Surface weathering and changes in components of microplastics from estuarine beaches

Microplastics are a type of emerging environmental pollutant that has been the subject of increasing concern worldwide. The surface morphology, composition and changes in the distribution of microplastics in the environment are poorly understood. The corresponding research methodology is also at the exploratory stage. Here, we examine typical estuarine sediments from Shandong Province, east China, that are influenced by intensive human activity. The microplastics are separated from the sediments using an apparatus of continuous flow and floating separation. The microplastics samples are processed to determine the types, morphology and changing composition of microplastics present using a range of advanced microscopic and micro- analytical methods. The aim is to understand the weathering and subsequent surface changes in the microplastics under the environmental conditions of estuarine sediments. Optical microscope and scanning electron microscope-energy spectrum (SEM-EDS) analysis shows that foams and pellets, together with fragments, fibers and films, are present in the estuarine sediments. The five shape types of microplastics had different weathering surface morphologies compared to the corresponding large plastic debris from the same sampling sites and to the corresponding commercial plastic products. The surfaces of the commercial products are smooth. The surfaces of plastic debris appear to be slightly broken and aged while the surface of microplastics from the same sites show many more microholes, cracks or protuberances. This indicates that the surfaces of microplastics on the estuarine beaches have been strongly weathered. Scratches, creases, microholes, cracks, either concave or convex and of various shapes and sizes were found on the surfaces of microplastics from the coastal environment, possibly due to mechanical friction, chemical oxidation and/or biological attack. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR) was used to find oxygen-containing functional groups such as carboxylic acids, aldehydes and esters or ketones on the microplastic surfaces from the tidal flats on the basis of polymer component analysis. The two selected shape types, namely foams and fragments from soft plastic woven bags, had different infrared spectra than their corresponding large plastic debris from the same sampling sites and to the original commercial plastic products. The surfaces of these two microplastics had more complicated infrared spectra near the fingerprint area. This implies that the aging process of large plastic debris may be an important source of microplastics in the environment. A polymer blend of both polyethylene and polypropylene was identified in the fibers using pyrolysis gas chromatography-mass spectrometry (pyr-GC-MS). The pyr-GC-MS analysis also indicates that the pyrolysis products were much common on the aged surfaces of foams from the beach than on the inner part of foams after removal of the aged surfaces, including mainly compounds containing oxygen or nitrogen such as oleanitrile, trans-13-docosenamide, α-n-normethadol; 1,1-diphenyl-spiro[2,3] hexane-5-carboxylic acid, methyl ester, hexadecanoic acid, octadecyl ester and hexadecanoic acid, hexadecyl ester. The surficial morphology, composition and possible properties of microplastics from the estuarine beaches were clearly different from those of the original commercial plastic products. We suggest that surface weathering can cause changes in the surface components of microplastics under the actual conditions prevailing on coastal beaches. By observation of the weathering of microplastic surfaces it is difficult, using a single identification method, to distinguish between finer polymer components, particularly in samples of blends or copolymers. This study shows that pyr-GC-MS can be used directly to obtain and verify the specific weathering products of microplastics and the ATR-FT-IR may be used as an ancillary tool for recognition of microplastics in the environment. Much effort needs to be devoted in the future to understanding the changes in surface processes, ecological and environmental effects, and methods of identification of microplastic particles in coastal and oceanic environments.