Influence Of Aging Process On Surface Properties Of Microplastics And Leaching Behavior Of Azo Pigment

As a kind of man-made material,the use of plastics is increasing year by year,and it plays an important role in human society.Plastic fragments and particles with a diameter of less than 5 mm were defined as microplastics.Microplastics were affected by aging in the environment.The microplastics before and after aging had different physical and chemical properties,and the plastic additives（such as azo pigments）contained in them were also susceptible to aging.Such microplastics migrated into the environment and contact with biological organisms for a long period of time,causing oxidative stress,inflammation and other hazards.What’s more,the leaching of the azo pigment and its role with the body need further evaluation.In this paper,low-density polyethylene microplastics containing azo pigments were mainly used as the research object.First,the xenon lamp aging box was used to simulate the aging of microplastics in the natural environment,and then the microplastics and their azo pigments were characterized.Secondly,the microplastics with different aging degrees were exposed to the simulated digestive system medium to study the effect mechanism of the aging process on the leaching behavior of the azo pigment in the microplastics,and then the fluorescence quenching titration was used to explore the interaction of azo pigment and enzyme（contain pepsin and pancreatin）,characterize the agglomerates formed during the quenching process and analyze its formation mechanism.Finally,different advanced oxidation processes were used for microplastic aging,to study the release and degradation of azo pigments in advanced oxidation treatment of microplastics,and to characterize physical and chemical properties of the microplastic micro-area using nano infrared atomic force microscope（nano IR2 AFM-IR）.Advanced oxidation treatment expanded research ideas related to microplastic aging.The main conclusions obtained in this article were as follows:（1）Under the aging of xenon lamps,the specific surface area of microplastics increased,cracks and debris appear on the surface,and the carbonyl index and color difference value increased with the aging degree.The aging was more than 6 weeks,and the growth was stable.The cracking of microplastics promoted the entry of light and oxygen into the inner layer,which accelerated the aging process.A large number of active sites were available in the early stage,so the oxidation was rapid,the available sites in the later stage were reduced,and the surface oxidation became saturated;The analysis of thermogravimetric-infrared-GC/MS instrument showed that the high temperature resistance of microplastics decreased after aging.However,the pigment was relatively stable during the aging process,and no other intermediates were formed.What’s more,due to UV radiation and other effects,polyethylene was degraded into other compounds,which made the content of azo pigment increased in the aging process from 0.131 ± 0.010 g·g-1 to 0.252 ± 0.007 g·g-1.（2）Azo pigments in microplastics caught migrate and release into the simulated medium,and prolonging the aging time caught release more pigments.The main reason was that the relative proportion of azo pigments increases after aging;in addition,the specific surface area of microplastics increased after aging,The contact area between the microplastic and the medium was increased,thereby promoting the release of the pigment.Once the azo pigments were released from the microplastics into the simulated medium,they would undergo complex interactions and caused the static fluorescence of the enzyme to quench,which mainly affects the intensity of fluorescence peak of pepsin(λ_ex / λ_em = 280/335 nm)and pancreatin(λ_ex / λ_em = 280/340 nm),the binding capacity of azo pigment to pancreatin was greater than that of pepsin;Raman spectroscopy and turbidity index confirmed that the azo pigment released by microplastics bound with the enzyme to form agglomerates.The average diameter of the agglomerates in the simulated gastric fluid was larger than that in the simulated intestinal fluid.The aggregates in the simulated gastric fluid were formed due to charge neutralization,while the formation of the aggregates in the simulated intestinal fluid was due to the bridging of the polymer.（3）At the current dose,the advanced oxidation process was used to process the azo pigments released by the microplastics.The degradation effect follows: thermally activated persulfate treatment > ozone treatment > untreated group;the surface morphology of the microplastics treated by the advanced oxidation process was relatively rough and uneven,in which the infrared signal of the azo pigment appears uneven compared with the untreated microplastics,and some areas were concentrated;the oxidation degree of the corresponding surface: thermally activated persulfate treatment > Fenton treatment > ozone treatment > untreated group.In microplastics containing azo pigments,the degree of oxidation was proportional to the mechanical hardness and inversely proportional to the glass transition temperature.The above changes in physical and chemical properties were due to the oxidation of free radicals on the surface of microplastics and the degradation of azo pigments.