Screening Of Hexavalent Chromium Reducing Bacteria And Its Application In Remediation Of Polluted Groundwater

With the development of modern industry,hexavalent chromium in industrial waste will cause great harm to human health and ecological environment after entering into groundwater environment.Compared with physical and chemical remediation,microbial remediation is more environmentally friendly,but it is difficult for free microorganisms to play a direct role in hexavalent chromium contaminated groundwater.Current research mainly focuses on the static simulation remediation experiment of high-efficiency hexavalent chromium reducing bacteria,which can not make a reasonable judgment or provide scientific basis for the microbial treatment of actual hexavalent chromium polluted groundwater.In order to solve the above problems,this study combined classical analytical techniques with advanced techniques such as electron microscopy analysis through static batch experiments and dynamic simulation experiments in the laboratory to study:(1)the domestication,screening and identification of hexavalent chromium reducing bacteria;(2)the growth and reduction characteristics of reducing bacteria and the reduction mechanism of hexavalent chromium;(3)the preparation optimization and static simulation application of immobilized microbial agent;and(4)the dynamic simulation application and main controlling factors of microbial agent in microbial PRB.Our aims were to screen highly efficient hexavalent chromium reducing bacteria suitable for groundwater remediation,to develop immobilized microbial agent for remediation of hexavalent chromium polluted groundwater,and to clarify the key regulatory factors and effects of microbial agents in the dynamic application of microbial PRB.The main results of this study were as follows:(1)A strain of reducing hexavalent chromium was isolated from activated sludge of a sewage treatment plant in Qingdao.The strain could tolerate 100 mg/L hexavalent chromium.When the concentration of hexavalent chromium was 20 mg/L,the bacterial strains in the medium grew fastest at 16 h-20 h,and the removal rate reached 100%within72 h.The strain was named ZY-2009.Some physiological and biochemical characteristics of this strain were compared with the relevant strains in the"Bergey's Manual of Systematic Bacteriology",and the homology of the gene sequences measured by 16S r DNA was compared with that of the strains in Gene Bank.Then the strain was identified to be Bacillus cereus ZY-2009.(2)The growth and reduction characteristics of Strain ZY-2009 were studied,and its adaptability and function in different environments were understood.Results showed that the optimum p H for removing hexavalent chromium by this strain was 7.0 and the temperature was 30?,and the removal rate of 20 mg/L hexavalent chromium solution reached 80.8%after 48 h of reaction.The removal rate of hexavalent chromium by the strain gradually increased with the increase of inoculation amount.The strain also had a good removal ability for hexavalent chromium coexisting with various ions(Fe³⁺,Cu²⁺,SO₄²⁺,and NO^3-).The dead cells and permeable cells of ZY-2009 were prepared,and the components of the cells were separated by ultrasound and filtration.The removal effect of hexavalent chromium in each treatment group was determined.The results showed that the removal of hexavalent chromium by Strain ZY-2009 was mainly via bioreduction,and the contribution of bioadsorption was negligible(3)Immobilized microbial technology combined ZY-2009 with carrier material to solve the problem of limited effect of free strain in groundwater environment.The experimental results showed that the removal effect of hexavalent chromium by activated carbon as carrier material was better than that by biochar and diatomite,and the bacteria prepared by compound immobilization method was better than that by adsorption method and free bacteria.The removal rate of hexavalent chromium could be improved by adding glucose as additional nutrient in the preparation process.The preparation process of compound immobilized bacterial agent was optimized.The results showed that the best effect was obtained when of polyvinyl alcohol was used to 5%(w/v),sodium alginate at 3%(w/v)and Ca Cl₂ at 4%(w/v).When the calcification time was 2 h and the ratio of bacteria to glue was 1/10,the removal rate of 35 mg/L hexavalent chromium solution reached 75.08%at 48 h.When the concentration of hexavalent chromium increased from 20 mg/L to 100mg/L,the removal amount of hexavalent chromium increased from 0.18 mg/g to 0.47 mg/g.Under the dual action of carrier material and strain,bacterial agent was suitable for bioremediation in most p H ranges.Scanning electron microscopy and BET analysis showed that the bacterial agent and carrier materials were mainly mesoporous below 10 nm.The bacterial agent prepared by compound immobilization method could encase the bacteria inside and reduce their loss.(4)Microbial PRB column was constructed with immobilized microbial agent as filler to explore the key influencing factors and reaction mechanism of microbial agent on dynamic remediation of hexavalent chromium contaminated groundwater.Compared with the adsorption method,the compound immobilization method produced less outflow of bacteria,higher removal rate of hexavalent chromium,but increased amount of COD in water samples.With the increase of hexavalent chromium concentration,the removal rate of hexavalent chromium in the solution by bacterial agent decreased gradually,but the removal amount did not decrease significantly.The removal amount of hexavalent chromium per unit bacterial agent in the dynamic simulation remediation of microbial PRB was slightly lower than that in the static experiment.The flow rate was an important factor affecting the remediation effect of the bacterial agent.When the concentration of hexavalent chromium was 15 mg/L and the operation was performed for 144 h,the removal rate(78.33%)in the 8.34 m L/min treatment group was 19.78%lower than that in the 2.78m L/min treatment group.With the increase of usage times,the removal efficiency of the bacterial agent gradually decreased.In conclusion,Strain ZY-2009 was feasible for remediation of hexavalent chromium pollution in groundwater.The compound immobilization method could ensure the stability of the bacterial agent and the effect of Strain ZY-2009,but in the microbial PRB device,the immobilization agent in the microbial agent would dissolve and release COD,which will affect the groundwater environment.Therefore,in practical application,the preparation of microbial agent and the operation parameters of microbial PRB structure design should be optimized according to the environmental conditions of contaminated sites,so as to improve the remediation efficiency and reduce environmental risks.The results can provide scientific basis for microbial remediation of groundwater contaminated by hexavalent chromium.