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Abstract: This paper mainly introduces the characteristics, principles, recent research progress, and applications of physical methods (adsorption and membrane separation), chemical methods (chemical coagulation, chemical oxidation, wet air oxidation, and photocatalytic oxidation), biochemical methods, and electrochemical methods in the treatment of dye wastewater. The dye production process is complex, generates large amounts of wastewater that is difficult to treat, and the costs associated with pollution control are high. The only sustainable solution is to reduce waste discharge at its source. Keywords: dye wastewater; physical methods; chemical methods; biochemical methods; electrochemical methods. The textile dye industry has developed rapidly in recent years; currently, the production volume of various dyes in China has reached 900,000 tons, and dye wastewater has become one of the major sources of environmental pollution. The dye industry features a wide variety of products and complex manufacturing processes. Its wastewater contains large amounts of organic matter and salts, featuring high CODCr values, a dark color, and strong acidity or alkalinity, making it a persistent challenge in wastewater treatment. This article mainly introduces the physical, chemical, electrochemical, and biochemical methods used in the treatment of dye wastewater, as well as the characteristics and principles of these methods, along with recent research progress and applications. 1 Physical methods 1.1 Adsorption method The adsorption method involves bringing porous solids (such as activated carbon and adsorption resins) into contact with dye-containing wastewater; utilizing the surface activity of these adsorbents to absorb and concentrate the organic substances and metal ions present in the wastewater on their surfaces, thereby achieving water purification. Activated carbon possesses strong adsorption capabilities; it is effective at adsorbing water-soluble dyes such as cationic dyes, direct dyes, acid dyes, and reactive dyes. However, it is expensive and difficult to regenerate. The dye adsorbent made from a mixture of chitosan, activated carbon, and cellulose exhibits excellent adsorption capacity for reactive dyes and acid dyes, with adsorption capacities of 264 and 421 mg/g respectively (the adsorption capacity of coconut activated carbon is less than 80 mg/g). This adsorbent exhibits excellent dispersibility in water and can be treated using a simple and inexpensive contact filtration method. Macroporous adsorption resins are polymer bead-like particles with a cross-linked network structure inside, featuring an excellent pore structure and a high specific surface area. Adsorption resins can be used to remove aromatic sulfonates and naphthol compounds that are difficult to biodegrade. It is easily regenerable and possesses good physicochemical stability; the resin adsorption method has become one of the effective approaches for treating dye wastewater. 1.2 Membrane separation: The membrane separation techniques applied to the treatment of dye wastewater are mainly ultrafiltration and reverse osmosis. It is reported that the decolorization rate of vat dye wastewater treated using tubular and hollow fiber polysulfone ultrafiltration membranes ranges from 95% to 98%, the CODCr removal rate is 60% to 90%, and the dye recovery rate is greater than 95%. In recent years, new membrane materials such as chitosan ultrafiltration membranes and porous carbon membranes have achieved good results in treating printing and dyeing wastewater. Xia Zhening et al. studied the water permeability and salt permeability of dye wastewater through cellulose acetate membranes under ultrasonic treatment, and found that ultrasound significantly accelerates mass transfer and reduces \"concentration polarization\" in membrane separation. With ultrasound applied, the permeability was 1.5 times that without it; the effect on salt permeability was even greater, with retention rates of 94% and 67%, respectively. 2 Chemical methods 2.1 Chemical coagulation method The chemical coagulation method mainly includes precipitation and air flotation; it is economical and effective, but the chemical sludge generated requires further treatment. Commonly used ones include inorganic iron complex salts. In recent years, the use of polymer coagulants has been increasing both domestically and internationally. Natural polymer flocculants mainly fall into three categories: starch and starch derivatives, chitin derivatives, and lignin derivatives. Cationic starch CST, prepared by Zeng Shulan et al. through the reaction of corn starch with the etherifying agent M using NaOH as a catalyst, achieves a decolorization rate of over 90% for acid dyes and reactive dyes at a concentration of 7–15 mg/L. Wu Bingyan et al. used lignin quaternary ammonium salt flocculants prepared by graft polymerization to treat J-acid dye wastewater. The quaternary ammonium ions in the flocculants reacted with the sulfonic acid groups in the wastewater to form water-insoluble substances; at a dosage of 20 mg/L, a color removal rate of 90% was achieved. Fang Xinlan used chitosan produced from shrimp and crab shells as a material for treating printing and dyeing wastewater, achieving a CODCr removal rate of over 85%. Natural polymer flocculants have a low charge density and low molecular weight, and they are prone to biodegradation, resulting in the loss of their flocculating activity. Synthetically produced organic polymer flocculants have a high molecular weight, numerous