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Clay-humus complexation: Effect of pH and the nature of bonding
Soil Eiol. Biorhem. Vol. 26, No. 9, pp. 1145-l 149, 1994 Copyright 0 I994 &lsevier Science Ltd Printed in Great Britain. All riehts reserved 0038-0717~94~EOO56-6 . , 0038-07 I7/94-$7.00 + 0.00
J
Pergamon
CLAY-HUMUS
COMPLEXATION: EFFECT OF pH AND THE NATURE OF BONDING
CHANDRIKA VARADACHARI, Department
AJIJUL HAQUE MONDAL,*
of Agricultural
DULAL C. NAYAK-/ and
Chemistry & Soil Science, University Calcutta 700 019, India
of Calcutta,
KUNAL GHOSH$ 35 B.C. Road,
(Accepted 24 February 1994) Summary-The effects of the pH variation on the complexation of humic substances by dried clay--humus systems were investigated. The amounts of FA (fulvic acid) fixed, when FA solutions at various pH values were complexed with Ca-montmorillonite, Ca-illite and Ca-kaolinite, were determined. The amounts of FA extractable at different pH values from FA@H 7.0)-clay-complexes were determined; the variation of extraction of HA (humic acid) at two different temperatures, from Ca-clay-HA complexes were also studied. A decrease in pH favoured increased bonding of FA. Illite fixed the largest amounts of FA at all pH values. At pH 2.0, the order of FA fixation was illite > kaolinite > montmorillonite. The fixation of FA by montmorillonite at pH 2.0 was even less than that of HA at pH 7.0. The more numerous negative charges on FA and its more hydrophilic character were apparently responsible for the poorer complexation of FA than of HA. The exchangeable cation-FA link is strongest in illite. Physical forces of bonding appeared to be dominant in kaolinite complexes, less so with illite and almost negligible in montmorillonite complexes. It was deduced from chemical bonding concepts that interaction of the acidic groups of humus with exchangeable cations is mainly electrostatic; water-bridges may exist between such links, even when the complexes are dry.
INTRODUCTION Investigations
by
Nayak
et
al.
(1990)
and
et al. (1991) on the complexation of humic substances by different kinds of clay minerals under low moisture conditions revealed new aspects of this phenomenon. A decrease in the acidic groups of humic acid (HA) by methylation increases fixation of HA by montmorillonite (Nayak et al., 1990). It was proposed that this was due to a lowering of interparticle repulsive forces as well as a reduction in the hydration energies of HA. Moreover, Varadachari et al. (1991) found that linkage through exchangeable cations is not only an important mode of interaction in montmorillonite-HA complexes but also in illitc-HA and kaolinite-HA complexes; direct bonding of HA to the exposed crystal edges of illite or kaolinite was not significant. With reduced-charge montmorillonites (RCM), electrostatic repulsive forces provide a major barrier to clay-HA bonding (Varadachari et al., 1991). The pH effect is obviously an important factor in clay-humus complexation. The effect of pH on the Varadachari
*t Present addresses: *College of Agriculture, North-East Hill University, Medziphema, Nagaland 797 106, India; tNationa1 Bureau of Soil Survey and Land Use Planning, Regional Centre, Block DK, Sector II, Salt Lake City, Calcutta 700 091, India. $Author for correspondence.
adsorption of FA (fulvic acid) by montmorillonite was studied by Schnitzer and Kodama (1966, 1967), who found a linear increase in the amount adsorbed with a decrease in pH; they also observed interlamellar complexation at low pH. There are numerous reports on various other aspects of complexes of montmorillonite with humic substances (Greenland, 1971; Theng, 1979; Stotzky, 1986). However, nothing is known about the effects of pH on complexation, particularly by illite and kaolinite, and the nature of interaction in dried systems. Drying promotes complexation by bringing the interacting species closer to each other. Dried complexes also stimulate natural conditions more closely. Most investigations have been focused on interaction in the suspension state. As in our previous studies (Nayak et al., 1990; Varadachari et al., 1991), we have used dried clay-humus complexes. Our objective was to understand how humus complexation by clay minerals may be influenced by pH in an attempt to derive some fundamental concepts of clay-humus bonding so as to extend our knowledge of this topic. We studied the complexation of FA at different pH by montmorillonite, illite and kaolinite. FA was extracted from these complexes by solutions varying pH and HA was extracted at two different temperatures. Chemical concepts of ionic bonding were used to deduce the nature of the linkage between the exchangeable cation and humic molecule.
1145
CHANDRIKA VARADACHARI er al.
1146 Table
1,Chemical
Constituents
analvsis
and X-rav diffraction
Montmorillonite
(%)
Al,& SiO Fe,O, + Fe0 TiO, cao M@ K,D Na,O H,OHzO+
51.59 16.92 3.80 0.37 9.19 5.62 0.11 0.15 5.91 6.06
Total
99.12
XRD bands (x IO~‘“rn)
14.90, 9.20, 5.06, 4.52, 3.37, 3.08
(XRD)
data of the clav minerals
Illite
Kaolinite
41.25 22.42 6.37 0.61 6.87 4.00 6.61 1.13 2.30 7.58
45.86 37.58 0.24 I .42 ND 0.76 0.05 ND 0.63 14.09
99.14
100.63
10.16, 6.41, 5.15 (weak), 4.62, 3.40. 3.06
7.25, 4.50, 4.39, 4.23, 3.90, 3.58, 3.06 (weak)
ND = not detectable.
