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a School of Nat. Resour., The Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210
b Currently at the Univ. of Mississippi, duty station: U.S. Army Eng. Res. and Dev. Cent., 3909 Halls Ferry Rd., Vicksburg, MS 39180
* Corresponding author (Kateryna.Dontsova{at}gmail.com)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AEC, anion exchange capacity • CEC, cation exchange capacity • DI, deionized • EGME, ethylene glycol monoethyl ether • OM, organic matter • SA, surface area • SOM, soil organic matter • TOC, total organic carbon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A few researchers have compared organic matter (OM) properties and turnover in adjacent soils with different mineral composition (Arai et al., 1996; Parfitt et al., 2002) or in different particle-size fractions of a single soil (Gonzalez and Laird, 2003; Kahle et al., 2003), but the limited number of such comparisons has made firm conclusions about mineral–OM relationships difficult to achieve.
Feller (1995) (as quoted in Feller and Beare, 1997) and Wattel-Koekkoek et al. (2001) analyzed large populations of soils in the tropics and found no difference in total soil OM (SOM) content between kaolinitic and smectitic soils. On the other hand, Wattel-Koekkoek et al. (2001) observed that kaolinitic materials were enriched with polysaccharides; whereas, smectite-associated OM contained more aromatic compounds.
Polysaccharides comprise about 10% of the OM in soils (Hayes and Swift, 1978; Cheshire, 1979) and are thought to play an essential role in the stabilization of soil structure (Tisdall and Oades, 1982; Robert and Chenu, 1992). Polysaccharides originate from both plants and microorganisms (Cheshire et al., 1979), but soil clay fractions are dominated by microbial rather than plant-derived polysaccharides (Feller and Beare, 1997). According to Foster (1981), microbial polysaccharides coat clay platelets, occupy crevices of submicron size within mineral aggregates, and bind clay particles together. Their position in small pores and their association with the clay fraction are believed to protect polysaccharides from degradation (Chenu and Stotzky, 2002).
Microbial polysaccharides are mostly net negatively charged compounds (Finch et al., 1967; Clapp and Emerson, 1972), particularly in neutral and alkaline environments because of an uronic acid component (Cheshire, 1979). Studies on the sorption of soil polysaccharides (Finch et al., 1967; Guckert et al., 1975) and anionic polysaccharides produced from laboratory cultures (Parfitt and Greenland, 1970; Clapp and Emerson, 1972; Parfitt, 1972; Labille et al., 2003) have largely been done using montmorillonite. Finch et al. (1967) showed that more polysaccharide was adsorbed by montmorillonite than kaolinite, and they attributed the difference to the greater surface area (SA) of the montmorillonite. Their comparison was not conducted under identical conditions, and it has been demonstrated that pH (Finch et al., 1967), exchangeable cations (Santoro and Stotzky, 1967; Parfitt and Greenland, 1970; Guckert et al., 1975), and electrolytes (Labille et al., 2003) can also affect the sorption of anionic polysaccharides.
In general, the published literature does not provide a complete picture of the effects of clay mineral composition on the accumulation of SOM, and there is a need for systematic studies under controlled laboratory conditions using defined organic substances and well-characterized clay minerals to resolve important chemical and mineralogical factors affecting retention of SOM. Therefore, the objective of this study was to evaluate the effect of clay mineral composition on the sorption of an anionic polysaccharide, representing one of the components of SOM, under different pH, electrolyte, and cation conditions typical of soil environments.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-D-mannosyl residue and pyruvate ketal on the side-chain-terminal ß-D-mannosyl residue, which would add another –COOH group. About 60% of the terminal mannose residues are 4,6-pyruvated, and the inner mannose is mostly 6-acetylated (Fig. 1)
. The molecular mass of xanthan varies between 0.9 and 1.6 x 106 Da (Sutherland, 1994).
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Clay Minerals
Specimen clay minerals (van Olphen and Fripiat, 1979) were obtained from The Clay Minerals Society Source Clays Repository (Purdue Univ., Dep. of Agron.). The samples represented a range of properties consistent with the mineralogical diversity of soils in the central United States (Table 1). The clays were fractionated by sedimentation using an automatic fractionator following pretreatment with 1 mol L–1 Na acetate buffer at pH 5 to remove carbonates, 30% (w/w) H2O2 to remove OM, and 0.25 mol L–1 Na2CO3 to achieve dispersion with sonication. The fractionated clays (<2 µm) were exchanged with Ca and washed free of excess salt by centrifugation. The clays were exchanged with Ca because it is the dominant exchangeable cation in most Midwestern soils. After freeze drying, the <2 µm materials were analyzed for both external (Brunauer–Emmett–Teller [BET] N2) and total (ethylene glycol monoethyl ether [EGME]) SA. In addition, the cation exchange capacity (CEC) at pH 7 was determined using Ca as the index cation. In this process, the clays were expanded with NaCl, then saturated with CaCl2, and washed with deionized (DI) water to remove excess salt. The exchangeable Ca was then displaced with Mg, and the leachate was analyzed for Ca.
