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a Dep. de Ciencias Químicas, Univ. de La Frontera, Temuco, Chile
b Dip. di Scienze del Suolo, della Pianta e dell'Ambiente, Univ. di Napoli Federico II, Napoli, Italy
* Corresponding author (aljara{at}ufro.cl)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AlSi, aluminosilicate • AlSiFe, allophane coated with iron oxide • CIT, citrate • HPLC, high performance liquid chromatography • LMMOAs, low-molecular mass organic acids • OX, oxalate • PO4, phosphate • SO4, sulfate • ZPC, zero point charge
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Many authors have claimed that the mechanisms of SO4 and PO4 adsorption onto variable charge minerals and soils are quite similar, although SO4 is adsorbed less than PO4 and does not compete strongly with PO4 (Pasricha and Fox, 1993, and references therein). Phosphate is specifically adsorbed and forms inner-sphere complexes by replacing the coordinated–OH and–OH2 groups of variable charge minerals (Goldberg and Sposito, 1984a, 1984b; Mora et al., 1992). Sulfate adsorption on these minerals is quite complex; it can be adsorbed as an outer-sphere (Hayes et al., 1987; Karltun, 1997) or inner-sphere complex (Zhang and Spark, 1990; Wolt et al., 1992; He et al., 1997), but only a few studies have provided direct evidence to suggest possible inner-sphere complexes, mainly at low pH values and high concentrations (Turner and Kramer, 1991; Peak et al., 1999, 2001). Competition in adsorption between SO4 and PO4 has received attention (Dinh and Dufey, 1995; Hiradate and Inoue, 1998; Pigna and Violante, 2003; Mora et al., 2005). Pigna and Violante (2003) and Mora et al. (2005), observed that PO4 adsorption on Andisols and Ultisols was slightly affected by the presence of SO4, whereas SO4 adsorption was greatly inhibited by the presence of PO4.
Low-molecular mass organic acids (LMMOAs) occur widely in soils; they are secreted from plant roots and produced through the decomposition of plant residues (Jones, 1998; Jones and Brassington, 1998; Xu et al., 2003). Organic acids have the capacity to complex metals in solution. The degree of complexation depends on: (i) the nature of the organic acid (number and proximity of carboxyl and hydroxyl groups), (ii) the concentration of organic acid, (iii) type of surface sites, and (iv) the pH and ionic strength of the soil solution. Oxalic, citric, and malic acids are strong chelating agents with a high affinity for trivalent metals such as Al3+ and Fe3+ and affect their mobility in soil environments (Jones, 1998). These ligands can be adsorbed onto variable charge minerals by inner-sphere coordination involving exposed aquo and hydroxyl groups (Hue, 1991; Violante et al., 1991; Evanko and Dzombak, 1998; Geelhoed et al., 1999; Liu et al., 1999). Hanudin et al. (2002) studied the adsorption of acetate, OX, and CIT onto natural allophanic samples collected from volcanic ash soils. They found that OX and CIT were adsorbed in bidentate and/or binuclear form, but the binuclear form is more stable for OX and the bidentate form is more stable for CIT.
Phosphate is very strongly adsorbed on variable charge minerals and soils, but humic and fulvic acids, LMMOAs and inorganic ligands (including SO4) may compete with PO4 for adsorption sites. Some authors (Lopez-Hernandez et al., 1986; Violante et al., 1991; Geelhoed et al., 1998; Dolfing et al., 1999; He et al., 1999) have demonstrated that some carboxylic acids prevent, at least in part, PO4 adsorption by soils, kaolinite, iron or aluminum oxides, and hydroxy-Al-montmorillonite complexes.
