Elsevier

Desalination

Volume 446, 15 November 2018, Pages 31-41
Desalination

Equilibrium ion partitioning between aqueous salt solutions and inhomogeneous ion exchange membranes

https://doi.org/10.1016/j.desal.2018.08.018Get rights and content

Highlights

  • Inhomogeneous ion exchange membranes synthesized via free radical polymerization.

  • Equilibrium NaCl, KCl, MgCl2 sorption in membranes measured experimentally.

  • Ion sorption data interpreted via Donnan/Manning thermodynamic model.

  • Model predicted KCl and MgCl2 sorption in membranes without adjustable parameters.

Abstract

This study investigates the ability of a recently proposed thermodynamic framework (i.e., Donnan/Manning model) to quantify equilibrium ion partitioning between aqueous salt solutions and inhomogeneous (i.e., phase separated) ion exchange membranes. Two inhomogeneous cation exchange membranes (CEMs) and one homogeneous anion exchange membrane (AEM), which served as a basis for comparison, were synthesized via free radical polymerization. Surface characterization of the membranes via cryogenic scanning electron microscopy revealed the inhomogeneous structure of the CEMs and the reasonably homogeneous structure of the AEM. Equilibrium ion concentrations in membranes equilibrated with NaCl, KCl, or MgCl2 solutions were experimentally measured. The experimental results were interpreted via a thermodynamic model based on Donnan theory and Manning's counter-ion condensation theory. The framework predicted NaCl partitioning in the AEM, presumably due to its reasonably homogeneous structure, but failed to predict NaCl partitioning in the CEMs when the Manning parameter, ξ, was calculated assuming a homogeneous membrane. For the inhomogeneous CEMs, the Manning parameter was used as an adjustable constant, which was extracted from the NaCl sorption results. Remarkably, the value of ξ obtained from the NaCl sorption results accurately predicted KCl and MgCl2 sorption in the two CEMs over the entire salt concentration range considered.

Introduction

Ion exchange membranes (IEMs) (i.e., ionomers, ion containing polymers, charged membranes, etc.) are a broad class of materials comprised of polymers bearing ionizable functional groups covalently bound to their backbone [1]. Recently, IEMs have attracted considerable interest in various membrane-based technologies for water purification and energy applications due to a combination of favorable ion transport properties, chemical stability, and diversity of chemical structures used to make such materials [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]]. Appending ionic moieties to a polymer backbone drastically influences ion transport in such materials, largely due to strong electrostatic interactions between ionic species, and it provides an avenue for tuning ion and water transport properties of such membranes [1,5,6,14,15]. Perhaps most importantly, ion exchange membranes allow selective transport of counter-ions (i.e., ions with opposite charge to that of charged groups) over co-ions (i.e., ions with similar charge to that of fixed charge groups), a property required for technologies such as electrodialysis, reverse electrodialysis, batteries, etc. [1,10,16,17]

IEMs are often fabricated from co-polymers consisting of hydrophilic monomer units, which bear ionic functional groups, and typically hydrophobic monomer units, which regulate membrane stability in aqueous environments [1,14,16,17]. Traditionally, IEMs have been synthesized via a two (or more) step procedure. For example, a hydrophobic, often cross-linked, polymer network (e.g., poly(styrene‑divinylbenzene)) is formed first, and ionic functional groups are subsequently introduced into the polymer network via chemical reactions (e.g., sulfonating the aromatic rings in poly(styrene‑divinylbenzene)) [1,16,17]. This multi-step synthesis procedure can lead to non-uniform distribution of fixed charge groups within the polymer matrix, resulting in membranes having inhomogeneous structures. Additionally, IEMs can be prepared by copolymerizing an ion containing monomer with a cross-linker (or hydrophobic mono-functional monomer) in a one pot synthesis [8,14,[18], [19], [20], [21]]. However, this procedure often requires a solvent that can dissolve both monomers, which may be difficult to find since charged monomers are hydrophilic and cross-linkers are often hydrophobic. Additionally, large differences in monomer hydrophobicity could lead to inhomogeneous membranes in which the fixed charge groups are non-uniformly distributed within the material.

