Degradation mechanism, kinetics, and toxicity investigation of 4-bromophenol by electrochemical reduction and oxidation with Pd–Fe/graphene catalytic cathodes

This study aimed to elucidate the electrodegradation intermediates, kinetics, and toxicity evolution of 4-bromophenol (4-BP), a compound abundant in water. In this study, 4-BP was subjected to electrochemical reduction and oxidation using a divided cell with two prepared Pd–Fe/graphene catalytic cathodes and a Ti/IrO2/RuO2 anode. In the cathodic compartment, reduction debromination in the presence of a Pd–Fe/graphene catalyst was processed with adsorbing hydrogen on the cathode. The cathode also generated hydroxyl radicals in aid of the feeding air to promote the oxidative degradation of 4-BP. At 60 min, the removal rate of 4-BP was 100% in the two catholytes and 99.5% in the anolyte. Most of the intermediate products formed during degradation were identified using liquid chromatography/mass spectrometry. The concentration evolution of three aromatic by-products and four carboxylic acids were detected by high-performance liquid chromatography and ion chromatography. Two possible pathways were proposed on the basis of the reduction and oxidation reaction. A kinetics experiment was carried out to evaluate significant degradation steps. A scheme for the complete mineralization of 4-BP was elucidated. Furthermore, the time course of 4-BP and intermediate concentrations satisfactorily correlated with the toxicity profiles determined from the inhibition of Photobacterium phosphoreum luminescence. Two toxicity evaluation methods demonstrated that a large amount of toxic substance (benzoquinone) was produced in the anolyte at 20 min. However, benzoquinone was not detected in the catholytes and the toxicity of the catholytes decreased with electrolysis time. The formation of the carboxylic acids induced a sharp toxicity decrease, thus ensuring overall detoxification.