NSF-BSF: Ion transport and selectivity in salt-rejecting membranes operating at elevated salinities and pressures

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Extremely salty water is produced in enormous quantities as waste from brackish water and seawater desalination, and oil and gas production. Purifying these wastewaters is a significant priority because it (1) advances human welfare and prosperity by increasing water supplies and (2) protects public health by reducing the discharge of potentially harmful wastewaters. Despite the importance of purifying very salty waters, current purification processes require substantial amounts of thermal energy or heat. This research project will investigate how to use polymer membranes to treat extremely salty water sources. Such membrane treatments can use less than one-tenth of the energy compared to conventional thermal systems. Novel experiments and molecular simulations will be used to better understand how polymer membranes perform under the high salt and high-pressure conditions needed for purifying this type of wastewater. The knowledge gained by these experiments will make it possible to develop polymer membranes specifically for high-salt wastewaters. Students researchers will collaborate directly with international partners, including travelling to Israel to improve cultural and scientific exchange. Undergraduate education will be improved through the creation of an innovative “Engineered Solutions to Water Scarcity” module where collaborators serve as guest lecturers. The interdisciplinary nature of this project will enhance mentorship of underrepresented and first-generation university undergraduate students through the CU Boulder BOLD Center by motivating interest in chemical engineering, environmental engineering, and separations science. Outreach to high-school students will be accomplished by developing and implementing classroom lessons, hands-on activities, and a short video that will be made publicly available on global water scarcity challenges and engineered solutions. The research project will elucidate mechanisms governing ion transport in membranes during high-salinity brine treatment and create a framework for designing membranes with improved water-salt selectivity by tuning chemistry and structure. The central hypothesis is that elevated salinity and pressure cause significant variations in the intrinsic ion transport properties through polymer deswelling, electrostatic charge shielding, membrane compaction, and fundamental changes in ionic hydration properties. The impact of extreme salinity and pressure conditions on these phenomena will be comprehensively investigated using advanced transport characterization techniques. In particular, transition state theory will be applied to membrane permeability to elucidate molecular-level enthalpy- and entropy-related effects that occur during ion transport and stem from extreme salinity and pressure conditions. Such molecular-level effects will be further explored using molecular simulations of ion transport through the membrane. Based on the insights gained, increasing water-salt selectivity of membranes at elevated salinity and pressure using tailored charge, hydrophobicity, and crosslinking density will be explored. Ultimately, the results of this project will reveal the effect of high salinity and pressure on molecular transport under the extreme confinement of reverse osmosis and nanofiltration membrane pores, improve the fundamental understanding of ion transport in polymers, and create design recommendations that will aid in the development of membrane-based processes for high-salinity brine treatment.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.