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DIVISION S-2—SOIL CHEMISTRY

Interaction of Iron Chelating Agents with Clay Minerals

^a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, the Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
^b Dep. of Microbiology and Plant Pathology, Faculty of Agricultural, Food and Environmental Quality Sciences, the Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel

^* Corresponding author (yonachen{at}agri.huji.ac.il).

	ABSTRACT

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Organic ligands play an important role in metal transport and availability in soils. However, their interaction with the solid phase of soils has not been sufficiently investigated. Two efficient Fe chelators were investigated in this study, as free ligands as well as Fe complexes: (i) the microbial siderophore ferrioxamine B (desferrioxamine B [DFOB] and ferrioxamine B [FOB], respectively); (ii) the synthetic chelating agent ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA and FeEDDHA, respectively). Batch experiments were conducted to characterize their adsorption to Ca- and Na-montmorillonite as well as to Fe-montmorillonite (for EDDHA and FeEDDHA) and Ca-kaolinite. Kinetics of adsorption, equilibrium adsorption isotherms, and effects of pH on adsorption were measured. Adsorption of DFOB and FOB to montmorillonite was rapid and adsorption isotherms indicated high affinity. The type of saturating cation affected the adsorption of DFOB but not that of FOB. Significant hysteresis between adsorption and desorption was exhibited. The effect of Ca²⁺ in solution on desorption compared with that of Na⁺, was found to vary with their solution concentrations. Ethylenediamine di-o-hydroxyphenylacetic acid adsorption to montmorillonite exhibited a linear adsorption isotherm and significantly higher affinity than that of FeEDDHA. The adsorption decreased significantly with the following order of saturating cation: Ca²⁺ > Fe³⁺ > Na⁺. Adsorption of all examined substances to kaolinite was extremely low. Adsorption mechanisms of DFOB and EDDHA and their Fe complexes to clays and the environmental implications are discussed.

Abbreviations: CAS, Chrome Azurol S • CEC, cation-exchange capacity • DFOB, desferrioxamine B • EDDHA, ethylenediamine di-o-hydroxyphenylacetic acid • FOB, ferrioxamine B

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
IRON IS A vital nutrient for living organisms. Despite its abundance in soils, plants growing in neutral to alkaline soils are often subjected to Fe deficiency due to its low solubility under the pH and redox conditions prevailing in these environments (Chen and Barak, 1982; Chen and Shenker, 2003). Iron concentration in soil solution is often higher than that expected from chemical equilibria equations of soil Fe minerals. This enhancement is partially ascribed to the presence of organic molecules exhibiting various extents of Fe-chelation abilities. Representatives of two important groups of these molecules will be examined in this study:

	Microbial siderophores—

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Low-molecular-weight chelating agent, excreted by microorganisms under Fe deficiency and possessing high affinity to Fe. Siderophores have been found to promote Fe solubilization from various soil minerals. Their function was recently reviewed by Kalinowski et al. (2000) and remains the subject of further investigation (e.g., Cheah et al., 2003). Another Fe source in soils is Fe-humic complexes (Solinas et al., 1994). Siderophore concentrations that were high enough to positively affect plant nutrition were found in soil extracts (Powell et al., 1980; Bossier and Verstraete, 1986; Crowley et al., 1987). In soils enriched with macronutrients, especially in the plant rhizosphere, the concentrations of hydroxamate siderophores were reported to be significantly higher (Powell et al., 1982; Bossier and Verstraete, 1986). The microbial siderophore FOB was chosen in this study because of its extensive use in siderophore research and its presence in soils. It is produced by Streptomyces and Nocardia bacteria (Leong and Raymond, 1975) and belongs to the group of hydroxamate siderophores. Iron carried by FOB has been found to be an efficient source of Fe for many microorganisms (Winkelmann, 1991), as well as for a variety of plants (e.g., Cline et al., 1984; Jurkevitch et al., 1988; Crowley et al., 1991; Wang et al., 1993; Siebner-Freibach et al., 2003a). The bacteria excrete the protonated linear siderophore (Fig. 1a) as a free ligand (DFOB). Iron is bound to this ligand to form a highly stable complex (FOB). Upon chelation, the three hydroxyls of the hydroxamate groups are deprotonated and bound to Fe along with the carbonyl oxygens. Thus, the resultant complex is rather hydrophobic compared with the free ligand. The molecule is fairly wide and flat (total thickness 5.5 Å) except for the free chain containing the protonated amine, which can move freely (Dhungana et al., 2001, Fig. 1b). The net charge of both DFOB and FOB in acid to slightly alkaline solutions is positive (Table 1).

View larger version (18K): [in this window] [in a new window]   Fig. 1. Schematic diagrams of (a) DFOB–desferrioxamine B, the free ligand of the siderophore ferrioxamine B; (b) FOB–the ferric complex of DFOB; (c) EDDHA–the free ligand of the synthetic ethylenediamine di-o-hydroxyphenylacetic acid; (d) FeEDDHA–the ferric complex of EDDHA.

