Research of advanced treatment technologies for chlorophenol wastewater

. Chlorophenol wastewater belongs to highly toxic industrial wastewater, which can cause serious harm to the environment and human health if not treated effectively. The current treatment methods for chlorophenol wastewater are divided into physical, chemical, and biological methods. The physical adsorption experiment found that the monodisperse magnetic nano-column was prepared by the solvothermal method, which could remove 91.5% of pentachlorophenol within 30 mins. More than 90% of chlorophenol can be extracted using an ionic liquid as an extractant. When the chlorophenol reaches 15 g/L, the extraction rate only slightly decreases. CaO and SiO2 can achieve 99.0% 2,4,6-trichlorophenol in 6 hours as ball milling materials by physical and chemical methods. The electrochemical reduction test found that using Pd/Py SDBS/Ti as the cathode, 2-chlorophenol, 4-chlorophenol, and 3-chlorophenol could be reduced and dechlorinated, in which 100 mg/L of 2-chlorophenol could be completely degraded within 60 minutes. The biochemical index BOD 5 /COD of wastewater is less than 0.3 if the biological treatment technology based on co-metabolism is adopted to realize the chlorophenol treatment. In treating chlorophenol wastewater, the application scenario should be fully considered to achieve the best treatment effect of chlorophenol wastewater.


Introduction
Chlorophenol wastewater belongs to a class of toxic industrial wastewater with poor biochemical properties, mainly generated in industrial industries such as coking, pharmaceutical, petrochemical, and synthetic phenol [1][2][3] .Due to the inadequate treatment of this type of productive wastewater, chlorophenol compounds enter natural water and soil with industrial drainage.Chlorophenols, as a class of synthetic compounds, lack microorganisms in nature that metabolize them, leading to their long-term presence and accumulation in water and soils.In recent years, the presence of chlorophenols has been frequently detected in drinking water, and their long-term consumption by humans can cause severe toxic effects on health [4] .Considering the harm of chlorophenol wastewater to the environment and human health, the search for effective treatment methods has become a hot spot for research.
This review provides a comprehensive overview of chlorophenol wastewater treatment methods, which will provide a reference and basis for developing and engineering applications of subsequent chlorophenol treatment technologies.

Chlorophenol wastewater treatment technology 2.1 Physical methods
Physical methods can achieve effective removal without changing the chemical nature of the target pollutant.Several physical treatment methods include adsorption, extraction, incineration, and membrane separation.The incineration method is gradually decreasing in application because of the inability to control the combustion temperature precisely and the tendency to produce secondary pollutants, such as dioxins while polluting the atmosphere.Adsorption and extraction methods are more mature physical treatment methods used in engineering treatment.

Adsorption method
The adsorption method uses a porous nature and large specific surface area material, which can adsorb chlorophenols on the material and then recover the adsorbent to remove chlorophenol compounds.Many adsorbent materials exist activated carbon adsorbent, chitosan adsorbent, zeolite adsorbent, and clay adsorbent.These are the original adsorbent materials.In the engineering treatment, to improve the adsorption performance and enhance the recovery capacity of the adsorbent, there are mainly mixed with other materials based on the original adsorbent to improve the adsorption performance and recovery performance.This way of synthesizing new materials has also achieved a good removal effect.Liu et al. chose chitosan, a degradable organic material, as a carrier material.They loaded ferric tetroxide on the chitosan surface to produce monodisperse magnetic nanopillars by the solvothermal method, which was tested and found to achieve 91.5% removal of pentachlorophenol by adsorption within 30 min [5] .Adsorption is a more mature technology in practical wastewater treatment, and a suitable adsorbent has high adsorption performance and high recovery, which is a priority factor when selecting an adsorbent.Otherwise, it will cause secondary pollution, but adsorbent preparation is a complex process, and achieving mass production and cost reduction is also essential to improve the treatment effect and applicability.However, when faced with large volumes of chlorophenol wastewater, the cost of adsorption and removal increases significantly, and this becomes a significant factor limiting its application.

