Environmental Migration Behavior Of Nanoplastics And Their Toxic Effects On Aquatic Organisms In Combination With Heavy Metals

Environmental problems caused by nanoplastics have attracted increasing attention.Nanoplastics can exist stably in the water environment for a long time and migrate long distances with the flow of water.At the same time,nanoplastics can cause different degrees of toxic effects on aquatic organisms.Due to their extremely small particle size and variable surface charge,Nanoplastics have great adsorption capacity and stronger scale effect than microplastics,which can affect aquatic organisms together with other pollutants in the water environment.As one of the most serious pollution in water environment,heavy metals can cause irreversible toxic effects on aquatic organisms only if they exist in trace amounts.However,there are few studies on the relationship between the inherent properties of Nanoplastics and sedimentation behavior,and the toxic effects of nanoplastics on microalgae when exposed to heavy metals.Therefore,it is of great significance to study the effects of different surface charges and particle sizes on the sedimentation behavior of Nanoplastics and the resistance of microalgae to the toxic effects of heavy metals for the environmental impact assessment and ecological risk assessment of Nanoplastics.In this study,first,polystyrene was selected as a typical nanoplastic,and six target Polystyrene nanoplastics（PSNPs）with particle sizes below 100 nm,100-200 nm and above200 nm were obtained by adjusting the parameters of the synthesis process.The effects of the inherent properties of PSNPs on the aggregation and sedimentation behavior of PSNPs under different water environmental factors were investigated by controlling p H,ionic strength（IS）,electrolyte type and humic acid（HA）as changing water environmental factors.Chlorella was selected as the model microalgae and Cd as the typical heavy metal.The effects of the natural properties of PSNPs on the toxic effects of Chlorella exposed to PSNPs and Cd were studied.Research contents and main conclusions are as follows:1.Study on controlled synthesis of model nanoplasticsStyrene is used as the main raw material,by emulsion polymerization method,sodium dodecyl sulfate（SDS）/dodecamine hydrochloride is used as surface active,potassium persulfate（KPS）/2,2-azo（2-methylproamidine）dihydrochloride（AIBA）is used as initiator to synthesize surface with positive/negative charges,six target PSNPs with particle sizes below 100 nm,100-200 nm,and above 200 nm were selected,and the physicochemical properties of the synthetic PSNPs were determined.Finally,six PSNPs were selected:PSNPs-50^-,PSNPs-150^-,PSNPs-250^-,PSNPs-50⁺,PSNPs-150⁺and PSNPs-250⁺.The particle sizes were 27.69 nm,137.64 nm,261.93 nm,38.50 nm,188.40 nm and 249.45 nm,respectively.The surface charges were-48.35m V,-60.89 m V,-39.27 m V,44.97 m V,50.51 m V and 37.90 m V,respectively,which were used as sample materials for subsequent sedimentation experiments and toxicity tests.2,the effect of particle size and surface charge on the settlement of nanoplastics in the aqueous environmentThe six synthesized PSNPs were used as the experimental objects,and the effects of different surface charges and particle sizes on the aggregation and sedimentation behavior of PSNPs under different water environmental factors were observed by changing four water environmental factors:p H,IS,electrolyte type and HA.The results showed that the sedimentation rate of small particle size PSNPs-50 with IS_{Na Cl}=100 m M and IS_{Ca Cl2}=30 m M increased by 6.45%-66.86%,and the particle diameter of large particle size PSNPs-150 and PSNPs-250 increased significantly after 24 hours of sedimentation.The aggregation and sedimentation behaviors of PSNPs with small particle size were more sensitive to IS changes.In addition,the sedimentation rate of negatively charged PSNPs increased by more than 60%with the increase of IS and the increase of electrolyte valence,while the sedimentation rate of positively charged PSNPs fluctuated within 10%,indicating that the stability of negatively charged PSNPs in water environment was more susceptible to the changes of IS and electrolyte types.All the six PSNPs showed good stability when HA was added.Compared with the HA=0mg/L group,the final sedimentation rate of high and low concentration PSNPs decreased from-6.82%to+36.06%.According to the Zeta potential detection 24 hours after settling,the surface electronegativity of negatively charged PSNPs was enhanced,and the surface charge of positively charged PSNPs changed from positive to high negative potential,which indicated that negatively charged PSNPs increased steric hindrance to enhance the stability.The positively charged PSNPs are due to the stabilization effect of the high electronegativity obtained by the large amount of adsorption of HA on the surface.3.Combined toxic effects of different properties of nano-plastics and cadmium on ChlorellaFour kinds of PSNPs with different properties were used as experimental materials to conduct toxicity experiments on Chlorella’s exposure to PSNPs alone and to PSNPs and Cd together,and to analyze the effects of particle size and surface charge on the mitigation or enhancement of the toxicity of heavy metal Cd in Chlorella.The results showed that after 7days of single PSNPs exposure and co-exposure with Cd,the growth inhibition rate of Chlorella with high concentration of PSNPs was higher than that of Chlorella without or with low concentration of PSNPs.The growth inhibition rate of small particle size PSNPs-50 with high concentration was always higher than that of large particle size PSNPs-250,and the growth inhibition rate with positive and negative charges was increased by 5.89%-20.23%and 9.00%-15.79%,respectively,indicating that the growth inhibition rate of small particle size PSNPs-50was stronger than that of large particle size PSNPs-250.PSNPs and Cd had synergistic toxic effect.The chlorophyll a content,EPS content,MDA content,SOD and CAT activities of Chlorella were measured.The chlorophyll a content of Chlorella decreased by 13.19%-61.37%,and MDA content,SOD and CAT activities increased after the addition of small particle size PSNPs-50 alone.These results indicated that small particle size PSNPs-50 could inhibit the production of photosynthetic pigments and induce oxidative stress in algal cells.When PSNPs were co-exposed to Cd,small particle size PSNPs-50 showed the same effect.Combined with the measurement of Cd concentration in and out of the algal cells and SEM images,it was found that the algal cells were ruptured after adding PSNPs-50^-,and the intracellular Cd content in the algal cells treated with low and high concentrations of PSNPs-50^-was 1.38 and 1.33 times higher than that of the Cd-exposed group,respectively.These results indicated that PSNPs-50^-could adsorb Cd on the basis of rupture of algal cells and bring it into algal cells,thus enhancing the toxic effect of Cd.At high concentrations of large-particle size PSNPs-250 and Cd co-exposure,a large increase in PSNPs-250 adsorption on algal cells was observed from SEM,which would enhance the restriction of energy and material exchange between algal cells and medium,thereby reducing the growth rate of Chlorella.