Metal association and bioaccessibility in rope-derived fibrous microplastics from remote coastal environments of Northern Scotland

Microplastic pollution is increasingly recognised as a pervasive environmental issue, including in remote coastal regions that experience relatively low direct anthropogenic pressure. Beyond their physical presence, microplastics can act as carriers of environmentally acquired and manufacturing-derived metals, influencing metal mobility and bioaccessibility. In this study, fibrous microplastics collected from sandy beaches at 24 sites across Northern mainland Scotland and the Orkney Islands were characterised, and plastic rope materials (both virgin and beach-collected), identified as the dominant source of these fibres, were analysed to assess associated metal burdens and bioaccessibility. Polymer composition and surface weathering were determined using Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), while metal concentrations and partitioning in rope materials were evaluated using sequential extraction and pseudo-total digestion methods. Polypropylene was the dominant polymer type, followed by polyethylene, consistent with fishing-related inputs. Weathered ropes exhibited surface oxidation and structural degradation, indicative of enhanced metal association. Sequential extraction showed that Mn exhibited the highest bioaccessible fraction, followed by Pb and Zn, whereas pseudo-total digestion revealed additional metal contributions originating from pigments and additives incorporated during manufacture. Overall, this study provides a regional-scale mechanistic assessment of metal association and bioaccessibility in rope-derived fibrous microplastics from remote coastal environments of Northern Scotland.