Pollution of soils and ecosystems by a permanent toxic organochlorine pesticide: chlordecone—numerical simulation of allophane nanoclay microstructure and calculation of its transport properties

Abstract

Pest control technology was introduced into the tropics without considering the specificity of their ecosystems and the risk of pollution was underestimated. Some volcanic soils (andosols) contain nanoclay (allophane) with a unique structure and porous properties compared to crystalline clays. Andosols are characterized by large pore volume and pore size distribution, a high specific surface area, and a fractal structure. These soils are more polluted than the other kinds of tropical soils but release less pollutants (chlordecone) to water and plants. The literature shows that the allophane microstructure favors accumulation and sequestration of chlordecone, an organochlorine pesticide, in andosols.<br />We used a numerical model to simulate the structure of allophane aggregates. The algorithm is based on a cluster-cluster aggregation model. From the simulated data, we derived the structural features, pore volume and tortuosity, and its transport properties, hydraulic conductivity and diffusion. We show that transport properties decrease because of the presence of allophane. We propose that low hydraulic conductivity and diffusion are important parameters to explain the high concentrations and trapping of pollutants in andosols. (Résumé d'auteur

The pesticide chlordecone is trapped in the tortuous mesoporosity of allophane clays

“…The value of the effective diffusion coefficient (for Cs + ion) in allophane was estimated to 10 −16 m 2 .s −1 , (Mon et al 2005), 6 orders of magnitude smaller than the diffusion coefficient in aqueous solution. Thus, allophane aggregate structure might be highly porous but has a low permeability and diffusion coefficient; fluids and chemical species migrate with difficulty inside the fractal structure (Woignier et al 2015). In addition, the fractal pore structure imposes a range of pore sizes including small as well as large ones.…”

“…These results are consistent with results on soil to crops contamination found in the literature. Contamination of crops by soil CLD depended on the type of soil: andosols were less contaminating than nitisols (Cabidoche and Lesueur-Jannoyer 2012;Woignier et al 2015). Studies on chlordecone transfer to water and crops led to the general assessment that andosol tends to retain the pesticide more than other soils.…”