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Soil Science Society of America Journal 66:406-412 (2002)
© 2002 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Rheology of Sodium-montmorillonite suspensions

Effects of humic substances and pH

J. Tarchitzkya and Y. Chen*,b

a Field Service, Extension Service, Ministry of Agriculture, P.O. Box 28, Bet-Dagan 50250, Israel
b Department 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

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


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Organic matter (OM) is considered to act as a soil structure stabilizer. However, under certain conditions, either in suspension or in soils, addition of low concentrations of humic substances (HS) can result in particle dispersion. In this research, the rheology of montmorillonite suspensions was studied as a function of exchangeable cation, HS concentration (0–4000 mg L-1) and pH (4–10). The Na-montmorillonite suspensions exhibited non-Newtonian rheology at all pH values. The differential viscosity of the clay suspension decreased with increasing pH. The pseudoplastic non-Newtonian flow resulted from the associations between clay platelets. Addition of HS to clay suspensions changed the flow behavior from non-Newtonian to Newtonian as the HS concentration increased. At a shear stress of 0.1 Pa, the differential viscosity of the Na-montmorillonite was 33.7 mPa s. At a HS concentration of 100 mg L-1, the suspension showed a decrease in differential viscosity to 11.6 mPa s, and a further increase in HA concentration to 400 mg L-1 reduced the differential viscosity to 3.92 mPa s. Addition of 100 mg L-1 HS at the lower pH values (4, 6, and 8) caused a decrease in the attraction forces between the clay particles resulting in reduced differential viscosity at low shear stress (0.1 Pa). This phenomenon is in accordance with the edge-charge reversal mechanisms (from positive to negative) reported previously. An additional mechanism influences the differential viscosity of the suspensions. This mechanism is possibly associated with the formation of mixed micelles of HS with the clay platelets.

Abbreviations: DOM, dissolved organic matter • HA, humic acid • HS, humic substances • FA, fulvic acid • FV, Flocculation value • OM, organic matter • PZC, Point of zero charge


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ORGANIC MATTER is considered to act as a soil structure stabilizer (Stevenson, 1994). A number of investigators, however, have shown that under certain conditions, either in suspension or in soils, addition of low concentrations of HS can result in particle dispersion, which can lead to the disintegration of soil aggregates (Tarchitzky et al., 1999). Narkis et al. (1968) investigated surface waters containing clay and OM and reported an increase in suspension stability relative to suspensions of reference clays. In coagulation experiments, Gibbs (1983) found that natural organic coatings on a mixture of minerals suspended in river water (consisting of kaolinite, illite, chlorite, and montmorillonite) increased the flocculation value (FV) of the clay four fold compared with uncoated samples. Bloomfield (1954)( 1956, 1957) described deflocculation and reflocculation of kaolinite by aqueous leaf extracts of various plant species and Gillman (1974) explained changes in water-dispersible clay of basaltic soils in terms of the variation in the point of zero charge (PZC) of the clay edge. Shanmunagathan and Oades (1983) found that, increasing the concentrations of some anions in the soil solution resulted in the dispersion of the clay fraction, because of the fact that adsorbed anions lowered clay PZC. Similarly, HA was reported to disperse montmorillonite tactoids in suspension (Narkis et al., 1970) as well as soil particles from a humic gleysol (Visser and Caillier, 1988).

Goldberg et al. (1990) reported that the removal of OM from arid soils causes a decrease in clay dispersivity. This suggests that at the clay-particle level, negative charges of organic anions enhance clay dispersion. Goldberg and Foster (1990) found lower critical coagulation concentrations for reference clays than for soil clays, suggesting that the content of OM and that of Al and Fe oxides influences soil-clay dispersion. The natural abundance of soil humus may be sufficient to impart a high degree of dispersion to soil-clay fractions (Frenkel et al., 1992). Moreover, results reported by Kretzschmar et al. (1993) support the hypothesis that naturally occurring HS increase the colloidal stability of kaolinitic fine clays in aqueous suspensions. These authors suggested that a combination of electrostatic and steric stabilization is responsible for this effect. Kretzschmar et al. (1997)( 1998) described the effect of additions of small amounts of HA to kaolinite suspensions and found that at low pH flocculation was inhibited because of charge reversal from positive to negative. Similarly, Jekel (1986) reported that HS stabilize dispersed silica and kaolinite particles in suspension. Supportive evidence for potential dispersive properties of organic molecules was also reported by Gu and Doner (1993) who studied the influence of organic polyanions on soil properties (a soil HA, a soil polysaccharide, and a commercial anionic polysaccharide) and found them to serve as effective dispersing agents for Na-clays and soils. These investigators did not describe in detail the mechanism involved in the clay dispersion, although they suggested that organic polyanions are adsorbed on the clay edges, and steric interferences are involved.

