Degradation of 2,4-dichlorophenol in wastewater by low temperature plasma coupled with TiO₂ photocatalysis

Abstract

Degradation of 2,4-dichlorophenol (2,4-DCP) in wastewater was conducted in a dielectric barrier discharge (DBD) reactor coupled with TiO₂ photocatalysis. The main advantage of the system is that ultraviolet (UV) light produced by the DBD and reactive species like ozone (O₃) can be used for the treatment of wastewater. In this study, the effect of discharge voltage, initial concentration and initial pH on the degradation of 2,4-DCP was studied. TiO₂ and I–TiO₂ were introduced to enhance the removal efficiency of 2,4-DCP. We also investigated the effect of adding tert-butanol to probe the role of hydroxyl radicals in the reaction. The results indicated that 2,4-DCP could be removed effectively and hydroxyl radicals played an important role during the degradation process by the low temperature plasma. The removal efficiency of 2,4-DCP with I–TiO₂ was better than with TiO₂. The degradation efficiency with 10% I–TiO₂ was 89.59% after 120 min when the discharge voltage was 75 V, pH 5.32 and 50 mg L⁻¹ was selected as the initial concentration. The removal efficiency of 2,4-DCP decreased with the increasing concentration of tert-butanol because alcohols are excellent radical scavengers that inhibit the generation of hydroxyl radicals during the DBD process. The degradation products of 2,4-DCP were characterized qualitatively and quantitatively using Mass spectrometry and UV-Vis spectroscopy. Besides, the degradation mechanism, the degradation pathway and the structures of intermediate products were also examined and discussed in detail.

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1. Introduction

Chlorophenol compounds are a major class of environmental pollutants and potential human carcinogens.¹ They were widely used in the manufacturing processes of plastics, dyes, drugs, pesticides and papers.² Due to the high toxicity, recalcitrance, bioaccumulation, strong odor emission, persistence in the environment, suspected carcinogenicity and mutagenicity to living organisms, chlorophenol compounds cause a serious ecological problem as environmental pollutants.³ They have been listed by the US environmental protection agency as a priority environmental pollutant and their disposal may contaminate soil and water.⁴ Among them, 2,4-DCP is a highly toxic substance and an important reagent in the manufacture of pesticides and herbicides.⁵ Although there have been attempts to regulate the use of 2,4-DCP, large quantities of wastewater containing it continue to be discharged into water bodies. Consequently, the removal of 2,4-DCP is currently relevant.

Over the past few decades, the conventional technologies used for the degradation of chlorophenols compounds have not been found effective for this purpose. The reported methods for removing chlorophenols compounds from wastewater include activated carbon adsorption,⁶ microbial degradation,⁷ chemical oxidation,⁸ corona discharge,⁹ and enzymatic degradation.^10–12 All these methods have some disadvantages, such as high operative costs, long reaction time, poor maneuverability and secondary pollution. Therefore, new techniques are needed for the degradation of chlorophenols compounds. The advanced oxidation process has been proposed as an effective technology for treating organics, but it has drawbacks in terms of economics, equipment and efficiency.^13,14 In recent years, the low temperature plasma technology has been applied and the results show that it is effective for the degradation of chlorophenols compounds.¹⁵

Low temperature plasma is a new wastewater treatment technology combining high-energy electron irradiation, radical particles, and O₃ ultraviolet radiation.¹⁶ When low temperature plasma degrades organic compounds, both physical and chemical processes take place.¹⁷ The former can generate high-energy electrons and large quantities of ultraviolet light, and the latter can cause the formation of active particles. These active particles play an important role in degrading toxic pollutants. Especially, the formation of ˙OH is critical since it is strong oxidizers and its reaction with organic compounds is characterized by very high reaction rates. The detailed formation mechanisms for these particles during discharge process are as follows (eqn (1)–(10)):^18–20

O˙ + O₂ → O₃ (2)

O˙ + H₂O → ˙OH +˙OH (3)

O⁻ + H₂O → ˙OH + OH⁻ (4)

O₂⁻ + H₂O → HO₂˙ + OH⁻ (5)

O₃ +H₂O₂ →˙OH + O₂ + HO₂˙ (7)

