*2.7.2.1. Bacterial degradation of 4-nitrotoluene*

Nitrotoluenes may be subjected to reductive pathways (formation of aminotoluenes) [70] or partial reductive pathway, wherein a nitro group is reduced to hydroxyl amino group and finally releases ammonia [71–72]. For example, during the degradation of 4-NT by *Pseudomo‐ nas* sp., initially 4-NT is converted into 4-nitrobezoic acid via the formation of 4-nitrobenzyl alcohol and 4-nitrobenzaldehyde. Then the nitro group is partially reduced to hydroxylamino derivative (rather than amino derivative), which is further converted to protocatechuate without the utilization of oxygen and release of ammonia [71, 72]. This type of mechanism was first reported for the degradation of 4-nitrobenzoate by *C. acidovorans* NBA-10 [69]. In yet another mechanism of 4-NT metabolism by *Micobacterium* sp., 4-NT was first converted to 4 hydoxyl aminotoluene followed by 6-amino-*m*-cresol. Here ammonia is released only after the

An oxidative pathway is reported for 2-NT degradation by *Acidovorax* JS42 (Figure 7), wherein the initial oxidation of the aromatic ring takes place to form methylcatechols by simultaneous incorporation of both atoms of molecular oxygen and subsequent removal of the nitro group

The role of mono-oxygenases and dioxygenases in the removal of nitro group from *p*-nitro‐

The toluene mono-oxygenase encoded by TOL plasmid oxidizes only the methyl group of 3- NT and 4-NT but not of the 2-NT [77]. Toluene dioxygenase from *Pseudomonas putida* F1 and *Pseudomonas* sp. strain JS-150 oxidatively attacks on the methyl group of 2- and 3-NT to form corresponding nitrobenzylalcohols. The enzyme, however, attacks on the aromatic ring of 4- NT to produce 2-methyl-5-nitrophenol and 3-methyl-6-nitrocatechol [81]. In both cases (either with toluene mono-oxygenase or dioxygenase as described above), the nitro group was not removed from the benzene ring and mononitrotoluene isomers did not serve as growth substrate. Degradations of monosubstituted 2-, 3-, and 4-nitrotoluenes were also reported from

The two strains of C*omamonas* JS47 and JS46 capable of degrading 4-nitrobenzoate and 3 nitrobenzoate respectively were immobilized on alginate beads jointly and separately, and these beads were loaded in the reactor and fed to different regimes of alternating nitrobenzoate isomer or mixed nitrobenzoate isomer. Through this experiment, it was deduced that same beads containing both strains were able to recover faster from change in input composition

as nitrite by the action of a dioxygenase enzyme [64, 65, 73].

phenol has also been reported from a *Pseudomonas* sp. [74–76].

**Figure 7.** Degradation of 2-NT by the formation of 3-methylcatechol in *Acidovorax* sp. JS 42.

an adapted activated sludge system [79].

than different beads containing different strains [80].

ring cleavage [72].

60 Wastewater Treatment Engineering

Two different pathways are reported in bacterial strains (as shown in Figure 8). In aerobic methyl group oxidation (*Pseudomonas* sp. strain TW3), initially 4-NT is converted to 4 nitrobenzoic acid, and then the nitro group is partially reduced to hydroxylamine derivative, which is further converted to protocatechuate without the utilization of oxygen but with concomitant release of ammonia [81]. In another mechanism, 4-NT degradation is followed by the formation of 4-hydroxylaminotoluene.

**Figure 8.** Degradation pathways for 4-nitrotoluene. Aerobic partial reduction rout is present in *Mycobacterium* sp. strain HL-4NT-1. Aerobic methyl group oxidation rout is reported in *Pseudomonas* strains TW3 and strain 4NT.

There are few well-characterized bacterial strains that degrades or biotransforms more than one mono-nitro compounds. 2-NT and 4-NT could transform to their corresponding amino‐ toluenes and hydroxyl nitrotoluenes (pathways A and B, Figure 9) in *P. putida* OU83. Both oxidative as well as reductive attack is reported during metabolism of 3-NT [71]. Here 70% of the 3-NT was reduced to aminotoluene, whereas 30% was converted to 3-nitrophenol via the formation of 3-nitrobenzylalcohol, 3-nitrobenzaldehyde, and 3-nitrobenzoic acid. 3-Nitrophe‐ nol was further metabolized with the release of nitrite (pathway C, Figure 9).

**Figure 9.** Degradation of mononitrotoluenes by *Pseudomonas putida* strain OU83 [82]. The strain converts 2- and 4-nitro‐ toluenes to corresponding aminotoluenes and hydroxyl nitrotoluenes (pathways A and B), whereas 70% of 3-NT was converted to 3-aminotoluene and 30% was degraded via the formation of the 3-nitrophenol (pathway C).
