Intrapore energy barriers govern ion transport and selectivity of desalination membranes

Abstract

State-of-the-art desalination membranes exhibit high water-salt selectivity, but their ability to discriminate between ions is limited. Elucidating the fundamental mechanisms underlying ion transport and selectivity in subnanometer pores is therefore imperative for the development of ion-selective membranes. Here, we compare the overall energy barrier for salt transport and energy barriers for individual ion transport, showing that cations and anions traverse the membrane pore in an independent manner. Supported by density functional theory simulations, we demonstrate that electrostatic interactions between permeating counterion and fixed charges on the membrane substantially hinder intrapore diffusion. Furthermore, using quartz crystal microbalance, we break down the contributions of partitioning at the pore mouth and intrapore diffusion to the overall energy barrier for salt transport. Overall, our results indicate that intrapore diffusion governs salt transport through subnanometer pores due to ion-pore wall interactions, providing the scientific base for the design of membranes with high ion-ion selectivity.

Fabrication of desalination membranes by interfacial polymerization: history, current efforts, and future directions

“…Water-solute selectivity of TFC membranes is determined by the combined mechanisms of steric hindrance, Donnan exclusion, and dielectric effect. 13 Steric hindrance is related to the pore size (or free volume) of polyamide, which allows molecules with smaller sizes to permeate while keeping larger ones from doing so. Donnan exclusion is governed by the fixed charge density on the membrane surface.…”

Evaluation of Performance of Existing RO Drinking Water Stations in the North Central Province, Sri Lanka

“…Similarly, at lower alkalinity, groundwater has lower inorganic ions. Hence, resistance to entering the TFC layer and migrating to the permeate side is significantly lower, except in terms of the slight repulsive forces exerted by the negatively charged polyamide surface functional groups while giving lower alkalinity rejections [ 39 ].…”

“…Monovalent ions such as K + showed a moderate negative correlation (r = −0.41, p = 0.062) with the operating pressure, while other ions showed a very weak negative correlation ( Figure 4 d). Smaller hydration radius and higher mobility of the monovalent ions gave comparatively lower resistance for intrapore diffusion through the polyamide matrix [ 39 ]. Similarly, slight expansion (swelling) of polymer (polyamide) matrix on the TFC layer at higher pressure could lead to increase in the porosity, allowing for permeation of more ions with smaller hydration radius, such as K + , through the membrane [ 41 , 42 ].…”

“…This is because the electrostatic repulsion forces exerted by attached divalent ions (Ca 2+ and Mg 2+ ) to the functional groups on the surfaces of the membrane is lower when feedwater contains dilute level of Ca 2+ ions. The only resistance that affected the molecular transport of calcium is the energy barrier of the intrapore diffusion [ 39 , 44 ], resulting from the cations’ interaction with the negatively charged internal pore walls. When the ion concentration becomes higher (especially Ca 2+ ), more divalent ions attach to the membrane surface and cover a significant portion of the membrane surface [ 36 ] while providing enough repulsive force on the cations in solution.…”

Comparative Analysis of Conventional and Emerging Technologies for Seawater Desalination: Northern Chile as A Case Study

“…Conceptual diagram of the conventional membrane technologies applied to desalination (Mentioned references[53][54][55][56][57][58][59][60][61][62][63]). …”