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DIVISION S-2-SOIL CHEMISTRY

Magnesium- and Silicon-Induced Phosphate Desorption in Smectite–, Palygorskite–, and Sepiolite–Calcite Systems

^a Saskatchewan Centre for Soil Research, Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada

mermut{at}sask.usask.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Carbonates are one of the most important group of minerals that fix phosphates in soils. Magnesium and silicon are known to reduce the P fixation of calcite. The effect of Mg and Si addition on P sorption–desorption of a mixture of 90% clay and 10% calcite was studied. The mixtures were montmorillonite–calcite (Mont–CaCO₃), palygorskite–calcite (Pal–CaCO₃), and sepiolite–calcite (Sep–CaCO₃). Concentrations of 0.0, 5.0, and 10.0 mg L^-1 of dissolved Mg or Si were introduced into P sorbing systems. The chemical speciation of P sorption solutions was predicted using the SOILCHEM computer program. Desorption of sorbed P on different samples was studied in successive extractions with 0.01 M KCl. The P sorption of Mont–CaCO₃, Pal–CaCO₃, and CaCO₃ were similar (10 cmol kg^-1). Addition of 10.0 mg L^-1 Mg or Si sharply reduced the P sorption of calcite to <1 cmol kg^-1, while it induced only a 20% reduction in P sorption of the mixtures. Magnesium and silicon treatments increased the desorption of sorbed P. The Sep–CaCO₃ mixture showed a P sorption 60% lower than that of pure CaCO₃. The desorption of sorbed P by Sep–CaCO₃ was far above that of other clay–CaCO₃ combinations. Addition of Mg and Si inhibited the formation of Ca–phosphates, which would otherwise make major contributions to P retention in all samples used in this study. Phosphate sorbed by silicate clays was the second major fraction of retained P, limiting the effects of Mg and Si treatments. As a slow release source of Mg and Si, sepiolite increased the desorption of P sorbed by CaCO₃. This has an important implication for P availability in arid soils containing sepiolite. This mineral may be mixed thoroughly with commercial P fertilizers before P application into soil.

Abbreviations: BET, Brunauer-Emmett-Teller • EC, electrical conductivity • Mont, montmorillonite • Pal, palygorskite • Sep, sepiolite

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
CALCITE (CaCO₃) is common in arid and semiarid soils world-wide. In the absence of Fe– and Al–oxides, CaCO₃ is considered to be the most effective mineral for fixing phosphate in soils. The presence of this mineral induces the precipitation of Ca–phosphate compounds and consequently decreases the availability of P in calcareous soils. It is well established that dissolved Mg inhibits the P fixing capacity of CaCO₃ (Bischoff, 1968; Martens and Harriss, 1970; Ferguson and McCarty, 1971; Ferguson et al., 1973; Marion and Babcock, 1977; Kuo and Mikkelsen, 1979; Yadav et al., 1984; Yadav and Paliwal, 1988); however, quantitative information about the effects of Mg on the P sorption of CaCO₃ in the presence of silicate clay minerals is lacking.

Sorption is the process by which atoms, molecules, or ions are taken up and retained on the surfaces of solids by chemical or physical binding. Many researchers have shown that silicate strongly competes with phosphate for sorption sites on different soil components (Obihara and Russell, 1972; Alvarez et al., 1980; Smyth and Sanchez, 1980; Ryden et al., 1987; Pardo and Guadalix, 1990); however, information on the effect of dissolved Si on sorption of P by calcite is limited. Soil solutions usually contain notable levels of Mg and Si ions that may interfere with phosphate retention by calcite. The interference is expected to be more pronounced in soils of arid regions with Mg–silicate clays such as palygorskite and sepiolite that provide high amounts of Mg and Si ions to the soil solution (Shadfan et al., 1985; Singer, 1984 and 1989).