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Adsorption Experiments
Adsorption experiments were conducted to assess the effects of polysaccharide concentration, pH, electrolyte concentration, and cation type on sorption of xanthan by different clay minerals. Subsamples of the clays (0.15 g) were ultrasonically dispersed in water or a salt solution, adjusted to the desired pH using 0.1 mol L–1 HNO3 or saturated (less than 0.025 mol L–1) Ca(OH)2 solutions, and mixed with the appropriate xanthan solution to form 40 mL of a final suspension with 3.75 g L–1 of clay. Preliminary experiments and literature reports (Wen, 2002) showed that the amount of xanthan adsorbed was proportional to the quantity of clay; therefore, the same amount of clay was used in all experiments.
After 12-h equilibration on a reciprocal shaker (100 rpm) at room temperature (18°C), the clay–xanthan suspensions were centrifuged for 30 min at 6328 x g, and total organic C (TOC) in the supernatant was measured using a Rosemount Dohrmann DC-190 Total Organic Carbon Analyzer (Tekmar-Dohrmann, Mason, OH). The equilibration time of 12 h was in excess of the time needed to reach equilibrium, which has been reported to be less than 1 h (Moavad et al., 1974; Guckert et al., 1975; Wen, 2002). The quantity of xanthan adsorbed was determined by difference between the concentration of TOC in the supernatant liquid of samples with and without clay. Control measurements (without clay) showed the C content of xanthan in the supernatant to vary between 42% without electrolyte and 31% in the presence of electrolyte. These values were less than calculated from the molecular formula (43%), probably as a result of the formation of some gel-like structures that sedimented during centrifugation (Sutherland, 1994). Any change in xanthan concentration caused by dissolved salt was corrected by using a control (no clay added) for each treatment. The pH of the supernatant was measured (Orion 420A Benchtop pH Meter, Thermo Electron Corp., Waltham, MA) immediately after TOC was determined.
Xanthan sorption isotherms for all specimen clays were obtained at pH 4 in suspensions with and without electrolyte [10 mmol L–1 Ca(NO3)2]. Xanthan concentrations ranged between 0 and 200 mg L–1 with a 25 mg L–1 step interval.
To study the effect of pH on sorption of xanthan, three clays with different charge characteristics (SAz-1, SWy-1, and KGa-1) were equilibrated with 100 mg L–1 xanthan solutions with and without electrolyte [10 mmol L–1 Ca(NO3)2]. The pH range was 3 to 8 with a 0.5-pH step interval.
The effect of electrolytes on sorption was evaluated for all clays by equilibrating them with 100 mg L–1 xanthan solutions in 0, 1, 2.5, 5, 7.5, and 10 mmol L–1 Ca(NO3)2 under two pH regimes, 4 and 7, representing acid and neutral conditions. Concentrations of electrolyte were within the range previously observed for saturation extracts from common soils in the central USA (Dontsova and Norton, 2002).
To evaluate the effect of cation type on the sorption of xanthan, samples of SAz-1 were washed with nitrate salts of Li+, Na+, K+ (1 mol L–1), Mg2+, Ca2+, Sr2+ (0.5 mol L–1), and Al3+ (0.33 mol L–1) three times, and excess salt was removed by multiple centrifugations with DI water. The samples were then equilibrated with 100 mg L–1 solutions of xanthan with salt concentrations ranging between 0 and 20 mmol L–1.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The values obtained for xanthan sorption on smectites in this study were in reasonable agreement with previous reports. Sorption by Ca-exchanged SWy-1 was about 2 g kg–1 TOC (5 g kg–1 xanthan) under neutral conditions from a 100 mg L–1 xanthan solution in 1 mmol L–1 Ca(NO3)2 and 9 g kg–1 TOC (22.5 g kg–1 xanthan) from a 200 mg L–1 xanthan solution in 10 mmol L–1 CaCl2. Clapp and Emerson (1972) reported the adsorption of 4.8 g kg–1 of xanthan from a 50 mg L–1 solution and 27.8 g kg–1 from a 250 mg L–1 solution by Ca montmorillonite with a background of <1 mmol L–1 CaCl2. Reported results for other anionic polysaccharides vary between 20 and 30 g kg–1 for Na montmorillonite (Finch et al., 1967; Labille et al., 2003), 90 to 150 g kg–1 for H montmorillonite (Finch et al., 1967), and up to 115 g kg–1 for Ca montmorillonite (Labille et al., 2003). The reported values depended on experimental conditions, and conditions varied between experiments.
