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Experimental content of ozone oxidation
1. The purpose of the experiment
Ozone can oxidize unsaturated organic matter in wastewater, and it can also open and partially oxidize aromatic hydrocarbon compounds, degrade some macromolecular organic matter into small molecules, and improve the biodegradability of wastewater. Ozone is extremely unstable and decomposes into oxygen at room temperature.
1. Understand the characteristics and master the method of measuring ozone concentration;
2. Through the treatment of dyeing wastewater, understand the basic process of ozone treatment of industrial wastewater.
2. Experimental instruments and equipment
Experimental instruments: ozone generator, gas scrubber, basic burette, measuring cylinder, conical flask, pH test paper, spectrophotometer, large beaker;
Experimental reagents: 2% KI solution, acetic acid, 0.005mol/L Na2S2O3, 1% starch solution, distilled water, disperse blue solution
Three, experimental principle
Ozone is a strong oxidant, and its oxidizing power is second only to fluorine among natural elements. Ozone can be used to deodorize, decolorize, sterilize, disinfect, reduce phenol, degrade COD, BOD and other organic matter in sewage treatment.
The strong oxidation of ozone in aqueous solution is not caused by ozone itself, but mainly caused by the intermediate products of ozone decomposition in water ·OH groups and HO2 groups. Many organic substances react easily with ozone. For example, ozone has a strong oxidative degradation effect on water-soluble dyes, proteins, amino acids, organic ammonia and unsaturated compounds, phenols and aromatic derivatives, heterocyclic compounds, lignin, humus and other organic substances; it also has strong bactericidal and disinfection effects. .
Advantages of ozone oxidation:
1. Ozone can oxidize other pollutants that are not easily handled by chemical oxidation and biological oxidation, and has significant effects on deodorization, decolorization, sterilization, and degradation of organic and inorganic substances;
2. After the sewage is processed, the remaining ozone in the sewage is easy to decompose, does not produce secondary pollution, and can increase the dissolved oxygen in the water;
3. Preparation of ozone using air as raw material, easy to operate.
In industry, high-voltage (1.5-3 million volts) high-frequency discharge is used to produce ozone. Usually, a mixed gas containing 1-4% ozone is produced, which is called ozonized gas.
1. The mechanism of ozone oxidation of organic matter:
1) Direct reaction mechanism-the reaction of ozone molecules directly attacking organic matter
(1) Open the double bond, and an addition reaction occurs: With the aid of its dipole structure and the unsaturated bond of the organic matter, an addition reaction occurs to form an intermediate product of ozonation and further decompose.
R1C=CR2+O3 is decomposed into R1GCOOH+R2C=O G stands for -OH, -OCH3, -OCOCH3
(2) Electrophilic reaction
(3) Nucleophilic reaction
Ozone molecular form
Four typical forms of resonance hybrid molecules
The resonance triangle structure of ozone molecule shows that it can be used as dipolar reagent, electrophile reagent and nucleophile reagent to form three kinds of reactions with organic matter.
2) Indirect reaction mechanism-ozone decomposes to form a reaction between free radicals and organic matter
Soluble ozone decomposes to form hydroxyl radicals, and HO · oxidizes dissolved inorganic and organic substances through different reactions.
(1) Decomposition mechanism of soluble ozone
HSB (Hoigne, Staehelin and Bader) mechanism
GTF (Gorkon, Tomiyasn and Futomi) mechanism
(2) The reaction between HO· and dissolved compounds
Electron transfer reaction-extract electrons from other substances and reduce themselves to OH-;
Hydrogen extraction reaction-extracting H from the different substituents of the organic substance to turn the organic substance into an organic substance free radical, which itself is converted into H2O;
OH·addition reaction — OH·addition to olefin or aromatic hydrocarbon double bond.
The mechanism of ozone decolorization is that ozone and the active free radical OH produced by ozone breaks the unsaturated bond (aromatic group or conjugated double bond) in the dye chromophore group to generate small molecular weight acids and aldehydes, resulting in low molecular weight organics , Resulting in a significant reduction in the chromaticity of the water body. Ozone has a fast decolorization rate for hydrophilic dyes and good effect; it has a slow decolorization rate for hydrophobic dyes and a poor effect, and requires a large amount of ozone. Ozone can oxidize inorganic colored ions such as iron and manganese as insoluble substances. The micro-flocculation effect of ozone also helps the coagulation of organic colloids and particulate matter, and removes color-causing substances through filtration.
2. Influencing factors of ozone oxidation performance-solution pH
The change of pH will change the mechanism and removal effect of ozone oxidation reaction
1) The oxidizing ability of ozone itself is related to the pH value
The decomposition rate of ozone in water increases with the increase of pH value. At pH<4, the decomposition of ozone in the aqueous solution is negligible, and the main reaction is the direct reaction of dissolved ozone molecules with reducing substances in the aqueous solution to be treated; At pH>4, the decomposition of ozone cannot be neglected. At higher pH, ozone is mainly decomposed into free radicals with high reactivity through a series of chain reactions under the catalysis of OH, which will affect reducing substances. Carry out non-selective oxidative degradation. If the pH is increased by one unit, ozone decomposes approximately 3 times faster.
