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Surface modification of clay minerals by organic polyions
Colloids and Surfaces A: Physicochemical and Engineering Aspects 141 (1998) 379–384
Surface modification of clay minerals by organic polyions E. Tomba´cz a,*, M. Szekeres a, L. Baranyi a, E. Miche´li b a Department of Colloid Chemistry, Attila Jo´zsef University, H-6720 Szeged, Aradi Vt. 1, Hungary b Department of Soil Science, Go¨do¨llo˝ University, H-2100 Go¨do¨llo˝, Hungary Received 17 January 1997; accepted 17 September 1997
Abstract Parallel studies were made of the solid/gas and solid/liquid interfacial properties and the colloidal behaviour of aqueous dispersions of two types of material: soil mineral grains which had been coated with natural organic matter (humus) and clay minerals (montmorillonite and kaolinite) whose surfaces had been modified synthetically with an organic polyacid, as models for organo-mineral complexes. Solid samples with and without a polyionic organic coating, and also their aqueous suspensions, were investigated by means of nitrogen gas adsorption, small-angle X-ray scattering, potentiometric acid–base titration and rheological methods. The organic coating was found to exist as a rough layer on the surface of the mineral grains, and to plug their pores. Both the natural and the synthetic surface modification resulted in steric and electrostatic stabilization of the clay particles, because of the highly charged, polyionic character of the surface-modifying organic matter. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Soil colloids; Humus coating; Organo-mineral complexes; Organic surface modification; Surface charge titration; Rheology
1. Introduction Clay minerals are crystalline aluminosilicates which form the major constituents of the colloidal fraction of soils. The finely divided minerals are probably most active towards the organic constituents of soils. It has become evident that clays in soils do not present a clean surface for interactions: they are contaminated with amorphous oxyhydroxides and silicates, and with organic matter [1]. Fine grains of soil involve random aggregations of clay, oxides and organic matter, they include organo-mineral complexes. The inorganic soil constituents, and among them the clays in particular, are presumed to play a basic role in the * Corresponding author. Fax: +36 623 129 21; e-mail: [email protected]
formation of the natural organic humus coating on the minerals, due to their heterogeneous catalytic effect on the polymerization processes in which various mixtures of negatively charged polycondensed aromatic compounds (humic substances) are formed from the residues of plant decomposition [1] or the dead biomass [2]. Although tremendous work has been done on the structural and functional characterization of organo-mineral complexes, wide-ranging systematic interfacial and colloidal investigations are still needed. A membrane–micelle model for humus coatings in soils has been proposed by Wershaw [2]. Visual evidence of the surface roughness of organic-coated clay grains has recently been provided by scanning force microscopy [3]. The rough surface of soil grains (the fractal character of the surface) has been proved by small-angle X-ray
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scattering (SAXS) [4]. The steric stabilizing effect of the amorphous rough coating on clay particles has been reported [5]. Two basically different approaches have been used to study the organo-mineral complexes of soils. One involves the gradual removal of the humus from the soil, and comparative investigation of the residual samples; and the other is to study the interactions between purified inorganic and organic soil constituents [1,5–13]. A new technique, the synthesis of organic coatings, has recently been introduced to simulate the formation of humic coatings on mineral surfaces in the laboratory [14,15]. Heterogeneous catalytic effects of clay minerals such as montmorillonite and illite, and also clay and silt fractions from soils, in the formation of humic-like material formation due to the polymerization of phenolic compounds (e.g. gallic acid, pyrogallol and protocatechuic acid) were described many years ago [16–18]. In the present work, the solid/gas (S/G) and solid/liquid (S/L) interfacial properties and the colloidal behaviour of aqueous dispersions of soil mineral grains coated with natural organic matter, and of clays (montmorillonite and kaolinite) whose surfaces had been modified synthetically, to serve as models for organo-mineral complexes, and also of the inorganic constituent of each sample, were investigated in parallel to clarify the role of the organic coating in the interfacial and colloid properties.
