Adsorption and desorption kinetics of diflubenzuron and fenitrothion in two different boreal forest soils

Abstract

Adsorption and desorption of two insecticides, diflubenzuron (DFB) and fenitrothion (FEN), were studied by batch equilibrium method under laboratory conditions (pH 5.5, 20°C), using an organic soil (soil 1) and a silty clay loam soil (soil 2), collected from a boreal forest in northern Ontario, Canada. Adsorption parameters were calculated using the linear, Freundlich and Langmuir equations. Maximum amount adsorbed and the time required for maximum adsorption of DFB were 88 μg/g and 18 h (soil 1), and 73 μg/g and 24 h (soil 2), respectively. Corresponding values for FEN were 92 μg/g (soil 1) and 81 μg/g (soil 2), and maximum adsorption time for both soils was 30 h. DFB was more strongly adsorbed to both soils compared to FEN, however between the two soils, soil 1 showed stronger affinity for the chemicals than soil 2. The KD, KF and n values corroborated these findings. The Langmuir model showed deviation, probably due to heterogeneity of the soil surface and multi‐layer adsorption. Desorption studies showed, after six consecutive washes, that the adsorbed DFB was firmly retained by the soil matrix and only about 16 to 27% was desorbed. However, the desorption of FEN in the soils, under similar conditions, was higher and ranged from 38 to 49%. The soil parameters, KOC and KOM, were determined using the KD and the measured %OC (organic carbon) and %OM (organic matter) values. Moreover, KOC was calculated (KOC(calc)), using the KOM and the van Bemmelen factor. Reasonably good agreement was found between the measured KOC and the KOC(calc) value. Combining the average values of Koc and T1/2 (half‐life in soil), the groundwater ubiquity score (GUS) was computed to estimate the mobility of these two insecticides in the forest soil. The GUS values indicated that both insecticides could be considered as non‐leachers.