Introduction

Due to the rapid development of industries, toxic and harmful organic pollutants are increasingly being discharged into rivers, seriously polluting the environment and endangering human health (Basheer 2018; Ali et al. 2019; Gui et al. 2019; Huang et al. 2020). In the matter of water resources, advanced oxidation processes (AOPs) are excellent methods for treating water and wastewater (Ali et al. 2020, 2021a, Ali et al. 2021b). AOPs were originally defined as treatment technologies involving the production of strong oxidants, i.e., ·OH radicals to oxidize non-biodegradable, refractory, and toxic organic pollutants under environmental conditions (Yang and Wang 2018; Smith et al. 2019; Li et al. 2019b; Ren et al. 2021). AOPs include H2O2/Fe2+ (Behnajady et al. 2007; de Luna et al. 2013), O3/UV (Ghafoori et al. 2014), H2O2/UV (Dai et al. 2018), O3/H2O2 (Chen et al. 2020b), and other processes (Bian et al. 2019; Agu et al. 2020; Aghayi-Anaraki and Safarifard 2020; Ren et al. 2021), which involve the generation of hydroxyl radicals (·OH) by the decomposition of the added O3 or H2O2. Photocatalytic materials and photocatalytic reactions have significant application potential in the production of clean energy and efficient removal of environmental pollutants (Strieth-Kalthoff et al. 2018; Li et al. 2019a; Zhao et al. 2020; Wang et al. 2020b). Therefore, the development of a new type of catalyst with industrial value, low cost, and high efficiency is crucial (Gao et al. 2017; Ali et al. 2018; Chen et al. 2018, 2020a; Zhang et al. 2020; Basheer 2020). Since the discovery of photocatalytic water splitting by Fujishima and Honda in the early 1970s, photocatalytic processes using semiconductor materials have been regarded as promising methods because of their outstanding advantages: low cost, environmental friendliness, and sustainability.

Semiconductor photocatalysis has significant potential for reducing environmental pollution by using sunlight (Colmenares et al. 2017; Zhou et al. 2018; Wang et al. 2018). As a significant oxidation technology, photocatalytic oxidation has received considerable attention from researchers in recent years because of its advantages: photocatalytic oxidation can be carried out under mild reaction conditions, and it is a green method; furthermore, it involves the generation of strong oxidants, and it exhibits high efficiency (Kamat 2017; Kisch 2017; Hodges et al. 2018). In recent years, many novel catalysts have been reported for the degradation of organic pollutants (Li et al. 2012, 2020b; Zhao et al. 2017; Yang and Wang 2018; Zhou et al. 2020; Dai et al. 2020; Liang et al. 2020a). However, there are still a few high-efficiency and easy-to-prepare photocatalytic materials that can be used on a large scale for the removal of environmental pollutants. Among the studied photocatalytic materials, copper oxide (CuO) is particularly important due to its availability, high chemical inertness, high photocorrosion resistance to light irradiation, and long-term stability (Ding et al. 2018; Raees et al. 2021; Islam et al. 2021). The current methods used for preparing catalysts include the impregnation-calcination method, template method, emulsion polymerization method, and self-assembly method (Xiao et al. 2019; Liang et al. 2020b). However, it is difficult to control the diameter and microstructure of microspheres; therefore, these methods have some limitations. Moreover, the prepared catalysts can easily agglomerate; this reduces the specific surface area of the catalyst, thereby affecting the catalytic performance. To solve these problems, we used spray drying and calcination methods to prepare a CuO@γ-Al2O3 nanocomposite catalyst having a special structure. Spray drying combined with calcination has the advantages of excellent repeatability, fast speed, and simple operation, and it is an effective method for preparing uniformly dispersed catalysts. Nano γ-Al2O3 exhibits high adsorption capacity and catalytic activity as well as excellent mechanical properties; moreover, it is a very suitable carrier for the active components of a catalyst (Li et al. 2020a, 2021a; Wang et al. 2021).

On the basis of the abovementioned analysis, a CuO@γ-Al2O3 nanocomposite catalyst was prepared by spray drying and calcination methods. Furthermore, the effects of the calcination temperature, mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, pH value, catalyst dosage, H2O2 dosage, and dark conditions were investigated to optimize the performance of the photocatalytic Fenton-like system for the degradation of PVA, rhodamine B, and reactive red X-3B. Importantly, the synergistic effect of the photocatalytic reaction and the Fenton-like reaction and its possible mechanism were systematically explained. Therefore, this study can provide insights for constructing novel catalysts and developing an efficient electron transfer platform to accelerate the removal of environmental pollutants.