	Synthetic Fe chelating agents—

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

The transport and availability to soil biota of microbial, as well as synthetic chelates, are strongly influenced by their interactions with the solid phase. Clay minerals comprise a major part of the specific surface area in soils, thus their interaction with chelating agents and their Fe complexes is of great importance. An example was reported for the human pathogenic fungus Histoplasma capsulatum (Lavie and Stotzky, 1986). This fungus does not develop in montmorillonite rich soils due to the strong adsorption of its hydroxamate siderophore to the clay. Hoper et al. (1995) showed that the levels of suppressiveness to Fusarium wilt were increased in the presence of montmorillonite and illite, while the presence of kaolinite had no effect. Various studies have demonstrated a strong interaction of the cationic FOB with clayey soils (Powell et al., 1982; Cline et al., 1983; Reid et al., 1985). Neubauer et al. (2000) studied the effects of the free ligand DFOB on Cu²⁺, Zn²⁺, and Cd²⁺ adsorption to clay minerals. They found that in the absence of these metal ions DFOB sorbs to montmorillonite to a significantly greater extent than to kaolinite. Binding of synthetic chelates to soils has been found to vary in accordance with the complexed metal (Wallace and Lunt, 1956; Lahav and Hochberg, 1975). Their adsorption has been attributed to the clay fraction in soils (Lunt et al., 1956; Wallace and Lunt, 1956), but very little is known about the contribution of clay minerals to adsorption and its mechanism. Recently, Alvarez-Fernandez et al. (2002) studied the interactions of various agricultural Fe complexes with soils and soil components and measured their losses from the solution. Calcium-montmorillonite was not found as influential as peat and ferrihidrite. Nevertheless, we chose to study the sorption of the abovementioned ligands and chelates to montmorillonite, saturated with Ca²⁺, Na⁺, and Fe³⁺ because it represents the major component of the clay fraction in arid and semi-arid soils, where Fe deficiency is common. Montmorillonite is a 2:1 clay mineral that exhibits a high density of negatively charged surfaces. Adsorption to montmorillonite was compared with that of kaolinite.

Our study focused on both forms: the free ligands (DFOB and EDDHA) and their ferric complexes (FOB, FeEDDHA). The composition of the sorbing complex, soil conditions such as pH changes and various concentrations and compositions of soil solution, were expected to play an important role in the interactions of these substances with clay minerals in soil, and were examined in this study.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Chelating Agents
The microbial siderophore DFOB was purchased as the medical reagent Desferal from Ciba-Geigy Ltd. (Basel, Switzerland). Ethylenediamine di-o-hydroxyphenylacetic acid was purchased as an analytical reagent (Sigma Chemical Co., St. Louis, MO).

Clays
Montmorillonite (Wyoming Bentonite)—the clay fraction (<2 µm) was separated from the bulk, using the sedimentation method: the clay was dispersed in distilled water, and then allowed to settle down for the appropriate time according to Stokes law. For the measurement of cation exchange capacity (CEC) the clay was saturated with Na (using CH₃COONa) then washed thoroughly. The CEC of the clay was determined by measuring Na⁺ desorbed by CH₃COONH⁺₄ (Rhoades, 1982) and found to be 86 cmol Kg^–1. Cation saturation of the monoionic Na⁺– and Ca²⁺–montmorillonite was achieved by washing the clay three consecutive times with an excess of the appropriate Cl salt solution, and then with distilled water until the suspension was free of Cl as detected using AgNO₃. Ferric–montmorillonite was prepared using the titration method (Shaked, 1989): The clay was washed with HCl to transform it to H⁺ homoionic clay. Excess acid was removed using an OH^– saturated anion exchanger to achieve an acid-free H⁺–montmorillonite suspension. The suspension was titrated with a FeCl₃ solution. Fe³⁺ ions were exchanged with the H⁺ adsorbed to the clay and H⁺ excess was neutralized with the OH^– saturated resin. The titration was conducted until the pH reached values of 5.0 to 6.0 and the electrical conductivity started to increase. The clay was separated from the suspension by centrifugation, freeze-dried, and ground.

Kaolinite (Georgia KGa-1)—separation of the clay fraction was achieved by dispersion in 10^–4 M NaHCO₃ at pH 9 (adjusted with NaOH), followed by sedimentation. The clay was separated from the suspension by centrifugation, freeze-dried, and ground. The CEC according to the CMS data handbook (Van Olphen and Fripiat, 1979) was 2 cmol Kg^–1. Saturation with Ca²⁺ was achieved as described above.

Stock Solution and Suspensions
All the solutions and suspensions were adjusted to the desired pH with HClO₄ and KOH and then the volume was adjusted with distilled water to obtain the appropriate concentration. Clay suspensions were prepared with water and KClO₄ to achieve a clay concentration of 5 g L^–1 and an ionic strength of 0.01 M in the final reaction flask. Desferrioxamine B was dissolved in distilled water, then the pH was adjusted to the desired level. Ethylenediamine di-o-hydroxyphenylacetic acid was dissolved in water, stirred vigorously and a 1 M NaOH solution was added continuously to raise the solution pH to 9, until complete dissolution was achieved. Then the pH was adjusted to the final experimental level. The Fe complexes of both chelates were prepared by adding FeCl₃ to the appropriate free ligand solution with a 10% excess of Fe, then centrifuged at 106000 x g and filtered through 0.1-µm filters, to remove Fe colloids.

