Metolachlor dechlorination by zerovalent iron during unsaturated transport

Permeable zerovalent iron (Fe0) barriers have become an established technology for remediating contaminated ground water. This same technology may be applicable for treating pesticides amenable to dehalogenation as they move downward in the vadose zone. By conducting miscible displacement experiments in the laboratory with metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; a chloroacetanilide herbicide] under unsaturated flow, we provide proof-of-concept for such an approach. Transport experiments were conducted in repacked, unsaturated soil columns attached to vacuum chambers and run under constant matrix potential (-30 kPa) and Darcy flux (approximately 2 cm d(-1)). Treatments included soil columns equipped with and without a permeable reactive barrier (PRB) consisting of a Fe0-sand (50:50) mixture supplemented with Al2(SO4)3. A continuous pulse of 14C-labeled metolachlor (1.45 mM) and tritiated water (3H2O) was applied to top of the columns for 10 d. Results indicated complete (100%) metolachlor destruction, with the dehalogenated product observed as the primary degradate in the leachate. Similar results were obtained with a 25:75 Fe0-sand barrier but metolachlor destruction was not as efficient when unannealed iron was used or Al2(SO4)3 was omitted from the barrier. A second set of transport experiments used metolachlor-contaminated soil in lieu of a 14C-metolachlor pulse. Under these conditions, the iron barrier decreased metolachlor concentration in the leachate by approximately 50%. These results provide initial evidence that permeable iron barriers can effectively reduce metolachlor leaching under unsaturated flow.