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Soil Science Society of America Journal 65:753-760 (2001)
© 2001 Soil Science Society of America

DIVISION S-2 - SOIL CHEMISTRY

Effects of pH and Electrolytes on Inositol Hexaphosphate Interaction with Goethite

L. Celi, M. Presta, F. Ajmore-Marsan and E. Barberis

Università di Torino, DIVAPRA-Chimica Agraria via Leonardo da Vinci 44, Grugliasco 10095 (TO) Italy

Corresponding author (celi{at}agraria.unito.it)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The pH and the nature of electrolytes are important factors affecting the sorption of anions by soil components. The effects of pH, K, and Ca on the interaction of inositol hexaphosphate (IHP) and inorganic phosphate (Pi) with goethite were investigated by sorption experiments. Laser Doppler Velocimetry-Photon Correlation Spectroscopy was employed to determine zeta potential ({zeta}) and particle size before and after sorption. In the presence of KCl, the amount of adsorbed P decreased, with increasing pH, from 4.5 to 0.18 µmol P m-2 for IHP, and from 2.5 to 0.67 µmol P m-2 for Pi. A more pronounced decrease was observed in the amount of IHP adsorbed compared with Pi, because of the higher negative charge of IHP and to the lower tendency of phosphate groups of IHP to neutralize the OH- released from the surface during adsorption. In CaCl2, sorption increased from 4.5 to 4.9 µmol P m-2 for IHP, and from 2.7 to 4.8 µmol P m-2 for Pi with increasing pH. For both P compounds, however, the sorption increased even beyond the maximum adsorption capacity of the mineral. At the used concentrations of anions and Ca2+ and in a pure system, the reaction with goethite may involve adsorption at low pH, but precipitation of Ca salts at pH >5 must be taken into account. The sorption of IHP caused a high change of the surface charge that became negative at all pH values in K+, causing dispersion of particles, while in Ca2+ the slightly negative phosphated surface determined particle aggregation.

Abbreviations: dz, hydrodynamic diameter • IHP, inositol hexaphosphate • Pi, inorganic phosphate • {zeta}, zeta potential


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE IMPORTANCE OF SOIL ORGANIC P in the total P cycle has been recognized, especially in soils where it represents the main reserve of P. Although several forms of organic P can be present in soils, inositol phosphates are the most abundant compounds, making up to 50% of organic P (Anderson, 1980). The preferential accumulation of these compounds has been attributed to sorption by soil colloids (Stewart and Tiessen, 1987). In acid soils the sorption of inositol phosphates was reported to be dependent on the contents of amorphous Al and Fe oxides (Anderson et al., 1974), while in neutral and basic soils this was governed by clays and organic matter (McKercher and Anderson, 1989). The adsorption extent depends on the nature and properties of the mineral surfaces, as observed by Shang et al. (1990)(1992) on short range ordered Al and Fe precipitates. We recently found that Fe oxides retained more inositol phosphates than kaolinite and illite (Celi et al., 1999). All these minerals showed a higher affinity for inositol phosphate than for Pi, as deduced by the Langmuir K values. The adsorption occurred through the phosphate groups of IHP, similarly to the free orthophosphate ion, forming a binuclear complex. The organic moiety affected the mechanism only in terms of conformational hindrance (Ognalaga et al., 1994; Celi et al., 1999).

The adsorption of P involves specific chemical bonding with Fe oxides (Torrent et al., 1990; Goldberg and Sposito, 1985; Parfitt et al., 1976), but is also affected by pH through variations induced in the charge and in the electric potential of the reacting surfaces (Barrow, 1993; Bolan et al., 1986; Barrow et al., 1980). When ions react with charged surfaces, the electric potential plays a role closely analogous to that played by energy of adsorption for reactions between molecules and uncharged surfaces (Barrow, 1993). Several authors (Barrow, 1993; Barrow and Whelan, 1989; Barrow et al., 1980; Van Olphen, 1977) have shown that the electric potential of the reacting surfaces changes with the pH in a different way, depending on the nature of the electrolyte, and that P adsorption decreases as the pH increases when solutions containing monovalent instead of divalent ions are used. Another parameter to be considered is the electrolyte concentration; on its increase, P adsorption decreases at pH lower than the point of zero charge and increases at higher pH, because of the decrease of the absolute value of the surface charge (Barrow, 1993).

