Chemical Transformation Mechanism Of Microplastics During Chlorination And The Effects Of UV Aging

Microplastics（MPs）are widely present in various water bodies such as surface water,groundwater,drinking water and sewage,and are currently one of the most concerned emerging contaminants.The traditional water treatment front-end process is difficult to completely remove the MPs in the raw water,resulting in its reac tion with chlorine in the subsequent chlorination disinfection process.However,the mechanism of chemical transformation during chlorination is still unclear.In this paper,the change of functional groups,the release of organic matter and disinfection by-products（DBPs）formation in the chlorination process of MPs were systematically investigated.At the same time,the effect of ultraviolet（UV）aging on the chemical transformation of MPs during chlorination was also studied.This paper provides a theoretical basis for clarifying the fate and transformation of MPs in water treatment processes.The main conclusions of this paper are as follows:（1）The chlorine consumption of MPs followed polyamide（PA6）>>poly（butyleneadipate-co-terephthalate）PBAT>polylactic acid（PLA）>polyethylene（PE）>polyethylene terephthalate（PET）/polyvinyl chloride（PVC）.The chlorine consumption of PA6 was 915.5±32.4μmol Cl₂/g MP,significantly higher than that of other types of microplastic polymers（<14.0μmol Cl₂/g MP）.After UV irradiation,the chlorine consumption of PBAT and PLA aged for 30 days was 1.7 times and 1.6times higher than that of the pristine PBAT and PLA,respectively.However,the chlorine consumption of PET,PE and PVC did not change significantly before and after UV aging treatment,showing a relatively inert.Further study on the decay kinetics of chlorine found that the decay rate of chlorine followed PA6>>PBAT>PLA>PET/PE/PVC in the presence of MPs with different aging conditions（0-30days）.（2）After chlorination,Fourier transform infrared spectroscopy（FTIR）results showed that the intensity of C=O(1635 cm^-1)and N-H and C-N bonds(1538 cm^-1)in PA6 decreased;the strength of fatty ester O=C-O-C(1165 cm^-1)and C-O(1250 cm^-1)bonds in PBAT and C=O(1267 cm^-1)bond in PLA decreased slightly after chlorination,implying that chlorination treatment caused the destruction of amide group and ester group in MPs and fatty ester was easier to hydrolyze than aromatic ester.For the aged MPs,the reduction of functional group strength after chlorination was more significant.However,for the pristine and UV aged polyolefin MPs,the strength of CH₂ in PE and C-Cl/C-H bonds in PVC remained almost unchanged after chlorination.（3）The amount of organic matter released from the MPs after chlorination was1.3-2.7 times of that released from MPs without chlorination.Notably,PA6 released the highest concentration of organic matter（22.6±0.7 mg C/g MP）.Through high-resolution mass spectrometry analysis,it was found that these organic compounds were polymer monomers,oligomers,additives（dimethyl phthalate and butyl octyl phthalate）and so on.After UV aging（30 days）,the content of organic matter released from the aged MPs during the chlorination process is 2.4 times higher than that of the pristine MPs.Indicating that UV photoaging led to the breakage of the backbone structure of microplastic polymers and accelerated the release of non-chemical bonding additives.（4）Microplastic polymers could lea d to the formation of DBP during chlorination process.Trihalomethanes（THMs）and haloacetic acids（HAAs）were the two main DBPs generated during the chlorination process of MPs,and the amount of HAAs was 2.6-5.6 times higher than that of THMs.After UV aging,the DBP formation potential in the chlorination process of MPs was enhanced.Among them,the yields of total DBPs detected in PBAT and nitrogenous DBPs（N-DBPs）detected in PA6increased by 10.8 times and 2.7 times respectively.The results showed that UV aging treatment enhanced the release of organic matters in the chlorination process of MPs,thus increasing the formation of DBPs.