Degradation of p-nitrophenol using CuO/Al₂O₃ as a Fenton-like catalyst under microwave irradiation

Abstract

CuO/Al₂O₃ was synthesized successfully by the impregnation-deposition method and used as a heterogeneous catalyst in a microwave assisted Fenton-like process (MW/FL). The morphology and physico-chemical properties of the CuO/Al₂O₃ catalyst were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) analysis. The degradation of p-nitrophenol (PNP) was investigated by different processes, including microwave irradiation (MW) alone, hydrogen peroxide (H₂O₂) oxidation under microwave irradiation without the catalyst (MW/H₂O₂), a Fenton-like (FL) process, a MW/FL process and a thermally-assisted Fenton-like (TH/FL) process. The results showed that the CuO/Al₂O₃ catalyzed MW/FL could generate more hydroxyl radicals (˙OH) and remove PNP more effectively compared to other processes. The effects of initial pH, dosage of H₂O₂ and catalyst, microwave power and radiation time, and PNP concentration on the removal efficiency were also studied. The results showed that 93% removal efficiency of PNP was obtained within 6 min under optimized conditions. Moreover, the catalyst had a good stability and reusability, thus expanding the scope of non-iron based catalysts in the Fenton-like reactions.

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1. Introduction

PNP is one of the most important nitroaromatic compounds which are a major class of environmental contaminants and widely used in the manufacture of agrochemicals, dyes and explosives.^1–3 Because of its acute toxicity and mutagenic potential, PNP contamination can pose a significant environmental and public health risk.^2,4,5 The United States Environmental Protection Agency has listed PNP as a priority pollutant.¹ Due to its stability to chemical and biological degradation, it is difficult to purify PNP-contaminated wastewater.^5–7

The advanced oxidation processes (AOPs) based on ˙OH have attracted considerable attention to remove toxic and persistent contaminants.^8–12 The Fenton process, as one of the most popular AOPs, has been investigated extensively in the treatment of non-biodegradable organic pollutants, in which the iron(II) catalyzed decomposition of H₂O₂ can produce ˙OH easily.^8,13,14 Nevertheless, the homogeneous Fenton systems have a couple of disadvantages with which the overall efficiency is limited: (i) the presence of large quantities of dissolved iron in the solution after the treatment may demand a secondary treatment^15,16 and (ii) Fenton reactions need a limiting pH range (2.5–3.5), which requires extra conditioning before and after the treatment.^17–20

To overcome the drawbacks raised from the homogeneous Fenton process, more and more reports have focused on the development of a heterogeneous catalyst as an alternative for the degradation of organic pollutants, which is a so-called heterogeneous Fenton-like process.²¹ During the last decades, it has been extensively reported that transitional metals or metal oxides as a heterogeneous catalyst pose high activity in Fenton-like oxidation of hazardous pollutants.²² And among a variety of heterogeneous catalysts, copper-based catalysts have attracted significant interest because of their outstanding catalytic properties and reaction can be performed at near neutral pH value.^11,23–25

On the other hand, researchers used additional assistance such as photo,^26–28 ultrasound²⁹ and microwave^30–33 to promote the Fenton oxidative degradation of wastewater, because it's swift, high efficient, and environmental friendly. The applications of microwave energy to enhance chemical reactions are well known.³⁴ There are researches suggested that in AOPs with microwave irradiation a better degradation efficiency could be obtained comparing with traditional treatment methods.^35–37

In this work, the applicability of CuO/Al₂O₃ as a heterogeneous catalyst for MW/FL degradation of PNP was investigated. The influence of various parameters on the degradation efficiency was investigated and the beneficial effect of microwave field in promoting the reaction efficiency of the Fenton-like processes was emphasized.

2. Experimental

2.1. Materials

The γ-Al₂O₃ particles (2–3 mm in diameter) used as carriers were purchased from Zibo Henghuan Chemical Co., Ltd. All other chemicals used in the experiments were purchased from Aladdin Reagent Co., Ltd. They were all of analytical grade and used without further purification. All solutions were prepared using deionized (DI) water at room temperature.

