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How do fixed positive charges in a resin exchange negative ions?

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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).

An ion-exchange resin with fixed positive charges functions as an anion exchanger by attracting and binding negatively charged ions (anions) from a surrounding solution.12

The process works through the following principles:

Fixed Ionic Sites: The resin contains a polymeric framework with permanently attached positive groups, such as quaternary ammonium salts.12
Mobile Ions: These fixed positive charges are balanced by mobile, negatively charged ions (such as hydroxide or halide ions) that are loosely held within the resin's structure.12
Exchange Mechanism: When the resin is immersed in a solution, it is permeable to water and small ions. If the solution contains other anions with a greater affinity for the resin, these ions will displace the original mobile anions, effectively exchanging them.23

Essentially, the resin acts as a solid-phase medium where the fixed positive charges maintain electrical neutrality by holding onto mobile negative ions, which can be swapped for other negative ions present in the surrounding liquid.12