Assessing Microplastic-Pollutant Interactions in Coastal Sediments and Biofilms

Thesis Abstract

Coastal ecosystems increasingly contend with microplastics and associated pollutants, yet the interactive dynamics of microplastic particles with hydrophobic contaminants in sediments and resident biofilms remain poorly understood, limiting accurate assessment of ecosystem risk and design of mitigation strategies. This study aims to quantify how microplastics of varying polymer types and sizes influence the sorption, desorption, and bioavailability of persistent organic pollutants (POPs) and heavy metals within coastal sediments and their attached biofilms, thereby elucidating combined exposure pathways for benthic organisms. Specific objectives include (i) characterizing microplastic abundance, polymer composition, particle size distribution, and associated pollutant load in sediment cores from three representative coastal zones; (ii) determining sorption/desorption kinetics of representative POPs (PCB-153, PAH-ochratene, and PBDE-47) and metals (Cu, Pb, and Zn) on collected microplastics under simulated tidal and sediment–water conditions; (iii) assessing pollutant transfer between microplastics and biofilms through controlled microcosm experiments using native coastal biofilms; (iv) evaluating how sediment characteristics (grain size, organic matter content) modulate pollutant partitioning and biofilm uptake; and (v) developing a predictive model integrating microplastic concentration, pollutant type, and sediment parameters to estimate relative bioavailability to benthic organisms. The study adopts a mixed-methods approach. In the field component, sediment cores (n=60) and surface biofilm samples (n=30) are collected from three zones with distinct anthropogenic pressures, ensuring replicate temporal coverage across wet and dry seasons. Laboratory analyses involve Fourier-transform infrared spectroscopy (FTIR) and micro-Raman spectroscopy for polymer typing; pyrolysis-GC/MS for detailed additive and pollutant profiling; gas chromatography–mass spectrometry (GC-MS) for selected POPs; inductively coupled plasma mass spectrometry (ICP-MS) for metal concentrations; and thermogravimetric analysis (TGA) for microplastic quantification. Sorption/desorption experiments employ batch kinetics with varying contact times (0.5–72 hours) and environmental conditions (pH 7.5–8.5, salinity 20–35 g/L) to derive isotherms and rate constants. Biofilm-pollutant interactions are investigated in microcosms (solid–water ratio 14) under light-dark cycles, with monitoring over 28 days to quantify pollutant uptake using LC-MS/MS and compare with free-dlood pollutant controls. Statistical analyses include multivariate redundancy analysis (RDA) to relate pollutant loads to sediment properties, nonlinear regression to fit sorption isotherms (Freundlich and Langmuir), and mixed-effects models to account for site and temporal variability. The theoretical framework integrates the concepts of particle-mediated transport and the bioavailability paradigm, drawing on the NER (net exchangeable release) concept and the partitioning theory for hydrophobic organic compounds, with explicit reference to the Michaelis–Menten–type uptake kinetics in biofilms where appropriate, and incorporating pristine and weathered microplastic characteristics. Anticipated findings indicate that microplastic presence enhances sorption capacity for hydrophobic organic contaminants, with sorption affinity increasing with smaller particle size, higher surface area, and weathered polymer surfaces; biofilms act as secondary sorption sites and may modify desorption dynamics, leading to altered bioavailability profiles compared with particle-free sediments. It is expected that sediment organic matter content and grain size will significantly modulate pollutant partitioning, reducing desorption in finer, organic-rich sediments, while co-occurring metals may co-sorbed onto microplastics via complexation, enhancing co-transport potential. The study contributes to knowledge by providing mechanistic insight into dual habitats (sediments and biofilms) as integrated reservoirs and vectors for microplastic-associated pollutants, offering a data-driven framework for risk assessment in coastal management. Policy-relevant recommendations include prioritizing microplastic reduction strategies in high-sediment-organic-matter zones, refining sediment quality guidelines to incorporate microplastic-mediated pollutant mobility, and informing monitoring programs with standardized metrics for microplastic–pollutant interactions. The main conclusion anticipates that microplastics substantially influence pollutant fate in coastal sediments through coupled sorption-desorption-biofilm processes, necessitating integrated management approaches that address both plastic pollution and chemical contaminants. Recommendations emphasize targeted source reduction, site-specific sediment remediation considerations, and expanded temporal monitoring to capture seasonal variability in microplastic-pollutant interactions.