Present study proposed the synthesis of mixed p-type and n-type nanocomposite heterostructures by co-precipitation method. The as-synthesized heterostructures were characterized through different characterization techniques. The as-synthesized Bi2WO6 and Bi2O3-ZnO heterostructures were tested as photocatalysts during the photodegradation of Bisphenol A (BPA). The Bi2O3-ZnO heterostructure nanocomposite was found to be a more effective photocatalyst than Bi2WO6. The effect of operating parameters including catalytic dose (0.02–0.15 gL-1), initial BPA concentration (5–20 mgL-1), temperature change (5–20 °C) and solution pH changes (4, 5, 7, and 8) were evaluated with Bi2O3-ZnO under UV-light irradiation by selecting a 300 W Xe lamp. More than 90% BPA was degraded with 0.15 gL−1 Bi2O3-ZnO, keeping 1.0 mM H2O2 concentration fixed in 250 mL of reaction suspension. The HPLC and GC-MS were used to detect the reaction intermediates and final products. A plausible degradation pathway was proposed on the basis of the identification of reaction intermediates. Repeatability test analysis confirmed that the as-synthesized catalyst showed superb catalytic performance on its removal trend. The kinetics of degradation of BPA were well fitted by the power laws model. With the order of reaction being 0.6, 0.9, 1.2, and 1.3 for different operating parameters, i.e., catalyst dose, initial pH, temperature, and initial BPA concentration.

  • A photocatalytic process to intensify the degradation of Bisphenol.

  • XRD, TEM and FE-SEM studied the p-n type heterostructure nanocomposite.

  • Parametric study for optimization of removal efficiency of Bisphenol.

  • Kinetics analysis of degradation of Bisphenol A Degradation pathway of Bisphenol.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Due to the presence of toxic colorants, herbicides, insecticides, and other chemicals, industries are producing recalcitrant organic pollutants that pose a major threat to the environment. Bottles, food packaging, polycarbonates, and phenolic resins all are made using the organic plasticizer Bisphenol A (BPA). It interferes with the physiological processes of the brain and is a heart disease-causing agent and endocrine disruptor (Abdul et al. 2021). High levels of BPA were found in the influent (138 ng L−1) and effluent (60.5−1,960 ng L−1) of wastewater treatment plants (Zhang et al. 2009). It has been shown that creating effective photocatalysts is crucial for many emerging applications, such as the removal of resistant organic contaminants in wastewater and the conversion of solar energy into vital chemical feedstock (Nie et al. 2019).

Semiconductor photocatalysis offers a potential solution to the issues of energy shortages and environmental pollution. Photocatalytic degradation of organic pollutants by semiconductor photocatalysts has been a useful technology for environmental purification. However, due to the quick electron-hole recombination, the photo-efficiency of the bare semiconductor catalyst is inhibited. Therefore, it is urgent to develop highly efficient photocatalytic materials for pollutant degradation. Semiconductor oxide's physical and chemical characteristics determine its photocatalytic activity. Crystallinity, or the degree of crystallization, is one of the key determinants of the effectiveness of photocatalytic reactions because it affects the recombination of photo-excited electrons and holes at crystal lattice defects (Parvaz et al. 2021). It is also found that photo-generated charge carriers can be effectively separated inside semiconducting composite materials according to the different band gap structures of their components (Xu et al. 2009).

In the field of photocatalysis, ZnO is believed to be an efficient photocatalytic material alternative to TiO2 because both have similar band gaps and photocatalytic mechanisms. The enhanced photocatalytic activity of the nanomaterials was ascribed to the larger surface area, increased oxygen vacancy, and the facilitation of diffusion and mass transportation of the reactant molecules (Yasin et al. 2022). The synthesis of zinc oxide nanomaterial has received great attention due to its size-dependent properties and photocatalytic applications (Kuo et al. 2007). Bismuth oxide (Bi2O3) is an important p-type semiconductor with four main crystallographic polymorphs denoted by α-, β-, γ-, and δ-Bi2O. Bismuth-based oxides appear to be good candidates because they have a band gap in the visible range (Zhang et al. 2006). Bi2O3, with a band gap of 2.8 eV, accounts for its ability to oxidize water and possibly generate highly reactive species, such as O2−• and OH radicals, for initiating oxidation reactions (Singh et al. 2017).

One of the effective ways to enhance electron-hole separation is to use coupled semiconductors. A heterojunction interface is built between the coupled semiconductors’ matching band potential semiconductors. In this way, the electric-field-assisted charge transport from one particle to the other via interfaces is favorable for the electron-hole separation in the coupled materials and for the consequent electron or hole abundance on the surfaces of the two semiconductors (Jiang et al. 2007). Hence, in linked semiconductors, advantages such as improvement of charge separation, increase in the lifespan of the charge carrier, and augmentation of the interfacial charge transfer efficiency to an adsorbed substrate can be realized. Cabot et al. (2004) and Hernández-Alonso et al. (2009) have also reported ZnO film/Bi2O3 microgrid heterojunction using the microsphere lithography technique for the degradation of methyl orange dye. Although, the wide band gap and fast recombination of electron-hole is a challenge in semiconductor metal oxides. Nowadays, more attention has been paid to decreasing the recombination ability by coupling the different semiconductors’ metal oxides to generate a heterojunction interface among different semiconductors. The present study proposed the synthesis of the coupled semiconductor containing p-n heterojunctions that decrease the recombination of electron-hole pairs. The as-synthesized heterostructures were characterized through different characterization techniques such as X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and Fourier transfrom infrared (FTIR) photocatalytic properties of as-synthesized materials were evaluated by degradation study of Bisphenol A. Parametric analysis was carried out by considering catalytic dose (0.02–0.15 g L−1), initial BPA concentration (5–20 mg L−1), temperature change (5–20 °C) and solution pH changes (4–8). The kinetics analysis and reaction intermediates and final products have also been identified.

Chemicals

All chemicals are analytical grades. Bisphenol-A (BPA), bismuth nitrate (Bi(NO3)3.5H2O), zinc nitrate (Zn(NO3)3.6H2O), sodium tungstate (Na2WO4.2H2O) with > 99% purity were purchased from Sigma Aldrich. Dichloromethane (CH2Cl2), acetic acid (CH3COOH), hydrogen peroxide (H2O2), and NaHCO3 were obtained from Hi Media, Mumbai; sulfuric acid (H2SO4), NaOH, and acetonitrile (>99% purity) were purchased from Fine Chemicals, New Delhi. The laboratory scale (Milli-Q Biocel) system was used for the Millipore water.

