Graphite- and Soot-Mediated Reduction of 2,4-Dinitrotoluene and Hexahydro-1,3,5-trinitro-1,3,5-triazine

Abstract

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.

Biochar‐mediated reductive transformation of nitro herbicides and explosives

“…The bottles were then filled completely with deoxygenated phosphate buffer solution (20 mM, pH 7). Phosphate is known to be a good buffer and to not significantly affect the reductive reaction in the presence of BC [23,25,26]. The bottles were prepared in duplicate to ensure reproducibility and to estimate experimental errors.…”

“…Due to the low volatility of DNT and RDX, approximately 50 ml of headspace remained in the vial. Conditions for batch reduction experiments with DNT and RDX have been described in detail previously [23].…”

“…Also using high-performance liquid chromatography, we determined DNT, RDX, and their reduction products (2A4NT, 4A2NT, and DAT for DNT, formaldehyde for RDX). The analytical methods and conditions for the quantification of DNT, RDX, and their daughter products are described in detail elsewhere [23].…”

“…Graphite and GAC are two BC materials that had been shown previously to promote the reduction of nitro and halogenated aromatic compounds [23][24][25][26]. Without graphite or GAC, direct reduction of pendimethalin by dithiothreitol in the aqueous phase was relatively slow, with approximately 20% of pendimethalin transformed over 120 min (Fig.…”

“…We showed that reduction of nitroaromatic compounds, such as DNT, and heterocyclic nitramines, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), by a thio reductant was greatly enhanced by BC materials such as soot or graphite [23]. More recently, we observed that reduction of DNT and 2,4-dibromophenol by H 2 S was enhanced by soot and granular activated carbon (GAC) [24].…”