Abstract
Adsorption characteristics of the radiolabeled pesticide aldicarb (2-methyl-2-(methyl-14C-thio)-propionaldehyde-O-methycarbamoyO-oxime) on loamy sand and its mobility through a water-saturated soil column were studied. This study emphasizes the effect of soil organic carbon content and of dissolved humic materials on the sorption characteristics of aldicarb. Dissolved humic substances at 25 mg/L present a very small binding affinity for aldicarb (“association constant” ≈ 20 [nmol/g carbon]/[nmol/g water]) and do not significantly affect the sorption rate for this pesticide. Freundlich isotherms were determined for aldicarb sorption on the original soil (% o.c. = 2.2) and on soil samples treated to reduce the initial soil organic carbon content (% o.c. = 0.8). If linear isotherms are assumed, the adsorption coefficient KD is nearly three-fold higher for the original soil than for the treated soil samples. The ratios of the KD values obtained for adsorption in the presence and in the absence of dissolved humic substances (25 mg/L) were 1.3 and 1.4, respectively, for the original and for the treated soil samples: this indicates that humic substances could modify only slightly aldicarb transport in soil. Effluent concentration profiles for the movement of the radiolabeled pesticide through a 30-cm long saturated soil column indicate a retardation factor R = 1.4. This R value is related to hydrolysis product of aldicarb, since no detectable parent compound was found in the column effluent. Results showed 45% loss of the initial radioactivity applied on the soil column: this could be attributed to irreversible adsorption and to biodegradation of the pesticide compounds.