Global sources, abundance, size, and distribution of microplastics in marine sediments - A critical review

Microplastics (<5 mm) are a pollutant of growing ecotoxicological concern for marine and human health. This critical review aims (i) to systematically evaluate the findings on microplastics sources, occurrence, and spatial distribution in marine sediments, (ii) to examine the characteristics of keywords, authors, co-authors, country, bibliographical analysis, and publication results using VOSviewer. We identified around 350 papers from the Web of Science regarding microplastic pollution published between 2010 and 2020. Original research articles (95.62%) in high quality journals such as Marine Pollution Bulletin (54.79%), Environmental Pollution (15.07%), Science of the Total Environment (7.40%), Environmental Science and Technology (4.93%), Marine Environmental Research (4.66%), Environmental Science Pollution Research (2.7%), Estuarine, Coastal and Shelf Science (2.19%), Archives of Environmental Contamination and Toxicology (1.65%), Environmental Monitoring and Assessment (1.37%), and Environment International (0.82%) were chosen. Different types of polymers are found in the environment, the most common being polyethylene (PE), polypropylene (PP), and polystyrene (PS). Possible sources, abundances, composition, and size of microplastics in marine sediments, as well as keywords (1000), authors, and co-authors (1445), are discussed. For the identification of microplastics in marine sediments, most researchers used Fourier Transform Infrared Spectrometry (FTIR) (74%), Raman Spectroscopy (14%), and Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS) (12%). The current review offers an extensive range of detailed information to help the scientific community understand the problems and devise control measures to minimize both plastic consumption and the resulting risks to the marine ecosystem.