Synthesis of photocatalysts

The co-precipitation method was used for the synthesis of Bi2WO6 and Bi2O3-ZnO nanocomposite heterostructure. During the preparation of the Bi2WO6 heterostructure, the 0.1 M Na2WO4.2H2O and 0.1 M Bi(NO3)3.5H2O was prepared in 100 mL DI water. The 1.0 M aqueous solutions of NaOH were dropwise added into the solution until the solution obtained a pH of 7. The resulting suspension was stirred at 80 °C for 1 h by using the magnetic stirrer. After a continuous stirring, a light-yellow suspension was formed, which was cooled at room temperature and filtered out. The yellow precipitate was collected and washed with DI water and ethanol. As obtained, precipitates were dried for 8 h in an oven at 80 °C. The dry precipitates were calcined at 550 °C for 5.0 h. During the preparation of Bi2O3-ZnO nanocomposite heterostructure, the 0.1 M Zn (NO3)3.6H2O and Bi (NO3)3.5H2O solution was prepared in 100 mL DI water, 1.0 M aqueous solution of NaOH was dropwise added into the solution until the solution obtained a pH of 7; then the resulting suspension was stirred at 80 °C for 1 h by using the magnetic stirrer. After a continuous stirring, a light-yellow suspension was formed, which was cooled at room temperature and filtered out. The yellow precipitate was collected and washed with DI water and ethanol. As obtained, precipitates were dried for 8 h in the oven at 80 °C, and the dry precipitates were calcined at 450 °C for 4.5 h. After that, both as-synthesized materials were collected and characterized.

Analytical procedures

The morphology and the structure of as-synthesized nanocomposites were examined by using several characterization techniques. The crystalline structure of as-synthesized materials was detected with powder X-ray diffraction (XRD: D8, Bruker AXS diffractometer) having Cu Kα radiation scan rate of 5° min−1 at the wavelength (λ = 1.5406 Å) for 30 min within the scanning range of 10–90° and applied voltage of 40 kV. The elemental composition and morphological characteristics of as-synthesized material were determined by using field emission scanning electron microscopy (FE-SEM: EVO MA, 15 Carl Zeiss) equipped with an energy dispersive X-ray (EDX) analyzer. The Fourier transform infrared spectroscopy (FTIR: Shimadzu 8400 model) recorded the spectrum to explain the bond stretching frequency of the materials.

The BPA concentration before and after the treatment was detected by using high-performance liquid chromatography (HPLC: Waters India) equipped with a UV detector at λmax of 276 nm for BPA and C18 column having a high-pressure pump for BPA quantification. A mixture of acetonitrile and Milli Q-water (50/50; v/v) was selected as the mobile phase in the present study at 1 mL min−1 flow rate. GC-MS analysis was done by using the Perkin Elmer GC-MS (Clarus − 680) having an MS capillary column − Elite 5 with the dimension of 30 m × 0.25 mm coupled with the Clarus SQ8C MS model. 1 μL aliquots amount was injected at 493 K in split less mode and at a fixed flow rate 1.5 mL min−1 of carrier helium gas. To identify the unknown peak of the sample, NIST and Wiley library are used.

Experimental set-up

Photocatalytic experiment of as-synthesized Bi2O3-ZnO nanocomposite was performed to detect the photocatalytic activity. For this purpose, BPA was selected as a model compound in an aqueous medium under UV-light irradiation. A 500 mL borosilicate glass beaker was selected as a photo-reactor (300 W Xe lamp) in a batch mode study keeping 250 mL BPA solution. The 0.1 N NaOH or 0.1 N H2SO was used to adjust the initial pH of the solution during each experiment, as required. Adsorption-desorption equilibrium was attained before the start of the main photocatalytic experiments of BPA degradation. After that, the suspension shifted in a photocatalytic reactor, a solution was irradiated, and at a fixed interval of time, a 3 mL sample was taken out during the entire experiment. The concentration change of BPA during all the experiments was calculated by using Equation (1).
formula
(1)

Co and Cf are the initial and final concentration change BPA in aqueous solution at t = 0 and after time t = t (min). The experiments were performed multiple times, and the averages of three replicate experiments were reported in all the results. A smaller than 5% deviation was considered for the average value of all experimental results.

Characterization of the materials

XRD analysis

XRD measurements were used to analyze the crystallinity of as-synthesized heterostructure in the range of 5–90° at room temperature. As shown in Figure 1, the multiple diffraction data, including multiple diffraction peaks at 2θ with their corresponding crystallographic planes at 28.12° (113), 32.52° (020), 47.32° (220), 55.12° (313), 58.21° (226), 68.42° (400), 75.12° (102) and 78.23° (204) perfectly reflect of Bi2WO6 orthorhombic crystal structure (JCPDS card no. 79 − 2381), and no other impurities peaks were observed, showing the higher phase purity of as-prepared sample which increased with increasing the calcination. The average crystallite size of Bi2WO6 was about 48 nm for the high-intensity peak at 32°, calculated by using the Debye-Scherrer (Liu et al. 2015). For Bi2O3, the sequence of sharp and strong diffraction peaks consisted of the α-phase of monoclinic Bi2O3 along with O2 deficient Bi2O3, and the corresponding (hkl) values of different peaks were indexed to compare with the standard JCPDS data files. Figure 1 presents the highest peak intensity at ∼27.55°, corresponding to (hkl) value (120) for pure Bi2O3 monoclinic (Bera et al. 2020). The occurrence of new peaks in considerable proportions perfectly matches oxygen-deficient Bi2O3. The perfect peaks at 56.41° (202), 55.219° (006), 45.92° (101), 44.02° (006), 32.12° (311), 33.02° (101), 29.6° (107), 28.1° (101), and 23.6° (101) corresponding to tetragonal, and hexagonal phases, respectively. Higher O2 deficient phases in the Bi2O3 sample recommend n-type carrier concentrations (higher donor density) and excellent photocatalytic performance of as-prepared nanocomposite heterostructure. The two-phase mixture of Bi2O3 and ZnO occurred in the bismuth-zinc oxide sample. The signal of a two-phase mixture of Bi2O3 and ZnO occurred in the sample, which is a good arrangement with Bi2O3 (ICDD 98–008–5622) (Landge et al. 2021). The XRD pattern of calcined ZnO and the sequence of sharp and each strong Bragg diffraction peaks confirm the polycrystalline nature of the as-prepared material. The ZnO sample illustrates the diffraction peaks planes (110), (102), (101), (002), and (100) correspondingly to the diffraction angle peaks 56.78, 47.68, 36.58, 34.58, and 31.98, respectively (Singh et al. 2017; Khoshnam & Salimijazi 2021). The planes at 110, 101, and 002 support confirm the crystalline structure of ZnO is a wurtzite structure (JCPDS file no. 79 − 0208) with a = 3.2190 Å c = 5.1489 Å (Saha et al. 2018; Alavi et al. 2019). The XRD pattern of ZnO in pure crystalline Bi2O3 shows the presence of perfect crystalline phases in ZnO, the average crystallite size of about 45 nm of as-synthesized Bi2O3 − ZnO nanocomposites heterostructure was calculated by using the Debye-Scherrer equation (Xu et al. 2009; Qin et al. 2020).
Figure 1