The dispersive effect of Na-humate on clay suspensions was explained by Zhang et al. (1991) whose interpretation involved adsorption of humic polyanions onto the edges of clay particles resulting in a reduced attractive force between the particles. Van Olphen (1977), Frey and Lagaly (1979), and Tombacz et al. (1984) proposed a similar mechanism. Tarchitzky et al. (1993) explained the effect of HS on the stability of Na-montmorillonite suspensions at various pH values by an analogous edge-charge reversal (from positive to negative) mechanism, and by a second flocculation mechanism based on the random distribution of HS macromolecules among the montmorillonite tactoids (mutual flocculation). The measured effects on suspension stability were strongly dependent on the concentration of the HS as well on the pH. Tarchitzky et al. (1999) described the influence of dissolved organic matter (DOM) contained in reclaimed wastewater effluents (mainly HS) on the flocculation of montmorillonite. Flocculation values for Na-montmorillonite increased with increasing concentrations of DOM at all pH levels analyzed (5.5, 7.0, and 8.5). Maximum FV levels were exhibited for Na-montmorillonite at the highest DOM concentrations. The effect of DOM on FV can be explained by the same mechanisms of edge-charge reversal and mutual flocculation (Tarchitzky et al., 1993).

The rheological properties of either pastes or suspensions of clays are known to correlate with intraclay-particle interactions (Van Olphen, 1977). The effect of anionic macromolecules on the rheological properties of montmorillonite was studied for various organic and inorganic substances: Na-fulvate and Na-salycilate (Tombacz et al., 1984), Na-polyphosphate (Keren, 1988), and Na-humate (Zhang et al., 1991). In general, these researchers found a decrease in the viscosity of the suspension with an increase in the concentration of the anionic molecule. Edge adsorption of the anionic molecule was usually the explanation given for the results. In contrast, addition of Fe oxides to illite increased the yield stress of the suspensions of Fe-illite complexes because of smaller interparticle repulsion (Ohtsubo et al., 1991). The findings of Tombacz et al. (1984) and Zhang (1991) obtained by rehological measurements did not include observations on the effects of pH which are of major importance especially when the mechanisms involved are of concern. Therefore the aim of the present research was to substantiate the observations formerly made by measurement of flocculation values, using their rheological characterization. The influence of pH and HS concentration on these properties was studied.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Clay Preparation
Montmorillonite (Wyoming bentonite obtained from Fisher Scientific Corp., West Haven, CT) was fractionated by allowing larger particles to settle out of suspension and collecting the <2.0-µm fraction. This fraction was used to prepare homoionic clay of Na- and Ca-montmorillonite by washing the clay three times with a 1 M solution of the appropriate Cl salt. Excess salt was removed by washing the clay with distilled water until the supernatant was free of chlorides (<0.1 mM). The salt-free clay was freeze-dried and stored at room temperature. The homoionic clays were resuspended in distilled water when ready for laboratory tests and adjusted to the desired pH values with dilute HCl, NaOH, or Ca(OH)2 for Na- and Ca-montmorillonite, respectively.

