『Weekly Topic』—What are the mainstream treatment methods for phenolic wastewater? (2015.9)

For the treatment of phenol-containing wastewater, the most effective approach is to control the source of pollution. This involves selecting appropriate process flows, developing pollution-free processes and catalysts, as well as using reagents that do not cause pollution in order to implement cleaning techniques, thereby reducing the amount of wastewater generated or lowering the concentration of phenols in it. For example, the current method for producing paracetamol is primarily the iron reduction process, which generates 44 tons of wastewater per ton of finished product produced; thus, the amount of wastewater is large and the pollution resulting from it is severe. In recent years, new processes have been developed using *** catalytic oxidation to produce p-aminophenol; for every ton of finished product produced, only 10 tons of phenol-containing wastewater are generated, thereby reducing pollution. Secondly, effective operating conditions and production equipment should be selected to develop a closed-loop system for the production of phenolic compounds, thereby minimizing and reducing the release of pollutants into the environment and achieving clean production with \"zero emissions\". Third, strengthen enterprise management by implementing effective treatment, recovery, and comprehensive utilization of phenol-containing wastewater. Due to the differences in the composition, acidity or alkalinity, and concentration of phenol-containing wastewater, different treatment methods are employed. Currently, industrial methods for treating such wastewater are generally divided into three categories: physical-chemical methods, chemical methods, and biochemical methods. It mainly introduces the most common methods. 1. Physicochemical method: The physicochemical method is a technique for treating wastewater by employing physicochemical processes to remove pollutants; it is widely used and has seen rapid development in recent years. Its main methods include: adsorption, extraction, reverse osmosis, electrodialysis, liquid membrane, air lift, ultrafiltration, and other methods. 1.1 Adsorption method The adsorption method is widely used in the treatment of phenol-containing wastewater. The adsorption method utilizes porous solid materials as adsorbents, such as activated carbon, diatomite, activated alumina, exchange resins, and sulfonated coal, to absorb phenol pollutants (in the liquid phase) present in wastewater on the surface of these adsorbents (the solid phase). Depending on the forces acting between the adsorbent and phenolic compounds, the adsorption mechanism can involve physical adsorption, chemical adsorption, or exchange adsorption. In the treatment of phenol-containing wastewater, physical adsorption is the main mechanism, and sometimes it is a combination of several adsorption processes. Choosing adsorbents with good adsorption performance, high adsorption capacity, easy regeneration, and durability is key to ensuring effective separation. 1.2 Extraction method: There are two approaches for treating phenol-containing wastewater using the extraction method. One involves using an extraction process with a high distribution coefficient, employing specific extraction techniques and equipment; this approach takes advantage of the different solubilities of phenolic compounds in the organic and aqueous phases, as well as the fact that these two phases are immiscible, in order to separate the phenols. The other approach relies on coordination reactions, allowing a single extraction step to reduce the phenol content in the wastewater to levels below the **allowed emission standards**. 1.3 Liquid film method: The liquid film method is a new type of wastewater treatment and separation technology that has been developed in recent years. The removal of phenol using the liquid film approach relies on a water-in-oil-in-water (W/O/W) system. The liquid film is composed of a solvent (such as kerosene) and a surfactant. During the separation process, the substance to be separated (phenol) undergoes both extraction and back-extraction, and separation and concentration are achieved through liquid film transfer. The process of removing phenol via a liquid film involves the emulsion being stirred to form many small droplets, which are dispersed in the phenol-containing wastewater. At this time, the internal aqueous phase is an NaOH aqueous solution, while the external phase is phenol-containing wastewater. The aqueous phase within the liquid film is separated from the external phase. Phenol in wastewater can pass through the liquid film into the inner aqueous phase, where it reacts with NaOH as a weak acid to form sodium phenolate. Sodium phenolate is insoluble in oil and therefore diffuses into the aqueous phase (the closed phase); it does not return to the outer aqueous phase nor spread into the wastewater being treated, thereby achieving the purpose of separation. The liquid film process is divided into three steps: floating formation, contact removal, and demulsification. It has the advantages of simple process, high efficiency and speed, high selectivity, high separation efficiency, and the ability to reuse the emulsion after demulsification. The liquid film method is suitable for treating wastewater containing phenols at both high and low concentrations, with a phenol removal rate of up to 99.9%; it has been reported to be effective for concentrations as low as 10–47 g/L of phenols. In recent years, considerable progress has been made at home and abroad in the research on using liquid film methods to treat phenol-containing wastewater. In the early 1990s, China built industrial liquid film treatment units for high-concentration phenol-containing wastewater with a capacity of 50 t/d, which have been used to treat phenol-containing wastewater from plastic factories, petrochemical plants, and other similar facilities. In recent years, processes have been developed to select the optimal rotation speed for the transfer tower, to adjust the flow rates of wastewater and emulsion for separation, and to reduce the phenol content in wastewater to below 0.5×10-6 through liquid film treatment. However, due to the high technical requirements for operating this liquid film method and the fact that the stability of the liquid film has not yet been fully resolved, this new technology has not been widely adopted in industry. It is reported that the issue of liquid film stability has been largely resolved recently, and the widespread use of this technology is just around the corner. 2. Chemical method: The chemical treatment method involves using chemical reactions between substances; it is an efficient approach with great potential for treating petrochemical wastewater. In chemical methods, commonly used ones include neutralization, precipitation, oxidation, reduction, electrolysis, photocatalysis, etc. 2.1 Precipitation method: Chemical substances are added to the wastewater to cause precipitation with phenol. The method is simple and cost-effective, but the phenol concentration in the wastewater appears to be high after treatment; it yields better results when used in combination with other methods. The recently developed copolycondensation method is an effective phenol removal technique among chemical precipitation methods. The phenol-polymerization method involves adding formaldehyde to high-concentration phenol-containing wastewater and adjusting the molar ratio of phenol to formaldehyde in the presence of acidic or basic catalysts, thereby polycondensing the phenol in the wastewater into low-molecular-weight thermoplastic or thermosetting resins. After separating the resin, urea is added to the wastewater for a second reaction; the residue is harmless and can be discarded or incinerated. The phenol concentration in the wastewater before treatment can be as high as over 30,000 mg/L. After treatment, the wastewater is sent to a sedimentation and filtration tank; once the samples taken for testing meet the required standards, the wastewater can be discharged. The residue in the tank is then removed. When using the phenol-aldehyde-urea polycondensation method, it is necessary to adjust the ratio of phenol:aldehyde:urea to 1:3:1 and maintain a pH level of 9.7–10.0. Heating is used to produce an acidic resin, and formaldehyde is recovered from the wastewater. As a result, the phenol content in the wastewater can be reduced to (10–50)×10-6. Further biological treatment or dilution is then applied to bring the wastewater up to the discharge standards. Another method is the acid digestion and precipitation approach, in which polycondensation reactions are carried out under heating with salts and acids in phenol-containing wastewater, allowing for the recovery of resin and reducing the phenol content from 3% to one ten-thousandth of its original level. 2.2 Oxygen method: Oxidants such as Cl2, ClO2, O3, H2O2, and KmnO4 are added to wastewater to oxidize and decompose phenols, while also oxidizing the reducing properties in the water. Chemical oxidants are scarce and expensive. It is commonly used for the treatment of low-concentration phenol-containing wastewater. 2.3 Electrolysis method: An appropriate amount of electrolyte is added to the wastewater, and through a complex oxidation process during electrolysis, phenol is purified. Its features are: no need to use chemical reagents such as oxidants or reducers, thus eliminating the need for post-treatment ； Secondly, the equipment has a high processing capacity per unit volume ； Furthermore, the reaction rate and type can be easily controlled by adjusting current and voltage, and the operation is also simple. However, the electrolytic method is only suitable for the advanced treatment of low-concentration phenol-containing wastewater; it has high energy consumption and treatment costs, and it also causes some side reactions. 2.4 Photocatalytic method: This is a technology newly developed in China for treating phenol-containing wastewater, and its advantage is that it can handle wastewater with relatively high concentrations of phenol ； Fast degradation rate, no secondary pollution ； The catalyst is inexpensive and readily available ； It can be recycled and reused, with low operating costs ； The photocatalytic method features simple equipment, low investment, and good efficiency. It is primarily used to treat low-concentration phenol-containing wastewater resulting from the recovery of resin via copolymerization; a photocatalyst is added to this wastewater, which is then exposed to light (ultraviolet light or sunlight), heated to 600°C, and stirred while air is introduced. Samples are taken after two hours for testing, and once the phenol content meets the emission standards, the reaction can be stopped. The catalyst can be recycled after recovery.

