The unsaturated double bonds of the chromatic group in the dye molecule can be oxidized and disconnected to form organic or inorganic substances with a small molecular weight, so that the dye loses its ability to develop color. Oxidation methods include chemical oxidation, photocatalytic oxidation, and ultrasonic oxidation. Although the specific processes are different, the decolorization mechanism is the same. Chemical oxidation is a relatively mature method at present. The oxidizing agent generally uses Fenton reagent (Fe2+-H2O2), ozone, chlorine, sodium hypochlorite and so on. Fenton reagent was used to catalyze H2O2 formation at pH4~5· OH, the dye oxidation and decolorization, the resulting new ecological Fe2+ also has a pro-coagulation effect.
The treatment of printing and dyeing sewage with iron filings-H2O2 can generate new ecological Fe2+ at pH1~2, and its hydrolysis products have a strong adsorption flocculation effect, which can remove more than 99% of the color of nitrophenols and anthraquinone printing and dyeing sewage; When using iron powder -H2O2 to print and dye decolorization, when the iron powder content is 1g/L, H2O2 is 1mmol/L, pH2~3, the decolorization effect is excellent. Because the surface of activated carbon contains a large number of acidic groups such as hydroxyl, phenolic hydroxyl and alkaline groups, it has both adsorption capacity and catalytic effect on dye molecules.
H2O2 can liberate O or generate on the surface of activated carbon· OH, so that the dye molecules adsorbed on the surface of activated carbon rapid oxidation, reduce the color and COD of printing and dyeing sewage. Some authors used Fenton reagent to treat the production waste liquid of the dye intermediate H acid, and found that the SO42- content in the waste liquid increased rapidly with the increase of H2O2, indicating that the sulfonic acid group (hydrophilic group) was oxidized and removed from the dye intermediate, and the result was to increase the hydrophobicity of the molecule, which was extremely beneficial to flocculation and decolorization. The use of small doses of Fenton reagents to treat phenol-containing sewage, so that the organic pollutants in the sewage polymerize to change its water solubility, is also conducive to flocculation and decolorization. For the decolorization of water-soluble dye sewage containing more hydroxyl groups, it can also be prepared in the same way.
Photocatalytic oxidation method uses certain substances (such as iron complexes, simple compounds, etc.) to generate free radicals under the action of ultraviolet light, and oxidize dye molecules to achieve decolorization. TiO2 photocatalytic oxidation produces O and · at pH3~11 OH, so that the dye molecules decompose rapidly and obtain a good decolorization effect, such as methylene blue solution and photocatalytic decolorization and degradation of wool spinning dyeing and finishing sewage; Using iron-oxalic acid, iron-citric acid or iron-succinate complex as catalysts, under ultraviolet light irradiation and pH2~4, the decolorization experiment of printing and dyeing sewage is carried out, and the iron-carboxylic acid complex can generate alkyl, hydroxyl and other free radicals to make the printing and dyeing sewage oxidative and decolorized; Ultraviolet light also enhances the decolorization effect of O3 on diazo dyes. Iron-oxalate complexes can be used for photolysis of active bright red X-3B, and their photolysis mechanisms have been fully discussed. Ultrasonic treatment of printing and dyeing sewage is based on ultrasonic waves that can produce local high temperature, high pressure and high shear force in the liquid, induce water molecules and dye molecules to crack and produce free radicals, trigger various reactions and promote flocculation. The degradation concentration of 44.4mg/L acid red B sewage was degraded by ultrasonic technology, and the decolorization rate of acid red B sewage was nearly 90% when naCl was added to about 1g/L and treated for 50min.
In short, the oxidation method is an excellent method of decolorization of printing and dyeing sewage, but if the degree of oxidation is insufficient, the chromophore of the dye molecule may be destroyed and decolorized, but the COD in it has not been removed; If the dye molecules are fully oxidized, the energy and drug dosage consumption may be too large and the cost is too high, so the oxidation method is generally used for oxidation-flocculation or flocculation-oxidation processes. The purpose of the oxidation-flocculation process is to turn the water-soluble dye molecules into hydrophobic or the cationic dye molecules by oxidation.