MATERIALS
AND METHODS
The origin and characteristics of the HA sample we used were described by Varadachari et al. (1991). Elemental analysis of the HA shows 52.6% C, 3.6% H, 3.6% N, trace S, 40.2% 0 and 1.8% ash; its Ed/E6 ratio (absorbance at 465 nm/absorbance at 665 nm) was 4.94 and total acidity 7.2 x lo2 C mol (p’) kg-‘. The FA sample was kindly supplied by Dr Morris Schnitzer (LRRI, Agriculture Canada, Ottawa) and its detailed analytical characteristics have been described elsewhere (Schnitzer, 1978). The clay mineral samples used were those used by Varadachari et al. (1991); montmorillonite (SWy-1) from Cook County, WY., U.S.A.; an illite (IMt-1) from Silver Hill, Mont., U.S.A. and a well-crystallized kaolinite (KGa-1) from Washington Country, Ga, U.S.A., obtained from the Source Clay Repository of the Clay Minerals Society, U.S.A. Clay-size fractions were separated and converted to both Ca- and Na-forms. Chemical analysis data and the major X-ray diffraction bands the Ca-clay (under room humidity conditions) are shown in Table 1. For pH variation studies, the FA sample was used and the clay minerals in the Ca-form were used. The clay-FA complexes, all in 20-to-l ratios (clay:FA), were prepared as follows: to FA solution adjusted to pH 2.0,4.0, 5.0,7.0 or 9.0 using dilute NaOH or HCl, the required amount of clay was added to obtain a clay-FA ratio of 20-to-1 (w/w), gently shaken for 1 h, stood overnight, dried under vacuum at room temperature, powdered and stored. The clay-HA complex was similarly prepared from a HA solution at pH 7.0 (Nayak et al., 1990). With the systems under study, “fixed” humus has been defined (Nayak et al., 1990) as that which is not removed by washing with water at pH 7.0. The extent of fixation was determined in the following manner: to weighed amounts of the complex, 10 ml of water was added and the pH adjusted to 7.0. The suspension was shaken for 1 h and centrifuged (at 5000 revmin-’ for 30min), and the washings collected. This operation was repeated twice more. All three washings were collected and made up to 100 ml
with 0.05 N NaHCO,. Amounts of HA/FA were obtained from a standard curve constructed at 465 nm from the same HA/FA as those used for the complex preparation. Amounts of “fixed” HA were determined by the difference between the amounts added for preparing the clay-humus complexes and the amounts extracted. Montmorillonite-FA, illite-FA and kaolinite_FA complexes prepared at pH 7.0 were similarly extracted at pH 2.0 and 9.0. The effect of heating on the extraction of HA was studied with the Ca-montmorillonite-HA, Caillite-HA and Ca-kaolinite-HA complexes. The method of extraction was similar to that for “fixed” HA, except that after the addition of 10 ml water to the complex, the suspension was heated on a waterbath (at 100 + 1°C) for 1 h before centrifugation. The HA sample was used instead of FA because fixation of the FA by kaolinite, was so low that the effects of temperature could not be studied. All analyses were done in triplicate. RESULTS
AND DISCUSSION
Illite showed the highest fixation of FA at all pH values, followed by montmorillonite and then kaolinite; kaolinite, however, appeared to provide stronger bonding sites for FA at pH 2.0 than montmorillonite (Table 2). As the FAs from all complexes were extracted at pH 7.0, the data only concern the bonds that were formed as the complexes were prepared. The pH during extraction was the same for all complexes and there should be no disparity in the conditions of extraction between various complexes; Table 2. Effect of pH of FA solution on its complexation minerals (SEM given in parentheses)
by the clay
% Of total FA fixed (Ca-clay-FA = 20-to-i complex) pH Of FA solution 2.0 4.0 5.0 7.0 9.0
Montmorillonite 37 (0.43) 41 (0) 24 (0.48) 18(O) 13(O)
Illite
Kaolinite
69 (0) 56 (0) 32 (3.90) 24 (0) 26 (0)
54 (3.34) 25 (2.32) I5 (0.40) 0 (0) 7 (0)
Clay-humus complexation Table 3. Extraction of HA from the clay-HA complexes at different temperatures: the pH of the HA solution used for complexation was 7.0; pH during extraction was 7.0 (SEM given in parentheses) Temperature extraction (“0 27 100
during
% Of total HA fixed (Ca-clay-HA = 20-to-l complex) Montmorillonite 50 (0) 51 (0.44)
Illite 42 (0) 35 (3.30)
Kaolinite 20 (0) 10 (3.30)
therefore, the nature of the FA-clay bond being broken, would be the same in all samples. The decrease of fixed FA on montmorillonite with increasing pH (Table 2) may be attributed primarily to differences in interparticle repulsion as the complexes were formed. The drop in fixation at pH 2.0 over that at pH 4.0 may be the result of increased bonding by NH groups which are protonated at low pH to form NH:. Bonds formed with NH: may be readily broken when the complex is extracted at pH 7.0, where the NH: ion would revert to the non-ionic, NH-form. There is a 24% decrease in fixation of FA by montmorillonite when the pH of FA was increased from 2.0 to 9.0 (Table 2). Although greater dissociation at pH 9.0 should make available a larger number of interacting sites, interm_olecular repulsion was apparently sufficient to prevent the colloids from coming close enough to form a stronger bond. This was despite the fact that the complex had been dried to increase interaction between the groups. According to Varadachari et al. (1991), two platelets of a montmorillonite stack, sharing an interlayer cation, may be forced completely apart by the prying action of the HA/FA which then bonds to one of the platelets leaving the other free. A highly negatively-charged molecule like FA at pH 9.0, may not be able to approach close enough to the silicate layers to force them apart and can only bond to already available basal surface (which contain cations). Moreover, since forcing apart of the layers can only take place in a suspension state, drying the complex would not promote such a process of layer separation. Thus, increasing pH would cause greater dissociation of acidic groups of HA/FA, thereby, increasing clay-humus repulsion and reducing the prying action of HA/FA on montmorillonite stacks. Consequently complexation would reduce with an increase in pH, as observed in Table 2. By a similar reasoning, it is also possible to understand why less of FA is fixed than HA under comparable conditions. Thus, at pH 7.0, %FA fixed by montmorillonite is 18% (Table 2), whereas the %HA fixed by the same clay is 50% (Table 3). Since FA contains more acidic groups than HA, it is more negatively charged and for the above reasons, its interaction with montmorillonite is less effective. A similar increase in fixation of HA with a decrease in the acid group content was noted by Nayak et al. (1990), not only amongst different humic substances but also with a series of methylated derivatives of a
1147
