The impact of alkaline earth oxides on Bi2O3 and their catalytic activities in photodegradation of Bisphenol A

The BPA into wastewater has posed a threat to environment and human health. Hence, we aimed to eliminate BPA in a short time and with a rapid degradation rate from food wastewater. Herein, the effects of different alkaline-earth oxide doped with Bi2O3 nanoparticles on the photocatalytic degradation of bisphenol A were investigated. SrO-Bi2O3, CaO-Bi2O3, and MgO-Bi2O3 binary oxides were prepared by wet-impregnation method. The structural and optical features of catalysts were clarified BET, XRD, DRS, FT-IR, PL, and SEM techniques. The photocatalytic activities of catalysts were compared for different light sources. Considering that the characterization analysis and experimental results, the highly improved photocatalytic activity was mainly attributed to the effective structure of the SrO-Bi2O3 binary oxide and the strong alkali properties in the nanocomposite. Obviously, 5wt% SrO-Bi2O3 photocatalyst showed more excellent degradation performance and highest degradation reaction rate (0.21 mg l–1 min–1) within 30 min. It was observed that the photocatalytic activity improved by the additive of alkaline oxide on Bi2O3.

Previous studies have shown that photodegradation gives good results. So, heterogeneous photocatalytic oxidation has been seen as potential effective technique to deteriorate environmental pollutant. With this in mind, we prefer heterogeneous photocatalytic degradation which is more advantageous than the other methods since it is low cost, reusable, provides complete degradation, and it is eco-friendly method.
BPA can be co-existing with other contaminants, so more selective catalysts should be used. During the study we used alkali metal oxides on bismuth oxide. The alkaline earth metals used in this study are often white in color, soft, and workable. In addition to being less reactive (prone to reactions) than alkali metals, their melting and boiling temperatures are also lower. Ionization energies are also higher than alkali metals. SrO impregnated with different proportions on Bi 2 O 3 showed great selectivity and was successful in a short time. In addition to these different catalysts were used as the main catalyst [13][14][15]. When the previous studies were examined, titanium oxide was used many times as the main catalyst, as it has an extended band gap [14][15][16][17]. In particular, Bi 2 O 3 has been used since it is a nontoxic and noncarcinogenic compound and has high photocatalytic activity with a bandwidth ranging from 2.0-2.8 eV [18][19][20][21].
The use of binary metal oxides as photocatalysts has been made widely for decades because of the fact that the morphological properties of the individual oxides can be changed due to the formation of new sites in the interface between the components, or by the incorporation of one oxide into the lattice of the other. They also found that this enhancement was attributed to gradually increasing shift of the conduction bands with increasing metal oxide contents, so resulting in a stronger reduction power of photogenerated electrons and promoting the improved photocatalytic activity.
In the current study, MxOy-Bi 2 O 3 (M: Ca, Mg, Sr) photocatalysts with various loading of the metal oxides were prepared by impregnation method. The catalytic activities of synthesized materials were investigated by using UV irradiation. The complete degradation of pollutants such as BPA by photocatalytic methods is a promising solution for environmental problems and human health. In this study, the purpose is to explain the impact of parameters such as the use of different types of metal oxides and the percentage of metal oxides on the photooxidation of BPA. Another aim is to reduce the concentration of BPA. This is due to the fact that BPA has pollutants for the environment and harmful effects on human health. Moreover, the relationships between the catalyst morphologies and the photocatalytic activities were also investigated by using varied characterization methods such as scanning electron microscope (SEM), diffuse reflectance spectroscopy (DRS), Brunauer, fourier transform infrared (FTIR), Emmet and Teller (BET), and X-Ray diffraction (XRD).

Catalyst synthesis methods
Co-precipitation method was utilized to synthesis Bi 2 O 3 catalyst. 1.94 g of Bi(NO 3 ) 2 .5H 2 O weighed by precision balance. In addition, 1.12 M 20 mL HNO 3 (nitric acid-65%) solution and 0.2 M NaOH solution were prepared and kept in an ultrasonic bath for 15 min to ensure better dissolution. Finally, 20 mL of 1.12 molar HNO 3 solution was added to 1.94 of bismuth, which was weighed and then kept in the ultrasonic bath for 15 min. The solution of resulting were mixed by magnetic mixer at the room temperature. Subsequently, 0.2 M NaOH solution by adding drop by drop into the bismuth (III) nitrate solution was provided to reach pH value 11 and then mixed for 2 h at 75 °C to make a homogenous yellowish mixture. Then this mixture was filtered as well as washed with distilled water and absolute ethanol several times. Obtained this matter was dried in a oven at 80 °C for 2 h, then calcined at 450 °C for 2 h.
