Diffusion Dialysis Technology Based on Anion Exchange Membrane

In the mining, metallurgy, metal processing and nuclear fuel reprocessing industries, the production of waste acidic liquids containing toxic metal ion complexes is often involved. Diffusion dialysis technology (DD) based on anion exchange membrane (AEM) is considered an effective technology with low energy consumption and less environmental pollution for recovering acids and valuable metals from industrial wastewater.

How Diffusion Dialysis Technology Works

DD technology relies on the principle of selective ion transport through anion exchange membranes. In the DD process, acid migration through the AEM can usually be described by two models, namely the solution diffusion model and the three-phase membrane model. In acid recovery applications, the DD process can be roughly divided into the following steps:

• Ions in the acidic waste solution (feed solution) are driven by an activity gradient to migrate through the AEM to the other side filled with deionized water (receiver solution).
• Due to the presence of positively charged functional groups in AEM, anions (Cl^-, SO₄^2-, NO₃^-, etc.) can freely migrate through the membrane, while most cations are repelled by the membrane blocking common ions due to the Donnan criteria.
• It should be mentioned that although H⁺ is positively charged, it is easier to migrate through AEM than other cations due to its small size, low valence and high mobility. Therefore, H⁺ can migrate from the feed solution to the receiving solution through the drag effect with anions to maintain the electroneutrality of the solution.
• Ultimately, the acid can be collected on the receiving side, while the metal ions can be retained on the feed side.

Important Factors Affecting Acid Recovery

• Acid Permeability: The type of functional groups in AEM and the structure of AEM will affect its acid permeability.
• Acid Selectivity: The type and size screening effect of functional groups in AEM are important factors affecting acid selectivity in the DD process.
• Membrane Stability: Membrane stability determines the service life of the AEM. Higher membrane stability means it can be used for long-term DD acid recovery. Functional groups, cross-linking and incorporation of inorganic components into AEM are all ways to improve membrane stability.

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Reference

1. Zhang, Chengyi, et al. Membranes, 2020, 10(8), 169.