Decolorization technology of printingdyeing wastewater will become the mainstream

Adsorption decolorization technology relies on the adsorption of adsorbent 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), etc. At present, the adsorbents used for adsorption and decolorization mainly rely on physical adsorption, but ion exchange fiber and modified bentonite also have chemical adsorption.

Activated carbon is the first solid adsorbent that has been applied in industry and studied most thoroughly. Activated carbon has many micropores, insufficient large and medium pores and strong hydrophilicity, which limits the internal diffusion of macromolecules and hydrophobic dyes. It is suitable for the decolorization of water-soluble dyes with molecular weight no more than 400, and has poor decolorization effect on macromolecules or hydrophobic dyes. Due to the great difference between intermolecular dipole and deformability (the main factor determining the size of induced dipole), the physical adsorption also shows a certain selectivity. For example, the decolorization rate of activated carbon for alkaline dye wastewater is more than 90%, while that for acid dye wastewater is only 30% ~ 40%. As a flocculant widely used in water treatment, bentonite has been widely used in the field of decolorization of printing and dyeing wastewater. Recently, a variety of composite and modified bentonite have been further developed [37]. At present, ion exchange fiber is widely noticed. It is mainly used to adsorb heavy metals and pigments [38]. It has large specific surface, fast ion exchange speed and easy regeneration. It has a good decolorization effect on refractory reactive dye wastewater; Some adsorbents integrating adsorption and flocculation functions, such as diatomite composite water purifying agent, have also been developed [39]. Modified fly ash with flocculation performance is made from fly ash of power plant, which has a high decolorization rate for hydrophobic and hydrophilic dye wastewater [40].

Flocculation decolorization technology of printing and dyeing wastewater is a widely used decolorization technology with low investment cost, less equipment occupation and large treatment capacity. The flocculation decolorization mechanism of printing and dyeing wastewater is based on the theory of colloidal chemistry. As far as inorganic flocculants are concerned, it is the hydrolysis and polymerization of iron series, aluminum series and other flocculants to generate high valence polyhydroxycations, compress the double electric layer with the colloid in water, neutralize and destabilize, adsorb and bridge, supplemented by sediment net capture and sweeping, so as to precipitate and remove the generated coarse flocs, so as to achieve the purpose of decolorization. For organic polymer flocculants, in addition to electric neutralization and bridging, there may be a flocculation mechanism similar to chemical reaction bonding. The modification of inorganic polymer flocculant and the introduction of inorganic acid radical or organic functional group with complexation ability have gradually become a new trend of decolorization of water-soluble dye wastewater.

The decolorization mechanism of inorganic polymer flocculant is different from that of low molecular inorganic flocculant. The development of new flocculants is also one of the ways to remove hydrophilic dyes, such as polysilicate flocculant, which has become one of the hotspots recently. At the same time, organic polymer flocculants are developing rapidly. For example, the removal rates of turbidity and chromaticity by starch modified cationic flocculants are more than 90% [41].

Some substances can react with dye molecules, mask or even interrupt the hydrophilic groups of dyes, or destroy the chromogenic structure of dye molecules, reduce the water solubility of dye molecules and turn them into hydrophobic molecules or ions. Some metal ions with empty orbits, such as Mg2 +, Fe2 +, Ca2 +, can accept lone pair electrons and complex with dye molecules containing lone pair electrons to form macromolecules with complex structure, 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 the effect of long-chain cationic surfactant dodecyl dimethyl ammonium chloride on water-soluble dye wastewater containing sulfonic acid groups [42].