In this work, the phosphate sorption–desorption behavior of silicate clay–calcite systems was examined in the presence of different levels of Mg and Si ions in order to determine the influence of these ions, released from silicate clay minerals, on P sorption by calcite. The work is part of an attempt to improve management of P in soils of arid regions that contain large amounts of CaCO₃.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Materials
The CaCO₃ used in this study was a Fisher Laboratory (Pittsburgh, PA) sample (Lot # 793932) and the silicate clay minerals were montmorillonite (STx-1) and palygorskite (PFl-1), both of which were obtained from Source Clay Mineral Repository (Univ. of Missouri, Columbia), and sepiolite from Eskishehir, Turkey. The phyllosilicate minerals were treated with pH buffered Na–acetate, 30% H₂O₂, and sodium dithionite bicarbonate, as suggested by Jackson (1979). Particles <2µm were separated by centrifugation and then freeze-dried. The minerals were homoionized with 0.5 M CaCl₂ and the excess salts were washed until an electrical conductivity (EC) level of 30 µS m^-1 was reached. Oriented samples on glass slides were examined with x-ray diffraction, which confirmed that they contained only montmorillonite, sepiolite, or palygorskite. A detailed description of the clay mineral characterization can be found in Shariatmadari (1998). The Brunauer-Emmett-Teller (BET) surface area of all minerals and the cation-exchange capacity of the silicate clay samples were then determined (Table 1) .

Mineral Suspensions
A 3.75-g sample (calcite or clay–calcite mixtures) was suspended in 80 mL deionized water in a 100-mL volumetric flask at room temperature for 48 h. The pH of the suspension was adjusted to 7.4 at 12, 24, and 48 h using 0.05 M HCl with a Mettler DL21 automated titrator (Mettler-Toledo International, Greifensee, Switzerland.). The titration curve of sepiolite, palygorskite and kinetic studies indicates little change in surface charge between pH 7 and 8 (Shariatmadari, 1998). After addition of electrolytes the initial pH of the systems were near 7.4 and, therefore, this pH level was chosen to conduct the experiments. Before the P sorption, the volume of each suspension was increased to 150 mL, while its EC and pH were adjusted to 2 dS m^-1 and 7.4, respectively. A 0.05 M KCl solution was used for EC adjustment. To suppress microbial activity in the suspensions, chloroform was added in the ratio of 1 mL to 1 L of suspension.

Phosphate Sorption and Desorption
Fifteen milliliters of a solution containing known concentrations of P, Mg, and Si, adjusted to pH 7.4 and EC of 2 dS m^-1, was added to duplicate 10-mL samples of each mineral suspension. The solid/solution ratio was 1/100. The suspension was shaken for 24 h at 24 ± 1°C in a tared 50-mL Nalgene centrifuge tube. The P solution concentrations were adjusted to 100% of P sorption capacity of each mineral suspension, based on previously established sorption isoterms (Shariatmadari, 1998). Duplicate control samples from each suspension were prepared in the same way, without added P. The final concentration of Mg and Si in the mineral suspensions were 0.0, 5.0, and 10.0 mg L^-1. Treatments containing 5.0 mg L^-1 of both Mg and Si were also included in the experiment. After 24 h, each suspension was centrifuged for 15 min at 20 000 g to obtain a clear solution for P measurements. Phosphorus was determined by the ascorbic acid method (Murphy and Riley, 1962), with the absorbance read at 720 nm on a Beckman Model DU spectrophotometer (Beckman Coulter, Fullerton, CA). Microbial interference was minimized by using a few drops of chloroform.

For the P desorption studies, 25 mL of the 0.01 M KCl solution adjusted to pH 7.4 and EC of 2 dS m^-1, with the 0.0, 5.0 and 10 10.0 mg L^-1 Mg and Si treatments, was added to the tubes containing 0.25 g of clay samples enriched with phosphate. The tubes were capped, shaken vigorously by multivortex to bring the clay into suspension, and then placed on a rotary shaker and shaken at 24 ± 1°C. After 24 h, the samples were centrifuged for 15 min at 20 000 g, and the clear supernatant was removed for P measurement. Then the tubes with the sample residue were weighed, 25 mL of the respective extractant was added, and the samples were vortexed and placed on the rotary shaker. This procedure was repeated for a total of seven extractions.

The tubes containing the samples were weighed after each desorption to determine the amount of P that was carried over to the next desorption. This amount was then subtracted from the subsequently desorbed P. The P concentrations in successive volumes were measured and the total P desorbed from the sample was calculated by summing (Raven and Hossner, 1993). The amount of phosphate desorbed from the samples was corrected for that amount from the control sample which received no phosphate (or Mg, Si) treatment.

