Heavy Metal Adsorption on Microplastics: Adsorption Mechanisms, Influencing Factors, Analytical Techniques, Toxicological Implications, and Remediation Strategies

Abstract

Microplastics (MPs) act as reactive surfaces and carriers in aquatic systems, influencing the mobility, bioavailability, and toxicity of heavy metals. Recent research has clarified how heavy metals adsorb onto MPs, focusing on adsorption pathways, physicochemical factors, analytical methods, toxicological impacts, and remediation strategies. Metal binding to MPs depends on factors such as polymer type, particle size, aging, surface properties, zeta potential, pH, salinity, dissolved organic matter, temperature, and the presence of biofilms. Adsorption occurs through mechanisms including electrostatic interactions, surface complexation, ion exchange, pore filling, hydrogen bonding, van der Waals forces, cation–π interactions, surface precipitation, and biofilm-mediated binding. MP–metal interactions vary with environmental conditions; for example, salinity and dissolved organic matter can enhance or reduce adsorption, depending on metal speciation, polymer characteristics, and experimental conditions. Toxicological studies show that MPs carrying heavy metals can increase bioaccumulation and combined toxicity by promoting transport and cellular uptake. However, strong adsorption and limited desorption may sometimes reduce the bioavailability of dissolved metals. The Mediterranean Sea is a key case study due to its semi-enclosed nature, high urbanization, maritime activity, wastewater discharge, and significant plastic pollution, all of which heighten the importance of MP–metal interactions. There is an urgent need for standardized adsorption protocols, thorough in situ and ex situ characterization, ecologically relevant toxicological studies, and integrated remediation strategies addressing both MPs and associated heavy metals.