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Change in microstructure of soils due to natural mineralization
Applied Clay Science 23 (2003) 169 – 177 www.elsevier.com/locate/clay
Change in microstructure of soils due to natural mineralization Masaharu Fukue *, Yoshio Sato, Mitsuaki Yamashita, Masahiro Yanai, Yuichi Fujimori Marine Science and Technology, Tokai University, 3-20-1 Orido, Shimizu 424-8610, Japan
Abstract If a very long-term stability of soil materials is concerned, it is necessary to consider natural mineralization processes resulting from physical, chemical and biological actions. Precipitation, dissolution and reprecipitation of minerals due to these actions will change the microstructure of soils in terms of void distribution, strength, permeability, etc. Precipitated minerals at the contacts between soil particles may play an important role as cement agent, while the formation of nodules will make soils unhomogeneous. On the other hand, dissolution of minerals produces very large pores. It may cause high permeability of the soils. Therefore, this paper describes natural mineralization under various conditions and discusses the possibility of mineralization in some crucial site, such as a deep excavated-buried ground. D 2003 Elsevier B.V. All rights reserved. Keywords: Natural mineralization; Precipitation; Permeability
1. Introduction Because of the very long half-life of radioactive wastes, smectitic backfill in tunnels is required to be stable for a very long term. Therefore, many studies have been made on the properties of buffer materials under various conditions. They contain physical and chemical reactions that may occur under various conditions to be considered. Buffer materials consisting of natural minerals and the groundwater infiltrated from rock wall may contain microorganism. The pore spaces of compacted buffer materials may not be too small for the proliferation of microorganism, because of the existence of macro-pores at contacts between
* Corresponding author. Tel.: +81-543-34-0411; fax: +81-54334-9768. E-mail address: [email protected] (M. Fukue).
bentonite and sand/silt fractions. Considering procedures of the construction of tunnel and the compaction of buffer materials, initial oxic condition will change to anoxic condition after the infiltration of groundwater. Under this situation, microorganism may proliferate and the consequent biological actions possibly occur.
2. Biomineralization Microorganisms live in even severe circumstances. Some bacteria like very high pressure. They are called barophilic bacteria (ZoBell and Morita, 1959; Vayanos et al., 1979). There are also barophobic, thermophilic or hyperthermophilic bacteria. ‘‘Biomineralization’’ is the formation of minerals by organisms (Lowenstam and Weiner, 1989). Biomineralization occurs in seawater, freshwater, lake ores, drinking water plant, sediments, desert varnish,
0169-1317/03/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-1317(03)00100-5
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M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
soils and rock varnish (Ghiorse and Ehrlich, 1992). There are a variety of microbes-Bacteria, fungi, algae and protozoa that exhibit an ability to promote mineralization, i.e., oxidation and reduction. There are also a variety of minerals associating with biological activity. According to Lowenstam and Weiner (1989), various minerals, such as carbonate, sulfate, silicate, iron and manganese oxides, sulfide, halide, oxalate and phosphate, are formed by biological activities. A typical example of natural mineralization under a very high hydrostatic pressure is formation of manganese nodules at ocean bottom, where water depth usually exceeds 2 km (Cronan, 1980). At present, two mechanisms on the formation of the nodules are advocated. One is due to inorganic reaction and the other is biological activity (Dugolinsky et al., 1977; von Stackelberg, 1984; Ehrlich, 1971).
3. Carbonates 3.1. Origin of carbonates in soils Aragonite CaCO3 is formed by bacteria, algae, coral and shell fishes, while calcite CaCO3 is formed as a shell of Coccolith, Foraminifera and Arthropoda. Dead
bodies of these living things deposit onto sea floors. Since the sedimentation rate of the dead bodies is almost constant in sea, the contents of carbonates in sediments depend on sedimentation rate of solids discharged from land. The higher the sedimentation rate of solid sediments from land, the lower the carbonate content will be, because of the buried effect (Fukue et al., 1996). At coastal regions except for coral reef, carbonate content is usually less than 10%, while it usually exceeds 50% at ocean bottoms (Fukue et al., 1996). It is well known that in sea, carbonates are originally formed by living thing, as the following reaction, Ca2þ þ 2HCO 3 ! CaCO3 þ H2 O þ CO2
ð1Þ
Many of mountainous regions have been formed from the upheaval of seabed. Therefore, some soils contain a large amount of carbonate. A typical example is mountains consisting of limestone. The limestone will be dissolve under certain conditions. For example, the reverse reaction to Eq. (1) can be considered. CaCO3 þ H2 O þ CO2 ! Ca2þ þ 2HCO 3
ð2Þ
The reaction (2) indicates that the carbonate will dissolve under the existence of H2O and CO2.