Experimental section

Materials

A PVA fabric was obtained from Baohualin Industrial Development Co. Ltd. Nano γ-Al2O3 (10 nm) was purchased from Beijing Enokai Technology Co. Ltd. Rhodamine B and reactive red X-3B were purchased from Jinan Haoxing Chemical Co. Ltd. Potassium iodide (KI), hydrogen peroxide (H2O2, 30%, in water), boric acid (H3BO3), iodine (I2), concentrated nitric acid (HNO3), and copper nitrate trihydrate (Cu(NO3)2·3H2O) were obtained from Sinopharm Group Chemical Reagent Co. Ltd. All the chemicals used in this research were of reagent grade, and they were used without further purification. Deionized water was used in all the experiments of this study.

Preparation of the CuO@γ-Al 2 O 3 nanocomposite catalyst

The CuO@γ-Al2O3 nanocomposite catalyst was prepared by spray drying and calcination methods. First, nano γ-Al2O3 and Cu(NO3)2·3H2O were weighed in proportion. Then, Cu(NO3)2·3H2O was dissolved completely under stirring in 800-mL water in a 1000-mL beaker. Next, nano γ-Al2O3 was added, and the suspension was continuously stirred for 3 h. The concentration of the resulting nano γ-Al2O3 and Cu(NO3)2·3H2O solution was obtained using the mass ratio of nano γ-Al2O3 to Cu(NO3)2·3H2O; for example, for 1:1, 1:5, 1:10, 1:15, and 1:20, 1, 5, 10, 15, and 20 represent the quantities of Cu(NO3)2·3H2O, whereas 1 corresponds to the quantity of nano γ-Al2O3 used for preparing the nano γ-Al2O3 and Cu(NO3)2·3H2O solution. The CuO@γ-Al2O3 nanocomposite catalyst was prepared by a spray drying method by using the nano γ-Al2O3 and Cu(NO3)2·3H2O solution as the starting material and passing it through a small spray dryer (QFN-8000S, Shanghai Guanmou Industrial Co., Ltd). The nano γ-Al2O3 and Cu(NO3)2·3H2O solution was sprayed at 180 °C using hot air as the carrier gas. The dried CuO@γ-Al2O3 nanocomposite catalyst was collected by a cyclone separator. The CuO@γ-Al2O3 nanocomposite catalyst was easily prepared via calcining the CuO@γ-Al2O3 nanocomposite particles in a high-temperature furnace (GSL-1600X, Hefei Kejing Material Technology Co., Ltd) under an N2 atmosphere. The calcination temperatures were set at 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, and 600 °C in a programmable tube furnace. The calcination time was 3 h. The resulting CuO@γ-Al2O3 nanocomposite catalysts were named as CuO@γ-Al2O3(1:15)-350, CuO@γ-Al2O3(1:15)-400, CuO@γ-Al2O3(1:15)-450, CuO@γ-Al2O3(1:15)-500, CuO@γ-Al2O3(1:15)-550, CuO@γ-Al2O3(1:15)-600, CuO@γ-Al2O3(1:5)-450, CuO@γ-Al2O3(1:10)-450, CuO@γ-Al2O3(1:15)-450, and CuO@γ-Al2O3(1:20)-450, where 350, 400, 450, 500, 550, and 600 represent the calcination temperatures; furthermore, the ratio in the brackets refers to the mass ratio of nano γ-Al2O3 and Cu(NO3)2·3H2O solutions, as described above.

Determination of photocatalytic reaction performance

First, an aqueous solution of the PVA fabric was prepared using deionized water (dissolution temperature: 95 °C), and the concentration of the solution was 2.5 g/L. Rhodamine B and reactive red X-3B were dissolved in deionized water via stirring in a 500-mL beaker; the concentration of all the aqueous solutions of Rhodamine B and reactive red X-3B was 200 μg/mL. All photocatalytic experiments were performed in a 500-mL glass beaker under sunlight, and the reaction temperature was room temperature (20 °C). The pH value of the reaction solution was adjusted via concentrated HNO3 or an aqueous solution of NaOH. The photocatalytic reaction was initiated by adding an appropriate amount of the CuO@γ-Al2O3 nanocomposite catalyst, followed by adding H2O2 (30%) into the reaction solutions. Then, the optimal reaction conditions for degrading PVA were obtained via photocatalytic experiments. At last, in order to further determine the catalytic performance of the CuO@γ-Al2O3 nanocomposite catalyst, rhodamine B and reactive red X-3B (aqueous solutions) were used as the target pollutants. The concentrations of PVA, rhodamine B, and reactive red X-3B were measured according to the equations provided in the supplementary material (Table S1 and Fig. S1). All experiments were repeated three times, and the experimental data were recorded as the mean and standard deviation. The removal rate can be calculated using Eq. (1):

$$\mathrm{Removal}\;\mathrm{rate}=\frac{C_0-C_t}{C_0}\times100\%$$
(1)

Here, C0 and Ct represent the concentrations of PVA, rhodamine B, and reactive red X-3B before and after degradation, respectively.