O₃ + HO₂˙ → ˙OH + O₂ + O₂⁻ (8)

O₃ + hv + H₂O → H₂O₂+ O₂ (9)

H₂O₂ + hv →˙OH + ˙OH (10)

The primary active species involved in the degradation of organic pollutants are ˙OH and H₂O₂.^21,22 Hydroxyl radicals are effective in breaking down organic pollutants due to their high oxidation potential, and they are formed by the decomposition of H₂O₂ by ultraviolet radiation.²³ In this experiment, 2,4-DCP was selected as the target contaminates to examine the degradation effect by DBD. The aim of this research is to investigate the degradation behavior and degradation mechanism of 2,4-DCP in aqueous solution. Meanwhile, several factors which affecting the removal efficiency of 2,4-DCP were also studied in detail.

2. Materials and methods

2.1 Experimental apparatus

Fig. 1 shows a schematic diagram of the experimental apparatus used in this experiment. It consists of the reaction system and the low temperature plasma power (CTP-2000 K) which could provide steady voltage and current. The total volume of the breaker is 1.0 × 10⁻³ m³. Isolation medium is quartz glass (diameter 90 mm) and the high voltage electrode is located above the quartz glass. The ground electrode is placed in the centre of the sink. The distance between the barrier and solution surface is about 5 mm. The solution is circulated continuously by means of a peristaltic pump (BT100-1L) connected in the inlet at a speed 400 mL min⁻¹.

Fig. 1 The diagram of the experimental DBD apparatus. (1. High voltage electrode 2. Ground electrode 3. Quartz glass 4. Discharge area 5. Overflow hole 6. Low-temperature plasma power 7. Peristaltic pump 8. Beaker 9. Storage tank 10. Outlet pipe 11. Inlet pipe 12. Sink).

2.2 Chemicals and reagents

2,4-DCP is obtained from Suzhou Outlet Chemical Corporation Limited and used without further purification. The molecular structure of 2,4-DCP is shown in Fig. 2. All chemicals are reagent grade with purity higher than 96%. 2,4-DCP solution (pure water is used as solvent) is prepared as simulated wastewater solution.

Fig. 2 The schematic diagram of 2,4-DCP molecular structure.

2.3 Analysis methods

The concentration of 2,4-DCP solution was analyzed by using UV spectrophotometry in accordance with the Beer–Lambert law. The degradation products were detected using Mass Spectrometry. The formula of the removal efficiency of 2,4-DCP was calculated from (11):where η is the removal efficiency (%) of 2,4-DCP, C₀ and C_t is the initial concentration (mg L⁻¹) and the concentration (mg L⁻¹) at time t of 2,4-DCP, respectively.

2.4 Preparation of I–TiO₂ and TiO₂ film by sol–gel method

In this experiment, TiO₂ and 10% I–TiO₂ film was prepared by sol–gel method. They were calcined at 500 °C for 5 h and Fig. 3 is their XRD diffraction spectrogram. As presented in Fig. 3, both 10% I–TiO₂ (a) and TiO₂ (b) had diffraction peaks when 2θ were 25.6°, 37.8°, 48.0°, and the main diffraction peak of XRD was at 25.6° (anatase). There were some diffraction peaks at other 2-theta, but the proportion was relatively low. It was indicated that the main structure of calcined 10% I–TiO₂ and TiO₂ was anatase, and anatase promoting photo catalysis reaction could be got under this condition.

Fig. 3 Diffraction spectrogram of TiO₂ and 10% I–TiO₂ film.

In general, anatase TiO₂ was better than rutile TiO₂ and brookite TiO₂ in terms of photo catalysis and photoelectric catalysis.²⁴ The reason was rutile has the feature of stable crystal shape, good crystal structure, less crystal defect and less oxygen vacancy, and the oxygen vacancy was the key of the photo catalysis reaction. In contrast, anatase had many oxygen vacancies and its forbidden band width was small (3.0 eV),^25,26 therefore, anatase TiO₂ could promote photo catalysis reaction better than rutile and brookite TiO₂.

