Microplastics in the Indian Riverine Systems: their Assessment, Toxicity and Remediation

Ganie, Zahid Ahmad (2024) Microplastics in the Indian Riverine Systems: their Assessment, Toxicity and Remediation. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Zahid Ahmad Ganie (19RS105)) 19RS105.pdf - Submitted Version Restricted to Repository staff only Download (15MB)

Abstract

Microplastics and nanoplastics are pervasive in all environmental compartments, and their distribution, toxicity, fate and potential remediation strategies remain understudied. The current thesis offers a comprehensive overview of microplastics in the aquatic environment, beginning with their assessment and characterisation in freshwater systems, following their toxicity studies in the presence of co-existing contaminants, and concludes with their removal alongside coexisting contaminants like toxic heavy metal ions and organic dye pigments, utilising eco-friendly composite materials. We have adopted an interdisciplinary approach to studying microplastics in aqueous environment. We aim to provide new insights into their distribution, interaction with co-existing contaminants, toxicity in the presence of these contaminants, and their remediation. The work done in this thesis began with assessment and characterisation studies of MPs, which provided a basic understanding of the distribution of microplastics in Indian freshwater systems. Microplastic assessment was done in the Mahanadi River, the third-largest river in peninsular India. The studies revealed a high concentration of microplastics in sediment, water, and fish samples, with abundant filaments and fragments of black colour potentially arising from vehicle tire abrasion, agricultural mulching film degradation and laundry effluents. Polystyrene, Polypropylene and Polyethylene polymers were the dominant with sizes < 1 and 0.5 mm. The risk assessment studies highlighted a potential high risk to freshwater ecosystems due to the high prevalence of microplastics in all three studied river compartments. Correlation studies marked similarities in traits between the microplastics found in sediment and water samples. The high prevalence of microplastics in all the river components creates an inevitable situation where microplastics can interact with co-existing contaminants like metal nanoparticles, etc. Furthermore, we conducted experiments to explore the interaction of microplastics (tire-ware microplastics due to their high persistence as revealed during the assessment studies) with Zinc -Oxide nanoparticles in the aqueous environment and later on, their combined toxic effects were investigated using a model organism Chlorella vulgaris. It was found that tire-ware microplastics have a high potential to adsorb zinc-oxide nanoparticles (312.49 mg/g) in the aqueous environment. This led to our curiosity to explore their combined toxic effects on Chlorella vulgaris. The toxicity studies revealed that the combined exposure to tire-ware microplastics and zinc-oxide nanoparticles have synergistic toxic effects on the model organisms compared to their separate exposures. Adverse physiological and biochemical effects were observed due to toxicity, including membrane damage, less production of photosynthetic pigments, and less antioxidant enzyme activity. The hyperproduction of ROS and lipid peroxidation enzyme MDA highlighted increased cellular oxidative stress due to toxicity. The findings revealed significant toxicological effects of microplastics and co-contaminants, with the associated risk potentially impacting overall aquatic ecosystems and human health, reflecting a much-needed and immediate solution to this problem. An eco-friendly material that can be a potential solution to microplastics in aqueous systems was developed. We synthesised biochar from agricultural waste sugarcane bagasse and explored it to remove microplastics from water. The synthesised biochar could remove more than 95 % of microplastics from deionised water within <5 minutes of interaction time. However, its removal capability was drastically reduced in bicarbonates and higher humic concentrations and when applied for microplastic removal in complex aqueous matrices like river water and wastewater. We further modified biochar by loading its surface with nano-Zerovalent iron particles to target the removal of toxic heavy metal cations, anions, and microplastics of different sizes and functionality. Our generated material did wonders by removing the target contaminants in all complex aqueous matrices, including river water, wastewater, etc. It also performed efficiently in various environmental parameters like pH, dissolved organic matter and co-existing ions. In addition, it could remove the mentioned contaminants in continuous column filtration mode. Finally, we aimed to simultaneously remove inorganic and organic contaminants (heavy metal ions and dyes) alongside microplastics from water using a novel Sulfadised-nZVI-Biochar. Sulfidised-NZVI-biochar is an excellent choice for eliminating dyes, metal ions and microplastics. Sulfate has an excellent reducing capability for organic contaminants, and nZVI can also contribute to degrading organic pollutants due to Fenton degradation. The designed material could simultaneously remove all the target contaminants in various environmental parameters (pH, dissolved organic matter and competing ions) and complex aqueous matrices (river water, tap water and wastewater). Furthermore, it could remove all target contaminants in continuous filtration mode more efficiently than nZVI-Biochar. The knowledge produced from this work will offer invaluable insights and preliminary data for assessing the risk posed by microplastics and potential preventive strategies to mitigate these risks. The work in this thesis emphasises the mechanisms and processes that govern the distribution of microplastics in freshwater systems, their interaction and toxicity in the presence of co-contaminants, and potential solutions for their removal from aquatic environments.

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