Photodegradation Of Polystyrene Nanoplastics Using C-14 Isotope Tracer

Plastics pollution has become a serious environmental problem for decades.Large amount of plastic products were abandoned into the environment after use.The low degradation rate makes plastics persist in environment.Polystyrene,one of the most used plastics,has been detected frequently in the environment.Previous studies have reported that ultraviolet(UV)being an essential factor for plastics degradation in the environment.Combined with physical,chemical and biotic effects,plastics debris break into small particles undergo UV irradiation.With the small sizes,these plastics particles may transport for long-distances among different environmental compartments,being ingested by organisms,and pose adverse effects.Among these small plastics particles,nanoplastics(<1?m,NPs)have been recently detected in the environment and many studies have indicated the wide existence and potential environmental risks of NPs.However,the generation and degradation mechanism as well as environmental persistency of NPs in the environment are still unclear.The small sizes of NPs facilitate them to be susceptible by environmental factors,such as interact with other particles or pollutants in the environment.A comprehensive knowledge about the degradation mechanism and the fate of nanoplastics in the environment is vital to understand the life cycle of plastics.In the present study,using¹⁴C isotope tracer,the photodegradation processes and influencing factors of PS NPs were studied to understand the degradation mechanisms of PS NPs under UV,and furthermore,the fate of PS NPs and its water soluble photodegradation products in different waters under natural conditions were investigated.The main findings of the present study are as follows:(1)¹⁴C-PS NPs with diameter sizes of 100-500 nm were obtained from ¹⁴C-styrene.The products were found to be uniformly spherical and the average molecular weight increased with increasing diameter.The composition of synthesized ¹⁴C-PS NPs was confirmed by Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy.The synthesized ¹⁴C-PS NPs facilitated the application of ¹⁴C isotope tracer in quantification of NPs degradation.(2)¹⁴C-PS NPs could be photodegraded under UV254 irradiation in air or suspended in water.The mineralization of the PS nanoplastics in air was 6.17±0.1%,the oxygen elements increased from 6.7%to 12.3%,and 4.3±0.06%of degradation products being washed out from the surface of nanoplastics,the molecular weight(MW)of the PS NPs increased in air after the irradiation,suggesting cross-linking of the PS chains;the mineralization of the PS NPs in water was 17.1±0.6%,the oxygen elements increased from 6.7%to 21.9%,11.0±0.1%of degradation products with small MW was detected in the water.The strong oxidation of NPs occurred while the morphology of NPs have no significant change indicated that the degradation the during UV irradiation occurred uniformly on the surface.Further,the presence of water significantly accelerated the photodegradation of NPs by the reaction rate with hydroxyl radicals and promote more products leach into water.(3)UV wavelength significantly affected the mineralization rate of ¹⁴C-PS NPs.The mineralization rate of ¹⁴C-PS NPs under UV254 nm was 84.9±12.3%in 60 d while only 0.18±0.03%under UV313 in 108 d.The different energy between UV254 and UV313 could cause the different degradation rate.The mineralization of ¹⁴C-PS NPs decreased from 11.0±2.6%to 5.2±1.2%with increased salinity,which could be related to the aggregation of NPs.DOM in lake water had no significant effect on the mineralization while decreased the amount of degradation products leaching out from the NPs,which may affect the degradation of NPs.(4)¹⁴C-PS NPs could be photodegraded under sunlight but with much slower rate compared with UV.The increasing of salinity and DOM decreased the degradation of¹⁴C-PS significantly.Mineralization of ¹⁴C-PS NPs in lakewater under sunlight for100 d was only 0.90±0.4%of initial applied radioactivity and 2.1±0.4%of dissolve¹⁴C was detected in the water.Different from the PS NPs,the mineralization of water-soluble photodegradation products under sunlight was very fast,with 49.3±1.0%of total ¹⁴C being mineralized after 10 days.This indicate that the leaching of photodegradation products could be the key process of degradation of NPs in aquatic environments.The present study systematically investigated the photodegradation of PS NPs.The photodegradation processed,mechanisms and influencing factors of the photodegradation were illuminated.The results from this study indicated the persist risks of NPs under natural conditions although the photodegradations of PS NPs were very fast under UV254.As one of the most important part of life-cycle of plastics in the environment,the fate of NPs could provide the basis for the research about the distribution and fate of PS plastics in the global circumstances.The present study could provide the theoretical support for the comprehensive risks assessment of NPs.