Why is the cation resin placed in front and the anion resin behind in the desalinated water system?

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In demineralization systems, whether it is the conventional double-bed process consisting of an anion exchange bed + carbon remover + cation exchange bed, or the pre-demineralization process that involves reverse osmosis followed by a mixed-bed, the design of ion exchange equipment follows one fundamental rule: water must first pass through the anion resin and then through the cation resin; the order cannot be reversed. This is not a arbitrarily determined sequence of processes; rather, it is determined by the chemical properties of ion exchange, resin protection, operational economy, and the quality of the effluent water. 1. To prevent scaling on the surface of anion resins, raw water contains large amounts of Ca²⁺ and Mg²⁺; if the water comes into contact with the anion resins (OH-type) first, these resins will release OH⁻, causing the local pH in the water to rise sharply to 11–12. At this point, a typical precipitation reaction occurs: Ca²⁺ + 2OH⁻ → Ca(OH)₂↓; Mg²⁺ + 2OH⁻ → Mg(OH)₂↓. At 25°C, the solubility product of Mg(OH)₂ is only 5.6×10⁻¹²; even if the concentration of Mg²⁺ in water is only 1 mmol/L, significant precipitation begins as long as the pH is greater than 10.4. These hydroxide precipitates will cover the surface of the anion resin, blocking the exchange pores and causing irreversible \"scaling poisoning,\" leading to a sharp increase in pressure difference and a significant loss of exchange capacity. By placing the cation resin at the front, all cations in the water are first exchanged for H⁺, resulting in highly acidic effluent (with a pH typically ranging from 2.4 to 3.0). At this point, Ca²⁺ and Mg²⁺ have been completely removed, and the water entering the anion bed contains almost no hardness components, thereby completely eliminating the possibility of scaling on the anion resin. 2. The load on the anion exchange bed is significantly reduced, resulting in a 40%～60% improvement in economic efficiency. The anions in the raw water consist of strong acid radicals (Cl⁻, SO4²⁻, NO3⁻) and weak acid radicals (HCO3⁻, HSiO3⁻), with HCO3⁻ typically accounting for the highest proportion. If the quality of certain surface water is as follows: total cations: 5.0 mmol/L (Ca²⁺ 2.5, Mg²⁺ 1.5, Na⁺ 1.0); anions: Cl⁻ 1.5 mmol/L, SO₄²⁻ 1.0 mmol/L, HCO₃⁻ 2.5 mmol/L. After passing through the cation exchange resin, HCO₃⁻ combines with H⁺ to form large amounts of H₂CO₃, which then decomposes into free CO₂: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂↑ + H₂O. The 2.5 mmol/L of HCO₃⁻ will produce 2.5 mmol/L of CO₂, resulting in a free CO₂ concentration in the water of approximately 110 mg/L. If not removed, these CO₂ will revert to HCO₃⁻ upon entering the anion bed, where they are exchanged by the strong-base anion resin, consuming a large amount of exchange capacity. By installing a decarburizer, CO₂ in water can be stripped down to below 5 mg/L, with a removal rate of over 95%. Comparison of the actual anion load that needs to be exchanged in the anion bed at this point: Without carbon removal: all of Cl⁻, SO4²⁻, and HCO3⁻ need to be exchanged → total anions of 5.0 mmol/L; After carbon removal: only Cl⁻ (1.5) + SO4²⁻ (1.0) + residual CO need to be exchanged₂