INTERACTION OF IRON OXIDES WITH CLAYS

Size: px

Start display at page:

Download "INTERACTION OF IRON OXIDES WITH CLAYS"

Corey Ashlee Hawkins
5 years ago
Views:

1 Clay Science 7, (1989) INTERACTION OF IRON OXIDES WITH CLAYS MASAMI OHTSUBO Department of Agricultural Engineering Kyushu University Fukuoka-shi, 812 Japan (Accepted April 13, 1989) ABSTRACT The present paper reviews the interaction of synthetic and natural iron oxides with clay surfaces in terms of the charge characteristics of iron and clay minerals, and the role of iron oxide in the clay soil behaviour. The association of iron oxide particles with clay surfaces in the synthesized iron oxide-clay complexes was due to coulombic attraction. For natural clay soils the specific adsorption of anions on iron oxides affected the association between iron oxides and clay minerals. Addition and extraction of iron hydroxides and oxides in clays indicated that iron minerals contribute to a decrease of sensitivity and an increase of Bingham yield stress. Key words: Iron Oxide, Charge Characteristics, Interaction Zero Charge, Marine Clays, Cementation, Point of INTRODUCTION Iron oxides, which is important as cementing agents, have been studied in relation to the behaviour of marine clays (Torrance, 1974; Quigley, 1980) and with respect to the aggregate stability of clay soils (El-Swaify, 1980). Numerous studies of the formation, morphology, and electrochemical characteristics of iron oxides have been conducted in the field of soil chemistry (Shwertmann and Taylor, 1977). However, the results obtained in this field have not fully served to evaluate the role of iron oxides in the clay soil behaviour. The present paper reviews the charge characteristics of iron oxides, the association of iron oxide with clay particles, and the role of iron oxides in the aggregate stability and the physical behaviour of clay soils. It is hoped that this paper would serve as useful input into the development of behavioural models of clay soils containing iron oxides. CHARGE CHARACTERISTICS AND WATER ADSORPTION OF IRON OXIDES The surface charge of iron oxides are determined by the concentration of H+ and OH- ions in solution. The surface charge is created by an adsorption or desorption of H+ (or a desorption or adsorption of OH-, respectively) in the potential determining layer consisting of surface O, OH, and OH groups. This mechanism may be represented

2 228 M. Ohtsubo schematically as In this model H+ and OH- ions are potential determining ions. Fig. 1 shows the adsorption density of potential determining ions on ferric oxide as a function of ph and FIG. 1. Adsorption density of potential determining ions on ferric oxide as a function of ph and ionic strength: temperature, 21 Ž; indifferent electrolyte KNO3. (From Parks and De Bruyn, 1962, J. Chem. Phys., 66. Reproduced by permission of the American Chemical Society.)

3 Iron Oxide-Clay Interaction 229 ionic strength. The point where the adsorption density versus ph curves cross each other gives the point of zero charge (PZC) of the interface. At the ph of PZC equal amounts of H+ and OH- ions are adsorbed on the iron surface. The net adsorption is given by H+ - COH-. Excess of H + ions must give the surface a positive charge, whereas C an excess of OH- ions must make the surface negative. The PZC of the ferric oxide in this figure is ph8.3. The PZC values of synthetic iron minerals lie in the range of ph for goethite and hematite, and 8.1 for ferrihydrite (Schwertmann and Taylor, 1977). Water vapor adsorption and desorption isotherms for goethite and amorphous hydrated ferric oxide at 25 Ž are shown in Fig. 2. The amorphous ferric oxide adsorbed approximately three times more water than goethite at a given relative pressure and showed a significant hysteresis effect. In contrast, goethite showed no detectable hysteresis. Surface areas of goethite and amorphous ferric oxide, calculated by BET equation were 32 and 320 m2/g, respectively. Higher water adsorption for the amorphous ferric oxide could be due to the higher surface area of this material. FIG. 2. Water vapor adsorption-desorption isotherms (relative pressure) at 25 Ž on goethite and amorphous hydrated ferric oxide outgassed at room temperature. (From Gast et al., 1974, Clays and Clay Minerals, 22. Reproduced by permission of the Clay Minerals Society.)

