Effect And Mechanisms Of Hexabromocyclododecane On The Adsorption Of Fulic Acid And Metal Ions On Polystyrene Microplastics

Microplastics（MPs）,an emerging and widespread pollutant,can significantly affect the migration and transformation of pollutants in the environment by virtue of their excellent physicochemical properties.Hexabromocyclododecane（HBCD）is a typical brominated flame retardant that is widely used in textiles,electronic products and construction materials,especially as a flame retardant additive in polystyrene（PS）plastics.Therefore,polystyrene plastics with hexabromocyclododecane（HCBD-PS）are common in human life.Although most studies have focused on the interaction mechanisms of microplastics and contaminants in the environment and their potential risk assessment;however,little research is known about microplastics containing flame retardants.Heavy metals are widespread in the natural environment,difficult to degrade,and easily enriched on plastic particulate matter.Fulvic acid（FA）,a natural organic substance in the aqueous environment,can be complex with heavy metal ions in the aqueous environment,which in turn affects the transport,translocation,and species morphology of heavy metal ions in the environment.In this study,we investigate the adsorption behaviors of typical pollutants（Pb（II）,Cu（II）,Cd（II）,Zn（II）,and FA）onto PS and HBCD-PS microplastics in the water environment.Further,adsorption kinetics,thermodynamics,and isotherm were used to characterize the environmental behavior of pollutants onto microplastics.In addition,the adsorption mechanisms of microplastics for pollutants were revealed by fitting the adsorption model and advanced characterization tools.The results showed that the adsorption of FA,Pb（II）,Cu（II）,Cd（II）,and Zn（II）ions onto the pristine PS microplastics was negligible.However,the presence of HBCD significantly promoted the adsorption of FA and the four heavy metal ions by HBCD-PS microplastics.The adsorption processes of FA and heavy metal ions by HBCD-PS microplastics were well consistent with the pseudo-second-order kinetic model.The final adsorption equilibrium time and maximum adsorption amounts were FA(36h,0.321 mg·g^-1),Pb（II）(24h,30.182μmol·g^-1),Cu（II）(24h,21.491μmol·g^-1),Cd（II）(24h,16.338μmol·g^-1),and Zn（II）(24h,14.996μmol·g^-1).The Freundlich and Langmuir isothermal models can be used to better describe the adsorption equilibrium phenomena of FA and heavy metal ions onto HBCD-PS,respectively.The Weber-Morris internal diffusion and Boyd’s membrane diffusion models suggested that the adsorption process of FA on HBCD-PS was mainly controlled by intraparticle diffusion;Pb（II）,Cu（II）,Cd（II）,Zn（II）on HBCD-PS was controlled by both intraparticle diffusion and membrane diffusion.Overall,intraparticle diffusion was the main rate control step during the adsorption processes.Thermodynamic studies showed that the adsorption of the four metal ions on HBCD-PS microplastics was a spontaneous heat adsorption process with increasing confusion.Environmental factors such as solution p H,salinity,and temperature all significantly affected the adsorption behavior of FA and four metal ions onto HBCD-Ps microplastics.The adsorption of FA on HBCD-PS microplastics was obviously promoted under low p H and high salinity conditions.However,low p H and high salinity inhibited the adsorption of these four metal ions on HBCD-PS microplastics.The adsorption capacity of HBCD-Ps on FA and metal ions increased with the increase in temperature,indicating that the adsorption process of HBCD-PS on the four metal ions was spontaneous and the disorder degree increased.In the binary system,the presence of metal ions（Pb（II）,Cu（II）,Cd（II）,Zn（II））was conducive to the adsorption behavior of FA on HBCD-PS microplastics,which was positively correlated with the complexation ability of FA and metal ions.Therefore,compared with Cd（II）and Zn（II）,the presence of Cu（II）and Pb（II）significantly promoted the adsorption of FA on HBCD-Ps microplastics.However,the presence of FA inhibits the adsorption behavior of these four metal ions on HBCD-Ps microplastics,and the higher the concentration of FA is,the stronger the inhibition effect is on the adsorption of HBCD-Ps microplastics,which was mainly attributed to the fact that FA can form complexes with metal ions in solution and thus form competitive effect.Among them,FA has the most obvious inhibition effect on Cu（II）adsorption of HBCD-PS microplastics,and the least inhibition effect on Pb（II）adsorption.In summary,the adsorption mechanisms of FA on HBCD-PS microplastics were hydrophobic and electrostatic interactions.The binding mechanism between the four metal ions and HBCD-PS microplastics was electrostatic interactions and surface complexation.It was noteworthy that the coexistence of metal ions with FA can significantly promote the adsorption behavior of FA on HBCD-PS microplastics;however,the adsorption of the metal ions onto HBCD-PS was inhibited.This study systematically revealed the adsorption behaviors of fulvic acid and heavy metal ions onto microplastics in the aqueous environment,which provided some theoretical basis for microplastics as carriers of pollutants in the environment and further emphasized the potential risk of added microplastics in the environment.In this thesis,there are 26 figures,10 tables,and 195 references.