The game changer of biofilm in microplastic pollution and potential environmental risks: Unveiling the pivotal roles on surface modification, metal adsorption, and biological uptake

Microplastics (MPs), as pervasive environmental pollutants, can act as vectors for heavy metals (HMs); however, their surface properties and environmental risks are profoundly altered by biofilm colonization under realistic aquatic conditions. In this study, we systematically investigated the adsorption behaviors of cadmium (Cd) and copper (Cu) on biofilm-developed MPs of polypropylene (PP), polyethylene terephthalate (PET), and polylactic acid (PLA), and explored the potential environmental risks associated with the enhanced vector effect of biofilm-developed MPs on HMs in aquatic environments. After 35 days of biofilm cultivation, successful biofilm colonization was confirmed by scanning electron microscopy (SEM) and crystal violet staining, with an order of PP > PLA > PET. Biofilm formation significantly altered surface physiochemical properties of MPs as evidenced by increasing specific surface area and introducing oxygen/nitrogen-containing functional groups. Batch adsorption experiments demonstrated that biofilm-developed MPs exhibited significantly enhanced adsorption capacities for Cd/Cu, with Cd adsorption on PP increasing by up to 101.34% compared with virgin MPs. Adsorption kinetics and isotherm analysis revealed that biofilm-developed MPs conformed to the pseudo-second-order model and the Langmuir model, and were jointly dominated by physical adsorption, chemical adsorption and biological adsorption. Importantly, zebrafish exposure experiments demonstrated that the biofilm-developed MPs increased intestinal Cd accumulation by 95.43%, suggesting that biofilm colonization amplified the vector role of MPs and enhanced their associated environmental health risks. This study demonstrates that biofilm colonization is a game changer transforming MPs into more reactive and hazardous composite pollutants, emphasizing the necessity of incorporating this biotic layer into environmental risk assessments of MP-HMs.