Mechanism And Characteristics Of Removing Microplastics From Water By Coagulation Process

Microplastics,as an emerging pollutant,are present in large quantities in the water environment.It not only threatens the safety of human drinking water but also has a negative impact on the whole ecosphere.Coagulation technology,as a conventional part of water treatment plant,has a certain effect on the removal of microplastics,and there is an urgent need to understand the performance,principle and influencing factors of coagulation to remove microplastics.In this study,the coagulation method was used to remove microplastics,using polymeric aluminum chloride and polyacrylamide as coagulant and coagulant aid,respectively,the common microplastics of polyethylene(PE),polypropylene(PP),polystyrene(PS)and polyethylene terephthalate(PET)in the water environment were used as removal objects,and fluorescence analysis was used as the detection method of microplastics to investigate the effect of coagulant and coagulant aid on the removal of microplastics at different dosages.The effect of coagulant and coagulant additives on the removal of microplastics was investigated,and the effect of different factors on the removal effect was investigated by changing the initial turbidity,p H,concentration and particle size of microplastics.In this experiment,the Nile red dye was made to enter the interior of microplastics that were expanding at 70°C by using the thermal expansion and contraction effect of microplastics,and was rapidly cooled to enhance the dyeing effect.The results showed that a dyeing time of 30 min and a dye concentration of 100 mg/L were the most suitable conditions for fluorescent dyeing of microplastics.The differences in the type of microplastics and the size of microplastics after dyeing affected the fluorescence intensity of microplastics.The staining success rates of the four microplastics were PE(100%),PP(98.82%),PS(97.1%),and PET(94.87%),respectively.The surface morphology,functional groups,and zeta potential of the dyed microplastics did not differ significantly from those of the pre-dyed microplastics and maintained the fluorescence intensity in a variety of complex water bodies,indicating that the microstructure of the microplastics was less affected by this dyeing method.The stained microplastics can be successfully applied to quantitative studies of coagulation removal.Overall,our proposed staining and quantification method based on laboratory studies is relatively simple,economical and rapid,and can be useful for studying the effects of coagulation,adsorption,sand filtration and membrane filtration techniques on microplastic removal in laboratory scale,as well as for tracer analysis of microplastics in water bodies.In this experiment,a combination of no coagulant,single coagulant,polymeric aluminum chloride and polyacrylamide was used to remove the four microplastics.The results showed that when no coagulant was added,the removal efficiencies of the four microplastics were PE(13.7%),PP(17.4%),PS(21.6%),and PET(39.6%)due to the difference of their own densities.The best removal efficiencies of the four microplastics were PE(37.8%),PP(26.7%),PS(36.4%),and PET(94.2%)when polymerized aluminum chloride was added singly.Zeta potential,Fourier infrared spectroscopy and microscopic analysis showed that the main mechanism of microplastic removal by polymeric aluminum chloride was charge neutralization and adsorption shelf.The main mechanism of microplastic removal by polyacrylamide is adsorption bridging and netting sweeping,while the main mechanism of microplastic removal by polyacrylamide is adsorption bridging and netting sweeping.Overall,this study demonstrated the ability of conventional coagulants in water treatment to remove microplastics,and explored the role of coagulation mechanism in different coagulants,which helps to scientifically regulate the practical application of microplastics removal in water treatment plants,and also provides some theoretical support for the subsequent study of microplastics removal mechanism.In this experiment,different factors were changed in order to investigate their effects on the removal effect.The results showed that the initial turbidity of the water sample and the concentration of microplastics had a partial effect on the removal effect,and the greater the initial turbidity of the water body and the lower the concentration of microplastics,the higher the removal efficiency of microplastics;while when the combination of aluminum chloride and polyacrylamide was used,the initial turbidity of the water body and the concentration of microplastics had little effect on the removal efficiency of microplastics.The initial p H of the water sample and the particle size of the microplastics had a greater effect on the removal efficiency of microplastics,and the removal efficiency of microplastics increased and then decreased with the increase of p H,and the larger the particle size of microplastics,the higher the removal efficiency of microplastics.Overall,this study analyzed the effect of different factors on the removal effect,which can help guide the practical application of microplastic removal in water treatment plants.