Clay Science 12 Supplement 2, 274-279 (2006)

Chemical and Physical Characteristics of with Noncrystalline and Poorly Crystalline Materials and Their Applications

MASAMINANZYO*

GraduateSchool of Agricultural Science, Tohoku University, Sendai 981-8555,

(ReceivedAugust 21, 2005. AcceptedDecember 28, 2005)

ABSTRACT

Typical noncrystalline and poorly crystalline materials in soils are allophane, imogolite, ferrihydrite and humus. These materials occur in and spodic horizons in relatively high amounts. The humus is complexed with Al in these soils and the humus-Al complex shows high resistance to decomposition. Chemical characteristics of the soils with these materials are variable charge, high phosphate fixation and so on. The variable charge sites show high preference to multivalent cations. Although silicon and some alkaline and alkaline-earth elements are removed, many heavy metals are retained in Andisols during formation of the Al and Fe-rich variable charge materials. These characteristics were problems for crop production but they were mostly amended by high input of fertilizers. Physical characteristics of the soils with these inorganic and organic materials stem from stable aggregates and porous structure, leading to low bulk density, high water retention, high water and air permeability. These physical characteristics are suitable for upland crop production. Allophane and imogolite are natural nano-porous materials expected also for industrial uses such as humidity control, dewing control and heat exchange at the temperature lower than 373 K.

Key words: , , Allophane, Imogolite, Al-humus

INTRODUCTION has a tubular structure with inner and outer diameters of 1 and 2nm, respectively. Well-developed imogolite has long Soils with noncrystalline and poorly crystalline materials tubular structure and it exists as bundles of many tubes under show distinctive properties different from other soils 1,2). a transmission electron microscope. The typical minerals that characterize these soils are Allophane and imogolite sometimes occur separate in a allophane and imogolite. Ferrihydrite is a poorly crystalline weathered pumice layer. For example, whitish imogolite iron mineral contained in these and some other soils, and it gel films can be observed with the naked eye in the contributes to the distinctive properties of these soils. intersticial pores of weathered pumice particles showing Humus is also important when the distinctive properties of several mm to 1 cm in diameter. Almost only allophane is these soils are discussed and is included in the noncrystalline observed in the fine-clay suspension prepared using inner materials here. part of the weathered pumice particles 3). In case of the Allophane has hollow spherical structure with outside ordinary A and Bw horizons of Andisols, it is difficult to diameter of abou 5 nm according to convincing observation separate allophane from imogolite using a transmission electron microscope 1). It has gibbsite In order to determine the content of Al-rich allophane in like sheet outside and monomeric silicate is bound- to 3 soils including imogolite,, oxalate-extractable Si (Sio) hydroxo-ligands inside the gibbsite-like sheet substituing 3 multiplied by 7.1 or 8 is frequently used 4). The Al-rich protons. Each silicate has one free hydroxo-ligand in an allophane is rather common in soils5). The matured Al-rich allophane that shows the same Si/Al ratio of 0.5 with Andisols in Japan have 150 to 300g kg-1of allophane. imogolite. Si-rich allopphane in which silicate is believed to Allophane and imogolite occur in Andisols and spodic be partially dimmers or more polymerized forms. The horizon soils in relatively high amount. Andisols are spherical structure appears to have several holes that may be normally distributed in the humid and well-drained uplands deficits. Allophane particles almost always exist as of volcanic areas. The spodic horizon soils are illuviated aggregated forms according to photographs taken by a high horizons of Spodosols that develops on the coarse-textured magnification transmission electron microscope. Imogolite parent materials under humid and cold climate in many cases. Andisols and Spodosols occupy 0.7 and 2.5% of the world land, respectively. Thus, 3.2% is the estimated maximum E-mail addressof the correspondingauthor: [email protected] percentage of the land where allophane and imogolite are 275 distributed. Major distinctive properties of Andisols were preference to multivalent cations compared to monovalent overviewed focusing on some recent developments in this ones. Multivalent cations can bridge the negatively charged paper. sites in these soils and the bridging structure appears stable. Among the divalent cations, Pb2+ and Cu2+ show higher CHEMICAL CHARACTERISTICS preference to allophanic soils than Ca2+ and Cd2+1). If the bridging structure is important for high preference, the Variable charge and neutralizing capacity degree of preference to multivalent cations may be dependent Soils rich in allophane and imogolite with the small on the surface density of negative charge sites and eventually amount of humus have zero point of charge (ZPC) at around on pH and electrolyte concentration. If the number of pH 6. The soils have amphoteric nature and their amount of high-preference site is limited, the weakly bound fraction negative and positive charge increases with increasing and may increase with increasing concentration of the decreasing pH, respectively. Comparing the amount of multivalent cation in . Hydroxides or carbonates of negative and positive charges of the allophanic soil at the heavy metal cations precipitate at around neutral or higher same pH, they increase with increasing concentration of an pH range. Both adsorption and precipitation of heavy metal indifferent electrolyte. Thus, the surface charge of the soil, cations take place in the soil suspension of this pH. both in magnitude and sign, can vary greatly with changes in In the soil suspension, cations are also adsorbed to organic the pH and ionic strength/composition of the ambient matters and multivalent cations are more preferentially solution 6).Uncultivated soils show pH (H2O)values at around adsorbed than monovalent cations. However, there are two their ZPC values. In a extreme case, pH (H2O)lower than 5 ways how organic matters affect the behavior of multivalent sometimes occur in the teagarden plow layer soils due to cations in soils. If the organic matter is insoluble or heavy application of nitrogen fertilizers7). On the other strongly retained in the soil, the sorbed cation is retained in hand, the cation exchange capacity (CEC) of soils is the soil. In contrast, translocation of multivalent cations is routinely determined at pH 7 using 1mol L-1of NH4 acetate. enhanced if the organic matter complexed with the cations is The pH and electrolyte concentration are higher than those soluble at ambient pH. for cultivated soils. Thus, CEC of the variable charge soils Recommended composition of exchangeable cations in the are overestimated than those in the fields. cultivated soils is in the order: Ca2+>>Mg2+>K+ at around Variable charge properties are directly related to the high pH 6.5. However, K+ is easily leached down due to its low neutralizing capacity of these soils. When protons or preference to Andisols. On the other hand, excessive K+ hydroxide ions are added to these soils, much of them retards uptake of Ca2+and Mg2+.Hence, care should be taken coordinate to the aluminol or dissociate silanol groups of the for management of K+ in Andisols. NH4+, a common N soils, respectively. Thus, the neutralizing capacity depends fertilizer, should be managed carefully because its behavior on the pH-charge relationships of the soils at the ambient is similar to K+ in Andisols and excessive application of N is electrolyte concentration. Allophane shows the high not necessarily appropriate for crop production and capacity to have charges of 100-300 cmolc kg-18). With an eutrophication control. increase in the amount of charge, allophane is gradually dissolved 9). Anion sorption Humus-rich allophanic soils show only small amount of Bonding strength between anions and allophanic soils positive charges10). Positive charge sites of allophanic soils depends on anionic species. The bond strength increases in may be disappeared by the negative charges from dissociated the order of NO3-, Cl-

