represents the mean of two replicate experiments, with standard deviation shown byerror bars

8 HU ET AL.

5 | CONCLUSIONS ANDIMPLICATIONS

The dissolved Al(III) concentration in the adsorption initia-tion stage increases with increasing IHP concentration atpH 4.0. Myo-inositol hexakisphosphate readily forms com-plexes with dissolved Al(III) in solution, which can promotethe further release of Al(III) from kaolinite. The releasedAl(III) combines with sorbed or free IHP, which can furtherpromote the sorption of IHP on kaolinite under weak acidiccondition. Aluminium phytate precipitates can be formedwithin the pH range of 2.5 to 5.5, with pH 4.0 being themost favourable condition for the precipitation. Adsorptionof IHP onto kaolinite and the formation of surface precipi-tates essentially limit the release of sorbed IHP from thekaolinite surface. At relatively high pH, the formation ofinner-sphere complexes becomes the dominant sorptionmechanism.

The results presented in this study indicate that in addi-tion to the formation of inner-sphere complexes on kaolinite,IHP can precipitate as aluminium phytate under acidic con-ditions. In highly weathered soils in tropical and subtropicalregions, kaolinite is the dominant phyllosilicate mineral,compared to other (2:1-type) phyllosilicate minerals. Theadsorption and precipitation reaction of IHP on kaoliniteindicate that, apart from iron and aluminium oxides, 1:1-typephyllosilicate minerals such as poorly crystalline kaolinitemay strongly retain IHP, playing an important role in regu-lating phosphorus mobility and bioavailability in soils.

ACKNOWLEDGEMENTS

This research is supported by the National Key Research andDevelopment Program of China (No. 2017YFD0200201), theNational Natural Science Foundation of China (Grant Nos.41471194 and 41603100) and the Fundamental ResearchFunds for the Central Universities (No. 2662017PY070) toX. H. Feng and U.S. National Science Foundation(No. 1709724) to D. P. Jaisi. The authors report no conflict ofinterests.

DATA ACCESSIBILITY

The data that support the findings of this study are availablefrom the corresponding author upon reasonable request.

ORCID

Yupeng Yan https://orcid.org/0000-0001-7965-6173Qiaoyun Huang https://orcid.org/0000-0002-2733-8066

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SUPPORTING INFORMATION

Additional supporting information may be found online inthe Supporting Information section at the end of this article.

How to cite this article: Hu Z, Jaisi DP, Yan Y,et al. Adsorption and precipitation of myo-inositolhexakisphosphate onto kaolinite. Eur J Soil Sci. 2019;1–10. https://doi.org/10.1111/ejss.12849

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