Printing and dyeing wastewater decolorization technology

Printing and dyeing wastewater refers to wastewater discharged from textile, wool, chemical fiber and other textile products during pretreatment, dyeing, printing and finishing. The printing and dyeing wastewater has complex composition, mainly composed of aromatic hydrocarbons and heterocyclic compounds, with chromogenic groups (such as -N═N-, -N═O) and polar groups (such as -SO3Na, -OH, - NH2). The dye molecules contain more -SO3H, -COOH, -OH groups such as reactive dyes and neutral dyes, which can form hydrogen bonds with water molecules. The dye molecules can be completely dissolved in wastewater; no or less -SO3H Dye molecules of hydrophilic groups such as -COOH, -OH exist in the form of hydrophobic aerosols; dye molecules containing a small amount of hydrophilic groups but having a large molecular weight or completely free of hydrophilic groups are often found in water. It exists in colloidal form. Printing and dyeing wastewater often also has the following additives: 1 neutral electrolyte such as NaCl, Na2SO4, etc.; 2 acid-base regulator such as HCl, NaOH or Na2CO3; 3 surfactant; 4 bulking agent such as urea; 5 adhesive such as modification Starch, urea-formaldehyde resin, polyvinyl alcohol, etc.; 6 stabilizers such as phosphates. Printing and dyeing wastewater has complex composition, large chroma and high COD, and has developed towards anti-oxidation and biodegradation. It has become an important source of pollution in China's major waters. At present, the decolorization of hydrophobic or water-insoluble dye wastewater has been basically solved. The difficulty lies in the decolorization of many hydrophilic or water-soluble dye wastewaters, which is one of the currently recognized industrial wastewaters that are difficult to handle. The decolorization of printing and dyeing wastewater is mainly to remove the color of the wastewater, that is, the dye molecules and COD. The decolorization methods widely used nowadays mainly include the following.

1, adsorption bleaching

   Adsorption decolorization technology relies on the adsorption of adsorbents to remove dye molecules. Commonly used adsorbents include regenerable adsorbents such as activated carbon, ion exchange fibers, and non-renewable adsorbents such as various natural minerals (bentonite, diatomaceous earth), industrial waste (cinder, fly ash), and natural waste (charcoal, Sawdust). At present, the adsorbent used for adsorption decolorization mainly relies on physical adsorption, but ion exchange fibers and modified bentonite also have chemical adsorption. Activated carbon is the first solid adsorbent to obtain the most thorough industrial application. Activated carbon microporous, large and medium pores, strong hydrophilicity, limit the internal diffusion of macromolecules and hydrophobic dyes, suitable for decolorization of water-soluble dye molecules with molecular weight not exceeding 400, decolorization effect on macromolecules or hydrophobic dyes Poor. Because the intermolecular dipole and deformability (the main factors that determine the induction of the dipole is small) are very different, the physical adsorption also shows a certain selectivity, such as the decolorization rate of activated carbon on the alkaline dye wastewater is more than 90%, and the acidity The decolorization rate of dye wastewater is only 30%-40%. As a flocculant widely used in water treatment, bentonite has been widely used in the decolorization of printing and dyeing wastewater, and has recently been developed into a variety of composite and modified bentonite [37]. The purpose of the note is ion exchange fiber, which is mainly used for adsorbing heavy metals and pigments [38] and is larger than surface, fast ion exchange rate, easy to regenerate, and has good decolorization effect on refractory reactive dye wastewater; some adsorption and flocculation A functionally integrated adsorbent such as a diatomaceous earth composite water purifying agent has also been developed. The 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 and bleaching