2.2. Preparation of catalyst

The CuO/Al₂O₃ catalyst was prepared by impregnation-deposition method.³⁸ Briefly, Cu(NO₃)₂·3H₂O was used as the source of Cu. Before the preparation, Al₂O₃ particles were pretreated by the following steps: Al₂O₃ particles were washed repeatedly with DI water at first, dried in an air drying oven at 383 K, and then calcined at 623 K for 3 h. In a typical synthesis, 20 g of the pretreated Al₂O₃ particles were immersed into 100 mL of 0.6 M Cu(NO₃)₂ solution in a constant temperature water bath at 328 K for 24 h. Then, they were filtered and immersed into 100 mL of 0.4 M NaOH solution to react at 328 K for 24 h. After the reaction was completed, the resulting blue granulated product was filtered, washed with DI water to remove possibly remaining NaOH, and then dried overnight at 383 K in the air drying oven. The CuO/Al₂O₃ catalyst was finally obtained after being calcined in air atmosphere at 623 K for 4 h using a muffle furnace with the heating ramp rate of 10 K min⁻¹.

To study the XRD, the unsupported CuO was prepared as follows: 0.06 mol of Cu(NO₃)₂ and 0.04 mol of NaOH were mixed with 100 mL of DI water and the mixture continued to be stirred at 328 K for 24 h. Then the resulting products were filtered and washed with DI water repeatedly and dried overnight at 383 K. Finally, CuO powder were obtained after calcining at 623 K for 4 h.

2.3. Characterization

The surface morphology of the CuO/Al₂O₃ catalyst was observed by field emission scanning electron microscopy (FESEM, FEI Helios Nanolab 600i) equipped with Oxford Instruments X-ray energy dispersion spectrograph (EDS). The XRD analyses for the CuO/Al₂O₃ and CuO were obtained by X-ray diffractometer (Rigaku, D-MAX-RB) with Cu K alpha radiation source (λ = 1.5418 Å) at 40 kV and 40 mA. The data were collected for 2θ from 20° to 80° with a scan speed of 0.02° s⁻¹. The BET specific surface area, pore volume and pore size distribution of the CuO/Al₂O₃ catalyst were measured by BET analyzer (Micromeritics, ASAP2020) using N₂ adsorption–desorption at 77 K.

2.4. Procedures and analyses

All the experiments were carried out in a 250 mL glass conical flask containing 100 mL of PNP solution. The microwave radiation source was a chemical reaction microwave oven (EXCEL, with a frequency of 2.45 GHz), which was purchased from PreeKem Scientific Instruments Co., Ltd. (Fig. 1). The initial pH of PNP solution was adjusted to the desired values using 0.4 M HCl and 0.4 M NaOH for the experiments at different pH conditions. A given amount of CuO/Al₂O₃ catalyst was added into the PNP solution, and the suspension was stirred for 30 min to reach adsorption–desorption equilibrium between the catalyst and PNP before the reaction. Then the appropriate amount of H₂O₂ was also added into the solution. The temperature of the PNP solution after 6 min of microwave irradiation of 100 W power reached about 343 K. The TH/FL process was also carried out as a reference condition, and a constant temperature water bath (358 K) was used as the thermal source. The removal efficiencies in other different processes were also investigated at ambient temperature (∼298 K). For the stability and reusability test of the catalyst, after each degradation experiment, the resulting suspension was centrifugalized and the catalyst was recycled. The experiments for the comparison of different processes and the catalyst's reusability test were performed three times to confirm their reproducibility. The presented data were mean values with standard deviations (SD) as error bars.

Fig. 1 Schematic diagram of experimental setup.

Samples (1.0 mL) taken at selected time intervals were immediately filtered through a 0.45 μm syringe micro-filter to separate the catalyst from the solution and diluted 1 : 25 with 0.4 M NaOH solution. Subsequently, the concentrations of residual PNP were determined by light absorbance measurement at 400 nm using a UV-vis scanning spectrophotometer (Shimadzu, UV2550). The TOC of the sample was measured by a TOC analyzer (Shimadzu, 5000A). The indirectly determination of ˙OH was conducted by a fluorescence method, as reported in our previous studies.^39,40 The dissolved copper concentration in the solution after reaction was determined using a flame atom absorption spectrophotometer (PerkinElmer, AAnalyst200).

3. Results and discussion

3.1. Characterization of catalyst

Fig. 2 shows the EDS spectra and FESEM images of Al₂O₃ support and the synthesized CuO/Al₂O₃ catalyst. It could be found obviously that, comparing with the pure Al₂O₃ (Fig. 2a), a large number of particles attached to outer surface after the CuO loading (Fig. 2b). According to the chemical EDS analysis, Al and O are the only elements in Al₂O₃ support, while the resulting CuO/Al₂O₃ catalyst was composed of copper, aluminium and oxygen without other elements, indicating that the CuO/Al₂O₃ catalyst was synthesized successfully and not contaminated with other elements during the preparation process. CuO particles with the flake structure are uniform and well defined, and about 50 nm thick and 150–400 nm in diameter (Fig. 2c).