Although the amounts of polysaccharide sorbed in the current study were small, they would probably contribute to the stabilization of soil structure. Clapp and Emerson (1972) showed that the adsorption of as little as 1 to 2 g kg–1 of xanthan was enough to prevent dispersion of montmorillonite. In other studies, maximum aggregate stability was achieved at 10 g kg–1 for neutral polysaccharides (Chenu et al., 1987) and 20 g kg–1 for anionic polysaccharides (Labille et al., 2003).
Electrolyte Concentration
Greater adsorption of xanthan was observed in the presence than in the absence of 10 mmol L–1 Ca(NO3)2 (Fig. 2). At pH 4, there was a 2.3- to 6.5-fold increase in adsorption with electrolyte (Table 2), and this increase was significant at the 0.05 probability level for all clays. At pH 7, the relative increase in adsorption of xanthan in the presence of Ca(NO3)2 was also significant and even greater than at pH 4 (4.5- to 141.5-fold). The positive response to electrolyte agrees with the one described for anionic polymers by Theng (1982), who attributed the effect to screening of the charge on the polyanion with a resulting decrease in electrostatic repulsion between surfaces.
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Electrolyte Type
Sorption of xanthan by smectite SAz-1 from 100 mg L–1 solutions using nitrate salts of various monovalent (K+, Na+, and Li+) and divalent (Sr2+, Ca2+, and Mg2+) cations as background electrolyte showed that an increase in ionic strength from 0 to 30 mmol L–1 led to an increase in adsorption, approaching a plateau in the upper concentration range (Fig. 4a)
. This behavior was similar to that observed previously for Ca2+. Divalent cations enhanced adsorption to a greater degree than monovalent cations at the same ionic strength.
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Despite the fact that at the same cationic strength, there are more unit charges in monovalent than in divalent cation solutions, sorption was greater in the divalent system. This result indicated that charge screening and compression of the double layer did not entirely explain the electrolyte effect observed in our study and supported active participation by cations in xanthan binding. McBride and Baveye (2002) stated that cations provide an attractive force between negatively charged colloidal particles because they are electrostatically attracted to both surfaces. Cation bridging has also been used to explain the sorption of negatively charged sugars by montmorillonite (Parfitt, 1972; Labille et al., 2003), and this mechanism was supported by spectroscopic evidence of associations between exchangeable cations and carboxylic groups of the polysaccharides (Parfitt, 1972).
Previous research has shown a decrease in sorption of anionic polymers with an increase in cation size and electronegativity for monovalent cations (Mortensen, 1962; Theng and Scharpenseel, 1975) and an increase in sorption with a decrease in cation size for multivalent cations (Mortensen, 1962; Theng and Scharpenseel, 1975; Theng, 1976). This behavior indicated that anionic polymers can remove water from monovalent cations and bind directly to the exchangeable cation, whereas with divalent cations, binding occurs through the water of hydration. The current study did not show differences between divalent cations of different size, but it did indicate a difference between more hydrated Li+ and less hydrated K+.
Aluminum caused flocculation of both the clay and xanthan at the minimum concentration used (0.3 mmol L–1), making sorption measurements impossible. This behavior agreed with observations of Sutherland (1994), who noted that in the presence of trivalent metal ions, xanthan solutions became cross-linked to form gels.
pH
Total sorption of xanthan was greater at pH 4 than at pH 7 for all clays (Table 2). A 1.1- to 2.6-fold difference in sorption was observed between pH 4 and 7 in 10 mmol L–1 Ca(NO3)2. This difference was significant at the 0.05 probability level for IMt-1, SAz-1, SCa-2, and SWy-1 (2.6, 2.1, 1.4, and 1.7 times increase, respectively) and not significant for KGa-1, NG-1, and SHCa-1 (1.2, 1.2, and 1.1 times increase). Such behavior was consistent with an increase in the negative charge of both xanthan and clay as the pH increased and variable charge sites dissociated. Finch et al. (1967) showed that a decrease in pH from 7 to 4.4 reduced the electrophoretic mobility of soil polysaccharides by a factor of about 10.
Xanthan sorption was studied in more detail for SAz-1, KGa-1, and SWy-1 over a pH range of 3 to 8. As before, sorption decreased with increase in pH (Fig. 5) . Without electrolyte, xanthan sorption was small, approximately 1 g kg–1 TOC for all three clays, and sorption increased only at pH values below 4. This behavior was similar to previous observations for polygalacturonic acid, which was not adsorbed by montmorillonite when the pH was 6 and above (Parfitt and Greenland, 1970). Low-charge smectite, SWy-1, displayed greater sorptive capacity at pHs < 4.0 than high-charge smectite, SAz-1, and kaolinite (Fig. 5). Burchill et al. (1981) noted increased sorption of humic substances by kaolinite as the pH decreased, but sorption never reached that of montmorillonite.
