Transformation Of Two New Carbon Nanomaterials In Typical Water Treatment Disinfection Processes:Mechanisms And Environmental Effects

Carbon nanomaterials（CNMs）have been widely used in the fields of electronics,chemical industry,energy,medicine,and aerospace due to their unique electronic,optical,thermal,mechanical and chemical propertiesm,inevitably,CNMs will enter the environment during their production,transportation,application and disposal process.Most of the CNMs released into the environment enters natural water bodies directly or indirectly through sewage treatment plants.CNMs will be physically,chemically and biologically transformed during the water treatment and disinfection processes.The physical and chemical properties of CNMs after transformation be changed,which would affect their environmental fate and risks.In recent years,graphitized carbon nitride（C₃N₄）andcarbon quantum dots（CD）have attracted the attention due to their unique structures and properties.Both of them have been broadlyapplied in environmental protection and biomedicalfields.Therefore,it is necessary and significant to explore the degradation and transformation of C₃N₄and CD in the process of water treatment and disinfection,and to evaluate the changes on their environmental effects.In this study,C₃N₄and CD were selected as the representative new CNMs,and their degradation and transformation mechanism in water treatment and disinfection processes were systematically studied.The main research contents and results are as follows:（1）The C₃N₄（CN）was prepared through thermo-polymerizationmethod,the photo-transformations and photocatalytic activity of bulk C₃N₄（CN）and oxidized C₃N₄（OCN）in UV disinfection wereexamined.Various characterization results revealed the structural and chemical properties changes of CNand OCN under UV irradiation.In addition,the influence of photo-transformation on the water stabilities of CN and OCN wereassessed by conducting sedimentation experiments.In this study,we proposed that under reactive oxygen species attack,CN and OCN weregradually broken into smaller fragments and finally mineralized into NO3^-、CO₂and H₂O,the potential photo-transformation pathways of CN and OCN werespeculated by identifying the react intermidates.Moreover,the photocatalytic activities of the photo-transformed OCNand CN substantially decreased,indicating that the structural changes might be the main reason for theirphotocatalytic activity loss.The stability of CN in water increased while the water stability of OCN decreased after photo-transformation.These results highlighted the non-negligible influenceof photo-transformation on the environmental fate of CN-based materialsafter they are released into environment.（2）Fluorescent CD was synthesized by hydrothermal calcination of urea and citric acid.The degradation and transformation of CD in three typical water treatment disinfection processes（UV,ozone and chlorine）were studied.The effects of water environmental factors including p H,dissolved organic matter and anions(Cl^-,CO₃^2-,HCO₃^-,SO₄^2-and NO₃^-)on the transformation of CD in water treatment disinfection processes were examined.The results indicated that CDcan be degraded and photobleached in three disinfection processes and various water parameters show different effects on the transformation of CD.Compared to chlorine disinfection,thedegradation and transformation effeciencies of CD in UV and ozone disinfection processes are higher,with the more obvious photobleaching phenomenon.Meanwhile,the elemental composition of CD also changed differently after the treatment by three disinfection processes.In addition,the degradation pathways of CD were proposed by identifying the degradation intermediates of CD,chlorine substitution and ring cracking steps,indicating that CDwill be transformed into small molecules in the water treatment and disinfection processes.