Study On The Biodegradation Characterristics Of Direct Dyes

Dyestuff wastewater contains a large amount of organic matter,with the characteristics of high content of COD_Cr,deep color,high acidity and alkalinity,etc.Dye is one of the characteristic pollutants of dyestuff wastewater,which will contribute most of the color and some organic pollutants in printing and dyeing wastewater.Direct dyes are widely used in the printing and dyeing industry.They can directly interact with protein fibers,cellulosic fibers,viscose fibers,etc.,to achieve the coloration of fibers.Therefore,in this paper,five typical direct dyes?direct yellow27,direct red 28,direct black BN,direct yellow 12 and direct blue 151?were selected to investigate their degradation mechanism under three different aeration conditions:aerobic,anaerobic,anaerobic/aerobic alternate.Firstly,the effects of initial dye concentration and additional carbon sources on the decolorization of five direct dyes and the removal of COD_Cr were investigated.At the same time,the biodegradation kinetics of the five dyes was studied.According to the results of UV-Vis spectrograms and gas chromatography-mass spectrometry?GC-MS?,the degradation pathways were further analyzed.The evolution of the microbial community structure was aslo analyzed by high-throughput sequencing in the bioreactors under different aeration conditions.The experimental results are as follows.?1?The removal rate of COD_Crr in five direct dyes wastewater is highest under aerobic condition.The aerobic condition is favorable to the continued degradation of the metabolic intermediates.There is also an anaerobic environment in the aerobic sludge flocs.The decolorization rate of direct yellow 27,direct red 28,direct yellow 12 and direct blue 151 dyes were the highest under anaerobic/aerobic condition.However,the decolorization rate of direct black BN dye was highest under anaerobic condition.The initial dye concentration has a great influence on the decolorization of dyes wastewater.When the initial dye concentration was100-300 mg/L,the decolorization rate of the five direct dyes increased with the increase of the initial dye concentration.When the initial dye concentration was higher than 300 mg/L,the decolorization rate of the five dyes increased slowly.The results showed that the suitable initial concentration of the five direct dyes was 300 mg/L.The carbon source also has a certain influence on the degradation of dyes.Compared with no external carbon source,the decolorization rate of the five direct dyes was significantly higher with the external carbon source.Therefore,the external carbon source will promote the degradation of the dyes.?2?The five dyes have the highest degradation efficiency under anaerobic/aerobic conditions.Howereve,the degradation of the direct yellow 27 dye approximately conforms to the second-order kinetics under the conditions of aerobic and anaerobic,and under anaerobic/aerobic alternating condition,the degradation of the direct yellow 27 dye approximately accords with the first-order kinetics.The degradation of the direct red 28 dye approximately conforms to the first-order kinetics under aerobic condition,and under anaerobic and anaerobic/aerobic alternating conditions,the degradation of the direct red 28 dye approximates the second-order kinetics.The degradation of the direct black BN dye approximately conforms to the zero-order kinetics under aerobic and aerobic/anaerobic alternation condition,and under anaerobic condition,the degradation of the direct black BN dye approximately accord with the second-order kinetics.The degradation of the direct yellow 12 dye approximately conforms to the second-order kinetics under aerobic condition,and under anaerobic condition,the degradation of the direct yellow 12dye approximately conforms to the zero-order kinetics and under anaerobic/aerobic alternating condition,the degradation of the direct yellow 12 dye approximately conforms to the first-order kinetics.The degradation of the direct blue 151 dye approximately conforms to the first-order kinetics?R²>0.96?under the aerobic,anaerobic,anaerobic/aerobic alternating conditions.?3?According to UV-Vis spectra and GC-MS determination results,the different molecular structures produced different intermediates.The direct yellow 27,direct red 28,and direct black BN dyes can be finally converted to CO₂ and H₂O,to achieve complete mineralization of the dye.But,the mineralized products of direct yellow 12,direct blue 151 dyes still contain toxic carcinogens such as nitrobenzene and aniline.Under the experimental conditions,the two dyes cannot be completely mineralized due to the limited biodegradation time.?4?By investigating the change of bacterial populations at the level of bacteria,genus and genus in the reactor,it can be seen that the activated sludge in the reactor was acclimated with the mixed dyes,and the bacterial populations at the level of bacteria,genus and genus have undergone significant changes.The dominant bacteria in the initial sludge are mainly Proteobacteria,Firmicutes,Bacteroidetes and Acidobacteria.The dominant bacteria class is mainly Alphaproteobacteria,Acidobacteria and Bacilli.Propionibacterium is the main dominant genus.After the activated sludge in the reactor is acclimated by the mixed dye,the dominant bacteria in the aerobic reactor are Acidobacteria,Bacteroidetes and Proteobacteria.The dominant bacteria class is Betaproteobacteria,Alphaproteobacteria,Sphingobacteria and Acidobacteria.The main dominant genus is Propionibacterium.The dominant bacteria in the anaerobic reactor are Nitrospirae,Bacteroidetes and Actinobacteria.The dominant bacteria class is Actinobacteria and Sphingobacteria.The main dominant genus is Roseiarcus and norank_fAnareolineaceae.The dominant bacteria in the anaerobic/aerobic alternating reactor are Actinobacteria,Bacteroidetes and Firmicutes.The dominant bacteria class is Actinobacteria,Sphingobacteria and Bacilli.The main dominant genus is Roseiarcus and Enterobacter.The results will provide important reference value for understanding the biodegradation process of direct dyes,and will also provide theoretical basis and data support for the optimization and improvement of dye wastewater and printing and dyeing wastewater treatment processes.