In order that preparing of binary catalyst, impregnation method was used. This method is based on impregnation of metal oxide solutions onto pure bismuth oxide catalyst. When preparing solutions, attention was paid to the weight percentages of metal oxides present in the binary catalyst. Firstly, nitrous forms of metal oxides (Ca(NO 3 ) 2 .4H 2 O, Mg(NO 3 ) 2 .6H 2 O, Sr(NO 3 ) 2 ) and 0.2 M solution of 5% by weight in binary catalyst were prepared. These solutions were added dropwise to the powdered pure bismuth oxide catalyst to give a wet mixture and then dried at 105 °C. This process was continued until the solutions were finished. It was then dried in oven at 105 °C for 16 h and then was calcined at 500 °C during 3 h.

Catalyst characterization
Total catalyst surface area of the catalysts was measured by nitrogen adsorption/desorption using a Quantachrome instrument. All catalysts were degassed under vacuum at 200 °C for 4 h.
Crystallographic structure of catalysts were determined by X-ray powder diffraction using CuKα radiation (λ = 1.54056 Å) with a Rigaku D/Max-2200 powder X-ray diffractometer. Before the analysis was run, the samples were gently granulated in an agate mortar to reduce the required orientation. Patterns were recorded at scan speed 2 degree of twotheta in the range of 10-90° 2θ. The average crystallite size (D avg ) was computed using the Debye-Scherrer equation.
The powders were examined with a high resolution scanning electron microscope (SEM) (JEOL/JSM-a6335F) for possible differences in morphologies and size distributions of the powders.
FT-IR spectra were examined by FT-IR spectroscopy (Perkin Elmer Precisely Spectrum One). KBr powder was used to prepare KBr pellets for samples. The samples were acquired as 100 scans with 4 cm -1 resolution Optical energy gap of nano powders were carried out by a doublebeam UV-Shimadzu 3600 UV-vis-NIR spectrophotometer equipped a diffuse reflectance (DR) accessory.
The energy of band gap for the catalyst was evaluated by using the Kubelka-Munk formula with Tauc's relation (Eq. (1)), which derived from DRS measurement.
(1) In this expression, hν is the energy of a single proton, α is the optical absorption co-efficiency, E g is the optical band gap energy, A is constant for direct band gap transitions, the value of exponent parameter n denotes the nature of sample transition (n = ½ is used for the catalyst). R is the absolute reflectance value; F(R) is proportional to the absorption coefficient (α).
Moreover, photoluminescence (PL) spectra were obtained on a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies, city, country). The catalyst was excited by a xenon lamp light source by 450 nm at the room temperature.

Studies on photocatalytic activity
The experiment of BPA degradation was carried out over MxOy-Bi 2 O 3 (M: Ca, Mg, Sr) binary oxide in a cylindrical quartz micro-photoreactor. Fifty mL of a 25 mg/L aqueous BPA solution was prepared and then 100 mg of catalyst was added in this mixture to initiate the reaction. Before exposure illumination, the solution was stirred in the dark for 1 h to establish an the adsorption-desorption equilibrium between the catalyst and the liquid. All reactions were conducted at ambient temperature under constant magnetic stirring and natural pH conditions. The photocatalytic activity of catalyst were compared using different light sources (UV-B, sunlight, and visible light irradiation). During the irradiation, sample was taken at regular intervals from the solution and filtered through a PTFE filter (pore size 0.45 mm for use total organic content (TOC) measurement (TOC-V, Shimadzu, city, country) and HPLC analysis. The analysis of BPA was performed by a HPLC (Thermo Scientific) using a C18 column. The mobile phase consists of a mixture of water and acetonitrile (40:60, v/v).

