Study On UV-H₂O₂/O₃ System On The Release Of DOM From Microplastics And The Formation Of Disinfection By-products

The global mass production of plastics has led to an increase in the amount of microplastics(MPs)released into aquatic ecosystems each year,posing a serious risk to the global ecosystem.A large number of studies have shown that a certain amount of MPs can still be detected in the effluent water of drinking water treatment system.Once in water,MPs is subject to non-biodegradation,including chemical and mechanical reactions,leading to plastic fragmentation.Although there are many studies on the detection of MPs in the ecological environment and water treatment system and its impact on the ecosystem,little attention has been paid to the dissolved organic matter(MPs-DOM)dissolved from MPs and its characteristics have also been studied.DOM leaching can be accelerated when MPs loses protective additives during transport and oxidation in water.This paper focuses on the generation,characteristics and control of MPs-DOM in typical water treatment oxidation process,comparatively studies the influence of typical oxidation methods on MPs,and analyzes the composition of MPs-DOM dissolved,the generation characteristics of disinfection by-products(DBPs)and related toxicity.DOM dissolution of polypropylene(PP),polyethylene glycol terephthalate(PET)and nylon 6(PA6)without additives was studied in the control group(dark condition),UV reaction,UV/H₂O₂ oxidation and UV/O₃ oxidation.On the whole,PA6 has the highest DOM generation(in DOC),followed by PET,and the lowest PP,which is not statistically significant.Therefore,PP-DOM will not be discussed in the future.By contrast,in the control group,UV oxidation and UV/O₃ oxidation processes,The DOC values of PET-DOM and PA6-DOM showed an increasing trend with the extension of reaction time and the increase of oxidizing agent concentration.In the UV/H₂O₂ system,the DOC production trend increased first and then decreased with the increase of reaction time.The oxidation effect of UV/O₃ system on MPs is stronger,the surface of MPs after oxidation produces holes and cracks,and the average particle size decreases.Oxidation did not change the functional groups on the surface of MPs,but the carbonyl index(CI)of MPs increased.Excitation emission matrix-parallel factor analysis(EEM-PARAFAC)model was used to characterize MPs-DOM.PA6-DOM and PET-DOM were similar in functional group composition and fluorescence components,mainly composed of aromatic organic matter/phenolic,humic acids and fulvic acids.Parallel factor analysis(PARAFAC)was used for semi-quantitative analysis of EEM scan results,which can be divided into three components C1,C2 and C3.Components C1 and C2 represent aromatic organics/phenols,while component C3 usually represents reduced quinones.Pearson correlation coefficient was used for correlation analysis.When R~2>0.9,it indicated that the correlation was good.Component C1 could be used to mark the generation of PA6-DOM,and components C1 and C2 could be used to mark the generation of PET-DOM.In order to determine the influence of MPs-DOM generation on the disinfection process,SDS disinfection method was used to simulate the disinfection experiment,and 0.1mg/L Br^-1was added to simulate the actual water matrix background.The formation of DBPs such as trihalomethanes(THMs),haloacetic acids(HAAs),haloacetaldehydes(HALs)and haloacetonitriles(HANs)was determined.On the whole,MPs-DOM was a potential source of DBPs precursors,and DBPs production was positively correlated with DOC concentration(R~2=0.92).For the two types of MPs-DOM,THMs and HALs had the highest yield,while HAAs had the lowest yield,which was ng grade.Advanced oxidation has a significant promoting effect on DBPs generation potential,in which UV/O₃ reaction has the strongest promoting effect,and the amount of DBPs generation increases by more than 2 times.The cytotoxicity(CTI)and genotoxicity(GTI)of DBPs were calculated,and the change trend of CTI and GTI was similar to the change of DBPs formation potential,and followed the change trend of DOC concentration.The CTI value of HALs was much higher than THMs and HAAs,and the GTI value of HALs was much higher than HAAs.In order to characterize the toxicological characteristics of MPs-DOM,microbial and phytoplankton were selected as research objects for toxicity tests.The inhibitory effect of MPs-DOM on VIbrio fischeri was enhanced with the increase of DOC concentration.Under the same DOC concentration,the inhibitory effect of PET-DOM on VIbrio fischeri was stronger than PA6-DOM.For phytoplankton,MPs-DOM has certain growth inhibition effect on Chlorella vulgaris and Microcystis aeruginosa,and PA6-DOM has stronger growth inhibition effect on them than PET-DOM.On day 6,Microcystis aeruginosa basically died and the growth degree of Chlorella vulgaris was inhibited,indicating that MPs-DOM had a stronger inhibitory effect on Microcystis aeruginosa.