NSF-BSF: Selective Transport of Divalent Cations through Polymeric Membranes Using Host-Guest Chemistry

Producing drinking water from unconventional water sources, such as seawater, brackish water, and municipal wastewater effluent, is crucial for alleviating global water scarcity. Polymeric membranes, such as thin-film composite reverse osmosis (RO) membranes, have been at the forefront of water purification and desalination processes since their advent in the early 1980s.

While RO systems are energy efficient and consume only ~25% more than the practical minimum theoretical energy of desalination, RO membranes are susceptible to inorganic scaling caused by scale-forming ions such as sulfate and divalent calcium, magnesium, or barium. Source waters that have high concentrations of these ions and/or require high recovery rates, such as in inland desalination, have an exceptionally high propensity to produce inorganic scale on membranes.

Inorganic scaling is known to drastically lower membrane water flux, limit membrane lifetime, increase treatment costs, and increase the energy consumption of membrane processes. Economic and environmental effects of membrane scaling have led to various mitigation approaches, including adjusting solution pH and adding polymeric antiscalants to block crystal growth sites.

However, the primary limitation of established techniques is they require the addition of chemicals, such as polymers or strong acids/bases, that are environmentally unfriendly and costly. In collaboration with researchers at Ben-Gurion University, this project will address the imminent need for an alternative, chemical-free method to selectively remove scale-forming ions to mitigate scaling on RO membrane surfaces and improve the economics of desalination processes.

The overall goal of the research is to translate selectivity mechanisms of biological channels into polymeric membranes for the purpose of selectively removing scale-forming species in a continuous electrodialysis process. The investigators hypothesize that molecular binding sites that can selectively remove water shells from ions (as seen in some biological channels) will enable highly specific adsorption and transport through membranes.

To test this hypothesis, a selective membrane will be created by modifying the surface of conventional membranes with polymers comprising pendant groups with a high chemical affinity for target ions (Task 1). These functional groups are expected to yield unprecedented selectivity because they provide favorable host-guest complexes to selectively remove water shells of target ions (Task 2).

This functional prototype will then be used to elucidate selectivity mechanisms for membranes with host-guest chemistry (Task 3), as well as to assess the relationship between the structural properties of those membranes and their selective transport (Task 4). Finally, the insights from Tasks 1-4 will be used to develop a homogenous membrane (Task 5), which will then be tested in electrodialysis for selective removal of scale-forming ions (Task 6).

The specific objectives of the project include: (i) investigating the role of ion affinity to chemically tailored polymers in achieving selective transport, (ii) assessing how intrinsic membrane structural properties affect solute transport and selectivity, and (iii) fabricating robust membranes to reduce scaling potential using electrodialysis. The outcome of the project will be a new membrane technology capable of removing scale-forming ions prior to desalination to mitigate scaling on RO membranes.

This technology would be the first continuous approach for separating divalent ions from monovalent ions without requiring periodic use of chemicals, overcoming the limitations of existing approaches. This study will also advance the fundamental understanding of selective transport processes by applying transition-state theory to describe solute transport phenomena in terms of entropy and enthalpy.

These insights, along with design principles established from this study, will be relevant for separations of other solutes as well, which may later find application in reclaiming valuable resources or removing contaminants of concern from water. This research is jointly funded by NSF and The US-Israel Binational Science foundation through the special submission opportunity NSF 20-094.

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.