functional groups in their molecular chains, excellent flocculation properties, require low usage amounts, and can function effectively over a wide pH range. Representative synthetic organic polymer flocculants include PAN-DCD (polyacrylonitrile polyelectrolyte modified with dicyandiamide), the Wx series of polymer decolorization flocculants, and PDADMA-A (dimethyldiallylammonium chloride polymer) M. 2.2 Chemical oxidation method: Chemical oxidation involves the use of ozone, chlorine, and their oxides to destroy the chromophoric groups in dyes, thereby causing decolorization. Ozone oxidation can achieve good decolorization effects for most dyes. However, it has poor effectiveness on water-insoluble dyes such as sulfonated and reduced dyes. In the Fenton reagent oxidation method, the decolorization process is essentially driven by hydroxyl radicals generated from the reaction between H2O2 and Fe2+, which cause the breakdown of the dye molecules. In addition to its oxidizing effect, Fenton’s reagent also has a coagulating effect. Studies have shown that when this method is used to treat wastewater generated from sodium 2-naphthalenesulfonate, after coagulation and precipitation using FeCl3, oxidation is carried out at a pH of 1.5–2.5 using H2O2 at a dosage of 2 g/g CODCr and Fe2+ at a concentration of 4 g/L for 60 minutes; this results in the removal of 99.6% of CODCr and 95.3% of color. 2.3 Wet Air Oxidation Method The Wet Air Oxidation method (WAO) involves introducing air under high temperature (125–320°C) and high pressure (0.5–20 MPa) to directly oxidize the organic substances in wastewater. Supercritical water oxidation (SCWO) refers to the oxidation of organic substances in water under conditions where the temperature and pressure are higher than the critical temperature (374°C) and critical pressure (22.05 MPa) of water. It is essentially an enhancement and improvement of the wet oxidation method. The physicochemical properties of supercritical water undergo significant changes; the water-vapor interface disappears, resulting in a homogeneous oxidation system in which the oxidation rate of organic compounds is extremely fast. Models such as this achieved removal rates of 99.99% and 99.97% for organic chlorines and organic carbon, respectively, within 60 seconds at 550°C for organic wastewater with an organic carbon content of 27.33 g/L. Compared with traditional methods, supercritical water oxidation is more efficient, features faster reaction rates, and has a wider range of applications, allowing it to be used for various hard-to-degrade organic substances ； When the organic content is below 2% ； It can rely on its own heat exchange, eliminating the need for external heating; it features a simple reactor structure and high processing capacity. 2.4 Photocatalytic oxidation method: The photocatalytic oxidation method typically uses H2O2 or photosensitized semiconductors (such as TiO2, CdS, Fe2O3, and WO3 as catalysts). Under high-energy ultraviolet radiation, electrons transition from the valence band to the conduction band, creating holes in the valence band, which in turn triggers an oxidation reaction. This method has a high decolorization efficiency for dye wastewater, but its disadvantages are high investment costs and high energy consumption. Zhang Guilan et al. used a novel rotary photocatalytic reactor; under optimized conditions with suspended TiO2, the decolorization rate of azo dyes reached 98%. Cheng Cangcang and others conducted studies on the photocatalytic degradation of the organic dye Direct Blue GL using a fixed-bed photoreactor and a inclined-plate photoreactor, respectively; after 60 minutes of irradiation, the degradation rates were 83% and 81.4%, respectively. 3 Biochemical method: The biochemical method features low operating costs and minimal environmental pollution. However, dye wastewater features large fluctuations in quality, a wide variety of components, and high toxicity, making it difficult for microorganisms with strict requirements regarding temperature and pH conditions to adapt. The aerobic treatment method is simple to operate, achieves high removal rates for CODCr and BOD5, but its removal efficiency for color is not very satisfactory. The anaerobic treatment method has a high removal rate for the chroma in dye wastewater. The anaerobic treatment method results in less sludge production, and the gas generated is methane, which can be used as an energy source. But when used alone, the effect is not satisfactory. In the treatment of phthalocyanine blue wastewater, Huang Tianyin and others employed physical and chemical methods such as air lifting, stripping, and air flotation to remove most of the NH3-N and Cu2+ from the wastewater, thereby improving its biodegradability. After anaerobic treatment, all indicators meet the first-level standards of the comprehensive wastewater discharge regulations: the removal rate of CODCr is 90.0%, that of BOD5 is 88.9%, that of NH3-N is 99.1%, and that of Cu2+ is 99.7%. Due to the recent trend in dyes toward resistance to decomposition and biodegradation, it is difficult to achieve satisfactory results with a single processing method. Current treatment processes are moving towards an anaerobic-aerobic combined treatment process. Yan Qingsong et al. employed an anaerobic-aerobic process for dye wastewater. The anaerobic stage employs the UASB process for mesophilic digestion, with a residence time of 48 hours; the CODCr removal rate can reach 55%, and the BOD5/CODCr ratio of the effluent increases from 0.1 to 0.42. Granular sludge is formed within the system, which