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The mechanism and pathway through which 2,4-dinitrotoluene (DNT) is reduced with elemental iron were investigated through batch experiments performed utilizing the same iron surface area, with high-purity iron powder and Master Builders scrap iron. In addition to different kinetics and adsorption patterns, the distribution of two intermediates, 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT), contrasted sharply. This suggests that different mechanisms are involved in DNT reduction with pure iron and scrap iron. We hypothesized that exposed graphite in scrap iron transferred reductants from iron to adsorbed nitroaromatic molecules. This hypothesis was supported by an experiment conducted using two-compartment dialysis cells in which ONT and pure iron powder were separated by a graphite sheet. Results indicate that graphite-mediated, indirect reduction of DNT occurred primarily through reduction of the ortho nitro group to form 2A4NT, whereas DNT reduction at the iron (hydr/oxide) surface occurred via para nitro reduction to give 4A2NT. Based on pH and product analysis, atomic hydrogen probably accounted for most of the reducing equivalents that passed through the graphite, reacting with adsorbed DNT mainly through ortho nitro reduction. In contrast, electron was a minor fraction of the reducing equivalents, reducing DNT mainly through para nitro reduction. The implications of graphite as a reaction site and conductor of electron and atomic hydrogen are discussed with respect to treatment processes involving iron.

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Black carbon (BC) is an important class of geosorbents that influence the fate and transport of organic pollutants. It is commonly assumed that molecules sorbed to BC are chemically inert. Here we show that this is not true for redox-sensitive sorbates such as nitro-aromatic compounds. In the presence of graphite or n-hexane soot as a BC material, the reduction of 2,4-dinitrotoluene (DNT) to 2,4-diaminotoluene by dithiothreitol was greatly accelerated. The para and ortho nitro groups of graphite or soot-sorbed DNT had an approximately equal probability of being reduced. This (1:1) regio-selectivity is different from that when DNT is reduced in homogeneous solution. That is, sorption to BC altered both the kinetics and pathway of DNT reduction. Transformation of hexahydro-1,3,5-trinitro-1,3,5-triazine, a nonaromatic nitro compound, by dithiothreitol was also enhanced by graphite, with concurrent formation of formaldehyde. We propose that BC can catalyze the reduction of nitro compounds because it contains microscopic graphitic (graphene) domains, which are both sorption sites and electron conductors. The environmental significance and potential applications of these findings are discussed.

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