Fig. 1. Carbonate content and unconfined compressive strength in marine sediments of Tokyo Bay.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
171
Fig. 2. Carbonate nodules found in a boring core sample.
3.2. Marine sediments-reprecipitation of carbonate Fig. 1 shows the profiles of carbonate content and vane shear strength of Tokyo bay surface sediments. Both the profiles show similar trends. The slopes indicated in both the figures show the individual increments with depth. The comparison of the corresponding slopes shows that the vane shear strength increases with increasing of carbonate content, though the amount of carbonate content is within a relatively
small range. This means that the carbonate plays an important role as cement agents in soils. This type of cementation effect can also be seen in older sediments (Fukue et al., 1999). Deeper soils show a relatively high cementation effect with carbonate, because of the higher bulk density of soils. There are many facts that carbonate will strengthen the structure of marine soils (Fukue and Nakamura, 1996; Fukue et al., 1999). The origin of the carbonate is marine living things, such as coccolith and fora-
Fig. 3. Small size of carbonate nodules found in boring core samples.
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Table 1 Carbonate contents in various types of clay products Clay
CaCO3 (%)
Clay
CaCO3 (%)
Bentonite A (Fuji) Bentonite B (Haruna) Bentonite C (Hodaka) Sericitea
4.47 4.89 3.68 2.64
Montmorillonitea Kaolinitea Dickitea
6.29 0.28 0.27
a
Samples from the Clay Science Society of Japan.
minifer. It was described that these living things forms their carbonate bodies taking Ca2 + and HCO3-, as indicated by reaction (1). 3.3. Formation of sandy rock by cementation A similar effect of carbonate was found in Narita formation in Chiba Prefecture, which is also marine sediments in past. A floating sand rock layer was found in the diluvium sediments. The rock layer has a thickness of approximately 1 m and exists at 90 m in soil depth. The rock sample had a remarkable high strength, i.e., an unconfined compressive strength of 19.6 MPa. The upper and lower sand layers are still granular. It was found that the carbonate content of the granular sands is less than 1%, while the sandy rock contains 25% (Fukue et al., 1999). Since the
Narita formation contains a large amount of shells, it may be concluded that shells contained in Narita formation was partially dissolved and gradually condensed into the present rock layer. The formation of the rock layer may be due to repeat of reactions (1) and (2). Since the origin of carbonates in the sediments is dead bodies of coccolith and foraminifer which do not have cementation effects, the dissolution of the dead bodies is needed before reprecipitation. The reprecipitation may occur at contacts between particles. 3.4. Carbonate nodules Carbonate condenses sometimes to nuclei. This type of condensation of carbonate forms a spherical shape and it is called ‘‘nodule’’. Nodules of calcium carbonate, CaCO3, are often found in sediments and clay mines. The size of the nodules varies with site and seems to be dependent on soil depth, i.e., possibly the age of sediments. This indicates that the nodules will grow up with age. The formation of carbonate nodules may require first the dissolution of carbonate then precipitates to some part. In soils, the interface between groundwater surface and atmosphere is more possible for the dissolution and precipitation of carbonate. The movement of ground-
Fig. 4. Big size of carbonate nodules found in bentonite mines.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
173
Fig. 5. Formation of manganese oxides formed by bacteria on liquid and solid boundary under oxidation. (a) Vibrio sp. after 5 days, (b) after 1 month, (c) after 3 months and (d) after 10 months.