XRD spectrum analysis of Bi2WO6 and Bi2O3-ZnO.

Figure 1

XRD spectrum analysis of Bi2WO6 and Bi2O3-ZnO.

Close modal

Surface morphological analysis

FE-SEM analysis was used to confirm the surface morphology, including the shape and size of the as-synthesized heterostructure. Figure 2(a), 2(b), 2(d) and 2(e) show the lower and higher magnification images of as-synthesized Bi2WO6 and Bi2O3-ZnO nanocomposites, respectively. It can be seen that the Bi2WO6 contains a mixture of nanospheres in the range of 44 nm diameter with average breadths and lengths of 40 and 56 nm, respectively. The inter-particle boundaries are almost invisible and confirm the agglomerated nature of the particles from the images (Figure 2(a) and 2(b)). Spherical particles with an average diameter of 42 nm are displayed in Bi2WO6 images, which supports the particle size calculation formed by the XRD results (Hajra et al. 2019). The shape and size of the as-prepared Bi2O3-ZnO nanocomposites are illustrated in Figure 2(d)–2(e). The uniform size and spherical morphology of Bi2O3 and ZnO can be seen. The FE-SEM image of Bi2O3-ZnO nanocomposites demonstrates the particle size of about 50 nm fine distributed in a spherical shape, which supports by the XRD analysis of size calculations.
Figure 2

FE-SEM images of Bi2WO6 (a–b) and Bi2O3-ZnO (d–e) with high and low magnification. EDX spectrum: (c) Bi2WO6 and (f) Bi2O3-ZnO.

Figure 2

FE-SEM images of Bi2WO6 (a–b) and Bi2O3-ZnO (d–e) with high and low magnification. EDX spectrum: (c) Bi2WO6 and (f) Bi2O3-ZnO.

Close modal
The existence of Bi2WO6 and Bi2O3-ZnO nanocomposites was further confirmed by the elemental analyses using EDX analysis and elemental mapping, and as detected results illustrated in (Figure 2(c) and 2(f)) and (Figure 3). The EDX spectrum of Bi2WO6 shows that the 1.0% Bi, 0.5% W, and 17.7% O atom existed in 1: 1.2 ratio in the structure of Bi2WO6. The result and analysis confirm that the relative content of Bi is slightly lower than the corresponding oxygen in Bi2WO6 (Liu et al. 2015). However, the 1:1 ratio of bismuth and oxygen signifies an increase in the relative trend without a noticeable number of other elements present, and the diverse modification in the crystalline pattern and surface morphology supports the above results (Figure 3). The EDX profile of Bi2O3-ZnO mesosphere confirmed the higher ratio of Zn and O in the spherical particles (Figure 2(f)), while the mapping of the corresponding elements shows the distribution of carbon, bismuth, zinc, and oxygen in Figure 3. The quantitative analysis of the Zn/O distribution in the Bi2O3-ZnO sample showed a good agreement with the values described for Bi2O3-ZnO in the open literature (Zhang & Kong 2011). The quantitative analysis has shown the molar presence of bismuth (20.7%), zinc (11.4%), and oxygen (65.4%) in the nanocomposites. This demonstrates the effective synthesis of Bi2WO6 and Bi2O3-ZnO nanocomposites by the precipitation method. Particularly, the morphology and elemental distribution of the support can be exactly measured by using this precipitation method with modest operational conditions.
Figure 3

Elemental mapping for Bi2WO6 (a–e) and Bi2O3-ZnO (f–j) nanocomposites.

Figure 3

Elemental mapping for Bi2WO6 (a–e) and Bi2O3-ZnO (f–j) nanocomposites.

Close modal

FTIR analysis

FTIR analysis is used to determine the identity of as-synthesized Bi2WO6 and Bi2O3-ZnO nanocomposites shown in Figure 4. Many absorptions peaks are formed by the bending vibration below the 1,500 cm−1 also known as Fingerprint region. The strong absorption peak below 400 cm−1 shows the presence of metal oxide nanoparticles. For all samples, a broad peak detected between 3,200 and 3,500 cm−1 is recognized as the O–H stretching vibration frequency of water adsorbed on the material surface, and the bending vibration peak frequency at 1,635 cm−1 can be represented as the H–O–H bond (Muthukumaran & Gopalakrishnan 2012; Parvaz et al. 2021). The FTIR spectra of Bi2WO6 are illustrated in Figure 4. The main peak of Bi-O stretching frequency occurs at 667–1,600 cm−1. The peak ∼3,500 and 1,630 cm−1 can give the appearance of both -OH stretching and bending vibrations, respectively (Wang et al. 2015). The peak at 843.9 cm−1 and 845.6 cm−1 was assigned to the bending vibration of Bi-O bonds and Bi-O-Bi bonds, respectively, in the Bi2WO6. The results of FTIR show the final products as Bi2WO6, which again confirms the results of the XRD analysis. The FTIR spectra of Bi2O3-ZnO demonstrate the strong characteristic bending vibrations peaks at 682 and 679 cm−1, which represent the characteristic peaks of Zn-O bonds in zinc oxide nanoparticles (Figure 4). The stretching vibration peaks at 3,449 cm−1, 1,699 cm−1 and 1,581 cm−1 are attributed to the existence of remaining hydroxyl groups (O-H stretching) and C-O and C-C vibrations, respectively. The weak vibrational bands at 3,500 cm−1 and 1,040 cm−1 given to the O-H stretching frequency of water may occur due to the presence of moisture in the KBr matrix.
Figure 4

FTIR spectrum of (a) Bi2WO6 and (b) Bi2O3-ZnO nanocomposites.