A pseudo-first-order kinetic model was used to evaluate the photocatalytic degradation curve of PVA. The reaction rate constant, kobs, was calculated using Eq. (2) and Eq. (3):

$$-\frac{dC}{dt}={k}_{obs}\bullet C$$
(2)
$$ln(\frac{{C}_{0}}{{C}_{t}})={k}_{obs}\bullet t$$
(3)

Here, C0 is the initial concentration of PVA aqueous solution, and Ct is the concentration of PVA at time t.

Characterization

The morphologies and structures of CuO@γ-Al2O3 nanocomposite catalysts were characterized via scanning electron microscopy (SEM) with a working distance of 5 mm and at an accelerating voltage of 15 kV. The X-ray diffraction (XRD) patterns were obtained by using a D2 PHASER X-ray diffractometer (Bruker AXS GmbH, Germany); XRD was conducted at room temperature with Cu Kα radiation (λ = 1.540 Å). X-ray photoelectron spectroscopy (XPS) was conducted by using a VG ESCALAB MK II instrument, and the Al Kα X-ray radiation was 1486.6 eV. The Brunauer–Emmett–Teller–specific surface area of the CuO@γ-Al2O3 nanocomposite catalyst was studied using nitrogen adsorption–desorption isotherms. A UV-1800PC spectrophotometer (Shanghai Mapada Instruments Co., Ltd) was used to measure the concentrations of PVA, rhodamine B, and reactive red X-3B. The analyzed wavelengths of PVA, rhodamine B, and reactive red X-3B were 645 nm, 554 nm, and 539 nm, respectively. The total organic carbon (TOC, SHIMADZU TOC-VCPH) analysis confirmed the mineralization of polyvinyl alcohol. ICP-OES/MS (Perkin Elmer, USA) was conducted to obtain the concentration of the released metal ions. UV–vis diffuse reflectance spectroscopy (DRS) was conducted by using a UV-3600i Plus spectrophotometer.

Reusability experiments

To study the reusability of the CuO@γ-Al2O3 nanocomposite catalyst, the used CuO@γ-Al2O3 nanocomposite catalysts were recovered after PVA degradation for 120 min in the first run via centrifugation, separation, and drying with deionized water. The recovered CuO@γ-Al2O3 nanocomposite catalysts were reused to degrade PVA in the next run. After reaction for 120 min, the removal rate of PVA was determined by a UV-1800PC spectrophotometer. The degradation test was repeated ten times. The catalyst was characterized by XRD after 10 cycles.

Results and discussion

Characterization of the CuO@γ-Al 2 O 3 nanocomposite catalyst

SEM and EDS characterization

Figure 1 shows the SEM images of CuO@γ-Al2O3(1:1)-450, CuO@γ-Al2O3(1:5)-450, CuO@γ-Al2O3(1:10)-450, CuO@γ-Al2O3(1:15)-450, and CuO@γ-Al2O3(1:20)-450, the elemental mapping image, and the EDS spectrum of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst. The SEM images indicate the irregular structure and bulk morphology of the CuO@γ-Al2O3 nanocomposite catalyst, which were due to the crystallization of CuO on the surface of γ-Al2O3 after calcination. The particle size of the catalyst was about 5–10 μm. In comparison, as the proportion of Cu(NO3)2·3H2O increased, the number of CuO crystals on the surface of γ-Al2O3 also increased. Furthermore, the catalyst exhibited high dispersibility because of spray drying and calcination, and this irregular shape further increased the specific surface area. To further confirm the composition of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst, EDS was conducted. The EDS spectrum is shown in Fig. 1(h); it can be observed that the atomic masses of O and Cu were 26.43% and 57%, respectively, indicating the high content of the active component CuO on γ-Al2O3.

Fig. 1
figure 1

The SEM images (a–e) of CuO@γ-Al2O3(1:1)-450, CuO@γ-Al2O3(1:5)-450, CuO@γ-Al2O3(1:10)-450, CuO@γ-Al2O3(1:15)-450, and CuO@γ-Al2O3(1:20)-450; enlarged image (f); elemental mapping image (g); and EDS spectrum (h)

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XRD analysis

The crystal structures of the nanocomposite catalysts were verified by X-ray powder diffraction (XRD). Figure 2(a) shows the XRD patterns of CuO@γ-Al2O3(1:15)-350, CuO@γ-Al2O3(1:15)-400, CuO@γ-Al2O3(1:15)-450, CuO@γ-Al2O3(1:15)-500, CuO@γ-Al2O3(1:15)-550, and CuO@γ-Al2O3(1:15)-600; Fig. 2(b) exhibits the XRD patterns of CuO@γ-Al2O3(1:1)-450, CuO@γ-Al2O3(1:5)-450, CuO@γ-Al2O3(1:10)-450, CuO@γ-Al2O3(1:15)-450, and CuO@γ-Al2O3(1:20)-450. As shown in Fig. 2(a–b), when compared with the standard card of CuO (PDF#72–0629), the (110), (-111), (111), (-202), (-113), (113), and (220) characteristic peaks observed at approximately 32.534°, 35.558°, 38.753°, 48.752°, 61.569°, 67.971°, and 68.142°, respectively, indicate the formation of CuO (Wang et al. 2021). From the XRD analysis, we concluded that CuO was generated irrespective of the calcination temperature (350 °C, 400 °C, 450 °C, 500 °C, 550 °C, or 600 °C) and the mass ratio of γ-Al2O3:Cu(NO3)2·3H2O (1:1, 1:5, 1:10, 1:15, or 1:20).