Fig. 2 EDS spectra and FESEM images of (a) the Al₂O₃ support and (b) the synthesized CuO/Al₂O₃ catalyst; (c) FESEM image of the synthesized CuO/Al₂O₃ catalyst.

The XRD patterns of the CuO/Al₂O₃ catalyst and unsupported CuO sample (prepared by the similar method) are presented in Fig. 3. The XRD pattern of the CuO/Al₂O₃ catalyst showed no obvious diffraction peak of crystalline CuO, indicating the high dispersivity of CuO on the surface of Al₂O₃. It also can be found that the pattern of the CuO sample show XRD reflections in the 2θ region of 20–80°, which were characteristic of the monoclinic phase of CuO (JCPDS 45-0937). These peaks exhibit narrow and intense characteristics, indicating a good crystallinity.

Fig. 3 XRD patterns of the synthesized CuO/Al₂O₃ catalyst and unsupported CuO sample.

Fig. 4 shows the nitrogen adsorption–desorption profiles obtained for the Al₂O₃ support and the synthesized CuO/Al₂O₃ catalyst. As shown in Fig. 4, both the Al₂O₃ and the CuO/Al₂O₃ have significant hysteresis loops that exhibited typical isotherms of mesoporous structure, where the curve obtained from the CuO/Al₂O₃ catalyst are found to shift down along the Y-axis. This suggested the decrease of pore size after the CuO loading, and also proved that the loaded CuO entered into channels of the Al₂O₃ support.¹⁸

Fig. 4 N₂ adsorption–desorption isotherms obtained for Al₂O₃ support and the synthesized CuO/Al₂O₃ catalyst.

The BET surface area, the average pore volume and the average pore diameter derived from nitrogen physisorption are listed in Table 1. It can be found that the loading of CuO on the Al₂O₃ leads to an increase in the specific surface area and a slight decrease in the average pore volume and the average pore diameter. A specific surface area of 262 m² g⁻¹, an average pore volume of 0.37 cm³ g⁻¹, and an average pore diameter of 5.3 nm were obtained for the resulting CuO/Al₂O₃ catalyst.

Table 1 Physico-chemical properties of Al₂O₃ support and the synthesized CuO/Al₂O₃ catalyst

Sample	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	Dp (nm)
Al2O3	246	0.40	5.8
CuO/Al2O3	262	0.37	5.3

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3.2. MW/FL oxidation of PNP

3.2.1. Performances of different processes. To compare the PNP removal efficiency in different processes, the PNP degradation experiments by MW alone, MW/H₂O₂, FL process, MW/FL process and TH/FL process were carried out. The results (Fig. 5) show that the removal of PNP was hardly observed by MW alone, which indicated that the energy of microwave irradiation is too weak to break the bonds of the PNP, and the volatilization of PNP in our experiments could be ignored. Furthermore, there was only 15% PNP removal in MW/H₂O₂ process. The PNP removal efficiency exceeded 90% in MW/FL process, which was nearly 80% higher than that in FL process. In addition, in TH/FL process the removal efficiency of PNP reached 71%, in which the temperature of water bath (358 K) is higher than the final temperature of the solution in MW/FL process (343 K). The order of the PNP removal efficiency was MW/FL > TH/FL ≫ MW/H₂O₂ > FL. This indicated that MW/FL process had a synergistic effect for degrading PNP. Compared with the conventional heating method, an obvious improvement of the removal efficiency was observed in the case of microwave irradiation.

Fig. 5 PNP removal in different processes (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

Fluorescence spectra of 2-hydroxyterephthalic acid generated in different processes were shown in Fig. 6. It could be observed that the order of the amount of ˙OH generated in different processes was same with that of the PNP removal efficiency (Fig. 5). The amount of ˙OH generated in the MW/FL process was much larger than that in FL process, indicating the generation of ˙OH in MW/FL process was enhanced by the addition of microwave irradiation. There was nearly no ˙OH generated in MW process. Therefore, it could be confirmed that the synergistic effect of microwave irradiation and Fenton-like reaction promoted the generation of ˙OH. The MW/H₂O₂ process had some contribution to the generation of ˙OH because microwave irradiation improves the generation of ˙OH by decomposition of H₂O₂:⁴¹

H₂O₂ + microwave energy → 2˙OH (1)

Fig. 6 ˙OH generated in different processes (terephthalic acid concentration = 800 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

It could be found that, although the temperature of the PNP solution after reaction in TH/FL process was higher than that in MW/FL process, the promoting effect of constant temperature water bath on the generation of ˙OH was not as much as that of microwave irradiation. This indicates that the heat effect is not the only cause of the promoting effect of microwave irradiation.