In turn, anion adsorption affects the charge and electric potential of colloidal particles, hence their dispersion–flocculation behavior. This can have important consequences on soil structural stability, colloid mobility in soils, and groundwater aquifers, as well as suspended sediment behavior in surface waters (Kretzschmar et al., 1993; McCarthy and Zachara, 1989; Van Olphen, 1977).

The aim of this work was to characterize the sorption of IHP on goethite as a function of pH and the nature of electrolytes. The resulting effects on surface charge and colloidal stability of goethite following the interaction were also considered. For comparative purpose the study was also carried out with Pi.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Goethite
Goethite was prepared as described by Schwertmann and Cornell (1991), by dissolving Fe(NO3)3 · 9H2O in deionized water and 5 M KOH rapidly and with stirring. Immediately the suspension was diluted with deionized water and kept at 343 K for 60 h. The suspension was then centrifuged, washed with deionized water and then freeze-dried. X-ray diffractograms and transmission electronic micrographs were done on the synthesized oxide. The specific surface area was measured by N2 adsorption.

Phosphate
Monopotassium myo-inositol hexaphosphate and monopotassium phosphate were used in this study. Stock solutions of organic phosphates were prepared in deionized water and refrigerated. The hydrolysis of organic phosphates in stock solutions was monitored by measuring orthophosphate level in solution before each use.

Effect of pH and Electrolytes
Prior to sorption experiments, the goethite was shaken for 24 h at 298 K in KCl or CaCl2 at an ionic strength of 0.01 M. Aliquots of each suspension were then adjusted to different pH values between 3 and 10 with HCl or KOH. After 24 h of further shaking, the pH was measured and 3.5 mL of each suspension, containing 30 mg of goethite, were then added to 3.5 mL of P-containing solutions. These solutions had a selected P concentration of 1.6 mM of IHP or Pi, both normalized as P content, in order to obtain the complete saturation of the goethite surface, and they had been prepared at the same pH of the corresponding goethite suspension. The solid/liquid ratio (1:233) was chosen on the basis of the ratio used for determining P adsorption in soils (1:20) (Fox and Kamprath, 1970) and the content of goethite in soils that usually does not exceed 10% (Cornell and Schwertmann, 1996). The suspensions were shaken for 24 h at 298 K in the dark, centrifuged (1600 g for 15 min), and filtered through a 0.22-µm Millipore membrane (Millipore Corp., Bedford, MA). A preliminary experiment had shown that the equilibrium was reached in {approx}16 h. The solutions containing IHP were subjected to a H2SO4 – HClO4 digestion at 473 K for 30 min (Martin et al., 1999) in order to hydrolyze IHP to orthophosphate prior to the colorimetric determination. The amount of orthophosphate in solution at the equilibrium was determined on the filtrates as described by Ohno and Zibilske (1991).

The amount of sorbed IHP and Pi, normalized as P content (µmol P m-2), was determined by the calculation

(1)
where Co is the initial concentration and Ce the residual concentration (mol P L-1), V is the solution volume (L), m is the mass of the sorbent (g), and SSA is its specific surface (m2 g-1). All experiments were duplicated. The experimental error was estimated by Eq. [2] (Thomas et al., 1989) and was always lower than 5%.

(2)

Blank samples (without absorbents) were run for each experiment and showed that no adsorption on the polyethylene containers had occurred.

All calculations involving activities and precipitation of Pi were carried out using MINTEQA2 (Allison et al., 1991).

