Transformation of Herbicide Propachlor by an Agrochemical Thiourea

Propachlor and other chloroacetanilide herbicides are frequently detected contaminants of groundwater and surface water in agricultural regions. The purpose of this work was to develop a new approach to remove propachlor residues from the environment via chemical remediation by the nitrification inhibitor thiourea. The transformation processes of propachlor and thiourea mixed in aqueous solution, sand, and soil were elucidated. Analysis of transformation products and reaction kinetics indicated that an S_N2 nucleophilic substitution reaction occurred, in which the chlorine of propachlor was replaced by thiourea, detoxifying the herbicide. It appears that propachlor undergoes a catalytic reaction in sand or soil amended with thiourea, which results in a significantly accelerated transformation rate as compared to the reaction in aqueous solution. The second-order reaction process was examined at different temperatures to investigate the role of the activation energy. The enthalpy of activation (ΔH) for the reaction of propachlor with thiourea was demonstrated to be significantly lower in sand than in aqueous solution, which provides evidence that a catalytic transformation mechanism occurs in thiourea-amended sand. The chemical reaction rate increased proportionally to the amount of thiourea added to the sand. Column experiments further suggested that the remediation strategy could be used to remove propachlor residues from sand or soil to reduce leaching and prevent contamination of surface water and groundwater.