Decolorization Mechanism Of Methylene Blue By Bacillus Thuringiensis And Its Immobilization

Due to the wide application of dyes and the increase in annual production,dye wastewater pollution has become one of the main environmental problems.The high chromaticity,toxicity,carcinogenicity and mutagenicity of the dye not only affect the photosynthesis of the aquatic system,but also harm to human health.Therefore,dye-containing wastewater must be treated before being released into the natural environment.Microbial degradation is considered to be an environmentally friendly,economical and effective method for decolorization and degradation of dyes.The purpose of this study is to isolate and screen the strains with high efficiency of decolorizing methylene blue dye,to investigate the effects of different decolorization conditions on the decolorization of the strains,to analyze the decolorization mechanism of the strains from the degradation enzymes and degradation products,and the decolorization effect and mechanism of immobilized strain F5 on methylene blue.The main results are as follows:(1)A ligninase-producing strain F5,which has a highly effective decolorizing effect on methylene blue,was isolated and screened from the virgin forest soil.Based on morphological and molecular biological identification,the strain F5 was identified as Bacillus thuringiensis.(2)The single factor experiment results showed that pH 6.0,temperature 30?,rotation speed 140 rpm,inoculation amount 6%,the decolorization efficiency of the strain to methylene blue was the highest,and the decolorization rate was about 95% at 12 h.In addition,the strain F5 had a certain tolerance to methylene blue and salt concentration.When the initial methylene blue concentration was 200 mg/L,the decolorization rate was 89.6% after 12 h.When the NaCl concentration reached 50 g/L,the decolorization rate was still 82.8%,which provided favorable conditions for the industrial treatment of high-salinity dye wastewater.(3)UV-vis,FT-IR and GC-MS were used to analyze the degradation intermediates of methylene blue before and after the treatment by strain F5.The disappearance of the main absorption peak and the appearance of new peaks in the UV-vis spectrum and FTIR spectrum indicating that the methylene blue molecular structure was destroyed and new substances were produced;GC-MS confirmed that strain F5 can change the structure of methylene blue and degrade it into small molecular compounds.During the degradation process,four main intermediate metabolites were produced.Based on GC-MS analysis,the possible degradation pathway of methylene blue was proposed.The strain F5 had a high-efficiency degradation ability for methylene blue,which was the the synergistic effect of laccase,manganese peroxidase,lignin peroxidase and NADH-DCIP reductase.Among them,laccase and manganese peroxidase were the key enzymes in the degradation of methylene blue.The results of the phytotoxicity of methylene blue dye and its metabolites to wheat and mung bean showed that the toxicity of the degradation products was significantly reduced compared with the parent compound,but there was still toxic.(4)The polyvinyl alcohol(PVA),sodium alginate(SA)and activated carbon(AC)were used as the immobilization materials of strain F5 to prepare immobilized cells,WT,HB,and WB.To analyze the removal mechanism of methylene blue by immobilized cells through SEM,BET,FT-IR,XPS,kinetics,isothermal models,and thermodynamics.The results showed that the immobilized cells can effectively remove methylene blue.The larger specific surface area and mesopores structure of the immobilized cells were conducive to the adsorption of methylene blue,and the surface of its contained abundant functional groups,which provided active sites for the adsorption of methylene blue.The results showed that the adsorption process of methylene blue on WT,HB,and WB followed the pseudo-second-order kinetic model.The adsorption of methylene blue on WT was described by the Langmuir model,with a maximum adsorption capacity of 182.82 mg/g,while the adsorption of methylene blue on HB and WB were better fitted the Freundlich model.In addition,the adsorption process of methylene blue by WT,HB,WB at different temperatures was a spontaneous endothermic process.