exhibits good sedimentation properties. The aerobic stage employs the contact oxidation method; after acclimatization, the sludge’s ability to degrade wastewater gradually improves. HighSolutionBacteria is a method that utilizes a composite microbial community to treat dye wastewater; there are now over 100 different species of such bacteria, including denitrifying alkaliphilic bacteria, thiobacillus denitratans, and thiobacillus oxidans. It can be tailored to different wastewater types by using specific microbial communities to break down various pollutants, offering a high degree of specificity. Efficient microbial communities break down organic matter into SO2, H2O, and many small organic molecules that have no impact on water quality. The H.S.B technology was used to treat wastewater containing disperse dyes and acid dyes produced by a dye factory in Wuxi (with a CODCr concentration of 2000–2500 mg/L); the CODCr level in the treated wastewater was less than 100 mg/L, achieving an average removal rate of 92.68%. The removal rates for aniline are 94%, for phenol they are 93%, and for ammonia nitrogen they are 92%; the color intensity is below 50 times. To increase the concentration of beneficial bacterial strains in biological treatment systems and improve the efficiency of treating dye wastewater, free bacteria are typically immobilized using chemical or physical methods to maintain their biological activity and enhance their utilization rate. Studies have shown that by fixing efficient decolorizing microbial communities on activated sludge, the activity of decolorizing enzymes increases by 70%. 4 Electrochemical method: The treatment of wastewater using the electrochemical method involves, either indirectly or directly, utilizing electrolysis to convert the toxic substances in dye-containing wastewater into non-toxic ones. In recent years, thanks to the development of the power industry, which has ensured an adequate supply of electricity and significantly reduced treatment costs, electrochemical methods have gradually become a highly competitive approach for wastewater treatment. The electrochemical purification of dye wastewater can be divided into internal electrolysis, electrocoagulation and electroflotation, electrocatalytic oxidation, etc., depending on the manner in which the electrode reactions occur. The most widely used internal electrolysis method is the iron scrap-carbon method. Jin Jianyong used the iron filings internal electrolysis method to decolorize 11 types of dye wastewater belonging to 5 categories. Studies show that for wastewater with moderate color intensity and concentration, the decolorization rate is over 96% ； Adding additives can achieve a CODCr removal rate of over 70% for wastewater. The advantage of the internal electrolysis method is that it can remove various pollutants and chroma from waste without consuming energy, while its disadvantages include slow reaction speed, easy clogging of the reaction column, and poor performance in treating wastewater with high concentrations. Under the action of an external voltage, a soluble anode (iron or aluminum) is used to generate a large number of cations, which cause coagulation of the colloidal wastewater; at the same time, numerous hydrogen bubbles are formed at the cathode, which attach to the flocs and cause them to rise to the surface. This method is called electrocoagulation electrophoresis. Compared to chemical coagulation, it results in about half as much material loss, less sludge, and no need for cumbersome chemical dosing methods. Its disadvantage is high power consumption and high material consumption. Electrocatalytic oxidation involves the direct degradation of organic compounds through anodic reactions, or the degradation of such compounds by oxidants such as hydroxyl radicals and ozone generated via anodic reactions. The advantage of electrocatalytic oxidation is complete oxidation of organic compounds with no secondary pollution. However, the actual application of this method in the industrial treatment of wastewater depends on inexpensive and efficient catalytic electrodes with a high oxygen evolution potential. At the same time, the structure of the electrodes and electrolyzer also plays an important role in reducing energy consumption. Jia Jinping et al. studied the composite electrodes of activated carbon fiber and iron for the degradation of various simulated printing and dyeing wastewater, achieving good results. 5 Conclusion The dye production process is complex, generates large amounts of wastewater that is difficult to treat, and results in high costs for pollution control. The sulfur-containing wastewater generated during the reduction of caustic sulfide is difficult to meet discharge standards using methods other than the expensive wet oxidation process. In recent years, the hydrogen reduction method has been used to completely eliminate sulfide pollution. The mercury-catalyzed sulfonation method for producing aminoanthraquinones has been replaced by a nitration-reduction method to completely eliminate mercury pollution. The research and application of various new technologies** have improved the efficiency of dye wastewater treatment and reduced treatment costs. However, it is better to address the root causes rather than just the symptoms; researching and developing economically viable clean production processes is just as important as developing efficient and cost-effective wastewater treatment methods. Fundamentally reducing pollution discharge is the long-term solution. This post was last edited by hesonchang214 on 2009-2-18 07:55.]