UV-ageing effects on polystyrene microplastics surface polarity and transport in soils

Microplastics (MP) in soils are considered an emerging environmental pollutant of global concern. The transport processes of MP in soils are poorly understood, which indicates a major knowledge gap regarding the environmental impact and behaviour of MP. Mobility and surface charge can be affected by ageing, mainly via UV irradiation. In the present study, 1 µm polystyrene microspheres (PS-MP) were aged with UV irradiation using an irradiance of 2.05 W m⁻² (ultraviolet A, UVA; 365 nm) and 5.58 W m⁻² (ultraviolet C, UVC; 254 nm) and different exposure times between 24 und 1176 hours to generate a gradient in MP ageing. The UV-ageing effects on PS-MP were characterised in terms of changes in particle size, zeta potential, and surface functional groups as determined by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Non-irradiated PS-MP microspheres were used as a control. Column tests were performed by percolating PS-MP microspheres suspended in CaCl₂ solution (ionic strength 7.5 mM) through a quartz sand matrix (0.3 mm particle diameter). PS-MP sizes steadily decreased with increasing ageing time from 24 h to 1176 h for both UVA and UVC treatment. ATR-FTIR results revealed that UV irradiation induced a transformation of PS-MP surfaces with especially the appearance of a carbonyl (C=O) peak due to C-H bond breaking during the oxidation process. The increasing presence of oxygen-containing functional groups in UV-treated PS-MP led to an increasing negative surface charge and zeta potential and enhanced PS-MP transport through the quartz sand column. With increasing irradiation time, the aged PS-MP showed continuously increasing transport rates compared to the transport rates of non-aged PS-MP, ranging from 46 to 85% and 48 to 91% for UVA- and UVC-treated samples, respectively. The fastest breakthrough was observed for highest negative zeta potential and amount of O-containing functional groups as compared to control PS-MP, which enhanced electrostatic repulsion between aged PS-MP and quartz sand particles as was demonstrated from calculation of the interaction free energy. Our findings imply that UV irradiation changes the PS-MP surface physicochemical characteristics and transport potential by increasing the mobility of PS-MP and hence the risk for contamination of deeper soil layers and water bodies.