REACTIONS BETWEEN HUMIFIED CLOVER EXTRACT AND IMOGOLITE AS A MODEL OF HUMUS-CLAY INTERACTION : PART II

Transcription

1 Clay Science 4, (1971) REACTIONS BETWEEN HUMIFIED CLOVER EXTRACT AND IMOGOLITE AS A MODEL OF HUMUS-CLAY INTERACTION : PART II TAKAHIRO INOUE and Koji WADA Faculty of Agriculture, Kyushu University, Fukuoka (Received October 30, 1971) ABSTRACT The adsorption complexes which consist of imogolite and humified clover extract were characterized by x-ray analysis, measurement for adsorption of water vapor, infrared absorption spectroscopy and thermogravimetry. The x-ray and water retention data indicated that considerable portions of humified material in the complexes, particularly that adsorbed from dilute solutions, were accommodated between the structure units of imogolite, resulting in displacement of water from the surfaces of the clay and humus molecules. This indication was also supported from very remarkable reduction of adsorption by preheating of imogolites in which the pores between the structure units were collapsed. A- bout 30 to 70 % of the adsorbed humified material was estimated to come into contact with 10 to 35 % of the imogolite surface. The difference infrared spectrum of the adsorbed humified material indicated removal of adsorbed water from the complex, but failed to show any sign of specific interactions between inorganic and organic functional groups. The complex formation resulted in changes on the DTG curve of the humified material similar to those observed for the Al-fulvic acid complex by Schnitzer et al. (1967). From the foregoing observations (Part I and II), the adsorbed humified material was divided into two fractions accommodated and unaccommodated into the imogolite thread. Two different bonding mechanisms were assumed to operate in this accommodation reaction ; the first is the co-ordination of the carboxyl groups of the humus molecules to the Al atoms on the surfaces of imogolite. The second is multiple surface-segment interactions primarily associated with hydrogen bonding and/or van der Wads forces. The difference in the energetics of interactions was interpreted in terms of different orientations of the humus molecules with respect to the thread axis of imogolite. INTRODUCTION The Part I of this paper (Inoue and Wada, 1971) provided the data on the adsorption of humified clover extract as a model substance of newly formed humus on imogolite. Two mechanisms seem to operate primarily in the adsorption. The first is a ligand-exchange reaction through which some carboxyl groups of the humus molecules are incorporated into the co-ordination shells of Al atoms on the clay surfaces. The second is interactions between the extensive and flexible surface segments of the humus molecules and the structure units of imogolite.

2 72 T. Inoue and K. Wada The assessment of the bonding mechanism as a whole, however, awaits characterization of the resulting complexes of imogolite and humified material, which forms the object of the present study. X-ray analysis, measurement for adsorption of water vapor, infrared absorption spectroscopy and thermogravimetry were carried out for the complexes and mixtures of imogolite and humified material, whenever possible. Imogolite has now been accepted as a valid mineral species (AIPEA Nomenclature Committee, 1970), but there remain different opinions and uncertainties regarding to the nature of its structure (Russell et al., 1969 ; Wada and Yoshinaga, 1969 ; Wada et al., 1970). At this point, therefore, it may be useful to state a summary of our present knowledge and interpretation on the structure of imogolite as a reactant : Imogolite consists of micron-length, flexible "tubular" structure units with the inner and outside diameters, about 10 and 20 A, respectively (Wada et al., 1970). The x-ray patterns of imogolite are interpreted as indicating that these units have a chain structure, align more or less in parallel and form a paracrystalline assembly with the mean inter-structure-unit separation 17.7A (Wada and Yoshinaga, 1969). Russell et al. (1969) postulated also a different chain structure, but did not mention a particular packing of the units. Chemical analyses gave the formula 1.1 SiO2 Al2O H2O (Wada and Yoshinaga, 1969) and 3 SiO2 E2 Al2O3 E5 H2O (Russell et al., 1969). Deuteration studies showed that all the OH groups are located at the surface (Wada, 1966 ; Russell et al., 1969). Imogolite heated at 250 Ž showed at first a sign of irreversible change in the arrangement of the structure units, which was interpreted in terms of their linking upon dehydroxylation. This change proceeds up to 300 Ž, and then a breakdown of the structural organization occurs progressively and completed at 400 Ž (Wada and Yoshinaga, 1969). MATERIALS AND METHODS a) Preparation of Humified Clover Extract-Imogolite Complexes The KaG- and 905- humus complexes were obtained from the corresponding residues of clay and humified material remained in the centrifuge tubes after adsorption experiments (Part I). A portion of the residues were spread and air-dried on glass slides for the x-ray analysis. The remaining portions were dried in a CaCl2-desiccator under suction with an aspirator, and pulverized for the measurement of water vapor adsorption. The KiG-humus complexes were also obtained from the residues after adsorption and desorption determinations described in Part I. A portion of the residues were used for the x-ray analysis, and the remains were freeze-dried and used for the measurement of water vapor adsorption. The KiG clay suspension and the humified clover extract alone were freeze-dried and used as references. b) X-ray Analysis The clay-humus complexes and the clays were x-rayed in a Geigerflex diffractometer at room temperature and after heating progressively at 120 to 350 Ž for 1 hour or longer. Rehydration of the heated samples during the x-raying was prevented by applying a small heater on the back of the glass slide and keeping its temperature at about 110 Ž.

