Compositional Profiles and Spatial Distribution of Microplastics Across Three Selected Rivers in Rivers State, Nigeria

Microplastics (MP) are ubiquitous and persistent contaminants in waterbodies and a pervasive and preventable threat to the health of marine ecosystems. These particles are defined as <5 mm in size and can be introduced into the environment via primary sources such as the use and disposal of microbeads in cosmetic, cleaning products and sandblasting, as well as secondary sources which include the fragmentation of litter by mechanical or UV light-induced degradation. This study was conducted to investigate the concentrations of microplastic contaminants in Imo River, Ntawogba River and New Calabar River as well as the physicochemical and microbial characteristics of the water bodies. Composite sampling technique was used in this study. The physicochemical analysis of the water samples was carried out both in-situ and ex-situ while the microbiological analysis was performed ex-situ to determine the load using APHA 9215B/9610B and ASTM D 5465-93 (Pour plate) test methods. The total heterotrophic bacteria were more predominant than the other microbial groups in samples obtained from the three rivers. The relatively higher hydrocarbon-utilizing bacteria counts obtained in samples collected from Ntwawogba River is suggestive of benign hydrocarbon contamination of the river. Two techniques (Fourier Transform Infrared Spectroscopy, FTIR and Gas Chromatography-Mass Spectrometry, GC-MS) were used to identify the microplastics and their concentrations. The reference spectra for the FTIR results were in the spectral range stretching from 4000–650cm-1 where the prominent absorption peak at 2920 cm-1 corresponded to aliphatic C-H (Hydrocarbon) stretching, indicating the presence of CH2 (Alkene) and CH3 (Alkane) groups in the microplastic samples, at 1725 cm-1 associated with carbonyl (C=O). The band at 1600 cm-1 is characteristic of aromatic C=C stretching, at 1200-1000 cm-1 range suggesting C-O stretching vibrations. The region between 800 and 700 cm-1 corresponded to out-of-plane bending vibrations commonly seen in aromatic groups while the GC-MS polymer identification library produced results of both qualitative and quantitative assessment of the microplastics. The eight (8) priority MP polymers found in the water samples were polyethylene (PE), polyethylene terephthalate, dioctyl terephthalate, polybrominated diphenyl ether, tetrabromobisphenol A, polypropylene, acrylic fibre, and polystyrene. SEM images of the MPs were acquired at various magnifications to capture their surface features, shapes, and sizes. PE was found to be the most ubiquitous MP (45.74%, 45.84% and 47.76%) across Imo, Ntawogba and New Calabar River, respectively.