The sources of chlorophenol in the environment are mainly man-made sources and natural sources. Man-made sources are mainly organic chemical wastewater containing CPs discharged into the environment by human production activities such as oil refining, coking, paper making and plastic processing. Natural sources mainly include two kinds: ① Primary chemicals used by human beings produce secondary CPs through natural biochemical processes, such as pesticides 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid widely used in agricultural production, which are degraded by natural microorganisms to produce intermediate products such as CPs; (2) Natural substances synthesize CPs under certain catalysis. For example, inorganic chloride salts and organic compounds in soil humus mud layer will generate CPs under the catalysis of peroxidase, such as 4-CP, 2,5-DCP, 2,4-DCP, 2,6-DCP and 2,4,5-TCP.
2. The environmental pollution level of chlorophenols.
Chlorophenols are a kind of persistent organic pollutants with wide uses and great toxicity. Once they are released into the environment without treatment or improper treatment, they will pollute the natural ecological environment and threaten human safety. At present, there have been a lot of reports about the existence and pollution of chlorophenol in water environment, sediment and soil environment and aquatic organisms.
2. 1 water environment
CPs is widely distributed on the water surface, and its content is related to the source of wastewater discharge. Precipitation and water flow also greatly affect the changes of various CPs concentrations. It has been reported that the concentrations of DCP and TCP in Lake Superior, Canada, will rapidly rise to 4 mg/L and 13 mg/L after being discharged into pulp mill wastewater. In the rivers and coastal waters of the Netherlands, the concentrations of TCP, Mono-CP and DCP reached 0.0030. 1 mg/L, 320 mg/L and 0.01.5 mg/L, respectively. Advanced research shows that the concentrations of 2,4-DCP and 2,4,6-TCP in the Yellow River, Huaihe River and Haihe River in northern China are higher, and the pollution in the north is more serious than that in the south. However, the pollution of pentachlorophenol in the Yangtze River basin is serious, and pentachlorophenol can be detected in 85.4% of surface water samples, with an average concentration of 50.0 ng/L. China's Water Quality Standard for Urban Water Supply (CJ/T 206-2005) lists chlorophenol as an unconventional inspection item, requiring total chlorophenol (including 2-CP, 2,4-DCP and 2,4,6-).
2.2 Sediment and soil environment
The octanol/water partition coefficient of CPs is large, which increases with the increase of chlorine atoms in benzene ring, resulting in its enhanced lipophilicity. Therefore, CPs in water phase can be easily transferred to sediments and soil environment. Therefore, the accumulation of CPs in river sediments is much greater than that in water, and the environmental pollution in sediments is also serious. In addition, the retention time and harm degree of CPs in sediments are directly proportional to the number of chlorine atom substituents on the benzene ring of CPs. A large amount of production wastewater containing CPs was discharged into the sea area of British Columbia, Canada, resulting in the cumulative total concentration of TCP and tetrachlorophenol (Tetra-CP) in seabed sediments reaching 96 mg/k, and the CPs content in sediments near nuclear power plants in Korea was as high as 0.14516.1g/kg (dry weight). 2,4-DCP was detected in the sediments of Thermaikos Bay and Loudia River in Greece. The concentrations of 2,4-DCP and 2,4,6-TCP in the sediments of Du Ze Reservoir in Dzierzno, Poland are close to 0.02 g/kg and 0.0 10.62 g/kg, respectively. In addition, the middle and lower reaches of the Yangtze River in China are threatened by schistosomiasis, and the long-term use of sodium pentachlorophenol to control schistosomiasis in various provinces has led to the accumulation of a large number of pentachlorophenol in soil and sediments. Xu Shifen et al. detected the content of CPs in the sediments of the lower reaches of the Yangtze River, and found that the concentration of pentachlorophenol was the highest, reaching 0.494.57 g/kg, accounting for 39.4% of the 18 chlorophenol to be detected, which was significantly higher than other chlorophenols in the sediments of the Yangtze River. In addition, Zhang Bing et al. determined that the content of pentachlorophenol in sediments in Dongting Lake area was as high as 48.3 mg/kg (dry sludge). According to the monitoring data, the content of 2-CP in soil environment in Kaohsiung City, Taiwan Province Province is 28 103.6 mg/kg[22]. Apajalahti et al. tested the soil samples around the wood processing factory preserved by CPs, and the results showed that the content of pentachlorophenol in the samples reached 1 g/kg.
