Study On The Degradation Of Polyethylene Terephthalate Microplastics By Three-dimensional Electrochemical Technology

Plastics with a particle size of less than 5 mm are called microplastics（MPs）.They accumulate in the environment,and endanger biological and human health.The current research on the degradation of MPs focuses on biological methods and photocatalytic methods.Biological method has little environmental pollution,but it depends on the types of microorganisms and the reaction conditions are strict.The photocatalytic method has low cost,but the loss of oxidant during the reaction will limit the reaction rate.The three-dimensional electrochemical reactor（3DER）is constructed on the basis of two-dimensional electrochemistry and filled with materials.Due to its high oxidation capacity,high energy utilization rate,low energy consumption and other advantages,three-dimensional electrochemical technology is widely used in the treatment of various wastewater,providing a new method for the degradation of MPs.Polyethylene terephthalate（PET）is one of the most commonly used polyester-based thermoplastics.It has the characteristics of high toughness,low temperature influence,and high stability.It is widely used in the manufacture of beverage bottles and food containers.This article selects PET MPs as the experimental object,and uses 3DER for the first time,using ceramsite tourmaline as a particle electrode to degrade PET MPs.KOH solution is used to thermally pretreat PET MPs to simulate the process of embrittlement,fragmentation,oxidation and chain scission on the surface of MPs in the natural environment,and destroy the surface structure and chemical composition of MPs.Finally,the operating conditions of the reactor are optimized and the degradation mechanism of PET MPs is explored in 3DER.The main findings are as follows:（1）In 3DER,the PET film is continuously degraded into fibrous and flake MPs under the action of an electric field and falls off into the reactor.As the reaction time increases,MPs with a large particle size are gradually degraded into MPs with a small particle size,which are then decomposed to form soluble organic matter and further mineralized into CO₂.Therefore,the total number of MPs decreases with the increase of reaction time,which proves the degradation effect of 3DER on PET MPs.（2）Pretreatment of PET MPs with 20%KOH solution at 60°C for 24 h can effectively destroy the surface structure and chemical composition.The surface of PET MPs after pretreatment was damaged by erosion and embrittlement occurred.Oxygen-containing groups such as carboxylic acid and ketone increased,and the carbonyl index increased from 1.412 to 1.437;the O/C atomic ratio increased slightly;the amorphous region was destroyed and rearranged and crystallized,and the crystallinity increased from 4.54 to 12.58.（3）The optimal operating conditions for 3DER degradation of PET MPs are obtained by controlling variables:the voltage is 12 V,the concentration of Na Cl electrolyte is 1.5 g/L,and the temperature is 85°C.The degradation rate of PET MPs within 24 h was 23.6%±4%.Under the action of an electric field,the PET MPs after24 hours of 3DER degradation treatment showed a powdered state.Oxygen-containing groups such as carboxylic acid,ketone and aldehyde on the surface of PET MPs increased greatly,and the carbonyl index rose from 1.437 to1.838.The O/C atomic ratio increased significantly from 0.38 to 0.78,and PET MPs were oxidized to a greater extent.The crystalline region of PET MPs was attacked by the electric field,and the crystallinity decreased from 12.58 to 5.29,indicating that PET MPs were oxidatively degraded.（4）·OH and·O₂^-play an important role in the process of 3DER degradation of PET MPs.The main degradation pathway of PET MPs in the electrochemical oxidation process is chain scission.The activated ketones are broken by NorrishⅠand NorrishⅡmechanisms.The·OH generated during the operation of 3DER attacks the ester groups and C-C bonds in the structure of PET MPs,and generates various intermediate products through NorrishⅠreaction.The Norrish typeⅡreaction extracts theγ-hydrogen on the CH₂-O bond through the ring intermediate,rearranges it in the molecule to form carboxyl and vinyl end groups,and then cracks into products such as alkenes and alkanols,and finally mineralizes them into CO₂ or other types of inorganic carbon.