3. Results and discussion

3.1 The ultraviolet spectral analysis of 2,4-DCP

The concentration of 2,4-DCP after degradation was analyzed by UV-vis spectrophotometry. The UV spectroscopy analysis of the degradation process is shown in Fig. 4. Large absorption peaks were observed at 285 nm and the peaks were stable at 285 nm. There was a slight blue-shift phenomenon at 225 nm as the discharge reaction continued, which may be due to the effects of pH and polarity of the solution,²⁹ therefore, 285 nm was selected as the maximum absorption wavelength in this research.

Fig. 4 UV spectra of 2,4-DCP degraded for different discharge times. (a: 0 min; b: 20 min; c: 60 min; d: 120 min; (1) 200–400 nm; (2) 250–400 nm).

3.2 Effect of the discharge voltage on 2,4-DCP degradation by low temperature plasma

Fig. 5 shows the change of removal efficiency of 2,4-DCP over time for different discharge voltage. It was indicated that 2,4-DCP could be removed effectively by low temperature plasma, and increasing discharge voltage favored the degradation process in a certain range. When discharge time was 120 min, 63.25%, 76.13% and 70.32% of 2,4-DCP was removed when the discharge voltage were 70 V, 75 V and 80 V respectively. The removal efficiency at 75 V was better than 70 V, because the low temperature plasma combines energetic electron radiation, ozone oxidation, UV photolysis and other functions²⁷ and increasing the discharge voltage increased the energy of the system. However, excessive increases of voltage (to 80 V) caused the temperature in the DBD reactor to rise, which was not conductive to generation of O₃, ·OH and other active substances. Besides, excessive high discharge voltage could lead to spark discharge easily. In conclusion, the optimal removal efficiency was achieved at 75 V.

Fig. 5 Effect of different discharge voltage on 2,4-DCP degradation. Condition: (concentration (2,4-DCP) = 50 mg L⁻¹; pH = 5.9; no loading TiO₂).

3.3 Effect of the initial concentration on 2,4-DCP degradation by low temperature plasma

The variation of the removal efficiency of 2,4-DCP with different initial concentration is shown in Fig. 6. The removal efficiency could be affected by the initial concentration and degradation time of 2,4-DCP at same discharge power. The highest removal efficiency of 2,4-DCP (76.13%) was achieved when 50 mg L⁻¹ was selected as initial concentration after 120 min. When the initial concentration was 25 mg L⁻¹ and 100 mg L⁻¹, the removal efficiency was 68.86% and 62.28% respectively. At lower concentrations, the amount of 2,4-DCP molecules was too small to react completely with ˙OH and other active particles which generated during the discharge process. In contrast, the number of 2,4-DCP molecules increased with the increasing initial concentration, but the amount of free radicals particles remained at same level under the same discharge voltage and discharge time. Consequently, these free radicals particles could not degrade a large number of 2,4-DCP molecules. In summary, both the larger and smaller initial concentration reduced the removal efficiency, therefore, 50 mg L⁻¹ was chose as optimal initial concentration in this experiment.

Fig. 6 Effect of different initial concentration on 2,4-DCP degradation. Condition: (discharge voltage = 75 V; pH = 5.9; no loading TiO₂).

3.4 Effect of the initial pH on 2,4-DCP removal efficiency by low temperature plasma

It was well known that the oxidation process was very sensitive to the pH of aqueous.²⁸ Fig. 7 shows the removal efficiency of 2,4-DCP at different initial pH. When the discharge time was 120 min, 73.07%, 76.13% and 71.2% of 2,4-DCP was removed when the initial pH of 2,4-DCP was 3, 5.32 and 9 respectively. The highest removal efficiency was achieved when 5.32 was selected as the initial pH. The reason might be that the number of ˙OH which generated during DBD was more than the amount of H₂O₂ at the condition of weak acid, and the oxidative ability of ˙OH was better than H₂O₂ (the oxidation potential of ˙OH and H₂O₂ was 2.80 V and 1.77 V (ref. 29)). There might be hydrogen bonds generated between molecules and molecules at the condition of strong acidic and they could inhibit the degradation of 2,4-DCP. At the condition of alkaline, HO₂˙ which generated during DBD could react with ˙OH and this reaction would reduced the number of ˙OH reacted with 2,4-DCP.³⁰ Therefore, 5.32 were selected as the optimal initial pH.

