Retention and mobility of deltamethrin in soils: 1. adsorption-desorption 1

Pesticide adsorption and desorption are important processes that influence the amount of pesticide retained by the soil matrix and its susceptibility to movement in the soil profile. In this study the adsorption-desorption behavior of the insecticide deltamethrin was investigated in soils with different physical and chemical properties. Four soils having different organic matter content, clay content, and pH were used along with acid-washed sand (reference material). An adsorption kinetic batch method was used for a wide range of deltamethrin concentrations and reaction times. Desorption was carried out following 360 h of adsorption using successive dilutions with 0.005 M CaCl2 background solution. This was followed by an extraction step using methanol. Based on the distribution coefficients (Kd) that were derived from adsorption isotherms, deltamethrin exhibited strong retention over time but varied extensively among soils. The Kd values after 1-day sorption ranged from 13 to 98 mL/g soil. With the exception of the reference sand material, adsorption was not kinetic. Deltamethrin adsorption was correlated positively with increasing cation exchange capacity and decreasing soil pH. Strong deltamethrin hysteresis was observed for all soils as depicted by discrepancies of adsorption from desorption isotherms. The extent of recovery or release of applied deltamethrin varied among soils and input concentrations, but it did not exceed 10.2% of the total amount adsorbed. Moreover, for all soils the total amount of deltamethrin retained following desorption ranged from 78 to 97% of total input. The only exception was for a nonacidic Vacherie soil (pH of 7.6), which showed the highest recovery with the lowest amount retained (39.5% of input). Low retention may be caused by solubilization of dissolved carbon. As a result of observed strong low recovery, we conclude that deltamethrin is not susceptible to leaching losses from the zone of application.