Stock solution concentrations of the free ligands (DFOB and EDDHA) were measured using the Chrome Azurol S (CAS) method (Shenker et al., 1995). Briefly: a solution containing a known concentration of Cu-CAS (Cu²⁺ complexed with Chrome Azurol S) was added to the measured solution. The free ligands (DFOB, EDDHA) removed Cu from the colored complex, as measured by its UV absorption at 582 nm. The concentration of the measured ligand in solution was calculated from the absorption at this wavelength. The concentration of the solutions of Fe complexes (FOB and FeEDDHA) was determined using atomic absorption spectroscopy (Perkin-Elmer 2380, Norwalk, CT).

Sorption Studies
The adsorption of both the free ligands (DFOB, EDDHA) and their complexes with Fe (FOB, FeEDDHA, respectively) was studied. These studies were conducted in polypropylene centrifuge tubes. The solution of each chelating agent or Fe complex was added to the clay suspension at the appropriate volume to achieve the desired concentrations. Then it was brought to a final volume of 10 mL (unless otherwise mentioned) with distilled water. The tubes were shaken for the appropriate time at 25°C, and then centrifuged at 25°C, 40000 x g for 10 min. The supernatant was filtered through a 0.45-µm filter of cellulose acetate. Adsorption to the various containers and filters used in our experiments was tested and found negligible. The concentration of the nonsorbed chelate in the supernatant solution was measured using a Hewlett Packard 8452A diode array spectrophotometer (Hewlett Packard, Palo Alto, CA) at the appropriate wavelength of the complex with Fe (428 for FOB and 480 for FeEDDHA). To facilitate measurements of the free ligand, Fe was added to the filtrate as FeCl₃, after which a HEPES buffer solution was added (0.2 M, pH 7.5). This method was used to ensure constant pH throughout the spectroscopic measurements on the one hand, and to prevent sorption of the siderophore to Fe oxides created by precipitation of the excess Fe, on the other. After a 1-h equilibration at each stage, the solutions were filtered through a 0.45-µm filter to remove Fe colloids. Adsorption was calculated as the difference between the initial and final concentrations.

Measurements of Sorption Kinetics
A stock suspension of Na, Ca, and Fe monoionic montmorillonite and Ca-kaolinite, was prepared and adjusted to pH 7.5, which is typical of calcareous soils. The suspension was divided into centrifuge tubes. Solutions of the chelating agents and Fe complexes were prepared at pH 7.5 and added to the tubes to obtain a volume of 10 mL and a final concentration of 1.5 mM for DFOB and FOB, or 0.54 for EDDHA and FeEDDHA. Blanks of clay suspensions and ligands and chelates solutions were prepared as well. The tubes were shaken for the following time periods: 0 and 2 h, 1, 2, 7, and 30 d. Two replicates for each time period were sampled and the nonsorbed concentration was measured. According to the results, the time required to obtain a steady state was determined and used for subsequent experiments.

Adsorption Isotherms
Adsorption isotherms were determined at pH 7.5, using the free ligands (DFOB, EDDHA) and the Fe complexes (FOB, FeEDDHA). The stock suspension of the monoionic clay was added to centrifuge tubes with the appropriate volumes of distilled water and ligand or chelate stock solution, to a total amount of 10 mL. The final clay concentration in the centrifuge tube was 5 g L^–1 and the concentrations of the adsorbates were spread over the desired range. The tubes were shaken for a 3-d period, which was found sufficient to achieve a steady state. Concentrations of the nonsorbed chelating agent or Fe complex were measured. Adsorption isotherms were measured on monoionic Na- and Ca-montmorillonite, as well as on Ca-kaolinite, for DFOB and FOB. To test chelation of exchangeable cations as a possible adsorption mechanism, Fe-montmorillonite was included in the sorption studies of EDDHA. To evaluate the potential influence of surface coating by Fe oxides on the adsorption, adsorption isotherms were compared with those obtained after removal of Fe oxides from the Ca-montmorillonite surface, using the citrate-bicarbonate-dithionite method as described by Loeppert and Inskeep (1996). Adsorption isotherms of DFOB and FOB were fitted to the Langmuir equation:

where q is the amount adsorbed (mmol g^–1), C_e is the adsorbate concentration at equilibrium, k is a constant related to the bonding energy and b is the adsorption maximum. Linear regressions were obtained using the linear transformation to obtain the constants k and b:

The Effects of pH on Adsorption
Variations in pH are common in the rhizosphere, and it is therefore important to assess their effect. Adsorption to Ca-kaolinite and Ca-montmorillonite was measured for the free ligands (DFOB, EDDHA) and the Fe complexes (FOB, FeEDDHA) at four pHs (4.0, 6.0, 7.5, and 9.0) with a concentration of 1.0 mM. The pH was adjusted using KOH and HClO₄ in all solutions as well as in the clay suspensions. The ionic strength was fixed as already described. The clay suspension was mixed with the appropriate solution in separate centrifuge tubes for each of the selected pH conditions and shaken for 3 d at 25°C. Adsorption levels were measured in the filtrate after centrifugation. To corroborate the mechanism of adsorption, aqueous speciation of the amphoteric sites of the chelating agents, Fe complexes and clays, were calculated using the Geochem software (Parker et al., 1995). Stability constants according the references mentioned in Table 1 were incorporated into the Geochem database and the experimental conditions were provided as data input. Estimates of the concentrations of amphoteric sites of montmorillonite were taken from Bradbury and Baeyens (1997). Those of kaolinite were calculated assuming a surface-site density of 1 site nm^–2 (0.6–6 sites nm^–2 according to Schroth and Sposito, 1998) and a specific surface area of 10 m² g^–1 for KGa-1 (Van Olphen and Fripiat, 1979).

