The Role Of Exogenous Electron Donors For Accelerating 2,4,6-trichlorophenol Biodegradation

2,4,6-trichlorophenol(TCP) is one of a toxic organics which has been widely used as a pesticide, herbicide, and lumber antimicrobial. The widespread use of TCP has led to its presence in the natural environment, creating a potential human-health risk. For treatment of wastewater containing TCP, conventional approaches are usually physical, chemical and biological methods. Biological treatment is economical and feasible method and has been widely employed for treatment of wastewater containing TCP. TCP is also inhibitory to microorganisms, and this has precluded its biodegradation in normal biological wastewater treatment, biological methods have a certain limitation when they are used for the treatment of TCP. In principle, the kinetics for the transformation of a slowly biodegraded hydrocarbon can be accelerated by a strategy called co-substrate.TCP belongs to one of recalcitrant organic compounds, but it’s biodegradation could be accelerated by adding some exogenous electron donors. In this work, some exogenous electron donors, acetate, formate, and phenol, were respectively added into solution of TCP to accelerate its biodegradation to varying degrees, and effectively improve the degree of-its mineralization. Based on the research, an internal circulation baffled biofilm reactor(ICBBR) which provides anoxic and aerobic reaction conditions was used for the biodegradation of TCP. At the same time, TCP’s biodegradation intermediates were analyzed to explore the mechanism how exogenous electrons accelerate the biodegradation of TCP. Experimental results were gotten:(1) Bacillus amyloliquefaciens grew under aerobic and anoxic + aerobic conditions, which gives similar trend. For 10 hours of biodegradation, TCP was degraded completely under aerobic + anoxic condition, and it was only 63% TCP removal percentage under aerobic condition. Protocol aerobic + anoxic always gives higher TCP removal percentage.(2) Acetate was compared with formate for accelerating TCP to suggest that acetate led faster TCP biodegradation because it has more electron equivalents, but the same electron equivalents gave the same rates.TCP removal rates were increased by 66% and 88% by adding 1m M formate and 1m M acetate, because acetate has 4 times electron equivalents more than formate. The experiments with different amount of formate and acetate added document that 0.25 m M acetate and 1m M formate had the same effect on TCP removal rates, and 0.5m M acetate and 2m M formate had the same effect on TCP removal rates, which suggested that amount of electron equivalents is proportional to TCP removal rates.(3) 1m M formate, acetate and phenol were respectively added into solution of TCP. The experimental results documented that acetate gave faster TCP biodegradation with the same amount of mol added than phenol and formate, which was faster than phenol. However, phenol gave the slower TCP biodegradation, although it has more electron equivalents than both acetate and formate. Acetate and formate were compared with phenol for TCP removal to suggest that there were only less than 7% electrons of phenol to be utilized for accelerating TCP biodegradation, while the most electrons of it for itself mono-oxygenation and cleaving ring.(4) Analysis on TCP biodegradation pathway suggested that biotransformation is initiated by three mono-oxygenation reactions that require molecular oxygen(O2) and an intracellular electron carrier(represented as 2H); they insert three –OH groups into the ring during TCP biodegradation.(5) The removal of COD of which containing formate、acetate and phenol was higher than the solution of TCP alone. The experimental result suggests that adding organics can accelerate the removal of COD to varying degrees, and effectively improve the degree of TCP’s mineralization.(6) TCP removal rate was faster after UV photolysis compared with original TCP without photolysis. TCP’s photolysis intermediates have a lower microbial inhibitory resistance, and they play the role of the exogenous electron donors to accelerate the biodegradation of TCP further. The results can be used in industrial and other practical wastewater treatment to improve the processing efficiency of wastewater containing TCP.(7) To analyze quantitatively the relative impacts of added and endogenous electron donor, we did an H-equivalent accounting for all experiments. Oxidation of a readily biodegradable exogenous donor had a stronger impact on accelerating initial reactions requiring H than did oxidation of the endogenous donor, maleic acid semialdehyde.(8) Anoxic and aerobic reaction conditions were made by means of the internal circulation baffled biofilm reactor, which effectively improve the degradation efficiency. Therefore a novel technology can be used for organic wastewater treatment.