Adsorption-Desorption of Hexaconazole in Soils with Respect to Soil Properties, Temperature, and pH

Abstract

The effect of temperature and pH on adsorption-desorption of fungicide hexaconazole was studied in two Malaysian soil types; namely clay loam and sandy loam. The adsorption-desorption experiment was conducted using the batch equilibration technique and the residues of hexaconazole were analysed using the GC-ECD. The results showed that the adsorption-desorption isotherms of hexaconazole can be described with Freundlich equation. The Freundlich sorption coefficient (K d ) values were positively correlated to the clay and organic matter content in the soils. Hexaconazole attained the equilibrium phase within 24 h in both soil types studied. The adsorption coefficient (Kd) values obtained for clay loam soil and sandy loam soil were 2.54 mL/g and 2.27 mL/g, respectively, indicating that hexaconazole was weakly sorbed onto the soils due to the low organic content of the soils. Regarding thermodynamic parameters, the Gibb's free energy change (ΔG) analysis showed that hexaconazole adsorption onto soil was spontaneous and exothermic, plus it exhibited positive hysteresis. A strong correlation was observed between the adsorption of hexaconazole and pH of the soil solution. However, temperature was found to have no effect on the adsorption of hexaconazole onto the soils; for the range tested. Keywords:Hexaconazole Adsorption Desorption Freundlich Kinetic IntroductionAdsorption and desorption are the main processes that affect the mobility, persistence, degradation, transport, and dissipation of pesticides and contaminants in the soil. Understanding the effects of these processes is important in order to obtain early information for predicting the environmental behaviour of pesticides and contaminants. Adsorption is a process in which the pesticide forms chemical bonds with colloidal materials such as soil organic matter and clay particles (Wauchope et al. 2002). Pesticide soil/solution distribution coefficients (K d values), commonly referred to as pesticide soil sorption values, are used to predict soil mobility of the compounds (Weber et al. 2004).Investigations on adsorption-desorption phenomena of pesticides in soil have been reported for various soil types (Liu et al. 2010, Singh and Singh 2015, Rani and Sud 2015. It has been shown that adsorption and desorption of pesticides in soils depend on soil properties such as pH, organic matter/clay content, chemical properties of the pesticide, and the environment Maznah 2006, Rani andSud 2015). Aside from organic matter and clay content, the adsorption process is influenced by the size, shape, configuration, molecular structure, chemical functions, solubility, polarity, environmental temperature, and acid-base nature of the pesticide molecule itself (Arias-Estevez et al. 2006, Wauchope et al. 2002.The effect of pH on the adsorption of pesticides depends on the soil composition and the characteristics of the compound (Kah and Brown 2006). The pH related dependence of sorption is partly derived from the different proportions of ionic and neutral forms of the pesticide pre...