A comparative evaluation of metal sorption across environmentally relevant microplastics: a focus on polyethylene terephthalate

Microplastics (MPs) are increasingly recognized as dynamic vectors for heavy metals, affecting contaminant mobility, bioavailability, and ecological risk across aquatic and terrestrial systems. This review systematically evaluates heavy metal sorption onto environmentally relevant MPs, with particular emphasis on polyethylene terephthalate (PET), while also comparing polyethylene (PE), polypropylene (PP), polystyrene (PS), and selected modified polymers. A preferred reporting items for systematic reviews and meta-analyses (PRISMA)-guided framework was used to identify key studies for synthesis and mechanistic evaluation. The reviewed evidence demonstrates that adsorption behavior varies widely and is controlled primarily by environmental medium chemistry, metal speciation, and surface aging rather than polymer identity alone. PET frequently exhibits relatively high affinity toward Pb, Cd, and Co under freshwater and low-ionic-strength conditions, whereas marine environments commonly suppress sorption because of ionic competition and metal-ion complexation. Many studies reported pseudo-second-order kinetic behavior, indicating the importance of surface-controlled interactions on heterogeneous and oxidized MP surfaces. Despite emerging mechanistic consistency, substantial variability in experimental design, particle characteristics, and reporting approaches limits direct comparison among studies. Consequently, the proposed sorption hierarchy should be interpreted as a qualitative evidence-based framework rather than a statistically validated ranking. Overall, this review highlights MPs as environmentally conditioned and dynamic sorbents requiring standardized methodologies for more reliable ecological risk assessment.