| Hexavalent chromium（Cr（Ⅵ））and dichlorobenzene are typical environmental pollutants with environmental persistence and bioaccumulation.They are often detected in wastewater discharged from chemical industrial parks,and their harmless treatment is a difficult problem in the environmental field.In this study,two dominant nickel-resistant patent strains（hereinafter referred to as dominant strains）,ie,Bacillus cereus FNXJ1-2-3（GCMCC9683,ZL201510291867.1）and Mucor circinelloides FNZJ3-2-2（GCMCC9684,ZL201510293835.5）,screened in the laboratory were used.Firstly,the tolerance and removal ability of two dominant strains to Ni（Ⅱ）and Cr（Ⅵ）were studied in LB medium.It was found that the two strains had high Ni（Ⅱ）tolerance.Although they could not effectively reduce the concentration of Ni（Ⅱ）in wastewater,they had good removal effect on Cr（Ⅵ）.Through the removal ability of Ni（Ⅱ）and Cr（Ⅵ）and the growth performance of the two dominant strains,it was found that they had the potential to treat Ni（Ⅱ）and Cr（Ⅵ）mixed wastewater and the ability to selectively remove Cr（Ⅵ）.Subsequently,the single-factor culture conditions for the removal of Cr（Ⅵ）by the two dominant microbial agents were studied.The removal performance and mechanism of Cr（Ⅵ）by the two dominant microbial agents were evaluated and analyzed by mathematical model fitting,scanning electron microscopy,XPS characterization analysis and other means.Then,under the appropriate conditions for removing Cr（Ⅵ）,their degradation performance for mixed dichlorobenzene was studied.The results showed that the removal ability of Cr（Ⅵ）and dichlorobenzene by FNZJ3-2-2 strain was stronger than that of FNXJ1-2-3 strain.Therefore,FNZJ3-2-2 strain was selected as the experimental strain for simultaneous removal of Cr（Ⅵ）and degradation of dichlorobenzene.Through single factor test and orthogonal test,the chromium removal and chlorine reduction characteristics of FNZJ3-2-2 strain were further optimized in Cr（Ⅵ）-containing inorganic medium with dichlorobenzene as the sole carbon source,and the ring-opening mechanism was determined.The main results are as follows:（1）The tolerance and removal ability of Ni（Ⅱ）and Cr（Ⅵ）to two strains Bacillus cereus FNXJ1-2-3 and Mucor circinelloides FNZJ3-2-2 screened by our research group were studied.The two strains can tolerate 90 mg/L and 200 mg/L Ni（Ⅱ）,50 mg/L and 100 mg/L Cr（Ⅵ）,respectively.It has removal effect on Cr（Ⅵ）),but neither can remove or enrich Ni（Ⅱ）,so they are called nickel-resistant strain.The two strains could maintain good biological activity in Ni（Ⅱ）-Cr（Ⅵ）mixed wastewater.The removal rate of Cr（Ⅵ）was the highest in 30 mg/L Cr（Ⅵ）and 30 mg/L Ni（Ⅱ）mixed wastewater,FNXJ1-2-3 strain and FNZJ3-2-2 strain were87.68%and 88.6%,respectively.When the two strains were used to construct a composite microbial agent,the removal effect of Cr（Ⅵ）by a single strain was higher than that of a composite microbial agent with different bacterial content ratios,so the two strains were not suitable for use as a composite microbial agent.（2）Under the suitable conditions of Cr（Ⅵ）removal,the removal ability of FNZJ3-2-2strain to Cr（Ⅵ）was stronger than that of FNXJ1-2-3 strain.The results of kinetic analysis showed that:The Cr（Ⅵ）removal rate by FNXJ1-2-3 strain was 3.401×10-2 mg L-1 h-1 at 10mg/L and 3.100×10-4 mg L-1 h-1 at 50 mg/L.The Cr（Ⅵ）removal rate by FNZJ3-2-2 strain was 7.310×10-3mg L-1 h-1 at 20 mg/L and 6.383×10-4 mg L-1 h-1 at 60 mg/L.The adsorption behavior of Cr by FNXJ1-2-3 strain fitted well with Langmuir isotherm model.,R2=0.998,and the maximum adsorption capacity was 21.008 mg/L.The adsorption behavior of FNZJ3-2-2 strain to Cr fitted well with Freundlich isotherm model,R2=0.999,1/n=1.015,which was easier to adsorb.FTIR results showed that the two strains were bound to chromium through coordination between functional groups,such as amino,methyl,carbonyl,phosphoryl and polysaccharide C-O stretching vibration.XPS results showed that there was a process of Cr（Ⅵ）→Cr（ⅡI）→Cr（0）inside and outside the cells of the two strains,and with the increase of Cr（Ⅵ）concentration,the chromium reductase ECrⅥ→ⅡI and ECrⅡI→0 of the two strains were inhibited to varying degrees.（3）The removal of Cr（Ⅵ）by different cell components of FNXJ1-2-3 strain showed there were at least four ways to remove Cr（Ⅵ）.（I）Adsorbed,chelated,and reduced by the FG and EPS on the cell surface;（Ⅱ）The transport of various extracellular Cr species into the cell through their special（integrated）channel proteins;（ⅡI）Reduced by the chromium reductases existing intracellular and extracellular cells;（IV）Reduced by various external electron donors in the medium.（4）The degradation rate of dichlorobenzene was investigated under the condition of two strains using dichlorobenzene as the sole carbon source.The results showed that the degradation ability of FNZJ3-2-2 strain to dichlorobenzene was stronger than that of FNXJ1-2-3 strain.The suitable conditions of FNZJ3-2-2 strain in Cr（Ⅵ）-dichlorobenzene mixed wastewater were determined by orthogonal test.The initial dichlorobenzene concentration was 50 mg/L,p H 7.0,cell dry weight 0.1 g,and initial Cr（Ⅵ）concentration was 20 mg/L.At this time,the removal rates of o-dichlorobenzene,m-dichlorobenzene,p-dichlorobenzene and Cr（Ⅵ）were 88.76%,85.43%,79.07%and 90.22%,respectively.The results of enzyme activity detection of the crude enzyme solution inside and outside cells showed that the strain degraded dichlorobenzene mainly by by ortho-ring-opening dechlorination of catechol.In this study,the tolerance and removal characteristics of FNXJ1-2-3 strain and FNZJ3-2-2 strain to Ni（Ⅱ）and Cr（Ⅵ）were discussed.The effects of different environmental factors on the removal of Cr（Ⅵ）by strains were explored,the changes of cell morphology and chemical structure in the process of Cr（Ⅵ）removal were analyzed and the pathway of Cr（Ⅵ）removal was deduced.Finally,a combined detoxification system of dichlorobenzene and hexavalent chromium was established.The characteristics of chromium removal and chlorine reduction were analyzed by orthogonal test and enzyme activity detection,which provided theoretical support and technical reference for microbial cell treatment of heavy metal-organic chlorine mixed wastewater. |
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