3 Reactions between Humified Clover and Imogolite 73 c) Measurement for Adsorption of Water Vapor The adsorption of water vapor at various relative humidities was determined by gravimetry after placing the airdried and pulverized samples for 10 days in desiccators containing appropriate concentrations of sulfuric acid at 25 Ž. The sample weight free from adsorbed water was obtained by drying in a P2O5- desiccator, or by heating at 110 Ž, and the reference clay weight was obtained by igniting the sample after all the determinations. d) Infrared Spectroscopy Appropriate quantities of the KiG clay suspension, the humified clover extract and their mixture were dried on watch glasses. The residues were scraped off as completely as possible, and then diluted with KBr to 600 mg. All the KBr mixtures were allowed to stand at least for 24 hours in the spectrophotometer room (R. H. 32 %) to equilibrate their moisture retention. The infrared spectrum was obtained from the KBr disc containing 4.0 mg of the clay, 2.0 mg of the humified material or the corresponding amount of their complex. Although an adsorption experiment showed that 30 per cent or less of the added humified material remained unadsorbed in the prepared complex, the preparation will simply be referred to as the KiG-humus complex in the text. The spectrum of the adsorbed humified material was obtained as the difference spectrum between those of the clay-humus complex and the clay. e) Thermogravimetry Thermogravimetry (TG) was carried out using a Thermoflex apparatus. Thirty milligrams of the KiG clay, 8 mg of the humified material or the corresponding amount of their complex was heated in static air with a heating rate at 20 Ž per minute. Differential thermogravimetric (DTG) curves were obtained from the TG curves by determining the weight losses per 5 Ž interval and plotting these versus temperature. RESULTS AND INTERPRETATION X-ray Analysis A preliminary x-ray study was carried out for the complex consisting of a non-deferrated 905 clay and a non-dialyzed humified clover extract (Inoue and Wada, 1968). The result indicated an inhibitory effect of adsorbed humified material on the development of A peak within a broad 13 to 18 A diffraction band of imogolite upon heating. This inhibitory effect was observed in the present study for the KaG- and 905- humus complexes as shown in Fig. 1, where the ratios of the diffraction intensity at 18 A to that at 14 A are plotted versus temperature. The development of the A peak upon heating was interpreted in terms of the rearrangement of the structure units in imogolite (Wada and Yoshinaga, 1969). The observed reduction in the intensity ratio between 100 to 300 Ž (Fig. 1) can be explained along the same line. Some of the humus molecules adsorbed would be present between the structure units of imogolite, and inhibit their rearrangement upon heating. When heated above 300 Ž, the humus molecules are burnt off and the rearrangement of the structure units therefore should take place. The reduction of the diffraction intensity ratio, however, was not parallel with the increase in adsoption, and was very small for the humified material adsorbed