Figure 4

FTIR spectrum of (a) Bi2WO6 and (b) Bi2O3-ZnO nanocomposites.

Close modal

Comparative photocatalytic activity of Bi2WO6 and Bi2O3-ZnO nanocomposite

To detect the photocatalytic activity of as-synthesized materials, BPA was selected as the model compound. As represented in Figure 5(a), almost 90% of BPA was degraded within 80 min with the Bi2O3-ZnO/H2O2 system, whereas < 10% and around 58% BPA were degraded by H2O2 and Bi2O3-ZnO alone, respectively, under identical treatment conditions. The comparative catalytic performance of both Bi2WO6 and Bi2WO6/H2O2 nanocomposite powder showed 53% and 75% BPA removal, respectively, in 80 min. These results proposed that Bi2O3-ZnO nanocomposite with H2O2 had a synergistic effect on the effective degradation of BPA. BPA degradation in both system Bi2WO6 and Bi2O3-ZnO nanocomposite and H2O2 alone system was followed by the pseudo-first-order kinetic with the seeming rate constant (kobs) of 1.4 × 10−3 min−1, 2.1 × 10−2 min−1, and 1.6 × 10−3, respectively (Figure 5(b)). The rate constant value of BPA degradation with Bi2O3-ZnO is comparatively higher than H2O2 alone system and Bi2WO6 decreased as the reaction process, which may be because the reactive sites on Bi2O3-ZnO surface occupied by the recycling of Bi2+ and Zn2+ (originated from the relative content of adsorbed oxygen increased and the reduction of relative content of lattice oxygen) and/or the oxidative by-products of BPA (Farzana & Meenakshi 2015; Landge et al. 2021). These results confirmed that compared to the Bi2WO6/H2O2 system, the Bi2O3-ZnO/H2O2 system shows excellent heterogeneous activator to produce radicals (e.g., OH and SO4) for the oxidation of pollutants. Therefore, Bi2O3-ZnO/H2O2 system was selected as a photocatalyst for the parameters optimization of the BPA degradation for further study.
Figure 5

(a) Decomposition of BPA with different catalytic systems at [H2O2]o = 1.0 mM, [BPA]o = 10 mg L−1, catalyst dosage = 0.15 g L−1, T = 298 K and pH = 7.0 and (b) kinetics of degradation at condition of H2O2 alone, Bi2WO6 and Bi2O3-ZnO with H2O2.

Figure 5

(a) Decomposition of BPA with different catalytic systems at [H2O2]o = 1.0 mM, [BPA]o = 10 mg L−1, catalyst dosage = 0.15 g L−1, T = 298 K and pH = 7.0 and (b) kinetics of degradation at condition of H2O2 alone, Bi2WO6 and Bi2O3-ZnO with H2O2.

Close modal

Effect of operating parameters

Photocatalytic degradation of BPA using as-synthesized Bi2O3-ZnO nanocomposite was optimized by knowing the effect on major treatment parameters, including catalyst dose, initial BPA concentration, reaction temperature, and solution pH. The effect of each parameter was systematically studied as follows.

Effect of catalyst dose

Catalyst's dose is an important factor that influences the pollutant degradation rate. As the catalyst dose increases, the rate of pollutant degradation successively increases at a certain level. To know the effect of catalyst dose on BPA degradation, the catalyst dose was tested from 0.02 to 0.15 g L−1 in 250 mL of BPA (10 mgL−1) concentration and pH = 7.0. As shown in Figure 6(a), ∼28% BPA was degraded with 0.02 gL−1 at 80 min, while more than 92% BPA was removed after 80 min when Bi2O3-ZnO dosages reached 0.15 g L−1. This improvement in BPA reduction might be ascribed to an increased dosage of Bi2O3-ZnO that could offer additional active sites for H2O2 activation with radical-radical reactions happening prior to radical-pollutant interactions. However, the BPA removal efficiency was not significantly enhanced when the catalyst dose was increased from 0.1 g L−1 to 0.15 g L−1 (86% to 92% BPA removal) because excess catalyst dose diminishes catalytic capacity because of an insufficient breakdown of pollutant with reactive oxidation species (ROS) at the surface of the catalyst (Yang et al. 2022).
Figure 6

Effect of operating parameters: (a) catalyst dose [(BPA)o = 10 mgL−1, pHo = 7.0 at 273 K] (b) BPA initial concertation [(B2O3-ZnO)o = 0.15 g L−1, pHo = 7.0 at 273 K]; (c) temperature [(BPA)o = 10 mgL−1, (B2O3-ZnO)o = 0.15 gL−1, pHo = 7.0]; and (d) Initial pH [(BPA)o = 10 mgL−1, [B2O3-ZnO]o = 0.15 gL−1, T = 303 K].

Figure 6

Effect of operating parameters: (a) catalyst dose [(BPA)o = 10 mgL−1, pHo = 7.0 at 273 K] (b) BPA initial concertation [(B2O3-ZnO)o = 0.15 g L−1, pHo = 7.0 at 273 K]; (c) temperature [(BPA)o = 10 mgL−1, (B2O3-ZnO)o = 0.15 gL−1, pHo = 7.0]; and (d) Initial pH [(BPA)o = 10 mgL−1, [B2O3-ZnO]o = 0.15 gL−1, T = 303 K].

Close modal
Figure 7

Effect of (a) catalyst dose; (b) initial BPA concertation; (c) temperature; and (d) initial pH of solution on the nth-order kinetic study (n is order of the reaction).

Figure 7

Effect of (a) catalyst dose; (b) initial BPA concertation; (c) temperature; and (d) initial pH of solution on the nth-order kinetic study (n is order of the reaction).

