Linear relationship between carbonyl index of weathered microplastics with a CC backbone and solar light photodegradation kinetics of nicotine

Abstract

In this study, we argue that the Carbonyl Index (CI) values of weathered microplastics with carbon–carbon backbones can reliably predict their ability to enhance indirect photodegradation of organics under solar light, an effect that is independent of the type of polymer used. Polystyrene (PS), polypropylene (PP), and low-density polyethylene (LDPE) pellets were artificially weathered under UV-C, either in ambient air or submerged in water. Surface changes were characterized using ATR-FTIR spectroscopy, and the CI values were calculated. PS exhibited a much higher weathering rate than PP and LDPE, with air exposure being more effective than water for all plastic types. Nicotine (NIC) was used as the model organic compound. NIC photodegradation (10 mg L⁻¹) was monitored in the presence of each weathered pellet type (50 g L⁻¹) for 6 h under simulated sunlight at pH 11. The apparent pseudo-first-order rate constants, k, were 0.025 ± 0.015 h⁻¹ (air-weathered LDPE), 0.036 ± 0.002 h⁻¹ (water-weathered PP), 0.079 ± 0.029 h⁻¹ (air-weathered PP), 0.216 ± 0.014 h⁻¹ (water-weathered PS), and 0.346 ± 0.016 h⁻¹ (air-weathered PS). A linear correlation (R² = 0.956) was found between CI and k, indicating that photodegradation is primarily governed by the degree of aging rather than the polymer type. This relationship was further validated using naturally weathered PS and PP items collected from beaches, which aligned with the established CI–k correlation. These findings suggest that the CI is a practical indicator for predicting the environmental reactivity of weathered microplastics.