An innovative electrochemical degradation of 1-diazo-2-naphthol-4-sulfonic acid in the presence of Bi2Fe4O9

Abstract

This paper explores the degradation of a model pollutant, 1-diazo-2-naphthol-4-sulfonic acid (1,2,4-Acid), by an advanced oxidation process that combines electrochemical degradation and Bi2Fe4O9 (BFO) oxidation. Experiments were done using a gas-diffusion cathode to produce in situ hydrogen peroxide by oxygen reduction. Column crystal semiconductor Bi2Fe4O9 was synthesized and used as a novel catalyst in the combined system. It was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Comparative experiment in two separated systems (BFO/H2O2 and BFO/Electrolysis) was conducted to determine the mechanism involved in the process. The effect of operating conditions such as applied current, solution pH, catalyst dosage on the efficacy of the process was investigated. It is shown that 1,2,4-Acid and its products can be effectively degraded by the OH radicals produced by the reaction between BFO and the electro-generated H2O2, which was further being confirmed with the electron spin resonance spin-trapping technique. Hydroxyl radicals generated in the process that led to the complete and rapid elimination of total organic carbon was observed even at low catalyst loadings. In this system, almost a complete removal of COD (89%) was achieved after 200min using 1.0gL−1 of BFO, whereas at the same time, only 39% and 32% of COD was removed by an individual process alone (electrochemical degradation and BFO adsorption, respectively). In this combined system, BFO has the principle role of adsorbing the initial substance on the surface of catalyst and catalyzing the electro-generated H2O2 to form active hydroxyl radicals. The LC-(ESI)-TOF-MS analysis indicated that the mineralization occurred simultaneous with the destruction of naphthalene ring, and 1,2,4-Acid was oxidized to colourless intermediates (mainly phthalic acid, small carbonyl species) and then to carbon dioxide.