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Adsorption of heavy metals by biofilm-coated microplastics in aquatic environments: Mechanisms, isotherm and kinetic processes, and influencing factors
Summary
This review synthesizes research on how biofilms—microbial coatings that naturally form on microplastics in water—alter the particles' ability to absorb heavy metals like lead, copper, and cadmium, finding that biofilmed microplastics generally adsorb more metal than bare plastic and that electrostatic forces and surface complexation are the dominant mechanisms. This matters because microplastics coated in both biofilm and toxic metals may deliver a double dose of contamination to organisms that ingest them. The review identifies key gaps, including how competitive metal mixtures and shifting biofilm composition over time affect this combined pollution risk.
Microplastics (MPs) and heavy metals (HMs) are pervasive co-contaminants in environmental systems, where their synergistic interactions may amplify ecological risks. Notably, biofilm-coated microplastics (B-MPs), ubiquitous in aquatic environments, exhibit distinct physicochemical properties that govern heavy metal (HM) adsorption behaviors. Despite a surge in research on B-MPs-mediated HM adsorption, mechanistic drivers, quantitative modeling, and multifactorial regulation are still lack of systematic elucidation. This critical review synthesizes current advances to systematically decode adsorption mechanisms, adsorption isothermal/kinetic models, hierarchical controls spanning biofilm traits, microplastic characteristics, metal properties, and environmental conditions. The difference in HM adsorption between by B-MPs and naked MPs (N-MPs) are also systematically discusses. Key findings reveal that electrostatic interactions and surface complexation generally dominate the adsorption of HMs onto B-MPs, with kinetics best described by pseudo-second-order models and isothermal processes fitting Freundlich or Langmuir models. Several key aspects necessitate further elucidation, including competitive adsorption phenomena and their interplay with microbial metabolic shifts in multimetallic systems, the influence of plastic-derived dissolved organic matter (DOM), dynamic adsorption processes during biofilm formation, and the repercussions of pH-induced alterations in biofilm architecture and extracellular polymeric substance (EPS) composition. By bridging current insights with environmental realism, this work identifies understudied knowledge-high-environmentally relevant research models, biofilm succession dynamics, long-term HM retention, multifactorial influencing effects, AI-assisted exploring approaches-that warrants prioritization in future research.