Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2004
Publication Date: 11/10/2004
Citation: Zheng, W., Yates, S.R., Papiernik, S.K., Guo, M. 2004. Transformation of herbicide propachlor by an agrochemical thiourea. Environmental Science and Technology. 38:6855-6860.
Interpretive Summary: Propachlor is a chloroacetanilide pesticide extensively used in the U.S. to control annual grasses, broad-leaved weeds in corn and soybeans, and several other crops. This has led to frequent detection of chloroacetanilide residues in ground and surface waters. Since chloroacetanilide pesticides are toxic to a wide range of organisms, exposure to these chemicals via contamination of water or agricultural products, even at very low concentration, would be a great threat to public health and environment. To minimize pesticide residue in the environment, it is important to develop and improve the current pesticide management strategies and develop methods to remediate contaminated soil and water. This study explored the potential of thiourea, a nitrification inhibitor, to degrade propachlor in aqueous solution and soil. This involved an investigation of propachlor hydrolysis and nucleophilic transformation in the presence of thiourea. The effect of temperature on propachlor degradation was also investigated. This information will be useful in developing approaches to reduce propachlor movement from the treatment zone.
Technical Abstract: A new approach was developed to remove propachlor residues, a commonly used herbicide, from the environment via chemical remediation by thiourea, an agrochemical used to inhibit nitrification. The reaction process and mechanism were elucidated for reaction of propachlor and thiourea in aqueous solution, soil, and sand. The transformation products and analysis of the kinetics indicate that a SN2 nucleophilic substitution reaction occurred, in which the chlorine attached to propachlor was replaced by thiuronium ion as conjugated product of thiourea. As a result of the reaction, the herbicide was degraded. In thiourea-amended soil, a catalytic reaction process leads to a significantly increased transformation rate compared to the rate in aqueous solution. Second-order reaction rates for propachlor with thiourea were examined at different temperatures to investigate the role of the activation energy in different reaction media. Values of enthalpy of activation (?H) for the reaction of propachlor with thiourea were demonstrated to be statistically different in aqueous solution and sand. This observation supports the hypothesis that, in sand, a catalytic transformation mechanism occurs that is absent in aqueous solution. Chemical remediation of propachlor increased proportionally with increasing level of thiourea amendment added to the sand. Column experiments further show that the reaction could be used to remove propachlor residues in soil, which would reduce propachlor leaching and prevent groundwater contamination.