Degradation And Application Of O-Cresol In Wastewater By Microalgae

Phenolic wastewater has a wide range of pollution and great harmfulness,which brings serious harm to human,water,fish and crops.As a representative phenolic compound in wastewater,o-Cresol has the characteristics of high toxicity,strong occurrence and difficult degradation.Microalgae can metabolize and degrade phenol and its derivatives into non-toxic and harmless substances.In this paper,the intelligent light incubator was used as the microalgae culture instrument under experimental conditions.The degradation of o-Cresol in different concentrations by different algae species was studied by artificial preparation of o-Cresol solution to simulate wastewater.The effects of different culture conditions（light or dark,different initial p H,different initial algal density）on the biodegradation of o-Cresol by microalgae were investigated under a specific concentration of o-Cresol for the optimum degradation species.Considering the growth of microalgae and the degradation of o-Cresol,the effects of adding different concentrations of carbon sources on the tolerance of microalgae and the removal rate of o-Cresol were investigated under the optimal degradation conditions.The metabolic pathway of o-Cresol degradation was analyzed.The effects of different influent concentrations and hydraulic retention time（HRT）of o-Cresol on the degradation of o-Cresol by microalgae in an alga-membrane bioreactor with optimal degradation species were investigated.The results show that:（1）The study of first selection was less and the strong ability of degradation of organic pollutants of three kinds of microalgae:Scenedesmus obliquus,Microcystis aeruginosa and Selenastrum capricornutum in the initial algal liquid after vaccination for alkali or alkaline conditions for experiments,the results showed that in different initial concentration of o-Cresol,Selenastrum capricornutum had the best degradation efficiency.When the initial concentration of o-Cresol was 50～200 mg/L,the o-Cresol could be completely degraded within 6 days.After 7 days,the corresponding microalgae biomass was greater than that of the control experiment.However,the degradation of o-Cresol in 7 days was 18.2%and 25.5%,respectively,at the minimum initial concentration of o-Cresol at 50 mg/L.When the initial concentration of o-Cresol was 400 mg/L,the growth of Scenedesmus obliquus was almost completely inhibited.Corresponding the initial concentration of o-Cresol was greater than 300mg/L,the biomass of Microcystis aeruginosa decreased linearly.Through the analysis of experimental data,it was determined that Selenastrum capricornutum was the best species for the degradation of o-Cresol.（2）This paper discussed the degradation efficiency of o-Cresol and tolerance of Selenastrum capricornutum under different culture environment:light factors,the different initial p Hand the different initial algal densities.The study found that under the condition of all the darkness of the 50 mg/L of o-Cresol within 7 d was completely degraded,however,the degradation rate under which were ignored under the full light condition.Microalgae grew better in full light than in dark;When the initial p H of algal solution was 6.0 and 7.0,100mg/L o-Cresol was completely degraded within 5 days.No matter the initial p H value was 6.0or 7.0,the growth of Selenastrum capricornutum was similar.When the initial density of algae OD₆₈₀ of 0.20,the degradation effect of o-Cresol was the best.When the initial concentration of 100 mg/L of o-Cresol within 5 d was fully degraded.Regardless of the initial algae density,when the p H of 7.0 Selenastrum capricornutum biomass were greater than the p H of 6.0.Through a series of studies,it was determined that the degradation of o-Cresol mainly occurred under dark conditions,the optimal initial p H of degradation was 7.0,and the OD₆₈₀ was 0.20.（3）Under the conditions of light:dark of 12h:12h,optimal initial p H and algal density for degradation,the effect of co-degradation of o-Cresol by adding different concentrations of organic nutrient carbon source（glucose）or organic pollution carbon source（phenol）and o-Cresol to medium was investigated.The experimental data showed that Selenastrum capricornutum preferentially degraded glucose to promote the growth at higher initial o-Cresol concentration.But this did not mean that higher glucose concentration was better;With the same initial concentration of glucose,the growth of microalgae would be inhibited under the stress of higher concentration of o-Cresol.The change of chlorophyll a content in the growth process of microalgae was basically consistent with the change of algae density OD₆₈₀.When the initial o-Cresol concentration was 300 mg/L,the degradation rate of o-Cresol in the experimental group with glucose was 2.2 times that of the control group.When the initial o-Cresol concentration was 400 mg/L,the degradation effect of o-Cresol by microalgae was not obvious,but the growth of microalgae was significantly promoted.At the same initial o-Cresol concentration,the growth of microalgae was gradually inhibited with the increase of phenol concentration.Selenastrum capricornutum could degrade both phenol and o-Cresol at the same time.When the initial concentration of o-Cresol was 100 mg/L and the initial concentration of phenol was 120 mg/L,the degradation could be completed within 8days.However,the degradation effect of the two pollutants was obviously inhibited at the higher initial concentration of phenol.（4）The degradation pathways of o-Cresol by Selenastrum capricornutum was studied.Using the qualitative analysis of GC-mass and LC-mass,the production of 2-hydroxybenzyl alcool,salicylic acid and 2,5-dihydroxybenzoic acid was detected in the degradation process of o-Cresol.After speculating that the degradation of o-Cresol was the gentisic acid pathway:o-Cresol undergoes a series of enzymatic reactions such as phenol hydroxylase during the degradation process,followed by methyl carboxylation to generate hydroxybenzoic acid,before ring opening and cleavage re-hydroxylation at the ortho position of the benzene ring hydroxyl group to produce 2,5-dihydroxybenzoic acid,which was then further degraded.（5）The simple application of o-Cresol degradation in algae-membrane bioreactor was studied.The results show that under the condition of constant hydraulic retention time,when the influent concentration of o-cresol was less than 200 mg/L,the removal rates of o-cresol and COD reach 90%and 83%,respectively;When the o-Cresol influent concentration was maintained at 300 mg/L and the hydraulic retention time was greater than 3 days,the removal rates of o-Cresol and COD would reach 85%and 80%,respectively.Scanning electron microscopy analysis showed that the microalgae only attached to the surface of the membrane element and did not enter the interior of the membrane structure,so it did not cause serious membrane pollution.