3.2.2. Effect of various parameters on PNP removal. In MW/FL process the removal efficiency of PNP depends on several factors such as initial pH, dosage of H₂O₂ and catalyst, microwave power and radiation time, and PNP concentration.

Conventional Fenton processes are influenced greatly by solution pH and can only achieve ideal removal efficiency when initial pH is 2.5–3.5.¹⁸ The effect of the initial pH on PNP removal under microwave irradiation was investigated first by varying the initial pH in the range of 2.0–10.0. The results (Fig. 7) show that PNP was almost completely removed from initial pH 2.0 to 3.0. And the removal efficiency of PNP decreased slowly when the initial pH increased from 3.0 to 8.0, which still remained more than 85%; A further increase in initial pH resulted in a faster decrease of the removal efficiency of PNP. At the initial pH of 10.0, the removal of PNP dropped to 62%. This phenomenon is similar with the reported literature^42,43 and can be attributed to several aspects. First, at lower pH, the H₂O₂ decomposition reaction was promoted to yield ˙OH in the solution whereas the recombination reaction of the free radicals decreases.^42,43 Furthermore, in alkaline solution the deprotonation of hydroxyl groups increases the stability of the C–NO₂ bond and consequently the PNP molecule.⁴² Obviously, MW/FL could greatly expand the pH range of reaction compared with conventional Fenton system. In particular, the removal of PNP reached 93% when the initial pH of the PNP solution was 6 (without adjustment).

Fig. 7 Effect of initial pH on PNP removal (PNP initial concentration = 50 mg L⁻¹; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

The effect of the H₂O₂ concentration on the PNP removal was investigated for the H₂O₂ concentration from 5 to 100 mM. The data illustrated in Fig. 8 show the similar influences of the H₂O₂ concentration on the processes with and without CuO/Al₂O₃. For both processes, the removal efficiency of PNP increased significantly with the increase of the H₂O₂ concentration until a concentration of 25 mM was reached. When the H₂O₂ concentration exceeded 25 mM, the increase of the removal efficiency slowed down greatly. At low concentrations, there was not enough H₂O₂ that generated enough ˙OH.⁴⁴ At rather high concentrations, H₂O₂ can act as a scavenger of ˙OH:^45,46

H₂O₂ + HO˙ → HO₂˙ + H₂O (2)

HO₂˙ + HO˙ → H₂O + O₂ (3)

Fig. 8 Effect of H₂O₂ concentration on PNP removal (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

Nevertheless, at the same H₂O₂ concentration of 25 mM, the removal efficiency of PNP was 93% in the process with CuO/Al₂O₃ while that was just below 15% in the process without CuO/Al₂O₃.

Fig. 9 shows the effect of the catalyst dosage on PNP removal. It can be observed that the PNP removal was largely enhanced by increasing the catalyst dosage from 0 to 4.0 g, mainly because higher dosage of catalyst can offer more active sites that can expedite generation of ˙OH and thereby promote the removal efficiency of PNP.⁴⁷ When the dosage of CuO/Al₂O₃ reached to 4.0 g, PNP could be removed 93% in 6 min. While further enhancing the catalyst amount, the PNP removal efficiency was hardly enhanced. The change occurred probably because the concentration of H₂O₂ is constant and there was not enough H₂O₂ to be decomposed by excess catalyst. Thus, it can be concluded that the optimum catalyst dosage was about 4.0 g under the present MW/FL system.

Fig. 9 Effect of catalyst dosage on PNP removal (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; microwave power = 100 W; radiation time = 6 min).

