Adsorption-desorption and leaching of phenylurea herbicides on soils

Adsorption-desorption equilibria of phenylurea herbicides (chlortoluron, isoproturon, metobromuron, chloroxuron, difenoxuron) were determined in two different soils. Organic carbon content of the soils ranged from 1.1-5.6% and the clay fraction from 25-30%. Spiked aqueous suspensions of soils were centrifuged and urea derivative concentrations in the aqueous phase were determined using a HPLC-UV system. Adsorption isotherms conformed to the Freundlich equation. Freundlich K-f values indicated that organic carbon was the main factor affecting urea sorption in the studied soils. The R(f) values calculated from the soil column displacement closely correlated with the adsorption K-f values. Results indicated that chloroxuron and difenoxuron were sorbed to a larger extent than the other three ureas on the two soils; as expected their mobilities through soil columns were very limited. The similarity between the molecular structures of chloroxuron and difenoxuron enables us to assume the phenoxy group to be the main structural feature in the adsorption phenomenon. The relationships between sorption, leaching and some characteristics of urea molecules such as water solubility and octanol-water partition coefficient were also examined.