Ultraviolet-C and vacuum ultraviolet inducing surface degradation of microplastics

Wastewater treatment plants (WWTPs) are considerable microplastics (MPs) contributors to environmental waters. Knowledge about the MPs degradation process under ultraviolet irradiation was crucial to understanding the fate of MPs during and after water disinfection. In this study, surface alternations of polystyrene (PS), polyethylene (PE), polyvinylchloride (PVC) and polyethylene terephthalate (PET) under 254 nm (UV-C) and 185/254 nm (vacuum ultraviolet, VUV) irradiation were estimated. One-way treatment of MPs by UV or VUV with the recommended dose for WWTPs (USEPA, ~180 mJ cm^-2) had little effect. In contrast, excessive exposure under twenty-times doses irradiation (3600 mJ cm^-2) resulted in significant alternations on surface morphology, chemical feature and hydrophobicity. Noticeably morphology alterations, including cracks, wrinkles and protuberances, were observed for PS, PVC and PET, while PE was relatively resistant. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that UV irradiation induced cleavage of chemical bonds. Besides, simultaneous radical oxidation was predominant during VUV treatment, which contributed to the increasing generation of oxygen bonds, such as CO and CO, on all MPs surfaces. Drastic decreases of contact angle (> 20°) were observed for PS, PVC and PET even after 180 mJ cm^-2 UV irradiation, indicating the UV disinfection in WWTPs can easily change their surface hydrophobicity. All these alternations weakened the adsorption capacity of non-polar benzene and polar ciprofloxacin on MPs. Generally, regular dose UV and VUV irradiation in water treatment disinfection can only induce slight effects on MPs surface characteristics and adsorption performance, while extreme dose irradiation can induce a potential reducing risk of organic contaminants migration along with MPs.