Abstract
More than 80 per cent of wastewater from industries is discharged into receiving water bodies without any pollution control. Microbial fuel cells (MFCs) are a promising technology for the simultaneous treatment of wastewater and electricity production. With regard to azo-dye containing wastewater (e.g. from textile manufacturing), the dye may be fed via the anode chamber containing electrochemically active bacteria or via the cathode chamber containing laccase enzyme as catalyst for oxygen reduction. This study investigated which of the two approaches is the best with regard to rate of decolourization of the dye (Acid orange 7), COD reduction and electricity production. The power density was higher for the MFCDye cathode (50±4 mW m-2, COD reduction 80.4±1.2%) compared with 42.5±2.6 mW m-2 (COD reduction 69±2%) for MFCDye anode. The time required for decolourization was longer in the MFCDye anode (Shewanella oneidensis) where only 20% decolourization was obtained after 24 h compared to 80% for the MFCDye cathode. The anodic dye degradation products were unstable when exposed to air resulting in regaining of colour. In case of degradation by laccase in the cathode chamber, the decolourization products were stable and simpler in chemical structure as determined by GC-MS. This work suggests that feeding azo dyes in cathode chambers of MFCs containing laccase is a better way of treating the dyes compared to the commonly used approach of feeding the dye in the anode chamber provided enzyme activity can be sustained.
Highlights
Azo dyes are the most common type of synthetic dyes used in the textile industry
The open circuit voltage (OCV) was highest for MFCControl with 1.3 V followed by MFCDye Cathode with 950 and 930 mV for Microbial fuel cells (MFCs) Dye Auto-Oxidation of S. oneidensis (Anode)
The lower power density in case of dye in the anode indicates that the presence of AO7 had a significant effect on the growth rate of S. oneidensis
Summary
Azo dyes are the most common type of synthetic dyes used in the textile industry. These dyes are known to be xenobiotic compounds that possess electron withdrawing groups that generates electron deficiency thereby making them resistant to degradation (Singh et al, 2014). There are various studies that have investigated azo dye decolourization in the anode and others in the cathode (Huang et al, 2017; Lai et al, 2017; Mani et al, 2018) It is not clear which approach is the best as different studies used different organisms, operating conditions, cathode catalyst etc., making a direct comparison of decolourization rates for each system difficult (Table 1). This is the first study to understand the mechanism of dye decolourization in both the processes (anode & cathode), and the nature of products formed, while operating under the same conditions. The rate of dye decolourization, power density, COD reduction, degradation products and their toxicity were used as performance indicators
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