Printing and dyeing wastewater treatment decolorization method summary: printing and dyeing wastewater decolorization is mainly to remove the chroma of wastewater, that is, dye molecules and COD, and there are five widely used decolorization methods: adsorption decolorization, flocculation decolorization, oxidation decolorization, biological decolorization and electrochemical decolorization.
1.Adsorption decolorization
Adsorption decolorization technology relies on the adsorption of adsorbents to remove dye molecules. Commonly used adsorbents include renewable adsorbents such as activated carbon, ion exchange fiber, etc., and non-renewable adsorbents such as various natural minerals (bentonite, diatomite), industrial waste (cinder, fly ash) and natural waste (charcoal, sawdust). At present, the adsorbents used for adsorption and decolorization mainly rely on physical adsorption, but ion exchange fiber, modified bentonite and so on also have chemical adsorption.
Activated carbon is the first solid adsorbent with industrial applications and the most well-studied. Activated carbon has many micropores, insufficient macropores and strong hydrophilicity, which limits the internal diffusion of macromolecules and hydrophobic dyes, and is suitable for the decolorization of water-soluble dyes with molecular weight of less than 400, and has poor decolorization effect on macromolecules or hydrophobic dyes. Due to the great difference in intermolecular dipole and deformability (the main factor that determines the induced dimaximum), the physical adsorption also shows a certain selectivity, such as the decolorization rate of activated carbon for alkaline dye wastewater is more than 90%, while the decolorization rate for acidic dye wastewater is only 30% to 40%. As a widely used flocculant in water treatment, bentonite has been widely used in the field of decolorization of printing and dyeing wastewater, and a variety of composite and modified bentonite has been further developed recently [37]. At present, ion exchange fibers have attracted wide attention, which are mainly used for adsorption of heavy metals and pigments [38] and are larger than the surface, faster than the ion exchange rate, easy to regenerate, and have good decolorization effect on refractory reactive dye wastewater. Some adsorbents such as diatomite composite water purification agent have also been developed. Modified fly ash with flocculation performance is made from power plant fly ash, which has high decolorization rate for both hydrophobic and hydrophilic dye wastewater.
2.Flocculation decolorization
The flocculation and decolorization technology of printing and dyeing wastewater is a widely used decolorization technology with low investment cost, small equipment area and large processing capacity. The mechanism of flocculation and decolorization of printing and dyeing wastewater is based on the theory of colloidal chemistry. As far as inorganic flocculants are concerned, hydrolysis and polymerization of iron series and aluminum series flocculants produce high-priced polyhydroxyl cations, which are compressed with colloid in water to double electric layer, electric neutralization and instability, adsorption bridge, and supplemented by sedimentation net trapping and rolling, precipitation removes the generated coarse floc, so as to achieve decolorization. For organic polymer flocculants, in addition to electric neutralization and bridging, there may also be flocculation mechanisms similar to chemical reaction bonding. The modification of inorganic polymer flocculants and the introduction of inorganic acid groups or organic functional groups with complexing ability have gradually become a new trend of decolorization of water-soluble dye wastewater.
The decolorization mechanism of inorganic polymer flocculants is different from that of low molecular inorganic flocculants, and the development of new flocculants is also one of the ways to remove hydrophilic dyes, such as polysilicate flocculants which have become one of the hot spots recently. At the same time, organic polymer flocculants are developing rapidly, such as the removal rate of turbidity and chroma of starch modified cationic flocculants are more than 90%.
Some substances can react with dye molecules, mask or even interrupt the hydrophilic groups of dyes or destroy the hair color structure of dye molecules, reduce the water solubility of dye molecules, and make them become hydrophobic molecules or ions. Some metal ions with empty orbitals, such as Mg2+, Fe2+, Ca2+, can accept lone pair electrons, and can be complexed with dye molecules containing lone pair electrons to form complex macromolecules, so that dye molecules have colloidal properties and are easy to be removed by flocculation. Some organic molecules can also form complexes with dye molecules to reduce the water solubility of dye molecules, such as long-chain cationic surfactant dodecyl dimethyl ammonium chloride to water-soluble dye wastewater containing sulfonic acid groups.