4 74 T. Inoue and K. Wada in excess of 10 g per 100 g clay (Fig. 1). This may suggest that there are limits for accommodation of the humus molecules between the structure units of imogolite and/or that relatively small number of the humus molecules adsorbed initially act as pillows. For comparison, KCH3COO was chosen as a compound with much smaller molecular weight, and was found to form a similar complex with imogolite. Fig. 2 shows the observed relationships between the diffraction intensity ratio and the rate of salt addition. The intensity ratio increased at first and then decreased with the addition of the salt. When KCH3COO was added at the rate of 120 g per 100 g clay, the clay film was moistened due to deliquescence of the excess salt. The resulting complex gave a symmetrical and fairly sharp band with maximum at 14.5 to 14.7 A, which was hardly affected by heating at 100 Ž. A similar observation was made with other organic compounds such as (CH3)4- NCl and (C2H5)4NCl (Wada and Henmi, unpublished). These salt-uptake phenomena were interpreted in terms of filling of the micropores in imogolite, and used for the pore space determination. When the pore space was expressed in the void ratio, the values 1.3 to 1.6 were obtained from their uptake maximum. The water adsorption data also gave the void ratio 1.2 to 1.3 (Wada and Yoshinaga, 1969 ; Wada and Henmi, unpublished). The percentage of the pore space filled with KCH3COO was then calculated by assuming accommodation of all the added KCH3COO and only that in the micropores, and using the value 1.3 for the void ratio. In this calculation, the density of the accommodated KCH3COO was assumed to be equal to that of the solid KCH3COO. The humified clover extract was also added to the same preparations of the KiG and KaG clays at the rate of 19.8 and 22.7 g per 100 g clay respectively, and the diffraction intensity ratios were measured at 110 Ž. If filling of the micropores in imogolite occurs in a similar way with the humified material and KCH3COO, the plots of the measured intensity ratios 1.2 (KiG) and 1.5 (KaG) on the curves shown in Fig. 2 FIG. 1. Effect of adsorption of humified material on x-ray diffraction of imogolite. The content of humified material in the complexes expressed in g per 100 g clay : KaG ; 0, 9.4, œ FIG. 2. Effect of addition of KCH3COO on x-ray diffraction of imogolite. 905 ; 0, 8.6, œ 24.0.

5 Reactions between Humified Clover and Imogolite 75 would give a rough measure of the micropore space occupied by the humus molecules. It turned out to be 10 to 15 % of the total micropore space. Measurement for Adsorption of Water Vapor Water retention data for the KiG, KaG and 905 clays and their humus complexes are shown in Fig. 3. It also contains the reference data on the corresponding mixtures obtained by simple addition of the retention values of the clay and humified material according to their content and assuming no interaction between them. The water retention is always lower in the complexes than the corresponding mixtures, reflecting an interaction between the surfaces of the clay and humified material in the complex, which results in reduction of surface area available to water adsorption. The difference in water retention between the complex and mixture increases at first with the increasing content of humified material and then remains almost constant (Fig. 3). It suggests that there are some limits in accommodation of humified material into imogolite threads, beyond which increasing parts of the adsorbed material extrude from the latter. The amount of humified material accommodated was calculated from the data assuming complete dehydration of the humus molecules accommodated into imogolite threads and an equality in the volume between the space occupied by them and that had been occupied by water thereby re- FIG. 3. Adsorption of water on imogolites and imogolite-humus complexes. Measurement was carried out at R.H. 95G/ with KiG and at R. H. 96 % with KaG and 905. Dot-dash lines show water adsorption on the corresponding imogolite-humus mixtures calculated by assuming no interaction between the constituents. placed from the clay surface. The results of calculation are shown in Table 1, which also includes estimates of the percentages of the accommodated out of total adsorbed humified material and those of the displaced out of total adsorbed water on the clay surface. These values may give measures of the extent of surface interaction between imogolite and humus molecules. Infrared Spectroscopy The infrared spectrum of the humified clover extract and the difference spec - trum between the KiG-humus complex and the KiG clay are shown in Fig. 4, together with the assignment of major absorption bands. If there is no change

6 76 T. Inoue and K. Wada TABLE 1. Accommodation of humified material into imogolite threads and displacement of water from surface of imogolite estimated from measurement of water adsorption at A.H. 95 % on KiG- humus complex and at B.H. 96 % on KaG- and 905-humus complexes 1) The density of humified material was assumed to be 1.4 (Bayer, 1956). 2) The percentage of the accommodated out of total adsorbed humified material. 3) The percentage of the displaced out of total adsorbed water of imogolite. in imogolite in the complex formation, the difference spectrum can be equated with the spectrum of the adsorbed humified material, but any change in imogolite w ould also appear on the difference spectrum. Thus, an apparent lack of the broad absorption bands with maxima at 3340 cm-1 and 1630 cm-1 on the difference spectrum would primarily reflect the effect of replacement of water both from humified material and imogolite as described in the preceding section. The loss and/or bonding of the OH groups in humus molecules as well as imogolite could lead to the similar change, but under this circumstance, it is difficult to prove these by infrared spectroscopy. FIG. 4. Infrared spectra of "free" and "adsorbed" humified material.

