Mechanism Of Microplastic Adsorption On Trace Heavy Metals And Polycyclic Aromatic Hydrocarbons And Its Effect On Microalgae In Rivers

Compound pollution of freshwater with microplastics(MPs),heavy metals and polycyclic aromatic hydrocarbons(PAHs)is a growing concern.Heavy metals and PAHs tend to flow into rivers,causing water pollution easily.Microplastics act as carriers of heavy metals and PAHs,accelerating the migration of pollutants in freshwater systems.In recent years,as the mechanism of adsorption between microplastics on different environmental pollutants has been increasingly studied.Although the adsorption of heavy metals and PAHs on MPs has been extensively studied,in most of the studies,the pollutant concentrations in these experiments were much higher than the realistic environmental concentrations,which led to a particularly prominent role of partitioning in the adsorption behavior between MPs and pollutants,while other adsorption mechanisms would be weakened.Moreover,there is a lack of research regarding the specificity of different MPs for heavy metals and PAHs adsorption capacity.And in aquatic ecosystems under realistic environmental conditions,the amount of pollutants adsorbed on MPs is generally much lower than those adsorbed on other media.Therefore,the joint effect in systems where trace contaminants and MPs coexist is debatable.Therefore,it is important to assess the interactions between the joint effect of typical plastic particles,heavy metals/PAHs and microalgae.We studied three microplastics with similar particle sizes,namely polypropylene(PP),polystyrene(PS)and polyvinyl chloride(PVC),and analyzed the sorption mechanisms of trace heavy metals(Pb,Cu,Cr and Cd)and trace PAHs(anthracene and naphthalene)using sorption kinetic and sorption isotherm models to investigate the effects of contaminant sorption by microplastics under different temperature and salinity conditions.Chlorella vulgaris(GY-D27)was selected as a trainee organism.In order to study the joint effect of microplastics in combination with heavy metals,microplastics and PAHs,the growth condition of algal cells and oxidation indicators such as SOD and CAT were measured.(1)Different MPs depict different affinities for different heavy metals.PP has high adsorption affinity for four kinds of heavy metals.The adsorption processes of MPs for different heavy metals mainly follow the preudo-second-order model and the Langmuir isotherm model,indicating that the adsorption of trace heavy metals by microplastics is dominated by the outer surface complexation,and the outer spherical surface complexes are non-specific adsorption.And the adsorption rate is mainly controlled by the intraparticle diffusion.In addition,higher temperature and lower salinity are favorable to the adsorption of trace heavy metals by MPs.FTIR and XPS analyses confirm that the adsorption mechanism of trace heavy metals on the surface of MPs is dominated by physical interactions,in which oxygen-containing functional groups,π-π interactions,halogen interactions and electrostatic interactions play an important role.(2)The affinity of MPs for anthracene adsorption is stronger than that of naphthalene.And the affinities of different types of microplastics for PAHs are different.The affinity of MPs for anthracene is: PVC>PP>PS;the affinity of MPs for naphthalene is: PP≈PS>PVC.The adsorption process of MPs for two trace PAHs is non-homogeneous surface adsorption,which mainly follows the preudo-second-order model and the mechanism of pore filling,which indicates that the adsorption rate is controlled by the chemisorption mechanism,in which partitioning effect plays an important role.In addition,higher temperature and lower salinity are favorable for the adsorption of trace PAHs by MPs.The FTIR and XPS analyses confirmed that the adsorption mechanism of trace PAHs on the surface of MPs may be physical adsorption,in which hydrogen bonding and oxygen functional groups play an important role and π-π stacking plays a crucial role in the adsorption of PAHs by microplastics,however,under trace PAHs,halogen interactions are not obvious.(3)By analyzing the growth inhibition and oxidative stress of Chlorella vulgaris(GYD27)by PP,PS and PVC;the single system MPs or Pb/Nap had inhibitory effect on Chlorella vulgaris;in the composite system,the growth inhibition of Chlorella vulgaris was weakened and the large size microplastics had the effect of alleviating the growth inhibition.And the joint effect of chemicals and microplastics at low concentrations on Chlorella vulgaris was antagonistic when using superoxide dismutase and catalase activities confirmed.