HA sample. Mukherjee (1956) studying methylated HA also observed such behaviour. Our results are compatible with such findings. The explanation for such behaviour would be that a decrease in the number of acidic groups causes two simultaneously opposing effects. On the one hand, there would be fewer interacting sites available for complexation; on the other hand, intermolecular repulsion between clay and humic molecules would decrease thereby facilitating complexation. The extent of fixation would then depend on the combined effect of these two factors. Although electrostatic repulsion could explain why fixation of FA by montmorillonite at pH 7.0 was less than that of HA at pH 7.0, it does not explain why a smaller amount of FA is fixed by montmorillonite even at pH 2.0 or 4.0 as compared with HA at pH 7.0 (Tables 2 and 3 ). As FA at low pH (being in a more or less undissociated state) is expected to have less negative charge than HA at pH 7.0, a larger amount of FA than HA should have been fixed by montmorillonite under these conditions. The relatively low fixation of FA, even at low pH, was probably the results of the more numerous hydrophilic groups on the molecule and its, consequently, more soluble character as compared with HA. Thus, if a molecule of FA and another of HA are bonded in the same manner and with the same energy to a clay platelet, it is reasonable to suppose that because of the greater hydration energy imparted by the hydrophilic groups of FA, the clay-FA bond will be more readily broken and the FA more easily extracted by water. The process may also be likened to coagulation of FA or HA solutions by (in this case) Ca2+ ions. Kononova (1966) observed that whereas about 5 meq of Ca2+ ions can completely coagulate a certain amount of chernozem HA at a particular pH, the same amount of the corresponding FA at that pH cannot be readily coagulated by even 40 meq Ca*+ ions. Coagulation by Ca*+ Ions, . like fixation by Ca-clay, is rendered more difficult with FA due to the increased number of acidic groups which must be bonded before the molecule can be removed from solution. By similar reasoning, it is possible to understand why illite fixed more FA than montmorillonite (Table 2). Varadachari et al. (1991) inferred that the fixation capacity for HA increases with the strength of clay-cation bonds. As Ca2+ is more strongly bonded on illite than on montmorillonite (Varadachari et al., 1991), Ca-illite can retain FA more effectively than Ca-montmorillonite against washing with water. Moreover, the larger negative charge and interparticle repulsion on montmorillonite, are of greater hindrance to bonding than on illite. The number of basal surfaces of montmorillonite available bonding will also not be very large owing to the effect of intermolecular repulsion (as explained earlier). At pH 2.0, illite fixed 69% of the added FA (Table 2). Fixation decreased with increasing pH and
1148
CHANDRIKA~ARADACHARI
reached a plateau above pH 7.0. The material that remains fixed even at pH 9.0, could be linked by covalent or very strong ionic bonds, such as in ligand-exchange reactions (Greenland, 1971), as at high pH, the competition from OH- will be significant. Moreover, as Ca*+ on illite provides the main bonding centres (Varadachari et al., 1991) and as covalent linkage to Ca2+ is unlikely, the data confirm the presence of strong ionic attractive forces between FA and Ca*+ on the illite basal surfaces. That these forces are weaker on montmorillonite was indicated by the lower retention of FA at high pH values than on illite (Table 2). With kaolinite, very little fixation of FA was observed (Table 2) at pH 7.0 or 9.0, indicating the presence of weak electrostatic forces. Lowering of pH had the most marked influence on kaolinite_FA complexes-the amount of FA fixed increased by 47% when the pH was decreased from 9.0 to 2.0 (Table 2). As kaolinite has a very little negative charge, the effect of pH cannot have been entirely the result of intermolecular repulsion. A decrease in pH would favour the formation of positively-charged edges to which the FA may bond (Theng, 1979). Such bonding, although expected to be reversible with increases in pH, on drying may become sufficiently strong to be irreversible. This irreversibility of the bonding can also be observed from a comparison of % FA fixation values in Tables 2 and 4. Kaolinite complexed with FA at pH 2.0 and extracted at pH 7.0 showed 54% fixation of FA (Table 2), whereas when complexed with FA at pH 7.0 and extracted at pH 2.0, only 45% of the FA was fixed (Table 4; their difference is statistically significant). This indicated that bonding at pH 2.0 was favoured by drying. At low pH, hydrated H+ probably act as the bridging cations in addition to Ca*+; once the water shell surrounding the H+ is removed by drying, these ions would be firmly trapped between the kaolinite and FA molecules and would not be easily removable. Therefore, retention of FA by kaolinite would be favoured at low pH, due to the enhanced effect of such H+-bridges (in addition to the Ca*+-bridges). On extracting at pH 7.0 and 9.0 (Table 4), illite showed the largest retention of FA, indicating, as before, the greater strength of the illite-FA bond. In addition to chemical forces of interaction, physical forces such as H-bonding and van der Waals interactions may also be involved in the formation of clay-humus complexes. The latter types of bonds are Table 4. Extraction of FA at different pH values of the extracting solution: the pH of the FA solution used for complexation was 7.0 (SEM given in parentheses) % Of total FA fixed (Ca-clay-FA = 20-61 complex) pH Of the extracting solution 2.0 7.0 9.0
Montmorillonite 45 (0.38) 18 (0) 9(-)
lllite 33 (3.88) 24 (0) 21 (0)
Kaolinite 45 (0) 0 (0) 4 (0)
et al
likely to be more susceptible to rupture by an increase of temperature, and some idea of the extent to which the physical bonds are involved in complexation may be obtained by raising the temperature during extraction. At lOO”C, the montmorillonite-HA complex showed no significant change in HA release over that at 27°C (Table 3). For a similar temperature increase from 27 to lOO”C, illite showed an increase of 7 and kaolinite 10% in the amount of released HA. The data indicate that weak physical forces of attraction such as H-bonding or van der Waals interactions, were dominant in the kaolinite complex (temperature-influenced HA release is largest); it is less so in illite and almost absent in the montmorillonite complex (increase in HA-release with temperature is observable in the former but insignificant in the latter). From the overall results of the experiments on the effects of pH on “fixation”, it appeared that except at low pH, the bonding of FA on the Ca-clays was electrostatic in character although physical bonds may also be involved to a small extent. In general, it is logical