Detection of microplastic hotspots in beach sand for national surveys using fluorescence microscopy and infrared spectroscopy: Case study on the Catalan coast.

The aim of this study is the characterization of microplastics (60 μm -5 mm) on Catalan beaches. It takes into consideration factors as sand size distribution, geomorphology, meteorological parameters and anthropogenic pressures. MPs were measured in seventy (n = 70) intertidal sand samples covering 580 km coastline. After dry sieving the fraction over 0.5 mm was analysed by visual sorting and FTIR techniques. The fraction below 0.5 mm was separated by wet density separation processes, oxidation of organic matter and then analysed by Nile Red staining and fluorescence microscopy. A subset of samples (n = 23) was also subjected to μ-FTIR techniques for compositional analysis in order to compare and confirm the results given by the staining method. The concentration of MP > 0.5 mm ranged from <0.55 to 56.8 MPs/kg (60 % fragments). For sizes 60 μm-0.5 mm it varied from 0 to 2013 MPs/kg (70 % fibres). Polyethylene, polypropylene, polyethylene terephthalate, and polystyrene composed most fragments. Polyester, polyamide, polypropylene and acrylic constituted fibres. Although spectroscopy (μFTIR) is needed to confirm the results, the staining method can be a screening strategy to detect hotspots and provided good approximations for MPs quantification. Only sand particle size is correlated with MPs concentration. Sewage treatment plant discharges, runoff, and local ocean currents might all be significant in the accumulation of MPs along the coast.