Humic Acid and Fulvic Acid Extraction and Purification
Humic substances were extracted from a peat soil from the Hula Valley, Israel according to the procedure recommended by the International Humic Substances Society (Swift, 1996). The soil was shaken overnight with 0.1 M NaOH under N2 with a 10:1 extractant/soil ratio. The alkaline supernatant was separated from the residue by centrifugation; acidified with 6 M HCl to pH 1 and allowed to stand overnight at room temperature. The supernatant fulvic acid (FA) (FA-first fraction) was separated from the coagulate (HA) by centrifugation. Suspended clays were removed by dissolving the HA in a minimum volume of 0.1 M KOH + 0.2 M KCl (total of 0.3 M K), centrifuging, decanting, and collecting the HS from the supernatant solution as follows: The solution was acidified to pH 1 and the HA was allowed to precipitate, then this second fraction of the FA was separated from the HA by centrifugation. The second fraction of the FA was combined with the first fraction. The HA precipitate was treated with 0.1 M HCl + 0.3 M HF for 7 d so that the ash content was reduced to below 1%. The purified HA was then dialyzed against deionized water to remove free acid and salt and then freeze-dried.

The supernatant (FA) was passed through a prewashed XAD-8 resin (Rohm and Haas Chemical Co., Philadelphia, PA) in a plastic column, followed by 0.65-column volumes of deionized water. The FA was back eluted with 0.1 M NaOH (one column of 0.1 M NaOH, followed by two column volumes of deionized water). The eluent was passed through H+-saturated cation-exchange resin and freeze-dried.

Stock solutions of Na-humate (or Na-fulvate) were prepared by dissolving HA (or FA) with 0.1 M NaOH and dialyzing these solutions against dionized water. Stock solutions of Ca-humate (or fulvate) were prepared by dialyzing Na-humate (or fulvate) against a CaCl2 solution, followed by deionized water to remove free salts. Humate (or fulvate) solutions were prepared close to their testing time and were adjusted to the desired pH values with dilute HCl, NaOH, or Ca (OH)2 for Na- or Ca-humate (fulvate) respectively. The COOH, OH, and total acidity of the HA and FA, respectively, were as follows: 3.64, 1.89, 5.53, and 7.16, 2.34, 9.50 (mol kg-1). These values were determined by acid-base titrations following the procedure described by Bowles et al. (1989).

Preparation of Clay-Humic Substances Suspensions
Mixed clay-HS suspensions were prepared by the addition of the HS solutions to montmorillonite suspensions. The concentration of the clay was maintained constant at 25 g kg-1 and increasing levels of the HS suspensions were added. The sequence of addition (HS to the clay suspension) was based on an earlier study to ensure that no artifacts were involved (Tarchitzky et al., 1993). Each suspension had a total volume of 0.015 L and was stirred overnight.

Rheological Measurements
A viscometer (Gebruder Haake, Model RV2, Berlin, Germany) with a rotating inner cylinder and a stationary outer cylinder (Model NV ST, Gebruder Haake, Berlin, Germany) was used for the viscosity measurements. The distance between the two cylinders was 0.4 mm. The inner cylinder (rotor) was rotated at a known speed (related to the shear rate, {alpha}) and the stress transmitted by the suspension to the outer cylinder was determined from the deflection of an indicator needle connected through a torsion spring to the outer cylinder. The rotation rate of the rotor could be adjusted by means of a synchronous motor determining the shear rate. The range of shear rate applied varied from 5 to 2000 s-1. Small increments of shear rate were test in the low shear rate range and then were increased with increasing shear stress (Fig. 1) . Nine milliliters of suspension were added to the inner cylinder prior to measurement. The shear stress ({tau}) was determined at each shear rate ({alpha}) by observing the deflection of the needle. The values of the shear rate and the shear stress were obtained respectively from the rotation rate of the inner cylinder and the reading of the instrument by multiplication with the viscometer constants. Readings were taken when constant values were reached. The viscometer was calibrated against a standard Newtonian fluid before use. The total clay concentration in suspension for viscosity measurements was 25 g kg-1, and the temperature was 25.0 ± 0.1°C. Three replicates were tested for each point on the graphs. The values obtained were practically the same.



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  		Fig. 1. Rheograms of Na-montmorillonite suspensions at a clay concentration of 25 g kg-1, and various pH values and humic acid (HA) concentrations.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Theory
The rheological properties of stable clay suspensions depend on the following factors: (i) viscosity of the dispersing medium; (ii) particle concentration in the suspension; (iii) size and shape of the suspended particles; and (iv) interaction forces between particles (Van Olphen, 1977).

