Field-scale remediation of a metolachlor-contaminated spill site using zerovalent iron

Pesticide spills are common occurrences at agricultural cooperatives and farmsteads. When inadvertent spills occur, chemicals normally beneficial can become point sources of ground and surface water contamination. We report results from a field trial where approximately 765 m³ of soil from a metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] spill site was treated with zerovalent iron (Fe⁰). Preliminary laboratory experiments confirmed metolachlor dechlorination by Fe⁰ in aqueous solution and that this process could be accelerated by adding appropriate proportions of Al₂(SO₄)₃ or acetic acid (CH₃COOH). The field project was initiated by moving the stockpiled, contaminated soil into windrows using common earth-moving equipment. The soil was then mixed with water (0.35-0.40 kg H₂O kg^-1) and various combinations of 5% Fe⁰ (w/w), 2% Al₂(SO₄)₃ (w/w), and 0.5% acetic acid (v/w). Windrows were covered with clear plastic and incubated without additional mixing for 90 d. Approximately every 14 d, the plastic sheeting was removed for soil sampling and the surface of the windrows rewetted. Metolachlor concentrations were significantly reduced and varied among treatments. The addition of Fe⁰ alone decreased metolachlor concentration from 1789 to 504 mg kg^-1 within 90 d, whereas adding Fe₀ with Al₂(SO₄)₃ and CH₃COOH decreased the concentration from 1402 to 13 mg kg_-1. These results provide evidence that zerovalent iron can be used for on-site, field-scale treatment of pesticide-contaminated soil.