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Soil Science Society of America Journal 64:1347-1355 (2000)
© 2000 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Adsorption of Phosphate and Tartrate on Hydroxy-Aluminum–Oxalate Precipitates

A. De Cristofaroa, J.Z. Heb, D.H. Zhoub and A. Violantea

a Dipartimento di Scienze Chimico-Agrarie, Università di Napoli Federico II, 80055 Portici (Napoli), Italy
b Dep. of Resources, Environment and Agrochemistry, Central China Univ. of Agriculture, Wuhan 430070, China

violante{at}unina.it


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 NOTES
 Conclusions
 REFERENCES
 
Sorption of phosphate in the presence of low molecular mass organic ligands on hydroxy-Al–organics coprecipitates may be the rule and not the exception in rhizospheric soils. We studied the competitive sorption of phosphate and tartrate on hydroxy-Al–oxalate precipitates obtained by coprecipitating Al and oxalate at pH 7.0 and initial oxalate/Al molar ratios of 0, 0.1, 0.2, and 0.5. The noncrystalline precipitates showed different chemical and physicochemical properties. Oxalate was not homogeneously distributed throughout the solids, and the greater its content in the samples, the higher the surface area, reactivity, and solubility of the solids. Oxalate was released from each complex mainly at pH <5.0 and >7.0 and more in the presence than in the absence of phosphate or tartrate. More phosphate than tartrate was sorbed on the samples, but tartrate appeared to be more specific than phosphate in replacing oxalate. The quantity of phosphate and tartrate sorbed on the organomineral complexes, containing greater amounts of oxalate, was little affected by pH (4.0–9.0), probably due to the release of oxalate from the surfaces of the complexes by increasing the pH. When mixtures of equimolar amounts of the two ligands were added at pH 4.0 to 9.0, more phosphate than tartrate was sorbed on the organomineral solids. The sorbed tartrate/sorbed phosphate molar ratio was always <1.0, but usually increased by decreasing pH and increasing the content of oxalate in the precipitates. At pH <=5.0 the presence of high tartrate concentrations reduced the phosphate sorption, releasing Al and Al-bounded oxalate. Only at pH >5.0 could the decrease in sorption of phosphate be attributed mainly to competition in sorption between the organic and inorganic ligands for different sorption sites.

Abbreviations: FTIR, Fourier transform infrared • LMMOA, low molecular mass organic acids • PZSC, point of zero salt charge • T/P, tartrate/phosphate molar ratio


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 NOTES
 Conclusions
 REFERENCES
 
THE RHIZOSPHERE is a zone of intense activity, where the interactions among plant roots, soil colloids, organics, nutrients, and microorganisms are very complex and still very poorly understood. Some studies have shown that the rhizosphere soil may have differences in physical characteristics, mineralogy, and weathering compared with the bulk soil (Lynch, 1990; Marschner, 1995).

Low molecular mass organic acids (LMMOA; i.e., oxalic, tartaric, succinic, malic, and citric acid), secreted by plant roots and released by microorganisms, play important roles in many processes at the soil–root interface. Oxalic and tartaric acid are widely represented in rhizosphere soil or deriving from the lysis of microbial cells or released in soil as root exudates of cereals and solonaceous groups of plants (Stevenson, 1967; Bruckert, 1970; Huang and Violante, 1986; Fox and Comerford, 1990).

The interactions between biochemical compounds and minerals, which are greatly enhanced in the rhizosphere, result in the precipitation of short-range ordered Al and Fe oxides (Sarkar et al., 1979; Huang and Violante, 1986; Vance et al., 1996). All the LMMOA may interact with hydrolytic products of Al and Fe, forming organomineral precipitates, which may differ tremendously in chemical composition, surface properties, and reactivity toward plant nutrients and organic compounds (Huang and Violante, 1986; Schwertmann et al., 1986; Barnhisel and Bertsch, 1989). Sarkar et al. (1979) found that the amounts of noncrystalline Al or Fe precipitation products extracted from rhizosphere soil were larger than those from nonrhizosphere soil, and extractable Al and Fe showed significant positive correlations with extractable C, suggesting chemical associations between these inorganic and organic components.