PNP removal can be enhanced by increasing microwave power. As mentioned before, H₂O₂ in solution generates extra ˙OH when it is exposed to MW irradiation (eqn (1)). Violent motion of H₂O₂ molecules (which are polar compounds) induced by microwave irradiation can result in an increase of collision numbers and excitation of molecules to a higher-excited state (higher vibrational and rotational energy levels), resulting that the strength of the molecular bonds reduce and consequently rupture.⁴⁸ Moreover, the excitation of PNP molecules also contributed to the higher PNP removal in MW/FL. Besides, localized superheating (hot spots) with high temperature above 1473 K on the surface of CuO/Al₂O₃ catalyst can be caused by Maxwell–Wagner effect of microwaves.⁴⁹ These hot spots could accelerate the reactions of H₂O₂ and PNP molecules which are adsorbed on the surface of CuO/Al₂O₃ catalyst. Since microwave energy efficiency is an important parameter in the MW/FL process, the effects of microwave energy consumption on the PNP removal were studied under different microwave powers. As shown in Fig. 10, under the same microwave energy consumption, the different microwave powers got the different PNP removal efficiencies. The PNP removal efficiency was enhanced in different degrees by increasing microwave energy consumption, and increased rapidly at low microwave powers (50 and 100 W) while slowly at high microwave powers (200, 300 and 400 W). Under a constant microwave energy consumption, lower microwave power means longer radiation time and consequently more completed PNP removal. Consequently, the removal efficiency increased by prolonging reaction time under the same microwave energy consumption and reached a peak of 93% after 6 min of reaction time under microwave radiation of 100 W.

Fig. 10 Effect of microwave energy consumption on PNP removal (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g).

The effect of initial PNP concentration on the MW/FL processes was investigated with different concentrations of PNP and the results were shown in Fig. 11. From the results it is possible to see that the increase of the initial concentration of PNP from 50 to 300 mg L⁻¹ decreases the PNP removal efficiency from 93% to 68%. This phenomenon might be due to the fact that with a constant concentration of ˙OH the increase of PNP concentration decreases the relative concentration of ˙OH, which led to reduced PNP removal efficiency.^47,50 However, 68% of PNP removal efficiency was still obtained even when the initial PNP concentration was as high as 300 mg L⁻¹.

Fig. 11 Effect of PNP concentration on PNP removal (initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

Fig. 12 shows the change of the UV-vis absorption spectrum for the PNP solution during the MW/FL process in a typical experiment. The gradual decrease of absorption peak at λ_max (400 nm) indicated that PNP was removed gradually with the degradation time. To get the degree of mineralization of the PNP, the TOC removal efficiency was measured at the end of degradation process. The TOC removal efficiency reached 68% within 6 min in MW/FL process. The final pH of the solution is about 5.3, and this decrease may be due to the generation of intermediate degradation products of PNP.

Fig. 12 The UV-vis absorption spectra of PNP with the reaction time, as indicated in the legend (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

The AOPs are mainly based on the generation of the ˙OH, which has a great oxidation power and is able to oxidize almost all organic compounds to carbon dioxide and water.⁵¹ Fluorescence spectra data given in Fig. 6 show that there were a large amount of ˙OH generated during the MW/FL process. In the MW/FL process the ˙OH that oxidize PNP are produced through the decomposition of H₂O₂ under microwave irradiation (eqn (1)) and the reaction between CuO and H₂O₂ adsorbed on the surface of CuO/Al₂O₃ catalyst.²³

≡Cu²⁺ + H₂O₂ → ≡Cu⁺ + HO₂˙ + H⁺ (4)

≡Cu⁺ + H₂O₂ → ≡Cu²⁺ + OH˙ + OH⁻ (5)

The PNP degradation pathway by ˙OH oxidation has been well discussed previously.^4,5,7 The intermediate degradation products of PNP mainly include 4-nitropyrocatechol, hydroquinone, 1,2,4-benzenetriol and benzoquinone, and the possible formation pathway of intermediate products in PNP degradation process is illustrated in Fig. 13. The electron-donor property of hydroxyl group on the aromatic ring favors the electrophilic attack of the OH˙ on the ortho- and para-positions.^7,35 Attack of ˙OH at ortho-position leads to the formation of 4-nitropyrocatechol. Meanwhile the para-position of PNP may be attacked by ˙OH and nitrite ion is eliminated from PNP to yield hydroquinone, which subsequently changes into 1,2,4-benzenetriol and benzoquinone. Subsequently, these intermediates further react with ˙OH leading to ring cleavage and final formation of carbon dioxide and water, and nitrous acid is oxidized into nitrate ion.

Fig. 13 Possible producing pathway of the intermediate degradation products of PNP.

