| Hexane was so hydrophobic that the removal efficiency of conventional biological treatment technology affected by mass transfer limitations was unsatisfied.An efficient hexane-degrading strain named NX-1 was isolated from the activated sludge which was collected from the sewage treatment tank in a local pharmaceutical plant in Zhejiang Province.The strain NX-1 was identified as Pseudomonas mendocina through the morphological and physiological identification and 16S rDNA sequence analysis.NX-1 could completely degrade 700mg·L-1 hexane within 73h,the degradation rate was 0.04mg·mg-1cell·h-1,which had obvious advantage on degradation rate of hexane compared with other hexane-degrading bacteria.Since hexane removal rate was closely related to cell surface hydrophobicity?CSH?,this study subsequently investigated the key factors influencing the CSH of P.mendocina NX-1.The method of the microbial adherence to hydrocarbon was employed to determine the value of CSH.It was found that CSH was significantly affected by pH,starch concentration and chitosan concentration.The Box-Behnken design and response surface methodology were conducted,and the optimal conditions were found to be pH 7.61,starch concentration 9.39 g·L-1 and chitosan concentration1 g·L-1,resulting in the CSH as high as 56.0%.In order to evaluate the relationship between cell surface hydrophobicity?CSH?and the degradation rate of the P.mendocina NX-1,we have compared hexane biodegradation rate of NX-1 with CSH of 56.0%and CSH of 15.0%.The biodegradation of hexane conformed to the Haldane inhibition model,the maximum degradation rate(?max)of the cells with 56.0%CSH was 0.72 mg·mgcell-1·h-1,in comparison with 0.47 mg·mg-1cell·h-1 for cells with 15.0%CSH.The hexane concentrations being absent of substrate inhibition were inferred theoretically and determined to be under about 5359 and 4659 mg.L-1,respectively.The increase of CSH resulted in a significant increase of cumulative oxygen uptake amount,the oxygen uptake rate of cells at higher CSH was obviously higher,which could reach287.08?L.h-1 between 18h30h,compared with the highest rate of 57.25?L.h-1by lower CSH cells.The study also showed CSH was closely related to surface properties of NX-1.The increase of extracelluar proteins and decrease of polysaccharides and Zeta-potential resulted in an increase of CSH.FT-IR spectra also showed new lipid aliphatic chains and proteins peaks appeared in the cell surface of NX-1 with higher CSH.For the effect of CSH on hexane degradation in two phase partitioning bioreactor system?TPPB system?,the results showed that when silicone oil/water was 0.5,enhancement of CSH and hexane degradation efficiency were the best.In TPPB system,with the increase of hexane concentration,the difference of degradation efficiency with different CSH increased significantly.At high concentration of hexane(1200 mg·L-1),average degradation rate of NX-1 with higher CSH was 13.57mg·L-1·h-1,which was 5.15 mg·L-1·h-1 higher than lower CSH,this difference value of degradation average rate was higher than 1.2 mg·L-1·h-1 at low concentration(500mg·L-1).In addition,the hexane distribution coefficient in the silicone oil,the gas phase and the aqueous phase was 2513:13:1,and the distribution ratio of NX-1 with CSH of 56.0%in water and non-aqueous phase was 1:14.7,which was much higher than that of NX-1 with CSH of 15.0%.This experiment could provide foundation to further investigation on the role of CSH in TPPB system. |
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