2. Experimental 2.1. Materials The organic matter was gradually removed from the fine fraction of chernozem soil samples. The mobile humic substances were extracted with 0.1 M NaOH solution at a soil to alkaline solution ratio of 1:4. The bound organics were then completely decomposed by wet oxidation with H O . 2 2 Montmorillonite obtained from Kuzmice bentonite (Czech Republic) and kaolinite from Zettlitz kaoline (Germany) were used after peptization with Na CO and fractionation to a size smaller 2 3 than 1 or 2 mm. Organic coating was synthesized
on the surface of both clays in a heterogeneous catalytic process involving oxidation and polymerization of gallic acid [16 ]. All samples were carefully purified by washing with distilled water, followed by dialysis against 0.01 M KNO . The 3 organic carbon contents of the samples are given in Table 1. The carbon content of the product synthesized from montmorillonite was roughly twice that of the organic-coated kaolinite sample, which lay in the range of carbon contents of chernozem soils. 2.2. Methods Freeze-dried solid samples with and without organic coatings were investigated by means of nitrogen gas adsorption at 77 K (Micromeritics Gemini 2375). The adsorption and desorption isotherms were evaluated by the BET method [19,20], and the pore distribution was calculated from the desorption by the BJH method. The SAXS curves were determined for the same freeze-dried samples (Philips PW 1830 generator, Kratky camera). The scattering vector varied over the range 0.04 to 1 nm, the sample chamber was loaded with He, and the temperature was 25±0.1°C. Potentiometric acid–base titration of aqueous suspensions of the original and oxidized soil, and of the original and organic-coated clay samples, was performed under well-defined conditions [21]. The background electrolyte was 0.01 M KNO , 3 and 0.1 M KOH and 0.1 M HNO solutions were 3 used as titrants. All samples were first titrated with acid and then with alkaline solution. Concentrated suspensions of the same samples (soils: 40 wt%, pH~7, I=0.01 M; kaolinite: 10 wt%, pH~5, I=0.01 M; montmorillonite: 7 wt%, pH~5, I=0.01 M ) were investigated rheologically. Flow curves were determined with a Haake Rotovisco RV-20, CV-100 apparatus over the shear rate range 0 to 100 s−1 at 25±0.1°C.
3. Results and discussion Some numerical data from the S/G and S/L interfacial investigations for the various samples
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Table 1 Changes in S/G and S/L interfacial character of soil and clay samples due to the natural or synthetic organic surface modification Samples
Chernozem soils
Kaolinite
Surface
Oxidized
Coated
Original
Coated
Original
Coated
Organic matter content (C%)
0.1–0.2
2.0–3.5
—
3.25
—
7.86
S/G interfacial properties Specific surface area a (m2/g) BET Pore volume (cm3/g) Surface fractal dimension
33–48 0.05–0.71 2.50–2.80
9–29 0.02–0.04 2.70–2.91
18.2 0.039 2.25
11.1 0.025 2.34
14.0 0.020 2.47
7.6 0.015 2.61
S/L interfacial properties Net proton excess at pH~5, 0.01 M (mmol/g) pznpc (pH ) Yield value (Pa)
0.04–0.05 7.3–9.2 25–60
0.06–0.08 7.0–8.7 2.0–3.0
0.01 ~7 1.9
0.08 6.1 0.2
0.40 — 2.0
0.35 7.3 0.32
are presented in Table 1. Four chernozem soil samples from different areas in Hungary were studied in parallel. Since the aim of this paper was to compare the effects of natural and synthetic organic coatings on the interfacial properties, and not to consider particular soil behaviour, the individual data of the soils are not given in Table 1, which reports only the range in which the measured data fell. Removal of the natural organic coating from the soil samples, and the synthetic polyionic surface modification of the clay samples, caused significant changes in the interfacial properties. Similar shifts in the numerical data can be observed for the samples containing natural or synthetic organic coatings. The S/G interfacial measurements revealed that the specific surface area and pore volume of the organo-mineral complexes were smaller than those of the inorganic constituents. The increase in nitrogen adsorption after the exposure of soils to H O [19,20] was explained in terms of the disin2 2 tegration of clay particles bound together by organic matter in the soil. In contrast with this explanation, we presume that the organic matter can plug the pores in the clays, since not only the BET surface areas but also the pore volumes of all the organic-free samples were significantly larger than those for the organic-coated ones. The SAXS investigations of powder samples demonstrated that the soil samples are disordered two-phase systems or fractal scatterers, similarly
Montmorillonite
to published results [4]. All the measured scattering curves could be fitted by a power law (I˜ –I˜ h−a, h h where a is the power law exponent) over a fairly broad region. The surface fractal dimension, which ranges from 2 (smooth surface) to 3 with increasing roughness of the surface, could be calculated from the slope of the scattering curve plotted on a logarithmic scale using the equation a=5−D [4]. s The surface fractal dimension values for soil samples ( Table 1) are somewhat larger than those in the literature [4], probably because of the insufficient fractionation of the soils, but these data are comparable. Since the surface fractal values for the inorganic residues of all four soil samples were definitely smaller than those for the original soils, it can be stated that particles with rough surfaces in the original soils become smoother on removal of the natural organic coating. This decrease in surface roughness of soil particles following the removal of organic matter harmonizes well with the suggested picture of organic coatings produced by weathering [1] and with the amorphous rough structure proved for illite particles by means of scanning force microscopy [3]. In the model systems, the surface roughness was also increased by the synthetic polyionic coating on clay surface, as shown by the larger surface fractal dimension for the coated sample than that for the original clay in Table 1. The S/L interfacial investigations revealed that the pH-dependent interfacial properties in aqueous