According to Van Wazer et al. (1963), the non-Newtonian rheology of colloid suspension can be described by the following power law Eq. [1]:

[1]
where {tau} is the shear stress (mPa), {alpha} is the shear rate (s-1), and a and b are the power law coefficients.

This rheogram can be transformed to its logarithmic form given in Eq. [2].


[2]

Thus, the values of a and b can be calculated from the linear plot of Eq [2].

Derivatization of Eq [1] is given in Eq [3]:

[3]

Where {eta}D is the differential viscosity (mPa s).

pH Effect on the Rheological Properties
The shear stress ({tau}) versus shear rate ({alpha}) curves for suspensions containing Na-montmorillonite and increasing concentrations of HA at four pH values are shown in Fig. 1. The Na-montmorillonite suspension exhibited a non-Newtonian rheology at all pH values, but the deviation from Newtonian flow was lower at pH = 10. In contrast, Keren (1988) showed a Newtonian flow behavior for Na-montmorillonite at this pH. The flow behavior of the Na-montmorillonite suspension at pH = 4 (Fig. 2a) exhibited a sharp decrease in differential viscosity with the increase in shear stress. The differential viscosity was about 35 and 8 mPa s at low shear stress and high shear stress, respectively. The differential viscosity of the clay suspension (without HS) was lower as the pH increased. Keren (1988) reported similar values. This behavior is characteristic of pseudoplastic flow. The pseudoplastic non-Newtonian flow derived from the associations between clay platelets (Keren, 1988). The particle association between Na-montmorillonite platelets was found to be pH-dependent, and at pH values equal or less than the PZC of the edges, Edge-Edge (E-E) and Edge-Face (E-F) associations take place and at higher pH values only Face-Face (F-F) associations occur (Swartzen-Allen and Matijevic, 1976; Arora and Coleman, 1979; Keren et al., 1988). Swartzen-Allen and Matijevic (1976) found that neutralization of the positive charge was reflected in an increase in the electrophoretic mobility toward the positive electrode as the pH was raised from 8 to 10 and as the broken edges turned negative. Greenland and Mott (1978) found a decrease in the positive charge with a pH increase because of the neutralization by OH- ions. These edge charge characteristics can explain the decrease in the pseudoplastic behavior of Na-montmorillonite suspensions since an increase in suspension pH results in a change in platelets association, from E-E and E-F to F-F.



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  		Fig. 2. The differential viscosity of Na-montmorillonite suspensions as a function of the shear stress at various humic acid (HA) concentrations and pH values: (A) pH = 4; and (B) pH = 6. Figures (a) and (b) present an enlargement of the area outlined in figures (A) and (B).

 
Effect of Humic Acid
The rheology of montmorillonite suspensions was affected by the presence of HA (Fig. 1). The extent of deviation from Newtonian flow was found to depend on the concentration of the HA: a decrease in the non-Newtonian behavior was observed with an increase in the HA concentration. The differential viscosity values of the Na-montmorillonite suspensions at increasing concentrations of HA at four pH values are shown in Fig. 2 (pH = 4 and 6) and 3 (pH = 8 and 10). Addition of HA to a clay suspension changed the flow behavior from non-Newtonian to Newtonian flow as the HA concentration increased. A comparison of the differential viscosity of Na-montmorillonite and the clay-HA suspension, at the same shear stress, showed that any increase in HA concentration caused a decrease in the differential viscosity. This difference was remarkable at low shear stress values. At a shear stress of 0.1 Pa, the differential viscosity of the Na-montmorillonite was 33.7 mPa s. At a HA concentration of 100 mg L-1, the suspension showed a decrease in differential viscosity to 11.6 mPa s, and a further increase in HA concentration to 400 mg L-1 reduced the differential viscosity to 3.92 mPa s. At higher shear stress (5 Pa) the values of the differential viscosity were 7.67, 6.36, and 5.08 mPa s for HA concentrations of 0, 100, and 4000 mg L-1.