7 Reactions between Humified Clover and Imogolite 77 Besides this, no significant change due to adsorption can be found on the difference spectrum (Fig. 4). It may be expected that the interaction between Al and carboxyl groups as suggested from the previous study (Part I) would result in changes of absorption bands related to the COO- and COOH groups. No detectable change appears, however, presumably because only a small fraction of these groups underwent any interaction at ph 5.6, where the complex was formed (Part I). Also, it has been known that the frequencies are not very sensitive to the changes in the bonding cations, unless the bonding becomes more covalent (Nakamoto, 1963). Thermogravimetry The DTG curves of the KiG clay, humified material and their complex and mixture are shown in Fig. 5. The DTG curve of the KiG clay shows two broad peaks at 60 to 100 Ž and at 370 Ž due to dehydration and dehydroxylation, respectively, while the DTG curve of the humified material shows three peaks at 60, 26 0 and 430 Ž. A DTG curve similar to the latter was obtained for fulvic acids separated from the Bh horizon of a Podzol (Kodama and Schnitzer, 1970). On the basis of their earlier pyrolysis experiments, Schnitzer and Hoffman (1965) ascribed the observed small peak at 100 Ž to dehydration, the minor peak at 180 Ž to partial decarboxylation, the medium peak at 280 Ž to dehydroxylation and de - carboxylation, and the principal peak at 420 Ž to the decomposition of the poorly condensed aromatic nucleus of the fulvic acid. FIG. 5. DTG curves of KiG clay, humified material and their mixture and complex. In Fig. 5, the effect of interaction between the clay and humified material is clearly seen on the major decomposition peak of the latter. In the complex, the reaction initiates at the lower temperature, almost merging to dehydroxylation of imogolite, and extends to the higher temperature without showing any sharp peak, which is found for the unadsorbed humified material. Little knowledge on

8 78 T. Inoue and K. Wada the thermal decomposition of humified material does not permit detailed interpretation, but there is one pertinent and interesting observation. Schnitzer and Hoffman (1967) examined the effects of metallic cation addition on the DTG curve of the Podzol fulvic acid described above. Addition of monovalent cations resulted in the shift of the principal peak at 420 Ž to higher temperatures, while addition of di- and tri-valent cations to lower temperatures. The shift and broadening of the principal peak observed with the Al-fulvic acid complex are very similar to those observed with the imogolite-humus complex in the present study. The Fe-fulvic acid gave a distinctly different DTG curve, which was utilized for identification of the metal-fulvic acid complex in an iron pan of a Humic Podzol (McKeague et al., 1967). The importance of Al in interaction between imogolite and humified material is generally consistent with all the foregoing observations. DISCUSSION AND CONCLUSIONS The x-ray and water retention data indicated that considerable portions of humified material adsorbed, particularly from dilute solutions, are accommodated between the structure units of imogolite, and that their interaction resulted in displacement of water from the surfaces of the reactants. This observation has an important bearing on the mechanism with which the increasing strength is conferred to the adsorption, since the adsorption and desorption studies (Part I) have shown that the humified material adsorbed initially has higher affinity to the clay than has the remaining humified material. The extent of surface interaction was also estimated from the x-ray and water retention data. The estimates were given as percentages of the accommodated out of total adsorbed humified material (Table 1), the occupied out of total micropore space in imogolite (p. 75) and the displaced out of total adsorbed water of imogolite (Table 1). Despite of the assumptions involved in these estimations, there was rather good agreement between the values obtained from the different analyses. Thus, we may estimate that about 30 to 70 per cent of the adsorbed material is coming into contact with 10 to 35 per cent of imogolite surfaces. The rate of accommodation of humified material into imogolite structure generally decreased with its increasing content in the complex, and the trend was not similar from one sample of imogolite to another. The observed difference between the samples of imogolite probably reflects the differences in the degree of its structural organization and in the amount and nature of the coexisting allophane. As described in Introduction, high resolution electron microscopy indicated that the units of imogolite are tubes with the inner and outside diameters, about 10 and 20 A, respectively (Wada et al., 1970). A corroborative evidence to support the presence of intra-unit-pores in imogolite has been obtained by determining retention of quaternary ammonium chlorides and water (Wada and Henmi, unpublished). Then, it may be interesting to know whether humified material is accommodated or not in the intra-unit-pores. The adsorption of humified material was measured with preheated imogolites, since preheating has been known to result in preferential destruction of its inter-unit-pores (Wada and Henmi, unpublished). The absorbance was determined at 600 mƒê with the supernatants obtained from the preheated KiG clay suspensions to which humified material

