Benzidines in tannery dye wastewater: Oxidative degradation by chlorine dioxide

To effectively remove toxic benzidine pollutants from tannery dyeing wastewater, a new method for chlorine dioxide oxidation was developed in a simulated wastewater treatment. In the experiment, the oxidative degradation behaviour towards chlorine dioxide of four toxic pollutants including benzidine, o-tolidine, 3.3'dimethoxylbenzidine and 3.3'-dichlorobenzidine were studied by detecting the concentrations of benzidines at regular intervals with HPLC. The effects of the reactant molar ratio, pH and time of reaction on the removals of COD,, and benzidines are discussed. The optimal reaction conditions are presented. The results show that oxidation with chlorine dioxide is feasible and efficient for benzidines degradation. The removals of COD,, reached 65%, and the degradations of benzidines were over 90%. Azobenzene, hydrazobenzene and benzoquinone were detected by IR and GC/MS as intermediates. A reaction mechanism for the oxidation of benzidine with chlorine dioxide in water has been proposed.