Study On The Sorption Behaviour Of Microplastics And Sediment On Lead And Cadmium In Water

As a new type of pollutant,microplastics are widely found in surface water and sediments in estuaries and near coasts of the ocean.Because of their small particle size,large specific surface area,hydrophobicity and difficulty in degradation in natural environment,they will interact with various pollutants in the environment(such as heavy metals,antibiotics,organic pollutants,etc.)and become new carriers of pollutants in water bodies.As the main material of rivers,especially in the Three Gorges reservoir area,the change of hydrodynamic conditions leading to sedimentation and water environment problems have been of great concern.Sediment is easy to adsorb pollutants(such as heavy metals,nitrogen,phosphorus,etc.)in the process of flocculation and sedimentation,and microplastics entering the water-sand environment will cause migration changes of pollutants in the original environment.The study of the environmental behavior of microplastics in the Three Gorges reservoir area and the migration changes of pollutants after microplastics enter the water-sand environment can provide a reference for solving microplastic pollution and water pollution problems in the reservoir area.In this paper,polystyrene(PS),polypropylene(PP),sediment and the mixed system of these two microplastics after the addition of sediment were used as the research objects.The adsorption characteristics of Pb and Cd under different gradient influence conditions were investigated by kinetic adsorption experiments and isothermal adsorption experiments based on kinetic adsorption model and isothermal adsorption model,respectively,using different mass concentrations and different water disturbance intensities as the influencing factors.(1)The adsorption process of PP and PS on Pb and Cd includes three stages: fast adsorption,slow adsorption and saturation adsorption.0-5h is the fast adsorption stage,and the adsorption amounts of PP and PS on Pb and Cd per unit mass change rapidly.The adsorption amount of PP to Pb can reach 55%-61% of the equilibrium state;the adsorption amount of PS to Cd can reach 45%-71% of the equilibrium state.5～36h is the slow adsorption phase,and after 36 h,it basically reaches the dynamic equilibrium.For PP,it is easier to adsorb Cd;while for PS,it is easier to adsorb Pb.(2)There is a "solid concentration effect" in the sorption of Pb and Cd by microplastics,sediment and microplastics in conjunction with sediment.(3)The equilibrium adsorption capacity was positively correlated with the intensity of water column disturbance,and with the increase of water column disturbance,the capacity of both microplastics,sediment and the mixture of microplastics and sediment to adsorb Pb and Cd was enhanced,and the equilibrium adsorption capacity was increased.The water column perturbation promoted the contact efficiency of the above-mentioned study objects with heavy metal ions in water,making them more capable of exchanging substances with the water column and thus increasing the adsorption capacity.In addition,changing the intensity of water column perturbation does not affect the adsorption mode.(4)In the mixture of microplastics and sediment,the adsorption capacity of the mixture of sediment and PP for Pb is weaker than that of the mixture of sediment and PS for Pb;the adsorption capacity of the mixture of sediment and PP for Cd is stronger than that of the mixture of sediment and PS for Cd.The addition of microplastics to the sediment reduces the adsorption capacity of the sediment itself for heavy metals.The addition of microplastics changes the movement of sediment particles in the aqueous phase,and in the process of flocculation and sedimentation,the microplastic particles are "wrapped" by net trapping and sweeping,forming larger sediment particles or flocs,thus increasing the specific surface area and reducing the number of adsorption sites.In addition,the addition of microplastics increases the collision probability between microplastics and sediment,which increases the mutual interference strength and leads to the decrease of adsorption amount.(5)The kinetic adsorption processes and isothermal processes fitted by adsorption kinetic model and isothermal adsorption model for polystyrene(PS),polypropylene(PP),fine-grained sediment,and mixtures of these two microplastics and sediment found that the quasi-secondary kinetic model could better describe their kinetic adsorption processes for Pb and Cd;the Langmuir model could better describe their isothermal adsorption processes.The other two kinetic adsorption models and isothermal adsorption models differed in their predictions of the maximum adsorption capacity parameters,equilibrium adsorption capacity parameters,reaction trends and reaction endpoints of the adsorption processes due to different algorithms.