Screening Of Highly Efficient Decolorization Bacteria Of Malachite Green And Its Decolorization Characteristics And Mechanism

Malachite green(MG)has been banned to use in aquaculture and dyeing industry due to its high toxicity,high residue,high carcinogenic,high teratogenicity and high mutagenicity.Nevertheless,owing to its low price and high efficiency,MG is still illegally employed for fishery disinfection worldwide,especially in developing countries.Biological treatment of MG through biosorption or biodegradation has been perceived as an efficient,environment-friendly and low cost alternative,which has received more and more attention.In this thesis,a novel strain,Pseudomonas sp.YB2,which was selected from activated sludge exhibited high decolorization ability towards MG.The performance conditions for decolorization process of MG were optimized and decolorization mechanism of malachite green by Pseudomonas sp.YB2 was explored.The main work of this paper is as follows:1)Activated sludge was obtained from a sewage treatment plant,and a novel strain was selected after enrichment,separation and purification.Based on the phenotypic characteristics and physiological and biochemical characteristics,together with the results of morphological characteristics and 16S rRNA gene sequence analysis,the strain was identified as a Pseudomonas sp.strain.The isolate was designated Pseudomonas sp.YB2.UV-vis spectrum analysis showed high decolorization efficiency of MG by Pseudomonas sp.YB2.2)Conditions for bacterial growth of the strain YB2 were optimized by single factor experiment.The results mainly showed appropriate temperature range for bacterial growth was between 25-35 ?,and the most suitable temperature was 25?.The strain could grow when pH was between 5.0-10.0,bacterial growth was significantly inhibited as pH<5.0,and it grew better in both weak acidity and neutral condition(pH=5.0-7.0),of which pH=6.0 was most suitable for bacterial growth.Meanwhile,YB2 was an aerobic strain which grew better at shaking condition with high air content.In addition,YB2 kept growing as NaCl concentration reached 50 g/L,which revealed that YB2 had high salt tolerance,but it did not stop growing until NaCl concentration reached 100 g/L.Finally,YB2 was found to tolerate all tested antibiotics(sulfadiazine,tetracycline,doxycycline hyclate,metacycline hydrochloride,chloramphenicol,streptomycin)at lower concentration,especially for sulfadiazine and chloramphenicol.3)Conditions for decolorization of MG by Pseudomonas sp.YB2 were optimized by single factor experiment.The decolorization rate of MG was over 90%when the temperature was between 25-35?,and the highest decolorization rate(99.21%)was obtained at 30?.There was more than 90%decolorization rate at pH ranging from 5.0 to 10.0.Likewise,higher decolorization rate was obtained at shaking condition.Metal ions such as Cu2+?Cd2+?Fe3+?Al3+at low concentration decreased decolorization rate while Mg2+slightly increased decolorization rate.Ni2+,Zn2+?Mn2+had no obvious influence on decolorization of MG.Remarkably,Pseudomonas sp.strain YB2 could efficiently decolorize 90.40%MG at a high concentration up to 1500 mg/L in 24 h,and concentrations of MG lower than 1000 mg/L could be completely decolorized in 12 h.4)The dye removal by strain YB2 was largely attibuted to biodegradation rather than biosorption.12 intermediate products of MG during different biodegradation phases were identified by GC-MS.They included aromatic compounds such as 4,4'-bis(dimethyla-mino)benzophenone,4-(dimethylamino)benzophenone,phenol,dimethylaniline,hydroquinone,4-hydroxybenzoic acid and 1,4-benzoquinone,as well as linear compounds such as maleic acid,isobutyric acid,glutaric acid and 3-hydroxypropionic acid.Thus,the possible degradation pathways of MG by the strain were proposed:The central carbon atom of MG was attacked,and MG was hydroxylated to malachite green carbinol or reduced to leucomalachite green.Then the former decomposed into 4,4'-bis(dimethyla-mino)benzophenone or 4-(dimethylamino)benzophenone,the latter into 4-(dimethylamino)benzophenone through benzene ring-removal reaction.These intermediate products were further degraded through benzene ring-removal reaction and continuous oxidation reaction,and eventually decomposed into small molecules.