Biodegradation Of Organic Pollutants By Microbial Cells/nanoparticles Biocomposites

Carbazole is a kind of nitrogen heterocyclic aromatic compounds found in fossil fuels and is also the model compound in the environmental pollution.Therefore,it is necessary to establish an effective,stable,simple and environmentally friendly method to chean up carbazole.The immobilization of microbial cells was considered to be one of the most promising approaches in the biodegradetion.Due to good biocompatibility,large specific surfaces,and super-paramagnetism,magnetic Fe3O4 nanoparticles can be used as a new carrier for microbial cells immobilization.However,the adaptability and stability of the immobilized microbial cells by Fe3O4 nanoparticles need to be further explored in the carbazole degradation.Recently,researchers focus on the effects and mechanisms of nanoparticles(NPs)toxicity against various bacterial in the immobilization process.There are different toxicity effects and toxicity mechanisms of different nanoparticles on microbial cells.Therefore,the toxicity effects and toxicity mechanisms of nanoparticles on microbial cells should be further explored in the specific situation.In this paper,Sphingomonas sp.XLDN2-5 was chosen as the research microbial which can degrade carbazole.Fe3O4 nanoparticles with super-paramagnetic were synthesized by the chemical coprecipitation method.Then,an efficient biocomposites was constructed successfully by assembling Fe3O4 nanoparticles onto the surface of Sphingomonas sp.XLDN2-5 cells.The resulted Sphingomonas sp.XLDN2-5/Fe3O4 nanoparticles biocomposites was characterized by transmission electron microscopy(TEM).We explored the carbazole degradation performance of immobilized cells under different environmental conditions,and found that free cells showed the maximum rate of carbazole degradation in 37℃ and pH7.0 conditions which degraded 3500 μg carbazole within 7 h.Compared to free cells,the biocomposites exhibited a wider range of pH and temperature.When we added dodecane with LogP of 6.6,the carbazole degradation rate did not decrease.In contrast,organic solvents with LogP less than 4 presented negative influence on the degrading activity for free cells and the biocomposites.Due to the super-paramagnetic properties of Fe3O4 nanoparticles coating,the biocomposites could be collected effectively by a magnetic field.This characteristic can be effectively applied in the reused experiment.As to the reused experiment,the biodegradation activity of microbial cell/Fe3O4 nanoparticles biocomposites increased obviously during the ten cycling processes.During the tenth cycle,3500μg carbazole can be degraded completely within 2 h.After stored at 4℃for a long time,the biocomposites showed excellent stability which can restore the degradation activity in a short time.These results provided the foundation for the long-term and repeated use of the biocomposites.In order to explore nanoparticles antibacterial effect,we chose Fe3O4 nanoparticles,Fe2O3 nanoparticles,Al2O3 nanoparticles,and multi-walled carbon nanotubes(MWCNT)as immobilization carriers.After cultured for 2 h,the toxicity effects of four kinds of nanomaterials on Sphingomonas sp.XLDN2-5 cells were evaluated.It is clear that the cytotoxicity of nanomaterials increased gradually with increasing of concentration nanomaterials.What’s more,nanomaterials at low concentrations did not caused obvious influence on the degradation activity of carbazole.This study explored possible mechanisms of nanoparticles’ cytotoxicity.TEM images showed a nanoparticles coating was observed due to the attachment of nanoparticles on the surface of microbial cells,suggesting that physical interaction may lead to microbial surface damage.Zeta potential measurement showed the NPs and bacterial have the similar electric potential.This phenomenon can explain the relatively low toxicity of nanomaterials at low concentration.To further explore the cytotoxicity of Fe3O4 nanoparticles,we detected the level of intracellular reactive oxygen molecules(ROS)and the surface damage of microbial cells after cultured with nanoparticles at different concentration.Results showed that the intracellular ROS and cell wall damage can be clearly detected at high concentrations of Fe3O4 nanoparticles.These results suggested that intracellular ROS and cells wall damage may be the possible mechanisms of the nanoparticles’ cytotoxicity at high concentration.