Bloom (1981) and Haynes and Swift (1989) have found that Al–organic precipitates in soil may fix large amounts of phosphate. Violante and Huang (1989) showed that the nature and amount of organic ligands coprecipitated in Al oxides, and the surface area and crystallinity of the minerals play a significant role in phosphate sorption. However, until now, little work has been carried out on the sorption of nutrients and LMMOA on hydroxy-Al–organic precipitates as a function of chemical composition and surface properties, and on the possible displacement of organic ligands present in the precipitates (Violante and Huang, 1989; Violante and Gianfreda, 1995). Furthermore, competition in sorption between nutrients and LMMOA on hydroxy-Al–organic precipitates has not received much attention.

The objective of this work was to study (i) the sorption of phosphate and tartrate on hydroxy-Al–oxalate precipitates as a function of oxalate content and (ii) the competitive sorption of phosphate and tartrate on the organo-Al precipitates and the associated release of oxalate.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 NOTES
 Conclusions
 REFERENCES
 
Preparation and Characterization of the Samples
An Al hydroxide [Al(OH)x] and three hydroxy-Al–oxalate [Al(OH)x–oxalate] precipitates [referred to as Al(OH)OX1, Al(OH)OX2, and Al(OH)OX5] were prepared by titrating stirred solutions of 0.1 mol L-1 AlCl3 with and without oxalic acid (oxalic acid/Al molar ratio [Ri] of 0.1, 0.2, and 0.5) with 0.1 mol L-1 NaOH at a rate of 2 mL min-1 to pH 7.0. The final suspension volumes were then adjusted with deionized water to bring the Al to 0.03 mol L-1. The suspensions were aged for 24 h, centrifuged at 10000 g, and washed twice with deionized water, dialyzed until Cl- free, and freeze-dried. The Al precipitates were lightly ground to pass a 0.1-mm sieve.

The specific surface of the samples was determined by the gravimetric method based on the retention of the ethylenglycol monoethyl ether (Carter et al., 1986). The x-ray diffraction patterns of the randomly oriented Al precipitation products were obtained with a Rigaku (Tokyo) diffractometer using Ni-filtered CoK{alpha} radiation at 40 kV and 30 mA. The Fourier transform infrared (FTIR) spectra of the Al(OH)x and Al(OH)x–oxalate precipitates were recorded on KBr disks containing 1% of sample by weight, using a 1720-X FTIR spectrometer (Perkin Elmer, Norwalk, CT). The oxalate present in the organomineral precipitates was determined by ion chromatography (described below) after dissolution of the precipitates in 0.5 mol L-1 NaOH (Jackson, 1969). The point of zero salt charge (PZSC) of the precipitates was determined according to the method of Sakurai et al. (1988).

Phosphate and Tartrate Sorption as a Function of pH
Twenty-three milliliters of 0.02 mol L-1 KCl were added to 25 mg of each sample in reaction flasks. The pH values (4.0–9.0) of the mixtures were adjusted with 0.1 or 0.01 mol L-1 HCl or KOH. Suitable volumes of 0.025 or 0.05 mol L-1 solutions of phosphate (as K2HPO4, reagent grade supplied by Applichem, Darmstadt, Germany) or tartaric acid (reagent grade supplied by Carlo Erba, Milan, Italy), whose pH was previously adjusted at 4.0 to 9.0, were pipetted into each flask in order to attain 50 to 1000 mmol of ligand per kilogram of sample. A few drops of toluene were initially added to each flask to inhibit microbial activity. The suspensions were shaken for 24 h at 20°C. The pH of the suspensions was periodically adjusted with 0.1 or 0.01mol L-1 HCl or KOH by a Radiometer automatic titrator (Copenaghen, Denmark). The final suspensions (25 mL) were centrifuged and filtered through 0.2-µm Millipore M.F. filters (Bedford, MA), and phosphate and tartrate concentrations in the solution were then determined. The amounts of phosphate and tartrate sorbed were calculated from the difference between the initial and final concentrations of the ligands in the solutions.