to expect that the nature of the bond formed by the humic substances to a bridging cation will be similar to that in the metal-humus complex containing the same cation. The alkali and alkaline-earth cations such as Li+, Nat, K+, Ca*+ and Ba2+, form chemical bonds which are mainly ionic (Cotton and Wilkinson, 1969). Therefore, this property must also be reflected in the bonding of humus to the bridging cation. On the other hand, A13+, Fe3+, Si4, Ti4+ etc., which have a large ionic potential (i.e. cation charge-cation radius) form bonds mainly of a covalent character (Lee, 1979). The corresponding clay-humus complexes are, therefore, likely to have a more covalent character in the metal-humus bond. Ionic potential also influences the hydration energy of a cation and this, in turn, will determine whether a water-bridge will be present in the cation-humus linkage. The hydration energies of some of the cations are (in kJmollI): Li+, -519; Na+, -406; K+, -322; Mg*+, -1921; Ca*+, -1577; Ba*+, -1305; and A13+, -4665 (Lee, 1979). These values differ from free energy (AG) values by the value (TAS), which is very small (Lee, 1979). On the other hand, the logarithm of stability constants (log K) for the formation of metal humates are approximately in the range of 24, with maximum values around 10 for the more highly-complexing ions such as Pb*+ and Cu*+ (Schnitzer and Hansen, 1970; Stevenson and Ardakani, 1972; Stevenson, 1982). Calculation of the in free energy from the equation, change AG = - RT In K, even for a high log K value of 10, gives a free energy change of about - 57 kJ mol-’ for the complexation reaction. As the change in AG accompanying hydration appears to be much greater than that resulting from complexation, the possibility that the water-shell of the cations is displaced by the complexing groups of the humic molecule is negligible. Therefore, in a suspension, water-bridges must
Clay-humus complexation
bond humic substances to the cations at the surfaces of the clay minerals. However, since the logarithms of the equilibrium constant or stability constant (log K) values represent an average for different kinds of binding sites, some sites may exist which have more strongly complexing characteristics. Within these sites, water molecules may be displaced from the cations to form a direct linkage between the cation and the HA. These postulates may be relevant to an aqueous environment, but may be different under dry conditions. Monovalent cations like Na+ of K+, which have low hydration energies and can be readily dehydrated, may be directly bonded to the anionic groups of the humic molecule when subjected to drying. However, cations like Li+, Mg2+, Ca2+ or Ba*+ usually exist as heavily-hydrated salts from which the coordinated water molecules cannot be easily removed at room temperature (Lee, 1979). Therefore, clays saturated with such cations must be bonded to the humic molecule through a waterbridge even when in a dried state, under conditions of ambient temperature and room humidity. Another factor that may influence the nature of the exchangeable cation-humus linkage is the proximity of the reactive groups to the cations. If some acidic groups are sterically hindered from approaching close to the cation, either owing to the position of the groups or to the configuration of the molecule, then the nature of the linkage may be greatly altered from, e.g. a direct ionic bond, such as M+--OOC, to one such as M+-H20.. . -0OC. The main conclusions we draw from our study, based on experimental data combined with chemical concepts of bonding, are as follows: when the pH was varied, compared with HA, FA showed greatly reduced fixation by all clay minerals. The two key factors that influence the fixation of FA are, its negative charge and hydrophilic nature. Illite formed the strongest FA-clay linkages, followed by montmorillonite and kaolinite. At pH 2.0, there was strong and irreversible bonding of FA to illite and kaolinite. From temperature effect studies, it appeared that physical forces of attraction are predominant with kaolinite, less so with illite and almost negligible with montmorillonite. It was also deduced from chemical concepts that, apparently, the interaction of acidic groups of humus with clay minerals is mainly electrostatic; bonding to the exchangeable cations is gener-
1149
ally through a water-bridge even in dry complexes under atmospheric conditions; a few centres with strong complexing properties and in stericallyfavourable positions may form direct ionic or covalent links. Acknowledgement-We are grateful to the Indian Council of Agriculture Research, New Delhi for financial support.
REFERENCES
Cotton F. A. and Wilkinson G. (1969) Adoanced Inorganic Chemistry. Wiley-Eastern, New Delhi. Greenland D. J. (1971) Interactions between humic and fulvic acids and clays. Soil Science 111, 3441. Kononova M. M. (1966) Soil Organic Matter. Pergamon Press, Oxford. Lee J. D. (1979) Concise Inorganic Chemistry. English Language Book Society, London. Mukherjee H. (1956) Studies on the nature of humus and clay-humus complex. Journal of the Indian Chemical Society 33, 744748.
Nayak D. C., Varadachari C. and Ghosh K. (1990) Influence of organic acidic functional groups of humic substances in complexation with clay minerals. Soil Science 149, 268-27 1. Schnitzer M. (1978) Humic substances: chemistry and reactions. In Soil Organic Matter (M. Schnitzer and S. U. Khan, Eds), pp. la. Elsevier, Amsterdam. Schnitzer M. and Hansen E. H. (1970) Organo-metallic interactions in soils: 8. An evaluation of methods for the determination of stability constants of metal-fulvic acid complexes. Soil Science 109, 333-340. Schnitzer M. and Kodama H. (1966) Montmorillonite: effect of OH on its adsorption of a soil humic compound. Science i53, 70-71. _ Schnitzer M. and Kodama H. (1967) Reactions between a nodzol fulvic acid and Na-montmorillonite. Soil Science Society of America Proceedings
31, 632636.
Stevenson F. J. (1982) Humus Chemistry. Wiley-Interscience, New York. Stevenson F. J. and Ardakani M. S. (1972) Organic matter reactions involving micronutrients in soils. In Micronutriems in Agriculture (J. J. Mortvedt, P. M. Giordano and W. L. Lindsay, Eds), pp. 79-114. American Society of Agronomy, Madison, Wise. Stotzky G. (1986) Influence of soil mineral colloids on metabolic processes, growth, adhesion, and ecology of microbes and viruses. In Interactions of Soil Minerals with Natural Organics and Microbes (P. M. Huang and M. Schnitzer, Eds), pp. 305-428. Soil Science Society of America, Madison, Wise. Theng B. K. G. (1979) Formation and Properties of Clay-Polymer Complexes. Elsevier, Amsterdam. Varadachari C., Mondal A. H. and Ghosh K. (1991) Some aspects of clay-humus complexation: effect of exchangeable cations and lattice charge. Soil Science lS1,220-227.