The following explanation was proposed by Goodwin (1975) for the decrease in the differential viscosity at high shear stress: The shear stress and the Brownian motion induce the breakdown of clay platelets association; in contrast, the buildup of the association between platelets depends only on the Brownian motion and thus, an increase in the shear stress results in a decrease in the number of bonds (the breaking force increases while the Brownian motion remains constant). In the clay suspension without HA, the shear stress caused the disruption of the E-E and E-F bonds between Na-montmorillonite platelets, and consequently a decrease in the differential viscosity.

Addition of 100 mg L-1 HA at the lower pH values (4, 6, and 8) caused a decrease in the initial number of bonds diminishing the differential viscosity of the suspension at low shear stress (0.1 Pa). These results are in accordance with earlier research on Na-FA (Tombacz et al., 1984), Na-polyphosphate (Keren, 1988), and Na-humate (Zhang et al., 1991). This change in the differential viscosity at low shear stress when the HA was added was a result of the disruption of E-E and E-F bonds by the adsorption of the negative HA macromolecules at the positive edges of the clay. This phenomenon is in accordance with the edge-charge reversal mechanisms (from positive to negative) reported by Tarchitzky et al. (1993) for HS and Tarchitzky et al. (1999) for DOM originating from reclaimed wastewater. At the lower pH values (4 and 6) a HA concentration lower than 200 mg L-1 was sufficient to achieve the Newtonian flow behavior from the beginning and the differential viscosity remained practically constant with an increase in the shear stress. This result could be an indication for the breakdown of all the E-E and E-F associations, because of edge charge reversal, which seems to be the major mechanism involved in HS effects on particle dispersion. However, a further increase in HA concentration resulted in an additional decrease in the differential viscosity (Fig. 2). The lower differential viscosity values obtained at high HA concentrations probably reflects a different mechanism influencing the flow behavior of Na-montmorillonite suspensions in the presence of HA.

The differential viscosity of the suspension as a function of the shear stress at the higher pH values (Fig. 3A) strengthens this hypothesis. At pH 8, the HA concentration which caused a transition to Newtonian flow was relatively high (400–800 mg L-1). This value was not expected when the edge-charge reversal mechanism was assumed as the only mechanism influencing the suspension flow. At this pH, the charge at the broken edges of the montmorillonite platelets is low and most of the carboxyl groups on the HA are dissociated. Under these conditions, the HA concentration needed to change the flow from pseudoplastic non-Newtonian to Newtonian was expected to be lower than that necessary at lower pH values (4 and 6).



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  		Fig. 3. The differential viscosity of Na-montmorillonite suspensions as a function of the shear stress at various humic acid (HA) concentrations and pH values: (A) pH = 8; and (B) pH = 10. Figures (a) and (b) present an enlargement of the area outlined in figures (A) and (B). A curve related to Ca-montmorillonite is individually indicated.

 
Furthermore, the edge-charge reversal mechanisms cannot explain the decrease in the differential viscosity when HA was added at pH = 10, because this pH value was considerably higher than the PZC of the clay edges and the association between platelets was only F-F. As mentioned above, our observations at high pH values (8 and 10) indicate that an additional mechanism, possibly the formation of mixed micelles, dominates the flow behavior of the studied suspensions. We hypothesize that in suspensions containing >=800 mg L-1 HA at pH 8 or >=400 mg L-1 at pH 10 (when Newtonian or dilatant flow occurs) mixed layers of HA and clay platelets are formed. The formation of such mixtures results in an increase in the repulsion forces between particles because of the negative charges of the surfaces of both the HS and the clay. A similar mechanism was proposed earlier as an explanation for the flocculation behavior of HA/Na-montmorillonite suspension at the same pH values (Tarchitzky et al., 1993; Tarchitzky et al., 1999).

Clay suspensions with high HA concentration showed a slight increase in the differential viscosity with an increase in the shear stress. This behavior is characteristic of a non-Newtonian dilatant flow, which is quite unusual in diluted suspensions. Similar results were obtained at all the pH values tested (4, 6, 8, and 10) (Fig. 2 and 3). Dilatant flow is defined as flow in a system where a shear rate increase caused a differential viscosity increase, or as a suspension showing a resistance to deformation (Van Olphen, 1977). Van Wazer et al. (1963) reported that a high concentration of negatively charged particles can transform a suspension flow from pseudoplastic to dilatant.