Structural, morphological, and optical properties
The composition of the catalyst was determined using Thermo Elemental X Series ICP-MS. The actual weight percentages of the catalysts in the binary oxide catalysts were evaluated by ICP-MS analysis. The calculated wt% of the catalysts and the ICP-MS values were almost similar. Furthermore, ICP-MS values of the two representative catalysts clearly suggest that there may not be any noticeable differences between the calculated values and ICP values for the other weight percentages of metal oxides.
The diffraction patterns of powder samples were examined to identify the phase structures. XRD patterns representing this are show in Figure 1 Table. After the impregnation of metal nitrate on Bi 2 O 3 , the lattice structure of Bi 2 O 3 did not change. However, the average crystallite size of Bi 2 O 3 changed with addition of metal oxide on Bi 2 O 3 . The crystallite size decreased with only the adding of metal oxide. After the XRD analysis, the crystallite sizes of samples were calculated as 41, 19, 17, and 15 nm for Bi 2 O 3 , 5MgO-Bi 2 O 3 , 5CaO-Bi 2 O 3 , 5SrO-Bi 2 O 3 , respectively. It has expressed that small crystallite size causes higher photocatalytic activity for the increased reactive sites and the promoted electron-hole separation efficiency [23].
Accordingly, the 5SrO-Bi 2 O 3 binary oxide is expected to show higher photocatalytic activity due to its low crystallite size.
The morphology and particle size of Bi 2 O 3 , 5MgO-Bi 2 O 3 , 5CaO-Bi 2 O 3 , and 5SrO-Bi 2 O 3 were observed by SEM in Figure  2a The change in the band gap energy of Bi 2 O 3 with alkaline earth oxide loading was investigated using UV-vis diffuse reflectance spectroscopy. Figure 4 corresponds to UV-vis DRS spectra of the catalysts. The band gap energies of the samples were calculated from graphical extrapolation by using Tauc Plot ((hnα) 1/n = A(hn-E g )) adapted for Kubelka-Munk function [23]. The calculated band gap values are given in Table. According to the results, the band gap of pure Bi 2 O 3 is 2.99 eV, whereas the band gap is decreased to 2.82 eV by the loading of 5 wt% SrO. The band gap of 5CaO/Bi 2 O 3 and 5Mg/Bi 2 O 3 are about 2.89, 2.92 eV, respectively. According these results, the loading of alkaline-metal oxide on Bi 2 O 3 powder could significantly shift the optical band gap width E g . It can be caused that the optical properties of the samples were affected by the quantum size, which is a consequence of the extent of the electron delocalization. As seen SEM image, MgO structure covered to Bi 2 O 3 external surface. Therefore, the diffuse reflectance spectrum of 5MgO/Bi 2 O 3 is more different than others. Because of the band gap value of Bi 2 O 3 , we studied the photocatalytic degradation of Bisphenol-A under UV irradiation.   Figure 5 shows the FTIR spectra from 4000-400 cm -1 for pure Bi 2 O 3 and alkaline earth oxide additive Bi 2 O 3 catalysts. Infrared spectroscopy technology is used to detect the presence of functional groups such as hydroxyl radical (•OH) adsorbed on the surface of synthesized nanoparticles. In the photocatalytic degradation experiments, the surface OH groups can not only embrace the photogenerated holes to form hydroxyl radical (•OH) but also serve as active sites for the adsorption of reactants [24]. The intensive signal at around 1430-1445 cm -1 for all alkaline-doped catalyst was attributed to the absorption of non-bridging O-H groups [25]. The concentration of the hydroxyl group was affected by the addition of alkaline oxide to Bi 2 O 3 .OH bending appears in the spectra for 5SrO/TiO 2 and 5CaO/Bi 2 O 3 catalyst at about 1439 cm -1 . However, this peak was not observed after MgO loading. These results are in line with the activity results. In addition, the infrared spectrum of Bi 2 O 3 was not observed any hydroxyl group. This result shows that the removing most of the adsorbed water from the surface of Bi 2 O 3 with calcination process. In the spectrum, the band arises at 1300 cm -1 from the weak band of Bi-O-(NBO) bond in BO 3 units for 5CaO/Bi 2 O 3 and 5SrO/Bi 2 O 3 . This peak was not observed for 5MgO/ Bi 2 O 3 . The low absorption band at around 830 cm -1 seemed in FTIR spectra of samples is the stretching vibration of Bi─O bonds in BiO 6 octahedral units [26]. The Bi─O bending vibration was observed at about 497, 537, and 830 cm -1 for Bi 2 O 3 . FTIR spectra of 5CaO/Bi 2 O 3 revealed the existence of peak at 874 cm -1 which are the characteristic bands of Ca-O [27]. In addition, the peak of the Sr-O band was observed at 857 cm -1 in the spectrum [28].