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medium are significantly influenced by the presence of either natural or synthetic organic coatings. When the surface charge properties of soils and soil constituents such as clays, oxides and humic materials are considered, it is obvious that different surface sites exist [1,22]. The isomorphic substitutions in the crystal lattice of clays hold permanent charges and the amphoteric sites (e.g. Al–OH and Fe–OH ) or the organic functional groups (e.g. –COOH and phenolic –OH ) are variable or pH-dependent charge sites. Surface protolytic processes can be measured by means of acid–base titration [1]. The pH-dependence of the net proton surface excess amount, as shown in Fig. 1, was calculated from the H+ or OH− consumption of suspensions relative to the background electrolyte solution. The most obvious feature is that the curves for kaolinite and montmorillonite become similar to those for the soil samples with respect to both shape and the magnitude of surface excess amounts only after they are coated with humus-like material. Numerous net proton surface excess vs. pH and also surface charge density vs. pH curves for amphoteric solid surfaces, and some for soil samples, are to be found in the literature [1,10,21– 23]. These curves can be explained if H+ and OH− are the potential-determining ions and the total consumption of these ions produces only charged sites on the surface. However, if specific adsorption or any other reaction, such as dissolution of solid or ion-exchange, takes place during the titration, evaluation of the titration data is problematic. Composite systems such as soil always involve these problems. Therefore, it cannot be assumed that the measured net proton excess amount is proportional to the surface charge density of the solid. This is obvious from the curve for montmorillonite (bottom Fig. 1), since the net proton surface excess values are positive over the whole range of pH, i.e. protons are accumulated on the montmorillonite platelets, while it is well known that the basal plane of montmorillonite contains a significant number of permanent negative charges [1,7,24]. The curves for the original kaolinite and montmorillonite samples show the essential differences between these clays, e.g. the 2:1 layer-type mont-
Fig. 1. Influence of polyionic organic coating on pH-dependence of net proton surface excess amount: typical figures for soil (top), kaolinite (middle) and montmorillonite (bottom).
morillonite has a high permanent layer charge, while the 1:1 layer-type kaolinite has a very low permanent charge, but a comparable variable charge [1,22]. The permanent negative charges are compensated by the exchangeable cations [1,22], which are K+ in the suspensions initially due to the exhaustive dialysis against KNO solution. 3
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383
K+ is exchanged by H+ in the following ionexchange process during the acidic titration cycle: K-clay+(H+) PH-clay+( K+) aq aq and they remain specifically adsorbed [25] during the alkaline cycle. The net proton excess amount (DC ) calculated as the difference between the surface excess amount of protons (C ) and that of H+ hydroxide ions (C ) will therefore be positive. OH− The measured net proton excess amount for montmorillonite at low pH, ~0.48 mmol/g, is close to the cation-exchange capacity for this clay, 0.59 meq/g [25]. The low net proton excess amount vs. pH curve for kaolinite (middle Fig. 1) accords well with the low charging of this clay. The number of pH-dependent charges is larger, the curves shift in the direction of lower pH, and the characteristic pH of zero net proton charge (pznpc) is lower in the presence of the organic coating for both the soil and clay samples ( Table 1). Similar shifts in pzc have been described for soil and oxidized soil samples [23]. These changes can readily be explained by considering the protolytic processes [8] assigned to the organic coatings, which contain numerous acidic groups bound chemically on the mineral surfaces. The presence of a polyionic rough layer of organic matter will influence the particle–particle interaction, the colloidal stability and the structure of suspensions. Rheological measurements on concentrated suspensions can provide information on these [21,26 ]. Clay suspensions are pseudo-plastic systems from a rheological aspect [26 ]. The effects of exchangeable cations, i.e. the structure of the electric double layer, on the rheological properties of a montmorillonite suspension were analysed earlier [24]. Evaluation of the pseudo-plastic flow curves according to the classical Bingham model (t=t +g c˙ , where t is the shear stress, t is the B pl B yield value, g is the plastic viscosity and c˙ is the pl shear rate) results in the yield value (t ) by extrapoB lation of the linear part of the flow curve back to zero shear rate. This rheological parameter bears a relation to the strength of the physical network built up from the particles. As shown in Fig. 2, pseudo-plastic flow behaviour is characteristic of all the concentrated aqueous suspensions measured, independent of the presence
Fig. 2. Effects of coating by organic polyions on the spontaneously formed structure of concentrated suspensions: flow curves for soil (top), kaolinite (middle) and montmorillonite (bottom) samples.
