Micro(nano)plastics from synthetic oligomers persisting in Mediterranean seawater: Comprehensive NMR analysis, concerns and origins

The presence in seawater of low-molecular-weight polyethylene (PE) and polydimethylsiloxane (PDMS), synthetic polymers with high chemical resistance, has been demonstrated in this study for the first time by developing a novel methodology for their recovery and quantification from surface seawater. These synthetic polymer debris (SPD) with very low molecular weights and sizes in the nano- and micro-metre range have escaped conventional analytical methods. SPD have been easily recovered from water samples (2 L) through filtration with a nitrocellulose membrane filter with a pore size of 0.45 μm. Dissolving the filter in acetone allowed the isolation of the particulates by centrifugation followed by drying. The isolated SPD were analysed by 1H nuclear magnetic resonance spectroscopy (1H NMR), identifying PE and PDMS. These polymers are thus persisting on seawater because of their low density and the ponderal concentrations were quantified in mg/m3. This method was used in an actual case study in which 120 surface seawater samples were collected during two sampling campaigns in the Mediterranean Sea (from the Gulf of Salerno to the Gulf of Policastro in South Italy). The developed analytical protocol allowed achieving unprecedented simplicity, rapidity and sensitivity. The 1H and 13C NMR structural analysis of the PE debris indicates the presence of oxidised polymer chains with very low molecular weights. Additionally, the origin of those low molecular weight polymers was investigated by analysing influents and effluents from a wastewater treatment plant (WWTP) in Salerno as a hot spot for the release of SPD: the analysis indicates the presence of low molecular weight polymers compatible with wax-PE, widely used for coating applications, food industry, cosmetics and detergents. Moreover, the origin of PDMS debris found in surface seawater can be ascribed to silicone-based antifoamers and emulsifiers.

The authors would like to express their sincere gratitude to Paolo Napodano from the University of Salerno for technical assistance. Deurex AG (Germany) is gratefully acknowledged for the kind supply of PE, oxidized PE and PP waxes. Financial support is acknowledged from the University of Salerno and the Ministero dell'Università e della Ricerca (FARB projects: FARB BUONERBA ORSA224812 and ORSA239513; 300393FRB20NADDE, 300393FRB21NADDE). The authors would like to extend their gratitude for the financial support provided by Ministry of Foreign Affairs and International Cooperation through the project coordinated by prof. V. Naddeo (grant number: KR23GR05). In addition part of this work was developed within the project "Innovative membrane technologies for advanced and sustainable wastewater treatment in view of boosting a cirvcular economy approach (CUP B53D23027250001) granded by the Italian Ministry of University and Research (MUR) through the research project of national interest PRIN 2022 PNRR (D.D. n. 1409 on 14/09/2022).

Peer reviewed