Selective ion separations are gaining increasing importance across water, energy, and environmental sectors. Ion-exchange membranes (IEMs), which are charged polymeric films, have been playing a crucial role in diverse applications, such as electrodialysis (ED) desalination, redox flow batteries, and chloralkaline processes. However, the growing demand for enhanced selectivity poses challenges to current IEMs, necessitating improved membrane separation capabilities beyond simple charge selection between cations and anions. The objective of this thesis is to advance the selectivity of IEMs, moving them closer to becoming ion-selective membranes. Specifically, the research deepens the fundamental understanding of transport phenomena in IEMs and develops novel membranes with improved specific ion selectivity, permselectivity, and ion/water selectivity.
In IEM processes, the correlation between conductivity and permselectivity, representing the selectivity of counterion oppositely charged to the membrane against like-charged co-ion, significantly impacts process performance. The dissertation work first investigates the tradeoff between conductivity and permselectivity of IEMs, arising from variations in solution concentrations (Chapter 2). These tradeoff patterns are broadly observed across diverse electrolytes, which are primarily influenced by factors including valencies of counter- and co-ions, as well as counterion diffusion coefficients. The research next delves deeper into the mobility of condensed counterions in IEMs (Chapter 3). An analytical model is introduced to depict the mobility of condensed counterions, facilitated by the novel utilization of a scaling relationship to accommodate the screening length in highly charged IEM matrices.
Upon integrating the contributions of condensed counterions, the Donnan-Manning transport framework accurately predicts IEM conductivities in monovalent counterions, aligning closely with experimental values (as small as within 7%) and devoid of adjustable parameters. The analysis underscores the greater mobility of condensed counterions compared to their uncondensed counterparts, as electrostatic interactions accelerate condensed counterions while impeding uncondensed counterions. The advancements in transport theories concerning conventional IEMs, as presented in Chapters 2 and 3, provide vital insights into the selectivity limitations of existing membranes, emphasizing the necessity for IEMs with improved selectivity.
The thesis then transitions to the development of IEMs tailored for specific ion selectivity. This endeavor involves engineering water-deficient sulfonated polystyrene membranes to leverage discrepancies in ion hydration free energy, thereby refining selectivity between counterions with identical valence (Chapter 4). The fabricated membranes prefer the transport of K+ over Li+ in ED, with the K/Li transport selectivity increasing from 2.5 to 3.1 as the membrane's water deficiency, represented by the number of water molecules per fixed charge site (λ), declined from 12 to 6.3. Further analysis of ion sorption behaviors highlights selective partitioning as the primary driving factor, with K+ showing a greater affinity into the membrane at lower λ levels as a result of its lower hydration free energy. In addition to polymeric membranes, the work also explores composite ceramic IEMs employing sol-gel chemistry to achieve tunable selectivity among different counterions (Chapter 5). Significantly, the resulting membranes display an impressive K/Li transport selectivity of up to 6.3 in the ED measurements, surpassing many research efforts focused on polymeric materials. This remarkable selectivity primarily stems from the preferential sorption of K+ into the ceramic matrix. Moreover, these membranes demonstrate excellent differentiation between monovalent and divalent counterions, with Li/Mg and Na/Ca transport selectivity values of 17 and 29, respectively, obtained in the ED process, rivaling or even leading commercial products.
ED shows great potential for cost-effective hypersaline desalination; however, its effectiveness has been limited by the absence of suitable IEMs with high permselectivity and ion/water selectivity in high-salinity conditions. This work presents the development of highly charged and low-swelling IEMs customized for high-salinity ED desalination, achieved through a facile sulfonation strategy of polystyrene (Chapter 6). The heightened fixed charge density enables fabricated membranes to sustain remarkable permselectivity, exceeding 0.96 in ED characterization with 4 M NaCl solution, which far transcends that of commercial IEMs. Furthermore, these membranes effectively suppress osmotic water permeability and electro-osmosis at lower water content. The performance of hypersaline ED desalination is improved by employing the developed membrane to treat synthetic brine, together with the successful desalination of practical feed.
This dissertation significantly enhances our comprehension of membrane transport phenomena and showcases the strategic development of innovative IEMs with advanced selectivity. The progression in transport theories offers crucial insights into the structure-property-performance relationships of IEMs, while highlighting the selectivity constraints of current membranes. The work demonstrates straightforward strategies for producing membranes capable of distinguishing between different counterions, including water-deficient polymeric membranes and composite ceramic membranes. The highly charged and low water content membranes customized to achieve improved permselectivity and ion/water selectivity enable efficient electromembrane processes under high-salinity conditions. The findings of the thesis contribute to the eventual realization and deployment of ion-selective membranes to address water, energy, and environmental concerns.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/vyy0-9476 |
| Date | January 2024 |
| Creators | Huang, Yuxuan |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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