The effect of organic ligands on SO4 and PO4 adsorption/desorption and on their mobility and phytoavailability has received attention (Inskeep, 1989; Evans and Anderson, 1990; Violante et al., 1996; Martinez et al., 1998; Karltun, 1998; Liu et al., 1999; Wijnja and Schulthess, 2000). Inskeep (1989) showed that various organic ligands, such as oxalic, humic, citric, and gallic acids, competed with SO4 for adsorption of kaolinite and iron oxide. Their competitive ability was related to the type of functional groups; organic ligands with functional groups containing more oxygen atoms were more competitive than those with functional groups containing more carbon atoms. Karltun (1998) also studied the competitive adsorption among SO4, OX, and fulvate and observed that OX and fulvate competed with SO4 for the adsorption sites of goethite and a soil material from a podzol. This was attributed mainly to the neutralization of the positive charge on the surface by the adsorption of OX and fulvate.
Liu et al. (1999) studied the competitive adsorption of SO4 and OX on goethite found that SO4 competed with OX particularly in acidic systems, mainly when added 24 h before OX. However, when OX was added before that SO4, the authors did not find a competitive effect between SO4 and OX.
Many investigations have been performed on the influence of competitive adsorption on anion partitioning in binary systems containing two inorganic and/or organic ligands. However, studies on the relative competition for sorption sites onto variable charge minerals and soils among various ligands (e.g., ternary systems) are rather scant (He et al., 1999; Liu et al., 1999; Wu et al., 2001). He et al. (1999) demonstrated that tartrate and OX added as a mixture in equimolar quantities onto an Al(OH)x–montmorillonite complex were much more effective in inhibiting PO4 sorption than tartrate or OX alone. In ternary systems tartrate competed with PO4 more than OX. Liu et al. (1999) reported evidence that the addition of PO4 after SO4 and OX adsorption strongly facilitated SO4 and, to a lesser extent, OX desorption in a wide pH range (4.0–8.0). However, at pH 3.0 SO4 remained, in part, on goethite surfaces (
25% of SO4 adsorbed when added with OX). In soil environments, particularly at the soil–plant interface, organic and inorganic ligands may compete with each other for sorption sites onto soil components. The study of the mutual interaction of two or more ligands is of paramount importance for understanding the factors that affect the mobility of a given ligand in soil.
The aim of this work was to study the mutual adsorption at different pH values of SO4, PO4, OX, and/or CIT in binary and ternary systems, both onto a synthetic allophanic compound and an Andisol containing more than 40% of allophane.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Synthetic and Natural Allophanic Samples
A synthetic allophane coated with iron oxide (AlSiFe) was prepared according to the procedure described by Mora et al. (1994). Briefly, an aluminosilicate was synthesized in a 1000-mL Teflon beaker, where 200 mL of doubly distilled water and 74.1 mL of 1.20 mol L–1 aluminum chloride solution were mixed. Then, 1.57 mol L–1 potassium silicate solution was added at a rate of 0.2 mL min–1 with stirring, until pH 5.0 was achieved. After this, the aluminosilicate (AlSi) synthesis was performed at pH 5.0 by simultaneous addition of 5 mol L–1 HCl (drop-wise) and potassium silicate (57.4 mL in total). To the synthetic AlSi, Fe3+ was added as Fe(NO3)3 and the new aluminosilicate, AlSiFe, was washed with distilled water until chloride-free and kept in suspension. As described elsewhere (Jara et al., 2005) this aluminosilicate showed a SiO2/Al2O3 molar ratio of 2.2, a percentage of Fe2O3 of 5.0%, and a surface area of 341 m2 g–1 using the retention method of ethylene glycol monoethylether (EGME), and had a ZPC of 4.8. The surface charge and the ZPC were determined by potentiometric titration (Jara et al., 2005).