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Mineral Composition
A highly significant effect (P > F < 0.0001) of clay mineral type on xanthan sorption was observed (Table 2). It can be concluded from Fig. 2 and 3 and Table 2 that at pH 4.0 and 10 mmol L–1 Ca concentration, sorption was smallest with kaolinite and greatest with NG-1, a low-charge smectite where layer charge originates mostly in the tetrahedral positions. The increase in average sorption from kaolinite to smectite was by a factor of two. At pH 7.0 without electrolyte, low-charge smectites continued to have the greatest sorption, whereas high-charge smectites adsorbed similar amounts or even less than kaolinite. Under all the environmental conditions studied, illite (IMt-1) behaved similar to the high-charge smectite, SAz-1, despite differences in total layer charge, CEC, and total and external SA. This similarity may be explained by weathering of external surfaces of the illite particles, resulting in a smaller charge density on these surfaces.
To quantitatively evaluate the effect of clay properties on xanthan sorption, a series of linear regressions was performed. The retention of TOC by sediments has been previously linked to the EGME SA of smectites (Kennedy et al., 2002), the N2 SA of clay fractions (Ransom et al., 1998; Adams and Bustin, 2001), and SA as measured by para-nitrophenol adsorption (Saggar et al., 1996, 1999). Intercalation has been observed for neutral polysaccharides in montmorillonite (Olness and Clapp, 1973) but not for anionic polysaccharides (Parfitt and Greenland, 1970). Therefore, external SA, as measured by N2 sorption, should be a more important property in determining xanthan sorption than total SA.
Guckert et al. (1975) observed that the extent of polysaccharide sorption on montmorillonite was related to the exchangeable cation but not the SA. In the current study, we also observed no statistically significant relation between the sorption of xanthan from 100 mg L–1 solutions and the N2 SA, EGME SA, or CEC of the minerals studied. The only exception was a negative correlation between sorption at pH 7 without background electrolyte and CEC (P > F = 0.0288). In general, sorption of xanthan without electrolyte was negatively correlated with N2 SA, EGME SA, and CEC; whereas, the correlation was positive in the presence of background electrolyte. When sorption values were calculated on the basis of external SA (N2 SA [µg m–2]), then the relationship between sorption and CEC became negative both with and without electrolyte and was significant for samples without electrolyte. This relationship improved when values for SWy-1 were removed (Fig. 6a) . Surface area measurements indicated that the smectites formed approximately 9- to 11-layer domains, but SWy-1 was an exception. It gave a low N2 SA/EGME SA ratio, probably as the result of a high affinity for water, which interfered with the N2 SA measurements. Theng (1995) also reported a small external SA for SWy-1 measured by N2.
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Steric considerations, i.e., a match between charge density on the clay and size of the organic molecule and its charge distribution, also affect sorption (Harter, 1977). Finch et al. (1967) observed preferential adsorption of the medium-charge (one uronic acid per six sugar residues) component from a mixture of soil polysaccharides. In this study, low-charge smectites apparently had a better match for the xanthan charge distribution than high-charge clays. Although correlations involving charge distribution used a limited number of samples and should be viewed with caution, the relationships were statistically significant. Anion exchange capacity (AEC) and the CEC/AEC ratio may also affect sorption (Stotzky, 1986); however, a lack of information on AEC in the current study did not permit an evaluation of this factor.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The sorption of xanthan was related to inherent properties of the studied clay minerals, such as N2 SA, CEC, and the amount of octahedral and tetrahedral charge. Negative correlations between sorption and CEC and octahedral charge indicated repulsion between negatively charged clay particles and the anionic xanthan molecules. Positive correlations between xanthan sorption and the amount of tetrahedral charge indicated that the mechanisms of sorption probably involved cation bridging between surface-charge sites on the clays and the polysaccharide molecules. Overall, low-charge smectites sorbed the greatest amount of xanthan, and kaolinite the least, with high-charge smectites having intermediate sorption values. Illite behaved like high-charge smectite.
A highly significant effect (P > F < 0.0001) of clay type was observed on the sorption of xanthan. Differences between minerals accounted for a two- to threefold difference in sorption, indicating that soil mineral composition may be important in determining the quantity of polysaccharide stored in the soil, its turnover time, and its flux to the atmosphere. However, the effect of clay mineral composition was not as pronounced as might be expected from the wide range of properties represented. Under field conditions where different minerals usually are present in mixtures, it should be expected that an effect of clay mineral composition on sorption of anionic polysaccharides will be less evident.
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ACKNOWLEDGMENTS |
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Received for publication June 21, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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