Pesticide-contaminated soil may require remediation to mitigate ground and surface water contamination. We determined the effectiveness of zerovalent iron (Fe(0)) 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 Fe(0), we found destruction kinetics were greatly enhanced when Al, Fe(II), or Fe(II) 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 Fe(0) + 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 Fe(0) also profoundly affected destruction rates. Metolachlor destruction rates with salt-amended Fe(0) 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 Fe(0) treatment may be enhanced by adding Fe or Al salts and creating pH and redox conditions favoring the formation of green rusts.

Soils at former munitions production facilities are often contaminated with hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX). Contamination can be excessive and soils often contain precipitated or solid‐phase RDX, resulting in soil solution concentrations at or near saturation. Sorption and long‐term fate must be understood to predict RDX availability and develop remediation strategies. We characterized RDX sorption and availability in Sharpsburg (a fine, montmorillontic, mesic Typic Argiudoll) surface soil by equilibrating the soil with 32 mg RDX L−1 (spiked with 14C‐labeled RDX) for 168 d; similar experiments were performed with contaminated and uncontaminated subsurface soils. Surface soils exhibited rapid RDX sorption with 34% of the 14C sorbed within 30 min. This sorbed fraction increased to only 37% at 168 d. During the 168‐d equilibration, readily available RDX (sorbed RDX extractable with 3 mM CaCl2) decreased from 75 to 52%, while potentially available RDX (acetonitrile‐extractable) increased from 24 to 32%. Carbon‐14 in the 0.5 M NaOH‐extractable organic matter fraction increased from 0.8 (T = 30 min) to 3.8% (T = 168 d). Little 14C was removed after eight extractions with 10% KOH in ethanol. Eight percent of the 14C‐label was unextractable (bound) residue at 168 d; no 14C‐bound residue formed in surface soil when solid‐phase RDX was present in the equilibrating solution. Our experiments indicated limited RDX sorption and transformation in the Sharpsburg surface and subsurface soils. Most of the sorbed 14C was potentially available for transport, indicating the importance of remediating RDX‐contaminated soil to protect groundwater quality.

In situ chemical reduction of clays and iron oxides in subsurface environments is an emerging technology for treatment of contaminated groundwater. Our objective was to determine the efficacy of dithionite-reduced sediments from the perched Pantex Aquifer (Amarillo, TX) to abiotically degrade the explosives RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and TNT (2,4,6-trinitrotoluene). The effects of dithionite/buffer concentrations, sediments-solution ratios, and the contribution of Fe(II) were evaluated in batch experiments. Results showed that reduced Pantex sediments were highly effective in degrading all three high explosives. Degradation rates increased with increasing dithionite/buffer concentrations and soil to solution ratios (1:80-1:10 w/v). When Fe(II) was partially removed from the reduced sediments by washing (citrate-bicarbonate buffer), RDX degradation slowed, but degradation efficiency could be restored by adding Fe(II) back to the treated sediments and maintaining an alkaline pH. These data support in situ redox manipulation as a remedial option for treating explosive-contaminated groundwater at the Pantex site. Published by Elsevier Ltd.

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