Natural Aerogels with Interesting Environmental Features: C-Sequestration and Pesticides Trapping

Abstract

Volcanic (allophanic) soils contain amorphous clays (allophanes), which present completely different structures and physical properties compared to usual clays. Allophane aggregates have peculiar physical features very close to that of synthetic gels: large pore volume and pore size distribution, a high specific surface area and very large water content. These volcanic soils have exceptional carbon (C) sequestration properties and are considered as sink for green house gases (C and N). Moreover, these peculiar clays have a large ability to trap pesticides found in soils. One proposes that these interesting environmental properties can be due to the peculiar structure of the allophane aggregates. Because of a large irreversible shrinkage during drying, the supercritical drying technique was used to preserve the porous structure and the solid structure of allophanic soils, and the fractal structure of these natural aerogels was determined at the nanoscale. It was found that the spatial extent of the fractal aggregates depends on the allophane content in soils. One also proposes that this fractal structure, analogous to the silica gel network, could explain the high carbon, nitrogen, and pesticides content in the allophanic soils. Because of high specific surface area and low transport properties, the tortuous structure of the allophane aggregates plays the role of a labyrinth which traps C, N, and pesticides in the porosity of allophane aggregates.