Fig. 7 Effect of different initial pH on 2,4-DCP degradation. Condition: (discharge voltage = 75 V; C (2,4-DCP) = 50 mg L⁻¹; no loading TiO₂).

3.5 Effect of the TiO₂ and I–TiO₂ on 2,4-DCP degradation by low temperature plasma

As shown in Fig. 8, the removal efficiency of 2,4-DCP is enhanced in presence of loading TiO₂ and I–TiO₂. When the discharge time was 120 min, 83.68% and 76.13% of 2,4-DCP was removed in the presence and absence TiO₂. The reason was the presence of TiO₂ could cause photo catalysis reaction by UV light which generated during DBD and generate more hydroxyl radicals.³¹ The hydroxyl radical was formed in the presence of TiO₂ through the following possible reactions:³²

TiO₂ + hv → TiO₂ + e⁻ + h⁺ (12)

OH⁻ + h⁺ →˙OH (13)

H₂O + h⁺ →˙OH + H⁺ (14)

Fig. 8 Effect of I–TiO₂ and TiO₂ on 2,4-DCP degradation efficiency. Condition: (discharge voltage = 75 V; C (2,4-DCP) = 50 mg L⁻¹; pH = 5.32).

Reaction (13) and (14) indicated that a large number of ˙OH were produced in the presence of loading TiO₂ during DBD.

The removal efficiency could raise to 89.59% as the introduction of 10% I–TiO₂. There were two reasons could explain this phenomenon, one was the introduction of iodine could enhance the thermal stability of TiO₂ and inhibit the change in the structure of TiO₂; another was that iodine could change the surface morphology of TiO₂ and generate more oxygen vacancies than without iodine.³³ All these could increase the generation efficiency of ˙OH in the surface of TiO₂ and then enhancing the removal efficiency of 2,4-DCP.

3.6 Effect of tert-butanol on 2,4-DCP degradation by low temperature plasma

Fig. 9 shows the change of removal efficiency of 2,4-DCP with different tert-butanol concentrations and increasing concentration of tert-butanol reduced the removal efficiency obviously. Removal efficiency of 76.13%, 42.78% and 20.68% was achieved when 0 mmol L⁻¹, 5 mmol L⁻¹ and 15 mmol L⁻¹ of tert-butanol was added into 2,4-DCP solution. Tert-butanol could inhibit the generation of ˙OH effectively during the DBD. It was because that alcohols were well known as typical hydroxyl radical scavenger,³⁴ and this indicated that ˙OH played a major role for the degradation of 2,4-DCP. The removal efficiency of 2,4-DCP was 13.32% at 20 min when 15 mmol L⁻¹ tert-butanol was added into solution, and it suggested that although ˙OH was the primary oxidant in the degradation of 2,4-DCP by DBD, but also there were other active substances for the degradation at the same time.

Fig. 9 Effect of tert-butanol on 2,4-DCP degradation. Condition: (discharge voltage = 75 V; C (2,4-DCP) = 50 mg L⁻¹; pH = 5.32; no loading TiO₂).

3.7 The analysis of degradation products and degradation pathway of 2,4-DCP

An attempt was made to identify the intermediate products formed during the discharge process through mass spectrometry analysis. We can seen from the mass spectra in Fig. 10 that the amount of 2,4-DCP decreased gradually as the discharge time increased. From Fig. 10a–c we can see that the relative molecular mass of 61.8 substance is always presented and its content changed little, therefore, we believed it was a kind of impurity. It can be concluded from Fig. 10a–c that 2,4-DCP was decomposed into other new substances whose relative molecular masses were 59, 91, 111 and 177 respectively. The relative molecular mass of 177 substance may be generated by the combination of 2,4-DCP and ˙OH. We can concluded from Fig. 10b that part of 2,4-DCP was decomposed into acetic acid by opening the benzene ring directly. Fig. 10c shows that there were few of 2,4-DCP in the solution after 120 min of treatment. It indicated that most of 2,4-DCP were decomposed into other substances. According to the relative molecular masses of intermediates and principles of organic degradation, the possible molecular structures of the intermediates were listed in Table 1.

