Heterogeneous electro-Fenton-like oxidation for the degradation of 4-nitrophenol characterized by immobilized Fe(III): Performance, mechanism and chlorinated organic compounds formation

Abstract

Degradation of 4-nitrophenol was carried out by a novel electrochemical process, in which a carbon nanotube-containing cathode was employed for the electro-generation of hydrogen peroxide (H2O2) and immobilized Fe(III) served as the heterogeneous catalyst. The catalyst of immobilized Fe(III) prepared by dipping molecular sieves into Fe(NO3)3 solution exhibited a well catalytic activity and a weak Fe(III) leaching. By the major function of electro-Fenton-like oxidation and the secondary function of anodic oxidation, TOC removal of 77 % and 4-nitrophenol removal of 100 % were achieved under the optimum conditions. The degradation efficiency decreased in the presence of chloride ions (Cl−), and this unfavorable effect was relevant to the electro-generation of chlorine (Cl2). It was confirmed that hydroxyl radical (OH) was the major oxidizing specie in the heterogeneous electro-Fenton-like oxidation. During the degradation, firstly nitro-group was replaced by hydroxyl forming quinol and the further oxidation generated p-benzoquinone. Afterwards, benzene ring was destroyed and oxidized leading to the mineralization. The generation of inorganic ions mainly presented as nitrate ions (NO3−) and ammonium ions (NH4+) at the end. Moreover, some chlorinated organic compounds were found during the degradation in the presence of chloride ion (Cl−), and they could be completely destroyed by the successive oxidation degradation. This paper indicates that the presented process is able to effectively degrade organic compounds as well as avoid the generation of iron sludge.