Water Treatment Methods and Principles: Deionization

The purpose of deionization is to remove dissolved inorganic ions from water. Similar to water softeners, it utilizes the principle of ion exchange resins. Two types of resins are used: cation exchange resin and anion exchange resin. The cation exchange resin uses hydrogen ions (H+) to exchange for cations; while the anion exchange resin uses hydroxide ions (OH-) to exchange for anions. Hydrogen ions and hydroxide ions combine to form neutral water. The reaction equations are as follows:

M+x+xH-Re→M-M-Rex+xH+1

A-z+zOH-Re→A-Rez+zOH-1

In the above equations, M+x represents the cation, and x represents the valence. The M+x cation exchanges with the hydrogen ions of H-Re on the cation exchange resin. A-z represents the anion, and z represents the valence. After A-z combines with the anion exchange resin, it releases OH- ions. The combination of H+ ions and OH- ions results in neutral water.

Once the adsorption capacity of these resins is exhausted, they need to be reduced. Cation exchange resins require strong acids to be reduced; conversely, anions require strong bases to be reduced. The adsorption capacity of cation exchange resins for various cations varies, and their relative strengths and relationships are as follows:

Ba²⁺ > Pb²⁺ > Sr²⁺ > Ca²⁺ > Ni²⁺ > Cd²⁺ > Cu²⁺ > Co²⁺ > Zn²⁺ > Mg²⁺ > Ag¹⁺ > Cs¹⁺ > K¹⁺ > NH₄¹⁺ > Na¹⁺ > H¹⁺

The affinity of anion exchange resins for various anions is as follows:

SO₄⁻ > I⁻ > NO₃⁻ > NO₂⁻ > Cl⁻ > HCO₃⁻ > OH⁻ > F⁻

If the anion exchange resin is completely depleted without reduction, fluoride, the weakest adsorbent, will gradually appear in the dialysis water, causing rickets, osteoporosis, and other bone diseases. If the cation exchange resin is depleted, hydrogen ions will also appear in the dialysis water, increasing the acidity of the water. Therefore, the effectiveness of the deionization function needs to be monitored frequently. This is generally determined by the water's resistivity or conductivity. The ion exchange resins used in deionization processes can also cause bacterial growth and lead to bacteremia, which is a point worth noting.