Ion Selectivity of Ion Exchange Membranes

INQUIRY

Ion exchange membranes (IEMs) are an important class of dense polymer membranes with fixed charges in the polymer matrix. These membranes can selectively allow ions with opposite charges (counter ions) to pass through while blocking ions with similar charges (co-ions). IEMs have become key components in industrial water desalination and electrolysis processes, and they are also being extensively studied and developed for emerging applications such as membrane capacitive deionization (MCDI), reverse electrodialysis (RED), microbial fuel cells (MFCs), and ion exchange membrane bioreactors (IEMBs).

What Is the Ion Selectivity of IEMs?

One of the key characteristics of ion exchange membranes is their ion selectivity, which refers to the ability of a membrane to selectively transport specific ions while blocking others.

As the application of IEMs continues to expand, permselectivity between counterions and co-ions and between counterions of different valences (monovalent and multivalent, such as Li⁺ and Mg²⁺) or equal valences (such as NO₃^- and Cl^-) is desirable to be achieved.

Ion Transport Mechanism of IEMs

The transport of ions through ion exchange membranes can occur via various mechanisms, including Donnan exclusion, electrostatic repulsion, size exclusion, and diffusion. Donnan exclusion is a significant mechanism where ions are selectively transported based on their charge interactions with the functional groups present in the membrane. Electrostatic repulsion occurs when ions with the same charge as the membrane functional group are repelled, allowing selective ion transport.

Donnan potentials at the CEM and AEM. Donnan effect of IEMs. [1]

Ion Selectivity Order of IEMs

The rate of ion transport through an IEM is determined by the ion concentration (ion exchange) and mobility in the membrane. Ion exchange is related to the valence and size of the ions: ions with higher valence, or ions with larger ionic radius when the valence is the same, are preferentially exchanged into the IEM. The mobility of ions in the membrane matrix depends on the Stokes radius of the ions and their interaction with the fixed ionic groups.

The selective permeation order of common anion exchange membranes (AEMs) transporting anions can be summarized as:

I^- > (NO3- ~ Br^-) > NO₂^- > Cl^- > OH^- > SO₄^2- >F^-

For common cations in electrodialysis (ED) with mixed binary salts, the general transport order through CEMs with fixed sulfonic acid groups is:

Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺ > H⁺ > (Cu²⁺ ~ Zn²⁺ ~ Ni²⁺) >K⁺ >Na⁺ >Li⁺ >Fe³⁺

Methods for Determining Ion Selective Permeability

Several methods are employed to determine the ion selective permeability of ion exchange membranes, including ion exchange capacity measurements, membrane potential measurements, chronoamperometry, and ion-selective electrode techniques.

Ion exchange capacity measurements involve determining the quantity of ions that can be exchanged with the membrane functional groups.
Membrane potential measurements provide information on the charge interactions influencing ion transport.
Chronoamperometry and ion-selective electrode techniques offer insights into ion flux and selectivity under different conditions.