The solvent extraction method, specifically solvent extraction for phenol removal, is a commonly used technique in industry for removing phenol. The steam dephenolization method is an earlier type of dephenolization technique; it is simple to operate and suitable for treating wastewater containing mainly volatile phenols. The most commonly used adsorption method is activated carbon adsorption. In the ion exchange method, phenol is removed using ion exchangers, with weakly basic anion exchange resins showing the best efficiency in adsorbing and regenerating phenol for recovery. The chemical precipitation method involves adding chemical reagents to cause phenol in wastewater to form precipitates, which can then be separated and recovered. Phenol-containing wastewater with low concentrations that have no value for recovery, or wastewater containing dozens to hundreds of milligrams of phenol per liter after recovery treatment, needs to be purified before it can be discharged or reused.

1. Solvent extraction method: The solvent extraction method for phenol removal is a commonly used technique in industry for removing phenol. There are mainly two types of solvent extraction for phenol removal: physical extraction technology and complexation reaction extraction technology. Physical extraction technologies for phenol removal primarily use benzene, heavy benzene, heavy solvent oils, ethyl acetate, isopropyl ether, etc. as extraction solvents, all of which exhibit relatively high equilibrium distribution coefficient D values for phenol. Complexing extractants are generally composed of a complexing agent, a cosolvent, and a diluent. In the complex extraction process, the role of cosolvents and diluents is very important. Common cosolvents include octanol, methyl isobutyl ketone, butyl acetate, diisopropyl ether, chloroform, etc. Common diluents include aliphatic hydrocarbons (n-hexane, kerosene, etc.) and aromatic hydrocarbons (benzene, toluene, heavy benzene, etc.). 2. The steam dephenolization method is an earlier developed technique for dephenolization; it is simple to operate and suitable for treating wastewater containing primarily volatile phenols. 3. Adsorption method: Activated carbon adsorption is the most commonly used approach. 4. Ion exchange method: Phenol is removed using ion exchangers, with weakly basic anion exchange resins showing the best efficiency in adsorbing and regenerating phenol. 5. Chemical precipitation method: Chemical reagents are added to cause the phenols in wastewater to form precipitates, which can then be separated and recovered. 6. Biological method: Wastewater containing phenols at low concentrations and of no value for recovery, or wastewater containing dozens to hundreds of milligrams of phenols per liter after recovery treatment, needs to be purified before it can be discharged or reused.

I need this too; please provide a more detailed explanation. Everything is the same: L:L:L