Effective electrochemical decolorization of azo dye on titanium suboxide cathode in bioelectrochemical system

Abstract

Azo dye presents severe pollution to water environment, which needs to be treated properly to minimize environmental impact. In this study, we developed a porous titanium suboxide electrode for electrochemical reduction of azo dye (Acid Red B) in dual-chamber bioelectrochemical system (BES). The decolorization efficiency could reach 91.9% on titanium suboxide, which followed the first-order reaction kinetics with apparent rate constant of kapp = 0.339 h−1, a value 89.4% higher than that (kapp = 0.179 h−1) for carbon cloth. The cyclic voltammogram and electrochemistry impedance spectroscopy (EIS) characterization indicated higher activity and lower charge-transfer resistance of titanium suboxide. The decolourization rate was positively related to cathode potential, reaching the maximum kinetic constant of 0.456 h−1 at − 1.15 V versus standard hydrogen electrode. Lower pH was favorable for decolorization because of requirement for excessive protons for the cleavage of azo bonds, indicated by 86.4% increase in kinetic constant when shifting pH from 9.0 (0.1643 h−1) to 5.0 (0.3064 h−1). The Tafel plot illustrated a higher corrosion resistance of titanium suboxide (the corrosion current of 1.28–1.84 mA/m2) than that of carbon cloth (6.19–11.43 mA/m2) in acidic condition, suggesting higher stability due to unique oxygen-deficiency crystalline structure. This study provides the first demonstration of titanium suboxide cathode to efficiently reduce azo dye via direct electron transfer in BES, which eliminates the need for time-consuming enrichment of electroactive biofilm and avoids the development of non-electroactive microorganisms.