water level may also be necessary to repeat the reactions (1) and (2). In fact, the formation of carbonate nodule is limited within some range of depth in soils. This may be because of the level of groundwater. Figs. 2 and 3 show carbonate nodules found in Alluvial sediments near the mouth of Ibi river in Mie Prefecture. The soil depth was approximately 19 m. Nodules found in this layer have a relatively small size and are spherical. The sediment age is possibly several thousand years, but it is considered that the level of groundwater was considerably changed after sedimentation, because of the change of seawater level associating with the post glacier age. Therefore, it is not obvious when the nodules started to form and if they can still grow up.
Relatively large carbonate nodules were found in Neogene soil layer in Shizuoka Prefecture, when the slope was cut (Fukue et al., 1999). The slope was failed with very small angle in a few months after the slope was cut. To investigate the reason, detail observation was made. The result showed that the failure occurred just below a tuff layer. It was found that the tuff layer contains approximately 40% calcium carbonate while the failed layer contained a very small amount of carbonate, i.e., less than 1%. Therefore, it was concluded that carbonate was dissolved and migrated towards the tuff layer for a long time. The strength of the tuff was increased, because of the cementation effects due to carbonate precipitation (Fukue et al., 1999).
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Fig. 6. Formation of iron oxides formed by fungi.
3.5. Possibility of nodule formation in bentonit clays Carbonate contents in commercially available clays are presented in Table 1. The measurement of carbonate content was made using gas pressure method (Fukue et al., 1999). The table shows that bentonite clays produced at different sites and montmorillonite contain a relatively high amount of carbonate. Kaolinite and dickite contain less amount of carbonate. This
may indicate that buffer materials, mixture of bentonite and sand/silt, may contain a relatively high amount of carbonate. Therefore, it is possible that carbonates in buffer materials will dissolve and/or precipitate under certain condition during a long period. In fact, large carbonate nodules were often found in bentonite mines. Fig. 4 shows nodules found in bentonite mines in Japan. The larger nodule has a diameter of 13 cm. The nodules are so hard that the mining
Fig. 7. Formation of vivianites found in the Lake Baikal sediments.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
excavator can be damaged during operation. The formation of the nodules in bentonite mines is not well understood. However, it is not dissimilar to those found in the natural marine sediments. Therefore, it is possible that dissolution and reprecipitation of carbonate occur in bentonite mixtures.
4. Oxidation 4.1. Mn –Fe oxides There are many evidences that manganese oxides and iron oxides are formed by microbial mediation under various oxidation conditions. There are a variety of microbes- Bacteria, fungi, algae and protozoa that exhibit an ability to promote mineralization. Fig. 5 shows the formation mechanism of Mn oxides in a pipe in which Mn-rich ground seawater is flowing. The oxidator was Vibrio sp. The pictures in Fig. 5 shows (a) Vibrio sp., and Mn oxides formed after (b) 1 month, (c) 3 months and (d) 10 months, respectively. This may be an example of rapid formation of biominerals. The formation of Fe oxides is very similar to the case of Mn. Fig. 6 shows oxidator for Fe oxides. In the figure, the formation of Fe oxides is seen.
175
The reduction of Mn oxides and Fe oxides occur under an anaerobic condition. In this case, as Mn and Fe oxides become solutes, the volume of solids (oxides) will decrease. In soils, the reduction of oxides will make the pores larger. From the engineering point of view, the structure change due to mineralization should be taken into account. Carbonate content strongly influences the soil strength (Fukue et al., 1999). The formation of manganese oxides and iron oxides also cements soil particles (Dixon and Skinner, 1992). Furthermore, the transformation from solids to solutes or from solutes to solids may provide the change in permeability of soils.
5. Reduction 5.1. Vivianite Fig. 7 shows vivianite, Fe3(PO4)28H2O, found in the sediments of L. Baikal. The sediment ages are 25,000, 230,000 and 270,000 years BP, from left. This type of mineral is usually found in mines and volcanic ashes. The case demonstrated in Fig. 7 may be a very slow biomineralization.