Electrochemical Measurements
The pH and the electrophoretic mobility were measured on the suspensions of goethite before and after P sorption. For these determinations, 50 µL of each suspension were transferred to a flask and then diluted with 4 mL of its own supernatant. The pH was measured electrometrically. The electrophoretic mobility was measured by Laser Doppler Velocimetry–Photon Correlation Spectroscopy using a DELSA 440 spectrometer (Beckman Coulter Electronics, Hialeah, FL) equipped with a 5-mW HeNe laser (632.8 nm). This technique takes advantage of the Doppler effect, where the movement of particles results in a slight shift in the frequency of scattered light. As a result, measurement of the Doppler shift for particles subjected to an electric field can be used to determine particle velocity and, from the magnitude of the applied field, electrophoretic mobility. The electrophoretic mobility data were converted to {zeta} using the Smoluchowski equation (Hunter, 1988). In an analogous fashion, the Doppler shift arising from Brownian motion can be used to calculate the average diffusion coefficient of particles, which is converted to an equivalent hydrodynamic diameter (dz) using the Stokes-Einstein equation. The apparent particle size can be used as an indication of dispersion–flocculation behavior.

All measurements were run in triplicate. Previous experiments (Ajmone-Marsan et al., 1997) have demonstrated that this procedure generates reproducible results.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The x-ray diffractograms confirmed the Fe oxide to be goethite, which appeared well crystallized and with acicular shape as observed at the transmission electronic micrograph (Fig. 1) . The specific surface area was 38 m2 g-1. Zeta potential and dz of goethite vs. pH are shown in Fig. 2 . Below pH {approx}7.5, the {zeta} curves for both K and Ca electrolytes follow similar trends. At higher pH values, K decreases {zeta} and dz more than Ca. In the presence of KCl, in fact {zeta} changed from +55 to -16 mV with a point of zero charge at pH 9.2. The apparent particle size of goethite was {approx}0.8 µm at pH 3 to 4.5, increasing to 1.5 µm at pH 5.5 to 9.5, and then returning to 0.8 µm at the highest pHs. In the presence of CaCl2, the goethite never showed a negative value of {zeta}, not even at the highest pH values. The {zeta} decrease, from +50 to +18 mV, was followed by a particle size increase from 0.8 to 1.7 µm.



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  		Fig. 1. Transmission electronic micrograph of goethite

 


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  		Fig. 2. Variation of (a) zeta potential ({zeta}) and (b) apparent particle size (dz) of goethite with pH in 0.01 M KCl and 0.0033 M CaCl2

 
In the presence of KCl, the amount of sorbed P decreased from 4.5 to 0.18 µmol P m-2 for IHP and from 2.5 to 0.67 µmol P m-2 for Pi with increasing pH from 3 to 10 (Fig. 3a) . The sorption of IHP caused a significant reduction of {zeta} that became negative (about -45 mV) at all pH values, while sorption of Pi caused a gradual pH-dependent decrease from +2 to -44 mV (Fig. 3b). The apparent particle size of goethite after IHP sorption was approximately constant for all pH values ({approx}0.8 µm) while it decreased from 1.4 to 0.8 µm upon Pi adsorption (Fig. 3c).



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  		Fig. 3. Effect of the pH on sorption of inositol hexaphosphate (IHP) and inorganic phosphate (Pi) by goethite in 0.01 M KCl: Variation of (a) the sorbed amount (Qa), (b) zeta potential ({zeta}), and (c) apparent particle size (dz) with the pH

 
When the experiments were carried out with CaCl2 the P sorption increased from 4.5 to 4.9 µmol P m-2 for IHP and from 2.7 to 4.8 µmol P m-2 for Pi with increasing pH (Fig. 4a) . After IHP sorption, {zeta} changed from -16 to -8 mV following the increasing pH, while after Pi sorption it showed values ranging between +2 and -4 mV (Fig. 4b); both sorbates caused particle size increase from the original 0.8 to 1.2 to 1.5 µm for all pH values (Fig. 4c).



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  		Fig. 4. Effect of the pH on sorption of inositol hexaphosphate (IHP) and inorganic phosphate (Pi) by goethite in 0.0033 M CaCl2: variation of (a) the sorbed amount (Qa), (b) zeta potential ({zeta}), and (c) apparent particle size (dz) with the pH

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The effect of the pH on sorption of phosphates can be firstly attributed to the variation of the surface charge of goethite. It is well-known that the surface sites, responsible for adsorption of phosphates, are differently charged because of the presence of amphoteric hydroxyl groups which can undergo reaction with either H+ or OH- depending on the pH (Fig. 2). In the presence of a monovalent electrolyte, such as K+, the excess positive charge decreased when the pH increased from pH 3 to 9, and then became negative at the highest pH values (Fig. 2a). Conversely, the presence of Ca2+ caused a surface excess positive charge at all pH values. This can be attributed to the fact that Ca2+ is more efficient than K+ to compensate the surface excess negative charge arisen by the increase in pH and has a specific affinity for the surface (Barrow et al., 1980; Kinninburgh et al., 1975).