4 M. 230 FIG. 3. Transmission micrographs Ohtsuha of kaolinite-amorphous iron hydroxide complexes: (a) ph 4.1 (initial ph 3.0); (b) ph 9.7 (initial ph 3.0); (c) ph 4.1 (initial ph 9.5); (d) ph 9.8 (initial ph 9.5).

5 Iron Line in each duced by print permission indicates of 0.5ƒÊm Elsevier (From Science Oxide-Clay Yong Publishers Interaction and Ohtsubo.) 231, 1987, Applied Clay Sci., 2. Repro-

6 232 M. Ohtsubo ASSOCIATION OF IRON OXIDES WITH CLAYS Synthesized iron oxides The association of iron oxides with day particles is thought to result from the adsorption and/or precipitation of hydrolyzed ferric species on the clay surfaces. This process has been studied using controlled laboratory synthethized techniques by the authors including Follett (1965), Greenland and Oades (1968), Greenland (1978), Saleh and Jones (1984), and Yong and Ohtsubo (1987) for the iron oxide-kaolinite system and also by Kavanagh and Quirk (1978) and Rengasamy and Oades (1977a, b) and Robert et al. (1981) for the iron oxide-montmorillonite and illite system. In general the results of these studies indicate that the presence of adsorbed iron oxides leads to a marked reduction in the cation exchange capacity of day minerals or even to the development of a net positive charge on clay surface. Figure 3 shows the electron micrographs of kaolinite-amorphous iron hydroxide complexes obtained by Yong and Ohtsubo (1987). The iron hydroxides in the complexes prepared by adding iron hydroxide to clay suspensions at ph 3 followed by increasing the ph to 4.1 were precipitated on the faces of kaolinite but not on the edges (Fig. 3a). This could be due to the adsorption of positively charged iron particles on the negatively charged kaolinite surfaces. Raising the ph of the mixture from 4.1 to 9.7 produced no electron-optically visible changes in the distribution of the iron hydroxide particles (Fig. 3b). It would appear that once formed, the kaolinite-amorphous iron hydroxide complex is extremely stable. The separation of the iron hydroxides from the kaolinite particles occurred by raising the ph of kaolinite-iron complexes to above 10. This suggests that the mechanism by which positively charged iron particles are associated with kaolinite FIG. 4. Zeta potential of kaolinite and the complexes of kaolinite and amorphous iron hydroxides as a function of ph. (From Yong and Ohtsubo, 1987, Applied Clay Sci., 2. Reproduced by permission of Elsevier Science Publishers.)

7 Iron Oxide-Clay Interaction 233 surface is one of adsorption by a coulombic force. At a ph 9.8, the iron hydroxide in the complex prepared by adding iron hydroxides to kaolinite suspensions at ph 9.5 is present as a separate phase (Fig. 3c). This could be due to the repulsion between the negatively charged iron hydroxide and negatively charged face of kaolinite particles. By lowering the ph to 4.1, some iron hydroxides are adsorbed on the surfaces of the kaolinite particles, but a significant proportion of the iron hydroxides remain as a separate phase. This would suggest that for iron hydroxides to be more fully associated with kaolinite surfaces, a lowering of the ph below 4 would be needed. The zeta potential determined by the electrophoretic mobilities of the clay particles gives information on charge reversal of clays associated with iron hydroxides and the isoelectric point of clays. Figure 4 gives the zeta potential curves for kaolinite-amorphous FIG. 5. Electrophoretic mobilities of bentonite as a function of ph before and after treatment with iron (III). (From Rengasamy and Oades, 1977, Aust. J. Soil Res., 15. Reproduced by permission of CSIRO Editorial and Publishing Unit.) FIG. 6. Electrophoretic mobilities of illite as a function of ph before and after treatment with iron (III). (From Rengasamy and Oades, 1977, Aust. J. Soil Res., 15. Reproduced by permission of CSIRO Editorial and Publishing Unit.)