J
Pergamon
CLAY-HUMUS
COMPLEXATION: EFFECT OF pH AND THE NATURE OF BONDING
CHANDRIKA VARADACHARI, Department
AJIJUL HAQUE MONDAL,*
of Agricultural
DULAL C. NAYAK-/ and
Chemistry & Soil Science, University Calcutta 700 019, India
of Calcutta,
KUNAL GHOSH$ 35 B.C. Road,
(Accepted 24 February 1994) Summary-The effects of the pH variation on the complexation of humic substances by dried clay--humus systems were investigated. The amounts of FA (fulvic acid) fixed, when FA solutions at various pH values were complexed with Ca-montmorillonite, Ca-illite and Ca-kaolinite, were determined. The amounts of FA extractable at different pH values from FA@H 7.0)-clay-complexes were determined; the variation of extraction of HA (humic acid) at two different temperatures, from Ca-clay-HA complexes were also studied. A decrease in pH favoured increased bonding of FA. Illite fixed the largest amounts of FA at all pH values. At pH 2.0, the order of FA fixation was illite > kaolinite > montmorillonite. The fixation of FA by montmorillonite at pH 2.0 was even less than that of HA at pH 7.0. The more numerous negative charges on FA and its more hydrophilic character were apparently responsible for the poorer complexation of FA than of HA. The exchangeable cation-FA link is strongest in illite. Physical forces of bonding appeared to be dominant in kaolinite complexes, less so with illite and almost negligible in montmorillonite complexes. It was deduced from chemical bonding concepts that interaction of the acidic groups of humus with exchangeable cations is mainly electrostatic; water-bridges may exist between such links, even when the complexes are dry.
INTRODUCTION Investigations
by
Nayak
et
al.
(1990)
and
et al. (1991) on the complexation of humic substances by different kinds of clay minerals under low moisture conditions revealed new aspects of this phenomenon. A decrease in the acidic groups of humic acid (HA) by methylation increases fixation of HA by montmorillonite (Nayak et al., 1990). It was proposed that this was due to a lowering of interparticle repulsive forces as well as a reduction in the hydration energies of HA. Moreover, Varadachari et al. (1991) found that linkage through exchangeable cations is not only an important mode of interaction in montmorillonite-HA complexes but also in illitc-HA and kaolinite-HA complexes; direct bonding of HA to the exposed crystal edges of illite or kaolinite was not significant. With reduced-charge montmorillonites (RCM), electrostatic repulsive forces provide a major barrier to clay-HA bonding (Varadachari et al., 1991). The pH effect is obviously an important factor in clay-humus complexation. The effect of pH on the Varadachari
*t Present addresses: *College of Agriculture, North-East Hill University, Medziphema, Nagaland 797 106, India; tNationa1 Bureau of Soil Survey and Land Use Planning, Regional Centre, Block DK, Sector II, Salt Lake City, Calcutta 700 091, India. $Author for correspondence.
adsorption of FA (fulvic acid) by montmorillonite was studied by Schnitzer and Kodama (1966, 1967), who found a linear increase in the amount adsorbed with a decrease in pH; they also observed interlamellar complexation at low pH. There are numerous reports on various other aspects of complexes of montmorillonite with humic substances (Greenland, 1971; Theng, 1979; Stotzky, 1986). However, nothing is known about the effects of pH on complexation, particularly by illite and kaolinite, and the nature of interaction in dried systems. Drying promotes complexation by bringing the interacting species closer to each other. Dried complexes also stimulate natural conditions more closely. Most investigations have been focused on interaction in the suspension state. As in our previous studies (Nayak et al., 1990; Varadachari et al., 1991), we have used dried clay-humus complexes. Our objective was to understand how humus complexation by clay minerals may be influenced by pH in an attempt to derive some fundamental concepts of clay-humus bonding so as to extend our knowledge of this topic. We studied the complexation of FA at different pH by montmorillonite, illite and kaolinite. FA was extracted from these complexes by solutions varying pH and HA was extracted at two different temperatures. Chemical concepts of ionic bonding were used to deduce the nature of the linkage between the exchangeable cation and humic molecule.
1145
CHANDRIKA VARADACHARI er al.
1146 Table
1,Chemical
Constituents
analvsis
and X-rav diffraction
Montmorillonite
(%)
Al,& SiO Fe,O, + Fe0 TiO, cao M@ K,D Na,O H,OHzO+
51.59 16.92 3.80 0.37 9.19 5.62 0.11 0.15 5.91 6.06
Total
99.12
XRD bands (x IO~‘“rn)
14.90, 9.20, 5.06, 4.52, 3.37, 3.08
(XRD)
data of the clav minerals
Illite
Kaolinite
41.25 22.42 6.37 0.61 6.87 4.00 6.61 1.13 2.30 7.58
45.86 37.58 0.24 I .42 ND 0.76 0.05 ND 0.63 14.09
99.14
100.63
10.16, 6.41, 5.15 (weak), 4.62, 3.40. 3.06
7.25, 4.50, 4.39, 4.23, 3.90, 3.58, 3.06 (weak)
ND = not detectable.