Fig. 2
figure 2

XRD patterns of CuO@γ-Al2O3(1:15)-350, CuO@γ-Al2O3(1:15)-400, CuO@γ-Al2O3(1:15)-450, CuO@γ-Al2O3(1:15)-500, CuO@γ-Al2O3(1:15)-550, and CuO@γ-Al2O3(1:15)-600 (a); XRD patterns of CuO@γ-Al2O3(1:1)-450, CuO@γ-Al2O3(1:5)-450, CuO@γ-Al2O3(1:10)-450, CuO@γ-Al2O3(1:15)-450, and CuO@γ-Al2O3(1:20)-450 (b); nitrogen adsorption–desorption isotherms (c); pore size distribution (d); and UV − vis DRS spectrum (e) of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst

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The N2 adsorption–desorption isotherms and pore size distribution of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst are shown in Fig. 2(c) and Fig. 2(d), respectively. The obtained isotherms, which exhibited type IV characteristics, and pore size distribution implied the existence of mesoporous structures in the synthesized nanocomposites (Ding et al. 2018; Raees et al. 2021). The mesoporous CuO@γ-Al2O3(1:15)-450 nanocomposite exhibited a large specific surface area and high pore volume, which were mainly due to the mesoporous structure; these values were 22.444 m2/g and 0.117 cm3/g, respectively. The large specific surface area and high pore volume of the mesoporous CuO@γ-Al2O3 nanocomposite were expected to increase the removal rate of PVA.

The UV–vis diffuse reflectance spectrum of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst was obtained to investigate its optical properties. As shown in Fig. 2(e), the catalyst exhibits excellent absorption in the visible wavelength range (500–700 nm).

XPS characterization

The chemical composition and surface chemical state of the prepared CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst (fresh and used) were studied by X-ray photoelectron spectroscopy (XPS). Figure 3 clearly shows that the Fenton-like reaction has no effect on the chemical elements in the CuO@γ-Al2O3(1:15)-450 catalyst. This shows that the CuO@γ-Al2O3(1:15)-450 catalyst has high chemical stability. The results of XPS analysis indicated that the sample contained O, Cu, and Al elements (Fig. 3(a)). The high-resolution Cu 2p XPS spectrum (Fig. 3(d)) further indicates that the peaks at 933.34 eV and 953.19 eV correspond to 2p3/2 and 2p1/2 of the Cu atoms in the CuO structure. Simultaneously, the peaks located at approximately 941.35 eV, 943.96 eV, 961.38 eV, and 962.38 eV corresponded to the strong Cu2+ satellite peaks. Figure 3(c) shows the spectra of pure CuO (black line) and the CuO@γ-Al2O3 nanocomposite catalyst (fresh: purple line; used: green line), and we can observe that the spectra basically coincide; this indicated that the CuO@γ-Al2O3 nanocomposites were formed after calcination. The O 1 s spectra are shown in Fig. 3(b). The peak at 531.1 eV was relatively strong, which might be due to the adsorption of hydroxyl groups; the peak at 529.74 eV may represent the lattice oxygen (O2−), and the peak at 533.2 eV may correspond to H2O (Shuang et al. 2020). Therefore, the XPS analysis further confirmed that this irregular structure was the CuO@γ-Al2O3 nanocomposite.

Fig. 3
figure 3

XPS spectra (before use and after use) of the CuO@γ-Al2O3(1:15)-450 nanocomposite catalyst (a); O 1 s (b), CuO (c), and Cu 2p (d) spectra

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Photocatalytic performance and reusability

The photocatalytic degradation of PVA was used to assess the photocatalytic performance of the synthesized CuO@γ-Al2O3 nanocomposite catalysts. In this study, we discussed the effect of calcination temperature, mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, pH value, catalyst dosage, H2O2 dosage, and different systems on PVA degradation. The reaction time was 120 min, and the reaction temperature was 20 °C for all degradation reactions. The results are discussed below.