9 Reactions between Humified Clover and Imogolite 79 was added at the rate of 20 mg per 40 mg clay. A very remarkable reduction of adsorption, almost to null, was shown with the clay preheated at 105 Ž slight negative adsorption with the clays preheated at 150, 200 and 300 Ž. The latter merely reflects slight dispersion of the clay, which was no longer kept in flocculated state unlike the air-dry clay. The sedimentation volume of the clay resuspened in water was also remarkably reduced by the preheat treatment, and a microscopic observation revealed aggregation of imogolite into thin flakes of micron size. Effect of this change of the clay on the adsorption of humified material is not known at present, but the magnitude of the observed reduction in adsorption makes good contrast with that in adsorption of water, (CH3) 4NCl, (C 2H5) 4NCl and (C4H9) 4NCl quoted above. The retention values of the latter reduced only to half or little less than half of those of the air-dry clay by preheating. Thus, it may be inferred that the intra-unit-pores accommodate water and the quaternary ammonium chlorides but not humus molecules in any significant amount. In conclusion, the adsorbed humified material can be divided into two fractions, accommodated and not accommodated into imogolite threads. This distinction is applied to the segments of single humus molecules rather than the groups of humus molecules as shown schematically in Fig. 6. The accommodation of humus molecules can be visuallized as a result of co-operation of the two different mechanisms. The first is incorporation of the carboxyl groups into the co-ordination shell of Al atoms which reside on the surface of the structure units of imogolite. The second is multiple, weak surface-segment interactions between the accommodated humus molecules and FIG. 6. Suggested scheme of accommodation of humus molecules into imogolite the structure units of imogolite, where hydrogen bonding and/or van der Waals thread. forces have important roles. Since the two bonding mechanisms are expected to work quite co-operatively, the number of the bonding points per humus molecule would primarily be determined by its orientation on and within the thread of imogolite. The orientation of humus molecules somehow in parallel to the tube axis of the structure units of imogolite allows their maximum contact with the clay surface, where humus molecules would probably penetrate into the inter-unitpores of imogolite from the ends of each thread (Fig. 6 ; left). The rapid rise of the initial portion of the adsorption curve and the difficult displacement of humified material adsorbed there suggest that humus molecules initially adsorb - h ed take this "parallel" orientation. Even in this orientation, the penetration of umus molecules would be restricted from their own three-dimensional structure, which was not shown for the sake of simplicity in Fig. 6, against the one-dimensional development of the inter-unit-pores of imogolite. Thus, considerable portions of humus molecules would be left on the outside of the imogolite thread. As, and

10 80 T. Inoue and K. Wada humified material is more taken up, there will be progressively less chance that humus molecules can take this energetically favorable orientation. Then, they will find their adsorption sites at intermediate portions of the thread and take "diagonal" orientation with respect to the thread axis of imogolite, which would result in increase of the proportion of the extruded segments of the humus molecules. REFERENCES BAVER, L. D. (1956) Soil Physics (3rd Edition). John Wiley & Sons, Inc. New York. p. 57. INOUE, T. and WADA, K. (1968) Trans. 9th Intern. Congr. Soil Sci., 3, INOUE, T. and WADA, K. (1971) Clay Sci. 4, KODAMA, H. and SCHNITZER, M. (1970) Soil Sci., 109, MCKEAGUE, J. A., SCHNITZER, M. and HERINGA, P. K. (1967) Can. J. Soil Sci., 47, NAKAMOTO, K. (1963) Infrared Spectra of Inorganic and Coordination Compounds. John Wiley & Sons, Inc. New York. p RUSSELL, J. D., MCHARDY, W. J. and FRASER, A. R. (1969) Clay Minerals, 8, SCHNITZER, M. and HOFFMAN, I. (1965) Geochim. Cosmochim. Acta, 29, SCHNITZER, M. and HOFFMAN, I. (1967) Geochim. Cosmochim. Acta, 31, WADA, K. (1966) Soil Sci. Plant Nutr. (Tokyo), 12, WADA, K. and YOSHINAGA N. (1969) Amer. Mineral., 54, WADA, K., YOSHINAGA, N., YOTSUMOTO, H., IBE, K. and AIDA, S. (1970) Clay Minerals, 8,