Sorption kinetics and degradation of chlorpyrifos in selected Uganda soils using mixed solvents

Abstract

The purpose of this study was to determine sorption kinetics using mixed solvents and degradation of Chlorpyrifos in tropical soils. The information is important to understand the behaviour and fate of Chlorpyrifos in Uganda soils. Batch sorption experiments were done to determine the sorption kinetics parameters; mass transfer coefficient (k2m) and equilibrium sorption coefficient (KM) of Chlorpyrifos. One site nonequilibrium model was used to optimise these sorption kinetics parameters for Chlorpyrifos for all the soils used. Sorption was found to be fast in systems where the fc had higher fraction cosolvency which lead to attainment of equilibrium before 12 h for all the soils. The sorption equilibrium for Chlorpyrifos was attained after 4 h and 6 h for fc = 0.8 and fc = 0.5, respectively. The mass transfer coefficient k2m of Chlorpyrifos was found to increase with increase in methanol fraction cosolvency except for TBGG 10 soil sample which remained constant despite the increase in methanol cosolvency in the system. Solvophobic theory was employed to predict the aqueous sorption coefficient (KW and k2w) of Chlorpyrifos for the soils which could not be accurately determined directly in water because of its strong hydrophobic nature. The normalized sorption coefficients to organic carbon, KOC of 1551 – 1890, found in this study were less than those reported in literature whose experiments were done in aqueous systems. Degradation studies were done in under submerged and field capacity conditions. Nine soils were used to study the effect of water hydrolysis on Chlorpyrifos degradation under submerged moisture conditions and in three soils for field capacity conditions. Sterile and unsterile soil samples were used to compare biotic and abiotic degradation of Chlorpyrifos. The degradation rate under submerged moisture regime was found to be slower than that of field capacity. The calculated half-lives of Chlorpyrifos in this study ranged from 25.3 – 86.6 days for sterilized soils and 17.8 – 132.4 days for unsterilized samples under submerged conditions. However, under field capacity, half-life ranged from 33.0 to 35.8 and 32.7 to 33.7 days for unsterilized and sterilized soils respectively. The groundwater ubiquity score index was calculated and found to range from 1.18 to 1.53, which showed that Chlorpyrifos has no potential to leach to underground water