Adsorption of organophosphorus pesticides in tropical soils: The case of karst landscape of northwestern Yucatan

- The tropical soil samples presented similar physicochemical properties.
- High organic matter content was uniform in all soil samples.
- Organophosphorus insecticides were weakly adsorbed on the soil samples.
- Organophosphorus pesticide can endanger the groundwater quality in karst regions.

This article discusses the adsorption of four organophosphorus pesticides-diazinon, dimethoate, methyl parathion, and sulfotep-in soil samples from four sites-Komchén, Xcanatún, Chablekal and Mocochá- in the northwest of Yucatan, Mexico. These pesticides have been detected in groundwater at concentrations greater than 5 (μg/L) during recent monitoring campaigns in the study area. In this region, groundwater contamination is exacerbated by its karst aquifer, which is susceptible to contamination and is considered very vulnerable. The experimental work was carried out using the batch equilibrium technique. Pesticide analyses by solid-phase extraction and gas chromatography were performed. The equilibrium adsorption data were analyzed by Henry, Langmuir and Freundlich models. The results indicate that the Freundlich model provides the best correlation of the experimental data. Freundlich adsorption coefficients Kf were in the range of 1.62-2.35 for sulfotep, 2.43 to 3.25 for dimethoate, from 5.54 to 9.27 for methyl parathion, and 3.22 to 5.17 for diazinon. Freundlich adsorption coefficients were normalized to the content of organic carbon in the soil to estimate the sorption coefficient of organic carbon (KOC). KOC values were in the range of 9.45-71.80, indicated that four pesticides have low adsorption on the four studied soils, which represents a high risk of contamination to the aquifer.