Occurrence and distribution of micro(meso)plastic-sorbed heavy metals and metalloids in sediments, Gulf of Guinea coast (SE Atlantic)

Abstract

The pervasive existence of microplastics (MPs) and toxic metals is raising environmental and health concerns. Plastics are essentially a complex mixture of chemicals, but exposure to the aquatic environment increases their complexity through contaminant desorption/sorption. The aim of this study was to establish baseline data on the elemental occurrence and distribution of potentially toxic and geochemical metals/metalloids in microplastics 1-5 mm and mesoplastics (> 5 mm - 1 cm) along designated coastlines of the Gulf of Guinea (Nigeria) in addition to enabling more comprehensive ecotoxicological risk assessment. The concentrations of twenty-six metals: aluminium (Al), antimony (Sb), arsenic (As), barium (Ba), boron (B), cadmium (Cd), calcium (Ca), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), potassium (K), selenium (Se), sodium (Na), silicon (Si), silver (Ag), strontium (Sr), thallium (Tl), titanium (Ti), vanadium (V), and zinc (Zn), associated with beach MPs, pristine, and lagoon plastics were determined after extraction in 10% nitric acid and analysis using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The distribution of MPs was variable, with 3680 particles identified along the drift and high waterlines across designated shoreline locations. The beach MPs were dominated by polyethylene (PE), polypropylene (PP), and polystyrene (PS), whereas lagoon-sourced plastics were characterized by polyethylene terephthalate (PET), polystyrene (PS), and polyurethane (PUR). Metal concentration was higher when associated with foam plastic (PS, PUR, PEVA) compared to hard plastic (PE, PP, PET) samples. The results showed that all samples had slightly elevated Al, Fe, Mn, and Zn concentrations, suggesting potential sorption interactions and plastic additive influences. Notably, foam MPs had a stronger affinity for metals. This study emphasizes the critical role of microplastics in serving as vectors for toxic metals. Except for Cd, pollution indices such as the potential contamination index (PCI), hazard quotient (HQ), and modified hazard quotient (mHQ) indicated low severity contamination of beach and lagoon MPs by heavy metals. However, considering long-term accumulation of sorbed metals, their potential toxicity to marine biota may be considerable.

Keywords: Beach sediments; Gulf of Guinea; Mesoplastics; Microplastics; Toxic metals.