Molecular structure-dependent bioelectrochemical decolorization of azo dyes

Abstract

Azo dyes, containing one or more azo bonds (–N=N–), are widely used but pose environmental and health risks due to their toxicity and resistance to degradation. Bioelectrochemical systems (BESs) offer a potential approach for their reductive degradation, yet the role of molecular structure in degradation remains unclear. In this study, nine representative azo dyes were examined to access how substituent type and position affect degradation kinetics and electron transfer under controlled cathodic potentials in BESs. Electron-withdrawing substituents (e.g., –SO₃⁻, –NO₂) and o−/m- substitution enhanced azo bond cleavage, while p-substitution or steric hindered degradation. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed that higher reduction currents and lower charge-transfer resistance correlated with faster degradation. Quantitative structure-activity relationship (QSAR) analysis identified that the –N=N– group and other molecular features such as atom count, are key determinants of azo dyes removal. Experimental and theoretical calculations showed that molecular structure regulates the electron transfer efficiency from electrode to dye by affecting the electron density and steric hindrance of the azo bond, thereby determining degradation kinetics. This study deepened the influence of the molecular structure on azo dyes bioelectrochemical removal, and provided optimized guidance for the treatment of wastewater containing azo dyes by BESs.