Competitive Sorption Experiments
Twenty-five milligrams of the Al(OH)x–oxalate precipitates were shaken for 24 h at 20°C with a series of 23 mL of 0.02 mol L-1 KCl solutions containing different amounts of phosphate and tartrate (500–2500 mmol of each ligand kg-1). The pH values (5.0 or 7.0) of the suspensions were adjusted with 0.1 or 0.01 mol L-1 HCl or KOH and were controlled by a Radiometer automatic titrator. The amounts of phosphate initially added to the complexes ranged from 500 to 1000 mmol kg-1, which resulted in nearly maximum sorption. The final suspensions (25 mL) were centrifuged and filtered, and phosphate and tartrate concentrations in the solution were then determined.

Ligands and Aluminum Analysis
The amounts of phosphate, tartrate, and oxalate in the supernatant solutions were determined by ion chromatography. Aluminum was determined by atomic absorption spectrophotometry (Perkin Elmer 3030B). A Dionex ion chromatograph (Model 2000i/SP, Dionex Co., Sunnyvale, CA) with an AS4A separator column was used with a guard column (AG4A). The eluent was 0.75 mmol L-1 NaHCO3 + 2 mmol L-1 Na2CO3 at a flow rate of 2 mL min-1. The phosphate (elution time {approx}6 min), tartrate (elution time {approx}8 min), and oxalate (elution time {approx}10 min) concentrations were calculated from calibration curves obtained with standard solutions, as described by Violante et al. (1991). All experimental data were conducted at least in duplicate and the relative standard deviation, calculated from experimental replicates, was lower than 4%.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
NOTES
 Conclusions
 REFERENCES
 
Chemical Composition and Properties of the Samples
All the samples were noncrystalline to x-ray diffraction (Fig. 1) . The FTIR analysis data (Fig. 2) also support the existence of amorphous precipitates, as is evident from the large band centered at {approx}3470 and the absence of bands at {approx}3650, 3620, 3545, 3460, 1025, 970, 770, and 530 cm-1 that are characteristic of Al(OH)3 polymorphs (Violante and Violante, 1980; Hsu, 1989). The organomineral precipitates showed peaks at {approx}1730, 1700, 1430, and 1300 cm-1 (Fig. 2), which are characteristic of oxalate–Al bonds (Fujita et al., 1962; Parfitt et al., 1977; Dynes and Huang, 1997) and similar to those reported by Parfitt et al. (1977) for oxalate sorbed on gibbsite. According to Dynes and Huang (1997), the infrared bands at {approx}1710 and 1700 cm-1 of oxalate sorbed on short-range ordered Al oxides indicate that COOH groups are present and are indicative of monodentate complexes. Two peaks at {approx}915 and 810 cm-1 appeared clearly only on the Al(OH)OX2 and Al(OH)OX5 precipitates (Fig. 2) and are attributable to C-O stretching (915 cm-1) and O-C=O bending (915 and 810 cm-1) of oxalate. The band at 810 cm-1 could be due to stretching of Al-O bonds present in Al and oxalate complexes (Fujita et al., 1962).



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  		Fig. 1 X-ray diffraction patterns of Al(OH)x and hydroxy-Al–oxalate precipitates generated from solutions with initial oxalic acid/Al molar ratios of 0.1 [Al(OH)OX1], 0.2 [Al(OH)OX2], and 0.5 [Al(OH)OX5]

 


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  		Fig. 2 Fourier transform infrared spectra of Al(OH)x, hydroxy-Al-oxalate precipitates generated from solutions with initial oxalic acid/Al molar ratios of 0.1 [Al(OH)OX1], 0.2 [Al(OH)OX2], and 0.5 [Al(OH)OX5], and a mixture of Al(OH)3 polymorphs (gibbsite and bayerite) well crystallized