or lack of an organic coating. However, it is also obvious that the yield values should be highly different, as indicated in Table 1. Since this rheological parameter is directly related to the attractive energy and the separation distance between the particles [21,24,26 ], the large decrease in it indicates the weakening of the physical network
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in the presence of the organic coating. It can be supposed that the attraction between the particles significantly decreases due to the combined steric and electrostatic stabilizing effects of the expanded layer of either natural or synthetic polyelectrolytes bound to the mineral surface.
4. Conclusions This work revealed that the polyionic organic coating exists on the surface of mineral grains as a rough layer plugging their pores. Both natural and synthetic coatings result in steric and electrostatic stabilization of mineral particles, because of the highly charged, polyionic character of the surface-modifying organic matter. The colloidal stability of aqueous systems, the particle–particle interaction and sediment formation are highly influenced by such surface modifications. This work was the first attempt to establish the role of a polyionic organic coating in the interfacial and colloidal properties of soil organo-mineral complexes and their synthetic models. Detailed investigations are needed to improve the evaluation by identifying the surface sites and interfacial process, by determining the structure of the interfacial layer. The calculation of ion distributions, potential functions and particle–particle interactions will perhaps be possible in the future to allow a quantitative characterization of these interesting systems from an environmental point of view.
Acknowledgment This work was supported by Hungarian National Science and Ministry Fund Grants (OTKA T006077, T022426, MKM 168/96).
References [1] G. Sposito, The Surface Chemistry of Soils, Oxford University Press, New York, 1984. [2] R.L. Wershaw, Environ. Sci. Technol. 27 (1993) 814. [3] D. Heil, G. Sposito, Soil Sci. Soc. Am. J. 59 (1995) 266. [4] M. Borkovec, Q. Wu, G. Degovics, P. Laggner, H. Sticher, Colloids Surf. A 73 (1993) 65. [5] D. Heil, G. Sposito, Soil Sci. Soc. Am. J. 57 (1993) 1241 and 1246. [6 ] M. Schnitzer, S.U. Khan, Humic Substances in the Environment, Marcel Dekker, New York, 1972. [7] B.K.G. Theng, Formation and Properties of Clay–Polymer Complexes, Elsevier, Amsterdam, 1979, p. 283. ´ braha´m, F. Sza´nto´, Appl. Clay [8] E. Tomba´cz, M. Gilde, I. A Sci. 3 (1988) 31. ´ braha´m, M. Gilde, F. Sza´nto´, Appl. Clay [9] E. Tomba´cz, I. A Sci. 5 (1990) 101. [10] J. Buffle, Complexation Reactions in Aquatic Systems: An Analytical Approach, Ellis Horwood, Chichester, 1988, p. 195. [11] E. Tipping, D. Cooke, Geochim. Cosmochim. Acta 46 (1982) 75. [12] E. Tomba´cz, E.K. To´th, M. Szekeres, E. La´mfalusi, E. Micheli, Euroclay’95, ECGA, Leuven, 1995, p. 464. [13] B. Gu, J. Schmitt, Z. Chen, L. Liang, J.F. McCarthy, Environ. Sci. Technol. 28 (1994) 38. [14] R.L. Wershaw, E.C. Llaguno, J.A. Leenheer, R.P. Sperline, Y. Song, Colloids Surf. A 108 (1996) 199. [15] R.L. Wershaw, E.C. Llaguno, J.A. Leenheer, Colloids Surf. A 108 (1996) 213. [16 ] T.S.C. Wang, S.W. Li, Z. Pflanz. Bodenkd. 140 (1977) 669. [17] T.S.C. Wang, M. Kao, P.M. Huang, Soil Sci. 129 (1980) 333. [18] T.S.C. Wang, M.C. Wang, Y.L. Ferng, P.M. Huang, Soil Sci. 135 (1983) 350. [19] K.D. Pennell, S.A. Boyd, L.M. Abriola, Soil Sci. Soc. Am. J. 59 (1995) 1012. [20] Y.A. Pachepsky, T.A. Polubesova, M. Hajos, G. Jozefaciuk, Z. Sokolowska, Soil Sci. Soc. Am. J. 59 (1995) 410. [21] E. Tomba´cz, M. Szekeres, I. Kerte´sz, L. Turi, Progr. Colloid Polym. Sci. 98 (1995) 160. [22] D.L. Sparks ( Ed.), Soil Physical Chemistry, CRC Press, Boca Raton, FL, 1986. [23] P.K. Basu, D.C. Nayak, A.K. Barman, C. Varadachari, K. Ghosh, J. Indian Soc. Soil Sci. 34 (1986) 24. [24] E. Tomba´cz, J. Bala´zs, J. Lakatos, F. Sza´nto´, Colloid Polym. Sci. 267 (1989) 1016. ´ braha´m, M. Gilde, F. Sza´nto´, Colloids [25] E. Tomba´cz, I. A Surf. 49 (1990) 71. [26 ] H.A. Barnes, J.F. Hutton, K. Walters, An Introduction to Rheology, Elsevier, Amsterdam, 1989, p. 115.