To assess the reversibility of sorbed P, the total desorbed P(Q) is plotted against the equilibrium P concentration (I) in the solution. To describe the Q/I relationship, a desorption isotherm suggested by Raven and Hossner (1993) as

(1)

was fitted to the experimental data. The a, b, and c were estimated using nonlinear regression procedures (SAS, 1985).

Chemical Analyses and SOILCHEM Assessments
The chemistry of the P sorption solution for selected samples was studied by elemental analyses (not reported here). In addition to P concentration measurement, total soluble Si was determined by colorimetry (Prince, 1965); Al, Ca, and Mg by atomic absorption; and K and Na by atomic emission spectrophotometery, using a Perkin Elmer (Norwalk, CT) 3100 atomic absorption spectrophotometer. Carbonate and chloride concentrations were determined by acid digestion (Tiessen et al., 1983) and colorimetry (Fuge, 1976), respectively.

The cations, anions, pH, and the ionic strength, as well as some information about the sorbents involved in the reaction (cation-exchange capacity, specific surface area, and sorption site density) were entered in the SOILCHEM computer program for a comprehensive speciation of the factors contributing to P sorption (Sposito and Coves, 1991).

Ion-speciation models were used to estimate the forms and concentrations of chemical species from measured total concentrations of all the components and appropriate thermodynamic equilibrium constants (Sposito, 1981; Allison et al., 1991; Kalbasi et al., 1995). In this study SOILCHEM was applied to predict the fate of phosphate introduced to different systems.

The program was run for specified values of pH and ionic strength. The ionic strength (I_e) was estimated with the equation (Marion and Babcock, 1976)

(2)

in units of mol L^-1. Both precipitation and adsorption were allowed; no redox reactions and mixed solids were considered. Metals in the solution were Al³⁺, Ca²⁺, Mg²⁺, K⁺, Na⁺, and SOH⁺₂ (specific sorption sites of anions). The major ligands were CO^2-₃, Cl^-, PO^3-₄, SiO₂^2-₂, XCON^- (the constant cation-exchange sites), XVAR^- (variable cation-exchange sites), and SO^- (specific sorption sites of cations). Surface charge of the clay minerals was based on experimental data (titration) and expressed in units of mol_c L^-1 (Shariatmadari, 1998).

For calcite, the sorption sites were assumed to be the surface Ca²⁺ and CO^2-₃, as suggested by Thompson and Pownall (1989). The surface density of these sites was estimated by Moller and Sastri (1974) and Compton and Pritchard (1990) as 8.3 x 10^-6 mol m^-2. On the basis of numerous parameters, Compton and Pritchard (1990) estimated that 1% of the surface sites are active sites. Taking this into account, the concentration of P sorption sites on calcite was estimated as 0.8 x 10^-7 M. The partial pressure of CO₂ was set to atmospheric pressure, 30 Pa (10^-3.52 atm).

	Results and discussion

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Phosphate Sorption of Clay–Calcite Mixtures in the Presence of Magnesium
The P sorption of different clay–calcite combinations in the presence of two Mg concentrations is illustrated in Fig. 1 . With no Mg in the solution, the three samples showed a P sorption of 10 cmol kg^-1. Addition of 5.0 and 10.0 mg L^-1 Mg sharply decreased the P sorption of calcite to <1 cmol kg^-1. The decrease in P sorption of Pal–CaCO₃ and Mont–CaCO₃ was not as sharp, but both samples experienced about a 2 cmol kg^-1 reduction in the amount of P sorbed.

View larger version (85K): [in this window] [in a new window]   Fig. 1 Effects of Mg on P sorption of clay–CaCO3 mixtures

Addition of 10.0 mg L^-1 Mg sharply reduced the calcite P sorption (Fig. 1). The introduction of Mg into the solution increased the Ca concentration, which is likely the result of higher calcite dissociation in the presence of Mg ions. According to thermodynamic constants used in SOILCHEM, the concentration of soluble P in the presence of Mg²⁺ was high enough to decrease the free Ca²⁺ to <2% of total dissolved Ca, while >55% of P remained as soluble complexes, mainly H₂PO^-₄and HPO^2-₄. A relatively small proportion of total P was bound by Mg²⁺ as soluble complexes. Due to the very small surface area of calcite, the contribution of surface sorption to total P complexation is negligible, even though >96% of the sorption sites on calciate should be occupied with phosphate ions due to the high concentration of P in the solution.