A soil sample (Andisol) was collected from a 2C horizon of a medial, ferrihydritic, mesic Typic Hapludand from the caldera of Roccamonfina volcano (south-central Italy). The classification and chemical and mineralogical properties of this soil are described elsewhere (Vacca et al., 2003). This soil showed a pH in H2O of 5.5 (1:2.5 soil/water) and in 1 M NaF of 11.0 (1:50 soil/solution), and a very low organic C content (2 g kg–1) and cation exchange capacity (1.85 cmol kg–1). The high pH value in 1M NaF must be attributed to the presence of allophanic materials into the soil (Fieldes and Perrot, 1996; Wada, 1980). This sample, which did not show crystalline materials to x-ray diffraction analysis, was characterized for a high content of allophanic materials (42%) with a Al/Si molar ratio of 2.96 and a content of Fe extracted by Na-dithionite-citrate (Fed) of 36.1 g kg–1 (Vacca et al., 2003).
Adsorption Isotherms
Appropriate quantities of the suspensions of AlSiFe (equivalent to 100 mg solid) or 100 mg of allophanic soil were equilibrated at 25°C with final volume of 30 mL and 0.1 mol L–1 KCl as background electrolyte. Predetermined quantities of freshly prepared SO4 (as K2SO4), PO4 (as KH2PO4), OX (as C2H2O4·2H2O), or CIT (as C6H8O7·H2O) solutions were added to give an initial concentration ranging from 0.1 to 5.0 mmol L–1. The pH of each suspension was kept constant at pH 5.5 (pH 5.0 for Andisol) for 24 h by addition of 0.01 or 0.1 mol L–1 HCl or KOH. The reaction flasks were put on a water bath at 25°C for 24 h with stirring. The suspensions were centrifuged at 12 000 x g for 12 min, and SO4, PO4, OX, and CIT were determined in the supernatants as described below.
Surface Charge Determination
The surface charge characteristics of the AlSiFe samples were measured by potentiometric titrations in absence and presence of PO4, OX, CIT, or SO4 (10–3 and 10–4 mol L–1). The AlSiFe suspensions were prepared by mixing 300 mg of solid samples with 100 mL of KCl at ionic strength determined. The titrations were done in an N2 atmosphere and at a constant temperature of 25°C. The titration was initiated from their original pH and 0.2 mL of 0.1 M KOH or HCl was added every 20 min. Zero point charge was determined by locating the common point of intersection of the potentiometric titration curves at different ionic strengths.
Simultaneous Adsorption of Sulfate with Oxalate, Citrate or Phosphate (Binary Systems)
The adsorption experiment on AlSiFe (100 mg) of binary systems SO4–OX, SO4–CIT, and SO4–PO4, containing equimolar initial concentrations of both the ions (0.3, 0.5, and 1.0 mmol L–1), was performed at different initial pH values ranging from 4.0 to 8.0. The reaction flasks were kept in a water bath at 25°C for 24 h.
Adsorption of SO4 onto the soil sample in the presence of increasing concentrations of PO4, CIT, or OX (initial PO4, CIT, or OX/SO4 molar ratio of 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) was determined by adding 150 mmol SO4 kg–1 at pH 4.5 in the presence of suitable amounts of PO4, CIT, or OX. The suspensions were shaken for 24 h, and their pH was periodically adjusted with 0.1 or 0.01 mol L–1 HCl or KOH to keep the pH at 4.5. Finally, the suspensions (20 mL) were centrifuged and filtered through 0.2-µm Millipore M.F. filters (Bedford, MA), and SO4 concentrations in solution were determined as described below.
To study the kinetics of adsorption of SO4 at pH 4.5 in the presence of equimolar amounts of PO4, CIT, or OX on the Andisol sample, suspensions containing 150 mmol kg–1 of SO4 and 150 mmol kg–1 of PO4, CIT, or OX were shaken for 0.16 to 360 h in darkness. The final suspensions, whose pH was kept constant during the entire reaction period, were centrifuged and filtered. Sulfate was determined in the solutions as described below.
