Adsorption of Cobalt onto Microplastics in Riverine Mesocosms: Influence of Biofilm Development

The plastisphere critically modulates interactions among microplastics (MPs), biofilms, and trace metals under environmentally realistic conditions, governing the interfacial reactivity of plastisphere-coated microplastics. Here, we investigated cobalt (Co) adsorption onto large polypropylene (PP) primary microplastics (~4 mm) exposed for 28 days in a flow-through riverine mesocosm across a gradient of Co concentrations (0–60 µg·L−1). A multi-technique analytical approach was employed, combining inductively coupled plasma mass spectrometry (ICP-MS), laser ablation ICP-MS (LA-ICP-MS), quantitative PCR (qPCR), and scanning electron microscopy (SEM). Cobalt accumulation increased linearly with time, consistent with apparent first-order dependence on aqueous Co concentration under constant exposure, reaching 38 ± 3 mg·kg−1 after 28 days at 60 µg·L−1, with no saturation observed. The concentration-dependent Co accumulation was well described by an empirical power-law model (Q = KC0n), with the empirical coefficient K increasing linearly over time, reflecting the progressive increase in Co accumulation at a given aqueous concentration. Microbial colonization developed rapidly on MPs, with 16S and 18S rRNA gene copy numbers stabilizing after 14 days, while surface-normalized Co signals increased sharply after day 21, indicating a time-dependent modification of biofilm properties influencing Co retention. SEM confirmed complex microbial structures, including diatom-like cells. No cobalt adsorption was observed under sterile conditions, confirming the key role of biofilm presence. These findings highlight the dynamic role of the plastisphere as a chemically and biologically active interface under environmentally realistic riverine conditions, with implications for contaminant fate, bioavailability, and risk assessment in freshwater systems affected by plastic pollution.