Decoding the Role of Valency in Multi-valent Ion Partitioning through Ion-Exchange Membranes: A Molecular Theory Approach

Ion-exchange membranes are critical in many modern water desalination technologies, such as reverse dialysis and pressure-retarded osmosis. Despite their importance in these water purification systems, the fundamental working principles of ion-exchange membranes remains poorly understood. As an open problem in the membrane science and water research communities, how ion valency modulates the transport property (i.e., ion partitioning) of these membranes stays mainly unresolved. Hence, the main objective here is to discover new knowledge that emerges when these membranes operate in multivalent salt solutions (e.g., impaired water, including river and seawater). In other words, this proposal wants to understand the underlying mechanism governing the behavior of mono-, di- and tri-valent ions in ion-exchange membranes, particularly at the microscopic level. This proposal will formulate a novel method to capture this multivalent ion partitioning process: an experimentally testable molecular theory based on statistical mechanics and thermodynamics principles. This framework can, for the first time, rationalize the (a) local (membrane solution and polymeric chains) and (b) global (between bulk solution and membrane) partitionings. With this molecular theory framework, a two-key question that emerged in recently published experiments can be addressed: (1) how does ion valency determine the fraction of condensed or free ions in the membranes? and (2) how does (mono-, di- and tri-valent) ion condensation impact the partitioning performance at the local and global levels? By answering these two questions, a vital link can be systematically established between ion valency and transport property (such as partitioning) of such membranes. Altogether, this project will produce novel fundamental insight and new theories for the rational design of ion-exchange membranes operating in multivalent salt solutions across different lengths scales without over-engineering, contributing to the United Nations' Sustainable Development Goal 6 for sustainable water management and sanitation in developing economic and high-performing water desalination materials.