Heavy metals in the “plastisphere” of marine microplastics: adsorption mechanisms and composite risk

Heavy metals that are adhered to the surfaces of microplastics in the natural marine environment pose potential threats to marine life. However, little is known about their adsorption mechanisms and composite risks. This study collected seawater, suspended particulate matter (SPM), plankton, and microplastic samples from the waters surrounding Hong Kong, established a method for analyzing 13 heavy metals in the plastisphere, and explored the extrinsic and intrinsic sources of the heavy metals by analyzing natural matrices (seawater, SPM, and plankton) and anthropogenic plastics. We found that heavy metals in the plastisphere were significantly affected by the surrounding matrices, as indicated by its positive correlations with the heavy metals in SPM and plankton at most of the sampling sites. Heavy metals in plankton usually occurred as traces of oxides such as As2O3 and ZnO. The partition coefficients of the studied heavy metals between seawater and the plastisphere were identical to those between the SPM and the plastisphere. No correlations were found between the concentrations of heavy metals in the plastisphere and the heavy metal concentrations, masses, quantities, or surface areas of the microplastics, although some polymer types contained extremely high Cd (676 mg/kg), Pb (1,742 mg/kg), and Zn (1,712 mg/kg) concentrations. Only the Cr, Cu, and Ni concentrations were positively correlated with the specific surface area of the microplastics. A cluster analysis showed that Cr, Cu, and Ni were derived from anthropogenic sources, indicating that adventive heavy metals could be adsorbed by plastics with large specific surface areas. We also established a new composite risk assessment method for heavy metals and microplastics that leverages the specific surface areas of microplastics. The assessment results showed that Cd from Tolo Harbor had a strong ecological risk, while As and Cd from the eastern waters off Hong Kong had moderate composite ecological risks.