Fig. 8. Formation of pyrites in marine sediments under reduction.
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5.2. Pyrite Pyrite Fe2S forms in anaerobic environment. Fig. 8 shows the pyrites formed in diatom skeleton in alluvial marine sediments near the mouth of Ibi river. The soil depth is 18 m and the age of the sediments is estimated to be several thousand years. Pyrite is often found in marine sediments, because of the abundant iron and sulfur ions in pore water.
6. Reduction – oxidation
and rocks. There is no evidence that microorganisms become active in buffer materials. However, it was found that in alcohol– bentonite clays, proliferation of bacteria is relatively rapid. Since some species of microorganism are definitely living in cracks and fissures even if it is a deep rock layer. They may play an important role in weathering of rock. Therefore, an important matter of concern is whether or not natural mineralization can occur in deep soils (Fig. 9). If natural mineralization can occur in buffer materials, the following two mechanisms become important in relation to the present problems.
Willett et al. (1992) observed oxidation of pyrites in a sample of acid sulfate soils. The results showed that the oxidation of pyrites provides dissolution of the minerals. As a result, large pore spaces are built in soils. It is obvious that the formation and dissolution of pyrite minerals provide a significant change in the structure of sediments, as well as other minerals.
(1) The pore spaces will be clogged by natural mineralization from solutes to solids. (2) Oxidation or reduction resulting in transformation from solids to solutes will make large pore spaces, as shown in Fig. 8.
7. Discussions
(1) groundwater from rock wall in terms of type and concentration of solutes, (2) species of microorganism in buffer materials and excavated rocks,
Since many species of micro-organisms have a very small size, they can live in small pores of soils
To confirm what kinds of biomineralization possibly can occur in buffer materials, the following knowledge is required.
particle biomineralization solution
Oxidation or reduction
microorganism
reduction or oxidation
pore
anaerobic or aerobic
aerobic or anaerobic
Fig. 9. Possible microstructure changes due to natural mineralization in soils.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
(3) mineralogical aspects of buffer materials and (4) redox potential.
8. Conclusions Natural or biological mineralization occurs under various conditions. There are a variety of mineralization in terms of oxidation and reduction associating with anaerobic and aerobic conditions. In addition, the activities of organism to promote mineralization are complicated in terms of species of microorganisms, i.e., bacteria, fungi, algae, etc. It is more likely that carbonate contained in bentonite clays results in cementation of soils. It is also possible that carbonate nodules form in bentonite clay. As a result of the precipitation of carbonate, the structure of the bentonite clay will be changed in terms of strength and permeability. In this case, the strength will increase and the permeability will decrease. The reverse reaction, i.e., dissolution of carbonate is also possible. The dissolution will decrease the strength and will increase the permeability.
Acknowledgements The authors wish to thank Prof. M. Nishimura, Tokai University and Mr. K. Suzuki, Kunimine Kogyo for their offer of samples.