Effect of calcination temperature

Figure 4(a–b), Table S2, Fig. S2(a), and Fig. S3(a) indicate the effect of the calcination temperature on the PVA removal rate and TOC removal rate. The other reaction conditions are as follows: mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; pH value, 7; catalyst dosage, 2.6 g/L; and H2O2 dosage, 0.2 g/mL. When the calcination temperature increased from 350 to 450 °C, the PVA removal rate, TOC removal rate, and k value increased from 92.7 to 99.21%, 87.69 to 94.57%, and 0.02357 to 0.04183 min−1, respectively. When the calcination temperature increased from 450 to 600 °C, the PVA removal rate, TOC removal rate, and k value were almost unchanged. This indicated that when the calcination temperature was higher than 450 °C, the catalytic activity of CuO was the highest, and a stable mesoporous structure could be formed on the catalyst surface (Ahn et al. 2016). When the calcination temperature was lower than 450 °C, the active components may not be able to form strong structures, resulting in reduced catalytic efficiency (Su et al. 2019; Dong et al. 2020). To test this inference, we performed metal leaching experiments. The experimental results are shown in Table S3. When the calcination temperature was higher than 450 °C, the concentration of leached Cu ions significantly decreased to 0.12 mg/L. This value was less than the concentrations observed at 350 °C (0.183 mg/L) and 400 °C (0.145 mg/L).

Fig. 4
figure 4

Effects of calcination temperature (a–b) and mass ratio of γ-Al2O3:Cu(NO3)2·3H2O (c–d) on PVA degradation

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Effect of the mass ratio of γ-Al 2 O 3 :Cu(NO 3 ) 2 ·3H 2 O

The mass ratio of γ-Al2O3:Cu(NO3)2·3H2O always has a significant effect on photocatalytic activity, especially for catalysts. Therefore, to determine the effect of the mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, five mass ratios, i.e., 1:1, 1:5, 1:10, 1:15, and 1:20, were investigated under visible light irradiation. The other reaction conditions are as follows: calcination temperature, 450 °C; pH value, 7; catalyst dosage, 2.6 g/L; and H2O2 dosage, 0.2 g/mL. As illustrated in Fig. 4(c–d), Table S4, Fig. S2(b), and Fig. S3(b), after 120 min, the PVA removal rate, TOC removal rate, and k value increased to 99.21%, 94.57%, and 0.04183 min−1, respectively. When the mass ratio of γ-Al2O3:Cu(NO3)2·3H2O was further increased, the PVA removal rate and TOC removal rate did not increase further. Theoretically, the more the Cu(NO3)2·3H2O added, the more the active sites produced (Liang et al. 2021; Li et al. 2021b). However, the excessive addition of Cu(NO3)2·3H2O did not further increase the PVA removal rate. This was because the active sites of the γ-Al2O3 carrier were saturated. When the active sites of a carrier are saturated, the catalytic activity does not increase further. Accordingly, based on the catalytic efficiency, the best mass ratio of γ-Al2O3:Cu(NO3)2·3H2O was considered as 1:15 in the subsequent catalytic experiments.

Effect of pH value

An appropriate pH value of the reaction solution is of great significance for the practical applications of photocatalysis experiments; thus, the pH values of 3, 4, 5, 6, 7, 8, 9, and 10 were explored to investigate the effect of pH on the catalytic reaction. The other reaction conditions are as follows: calcination temperature, 450 °C; mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; catalyst dosage, 2.6 g/L; and H2O2 dosage, 0.2 g/mL. Figure 5(a–b), Table S5, Fig. S2(c), and Fig. S3(c) indicate that when the pH value increased from 3 to 7, the PVA removal rate, TOC removal rate, and k value increased from 98.224, 94.71, and 0.03382 to 99.21%, 94.57%, and 0.04183 min−1, respectively. When the pH value increased from 7 to 10, the PVA removal rate, TOC removal rate, and k value decreased to 32.374%, 28.79%, and 0.00325 min−1, respectively. Thus, the increase in the pH value from 7 to 10 considerably reduced the PVA removal rate, TOC removal rate, and k value, indicating that the CuO@γ-Al2O3 nanocomposite catalyst has very low catalytic activity under alkaline conditions. Under light irradiation, the CuO@γ-Al2O3 nanocomposite catalyst produced oxidative species, such as ·OH radicals. These free radicals and H2O2 easily decompose under alkaline conditions but exhibit better stability under acidic or neutral conditions (Li et al. 2019b; Kanjana et al. 2020). Therefore, the catalyst could provide the best oxidation performance under acidic or neutral conditions and improve the photocatalytic removal rate (Shang et al. 2012; Wang et al. 2020a). The pH experiments showed that the CuO@γ-Al2O3 nanocomposite catalyst has excellent catalytic activity under acidic or neutral conditions.

