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Why can insoluble ion-exchange materials attract ions from a solution?

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chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

Insoluble ion-exchange materials attract ions from a solution because they contain fixed ionic groups within their molecular framework or on their surface. These materials are permeable to water and small ions, allowing mobile ions from the surrounding solution to enter the material's structure.12

The fundamental mechanism of attraction is based on electrostatic force: the fixed ionic groups on the material carry a specific electrical charge, and they attract mobile ions of the opposite charge from the solution. For example, materials with fixed negative charges perform cation exchange by attracting and binding positively charged ions, while those with fixed positive charges perform anion exchange by attracting negatively charged ions.123

Because these materials are porous or possess an open lattice structure, the mobile ions can move in and out, enabling them to change places with other ions in the solution. This process is reversible, allowing the material to effectively capture, hold, or exchange ions based on the chemical equilibrium between the solid material and the surrounding liquid.124