Extraction
Extraction is a typical production process in chemicals, materials, wet metallurgy, and other industrial production.In recent years, extraction is also primarily used in the field of environmental pollutant removal, making full use of the different partition coefficients of the target pollutant in the extractant and water so that the target pollutant is transferred from the polluted water and diffused into the extractant to achieve the purpose of removing the target pollutant [6] .A good extractant is a key to improving extraction efficiency, and several types of organic extractants are commonly used, such as alcohols, hydrocarbons, ethers, and ketones [7,8] .Olalla et al. used ionic liquids as extractants to remove chlorophenol compounds from wastewater and achieved more than 90% efficient extraction, which only slightly decreased when the total chlorophenol concentration reached 15 g/L [9] .However, the extraction method is prone to the problems of extractant contamination, poor regeneration efficiency effect, and low recyclability due to the complex water quality in the application process.The extracts need subsequent treatment and are prone to secondary contamination, which limits the extraction method's practical application.

Chemical technique
The chemical method can be divided into reagent dosing and advanced oxidation methods.The reagent dosing method is the chlorophenol decomposition by artificial dosing with strong oxidizing compounds; the advanced oxidation method uses chemical means to produce strong oxidizing substances to engage in redox reactions with chlorophenol or to directly oxidize chlorophenols to carbon dioxide and water [10] .

Mechanochemical
The mechanochemical is a relatively new treatment method in which external physical effects are used to change the target pollutant's physical properties and enhance the compound reactivity so that it is more likely to undergo chemical reactions spontaneously [11] .Mechanochemical usually employ enclosed ball mills as the main reaction device and use metal oxides as ball abrasives to enhance their mechanical collision strength.Lu et al. used CaO and SiO 2 as ball milling materials and achieved more than 99.0% removal of 2,4,6trichlorophenol within 6 h.The results indicated that the chloride ions generated from 2,4,6-trichlorophenol dechlorination reacted synthetically with CaO to form CaOHCl, and complete removal was achieved by reductive dechlorination, complete degradation, carbonization and carbonation [12] .However, the mechanochemical has a typical characteristic that chlorophenol must exist in solid form.The dissolved chlorophenols must be pretreated (purified, solidified) before they can be degraded by ball milling.The ball milling must be added to achieve the desired removal effect, and the reaction products must be rendered harmless.Furthermore, the ball mill operation is energyintensive, and the chlorophenol treatment capacity is limited.Therefore, the use conditions of the mechanochemical are limited, and it is not suitable for treating a large number of phenolic wastewater.

Electrochemical method
Electrochemical can be divided into oxidation and reduction methods.Synergistic anodic oxidation and cathodic reduction within the electrochemical system can improve chlorophenol removal efficiency and complete mineralization of the target pollutant.The basic principle of electrochemical oxidation is direct chlorophenol oxidation at the electrochemical anode or chlorophenol oxidation at the anode by generating highly oxidizing radicals under an applied electric field.In order to improve the oxidation efficiency of the anode for chlorophenol, many researchers have worked on developing new electrode materials, for example, noble metal electrodes (platinum sheets), metal oxide electrodes, and diamond electrodes (BDD).Electrochemical reduction indicates that the chlorophenol atoms gain electrons in the electrochemical system; while gaining electrons, the chlorine ions in chlorophenols are released into the environment, reducing the molecule's stability.In general, the electrochemical reduction is more difficult to achieve complete chlorophenols mineralization, and the dechlorination products are more easily removed by oxidation.Zhao [13] prepared a Pd/PPy-SDBS/Ti cathode and performed electrochemical reduction dechlorination tests on 2-chlorophenol, 4-chlorophenol, and 3chlorophenol, respectively, in which a 100 mg/L 2chlorophenol could achieve complete degradation within 60 min.Electrochemistry improves the influent's biochemical properties, which results in both energy savings and higher wastewater treatment rates.However, electrochemical applications have some limitations, such as electrodes being prone to passivation; poor electrode reusability; electrodes being challenging to cope with large wastewater; and electrodes requiring higher influent quality.Zhu et al. used SBR effluent to configure wastewater containing the target pollutant, and the results showed that the pollutant removal efficiency decreased from the initial 100% to 84% [14] .In addition, high electrolytes are often added to improve the removal efficiency, which increases the possibility of secondary contamination and the subsequent treatment difficulty.; (b) Diuron removal efficiency in real effluent and tap water by AO.Raw diuronconcentration: 20 mg L -1 ; applied current density 3 mA cm -2 ; Na2SO4 0.05 mol L -1 [14]

