Biodecolorization Of Wasterwater Containing Azo Dye Cation Red X-GRL By Shewanella Oneidensis MR-1

The production and usage of azo dyes not only brings serious ecological pollution problems,but also poses a serious threat to human health.Therefore,the treatment of azo dye wastewater is imperative.In order to deal with such wastewater,many methods have been reported to be effective,but biological methods are known to be more eco-friendly and less costly than traditional physical or chemical methods.Electrogenic microorganisms degrade environmental pollutants through the electrons released during their metabolism.They have a broad-spectrum degradation performance and are superior to ordinary microorganisms that degrade pollutions by specific enzymes.These microorganisms have become the new favourites for treatment of environmental pollutions.Therefore,in this study,Shewanella oneidensis MR-1,a model electrogenic microorganism,was used to decolorize cationic red X-GRL(X-GRL),a typical cationic azo dye.The decolorization of X-GRL by suspension of S.oneidensis MR-1 and the immobilization of S.oneidensis MR-1 were systematically studied.The effects of different conditions on the decolorization of X-GRL were investigated.Its decolorization mechanism was explored.The main conclusions are as follows:(1)When the concentration of X-GRL was less than 200 mg/L,S.oneidensis MR-1 could effectively decolorize X-GRL with a decolorization efficiency of 96% in 11 h.While high concentration of X-GRL showed toxicity to bacteria.It was very important to control the concentration of azo dyes in the actual biological treatment of wastewater.(2)Decolorization of X-GRL by S.oneidensis MR-1 was less affected by the pH and temperature in the system;it was better in a wider pH range(5.5-8.0)and temperature range(25-40°C).However,the type of carbon source(electron donor)used,also had a great influence on the decolorization efficiency.Lactate and citric acid were suitable carbon sources for X-GRL decolorization.In addition,S.oneidensis MR-1 can tolerate wastewater with high salinity.When the NaCl concentration was as high as 40 g/L,the decolorization efficiency of X-GRL could still approach 100% with 36 h,which is beneficial to its application in the actual biological treatment of dye wastewater.(3)By making immobilized cells of S.oneidensis MR-1,biodecolorization of X-GRL can be achieved in the presence of oxygen.The beads of immobilized S.oneidensis MR-1 could help in simplifying the operating conditions,facilitating the separation,recycling of bacterial cells and enhancing the decolorization ability of X-GRL by S.oneidensis MR-1.In addition,the immobilized cells of S.oneidensis MR-1 showed a high stability potential,that is,after 4 cycles(192 hours of continuous decolorization),the decolorization efficiency of X-GRL could be maintained at more than 90%,and the recycling of the beads can be achieved.(4)X-GRL could be simultaneously decolorized extracellularly and intracellularly by S.oneidensis MR-1.The extracellular decolorization could be as a result of the actions of the Mtr respiratory pathway or the indirect reduction of riboflavin.On the other hand,intracellular decolorization might be as a result of the intracellular reduction involving the CymA pathway and independent of the CymA pathway.(5)The decolorization mechanism of X-GRL was mediated by environmental pH.The intracellular and extracellular decolorization of X-GRL by S.oneidensis MR-1 was regulated by environmental pH conditions.Within a certain range,the proportion of extracellular reduction depending on the Mtr respiratory pathway and intracellular reduction depending on CymA pathway decreased with the pH increasing,while the decolorization efficiency of X-GRL increased.(6)The cleavage of the azo bond led to the decolorization of the azo dye.After decolorization by S.oneidensis MR-1,the azo bond of X-GRL was broken,resulting in the production of an aromatic compound.The phytotoxicity of X-GRL after decolorization was decreased while the genotoxicity was increased.