The inhibitory effect of Mg on the sorption of P by calcite results mainly from its interference with the formation of Ca–phosphate complexes and, therefore, the precipitation of Ca–phospahate. Marion and Babcock (1977) explained that this occurs through the substitution of Mg²⁺ for Ca²⁺ in calcium phosphate and apatite precipitates, which leads to disruption of the crystal lattice of calcite by the highly hydrated Mg²⁺. Since SOILCHEM predicted that formation of Ca–phos-phates was the major mechanism of P sorption (Table 2), Mg²⁺ inhibition should increase P availability in the calcium-bearing systems.

It has also been established that Mg²⁺ inhibits calcite nucleation, and that with time this ion is incorporated in the calcite lattice. This may result in the formation of a dolomitic structure at the surfaces of calcite crystals that has a much lower affinity for phosphate sorption (Yadav and Paliwal, 1988; Coale et al., 1994). However, due to the short time scale of this experiment and also, the very low predicted contribution of surface complexation to total P sorption (Table 2), this mechanism may not be significant.

The P sorption capacities of the clay–CaCO₃ combinations were almost as high as that of pure calcite. However, as shown by the speciation results (Table 2), the amount of available P the in montmorillonite–calcite system was slightly raised to 3.5% (PO₄ bound with different cations in calcite is 1.6% and Mont–CaCO₃ 5.1%), which is twice as much as that of pure calcite under the same conditions. Therefore, one may conclude that, although calcite controls the P sorption in the carbonatic systems, the presence of silicate clays increased the phosphate desorption of the system to a level above that of pure calcite. This may be due to clay–CaCO₃ interactions that increase the solubility of calcite or newly formed Ca–phosphates in the system. The higher concentration of soluble Ca in the montmorillonite–CaCO₃ solution (93.1%) over that of pure calcite (79.4%) observed in this experiment supports this view.

The addition of Mg also decreased the P sorption capacity of the calcite and clay–CaCO₃ systems; however, the reduction was not as sharp in the clay–calcite as in the pure calcite system (Fig. 1). The presence of 90% clay in the mixture to some extent buffers the inhibitory effects of Mg on P sorption by calcite in the system. Sorption of Mg on cation-exchange sites of silicate clays may be one factor that decreases the activity of this ion in solution; however, this was not predicted by the speciation. The SOILCHEM predicted that cation-exchange sites should be totally occupied by Ca²⁺ and K⁺ ions. Another explanation for the buffering behavior of silicate clays is an increase in the contribution of surface sorption sites to the complexation of dissolved P.

SOILCHEM predicted that the surface sorption sites may complex 13% of total dissolved P, making them the second major contributor to P retention in the system. Since the P sorbed by silicate clay surfaces is more easily available to the plant than Ca–phosphates precipitates (Brady, 1990), it may be concluded that the addition of Mg to a clay–calcite mixture favors the transformation of precipitated P to more available forms. The P sorption behavior of Pal–CaCO₃ was similar to that of Mont–CaCO₃. The Mg concentration in the Pal–CaCO₃ suspension was 1 mg L^-1 higher than that of Mont–CaCO₃, but apparently it did not cause a detectable difference in the P sorption of the mixture.

The concentration of total dissolved Al was 10 mg L^-1 in the Mont–CaCO₃ suspension; however, the precipitation of Al–phosphates at the pH of the experiment (7.4) was not predicted by SOILCHEM. As Table 2 shows, the silicate clay systems are saturated with respect to the formation of Al–SiO₃ and Al–OH precipitates. With a high P-sorption capacity, the fresh Al precipitates may also play a significant role in retention of phosphates by silicate clay mixtures; however, due to a lack of data on the surface properties of these compounds, their contribution to P sorption was not considered.

