Complete reduction of TNT and other (poly)nitroaromatic compounds under iron reducing subsurface conditions

Contamination of soils and aquifers with (poly)nitroaromatic compounds ((P)NACs) is a widespread problem. This work demonstrates that (P)NACs su ch as the explosive 2,4,6-trinitrotoluene (TNT) can be completely reduced to the corresponding aromatic polyamines by Fe(ll) present at the surface of Fe(III)(hydr)oxides or, less efficiently, by hydroquinone moieties of (natural) organic matter in the presence of H2S. The reduction kinetics of (P)NACs were investigated in sterile batch systems as well as in columns containing either FeOOH-coated sand and a pure culture of the iron-reducing bacterium Geobacter metallireducens or ferrogenic consortia in aquifer sediments. The relative reactivities as well as the competition behavior of (P)NACs in batch and column systems, respectively, correlated well with their one-electron reduction potentials, E-h(1)', which we determined for TNT and its aminonitrotoluene transformation products. A similar reactivity pattern of (P)NACs was found irrespective of the processes that (re)generated the surface-bound Fe(II), i.e., adsorption of Fe(II) from aqueous solution or microbial reduction of Fe(III)(hydr)oxides. The apparent stability of the toxic arylamine products under ferrogenic conditions may compromise intrinsic attenuation as an acceptable remediation option for (P)NAC contaminated anoxic aquifers. Iron-reducing conditions would, however, be favorable as a first step in a two-stage anaerobic/aerobic treatment of PNAC contaminated sediments since aromatic polyamines are biodegradable and/or bind irreversibly to the solid matrix under oxic conditions.