3.3. Stability tests of CuO/Al₂O₃ catalyst

In order to evaluate the stability of catalyst, the PNP removal efficiencies over CuO/Al₂O₃ in MW/FL process were assessed and the results were showed in Fig. 14. After each cycle, the catalyst was just recycled by filtration after the former four cycles. Furthermore, the catalyst was regenerated by washing with DI water and drying in oven at 110 °C after the fifth, sixth and seventh cycles. It can be found that the decay of catalyst activity was quite slight between the former four cycles. When the catalyst was reused four times, the PNP removal efficiency still remained more than 90%. Although a significant decline in the PNP removal efficiency was observed at the fifth reuse, high removal efficiency was restored by regeneration. The PNP removal efficiency remained more than 90% in 3 more times of reuse with regeneration. The concentrations of dissolved copper in the solution after each reaction were determined and the results indicated that the total amount of dissolved copper after five cycles of reaction was less than 1%. So the decrease of the catalyst activity may be due to the occupying the active sites of CuO/Al₂O₃ by some organic intermediates.⁴⁵

Fig. 14 Oxidation of PNP on CuO/Al₂O₃ in eight batch reaction cycles (PNP initial concentration = 50 mg L⁻¹; initial pH = 6.0; H₂O₂ dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).

4. Conclusion

In this study, the CuO/Al₂O₃ catalyst was synthesized and used at the heterogeneous catalytic system for degradation of PNP in MW/FL process. The catalyst was characterized by FESEM, XRD and BET analyzer. The results showed that, the CuO attached to the surface and pores of Al₂O₃ with the flake structure and there is an increase of specific surface area. The results also showed that the MW/FL process could degrade PNP quickly. In contrast to FL process, the MW/FL could generate larger amount of ˙OH, and remove PNP more effectively. The initial pH, dosage of H₂O₂ and catalyst, microwave power and radiation time, and PNP concentration had strong effect on the removal efficiency of PNP in MW/FL process, and the optimum parameters were a H₂O₂ dosage of 25 mM, a catalyst dosage of 4.0 g, a microwave power of 100 W without adjustment of the initial pH (pH = 6). Under these conditions, the PNP removal efficiency reached 93% after a reaction time of 6 min for 100 mL of 50 mg L⁻¹ initial concentration. The results also showed the catalyst had a good stability and reusability by catalyst regeneration. Notably, this study proved that the MW/FL process catalyzed over CuO/Al₂O₃ could overcome the drawback of the homogeneous catalyst, widen the applicable initial pH range of reaction and improve greatly the degradation efficiency.

Acknowledgements

The authors sincerely thank the National Water Pollution Control and Management Technology Major Projects (2012ZX07205-005) and Creative Research Groups of the National Science Foundation of China (5112106) for the financial support. We also acknowledge the technical support of State Key Laboratory of Urban Water Resource and Environment at Harbin Institute of Technology.