   The flocculation and decolorization technology of printing and dyeing wastewater has low investment cost, small equipment occupation and large processing capacity, and is a commonly used decolorization technology. The flocculation and decolorization mechanism of printing and dyeing wastewater is based on colloidal chemistry theory. In the case of inorganic flocculants, the flocculating agents such as iron and aluminum are hydrolyzed and polymerized to form high-priced polyhydroxy cations, which are compressed with water colloids, electrically neutralized, destabilized, adsorbed and bridged, and supplemented by The sediment net is trapped and rolled, and the resulting coarse flocs are removed by precipitation to achieve the purpose of decolorization. For organic polymer flocculants, in addition to electrical neutralization and bridging, there may be a flocculation mechanism similar to chemical reaction bonding. The modification of inorganic polymer flocculant introduces inorganic acid or organic functional groups with complexing ability, which gradually becomes a new trend of decolorization of water-soluble dye wastewater. The decolorization mechanism of inorganic polymer flocculants is different from low molecular inorganic flocculants. The development of new flocculants is also one of the ways to remove hydrophilic dyes, such as polysilicate flocculants, which have recently become one of the hot spots. At the same time, organic polymer flocculants are rapidly developing. For example, starch-modified cationic flocculants have turbidity and chroma removal rates above 90%. Certain substances can react with dye molecules, masking or even breaking the hydrophilic groups of the dye or destroying the chromonic structure of the dye molecules, reducing the water solubility of the dye molecules, making them hydrophobic molecules or ions. Some metal ions with empty orbitals, such as Mg2+, Fe2+, and Ca2+, can accept lone pairs of electrons, and can complex with dye molecules containing lone pairs of electrons to form macromolecules with complex structures, so that the dye molecules have colloidal properties and are easily flocculated. . Some organic molecules can also form complexes with dye molecules to reduce the water solubility of dye molecules, such as the long-chain cationic surfactant dodecyldimethylammonium chloride for water solubility of sulfonic acid groups. Dye Wastewater. In recent years, it has been found that oxidation also promotes flocculation. The mechanism is that the organic molecules are coupled to some extent by the oxidant [43] or the oxidant breaks the hydrophilic groups of the dye molecules. For the printing and dyeing wastewater containing cationic dyes, iron and aluminum are used. The representative inorganic flocculant is basically ineffective for decolorization because the polyhydroxy cation formed by the hydrolysis of these inorganic flocculants has the same charge as the complex dye cation in the water. Due to the homophobic repulsive, the flocculation and decolorization by cationic coagulation The flocculants, including inorganic flocculants, most of the positive polymer flocculants, are naturally incapable of cationic dyes. If the dye cations in the water can be converted to anionic or neutral molecules in some way, they can be removed by an inorganic flocculant or a cationic high-molecular flocculant. It has been reported that the γ-ray radiation flocculation process has greatly improved the removal rate of cationic dyes. Regardless of the oxidation or the gamma ray radiation flocculation process, the cationic dye is changed to neutral or negative, and further processed to obtain a good decolorization effect.

3, oxidative decolorization

   The unsaturated double bond of the chromophoric group in the dye molecule can be oxidatively cleaved to form an organic or inorganic substance having a smaller molecular weight, thereby causing the dye to lose color development ability. Oxidation methods include 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 at present. The oxidant is generally a Fenton reagent (Fe2+-H2O2), ozone, chlorine gas, sodium hypochlorite or the like. Using Fenton's reagent to catalyze the formation of H2O2 at pH 4-5? OH, the dye is oxidatively decolored, and the new ecological Fe2+ produced also has a coagulation effect. Treatment of printing and dyeing wastewater with iron filings H2O2, new ecological Fe2+ can be formed at pH 1-2, and the hydrolysate has strong adsorption flocculation effect, which can remove the chromaticity of nitrophenol and anthraquinone printing and dyeing wastewater by more than 99%; When the printing and dyeing wastewater is decolorized by iron powder H2O2, when the iron powder content is 1g/L, H2O2 is 1mmol/L, and pH is 2-3, the decolorization effect is excellent. The photocatalytic oxidation method utilizes certain substances (such as iron complexes, simple compounds, etc.) to generate free radicals under the action of ultraviolet light, and oxidizes the dye molecules to achieve decolorization. Such as methylene blue solution and wool spinning and finishing wastewater [47] photocatalytic decolorization and degradation; using iron oxalate, iron citrate or iron succinic acid complex as catalyst, under ultraviolet light and pH 2 ~ 4 when printing wastewater In the decolorization experiment, the iron carboxylic acid complex can generate various radicals such as alkyl group and hydroxyl group to oxidize and decolorize the printing and dyeing wastewater; ultraviolet light can also strengthen the decolorization effect of the diazo dye [45]. The iron oxalate complex can be used for light. The solution of Reactive Red X-3B has been fully discussed. Ultrasonic treatment of printing and dyeing wastewater is based on ultrasonic energy to produce local high temperature, high pressure, high shear force in the liquid, induce water molecules and dye molecules to crack free radicals, trigger various reactions and promote flocculation. The concentration of 44.4mg/L acid red B water was degraded by ultrasonic technology. When the dosage of NaCl was about 1g/L and the treatment was carried out for 50min, the decolorization rate of acid red B wastewater was nearly 90%. In short, the oxidation method is an excellent method for decolorization of printing and dyeing wastewater, but if the degree of oxidation is insufficient, the chromophoric group of the dye molecule may be destroyed and decolored, but the COD therein is still not removed; if the dye molecule is fully oxidized, the energy The consumption of the drug may be too large and the cost is too high, so the oxidation method is generally used for the oxidation flocculation or flocculation oxidation process. The oxidation flocculation process is adopted to change the water-soluble dye molecules into hydrophobicity or to convert the cationic dye molecules into neutral and negative molecules by oxidation to facilitate flocculation removal. Conversely, the flocculation oxidation process uses oxidation as a post-treatment step to further treat the dyeing wastewater to further remove residual color and COD.

