Thermodynamics of fluridone adsorption and desorption on three California soils

The thermodynamic parameters associated with fluridone [1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone] adsorption and desorption on three California soils were calculated from the temperature dependence of the adsorption reaction. The standard free energy change for the adsorption reaction on all three soils was negative, signifying a spontaneous reaction. The isosteric heat of adsorption and the partial molar enthalpy change indicated that van der Waals or physical forces were involved in fluridone adsorption. The enthalpy changes also showed that fluridone adsorption became weaker as the amount adsorbed increased on Yolo sandy clay loam (Typic Xerorthents) and Hesperia fine sandy loam (Typic Xerorthents) while the reverse was true on Stockton clay (not classified). Fluridone adsorption was correlated with clay content and increased as the time of exposure to the soils increased (i.e., the desorption isotherms exhibited hysteresis). The isosteric heats of adsorption calculated from the desorption isotherms were similar to those calculated from the adsorption isotherms indicating that there was no change in adsorption mechanisms between adsorption and desorption equilibrations. Thus, the thermodynamic parameters provided evidence that fluridone adsorption on the three soils was reversible and that the hysteresis between adsorption and desorption isotherms was not due to an irreversible adsorption reaction (chemical or coulombic adsorption). The increased retention of fluridone with time was attributed to the diffusion of fluridone molecules to relatively less accessible binding sites on clay particles as the time of fluridone exposure to the soils increased.