Fenton pre-treatment of wastewater containing nitrobenzene using ORP for indicating the endpoint of reaction

Because of their high toxicity and refractory to microorganisms, high concentration aromatic compounds like nitrobenzene (NB) have long been considered to be disadvantageous to stable operation of biological wastewater treatment systems in petrochemical industries. The pre-treatment of NB-bearing dye producing wastewater using Fenton’s oxidation was studied with the objective to remove NB and related aromatic compounds, and therefore improve the biodegradability of wastewater. Under the conditions of initial pH = 3, H2O2 = 500 mg l−1, H2O2/Fe2+ (in weight) = 6, and reaction time = 150 min, the BOD/COD ratio of wastewater was increased from 0.03 to 0.47. The redox potential (ORP) was found to be an effective indicator to monitor the Fentonʹs oxidation process. H2O2 was almost depleted at the point where the ORP reached the maximal value, and the lowest point of ORP was accompanied with the completion of Fentonʹs oxidation and the maximal BOD/COD. Increase of biodegradability continued for about 60 min even after the depletion of H2O2, suggesting the existence of some unknown oxidizing species.