Competitive Adsorption of Sulfate-Oxalate-Phosphate or Sulfate-Citrate-Phosphate (Ternary Systems)
The adsorption experiment on AlSiFe (100 mg) of ternary systems SO4–OX-PO4, SO4–CIT-PO4, containing equimolar initial concentrations of both the ions (0.3, 0.5, and 1.0 mmol L–1), was performed at different initial pH values ranging from 4.0 to 8.0. The reaction flasks were put on a water bath at 25°C for 24 h and the final pH of the suspensions was determined.
Sulfate and PO4 present in the final solutions were determined by high performance liquid chromatography (HPLC) using a Lachrom Chromatograph, a polyspher IC AN-1 column, an eluent of 1.5 mmol L–1 p-hydroxybenzoic acid + 10% methanol, pH 7.8 (adjusted with N,N-diethylethanolamine), and a conductivity detector. Citrate and OX were determined by HPLC using a Lachrom Chromatograph coupled with UV-Vis (
= 210 nm) detector. The mobile phase was 200 mM phosphoric acid, pH 2.1 (adjusted with NaOH), and the column was an RP-18 (particle size 5 µm). In the experiments performed using a soil (Andisol) sample, SO4, PO4, and OX present in the final solutions were determined by ion chromatography using a Dionex 2000i ion chromatograph (Dionex Co., Sunnyvale, CA) as described by Liu et al. (1999). The concentration of Si, Al, and Fe present in the final solutions was determined by atomic absorption spectroscopy using a Unicam 969AA spectrometer Solaar 594.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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OX SO4, while smaller it is this value, the interaction of the ion it will be higher with the surface.
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Adsorption isotherms of PO4, OX, and SO4 onto the natural soil sample at pH 5.0 also showed that PO4 adsorption capacity was much greater than that of OX and SO4 (Fig. 1b). On the other hand, the parameters of the Freundlich equation showed the same tendency for the adsorption on AlSiFe.
Surface Charge and Zero Point Charge
The excess surface charges and ZPC of the allophanic material AlSiFe determined both in the absence of or presence of increasing concentrations of the organic ligands are presented in Fig. 2 . The ZPC of the synthetic allophane occurred at pH 4.82. The presence of increasing concentrations of CIT, OX, and PO4 shifted the ZPC toward lower or higher pH values. In our experiments CIT decreased ZPC more than OX (Fig. 2a and 2b), PO4 or SO4. In fact, in the presence of 10–4 or 10–3 mol L–1 OX the ZPC shifted to 4.14 and 4.05, respectively, whereas in the presence of 10–4 or 10–3 mol L–1 CIT it shifted toward lower pH values, precisely 3.80 and 3.57.
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Adsorption of the Ligands
Experiments on the adsorption of SO4, PO4, CIT, and/or OX added alone or as a mixture in binary or ternary systems were performed at different initial pH values (4.0–8.0). The ligands concentrations used were 0.3, 0.5, and 1.0 mmol L–1, but for the sake of clarity only the data using a concentration of 0.3 mmol L–1 are shown in Fig. 3
through 5
and Table 2.
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Binary Systems
The adsorption of SO4 in the presence of equimolar amounts of PO4 in the range of pH 4.0 to 7.0 decreased significantly, particularly at low pH values. The efficiency of PO4 to depress SO4 adsorption was calculated according to the expression of Deb and Datta (1967):
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This expression was also used for calculating the efficiency of each ligand to depress the adsorption of the other anions.
The presence of PO4 decreased SO4 adsorption by 48% at pH 4.5, 35% at pH 5.5, and 25% at pH 6.0 (Table 2). Conversely, the efficiency of SO4 in preventing PO4 adsorption was less at all the pH values as reported by Mora et al. (2005) on Chilean Andisols. At pH 6.0, SO4 reduced PO4 adsorption of 18%, which is comparable with the percentage decrease of SO4 adsorption affected by PO4 at pH 6.0 (Table 2).