The pH also modifies the relative concentrations of the anionic forms of Pi and IHP in solution, their average negative charge increasing with increasing pH (Fig. 5) . It is often assumed that the adsorbing ion is that which is predominant in solution at a given pH. Thus, at acid pH, H2PO-4 rather than HPO2-4 should be the main adsorbing species; however, HPO2-4, with two nucleophilic centers and the potential to act as a bidentate ligand, has been reported to express a greater affinity for goethite than H2PO2-4 (Bowden et al., 1980). It follows that the dissociation equilibrium will be displaced by the adsorption of one ion type out of proportion to its concentration in solution at a given pH. Similarly, in the case of IHP, as the adsorption on goethite occurs through its phosphate groups, C6H6-2-4, abbreviated as (IHP8-) is supposed to be the species with the greater affinity for the surface.



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  		Fig. 5. Aqueous speciation of inositol hexaphosphate (IHP) and inorganic phosphate (H2PO-4) at increasing pH

 
In the presence of KCl, the adsorption of both IHP and Pi was high at low pH.

The mechanism of IHP adsorption on goethite had been deduced by the 3:2 sorption ratio between IHP and Pi, observed at pH 4.5 and at the maximum of adsorption (3.84 µmol P m-2) (Celi et al., 1999). It provided for the formation of a monolayer with four of the six phosphate groups of IHP bound to goethite, while the other two remained free, according to the configuration proposed by Ognalaga et al. (1994) (Fig. 6) . At pH < 4.5 the adsorbed amount of IHP exceeded the value of 3.84 µmol P m-2 (Fig. 3a). This can be due to a different rearrangement of the molecule on the goethite surface, leaving more than two phosphate groups free. In the case of Pi, the amount adsorbed in the range of pH 3.0 to 5.5 corresponds, instead, to the maximum capacity of the single-coordinated surface -OH groups to form a bidentate monolayer (Goldberg and Sposito, 1985; Parfitt et al., 1976) at the (110) face of goethite equaling 2.5 µmol m-2 (Torrent et al., 1990).



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  		Fig. 6. Mechanism of adsorption of myo-inositol hexaphosphate on the goethite surface at pH 4.5. For the sake of simplicity, the H and OH of the phosphate groups were omitted (by Ognalaga et al., 1994)

 
At pH > 5.5, the adsorption of both IHP and Pi decreased because of the repulsive forces between the anions and the surface. The positive contribution due to the predominance of the IHP8- and HOPO2-3 species in this range of pH (Fig. 5) does not appear to be sufficient to overcome the repulsive coulombic forces raised by the increasing surface excess negative charge. Compared with Pi, a more pronounced decrease was observed in the amount of adsorbed IHP with increasing pH. The higher charge due to the dissociation of multiple phosphate groups of IHP (Table 1) makes it more difficult to approach the negatively charged goethite at high pH values. Moreover, according to Sigg and Stumm (1981), adsorbed proton-containing anions can act as proton donors by neutralizing hydroxyl ions released from the surface during adsorption. This deprotonation and neutralization can favor adsorption. Shang et al. (1992) reported that IHP has lower buffer capacity for OH- than Pi, according to its acid dissociation constants (pK7,8 = 6.3 and pK9–12 = 9.7). The lower tendency of phosphate groups of IHP to dissociate their second protons leads IHP to be less effective than Pi for neutralizing the hydroxyl ions released from the surface.


