Recalcitrant Nitroaromatics

• Nitroaromatics are common military explosives such as Trinitro toluene-TNT, solvents such as nitrobenzene and pesticides such as nitro phenols.
• Nitrosubstitution of aromatic rings decreases biodegradability.
• These compounds are highy toxic and mutagenic
• Compounds with several nitrosubstituents are recalcitrant

• Biodegradation of nitroaromatic compounds tend to be slow and leads to bound or polymerized residues in soils and sediments
• Extensively nitro-substituted aromatics are more easily transformed under anaerobic conditions than aerobic conditions.

Oxidative pathway/Aerobic

• Nitrobenzene is converted to diol by the action of dioxygenases which gets converted to catechol. Catechol ring is opened by meta-cleavage later.
• Nitrobenzene ^dioxygenaseàNitrobenzene diolà Catechol (ring opening by metacleavage) àoxalocrotonaldehyde--àOxalocrotonate                                                           

 

Reductive pathway/Anaerobic

• Release more energy
• Does not require strict anaerobic conditions
• Nitro group is stepwise reduced through nitrobenzene to phenylhydroxylamine to 2-aminophenol. The phenol ring subsequently opens up and oxalocrotonate is formed with the release of ammonia

Trinitro toluene -TNT

• no efficient biodegradation pathway
• Under anaerobic and microaerophilic conditions, nitro groups of TNT are reduced one by one to amino groups, each subsequent reduction slower and less complete 
• If conditions are aerobic, the partially reduced intermediates form very complex and mutagenic azo condensation products 
• No mineralization, but only polymerization or binding of residues

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