Desorption of the Adsorbed Siderophore
The influence of salt concentration and composition on desorption of the sorbed chelating agents and Fe complexes was studied to simulate different conditions of salinity in soil solution. Clay with the adsorbed DFOB and FOB (0.28 mmol g^–1) was prepared at pH 7.5, using the same method as in the former experiments. It was then washed three times with distilled water until the solution concentration reached the level of blank solutions. The clay was then freeze-dried and the powder was kept in a desiccator. The influence of the following solutions was examined: (i) distilled water; (ii) CaCl₂ at 0.001 M, 0.01 M (representing a nonsaline soil solution concentration) and 1.0 M; and (iii) NaCl at 0.001, 0.01, 0.1, and 1.0 M. Treated clay (0.025 g) was weighed, placed into centrifuge tubes and suspended in 5 mL of the salt solution (final clay concentration of 5 g L^–1). The suspension was shaken overnight, then centrifuged at 40000 x g at 25°C and 4.5 mL of the supernatant solution were carefully taken out and filtered. Another dose of 4.5 mL was added, and the whole process was repeated five times. The quantity desorbed was calculated from the filtrate concentration. The contribution of the former residual solution was calculated and subtracted from the value obtained from each measurement.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Adsorption of the Siderophore
Kinetics Measurements
The adsorption of both DFOB and FOB to Ca- and Na-montmorillonite was rapid and high. Most of the reaction occurred within the first hour. The levels of adsorption remained close to constant from Day 3 to as long as one month. Desferrioxamine B rapidly reached a constant level of adsorption whereas FOB adsorption required two more days to reach its maximum (Fig. 2a) . The differences in the adsorption kinetics between DFOB and FOB can be explained by the steric structure of the molecule: the free ligand has a long linear structure with a positive charge at its end (Fig. 1a). To create the Fe complex, the molecule changes its configuration to a large sphere around the metal ion (Fig. 1b), and steric interferences in adsorption can occur. Reorganization of the FOB molecule is probably the reason for the final sluggish stage of adsorption. This hypothesis is strengthened by the advantage FOB sorption to Na-montmorillonite has over that to Ca-montmorillonite (Fig. 2b). Clays saturated with monovalent cations, such as Na⁺, are well dispersed in solution and an extensive fraction of the adsorbing surfaces are exposed. In contrast, Ca²⁺–saturated montmorillonite forms tactoids consisting of several platelets organized in a face-to-face arrangement (Sposito, 1984). In this context it should be mentioned that our assumption was that FOB adsorbs to the clay surface as a complex with Fe creating a ternary complex. In a recent report (Siebner-Freibach et al., 2003b) we confirmed that this is indeed the adsorption mechanism.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Changes in solution concentration vs. time. (a) 1.98 mM desferrioxamine B (DFOB) and 2.04 mM the ferric complex of DFOB (FOB) with Ca-montmorillonite (results with Na-montmorillonite exhibited similar relations between DFOB and FOB). (b) FOB with Ca- and Na-montmorillonite. Experimental conditions: two replicates; 5 g L–1 clay minerals; pH 7.5; I = 0.01 M (KClO4).

View larger version (17K): [in this window] [in a new window]   Fig. 3. Adsorption isotherm on Na- and Ca-montmorillonite of: (a) desferrioxamine B (DFOB); (b) the ferric complex of DFOB (FOB). Symbols represent measured values, lines represent calculated values. Experimental conditions: two replicates; 5 g L–1 clay minerals; pH 7.5; I = 0.01 M (KClO4).

Determination of DFOB and FOB adsorption to Ca-kaolinite revealed a very low affinity; most of the data obtained under the experimental conditions were within the error range (complete adsorption isotherms are not shown). Additional details can be found in the following section and in Fig. 4b .

View larger version (28K): [in this window] [in a new window]   Fig. 4. Calculated charge of amphoteric sites of clay minerals (bold lines represent the sum of all species bearing the same type of charge—positive, negative or neutral) compared with measured values of adsorption to these minerals (dotted lines) vs. pH. (a) Ca-montmorillonite; (b) Ca-kaolinite (note different scales compared with montmorillonite in Fig. 4a). Experimental conditions: two replicates; 1.0 mM of desferrioxamine B (DFOB), the ferric complex of DFOB (FOB), ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA), and FeEDDHA; 5 g L–1 clay minerals; pH values 4.0, 6.0, 7.5, 9.0; I = 0.01 M (KClO4).