Effect of Silicon Addition on Phosphorus Sorption of Clay–Calcite Mixtures
Additions of 5.0 and 10.0 mg L^-1 of Si decreased the P sorption of clay–CaCO₃ combinations to almost the same extent as the Mg treatments (Fig. 2) . A supply of 5.0 and 10.0 mg L^-1 Si sharply reduced the sorption of phosphate on pure calcite; however, the effect of this ion on the P sorption capacity of calcite was less than that of the Mg treatment. Total soluble Ca concentration also increased when Si was added to the system. Therefore, the introduction of Si may enhance the solubility of both calcite and the newly formed Ca–P compounds in the system, which results in a higher concentration of P remaining in the solution. The effect of different ligands (e.g., Cl^-, HCO^-₃, and SO^2-₄) on the dissolution of calcite is well known. These ligands may enhance the solubility of calcite by forming Ca ion-pair complexes in the solution, so the higher solubility of Ca–phosphates and calcite in the presence of Si ions may also be attributed to the same mechanism.

View larger version (89K): [in this window] [in a new window]   Fig. 2 Effects of Si on P sorption of clay–CaCO3 mixtures

Sorption of Phosphorus by the Sepiolite–CaCO₃ Mixture
The effects of Si and Mg from the dissolution of sepiolite on a mixture of 90% sepiolite and 10% calcite (Sep–CaCO₃), was investigated. Sepiolite is a Mg-bearing silicate clay with relatively low stability that is capable of supplying significant amounts of Mg and Si to the soil solution. The sepiolite–calcite system showed an 60% reduction of P sorption compared with that of pure calcite (Fig. 3) . The total dissolved Mg and Si in the P sorption solution were 3 and 7 mg L^-1, respectively, while Al was below detection. Sepiolite, compared with montmorillonite, has a lower Al content, which reduces the affinity of sepiolite for P, while the release of Mg from sepiolite reduces the P retention capacity of calcite in the mixture.

View larger version (65K): [in this window] [in a new window]   Fig. 3 Phosphate sorption of CaCO3 and Sep–CaCO3 mixture

View larger version (20K): [in this window] [in a new window]   Fig. 4 Magnesium-induced desorption of P sorbed by different clay–CaCO3 mixtures

The Si treatments also increased the desorption of P sorbed on both Pal– and Mont–CaCO₃ (Fig. 5) . The effect of Si on P desorption may be explained by the high affinity of the silicate ligand for P sorption sites, as well as its interference with the formation of Ca– and Al–phosphate complexes. The 10.0 mg L^-1 Si treatment increased the desorption of sorbed P by the Mont–CaCO₃ mixture, but the increase with 5.0 mg L^-1 treatment was not as sharp. The general pattern was similar to that of P desorption induced by Mg treatments. In the Pal–CaCO₃ mineral mixture, however, the effect of the two levels of Si addition was almost the same.

View larger version (20K): [in this window] [in a new window]   Fig. 5 Silicon-induced desorption of P sorbed by different clay–CaCO3 mixtures

View larger version (18K): [in this window] [in a new window]   Fig. 6 Desorption of P sorbed by Sep–CaCO3 and Mont–CaCO3 mixtures

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

Application of SOILCHEM confirmed that formation of Ca–phosphates is the major mechanism immobilizing soluble P in the clay–calcite systems. The P sorbed on the clay mineral surfaces was predicted to be the second largest contributor to retention of P by the systems. Since the P sorbed on clay surfaces is more labile than P in Ca–phosphate precipitates, it seems that the presence of silicate clays favored the desorption of P in the clay–calcite systems. Formation of Al–phosphates was ruled out based on the SOILCHEM prediction, but the Mont–CaCO₃ and Pal–CaCO₃ systems were saturated with respect to Al–silicate compounds. The newly formed Al–silicates may have contributed greatly to the retention of phosphate, especially in the Mont–CaCO₃ system, in which the concentration of dissolved Al was relatively high. Because of the lack of thermodynamic data on these solids, their contribution to P retention could not be estimated by the chemical speciation program.

As a slow-release source of Mg and Si, sepiolite in the clay–calcite mixture induced a lower P sorption in comparison with other clay–calcite combinations. The desorption of P sorbed by sepiolite–calcite was also much higher than in other combinations. This suggests that the presence or application of sepiolite may increase P desorption and availability in calcareous soils.SAS Institute 1985

Received for publication May 4, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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