4, biological method bleaching

   Biological decolorization is the use of microbial enzymes to oxidize or reduce dye molecules, destroying their unsaturated bonds and chromophores. Decolorizing microorganisms are specific to the dye, and the degradation process is completed in two stages, first the adsorption and enrichment of the dye molecules, followed by biodegradation. Dye molecules are eventually degraded into simple inorganic substances or converted into various nutrients and protoplasms through a series of life activities such as oxidation, reduction, hydrolysis, and chemical combination. Subtle structural changes of dye molecules will greatly affect the decolorization rate. For example, some algae have high decolorization rate of dyes containing -OH and -NH2, but almost impossible to degrade dye molecules containing -CH3, -OCH3, -NO2; dye concentration Decolorization rate also has a certain effect, high concentration of dye will inhibit microbial activity, affecting the decolorization rate or decolorization effect. Microorganisms use in vivo plasmids to control the decolorization of dyes of different structures, and the effective way to improve the application value of decolorizing microorganisms is to screen or construct super-multi-species and improve the biodegradability of dyes, and vigorously develop bioflocculation with broad-spectrum flocculating activity. Agent. The aerobic process is a common treatment process, but due to the strong biodegradability of the dye molecules, the BOD5/COD ratio decreases during the treatment process (the biodegradability is deteriorated), so that the ordinary aerobic process does not affect the color and COD of the wastewater. High (60% to 70%). By adding Fe(OH)3 to the aeration tank and prolonging the residence time of the refractory substance in the system, the activated sludge concentration of the aeration tank can be greatly increased, and the sludge load and the unit number of the bacterial group can be reduced. The amount of organic matter degradation increases the decolorization rate and COD removal rate of the system. Good results can also be obtained by applying immobilized cell technology to an aerobic process. The anaerobic aerobic treatment process can make up for the deficiency of the aerobic process to a certain extent. The refractory dye molecules and their auxiliaries are hydrolyzed and acidified by anaerobic bacteria to decompose into small molecular organic substances, which are then decomposed into small inorganic molecules by aerobic bacteria. In short, the decolorization rate and COD removal rate of the biological treatment of printing and dyeing wastewater are not high, and the reaction time is long, which is generally not suitable for single application, and can be used as a pretreatment or deep treatment step. The key to the current biological decolorization is to screen highly degrading bacteria and construct strains with degradability and flocculation activity, so that degradation, flocculation and decolorization can be completed in a short time, in order to improve the processing efficiency, reduce the cost, and actively explore the dye molecules or Pretreatment methods for printing and dyeing wastewater, such as electrolysis and low-dose oxidation, to improve the biodegradability 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 mode, the electrochemical method can be subdivided into internal electrolysis, electroflocculation and electrical floatation, and electrooxidation. The most famous internal electrolysis method is the iron filing method, which uses the cast iron chips as a filter material to immerse or pass the printing and dyeing wastewater. The potential difference between Fe and FeC and the solution is used to generate an electrode reaction, resulting in a higher chemical activity and a new ecological H. The redox reaction of various components of printing and dyeing wastewater destroys the chromonic structure of the dye, while the new ecological Fe2+ produced by the anode has strong adsorption and flocculation. In order to further improve the treatment effect of the traditional iron filing method, the iron filings are modified or added to the iron filings to increase the number of micro-batteries in the printing and dyeing wastewater or to prolong the residence time of the dye particles in the iron filings, so that the modified iron The shaving method increases the chroma and COD removal of insoluble dyes by 20% to 30%. Fe and Al are used as anodes, and the flocs are floated by H2 generated by the cathode, which is called electric floatation method; Fe2+ and Al3+ generated by electrode reaction are used to achieve flocculation and decolorization, which is called electrocoagulation. Due to the application of the pulse electric signal, the electrode reaction is intermittent, which can reduce the overpotential and the diffusion resistance, thereby reducing the energy consumption and the iron loss. Similarly, when alternating current is applied, both poles can generate cations, which is more conducive to the action of metal ions and colloids. And the polarity of the poles often changes, which is also beneficial to prevent electrode passivation, so the recent development of electroflocculation is pulse electric flocculation and alternating current flocculation. The electrical floatation method using activated carbon fiber as electrode utilizes the comprehensive performance of conductivity, adsorption, catalysis, redox and air floatation of the electrode to realize the one-stop treatment process of adsorption-electrode reaction-flocculation and desorption. Graphite, titanium plate, etc. are used as the plate, and the original salt of NaCl, Na2SO4 or water is used as the conductive medium to electrolyze the dye wastewater, the anode produces O2 or Cl2, the cathode produces H2, and the oxygen atom oxidizes and hydrogen atoms The reduction destroys the dye molecules to decolorize the printing and dyeing wastewater. Using activated carbon as an electrode, the dye molecules are enriched by its adsorption performance, and the chromophore group is oxidized under the action of an external electric field, the decolorization rate is over 98%, and the COD removal rate is over 80%. Further improving the catalytic performance of the electrode material, increasing the current efficiency, and reducing the polarization of the electrode to reduce the energy consumption are still the main directions in the future.