Study On The Self-mediated Extracellular Azo Dyes Reduction

The anaerobically reductive decolorization of azo dyes is believed to be the rate-limiting step due to its low reaction rate. The addition of redox mediator is an effective approach to improve the performance of anaerobic decolorization. However, externally added mediator is usually cost-ineffective, refractory and toxic, which results in secondary pollution.This thesis focused on the finding of bacterial strains which can decolorize azo dyes effectively without adding exogenous mediator, and revealed the procedure of self-mediated extracellular azo dye reduction. The self-mediated decolorization process can increase the anaerobic biotransformation rate of dye wastewater and overcome the disadvantage generated by externally added mediator. The isolation and characterization of the azo-dye-decolorization strain were studied and the performance and mechanism of self-mediation were revealed.Shewanella sp. XB was isolated from a quinone-reducing microbial community. The strain could decolorize as much as 97% of 200 mg/L Direct Blue B2RL. The optimal decolorization conditions were determined to be as follows,30-37℃, pH 7-9, inocula higher than 0.15 g/L,20 mmol/L lactate as carbon source. Over 95% Direct Blue B2RL(100-5000 mg/L) could be removed by Shewanella sp. XB. Proteins secreted by XB during the decolorizaiton processe could improve the decolorization of high concentrations of azo dye.The self-mediated decolorization and stimulating effects on other bacterial decolorization processes by Shewanella sp. XB were investigated. It was demonstrated that whole Shewanella sp. XB cells cultivated under aerobic or anaerobic conditions could transfer electrons between NADH and dye, and 95% Acid Red B(50mg/L) was decolorized in 60 min. The supernatants of aerobic or anaerobic XB cultures could enhance the decolorizing capacities quinone-reducing bacteria and activated sludge. The decolorization efficiency of 200 mg/L Direct Blue B2RL by quinone-reducing strain QR-1 was increased from 5% to 95%. The decolorizing time of by activated sludge was reduced from 60 h to 26 h.Riboflavin was secreted by Shewanella sp. XB when organic compounds such as lactate and formate were used as carbon sources under anaerobic conditions. Lactate was found to be the best carbon source for riboflavin secretion and 0.24 mg/L riboflavin was detected in the supernatant in 24 h. The self-mediated extracellular decolorization process was deduced to be composed of two steps, i.e. strain XB secretes riboflavin under anaerobic conditions and then utilizes the secreted riboflavin as redox mediator to enhance the reduction of azo dye.