MATERIALS
AND METHODS
The origin and characteristics of the HA sample we used were described by Varadachari et al. (1991). Elemental analysis of the HA shows 52.6% C, 3.6% H, 3.6% N, trace S, 40.2% 0 and 1.8% ash; its Ed/E6 ratio (absorbance at 465 nm/absorbance at 665 nm) was 4.94 and total acidity 7.2 x lo2 C mol (p’) kg-‘. The FA sample was kindly supplied by Dr Morris Schnitzer (LRRI, Agriculture Canada, Ottawa) and its detailed analytical characteristics have been described elsewhere (Schnitzer, 1978). The clay mineral samples used were those used by Varadachari et al. (1991); montmorillonite (SWy-1) from Cook County, WY., U.S.A.; an illite (IMt-1) from Silver Hill, Mont., U.S.A. and a well-crystallized kaolinite (KGa-1) from Washington Country, Ga, U.S.A., obtained from the Source Clay Repository of the Clay Minerals Society, U.S.A. Clay-size fractions were separated and converted to both Ca- and Na-forms. Chemical analysis data and the major X-ray diffraction bands the Ca-clay (under room humidity conditions) are shown in Table 1. For pH variation studies, the FA sample was used and the clay minerals in the Ca-form were used. The clay-FA complexes, all in 20-to-l ratios (clay:FA), were prepared as follows: to FA solution adjusted to pH 2.0,4.0, 5.0,7.0 or 9.0 using dilute NaOH or HCl, the required amount of clay was added to obtain a clay-FA ratio of 20-to-1 (w/w), gently shaken for 1 h, stood overnight, dried under vacuum at room temperature, powdered and stored. The clay-HA complex was similarly prepared from a HA solution at pH 7.0 (Nayak et al., 1990). With the systems under study, “fixed” humus has been defined (Nayak et al., 1990) as that which is not removed by washing with water at pH 7.0. The extent of fixation was determined in the following manner: to weighed amounts of the complex, 10 ml of water was added and the pH adjusted to 7.0. The suspension was shaken for 1 h and centrifuged (at 5000 revmin-’ for 30min), and the washings collected. This operation was repeated twice more. All three washings were collected and made up to 100 ml
with 0.05 N NaHCO,. Amounts of HA/FA were obtained from a standard curve constructed at 465 nm from the same HA/FA as those used for the complex preparation. Amounts of “fixed” HA were determined by the difference between the amounts added for preparing the clay-humus complexes and the amounts extracted. Montmorillonite-FA, illite-FA and kaolinite_FA complexes prepared at pH 7.0 were similarly extracted at pH 2.0 and 9.0. The effect of heating on the extraction of HA was studied with the Ca-montmorillonite-HA, Caillite-HA and Ca-kaolinite-HA complexes. The method of extraction was similar to that for “fixed” HA, except that after the addition of 10 ml water to the complex, the suspension was heated on a waterbath (at 100 + 1°C) for 1 h before centrifugation. The HA sample was used instead of FA because fixation of the FA by kaolinite, was so low that the effects of temperature could not be studied. All analyses were done in triplicate. RESULTS
AND DISCUSSION
Illite showed the highest fixation of FA at all pH values, followed by montmorillonite and then kaolinite; kaolinite, however, appeared to provide stronger bonding sites for FA at pH 2.0 than montmorillonite (Table 2). As the FAs from all complexes were extracted at pH 7.0, the data only concern the bonds that were formed as the complexes were prepared. The pH during extraction was the same for all complexes and there should be no disparity in the conditions of extraction between various complexes; Table 2. Effect of pH of FA solution on its complexation minerals (SEM given in parentheses)
by the clay
% Of total FA fixed (Ca-clay-FA = 20-to-i complex) pH Of FA solution 2.0 4.0 5.0 7.0 9.0
Montmorillonite 37 (0.43) 41 (0) 24 (0.48) 18(O) 13(O)
Illite
Kaolinite
69 (0) 56 (0) 32 (3.90) 24 (0) 26 (0)
54 (3.34) 25 (2.32) I5 (0.40) 0 (0) 7 (0)
Clay-humus complexation Table 3. Extraction of HA from the clay-HA complexes at different temperatures: the pH of the HA solution used for complexation was 7.0; pH during extraction was 7.0 (SEM given in parentheses) Temperature extraction (“0 27 100
during
% Of total HA fixed (Ca-clay-HA = 20-to-l complex) Montmorillonite 50 (0) 51 (0.44)
Illite 42 (0) 35 (3.30)
Kaolinite 20 (0) 10 (3.30)
therefore, the nature of the FA-clay bond being broken, would be the same in all samples. The decrease of fixed FA on montmorillonite with increasing pH (Table 2) may be attributed primarily to differences in interparticle repulsion as the complexes were formed. The drop in fixation at pH 2.0 over that at pH 4.0 may be the result of increased bonding by NH groups which are protonated at low pH to form NH:. Bonds formed with NH: may be readily broken when the complex is extracted at pH 7.0, where the NH: ion would revert to the non-ionic, NH-form. There is a 24% decrease in fixation of FA by montmorillonite when the pH of FA was increased from 2.0 to 9.0 (Table 2). Although greater dissociation at pH 9.0 should make available a larger number of interacting sites, interm_olecular repulsion was apparently sufficient to prevent the colloids from coming close enough to form a stronger bond. This was despite the fact that the complex had been dried to increase interaction between the groups. According to Varadachari et al. (1991), two platelets of a montmorillonite stack, sharing an interlayer cation, may be forced completely apart by the prying action of the HA/FA which then bonds to one of the platelets leaving the other free. A highly negatively-charged molecule like FA at pH 9.0, may not be able to approach close enough to the silicate layers to force them apart and can only bond to already available basal surface (which contain cations). Moreover, since forcing apart of the layers can only take place in a suspension state, drying the complex would not promote such a process of layer separation. Thus, increasing pH would cause greater dissociation of acidic groups of HA/FA, thereby, increasing clay-humus repulsion and reducing the prying action of HA/FA on montmorillonite stacks. Consequently complexation would reduce with an increase in pH, as observed in Table 2. By a similar reasoning, it is also possible to understand why less of FA is fixed than HA under comparable conditions. Thus, at pH 7.0, %FA fixed by montmorillonite is 18% (Table 2), whereas the %HA fixed by the same clay is 50% (Table 3). Since FA contains more acidic groups than HA, it is more negatively charged and for the above reasons, its interaction with montmorillonite is less effective. A similar increase in fixation of HA with a decrease in the acid group content was noted by Nayak et al. (1990), not only amongst different humic substances but also with a series of methylated derivatives of a
