Study On Aging Mechanism Of Microplastics,its Adsorption Behavior And Compound Toxicity With Heavy Metals

In recent years,more and more studies have shown that microplastics(MPs)are widely distributed in the environment,which has aroused worldwide concerns.MPs,as carriers and emitters of pollutants,can migrate and being aged in the environment which would seriously threat the ecosystem balance.It is reported that the surface structure and functional groups of aged MPs are significantly different from the virgin MPs,however the aging mechanism of MPs in the environment remains to be elucidated.Furthermore,the adsorption mechanism of heavy metals on MPs particularly aged MPs is still uncertain,although some previous studies have demonstrated that MPs can adsorb heavy metals in the environment.In addition,the biological toxicity of single MPs and its complex pollution system with heavy metals needs further study.Under such circumstance,studies on the aging mechanism of MPs,the adsorption behavior of heavy metals onto MPs,and their composite biological toxicity can provide a theoretical basis for further assessment on the potential environmental risks of MPs.In this paper,the common polystyrene(PS)and polyvinyl chloride(PVC)in the environment were selected to study their aging mechanism under five types of simulated environmental conditions.Furthermore,copper and cadmium were used as co-existed pollutants,while the adsorption behavior of MPs(PS and PVC)with heavy metals(Cd and Cu),and their compound biological toxicity to microalga Chlorella vulgaris(C.vulgaris)were studied by adsorption experiments and biological toxicity experiments.The main results of this paper are as follows:(1)The aging degree order of MPs under five types of conditions is different,the aging degree of MPs in air and sea water was more obvious but weaker in pure water.After being aged,obvious changes can be observed on the surface morphology of aged MPs,with many cracks,faults,fragments,granular oxides.Meanwhile,the content of surface functional groups such as-OH or C=O increased while the C-H decreased on aged MPs.(2)Chemisorption of multi-molecular layers,homogeneous adsorption of single molecular active sites and electrostatic adsorption included in the adsorption process of heavy metals on MPs,can be described by pseudo-second-order kinetic model and Freundlich model,which has a rule of initial rapid,slow and finally equilibrium.What’s more,the adsorption capacity of aged MPs to heavy metals was larger than that of virgin MPs,which indicated that the affinity of MPs to heavy metals can be enhanced in the process of aging.(3)All tested MPs were demonstrated to inhibit the cell growth of C.vulgaris but with an order of toxicity such as aged PVC<virgin PS<aged PVC<aged PS,indicating that a higher aging degree of MPs could promote their inhibition on microalgae.However,the biological toxicity of MPs to C.vulgaris decreased with the increase of MPs concentration,which may be attributed to the aggregation of MPs.Furthermore,different MPs caused different oxidative stress responses of C.vulgaris.For instance,the SOD content of C.vulgaris in PVC was higher than that of PS,which indicated that higher SOD activity of C.vulgaris can be triggered for responding to the stresses result from PVC as compared with PS.Contrarily,the toxicity of heavy metals to C.vulgaris increased with heavy metal concentration.(4)For composite systems,the toxicity of heavy metals to C.vulgaris was reduced by the adsorption of MPs with heavy metals,and meanwhile the toxicity of PVC to C.vulgaris was greater than that of PS,which is contrary to the single system.The results may be attributed to greater adsorption capacity of PVC to heavy metals than that of PS.