Mobilisation kinetics of Br, Cd, Cr, Hg, Pb and Sb in microplastics exposed to simulated, dietary-adapted digestive conditions of seabirds

Samples of beached plastics and historical and contemporary consumer plastics containing hazardous elements derived from reaction residues or functional additives have been micronised and subject to extraction conditions representative of the digestive environment of seabirds. Mobilisation of Br, Cd, Cr, Hg, Pb and Sb into NaCl solution, an avian physiologically-based extraction test (PBET) and a dietary-adapted PBET (DA-PBET) incorporating fish oil as part of the avian diet was monitored by ICP-MS over a 168-h period. Kinetic data were subsequently fitted using pseudo-first-order and parabolic diffusion models in order to derive rate constants for the release of hazardous elements during avian digestion of microplastics. Rate constants were variable and dependent on the nature and origin of plastic, type of residue or additive, extractant solution employed and model applied. Resulting estimates of bioaccessibility, defined as the equilibrium or maximum concentration of an element mobilised over the time course relative to its total concentration, were variable but considerable in many cases. Specifically, maximum values of about 65% of Cd and 100% of Pb were observed in consumer polycarbonate-acrylonitrile butadiene styrene exposed to the avian PBET and beached polyurethane exposed to the DA-PBET, respectively. The potential health risks of hazardous elements in microplastics are addressed and criteria for classification based on the European Toy Safety Directive migration (mobilisation) limits are proposed.