I II I II

Fig. 1. Diffuse reflectance IR spectra of Al gel and NO3- sorbed by Al gel. Fig. 2. Diffuse reflectance IR spectra of H2PO4- and vibration under the air-dried conditions at 298K (Fig.1-IIb) H2AsO4- sorbed by Al gel. were close to those in the aqueous solution (Fig.1-IIa). The IR spectra of sorbed anions were obtained as subtraction sorbed by 1060 and 539 mmol P kg-1followed by air-drying, spectra between with and without the anion sorption. Under respectively. Fig.3b, d,f, h and Fig.3a, c, e, g are with and the reduced pressure and heating, the additional absorption without cross polarization, respectively. Among these, side band at 1490cm-1 appeared (Fig.1-IIc, d). The splitting of bands of Al (H2PO4)3 only was enhanced by CP. Thus, it the absorption band is due to an increase in interaction was estimated that noncrystalline Al phosphate and P sorbed between the sorbed NO3- and Al gel surface. The similar by Al gel contain little P-O-H partial structure. changes were observed for the sorbed sulfate on the Al gel. The chemical shifts of P sorbed by Al gel from H3PO4, a In contrast, the split IR absorption bands of H2PO4-were reference material, are in the range between -2 and -4 ppm observed in aqueous solution (Fig.2-Ia). After sorption on and that of noncrystalline Al phosphate ranges between -11 the Al gel, the absorption band appeared as a broad band and -12 ppm in Fig.3. According to Lookman et al. 17),the even in the air-dried conditions at 298K (Fig.2-Ib) and it did chemical shift value of P sorbed by Al gel moved from -6 not change under reduced pressure and heating at 393K ppm 3 days after sorption to -10 ppm after the elapse of 120 (Fig.2-IIc, d). Thus, a change in the chemical state of days at 301K. The -10 ppm is close the chemical shift H2PO4-took place when H2PO4 was sorbed by the Al gel. value for noncrystalline Al phosphate. Similar to H2PO4-, the sorbed H2AsO4- did not change with reducing pressure and heating, suggesting that the chemical reaction took place with sorption by the Al gel 14). An IR spectrum of H2AsO4-in aqueous solution adjusted to pH4.5 was not obtained because of interference by the strong absorption bands of H2O. Similar changes in IR spectra are observed when H2PO4- is sorbed by allophanic clay and Andisols. Although the IR spectra of the sorbed phosphate are close to that of noncrystalline Al phosphate, the absorption band due to P-O stretching vibration is a little broad compared with that for noncrystalline Al phosphate. Thus, the sorption product is a material close to noncrystalline Al phosphate15) Phosphate sorbed by Al gel exhibits also similar characteristics to noncrystalline Al phosphate in 31P solid state high resolution nuclear magnetic resonance spectroscopy (31P-NMR). Fig.3 shows the results of 31P-NMR study if the P sorption product has a partial 31P MAS-NMR spectra (Jan structure of P-O-H or not, applying the method used for . 1986 JEOL) noncrystalline Ca phosphate 16). If the material has the a, c, e: Without CP. b, d, f: CP with H, contact time=1ms. partial structure of P-O-H, side bands of 31Pare enhanced by cross polarization (CP) with 1H. Fig.3a,b show 31P-NMR spectra of Al (H2PO4)3 as a phosphate having the P-O-H Fig. 3. High resolutionsolid state 31 P NMR spectraof referenceAl phosphates and H2PO4- sorbed by Al group. Fig.3c and d show those for the air-dried noncrystalline Al phosphate, Fig.3e, f and Fig.3g, h Al gel gel with and without crosspolarization (CP). 277