8 234 M. Ohtsubo iron hydroxide complexes shown in Fig. 3. The isoelectric point of the complexes are different depending on whether or not the iron hydroxide is associated with kaolinite surfaces; the kaolinite particles coated by iron hydroxides (Fig. 3a, b) gave the higher isoelectric point than the complexes in which iron hydroxides are not associated with kaolinite surfaces (Fig. 3c, d). Figures 5 and 6 show electrophoretic mobilities of bentonite and illite as a function of ph before and after treatment with iron hydroxides. The sodium-clay particles have negative mobilities over the ph range of 2 to 10, whereas treated clays behaved electrokinetically in a manner similar to kaolinite-iron hydroxide complexes in Fig. 4. The bentonite and illite treated with iron hydroxides gave the isoelectric point of ph 5.4 and 6.5, respectively. FIG. 7. Positive, negative, and resultant charges on Pasar Minggu lateritic red earth: A-after peroxidation only; B-after peroxidation and deferration with dithionite and 0.05N HCl; C- after peroxidation and treatment with 0.1N HCl. (From Deshpande et al., 1964, 8th Intern. Congress of Soil Sci. Reproduced by permission of the authors.)

9 Iron Oxide-Clay Interaction 235 Iron oxide in soils Deshpande et al. (1964) measured the charge characteristics of peroxidated soils containing free iron oxides using Schofield's method (1949). The charge characteristics for Pasar Minggu lateritic red soil after peroxidation is shown in Fig. 7. The soil sample contains dithionite soluble iron of 11.4%. Positive and negative charges at different ph values were determined from the amount of chloride and ammonium ions adsorbed on the soil sample respectively. The clay mineral surfaces associated with synthetic iron hydroxides have net positive charges at lower ph values (Figs. 4, 5, and 6). However, the net surface charge of Pasar Minggu soil before deferration was negative in the whole ph range (Fig. 7A). Figure 7 also shows that the removal of free iron oxides from the soil sample by treatment with sodium dithionite and 0.05N hydrochloric acid does not affect the chloride retention by the sample. In contrast to such negligible effect of dithionite and the brief washing with 0.05N hydrochloric acid, shaking with 0.1N hydrochloric acid for 24 hours removed the positive charge completely from the soil sample examined. The predominant cation in the 0.1N hydrochloric acid extracts was aluminum. Therefore, Deshpande et al. (1964) suggested that the loss of positive charge is due to the removal of aluminium oxides or hydroxides, and iron oxides are present as negatively charged particles because of the adsorption of silicate, phosphate, or organic anions. The Pasar Minggu soil contains kaolinite as the dominant clay mineral. Positive charges are normally present on kaolinite crystals over the ph range encountered in soils. Nevertheless, no positive charges could be detected after 0.1N acid treatment. The failure to observe positive charges was again attributed to adsorbed anions such as silicate or phosphate on the kaolinite particles. Parks (1965) attributed the negative charge on soil iron oxides to the specific adsorption of anions which lowered the point of zero charge of the iron oxides. For example, the lowering of the PZC was reported when silicate was on ferrihydrite (Pyman et al., 1979; Schwertmann and Fechter, 1982) and on gibbsite (Jepson et al., 1976). Figure 8 shows that the PZC decreased with an increasing molar ratio Sio/(Sio + Feo). Sio and Feo in the figure indicate the amount of oxalate-extractable Si and Fe in the soil, respectively, and correspond essentially to the Si and Fe of the ferrihydrite in the samples. Jepson et al. (1976) measured electrophoretic mobilities of a number of silica-coated gibbsite, differing in amount of silica as a function of ph. The isoelectric point fell with increasing SiO2 adsorption reaching a constant value at a ph around 3 when the surface coverage was about 15 p mole SiO2/m2 (Fig. 9). Therefore, natural iron oxide and kaolinite both should possess a negative charge at ph values commonly encountered in soils, and one would not expect a coulombic association between soil kaolinite and iron oxide. However, a natural kaolinite stained with iron oxide has been reported by Jefferson et al. (1975), and the presence of iron oxide concretion rich in kaolinite suggests an association under a limited set of environmental conditions. Based on these considerations, Golden and Dixon (1985) proposed to determine if iron oxides do in fact associate with kaolin minerals in the presence of silicate using electron micrographs and Mossbauer spectroscopy. They have shown that the silicate treatment disrupted the attractive forces between kaolinite and iron oxides and effectively dispersed the system in most cases. This was taken as an evidence for the existence of coulombic

