A Comprehensive Analysis of Microplastics in the Tidal Forest of Coastal South India─Characteristics, Aging, and Heavy Metal Adsorption

Tidal forests are ecologically and economically important due to their tolerance of salty water and soil erosion, as well as their ability to sequester carbon. As ubiquitous environmental pollutants, microplastics (MPs) are invading several ecosystems, including tidal forests. The current study aims to identify and classify MPs and the pollutants they transport, collected from a tidal forest in Kota on the west coast of Karnataka, India. MPs in the size range of 0.1 mm to 5 mm were extracted and characterized under a custom-built laser-induced breakdown spectroscopy (LIBS)–Raman spectroscopic system to identify the polymer class as well as the surface-adsorbed heavy metals. The Raman analysis identified MPs of polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). The LIBS study confirmed the presence of Fe, Ni, Ti, Cr, and Zn. Furthermore, the contact angle study of the larger MPs confirmed that they originated from diverse sources and had varying residence times across different environments. The study also evaluates the relationships among surface morphology, aging, and the adsorption of heavy metals, providing further insights into other environmental pollutants. The MPs with a lower contact angle exhibited a highly degraded surface and the possible presence of hydrophilic carboxyl groups, whereas the MPs with heavy metal contaminants showed a higher contact angle, as cross-validated by the LIBS study. The partial least-squares discriminant analysis (PLS-DA) classifier, coupled with Raman spectral data, achieved the highest accuracy with 4-fold CV. In contrast, the model failed to classify LIBS data using PLS regression (R2 = 0.55) within the studied range due to random variation in spectral features caused by pollutants.