Hexahydro-1,3,5-trinitro-1,3,5-triazine mineralization by zerovalent iron and mixed anaerobic cultures.

Soil microcosms were used to evaluate the potential benefits of an integrated microbial-Fe0 system to treat groundwater contamination by RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). Microcosms amended with both Fe0 filings and municipal anaerobic sludge mineralized RDX faster and to a greater extent than separate treatments, with up to 51% 14CO2 recovery after 77 d. The nitroso byproducts 1,3-dinitro-5-nitroso-1,3,5-triazacyclohexane (MNX), 1,3-dinitroso-5-nitro-1,3,5-triazacyclohexane (DNX), and 1,3,5-trinitroso-1,3,5-triazacyclohexane (TNX) were detected in all microcosms, although these compounds never accumulated above 5% of the added RDX on a molar basis. A soluble intermediate that was tentatively identified as methylenedinitramine [(O2NNH)2CH2] was relatively persistent, although it accumulated to a much lower extent in combined-treatment reactors than in sets with Fe0 or sludge alone. Some of the radiolabel was bound to soil and Fe0 and could not be extracted with CH3CN. This fraction, which was recovered by combustion with a biological oxidizer, was also found at lower concentrations in combined-treatment reactors. This work suggests that permeable reactive Fe0 barriers might be an effective approach to intercept and degrade RDX plumes and that treatment efficiency might be enhanced by biogeochemical interactions through bioaugmentation.

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.

Potential of activated carbon to decrease 2,4,6-trinitrotoluene toxicity and accelerate soil decontamination.

Activated carbon can be used to decrease 2,4,6-trinitrotoluene (TNT) toxicity and promote bioremediation of highly contaminated soil. Adding activated carbon at 0.25, 0.75, and 1.0% (w/w) to Sharpsburg soil contaminated with 500, 1,000, and 2,000 mg TNT/kg decreased concentrations of TNT and its transformation products in soil solution to 5 mg/L or less, resulting in low toxicity to corn plants (Zea mays L.) and soil microorganisms. As much as 50% of the added TNT was rapidly bound to the soil-activated carbon matrix. Simultaneous accumulation of 2,4,6-trinitrobenzaldehyde (TNBAld) indicated that the activated carbon promoted oxidation of TNT. Some of the TNBAld was further oxidized to 1,3,5-trinitrobenzene, followed by reduction to 3,5-dinitroaniline. Reversibly bound TNT was gradually transformed to 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene, and both were bound to the soil-activated carbon matrix. The transformation and binding of TNT to soil were further promoted by incorporating shredded corn plants after growing for 52 d in the activated carbon-amended soil. After 120 d, these amendments reduced extractable TNT and transformation products by 91% in soil containing 2,000 mg TNT/kg, compared to 55% in unamended soil. These results demonstrate the potential use of activated carbon in combination with plants to promote in situ bioremediation of soils highly contaminated with explosives.