Improved retention of imidacloprid (Confidor®) in soils by adding vermicompost from spent grape marc

Batch sorption experiments of the insecticide imidacloprid by ten widely different Spanish soils were carried out. The sorption was studied for the active ingredient and its registered formulation Confidor®. The temperature effect was studied at 15 °C and 25 °C. The addition of a vermicompost from spent grape marc (natural and ground), containing 344 g kg−1 organic carbon, on the sorption of imidacloprid by two selected soils, a sandy loam and a silty clay loam, having organic carbon content of 3.6 g kg−1 and 9.3 g kg−1, respectively, was evaluated. Prior to the addition of this vermicompost, desorption isotherms with both selected soils, were also performed. The apparent hysteresis index (AHI) parameter was used to quantify sorption–desorption hysteresis. Sorption coefficients, Kd and Kf, for the active ingredient and Confidor® in the different soils were similar. Sorption decreased with increasing temperature, this fact has special interest in greenhouse systems. A significant correlation (R2=0.965; Pb0.01) between Kf values and the organic carbon (OC) content was found, but some soils showed higher sorption coefficients than that expected from their OC values. The normalized sorption coefficients with the soil organic carbon content (Koc) were dispersed and low, implying that other characteristics of soils could contribute to the retention capacity as well. The spent grape marc vermicompost was an effective sorbent of this insecticide (Kf=149). The sorption of imidacloprid increased significantly in soils amended with this vermicompost. The most pronounced effect was found in the sandy loam soil with low OC content, where the addition of 5% and 10% of vermicompost increased Kf values by 8- and 15-fold, respectively. Soil desorption of imidacloprid was slower for the soil with the higher OC and clay content.