Notes and references

1. A. Chauhan, et al., p-Nitrophenol Degradation via 4-Nitrocatechol in Burkholderia sp. SJ98 and Cloning of Some of the Lower Pathway Genes, Environ. Sci. Technol., 2010, 44(9), 3435–3441.
2. S. Yi, et al., Biodegradation of p-nitrophenol by aerobic granules in a sequencing batch reactor, Environ. Sci. Technol., 2006, 40(7), 2396–2401.
3. S. Labana, et al., Pot and field studies on bioremediation of p-nitrophenol contaminated soil using Arthrobacter protophormiae RKJ100, Environ. Sci. Technol., 2005, 39(9), 3330–3337.
4. T. C. Wang, et al., Plasma-TiO₂ Catalytic Method for High-Efficiency Remediation of p-Nitrophenol Contaminated Soil in Pulsed Discharge, Environ. Sci. Technol., 2011, 45(21), 9301–9307.
5. L. X. Yang, et al., High Efficient Photocatalytic Degradation of p-Nitrophenol on a Unique Cu₂O/TiO₂ p–n Heterojunction Network Catalyst, Environ. Sci. Technol., 2010, 44(19), 7641–7646.
6. L. Li, et al., Removal of methyl parathion from artificial off-gas using a bioreactor containing a constructed microbial consortium, Environ. Sci. Technol., 2008, 42(6), 2136–2141.
7. M. A. Oturan, et al., Complete destruction of p-nitrophenol in aqueous medium by electro-Fenton method, Environ. Sci. Technol., 2000, 34(16), 3474–3479.
8. F. Duarte, et al., Fenton-like degradation of azo-dye Orange II catalyzed by transition metals on carbon aerogels, Appl. Catal., B, 2009, 85(3–4), 139–147.
9. S. S. Chou, et al., Factors influencing the preparation of supported iron oxide in fluidized-bed crystallization, Chemosphere, 2004, 54(7), 859–866.
10. M. H. Zhou and L. C. Lei, An improved UV/Fe³⁺ process by combination with electrocatalysis for p-nitrophenol degradation, Chemosphere, 2006, 63(6), 1032–1040.
11. H. C. A. Valdez, et al., Degradation of paracetamol by advance oxidation processes using modified reticulated vitreous carbon electrodes with TiO₂ and CuO/TiO₂/Al₂O₃, Chemosphere, 2012, 89(10), 1195–1201.
12. S. G. Kumar and K. S. R. K. Rao, Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications, RSC Adv., 2015, 5(5), 3306–3351.
13. L. G. Devi, K. S. A. Raju and S. G. Kumar, Photodegradation of methyl red by advanced and homogeneous photo-Fenton's processes: a comparative study and kinetic approach, J. Environ. Monit., 2009, 11(7), 1397–1404.
14. C. C. Su, et al., Effect of operating parameters on decolorization and COD removal of three reactive dyes by Fenton's reagent using fluidized-bed reactor, Desalination, 2011, 278(1–3), 211–218.
15. M. A. De Leon, et al., Catalytic activity of an iron-pillared montmorillonitic clay mineral in heterogeneous photo-Fenton process, Catal. Today, 2008, 133, 600–605.
16. J. B. Zhang, et al., Decomposing phenol by the hidden talent of ferromagnetic nanoparticles, Chemosphere, 2008, 73(9), 1524–1528.
17. J. Deng, et al., CoFe₂O₄ magnetic nanoparticles as a highly active heterogeneous catalyst of oxone for the degradation of diclofenac in water, J. Hazard. Mater., 2013, 262, 836–844.
18. L. X. Hu, X. P. Yang and S. T. Dang, An easily recyclable Co/SBA-15 catalyst: heterogeneous activation of peroxymonosulfate for the degradation of phenol in water, Appl. Catal., B, 2011, 102(1–2), 19–26.
19. L. G. Devi, et al., Kinetic modeling based on the non-linear regression analysis for the degradation of Alizarin Red S by advanced photo Fenton process using zero valent metallic iron as the catalyst, J. Mol. Catal. A: Chem., 2009, 314(1–2), 88–94.
20. L. Gomathi Devi, et al., Photo degradation of methyl orange an azo dye by advanced Fenton process using zero valent metallic iron: influence of various reaction parameters and its degradation mechanism, J. Hazard. Mater., 2009, 164(2–3), 459–467.
21. A. N. Soon and B. H. Hameed, Heterogeneous catalytic treatment of synthetic dyes in aqueous media using Fenton and photo-assisted Fenton process, Desalination, 2011, 269(1–3), 1–16.
22. S. Hocevar, et al., CWO of phenol on two differently prepared CuO–CeO₂ catalysts, Appl. Catal., B, 2000, 28(2), 113–125.
23. X. Zhong, et al., Modulating the copper oxide morphology and accessibility by using micro-/mesoporous SBA-15 structures as host support: effect on the activity for the CWPO of phenol reaction, Appl. Catal., B, 2012, 121, 123–134.
24. J. K. Kim, F. Martinez and I. S. Metcalfe, The beneficial role of use of ultrasound in heterogeneous Fenton-like system over supported copper catalysts for degradation of p-chlorophenol, Catal. Today, 2007, 124(3–4), 224–231.