In recent years, it has been found that oxidation can also promote flocculation. The mechanism is that organic molecules are coupled to a certain extent under the action of oxidants [43] or oxidants interrupt the hydrophilic groups of dye molecules [44]. For the printing and dyeing wastewater containing cationic dyes, the inorganic flocculants represented by iron series and aluminum series are basically ineffective for decolorization, because the polyhydroxycations generated by the hydrolysis of these inorganic flocculants have the same charge as the complex dye cations in the water. Due to the same-sex repulsion, the flocculants that rely on the aggregation and sedimentation of cations for flocculation and decolorization include inorganic flocculants and most positive polymer flocculants, There is nothing natural to do with cationic dyes. If the dye cations in water can be converted into anions or neutral molecules in some way, they can be removed by inorganic flocculant or cationic polymer flocculant. It is reported that it is adopted abroad γ X-ray radiation flocculation process greatly improves the removal rate of cationic dyes. Whether oxidized or γ X-ray radiation flocculation process is to change cationic dyes into neutral or negative, and then further treatment to obtain good decolorization effect.

The unsaturated double bonds of chromogenic groups in dye molecules can be oxidized and broken to form organic or inorganic substances with small molecular weight, so that the dye loses its chromogenic ability. 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 mature method at present. The oxidant generally adopts Fenton Reagent (Fe2 + - H2O2), ozone, chlorine, sodium hypochlorite, etc.

Fenton reagent is used to catalyze H2O2 to generate • Oh at pH 4 ~ 5 to oxidize and decolorize the dye. The generated new ecological Fe2 + also has the function of promoting coagulation. When iron filings H2O2 is used to treat printing and dyeing wastewater, new ecological Fe2 + can be generated at pH 1 ~ 2. Its hydrolysate has strong adsorption and flocculation effect, which can remove more than 99% of the chromaticity 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 content of iron powder is 1g / L, H2O2 is 1mmol / L and pH 2 ~ 3 [45]. Photocatalytic oxidation uses some substances (such as iron complexes, simple compounds, etc.) to produce free radicals under the action of ultraviolet light to oxidize dye molecules to realize decolorization. Such as photocatalytic decolorization and degradation of methylene blue solution [46] and wool dyeing and finishing wastewater [47]; Using ferric oxalic acid, ferric citric acid or ferric succinic acid complex as catalyst, the decolorization experiment of printing and dyeing wastewater is carried out under UV irradiation and pH 2 ~ 4. The ferric carboxylic acid complex can generate alkyl, hydroxyl and other free radicals to oxidize and decolorize printing and dyeing wastewater [48]; UV can also enhance the decolorization effect of diazo dyes [45]. Iron oxalate complex can be used for photolysis of reactive brilliant red X-3B, and its photolysis mechanism has been fully discussed [49]. 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. Ultrasonic technology is used to degrade 44.4mg/l acid red B water. When NaCl is added for about 1g / L and treated for 50min, the decolorization rate of acid red B wastewater is nearly 90% [50].

In short, oxidation is an excellent decolorization method for printing and dyeing wastewater, but if the degree of oxidation is insufficient, the chromophore group of dye molecules may be destroyed and decolorized, but the COD in it has not been completely removed; If dye molecules are fully oxidized, the energy and reagent consumption may be too large and the cost may be too high. Therefore, oxidation method is generally used in oxidation 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 method, so as to facilitate flocculation removal. On the contrary, the flocculation oxidation process takes oxidation as a post-treatment step to further treat the printing and dyeing wastewater to further remove the residual chromaticity and cod.

Biological decolorization uses microbial enzymes to oxidize or reduce dye molecules and destroy their unsaturated bonds and chromogenic groups. Decolorizing microorganisms are specific to dyes, and their degradation process is completed in two stages, first the adsorption and enrichment of dye molecules, and then biodegradation. Dye molecules are finally degraded into simple inorganic substances or transformed into various nutrients and protoplasm through a series of life activities such as oxidation, reduction, hydrolysis and combination.

Slight structural changes of dye molecules will greatly affect the decolorization rate. For example, some algae have high decolorization rate for dyes containing - oh and - NH2, but can hardly degrade dye molecules containing - CH3, - OCH3 and - NO2; Dye concentration also has a certain impact on the decolorization rate. High concentration dyes will inhibit microbial activity and affect the decolorization rate or decolorization effect. Microorganisms regulate the decolorization of dyes with different structures through in vivo plasmids. The effective way to improve the application value of decolorizing microorganisms is to screen or construct multifunctional super strains and improve the biodegradability of dyes [44], and vigorously develop biological flocculants with broad-spectrum flocculation activity [51].