Fig. 4. Changes in element concentration with oxalate-extractable Al (Al0) of andesitic, allophanic Andisols in Japan. Regression lines were drawn if correlation is significant at p=0.05.

The allophanic clay prepared from the weathered Kanuma elemental composition is greatly changed during the Andisol pumice released Si and the molar ratio of the released Si to formation process. sorbed P was about 0.2 18). This ratio is lower than that for Fig.4 summarizes the changes in concentrations of 56 Si/Al ratio of 0.5 for Al-rich allophane. If elements with oxalate-extractable Al (Al0) of allophanic and dithionite-citrate-bicarbonate extractable or 2% Na carbonate andesitic Andisols. Al0 shows an extent of Andisol extractable fractions, more Al-rich fraction than Al-rich formation from tephras. Andesitic Andisols, that was allophane, sorb phosphate forming noncrystalline Al estimated to have formed from andesitic tephras, was chosen phosphate, the obtained molar ratio of 0.2 for released because this group of soils tended to show less scattering in Si/sorbed P is reasonable. Other possibility may be that the element concentration than others. There are increasing and P sorption product is a mixture including secondary precipitate, monodentate and bidentate surface complexes. The molecular orbital method was applied for the sorption studies of many anions including phosphate, sulfate, molybdenate, borate and silicate 19). Further studies are needed to obtain effective management ways to control sorption and desorption of anions by these soils in agricultural fields.