Close modal

Effect of initial concentration of BPA

The impact of the initial concentration pollutant on the BPA degradation was studied by altering the initial BPA concentration from 5 to 20 mg L−1 while keeping a constant amount of Bi2O3-ZnO (0.15 g L−1) and H2O2 (1.0 mM) at pH = 7.0 (Figure 6(b)). BPA concentration of 5 mg L−1 and 10 mg L−1 were 100% and 90% eliminated after ∼80 min. But BPA degradation reduced from 100% to 54% when the BPA concentration was increased to 20 mg L−1 because with increases in concentration, the ROS interaction with the pollutant reduced, resulting in less degradation. At a higher initial pollutant concentration, the number of free radicals and the availability of the catalyst surface to adsorb the excess pollutant molecules is lower; consequently, the pollutant removal decreases (Sharma et al. 2015b; Cherifi et al. 2019). The extra number of pollutants needs more ROS and intermediates by-products in the reaction mixture; additionally, competition of ROS between intermediates by-products and BPA molecules may describe the decreases in BPA reduction. Moreover, at higher pollutant concentrations, repulsion between imminent particles is big enough; therefore, they continue to separate from one another and happen in a dispersion state. Consequently, pollutant removal decreases with increasing the pollutant concentration.

Effect of temperature

Temperature is another important parameter in knowing the effect of BPA reduction on the Bi2O3-ZnO/H2O2 system. The BPA reduction increased with increased temperature from 5 to 20 °C. A maximum of about 72% BPA was reduced after 80 min at 5 °C. The complete BPA degradation occurred within 80 min at 20 °C (Figure 6(c)). This might depend upon the activation energy (Ea) of the molecule. Moreover, the higher temperature helps the reaction to complete more efficiently with electron-hole recombination (Chen 2009). Further increases in the temperature increase the oxidation rate of the pollutant and also reduces the adsorptive capacities of the pollutant over the surface of the photocatalyst.

Effect of pH

The solution pH is an important factor in the photocatalytic degradation of pollutant initial solution pH impact was investigated at different pH range between 3.0 and 8.0 on BPA reduction (Figure 6(d)). It was observed that the BPA reduction increased with an increase in the solution pH from 4.0 to 8.0, but higher removal efficiency was observed at pH 5 and 7.0. The best results of BPA reduction occurred at pH 7.0; this might be due to its nature. BPA is a weak organic acid. Therefore, the pH lower than the zero-point charge improves the adsorption of BPA molecules onto the surface of photocatalysts. The zeta potential (pHpzc) of Bi2O3-ZnO is 8.32 at pH 5.8. As a result, the Bi2O3-ZnO surface most likely attained the positive charge at pH < 5.8 that helps maximize interaction between positively charged catalyst and less acidic BPA; these results favored the highest BPA degradation in neutral conditions than higher acidic and basic conditions (Abdul et al. 2021). On the other hand, photocatalytic degradation of the BPA in a slightly acidic and neutral medium favored the formation of hydroxy radicals, as can be assumed from the following reaction Equations (2)–(5).
formula
(2)
formula
(3)
formula
(4)
formula
(5)

As shown in Figure 6(d), the maximum BPA removal with Bi2O3-ZnO at a pH of 7.0 was found to be ∼ 92% because UV light activates the H2O2 for enhanced BPA degradation increasing the concentration of ROS at its optimal treatment condition of BPA reduction which greatly affected the three main factors, i.e., the rate of radical reaction with BPA deprotonation, conversion ROS itself at different pH, and absorption coefficient between the catalyst and BPA (Sharma et al. 2015a). The difference in initial and final pH was also tested, and it was observed that the final pH was slightly increased in most of the initial pH from acidic to alkaline, which might be ascribed to the high concentration of ROS species in the solution buffer (Qiao et al. 2021).

Kinetic study for photocatalytic degradation of BPA

A kinetic study was also performed for the photocatalytic degradation of BPA-containing wastewater at different operating parameters like catalyst dose, initial BPA concentration, solution pH, and temperature. The power law model, as well as the first-order model, were applied for kinetic study. First, second and nth order kinetic expressions are as follows (Patidar & Srivastava 2021; Ersoy et al. 2010):
formula
(6)
formula
(7)
formula
(8)
where k1 and kn are the 1st order (min−1) and nth order (mol L−1) (1−n) min−1 kinetic rate constant, respectively. Errors were minimized by using the nonlinear regression analysis method, and average relative error (ARE) was calculated as follows:
formula
(9)
where, and are the experimental and calculated concentration values, respectively, of fitting by corresponding kinetic model, of the BPA at time t. The kinetic study analysis of the experimental data of photocatalytic degradation of BPA was carried out using different kinetics models such as pseudo-first-order, pseudo-second-order, and nth-order kinetics models by the equation shown in the material and methods section. ARE was also calculated by using Equation (9). Kinetics parameters such as the first-order rate constant (kf), the second-order rate constant (ks), nth order rate constant (kn), and the order of reaction (n) (power law model), are shown in Table 1. It was found that BPA degradation least fit the second-order kinetics model, and the power law model well described the fitting of the kinetics. A fitting of the experimental data and calculated data by the power law model is shown in Figure 7 for the BPA removal with time. With the order of reaction being 0.6, 0.9, 1.2, and 1.3 for different operating parameters, i.e., catalyst dose, initial pH, temperature, and initial BPA concentration.
Table 1

Study of the pseudo-first-order, pseudo-second-order, and nth-order kinetics parameter for the photocatalytic treatment of BPA under different range of the operating parameter

