Title: Developing Ion-Selective Membrane Technologies at the Water-Energy Nexus
Description: Providing sustainable access to water and energy is among the grand engineering challenges in the 21st century. Notably, many pressing issues at the water-energy nexus can be addressed by effective ion separations, such as desalination, nutrient recovery from wastewater, and extraction of energy-related elements from unconventional sources. One tool for such separations is ion-exchange membranes (IEMs), which are charged polymeric thin films. However, conventional IEMs face several performance constraints and fail to achieve ion-specific selectivity. This thesis aims to advance the potential of IEMs for separations at the water-energy nexus. Present-day IEM processes, e.g., electrodialysis (ED) desalination and reverse electrodialysis (RED) power generation, commonly employ the membranes as charge-permselective barriers, transporting oppositely-charged counterions while retaining like-charged co-ions. However, the increase in charge permselectivity is accompanied by a decrease in ionic conductivity, which forms a conductivity-permselectivity tradeoff and crucially limits separation efficiency.

This work models IEM conductivity and permselectivity as functions of intrinsic membrane chemical and structural properties, simulating the performance of IEMs in a range of ED and RED operations (Chapter 2). Bulk solution concentration is identified as an external cause for the tradeoff, which confines current IEM applications to sub-seawater salinities. The structure-property-performance analyses reveal membrane water sorption as an intrinsic determinant of the tradeoff, while indicating that increasing ion-exchange capacity and reducing thickness can yield highly selective and conductive IEMs. To depart from this tradeoff, nanocomposite cation-exchange membranes (CEMs) with percolating 1-D sulfonated carbon nanotube (sCNT) network are fabricated (Chapter 3). Membrane conductivity is raised with greater sCNT blending in the polymer matrix (increasing by ≈30% with 20 w/w% incorporation of sCNT), while permselectivity is effectively unchanged (within 2% variation). Further characterization displays sCNT percolation beyond 10 w/w% blending, attributing the conductivity improvement to the interconnected sCNT network.

The results imply the potential to advance the conductivity-permselectivity tradeoff with rationally designed nanostructures. Next, this thesis moves from the conventional charge-discriminating selectivity to ion-specific selectivity (Chapter 4), which is urgently needed but underdeveloped due to insufficient understanding of the fundamental transport phenomena. Here, a transport framework is presented to describe counterion migration mobility using an analytical expression based on first principles. The two governing mechanisms are: spatial effect of available fractional volume for ion transport and electrostatic interaction between mobile ions and fixed charges. Mobilities of counterions with different valencies are experimentally characterized, showing high regression R²s with the mobility model. The influence of membrane swelling caused by different counterions is further accounted for, while the frictional effect of electrostatic interaction is quantitatively linked to fixed charged density and dielectric constant of membranes.

Additionally, the anion-exchange membrane (AEM) exhibits a weaker electrostatic effect compared to CEMs, which is attributed to the steric hindrance of the quaternary ammonium functional groups. Last, the membrane-level knowledge is extended to two process-level applications, coupling desalination with sustainable energy and desalinating hypersaline brines. This thesis presents a novel low-grade-heat-driven desalination process (Chapter 5), using CEM and AEM Donnan dialysis (DD) stepwise to remove salt ions, e.g. Na+ and Cl-, with a receiver solution of thermally-recoverable solute NH₄HCO₃. NH₄HCO₃ in the streams is later recycled by low-grade heat. The concept is experimentally validated by desalinating brackish water (100mM NaCl) to freshwater salinity (< 17mM). DD desalination of larger ranges of feed and receiver concentrations was then demonstrated, and module-scale analysis quantified the improvements of countercurrent operation to desalination efficiency. Another challenge in water management is the desalination of hypersaline brines.

While demand is rapidly increasing, the wider application of hypersaline desalination is held back by considerable technical obstacles. Here, theoretical analyses are carried out to assess the potential of hypersaline ED desalination (Chapter 6). We show that desalination performance is impacted by the interrelated charge-discriminating selectivity and ion-water selectivity of IEMs. The work demonstrates lower energy costs of ED compared to thermal processes, when desalinating 1.0-1.5 M NaCl, and identifies three key performance-determining tradeoffs: conductivity-permselectivity, conductivity-water resistivity, and energy cost-volume reduction factor. To enable highly efficient hypersaline ED, ultra-low swelling IEMs need to be developed. Overall, this work advances mechanistic understanding, membrane development, and process design of IEMs. The thesis contributes insights to breaking conductivity-permselectivity performance constraints and developing highly valued ion-specific selectivity.

The informed membrane fabrication and process design provide access to unconventional water sources with less energy input. The findings of the thesis will enable the systematic development of more ion separation processes to address challenges at the water-energy nexus.