Fig. 5
figure 5

Effects of initial pH (a–b), catalyst dosage (c–d), H2O2 dosage (e–f), and different systems (g–h) on PVA degradation

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Effect of catalyst dosage

To investigate the effect of catalyst dosage on photocatalytic activity, five catalyst dosages, i.e., 1.4 g/L, 1.8 g/L, 2.2 g/L, 2.6 g/L, and 3 g/L, were used for PVA degradation under visible light irradiation for 120 min. The results are shown in Fig. 5(c–d), Table S6, Fig. S2(d), and Fig. S3(d). The other reaction conditions are as follows: calcination temperature, 450 °C; mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; pH value, 7; and H2O2 dosage, 0.2 g/mL. When the catalyst dosage was increased from 1.4 to 2.6 g/L, the PVA removal rate, TOC removal rate, and k value increased from 87.64, 83.36, and 0.01777 to 99.21%, 94.57%, and 0.04183 min−1, respectively. When the catalyst dosage was further increased, the PVA removal rate and TOC removal rate did not increase further. This indicated that with the increase in the catalyst dosage, the active sites of the catalytic reaction increased, and the ·OH radicals generated per unit time also increased correspondingly (Raees et al. 2021; Islam et al. 2021). When the catalyst dosage was more than 2.6 g/L, the PVA removal rate and TOC removal rate did not increase further because of the low H2O2 concentration in the reaction solution, due to which more ·OH radicals could not be generated (Kadi et al. 2020; Li et al. 2021a). In summary, these results confirmed that the best catalyst dosage was 2.6 g/L.

Effect of H 2 O 2 dosage

The concentration of the H2O2 oxidant plays a vital role in the catalytic process because it is directly related to the number of generated ·OH radicals. The effects of H2O2 dosage (0.1 g/mL, 0.12 g/mL, 0.14 g/mL, 0.16 g/mL, 0.18 g/mL, 0.2 g/mL, and 0.22 g/mL) on the PVA removal rate and TOC removal rate were investigated in this study. The other reaction conditions are as follows: calcination temperature, 450 °C; mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; pH value, 7; and catalyst dosage, 2.6 g/L. The results are shown in Fig. 5(e–f), Table S6, Fig. S2(e), and Fig. S3(e). When the H2O2 dosage was increased from 0.1 to 0.2 g/mL, the PVA removal rate, TOC removal rate, and k value increased from 75.31, 70.89, and 0.00683 to 99.21%, 94.57%, and 0.04183 min−1, respectively. When the H2O2 dosage was further increased, the PVA removal rate and TOC removal rate did not increase further. In general, the amount of ·OH radicals produced is proportional to the concentration of H2O2; that is, the higher the concentration of H2O2, the higher the amount of the ·OH radicals produced; consequently, the removal rate of the pollutant increases (Gholizadeh et al. 2020; Bagtache et al. 2021). However, when the concentration of H2O2 exceeds the critical value, the excess H2O2 reacts with hydroxyl radicals and converts the hydroxyl radicals into hydroperoxyl radicals; consequently, the removal rate may decrease (Esmaeili et al. 2020; Chen et al. 2020c, p.; Muthukrishnaraj et al. 2020). The optimal concentration of H2O2 was 0.2 g/mL because of its fast removal rate and no excessive consumption of oxidants.

Effect of different systems

In this study, we also investigated the effects of three different systems (catalyst + H2O2, catalyst + irradiation, and catalyst + H2O2 + irradiation) on the removal rate of PVA; the experimental results are exhibited in Fig. 5(g–f), Fig. S2(f), and Fig. S3(f). For the catalyst + H2O2 system, the PVA removal rate, TOC removal rate, and k value were 67.35%, 62.47%, and 0.0094 min−1, respectively, after 120 min. This indicated that the CuO@γ-Al2O3 nanocomposite catalyst exhibited unsatisfactory performance for the degradation of PVA under dark conditions. Under the catalysis of CuO, only a small amount of H2O2 decomposed to produce ·OH radicals (Ding et al. 2018; Su et al. 2019; Akram et al. 2021), as shown in Eqs. (4) and (5):

$${\mathrm{Cu}}^{2+} + {\mathrm{H}}_{2}{\mathrm{O}}_{2} \to {\mathrm{Cu}}^{+} + {\mathrm{O}}_{{2}^{-}} + {2\mathrm{H}}^{+}$$
(4)
$${\mathrm{Cu}}^{+} + {\mathrm{H}}_{2}{\mathrm{O}}_{2} \to {\mathrm{Cu}}^{2+} + \cdot \mathrm{OH }+ {\mathrm{OH}}^{-}$$
(5)

Without the addition of H2O2, it was observed that the PVA removal rate, TOC removal rate, and k value were 23.46%, 18.56%, and 0.00223 min−1, respectively, after 120 min. This shows that without adding H2O2, the catalyst can generate free radicals through irradiation to achieve the degradation of PVA. For the catalyst + H2O2 + irradiation system, the PVA removal rate, TOC removal rate, and k value were 99.21%, 94.57%, and 0.04183 min−1, respectively. The experimental results indicated that under the conditions of both visible light irradiation and H2O2, the reaction system exhibited excellent performance for the degradation of PVA.