In recent years, it has been found that oxidation can also promote flocculation, and the mechanism is that the organic molecules have a certain degree of coupling under the action of oxidizing agents or oxidizing agents interrupt the hydrophilic groups of dye molecules. For printing and dyeing wastewater containing cationic dyes, inorganic flocculants represented by iron and aluminum are basically ineffective for decolorization, because the polyhydroxyl cations generated by the hydrolysis of these inorganic flocculants have the same charge as the complex dye cations in the water. Due to the reason of homosexual repulsion, all flocculants, including inorganic flocculants, that rely on the flocculation of cations for flocculation and decolorization have the same charge. Most positive polymer flocculants are naturally powerless against cationic dyes. If the dye cation in water can be converted into anion or neutral molecules in some way, it can be removed by inorganic flocculants or cationic polymer flocculants. It has been reported that the removal rate of cationic dyes has been greatly improved by γ-ray radiation flocculation process abroad. Whether oxidation or γ-ray radiation flocculation process, the cationic dye is turned into neutral or negative, and then further processed to obtain good decolorization effect.
3.Oxidation decolorization
The unsaturated double bond of the chromophore group in the dye molecule can be broken by oxidation to form organic matter or inorganic matter with small molecular weight, so that the dye loses its chromophore ability. The oxidation method includes chemical oxidation, photocatalytic oxidation and ultrasonic oxidation. Although the specific process is different, the decolorization mechanism is the same. Chemical oxidation is a relatively mature method. Oxidants generally use Fenton reagent (Fe2+-H2O2), ozone, chlorine gas, sodium hypochlorite and so on.
The formation of H2O2 was catalyzed by Fenton reagent at pH4 ~ 5. OH can oxidize and decolorize the dye, and the new ecological Fe2+ produced can also promote coagulation. Treating printing and dyeing wastewater with iron scrap H2O2 can produce a new ecological Fe2+ at pH1 ~ 2, and its hydrolyzed products have strong adsorption flocculation, which can remove more than 99% of the color of nitrophenol and anthraquinone printing and dyeing wastewater. When iron powder H2O2 is used to decolorize printing and dyeing wastewater, the decolorization effect is excellent when the iron powder content is 1g/L, H2O2 is 1mmol/L and pH2 ~ 3. Photocatalytic oxidation method uses some substances (such as iron complexes, simple compounds, etc.) under the action of ultraviolet light to produce free radicals and oxidize dye molecules to achieve decolorization. Such as methylene blue solution and wool dyeing and finishing wastewater photocatalytic decolorization and degradation; Ferrous oxalic acid, ferric citric acid or ferric malic acid complex was used as catalyst to decolorize printing and dyeing wastewater under UV irradiation and pH2 ~ 4. Ultraviolet light can also enhance the decolorization effect of diazo dyes. Iron oxalate complex can be used to photodissociate reactive brilliant red X-3B, and its photolysis mechanism has been fully discussed. Ultrasonic treatment of printing and dyeing wastewater is based on the fact that ultrasonic can produce local high temperature, high pressure and high shear force in the liquid, induce the cracking of water molecules and dye molecules to produce free radicals, trigger various reactions and promote flocculation. The acid red B water with concentration of 44.4mg/L was degraded by ultrasonic technology. When NaCl was added about 1g/L and treated for 50min, the decolorization rate of acid red B wastewater was nearly 90%.
In short, oxidation is an excellent decolorization method for printing and dyeing wastewater, but if the oxidation degree is insufficient, the chromophore group of dye molecules may be destroyed and decolorized, but the COD in it is still not removed. If the dye molecules are fully oxidized, the energy and drug dose consumption may be too large and the cost is too high, so the oxidation method is generally used for oxidative flocculation or flocculation oxidation process. The oxidation flocculation process is used to change water-soluble dye molecules into hydrophobic or cationic dye molecules into neutral and negative molecules by oxidation to facilitate flocculation removal. On the contrary, the flocculation oxidation process uses oxidation as a post-treatment step to further treat printing and dyeing wastewater to further remove residual chroma and COD.
4.Biological decolorization
Biological decolorization is the use of microbial enzymes to oxidize or reduce dye molecules to destroy their unsaturated bonds and color groups. Decolorizing microorganisms are specific to dyes, and the degradation process is completed in two stages, first adsorption and enrichment of dye molecules, and then biodegradation. Through a series of life activities such as oxidation, reduction, hydrolysis and combination, dye molecules are finally degraded into simple inorganic substances or transformed into various nutrients and protoplasm.