Pseudo-first-order kinetics
Pseudo-second-order kinetics
nth-order kinetics
ParameterkfR2ARE (%)ksR2ARE (%)nknR2ARE (%)
Catalyst dose (g L−1) Other conditions: (BPA)o = 10 mgL−1, pHo = 7.0 
0.02 7.9 × 10−3 0.98 2.6 1.1 × 10−3 0.97 3.1 0.6 1.7 × 10−2 0.99 1.98 
0.05 1.2 × 10−2 0.99 2.2 2.0 × 10−1 0.96 5.2 0.9 1.4 × 10−2 0.99 1.97 
0.1 3.5 × 10−2 0.97 1.2 1.7 × 10−3 0.96 4.4 0.6 5.3 × 10−2 0.99 4.65 
0.15 4.2 × 10−2 0.95 1.7 3.1 × 10−2 0.98 2.1 0.6 5.7 × 10−2 0.99 5.26 
Initial BPA Concentration (mg L−1) Other conditions: (B2O3-ZnO)o = 0.15 g L−1, pHo = 7.0 
3.5 × 10−2 0.97 1.3 1.7 × 10−3 0.90 3.9 0.6 5.5 × 10−2 0.99 4.36 
10 3.0 × 10−2 0.97 6.1 3.3 × 10−1 0.94 3.4 0.6 4.9 × 10−2 0.99 4.11 
15 3.3 × 10−2 0.99 2.2 8.5 × 10−2 0.98 2.1 1.3 4.7 × 10−2 0.99 2.26 
20 7.4 × 10−3 0.99 1.6 3.9 × 10−2 0.98 3.2 2.4 9.1 × 10−5 0.96 3.53 
Initial pHo Other conditions: (BPA)o = 10 mgL−1, (B2O3 − ZnO)o = 0.15 g L−1 pHo = 7.0 
1.3 × 10−2 0.98 2.5 2.1 × 10−6 0.93 7.9 2.3 8.5 × 10−4 0.94 8.68 
2.4 × 10−2 0.93 1.2 6.6 × 10−6 0.98 2.6 0.1 8.9 × 10−2 0.99 7.30 
3.0 × 10−2 0.88 2.1 1.2 × 10−6 0.95 4.3 0.3 7.7 × 10−2 0.99 6.18 
8.8 × 10−3 0.97 2.4 1.3 × 10−6 0.94 5.3 3.3 6.3 × 10−5 0.93 6.65 
Temperature (°C) Other conditions: (BPA)o = 10 mgL−1, (B2O3 − ZnO)o = 0.15 gL−1, pHo = 7.0 
3.6 × 10−2 0.98 1.4 1.8 × 10−4 0.97 4.1 0.6 5.56 × 10−2 0.99 5.61 
10 4.5 × 10−2 0.97 2.3 4.3 × 10−3 0.97 6.2 0.5 6.68 × 10−2 0.99 1.23 
15 5.6 × 10−2 0.95 2.1 8.8 × 10−2 0.96 3.2 0.7 6.25 × 10−2 0.99 9.39 
20 5.8 × 10−2 0.97 1.5 9.1 × 10−2 0.94 2.1 0.8 6.02 × 10−2 0.99 12.99 
Pseudo-first-order kinetics
Pseudo-second-order kinetics
nth-order kinetics
ParameterkfR2ARE (%)ksR2ARE (%)nknR2ARE (%)
Catalyst dose (g L−1) Other conditions: (BPA)o = 10 mgL−1, pHo = 7.0 
0.02 7.9 × 10−3 0.98 2.6 1.1 × 10−3 0.97 3.1 0.6 1.7 × 10−2 0.99 1.98 
0.05 1.2 × 10−2 0.99 2.2 2.0 × 10−1 0.96 5.2 0.9 1.4 × 10−2 0.99 1.97 
0.1 3.5 × 10−2 0.97 1.2 1.7 × 10−3 0.96 4.4 0.6 5.3 × 10−2 0.99 4.65 
0.15 4.2 × 10−2 0.95 1.7 3.1 × 10−2 0.98 2.1 0.6 5.7 × 10−2 0.99 5.26 
Initial BPA Concentration (mg L−1) Other conditions: (B2O3-ZnO)o = 0.15 g L−1, pHo = 7.0 
3.5 × 10−2 0.97 1.3 1.7 × 10−3 0.90 3.9 0.6 5.5 × 10−2 0.99 4.36 
10 3.0 × 10−2 0.97 6.1 3.3 × 10−1 0.94 3.4 0.6 4.9 × 10−2 0.99 4.11 
15 3.3 × 10−2 0.99 2.2 8.5 × 10−2 0.98 2.1 1.3 4.7 × 10−2 0.99 2.26 
20 7.4 × 10−3 0.99 1.6 3.9 × 10−2 0.98 3.2 2.4 9.1 × 10−5 0.96 3.53 
Initial pHo Other conditions: (BPA)o = 10 mgL−1, (B2O3 − ZnO)o = 0.15 g L−1 pHo = 7.0 
1.3 × 10−2 0.98 2.5 2.1 × 10−6 0.93 7.9 2.3 8.5 × 10−4 0.94 8.68 
2.4 × 10−2 0.93 1.2 6.6 × 10−6 0.98 2.6 0.1 8.9 × 10−2 0.99 7.30 
3.0 × 10−2 0.88 2.1 1.2 × 10−6 0.95 4.3 0.3 7.7 × 10−2 0.99 6.18 
8.8 × 10−3 0.97 2.4 1.3 × 10−6 0.94 5.3 3.3 6.3 × 10−5 0.93 6.65 
Temperature (°C) Other conditions: (BPA)o = 10 mgL−1, (B2O3 − ZnO)o = 0.15 gL−1, pHo = 7.0 
3.6 × 10−2 0.98 1.4 1.8 × 10−4 0.97 4.1 0.6 5.56 × 10−2 0.99 5.61 
10 4.5 × 10−2 0.97 2.3 4.3 × 10−3 0.97 6.2 0.5 6.68 × 10−2 0.99 1.23 
15 5.6 × 10−2 0.95 2.1 8.8 × 10−2 0.96 3.2 0.7 6.25 × 10−2 0.99 9.39 
20 5.8 × 10−2 0.97 1.5 9.1 × 10−2 0.94 2.1 0.8 6.02 × 10−2 0.99 12.99 

a: kf is Pseudo-first-order kinetic constant (min−1). b: ks is pseudo-second-order kinetic constant ((mg L−1)−1 min−1). c: kn is nth order kinetic constant ((mg L−1) (1−n) min−1). d: ARE is average relative error (%).

