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Cation exchange resin is mainly used to remove cations from natural water in water treatment systems. Since the cation exchange resin is located relatively early in the treatment system, the contamination it receives is different from that of the anion exchange resin. The polluted cation exchange resin usually reduces the periodic water production volume, the working exchange capacity decreases, and the effluent water quality deteriorates. It will also have adverse effects on the subsequent water production process of the anion exchange resin. Timely diagnosis and effective recovery of contaminated resin are of great significance to the operation of water treatment systems. 1 Introduction to the pollution mechanism Resin has a porous network three-dimensional structure. The porous network is a channel for ions to diffuse in and out of the resin. The inner wall of the channel has numerous functional groups, which are active points for the ion exchange reaction. Once this active point is covered, the ion exchange process cannot proceed. During the ion exchange process, substances such as ions or macromolecules with high exchange potential energy and strong adhesion are easily exchanged or adsorbed to the resin, but are difficult to elute during regeneration, thus hindering the progress of the ion exchange reaction or forming insoluble sediments during the ion exchange reaction, which are deposited inside the resin and block the ion exchange channels. 2 Different forms of contamination of cation exchange resin and solutions 2.1 Pollution caused by excessive coagulant In order to solve the problem of suspended solids in water, coagulant is usually added during pretreatment. Once the amount of coagulant added is inappropriate, it will cause pollution to the subsequent cation exchange resin. According to reports, when using epi-DMA (dimethylamine-epoxyhalohydrin) and poly-DADMAC (homopolymer of diallyldimethylamine chloride) as coagulants, if the effluent contains 1 mg/L of the above coagulants, it will cause serious contamination of the cation exchange resin, and it was found that coagulants with linear structures are more likely to contaminate the resin and can enter the interior of the resin particles. When the above-mentioned contamination occurs in the resin, if the degree of contamination is not very serious, methods such as increasing the backwash flow rate, extending the backwash time, or passing in compressed air can be used to revive it. If the degree of pollution is serious, the method of adding surfactants and dispersants can be used. Among them, surfactants can increase the hydrophilicity of the resin surface. ； The dispersant can ensure that the particles detached from the resin can be dispersed into the aqueous solution. According to reports, Rohm and Haas's nonionic surfactant TritonCF-54 and dispersant Orotan 731 have a good effect on solving this problem. Nalco Company used a resuscitation agent compounded with surfactants and dispersants to resuscitate the resin during the backwashing process of the contaminated resin layer, and achieved good results. If the cationic polyelectrolyte contaminates the cation exchange resin, it can also be treated with 4% sodium hydroxide solution to dissolve the polyelectrolyte to resuscitate the resin. 2.2 Contamination by iron ions Cation exchange resin is susceptible to contamination by iron ions, especially in water treatment systems that use well water as the water source. There are three different situations of resin contamination by iron ions. ①If the iron ions are present in colloidal suspension, they will leak through the filter and contaminate the cation exchange resin. ②Iron is exchanged to the resin in the form of ferrous iron ions, which are subsequently oxidized to ferric iron ions, thereby forming gelatinous water-insoluble iron hydroxide on the resin particles. ③The divalent iron ions that may be exchanged to the resin are directly converted into ferric ions on the exchange group of the resin, but they cannot be completely removed during the regeneration process and remain in the resin. If the first situation occurs, backwashing can be used to remove the accumulated colloidal suspension in the resin layer. If iron ions accumulate throughout the resin layer, the resin can be treated with sodium sulfite or sodium bisulfite, which reduces the ferric ions to the more soluble ferrous ions, which have less affinity for the resin than the former. The degree of contamination of the resin by iron and other ions can be determined by burning the resin or analyzing the iron content of the wet resin. See Table 1 and Table 2 for details. Table 1 The relationship between the mass ratio of elements in the burned ash of resin and the degree of resin pollution. The mass ratio of each element in the resin at different pollution levels/% is very low, medium, severe, very serious, extremely severe. Calcium 7 Magnesium 2.0 Aluminum 1.5 Iron 1.5 Barium 1.5 The mass of the total ash 13.5 The following method can also be used to determine whether the resin is contaminated by iron ions.: Table 2 Iron pollution degree classification Iron ion mass ratio/% Pollution degree 7.5 Extremely serious Note: The iron ion mass ratio in the table refers to the mass (mg) of iron ions per gram of resin. Wet resin is used as the analytical basis. Clean the contaminated resin with desalted water, soak it in 10% salt solution for 30 minutes, pour out the salt water, and then clean it with desalted water. Take out about one-tenth of the resin sample into the test tube, and then add 2 times the resin volume of 6 mol/L Hydrochloric acid solution, after sealing and shaking for 15 minutes, take out the acid solution and pour it into another cleaned test tube, and add a drop of saturated amine thiocyanide. From the depth of the Prussian blue generated (from light blue to opaque brown-black), the severity of iron contamination of the resin can be judged. One thing worth noting is that the iron ions in the water will form complex complexes with organic matter or silicon, and this complex is negatively charged, which can pass through the cation exchange resin and contaminate the anion resin behind it. 2.3 Contamination and recovery of organic matter Soviet scholars once conducted research on organic contamination of cation exchange resin. Studies have shown that dissolved organic matter in water is mainly adsorbed on cation exchange resin by van der Waals force. What is adsorbed at this time is basically organic matter with acidic groups, and these organic matter account for the main components of dissolved organic matter in water. There are not many research reports on the contamination of cation exchange resin by organic matter in the primary desalination system, but there have been reports on the contamination of cation exchange resin by organic matter in the condensate water treatment system. Harries once studied the contamination of the cation exchange resin by organic matter in the mixed bed of the condensate water treatment system. It was found that the decrease in the exchange capacity of the cation exchange resin is related to the low molecular weight polymers released by the anion exchange resin during operation. It is these low molecular weight polymers that contaminate the cation exchange resin. The effects of different forms of anion exchange resins in the mixed bed on the mass transfer coefficient of the cation exchange resin were measured. The details are shown in Table 3. In order to prove whether the nitrogen-containing low molecular weight polymer remaining in the anion resin contaminates the cation resin, Harries et al. once used X-ray light energy spectroscopy to analyze and test new and old resins. The results show that these low molecular weight polymers do contaminate the cation resin. For resins contaminated by organic matter, oxidizing agents such as H2O2 and Na2O2 can be used to decompose the organic matter adsorbed on the resin into water-soluble substances and peel them off from the resin. Table 3 Effect of anion resin on cation resin exchange capacity after mixed bed regeneration Cation resin Anion resin (initial form) Cation resin mass transfer coefficient/(10-4m·s-1) Sodium type cation resin Amine type cation resin Single cation bed 1.8 2.1 Mixed bed (1: 1) Macroporous resin (Cl-) 1.6 1.9 Mixed bed (1: 1) Macroporous resin (OH-) 1.5 1.8 Mixed bed (1: 1) Gel type resin (Cl-)