Surface modification of clay minerals by organic polyions E. Tomba´cz a,*, M. Szekeres a, L. Baranyi a, E. Miche´li b a Department of Colloid Chemistry, Attila Jo´zsef University, H-6720 Szeged, Aradi Vt. 1, Hungary b Department of Soil Science, Go¨do¨llo˝ University, H-2100 Go¨do¨llo˝, Hungary Received 17 January 1997; accepted 17 September 1997
Abstract Parallel studies were made of the solid/gas and solid/liquid interfacial properties and the colloidal behaviour of aqueous dispersions of two types of material: soil mineral grains which had been coated with natural organic matter (humus) and clay minerals (montmorillonite and kaolinite) whose surfaces had been modified synthetically with an organic polyacid, as models for organo-mineral complexes. Solid samples with and without a polyionic organic coating, and also their aqueous suspensions, were investigated by means of nitrogen gas adsorption, small-angle X-ray scattering, potentiometric acid–base titration and rheological methods. The organic coating was found to exist as a rough layer on the surface of the mineral grains, and to plug their pores. Both the natural and the synthetic surface modification resulted in steric and electrostatic stabilization of the clay particles, because of the highly charged, polyionic character of the surface-modifying organic matter. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Soil colloids; Humus coating; Organo-mineral complexes; Organic surface modification; Surface charge titration; Rheology
1. Introduction Clay minerals are crystalline aluminosilicates which form the major constituents of the colloidal fraction of soils. The finely divided minerals are probably most active towards the organic constituents of soils. It has become evident that clays in soils do not present a clean surface for interactions: they are contaminated with amorphous oxyhydroxides and silicates, and with organic matter [1]. Fine grains of soil involve random aggregations of clay, oxides and organic matter, they include organo-mineral complexes. The inorganic soil constituents, and among them the clays in particular, are presumed to play a basic role in the * Corresponding author. Fax: +36 623 129 21; e-mail: [email protected]
formation of the natural organic humus coating on the minerals, due to their heterogeneous catalytic effect on the polymerization processes in which various mixtures of negatively charged polycondensed aromatic compounds (humic substances) are formed from the residues of plant decomposition [1] or the dead biomass [2]. Although tremendous work has been done on the structural and functional characterization of organo-mineral complexes, wide-ranging systematic interfacial and colloidal investigations are still needed. A membrane–micelle model for humus coatings in soils has been proposed by Wershaw [2]. Visual evidence of the surface roughness of organic-coated clay grains has recently been provided by scanning force microscopy [3]. The rough surface of soil grains (the fractal character of the surface) has been proved by small-angle X-ray
