3.1 Characterization
The XRD patterns of CaWO4 were shown in Fig. 1a. The reflections at 2θ = 18.60o, 28.72o, 31.44o, 34.17o, 39.20o, 47.10o, 54.30o, and 58.05o corresponding to scheelite (JCPDS#41–1431). The results indicated that the sample synthesized by coprecipitation method in this experiment is scheelite (CaWO4). (Farsi et al., 2015).
To better explore the morphology and structure of CaWO4 samples, TEM were further carried out and the results were shown in Fig. 1c-f. It is seen that the synthesized CaWO4 samples were granular and the particle size of the sample was between 20 and 30 nm. According to mapping analysis (Fig. 1g-j), W, O, and Ca are evenly distributed, indicating from another angle that CaWO4 was successfully prepared. In addition, according to the calculation of element proportion (Fig. S2), the element distribution conforms to the CaWO4 molecular formula (Zhang et al., 2019).
The photoresponse characteristics of catalysts was characterized by
UV-vis DRS. As illustrated in Fig. 1b, the absorption onset of nano CaWO4 was around 400 nm, corresponding to the band gap energy (Eg) of 3.45 eV (Ayappan et al., 2019). In the wavelength range below 400 nm, the absorption capacity of the sample is good, especially at 240 nm and 330 nm. The result explained why the light absorption capacity of CaWO4 under ultraviolet light is stronger than that under visible light.
The adsorption volume and pore size distribution of catalysts also have important effects on catalytic performance. As can be seen in Fig. S3a and Fig. S3b, the BET-specific surface area was measured. The nitrogen adsorption-desorption isotherms of CaWO4 showed a type IV isotherm and type H3 hysteresis loops, indicating that the samples possessed a high proportion of mesopores and the pore shape belonged to slit-shape. And pore size distribution of CaWO4 shows that CaWO4 samples are mainly composed of 2-4 nanoparticles. The BET-specific surface of CaWO4 is 76.44 m2/g. It can draw a conclusion that adsorption capacity and pore size distribution of CaWO4 samples are favorable for catalytic reaction (Zhang et al., 2017).
Hence, through the characterization, it can be seen that nano CaWO4 material was synthesized by simple coprecipitate method. The sample has good dispersibility, large specific surface area, and good photoresponse performance under UV light. Deservedly, samples were used for photocatalytic experiments.
3.2 Degradation of MB
3.2.1 Comparation xperiment and TOC removal
The photocatalytic degradation of MB was verified by comparation experiments. Fig. S4 demonstrated that the MB was hardly removed under UV irradiation or bare CaWO4, with a degradation efficiency of about 15% in 3 h. It indicates the adsorption of CaWO4 and the photolysis of MB could be ignored. In contrast, in the presence of CaWO4 and UV irradiation, 100 mg/L of MB could be compeletely degraded in 3 h. It proved that CaWO4 has superior photocatalytic degradation efficiency of MB without adding oxidants. (Wen et al., 2020). As seen in Fig. S5, the TOC removal efficiency reached 58% after 3 h reaction when the MB removal efficiency was 99%. It indicates that the mineralization rate of pollutants is moderate. The reason why the mineralization efficiency is lower than the degradation efficiency may be due to the intermediates produced in the degradation process of MB rather than the complete conversion of CO2 and H2O.
3.2.2 Effect of reaction conditions
The photocatalytic degradation of MB is affected by different reaction conditions. The effects of solid-liquid ratio, light intensity, pollutant concentration, and initial pH values on the degradation of MB were investigated and the results were illustrated in Fig. 2a-d. Different experimental results were obtained under different experimental conditions. It is universal that the increase of solid-liquid ratio and light intensity can promote the degradation of pollutants in the photocatalytic reaction process (Nguyen et al., 2018). When the solid-liquid ratio increased from 0.2 g/L to 1.0 g/L, the degradation efficiency in 3 h of MB increased from 69% to 96%. And when the light intensity increased from 20 W to 80 W, the degradation efficiency of MB increased from 26% to 96%. Then, another common situation is that increasing concentrations of contaminants reduce degradation. When the concentration of MB is 200 mg/L, its degradation efficiency is only 25%, and when the concentration of MB is 30 mg/L, 50 mg/L and, 100 mg/L, its degradation efficiency can reach more than 96%. Interestingly, the influence of pH on MB is not a single rule (Wen et al., 2015). MB was degraded best at pH of 8.2, and worse at pH above or below 8.2. This may be due to MB solution itself contains cationic dye ionization in water formed by positive ions (Kedves et al., 2022). The positively charged ions are attracted to photogenerated electrons, which inhibits the recombination of electron hole pairs and promotes the photocatalytic reaction effect. Therefore, under the original pH condition of MB solution, the optimal MB degradation degree is obtained.
