Why is it necessary to remove residual chlorine before using ion exchange resins?

Before using ion exchange resins (whether for water softening, desalination, or other polishing processes), residual chlorine must be rigorously removed from the water. This is not just a standard operating procedure; it's crucial for protecting resin performance, ensuring long-term stable operation, and ensuring economic efficiency.

The core reason: Residual chlorine is a "chronic poison" and a "powerful destroyer" for ion exchange resins.

Residual chlorine (usually referring to free chlorine, such as hypochlorous acid (HOCl) and hypochlorite (OCl⁻)) plays a crucial role in disinfection in Water Treatment. However, its strong oxidizing properties are extremely destructive to the polymer backbone and functional groups of ion exchange resins. This damage primarily manifests itself in the following ways and is often irreversible:

1. Oxidative degradation of the resin backbone (polymer matrix)

The main component of resin particles is a molecular polymer, such as a crosslinked network of polystyrene and divinylbenzene. Residual chlorine, as a strong oxidant, attacks and breaks the carbon-carbon and carbon-hydrogen bonds within the resin backbone. In particular, the crosslinking points provided by divinylbenzene are destroyed, resulting in a decrease in the resin's crosslinking density. Oxidative degradation of the resin backbone can lead to:

Increased swelling: Reduced crosslinking causes the resin particles to absorb more water in the solution, leading to excessive swelling.

Decreased mechanical strength: Excessive swelling and backbone fractures make the resin particles soft and brittle. During operation and backwashing, particles can easily break due to friction, collision, and shear forces from the water flow.

Volume increase/deformation: Excessive swelling can significantly increase the resin's volume, occupying the exchange column space and even causing "clumping" or blocking the water flow path.

Increased pressure drop: Broken fine particles can clog the screen below the resin bed or accumulate in the bed voids, significantly increasing the resistance (pressure drop) to water flow through the resin bed, reducing water production and increasing energy consumption.

2. Oxidative degradation of functional groups

Cationic resins: Sulfonic acid groups (-SO₃H) are relatively stable, but long-term exposure to residual chlorine can also lead to slow oxidative decomposition. Quaternary ammonium groups (-N⁺(CH₃)₃) are very sensitive to oxidants.

Anionic resins: The quaternary ammonium group, the core functional group of anionic resins, is extremely sensitive to oxidants (including residual chlorine) and is their weakest link.

Residual chlorine oxidatively attacks the nitrogen atom in the quaternary ammonium group or the carbon chain attached to it.

Oxidative degradation of functional groups can lead to:
Functional group decomposition/loss: Oxidative degradation of the quaternary ammonium group may generate amines (such as tertiary and secondary amines), aldehydes, and organic acids, resulting in a loss of strong alkaline ion exchange capacity.

Permanent reduction in exchange capacity: The destruction of the functional group directly leads to an irreversible loss of the resin's theoretical exchange capacity. This means that the resin will need to be regenerated more frequently to process the same amount of water, or the product water quality may deteriorate prematurely.

Deterioration in effluent quality: Organic matter (such as organic acids and amines) produced by degradation may leak into the product water, affecting water quality (e.g., increased TOC, abnormal conductivity, pH changes, and odor). For high-purity water production (e.g., semiconductor and pharmaceutical applications), such organic leakage can be catastrophic.

Reduced regeneration efficiency: Damaged functional groups may affect the effective utilization of regenerants (such as NaCl and NaOH) and the recovery of exchange sites during the regeneration process.

3. Accelerated organic contamination:

Residual chlorine itself or the organic fragments produced by oxidative degradation of resins react with natural organic matter in the water to produce larger molecular weight, more hydrophobic, and more viscous oxidation products.

These products are more easily adsorbed and firmly deposited on the surface and internal pores of resin particles, forming irreversible organic fouling. This further hinders ion diffusion, reduces exchange rate and effective capacity, and increases cleaning difficulty.

How to remove residual chlorine?

Common and effective methods include:

Activated carbon filtration: The most commonly used and cost-effective method. Activated carbon removes residual chlorine through adsorption and catalytic reduction reactions (reducing HOCl/OCl⁻ to Cl⁻).

Chemical reduction (sulfite dosing): Sodium sulfite (Na₂SO₃) or sulfur dioxide (SO₂) is added to the influent, causing a rapid reduction reaction with the residual chlorine (Cl₂ + SO₃²⁻ + H₂O → 2Cl⁻ + SO₄²⁻ + 2H⁺).

Ultraviolet irradiation (specific wavelength): Effectively decomposes free chlorine.