Effect of light irradiation on heavy metal adsorption onto microplastics

Microplastics are frequently found in many environmental media. Polypropylene (PP) is one of the plastics commonly used, resulting in more and more PP fragments in natural waters. Contaminants, such as lead (Pb), could get adsorbed onto microplastics after the exposure to sunlight, and pose a larger threat to aquatic species. In this study, the oxidative indices of PP pellets after different exposure times to a Xenon lamp were evaluated by Fourier transform infrared (FTIR) and energy-dispersive X-ray spectrometry. The results show that the percentage of oxygen content increased from 2.80 to 20.95 wt% and changes of characteristic peaks of the FTIR pattern, implying that the exposure to the Xenon lamp could initiate oxidation. Due to the changes of functional groups after the exposure to the Xenon lamp for 28 days, the adsorption capacities of the PP pellets were up to 274.4 mg⋅kg^-1, 1.7 to 2.5 times higher than that of the raw PP pellets depending on the solution pHs. The adsorption behavior can be described by a pseudo-second-order model with rate constants of adsorption of 0.00212-0.01404 kg⋅mg^-1⋅h^-1. The increase of adsorption capacity due to changes of the PP pellets after the Xenon lamp exposure increased the potential risk to the aquatic species.