Soil microstructure and added organic matter: keys for chlordecone sequestration

Soil microstructure and added organic matter:

keys for chlordecone sequestration

P. Fernandes

, M.Jannoyer-Lesueur

, F. Clostre

, A.Soler

L. Rangon

, T.Woignier

a

UPR Fonctionnement agroécologique et performances des systèmes de culture horticoles, CIRAD/PRAM , Le Lamentin, Martinique

b

UPR Fonctionnement Systèmes de cultures bananes plantains et ananas, CIRAD/PRAM , Le Lamentin, Martinique

c

UMR CNRS 6116 UMR IRD 193- IMEP Institut Méditerranéen d’écologie et de paléoécologie, PRAM , Le Lamentin, Martinique

Abstract

The scientific and economic context of our study is related to the pollution of the soils, fresh and marine water by a persistent organochlorine pesticide (chlordecone) in a tropical context. The former application of chlordecone results today in a diffuse pollution in agricultural soils, which are sources of

contamination for cultivated roots, tubers, vegetables and terrestrial and marine ecosystems. Chlordecone is a very though and stable

molecule (considered as a POP), it is mainly present in solid phase and has a strong affinity with organic matters. To prevent consumers and ecosystems exposure, it is thus necessary for us to evaluate the factors that influence chlordecone migration in the environment.

In our research, we studied the impacts of clay microstructure on the chlordecone retention. We showed that allophane aggregates had a great ability to trap chlordecone mainly due to their fractal structure. We also measured the effects of added composts on chlordecone la-bility and transfer rate from soil to plant . We show that compost addition modifies the fractal structure of allophane clays favouring the chlordecone retention. These results allow us to propose a new strategy in opposition to the complete soil decontamination: the

chlor-decone sequestration. Molecule of chlordecone

Material and methods

SEM of halloysite clay 1/ 1

SEM and TEM of allophanes clay ( fractal structure) : Hierarchi-cal aggregation of particles with diameter between 3 and 5 nm

Treatments :

• Addition of 5 % (w/w) of

compost Vegethumus in field ( allophanic soils).

- control : 0%

Measures :

• Chlordecone concentration( CLD) in volcanic soils versus SOC . • Chlordecone concentration( CLD) in the fine fraction of

allopha-nic soils ( allophane clay) versus SOC.

• Chlordecone concentration( CLD) in radish tissues (small roots ;

main root ; leaves) and cucumber (fruit and leaves) measured af-ter compost addition (1-6 months) grown in fields.

• Specific surface area and pore surface distribution of allopha-nic soils from 0 to 90 days after compost addition determined by

adsorption desorption of N 2 (Micromeritics) .

Results

The Soil Organic Carbon (SOC) and allophane structure favour the CLD retention .

Dramatic reduction of specific area and of pore surface distribution in the range of 5-100 nm ( allophane microstructure) by the addition of organic matter.

Chlordecone content (µg/kg of fresh material) of radish (in red : soil without compost ; in green : soil with compost 7 kg fresh product/m²) cultivated on contaminated andosol.

The addition of compost reduces the contamination of plant tissues, 1-6 months after compost addition

CONCLUSION

Common soil decontamination techniques such as solvent extraction of chlordecone , incineration, microbial degradation are not yet even a promising technique (costly, slow degradation rates, metabolites whose toxicity is similar to that of chlordecone) . This is why we consi-dered increasing the sequestration of CLD in soils by adding compost as an alternative solution to phyto-extraction or microbial degrada-tion to reduce the diffusion of CLD in harvest products and environ-ment.

Our preliminary results in fields on CLD sequestration offer new op-portunities for the management of soil contamination. They show

that adding compost to contaminated soils increases chlordecone se-questration because it is associated with micro-structural changes,

pore collapse and closure of the fractal structure in allophane. _{This work was co-funded by the French Ministry of Outermost Region, the European Regional}

Develop-ment Fund (ERDF – 2007/2013) and the French National Research Agency through the CES program

y = 0,342x + 1,326 R² = 0,681 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 C L D ( m g / k g s o il ) SOC (%) y = 0,342x + 1,326 R² = 0,681 0 1 2 3 4 5 6 0 2 4 6 8 10 C L D ( m g / k g s o il ) SOC (%) Allophane clays Volcanic soils Composts characteristics Vegethumus Water content 24.8 %

Organic matter content 46.6 %

Humic yield (CBM) 577 kg C/t of fresh

product Humic potential ( K1) 0.70

0

50

100

150

Cucumber

0

5%

small root mean root leaves

Radish

Evolution of the pores surface distribution according to the incubation time (blue : 0 day ; red : 30 days ; green : 90

days) after addition of organic matter.

Evolution of the specific surface area (3 allophanic soils differ-ing from their allophanic content : 18%, 8% and 6%) versus in-cubation time after addition of organic matter.

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0 50 100 150 200 ,,,, S ( m 2/g / n m -1) R (nm) 0J 30J 90J 18% 8% 6% C L D ( m g /k g s o il ) ∆∆∆∆ S ( m ²/ g /n m -1 ) S p e c if ic s u rf a c e a re a (m ²/ g ) main root