Physical characteristics and Freundlich model of adsorption and desorption isotherm for fipronil in six types of Egyptian soil

The soil type and temperature are considered important parameters that can influence the rates and equilibria of different environmental processes. Therefore, the adsorption and desorption isotherms of fipronil in clay loam, clay, sandy loam, sandy clay loam, sand and loamy sand soils at 25 and 50˚C was studied. The amount of fipronil adsorbed and desorbed by different soils was significantly influenced by the temperature. Adsorption was higher in clay loam, clay, sandy clay loam and sandy soil at 25°C, while sand soil and loamy sand soil at 50°C. The non-desorbed amount was greater at 25°C in different types of soil except for clay loam soil. The negative ΔG˚ indicated that the adsorption/desorption in different types of soil was spontaneous at different temperatures. The value of standard enthalpy change (ΔH˚) was positive in clay soil, sandy loam soil, sandy clay loam soil and loamy sand soil for adsorption and sandy loam soil, sand soil and loamy sand soil for desorption. Moreover, the standard entropy change (ΔS˚) was negative in soils for adsorption and desorption isotherms except clay loam soil. Adsorption and desorption isotherms trends as well as the values of the correlation coefficients indicated that the adsorption and desorption isotherms of fipronil in tested soils were fitted to the Freundlich model because the correlation coefficient is very close to 0.999.