Aerobic process is a common treatment process, but due to the strong anti biodegradability of dye molecules, the BOD5 / COD ratio in the treatment process decreases (the biodegradability becomes worse), resulting in the low removal rate of chroma and COD of wastewater by ordinary aerobic process (60% ~ 70%). By adding Fe (OH) 3 to the aeration tank and prolonging the residence time of refractory substances in the system, the activated sludge concentration of the aeration tank can be greatly increased, the sludge load and the degradation amount of organic matter borne by unit number of bacteria can be reduced, and the decolorization rate and COD removal rate of the system can be improved. The application of immobilized cell technology in aerobic process can also achieve good results. Anaerobic aerobic treatment process can make up for the deficiency of aerobic process to a certain extent. Refractory dye molecules and their auxiliaries are hydrolyzed and acidified under the action of anaerobic bacteria to decompose 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 biological treatment of printing and dyeing wastewater are not high, and the reaction time is long. It is generally not suitable for separate application, and can be used as pretreatment or advanced treatment steps. At present, the key of biological decolorization is to screen high-efficiency degrading bacteria and construct strains with degradation ability and flocculation activity, so as to complete the degradation, flocculation and decolorization in a short time, so as to improve the treatment efficiency and reduce the cost, and actively explore the pre-treatment methods of dye molecules or printing and dyeing wastewater, such as electrolysis and low-dose oxidation, so as to improve the biodegradability of printing and dyeing wastewater.

Electrochemical method is to purify printing and dyeing wastewater through electrode reaction. The method can be divided into electrode method, electrochemical reaction method and internal oxidation method according to the subdivision method. The most famous internal electrolysis method is the iron chip method, that is, the iron chip is used as the filter material to immerse or pass the printing and dyeing wastewater. The potential difference between Fe and FEC and the solution is used to produce electrode reaction and produce new ecological h with high chemical activity. It can have redox reaction with various components of printing and dyeing wastewater and destroy the dye chromogenic structure. The new ecological Fe2 + produced by the anode has strong adsorption and flocculation effect on its hydrolysate. In order to further improve the treatment effect of the traditional iron filings method, the iron filings are modified or auxiliary fillers are added to the iron filings, which increases the number of micro cells in the printing and dyeing wastewater or prolongs the residence time of dye particles in the iron filings, so that the chromaticity and COD removal of insoluble dyes by the modified iron filings method are increased by 20% ~ 30%.

Using Fe and Al as anodes and H2 produced by the cathode to float the flocs, which is called electrical float; Using Fe2 + and Al3 + produced by electrode reaction to realize flocculation and decolorization, which is called electrocoagulation. The application of pulse electric signal makes the electrode reaction intermittent, which can reduce the overpotential and diffusion resistance, so as to reduce the energy consumption and iron consumption; Similarly, when AC is applied, cations can be generated at both poles, which is more conducive to the interaction between metal ions and colloids, and the polarity of the two poles often changes, which is also beneficial to prevent electrode passivation. Therefore, the recent development of electrocoagulation method is pulse electrocoagulation and AC flocculation. The electric float method with activated carbon fiber as electrode realizes the one-stop treatment process of adsorption electrode reaction flocculation desorption by using the comprehensive properties of conductivity, adsorption, catalysis, redox and air flotation of the electrode [53]. Using graphite and titanium plates as polar plates and NaCl, Na2SO4 or the original salt in water as conductive medium, the dye wastewater is electrified and electrolyzed, the anode produces O2 or Cl2 and the cathode produces H2. The dye molecules are destroyed by the oxidation of oxygen atoms and the reduction of hydrogen atoms to decolorize the dye wastewater. Using activated carbon as electrode, enriching dye molecules with its adsorption performance, oxidizing chromogenic groups 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 electrode materials, improving current efficiency and weakening electrode polarization to reduce energy consumption are still the main direction in the future.