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scattering (SAXS) [4]. The steric stabilizing effect of the amorphous rough coating on clay particles has been reported [5]. Two basically different approaches have been used to study the organo-mineral complexes of soils. One involves the gradual removal of the humus from the soil, and comparative investigation of the residual samples; and the other is to study the interactions between purified inorganic and organic soil constituents [1,5–13]. A new technique, the synthesis of organic coatings, has recently been introduced to simulate the formation of humic coatings on mineral surfaces in the laboratory [14,15]. Heterogeneous catalytic effects of clay minerals such as montmorillonite and illite, and also clay and silt fractions from soils, in the formation of humic-like material formation due to the polymerization of phenolic compounds (e.g. gallic acid, pyrogallol and protocatechuic acid) were described many years ago [16–18]. In the present work, the solid/gas (S/G) and solid/liquid (S/L) interfacial properties and the colloidal behaviour of aqueous dispersions of soil mineral grains coated with natural organic matter, and of clays (montmorillonite and kaolinite) whose surfaces had been modified synthetically, to serve as models for organo-mineral complexes, and also of the inorganic constituent of each sample, were investigated in parallel to clarify the role of the organic coating in the interfacial and colloid properties.
2. Experimental 2.1. Materials The organic matter was gradually removed from the fine fraction of chernozem soil samples. The mobile humic substances were extracted with 0.1 M NaOH solution at a soil to alkaline solution ratio of 1:4. The bound organics were then completely decomposed by wet oxidation with H O . 2 2 Montmorillonite obtained from Kuzmice bentonite (Czech Republic) and kaolinite from Zettlitz kaoline (Germany) were used after peptization with Na CO and fractionation to a size smaller 2 3 than 1 or 2 mm. Organic coating was synthesized
on the surface of both clays in a heterogeneous catalytic process involving oxidation and polymerization of gallic acid [16 ]. All samples were carefully purified by washing with distilled water, followed by dialysis against 0.01 M KNO . The 3 organic carbon contents of the samples are given in Table 1. The carbon content of the product synthesized from montmorillonite was roughly twice that of the organic-coated kaolinite sample, which lay in the range of carbon contents of chernozem soils. 2.2. Methods Freeze-dried solid samples with and without organic coatings were investigated by means of nitrogen gas adsorption at 77 K (Micromeritics Gemini 2375). The adsorption and desorption isotherms were evaluated by the BET method [19,20], and the pore distribution was calculated from the desorption by the BJH method. The SAXS curves were determined for the same freeze-dried samples (Philips PW 1830 generator, Kratky camera). The scattering vector varied over the range 0.04 to 1 nm, the sample chamber was loaded with He, and the temperature was 25±0.1°C. Potentiometric acid–base titration of aqueous suspensions of the original and oxidized soil, and of the original and organic-coated clay samples, was performed under well-defined conditions [21]. The background electrolyte was 0.01 M KNO , 3 and 0.1 M KOH and 0.1 M HNO solutions were 3 used as titrants. All samples were first titrated with acid and then with alkaline solution. Concentrated suspensions of the same samples (soils: 40 wt%, pH~7, I=0.01 M; kaolinite: 10 wt%, pH~5, I=0.01 M; montmorillonite: 7 wt%, pH~5, I=0.01 M ) were investigated rheologically. Flow curves were determined with a Haake Rotovisco RV-20, CV-100 apparatus over the shear rate range 0 to 100 s−1 at 25±0.1°C.
