Reduction of nitroaromatic pesticides with zero-valent iron

Reduction of eleven nitroaromatic pesticides was studied with zero-valent iron powder. Average half-lives ranged from 2.8 to 6.3 h and the parent compounds were completely reduced after 48-96 h. The di-nitro groups of the 2,6-dinitroaniline herbicides were rapidly reduced to the corresponding diamines, with a negligible amount of partially reduced monoamino or nitroso products. Low levels of de-alkylated products were observed after 10 days. The nitro group of the organophosphorus insecticides was reduced dominantly to the monoamines but in a slower rate than the 2,6-dinitroanilines. A trace amount of oxon products was found. Reduction of nitro to amino was also the predominant reaction for the diphenyl ether herbicides. Aromatic de-chlorination and de-alkylation were minor reactions. These amine products were more stable than the parent compounds and 60% or more of the amines were detected after two weeks. Humic acid decreased the reduction rates of pendimethalin, and dichlone (a known quinone redox mediator) counteracted the effect of humic acid on the reactivity. Storage of iron powder under air decreased the reactivity very rapidly due to iron oxidation. Repeated use of iron powder also showed similar results. The reduced activity of air-oxidized iron was recovered by purging with hydrogen, but not nitrogen. Integration of iron powder with hydrogen- and quinone-producing microbial technologies may be a viable mean for remediation of highly oxidized xenobiotics in the environment.