10 236 M. Ohtsubo FIG. 8. Potentiometric titratin curves of 3 natural ferrihydrites in 1, 0.1 and 0.01 M KNO3 solutions. (From Schwertmann and Fechter, 1982, Clay Minerals, 17. Reproduced by permission of Mineralogical Society.) type attractions. Attractions which could not be disrupted by silicate treatment were observed in cases where goethite was formed by the oxidation of Fe2+ ions in the presence of kaolinite and halloysite. EFFECTS OF IRON OXIDES ON THE PHYSICAL PROPERTIES AND BEHAVIOUR OF SOILS Iron oxides contribute to structural stability of soils (Bayer et al., 1972) and in a number of instances significant correlations between free iron oxides and properties such as aggregate stability (Chesters et al., 1957; Deshpande et al., 1968) and porosity (Arca and Weed, 1966) have been obtained. Experiments with synthetic iron oxides demonstrate that

11 Iron Oxide-Clay Interaction 237 FIG. 9. Electrophoretic mobility of silica-coated gibbsite showing the change of isoelectric point with amount of silica adsorbed. Experimental conditions: 0.02 wt% solids, M NaCl. (From Jepson et al., 1976, J. Colloid and Interface Sci., 55. Reproduced by permission of Academic Press Inc.) small additions of ferrihydrite (up to 3%) produces strong aggregation of silt while crystalline oxides (as measured from the ratio of oxalate soluble to total Fe) are much less effective (Table 1). Dehydration of soils changes their physical properties because of the irreversible hardening of iron oxides (Bayer, 1972). Ohtsubo and Wada (1988) have shown that drying of the complexes of kaolinite and synthetic amorphous iron hydroxides reduced their liquid limit. Investigations of the behaviour of complexes of pure clay minerals and synthesized materials are useful to provide a means for a better understanding of the role of iron oxides in natural soils. Figure 10 shows the Bingham yield stress curves tested at various ph values for kaolinite, amorphous iron oxides, and kaolinite-amorphous iron oxide com- TABLE 1. The influence of synthetic iron oxide addition to a loess soil (86% silt) on the formation of 1-2mm water stable aggregates after air drying. (From Schwertmann and Taylor, 1977, Minerals in Soil Environments. Reproduced by permission of Soil Science Society of America, Inc.)

12 238 M. Ohtsubo FIG. 10. Bingham yield stress for kaolinite, amorphous iron hydroxide and kaolinite-amorphous iron hydroxide complexes of initial ph 3.0 and 9.5 at various ph values. (From Yong and Ohtsubo, 1987, Applied Clay Sci., 2. Reproduced by permission of Elsevier Science Publishers.) plexes. The complexes of initial ph of 3.0 and 9.5 were prepared by adding iron hydroxides to clay suspensions at ph 3 and ph 9.5 respectively followed by adjusting to appropriate ph values in the range of ph 3 to 10. The addition of as small as 5% of iron hydroxides by solid weight to kaolinite enhanced its yield stress. The increase of the yield stress was more significant for initial ph 9.5 than for initial ph 3.0. Iron oxides in the complexes was present independent of kaolinite for initial ph 9.5 (Fig. 3c, d), but it was precipitated on the kaolinite surfaces for initial ph 3.0 (Fig. 3a, b). The maximum yield stresses of the complexes prepared at initial ph values of 3.0 and 9.5 occurred at ph 7.5 and 5.9, respectively, roughly in accordance with the ph values of the isoelectric point of the complexes (Fig. 4). The presence of amorphous material as cementing agents in marine clays has been a matter for speculation for years (Kenny et al., 1967; Quigley, 1968). More recently its importance has been emphasized by authors such as Loiselle et al. (1971), Sangrey (1972), McKyes et al. (1974), Yong and Sethi (1977), Yong et al. (1979), Bentley and Smalley (1978), Hanzawa and Adachi (1983), and Torrance (1987).

