Pore Properties and Fractal Structure

Transmission electron microscopy shows that allophane particles are nearly spherical with diameters in the range of 3.5-5 nm (by the way of comparison of the platelet-like 

The power law part appearing as linear in a log-log scale has a slope related to -D, D the fractal dimension, which expresses the cluster compactness. The position of the two intersects (black and white arrows) allows to calculate the cluster size and the particle size a. For q higher than 2 nm the curve shows a hump which can be attributed to the particle size (3-4 nm, white arrow). This description is in agreement with the transmission electronic microscopy results [4, 17, 23, 24]. Table 12.2 lists the evolution of D and la (the fractal extent) versus allophane content drawn from the experimental curves. The fractal extent can be considered as the size of a tortuous nano labyrinth. Our results show that the size of the fractal labyrinth increases with the allophane content which suggests that the accessibility inside the soil microstructure will decrease. 

Carbon Nitrogen and Pesticides Content in Aiiophanic Soils 

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]. 

In the case of pesticides retention, preliminary results [45] have shown that the chlorde-cone content increases with the allophane content but the chlordecone lability, the percent of chlordecone released by lixiviation experiments, decreases. So, allophanes favor the pesticides accumulation in volcanic soils but hinder the chlordecone release by weathering. 

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