Selective Ion Separations with Polymeric Membranes for Water and Energy Applications

Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Elimelech, Menachem

Abstract

Synthetic membranes with an ability to separate ions with similar physicochemical characteristics would have broad implications in the water and energy sectors. Ion-selective membranes that allow passage of a specific ion type but reject other species could enable recovery and reuse of valuable elements as well as targeted removal of problematic pollutants from water. In energy conversion and storage technologies, membranes are needed between electrodes to transport desired charge carriers while isolating electrochemical reactions. While ion-selective membrane technologies are desirable for these applications, state-of-the-art membranes cannot ensure efficient passage of select ions while also retaining similar ions. This dissertation research aims to establish principles, materials, and methods for designing polymeric membranes with exceptional selectivity between nearly identical species. Despite decades of research on membrane technology, the properties and working mechanisms that could provide membranes with precise ion selectivity are undefined. This work discusses the molecular-level mechanisms that may be leveraged for improving membrane selectivity and gathers insight from biological ion channels to provide guidelines for designing highly selective membranes. Specifically, tailored pore size, pore length, binding site chemistry, binding site spacing are identified as important membrane features for precise separations. Both conventional and advanced membrane materials are assessed for their ability to provide these features. Three advanced material classes — porous crystalline materials, two-dimensional materials, and discrete biomimetic channels — are specifically highlighted as potential materials for ion-selective membranes because of their molecular-level control over physical and chemical properties. Advanced materials have the potential to offer high selectivity factors, but they are prone to formation of performance-limiting defects during membrane fabrication. If advancements were instead made to the ion-specific selectivity of polymeric materials, such as through tailoring pore size or ion–membrane interactions, their processability and durability make them easier to scale-up. In this dissertation, a method for tuning the pore structure of polymer films is demonstrated using layer-by-layer assembly. Systematic control over the concentrations of polyelectrolyte and background salt in fabrication solutions allows the thickness and pore size of the thin film to be tuned. This simple method could allow for tailored selectivity and enable fundamental studies on size-based exclusion in membrane separations. For ionic species of nearly identical size, polymeric membranes have limited selectivity because of their nonuniform pore architecture and chemistry at the sub-nanometer scale. To leverage polymeric materials for challenging ion separations, selective binding sites within a membrane could provide molecular recognition for a specific species, but the strong binding interactions could pose considerable resistance to ion transport. This dissertation investigates transport of similarly sized metals through multilayered polymer membranes that contain selective binding sites. Experiment and atomistic simulations demonstrate that a difference in binding strength to the membrane can produce selectivity between two similar species. Metals with strong binding affinity to the membrane selectively permeated through the membrane, whereas metals with weaker binding affinity were excluded in proportion with their likelihood to complex with the membrane. Accordingly, the membrane selectivity was largest for thinner films because they posed the least diffusive resistance toward stronger binding species. These results reinforce the critical influence of binding site chemistry and pore length on membrane selectivity. Ultrathin polymer layers with selective binding sites may offer high selectivity between similar species, but thin films are typically placed on a supportive, non-selective material to form a stable composite. The composite material may not uphold the selectivity of the free-standing selective layer, however, if the mass transfer resistance of the support layer is considerable. In this dissertation, the resistance of each component of a composite ion-exchange (IEX) membrane is investigated to determine whether the resistance of the IEX membrane limits the selectivity of the composite. In electrodialysis, composite membrane selectivity is relatively unaffected by IEX membrane resistance, but the properties of the IEX membrane become increasingly important for ultrathin or highly conductive selective layers. Development of low-resistance IEX membranes may be crucial as the design of polymer selective layers advance. Overall, this research shows that with appropriate membrane design, high-precision separations may be possible with polymeric membranes. The fundamental understanding of ion transport captured within this dissertation provides design guidelines that may expedite the development of polymeric membrane technologies with ion-specific selectivity. With further development, membranes with single-species selectivity may find use as a sustainable method for extracting valuable elements from highly concentrated wastewaters or brines.

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