View larger version (21K): [in this window] [in a new window]   Fig. 5. (a) Desferrioxamine B (DFOB); (b) the ferric complex of DFOB (FOB). Calculated charge (bold lines represent the sum of all species bearing the same type of charge: positive, negative or neutral) compared to measured values of adsorption of DFOB and FOB to Ca-montmorillonite and Ca-kaolinite (dotted lines) vs. pH. Experimental conditions: 1.0 mM DFOB or FOB; 5 g L–1 clay minerals; pH values 4.0, 6.0, 7.5, 9.0; I = 0.01 M (KClO4); two replicates.

Desorption of the Adsorbed Siderophore
Following sequential washes with various concentrations of the salts NaCl and CaCl₂, the desorbed amount of sorbed DFOB and FOB was calculated from concentrations measured on the filtrates. The results (Fig. 6) strengthen our hypothesis that cation exchange is the main sorption mechanism. At low solution concentration (0.01 M ), the efficacy of divalent cations (Ca²⁺) in siderophore desorption was higher than that of the monovalent ones (Na⁺). The amount desorbed by Na⁺ under these conditions (similar to soil solution concentrations) resembled that with distilled water. The differences in the desorbed amount cannot be explained by the differences in cation valency alone. They are indeed related to the preferential energy of the divalent cation based on its charge density. However, at the highest concentration, desorption efficacy was higher for Na⁺ than for Ca²⁺ in the initial washing cycles, probably due to the dispersion effects of Na⁺ on montmorillonite platelets. The levels of desorbed siderophore by all salt solutions decreased from one washing cycle to the next, with the exception of the 0.1 M NaCl treatments. These solutions caused a moderate but constant release of sorbed DFOB and FOB (Fig. 6a and 6c, respectively). This might be explained by the gradual conversion of Ca-montmorillonite to Na-montmorillonite, which opens the clay structure and facilitates the continuation of desorption. At the highest concentrations of NaCl ( 0.1 M), the desorbed fraction of DFOB was much higher than that of FOB (Table 3). Although high concentrations of desorbing solutions and vigorous shaking were employed, the desorbed siderophore levels were much lower than the adsorbed ones (Table 3). This hysteresis effect may have resulted from the difference in the kinetics of the desorption and the adsorption processes.

View larger version (29K): [in this window] [in a new window]   Fig. 6. Desorption of desferrioxamine B (DFOB) and the ferric complex of DFOB (FOB) from Ca-montmorillonite by NaCl and CaCl2. (a) DFOB desorbed by NaCl; (b) DFOB desorbed by CaCl2; (c) FOB desorbed by NaCl; (d) FOB desorbed by CaCl2. Experimental conditions: two replicates; 0.28 mmol g–1 adsorbed DFOB or FOB, at pH 7.5; 5 g L–1 Ca-montmorillonite.

Kinetics Measurements
Adsorption of the synthetic ligand EDDHA to montmorillonite was low but rapid, reaching a steady state after 4 h (Fig. 7) . Adsorption of FeEDDHA was even lower, and changes with time were within the range of analytical error (data not shown).

View larger version (17K): [in this window] [in a new window]   Fig. 7. Changes in ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA) solution concentration vs. time. Experimental conditions: two replicates; 0.544 mM EDDHA; 5 g L–1 Ca-montmorillonite. pH 7.5; I = 0.01 M (KClO4).

View larger version (24K): [in this window] [in a new window]   Fig. 8. Ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA) and FeEDDHA adsorption isotherms on Ca-, Na-, and Fe-montmorillonite. (a) EDDHA; (b) FeEDDHA. Experimental conditions: two replicates; 5 g L–1 clay minerals; pH 7.5; I = 0.01 M (KClO4).

The strong dispersive effect observed during this research with Na-montmorillonite support adsorption to clay edges. On the other hand, the low adsorption to Na-montmorillonite and the extent of EDDHA adsorption to Ca-montmorillonite under the experimental conditions are hard to explain by the capacity of edge sites alone. Isotherms were measured at pH 7.5, at which the ligand is negatively charged (Fig. 9a) . At this pH, most of the amphoteric sites of the clay will be electrically neutral and only a small fraction will carry negative charges (Fig. 4a), thereby effectively overruling the possibility of electrostatic bonding. Hydrophobic sorption and the effect of van der Waals forces can be ruled out, since there was a large difference between EDDHA and FeEDDHA adsorption (one order of magnitude; note the different scales in Fig. 8a and 8b). Suggested adsorption mechanisms in this context are (i) H-bonding with oxygen or OH groups of clay surfaces and polarized hydration water of exchangeable cations (water bridge), (ii) salt bridges (Stevenson, 1994) formed through the exchangeable cation (clay-M-OOC-R), and (iii) ligand exchange. All suggested mechanisms are more likely to occur with the EDDHA molecule than with FeEDDHA, which is in full agreement with our results. While the suggested mechanisms explain well the difference between Fe- and Ca-montmorillonite, they do not provide an explanation for the low level of adsorption to Na-montmorillonite. This fact strongly implies the significance of salt-bridge formation as an important adsorption mechanism of EDDHA to montmorillonite, since it requires the involvement of polyvalent cations.