2.3 aquatic organisms
The enrichment effect of pollutants in organisms can be evaluated by BCF. Aquatic plants generally need 1020 min to fully absorb CPs. For most plants, the absorption rate of CPs decreases with the increase of pH value and increases with the increase of temperature. For aquatic animals or microorganisms, animal types, compound types and enrichment conditions have certain effects on BCF of CPs in water or food. Is the BCF from clam plum to PCP 4 1? The bioconcentration coefficient of snail to 2,4,6-TCP can reach 7403 020. The bioconcentration coefficients of trout and goldfish to 2,4-DCP in water are 10 and 34, respectively, while the bioconcentration coefficient of algae to 2,4-DCP is as high as 257. Kondo et al. reported that the bioconcentration coefficient of 2,4-dichlorophenol in medaka is different due to different types and concentrations of chlorinated paraffin. For example, the accumulation capacity of PCP is higher than that of 2,4 4- DCP and 2,4,6-TCP; When the exposure concentration of 2,4-DCP was 0.23 g/L and 27.3 g/L, its bioconcentration coefficient for medaka was 340 and 92, respectively. When the exposure concentration of PCP was 0.07 g/L and 9.7 g/L, the bioconcentration coefficients of PCP to medaka were 4 900 and 2 100, respectively. The BCF value of 2,4,6-TCP varies from fish to fish and generally fluctuates between 2503 10. Wang Fang and others conducted toxicity tests on crucian carp. The results showed that the gallbladder, liver, kidney and muscle of Carassius auratus obviously absorbed CPs, among which the gallbladder absorbed CPs the most, and its BCF value was as high as 2 0006 300.
3 removal method of chlorophenol
At present, the methods of treating CPs pollutants mainly focus on biological treatment technology, physical and chemical methods, chemical reduction methods and chemical oxidation methods.
3. 1 biological treatment technology
The biological treatment technology of CPs is mainly that microorganisms use CPs as carbon source and energy source to decompose and remove CPs during metabolism, including aerobic biological method, anaerobic biological method, anaerobic/aerobic combined method and so on. There are two main theories about the mechanism of degrading CPs by aerobic method: ① The mechanism of dechlorination by oxidative ring opening: for example, 4-CP undergoes ortho-oxidation under the catalysis of monooxygenase of Pseudomonassp to produce 4- chlorophenol. Then 4- chlorophenol was ring-opened in the ortho position under the catalysis of 1, 2- dioxygenase to produce chlorinated cis-muconic acid. Chlorinated cis-cis-mucic acid then removes chlorine atoms through lactonization, oxidizes into maleylacetic acid, enters tricarboxylic acid cycle (TAC), and finally mineralizes into CO2 and H2O. ② Mechanism of oxidative dechlorination-ring opening: Flavobacterium. The chlorophenol bacteria of Rhodococcus can oxidize the benzene ring of CPs under aerobic conditions to generate chlorinated diphenol, and then gradually remove the chlorine substituent to generate monochlorodiphenol or p-phenol, and then oxidize and open the ring, further mineralize into CO2 and H2O, and PCP is oxidized by aerobic bacteria Flavobacterium. In addition, aerobic microorganisms can successfully treat industrial wastewater with CPs concentration of 0. 1 1.2 g/L under aerobic conditions.
The reaction mechanism of microbial degradation of pentachlorophenol is mainly reduction dechlorination and anaerobic fermentation by anaerobic microorganisms under anaerobic conditions. The main ways of anaerobic degradation include front-end reduction dechlorination and subsequent anaerobic fermentation, that is, pentachlorophenol is reduced and dechlorinated under anaerobic conditions to produce low chlorophenol and phenol. Then, phenol is converted into acetic acid by the action of acetic acid-producing bacteria, and acetic acid is finally converted into methane and CO2 by the action of methanogenic bacteria. Zhou Yuexi and others used the upflow anaerobic sludge bed reactor (UASB) to treat pentachlorophenol wastewater at medium temperature. It was found that pentachlorophenol was dechlorinated at meta position to produce 2,3,4,6-tetrachlorophenol, followed by 2,4,6-trichlorophenol, 2,4-dichlorophenol and 2- monochlorophenol under anaerobic conditions. Armenante et al. studied the treatment of 2,4,6-TCP wastewater by anaerobic/aerobic combined process. The results show that in the anaerobic stage,
With formic acid, acetic acid and succinic acid as electron donors, 2,4,6-TCP was reduced and dechlorinated to produce 2,4-DCP and 4-CP under the action of aerobic microorganisms. In aerobic stage, dechlorination products 2,4-DCP and 4-CP are completely degraded by aerobic microorganisms under aerobic conditions. Arora and others studied the degradation mechanism of CPs under aerobic and anaerobic conditions respectively, and pointed out that under aerobic conditions, CPS formed corresponding chlorophenol or (chloro) hydroquinone under the action of bacteria, and then entered the tricarboxylic acid cycle; Under anaerobic conditions, CPs is dechlorinated by reduction to form phenol, which is further converted into benzoic acid and finally mineralized into CO2.
