Dechlorination of the Chloroacetanilide Herbicides Alachlor and Metolachlor by Iron Metal

Granular iron metal has been found to cause the reductive dechlorination of two important chloroacetanilide herbicides, alachlor and metolachlor. Aqueous solutions (113 mL) of the herbicides were contacted with 40 g of granular cast iron (CCI coarse, 40 mesh) with mild agitation at room temperature. First-order degradation rate constants were 0.12 and 0.10 h - 1 for 10 mg/L solutions of alachlor and metolachlor, respectively. A two-site, rate-limited sorption and first-order degradation model was applied to both batch data sets, with excellent agreement for alachlor and fair agreement for metolachlor. The products of the reaction were chloride (84% mass balance for alachlor and 68% for metolachlor) and dechlorinated acetanilides. Supported by GC/EIMS analysis, two sequential reactions may have occurred for alachlor, hydrogenolysis of the chloroacetyl group followed by an N-dealkylation reaction. However, only one product was confirmed by GC/FID, and the mechanism for the N-dealkylation reaction is unknown. Metolachlor was found to produce one dechlorinated product consistent with hydrogenolysis. These results are encouraging, as granular iron may be used at spill sites contaminated with these herbicides and related compounds.