Stimulation Effects And Mechanisms Of Biocahr In Biodegradation Of Phenol

The complete removal of organic pollutants in the environment depends mainly on the degradation of microorganisms.However,this method is partly limited in its application due to constrains in processing efficiency,tolerance to pollutant concentration,and adaptability of environmental conditions.In recent years,with biochar showing potential on promoting microbial degradation of organic pollutants such as azo dyes,phenanthrene and 2,6-dichlorophenol,the material is receiving more and more attention from researchers.Nevertheless,studies on the specific use conditions of biochar,the variance in effectiveness in different media and the promotion mechanisms are still in their infancy.In this paper,phenol was used as a model organic pollutant,and Pseudomonas citronellolis was selected as the phenol degrading strain.We specifically investigated the effects of different biochar addition quantities,pyrolysis temperature and microbial immobilization method on phenol degradation in aqueous phase;the effects of different strategies of biochar-microbe utilization on phenol degradation in soil;and finally,the biochar promotion mechanisms.The main conclusions drawn are as follows:(1)Pseudomonas citronellolis could completely degrade 110 mg/L phenol at 30 ? in 24 hours.When the concentration was increased to 420 mg/L,significant substrate inhibition occurred,and the degradation rate reduced to 45.7%.Although the microorganism is most adaptable in a neutral pH environment,a slightly alkaline condition generally contributes to an increase in phenol degradation rate.The best promotion effect was obtained by adding 0.6%(w/v)and 550 ? pyrolyzed biochar.At 800 mg/L phenol concentration,the degradation rate reached 100%,which was much higher than that of microorganisms alone(only 25.4%).(2)Using the method of calcium alginate gel fixation,the tolerance of microorganisms to phenol in the aqueous phase reached 1200 mg/L together with a further improved phenol degradation rate(33.7% to 62.5%).Gel fixation can significantly impede the diffusion of phenol,while biochar addition can alleviate this negative effect and weaken the toxicity of phenol to microbes by adsorption.(3)Biochar pyrolyzed at 550 ? can significantly improve the degradation of phenol by indigenous microorganisms in loessial soil and paddy soil(25.2% to 38.8%,16.6% to 34.1%,respectively)and promote complete phenol mineralization.It is supposed that,biochar may reduce soil toxicity and weaken the mass transfer resistance in the soil medium by its adsorption capability,and stabilizes soil pH during degradation by its alkalinity.Exogenous microorganisms and microbial-loaded biochar didn't show a substantial promoting effect because of the fierce competition with indigenous flora.(4)Biochar exerts multiple effects in the microbial degradation of phenol in solution.Firstly,biochar's alkalinity neutralizes acidic degradation intermediates.The alkalinity of 350 ? pyrolyzed biochar is mainly derived from hydroxyl groups and carboxylate anions;while that of 550 ? and 750 ? pyrolyzed biochar mainly originates from inorganic minerals such as calcium carbonate and calcium oxide.Secondly,the adsorption-desorption behavior of phenol by biochars affect the efficiency of microbial degradation.Biochar pyrolyzed at 350 ? mainly relies on partitioning and the "electron donor-acceptor" complex formed by its surficial electron donor groups with the phenol molecule,so that phenol is easily desorbed back into the solution;biochar pyrolyzed at 750 ? mainly depends on the dispersive force between the ?-electron of the graphite layer and aromatic ring of phenol,and the microporous structure;biochar pyrolyzed at 550 ? possesses both.Adsorption by micropores makes phenol difficult to desorb,reduces phenol bioavailability,and may pose a continuing risk of contamination.Thirdly,350 ? pyrolyzed biochar releases more organics in its water extracts,which may be directly dissimilated by microorganisms or synergistically degraded with phenol contributing to microbial proliferation and removal of organic pollutants.The amounts of organics released by biochars pyrolyzed at 550 ? and 750 ? are relatively smaller,and with the probable high toxic substances such as polycyclic aromatic hydrocarbons,their water extracts inhibited the activity of microorganisms and prolonged the lag phase.