View larger version (20K): [in this window] [in a new window]   Fig. 9. (a) Ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA); (b) FeEDDHA. Calculated charge (bold lines represent the sum of all species bearing the same type of charge–positive, negative or neutral) compared with measured values of adsorption of EDDHA and FeEDDHA to Ca-montmorillonite and Ca-kaolinite (dotted lines) vs. pH. (points for both clay minerals overlap). Experimental conditions: two replicates; 1.0 mM EDDHA or FeEDDHA; 5 g L–1 clay minerals; pH values 4.0, 6.0, 7.5, 9.0; I = 0.01 M (KClO4).

The adsorption of FeEDDHA to montmorillonite was very low and exhibited a stepwise isotherm (Fig. 8b). The wide spheric structure of FeEDDHA could create a steric limitation for its adsorption. In addition, H-bonding with FeEDDHA is very limited compared with that of the EDDHA molecule and other mechanisms, such as salt bridge formation and ligand exchange, are not possible with the FeEDDHA complex. Adsorption isotherms of both EDDHA and FeEDDHA to kaolinite were very low within the error range.

pH Effects on Adsorption
The measured adsorption of FeEDDHA at a fixed concentration was close to zero over the entire pH range examined, for both clay minerals (Fig. 4 a,b). In contrast, EDDHA adsorption was higher and influenced by pH (Fig. 4 a,b). The sharp increase in EDDHA adsorption to Ca-montmorillonite at low pH requires an explanation. The absence of a charge for the adsorbate (Fig. 9a) could facilitate the proximity of the organic molecule to the mineral surface, thereby facilitating the formation of other type of bonding. At pH 6.0 we would have expected, yet did not observe, an electrostatic attraction between the fraction of EDDHA that is negatively charged (Fig 9a) and the fraction of amphoteric sites on the clay that are positively charged (Fig. 4a). The increase in adsorption of EDDHA at the higher pH range is hard to explain. The electrostatic repulsion was expected to reduce adsorption significantly, but instead, there was an increase in adsorption. Chelation of exchangeable Ca²⁺, which becomes significant above pH 8, may provide an explanation. A general conclusion is that the charge of the amphoteric sites of montmorillonite is not the main factor affecting EDDHA adsorption.

The pH dependence of EDDHA adsorption to kaolinite, in contrast, did react as expected based on amphoteric-site charge (Fig. 4b). Adsorption increased when the system was mainly neutral, allowing the molecules to approach functional groups on the mineral surface. Adsorption decreased on repulsion due to the negative charge of both the adsorbent and adsorbate.

A certain degree of clay dissolution at low pH and release of Al³⁺, consequently adsorbed as an exchangeable cation, cannot be excluded, and may contribute to the elevated adsorption to both clay minerals at lower pH values. Ligand exchange is also more effective in this range (Sposito, 1984)

	SUMMARY

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Montmorillonite was found to be highly important to DFOB and FOB behavior in soils, while kaolinite was not as effective. Desferrioxamine B and FOB exhibited a strong affinity to montmorillonite. Depending on the salinity and ion composition of the soil solution, adsorbed siderophore could then be partially desorbed, while the remainder continued to be strongly retained. It appears from the results that in a large part of mineral soils, a siderophore released to the soil solution by its microbial synthesizer will sorb to the solid phase and will not leach with the mass flow, as do most synthetic chelates. By binding Fe from its environment, it can therefore form a reservoir of sorbed Fe complex, facilitating continuous Fe supply to plants according to the composition and concentration of the soil solution. Examples of typical soil concentrations of siderophores and the corresponding amounts adsorbed are given in Table 4 (according to Langmuir constants, calculated previously). Plant-growth experiments, performed in the second part of this research, indeed proved that plants could use sorbed FOB as an Fe source, directly or by ligand exchange with free ligands such as EDDHA in solution (Chen, 2000; Siebner-Freibach et al., 2003a). The high preference of FOB adsorption, its slow release and pH stability, in addition to the high stability constant with Fe and its resistance to microbial degradation (Winkelmann et al., 1999) are important advantages in terms of its use as an Fe fertilizer.

Understanding the role of clay interactions with chelating agents as well as their Fe complexes can facilitate the prediction of retention/desorption processes under varying environmental conditions, such as soil–solution composition, pH and concentration, as well as water quality and application regime. These processes are of importance for the use of chelates in plant nutrition.

Received for publication June 18, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION Microbial siderophores— Synthetic Fe chelating... MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 