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  		Table 1. Dissociation acid constants of IHP (Cosgrove, 1980) and H3PO4 (Corbridge, 1985)

 
The goethite surface charge is changed upon P adsorption in different ways depending on the nature of the adsorbed anion. The adsorption of IHP caused the {zeta} to be negative at all pH values, even at the lowest ones. According to the reaction mechanism (Celi et al., 1999; Ognalaga et al., 1994), the phosphates that are not bound to the surface (Fig. 6) have each one an -OH group dissociated at pH > 1.84 (pK4–6, Table 1). The consequent negative charge caused dispersion of aggregates, whose apparent size was halved. Conversely, each Pi adsorbed on the surface leaves free only a P-OH whose pK is 12.3 (Table 1). This OH group is more acidic when Pi is bound to Fe oxide than when it is in solution, because of the strong electropositive effect of Fe (Barrow, 1985). However, at pH{approx}3 it is undissociated and makes the surface charge close to zero while it dissociates, with increasing pH accounting for the excess negative charge (Fig. 3b). This turns into flocculation of particles at pH 3 and dispersion of aggregates at the higher pH (Fig. 3c).

When Ca2+ is used as an electrolyte, the electric potential of goethite remains positive at all pH values (Fig. 2a) and favors the adsorption of anions. For both P compounds the sorption increased with increasing pH, even beyond the maximum adsorption capacity of the mineral. According to Graf (1983), IHP can complex Ca2+ and form two soluble Ca-phytate species, Ca1-IHP and Ca2-IHP, whereas the reaction with a third Ca2+ ion would result in the precipitation of Ca3-IHP. Ca-IHP precipitation (Fig. 7) has been calculated by using the apparent association constants reported in Table 2. The precipitation of Ca salts at the used concentration of IHP (0.8 mM, expressed as P moles) and Ca2+ (3.3 mM) starts at about pH 5 to 6 and accounts for the complete disappearance of IHP from the solutions at pH > 6. In the case of Pi, precipitation of CaHPO4 occurs at pH >7 [The solubility product constant (Ksp) for the reaction CaHPO4 {rightleftharpoons} Ca2+ + HPO2-4] is 0.9 x 10-6. However, even at low pH (3–5), the sorption of IHP and Pi exceeded the amount of P retained when potassium was used as an electrolyte. In soils the higher P sorption induced by Ca than would be expected from its ionic strength has been attributed to a specific effect of Ca in increasing the electric potential near the surface and thereby permitting greater sorption of anions (Pardo et al., 1992; Barrow and Whelan, 1989; Barrow et al., 1980). However, in our case the {zeta} of goethite was not affected by the nature of the electrolytes in the range of pH 3 to 6 (Fig. 2). Thus, the higher retention could be attributed to the fact that the charge at the adsorption plane are rendered progressively more negative as a result of the reaction with IHP or Pi, but this effect is compensated by the cations, with Ca2+ being more efficient than K+. Consequently, further adsorption of anions could be less restricted (Bowden et al., 1980).



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  		Fig. 7. Precipitate formation of Ca3-IHP and CaHPO4 at increasing pH. On the y axis the activity of IHP6- or H2PO-4 is reported

 

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  		Table 2. Values of the apparent association constants (K, mM-1) determined at different pH values in 50 mM pH buffer, containing 3.04 mM of IHP or 0.15 mM of IHP (pH 8.4), by potentiometric titration with 1M CaCl2 at 20°C (Graf, 1983)

 
The sorption of IHP on goethite makes the surface more negative than Pi, but the charge is not sufficient to disperse aggregates: the presence of Ca2+ caused aggregation of the P–goethite colloids.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The sorption of IHP and Pi by goethite is affected by pH and background ionic composition. In particular, by increasing the pH, the retention of IHP decreased in the presence of K+ and increased with Ca2+ in a more pronounced way compared with Pi. This behavior can affect the accumulation of this molecule in soils and, hence, hamper its degradation.

The high charge density of IHP is responsible for the negative charge that is formed on goethite surfaces after adsorption. In the presence of K+, IHP adsorption causes a net increase of the negative charge that results in the dispersion of the particles, while with Ca2+ the negative charge is not sufficient to disperse particles. These effects can have important consequences on the dispersion–flocculation behavior of colloids and, hence, on soil structural stability and colloid mobility in soils and aquifers.


   ACKNOWLEDGMENTS
 
Funding of this work from the Italian Ministry of University and Scientific Research (MURST 40 and 60%) is gratefully acknowledged.

Received for publication December 3, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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