Dicamba (3,6-dichloro-2-methoxy benzoic acid) is a highly mobile pre- and post-emergence anionic herbicide that has been detected in ground water throughout the U.S. We determined the potential of zerovalent iron (Fe⁰) to remediate ground water contaminated with dicamba and its common biological degradation product, 3,6-dichlorosalicyclic acid (DCSA). Mixing an aqueous solution of 100 mM dicamba with 1.5% Fe⁰ (w/v) resulted in an 80% loss of dicamba within 12 h. Solvent extraction of the Fe⁰ revealed that dicamba removal was primarily by adsorption. Dicamba dechlorination was accomplished when the Fe⁰-dicamba mixture was augmented with either Al or Fe(III) salts (4 mM). Extracting this Fe⁰ treatment yielded an unknown degradation product. Fe⁰-treatment of DCSA removed 96% from solution with 20% molar release of Cl^-. When DCSA was treated with Fe⁰ plus Al or Fe(III) salts, DCSA destruction was 100% and we recovered 50% of DCSAs chlorides in solution. Extracting this Fe⁰ yielded the same unknown degradation product observed from Fe⁰ + Al or Fe(III) treatment of dicamba. These data indicated that Fe⁰ can either adsorb and/or dechlorinate dicamba and DCSA but removal mechanisms are dependent on Fe⁰ composition and may be related to the type of coordination complex formed on the iron surface. Mineralization experiments using ¹⁴C-labeled dicamba verified that 11% of the Fe⁰-adsorbed dicamba residues were available for subsequent biodegradation. This research indicates that anionic pesticides like dicamba and DCSA can react with Fe⁰ and possibly be treated with a PRB.

Abbreviations: Dicamba, 3,6-dichloro-2-methoxybenzoic acid; Fe⁰, zerovalent iron; DCSA, 3,6-dichlorosalicylic acid; Koc, organic carbon partition coefficient; HPLC, high performance liquid chromatography; LSC, liquid scintillation counting; ZPC, zero point of charge; IC, ion chromatography; a.i., active ingredient; VOCs, volatile organic compounds; PRB, permeable reactive barrier.