Biotreatment of sulfonated dyestuffs with energy recovery in microbial fuel cell: Influencing parameters, kinetics, degradation pathways, mechanisms, and phytotoxicity assessment

Removal of recalcitrant sulfonated dyestuff intermediates from wastewater has been an urgent challenge for environmental technologies. In this regard, the biodegradations of monoazo Methyl Orange (MO) and diazo dyes Reactive Black 5 (RB5) towards wastewater treatment and bioelectricity generation in microbial fuel cell were investigated and compared through the studies on azo dye concentration, aeration, sampling points arrays, and electrode spacings. The degradation of diazo RB5 yielded higher chemical oxygen demand removal, decolourization efficiencies, and power generation over monoazo MO. The decolourization efficiency of RB5 (97.62%) increased with an increase of RB5 concentration (50 mg/L), suggesting that the system has the capability of removing higher RB5 concentration. However, contrary results were obtained with MO due to its toxicity. This study also demonstrated that the decolourization rate of diazo RB5 (0.1533 h-1) was ≈ 53% higher than monoazo MO (0.0727 h-1). The findings revealed that the degradation kinetic was remarkably influenced by the chemical structure of dye, where dye with more electron-withdrawing groups at para position are more susceptible to be reduced. Higher output voltage (568.59 mV) and power generation (108.87 mW/m2) were attained with RB5 due to electron donor availability and electron-shuttling characteristics of RB5 decolourized intermediates. Furthermore, detailed degradation pathways of MO and RB5 were presented based on the UV-vis and GC-MS results. The phytotoxicity assessment via Sorghum bicolor seeds had further verified the reduction in toxicity after the treatment of azo dyes.