2D/3D-Functional Porous Materials for Efficient Water Purification

Abstract

As the primary source of life, water plays an important role in the sustainable and continuous development of society. The available water resources are currently under severe contamination due to various anthropogenic activities. The increased global demand for clean water and sanitation signifies the urgent requirement for efficient water purification techniques. As wastewater often contains harmful contaminants, special attention must be given to sustainable purification processes, which demand less energy and minimize the potential to generate secondary waste. Meanwhile, wastewater isn’t just a waste; it’s also a potential source of valuable resources including expensive and precious metals. A water purification strategy with precious element recovery can address the pressing demand for clean water, minimize energy consumption, environmental impact, and the overall idea of a circular economy. Porous materials with appreciable chemical and physical properties are better adsorptive platforms for the micropollutants present in contaminated waters. Covalent organic frameworks (COFs), a class of crystalline organic polymers, are found to be potential materials for water purification because of their inherent properties, including structural tunability, molecular precision, chemical stability, and formation of macroscopic architecture. Strategic and selective removal of molecules from wastewater resources using functionally superior COFs can also act as a promising new-generation material for recovering valuable resource materials. In our study, we approached water purification from a dual perspective so that it can address the challenge of water purification from complex wastewater sources and the simultaneous recovery of precious resource materials from a circular economic point of view.
In the first part of this thesis, we focused on the design and fabrication of COFs with different macroscopic forms, specifically, the membranes and the powders. The pristine three-dimensional (3D) COF membrane we developed exhibited 90 % uptake for environment-relevant perfluorooctanoic acid (PFOA) feed concentrations with a water permeance of ~40 L m-2 h -1 bar-1 . Moreover, the membrane showed effective separation of PFOA in water under varied pHs with good recyclability. In the second part of the thesis, we have focused on COF materials for resource recovery from wastewater. Firstly, we investigated the effect of hydrazone-linked 2D and 3D COFs for the efficient recovery of precious metal ions from complex wastewater with good selectivity. The 2D hydrazone-linked COF possessing an electron-rich backbone (Tfpa-Od) assisted in fabricating a heterogeneous catalyst by efficiently removing palladium (Pd) from wastewater. The Tfpa-Od COF exhibited high-affinity adsorption of Pd2+ ions from wastewater, achieving a distribution coefficient (Kd) of 3.62 × 106 mL g–1 . The electron-rich backbone of the COF contributes to its excellent selective removal efficiency (up to 100%) and adsorption capacity of 372.6 mg g–1 . The Pd-adsorbed COF was evaluated as a sustainable catalyst for the Suzuki–Miyaura coupling reaction, demonstrating good catalytic conversion and recyclability. Secondly, a mechanochemically synthesized crystalline 3D hydrazone-linked Tfm-Od COF exhibited a four-fold interpenetrated diamondoid topology and a high BET surface area of 1254.6 m2 g -1 . As TfmOd consists of excellent electron-rich moieties, we utilized them for the quantitative uptake of lithium with the highest adsorption capacity (1788.3 mg g-1 ) as per available literature. Moreover, the comparatively higher distribution coefficient of Li+ ions resulted in its high separation factor with respect to both monovalent and divalent competing ions. Finally, we did the efficient design of COF with functionally specific backbones for highly selective recovery of gold from complex e-waste with superior adsorption capacity. A quasi-planar benzoxazine core-based COF with synergic heteroatom was introduced to the aqueous e-waste solution for selective gold recovery (Uptake efficiencies >92 % and adsorption capacity = 3467 mg g–1 ). These results signify the importance of molecular-level engineering of COFs to enable the practical removal and recovery of desired molecules/ions from wastewater. Overall, this thesis outlines the current scenario of water purification techniques by introducing functionally diverse materials, with special attention to the COF research, from the very design to their fabrication as a new-generation material with enhanced performance in molecular separation and precious element recovery from wastewater.

Date of Award	2025
Original language	American English
Supervisor	Dinesh Shetty (Supervisor)