Mixed-Solvent Sorption and Moisture-Regime-Dependent Degradation of Chlorpyrifos in Selected Tropical Soils

The adsorption to container walls, syringes, injectors and analytical columns by strongly hydrophobic organics in aqueous media presents challenges in accurate estimation of sorption parameters of chemicals such as chlorpyrifos (CPF). To minimize this phenomenon, mixed solvents and Teflon-lined centrifuge tubes were used. The study aimed at investigating the sorption kinetics and equilibrium parameters in tropical soils. In addition, the persistence and leaching potential of CPF under submerged and field capacity moisture conditions were studied. Batch sorption studies utilizing the Solvophobic theory revealed time-dependent kinetics on Teflon container walls, where CPF sorption diminished exponentially with increasing methanol fraction. Sorption parameters for soils showed diverse kinetics and equilibrium times across soils and methanol fractions. The Solvophobic theory was used to predict the soil-sorption coefficients K^W and K_OC. Chlorpyrifos sorption exponentially decreased with increasing methanol fraction, reaching equilibrium in 4–8 h. Container wall K^W measured was 0.19 mL/g, while soil K^W values ranged from 46.53 to 56.71 mL/g. Chlorpyrifos K_OC values varied from 1551 to 1890. The degradation studies under submerged and field capacity conditions indicated microbial and abiotic influences on chlorpyrifos persistence, resulting in half-lives ranging from 18 to 52 days in submerged conditions and 18 to 33 days at field capacity. The Groundwater Ubiquity Index suggested no leaching potential in the examined soils. This study represents the first investigation of chlorpyrifos sorption kinetics only Teflon-lined centrifuge tube container walls, revealing that chlorpyrifos sorption is not instantaneous but rather time-dependent. Future analyses should explore CPF's environmental fate, considering microbial interactions and organic matter content, to contribute to a comprehensive understanding and develop sustainable pest management strategies in tropical regions.