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Microplastics (MPs) quantification in benthic marine sediments is typically performed by time-consuming and moderately accurate mechanical separation and microscopy detection. In this paper, we describe the results of our innovative Polymer Identification and Specific Analysis (PISA) of microplastic total mass, previously tested on either less complex sandy beach sediment or less demanding (because of the high MPs content) wastewater treatment plant sludges, applied to the analysis of benthic sediments from a sublittoral area north-west of Leghorn (Tuscany, Italy). Samples were collected from two shallow sites characterized by coarse debris in a mixed seabed of Posidonia oceanica, and by a very fine silty-organogenic sediment, respectively. After sieving at <2 mm the sediment was sequentially extracted with selective organic solvents and the two polymer classes polystyrene (PS) and polyolefins (PE and PP) were quantified by pyrolysis-gas chromatography-mass spectrometry (Pyr-GC/MS). A contamination in the 8–65 ppm range by PS could be accurately detected. Acid hydrolysis on the extracted residue to achieve total depolymerization of all natural and synthetic polyamides, tagging of all aminated species in the hydrolysate with a fluorophore, and reversed-phase high performance liquid chromatography (HPLC) (RP-HPLC) analysis, allowed the quantification within the 137–1523 ppm range of the individual mass of contaminating nylon 6 and nylon 6,6, based on the detected amounts of the respective monomeric amines 6-aminohexanoic acid (AHA) and hexamethylenediamine (HMDA). Finally, alkaline hydrolysis of the residue from acid hydrolysis followed by RP-HPLC analysis of the purified hydrolysate showed contamination by polyethylene terephthalate (PET) in the 12.1–2.7 ppm range, based on the content of its comonomer, terephthalic acid. [ABSTRACT FROM AUTHOR]

• A simple, sensitive & accurate analytical method for PET microplastics in sediments. • Polyester microplastics are found and quantified in marine and freshwater sediments. • The analytical protocol can be extended to complex environmental matrices. Microplastics are ubiquitous pollutants in marine and freshwater bodies. Poly(ethylene terephthalate) microfibers (PMFs) are among the main primary microplastics (as-produced polymer microparticles). Released in large amounts in laundry wastewaters, PMFs end up in freshwater and marine sediments due to their high density. PMFs are potentially hazardous pollutants for ecosystems and human health, being a deceiving food source for animal organisms at the base of the food chain (e.g. sediment and water filtrators, including edible shellfish and small crustaceans). This study describes a simple, sensitive and versatile procedure for quantifying the total mass of PET micro- and nano-particles in sediments. The procedure involves aqueous alkaline PET depolymerization with phase transfer catalysis, oxidation and fractionations to remove interfering species and pre-concentrate the terephthalic acid (TPA) monomer, and TPA quantification by reversed-phase HPLC. Recovery of TPA from a model sediment spiked with 800 ppm PET micropowder was 98.2 %, with limits of detection/quantification LOD = 17.2 μg/kg and LOQ = 57.0 μg/kg. Analyses of sandy sediments from a marine beach in Tuscany, Italy, showed contamination in the 370–460 μg/kg range, suggesting that a not negligible fraction of PET microfibers released in surface waters ends up in shore sediments. [ABSTRACT FROM AUTHOR]