1147
HA sample. Mukherjee (1956) studying methylated HA also observed such behaviour. Our results are compatible with such findings. The explanation for such behaviour would be that a decrease in the number of acidic groups causes two simultaneously opposing effects. On the one hand, there would be fewer interacting sites available for complexation; on the other hand, intermolecular repulsion between clay and humic molecules would decrease thereby facilitating complexation. The extent of fixation would then depend on the combined effect of these two factors. Although electrostatic repulsion could explain why fixation of FA by montmorillonite at pH 7.0 was less than that of HA at pH 7.0, it does not explain why a smaller amount of FA is fixed by montmorillonite even at pH 2.0 or 4.0 as compared with HA at pH 7.0 (Tables 2 and 3 ). As FA at low pH (being in a more or less undissociated state) is expected to have less negative charge than HA at pH 7.0, a larger amount of FA than HA should have been fixed by montmorillonite under these conditions. The relatively low fixation of FA, even at low pH, was probably the results of the more numerous hydrophilic groups on the molecule and its, consequently, more soluble character as compared with HA. Thus, if a molecule of FA and another of HA are bonded in the same manner and with the same energy to a clay platelet, it is reasonable to suppose that because of the greater hydration energy imparted by the hydrophilic groups of FA, the clay-FA bond will be more readily broken and the FA more easily extracted by water. The process may also be likened to coagulation of FA or HA solutions by (in this case) Ca2+ ions. Kononova (1966) observed that whereas about 5 meq of Ca2+ ions can completely coagulate a certain amount of chernozem HA at a particular pH, the same amount of the corresponding FA at that pH cannot be readily coagulated by even 40 meq Ca*+ ions. Coagulation by Ca*+ Ions, . like fixation by Ca-clay, is rendered more difficult with FA due to the increased number of acidic groups which must be bonded before the molecule can be removed from solution. By similar reasoning, it is possible to understand why illite fixed more FA than montmorillonite (Table 2). Varadachari et al. (1991) inferred that the fixation capacity for HA increases with the strength of clay-cation bonds. As Ca2+ is more strongly bonded on illite than on montmorillonite (Varadachari et al., 1991), Ca-illite can retain FA more effectively than Ca-montmorillonite against washing with water. Moreover, the larger negative charge and interparticle repulsion on montmorillonite, are of greater hindrance to bonding than on illite. The number of basal surfaces of montmorillonite available bonding will also not be very large owing to the effect of intermolecular repulsion (as explained earlier). At pH 2.0, illite fixed 69% of the added FA (Table 2). Fixation decreased with increasing pH and
1148
CHANDRIKA~ARADACHARI
reached a plateau above pH 7.0. The material that remains fixed even at pH 9.0, could be linked by covalent or very strong ionic bonds, such as in ligand-exchange reactions (Greenland, 1971), as at high pH, the competition from OH- will be significant. Moreover, as Ca*+ on illite provides the main bonding centres (Varadachari et al., 1991) and as covalent linkage to Ca2+ is unlikely, the data confirm the presence of strong ionic attractive forces between FA and Ca*+ on the illite basal surfaces. That these forces are weaker on montmorillonite was indicated by the lower retention of FA at high pH values than on illite (Table 2). With kaolinite, very little fixation of FA was observed (Table 2) at pH 7.0 or 9.0, indicating the presence of weak electrostatic forces. Lowering of pH had the most marked influence on kaolinite_FA complexes-the amount of FA fixed increased by 47% when the pH was decreased from 9.0 to 2.0 (Table 2). As kaolinite has a very little negative charge, the effect of pH cannot have been entirely the result of intermolecular repulsion. A decrease in pH would favour the formation of positively-charged edges to which the FA may bond (Theng, 1979). Such bonding, although expected to be reversible with increases in pH, on drying may become sufficiently strong to be irreversible. This irreversibility of the bonding can also be observed from a comparison of % FA fixation values in Tables 2 and 4. Kaolinite complexed with FA at pH 2.0 and extracted at pH 7.0 showed 54% fixation of FA (Table 2), whereas when complexed with FA at pH 7.0 and extracted at pH 2.0, only 45% of the FA was fixed (Table 4; their difference is statistically significant). This indicated that bonding at pH 2.0 was favoured by drying. At low pH, hydrated H+ probably act as the bridging cations in addition to Ca*+; once the water shell surrounding the H+ is removed by drying, these ions would be firmly trapped between the kaolinite and FA molecules and would not be easily removable. Therefore, retention of FA by kaolinite would be favoured at low pH, due to the enhanced effect of such H+-bridges (in addition to the Ca*+-bridges). On extracting at pH 7.0 and 9.0 (Table 4), illite showed the largest retention of FA, indicating, as before, the greater strength of the illite-FA bond. In addition to chemical forces of interaction, physical forces such as H-bonding and van der Waals interactions may also be involved in the formation of clay-humus complexes. The latter types of bonds are Table 4. Extraction of FA at different pH values of the extracting solution: the pH of the FA solution used for complexation was 7.0 (SEM given in parentheses) % Of total FA fixed (Ca-clay-FA = 20-61 complex) pH Of the extracting solution 2.0 7.0 9.0
Montmorillonite 45 (0.38) 18 (0) 9(-)
lllite 33 (3.88) 24 (0) 21 (0)
Kaolinite 45 (0) 0 (0) 4 (0)
et al