2) The physical and chemical properties of organic or inorganic substances in sewage are closely related to the pH value
3) Ozone absorption rate has a certain relationship with pH value
The pH value changes during the entire ozone oxidation process, mainly under neutral or alkaline conditions, the pH value will decline with the oxidation process, the reason is that organic matter is oxidized into small molecular organic acids or aldehydes and other substances. The removal rate of pollutants under alkaline conditions is higher than that under acidic conditions
3. Influencing factors of ozone oxidation performance-solution temperature
Increasing the temperature of the reaction solution will reduce the activation energy of the reaction, which is beneficial to increase the rate of the chemical reaction. However, as the temperature increases, the decomposition of ozone will accelerate and the solubility will decrease, thereby reducing the concentration of ozone in the liquid phase and slowing down the rate of chemical reactions. At the same time, since the reaction of ozone to oxidize organic matter is a series of reactions, the oxidation intermediate product must be deeply oxidized while the organic matter is degraded, which consumes ozone in the liquid phase and slows down the degradation rate of the target organic matter. In order to be close to the actual industrial wastewater, the temperature range of the experiment is 3 to 30 degrees.
4. Influencing factors of ozone oxidation performance-catalyst
Base-catalyzed ozone oxidation: such as O3/H2O2, they degrade organic matter by catalyzing the production of OH through OH-; photocatalytic ozonation: such as O3/UV, O3/H2O2/UV, here it is important to note that UV has a certain wavelength ; Heterogeneous catalytic ozone oxidation: such as O3/solid catalysts (such as activated carbon, metals and their oxides, the manganese dioxide composite oxide jointly developed by the Chinese Academy of Coal Science and Beijing Tonglin, has a good catalytic effect, and the efficiency can be improved 1.5 times).
5. Problems in the application of ozone technology
When low-concentration ozone treats organic matter, it cannot be completely oxidized to carbon dioxide and water, but generates a series of intermediate products, such as aldehydes and carboxylic acids. The low solubility of ozone limits the application of ozone in water treatment. In ozone production, the air quality that enters the generator is required to be high, and ozone is corrosive. The equipment and pipelines are required to use corrosion-resistant materials or anti-corrosion treatment. ) The half-life in the air is 16 hours, and when the ozone concentration in water is 3 mg/L, the half-life is only about 30 minutes.
Thinking of experimental principle:
1. After the wastewater is reacted by ozone, will CODcr increase and why?
Answer: It may increase.
1) The role of ozone is to decompose macromolecular groups and turn some substances that could not be detected into dominant ones. Therefore, if the ozone dosage is small in the early stage of the action, or the mass transfer efficiency of ozone in the ozone contact pool is low, or the residence time is insufficient, it may affect the degradation effect of COD, making it relatively stable and difficult to degrade. Macromolecular substances (not showing COD) become easily degradable small molecules (significant COD). In this way, CODcr will increase. This phenomenon has been confirmed in many school experiments. There are always students who are very anxious and say: Your ozone generator has added COD to my wastewater!
2) Caused by experimental equipment, such as: the latex tube can be oxidized when inserted in water to increase the CODcr. In order to avoid this kind of phenomenon, it is necessary to use oxidation-resistant silicone tube or stainless steel tube for testing.
2. When some wastewater is treated with ozone, there will be a lot of foam, how to deal with it?
Answer: Some waste water will contain foaming agents. This type of test requires coagulation pretreatment with ferric chloride, aluminum sulfate or polyaluminum chloride, and then ozone treatment.
Fourth, the experimental steps
1. Determination of ozone concentration:
Use a graduated cylinder to take 250ml of 2% KI solution into the gas absorption flask, then pass in ozone for 90s, take 100ml of the reaction solution in an Erlenmeyer flask, acidify with acetic acid to pH=4, and use 0.005mol/L Na2S2O3 to drop to light yellow , Add 1% starch solution, continue titration until colorless, and record the volume of Na2S2O3 consumed by the titration.
2. Determination of ozone decolorization efficiency:
(1) Take two gas absorption bottles and fill them with 250ml direct blue solution and 2% KI solution respectively, and connect them with a rubber tube to pass ozone for 45s.
(2) Take the dispersion blue solution reaction solution and measure the absorbance at the maximum absorption wavelength of 580nm, calculate the overflow of ozone, and find the decolorization efficiency and absorption rate of ozone.
(3) Take 100ml of the KI solution after the reaction in an Erlenmeyer flask, acidify with acetic acid to pH=4, use 0.005mol/L Na2S2O3 to drop to light yellow, add 1% starch solution, continue titration until it is colorless, record the titration consumption The volume of Na2S2O3.
(4) Adjust the pH of the raw water to 3 and 10, and repeat (1) (2) (3).
Determine the concentration of ozone in water, using iodometric method
Ozone is first absorbed by KI solution, and I2 is generated and titrated with Na2S2O3 standard solution
1. Ozone absorption: Take 250mL of 2% KI solution in the absorption bottle, and let in ozone gas for 1.5 minutes.
2. Take 100ml of the ozone-absorbing KI solution (2%) solution in an Erlenmeyer flask, acidify with glacial acetic acid to adjust pH=4 and titrate with 0.005mol/L Na2S2O3 standard solution to light yellow, then add 1% starch indicator. When the solution is blue, it is quickly titrated until the blue disappears as the end point.
Note: For colored solutions with different pH values, pay attention to the relationship between the ozone absorption of the solution and the decolorization rate.