Adsorption behavior of polyamide microplastics as a vector of the cyanotoxin microcystin-LR in environmental freshwaters

Microplastics are ubiquitous environmental contaminants, and concern about microplastics functioning as vectors for coexisting environmental contaminants has been increasing. In this study, we evaluated the potential of microplastics as a vector for microcystins (MCs) in an aquatic environment. Six microplastics—polyvinylidene chloride, polystyrene, polyamide-6 (PA-6), polyvinyl chloride, poly(ethylene terephthalate), and polyethylene—were used in the experiments, and the PA-6 microplastics showed strong affinity toward the cyanotoxin microcystin-leucine arginine (MC-LR) with an adsorption efficiency of 89.5 ± 0.1 %. The adsorption of MC-LR onto PA-6 microplastics was well described by the pseudo-first-order kinetics and Langmuir isotherm models, and the adsorption was considered to be driven mainly by polar–polar interactions. The maximum adsorption capacity (q_m) of MC-LR onto PA-6 microplastics was estimated to be 85.64–129.05 μg per g of PA-6 microplastics. Coexisting ions of NaCl, MgSO₄, KH₂PO₄, CaCO₃, and Na₂HPO₄ marginally affected the adsorption of MC-LR onto the PA-6 microplastics. However, water-quality parameters of conductivity and total-nitrogen content in environmental freshwaters influenced the adsorption of MC-LR onto PA-6 microplastics. The adsorption capability of PA-6 microplastics was evaluated using extracellular MCs (i.e., MC-LR, MC-YR, MC-RR, and total MCs) released from Microcystis aeruginosa cells during their growth.