Identification of Microplastics in Seafloor Sediment Samples – Practical Experience and Problem Issues of Research Methodology

Problem Statement and Purpose. In recent years, specialists of the State Scientific Institution “MorGeoEcoCenter of the NAS of Ukraine” have been actively developing a geoecological research direction that focuses on studying the distribution of microplastics (MP) in the seafloor sediments of the seas, oceans and river systems. The procedure for selecting natural material, as well as the laboratory sphere of processing and interpretation of the obtained samples, is quite complex in terms of the number of reagents used, the necessary stages of processing and identification of MP particles in bottom sediments, and is based both on various foreign methodological guides and relatively short practical experience of conducting such work by the institution’s specialists. Usually this concerns the study of small-volume bottom sediment samples, comparison of results for samples with different lithological composition, probable MP fragments too small for verification, etc. In recent years, due to the active accumulation of new factual material and research experience in world practice, international guidelines on the procedure for the selection and analysis of artificial polymers in surface seafloor sediments of water areas have been constantly improved. The purpose of this study was to clarify the problematic issues of the recommended methods for studying microplastics in seafloor sediments and to try to find ways to solve these issues. Data & Methods. Typically, marine sampling for microplastics (MP) is carried out within the framework of established methodological guidelines, using a variety of equipment, with subsequent sampling of the upper part (0–5 cm vertical interval) of the raised bottom sediment sample for analysis. Samples are transported chilled or frozen in aluminum or glass containers. Sample preparation includes such stages as drying at 60 °C; flotation in a saturated salt solution to separate the mineral component from microplastics (in our practice – ZnCl2, with a solution density of 1.8 g/cm3); treatment with 35% H2O2 to remove suspended organic matter, interstage washing with alcohol and distilled water. After filtering through a polyamide mesh with a cell size of 26–46 μm (used in various studies), potential MP fragments were identified by known characteristics, an available representative component was selected for verification on a Raman spectrometer and recalculation according to methodological guidelines. The study analyzed bottom sediment samples from the shelf and deep-water areas of the Black Sea (areas within the territorial waters of Ukraine, Romania and Georgia), the Danube Delta and a test area within the waters of the Southern Ocean adjacent to the Antarctic Peninsula. Results. This paper addresses critical issues in sampling techniques, data calculation, particle aggregation, and instrumental identification, all impacting reliable MP assessments. A significant problem is sampling. Our studies show sampler type (e. g., grab vs. multicorer) impacts sample quality. Grab samplers often lose the semi-liquid top layer, where low-density MP, like fibers, accumulate, distorting results. Correlation coefficients are needed to harmonize data. Calculating MP content is also complex due to varying lithology and wet/dry ratios. We propose basing calculations on non-carbonate substance weight or horizontal sampling area for accuracy, noting proportional extrapolation from small samples is debated. Moreover, aggregation and biofouling of MP particles during lab processing complicate visual identification and cause underestimations. High-density solutions can co-flotate mineral/organogenic matter. Mechanical de-fragmentation might damage fragile MP, while chemicals can dissolve polymers. Instrumental identification, notably with Raman spectroscopy, is hampered by physicochemical changes to MP surfaces, reducing signal intensity. Fibers, a widespread MP, are challenging due to their small cross-section (10–30 µm), often relying on less specific “hot needle” tests. Pyrolastics also present unique identification issues. In conclusion, robust and comparable MP research urgently demands methodological standardization. This includes unified sampling protocols, refined calculation methods, effective aggregation solutions, and overcoming instrumental identification limits. Continued experimental research and investment in advanced analytical equipment are essential for objective plastic pollution assessments.