With the continuous development of the industrialization process of human society, a large amount of industrial waste water is produced, which poses a serious threat to human health and survival. Organic sewage is the main part of industrial wastewater. Its composition is complex, toxic, and difficult to degrade, which is a difficult point in environmental governance.
Bisphenol A is an important chemical raw material for the production of polycarbonate and epoxy resins with a wide range of uses (such as plastic bottles, pipes, toys, medical equipment and electronic products) and other polymer materials1. It can also be used in fine chemical products such as plasticizers, flame retardants, heat stabilizers, antioxidants, and pesticide coatings.
Bisphenol A is also a hormone disruptor, which can cause disorders of the endocrine system2, and be exposed to the human body through diet or non-dietary, causing harm to human health3. After bisphenol A enters the human body, it produces estrogen-like or anti-estrogen effects through a variety of reaction mechanisms, thereby affecting the endocrine, reproductive and nervous systems, and producing adverse effects such as cancer promotion. Toyohira, Y used animal experiments to prove that bisphenol A can affect the secretion of hormone levels in the body and affect the normal metabolism of animals4.
In addition, bisphenol A also has certain embryo toxicity and teratogenicity. Animal and cell tests have confirmed that bisphenol A can increase the incidence of breast, prostate, and ovarian cancer in the body5-7.
Since the 1980s, Advanced Oxidation Process (AOPs), which is marked by the generation of hydroxyl radicals (•OH), has attracted wide attention because of its advantages such as fast reaction rate, complete treatment, no pollution, and wide application range.
The Fenton method is one of the classic AOPs processes, and the •OH generated by the Fenton system has a higher redox potential8, which can perform non-selective oxidative degradation of organic pollutants. Once hydroxyl radicals are formed, they will induce a series of free radical chain reactions, attack various organic pollutants in water, and finally mineralize them into H2O, CO2 and inorganic salts9. Fenton process has the advantages of high pollutant removal efficiency, mild reaction conditions, simple operation and low cost10, and has been widely used in sewage treatment in recent years. However, there are some problems in the traditional Fenton process, including the difficulty in separating and recovering the catalyst, and prone to secondary pollution and so on. In addition, the traditional Fenton reaction can be applied to a very narrow pH range, generally from 3 to 5. When treating organic wastewater, the wastewater needs to be pre-acidified. Therefore, reducing the cost of polluting wastewater treatment and simplifying the treatment process while ensuring that the water quality treatment requirements meet the standards have become an important direction for the development of industrial wastewater treatment technology.
In recent years, there have been many studies on the degradation of bisphenol A, Matz Dietrich11 et al. used a low frequency (24 kHz) ultrasonic horn and two boron-doped diamond electrodes to study the degradation of bisphenol A by a electrochemical hybrid system. It is found that under the synergistic effect of ultrasound and electrochemical oxidation, the degradation rate of bisphenol A with an initial concentration of 1 mg/L can reach 90% within 30 minutes. Anakovai et al.12 had studied the degradation of bisphenol A from lab-scale to pilot-scale, in which the degradation rate of bisphenol A can reach 90% under certain conditions.
Cai et al.13 proposed an effective method to remove bisphenol A from water using HP-β-CD polymer, which can quickly remove bisphenol A. Yiguang Qian14et al. used the activated sludge biodegradation method to treat bisphenol A and achieved good results.
In the past, many researches on the removal of bisphenol A also have some shortcomings, such as the high cost of material synthesis, some need to add more acid to adjust the pH, the treatment process is complicated, and the treatment time is long and so on.
In this experiment, the configured bisphenol A solution was used to simulate organic wastewater. This paper aims to overcome the shortcomings of the traditional Fenton method. A simple method is used to synthesize a green and pollution-free catalyst, and under neutral conditions, can achieve a good treatment effect on bisphenol A with less amount of catalyst and H2O2. In this way, the treatment time and treatment cost is reduced, and the organic wastewater treatment process is simplified.
MnO2 is one of the most effective transition metal oxides to degrade organic pollutants, and it is also favored because of its low cost and environmental friendliness15. MnO2 has a good adsorption and degradation effect on phenolic organic matter in water. Among the many crystal phases of MnO2, α-MnO2 has better catalytic activity than other crystal phases. The catalytic activity in the many crystal phases of MnO2 has the following order: α-MnO2> γ-MnO2> λ-MnO2> β-MnO216. Saputra et al. tested the catalytic activity of different MnO2 crystals (α-MnO2, β-MnO2, γ-MnO2) in PMS solution, and found that α-MnO2 nanowires presented the highest activity, which was attributed to its high surface area and better crystal structure17.
Coconut shell activated carbon has a good adsorption effect due to its large specific surface area. Combining the advantages of α-MnO2 and coconut shell activated carbon to prepare a composite catalyst α-MnO2/AC can better catalyze the degradation of bisphenol A. Activated carbon can adsorb bisphenol A on the surface of the catalyst, then the effective component α-MnO2 on the surface of the catalyst degrades bisphenol A to achieve the purpose of rapid degradation of bisphenol A.
This experiment used bisphenol A as the target pollutant, the α-MnO2 and α-MnO2/AC were synthesized by a simple method, the degradation efficiency of α-MnO2 and α-MnO2/AC on bisphenol A was studied. The mechanism of degradation of bisphenol A by α-MnO2 is seen in Fig. 1 and possible reaction is as follows :
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Mn(Ⅳ)+H2O2 → Mn(Ⅱ)+HO2 • +H+
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(1.1)
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Mn(Ⅱ)+H2O2 → Mn(Ⅳ)+HO • +OH-
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(1.2)
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HO2 • → H++ •O2-
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(1.3)
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HO • (•O2 -) + bisphenol A → H2O+CO2
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(1.4)
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Mn(Ⅳ) + bisphenol A → Mn(Ⅱ)+H2O+CO2
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(1.5)
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Abdul et al.18 took a certain solution during the degradation of bisphenol A and analyzed the intermediate products of bisphenol A degradation by LC-MS. From the LC-MS results, several products were identified, including p-isopropenylphenol, 4-isopropylphenol, benzophenone, dihydroxylated bispheol A and formic acid. In conjunction with other papers 19-21, possible pathways for the degradation of bisphenol A have been proposed, as shown in Fig. 2.