• Alvarez-Fernandez, A., M.A. Sierra, and J.J. Lucena. 2002. Reactivity of synthetic Fe chelates with soils and soil components. Plant Soil 241:129–137.
• Bossier, P., and W. Verstraete. 1986. Ecology of Arthrobacter JG-9-detectable hydroxamate siderophores in soils. Soil Biol. Biochem. 18:487–492.
• Bradbury, M.H., and B. Baeyens. 1997. A mechanistic description of Ni and Zn sorption on Na-montmorillonite. Part II: Modelling. J. Contam. Hydrol. 27:223–248.
• Calvet, R. 1989. Adsorption of organic chemicals in soils. Environ. Health Perspect. 83:145–177.[ISI][Medline]
• Cheah, S.F., S.M. Kraemer, J. Cervini-Silva, and G. Sposito. 2003. Steady-state dissolution kinetics of goethite in the presence of desferrioxamine B and oxalate ligands: Implications for the microbial acquisition of iron. Chem. Geol. 198:63–75.
• Chen, Y. 2000. Soil and foliar strategies for the remediation of iron deficiency. p. 28. In Abstracts, 10th international symposium on iron nutrition and interactions in plants. Houston, TX, 14–19 May 2000. USDA/ARS CNRC, Houston, TX.
• Chen, Y., and P. Barak. 1982. Iron nutrition of plants in calcareous soils. Adv. Agron. 35:217–240.
• Chen, Y., and M. Shenker. 2003. Agronomic approaches for increasing iron availability to food crops. Agricultural and molecular genetic approaches to improving nutrition and preventing micronutrient malnutrition globally. In I. Cakmak et al. (ed.) Encyclopedia of Life Support Systems (EOLSS). Developed under the auspices of the UNESCO, EOLSS Publisher, Oxford, UK. Available online at: http://www.eolss.net (verified 13 Nov. 2003).
• Cline, G.R., P.E. Powell, P.J. Szaniszlo, and C.P.P. Reid. 1983. Comparison of the abilities of hydroxamic and other natural organic acids to chelate iron and other ions in soil. Soil Sci. 136:145–157.
• Cline, G.R., C.P.P. Reid, P.E. Powell, and P.J. Szaniszlo. 1984. Effects of hydroxamate siderophore on iron absorption by sunflower and sorghum. Plant Physiol. 76:36–39.[Abstract/Free Full Text]
• Crowley, D.E., C.P.P. Reid, and P.J. Szaniszlo. 1987. Microbial siderophores as iron sorces for plants. p. 370–386. In G. Winkelmann, D. Van der Helm, and J. B. Neilands (ed.). Iron transport in animals, plants, and microorganisms. VCH Chemie, Weinheim, Germany.
• Crowley, D.E., Y.C. Wang, C.P.P. Reid, and P.J. Szaniszlo. 1991. Mechanisms of iron acquisition from siderophores by microorganisms and plants. Plant Soil 130:179–198.
• Dhungana, S., P.S. White, and A.L. Crumbliss. 2001. Crystal structure of ferrioxamine B: A comparative analysis and implications for molecular recognition. J. Biol. Inorg. Chem. 6:810–818.[ISI][Medline]
• Giles, C.H., D. Smith, and A. Huitson. 1974. General treatment and classification of solute adsorption isotherm. I. Theoretical. J. Colloid Interface Sci. 47:755–765.
• Hochberg, M., and N. Lahav. 1978. Movement of iron and zinc applied as EDTA complexes in soil columns. Plant Soil 50:221–225.
• Hoper, H., C. Steinberg, and C. Alabouvette. 1995. Involvement of clay type and pH in the mechanisms of soil suppressiveness to fusarium-wilt of flax. Soil Biol. Biochem. 27:955–967.
• Jurkevitch, E., Y. Hadar, and Y. Chen. 1988. Involvement of bacterial siderophores in the remedy of lime-induced chlorosis in peanut. Soil Sci. Soc. Am. J. 52:1032–1037.[Abstract/Free Full Text]
• Kafkafi, U., B. Bar-Yosef, R. Rosenberg, and G. Sposito. 1988. Phosphorus adsorption by kaolinite and montmorillonite: II. Organic anion competition. Soil Sci. Soc. Am. J. 52:1585–1589.[Abstract/Free Full Text]
• Kalinowski, B.E., L.J. Liermann, S. Givens, and S.L. Brantley. 2000. Rates of bacteria-promoted solubilization of Fe from minerals: A review of problems and approaches. Chem. Geol. 169:357–370.
• Lahav, N., and M. Hochberg. 1975. Kinetics of fixation of iron and zinc applied as FeEDTA, FeHDDHA, and ZnEDTA in the soil. Soil Sci. Soc. Am. Proc. 39:55–58.
• Lavie, S., and G. Stotzky. 1986. Interactions between clay minerals and siderophores affect the respiration of Histoplasma capsulatum. Appl. Environ. Microbiol. 51:74–79.[Abstract/Free Full Text]
• Leong, J., and K.N. Raymond. 1975. Coordination isomers of biological iron transport compounds. IV: Geometrical isomers of chromic desferrioxamine B. J. Am. Chem. Soc. 97:293–296.[ISI][Medline]
• Loeppert, R.L., and W.P. Inskeep. 1996. Iron. p. 639–664. Methods of soil analysis. Part 3. SSSA Book Ser. no. 5. SSSA, Madison, WI.
• Lunt, O.R., N. Hemaidan, and A. Wallace. 1956. Reactions of some polyamine polyacetate iron chelates in various soils. Soil Sci. Soc. Am. Proc. 20:172–175.
