| The degradation of microorganisms is the main pathway for the removal of PAHs,but the free bacteria will face the cometition of indigenous bacteria,the toxic effects of interimediates,and even the adverse environmental conditions,thus casuing the low survival rate and low density per unit volume as well as slow dagradation rate of the effective degrading bacteria.However,microbial immobilization technology precisely overcome these drawbacks.Selection of low-temperature resistance highly efficient PAHs-degrading bacteria and immobilization carriers is of vital importance,followed by selection of immonilization methods in order to achieve large scale popularization and application of microbial immobilization technology to repair freeze-thaw soils contaminated by PAHs in cold regions of northern China.In this paper,five strains?J4,L1,L2,S3,S4?of low temperature-resistant PAHs degrading bacteria that have been selected in the previous period of this research group,degradation of phenanthrene?Phe?,pyrene?Pyr?,and benzo?a?pyrene?Bap?in the contaminated soil was studied,and Michaelis-Menton and Mond kinetic models were applied for the fitting of the degradation results,obtained 3 strains?J4,L1,andS4?of low-temperature-resistance PAHs-degrading bacteria.Selected bacteria and two groups of fungi?J7,S1?were arranged and combined to obtain 6 groups of mixed bacteria?J4J7,L1J7,S4J7,J4S1,L1S1,S4S1?,and the optimal mixed bacteria?S4J7?were selected and identified.Corn cobs,peanut shells,vermiculite,and peat soils were selected as test carriers to select the best immobilized carrier materials and optimize and determine the immobilization process.Furthmore,the ability of immobilized microorganisms to repair PAHs-contaminated soil was investigated through laboratory simulation experiments.The optimum conditions for immobilized mixed bacteria were determined through environmental factors tests.Finally,the mechanism of immobilized repairing was performed preliminary discussion through scanning electron microscope?SEM?analysis.The main conclusions were as follows:?1?S4 had the best degradation effect on Phe,and could degrade 59.18%of Phe in 60 days.For the high-ring Pyr,the two strains had slightly superior degradation effect.J7 could degrade 38.11%of Pyr.The two fungi J7,S1 had a better degradation effect on Bap than bacteria,with the degradation rates being 28.26%and 25.47%,respectively.?2?From the three indicators of degradation rate,degradation rate and kinetic equation,most of the mixed bacteria had better effect on the degradation of PAHs in soil than some fungi and bacteria under low temperature conditions.Among them,S4J7 hads a slightly better degradation effect on Pyr and Bap than on single strains S1and S4.After 60 days,S4J7 could degrade 37.15%of Pyr and 27.41%of BaP.The bacterial S4 was identified as Pseudomonas sp.?SDR4?;the fungus J7 was Mortierella alpina sp.?JDR7?.?3?Corn cob?Y?,peanut hull?H?,vermiculite?Z?and peat soil?N?were used as test carrier to adsorb immobilized mixed bacteria S4J7?h?.The order of removal rate of PAHs in soil after 60 days was as follows::Z-h>Y-h>H-h>N-h>h,in which there was no significant difference between Z-h and Y-h,which could degrade 64.38%and58.49%Phe,48.71%and 45.91%of Pyr,40.19%and 37.07%of BaP,respectively.?4?Generally,10-15%inoculation of immoblized mixed bacteria coformed to the pratical application.The best degradation temperature for Y-S4J7 to PAHs was10°C.The degradation effect was better when the moisture content was 20%-60%.When the initial concentration of BaP was 30 mg·kg-1,Y-S4J7 had the best utilization and the best degradation effect.?5?In the presence of indigenous bacteria,the free bacteria occupied an absolute advantage,and the immobilized mixed bacteria treatment effect was better.After 60days,the residual rates of Phe,Pyr and BaP were only 41.51%,54.09%and 62.93%,respectively.?6?The mycelium of the carrier corn cob through SEM was more dense and the bacteria moved along with the growth of mycelia,exerted the synergistic action of bacteria and fungi,and showed great advantages in the degradation of PAHs.In addition,the corn cob was widely sourced in the northern regions,non-toxic,and has a certain degree of biodegradability,which was conducive to the growth of microorganisms.Therefore,the corn cob could be selected as a carrier for the immobilization of bacteria-fungal symbiosis system to repair PAHs-contaminated soil under freeze-thaw alternate conditions in northern cold regions. |
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