According to Van Olphen (1977), the dilatant behavior of the clay suspension can be explained as follows: since the particles repel each other, they are easily shifted with respect to each other by a small shearing stress. Simultaneously, since the particles are closely packed, liquid must flow through the interstitial spaces during the displacement of the particles. If the displacement is slow, the liquid is able to follow the change in geometry; during a rapid displacement, however when a high shearing stress is suddenly applied, the capillary flow of the liquid becomes the bottleneck in the shearing motion of the suspension, and a high resistance against shear develops. This mechanism can explain the dilatant behavior at high concentrations of HA and FA. The irregularity and randomly coiled structure of the HS at higher concentrations adds a component of interference to the suspension flow thereby enhancing its dilatant properties. The dilatant condition seemed to be more dominant at lower pH values (Fig. 2). This phenomenon can be explained by the spatial structure and micellar or aggregation condition of the HA macromolecule and its variation with the pH. Humic substances are macromolecules with a pH-dependent charge; They are slightly charged at pH 4 since the carboxyl groups of the HS exhibit pKa (the negative logarithm of the apparent dissociation constant of this group) values around pH 4.5. At higher pH values more carboxyl groups dissociate and an increase in the negative charge density takes place. These changes in charge affect the molecular conformation in solution, from that of a compact coil or sphere at low pH to an extended chain at higher pHs (because of intramolecular charge-charge repulsion) (Gosh and Schnitzer, 1980; Stevenson, 1994; Chen and Schnitzer, 1989; Barak and Chen, 1992). As a result of these configuration characteristics, we suggest that the HS molecules disturb the flow to a great extent at low pH values. Evidence of this can be found in the high viscosity of HA suspensions at low pH values (Chen and Schnitzer, 1976).

It should be stressed however, that the flow properties of Ca-montomorillonite were not influenced by the addition of Ca-humate (Fig. 3a), in accordance with a former observation obtained in flocculation experiments.

Effect of Fulvic Acid
When Na-FA was added to Na-montmorillonite suspensions at pH = 8, a transition from a non-Newtonian pseudoplastic flow to a Newtonian behavior was observed, at a FA concentration of 100 mg L-1 (Fig. 4) . This value is lower than that of the HA concentration needed to change the flow behavior of the clay suspension at the same pH. This is in accordance with the change in the intensity of the mixed micelles mechanism which was explained above. A lower concentration of FA was needed to reach this stage because the number of dissociated functional groups at any pH value for the FA is higher than the number for the HA. We do not show more data for the FA because individual observations that were made showed that FA had in principal the same effect on the clay suspension as the HA.



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  		Fig. 4. The differential viscosity of Na-montmorillonite suspensions as a function of the shear stress at various fulvic acid (FA) concentrations at pH = 8. Figure (a) presents an enlargement of the area outlined in figure (A).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The rheological properties of montmorillonite suspensions in the presence of HS show a decrease in the differential viscosity at all the pH values tested. These results can be explained by the edge-charge reversal mechanism. An additional mechanism influences the differential viscosity of the suspensions. This mechanism is possibly associated with the formation of mixed micelles of HA (or FA) with clay platelets. Repulsion forces within these mixtures are higher than those existing between the clay platelets and therefore the suspensions flow curves become Newtonian or dilatant.

The recorded changes in viscosity and flocculation studies published earlier by our group provide information on the dispersive effects of HS on Na affected clays and soils. A decrease in particle association strength can result in a decrease in hydraulic conductivity and infiltration as well as an increase in runoff an erosion (Tarchitzky et al., 1999).


   ACKNOWLEDGMENTS
 
The authors thank the Israel Ministry of Science (MOS), and the Bundesministerium fuer bildung, Wissenschaft, Forschung und Technologie (BMBF-Germany) for their financial support.

Received for publication January 2, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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