Complete Reduction of TNT and Other (Poly)nitroaromatic Compounds under Iron-Reducing Subsurface Conditions

Abstract

Contamination of soils and aquifers with (poly)nitroaromatic compounds ((P)NACs) is a widespread problem. This work demonstrates that (P)NACs such as the explosive 2,4,6-trinitrotoluene (TNT) can be completely reduced to the corresponding aromatic polyamines by Fe(II) 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, ‘, 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.

SERDP ER-1421 Abiotic and Biotic Mechanisms Controlling In Situ Remediation of NDMA: Final Report

“…The role of the 2:1 smectite clays may be significantly different for NDMA degradation, since there is no direct link between NDMA degradation and adsorbed ferrous iron (which there is for TCE). In a similar study, reduction of nitroaromatics was caused both by adsorbed iron on edge surfaces of clays as well as structural iron in the 2:1 smectite clays (Hofstetter et al 1999(Hofstetter et al , 2003.…”

“…The role of the 2:1 smectite clays may be significantly different for NDMA degradation, since there is no direct link between NDMA degradation and adsorbed ferrous iron (which there is for TCE). In a similar study, reduction of nitroaromatics was caused both by adsorbed iron on edge surfaces of clays as well as structural iron in the 2:1 smectite clays (Hofstetter et al 1999(Hofstetter et al , 2003 There may be a small amount of H 2 production at magnetite surfaces in the reduced sediment, which could catalyze NDMA degradation rate through reaction with H 2 . Our previous research has shown that limited quantities of H 2 can be produced in the natural subsurface abiotically by release from mafic minerals (Stevens and McKinley 2000).…”

Groundwater Annual Status Report for Fiscal Year 1999

“…They are mobile in groundwater and adsorption onto aquifer material is not significant (Schwarzenbach et al 1993). The highest concentrations of RDX (84 pg/L) and HMX (12 pg/L) were measured in a groundwater sample collected from 1047 ft, whereas the highest concentration of TNT (19 pg/L) was measured at 867 ft. Degradation products of TNT that were detected include 4-A-2,6-DNT and 2-A-4,6-DNT, which are stable under anaerobic conditions (Hofstetter et al 1999). Other HE-related compounds analyzed for but not detected include 1,3-dinitrobenzene, tetryl, nitrobenzene, 2,6-dinitrotoluene, 2,4-dinitrotoluene, 2-nitrotoluene, 4-nitrotoluene, and 3Znitrotoluene.…”

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

“…67,68 The degradation process can also be affected by a number of environmental conditions, such as pH, and the presence of cations or organic material in the system. 69 Alternatively, RDX undergoes both photolysis and hydrolysis to form several byproducts. 70,71 Several of these byproducts (e.g., 4-nitro-2,4-diazabutanal) are soluble and do not hydrolyze; however, once RDX degrades into amide intermediates, the amides rapidly undergo hydrolysis.…”