Extraction rate for carrot roots varied from:
• 6,4 to 10,0% for andosol • 31,1 to 32,1% for nitisol.
Pesticide transfer from soils to plants in tropical
soils: influence of clay microstructure
LesueurJannoyer M1, Woignier T2,3, Achard R1, Calba H4
contact: [email protected]
1 CIRAD, PRAM, BP 213, Petit Morne, 97232 Le Lamentin (Martinique) 2 IRD-Eco&sol, PRAM, BP 213, Petit Morne, 97232 Le Lamentin (Martinique).
3 CNRS-Université de Montpellier.
4 CIRAD/PERSYST, “Recyclage et gestion des risques", TA B-95/16,Av Agropolis, F-34398 Montpellier Cedex 5, France
← 600 nm →
Electronic microscopy
(TEM) of allophane aggregates
0 20 40 60 80 100 120 140 160 180 200 0 5 10 15 20 25 30 allophane (%) S (m 2 /g ) -0,5 0,5 1,5 2,5 V p ( c m 3 /g )
x
a y = 0,558x + 2,1248 R2 = 0,8528 0 5 10 15 20 0 5 10 15 20 25 30 allophane content (%) e x t e n t o f t h e f r a c t a l a g r e g a t e t s ( x / a ) 0,0001 0,001 0,01 0,1 1 0 10 20 30 40 50 échelle l (nm) p e rm e a b il it é r e la ti v e e t d if fu s iv it é r e la ti v eFractal structure modelling for allophane aggregates
→
→
Evolution of the labyrinth size
x/ a according to allophane content % (SAXS → Df= 2,7)
Modelling
→
Conclusion
Molecule transport in poral material depends on 2 parameters :
permeability K and diffusivity D.
•K depends on relative density
r and on pores mean size d : K (1-
r) d2•D depends on P, the porosity and on t, the tortuosity: D P/t
In the case of fractal aggregates, K(l) and D (l) decrease of several order ranges when the lenght scale decreases.
Thus, chemical species inside the fractal aggregates are not accessible
and are trapped by confinement.
Allophane concentration in andosols favours
chlordecone trapping. The trapping ability of this poral
material (andosol) could explain the low chlordecone
lability and thus accessibility and availability for crops
and environment in these soils.
Transfer experimentations demonstrated that even if
andosol chlordecone content was higher than nitisol,
andosol were less contaminant for crops.
The knwoledge of this allophanic clay structure
should help us to understand the environemental factors
to take in account in the general question of pesticide
trapping, transfer and remediation in these specific soils.
K
D
Chlordecone is a very though pesticide which was used from 1971 to 1993 mainly for the control of the banana weevil in French West Indies (Guadeloupe and Martinique islands). Chlordecone pollution is now diffuse becoming new contamination source for
crops and environment (water, trophic chain). Volcanic soils like andosols contain amorphous clays (allophanes), issued from the
transformation of volcanic materials with very specific properties. Our hypothesis is that the clay microstructure characteristics could be a crucial physico-chemical factor governing the fate of the pesticide in the environment.
Introduction
0 0,1 0,2 0,3 0,4 0,5 0,6 0 5 10 15 allophane (%) c h lo rd e c o n e c o n te n t (m /K g ) / C c o n te n t (g /K g )Chlordecone content doesn’t depend only on high organic matter content in our volcanic soils. Allophane content is a key parameter.
Allophanes present drastically different structures and physical
properties compared to usual clays: a very high poral volume (Vp) and an important pores surface too (specific surface S).
Soil pollutant content and properties vary with soil allophane
content
Evolution of the specific surface S and poral volume
Vp, according to allophane content %
Crop transfer
The fractal range increased with the allophane content.
Fractal structure of allophane aggregates
At harvest, the mean transfer rate to sweet potatoes was proportional
with the chlordecone soil content
and varied from 1,5% for andosol to
3,1% for nitisol.
Soil type had a great effect on the chlordecone availability in soil solution and its translocation into roots. We showed that allophane clay retained more pesticides than halloysite.
Controlled conditions, after 20 days of plantule growth in soil cells