• Martell, A.E., R.M. Smith, and R.J. Motekaitis. 1997. NIST critically selected stability constants of metal complexes database (v. 4). Standard Reference Data Program. Gaithersburg, MD.
• Neubauer, U., B. Nowack, G. Furrer, and R. Schulin. 2000. Heavy metal sorption on clay minerals affected by the siderophore desferrioxamine B. Environ. Sci. Technol. 34:2749–2755.
• Parker, D.R., W.A. Norvell, and R.L. Chaney. 1995. GEOCHEM-PC—A chemical speciation program for IBM and compatible personal computers. p. 253–269. In R.H. Loeppert et al. (ed.). Chemical equilibrium and reaction models. SSSA Spec. Publ. No. 42, SSSA, Madison, WI.
• Powell, P.E., G.R. Cline, C.P.P. Reid, and P.J. Szaniszlo. 1980. Occurrence of hydroxamate siderophore iron chelators in soils. Nature 287:833–834.
• Powell, P.E., P.J. Szaniszlo, G.R. Cline, and C.P.P. Reid. 1982. Hydroxamate siderophores in the iron nutrition of plants. J. Plant Nutr. 5:653–673.
• Reid, R.K., C.P.P. Reid, and P.J. Szaniszlo. 1985. Effects of synthetic and microbially produced chelates on the diffusion of iron and phosphorus to a simulated root in soil. Biol. Fertil. Soils 1:45–52.
• Rhoades, J.D. 1982. Cation exchange capacity. p. 149–157. In A. Page, R.H. Miller and D.R. Keeney (ed.) Methods of Soil Analysis. Part 2. 2nd ed., Agron. Monogr. 9. ASA and SSSA, Madison, WI.
• Schroth, B.K., and G. Sposito. 1997. Surface charge properties of kaolinite. Clays Clay Miner. 45:85–91.[Abstract]
• Schroth, B.K., and G. Sposito. 1998. Effect of landfill leachate organic acids on trace metal adsorption by kaolinite. Environ. Sci. Technol. 32:1404–1408.
• Shaked, D. 1989. Chemical state and exchange reactions of cationic micronutrient elements adsorbed on montmorillonite. PhD Thesis, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot.
• Shenker, M., Y. Hadar, and Y. Chen. 1995. A rapid method for accurate determination of colorless siderophores and synthetic chelates. Soil Sci. Soc. Am. J. 59:1612–1618.[Abstract/Free Full Text]
• Siebner-Freibach, H., Y. Hadar, and Y. Chen. 2003a. Siderophores sorbed on Ca-montmorillonite as an iron source for plants. Plant Soil 251:115–124.
• Siebner-Freibach, H., Y. Hadar, S. Yariv, I. Lapides, and Y. Chen. 2003b. Thermo-FTIR-spectroscopy analysis of the siderophore ferrioxamine b and its ferric form using curve-fitting technique. p. 98. In Book of abstracts; MEDICTA 2003—6th Mediterranean conference on calorimetry and thermal analysis. Porto, PT. 27–30 July. 2003. University of Porto, Portugal.
• Solinas, V., N. Senesi, and T.M. Miano. 1994. Cation effects on the adsorption of deferoxamine-B (DFOB) by humic acid. p. 1183–1188. In N. Senesi, and T.M. Miano (ed.) Humic substances in the global environment and implications on human health. Proceedings of the 6th International Meeting of the International Humic Substances Society, 20–25 Sept. 1992, Monopoli (Bari), Italy. Elsevier, Amsterdam.
• Sposito, G. 1984. The surface chemistry of soils. Oxford University Press, New York.
• Stevenson, F.J. 1994. Clay-organic complexes and formation of stable aggregates. p. 429–452. In Humus chemistry. 2nd ed. John Wiley & Sons, New York.
• Tarchitzky, J., and Y. Chen. 2002. Polysaccharides and pH effects on sodium montmorillonite: Flocculation, dispersion, and rheological properties. Soil Sci. 167:791–801.
• Van Olphen, H., and J.J. Fripiat. 1979. Data handbook for clay materials and other non-metallic minerals: Data prepared under the auspices of the OECD and the Clay Minerals Society. Pergamon Press, Oxford.
• Violante, A., and L. Gianfreda. 1993. Competition in adsorption between phosphate and oxalate on an aluminium hydroxide montmorillonite complex. Soil Sci. Soc. Am. J. 57:1235–1241.[Abstract/Free Full Text]
• Wallace, A., and O.R. Lunt. 1956. Reactions of some iron, zinc, and manganese chelates in various soils. Soil Sci. Soc. Am. Proc. 20:479–482.
• Wang, Y., H.N. Brown, D.E. Crowley, and P.J. Szaniszlo. 1993. Evidence for direct utilization of a siderophore, ferrioxamine B, in axenically grown cucumber. Plant Physiol. 16:579–585.
• Winkelmann, G. 1991. Specificity of iron transport in bacteria and fungi. p. 65–105. In G. Winkelmann (ed.) CRC handbook of microbial iron chelates. CRC Press, Boca Raton, FL.
• Winkelmann, G., B. Busch, A. Hartmann, G. Kirchhof, R. Suessmuth, and G. Jung. 1999. Degradation of desferrioxamines by Azospirillum irakense: Assignment of metabolites by HPLC/electrospray mass spectrometry. Biometals 12:255–264.[ISI][Medline]

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