3.2 Physical and chemical methods
Physical and chemical methods are used to remove CPs, which is mainly based on the adsorption removal of adsorption materials. Hameed et al. prepared coconut shell activated carbon to remove 2,4,6-TCP, and found that its adsorption isotherm accords with Langmuir model. At 30℃, the maximum single-layer adsorption capacity reached 765,438+06.10 mg/g. Ren et al. prepared coconut shell activated carbon with large specific surface area (890.27 m2/g) and various functional groups (hydroxyl, lactone, etc. ) 2,4-DCP and 2,4,6-TCP in water can be effectively removed by activating the fiber precursor of Typha angustifolia with phosphoric acid. Nourmoradi et al. used cationic surfactants hexadecyl trimethyl ammonium bromide (HDTMA) and tetradecyl trimethyl ammonium bromide (TTAB) to modify montmorillonite (Mt) to remove 4-CP from water. The results show that the adsorption capacities of HDTMA-Mt and TTAB-Mt are 29.96 mg/g and 25.90 mg/g, respectively. In contrast, HDTMA-Mt is more beneficial to 4-CP in water. Mubarik et al. prepared cylindrical porous biochar materials with large specific surface area from bagasse for adsorption and removal of 2,4,6-TCP. The results show that biochar can also effectively remove 2,4,6-TCP under the conditions of various organic pollutants, and the maximum adsorption capacity is 253.38 mg/g.
3.3 Chemical Reduction Method
Chemical reduction method is mainly based on the reduction dechlorination of zero-valent metal system. Morales et al. can completely dechlorinate 4-CP, 2,6-DCP, 2,4,6-TCP and PCP in isopropanol/water solution at normal temperature and pressure, especially PCP with extremely stable chemical properties. The results show that 2.48 mmol/L of pentachlorophenol can be completely dechlorinated within 48 h by using a 20-mesh Pd/Mg bimetallic alloy with a concentration of 1.0g, and only cyclohexanol and cyclohexanone which are easy to be further oxidized and degraded can be detected in the product. The reductive dechlorination effect of zero-valent iron infiltrated silica mixture on chlorophenols such as 2,4,6-TCP, 2,4-DCP and 4-CP is directly proportional to the number of chlorine substituents on the benzene ring of CPs, that is, the dechlorination effect is enhanced with the increase of chlorine substituents. The product identification and reaction mechanism study show that zero-valent iron permeates silica to catalyze the reduction and dechlorination to degrade CPs, mainly because zero-valent iron provides electrons to attack C-Cl bonds, one by one. In addition, Zhou et al. also comparatively studied the reductive dechlorination effects of Pd/Fe bimetallic nano-alloy and Pt/Fe, Ni/Fe, Cu/Fe and Co/Fe bimetallic nano-particles on 4-CP, 2,4-DCP, 2,4,6-TCP and other chlorophenols. The results show that the reduction dechlorination effect of Pd/Fe alloy nanoparticles is obviously better than that of other bimetallic systems. 2，4-DCP & gt； 2,4,6-TCP. This study is contrary to the dechlorination effect of zero-valent iron permeating silica mixture to reduce the degradation of chlorinated paraffin.
4 Summary and prospect
At present, remarkable achievements have been made in the study of CPs pollutant degradation and removal technology, but each technology has its own advantages and disadvantages. The investment and operation cost of biological method are relatively low, but it needs specific population domestication and the treatment cycle is relatively long. In addition, CPs is highly toxic, which may adversely affect the growth and metabolism of microorganisms. Physical and chemical adsorption method takes short time and has good treatment effect, but adsorption is only a phase transfer process of pollutants, and it does not fundamentally eliminate pollutants; At the same time, the adsorbed solid adsorption materials, whether regenerated or treated, will cause secondary pollution to the environment to some extent. Furthermore, activated carbon, a common adsorption material, can effectively adsorb and remove CPs from water, but the regeneration of activated carbon after adsorption is relatively difficult, which will indirectly increase the cost of wastewater treatment. The toxicity of chlorinated compounds increases with the increase of chlorine atoms. Chemical reduction dechlorination can achieve effective dechlorination and detoxification of CPs, but the ultimate goal of harmless treatment of pollutants is to realize its mineralization, and chemical reduction dechlorination only stays in the dechlorination process, which can not realize the ring opening and mineralization of CPs. AOPs based on free radical reaction has the advantages of high oxidation efficiency, fast reaction speed and mild reaction conditions, and has developed rapidly in degrading organic pollutants, especially CPs pollutants. However, these commonly used AOPs have certain limitations. For example, O3 oxidation technology needs to prepare oxidant O3 on site, and the yield is low, which will further increase energy consumption and indirectly increase operating costs. The input of H2O2, persulfate and other oxidants also needs high cost, and persulfate is converted into sulfate through redox process, which increases the ionic strength and salinity of the system and may have adverse effects on the subsequent treatment process; Metal ion catalysts such as cobalt, nickel and silver are toxic heavy metals, and introducing them into the reaction system will inevitably increase environmental risks or cause secondary pollution; In the process of degrading CPs by free radical reaction, more toxic "polychlorinated secondary pollutants" may be produced. Therefore, it is necessary to develop green, efficient and cheap unit treatment technology or combined process to realize the harmless treatment of chlorophenol pollutants. For example, cultivate and domesticate bacteria with high toxicity and high reaction efficiency; Developing renewable adsorbents; Coupling chemical reduction dechlorination with advanced oxidation technology to form segmented advanced reduction oxidation technology to realize reduction dechlorination and oxidation mineralization step by step to avoid secondary polychlorinated pollution; Coupling biological reduction dechlorination and advanced oxidation technology to realize efficient and harmless treatment of CPs pollutants.