Sorption Characteristics of Chlordecone and Cadusafos in Tropical Agricultural Soils

Banana plantations in tropical humid regions require a high input of pesticides. Given the long history of this practice, a number of pesticides that are now banned have accumulated in these soils. Little is known about the sorption of two banned insecticides, chlordecone and cadusafos, which are known to cause adverse environmental and health effects. We studied the sorption-desorption characteristics (Freundlich sorption-desorption coefficient, Kf, and partition coefficient, Kd) of these two molecules in tropical volcanic soils with different soil properties. In particular, we observed the effect of the chemical nature of the soil organic carbon (SOC). The sorption of chlordecone (35.56<d<144.96 L kg^-1) and the desorption hysteresis (apparent hysteresis index, H<0.43) were very high. Sorption was significantly lower for cadusafos (1.47< Kd 19.94 L kg^-1) and less hysteretic (0.59< H< 1.08). The correlation between Kd and Kfsor values and SOC content was statistically significant for both molecules (p<0.01). KOC anged between 1218-2547 L kg^-1 for chlordecone and between 67 L kg^-1 and 167 L kg^-1 for cadusafos. The chemical composition of the soil organic matter was determined using cross-polarization magic-angle-spinning NMR spectroscopy (CP/MAS NMR). Chlordecone was found to display a higher affinity to soils with shorter alkyl chains and fewer carboxyl groups, while cadusafos had a higher affinity for soils with more oxidized OC (methoxy and carboxyl groups) and longer alkyl chains. This highlights the complex role of SOC chemistry on the sorption of chlordecone and the fact that soil practices, such as the addition of fresh organic amendments, may not efficiently enhance the sorption of chlordecone.