Effect Of Ultraviolet And Chlorine Disinfection On The Adsorption Performance Of Typical Microplastics

Microplastic is an emerging contaminant,which is ubiquitous in aquatic environments worldwide.Wastewater treatment plants were proved as one of the pathways of microplastics entering the aquatic environment.Knowledge about the alteration of microplastics during disinfection processes is vital for investigating the environmental behaviors of microplastics.In this paper,four typical microplastics,polystyrene(PS),polyethylene(PE),polyvinyl chloride(PVC)and polyethylene terephthalate(PET)were selected to investigate the alteration on surface properties and adsorption performances of microplastics under disinfection processes including ultraviolet(UV)disinfection,vacuum ultraviolet(VUV)disinfection and chlorine disinfection.The main results include:The surface properties of the four microplastics treated according to the recommended disinfectant dosage of the wastewater treatment plant had little change.However,while the treatment dosage reached 20 times of the recommended dosage in the wastewater treatment plant,significant alteration on the surface morphology,chemical characteristics and hydrophobicity of the microplastics occurred.Among the microplastics,PS showed the most obvious change,followed by PVC and PET,the change of PE was the weakest,which is related to the chemical stability of polymer.The treatment of UV,VUV and sodium hypochlorite leaded to alteration on the surface morphology of microplastics,including cracks,melting,curling and protrusions.In addition,oxygen-containing functional groups O-C and O=C were generated on the surface of the microplastics as well as hydrophobic property of particle surfaces was changed.Three disinfection processes had different degradation mechanisms on microplastics.The main mechanism of UV disinfection on microplastic surfaces was photolysis,including cutting off and degrading the chemical groups on the surface.The degradation mechanism of VUV on the surface of microplastics contained photolysis and oxidation,resulting in damage of chemical functional groups.Oxidization occurred on the surface of microplastics and groups containing hydroxyl and carbonyl were generated.Furthermore,chlorine disinfection induced the formation of C-Cl bond and O-H bond on the surface of microplastics,suggesting chlorination and oxidation reaction occurred on microplastic surfaces.The alteration on surface properties and chemical structures weakened the adsorption capacity of polar ciprofloxacin by PS,PE and PET.The adsorption isotherms of ciprofloxacin by the microplastics before and after disinfection treatment were conformed to follow the Freundlich adsorption model(R~2>0.95).In short,the conventional dosage(low dosage)disinfection process in the water treatment disinfection had a slight effect on the surface properties and adsorption performance of the microplastics,while the high dosage(20 times)disinfection induced significant change in the characteristics of the microplastics,resulting in the decrease of their adsorption capacity in aquatic environment.