PhD Oral Exam - Zhikun Chen, 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

Sea ice can act as a temporary sink for microplastics (MPs), which also function as secondary sources and transport mediums for MPs. While field investigations have confirmed the widespread presence of MPs in ice, knowledge about their environmental fate in cold regions remains limited. This study investigates the entrainment of MPs during ice formation processes and how freezing alters their behavior in water. Additionally, an electrochemical removal technology was developed to separate MPs from water, addressing the issue at its source.

The study first explored the effects of various MP properties and environmental conditions on the entrainment and enrichment of MPs in ice under different turbulence scenarios. Results revealed that high turbulence in freshwater significantly increased the entrainment of hydrophobic MPs in ice, due to the combined influence of frazil ice and air bubbles. The hydrophobic nature of MPs led them to concentrate at the water/air or water/ice interface. However, in saline water, high turbulence inhibited the entrainment of all MP types, as loose ice structures allowed for the exchange of MPs between ice and water, resulting in rapid expulsion. In calm saline conditions, MP enrichment factors were higher compared to calm freshwater.

Next, the study examined the interactions between aquatic organic matter and MPs, focusing on how these interactions influence MP behavior during ice formation. A 28-day experiment evaluated the effects of dissolved organic matter on MP properties under UV and dark conditions. Findings revealed that aquatic organic matter adhered to MPs, with varying impacts on different MP types. Surface damage caused by physical abrasion and UV radiation improved the entrapment of MPs in ice, highlighting the critical role of MP properties in their environmental behavior.

As temperatures fluctuate, MPs stored in ice can be released, and freezing conditions can alter their properties, affecting their fate in the environment. Freezing in freshwater led to MP aggregation through physical compression, increasing particle size upon release. This aggregation enhanced buoyancy, accelerating the settling or rising velocity of MPs in water. In contrast, the presence of salt mitigated freezing effects due to the formation of a brine network within the ice structure, reducing pressure on entrapped MPs.

To address the issue of MP release into the environment via wastewater, this study developed an electrosorption (ES) technology using graphite felt electrodes to remove MPs from water. Preliminary experiments investigated the effects of flow rate, applied voltage, and electrolyte concentration on MP removal efficiency. Under optimal conditions, 96.9% of MPs were removed after 150 minutes. By analyzing the factors influencing MP removal efficiency and the underlying mechanisms using DLVO theory, this study provides a foundation for future advancements in ES technology for MP removal, with potential for real-world applications in complex environments.