Changes in element concentration during Andisolformation The atomic ratio of Si/Al in unweathered tephras changes with rock types and ranges between 2.4 and 4.8 20),meaning Si-rich. The Si/Al ratio of of non-colored volcanic glass contained in the rhyolitic, dacitic and andesitic tephras is about 4.7. The colored volcanic glass in the basaltic tephra Fig. 5. Relationship between the estimated weight change shows the Si/Al ratio of about 2.321). In contrast, imogolite (Wp/Ws-1) and Al or La concentration of andesitic, and Al-rich allophane have the atomic Si/Al ratio of 0.5. allophanic Andisols. WP/WS-1 was obtained The Si/Al atomic ratio of Si-rich allophane and Al-humus are assuming that Al was derived from volcanic glass. 0.5-1 and almost zero, respectively. All of these are the soil Solid lines show element enrichment due to estimated formation products in Andisols from tephras. Thus, the weight loss. Dotted lines show a regression equation. 278 decreasing tendencies in element concentration with Andisol Dispersion and coagulation formation 22). Dispersion and coagulation is related to charge properties Average content of C, N, Na, Mg, Al, Si, P, K, Ca, Ti, Mn and allophanic clays hardly disperse at pH around its ZPC. and Fe is about 1g kg-1or more in Andisols. Among these Allophanic clays disperse in the acid or alkaline pH regions. elements, C and N accumulated in soil with biological With phosphate fertilizer application, dispersion in the acidic activities, showing strong correlation between these two pH range is diminished due to a decrease in positive charge. element concentrations. Some essential elements for plant Sulfate, a divalent anion, flocculates allophanic clays in the growth such as Cu, Zn and P may show enrichment in the acid pH range. In the humus-rich horizon, aggregate humus-rich horizons. Na, Ca and Si concentration stability including allophane and imogolite is very high. significantly decrease with increasing Al0 indicating these Allophanic soils are irreversibly coagulated with drying. elements are depleted during Andisol formation. Thus, Accordingly, it is difficult to determine clay content by dissolution of volcanic glasses, formation of noncrystalline dispersion and sedimentation. The natural irreversible and poorly crystalline minerals, and accumulation of humus coagulation of Andisols is common under semi-dry climates strongly affect the concentration of some major elements in especially in the surface horizons. In that cases, content of Andisols. allophane-imogolite and ferrihydrite can be calculated based Regarding minor elements, many elements such as Cu, Y, on oxalate-extractable Si (Si0) and Fe0, respectively4). Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Allophanic soils have higher liquid and plastic limits Lu, Hf, Ta, Th and U show significant positive correlation compared with crystalline clay soils when they are compared with Al0 (Fig.4). These elements are concentrated during as a function of clay content4). Allophanic clays appear Andisol formation. Possible mechanisms are sorption of small aggregates even when they dispersed in water and have these elements in the noncrystalline and poorly crystalline some water inside the aggregates and it is not directly related minerals and humus, and residual as constituents in the to consistence. The liquid and plastic limits also show an highly weathering-resistant minerals. The almost similar irreversible decrease with drying. slopes of the regression lines for the increase in element concentration of these elements can be explained by weight APPLICATIONS OF THE UNIQUE PROPERTIES loss of the parent material. The slopes of the regression lines are close to the enrichment line obtained assuming P removal from wastewater weight loss of parent material during Andisol formation (Fig. High P retention capacity of Andisols was tested to remove 5). The Wp/WS-1 in Fig.5 was adapted after Kurz et al.23). P from wastewater. However, it was not always very The extent of element enrichment without illuviation was successful using soil columns due to clogging by colloidal estimated to be at most 2.7 times. Some elements among materials in the wastewater or bypass flow due to crack the first transition metals correlate more strongly with formation with repetition of drying and wetting. oxalate-extractable Fe (Fe0) than with Al0, suggesting preference of these elements to ferrihydrite. As a result of Control of soil-borne diseases residual element enrichment, Andisols show high minor Acidic soil pH is in general effective to control potato element concentration compared with other soils in Japan24) common scab. However, soil pH is not necessarily an in spite that Andisols form under intensive effective index for the scab control especially at around pH conditions. There sometimes are gibbsite-rich old layers 5-5.5. Nonallophanic Andisols show the scab control effect that received more intensive leaching and weathering than at this pH range whereas allophanic Andisols do not. The allophane-rich horizons. The element concentration may appropriate level of exchangeable Al as well as water-soluble less intensive in the gibbsite-rich layers than allophane-rich Al of the nonallophanic Andisols appears a key factor horizons because of the weaker sorptive properties. whereas allophanic Andisols do not have at this pH range25, 26). Bean root rot can be controlled similarly in the nonallophanic PHYSICAL CHARACTERISTICS Andisols in the similar pH range27).

High porosity Natural nano-porous materials Andisols rich in allophane and imogolite and/or humus are Allophane and imogolite are natural nano-porous materials highly porous with micro, meso and macro pores. The solid expected also for industrial uses such as humidity control, phase ratio is only 20 to 25% and bulk density is also low dewing control and heat exchange at the temperature lower ranging 0.4 to 0.9, which is the lowest except . than 373 K28). The bulk density of Andisols decreases with increasing allophane and imogolite content and/or humus content. Others Due to the highly porous structure, Andisols show high air Brassica plants show high preference to P fertilizers and and water permeability as well as high water holding they cover P fertilizer particles completely in P deficient capacity. The Andisols are suitable for upland crop nonallophanic Andisols 29). The P foraging root production after appropriate amendment of nutrient development like this appears useful to save P resources. deficiency 4) and high acidity of nonallophanic Andisols24). mosaic virus is adsorbed allophanic soils with low The water holding capacity of Andisols irreversibly decrease humus content and this soil has suppressive effect to this with drying. virus 30). Under flooding, plow layer soils of the paddy fields are reduced in the rice-growing season. However, excessive reduction is not good even for rice plant growth. 279

The excessive reduction may be controlled with the addition 27) Takahashi, T. (2003) J. Clay Sci. Soc. J., 42, 154-157. of Andisols 4). 28) Suzuki, M. (2003) J. Clay Sci. Soc. J., 42, 144-147. 29) Nanzyo, M. Shibata,Y. and Wada,N. (2002) Soil Sci. ACKNOWLEDGEMENT Plant Nutr. 48, 847-853. 30) Toriyama, S., Okabe, I., Nanzyo, M., Mitsuchi, M., This study was supported in part by a Grant-in-Aid for Kameya, M. (1995) Bull. Natl. Inst. Agro-Environ. Sci., 12, Scientific studies from Japan Society for Promotion of 75-86. Sciences (#17405024)

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