As a result of the FT-IR study, it was understood that Sr 2+ and Ca 2+ , except Mg 2+ , entered into the lattice of nano Bi 2 O 3 . The ionic radiuses of Sr 2+ (1.21 Å) and Ca 2+ (1.08) are larger than Bi 3+ (1.03 Å) but less than O 2-(1.31 Å). These ions can homogenously substituted or introduced into the nano Bi 2 O 3 matrix to produce oxygen vacancies that accelerate the transition and nanocrystalline growth of Bi 2 O 3 [29]. The occurrence of Bi-O-Sr prevents the transition of Bi 2 O 3 phase and prevents the agglomeration of nano Bi 2 O 3 particles. There is no evidence for an isomorphic settlement of Mg 2+ to the Bi 2 O 3 structure observed due to the ionic radius of Mg 2+ (0.86 Å) are lower than Bi 3+ by adding Mg 2+ to the nano Bi 2 O 3 structure [30].
Photoluminescence spectroscopy is a widely used technique for characterization of optical and electronic properties of semiconductors and molecules. Photoluminescence can measure the purity and crystal quality of semiconductors and give some information oxygen vacancies, photo-induced charge carrier separation, and recombination processes surface states in nano-sized semiconductor materials. At low Pl density, the recombination rate of the electron hole is also low [31].
PL spectra of the catalysts are shown in Figure 6. It was understood that the PL emission spectra of the catalysts were at the same peak maximum but different densities. A strong emission peak at about 393 nm was obtained in the PL spectrum of the catalysts. It was determined that the density of the 5SrO-Bi 2 O 3 catalyst in the emission spectrum was the lowest and this decrease showed the low recombination rate of the holes. As a result, it can be said that the 5SrO-Bi 2 O 3 catalyst helps inhibit the recombination of electrons and holes and improve photocatalytic activity.

Photocatalytic activity results
The photocatalytic activity of alkaline earth oxides loaded Bi 2 O 3 was examined for Bisphenol A degradation. We based on Langmuir-Hinshelwood (L-H) kinetic model [32] in our experiments. It can be expressed by Equation 2 below: ln(C o /C) = k obs t (2) Assuming that C = C o at t = 0 with low initial BPA concentration, where t is the given irradiation time and k obs is the rate constant of the observed pseudo-first order reaction. After experiments, we calculated the photocatalytic degradation  Table. The degradation efficiency (R%) of BPA was calculated by Equation (3): R% = (3) Firstly, we tested UV-B light and visible light as the different light source for degradation of BPA by using pure Bi 2 O 3 and the degradation efficiencies of pure Bi 2 O 3 were 76% and 47% for 30 min, respectively. The activity results show that although Bi 2 O 3 can be activated under different irradiation, it is not sufficient for complete of BPA.
Moreover, we studied with SrO-Bi 2 O 3 binary oxide prepared with varying SrO loadings. The degradation efficiency for the SrO-Bi 2 O 3 binary oxide increased with the rise in the amount of SrO charges to Bi 2 O 3 . It showed the highest percentage of BPA degradation (100%) in 30 min with a loading of 5% by weight SrO. However, there was no significant change in activity after this weight percentage and the value remained the same as the higher loading of SrO. The numerical values of gain showed that the structural and optical characterization of the samples and SrO dispersion on the surface affected with the loading of SrO in the binary oxide.
In addition, the performance of 5SrO-Bi 2 O 3 catalyst was studied with UV-B light and visible light for BPA degradation. According the experimental results, binary metal oxide nanoparticles are more photoactivated under UV light than visible light irradiation. The degradation of BPA under visible light and UV-B light (64 W) showed 35% and 100% for 30 min, respectively.
