Effects Of Fe³⁺,Zn²⁺and Surfactant On Extracellular Polymeric Substances Of Chlorobenzene Biodegradation

Volatile organic compounds?VOCs?,derived from natural and various anthropogenic sources,are the air pollutants of great concern in China.As the precursors to atmospheric photochemical reactions,they can be converted into secondary pollutants such as ozone and nitrogen oxides,which adversely affect the ecological environment and its functions.Chlorinated volatile organic compounds?CVOCs?are one of the most refractory organic compounds in VOCs due to their poor degradability,hydrophobicity,and other toxic by-products during degradation and transformation.Biological treatment of low-concentration CVOCs has the advantages of high economic efficiency and environmental friendliness.How to efficiently remove CVOCs in the environment is an important issue that needs to be urgently soveld by biological methods.In this paper,chlorobenzene,a typical representative of aromatic hydrocarbons in CVOCs,was selected as the target pollutant.The activated sludge from coking plant wastewater was inoculated into biotrickling filter?BTF?as the bacterial source,and the domestication of predominant chlorobenzene degrading bacteria was completed by adding the intermediate catechol to the BTF.Meanwhie,the effect of chlorobenzene initial concentration on the growth of microorganisms was investigated.To enhance the hydrophobility of chlorobenzene,an optimum surfactant was selected,and the critical micelle concentration and optimum concentration of the surfactant were also investigated.Based on the above reseach,the promoting effect of metal ions on microbial growth and metabolism was investigated,and the changes of microbial community structure were studied by high-throughput sequencing technology.Combined with Fourier Transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?,the influence of Fe³⁺and Zn²⁺on the chemical composition and functional group characteristics of extracellular polymeric substances?EPS?were studied to further reveal the mechanism of enhancement of chlorobenzene biodegradation by Fe³⁺and Zn²⁺.It was found that initial chlorobenzene at 150 mg/L could provide a great quantity carbon besides less toxicity inhibition.The micelles were formed when concentration was 74.98 mg/L,and the corresponding surface tension value of rhamnolipid was26.65 mN/m while Tween-80 was 53.1mN/m.The rhamnolipid at 2CMC could primely promote the growth and metabolism of microorganisms,the increment of OD₆₀₀ reached 0.144 when incubation time was 36h,and the degradation rate of chlorobenzene at 72 h was 66.4%.In contrast,there were also some toxicity inhibition of high concentration of rhamnolipid on the microbial growth,and the degradation of chlorobenzene would decrease with the increasing concentration of rhamnolipid due to the competition of carbon.On the base of these,ferric iron and zinc ion were investigated to promote the growth and metabolism of microorganism.The experiment results showed that both3mg/L Fe³⁺and 2mg/L Zn²⁺could effectively promote the growth of microorganisms and the degradation of chlorobenzene.Adding Fe³⁺and Zn²⁺simultaneously was able to accelerate the growth of microorganisms in logarithmic and stable period,interaction between different valence states of metal ions was confirmed that the metabolic ability of microorganisms was enhanced in carbon deficiency and intermediates accumulation,and the degradation rate of chlorobenzene at 72 h reached 66.4%.Among the effects of various trace elements under 2CMC rhamnolipid,combination of Fe³⁺and Zn²⁺>Zn²⁺>Fe³⁺>No addition.By observing the microscopic morphology of the microbial surface,it was found that the addition of Fe³⁺and Zn²⁺could make the surface of the microbial cell more dense,which make it easier to capture the pollutants and increase residence time,resulting in improving the mass transfer and degradation efficiency of the contaminants between the membranes.Quantitative analysis of the extracted EPS components by heating method also found that Fe³⁺and Zn²⁺promoted the secretion of a large number of polysaccharides and proteins by microorganisms.The increase of polysaccharide content in S-EPS enhanced its interaction with metal ions,besides the increase in the proportion of TB-EPS was conducive to the stability of the biofilm structure.The proportion of dominant strains Brevibacterium and Gordonia in BTF was increased to by 4.16%and 4.25%respectively,under the synergistic combination of rhamnolipid,Fe³⁺and Zn²⁺.Correspondingly,the degradation rate of chlorobenzene increased from 66.4%to 75.6%when mixed bacteria was isolated and cultured for 72h.FTIR and XPS analysis of EPS from various periods showed that the band strength of carboxyl and amine increased under the stress of Fe³⁺and Zn²⁺,both content and peak area of carboxyl had increased.All these indicated that Fe³⁺and Zn²⁺participated in the synthesis of protein and promoted the secretion of protein.The stretching vibration of C-H and C-Cl were enhanced which verified oxidative ring-opening reaction and dechlorination of chlorobenzene became more rapidly,the performance of BTF was positively affected by these metal ions.The shifts of electron binding energy of O1s and Zn2p_2/3 and the change of characteristic peaks of Fe2p indicated that Fe³⁺and Zn²⁺have interacted with the active chemical groups of polysaccharide and proteins in EPS,thus enhancing the biodegradability of chlorobenzene by mixed bacteria.