Partially reduced nitrotoluenes

Figure 8. Perspectives on the biodegradation of TNT. Heavy arrows indicate a remediation concept which is suggested on the basis of the current knowledge. Intermediates of nitro group reduction give rise to partially reduced nitrotoluenes (PRN), which are depicted in Fig. 10. Other reactions of denitration of TNT are discussed in the text.

To our present knowledge such partially reduced nitrotoluenes and azoxy compounds are not utilized by aerobic microorganisms. The great number of potential products are illustrated in Figure 10. Those, which were actually identified are marked by shaded aromatic rings. The insert indicates the formation of secondary products such as azoxy compounds. 

Figure 12. Under aerobic or limited oxygen conditions partially reduced nitrotoluenes (PRN) and secondary condensation products are generated. These are completely reduced to TAT under strict anaerobic conditions.

Complete reduction of TNT and its partially reduced derivatives can also be accomplished when these compounds are present in soil under strict anaerobic conditions (Fig. 14). Notably, after 4 days TNT and partially reduced nitrotoluenes have completely disappeared from the aqueous phase, whereas considerable amounts of these contaminants (approx. 180 mg TNT and 40 mg ADNT per kg ) are still present in soil. Obviously, at these lower levels of contamination the process of desorption becomes slower than the reduction reaction. The reduction of these compounds in heaps or compost piles, where aerobic or semiaerobic conditions prevail, remains incomplete. Products of partial reduction such as mononitro-diamino- and dinitro-monoamino-toluenes and azoxy compounds are extractable from soil. In contrast to the observations made in the absence of soil (Fig. 13), TAT as a product of complete reduction of TNT is irreversibly bound to the soil matrix and thus remains undetectable in the aqueous phase (23). Interestingly, this irreversible binding does not require the participation of O2. Therefore, primary adsorption of TAT corresponds to the well-known mechanism of intercalation of diamines into clay minerals such as montmorillonite (18,48). 

Nitroaromatic compounds can be partially reduced to the corresponding hydroxy-lamines, which are then hydrolysed to a catechol and ammonium (Fig. 4A). This degra-dative pathway, which will be discussed in detail below, has been described for the mineralization of 4-nitrobenzoate by Comamonas acidovorans NBA-10 (20, 21) and for degradation of 4-nitrotoluene by two Pseudomonas species (26, 51). In the latter case, the substrate is first oxidized to 4-nitrobenzoate and further degraded as depicted in Figure 4A. 

Dinitrotoluene is oxidized to 2,4-dinitrobenzoic acid [610-30-0] by potassium permanganate or chromic acid, and is reduced to 2,4-diaminotoluene by iron and acetic acid. It is reduced partially by zinc chloride and hydrochloric acid to 2-amino-4-nitrotoluene [99-55-8] and by ammonium sulfide to 4-amino-2-nitrotoluene [119-32-4],... 

The reduction of 2,4-diamino-6-nitrotoluene was therefore investigated with hydrogenase purified from Clostridium pasteurianum and with carbon monoxide dehydrogenase partially purified from Clostridium thermoaceticum (39). In contrast to methods described in earlier publications, a test system was applied by which the nitro-reducing activity could be directly measured the consumption of DANT was assayed spectrophotometrically at 325 nm. 

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