 
The chemical composition and the surface properties of the samples [Al(OH)x, Al(OH)OX1, Al(OH)OX2, and Al(OH)OX5] are reported in Table 1 . The organic C content of the precipitates increased from 0 to 64 g kg-1, whereas the Al2O3 content decreased from 650 to 442 g kg-1, with increasing Ri. The PZSC of the samples generally corroborates the existing information in the literature: anionic impurities shift the PZSC to lower values (Huang and Violante, 1986). Furthermore, the surface area increased from 192 to 698 m2 g-1 with increasing oxalate content. It is well known that organic ligands complex Al in aqueous solution and may also inhibit the crystallization of Al hydroxides. The resulting increasing specific surface areas with increasing organic ligand content has also been reported (Violante and Violante, 1980; Violante and Huang, 1984; Huang and Violante, 1986; Hsu, 1989). Violante and Huang (1984, 1992) revealed that the specific surface of the precipitation products of Al formed in the presence of tannic or tartaric acid increased with increasing initial ligand/Al molar ratio (from 0 to 0.01 for tannate and to 0.1 for tartrate). Further increase in the ligand/Al molar ratio decreased the specific surface of the precipitates because of the capacity of the polydentate anions to promote aggregation of the precipitates above certain critical concentrations. According to these authors, structural distortion and aggregation occur simultaneously, but aggregation may be more prominent when the amount of certain ligands in the solid phase is sufficiently high. Evidently, oxalate, even at oxalate/Al molar ratio of 0.5 (Table 1), promoted more distortion than aggregation of Al precipitation products.


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  		Table 1 Chemical composition, surface area (SA), and point of zero salt charge (PZSC) of the Al precipitation products formed in the presence of different initial oxalic acid/Al molar ratios (Ri).{dagger}

 
Sorption of Phosphate and Tartrate
Phosphate was almost completely sorbed on the Al(OH)OX5 precipitate in the pH range examined, whereas much lower quantities of phosphate were retained by Al(OH)OX2 and Al(OH)OX1 (Table 2) . Tartrate was sorbed to a lesser extent than phosphate on all the Al precipitation products (Table 2), consistant with the findings of He et al. (1999). The sorption capacity of Al(OH)x was similar to that of Al(OH)OX1 (Table 2).


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  		Table 2 Amounts of phosphate (P) and tartrate (T) sorbed on hydroxy-Al–oxalate precipitates (1000 mmol P or T added kg-1) and oxalate (OX) released from the solids at different pH values

 
In the pH range examined and on the basis of the millimoles of phosphate sorbed per kilogram of sample, Al(OH)OX5 retained on average 1.7 and 9 times more phosphate and 1.7 and 7.5 times more tartrate than the Al(OH)OX2 and Al(OH)OX1, respectively. This result may be associated with the different surface areas of the sorbents (698 vs. 465 and 200 m2 g-1). On the basis of millimoles of phosphate and tartrate sorbed per square meter of sample, Al(OH)OX5 on average retained 1.1 and 2.7 more phosphate and 1.1 and 2.1 more tartrate than the Al(OH)OX2 and Al(OH)OX1.

These results seem to contradict previous studies that demonstrated that oxalate, as well as other LMMOA with a high affinity for Al or Fe oxides, are able to inhibit the sorption of phosphate, mainly in acid environments (Nagarajah et al., 1970; Earl et al., 1979; Violante et al., 1991; Violante and Gianfreda, 1993). A possible explanation of our findings is that, when oxalate and Al are coprecipitated, the presence of oxalate in the solid promotes an increase in the specific surface and enhances the exposure of AlOH0.5- and AlOH0.5+2 by promoting phosphate and tartrate sorption (Huang and Violante, 1986).

It is also noteworthy that the amounts of phosphate sorbed on the Al oxalates [mainly Al(OH)OX5] remained nearly constant in the pH 4.5 to 9.0 range (Table 2). This behavior is surprising because the sorption of phosphate by amphoteric clay minerals usually substantially decreases as the pH increases (Parfitt, 1978; Hsu, 1989; Violante et al., 1991; Violante and Gianfreda, 1993). Conceivably, the release of oxalate from the surfaces of the coprecipitates by increasing the pH (as will be discussed below) created new adsorbing sites for phosphate sorption. Haynes and Swift (1989) found that raising the pH of Al–organic matter solid associations greatly increased the phosphate sorption capacity. Liming has been reported to increase, decrease, or not affect the phosphate sorption by highly weathered acid soils (Haynes, 1982 and references therein reported). Our data seem to strengthen the observation that phosphate sorption may remain constant on certain organomineral solids with increasing pH, as ascertained by Bloom (1981). This author showed that in a pH range 4.7 to 6.1, phosphate sorption on an Al–peat complex and on a P-fertilized soil was not affected by pH.