3. Results and discussion Some numerical data from the S/G and S/L interfacial investigations for the various samples
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Table 1 Changes in S/G and S/L interfacial character of soil and clay samples due to the natural or synthetic organic surface modification Samples
Chernozem soils
Kaolinite
Surface
Oxidized
Coated
Original
Coated
Original
Coated
Organic matter content (C%)
0.1–0.2
2.0–3.5
—
3.25
—
7.86
S/G interfacial properties Specific surface area a (m2/g) BET Pore volume (cm3/g) Surface fractal dimension
33–48 0.05–0.71 2.50–2.80
9–29 0.02–0.04 2.70–2.91
18.2 0.039 2.25
11.1 0.025 2.34
14.0 0.020 2.47
7.6 0.015 2.61
S/L interfacial properties Net proton excess at pH~5, 0.01 M (mmol/g) pznpc (pH ) Yield value (Pa)
0.04–0.05 7.3–9.2 25–60
0.06–0.08 7.0–8.7 2.0–3.0
0.01 ~7 1.9
0.08 6.1 0.2
0.40 — 2.0
0.35 7.3 0.32
are presented in Table 1. Four chernozem soil samples from different areas in Hungary were studied in parallel. Since the aim of this paper was to compare the effects of natural and synthetic organic coatings on the interfacial properties, and not to consider particular soil behaviour, the individual data of the soils are not given in Table 1, which reports only the range in which the measured data fell. Removal of the natural organic coating from the soil samples, and the synthetic polyionic surface modification of the clay samples, caused significant changes in the interfacial properties. Similar shifts in the numerical data can be observed for the samples containing natural or synthetic organic coatings. The S/G interfacial measurements revealed that the specific surface area and pore volume of the organo-mineral complexes were smaller than those of the inorganic constituents. The increase in nitrogen adsorption after the exposure of soils to H O [19,20] was explained in terms of the disin2 2 tegration of clay particles bound together by organic matter in the soil. In contrast with this explanation, we presume that the organic matter can plug the pores in the clays, since not only the BET surface areas but also the pore volumes of all the organic-free samples were significantly larger than those for the organic-coated ones. The SAXS investigations of powder samples demonstrated that the soil samples are disordered two-phase systems or fractal scatterers, similarly
Montmorillonite
to published results [4]. All the measured scattering curves could be fitted by a power law (I˜ –I˜ h−a, h h where a is the power law exponent) over a fairly broad region. The surface fractal dimension, which ranges from 2 (smooth surface) to 3 with increasing roughness of the surface, could be calculated from the slope of the scattering curve plotted on a logarithmic scale using the equation a=5−D [4]. s The surface fractal dimension values for soil samples ( Table 1) are somewhat larger than those in the literature [4], probably because of the insufficient fractionation of the soils, but these data are comparable. Since the surface fractal values for the inorganic residues of all four soil samples were definitely smaller than those for the original soils, it can be stated that particles with rough surfaces in the original soils become smoother on removal of the natural organic coating. This decrease in surface roughness of soil particles following the removal of organic matter harmonizes well with the suggested picture of organic coatings produced by weathering [1] and with the amorphous rough structure proved for illite particles by means of scanning force microscopy [3]. In the model systems, the surface roughness was also increased by the synthetic polyionic coating on clay surface, as shown by the larger surface fractal dimension for the coated sample than that for the original clay in Table 1. The S/L interfacial investigations revealed that the pH-dependent interfacial properties in aqueous
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medium are significantly influenced by the presence of either natural or synthetic organic coatings. When the surface charge properties of soils and soil constituents such as clays, oxides and humic materials are considered, it is obvious that different surface sites exist [1,22]. The isomorphic substitutions in the crystal lattice of clays hold permanent charges and the amphoteric sites (e.g. Al–OH and Fe–OH ) or the organic functional groups (e.g. –COOH and phenolic –OH ) are variable or pH-dependent charge sites. Surface protolytic processes can be measured by means of acid–base titration [1]. The pH-dependence of the net proton surface excess amount, as shown in Fig. 1, was calculated from the H+ or OH− consumption of suspensions relative to the background electrolyte solution. The most obvious feature is that the curves for kaolinite and montmorillonite become similar to those for the soil samples with respect to both shape and the magnitude of surface excess amounts only after they are coated with humus-like material. Numerous net proton surface excess vs. pH and also surface charge density vs. pH curves for amphoteric solid surfaces, and some for soil samples, are to be found in the literature [1,10,21– 23]. These curves can be explained if H+ and OH− are the potential-determining ions and the total consumption of these ions produces only charged sites on the surface. However, if specific adsorption or any other reaction, such as dissolution of solid or ion-exchange, takes place during the titration, evaluation of the titration data is problematic. Composite systems such as soil always involve these problems. Therefore, it cannot be assumed that the measured net proton excess amount is proportional to the surface charge density of the solid. This is obvious from the curve for montmorillonite (bottom Fig. 1), since the net proton surface excess values are positive over the whole range of pH, i.e. protons are accumulated on the montmorillonite platelets, while it is well known that the basal plane of montmorillonite contains a significant number of permanent negative charges [1,7,24]. The curves for the original kaolinite and montmorillonite samples show the essential differences between these clays, e.g. the 2:1 layer-type mont-
Fig. 1. Influence of polyionic organic coating on pH-dependence of net proton surface excess amount: typical figures for soil (top), kaolinite (middle) and montmorillonite (bottom).