likely to be more susceptible to rupture by an increase of temperature, and some idea of the extent to which the physical bonds are involved in complexation may be obtained by raising the temperature during extraction. At lOO”C, the montmorillonite-HA complex showed no significant change in HA release over that at 27°C (Table 3). For a similar temperature increase from 27 to lOO”C, illite showed an increase of 7 and kaolinite 10% in the amount of released HA. The data indicate that weak physical forces of attraction such as H-bonding or van der Waals interactions, were dominant in the kaolinite complex (temperature-influenced HA release is largest); it is less so in illite and almost absent in the montmorillonite complex (increase in HA-release with temperature is observable in the former but insignificant in the latter). From the overall results of the experiments on the effects of pH on “fixation”, it appeared that except at low pH, the bonding of FA on the Ca-clays was electrostatic in character although physical bonds may also be involved to a small extent. In general, it is logical to expect that the nature of the bond formed by the humic substances to a bridging cation will be similar to that in the metal-humus complex containing the same cation. The alkali and alkaline-earth cations such as Li+, Nat, K+, Ca*+ and Ba2+, form chemical bonds which are mainly ionic (Cotton and Wilkinson, 1969). Therefore, this property must also be reflected in the bonding of humus to the bridging cation. On the other hand, A13+, Fe3+, Si4, Ti4+ etc., which have a large ionic potential (i.e. cation charge-cation radius) form bonds mainly of a covalent character (Lee, 1979). The corresponding clay-humus complexes are, therefore, likely to have a more covalent character in the metal-humus bond. Ionic potential also influences the hydration energy of a cation and this, in turn, will determine whether a water-bridge will be present in the cation-humus linkage. The hydration energies of some of the cations are (in kJmollI): Li+, -519; Na+, -406; K+, -322; Mg*+, -1921; Ca*+, -1577; Ba*+, -1305; and A13+, -4665 (Lee, 1979). These values differ from free energy (AG) values by the value (TAS), which is very small (Lee, 1979). On the other hand, the logarithm of stability constants (log K) for the formation of metal humates are approximately in the range of 24, with maximum values around 10 for the more highly-complexing ions such as Pb*+ and Cu*+ (Schnitzer and Hansen, 1970; Stevenson and Ardakani, 1972; Stevenson, 1982). Calculation of the in free energy from the equation, change AG = - RT In K, even for a high log K value of 10, gives a free energy change of about - 57 kJ mol-’ for the complexation reaction. As the change in AG accompanying hydration appears to be much greater than that resulting from complexation, the possibility that the water-shell of the cations is displaced by the complexing groups of the humic molecule is negligible. Therefore, in a suspension, water-bridges must
Clay-humus complexation
bond humic substances to the cations at the surfaces of the clay minerals. However, since the logarithms of the equilibrium constant or stability constant (log K) values represent an average for different kinds of binding sites, some sites may exist which have more strongly complexing characteristics. Within these sites, water molecules may be displaced from the cations to form a direct linkage between the cation and the HA. These postulates may be relevant to an aqueous environment, but may be different under dry conditions. Monovalent cations like Na+ of K+, which have low hydration energies and can be readily dehydrated, may be directly bonded to the anionic groups of the humic molecule when subjected to drying. However, cations like Li+, Mg2+, Ca2+ or Ba*+ usually exist as heavily-hydrated salts from which the coordinated water molecules cannot be easily removed at room temperature (Lee, 1979). Therefore, clays saturated with such cations must be bonded to the humic molecule through a waterbridge even when in a dried state, under conditions of ambient temperature and room humidity. Another factor that may influence the nature of the exchangeable cation-humus linkage is the proximity of the reactive groups to the cations. If some acidic groups are sterically hindered from approaching close to the cation, either owing to the position of the groups or to the configuration of the molecule, then the nature of the linkage may be greatly altered from, e.g. a direct ionic bond, such as M+--OOC, to one such as M+-H20.. . -0OC. The main conclusions we draw from our study, based on experimental data combined with chemical concepts of bonding, are as follows: when the pH was varied, compared with HA, FA showed greatly reduced fixation by all clay minerals. The two key factors that influence the fixation of FA are, its negative charge and hydrophilic nature. Illite formed the strongest FA-clay linkages, followed by montmorillonite and kaolinite. At pH 2.0, there was strong and irreversible bonding of FA to illite and kaolinite. From temperature effect studies, it appeared that physical forces of attraction are predominant with kaolinite, less so with illite and almost negligible with montmorillonite. It was also deduced from chemical concepts that, apparently, the interaction of acidic groups of humus with clay minerals is mainly electrostatic; bonding to the exchangeable cations is gener-
1149
ally through a water-bridge even in dry complexes under atmospheric conditions; a few centres with strong complexing properties and in stericallyfavourable positions may form direct ionic or covalent links. Acknowledgement-We are grateful to the Indian Council of Agriculture Research, New Delhi for financial support.
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Nayak D. C., Varadachari C. and Ghosh K. (1990) Influence of organic acidic functional groups of humic substances in complexation with clay minerals. Soil Science 149, 268-27 1. Schnitzer M. (1978) Humic substances: chemistry and reactions. In Soil Organic Matter (M. Schnitzer and S. U. Khan, Eds), pp. la. Elsevier, Amsterdam. Schnitzer M. and Hansen E. H. (1970) Organo-metallic interactions in soils: 8. An evaluation of methods for the determination of stability constants of metal-fulvic acid complexes. Soil Science 109, 333-340. Schnitzer M. and Kodama H. (1966) Montmorillonite: effect of OH on its adsorption of a soil humic compound. Science i53, 70-71. _ Schnitzer M. and Kodama H. (1967) Reactions between a nodzol fulvic acid and Na-montmorillonite. Soil Science Society of America Proceedings
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Stevenson F. J. (1982) Humus Chemistry. Wiley-Interscience, New York. Stevenson F. J. and Ardakani M. S. (1972) Organic matter reactions involving micronutrients in soils. In Micronutriems in Agriculture (J. J. Mortvedt, P. M. Giordano and W. L. Lindsay, Eds), pp. 79-114. American Society of Agronomy, Madison, Wise. Stotzky G. (1986) Influence of soil mineral colloids on metabolic processes, growth, adhesion, and ecology of microbes and viruses. In Interactions of Soil Minerals with Natural Organics and Microbes (P. M. Huang and M. Schnitzer, Eds), pp. 305-428. Soil Science Society of America, Madison, Wise. Theng B. K. G. (1979) Formation and Properties of Clay-Polymer Complexes. Elsevier, Amsterdam. Varadachari C., Mondal A. H. and Ghosh K. (1991) Some aspects of clay-humus complexation: effect of exchangeable cations and lattice charge. Soil Science lS1,220-227.