177
References Cronan, D.S., 1980. Underwater Minerals. Academic Press, London. 362 pp. Dixon, J.B., Skinner, H.C.W., 1992. Manganese minerals in surface environments. In: Skinner, Fitzatrick (Eds.), Biomineralization. Catena Supplement, vol. 21, pp. 31 – 50. Dugolinsky, B.K., Margolis, S.V., Dudley, W.C., 1977. Biogenic influence on growth of manganese nodules. J. Sediment. Petrol. 47, 428 – 445. Ehrlich, H.L., 1971. Bacteriology of manganese nodules: V. Effect of hydrostatic pressure on bacterial oxidation of Mn and reduction of MnO2. Appl. Microbiol. 21, 306 – 310. Fukue, M., Nakamura, T., 1996. Effects of carbonate on cementation of marine sediments. Mar. Georesour. Geotechnol. 14, 37 – 45. Fukue, M., Nakamura, T., Kato, Y., Naoe, K., 1996. Correlation among carbonate content, accumulation rate and topography of seabed. Soils Foundations 36 (1), 51 – 60. Fukue, M., Nakamura, T., Kato, Y., 1999. Cementation of soils due to calcium carbonate. Soils Foundations 39 (6), 55 – 64. Ghiorse, W.C., Ehrlich, H.L., 1992. Microbial biomineralization of iron and manganese. In: Skinner, Fitzatrick (Eds.), Biomineralization. Catena Supplement, vol. 21, pp. 75 – 99. Lowenstam, H.A., Weiner, S., 1989. On Biomineralization. Oxford Univ. Press, Oxford. Vayanos, A.A., Dietz, A.S., van Boztel, R., 1979. Isolation of a deep-sea barophilic bacterium and some of its growth characteristics. Science 205, 808 – 810. von Stackelberg, U., 1984. Significance of benthic for the growth and movement of manganese nodules, equatorial North Pacific. Geo-Marine Lett. 4, 37 – 42. Willett, L.R., Crockford, R.H., Milnes, A.R., 1992. Transformation of iron, manganese and aluminium during oxidation of sulfidic material from an acid sulfate soil. In: Skinner, Fitzatrick (Eds.), Biomineralization. Catena Supplement, vol. 21, pp. 287 – 302. ZoBell, C.E., Morita, R.Y., 1959. Barophilic bacteria in some deepsea sediments. J. Bacteriol. 73, 563 – 568.
Change in microstructure of soils due to natural mineralization Masaharu Fukue *, Yoshio Sato, Mitsuaki Yamashita, Masahiro Yanai, Yuichi Fujimori Marine Science and Technology, Tokai University, 3-20-1 Orido, Shimizu 424-8610, Japan
Abstract If a very long-term stability of soil materials is concerned, it is necessary to consider natural mineralization processes resulting from physical, chemical and biological actions. Precipitation, dissolution and reprecipitation of minerals due to these actions will change the microstructure of soils in terms of void distribution, strength, permeability, etc. Precipitated minerals at the contacts between soil particles may play an important role as cement agent, while the formation of nodules will make soils unhomogeneous. On the other hand, dissolution of minerals produces very large pores. It may cause high permeability of the soils. Therefore, this paper describes natural mineralization under various conditions and discusses the possibility of mineralization in some crucial site, such as a deep excavated-buried ground. D 2003 Elsevier B.V. All rights reserved. Keywords: Natural mineralization; Precipitation; Permeability
1. Introduction Because of the very long half-life of radioactive wastes, smectitic backfill in tunnels is required to be stable for a very long term. Therefore, many studies have been made on the properties of buffer materials under various conditions. They contain physical and chemical reactions that may occur under various conditions to be considered. Buffer materials consisting of natural minerals and the groundwater infiltrated from rock wall may contain microorganism. The pore spaces of compacted buffer materials may not be too small for the proliferation of microorganism, because of the existence of macro-pores at contacts between
* Corresponding author. Tel.: +81-543-34-0411; fax: +81-54334-9768. E-mail address: [email protected] (M. Fukue).
bentonite and sand/silt fractions. Considering procedures of the construction of tunnel and the compaction of buffer materials, initial oxic condition will change to anoxic condition after the infiltration of groundwater. Under this situation, microorganism may proliferate and the consequent biological actions possibly occur.
2. Biomineralization Microorganisms live in even severe circumstances. Some bacteria like very high pressure. They are called barophilic bacteria (ZoBell and Morita, 1959; Vayanos et al., 1979). There are also barophobic, thermophilic or hyperthermophilic bacteria. ‘‘Biomineralization’’ is the formation of minerals by organisms (Lowenstam and Weiner, 1989). Biomineralization occurs in seawater, freshwater, lake ores, drinking water plant, sediments, desert varnish,
0169-1317/03/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-1317(03)00100-5
170
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
soils and rock varnish (Ghiorse and Ehrlich, 1992). There are a variety of microbes-Bacteria, fungi, algae and protozoa that exhibit an ability to promote mineralization, i.e., oxidation and reduction. There are also a variety of minerals associating with biological activity. According to Lowenstam and Weiner (1989), various minerals, such as carbonate, sulfate, silicate, iron and manganese oxides, sulfide, halide, oxalate and phosphate, are formed by biological activities. A typical example of natural mineralization under a very high hydrostatic pressure is formation of manganese nodules at ocean bottom, where water depth usually exceeds 2 km (Cronan, 1980). At present, two mechanisms on the formation of the nodules are advocated. One is due to inorganic reaction and the other is biological activity (Dugolinsky et al., 1977; von Stackelberg, 1984; Ehrlich, 1971).
