Ion-exchange materials: laboratory and water-treatment cation exchangers (0.3–1 mm beads) are commonly prepared from which resin family?

Introduction / Context:
Ion-exchange resins are crosslinked, insoluble polymers bearing ionic functional groups. Cation exchangers carry acidic groups (often sulfonic acids) fixed to the polymer matrix. While modern large-scale plants primarily use sulfonated polystyrene–divinylbenzene matrices, sulfonated phenolic resins have also been widely used, especially in laboratory and specialty applications.

Given Data / Assumptions:

• Bead size: 0.3–1 mm typical for packed beds.
• Functional requirement: strong or weak acid groups for cation exchange.
• Options list traditional thermoset resins.

Concept / Approach:
Phenol–formaldehyde networks can be sulfonated to introduce –SO3H groups, producing cation-exchange capability. Urea– and melamine–formaldehyde resins are not commonly used as ion-exchange matrices. Epoxies lack the appropriate backbone/chemistry for efficient sulfonation and exchange capacity in standard practice.

Step-by-Step Solution:
Identify which resin can be post-functionalised to strong acid groups and shaped into beads.Select sulfonated phenolic (phenol–formaldehyde) as the correct answer.

Verification / Alternative check:
Historical and specialty resin catalogs list phenolic-based ion exchangers alongside polystyrene–DVB-based products, especially for high-temperature or oxidative environments.

Why Other Options Are Wrong:
Epoxy, urea–formaldehyde, and melamine resins are not standard cation-exchange bead matrices.

Common Pitfalls:
Assuming 'ion-exchange resin' always means polystyrene–DVB; phenolic-based cation exchangers are well-established alternatives.

Final Answer:
Phenol–formaldehyde resins (sulfonated phenolic)