PhD Oral Exam - Xiaohan Yang, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The environmental fate and transport of microplastics (MPs) in aquatic ecosystems have become critical concerns due to their persistence and widespread distribution, particularly in coastal environments. In these dynamic regions, where land, rivers, and oceans interact, present complex hydrodynamic and chemical conditions that significantly influence the environmental fate and transport of MPs. Knowledge about the interactions between MPs and salinity gradients, suspended particulate matter (SPM), organic content, and external hydrodynamic forces remains limited. This study investigates how these environmental conditions influence MP aggregation and vertical transport in coastal systems.

Firstly, this study reviews our current understanding of the environmental behavior of MPs in coastal aquatic environments. A comprehensive overview of the main features of coastal zones is provided. The recently reported MP fluxes from different watersheds to the coastal ocean are summarized, and the controversy in estimating riverine MP fluxes is discussed. It then analyzes the dynamics, eventual fate, interaction with other particles and organisms, and natural weathering of MPs in coastal aquatic environments, along with their driving mechanisms. These findings are discussed by combining insights from the major coastal features related to hydrology, meteorology, topography, and geochemistry.

To begin addressing the complexities of MP environmental behavior, the study investigates the aggregation behavior of MPs with SPM under varying coastal environmental conditions, including salinity gradients, humic acid concentrations, and wave energy. The aggregation was influenced by the repulsive energy barrier between particles and the external energy transferred to the system. Low energy associated with mild wave conditions favored aggregation, while high energy under intense wave action hindered aggregation and caused the fragmentation of formed aggregates. The analysis for the environmental fate of MPs suggests that areas close to the shoreline are likely to serve as a sink for most MPs.

Next, the study examines the vertical transport of MPs with diverse aging pathways in coastal waters. It highlights how aging affects the physicochemical properties of MPs and influences their transport patterns. Opposite effects of aging on the vertical transport of hydrophobic and hydrophilic MPs were observed, with aging appearing to promote the dispersion of hydrophobic MPs but enhance the vertical transport of hydrophilic ones. Salinity played a positive role in vertical transport, while humic acid concentrations inhibited it. Intense turbulence promoted the floating of positively buoyant MPs and could even reverse the transport of negatively buoyant MPs from downward to upward. Additionally, Spearman correlation analysis confirmed that MPs' inherent characteristics and environmental features were the primary determinants of their vertical transport behavior.

Extending this investigation into more complex estuarine environments, the study explores MP settling in estuarine environments characterized by varying SPM types and fluid dynamics. It was found that the settling of positively buoyant MPs was more dependent on cohesive SPM compared to that of negatively buoyant MPs. A significant positive correlation was observed between salinity and MP settling percentage, following a logarithmic growth pattern with cohesive SPM and a linear growth pattern with non-cohesive SPM. Meanwhile, MP settling percentage was negatively correlated with fluid shear stress for both types of MPs, with negatively buoyant MPs exhibiting greater resistance to hydraulic disturbances.

Building on the understanding of MP settling, the study further explores the resuspension dynamics of micro- and nano-plastics (MNPs) from different types of sediment substrates under hydrodynamic disturbances. The role of cohesion and erosion resistance of sediments in governing MNP resuspension was highlighted. The grain size class and the mineralogical structure of the sediment particles played a dominant role in governing the resuspension of MNPs. The distinction between the resuspension behavior of micron-sized plastic particles and nano-sized plastic particles was identified. The resuspension dynamics of MNPs demonstrated a nonlinear response to changes in salinity. An increase in salinity level can largely constrain the resuspension of MNPs, particularly low-salinity range, where MNP resuspension is highly sensitive to changes in salinity. The U-shaped pattern was observed in PSNP resuspension behavior, where the resuspension rate initially decreased with water content loss, followed by an increase as sediments became drier. Higher fluid shear rates promoted MNP resuspension, releasing them from the sediment layer into water bodies. These findings offer valuable insights into the environmental fate and transport of MPs in coastal environments, aid in pollution hotspot identification, and support targeted mitigation strategies.