Reusability

The stability and reusability of materials are important aspects to evaluate their practical applications. To investigate the reusability of the CuO@γ-Al2O3 nanocomposite catalyst, ten photodegradation experiments were conducted (calcination temperature, 450 °C; mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; pH value, 7; catalyst dosage, 2.6 g/L; reaction temperature, 20 °C; and H2O2 dosage, 0.2 g/mL). The results are shown in Fig. 6. For the ten cycles, the PVA removal rates were 99.21%, 99.07%, 98.94%, 98.83%, 98.616%, 98.543%, 98.327%, 98.105%, 97.897%, and 97.582%, respectively (Fig. 6(a)); furthermore, the TOC removal rates were 94.57%, 94.31%, 93.97%, 93.68%, 93.41%, 93.19%, 92.99%, 92.71%, 92.49%, and 92.18%, respectively (Fig. 6(b)). Even though there was a slight decrease in the PVA removal rate and TOC removal rate during the ten cycles, the values were still higher than 97% and 92%, respectively. The slight decrease in the PVA removal rate was probably due to the following two reasons: (1) The degradation intermediates blocked the mesopores of the catalyst; (2) during the photodegradation process, the quantity of the CuO@γ-Al2O3 nanocomposite catalyst decreased after centrifugation, separation, and drying, thereby reducing the number of active sites. However, the satisfactory PVA removal rate and TOC removal rate after ten cycles indicated the excellent reusability of the CuO@γ-Al2O3 nanocomposite catalyst. The concentrations of leached Cu ions in the solutions were measured after each degradation experiment. Figure 6(c) shows that about 0.12 mg/L of Cu ions leached in 120 min in the first cycle, accounting for 0.005% of the total amount in the CuO@γ-Al2O3 nanocomposite catalyst. Furthermore, about 0.032 mg/L Cu ions leached in the tenth cycle. The XRD results (Fig. 6(d)) indicated that the reused CuO@γ-Al2O3 nanocomposite catalyst could maintain its crystalline phase after 10 catalytic cycles.

Fig. 6
figure 6

Reusability of CuO@γ-Al2O3 (a–b); leaching of Cu ions during the PVA degradation process (c); and XRD patterns (d: fresh and used)

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Practical applications of the CuO@γ-Al 2 O 3 nanocomposite catalyst

According to the abovementioned characteristics and performance analysis, it can be inferred that the CuO@γ-Al2O3 nanocomposite catalyst has high photocatalytic activity. In order to further verify this inference, the highly toxic printing and dyeing wastewater containing rhodamine B and reactive red X-3B was obtained for the degradation experiments, and the CuO@γ-Al2O3 nanocomposite catalyst was used for the photocatalytic degradation of rhodamine B and reactive red X-3B. The reaction conditions are as follows: calcination temperature, 450 °C; mass ratio of γ-Al2O3:Cu(NO3)2·3H2O, 1:15; pH value, 7; catalyst dosage, 2.6 g/L; reaction temperature, 20 °C; and H2O2 dosage, 0.2 g/mL. After photocatalytic degradation for 120 min, the removal rates of rhodamine B and reactive red X-3B were 99.91% and 99.25%, respectively (Fig. 7(e)). Figure 7(a–b) and Fig. 7(c–d) show the temporal changes in the UV–vis spectra of the aqueous solutions of rhodamine B and reactive red X-3B in the range of 200–700 nm. The absorption peaks detected at 554 nm and 539 nm correspond to rhodamine B and reactive red X-3B; it can be seen that the intensities of both absorption peaks decreased with the reaction time, and the peaks almost disappeared after 120 min. Therefore, it can be concluded that the CuO@γ-Al2O3 nanocomposite catalyst has high photocatalytic activity for the treatment of printing and dyeing wastewater. Furthermore, rhodamine B and reactive red X-3B can be effectively degraded and decolorized and finally completely mineralized into small molecules such as CO2 and H2O through photocatalytic redox reactions (Chen et al. 2020d).

Fig. 7
figure 7

Time-dependent UV–vis absorption spectra and images of rhodamine B (a–b, respectively) and reactive red X-3B (c–d, respectively); removal rates of rhodamine B and reactive red X-3B (e)

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Possible catalytic mechanism

On the basis of the experimental results, a mechanism for the photocatalytic degradation of PVA, rhodamine B, and reactive red X-3B by the CuO@γ-Al2O3 nanocomposite catalyst was proposed (Fig. 8). In the photocatalytic reaction, the photogenerated electrons transfer from the valence band (VB) to the conduction band (CB) (Dong et al. 2020; Raees et al. 2021). The transferred electrons react with H2O2 and O2 to generate ·OH and ·O2− radicals (Li et al. 2020b; Chen et al. 2020c; Abdelaal et al. 2020; Muthukrishnaraj et al. 2020), which participate in the removal of PVA, rhodamine B, and reactive red X-3B. At the same time, the holes accumulated in the VB of the CuO@γ-Al2O3 nanocomposite catalyst have sufficient energy to oxidize H2O to generate ·OH radicals (Abdelaal et al. 2020; Muthukrishnaraj et al. 2020; Bielan et al. 2021). The photocatalytic process may include the following elementary steps (Kamat 2017; Colmenares et al. 2017; Strieth-Kalthoff et al. 2018; Xiao et al. 2019; Zhang et al. 2020):

