Effect of direct electrical stimulation on decolorization and degradation of azo dye reactive brilliant red X-3B in biofilm-electrode reactors

• Electrical stimulation on cell growth on biofilm-electrode was studied.
• Effects of voltage drop on decolorization and COD removal in the BER were studied.
• Higher cultivating voltage helps to economize the operating voltage of the BER.
• The synergistic effect of electrode reaction and biodegradation existed in BER.
• Dye degradation products and degradation mechanism by the BER were analyzed.

The degradation of azo dye Reactive Brilliant Red (RBR) X-3B in biofilm-electrode reactors (BERs) with graphite rod as anode and activated carbon fiber attached to steel as cathode was studied. The electrical stimulating effect in terms of cell growth was obtained when the cultivating voltage was between 1.0 and 2.5 V, and higher decolorization efficiencies and COD removal were also obtained in this condition. There was long-term effect of cultivating voltage on performance of BERs even if the electrical power was shut off. As the dye concentration was higher than 100 mg L−1, the decolorization efficiency and COD removal in the BER were significantly higher than the sum of the same values in a single biological reactor and a single electrochemical reactor, which indicated that there was a synergistic effect between the electrode reaction and biodegradation. Kinetic analysis of color removal of RBR X-3B indicated that the rapid adsorption by microorganisms was weak and the decolorization process with time followed first-order kinetics. GC–MS analysis of the degradation products suggested that the main reaction mechanism of RBR X-3B degradation was the reduction of N=N bonds on the biofilm cathode.