Increasing global needs for clean water and renewable energy are key challenges in the 21st century. Membrane-based technologies offer cost-effective separations and show great promise of providing sustainable solutions to challenges in the water-energy nexus. This dissertation aims to advance membrane technologies for sustainable production of energy and water. The work advances the novel vapor pressure-driven osmosis (VPDO) technology for efficient energy conversion from low-temperature heat resources, proposes and develops an innovative cascading osmotically mediated reverse osmosis (COMRO) for energy-efficient desalination of high-salinity brines, and provides a framework for better understanding of the permeability-selectivity tradeoff relationship in thin-film composite polyamide (TFC-PA) membranes widely used in aqueous separations. Abundant low-temperature (< ≈100 ℃) heat resources remain untapped in industries and the natural environment. The emerging vapor pressure-driven osmosis (VPDO) membrane technology enables direct conversion of the low-temperature heat to energy. However, enhanced fundamental understanding of the process is still needed to bring the technology to practical applications. Work in this dissertation presents a theoretical model to understand mass and heat transfer in VPDO. The performance of two hydrophobic nanoporous membranes, polypropylene (PP) and polytetrafluoroethylene (PTFE), of different chemical and structural properties were evaluated. Knudsen diffusion was demonstrated to dominate the mass transfer in VPDO, and the work showed that evaporative heat transfer is significantly greater than conductive heat losses. A large amount of high-salinity brines (>≈70,000 ppm total dissolved solids, TDS, approximately 2× seawater salt concentration) are generated from various industrial processes and operations. Prevailing evaporation-based methods used to desalinate the hypersaline brines are highly energy intensive. Reverse osmosis (RO) is the most efficient method for seawater desalination, but is unsuitable for treating high salinities. This dissertation proposes and develops an innovative cascading osmotically mediated reverse osmosis (COMRO) technology to achieve energy-efficient desalination of high-salinity brines. Theoretical analysis in this work showed that COMRO requires only 68.3 bar to desalinate hypersaline feed of 70,000 ppm TDS, whereas conventional RO needs the exceedingly high pressure of 137 bar. Furthermore, up to −17% energy saving can be attained by COMRO. To develop the COMRO technology, this dissertation presents systematic investigations of transport and structural properties of osmotic membranes in COMRO during high-salinity desalination. The impacts of hydraulic pressures and high salinities on water and solute permeabilities of membranes were studied, and the membrane structural parameter was demonstrated to be consistent at different salinities. Governing equations for water and salt fluxes in COMRO were established and the transport model was validated with experimental results. The solution-diffusion (S-D) theory is currently the most widely accepted transport framework for salt-rejecting membranes. Importantly, it predicts a tradeoff relationship between permeability and selectivity in membrane transport. This study presents a framework to better understand the first principles governing the permeability-selectivity tradeoff in thin-film composite polyamide (TFC-PA) membranes used in aqueous separations. Tradeoff trends predicted by the conventional S-D was observed for nonelectrolyte solutes of different molecular sizes, and the thesis identified a second transport regime which deviates from the current understanding of S-D. The work showed that general principles of the S-D framework are applicable to TFC-PA membranes, and solute size is a principal factor governing the conventional S-D transport. Transition between the “deviation” regime and the conventional S-D transport was demonstrated to be governed by the solute size, and transport features of the “deviation” regime were also analyzed. Further, the dissertation elucidates the roles of structural properties of the polyamide layer in the tradeoff. The work showed that the tradeoff cannot be completely explained by the changes in free volume sizes of the PA layer, and the shortening of the effective transport pathway also accounts for the permeability-selectivity behavior of permeants. The dissertation proposes mechanisms to elaborate how the membrane structural properties affects the permeability-selectivity behavior of permeant with a certain size. Overall, this dissertation presents pioneering advancements of membrane technologies for sustainable production of energy and water. Fundamental understanding of the heat and mass transport in VPDO was advanced, and the study highlights future directions of developing the technology for power generation. An innovative COMRO technology is proposed for hypersaline desalination, and the work established foundations for understanding the transport phenomena of the process. The dissertation sheds light on fundamental transport mechanisms of the thin-film composite polyamide membranes, and serves to improve understanding of all the osmotic membrane processes. Findings of the work provide important insights into future design and development of high-performance membrane materials for water-energy application.

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.