Enabling Tailored Selectivity of Ion-Selective Membranes with Dual-Driving Force Operation

Technologies that enable the selective separation of different ions in aqueous mixtures are required for many essential energy, water, and environmental separations processes. Ion-selective membranes (ISMs) are polymeric films with charged ionic functional groups that allow the transport of oppositely-charged species (counterions) through the membrane while retaining like-charged ions (co-ions). A driving force (electrostatic potential) is applied to transport ions through the membrane. ISMs have been used in commercial applications such as fuel cells and electrodialysis desalination, but they exhibit performance limitations that preclude their application under many industrially-relevant conditions. First, ISMs suffer from diminished selectivity between counter- and co-ions at higher electrolyte concentrations. ISMs are also unable to discriminate between counterions with the same valency. This project will investigate using a second, simultaneous driving force (hydraulic pressurization) to alter the relative permeation of different ions. By coordinating the magnitude and direction of the two driving forces, termed “dual-driving force” operation, the two shortcomings in ISM selectivity above may be overcome. The development of more selective ion separation technologies can unlock opportunities for resource recovery from nontraditional source waters, including recovering lithium from geothermal brines, nitrogen and phosphorus from municipal and industrial wastewaters, rare earth elements from mining waste streams, and uranyl ions from seawater.The overall goal of this project is to achieve enhanced selectivities in ISM separations by purposeful control of two opposing driving forces for ion transport, i.e., dual-driving force operation. The specific objectives of the research are: 1) develop a membrane fabrication platform for composite ion-selective membranes in dual-driving force operation, 2) demonstrate the rational control of dual-driving force operation to enhance ISM selectivity for counterions with different mobilities, and 3) engineer the suppression of co-ion leakage using dual-driving force operation to improve ISM permselectivity. Successful execution of the project will enable the ISMs to i) depart from the affinity-mobility constraint on counterion transport and ii) attain improved exclusion of co-ions even at elevated solution ionic strengths. The research will systematically study the underlying factors governing transport and selectivity in ISMs and provide mechanistic insights into membrane design and operation strategies. The project will integrate education and research to train and prepare graduate, undergraduate, and high school students in STEM. The PI will engage in activities including: (i) developing modules for K-12 education and public outreach, (ii) enhancing undergraduate and graduate courses by integrating scientific concepts and technical principles of the research into class syllabus, and (iii) providing undergraduate research opportunities through structured programs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.