In our study Al was released from the solids, mainly at pH <6.0, both in the absence and in the presence of ligands (particularly tartrate) (Table 3) . Lower quantities of Al were found in solution in the presence relative to that found in the absence of phosphate (Table 3). This may be explained by the possible formation of sparingly soluble hydroxy-Al–phosphate complexes (Bloom, 1981).


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  		Table 3 Aluminum released from Al(OH)OX2 and Al(OH)OX5 in the absence and in the presence of 1000 mmol of ligands kg-1.{dagger}

 
Release of Oxalate from the Hydroxy-Aluminum–Oxalate Precipitates
In the absence of phosphate and tartrate relatively high amounts of oxalate were released from the organomineral solids at pH <5.0 and >7.0, and much more from Al(OH)OX5 than from Al(OH)OX2 and Al(OH)OX1 (Table 2). The Al(OH)OX5 solid was more soluble than Al(OH)OX2 and Al(OH)OX1, as is evident from the greater quantities of Al found in solution (Table 3). Larger quantities of Al were solubilized from each complex at acid pH values, and consequently, proportionally higher amounts of oxalate were also found in solution (Table 2). Conversely, at pH >7.0, in spite of the negligible amounts of Al solubilized (Table 3), the quantities of oxalate released from the complexes tremendously increased with increasing pH [i.e., 1300 and 2000 mmol oxalate kg-1 were released from Al(OH)OX5 at pH 8.0 and 9.0]. The release of oxalate at pH >7.0 may be attributed to deplacement by hydroxyl ions. In previous works Violante et al. (1991, 1996) and Violante and Gianfreda (1993, 1995) demonstrated that oxalate ions are poorly sorbed in alkaline systems on variable-charge minerals or are easily removed from the surfaces of Al oxides or oxyhydroxides, on which they were previously sorbed, by increasing pH.

Table 2 also shows (data in parentheses) the percentages of oxalate brought in solution at different pH values relative to the total content of oxalate coprecipitated with Al in each sample (Table 1). At pH 9.0, only 14% of oxalate initially present in Al(OH)OX1 was released vs. 51 and 75% of that present in Al(OH)OX2 and Al(OH)OX5. The much greater percentage of oxalate replaced from the solids containing greater amounts of these organic anions may be explained by considering that the higher the amounts of oxalate initially coprecipitated with Al, the greater the percentage of oxalate present on the external surfaces of the samples (Violante and Huang, 1992). In other words, the Al(OH)OX2 and Al(OH)OX1 complexes had a greater percentage of oxalate present within the network of the organomineral complexes, where the organic ligand was much more protected and not easily replaced by OH- ions. Also the more open structure of Al(OH)OX5 probably promoted the release of oxalate, whereas the greater aggregation of the other solids (Table 1) prevented or reduced the replacement of the organic ligand.

Another possible explanation of our findings is that oxalate sorbed on the external surfaces of Al precipitates forms both monodentate and bidentate complexes. However, as reported by Parfitt et al. (1977), at higher surface coverages, greater amounts of oxalate ions are adsorbed less strongly as monodentate complexes (AlOOC–COOH) to permit increased adsorption of organic ligands. Therefore, since the percentage of oxalate ions adsorbed as monodentate complexes are greater on Al(OH)OX5 than on Al(OH)OX2 and Al(OH)OX1 (in the order listed), the percentage of oxalate replaced by OH- (or other ligands) increased in the order: Al(OH)OX5 > Al(OH)OX2 > Al(OH)OX1. It is likely that all these factors influence the release of oxalate from the hydroxy-Al–oxalate precipitates.

