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Research trends in the treatment of high-concentration organic chemical wastewater — Catalytic oxidation. Organic chemical wastewater features high pollutant concentrations, high toxicity, and poor biodegradability, which makes biological treatment generally difficult. Therefore, how to treat high-concentration organic wastewater efficiently and economically has become a challenging and hot topic in the field of water treatment. Among the numerous treatment methods that have emerged, catalytic oxidation technology is a focal point; it involves using an oxidant in the presence of a catalyst to oxidize the organic substances in wastewater into carbon dioxide and water, thereby achieving removal. This method features a wide range of applications, high processing efficiency, and minimal secondary pollution. Experts at home and abroad agree that this technology is an effective method for treating high-concentration organic chemical wastewater, and various oxidation techniques have been developed, including photocatalytic oxidation, wet catalytic oxidation, supercritical water oxidation, electrocatalytic oxidation, and chemical catalytic oxidation. 1 Photocatalytic oxidation method The photocatalytic oxidation method is a type of advanced oxidation technology that has been developed over the past few decades. It is a process in which a specific light source is combined with a catalyst to degrade organic wastewater. In recent years, it has been widely used in experimental studies on various types of refractory organic wastewater. The mechanism involves the absorption of light energy by a photosensitized semiconductor (catalyst); when this energy exceeds the band gap energy, free electrons and holes are generated. The holes react with water, while the electrons react with dissolved oxygen, producing HO· and O2- respectively. Since both HO· and O2- possess strong oxidizing properties, they facilitate the degradation of organic substances. Some studies have been conducted on the degradation of phenol in water using the enhanced UVFenton method; when the ratio of the reagents F2+e:H2O2 was 1:4 and the initial concentration of phenol was 50 mg/L, a 375 W high-pressure mercury lamp was used, and after 20 minutes of reaction, 99% removal of phenol was achieved. The photocatalytic degradation of p-dimethylaminobenzaldehyde wastewater was studied using an anatase-type TiO2 catalyst. The results showed that at a pH of 2 and with a catalyst concentration of 2 g/L, 98% removal of CODcr was achieved after 3.5 hours of irradiation using a 300 W high-pressure mercury lamp. Studies on the photocatalytic degradation of butyraldehyde wastewater have shown that at a reaction temperature of 40°C, an acidic pH, and a catalyst concentration of 5 g/L, with exposure to a 300W high-pressure mercury lamp for 3 hours, the removal rate of COD is 85%–92%. 2 Wet catalytic oxidation method The wet catalytic oxidation method is a treatment technology that has been developed based on the traditional wet oxidation method. The traditional wet oxidation method involves using oxidants to oxidize the organic substances in wastewater into carbon dioxide and water under high temperature and pressure, thereby removing pollutants. Compared with conventional methods, it features a wide range of applications, high processing efficiency, very few secondary pollutants, and fast oxidation speed. However, the wet oxidation method generally requires operation under high temperature and pressure conditions, thus posing higher demands on the materials used for equipment. Wet catalytic oxidation involves the use of appropriate catalysts in traditional treatment processes to reduce the temperature and pressure required for the reactions, shorten the reaction time, and thereby lower the treatment costs. This technology has been industrialized abroad, and is mainly used in the regeneration of activated carbon, as well as in the treatment of cyanide-containing wastewater, coal gasification wastewater, papermaking black liquor, and urban sludge and landfill leachate. There are also relevant studies in China; for example, Du Hongzhang and others investigated catalytic wet oxidation techniques for treating coking wastewater, and developed precious metal catalysts with high oxidation activity and stability, enabling the removal of NH3-N at a rate of over 99%. The Ru/Al2O3 catalyst developed jointly by Qin et al. achieved a removal rate of 99% for ammonia nitrogen at 503 K, 15 MPa, and pH=12. 