| Microplastics is defined as small plastic particles less than 5mm in diameter.At present,the United States,the United Kingdom,Australia,Sweden,Finland and other countries have done research on microplastics in sewage treatment plants.Most wastewater treatment plants have no ability to effectively remove microplastics,lead to microplastics enter the natural water environment with wastewater discharge.Heavy metal pollution has also been the focus of people's attention.Scholars of many countries have carried out a lot of studies on the adsorption of heavy metals in environmental media,but most of studies focused on the adsorption between antibiotics and heavy metals in soil,water and sediment.It has been reported that plastics in the environment could absorb pollutants,and had an impact of no neglect for migration and transformation of pollutants,but the study of adsorption mechanism is still in the exploratory stage.Therefore,this thesis starts from microplastics and heavy metal to study on microplastics characteristics in domestic sewage and the adsorption mechanism of heavy metals onto microplastics,and to investigate the contribution of microplastics in wastewater to marine pollution.In this work,common plastics such as polyethylene(PE)and polypropylene(PP)and heavy metals such as lead,cadmium and arsenic were selected as study objects.Samples of domestic sewage were collected from Dalian Ling-shui-he sewage treatment plant.Firstly,the number and types of microplastics in sewage samples were analyzed,the influence of microplastics carried in wastewater on seawater pollution was discussed.Secondly,the adsorption mechanism of lead,cadmium and arsenic onto microplastics in fresh water was investigated,the influence of heavy metals adsorbed onto microplastics on marine pollution was analyzed.The experimental results are as follows:1.The average concentration of microplastics in primary treatment sewage was obviously higher than that in secondary treatment sewage,this indicated that the removal rate of microplastics in secondary treatment was significantly higher than that in primary treatment.2.The adsorption of lead,cadmium and arsenic onto microplastics in freshwater can be divided into three time periods:from 0 to 10 hours,these metals have adsorption on microplastics,and the adsorption process was very fast.The adsorption amounts of lead,cadmium and arsenic onto PE and PP can reach about 1/2 of the total adsorption amounts.Within 10?30 hours,the adsorption rate of lead,cadmium and arsenic onto PP and PE gradually decreased.After 30 hours,the adsorption of lead,cadmium and arsenic on the microplastics reached an equilibrium.The adsorption capacity of PE to metal was less than that of PP.3.The fitting results of kinetic adsorption model and isothermal adsorption model showed that quasi-second-order kinetic adsorption model fitted well the adsorption behavior of heavy metals onto microplastics in pure water,the adsorption process was jointly controlled by several adsorption periods.Three isothermal adsorption models all fitted well,among them,Freundlich model had the best fitting effect on the adsorption of three heavy metals on microplastics. |
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