morillonite has a high permanent layer charge, while the 1:1 layer-type kaolinite has a very low permanent charge, but a comparable variable charge [1,22]. The permanent negative charges are compensated by the exchangeable cations [1,22], which are K+ in the suspensions initially due to the exhaustive dialysis against KNO solution. 3
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K+ is exchanged by H+ in the following ionexchange process during the acidic titration cycle: K-clay+(H+) PH-clay+( K+) aq aq and they remain specifically adsorbed [25] during the alkaline cycle. The net proton excess amount (DC ) calculated as the difference between the surface excess amount of protons (C ) and that of H+ hydroxide ions (C ) will therefore be positive. OH− The measured net proton excess amount for montmorillonite at low pH, ~0.48 mmol/g, is close to the cation-exchange capacity for this clay, 0.59 meq/g [25]. The low net proton excess amount vs. pH curve for kaolinite (middle Fig. 1) accords well with the low charging of this clay. The number of pH-dependent charges is larger, the curves shift in the direction of lower pH, and the characteristic pH of zero net proton charge (pznpc) is lower in the presence of the organic coating for both the soil and clay samples ( Table 1). Similar shifts in pzc have been described for soil and oxidized soil samples [23]. These changes can readily be explained by considering the protolytic processes [8] assigned to the organic coatings, which contain numerous acidic groups bound chemically on the mineral surfaces. The presence of a polyionic rough layer of organic matter will influence the particle–particle interaction, the colloidal stability and the structure of suspensions. Rheological measurements on concentrated suspensions can provide information on these [21,26 ]. Clay suspensions are pseudo-plastic systems from a rheological aspect [26 ]. The effects of exchangeable cations, i.e. the structure of the electric double layer, on the rheological properties of a montmorillonite suspension were analysed earlier [24]. Evaluation of the pseudo-plastic flow curves according to the classical Bingham model (t=t +g c˙ , where t is the shear stress, t is the B pl B yield value, g is the plastic viscosity and c˙ is the pl shear rate) results in the yield value (t ) by extrapoB lation of the linear part of the flow curve back to zero shear rate. This rheological parameter bears a relation to the strength of the physical network built up from the particles. As shown in Fig. 2, pseudo-plastic flow behaviour is characteristic of all the concentrated aqueous suspensions measured, independent of the presence
Fig. 2. Effects of coating by organic polyions on the spontaneously formed structure of concentrated suspensions: flow curves for soil (top), kaolinite (middle) and montmorillonite (bottom) samples.
or lack of an organic coating. However, it is also obvious that the yield values should be highly different, as indicated in Table 1. Since this rheological parameter is directly related to the attractive energy and the separation distance between the particles [21,24,26 ], the large decrease in it indicates the weakening of the physical network
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in the presence of the organic coating. It can be supposed that the attraction between the particles significantly decreases due to the combined steric and electrostatic stabilizing effects of the expanded layer of either natural or synthetic polyelectrolytes bound to the mineral surface.
4. Conclusions This work revealed that the polyionic organic coating exists on the surface of mineral grains as a rough layer plugging their pores. Both natural and synthetic coatings result in steric and electrostatic stabilization of mineral particles, because of the highly charged, polyionic character of the surface-modifying organic matter. The colloidal stability of aqueous systems, the particle–particle interaction and sediment formation are highly influenced by such surface modifications. This work was the first attempt to establish the role of a polyionic organic coating in the interfacial and colloidal properties of soil organo-mineral complexes and their synthetic models. Detailed investigations are needed to improve the evaluation by identifying the surface sites and interfacial process, by determining the structure of the interfacial layer. The calculation of ion distributions, potential functions and particle–particle interactions will perhaps be possible in the future to allow a quantitative characterization of these interesting systems from an environmental point of view.
Acknowledgment This work was supported by Hungarian National Science and Ministry Fund Grants (OTKA T006077, T022426, MKM 168/96).
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