Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Elimelech, Menachem

Abstract

Water scarcity is among the most prominent humanitarian crises of our time. Achieving equitable access to safe drinking water for all humankind necessitates the treatment of unconventional and underutilized sources. Pressure-driven desalination technologies are particularly well-suited for augmenting global water supplies due to their energy efficiency. In these technologies, the membrane is the critical component driving process efficacy and requires material breakthroughs to achieve paradigm-shifting performance. Attempts to improve conventional membrane materials and investigations into numerous novel materials have failed to produce the next generation of desalination membranes due to our limited understanding of the mechanisms that govern water and ion transport under nanoconfinement. Hence, the objective of this dissertation research is to elucidate fundamental synthesis–structure–performance relationships in conventional- and novel-material based membranes, in the context of how these relationships can be exploited to improve practical water treatment processes. Electrostatic interactions between the membrane and charged constituents in the feed water are central to membrane performance. These interactions depend on the extent to which the membrane is charged, yet the reasoning behind the ionization behavior of state-of-the-art polymers used for desalination has been unknown for years. This dissertation unravels the anomalous ionization behavior of nanoporous polyamide, revealing that a large portion of the ionizable moieties in polyamide films remain uncharged, and thus, unutilized, during typical operation due to extreme confinement effects. Several approaches to exploit these ionizable groups for improved performance are highlighted. Electrostatic interactions may also depend on the physicochemical characteristics of the charged constituents in solution, such as the molecular shape of ions. To help guide the development of ion-ion selective membranes, this work utilizes machine learning to assess molecular-level features that influence ion transport in nanoporous cellulose acetate membranes. The findings suggest that attention should be redirected from the ion’s bulk solvation properties to their intrinsic electrical properties for selective separations. Ion-specific adsorption reactions can also dictate the effective pore charge through charge regulation. Cation-specific transport in charged nanochannels is thus rationalized from a charge regulation perspective, representing the phenomenon as an adsorption equilibrium process. This approach enables the use of conductance measurements to indirectly probe ion-surface reactions within the nanochannels—previously inaccessible experimentally. The role of defects in novel material-based membranes, which is frequently overlooked, are identified in this dissertation. This work focuses primarily on framework defects found in two-dimensional (2D) materials. The overlapping of framework defects are found to create percolation networks that greatly hinder the separation performance of 2D lamellar membranes. It is therefore emphasized that the mitigation of defects in novel material-based membranes is imperative to achieve practically viable desalination membranes that can compete with materials already commercially available. If unavoidable, the impact of defects in 2D material-based membranes on membrane performance can be masked if the water permeability remains high for thick membranes. This would require significant slip flow through the nanochannels, a phenomenon that can be experimentally resolved by the nanofluidic devices developed in this work. An interferometry-based apparatus is also designed to enable measurement of the ultralow flow rates (~few nL h-1) that are expected from these nanochannels. Overall, this dissertation investigates transport phenomena unique to extremely confined environments that are relevant to real-world water purification technologies. Electrostatic interactions between ions and the membrane are shown to be highly important for selective separations in conventional materials, and can even go as far as to influence the intrinsic charge of the membrane. For novel materials, fabrication-induced defects critically limit their future implementation in desalination technologies, making precisely controlled nanofluidic platforms essential for understanding defect-free transport within these frameworks. This dissertation demonstrates new approaches to studying nanoconfined transport and provides insight that may lead to next-generation membranes for desalination and water purification.

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