A Study On The Release Of Additives From Plastics In The Estuarine Environment And Their Toxic Effects

Plastics can remain in the environment for a long time and originate from various sources,and thus plastic pollution has become one of the hot topics in current environmental research.A large quantity of plastic debris has been accumulated in the estuarine area,which has become both the sink and source of plastic waste.The complex hydrodynamic conditions of the estuarine environment can accelerate the weathering of plastic debris,and plastic additives will continuously leach into the environment.In this study,Chongming Dongtan Region located in the Yangtze Estuary was selected as our field investigation area.We analyzed the distribution types and characteristics of plastic debris collected in the field,and explored the surface microscopic changes of plastics under common conditions.We also explored the occurrence and characteristics of plastic debris in this estuarine environment and the release concentrations of additives from plastics and leaching mechanisms under the common weathering conditions of estuaries,such as mechanical wear,hydrodynamic disturbance,and ultraviolet irradiation.We choose non-target contaminants to screen for additives in plastic leachates.Due to the different toxic effects of plastic leachate on model organisms,the toxic effects of the released additives on aquatic organisms were further investigated in the lab.This study comprehensively selected four model organisms for exposure experiments to explore the biotoxic effects of plastic leachate and its potential environmental risks.The main conclusions obtained in this study are as follows:(1)Plastic debris in the field included five main types of plastics:floating foam,plastic bottles,fishing nets,rigid floating balls,and plastic bags.Among them,floating foam and fishing nets had high concentrations of 4.8±5.1 g/L and 3.0±2.5 g/L,respectively.The concentration of plastic bags was lower 0.31±0.58 g/L because plastic bags were easily degraded into small pieces in the field and become difficult to be collected completely.This study observed the changes in the surface morphology of plastics after weathering.Under the wave scenario,plastics were more likely to cause deformation on the surface.(2)A total of 70 suspected additive substances were identified in the leachates of three typical estuarine plastic debris.Sixty-seven additives were suspected in the simulated wave scenario,while only 40 kinds were found in the shoal simulated scenario.There were at most 40 kinds of additives found in the leachate of plastic bags,followed by those in the foam and fishing net leachates.It was worth mentioning that a chemical with high toxicity and high concentration,nonylphenol polyethoxylate(PEGM-4-N₅),was found in the leachate of plastic bags,and whose response value was>2×10~7 and had obvious acute toxicity(LC₅₀ of 96 h fathead minnow is 0.76mg/L).According to quantitative analysis,we proved that the released PEGM-4-N₅concentration under the wave scenario was the highest,which could reach 2.4±0.8mg/L.(3)In laboratory simulation experiments,zebrafish embryos were exposed to different plastic leachates,which showed strong effects on the heartbeat rate of embryos.The plastic bag leachate exposed in the wave scenario had the greatest influence on heartbeat rate,with even the low-concentration exposure group showing a significant difference from that of the control group.Additionally,we also conducted an in-depth analysis of the key toxicant PEGM-4-N₅ found in the leachate of plastic bags.It was noteworthy that PEGM-4-N₅ can significantly reduce the hatching rate of zebrafish under the estuarine environmentally relevant concentrations.Meanwhile,the exposure to PEGM-4-N₅ also caused an increased heartbeat rate in zebrafish embryos and a phenomenon of spine curvature in the fish's juvenile stage.(4)This study further comprehensively analyzed and compared the toxic effects of the leachates of traditional plastics and emerging biodegradable plastics under the estuary wave scenario.The exposure experiments were carried out for four model organisms at different trophic levels.Our results showed that the acute toxicity(EC₂₀:16.2–796.1 g plastic/L)of the biodegradable plastic polydioxanone(PPDO)leachate to Caenorhabditis elegans,Scenedesmus obliquus,and Danio rerio was lower than that of traditional plastic leachates(EC₂₀<1.6 g plastic/L);whereas the acute toxicity(EC₂₀:0.04–1.9 g plastic/L)of C.elegans and Daphnia magna of the PPDO-octane leachate modified by C8 grafting was basically the same as that of traditional plastics.Moreover,we found that the plastic leachates had no obvious effects on the body length of D.rerio larvae,but had a significant impact on the body length of C.elegans.Among the traditional plastic leachates,fishing nets and plastic bag leachates had the greatest impacts on the growth of C.elegans,and showed significant growth inhibition at low concentrations(?0.001 g plastic/L).The leachate of biodegradable plastic PPDO can inhibit the growth of C.elegans at the high concentrations(?0.1 g plastic/L).However,the PPDO-octane leachate showed the most significant effect on the growth of the C.elegans.The body length of the C.elegans had decreased by 20%compared with the control group,even when the concentration was only 0.0001 g plastic/L for the PPDO-octane.In summary,this study illustrated the environmental plastic pollution concentrations in the estuary through field investigation.The leaching of additives from plastics was simulated in the laboratory,and it was found that the leached chemical numbers of additives were the largest in the wave environment.Through the nontargeted chemical screening,we found that plastic bags had the greatest leached numbers of chemicals in the estuary environment.Further,this study explored a key toxic chemical PEGM-4-N₅ in estuarine plastic leachate by coupling with bioassay analysis.Additionally,due to the sensitivity of different organisms to plastic leachates varies largely,the study also considered the application of a battery of baseline toxicities including four trophic-leveled organisms to comprehensively reveal the ecological effects of plastic leachates.Collectively,our research will provide an effective and feasible method for the environmental risk assessment of plastic leachates in the estuarine environment.