Fig. 8
figure 8

A schematic of the feasible mechanism of the photocatalytic reaction

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$$\mathrm{CuO}@\upgamma -{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\,\mathrm{ nanocomposite\, catalyst }+\mathrm{ hv }\to \mathrm{ CuO}@\upgamma -{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\,\mathrm{ nanocomposite\, catalyst }\,({\mathrm{h}}^{+} + {\mathrm{e}}^{-})$$
(6)
$${\mathrm{H}}_{2}\mathrm{O }+ {\mathrm{h}}^{+} \to {\mathrm{HO}}^{-} + {\mathrm{H}}^{+}$$
(7)
$${\mathrm{HO}}^{-} + {\mathrm{h}}^{+} \to \cdot \mathrm{OH}$$
(8)
$${\mathrm{O}}_{2} + {\mathrm{e}}^{-} \to \cdot {{\mathrm{O}}_{2}}^{-}$$
(9)
$${{\mathrm{O}}_{2}}^{-} + {\mathrm{H}}^{+} \to {\mathrm{HO}}_{2}$$
(10)
$${\mathrm{HO}}_{2}\cdot + {\mathrm{HO}}_{2}\cdot \to {\mathrm{H}}_{2}{\mathrm{O}}_{2} + {\mathrm{O}}_{2}$$
(11)
$${\mathrm{HO}}_{2}\cdot + \cdot {{\mathrm{O}}_{2}}^{-} \to {{\mathrm{HO}}_{2}}^{-} + {\mathrm{O}}_{2}$$
(12)
$${{\mathrm{HO}}_{2}}^{-}\cdot + {\mathrm{H}}^{+} \to {\mathrm{H}}_{2}{\mathrm{O}}_{2}$$
(13)
$${\mathrm{H}}_{2}{\mathrm{O}}_{2} + \cdot {{\mathrm{O}}_{2}}^{-} \to \cdot \mathrm{OH }+ {\mathrm{HO}}^{-} + {\mathrm{O}}_{2}$$
(14)
$${\mathrm{H}}_{2}{\mathrm{O}}_{2} + {\mathrm{e}}^{-} \to \cdot \mathrm{OH }+ {\mathrm{HO}}^{-}$$
(15)
$${\mathrm{H}}_{2}{\mathrm{O}}_{2} +\mathrm{ hv }\to 2\cdot \mathrm{OH}$$
(16)
$${\mathrm{HO}}_{2}\cdot \to {\mathrm{H}}^{+} + {\mathrm{O}2}^{-}$$
(17)
$${{2\mathrm{O}}_{2}}^{-} + {2\mathrm{H}}_{2}\mathrm{O }\to {}^{1}{\mathrm{O}}_{2} + {\mathrm{H}}_{2}{\mathrm{O}}_{2} + {2\mathrm{OH}}^{-}$$
(18)
$$\mathrm{h}/\mathrm{e}-/\cdot {{\mathrm{OH}/\mathrm{O}}_{2}}^{-}/{}^{1}{O}_{2} +\mathrm{ PVA}/\mathrm{rhodamine B}/\mathrm{Reactive Red X}-3\mathrm{B }\to {\mathrm{H}}_{2}\mathrm{O }+ {\mathrm{CO}}_{2}$$
(19)

Another important aspect of the prepared CuO@γ-Al2O3 nanocomposite catalyst is the ability to generate ·OH radicals, which can be generated in Fenton-like and photocatalytic reactions; these ·OH radicals can synergistically degrade PVA, rhodamine B, and reactive red X-3B. In the Fenton-like reaction, Cu2+ reacts with H2O2 to produce ·O2−. Then, H2O2 oxidizes Cu+ to Cu2+, generating ·OH. In this process, Cu+ and Cu2+ rapidly react with H2O2, which promotes the Cu+/Cu2+ cycle (Akram et al. 2021; Wang et al. 2021). In addition, the presence of electrons is conducive to the generation of ·OH and ·O2− radicals in the Fenton-like process (Yang and Wang 2018; Abudayyeh et al. 2021); they considerably increase the number of active groups and further enhance the photoactivity. The detailed mechanism of the Fenton-like reaction can be summarized as Eqs. (4) and (5).

Conclusion

In summary, a photocatalytic Fenton-like system was constructed, in which visible light and H2O2 were used as the external driving energy and raw material, respectively. This system exhibited excellent performance for the degradation of PVA. At the same time, the system also exhibited wide applications as well as stability for the degradation of rhodamine B and reactive red X-3B. The efficient utilization of H2O2 led to the continuous generation of abundant ·OH radicals, which resulted in high efficiency and high degradation and mineralization ability of the photocatalyst. This remarkable catalytic performance may be due to the synergistic effect of the photocatalytic reaction and the Fenton-like reaction. This study provides an efficient and easy-to-operate method for treating wastewater containing PVA and dyes.