Tiny plastic debris in the aqueous environment: mobility, toxicity, and remediation

Singh, Nisha (2021) Tiny plastic debris in the aqueous environment: mobility, toxicity, and remediation. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Nisha Singh (17RS048)) 17RS048_DES.pdf - Submitted Version Restricted to Repository staff only Download (18MB)

Abstract

In the last few decades, the vast application of plastics and their products with low recycling practices has resulted in a tremendous amount of waste generation. An enormous plastic load in the environment around the world has been reported in different sizes, viz. microplastics (MPs, <5 mm) or nanoplastics (NPs, <1μm). The fate, transport, bioavailability, toxicity, and impact of these plastic particles on the ecological process remain understudied. This study envisions an overall picture of MPs in the environment, their ubiquitous distribution, interaction with other co-existing contaminants, and disintegration into NPs. The thesis has also targeted NPs behavior in the aqueous environment focusing the aggregation studies on delineating their dispersion, distribution, and mobility with varying water chemistry as well as their eco-toxicity in the presence of other pollutants on the model organism (zebrafish). Finally, the development and potential practical applications of modified eco-friendly material for removing NPs in the environment based on the fundamental understanding developed from the previous sections are also highlighted. We have adopted an interdisciplinary approach to studying MPs/NPs to bring novel insight into their interaction with the aquatic environmental components. First of all, an understanding was developed on the distribution of MPs in the Indian freshwater system. MPs were estimated in the lower Ganga River, where a high dominance of white or transparent MPs was observed. Results show that polyethylene was the most prevalent polymer type at most of the sites. Statistical analysis showed a week negative correlation of MPs numerical value in water and sediments. Modeling of MPs flux in the Bay of Bengal displayed that they disperse horizontally to a vast distance. The likelihood of interactions of omnipresent MPs with co-present contaminants is inevitable in the environment. A part of the thesis has probed adsorption of emerging contaminant, CeO₂ nanoparticle (nCeO₂), with pristine and aged MPs. Weathered MPs showed higher sorption than pristine MPs. Environmental conditions such as salinity, pH, and organic matter content played a significant role in the sorption of nCeO₂, which moderated the mechanism of nCeO2 aggregation and surface charge. Ineluctable disintegration of plastic to nano-range is an environmental challenge. The aggregation profile, i.e., homoaggregation and heteroaggregation of NPs in the aquatic environment, depends upon their property and interaction due to water chemistry such as temperature, dissolved organic matter (DOM), electrolyte concentration, pH, and inorganic clay colloids. At higher salt concentrations, NPs aggregated rapidly. With an increase in kinetic energy at a higher temperature, NPs were unstable, whereas DOM showed a dramatic mechanism of stabilization and destabilization. It was delineated that inorganic colloid play a vital role in the heteroaggregation of NPs in the aqueous environment. Stability and bioavailability of NPs with co-existing contaminant leading to varying degrees of toxicity to zebrafish was evaluated. Metal oxide nanoparticles (nMOx) and polycyclic aromatic hydrocarbons (PAHs) were found to alter NPs stability such that critical charge reversal values lowered in the presence of nMOx while it remained stable with PAH. Combined exposure of NP and PAH altered the oxidative balance and caused DNA damage in zebrafish. NPs can have a severe and deleterious impact on ecology and human health as they can bypass the biological membranes. A much-needed solution to NPs removal from a water system was developed with an iron-biochar composite that attains a quick adsorption equilibrium after interaction (<10 min) and shows a high adsorption capacity. The size and functionality of NPs have been observed to govern their elimination by composite. A minimal impact of different environmental conditions on composite removal and its reusability endorses the robust technique applicability. The major phenomenon involved during the adsorption process was determined to be complexation and electrostatic attraction. Knowledge generated from the study will provide preliminary data for risk assessment and preventive measures to the potential hazards of MPs/NPs. The thesis highlighted the mechanism and phenomenon controlling fate and transport of MPs/NPs by unraveling their interaction in the aquatic environment.

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