Neutral Red Mediated Reductive Decolorization Of Metal Complex Azo Dye By Shewanella Oneidensis MR-1

Textile industries are major users of dye in the world.A large amount of dye discharged from textile industries has caused serious damage to our environment.Microbial remediation has been proved to be a desirable and cost-effective method to counter textile dye pollution.Some microorganisms have been found to be able to decolorize and metabolize dyes and are employed to remediate the environment polluted by textile dyes.Among them,electrogenic bacteria have received more attention due to their non-specific reducing ability to environmental pollutants which are degraded by electrons released during their metabolism.And their degradation capability can be enhanced by electron mediators,which is especially useful for bio-refractory dyes such as metal complex azo dye.There are some reports on dye biodecolorization improved by electron mediators,but the enhanced decolorization of metal complex azo dye by electrogenic bacteria is seldom reported.Therefore,in this study the biodegradability of a recalcitrant metal complex azo dye,acid blue 193(AB 193)was explored by using a model electrogenic bacterium,Shewanella oneidensis MR-1 with neutral red(NR)as an electron mediator.The individual and synergetic effects of different conditions on the decolorization of AB 193 were investigated and optimized using response surface methodology(RSM).The toxicity of its decolorization products was evaluated and its decolorization mechanism was explored.This study provides a new idea for the treatment of refractory metal composite azo dye wastewater,clarify the mechanism of azo dye degradation by electrogenic microorganism,and broadens the potential application of electrogenic microorganism in the field of environmental pollution bioremediation.The main conclusions are as follows:·The effective anaerobic decolorization of AB 193 by S.oneidensis MR-1 was highly dependent on the presence of NR as an electron mediator.At the presence of 4 ?M NR,S.oneidensis MR-1 could successfully decolorize AB 193,but hardly decolorized it without NR added.NR was found for the first time to be able to significantly enhance the capability of S.oneidensis MR-1 to decolorize the metal complex azo dye AB 193,and AB 193 was successfully biodegraded by S.oneidensis MR-1 for the first time.·Biodecolorization of AB 193 mediated by NR was significantly affected by the initial AB193 dye concentration,but slightly by incubation temperature and medium pH.When the initial concentration of AB 193 was up to 200 mg/L,it was hardly decolorized by S.oneidensis MR-1 despite adding NR.However,in a temperature range(25-40?)and pH range(6.0-8.0),good AB 193 decolorization effects could be achieved.·Biodecolorization of AB 193 by S.oneidensis MR-1 mediated by NR was optimized using RSM based on Box Behnken design(BBD).The optimal decolorization conditions obtained were initial AB 193 concentration of 52.7 mg/L,medium pH of 7.0,incubation temperature of 30 ?,and NR dosage of 3.5 ?M.Under the optimum conditions,the maximum decolorization rate of AB 193 for 144 h was 95.98%.Furthermore,the established regression equation was reliable and could effectively predict the removal of AB 193.And the interaction between initial AB 193 and NR dosage had the most significant effect on the removal of AB 193.·AB 193 was indirectly reduced and decolorized extracellularly by electrons mainly from the Mtr pathway of S.oneidensis MR-1.And NR was proven to act as an electron mediator during the reductive decolorization of AB 193 by S.oneidensis MR-1.NR promoted AB193 biodecolorization by facilitating electron transfer between bacteria cells and AB 193.Moreover,this accelerated electron transfer might be related to the extracellular polymeric substance(EPS)reduction of S.oneidensis MR-1 caused by NR,which could lead to a reduction in the resistance of electron transmission across the cell membrane.·Biodegradation of AB 193 by S.oneidensis MR-1 using NR as an electron mediator was confirmed through high-performance liquid chromatography(HPLC).The results of HPLC showed that during the decolorization of AB 193,the azo bond in AB 193 molecular structure was broken and aromatic compounds was produced.Furthermore,the phytotoxicity of metabolites from AB 193 to rice was significantly reduced during the decolorization process.