3. Carbonates 3.1. Origin of carbonates in soils Aragonite CaCO3 is formed by bacteria, algae, coral and shell fishes, while calcite CaCO3 is formed as a shell of Coccolith, Foraminifera and Arthropoda. Dead
bodies of these living things deposit onto sea floors. Since the sedimentation rate of the dead bodies is almost constant in sea, the contents of carbonates in sediments depend on sedimentation rate of solids discharged from land. The higher the sedimentation rate of solid sediments from land, the lower the carbonate content will be, because of the buried effect (Fukue et al., 1996). At coastal regions except for coral reef, carbonate content is usually less than 10%, while it usually exceeds 50% at ocean bottoms (Fukue et al., 1996). It is well known that in sea, carbonates are originally formed by living thing, as the following reaction, Ca2þ þ 2HCO 3 ! CaCO3 þ H2 O þ CO2
ð1Þ
Many of mountainous regions have been formed from the upheaval of seabed. Therefore, some soils contain a large amount of carbonate. A typical example is mountains consisting of limestone. The limestone will be dissolve under certain conditions. For example, the reverse reaction to Eq. (1) can be considered. CaCO3 þ H2 O þ CO2 ! Ca2þ þ 2HCO 3
ð2Þ
The reaction (2) indicates that the carbonate will dissolve under the existence of H2O and CO2.
Fig. 1. Carbonate content and unconfined compressive strength in marine sediments of Tokyo Bay.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
171
Fig. 2. Carbonate nodules found in a boring core sample.
3.2. Marine sediments-reprecipitation of carbonate Fig. 1 shows the profiles of carbonate content and vane shear strength of Tokyo bay surface sediments. Both the profiles show similar trends. The slopes indicated in both the figures show the individual increments with depth. The comparison of the corresponding slopes shows that the vane shear strength increases with increasing of carbonate content, though the amount of carbonate content is within a relatively
small range. This means that the carbonate plays an important role as cement agents in soils. This type of cementation effect can also be seen in older sediments (Fukue et al., 1999). Deeper soils show a relatively high cementation effect with carbonate, because of the higher bulk density of soils. There are many facts that carbonate will strengthen the structure of marine soils (Fukue and Nakamura, 1996; Fukue et al., 1999). The origin of the carbonate is marine living things, such as coccolith and fora-
Fig. 3. Small size of carbonate nodules found in boring core samples.
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Table 1 Carbonate contents in various types of clay products Clay
CaCO3 (%)
Clay
CaCO3 (%)
Bentonite A (Fuji) Bentonite B (Haruna) Bentonite C (Hodaka) Sericitea
4.47 4.89 3.68 2.64
Montmorillonitea Kaolinitea Dickitea
6.29 0.28 0.27
a
Samples from the Clay Science Society of Japan.
