Facile functionalization of nanomaterial for urea removal

• Main Author: Yen, Zhihao
• Other Authors: Lam Yeng Ming
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Chemistry; Engineering; Medicine, Health and Life Sciences; Urea adsorption; MXene; Nanomaterial; Cellulose nanocrystals
• Online Access: https://hdl.handle.net/10356/182618
• Institution: Nanyang Technological University

Urea poses a significant challenge to remove in most media due to its inert chemical nature and strong hydrophilicity, necessitating the design of a novel urea removal material. This thesis aims to make use of functionalized MXene nanosheets, which contain surface functional groups, with single Cu atoms to enhance urea adsorption capabilities. Previous work has demonstrated Cu affinity for interacting with urea molecules. The goal is to integrate this Cu-functionalized MXene into a hierarchical structure, forming a system that can be implemented at scale for real-world adsorption applications. By combining MXene nanosheet topology with isolated copper atoms, this research hypothesizes an improved material for overcoming urea's resistance to removal from aqueous solutions. Although there have been reports of using MXene for urea adsorption, the synthesis method employs highly hazardous HF to produce MXene with a urea adsorption capacity of 21.7 mg/g. Simulation studies have found that hydroxyl termination exhibits the best urea interaction, indicating that MXene could be an alternative sorbent material for urea adsorption. MXene can be synthesized using a HF-free method – Minimally Intensive Layer Delamination (MILD) but unfortunately MXene synthesized this way have functionalization that is less favorable for urea adsorption. The Cu functionalization onto these MXene results in an increase in its affinity for urea adsorption compared to pristine MXene due to the formation of Cu-urea complexes. The valence state of these Cu atoms in the MILD-synthesized MXene is between 0 and +1. Using XAS and XPS, it is shown that the Cu atoms are bound onto the MXene surface via Ti-O-Cu linkages. Optimal urea adsorption occurs on Cu when they exist as a single atomic site. Increasing the Cu content will result in Cu agglomeration, and this will not result in an increase in urea removal. Based on the adsorption behavior, it appears that Cu-doped MXene follows the monolayer adsorption on a homogeneous surface model. Since Cu-functionalized MXene has shown to improve urea adsorption compared to pristine MXene, a hierarchical structure with Cu-functionalized MXene as one of the components and bigger exposed surface area could be good for urea adsorption. CNCs could be used to induce spacing between MXene nanosheets and expose more sites for urea adsorption. In this work, an alginate hydrogel system was explored to create the macrostructure using ionic crosslinkers. Since it is a hydrogel system, the hydrophilicity of the hydrogel increases urea interaction and allows urea to diffuse through the hydrogel, maximizing the interaction of the material with urea, thereby promoting urea adsorption. After incorporating a mixture of MXene and cellulose into the formulation, the hydrogel was cast into beads. The urea adsorption capabilities of the hydrogel beads were found to be 170.5 mg/g in an aqueous solution and 67.9 mg/g in a simulated dialysate solution. The hydrogel hierarchical formulation has exhibited promising urea adsorption capabilities in both aqueous and complex environments. To further enhance the urea adsorption capacity of this system, the surface of CNC can be optimized through functionalization using compounds with a higher affinity for urea. Compounds with enhanced hydrogen bonding capabilities, such as dopamine, tannic acid, and melamine formaldehyde, were selected as potential candidates for the functionalization of the CNC surface. With the incorporation of functionalized CNC and Cu-functionalized MXene into the hydrogel formulation, the dopamine-functionalized CNC exhibited the best urea adsorption capacity. This optimized hydrogel system demonstrated remarkable urea adsorption capabilities, reaching up to 354.4 mg/g in aqueous solution and 115.1 mg/g in simulated dialysate.