Reusability of Bi2WO6 mesosphere

From the economic and potential applications point of view, the regeneration and reusability of photocatalyst materials is a significant aspect and is described in this study. Conventional separation methods, i.e., centrifugation and filtration, were used to separate the catalyst from the treatment solution (Xia et al. 2013). Consequently, five successive BPA degradation experiments were conducted to evaluate the reusability of the nanocomposite. Between each experiment, the samples were collected by centrifugation and filtration and then washed with ethanol and deionized water for some time and dried at 90 °C for 3 h. Figure 8(a) shows that photodegradation effectiveness is somewhat significantly reduced. These decreases in the percentage of removal efficiency perhaps were ascribed to little loss of Bi2O3-ZnO from the support surface through the washing procedure. The additional possible purpose is the accumulation of intermediate by-products formed by the organic pollutant degradation on the active surface of the photocatalyst, which results in diminished photocatalytic activity. Consequently, the photocatalyst was recovered after five cycles. The recovered catalyst was calcined at 400 °C for 3 h and then reused. The attained result proves that thermal recurrence leads to substantial regeneration of photocatalytic activity. Therefore, it can be stated that thermal treatment is an essential process for the used catalyst to the regeneration of its activity.
Figure 8

(a) Reusability test analysis of B2O3-ZnO in BPA degradation at [catalyst dose]: 0.15 g L−1, [BPA]: 10 mg L−1, pH: 7. (b) Effect of scavengers on BPA degradation with B2O3-ZnO system at [BPA]o = 10 mg L−1, [B2O3-ZnO] = 0.15 g L−1, [H2O2]o = 1.0 mM, pH = 7, T = 25 °C, t-BuOH]/[BQ]/[FFA] = 0.1 M). (c) Kinetics of the scavengers during BPA degradation.

Figure 8

(a) Reusability test analysis of B2O3-ZnO in BPA degradation at [catalyst dose]: 0.15 g L−1, [BPA]: 10 mg L−1, pH: 7. (b) Effect of scavengers on BPA degradation with B2O3-ZnO system at [BPA]o = 10 mg L−1, [B2O3-ZnO] = 0.15 g L−1, [H2O2]o = 1.0 mM, pH = 7, T = 25 °C, t-BuOH]/[BQ]/[FFA] = 0.1 M). (c) Kinetics of the scavengers during BPA degradation.

Close modal

Scavenger test analysis

Scavenger test analysis was used to investigate the main reactive oxygen species that participated in BPA degradation using the Bi2O3-ZnO/H2O2 system at pH 7. Among the scavenger analysis, 1 mM of tertbutanol (t-BuOH), furfuryl alcohol (FFA), and p-benzoquinone (BQ) were used as scavengers for OH, singlet oxygen (1O2), and superoxide anion radical (O2•−), respectively. The addition of a t-BuOH scavenger showed high effects on BPA degradation due to its hydrophilic nature, while BQ and FFA slightly decreased the BPA. This result suggests that OH was found to be major reactive species, whereas O2•− and 1O2 played a slight role in BPA degradation with Bi2O3-ZnO/H2O2 system (Figure 8(b)).

It is well documented that HO radicals were produced via the activation process with different composite metal oxides, as given in Equations (10) and (11) (Sharma et al. 2016). Recently, researchers have reported the role of non-radical mechanisms (i.e., 1O2) with radical mechanisms (HO and O2−•) with different phases of metal composite oxides (Landge et al. 2021). Scavengers analysis has been confirmed that OH involved in BPA reduction with Bi2O3-ZnO/H2O2, while the contributions of 1O2 (Equations (12) and (13)) and O2−• (Equations (14) and (15)) were found to be very less. In addition, the oxygen liability and surface permeability of the Bi2O3-ZnO composite influenced the catalytic activity of BPA (Sharma et al. 2016). These results suggested that the Bi2O3-ZnO composite contributed in H2O2 activation and yielded various reactive species that contribute to BPA degradation Equation (16) at a different rate (as shown in Figure 8(c)).
formula
(10)
formula
(11)
formula
(12)
formula
(13)
formula
(14)
formula
(15)
formula
(16)

Degradation mechanism of BPA

The production of intermediates during the breakdown of BPA (m/z: 228) with the Bi2O3-ZnO/H2O2 system was identified by performing GC-MS. According to past research, the possible degradation mechanism of BPA reduction consisted of the following steps: (a) BPA hydroxylation and dehydration to generate quinone, (b) radicals breaking the C-C link connecting two benzene rings, and (c) producing phenoxyl and para-isopropenyl phenol. The attack of OH radicals on an electron-rich site starts the degradation process. It is demonstrated that OH radicals have a particular selectivity for attack throughout the oxidation process. Both the addition and H-abstraction reactions are considered the reaction sites for the attack of •OH radicals. When it interacts with six-membered heterocyclic compounds, hydrogenated products, and resonance-stabilized organic radicals are produced (Lee & Von Gunten 2012). Radicals such as phenol and isopropyl phenol (C(CH3)2 C6H4OH) are produced by the β-scission of the C-C bond of the isopropyl group by the attack of OH radicals. Cyclohexanone (m/z: 228) and benzophenone (m/z: 228) are created when the BPA radical (CH2(C) CH3C6H6OH) is oxidized, which causes the destruction of a dimethyl group from the BPA (Takdastan et al. 2018).

It is well known that the H2O2 creates radical cations that then undergo hydrolysis to produce the transient OH adducts that further react with aromatic compounds directly via the direct electron transfer (Guan et al. 2013). The OH attack on BPA produces hydroxycyclohexadienyl radicals (m/z: 228) by electron transfer from the benzene ring (Singh et al. 2017). The quinone derivatives of monohydroxylated and hydroxylated BPA, i.e., quinone of monohydroxylated BPA (Q-MHBPA) (m/z: 247) and quinone of hydroxylated BPA (Q-DHBPA) (m/z: 252), respectively formed by the hydroxycyclohexadienyl radicals has been previously reported the similar observation during the ferrous ion-activated persulfate breakdown of BPA.

The addition and reduction reaction of the hydride ion (H-) and proton (H+) abstraction reaction takes place in BPA destruction, which is confirmed by the transformation of isopropyl radical into 4-isopropyl phenol and 4-hydroxy acetophenone, respectively. The 4-isopropenyl phenol is produced by the H-abstraction from the isopropyl radical, while 4-hydroxy acetophenone (m/z: 136) and 3-hydroxy-4-methoxy benzoic acid (m/z: 152) produced via the oxidation of 4-isopropyl phenol (m/z: 136). Phenol, hydroquinone, dimethyl-cyclohexanone, and hydroxy diphenyl ether were formed by the phenol radical destruction. Additionally, the hydroxylated aromatic intermediates are further degraded in low molecular weight substances like acids (acetic acid, oxalic acid, etc.) and aldehydes such as benzaldehyde (Sharma et al. 2016). BPA and its intermediates mineralization will be completed with the formation of CO2 and H2O. Figure 9 shows a plausible degradation pathway.
Figure 9

A schematic of BPA degradation pathway at the optimal treatment condition.