minifer. It was described that these living things forms their carbonate bodies taking Ca2 + and HCO3-, as indicated by reaction (1). 3.3. Formation of sandy rock by cementation A similar effect of carbonate was found in Narita formation in Chiba Prefecture, which is also marine sediments in past. A floating sand rock layer was found in the diluvium sediments. The rock layer has a thickness of approximately 1 m and exists at 90 m in soil depth. The rock sample had a remarkable high strength, i.e., an unconfined compressive strength of 19.6 MPa. The upper and lower sand layers are still granular. It was found that the carbonate content of the granular sands is less than 1%, while the sandy rock contains 25% (Fukue et al., 1999). Since the
Narita formation contains a large amount of shells, it may be concluded that shells contained in Narita formation was partially dissolved and gradually condensed into the present rock layer. The formation of the rock layer may be due to repeat of reactions (1) and (2). Since the origin of carbonates in the sediments is dead bodies of coccolith and foraminifer which do not have cementation effects, the dissolution of the dead bodies is needed before reprecipitation. The reprecipitation may occur at contacts between particles. 3.4. Carbonate nodules Carbonate condenses sometimes to nuclei. This type of condensation of carbonate forms a spherical shape and it is called ‘‘nodule’’. Nodules of calcium carbonate, CaCO3, are often found in sediments and clay mines. The size of the nodules varies with site and seems to be dependent on soil depth, i.e., possibly the age of sediments. This indicates that the nodules will grow up with age. The formation of carbonate nodules may require first the dissolution of carbonate then precipitates to some part. In soils, the interface between groundwater surface and atmosphere is more possible for the dissolution and precipitation of carbonate. The movement of ground-
Fig. 4. Big size of carbonate nodules found in bentonite mines.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
173
Fig. 5. Formation of manganese oxides formed by bacteria on liquid and solid boundary under oxidation. (a) Vibrio sp. after 5 days, (b) after 1 month, (c) after 3 months and (d) after 10 months.
water level may also be necessary to repeat the reactions (1) and (2). In fact, the formation of carbonate nodule is limited within some range of depth in soils. This may be because of the level of groundwater. Figs. 2 and 3 show carbonate nodules found in Alluvial sediments near the mouth of Ibi river in Mie Prefecture. The soil depth was approximately 19 m. Nodules found in this layer have a relatively small size and are spherical. The sediment age is possibly several thousand years, but it is considered that the level of groundwater was considerably changed after sedimentation, because of the change of seawater level associating with the post glacier age. Therefore, it is not obvious when the nodules started to form and if they can still grow up.
Relatively large carbonate nodules were found in Neogene soil layer in Shizuoka Prefecture, when the slope was cut (Fukue et al., 1999). The slope was failed with very small angle in a few months after the slope was cut. To investigate the reason, detail observation was made. The result showed that the failure occurred just below a tuff layer. It was found that the tuff layer contains approximately 40% calcium carbonate while the failed layer contained a very small amount of carbonate, i.e., less than 1%. Therefore, it was concluded that carbonate was dissolved and migrated towards the tuff layer for a long time. The strength of the tuff was increased, because of the cementation effects due to carbonate precipitation (Fukue et al., 1999).
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Fig. 6. Formation of iron oxides formed by fungi.
3.5. Possibility of nodule formation in bentonit clays Carbonate contents in commercially available clays are presented in Table 1. The measurement of carbonate content was made using gas pressure method (Fukue et al., 1999). The table shows that bentonite clays produced at different sites and montmorillonite contain a relatively high amount of carbonate. Kaolinite and dickite contain less amount of carbonate. This
may indicate that buffer materials, mixture of bentonite and sand/silt, may contain a relatively high amount of carbonate. Therefore, it is possible that carbonates in buffer materials will dissolve and/or precipitate under certain condition during a long period. In fact, large carbonate nodules were often found in bentonite mines. Fig. 4 shows nodules found in bentonite mines in Japan. The larger nodule has a diameter of 13 cm. The nodules are so hard that the mining
Fig. 7. Formation of vivianites found in the Lake Baikal sediments.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
excavator can be damaged during operation. The formation of the nodules in bentonite mines is not well understood. However, it is not dissimilar to those found in the natural marine sediments. Therefore, it is possible that dissolution and reprecipitation of carbonate occur in bentonite mixtures.
4. Oxidation 4.1. Mn –Fe oxides There are many evidences that manganese oxides and iron oxides are formed by microbial mediation under various oxidation conditions. There are a variety of microbes- Bacteria, fungi, algae and protozoa that exhibit an ability to promote mineralization. Fig. 5 shows the formation mechanism of Mn oxides in a pipe in which Mn-rich ground seawater is flowing. The oxidator was Vibrio sp. The pictures in Fig. 5 shows (a) Vibrio sp., and Mn oxides formed after (b) 1 month, (c) 3 months and (d) 10 months, respectively. This may be an example of rapid formation of biominerals. The formation of Fe oxides is very similar to the case of Mn. Fig. 6 shows oxidator for Fe oxides. In the figure, the formation of Fe oxides is seen.
