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TECHNICAL REPORTS

Organic Compounds in the Environment

Enhancing Metolachlor Destruction Rates with Aluminum and Iron Salts during Zerovalent Iron Treatment

^a Department of Environmental Science, Kasetsart University, Bangkok, Thailand 10900
^b School of Natural Resources, University of Nebraska-Lincoln, 256 Keim Hall, Lincoln, NE 68583-0915

^* Corresponding author (scomfort{at}unl.edu).

Received for publication September 19, 2002. Pesticide-contaminated soil may require remediation to mitigate ground and surface water contamination. We determined the effectiveness of zerovalent iron (Fe⁰) to dechlorinate metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl ethyl) acetamide] in the presence of aluminum and iron salts. By treating aqueous solutions of metolachlor with Fe⁰, we found destruction kinetics were greatly enhanced when Al, Fe(II), or Fe(III) salts were added, with the following order of destruction kinetics observed: Al₂(SO₄)₃ > AlCl₃ > Fe₂(SO₄)₃ > FeCl₃. A common observation was the formation of green rusts, mixed Fe(II)–Fe(III) hydroxides with interlayer anions that impart a greenish-blue color. Central to the mechanism responsible for enhanced metolachlor loss may be the role these salts play in facilitating Fe(II) release. By tracking Al and Fe(II) in a Fe⁰ + Al₂(SO₄)₃ treatment of metolachlor, we observed that Al was readily sorbed by the corroding iron with a corresponding release of Fe(II). The manufacturing process used to produce the Fe⁰ also profoundly affected destruction rates. Metolachlor destruction rates with salt-amended Fe⁰ were greater with annealed iron (indirectly heated under a reducing atmosphere) than unannealed iron. Moreover, the optimum pH for metolachlor dechlorination in water and soil differed between iron sources (pH 3 for unannealed, pH 5 for annealed). Our results indicate that metolachlor destruction by Fe⁰ treatment may be enhanced by adding Fe or Al salts and creating pH and redox conditions favoring the formation of green rusts.

Abbreviations: HPLC, high performance liquid chromatography • GR, green rusts • SEM, scanning electron microscopy

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