Biological aging of polyethylene and polystyrene microplastics by Chlorella vulgaris biofilm: enhanced malachite green adsorption on polystyrene

Biological growth on MPs in aquatic environments can significantly alter their physicochemical properties and interactions with pollutants. This study investigated how biofilm formation by C. vulgaris modifies the properties of PE and PS MPs and influences their capacity to adsorb MG dye. Initially, the biofilm formation potential of PE and PS MPs in the presence of C. vulgaris was evaluated. PS exhibited more stable and extensive biofilm development, along with higher biomass accumulation and EPS production compared to PE, and was therefore selected for subsequent experiments. The study then focused on the structural and chemical characteristics of C. vulgaris-mediated biofilms on PS. Following biofilm formation, the biofilm layer was removed using 30% hydrogen peroxide, producing bio-aged PS. The physicochemical properties of this bio-aged PS, including photosynthetic pigment content (chlorophyll a/b), EPS, surface charge, particle size, and surface chemistry (carbon content), were systematically quantified. Adsorption experiments were subsequently conducted using the biofilm-removed, bio-aged PS under varying MG concentrations and contact times to evaluate the impact of biofilm-induced modifications on dye adsorption. The results demonstrated that biofilm formation on PS induced pronounced structural and chemical changes. Biofilm-coated PS exhibited elevated chlorophyll and EPS levels, which, after removal, resulted in bio-aged PS particles with reduced size and a more negative surface charge (from − 45.6 mV for virgin PS to − 48.73 mV), along with a decreased carbon content (67.73%). Importantly, this biological aging significantly enhanced MG adsorption capacity, increasing from 4.1% for virgin PS to 38.3% for bio-aged PS within 90 min at 10 mg/L. These findings demonstrate that biofilm-mediated aging substantially enhances the adsorption capacity of MPs, emphasizing the critical role of microbial interactions in determining their environmental behavior and interactions with pollutants in aquatic systems.