In addition, the reusability of the 5SrO-Bi 2 O 3 catalyst was studied on fresh dye samples (5 trials). 5SrO-Bi 2 O 3 , when used for the first time, could degrade 98.52% BPA, with a small change (to 94.29%) in the efficiency when used for 5 times. This decrease in the efficiency for 5SrO-Bi 2 O 3 catalyst resulted probably from the photocorrosion effect.
In this study, the photodegradation of BPA includes three steps. In the first stage, the transmission of electrons excited by photons emitted from the UV-B source from the valence band to the conduction band takes place. In the second stage, the holes formed due to the excitation process act as decomposing agent or combine with the surface hydroxyl species on the binary oxide to form the hydroxyl radical. In the last stage, the contaminant is attracted by the holes or hydroxyl radicals by the photons from the UV-B source.    Figure 7a shows the photocatalytic degradation of BPA in the pure Bi 2 O 3 and alkaline earth oxide additive Bi 2 O 3 catalysts under UV-B illumination at different irradiation times. When the degradation results are evaluated, the photocatalytic degradation rates of the BPA in 30 min with the binary metal oxides are sorted in descending order: 5SrO─Bi 2 O 3 (ca. 100%), 5CaO─Bi 2 O 3 (ca. 88%), 5MgO─Bi 2 O 3 (ca. 84%), and Bi 2 O 3 (ca. 76%). Obviously, 5SrO─Bi 2 O 3 catalyst improved the degradation of BPA in a short time and with high efficiency compared to other catalysts and pure Bi 2 O 3 . The results show that •OH radical adsorbed on the catalyst was play an extreme role in the photodegradation of BPA. A small difference in the efficiency during the photocatalytic degradation of BPA can be elucidated by the free •OH radicals involved to a small extent in the photodegradation process [33].
The measured TOC removal results for the photodegradation of BPA with 5SrO─Bi 2 O 3 , 5CaO─Bi 2 O 3 , and 5MgO─Bi 2 O 3 are shown in the Figure 7b. Twenty-five ppm BPA completely decomposed in 30 min with 5SrO─Bi 2 O 3 binary oxide and 94% TOC removal was obtained in 30 min. It is clear that the total mineralization is completed in the presence of 5SrO─Bi 2 O 3 catalyst.
The acid-base properties of the oxide catalyst can also affect the photocatalytic activity as well as its structural and optical properties.
As known, alkalinity of alkaline earth oxide increases from MgO to SrO. It seems that the activity changes in parallel with the alkalinity. As a result, the addition of strong basic alkaline oxide caused an increase in both the photocatalytic activity and the final conversion in oxidative degradation of BPA over catalysts. In addition to this result, the tendency of MgO to form larger particles on Bi 2 O 3 surface also caused a decrease in BPA conversion.
In the study, it was understood that the changes in surface area, band gap energy, and crystallite size were not as much as the changes in catalytic activity. These differences in activity are thought to be due to the active species on the surface of the oxide mixtures.

Conclusions
The effects of different alkaline-earth oxide doped with Bi 2 O 3 nanoparticles on the photocatalytic degradation of bisphenol A were investigated. SrO-Bi 2 O 3 , CaO-Bi 2 O 3 , and MgO-Bi 2 O 3 binary oxides were prepared by wet-impregnation method. The photocatalytic activities of the catalysts were compared for different light sources. Considering that the characterization analysis and improved photocatalytic activity results are mainly varied in relation to the effective structure of SrO-Bi 2 O 3 binary oxide and the strong basic properties in the nanocomposite. Obviously, 5wt% SrO-Bi 2 O 3 photocatalyst showed more excellent degradation performance and highest degradation reaction rate (0.21 mg l -1 min -1 ) within 30 min. It was observed that the photocatalytic activity improved by the addition of alkaline-earth oxide on Bi 2 O 3 .
In this study, Sr 2+ played an important role in reducing the crystallite size of nano Bi 2 O 3 . The small particle size of 5SrO-Bi 2 O 3 and uniformly distribution of SrO on the pure nano Bi 2 O 3 surface were very effective in the short time and complete degradation of BPA. The findings of this study elucidated an approach for the removal of BPA in the water through photocatalytic of degradation by alkaline-earth oxide doped with Bi 2 O 3 .