13 Iron Oxide-Clay Interaction 239 Figure 11 shows a relationship between the sensitivity and total amorphous material for marine days from eastern Canada. The sensitivity is defined as the ratio of undisturbed strength of clay to its remoulded shear strength at natural water content. It is proposed by the authors that amorphous material functioning as bonding agents possess high water holding capability, and if present in large amount it would increase the remoulded shear strength and effectively reduce sensitivity. Torrance et al. (1986) investigated the effect of iron oxides on the Bingham yield stress of remoulded marine clays from Japan and Canada. Both samples showed a reduction in yield stress at a given water content and salt concentration after extraction with citratedithionite and even more reduction in yield stress after extraction with acid-base (Fig. 12). Chemical analysis indicated that Fe, Al, and Si were removed by the citrate-dithionite extraction (Table 2). The Mossbauer data indicated that Fe-bearing material extracted by the citrate-dithionite treatment was fine-grained hematite for the South Nation River landslide sample and ferrihydrite or other hydrous iron oxide as well as hematite for Ariake Bay sample. Ariake Bay sample contains smectite as the main day mineral, and this smectite i s unusual in that it responds to salinity and ion saturation in a manner similar to the low-activity minerals, illite and chlorite, rather than in the manner normally associated FIG. 11. Relationship between sensitivity and amorphous content NOTES: r = 0.89 significant at 1% level; y = x: Sy X is the standard error of estimates for the regression line of y on x and equals 2.56; and D denotes sample from a different borehole in Gatineau area. (From Yong et al., 1979, Can. Geotechnical J., 16. Reproduced by permission of National Res. Council of Canada.).

14 240 M. Ohtsubo FIG. 12. Yield stress vs. water content for the natural and extracted samples. The numbers for each indicate the pore water salinity, expressed as g/liter NaCl. (From Torrance et al., 1986, Clays and Clay Minerals, 34. Reproduced by permission of the Clay Minerals Society.) TABLE 2. Fe2O3, Al2O3 and SiO2 extracted by citrate-dithionite extractions. (From Torrance, 1984, Soils and Foundations, 24(2). Reproduced by permission of Japanese Society of Soil Mechanics and Foundation Engineering.) with smectites (Egashira and Ohtsubo, 1983). They suggest that this unusual behaviour is caused bythe occurrence of considerable substitution of Fe2+ for Al3 + in the octahedral layer and that this suppresses the swelling normally associated with smectites. The Mossbauer results indicate about 15% Fe2+ in the Ariake Bay sample (Torrance et al., 1986). In summary, iron hydroxides in synthesized iron hydroxide-clay complexes are associated with clay minerals due to coulombic attraction. For natural clay soils anions such as silicate and phosphate play an important role in the association between iron oxides and clay minerals.