Figure 9

A schematic of BPA degradation pathway at the optimal treatment condition.

Close modal

The present study shows that the Bi2O3-ZnO heterostructures using H2O2 under UV-light irradiation are a possible treatment technique for BPA elimination from the aqueous phase. It is observed that the BPA removal decreases with an increase in BPA concentration and at a higher catalyst dose. The optimum value of the catalyst dose was detected to be 0.15 gL−1 for the 10 mgL−1 BPA concentration. At the optimum treatment conditions, more than 90% of BPA was degraded. The enhancement of BPA degradation was formed at initial pH = 7. However, slightly less acidic and basic pH shows good results of BPA reduction than the higher acidic and alkaline pH because the conversion of H2O2 radicals to OH and the BPA degradation increases at the selected range of solution pH. The kinetics of degradation of BPA were well fitted by the power laws model. HPLC and GC-MS results confirmed that quinone(s) and hydroxylated products are detected during the BPA reduction to proceed with the electron transfer degradation mechanism. The phenoxyl type radicals’ formation confirmed the probable degradation pathway of BPA reduction involving oxidative skeletal rearrangement, hydroxylation, dehydration, demethylation, and ring opening.

The authors are thankful to the Ministry of Education (MOE) in Taiwan, National Taiwan University (NTUCCP-110L901003, NTU-110L8807), and National Natural Science Foundation of China (Grant No. 51978654), Qing Lan Project of Jiangsu Province, and the Uttaranchal University, Dehradun, Uttarakhand, India.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Cabot
A.
,
Marsal
A.
,
Arbiol
J.
&
Morante
J. R.
2004
Bi2O3 as a selective sensing material for NO detection
.
Sens. Actuators, B
99
(
1
),
74
89
.
Chen
C. Y.
2009
Photocatalytic degradation of azo dye reactive orange 16 by TiO2
.
Water Air Soil Pollut.
202
(
1
),
335
342
.
Cherifi
Y.
,
Addad
A.
,
Vezin
H.
,
Barras
A.
,
Ouddane
B.
,
Chaouchi
A.
,
Szunerits
S.
&
Boukherroub
R.
2019
PMS activation using reduced graphene oxide under sonication: efficient metal-free catalytic system for the degradation of rhodamine B, bisphenol A, and tetracycline
.
Ultrason. Sonochem.
52
,
164
175
.
Ersoy
B.
,
Sariisik
A.
,
Dikmen
S.
&
Sariisik
G.
2010
Characterization of acidic pumice and determination of its electrokinetic properties in water
.
Powder Technol.
197
(
1–2
),
129
135
.
Hajra
P.
,
Shyamal
S.
,
Mandal
H.
,
Sariket
D.
,
Maity
A.
,
Kundu
S.
&
Bhattacharya
C.
2019
Synthesis of oxygen deficient bismuth oxide photocatalyst for improved photoelectrochemical applications
.
Electrochim. Acta
299
,
357
365
.
Hernández-Alonso
M. D.
,
Fresno
F.
,
Suárez
S.
&
Coronado
J. M.
2009
Development of alternative photocatalysts to TiO2: challenges and opportunities
.
Energy Environ. Sci.
2
(
12
),
1231
1257
.
Landge
V. K.
,
Sonawane
S. H.
,
Sivakumar
M.
,
Sonawane
S. S.
,
Babu
G. U. B.
&
Boczkaj
G.
2021
S-scheme heterojunction Bi2O3-ZnO/Bentonite clay composite with enhanced photocatalytic performance
.
Sustainable Energy Technol. Assess.
45
,
101194
.
Liu
Y. J.
,
Cai
R.
,
Fang
T.
,
Wu
J. G.
&
Wei
A.
2015
Low temperature synthesis of Bi2WO6 and its photocatalytic activities
.
Mater. Res. Bull.
66
,
96
100
.
Singh
S.
,
Srivastava
V. C.
,
Lo
S. −L.
,
Mandal
T. K.
&
Naresh
G.
2017
Morphology-controlled green approach for synthesizing the hierarchical self-assembled 3D porous ZnO superstructure with excellent catalytic activity
.
Microporous Mesoporous Mater.
239
,
296
309
.
Xia
D.
,
Ng
T. W.
,
An
T.
,
Li
G.
,
Li
Y.
,
Yip
H. Y.
,
Zhao
H.
,
Lu
A.
&
Wong
P. K.
2013
A recyclable mineral catalyst for visible-light-driven photocatalytic inactivation of bacteria: natural magnetic sphalerite
.
Environ. Sci. Technol.
47
(
19
),
11166
11173
.
Xu
L.
,
Sithambaram
S.
,
Zhang
Y.
,
Chen
C. H.
,
Jin
L.
,
Joesten
R.
&
Suib
S. L.
2009
Novel urchin-like CuO synthesized by a facile reflux method with efficient olefin epoxidation catalytic performance
.
Chem. Mater.
21
(
7
),
1253
1259
.
Yasin
M.
,
Saeed
M.
,
Muneer
M.
,
Usman
M.
,
Haq
A.
,
Sadia
M.
&
Altaf
M.
2022
Development of Bi2O3-ZnO heterostructure for enhanced photodegradation of rhodamine B and reactive yellow dyes
.
Surf. Interfaces
30
,
101846
.
Zhang
L.
,
Wang
W.
,
Yang
J.
,
Chen
Z.
,
Zhang
W.
,
Zhou
L.
&
Liu
S.
2006
Sonochemical synthesis of nanocrystallite Bi2O3 as a visible-light-driven photocatalyst
.
Appl. Catal., B
308
,
105
110
.
Zhang
H.
,
Chen
G.
&
Bahnemann
D. W.
2009
Photoelectrocatalytic materials for environmental applications
.
J. Mater. Chem.
19
(
29
),
5089
5121
.
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