More oxalate was usually released in the presence than in the absence of phosphate or tartrate (Table 2). The percentage of oxalate removed from the complexes in the presence of phosphate at different pH values ranged from 3 to 19% for Al(OH)OX1, from 8 to 68% for Al(OH)OX2, and from 36 to 93% for Al(OH)OX5. Oxalate released from the solids in the presence of tartrate ranged from 6 to 17% for Al(OH)OX1, from 15 to 69% for Al(OH)OX2, and from 43 to 83% for Al(OH)OX5 (Table 2).

The ratios RP [(oxalate released in the presence of phosphate-oxalate released in the absence of phosphate)/phosphate sorbed] were always <1, but greater when phosphate was sorbed on Al(OH)OX5 (mean RP value of 0.64) than on Al(OH)OX2 (mean RP value of 0.29) and Al(OH)OX1 (mean RP value of 0.22) (Table 2). These data seem to indicate that on the Al(OH)OX1 and Al(OH)OX2 precipitates large amounts of phosphate were sorbed on oxalate-free sites. Conversely, the ratios RT [(oxalate removed in the presence of tartrate-oxalate removed in the absence of tartrate)/tartrate sorbed] were always much greater than RP and near to unity, irrespective of solid.

Oxalate brought into solution at a given pH for the partial solubilization of the OH-Al–oxalate precipitates, both in the absence and in the presence of phosphate and tartrate, may be calculated, at least approximately, from the amount of solubilized Al (Table 3), taking into account that the oxalate/Al molar ratios in the final precipitates Al(OH)OX1, Al(OH)OX2, and Al(OH)OX5 were 0.09, 0.15, and 0.31 (Table 1). As a consequence, the amount of oxalate desorbed by phosphate or tartrate may be calculated by subtracting the quantity of oxalate solubilized in the absence of phosphate and tartrate from the quantity of oxalate released in the presence of phosphate or tartrate at the same pH (Table 2). Also, oxalate desorbed in the absence of phosphate and tartrate may be calculated by subtracting from the amounts of oxalate removed at a given pH (Table 2) those released with Al in the absence of ligands (Table 3).

The ratios R'P (and R'T) {[oxalate desorbed in the presence of phosphate (or tartrate)–oxalate desorbed in the absence of phosphate (or tartrate)]/phosphate (tartrate) sorbed} were similar or slightly lower ({approx}10–15%) than RP and RT, respectively (data not shown).

Although RP and RT (or R'P and R'T) values may give only an indication of the replacement of oxalate by phosphate or tartrate, it appears clear that tartrate replaced oxalate from the surfaces of the complexes more selectively than phosphate. These results strengthen the observation of He et al. (1999), who found that oxalate and tartrate competed for many common sites on variable-charge minerals. Violante and Gianfreda (1995), Dynes and Huang (1997), and He et al. (1999) showed that the adsorption capacity of Al precipitation products for oxalate and tartrate was similar. In fact, these organic ligands have similar stability constants (KAl-L) for the 1:1 Al–organic complexes in aqueous solution (Nordstrom and May, 1996). According to Martell and Smith (1977), log KAl-L is 6.28 for oxalic acid and 5.84 for tartaric acid.

Competitive Sorption of Phosphate and Tartrate
Table 4 and Fig. 3 report the amounts of phosphate and tartrate sorbed on the organomineral solids, when mixtures of equimolar amounts of the two ligands (500–1000 mmol phosphate and tartrate kg-1) were added at pH 4.0 to 9.0. More phosphate than tartrate was sorbed on the organomineral solids. The sorbed tartrate/sorbed phosphate molar ratio (rf) was always <1.0, but usually decreased by increasing pH or concentration of phosphate and tartrate (Table 4). For example, by adding 1000 mmol of phosphate and tartrate to Al(OH)OX2, rf decreased from 0.39 at pH 5.0 to 0.11 at pH 7.0 (Table 4) and 0.06 at pH 9.0. Evidently tartrate prevented phosphate sorption more in acid than in alkaline environments. Similar results were obtained by other authors who studied the competitive sorption of phosphate and oxalate or tartrate on variable charge minerals (Kafkafi et al., 1988; Violante et al., 1991; Violante and Gianfreda, 1993, 1995; He et al., 1999).


