Allophanes surface area

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

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). 

In general, high-surface-area minerals such as layer silicate clays and allophane dissolve at significant rates in strongly acid soils ... 

Figure 12.4. Pore volume black symbols) and specific surface area red symbols) versus the allophane content.

However, recent results did not support the chemical protection of C bound to iron and aluminium complexes [38]. Some studies have suggested that it is the surface area of the mineral particles that can explain the slow C mineralization [39 2]. In previous studies one has shown that the carbon and nitrogen content is also related to the specific surface area [11, 17]. High specific surface areas will increase the possible adsorption sites for chemical species and certainly participate in the sequestration mechanism but the peculiar (fractal) structure of the allophane aggregate should play a role in the sequestration process. The mesopore protection hypothesis proposes that carbon is trapped in the tortuous porosity of the soils because the organic matter is protected from enzymatic degradation by sequestration within the mineral mesopores (2-50 nm diameter) [40, 42-44]. 

This investigation of the pore structure details allows one to propose a possible mechanism for the C and N sequestration and pesticides retention in allophanic soils. The large specific surface area is the signature of small pore sizes. Associated to the fractal... 

Although allophanes have great surface area and are therefore potentially highly reactive chemically, it is clear from the data of Table 5 that in the younger volcanic ash soils where free drainage removes bases which might otherwise react, allophane may persist for long periods of time. 

When clay fractions are removed from soils containing allophane using methods of clay dispersion, which are satisfactory for most soils, surface area measurements of the residual coarse fractions show that much clay-size material has not been effectively dispersed. Various degrees of dispersion of soil allophane are achieved by dispersion in aqueous alkaline media... 

Table 8. Values of Surface Areas of Allophanic Clays (Less Than 2 fim) from Volcanic Ash Soils. Areas were Determined by the Ethylene Glycol Adsorption Method of Dyal and Hendricks [1950] before and after Heating...

In a later study Birrell [1961a] studied the adsorption of various cations in allophane and showed that the quantity of cation adsorbed is inversely related to the radius of the hydrated cation, confirming that the apparent cation exchange capacity of allophane was a surface area effect. A similar conclusion was reached by Egawa, Watanabe, and Sato [1959]. 

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