15 Iron Oxide-Clay Interaction 241 Addition of amorphous iron hydroxides to kaolinite enhances its Bingham yield stress. The presence of iron oxides in marine clays contribute to a decrease of their sensitivity and an increase of their Bingham yield stress. ACKNOWLEDGMENTS The author wishes to thank Dr. K. Egashira and Dr. S. Wada for their critical and constructive comments on this manuscript. REFERENCES ARCA, M.N. and WEED, S.B.(1966) Soil Sci., 101, BAVER, L.D., GARDNER, W.H. and GARDNER, W.R.(1972) Soil Physics (4th Edition) John Wiley & Sons, BENTLEY, S.P. and SMALLEY, I.J.(1978) Sedimentalogy, 25, CHESTERS, G., ATTOE, O.J. and ALLEN, O.N.(1957) Soil Sci. Soc. Amer Proc., 21, DESHPANDE, D.J., GREENLAND, J. and QUIRK, J.P.(1964) 8th Int. Conf. of Soil Sci., Bucharest, Romania, DESHPANDE, T.L., GREENLAND, D.J. and QUIRK, J.P.(1968) J. Soil Sci., 19, EGASHIRA, K. and OHTSUBO, M.(1983) Geoderma, 29, EL-SWAIFY, S.A.(1980) Soils with variable charge (ed. by B.K.G. Theng), New Zealand Soc. of Soil Sci., FOLLETT, E.A.C.(1965) J. Soil Sci., 16, GAST, R.G., LANDA, E.R. and MERER, G.W.(1974) Clays and Clay Minerals, 22, GOLDEN, D.C. and DIXON, J.B.(1985) Soil Sci. Soc. Am. J., 49, GREENLAND, D.J. and OADES, J.M.(1968) Trans. 9th Int. Congr. Soil Sci., Adelaide, 1, GREENLAND, D.J.(1975) Clay Minerals, 10, HANZAWA, H. and ADACHI, K.(1983) Soils and Foundations, 23(4), JEFFERSON, D.A., TRICKER, M.J. and WINTERBOTTOM, A.P.(1975) Clays and Clay Minerals, 23, JEPSON, W.B., FEFFS, D.G. and FERRIS, A.P.(1976) J. Colloid and Interface Sci., 55, KAVANAGH, B.V. and QUIRK, J.P.(1978) Geoderma, KENNY, T.C., MOUM, J. and BERRE, T.(1967) Proc. Geotech. Conf., Oslo, 1, LOISELLE, A., MASSIERA, M. and SAINANI, U.R.(1971) Can. Geotechnical J., 8, MCKYES, E., SETHI, A. and YONG, R.N.(1974) Clays Clay Minerals,, 22, O HTSUBO, M. and WADA, S.(1988) J. Clay Sci. Soc. Japan, 28, (in Japanese with English summary). PARKS, G.A. and de BRUYN, P.L.(1962), J. Phys. Chem., 66, PARKS, G.A.(1965) Chem. Rev., 65, PYMAN, M.A.F., BOWDEN, J.W. and PONSNER, A.W.(1979) Clay Minerals, 14, QUIGLEY, R.M.(1968) Can. Geotechnical J., 3, QUIGLEY, R.M.(1980) Can. Geotechnical J., 17, RENGASAMY, P. and OADES, J.M.(1977) Aust. J. Soil Res., 15, RENGASAMY, P. and OADES, J.M. (1977) Aust. J. Soil Res., 15, ROBERT, M., BERRIER, J. and VICENTE, M.A.(1981) Proc. 7th Clay Conf., SALEH, A.M. and JONES, A.A.(1984) Clay Minerals, 19, SANGREY, D.A.(1972) Geotechnique, 22, SANGREY, D.A.(1972) Can. Geotechnical J., 9, SCHOFIELD, R.K.(1949) J. Soil Sci., 1, 1-8. SCHWERTMANN, U. and TAYLOR, R.M.(1977) In Minerals in Soil Environments (edited by J.B. Dixon and S.B. Weed) Soil Sci. Soc. Am., Madison, Wisc., SCHWERTMANN, U. and FECHTER (1982) Clay Minerals, 17,

16 242 M. Ohtsubo TORRANCE, J.K.(1974) Can. Geotechnical J., 12, TORRANCE, J.K.(1987) In Slope Stability-Geotechnical Engineering and Geomorphology (edited by M.G. Anderson and K.S. Richards), John Wiley & Sons, TORRANCE, J.K., GEDGES, S.W. and BOWEN, L.H.(1986) Clays Clay Minerals, 34, YONG, R.N. and SETHI, A.(1977) Proc. Specialty Session on Geotech. Eng. and Environmental Control, 9th Int. Conf. on Soil Mech. and Foundation Eng., Tokyo, YONG, R.N., SETHI, A.J. and LAROCHELLE, P.(1979) Can. Geotechnical J., 16, YONG, R.N. and OHTSUBO, M.(1987) Applied Clay Sci., 2,

Copyright SOIL STRUCTURE and CLAY MINERALS

Copyright SOIL STRUCTURE and CLAY MINERALS SOIL STRUCTURE and CLAY MINERALS Soil Structure Structure of a soil may be defined as the mode of arrangement of soil grains relative to each other and the forces acting between them to hold them in their