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  		Table 4 Amount of phosphate (P) and tartrate (T) sorbed on Al(OH)x–oxalate precipitates (Al(OH)OX1, Al(OH)OX2, Al(OH)OX5) at pH 5.0, 7.0, 9.0, when the ligands were added as a mixture

 


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  		Fig. 3 (a) Phosphate and (b) tartrate adsorption envelopes on the Al(OH)OX5 precipitate when the ligands either were added alone or were added in mixtures of equimolar amounts (1000 mmol P and T kg-1) to the organomineral precipitate

 
It is also interesting to note that greater values of rf were usually found when the ligands were added to Al(OH)OX5 than to Al(OH)OX2 and Al(OH)OX1 (in the order listed). For example, in the presence of 750 or 1000 mmol mixture of phosphate and tartrate per kilogram of sample at pH 5.0, rf values increased from 0.26 when the ligands were added to Al(OH)OX1 to 0.39 and 0.55 when the ligands were added to Al(OH)OX2 and to Al(OH)OX5, respectively. Probably, the precipitates containing greater amounts of oxalate coprecipitated with Al had more sites at high affinity for tartrate (as discussed before) and partially prevented phosphate sorption. The efficiency of tartrate in inhibiting phosphate sorption, calculated according to the expression of Deb and Datta (1967)1 , was greater when the ligands were added to Al(OH)OX5 than to Al(OH)OX2 and Al(OH)OX1 (in the order listed; Table 4). Obviously, phosphate prevented tartrate sorption much more on the Al(OH)OX1 complex than on the other precipitates and mainly at pH 7.0 or 9.0 (Table 4). At pH 5.0, the sorption of tartrate was reduced by 70% on Al(OH)OX1 vs. 35 and 2% on Al(OH)OX2 and Al(OH)OX5 by adding 750 or 1000 mmol of phosphate and tartrate (Table 4).

The effect of the combined addition of 1000 mmol kg-1 of phosphate and tartrate to Al(OH)OX5 compared with the addition of each ligand added alone, at pH 4.5 to 9.0, is shown in Fig. 3. In the presence of tartrate, a mean inhibition of phosphate sorption of {approx}20 to 25% at pH ranging from 5.0 to 6.5 was observed (Fig. 3a). In this pH range, the minimum release of Al from the organomineral solids was observed in the absence of ligands (Table 3). The lower apparent efficiency of tartrate in preventing phosphate sorption at pH <5.0 must be attributed to solubilization of Al (Table 3 and Fig. 4) and possible subsequent formation of sparingly soluble hydroxy-Al–phosphate–tartrate precipitates. In fact, tartrate sorption at pH <=5.0 was greater in the presence than in the absence of phosphate (Fig. 3b). However, it appears evident that phosphate strongly reduced tartrate sorption at pH >5.0.



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  		Fig. 4 Relationships between the amounts of phosphate sorbed on the Al–oxalate precipitate Al(OH)OX5 and the relative amounts of Al and oxalate solubilized, at pH 5.0 and in the presence of increasing tartrate/phosphate molar ratios (see Table 5). The number in parentheses indicates the initial tartrate/phosphate molar ratio. The linear equations describing the relationships are: Psorbed (mmol kg-1) = 1091.7 + -0.2827 Alsolubilized (mmol kg-1); R = 0.9906. Psorbed (mmol kg-1) = 1327.8 + -0.3356 Oxalatesolubilized (mmol kg-1); R = 0.98271

 

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  		Table 5 Competitive adsorption between phosphate (P) and tartrate (T) added to the Al(OH)OX5 precipitate as a mixture at pH 5.0

 
Table 5 shows the amounts of phosphate and tartrate sorbed on Al(OH)OX5 at pH 5.0 in systems containing 1000 mmol phosphate kg-1 sample and increasing quantities of tartrate up to an initial tartrate/phosphate (T/P) molar ratio of 2.5. The efficiency of tartrate in preventing phosphate sorption on Al(OH)OX5 increased from 15 to 46% by increasing the initial T/P molar ratios.