Subtle structural changes of dye molecules will greatly affect the decolorization rate. For example, some algae have a high decolorization rate for dyes containing -- OH and -- NH2, but can hardly degrade dye molecules containing -- CH3, -- OCH3 and -- NO2. The concentration of dye also has a certain effect on the decolorization rate, and the high concentration of dye will inhibit the microbial activity and affect the decolorization rate or the decolorization effect. Microorganisms regulate dye decolorization with different structures through in-vivo plasmids. The effective way to improve the application value of decolorizing microorganisms is to screen or construct multi-functional super strains and improve the biodegradability of dyes [44], and vigorously develop biological flocculants with broad spectrum flocculation activity.
Aerobic process is a common treatment process, but due to the strong resistance to biodegradation of dye molecules, the BOD5/COD ratio of the treatment process is reduced (biochemical deterioration), resulting in the ordinary aerobic process on the wastewater chroma, COD removal rate is not high (60% ~ 70%). By adding Fe (OH) 3 to the aeration tank, extending the residence time of refractory substances in the system and other measures, the activated sludge concentration of the aeration tank can be greatly increased, the sludge load and the organic matter degradation amount borne by the bacteria per unit number can be reduced, and the decolorization rate and COD removal rate of the system can be improved. The application of immobilized cell technology to aerobic technology can also achieve good results. Anaerobic and aerobic treatment process can make up for the deficiency of aerobic process to a certain extent. The refractory dye molecules and their auxiliaries are hydrolyzed and acidified under the action of anaerobic bacteria and decomposed into small molecular organic matter, and then decomposed into inorganic small molecules by aerobic bacteria.
In short, the decolorization rate and COD removal rate of printing and dyeing wastewater treated by biological method are not high, and the reaction time is long, which is generally not suitable for single application, and can be used as a pre-treatment or advanced treatment step. At present, the key to biological decolorization is to screen efficient degrading bacteria and construct strains with degradation ability and flocculation activity, so that degradation, flocculation and decolorization can be completed in a short time to improve treatment efficiency and reduce costs, and actively explore pre-treatment methods for dye molecules or printing and dyeing wastewater, such as electrolysis, small dose oxidation, etc., to improve the biochemical degradability of printing and dyeing wastewater.
5.Electrochemical decolorization
Electrochemical method is to purify printing and dyeing wastewater by electrode reaction. According to the electrode reaction method, the electrochemical method can be subdivided into internal electrolysis method, electric flocculation and electric float method, and electric oxidation method. The most famous internal electrolysis method is the iron filings method, which is to use cast iron filings as a filter material to immerse or pass the printing and dyeing wastewater, and use the potential difference between Fe and FeC and the solution to generate electrode reaction and produce a new ecological H with high chemical activity, which can REDOX reaction with various components of printing and dyeing wastewater and destroy the color structure of the dye, and the new ecological Fe2+ produced by the anode. The hydrolyzed products have strong adsorption and flocculation. In order to further improve the treatment effect of the traditional iron filings method, the number of microbatteries in printing and dyeing wastewater was increased by modifying the iron filings or adding auxiliary fillers to the iron filings, or the residence time of dye particles in the iron filings was extended, and the chroma and COD removal of insoluble dyes were increased by 20% ~ 30% by the modified iron filings method.
Fe and Al are used as the anode, and H2 produced by the cathode is used to float the floc, which is called electrical float method. The use of Fe2+ and Al3+ generated by the electrode reaction to achieve flocculation and decolorization is called electroflocculation. Because pulsed electrical signal is applied to make the electrode reaction intermittent, overpotential and diffusion resistance can be reduced, thus reducing energy consumption and iron consumption; Similarly, when alternating current is applied, cations can be produced at both poles, which is more conducive to the interaction of metal ions and colloids, and the polarity of the poles often changes, which is also good for preventing electrode passivation, so the recent development of electric flocculation method is pulsed electric flocculation and alternating current flocculation. The electroflotation process using activated carbon fiber as electrode realizes the process of adsorption-electrode reaction-flocculation and desorption by using the comprehensive properties of electrode conduction, adsorption, catalysis, REDOX and air flotation. Using graphite and titanium plates as plates, NaCl, Na2SO4 or the original salt in water as conductive medium, the dye wastewater is electrolysed with electricity, the anode produces O2 or Cl2, the cathode produces H2, and the dyeing wastewater is decolorized through the oxidation of oxygen atoms and the reduction of hydrogen atoms. Using activated carbon as electrode, the dye molecules are enriched by its adsorption properties, and the chromophore group is oxidized under the action of external electric field. The decolorization rate can reach more than 98% and the COD removal rate can reach more than 80%. Further improving the catalytic performance of the electrode material, improving the current efficiency, and weakening the electrode polarization to reduce energy consumption are still the main direction in the future.