Understanding the underlying mechanism governing ion partitioning and transport in ion exchange membranes is important for developing guidelines for rational design of such materials. Recently, we proposed a fundamental thermodynamic framework (i.e., Donnan/Manning model) for predicting equilibrium ion partitioning between aqueous salt solutions and highly charged, homogeneous ion exchange membranes [15,22,23]. The Donnan/Manning model predicted, with no adjustable parameters, partitioning of various electrolytes (e.g., NaCl and MgCl2) in a series of commercial ion exchange membranes [22]. In a later study, the Donnan/Manning model was used in concert with the solution-diffusion model to predict concentration gradient driven salt transport (i.e., salt permeability coefficients) in these materials [24].

The commercial IEMs used in our previous studies were considered to be reasonably homogeneous, a hypothesis supported by the good agreement between experimentally determined equilibrium membrane ion concentrations and values predicted by the Donnan/Manning model without using adjustable parameters. As mentioned previously, however, the synthetic conditions used to make ion exchange membranes more often result in inhomogeneous membranes rather than homogeneous ones. Additionally, there is widespread interest in using phase separated membranes (e.g., Nafion and other phase separated polymers) in fuel cell applications [5,7,8,[25], [26], [27], [28]]. Thus, there is a need to rigorously test the ability of the Donnan/Manning model to predict ion partitioning in inhomogeneous membranes, since this case is more practically relevant. This need for improved knowledge in this area served as motivation for the present study.

In this study, two relatively highly charged, highly cross-linked cation exchange membranes were synthesized via a one-pot free radical copolymerization reaction. It was found previously that these materials undergo polymerization induced phase separation, which results in inhomogeneous membrane morphologies [15]. Consequently, the Donnan/Manning model, which was originally proposed for homogeneous membranes, was unable to predict NaCl partitioning in the membranes without using an adjustable parameter. This study further explores ion partitioning in inhomogeneous membranes and demonstrates an avenue for using the Donnan/Manning model to predict partitioning of various electrolytes (e.g., KCl and MgCl2) in inhomogeneous membranes without previous detailed knowledge of membrane morphology.

Section snippets

Background

Equilibrium ion partitioning between ion exchange membranes and aqueous salt solutions is typically modeled using a thermodynamic framework referred to as ideal Donnan theory [1]. Recently, we improved this model by incorporating effects of thermodynamic non-idealities (i.e., ion activity coefficients) in the membrane and solution phases [22]. Accounting for thermodynamic non-idealities in the membrane and solution phases was critical for obtaining quantitative agreement between calculated and

Polymers

The synthesis protocol for the membranes used in this study was reported previously [15]. The main steps for synthesizing one of the CEMs, denoted as CA267, are summarized below for convenience. In a small glass scintillation vial, 8.7 g of 2‑acrylamido‑2‑methylpropane sulfonic acid (AMPS) (99% purity, Sigma Aldrich, St. Louis, MO) were combined with 6 g of ultrapure DI water (electrical resistivity of 18.2 MΩ-cm and <5.4 ppb TOC) generated by a Millipore RiOS and A10 water purification system

Membrane morphology

The physical structure of cross-linked polymers formed via free radical copolymerization of vinyl monomers with divinyl cross-linkers in the presence of a solvent depends on the relative amounts of the components in the prepolymerization mixture and the interactions between them [[41], [42], [43], [44], [45], [46], [47], [48], [49]]. Such systems could form non-porous (i.e., dense), as well as porous (i.e., heterogeneous) polymer structures [48]. According to Okay, the condition for forming

Conclusions

Two cation exchange membranes, CA238 and CA267, and one anion exchange membrane, AA267, were synthesized via a one-step free radical copolymerization reaction. Based on monomer reactivity ratios reported in the literature for chemically identical or similar monomers, the cation exchange membranes were presumed to be inhomogeneous, due to vast differences in monomer reactivity ratios, and the anion exchange membrane was presumed to be reasonably homogeneous. Surface characterization of

Acknowledgements

This material is based upon work supported in part by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1110007, the Welch Foundation Grant No. F-1924-20170325, and by the Australian-American Fulbright Commission for the award to BDF of the U.S. Fulbright Distinguished Chair in Science, Technology and Innovation sponsored by the Commonwealth Scientific and Industrial Research Organization (CSIRO). This study was also partially supported by the International

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