Potential and characteristics on nitrobenzene degradation by biological acidification.

Biological acidification, efficient and low-cost biotechnology, is crucial in treating pharmaceutical, pesticide water, and petrochemical wastewater. Nitrobenzene is a typical organic pollutant in petrochemical wastewater with high toxicity and long persistence. However, its effect on hydrolysis acidification is yet to be fully elucidated. The present study sought to investigate the inhibitory effect of nitrobenzene on biological acidification. Volatile fatty acid toxicity assays were performed to examine the acid production of sludge exposed to different concentrations of nitrobenzene over time. Extracellular polymeric substances (EPS) were measured by the phenol-sulfuric acid technique and Coomassie brilliant blue G250 to characterize the changes in extracellular polymers after exposure to different nitrobenzene concentrations. Enzyme-linked immunosorbent assay kits were employed to evaluate representative enzyme activities of acidified bacteria after exposure to nitrobenzene. Nitrobenzene and its products were respectively determined by liquid chromatography and gas chromatography-mass spectrometry, and the transformation properties of nitrobenzene were explored in the context of acid production, EPS, and changes in key enzymes. Results showed that nitrobenzene inhibited acid production at high concentrations (median effective concentration (EC) = 104.81 mg/L), and acetic fermentation was predominant. Furthermore, the amounts of EPS significantly dropped when the nitrobenzene concentration was above 100 mg/L. The contents of key enzymes decreased with an increase in nitrobenzene concentration. The process of nitrobenzene hydrolysis acidification was characterized as follows: EPS and anaerobic granular sludge adsorbed nitrobenzene, which is subsequently transformed to aniline by the joint action of microbial consortium reductase. Therefore, high concentrations of nitrobenzene should be pretreated before entering the biological treatment system since the capacity of bio-acidification to remove it is restricted.