Fig. 10 (a) Mass spectrogram of 2,4-DCP solution at 0 min. (b) Mass spectrogram of 2,4-DCP solution at 30 min. (c) Mass spectrogram of 2,4-DCP solution at 120 min. MS conditions: (ionization mode: ESI; detection ion: negative; MS range: 50–250; capillary temperature: 320 °C; cheath gas: 40; aux gas: 10; sweep gas: 0.)

Table 1 The molecular structure analysis of intermediates

Substance	m/z	Molecular structure	Formula
Main components	161		C6H4OCl2 exact mass: 161
Intermediate	59		C2H4O exact mass: 60
Intermediate	90		C2H2O4 exact mass: 90
Intermediate	110		C6H6O2 exact mass: 110

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The research of Quanfang Lu showed that part of 2,4-DCP were decomposed into CO₂ and H₂O by using glow discharge electrolysis;³⁴ The study of Ying Wang and Lifen Liu indicated that 2-chlorophenol and 4-chlorophenol may be the intermediate of the degradation of 2,4-DCP;^35,36 The study of Xiangling Liu showed that 1,4-dihydroxybenzene was also an intermediate product of the degradation of 2,4-DCP.³⁷ Lejin Xu's research suggested that there were acetic acid and formic acid generated during the process of Fenton-like degradation of 2,4-DCP when Fe₃O₄ magnetic nanoparticles as catalyst.³⁸ The study of Jiao Zhang showed that oxalic acid was a kind of degradation products of 2,4-DCP by using pulsed high voltage discharge.³⁹

Combining the studies of other scholars and mass spectrometry analysis, and the molecular structure of the intermediates, one probable degradation pathway of 2,4-DCP could be deduced as shown in Fig. 11.

Fig. 11 One probable degradation pathway of 2,4-DCP treated by low temperature plasma.

4. Conclusions

The research showed that 2,4-DCP in wastewater could be effectively removed by low temperature plasma and its removal efficiency was affected by initial concentration, the discharge voltage and the initial pH. The removal efficiency with TiO₂ was 83.68% at 120 min when the discharge voltage was 75 V, pH was 5.32 and 50 mg L⁻¹ was selected as the initial concentration. As the introduction of 10% I–TiO₂, the removal efficiency could raise to 89.59%. The degradation of 2,4-DCP was greatly inhibited by tert-butanol, demonstrating that the degradation was mainly achieved by the reaction of 2,4-DCP with ˙OH. The intermediate products and degradation pathway were investigated by analyzing the mass spectrometry of 2,4-DCP during discharge process. The result indicated that most of 2,4-DCP could be decomposed into other substances if the discharge time was long enough.

Acknowledgements

This research was supported by the Technological Progress Plan of Shandong, Grant no. 2011GGE27048, China. The authors are grateful to the anonymous reviewers for their reading of the manuscript, and for their suggestions and critical comments. The authors thank Dr Pamela Holt, Shandong University, for editing the manuscript.

