Occurrence characteristics of plastic waste on beaches and the influencing factors of mercury enrichment: a case study of Qingdao

Abstract Plastic pollution has become a research hotspot. To explore the occurrence characteristics and influencing factors of mercury (Hg) in plastic waste on beaches, plastic debris samples were collected from the intertidal and supratidal zones of four typical beaches in Qingdao. Polymer types of the plastics were identified, and Hg concentrations in the plastics were determined to investigate the effects of physicochemical properties (plastic color, polymer type, and degree of aging) on Hg enrichment. The results showed that plastic bags and food packaging dominated the beach plastic waste, and the main polymer types were polyethylene (PE) and polypropylene (PP). The average Hg concentration in plastics was 0.97 μg/g (0.27–2.17 μg/g). The Hg content in the plastic was generally higher in the intertidal zone than in the supratidal zone, indicating that tidal action may facilitate Hg enrichment in plastics. Significant differences in Hg content were found among plastics of different colors, with black and light-yellow plastics exhibiting higher Hg concentrations, which may be related to the raw materials and aging of the plastics. Among polymer types, polyethylene terephthalate (PET) plastics showed the highest Hg content. Plastic waste with a higher degree of aging contained higher Hg concentrations. The results indicated that the spatial distribution and physicochemical properties of plastics significantly influence their Hg enrichment. This study provides a scientific basis for understanding the interactions between beach plastic waste and Hg pollution, as well as their potential environmental risks.