Aqueous Electro-Driven Separations: Thermodynamic Limitations for Desalination and Opportunities for Selective Extraction of Target Species
Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Elimelech, Menachem
Abstract
While the planet’s limited freshwater resources incessantly shrink due to pollution and climate change, the global demand for freshwater continues to rapidly increase with population growth. Effectively combatting water scarcity requires the harnessing of unconventional, typically saline, water sources to augment dwindling freshwater supplies through desalination. Though desalination has already been widely deployed in an effort to alleviate global water scarcity, critical areas for further process development and supplementation remain — several of which are addressed through the following dissertation research. The process of water-salt separation inherently requires the consumption of energy, making minimization of energy consumption an essential goal for desalination. While utilizing brackish waters, which are considerably less saline than seawater, effectively reduces energy consumption, identification of which desalination technology provides the lowest energy consumption in this salinity range remained unclear. By utilizing rigorous microscopic- and process-scale modeling, we effectively addressed this knowledge gap. We begin by uncovering the thermodynamic limits of electrodialysis (ED) and capacitive deionization (CDI), a technology which has gained significant research momentum over the past decade under the premise of being energy efficient. Importantly, our results overturn these claims by demonstrating that the mechanism of water-salt separation in CDI inherently consumes significantly more energy than ED. Furthermore, through extensive review of the literature and mechanistic process modeling, we demonstrate that even with the development of novel electrode materials, the energy efficiency of CDI remains severely limited (< 10%). While CDI is shown to be an inadequate method for desalination, which requires removal of the majority of ions in solution, we demonstrate that the principle of electrosorption may still be leveraged as an efficient strategy for selective ion extraction. In the following chapters, we assess the energetic performance of electrodialysis (ED) with respect to the current state-of-the-art technology, reverse osmosis (RO). Specifically, we identify that both RO and ED are energy efficient for brackish water desalination, albeit for varying conditions. Thus, we precisely map out the sweet spots of operation of each technology the first time. Furthermore, we considered process economics, in addition to energy consumption, to project the potential of ED to overtake RO as the dominant technology in the brackish water desalination sector, highlighting the key technological improvements which will be required to lower the cost of desalination with ED. Notably, our combination of mechanistic Nernst-Planck ED modeling and technoeconomic analysis reveals clear structure-property-performance relationships for ion-exchange membranes, which allows for the identification of specific targets for future membrane design. While our above analyses demonstrate that electro-driven processes have inherent limitations for desalination, electrochemical processes, nonetheless, offer unique advantages when applied to the extraction of target species from complex aqueous systems. Hence, for the remainder of the dissertation we shift our focus towards the development of novel electro-driven technologies for the selective extraction of three species of particular interest — boron, silica, and lithium. Though RO membranes are highly effective at removing a broad range of contaminants, the rejection of boron remains insufficient, requiring the use of chemical and energy intensive post-treatment steps to achieve permissible boron concentrations. Another major remaining issue in RO desalination is the pervasive and detrimental formation of silica scale on the membranes, which ultimately limits the attainable water recovery and reduces energy efficiency. Current dissolved silica removal strategies, however, also require substantial chemical dosing and generally suffer from slow removal kinetics. Notably, both boron and silica are amphoteric species, existing as uncharged solutes in circumneutral pH and converting to anionic forms at high pH values. Accordingly, in this dissertation, we develop two novel electrochemical strategies which are capable of simultaneously regulating pH (without the use of external chemicals) and removing boron/silica via electrosorption. Specifically, with silica, we show that the local pH dynamics in a flow-through electrode electrosorption configuration can facilitate ionization of silica at the anode-separator interface, followed by electrosorption. Furthermore, we demonstrate that facile chemical functionalization of the anode enables high selectivity for silica over competing anions, even in saline waters. For the removal of boron, we introduce a novel electrosorption architecture, in which a bipolar membrane (BPM) is oriented between activated carbon electrodes, in effect directly coupling bipolar membrane induced water dissociation with electrosorption. The BPM-electrosorption process is shown to offer superior performance (i.e., sorption capacity and energy efficiency) compared to flow-through electrosorption, making it a promising technology for the eventual displacement of currently employed boron/silica removal methods. Lastly, while desalination aims to extract only water from saline sources, many feedwaters also contain substantial quantities of valuable resources. Hence, the development of supplemental technologies to RO, which are capable of selectively extracting species of interest, could revolutionize desalination plants, effectively dualizing their function as resource recovery facilities. With projected shortages within the next decade, lithium is a particularly critical species which is present in most desalination feedwaters, albeit at low concentrations (e.g., ~ 0.1 mg L-1 in seawater). As polymeric membranes have thus far failed to achieve sufficient selectivity for lithium, we assessed the use of inorganic solid-state electrolyte (SSE) materials as lithium-selective membranes in electrodialytic lithium extraction. We begin by investigating the lithium transport mechanisms in SSE-electrodialysis, revealing fundamental differences between transport in ion-exchange membranes and solid-state electrolytes. Furthermore, we show that the unique mode of lithium transport in SSEs facilitates nearly perfect ion-ion selectivity, even in the presence of overwhelming concentrations of competing ions. The mechanisms underlying such exceptional selectivity in SSEs are thoroughly explored, providing fundamental insights into the design of the next generation of selective membrane materials.
Recommended Citation
Patel, Sohum K., "Aqueous Electro-Driven Separations: Thermodynamic Limitations for Desalination and Opportunities for Selective Extraction of Target Species" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1413.
https://elischolar.library.yale.edu/gsas_dissertations/1413
