Study Of The Behaviour Of Phenolic Compounds Adsorption And Biodegradation By Chlorella Pyrenoidosa

Phenolic compounds can enter into the environment during the discharge of industrial wastewater,the use of pesticides and the disinfection of drinking water.Their wide existence in natural waters and sediments leads to disturbances of endocrine system and hormone level of aquatic organisms.Algae have a good potential in the biodegradation of phenolic compounds,however depend on environmental conditions.In this study,we selected Chlorella pyrenoidsa,a common green algae in freshwater,to remove 2,4-dichlorophenol(2,4-DCP),an ionizable phenolic compound.The research aimed to study the influences of different environmental conditions on the removal of 2,4-DCP by Chlorella pyrenoidsa in a closed-system,through short-term and long-term culture experiments,as well as the domestication.The main results are showen as follows:It was established in the study a method for determination of 2,4-DCP concentration in water and algae phases by gas chromatography–mass spectrometry(GC-MS)with the application of ultrasonic solvent extraction in the pretreatment of biological samples.The method detection limits were 0.390 μg/L and 0.010 μg/mg,respectively.In this study,the effects of initial p H and initial 2,4-DCP concentration on the removal of 2,4-DCP by Chlorella were investigated.The algae were cultured for 6 h in a dark and closed algae+DCP system at different initial pH of 5,6,7,8,9.The results showed the maximal 2,4-DCP removal occurred at pH 7~8.It was a consequence of the interaction between 2,4-DCP and the lipid molecules and polar parts of the algae cell membrane,ionizing or neutral form of 2,4-DCP in culture and the growth state of algae in buffer medium.Chlorella pyrenoidosa removed 2,4-DCP from water by absorption and transformation.The removal gradually increased with time as initial concentration of 2,4-DCP ranging from 30 to 350 μg/L during short-term.In addition,the amount of removal per unit mass of algae was increased as initial 2,4-DCP concentration increased.It was also observed that the bioaccumulation in Chlorella cells was very low,which was due to the property of 2,4-DCP.In the long-term experiment,the effect of initial pH(5,6,7,8,9)on the growth and biodegradation activity of Chlorella was investigated.In 120-h culture,with the significant increased alage density,the ability of Chlorella pyrenoidosa to degradate 2,4-DCP was highest at the initial pH 7.0,decreased in acid condition.It was correlated with the higher toxicity of 2,4-DCP in acid condition and adaptation of algae growth in the neutral or weak basic medium.At different light:dark ratio conditions(24:0,16:8,12:12,8:16,0:24 h),Chlorella pyrenoidosa was used to biodegrade 2,4-DCP during 120-h.The result showed,algae has an highest ability to remove 2,4-DCP at 12:12 h light:dark condition,while the ability decreased under other studied conditions,indicating that the participation of the photosynthetic apparatus and the respiratory mechanism of algae to biodegrade 2,4-DCP.In addition,the bioaccumulation of algae was only detected under 24 h absolutely dark and the amount was little.The effect of different carbon sources(absence of carbon source in medium,citric acid,Na2CO3,citric acid+ Na2CO3)on the removal abilities of algae in algae+DCP culture during 120-h was investigated.2,4-DCP can be used as sole carbon source for metabolism of Chlorella pyrenoidosa.The addition of external carbon source would promote the growth of algae,but also inhibite 2,4-DCP biodegradation.The effect of domestication on the degradation of 2,4-DCP by algae was also studied.The original and domesticated algae were both cultivated in the algae+DCP culture during 120 h.It was found that there was no 2,4-DCP detected in the algae cells,indicated that biodegration was the main way to remove 2,4-DCP by algae.Shortened of the lag period and faster growth rate of algae were also found after domesticated.The maximum specific growth rate and maximum specific degradation rate of domesticated algae were 0.47 and 1.44 d-1,which were 1.62 and 1.97 times of the original algae.The further analysis showed strong correlations among algal density,chlorophyll-a content and 2,4-DCP residual in the culture,indicating that 2,4-DCP removal is related to the growth of algae.The correlation between pH and the other three parameters mentioned above was stronger when the algae after domesticated,indicates domestication can enhance the the tolerance to 2,4-DCP of algae.As a result,the removal of 2,4-DCP by algae was mainly through biodegradation.Algae can use 2,4-DCP to growth.The best removal efficience of 2,4-DCP by Chlorella in closed system was obtained under the optimized conditions: initial pH 7.0,light:dark 12:12 h and no external carbon source added in the culture.This study had great significance to underatand the migration and transformation behavior of 2,4-DCP in water and the mechanisms of 2,4-DCP removal by freshwater algae,provided a theoretical basis for the operation conditions and selection of algae in the treatment of phenolic wastewater by algae.