The amounts of Al and oxalate solubilized were related to the amounts of phosphate sorbed on Al(OH)OX5. A linear relationship was found between the quantities of sorbed phosphate and the amounts of Al solubilized (Fig. 4). A similar relationship was also found between the amounts of phosphate sorbed on and the amounts of oxalate released from Al(OH)OX5 (Fig. 4). Earl et al. (1979) claimed that the linear relationship between the reduction in phosphate sorption on soils and synthetic Al and Fe oxide or hydroxide gels and the amounts of solubilized Al or Fe indicated an elimination of varying number of sites available for phosphate sorption on the gels and soil surfaces due to the presence of organic acids. Recently, Dynes and Huang (1997) found that dissolution of Al hydroxides by organic acids (including oxalic, citric, tartaric, malic, and salicylic acid) was a minor mechanism in decreasing selenite sorption at pH 5.0 in the concentration range of organic acid studied (0.01–0.1 mM).

From the results described above it seems reasonable to conclude that in acid systems (pH <=5.0) and in the presence of large tartrate concentrations, removing Al (and Al-bounded oxalate) from Al(OH)OX5 and forming soluble Al–tartrate complexes reduced phosphate sorption on the organomineral solid. Correspondingly, the decrease in sorption of phosphate and tartrate at pH >5.0 on the hydroxy-Al–oxalate samples must be attributed mainly to competition in sorption between the organic and inorganic ligands for different sorption sites. In fact, in this range of pH values the solubilization of the coprecipitates was usually negligible (Table 3).


   Conclusions
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 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 NOTES
 Conclusions
REFERENCES
 
Our data show:

  1. Hydroxy-Al–oxalate precipitates, containing different amounts of oxalate, showed different chemical and physicochemical properties and reactivity toward phosphate and tartrate anions. Oxalate was not homogeneously distributed throughout the solids, and the greater its content in the precipitates the higher the specific surface and solubility of the solids.
  2. The quantities of phosphate and tartrate sorbed on the Al(OH)x–oxalates were related either to the amount of oxalate coprecipitated with Al or to the specific surface.
  3. More phosphate than tartrate was sorbed on the precipitates, even when the initial concentration of tartrate was relatively high.
  4. Oxalate was released from the complexes mainly at pH <5.0 and >7.0 and more in the presence than in the absence of phosphate and tartrate. In neutral and alkaline environments the release of oxalate is attributed mainly to deplacement by hydroxyl, tartrate, and/or phosphate, whereas at pH <=5.0 the partial solubilization of the organomineral precipitates was evident.
  5. Phosphate sorption on the hydroxy-Al–oxalate precipitates that contained greater amounts of oxalate remained nearly constant in the range of pH 5.0 to 9.0, probably due to the release of oxalate from the surfaces of the solids by increasing the pH and formation of new sorption sites.
  6. Tartrate was more specific than phosphate in replacing oxalate from the hydroxy-Al–oxalate precipitates.
  7. The efficiency of tartrate in preventing phosphate sorption on the solids increased by increasing the initial tartrate/phosphate molar ratio and in the samples containing greater amounts of oxalate coprecipitated. Furthermore, tartrate prevented phosphate sorption more in acid than in alkaline systems.
  8. In acid systems (pH <=5.0) the presence of large tartrate concentrations reduced the phosphate sorption on the Al(OH)x–oxalate solids removing Al and Al-bounded oxalate. Only at pH >5.0 was decreased phosphate sorption attributed mainly to competition in sorption between the organic and inorganic ligands for different sorption sites.

Aluminum–organic coprecipitates (like Al–oxalates), which may be formed in the rhizosphere and in acid soils, may significantly influence phosphate sorption either in the absence or in the presence of low molecular mass organic acids.


   ACKNOWLEDGMENTS
 
This research was supported by the International Scientific Cooperation (Contract no. CI1*-CT94-0048) of the European Community. Contribution number 178 from the Dipartimento di Scienze Chimico-Agrarie (DISCA).


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 NOTES
Conclusions
 REFERENCES
 
1 Efficiency of tartrate (phosphate) (%) = 1 - {[Phosphate (tartrate) sorbed in the presence of tartrate (phosphate)/Phosphate (tartrate) sorbed when applied alone]100}. Back

Received for publication March 15, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 NOTES
 Conclusions
 REFERENCES
 




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