Biodegradation of organic micropollutants by anoxic denitrification: Roles of extracellular polymeric substance adsorption, enzyme catalysis, and reactive oxygen species oxidation

Abstract

The control of organic micropollutants (OMPs) in water environments have received significant attention. Denitrification was reported to exhibit good efficiency to remove OMPs, and the mechanisms involved in are too intricate to be well illustrated. In this study, we selected nitrobenzene [NB] and bisphenol A [BPA] as model pollutants and aimed to unravel the mechanisms of Paracoccus Denitrificans in the removal of OMPs, with a specific emphasis on aerobic behavior during denitrification processes. We demonstrated the formation of extracellular superoxide radicals, i.e., extracellular ^•O₂^-, using a chemiluminescence probe and found that extracellular polymeric substance adsorption, extracellular ^•O₂^-, and microbial assimilation contributed approximately 40 %, 10 %, and 50 % to OMPs removal, respectively. Transcriptome analysis further revealed the high expression and enrichment of several pathways, such as drug metabolism-other enzymes, of which a typical aerobic enzyme of polyphenol oxidase [PPO] participates in the degradation of NB and BPA. Importantly, all the immediate products showed a significant decrease in toxicity during the aerobic activity-related OMPs degradation process based on the proposed degradation pathways. This study demonstrates the formation of extracellular ^•O₂^- and the mechanisms of extracellular ^•O₂^-- and PPO-mediated OMPs biodegradation, and offers new insights into OMPs control in widely-used denitrification treatment processes.