Application of the DLCA model to “natural” gels: The allophanic soils

Volcanic soil comprises weathering products such as allophane, originating from a leaching process of volcanic ashes and glasses. These soils are interesting in terms of mitigation of the greenhouse effect (C sequestration), because they are known for accumulating more C than non-volcanic soils. Allophanes are natural amorphous silicates and have physical features very close to those of synthetic gels. Knowledge of the allophanic soil structure is required to understand the sequestration mechanism. In this paper, nitrogen adsorption-desorption experiments, measured on allophanic soil samples, show that the hydraulic diameter (Dh) is shifted towards smaller size while the pore volume (Vp) and specific surface area (S) increase, when the allophane content of the soil increases. We introduce a numerical model to simulate the structure of this “natural gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation in which larger particles hinder the DLCA. As a function of the relative content of allophane (gel) and larger particles, the textural properties (Vp, S, Dh) of the different simulated structure are calculated using a simple triangulation method. Numerical results are in good agreement with experimental data and from the simulated data, we can derive the permeability evolution as a function of the allophane content. We show that at the scale of the allophanic aggregates the calculated permeability is low and could be an important parameter to explain the larger C content of allophanic soils. Because of the low allophanic aggregate permeability, the fluid exchanges and chemical reactions are slow. This gel-allophane analogy and DLCA model allow proposing a different approach to describe the properties of these peculiar soils.