The strong capacity of PO4 in preventing SO4 adsorption appeared evident by adding increasing amounts of SO4 in solutions containing a constant concentration of PO4 (1.93 mmol L–1) at pH 4.5 to have an initial SO4/PO4 molar ratio ranging from 0 to 10 (Fig. 6
). To have a final SO4 adsorbed/PO4 adsorbed molar ratio 
1 it was necessary to reach an initial SO4/PO4 molar ratio > 4. This suggests increased competition between PO4 and SO4 for adsorption sites, through either the higher affinity or the effect of mass action of the increasing concentration of SO4 in solution.
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6.0 SO4 sorption in the presence of OX increased if compared with SO4 adsorbed when added alone. A possible explanation of our findings is that OX (as well as CIT) solubilized Al, Fe, and Si from the noncrystalline AlSiFe in the range of pH used (Table 3), facilitating at pH values > 5.5 to 6.0 the formation of precipitates of Fe and/or Al containing OX and/or SO4 (OX/Fe/SO4 or OX/Al/SO4) and then promoting a partial removal of SO4 from solution.
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Citrate strongly inhibited SO4 adsorption mainly at pH < 5.5 (Fig. 5). Its efficiency ranged from 63% at pH 4.5 to 22% at pH 6.0, but at pH values > 6.0 its efficiency was negligible (Fig. 5). The capacity of CIT to prevent SO4 adsorption was much greater than OX and similar to that of PO4. However, at pH < 5.0 CIT appeared to be a stronger inhibitor than PO4, probably because of the solubilization of Al and Fe from the surfaces of the allophanic material with the consequent removal of some surface sites for SO4 sorption. At low pH values CIT solubilized Al, Fe, and Si more than OX (Table 3).
Citrate inhibited SO4 adsorption more than OX in spite of the fact that CIT was adsorbed onto AlSiFe less than OX (Fig. 1). According to Barrow (1992), the competition for sorption sites may be due to changes in the electric potential of the surfaces after the addition of anions more than to competition for surface sites. This appears to be particularly true when the surface coverage of the sorbent by the ligands is low (as in our experiments) and when one of the two ligands that compete for sorption sites forms fairly weak complexes on the surfaces of the sorbent (outer-sphere complexes, as is the case of SO4). Probably because CIT lowered the ZPC and rendered the surface charge of AlSiFe more negative than did OX (Fig. 2) the sorption of SO4 was prevented and/or retarded (as discussed below) more in CIT than OX systems.
The effect of PO4, CIT, or OX on the adsorption of SO4 was also studied using as a sorbent a natural sample of an Italian Andisol, which contained 42% allophanic materials (Vacca et al., 2003). Figure 7 shows the adsorption of SO4 at pH 4.5 in the presence of increasing concentrations of PO4, CIT, or OX (initial PO4, CIT, or OX/SO4 [R] molar ratio ranging from 0 to 1.0). After 24 h of reaction CIT prevented SO4 adsorption more than PO4 or OX. At R = 1.0 only 8% of SO4 was adsorbed in the presence of CIT versus 28 and 31% in the presence of PO4 and OX, respectively. However, the extent of competition was also related to sorption kinetics (Fig. 8 ) as well as surface coverage of the sorbent by the ligands (data not shown).
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Furthermore, since the surface coverage of CIT (and OX) was greater than that of PO4 the competition with SO4 for common adsorption sites was greater for the organic ligands than for PO4.
Ternary Systems
Sulfate adsorption onto AlSiFe in the presence of both OX and PO4 (SO4 + OX + PO4 system) was lower than in the presence of OX (SO4 + OX system; Fig. 4; Table 2), particularly at pH > 5.0, but similar to adsorption in the presence of PO4 (SO4 + PO4 system; Fig. 3 and 4; Table 2), indicating that the addition of OX in the binary systems SO4 + PO4 did not significantly influence the adsorption of SO4. Vice versa, the addition of both PO4 and CIT (ternary system SO4 + CIT + PO4) significantly decreased SO4 adsorption as compared with the binary systems SO4 + PO4 and SO4 + CIT (Table 2; Fig. 3 and 5). In the ternary system SO4 + CIT + PO4 the efficiency of the ligands in preventing SO4 adsorption onto AlSiFe ranged from 43 to 70% in the pH range 4.5 to 6.0.
