Imogolite formation

The synthesis of imogolite occurs in mild conditions and in aqueous solution. However, its mechanism of formation is rather complex and involves self assembly. The AP+ ions in solution rapidly hydrolyze forming polynuclear species (Bi et ah, 2004). It has been pointed out that the thermodynamic equilibrium is not achieved rapadly and the kinetics is very slow (Casey, 2006). The silicates in solution are a very complicated system forming many polynuclear intermediates (Exley et ah, 2002 Schneider et ah, 2004). The imogolite formation... 

Figure 2.18. Proposed mechanism for the catalysis of hydroxylaluminosilicate ions in catechol humification. Reprinted from Liu, C., and Huang, P. M. (2002). Role of hydroxylaluminosilicate ions (proto-imogolite soil) in the formation of humic substances. Org. Geochem. 33, 295-305, with permission from Elsevier.

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Across a chronosequence of soils on the Hawaiian islands (Crews et al., 1995), Tom et al. (1997) found that both the quantity of stored carbon and its turnover time correlated with the noncrystalline (allophane, imogolite, and ferrihy-drite) mineral content of the soil (Figure 30). These amorphous minerals possess a unique geometry with a very high surface area (Table 13) which facilitates the formation of highly stable bonds with SOM (Oades, 1988). 

Imogolite can be a major component of clays in soils developed from volcanic ash, but has also been discovered as a minor constituent of spodosols formed in northern temperate climates. The fibrous appearance of imogolite under high magnification, with tubules that are as much as several microns in length but only 21 A in diameter, distinguishes it from allophane. Its formation is favored over that of allophane in acidic environments. 

At pH 6.2, the formation of the solid hydroxide phase gibbsite limits the level of dissolved aluminum to about 10-7 M. The formation of imogolite reduces this level to 10-11 M, so that the formation of hydroxyaluminosilicate phases reduces the concentration of biologically available aluminum to levels well below those producing toxic effects. 

Levard, C., Rose, J., Thill, A., Masion, A., Doelsch, E., Maillet, P., Spalla, O., Olivi, L., Cognigni, A., Ziarelli, F. Bottero, J.Y. (2010) Formation and growth mechanisms of imogolite-like aluminogermanate nanotubes. Chemistry Of Materials, 22, 2466-2473. 

The present chapter is structured as follows we will discuss the synthesis and formation mechanism of imogolites at Section 2, whereas Section 3 features the computational aspects of the approximate method Self-Consistent-Charge Density-Functional Tight-binding and its parameterization steps. The theoretical analysis for NT structures and stability will be presented at Section 4, followed by the electronic and mechanical properties at Section 5. Final remarks will be presented at Section 6. 

Tuschall JR, Brezonik PL (1983) Complexation of heavy metals by aquatic humus a comparative study of five analytical methods. In Christman RF, Gjessing ET (eds) Aquatic and terrestrial humic materials. Ann Arbor Science, Ann Arbor, pp 275-294 Ugolini FC, Dahlgren R (1988) Formation and role of imogolite in Spodosols. Am Soc Agron Abstr, Madison, WI, p 206... 

Su, C. and Harsh, J.B. (1994) Gibbs free energies of formation at 298 K for imogolite and gibbsite from solubility measurements. Geochim. Cosmochim. Acta, 58, 1667-1677. 

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