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Thierry Woignier G. Pochet H. Doumenc P. Dieudonné L. Duffours 《Journal of Sol-Gel Science and Technology》2007,41(1):25-30
Volcanic (allophanic) soils are interesting in terms of the control of the greenhouse effect and the knowledge of the porous
features is of importance to understand the mechanism of C and N sequestration. These soils contain a peculiar clay: allophane
aggregates quite close to the synthetic mineral gels aggregates. These volcanic materials behave as gels during drying with
a large irreversible shrinkage that can modify the soil physical properties. Consequently, as for silica gels, we use the
CO2 supercritical drying procedure (SD) to control the drying step and to preserve the structural and textural properties of
the soils. The experimental results show that the N and C content in the soils is clearly dependent on the allophane content.
We also show that the textural properties, such as specific surface area, are higher for the supercritically dried samples,
compared to the classically dried samples, and SAXS results confirm the preserving effect of the SD. With these data, we propose
possible effects of the specific surface area on the C and N content of the allophanic soils. 相似文献
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Thierry Woignier Juan Primera Anwar Hashmy 《Journal of Sol-Gel Science and Technology》2006,40(2-3):201-207
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. 相似文献
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