Ozone oxidation
Ozone is an isomer of oxygen, a light blue gas with strong oxidizing properties that can react with most organic substances.Ozone oxidation is the use of ozone with strong oxidation characteristics so that chlorophenols are oxidated to form small organic substances or directly oxidize to carbon dioxide and water.Feng et al. studied the effect of ozone influx on 2,4,6-trichlorophenol removal using the apparatus as shown in Figure 1. the results indicated that the removal of 2,4,6-trichlorophenol increased with the ozone addition, and finally, 10 mg/L 2,4,6-trichlorophenol could be removed [15] .Not only does ozone play a strong oxidizing role in the system, but the hydrogen peroxide, hydrogen peroxide, and hydroxyl radicals generated along with the oxidation also synergistically achieve the 2,4,6-trichlorophenol oxidation, which shows that the ozone oxidation process is a series of free radical reactions [16] .However, ozone is more likely to oxidize organic matter with double bonds.As the number of small molecules with open rings or short chains of organic matter increases, the efficiency of ozone oxidation will gradually decrease [17] .In addition, the oxidation process needs to prevent ozone leakage, which otherwise causes atmospheric pollution.

Fenton oxidation
The Fenton reaction is a more mature advanced oxidation technique, similar to ozone, which achieves complete mineralization by generating strong oxidizing substances (mainly free radicals) that react with organic matter [18] .
Fig. 3 Experimental setup for sequential pyrite-Fenton and biological treatment [18]   The principle of Fenton oxidation is that divalent iron ions react with hydrogen peroxide under acidic conditions to generate trivalent iron ions, hydroxyl groups, and hydroxyl radicals (Equation 1).Then the hydroxyl radicals dehydrogenate the target pollutant (Equation 2).The dehydrogenated pollutant is then oxidized by trivalent iron ions to small organic molecules, while divalent iron ions are generated, achieving a cycle of iron ions (Equation 3).Li et al. used Fe 2+ , nano-zero-valent iron ZVI, and nano-Fe 3 O 4 as non-homogeneous Fenton catalysts to generate sulfate radicals for 2,4-dichlorophenol removal, and the three catalysts increased the system persulfate radicals by 34.4%, 37.8%, and 5.8%, respectively [19] .Comparing the differences between laboratory and field treatments of the Fenton oxidation, it was found that Fe 2+ is highly susceptible to deactivation or oxidation.In order to avoid Fe 2+ deactivation, other chemical reagents (chelating agents) need to be introduced into the system to achieve the desired effect.However, introducing other chemical reagents is bound to cause secondary contamination.

Biological techniques
The biological method uses the metabolism of microorganisms to metabolize and decompose the target pollutants and use them as nutrients and energy substances to achieve removal or complete mineralization.The advantages of the biological are the large wastewater treatment capacity, thorough treatment, low investment, and operation costs.However, the biological method can only be used if the wastewater meets the biochemical index BOD 5 /COD greater than 0.3.Chlorophenol is poorly biochemical wastewater (BOD 5 /COD < 0.3).Therefore, the microbiological treatment process needs to be changed.
Researchers have proposed the concept of "co-metabolism, " adding a carbon source to chlorophenol wastewater.Cometabolism not only improves the biochemical properties of chlorophenol wastewater but also strengthens the resistance to toxicity and metabolic function of microorganisms.Thus, co-metabolism could achieve the degradation of high-concentration chlorophenol, a more effective biodegradation process for organic wastewater.

Conlusion
Chlorophenol wastewater belongs to highly toxic industrial wastewater, which can cause serious harm to the environment and human health if not treated effectively.The current treatment methods for chlorophenol wastewater are divided into physical, chemical, and biological methods.The adsorption and extraction methods are more mature physical treatment methods used in engineering treatment.The chemical method can be further divided into reagent dosing and advanced oxidation methods.The biological method uses the metabolism of microorganisms to metabolize and decompose the target pollutants.Biological methods have more significant advantages in terms of applicability and economy than physical and chemical methods.However, the biological approach to overcoming chlorophenols toxicity to microorganisms is still a primary issue among researchers.

Fig. 1 .
Fig. 1. (a)The effects of current on the conversion rate of 2-CP[13]