Notes and references

1. I. Alemzadeh and S. Nejati, J. Hazard. Mater., 2009, 166, 1082–1086.
2. J. Huang, Q. Chang, Y. B. Ding and X. Y. Han, Chem. Eng. J., 2014, 254, 434–442.
3. A. W. Chen, G. M. Zeng, G. Q. Chen, C. Zhang and M. Yan, Chemosphere, 2014, 109, 208–212.
4. J. B. Jia, S. P. Zhang, P. Wang and H. J. Wang, J. Hazard. Mater., 2012, 205, 150–155.
5. Y. L. Guo, M. Y. Li, X. S. Zhang, S. Yuan and H. L. Zhang, Diwen Gongcheng, 2014, 43, 1640–1643.
6. Z. Aleksieva, D. Ivanova, T. Godjevargova and B. Atanasov, Process Biochem., 2002, 37, 1215–1219.
7. S. Chen, J. B. Zhang, H. Chen and Y. X. Yang, Acta Sci. Nat. Univ. Pekin., 2014, 4, 490–495.
8. C. Z. Ma, F. Wang, J. J. Lu, M. C. Li and W. F. Zheng, Chem. Res. Chin. Univ., 2013, 34, 2850–2854.
9. J. Grimm, D. Bessarabov, W. Maier, S. Storck and R. D. Sanderson, Desalination, 1998, 115, 295–302.
10. M. Masuda, A. Sakurai and M. Sakakibara, Enzyme Microb. Technol., 2001, 28, 295–300.
11. C. Kinsley and J. A. Nicell, Bioresour. Technol., 2000, 73, 139–146.
12. G. Zhang and J. A. Nicell, Water Res., 2000, 34, 1629–1637.
13. L. Z. Zhu, B. L. Chen and S. Tao, Environ. Sci. Technol., 2003, 37, 4001–4006.
14. M. A. Malik, A. Ghaffar and S. A. Malik, Plasma Sources Sci. Technol., 2001, 1, 82–91.
15. Y. Chen, X. M. Cheng, L. L. Xu and L. Jin, J. Shihezi Univ., 2009, 27, 750–754.
16. J. B. Zhang, Z. Zheng, Y. N. Zhang and J. W. Feng, J. Hazard. Mater., 2008, 154, 506–512.
17. B. Sun, M. Sato and J. S. Clements, Environ. Sci. Technol., 2000, 34, 509–513.
18. X. J. Xu, Thin Solid Films, 2001, 390, 237–242.
19. N. Sano, T. Kawashima and J. Fujikawa, Ind. Eng. Chem. Res., 2002, 41, 5906–5911.
20. R. Peyrous, P. Pignolet and B. Held, J. Phys. D: Appl. Phys., 1989, 22, 1658–1667.
21. M. G. Nickelsen, W. J. Cooper, C. N. Kurucz and T. D. Waite, Environ. Sci. Technol., 1992, 1, 144–152.
22. A. A. Joshi, B. R. Locke and P. Arce, J. Hazard. Mater., 1995, 41, 3–30.
23. G. T. Li, H. Y. Song and B. C. Liu, Chin. J. Environ. Eng., 2012, 10, 3388–3392.
24. C. Wen, L. Sun, J. M. Zhang and H. Deng, Chem. Res. Chin. Univ., 2006, 27, 2408–2410.
25. A. L. Linsebigler, G. Lu and J. T. Yates Jr, Chem. Rev., 1995, 95, 735–758.
26. W. Choi, A. Termin and M. R. Hoffmann, Phys. Chem., 1994, 98, 13669–13679.
27. Y. S. Mok, J. Jo and J. Christopher Whitehead, Chem. Eng. J., 2008, 142, 56–64.
28. C. L. Chang and T. S. Lin, Plasma Chem. Plasma Process., 2005, 25, 227–243.
29. J. M. Poyatos, M. M. Muñio, M. C. Almecija, J. C. Torres and E. Hontoria, Water, Air, Soil Pollut., 2010, 205, 187–204.
30. J. W. Feng, Z. Zheng, Y. B. Sun and J. F. Luan, J. Hazard. Mater., 2008, 154, 1081–1089.
31. Y. J. Li, J. Li, M. Y. Ma, Y. Z. Ouyang and W. B. Yan, Sci. China, Ser. B: Chem., 2008, 11, 1036–1043.
32. A. Mills and S. Le Hunte, J. Photochem. Photobiol., A, 1997, 1, 1–35.
33. S. H. Yoon, S. E. Oh, J. E. Yang, J. H. Lee, M. Lee and S. Yu, Environ. Sci. Technol., 2009, 43, 864–869.
34. Q. F. Lu, J. Yu and J. Z. Gao, J. Hazard. Mater., 2006, 136, 526–531.
35. Y. Wang, Z. Y. Shen, Y. Li and J. F. Niu, Chemosphere, 2010, 79, 987–996.
36. L. F. Liu, F. Chen and F. L. Yang, Sep. Purif. Technol., 2009, 70, 173–178.
37. X. L. Liu, R. J. Pan and R. P. Li, J. China Three Gorges Univ., 2006, 28, 177–180.
38. L. J. Xu and J. L. Wang, Appl. Catal., B, 2012, 123, 117–126.
39. J. Zhang, D. Q. Liu, W. J. Bian and X. H. Chen, Desalination, 2012, 304, 49–56.

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