Low Water Transport in Fractal Microstructure of Tropical Soils: Application to Pesticides Trapping

1. Low water transport in fractal microstructure of tropical soils: application to pesticides trappingWoignier Thierry 1,2 , Morell Marc 3, Morell Olivija 4, Duffours Laurent51.UMR Eco&Sols IRD-PRAM,Le Lamentin, Martinique ,France2. CNRS-Université de Montpellier, Montpellier   France 3.IRD Caraïbes, Fort de France, Martinique, France 4 Meteo-Mak , Skopje, Republic of Macedonia 5. Prime Verre, avenue A. Einstein , Montpellier , France

2. Introduction • The scientific and economic context of this study is related to the pollution of water resources by a persistent chlorinated hydrocarbon pesticide (chlordecone) in a tropical contex. • Chlordecone is a very though pesticide which was used in the 1970’s mainly for the protection of banana plantation. • Today chlordecone is at the origin of the diffuse pollution in agricultural soils, becoming new sources of contamination for fresh water and fauna. (accumulates in the food chain)

3. Introduction • It is thus necessary to know the environmental mobility and fate of the pesticides; what content of chlordecone is present in the soils and is able to migrate through environment. • Previous studies have shown that pesticides diffusion in water differs with the soil type • Soils containing allophane clay are generally highly polluted but less contaminant for water resources.

4. Introduction • Allophane is an amorphous clay presenting peculiar physical features: large porosity (70%), a high specific surface area (100-200m2g-1) and very large water content (200 weight percent). • We propose that the peculiar structure of the allophane is responsible for pesticides affinity and slow transfer to water resources. • Influence of the allophane content on the porous and structural properties and correlate these features to the retention of chlordecone and trapping properties?

5. steps of this work • Pesticides affinity with allophane • Porous features of allophanic soils • Fractal Structure of the allophane aggregates • Modelisation of the allophane fractal aggregates • Conclusions

6. Pesticide affinity • the chlordecone contamination in soils is directly related to the soils allophane content • we can suppose that the pesticides retention properties could be dependant on allophane features.

7. Water content in allophanic soils • large increase of the water %t vs the allophane content • the allophane clay is like a sponge with water content as high as 180 weigh percent.

8. Porous features of allophanic soils • The allophane clay favors larger specific surface area (■) as high as 120 m2.g-1 and pore volume (O)close to 2 cm3.g-1) • The results suggest that the porous structure is made of micro and mesopores. The peculiar tortuous structure of the allophane is responsible for the pesticides retention?

9. Is this protecting behaviour related to theFractal Structure of the allophane aggregates?

10. Fractals in the nature

11. Fractals in the nature The natural fractal are randomly built

12. The fractal structure of allophane agregates The fractal structure of allophane agregates is defined between 2 limits :1)The size a of the primary particleswhich build the solid network 2) The size of the aggregates built by the aggregation of the primary particles.< allophane (600 nm) > < kaolinite (1500 nm) >

13. Fractal extent of allophane • SAXS experiments allows to calculate the fractal dimension Df (the clusters compactness and tortuosity) and the extent of the fractal aggregate  . • The size of the fractal labyrinths increases with the allophane content which suggests that the accessibility inside the clay microstructure will be poor

14. Discussion • Signature of a completely differentstructure of the allophane aggregate, compared to the usual clay. Allophane is a spongy structure made of aggregated small particles , forming a fractal network. • For all soils studied, the allophane particle size is quite constant (3-5nm), the fractal dimensions Df does not vary (Df =2.5-2.7) but the aggregates size increases with the allophane content. • This peculiar tortuous structure will have some incidences on the transport properties inside the allophane aggregates.

15. Discussion • The pore structure → possible mechanism for the high water content and pesticides retention in allophanic soils and for the poor chlordecone release in water resources. • The water inside the allophanic aggregates is quite trapped in the allophane porous structure. → pesticides can not move by flow from the interior of allophane particles to river water. • Permeability K and diffusion coefficient Di ?

16. Transport properties : perméability (K) and diffusion coefficient (Di) K ()  (1-( /a) Df-3) 2 Di (1-(l/a) Df-3) [1-2/3 (2-(l/a) Df-3) (l/a) Df-3)3/2]. K et Di decreases 10-4 -10-2 labyrinth structure= trapping

17. Permeability and diffusivity of the allophane aggregates

18. Conclusion • In french west indies water and drinking water are contaminated by a tough pesticides (chlordecone). • This contamination is the result of a diffuse pollution coming from agricultural soils. • The lixiviation and chlordecone transport in environment differs between soil types. • the porosity in allophanic soils can span a wide range of length scales including the micro-, meso- and macroporous regimes. →Role of the porosity structure?

19. Water and pesticides inside allophane aggregates occupy the pores→ the physical features of the clay structure control the pesticide release to water resources and environment. • The allophanic clay is fractal → clear correlation between the allophane content, the fractal features, the large water content and the comparatively low pesticide release in water. The peculiar structure of the allophane aggregates plays the role of a labyrinth which traps the pesticides molecules limiting the pesticides contamination in water resources and environment. The allophane fractal structure is fragile and future evolution of the clay structure could lead to a larger release of pesticide in water.