3 Supercritical water oxidation method The supercritical water oxidation technology (SCWO) is a novel oxidation technique developed in the mid-1980s by American scholar Modell; it enables the complete destruction of the structure of organic substances. Its principle involves using oxygen to break down the organic compounds present in wastewater into simple molecules such as water and carbon dioxide, all in a supercritical water state. The reaction conditions for SCWO are quite stringent, and generally high requirements are placed on the equipment materials. Therefore, in order to accelerate the reaction rate, reduce the reaction time, and lower the reaction temperature, catalysts have been introduced into SCWO. Currently, research on the use of catalytic supercritical oxidation for wastewater treatment has become an important area of focus in SCWO. In catalytic reactions, transition metal oxides and precious metals are generally used as active components, such as the oxides of Cu, Zn, Fe, Mn, Ni, Ti, Al, V, Cr, Co, and Pt. Mделл and colleagues studied the supercritical water oxidation of organic wastewater with an organic carbon content ranging from 27,000 to 33,000 mg/L using a continuous-flow system. They found that at temperatures above 550°C, the destruction rate of organic carbon exceeded 99.97%, with all organic substances being converted into carbon dioxide and inorganic substances. Ding et al. used SCWO to treat phenol-containing wastewater; under subcritical and supercritical conditions (T=400–500°C, P=25.3–30.4 MPa), the removal efficiency of phenol could reach over 96%. 4 Electrocatalytic oxidation method: Electrocatalytic oxidation involves the direct degradation of organic compounds through anodic reactions, or the generation of oxidants such as hydroxyl radicals and ozone via anodic reactions to degrade these organic compounds. This method enables more thorough decomposition of organic substances, reduces the formation of toxic intermediate products, and is thus more in line with environmental protection requirements. Studies by Kōji Murakami and others have shown that copper salts exhibit excellent catalytic activity in catalytic wet oxidation processes involving acids, amines, surfactants, and others. Li Yijiu et al. used composite chlorine oxidants to treat coking wastewater, reducing the color intensity from 140 times to below 60 times, and other pollutant levels also decreased significantly. 5 Chlorine dioxide catalytic oxidation method The basic principle of the chlorine dioxide catalytic oxidation method is to utilize the strong oxidizing properties of chlorine dioxide, in the presence of a catalyst, to catalytically oxidize organic pollutants in wastewater under normal temperature and pressure. During the degradation of COD, the conjugated double bond chromophores in organic molecules, such as azo groups, nitro groups, carbonyl sulfide groups, and imino groups, are broken apart, thereby achieving complete decolorization. At the same time, this method effectively increases the BOD5/COD ratio. Chlorine dioxide decomposes rapidly upon contact with water, producing various strong oxidizing agents such as HClO, HClO2, Cl2, H2O2, etc. It also generates a number of highly reactive radicals, which can activate the active hydrogen atoms in organic molecules. Through dehydrogenation reactions, these radicals yield R· radicals, which serve as initiators for further oxidation. Additionally, they can replace groups such as -SO2H and -NO2 on aromatic compounds through stepwise substitution reactions, resulting in unstable intermediate compounds. These carboxyl-substituted intermediates are prone to ring-opening decomposition until they are completely broken down into inorganic substances. Furthermore, ClO2 can also oxidize reducing substances such as S-2. Currently, the catalytic oxidation method using chlorine dioxide has been reported for the treatment of refractory wastewater. He Qihuan and others have conducted systematic research on the preparation of catalysts as well as the process conditions for catalytic oxidation, and applied these methods to the treatment of acidic scarlet dye wastewater and phenol-formaldehyde resin wastewater, achieving significant results. Xu Xibiao and others applied this method to the treatment of anisole amine wastewater, with a COD removal rate of over 90%. In addition, there are reports on the practical application of the chlorine dioxide catalytic oxidation method: The Zhonghua Chemical Group in Jiaxing City, Zhejiang Province, mainly produces vanilla aldehyde; its wastewater generation volume is 120 m3/day, the area required for the facility is 1,000 m2, and the investment amount is 5 million yuan. By using the chlorine dioxide catalytic oxidation reaction, organic substances can be degraded in a stable and efficient manner, and at the same time the B/C ratio increases from 0.154 to 0.359, which is beneficial for subsequent biochemical treatment. The catalytic oxidation method can accelerate the chemical reaction between organic substances and oxidants; during the degradation process, more reactive groups are generated. It exhibits high efficiency in treating certain types of refractory organic wastewater, and it also allows for further optimization of the combined use of wastewater treatment techniques. As research progresses, the catalytic oxidation method will become a very competitive new technology for dealing with refractory organic wastewater. This post was last edited by hw197358 on 2009-2-23 20:33.]