Reaction
When chlorination is applied, liquid chlorine or gaseous chlorine is added to the water, which rapidly undergoes a hydrolysis reaction and generates hypochlorous acid (HOCl), which is ionized into hypochlorite ions (OCl-) in the water. Hypochlorous acid and hypochlorite ion are strong oxidizing agents. The oxidizing properties of hypochlorous acid in the molecular state are stronger than those of hypochlorite ion in the ionic state. The ionization degree of hypochlorous acid increases with the increase of pH, when the pH is less than 2, the chlorine in water exists in molecular state; when the pH is 3-6, hypochlorous acid is dominant; when the pH is more than 7.5, hypochlorite ion is dominant; when the pH is more than 9.5, all hypochlorite ion. Therefore, the chlorination method is theoretically most effective in aqueous solutions with a low neutral pH.
The use of various hypochlorite as an oxidizing agent is to use it in aqueous solution ionization and hydrolysis of the formation of hypochlorite ions and hypochlorous acid oxidation properties. Chlorination of cyanide-containing wastewater is a practical and typical example of wastewater treatment. Because the cyanide is combined with *** valence bond, the bond energy is up to 225 kcal/mol, so it is not easy to decompose, and thus often use the strong oxidation method to promote its decomposition and destruction. In practice, alkaline chlorination is generally used. When using liquid chlorine or chlorine gas, the basic ionic reaction formula is as follows:
Partial oxidation:
CN-+HOCl-→CNCl+OH- (1)
CNCl+2OH-→CNO-+Cl-+H2O (2)
Complete oxidation:
2CNO-+3OCl-+H2O-→2CO2+N2+3CO2+N2+3CO2+N2+3CO2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N2+N3 2CO2+N2+3Cl-+2OH- (3)
Reaction (1) occurs at any pH and is almost instantaneous. In order for the toxic cyanogen chloride (CNCl) to be converted to cyanate in time for reaction (2), the pH of the wastewater needs to be adjusted above 10.5, under which conditions the reaction can be completed in a few minutes. Although the toxicity of the cyanate formed in the partial oxidation stage is only one thousandth of the original cyanide, however, it is usually necessary to further oxidize and decompose the cyanate into nitrogen and carbon dioxide according to the reaction (3), and it takes about 10-15 minutes to complete the complete oxidation reaction if the pH value of the wastewater is maintained at 7.5 to 8.0.
Chlorination is also widely used to treat phenol-containing wastewater, but it is not the perfect treatment method because of the high consumption of chlorine and the tendency to form chlorophenols, which release a strong odor. Under the condition of low pH, phenol can not be completely destroyed, it is easier to form chlorophenol. For this reason, before chlorination must be adjusted with lime pH, so that the pH of the chlorinated water is 7 to 10.
Chlorine in many kinds of industrial wastewater treatment is not only oxidizing agent, but also can affect the charge of colloidal particles, promote flocculation, improve the efficiency of particle precipitation and oil floating. Wool rinsing wastewater treatment with chlorination can destroy the emulsifiers in the wastewater, so that the suspended solids and emulsified fatty acids precipitate. After pretreatment by chlorination, the emulsion of wool oil and grease is rapidly separated, which can remove 80-90% of BOD, 95% of suspended solids and oil. This method is more expensive with large chlorine dosage, but 70% of the oil and grease can be recovered.
Industrial wastewater containing large amounts of ammonia or proteins, amino acids and other organic nitrogen compounds, chlorination will form chloramines or the corresponding organic derivatives, so that the consumption of chlorine is very large. In this way, the chlorination method is not economical.
In municipal wastewater treatment, a small amount of chlorine is often used to pre-chlorinate the sewage. Post-chlorination of effluent from sewage treatment plants. Pre-chlorination can prevent corrosion of sedimentation tanks and other treatment equipment, promote flocculation and precipitation, inhibit the use of activated sludge treatment of sewage in the process of filamentous bacteria and fungal reproduction, to avoid sludge expansion, and can prevent the formation of hydrogen sulfide, control the odor of the entire treatment plant. It also helps sludge digestion by preventing the formation of acid and foam in the digester. Post-chlorination sterilizes and reduces BOD. this treatment often works for industrial wastewater as well.
Chlorine dioxide (ClO2) is the product of the reaction between sodium chlorite and chlorine gas or hydrochloric acid.
2NaClO2+Cl2─→2ClO2+2NaCl
5NaClO2+4HCl─→4ClO2+5NaCl+2H2OIn order for the reaction to be complete, the dosage of hydrochloric acid and chlorine must be more than 2.5 times the theoretical value and 1.0 to 1.5 times the theoretical value respectively. Chlorine dioxide oxidizing ability in acidic solution exceeds that of chlorine, it can react quickly in a wide pH range compared with chlorine, it is most effective in killing spores and suitable for treating hospital sewage; wastewater such as phenol and nitrogen-containing compounds, it will not form chlorophenols, chloramines and other derivatives. Chlorine dioxide maintains a residual amount in water for a shorter time than chlorine and longer than ozone. It has a strong oxidative degradation of phenol and can be used to treat phenol-containing wastewater.
Commonly used oxidizing agents
①Chlorine, gaseous chlorine, liquid chlorine, sodium hypochlorite, calcium hypochlorite, chlorine dioxide, etc.;
②Oxygen, oxygen in the air, ozone, hydrogen peroxide, potassium permanganate and so on.
Oxidant selection
Selection of oxidants should take into account:
① specific pollutants in the wastewater has a good oxidation;
② reaction should be harmless or easy to separate from the wastewater after the product;
③ cheap, convenient source;
④ faster reaction at room temperature;
⑤ reaction does not require the reaction;
>The reaction is not necessary. p> ⑤ reaction does not need to greatly adjust the pH value, etc..
Oxidation treatment method can deal with almost all industrial wastewater, especially suitable for the treatment of wastewater difficult to be biodegradable organic matter, such as the vast majority of pesticides and insecticides, phenol, cyanide, and cause chromaticity, odor and other substances.