Mechanistic Investigation of Microplastic-Facilitated Transport of Polycyclic Aromatic Hydrocarbons in Niger Delta Estuarine Systems

The co-occurrence of microplastics and polycyclic aromatic hydrocarbons in Niger Delta estuarine sediments creates the potential for microplastic-facilitated contaminant transport and enhanced bioavailability to estuarine organisms through a Trojan horse mechanism, yet the thermodynamic, kinetic, and ecological dimensions of this interaction have not been investigated for the specific microplastic polymer types, polycyclic aromatic hydrocarbon congener profiles, and salinity-temperature regimes characteristic of Niger Delta estuaries. This study conducts a mechanistic investigation of the sorption, desorption, and bioaccessibility of priority polycyclic aromatic hydrocarbons onto and from five microplastic polymer types (polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and polyvinyl chloride) representative of those found in Niger Delta estuarine sediments. Field-collected microplastics from five Niger Delta estuaries (Bonny, Benin, Escravos, Imo River, and Calabar) are characterised by micro-Fourier transform infrared spectroscopy and Raman spectroscopy. Laboratory sorption kinetics and equilibrium isotherm experiments are conducted under simulated estuarine salinity (15 and 30 parts per thousand) and temperature (25 and 35 degrees Celsius) conditions for fifteen polycyclic aromatic hydrocarbon analytes measured by gas chromatography-mass spectrometry. Desorption hysteresis coefficients are calculated to assess contaminant sequestration. Bioaccessibility of sorbed polycyclic aromatic hydrocarbons to the estuarine bivalve Crassostrea gasar is quantified using Tenax extraction as a biomimetic method. Molecular dynamics simulations of polycyclic aromatic hydrocarbon sorption onto polymer surfaces provide mechanistic interpretation. An original risk model is derived to predict polycyclic aromatic hydrocarbon bioaccessibility enhancement by microplastics under Niger Delta estuarine conditions. Keywords: microplastics, polycyclic aromatic hydrocarbons, sorption mechanisms, Niger Delta, estuarine contamination