Effect Of Polystyrene Microplastics On Sunlight Transformation And Toxicity Of Ag NPs And ZnO NPs

Microplastics are considered to be plastic particles less than 5 mm in diameter,characterised by their small size,large specific surface area and difficulty in degradation,and their widespread occurrence has been detected in the aqueous environment.Microplastics can also be used as a carrier to modify the distribution and toxicity of other contaminants in the aqueous environment.silver nanoparticles（Ag NPs）and zinc oxide nanoparticles（ZnO NPs）are new environmental contaminants that tend to aggregate with microplastics and their toxicity can be modified by microplastics,but little research has been conducted on how microplastics modify silver nanoparticles and zinc oxide nanoparticles in the aqueous environment.In this study,the effect of PS MPs on the photoconversion of Ag NPs and ZnO NPs in the aqueous environment under simulated sunlight conditions was systematically investigated using polystyrene microplastics（PS MPs）as experimental materials,and the effects of the physicochemical properties（different particle sizes,different functional groups and aged PS MPs）,environmental impact factors（acidity,ionic strength and humic acid（HA））on the photoconversion of Ag NPs and The effects of photoconversion of Ag NPs and ZnO NPs were investigated.Then,the effects of PS MPs on phototransformation of Ag NPs and ZnO NPs in three natural water,namely Danjiangkou Reservoir（DJK）,Three Gorges Reservoir（SXSK）and Moshui Lake（M Lake）,were investigated.Finally,the environmental risk of Ag NPs and ZnO NPs under the action of PS MPs was assessed using Daphnia magna as the model organism.The main findings are as follows:1.Under dark conditions,Ag NPs underwent slow dissolution with or without the addition of PS MPs,while ZnO NPs were stably present in the aqueous environment.however,under light conditions,both Ag NPs and ZnO NPs underwent dissolution to release Ag⁺and Zn²⁺,and the presence of PS MPs significantly increased the ratios of Ag⁺/Ag NPs and Zn²⁺/ZnO NPs.Further studies showed that under sunlight,Ag NPs and ZnO NPs in the aqueous environment released certain amounts of O₂^·-,¹O₂,·OH and H⁺.The presence of PS MPs promotes the release of O₂^·-and H⁺,where ¹O₂and·OH promote the oxidative dissolution of Ag NPs and O₂^·-reduces the released Ag⁺to produce secondary Ag NPs particles.ZnO NPs are mainly proton and h⁺dependent for dissolution.Furthermore,it was observed by TEM that PS MPs in Ag NPs changed to a solvated state at96 h.The presence of PS MPs in ZnO NPs was not detected.2.Studies on the phototransformation of Ag NPs and ZnO NPs in aqueous environment by PS MPs with different particle sizes,different functional groups and ageing showed that:1)the production of secondary Ag NPs gradually decreased with increasing particle size of PS MPs（0.1-10μm）during prolonged sunlight exposure;while for ZnO NPs,compared with 0.1μm-PS MPs,10μm-PS MPs released large amounts of H⁺to promote the dissolution of ZnO NPs,and 1μm-PS MPs inhibited the dissolution of ZnO NPs by heterogeneous aggregation with ZnO NPs.2)With increasing sunlight duration,PS-NH₂and PS-COOH met with Ag NPs and ZnO NPs respectively and underwent heterogeneous aggregation,inhibiting the formation of secondary Ag NPs and dissolution of ZnO NPs.3)Aged PS MPs produced reactive oxygen species（ROS）and released H⁺,promoting the formation of secondary Ag NPs and dissolution of ZnO NPs.3.Studies on the effects of p H,HA,Cl^-,CO₃^2-and SO₄^2-in the aqueous environment on the phototransformation of Ag NPs and ZnO NPs showed that:1)with decreasing p H in the sunlight,high levels of H⁺were detrimental to the stability of secondary Ag NPs,making the production of secondary Ag NPs gradually decrease,while different initial p H values had little effect on the ZnO NPs dissolution.2)In sunlight exposure,the production of secondary Ag NPs and the dissolution of ZnO NPs gradually decreased with increasing HA concentration;the production of secondary Ag NPs was gradually inhibited with increasing concentration of Cl^-,while the effect on the dissolution of ZnO NPs was small;different concentrations of CO₃^2-had a slight effect on the secondary production of Ag The secondary production of Ag NPs was slightly promoted by different concentrations of CO₃^2-,while the dissolution of ZnO NPs was gradually inhibited;the effect of SO₄^2-on the dissolution of both Ag NPs and ZnO NPs was minimal at different concentrations.In addition,the promotion of the dissolution of Ag NPs and ZnO NPs by PS MPs can also occur in the natural water DJK,SXSK and M Lake.4.Toxicity experiments of Ag NPs and ZnO NPs on D.magna showed that:1)After dark treatment,the lethality of Ag NPs and ZnO NPs on D.magna without the addition of PS MPs was 8%and 20%,respectively,and the addition of PS MPs did not significantly increase the toxicity of Ag NPs and ZnO NPs on D.magna,the lethality was 10%and 16%,respectively.2)After 96 h of sunlight exposure,the lethality of Ag NPs and ZnO NPs to D.magna was 52%and 55%,respectively,when PS MPs were not added.The addition of PS MPs significantly increased the toxicity of Ag NPs（p<0.01）and ZnO NPs（p<0.05）to D.magna,with lethality rates reaching 95%and 83%,respectively.Also with or without the addition of PS MPs,the toxicity of Ag NPs and ZnO NPs to D.magna was significantly（p<0.05）greater under sunlight conditions than under dark conditions.Further studies showed that the toxicity of Ag NPs to D.magna was due to the release of Ag⁺and the secondary production of small particle size Ag NPs.The toxicity of ZnO NPs to D.magna was mainly due to the release of Zn²⁺and the toxicity of small particle size ZnO NPs to D.magna was minimal.In summary,by studying the phototransformation and toxicity effects of Ag NPs and ZnO NPs under the action of PS MPs in the aqueous environment,this paper tentatively demonstrates that PS MPs can increase the solubility of Ag NPs and ZnO NPs in the water column under sunlight,which exacerbates the biotoxicity of D.magna.The results of this experiment can provide some basis for subsequent studies on the environmental pollution by microplastics and nanomaterials.The results of this experiment may provide some theoretical basis for the subsequent research on environmental pollution by microplastics and nanomaterials.