Imidacloprid adsorption by soils treated with humic substances under different pH and temperature conditions

The mobility of a pesticide in soil is determined by the extent and strength of sorption, which is influenced by either the existing soil humus or exogenous humic substances. Exogenous humic acids (HAs) were added to soil to enhance the amount of soil organic carbon (SOC) by 2.5, 5.0 and 10.0 g kg(-1). Imidacloprid sorption of the treated soils was studied at three pH levels (4.5, 6.0 and 7.5) and two temperatures (15 and 25 degrees C). Soil imidacloprid adsorption was related to pH and the type and quantity of added HAs. Humic acid (HA) and fulvic acid (FA) derived from peat had different effects on adsorption of imidacloprid. When soil solution pH was 6, the amount of adsorbed imidacloprid was enhanced with increasing exogenous HA. On the contrary, the amount of adsorbed imidacloprid decreased with increasing quantity of exogenous FA. Adsorption of imidacloprid in the FA treatment at 5.0 and 10.0 g kg(-1) was lower than the controls (untreated soil or treatment with HAs at 0 g kg(-1)) when the soil solution pH was 6.0. However adsorption of imidacloprid in the HA treatment was higher than the controls. Imidacloprid adsorption was usually higher under lower pH and/or lower temperature at same condition. Imidacloprid sorption fitted the Freundlich isotherm, indicating that exogenous humic substances influenced adsorption of imidacloprid, which in turn was affected by environmental conditions such as pH and temperature. Thus, exogenous HA can be used to control the mobility of soil pesticide under appropriate conditions to decrease pesticide pollution diffusion and probably increase effectiveness of pesticides.