Source: Dissertation Abstracts International, Volume: 76-07(E), Section: B.

Adviser: Menachem Elimelech.

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The world population of the 21st century is facing an increasingly challenging energy landscape and declining water quality and availability, further compounded by a rapidly expanding global population against the backdrop of climate change. To meet the challenges of the water-energy nexus in a sustainable manner, existing methods need to be advanced and new technologies developed. Osmotically-driven and ion-exchange membrane processes are two classes of emerging technologies that can offer cost-effective and environmentally sensible solutions to alleviate the pressure on our water and energy demands. The objective of this thesis is to advance forward osmosis (FO), pressure retarded osmosis (PRO), and reverse electrodialysis (RED) for the sustainable production of water and energy.

A main hindrance restricting the progress of osmotically-driven membrane processes, FO and PRO, is the absence of adequate membranes. This work demonstrates the fabrication of thin-film composite polyamide FO membranes that can attain high water flux and PRO membranes capable of achieving power density of 10 W/m2, twice the benchmark of 5 W/m2 for PRO with natural salinity gradients to be cost-effective. A membrane fabrication platform based on mechanistic understanding of the influence of membrane transport and structural parameters on process performance was developed. The morphology and microstructure of the porous support layer, and hydraulic permeability and salt selectivity of the polyamide active layer were specifically tailored by thoughtful control of the fabrication and modification conditions.

The Gibbs free energy from the mixing of river water with seawater can potentially be harnessed for clean and renewable energy production. This work analyzed the thermodynamics of PRO power generation and determined that energy efficiencies of up to ∼91% can theoretically be attained. The intrinsic limitations and practical constraints in PRO were identified and discussed. Using a tenth of the annual global river water discharge of 37,000 km 3 for PRO could potentially produce electricity for over half a billion people, ascertaining natural salinity gradients to be a sizeable renewable source that can contribute to diversifying our energy portfolio.

However, fouling of the membrane support layer can diminish the PRO productivity by detrimentally increasing the hydraulic resistance. Analysis of the water flux behavior and methodical characterization of the membrane properties shed light on the fouling mechanism and revealed the active-support layer interface to play a crucial role during fouling. A brief osmotic backwash was shown to be effective in cleaning the membrane and achieving substantial performance recovery.

Reverse electrodialysis (RED) is an ion-exchange membrane process that can also extract useful work from salinity gradients. This dissertation research examined the energy efficiency and power density of RED and identified a tradeoff relation between the two performance parameters. Energy efficiency of &sim;33-44% can be obtained with technologically-available membranes, but the low power densities of < 1 W/m2 is likely to be impede the realization of the process. To further advance RED as a salinity energy conversion method, ion-exchange membrane technology and stack design need to be advanced beyond their current limitations.

When analyzed with simulated existing state-of-the-art membranes, PRO exhibited greater energy efficiencies (54-56%) and significantly higher power densities (2.4-38 W/m2) than RED (18-38% and 0.77-1.2 W/m 2). The drawback of RED is especially pronounced at large salinity gradients, where the high solution concentrations overwhelm the Donnan exclusion effect and detrimentally diminish the ion exchange membrane permselectivity. Additionally, the inherent different in driving force utilization (osmotic pressure difference for PRO and Nernst potential for RED) restricts RED from exploiting larger salinity gradients to enhance performance. Overall, PRO is found to be the more favorable membrane-based technology for accessing salinity energy.

This work presents pioneering advances for forward osmosis and pressure retarded osmosis membrane development. The fundamental studies of the osmotically-driven membrane processes and ion-exchange membrane processes yielded significant findings that enhanced our mechanistic and thermodynamic understanding of the technologies. The important insights can serve to inform the realization of the emerging membrane-based technologies for the sustainable production of water and energy. The implications of the thesis are potentially far-reaching and are anticipated to shape the discussion on FO, PRO, and RED.