Adsorption properties and mechanism of Cu(Ⅱ) on virgin and aged microplastics in the aquatic environment

Abstract Microplastics (MPs) bind to heavy metals in the aquatic environment and move into groups as carriers. They are prone to aging due to UV irradiation, which affects their adsorbability and mechanisms. The physicochemical properties Polyamide ( PA ) and polylactic acid ( PLA ) MPs were characterized. The impact and mechanism of UV aging on the adsorption of typical Cu(II) contaminants by MPs were investigated. The findings demonstrated that UV aging increased the specific surface area ( S BET ) and amount of oxygen-functional groups, decreased crystallinity, and generated pits on superficies of the MPs. Before and after aging, the adsorption behavior of the MPs on Cu(II) complied with the pseudo-second-order and Langmuir isotherm models. The primary adsorption processes, which were predominantly single-layer chemical reactions, were liquid-film and intra-particle diffusion. Compared to virgin MPs, the aged MPs had a higher adsorbability, which was primarily explained by the electrostatic attraction and complexation of the oxygen-functional groups with Cu(II). Cu(Ⅱ) adsorption by MPs was significantly affected by pH and salinity. At pH 5, the maximum adsorbability was noted, and increasing salinity reduced the MPs' ability to bind Cu(II). The equilibrium Cu(Ⅱ) adsorbability and MPs dosage were negatively correlated. These findings provide a scientific foundation for ecological environment risk assessment when MPs coexist with heavy metals.