Conversion Behavior And Risk Of Micro (Nano) Plastics Mediated By Plasma Oxidation

Widely distributed micro（nano）plastics in the water environment brings serious environmental problems and has drawn wide attention.Micro（nano）plastics exist in the environment for a long time,under the weathering of the environment,surface morphology and some physical and chemical properties have changed.However,at present,the information on the transformation process of micro（nano）plastics in the environment and the environmental risks in the process is very scarce.Due to the limitation of environmental forces,the transformation process of micro（nano）plastics in the actual environment is slow,which is not conducive to obtaining the transformation information.As a kind of advanced oxidation process（AOPS）,electrical discharge plasma has been widely concerned by people because of its advantages of fast,efficient equipment,simple,easy to operate and green environmental protection.In the process of electrical discharge plasma action,a large number of high-energy electronic active substances(·OH,·O,O₃,H₂O_2,etc.)and ultraviolet radiation exist in the system.These physical and chemical synergistic effects make organic macromolecules react in the system,such as ring opening and bond breaking.Therefore,discharge plasma plays a wide role in the mineralization removal of pollutants in water treatment and the modification of macromolecular materials.Therefore,in this paper,the electrical discharge plasma technology as a technical support to accelerate the micro（nano）plastic conversion process,nano polystyrene plastics（PSNPs）and four kinds of common microplastics polyvinyl chloride（PVC）,polystyrene（PS）,Polyethylene（PE）and polypropylene（PP）were used as the research objects.The conversion process of PSNPs under the action of discharge plasma,the change of surface characteristics of four kinds of microplastics and the release of dissolved organic matter（DOM）into the water phase were investigated.In addition,the environmental risks of microplastics in the process of transformation were comprehensively displayed by using the Scenedesmus obliquus and human non-small cell lung cancer A549 cells as the biological test and the generation capacity of disinfection by-products as the evaluation indexes.This study provided more information on the behavior and risks of microplastics in water environment.The main results of this paper are as follows:（1）PSNPs transform under the action of plasma oxidation.When the discharge voltage is 19 k V,the initial concentration of PSNPs is 30 mg/L,and the discharge treatment time is15 min,the fluorescence attenuation rate can reach more than 90%of the active substances(·OH、¹O₂and O₂^·-)played a key role in its conversion.These three active substances were all involved in the conversion of PSNPs.According to the results of the capture agent experiment,O₂^·-made the largest contribution among the three substances.The surface of PSNPs is eroded by plasma,making the surface rough.With the extension of plasma treatment time,the particle size of PSNPs gradually decreases and the electronegativity increases.Under the action of plasma oxidation,PSNPs are generated into hydroxylated and carboxylated intermediates,and the by-products of conversion are phenol,alcohol,esters and carboxylic acid.Plasma treated PSNPs showed a slight decrease in toxicity to the growth of Scenedesmus obliquus,but increased toxicity to human non-small cell lung cancer A549cells.In addition,the production of two typical disinfection by-products（DBPs）,trihalomethanes（THMs）and trihaloacetic acids（HAAs）,generally showed an increasing trend with the increase of plasma treatment time.（2）Four microplastics（PVC,PS,PE and PP）under the effect of plasma oxidation were released to the water phase of soluble organic matter（DOM）and the release of the DOM is similar characteristics.According to the result of ultraviolet full wavelength spectrum（UV-Vis）and the three dimensional fluorescence spectrum（3D-EEM）,the main components of DOM in aqueous phase is humic acid-like substances.DOM released by these microplastics can be used as the precursor of disinfection by-products in the generation of disinfection by-products.The total amount of disinfection by-products produced by PE microplastics is the highest compared with the other three kinds of microplastics,which is consistent with the highest amount of DOC released by the four kinds of microplastics,indicating that PE microplastics release the most DOM into the environment under the same environment and have the highest generation potential of disinfection by-products.In terms of the overall toxicity of disinfection by-products,PS microplastics produced lower toxicity than the other three kinds of microplastics.The ability and toxicity of disinfection by-products generated by DOM released by the four kinds of microplastics were measured and calculated,which indicated that microplastics under the action of plasma oxidation had a greater threat to the environment.