175
The reduction of Mn oxides and Fe oxides occur under an anaerobic condition. In this case, as Mn and Fe oxides become solutes, the volume of solids (oxides) will decrease. In soils, the reduction of oxides will make the pores larger. From the engineering point of view, the structure change due to mineralization should be taken into account. Carbonate content strongly influences the soil strength (Fukue et al., 1999). The formation of manganese oxides and iron oxides also cements soil particles (Dixon and Skinner, 1992). Furthermore, the transformation from solids to solutes or from solutes to solids may provide the change in permeability of soils.
5. Reduction 5.1. Vivianite Fig. 7 shows vivianite, Fe3(PO4)28H2O, found in the sediments of L. Baikal. The sediment ages are 25,000, 230,000 and 270,000 years BP, from left. This type of mineral is usually found in mines and volcanic ashes. The case demonstrated in Fig. 7 may be a very slow biomineralization.
Fig. 8. Formation of pyrites in marine sediments under reduction.
176
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
5.2. Pyrite Pyrite Fe2S forms in anaerobic environment. Fig. 8 shows the pyrites formed in diatom skeleton in alluvial marine sediments near the mouth of Ibi river. The soil depth is 18 m and the age of the sediments is estimated to be several thousand years. Pyrite is often found in marine sediments, because of the abundant iron and sulfur ions in pore water.
6. Reduction – oxidation
and rocks. There is no evidence that microorganisms become active in buffer materials. However, it was found that in alcohol– bentonite clays, proliferation of bacteria is relatively rapid. Since some species of microorganism are definitely living in cracks and fissures even if it is a deep rock layer. They may play an important role in weathering of rock. Therefore, an important matter of concern is whether or not natural mineralization can occur in deep soils (Fig. 9). If natural mineralization can occur in buffer materials, the following two mechanisms become important in relation to the present problems.
Willett et al. (1992) observed oxidation of pyrites in a sample of acid sulfate soils. The results showed that the oxidation of pyrites provides dissolution of the minerals. As a result, large pore spaces are built in soils. It is obvious that the formation and dissolution of pyrite minerals provide a significant change in the structure of sediments, as well as other minerals.
(1) The pore spaces will be clogged by natural mineralization from solutes to solids. (2) Oxidation or reduction resulting in transformation from solids to solutes will make large pore spaces, as shown in Fig. 8.
7. Discussions
(1) groundwater from rock wall in terms of type and concentration of solutes, (2) species of microorganism in buffer materials and excavated rocks,
Since many species of micro-organisms have a very small size, they can live in small pores of soils
To confirm what kinds of biomineralization possibly can occur in buffer materials, the following knowledge is required.
particle biomineralization solution
Oxidation or reduction
microorganism
reduction or oxidation
pore
anaerobic or aerobic
aerobic or anaerobic
Fig. 9. Possible microstructure changes due to natural mineralization in soils.
M. Fukue et al. / Applied Clay Science 23 (2003) 169–177
(3) mineralogical aspects of buffer materials and (4) redox potential.
8. Conclusions Natural or biological mineralization occurs under various conditions. There are a variety of mineralization in terms of oxidation and reduction associating with anaerobic and aerobic conditions. In addition, the activities of organism to promote mineralization are complicated in terms of species of microorganisms, i.e., bacteria, fungi, algae, etc. It is more likely that carbonate contained in bentonite clays results in cementation of soils. It is also possible that carbonate nodules form in bentonite clay. As a result of the precipitation of carbonate, the structure of the bentonite clay will be changed in terms of strength and permeability. In this case, the strength will increase and the permeability will decrease. The reverse reaction, i.e., dissolution of carbonate is also possible. The dissolution will decrease the strength and will increase the permeability.
Acknowledgements The authors wish to thank Prof. M. Nishimura, Tokai University and Mr. K. Suzuki, Kunimine Kogyo for their offer of samples.
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