25. C. R. Jung, et al., Selective oxidation of carbon monoxide over CuO–CeO(2) catalyst: effect of hydrothermal treatment, Appl. Catal., B, 2008, 84(3–4), 426–432.
26. D. Sempere, et al., Influence of pretreatments on commercial diamond nanoparticles on the photocatalytic activity of supported gold nanoparticles under natural sunlight irradiation, Appl. Catal., B, 2013, 142, 259–267.
27. B. Iurascu, et al., Phenol degradation in water through a heterogeneous photo-Fenton process catalyzed by Fe-treated laponite, Water Res., 2009, 43(5), 1313–1322.
28. J. X. Chen and L. Z. Zhu, UV-Fenton discolouration and mineralization of Orange II over hydroxyl-Fe-pillared bentonite, J. Photochem. Photobiol., A, 2007, 188(1), 56–64.
29. M. H. Entezari, A. Heshmati and A. Sarafraz-yazdi, A combination of ultrasound and inorganic catalyst: removal of 2-chlorophenol from aqueous solution, Ultrason. Sonochem., 2005, 12(1–2), 137–141.
30. Z. H. Ai, P. Yang and X. H. Lu, Degradation of 4-chlorophenol by microwave irradiation enhanced advanced oxidation processes, Chemosphere, 2005, 60(6), 824–827.
31. Y. Y. Hu, Y. H. Zhang and Y. Tang, Rapid detemplation of nanozeolite beta: microwave-assisted Fenton-like oxidation, RSC Adv., 2012, 2(14), 6036–6041.
32. G. Cravotto, et al., Decontamination of soil containing POPS by the combined action of solid Fenton-like reagents and microwaves, Chemosphere, 2007, 69(8), 1326–1329.
33. X. Y. Bi, et al., Degradation of remazol golden yellow dye wastewater in microwave enhanced ClO₂ catalytic oxidation process, J. Hazard. Mater., 2009, 168(2–3), 895–900.
34. Z. H. Ai, et al., Microwave-assisted green synthesis of MnO₂ nanoplates with environmental catalytic activity, Mater. Chem. Phys., 2008, 111(1), 162–167.
35. Y. Yang, et al., Microwave enhanced Fenton-like process for the treatment of high concentration pharmaceutical wastewater, J. Hazard. Mater., 2009, 168(1), 238–245.
36. S. Vilhunen and M. Sillanpaa, Recent developments in photochemical and chemical AOPs in water treatment: a mini-review, Rev. Environ. Sci. Bio/Technol., 2010, 9(4), 323–330.
37. N. Serpone, S. Horikoshi and A. V. Emeline, Microwaves in advanced oxidation processes for environmental applications. a brief review, J. Photochem. Photobiol., C, 2010, 11(2–3), 114–131.
38. X. Y. Bi, P. Wang and H. Jiang, Catalytic activity of CuO_n-La₂O₃/gamma-Al₂O₃ for microwave assisted ClO₂ catalytic oxidation of phenol wastewater, J. Hazard. Mater., 2008, 154(1–3), 543–549.
39. W. C. Liao, et al., Microwave-Assisted Photocatalytic Degradation of Dimethyl Phthalate Over a Novel ZrO_x Catalyst, Environ. Eng. Sci., 2010, 27(12), 1001–1007.
40. W. C. Liao and P. Wang, Microwave-Assisted Photocatalytic Degradation of Dimethyl Phthalate using a Microwave Discharged Electrodeless Lamp, J. Braz. Chem. Soc., 2009, 20(5), 866–872.
41. N. Li, et al., Microwave-Enhanced Fenton Process for DMSO-Containing Wastewater, Environ. Eng. Sci., 2010, 27(3), 271–280.
42. M. Capocelli, et al., Hydrodynamic cavitation of p-nitrophenol: a theoretical and experimental insight, Chem. Eng. J., 2014, 254, 1–8.
43. Z. Lei, et al., Electro-Fenton degradation of p-nitrophenol using the anodized graphite felts, Chem. Eng. J., 2013, 233, 185–192.
44. F. Ji, et al., Efficient decolorization of dye pollutants with LiFe(WO₄)(2) as a reusable heterogeneous Fenton-like catalyst, Desalination, 2011, 269(1–3), 284–290.
45. C. Bradu, et al., Removal of reactive black 5 azo dye from aqueous solutions by catalytic oxidation using CuO/Al₂O₃ and NiO/Al₂O₃, Appl. Catal., B, 2010, 96(3–4), 548–556.
46. S. G. Kumar and L. G. Devi, Review on Modified TiO₂ Photocatalysis Under UV/Visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics, J. Phys. Chem. A, 2011, 115(46), 13211–13241.
47. F. Ji, et al., Heterogeneous photo-Fenton decolorization of methylene blue over LiFe(WO₄)(2) catalyst, J. Hazard. Mater., 2011, 186(2–3), 1979–1984.
48. V. Homem, A. Alves and L. Santos, Microwave-assisted Fenton's oxidation of amoxicillin, Chem. Eng. J., 2013, 220, 35–44.
49. Z. H. Ai, et al., Microwave-induced catalytic oxidation of RhB by a nanocomposite of Fe@Fe₂O₃ core–shell nanowires and carbon nanotubes, J. Phys. Chem. C, 2008, 112(26), 9847–9853.
50. A. K. Abdessalem, et al., Treatment of a mixture of three pesticides by photo- and electro-Fenton processes, Desalination, 2010, 250(1), 450–455.
51. N. K. Temel and M. Sokmen, New catalyst systems for the degradation of chlorophenols, Desalination, 2011, 281, 209–214.

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