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About the removal principle and influencing factors of residual chlorine in water

by:BOQU     2023-03-16
Residual chlorine in water refers to total residual chlorine and free residual chlorine. Free residual chlorine is a very important disinfectant index in the domestic water sanitation standard, and it is also one of the daily necessary testing items in water plants. In water treatment, in order to prevent the growth of bacteria in water, fungicides are often added to the water. Chlorine is commonly used, and a certain excess amount is maintained. This is residual chlorine. Residual chlorine can be divided into free residual air and combined residual chlorine. , which refers to free residual chlorine. In industrial water treatment, the removal of residual chlorine in water is mainly required by subsequent treatment devices. In order to prevent the ion exchange resin from being oxidized, the subsequent ion exchange system requires the residual chlorine content in the influent to be less than 0.1mg/L. The subsequent reverse osmosis device In order to prevent the reverse osmosis membrane from being oxidized, the residual chlorine in the influent is required to be zero (composite membrane) or less than 0.3mg/L (acetate cellulose membrane). There are currently two ways to remove residual chlorine in water. One is to add certain chemicals to the water, such as NaHSO3, and the other is to pass the water through a granular fruit shell activated carbon filter. Both methods are currently in use. 1. The principle of removing residual chlorine in water It is generally believed that the dechlorination process is a comprehensive process of adsorption, catalysis and reaction of chlorine and carbon. Adsorption is the same as activated carbon adsorption of organic matter in water, except that the adsorbate molecules are smaller than the organic matter molecules. The reaction of chlorine and carbon means that residual chlorine exists in the form of hypochlorous acid in water, which undergoes a chemical reaction on the surface of carbon, and activated carbon acts as a reducing agent to reduce hypochlorous acid to chloride ions: Cl2+HO→HOCI+HCI In acidic or medium Under neutral conditions, residual chlorine mainly exists in the form of HOCI. When HOCl encounters activated carbon, it will oxidize the activated carbon, and generate oxides (or CO, CO2) on the surface of the shell activated carbon, and HOCl is reduced to h+ and cl-. After the water passes through the activated carbon filter bed, the residual chlorine in the water can be completely removed, and the residual chlorine in the filtered water can be close to zero. 2. Types of products for removing residual chlorine in water Select activated carbon for removing residual chlorine in water. Granular activated carbon or powdered activated carbon can be used. The flow rate of the filter bed can be designed as 20m/h, mainly because the activated carbon removes residual chlorine faster. Regarding the selection of activated carbon for removing residual chlorine, its physical properties are the same as the selection of activated carbon for adsorbing organic matter. There are three methods for selecting its adsorption performance: The first method is general adsorption according to the specific surface area of ​​activated carbon, iodine value, and carbon tetrachloride adsorption value. The main reason is that the chlorine molecule adsorbed by activated carbon is small, which is similar to the molecular size of iodine. It can enter the micropores of activated carbon and give full play to the role of all surfaces of activated carbon in participating in the adsorption. Therefore, the activated carbon with high specific surface area and iodine value is selected, and the adsorption performance of residual oxygen is also good. The second method is to measure the adsorption isotherm of activated carbon for residual chlorine, and select activated carbon with high adsorption capacity. The determination method of the adsorption isotherm is the same as that of the aforementioned activated carbon for the determination of the adsorption isotherm of organic matter in water. The third method is to measure the semi-dechlorination value of activated carbon to remove residual chlorine in water. The so-called semi-dechlorination value means that water containing residual oxygen passes through an activated carbon adsorption column to determine when the residual chlorine concentration in the effluent is just equal to the residual oxygen concentration in the influent water. Half of the required carbon layer height (cm) is the half-dechlorination value. According to regulations, activated carbon with a half-dechlorination value of less than 6cm is better for dechlorination. 3. Factors affecting the removal of residual chlorine (1) Although the particle size of activated carbon becomes smaller, it has little effect on the specific surface area of ​​activated carbon (about 0.02%), but the smaller particle size makes more pores inside open to the liquid phase, Facilitate the adsorption and reaction of residual chlorine. (2) pH Since the pH of water affects the form of residual chlorine in water, it also affects the effect of activated carbon on removing residual chlorine. Residual chlorine in water mainly refers to Cl2, HOCl, and OCl-, and the ratio of the three changes with the change of pH. The removal rate of activated carbon for molecular HOCl is faster than that of ionic OCI-, so low pH has a great effect on the removal of residual chlorine in water by activated carbon. favorable. (3) Temperature The increase in temperature is beneficial to the dechlorination of activated carbon. This law is different from the physical adsorption law of activated carbon, which also shows that the dechlorination process of activated carbon cannot be regarded as a pure physical adsorption process. (4) Effect of water turbidity High water turbidity may block some activated carbon pores, thus hindering the diffusion of residual chlorine molecules to the active surface, thus reducing the dechlorination speed.
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