In the binary systems SO4 + CIT and SO4 + OX, the adsorption of both the organic ligands was very poorly affected by the presence of SO4, as discussed before, but in the ternary systems the presence of PO4 strongly prevented their adsorption, particularly at pH > 5.5. Inhibition ranged from 38% at pH 4.5 to 48% at pH 6.0 for CIT and from 41% at pH 4.5 to 53% at pH 6.0 for OX (Table 2; Fig. 4 and 5). The efficiency of PO4 in preventing CIT or OX adsorption was similar. The efficiency of OX and CIT in inhibiting PO4 adsorption in ternary systems was lower than that of PO4 in preventing the adsorption of organic ligands onto allophanic material. Oxalate showed a slightly greater capacity to prevent PO4 adsorption than CIT. The exact reason for these findings is not clear, but it is possible that OX competed with PO4 for more sorption sites on AlSiFe than did CIT (Fig. 1). However, we have found that CIT prevented PO4 adsorption onto the Andisol slightly more than OX. Indeed, Balistrieri and Chao (1987) found that CIT inhibited selenite adsorption on goethite more than OX, but Dynes and Huang (1997) found a greater efficiency of OX in preventing selenite adsorption onto a poorly crystalline Al-oxide. Clearly, the nature of the sorbent also plays a significant role in the adsorption competition between selected LMMOAs and inorganic nutrients and pollutants.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The extent of competition was related not only to pH and surface coverage, but also to reaction time. The presence of CIT, OX, and PO4 strongly affected the kinetics of adsorption of SO4. The effectiveness of CIT, OX, or PO4 in preventing SO4 adsorption decreased with increases in the reaction time. However, even after 15 d CIT prevented SO4 adsorption more than did SO4 and OX.
Sulfate adsorption in the presence of both OX and PO4 was lower than in the presence of OX, but similar to the sorption in the presence of PO4, indicating that the addition of OX in the binary system SO4 + PO4 did not significantly influence the adsorption of SO4. Conversely, the addition of both PO4 and CIT significantly decreased SO4 adsorption as compared with the binary system SO4 + PO4 and SO4 + CIT.
In the ternary systems, the presence of PO4 strongly prevented the adsorption of OX and CIT. The efficiency of PO4 in inhibiting CIT or OX adsorption was fairly similar. In ternary systems the efficiency of OX and CIT in inhibiting PO4 adsorption was lower than that of PO4 in preventing the adsorption of organic ligands onto allophanic material.
This study provides useful information on the mutual influence of organic and inorganic ligands on their adsorption/desorption reactions in soil environments and particularly at the soil-root interface where LMMOAs may easily compete with nutrients for sorption sites onto soil colloids. However, the effect of pH, nature of the sorbent, kinetics of the reaction, and surface coverage of ligands on the mobility of selected ligands in binary and ternary systems deserve additional attention.
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ACKNOWLEDGMENTS |
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Received for publication March 17, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-alumina and kaolinite: Triple layer model. Soil Sci. Soc. Am. J. 61:784–793.
-FeOOH) particles. Geochim. Cosmochim. Acta 60:4385–4395.[CrossRef]-Al2O3 by the triple model. J. Colloid Interface Sci. 233:259–264.[CrossRef][Web of Science][Medline]This article has been cited by other articles:
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  B. D. Strahm and R. B. Harrison Controls on the Sorption, Desorption and Mineralization of Low-Molecular-Weight Organic Acids in Variable-Charge Soils Soil Sci. Soc. Am. J., September 30, 2008; 72(6): 1653 - 1664. [Abstract] [Full Text] [PDF]
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