3.2.3 Identification of reactive species
To investigate the mechanism of MB degradation via CaWO4, different scavengers were introduced to the system to identify investigate the working reactive oxygen species (ROS). According to previous studies, in the process of photocatalytic degradation of organic contaminants, four active substances are chiefly involved: electronic, hole, hydroxyl radical, and superoxide radical (Guo et al., 2022). In this study, EDTA-2Na, TBA, Tiron, and NaF were utilized to verify the contribution of h+, ·OH, O2·-, and e-, respectively. Fig. 3a-d shows the influence of different quenchers on the experimental effect. Firstly, photoexcited electron transitions produce photogenerated electrons. However, the addition of 0.5 mM, 1.0 mM, and 5.0 mM NaF have little effect on MB degradation, indicating that Electrons do not play a role in the photocatalytic degradation of MB. Then, photoexcited electron transitions leave electron holes. EDTA-2Na, an effective scavenger, was employed to scavenge its role. It shows a high reactivity towards hole. When 0.5 mM EDTA-2Na was used, photocatalytic degradation of methylene blue was suppressed. When the concentration of EDTA-2Na rose from 0.5 mM to 5.0 mM, the degradation efficiency of MB was reduced form 75% to 51%. It suggests that holes play an important role in photocatalysis. Then, water is oxidized to produce hydroxyl radicals. It is widely accepted in the studies that alcohols without an alpha-hydrogen, such as tert-butanol, is one of the best scavengers of hydroxyl radicals (Sun et al., 2021). After adding TBA, the photocatalytic effect becomes worse. The degradation efficiency of MB was inhibited only 40% when 0.5 mM TBA was added. When TBA concentration reached 5.0 mM, the degradation efficiency of MB was inhibited only 45%. Obviously, the degradation efficiency of MB was inhibited by TBA. This indicates that hydroxyl radical is one of the key radicals in the degradation of MB. The active species in the reaction system also include the superoxide anion produced by the reduction of oxygen. Tiron was selected for quencher of O2•- (Zhou et al., 2012). According to the results, the quenching of Tiron has an effect on photocatalysis, but the effect is not very obvious. When Tiron concentration was 0.5 mM, 1.0 mM, and 5.0 mM, the degradation rate of MB decreased by 22%, 25%, and 30%, respectively. Tiron inhibited the degradation of a small amount of MB. This indicates that superoxide anion also participates in the photocatalytic degradation of MB to a certain extent.
As a consequence, holes and hydroxyl radicals dominate the photocatalytic degradation of MB, and superoxide anions are also responsible for a few MB degradation, while electrons are not involved in this process.
3.2.4 UV–vis spectra analysis
To clarify the degradation of MB, the UV–vis spectra in the course of photocatalytic degradation was carried out. As seen in Fig. S6, the spectrum of dye initial solution shows a maxima absorption peak at 665 nm. This is due to the large conjugated structure of the nitrogen - sulfur heterocyclic ring of MB (Xia et al., 2020). And an absorption peak at 300 nm is related to the phenozezine structure. Additionally, relatively weak aromatic and PAHs absorption peaks appear at 250 nm. A progressive decrease in intensity of all peaks indicates that structure of MB is destructed during the photocatalytic process. At the same time, some new products are gradually formed, and the corresponding absorption peaks are exhibited near 200 nm.
3.3 Degradation of CR
3.3.1 comparation experiment and TOC removal
The photocatalytic degradation of CR was verified by comparation experiment. As seen from Fig. S7, CR was hardly removed under the condition of only UV light or CaWO4, with a degradation efficiency of about 20% within 3 h. It indicates that the adsorption of CR by CaWO4 and the photolysis of CR are both at low-level. In contrast, in the presence of CaWO4 and UV irradiation, 100 mg/L of CR can be completely degraded in 3 h. The results show that CaWO4 has good photocatalytic degradation of CR without oxidizing agent. The TOC removal efficiency reached 42% after reaction 3 h when the CR removal efficiency was 99% as displayed in Fig. S8. It indicates that the mineralization efficiency of pollutants is relatively low. Only a small part of CR is converted to CO2 and H2O during photocatalytic degradation.
3.3.2 Effect of reaction conditions
The photocatalytic degradation of CR was affected by different reaction conditions. Effects of solid-liquid ratio, light intensity, pollutant concentration, and initial pH values on CR degradation were shown in Fig. 4a-d. There is a clear trend, when the solid-liquid ratio and light intensity increased, the degradation effect of CR better (Othman et al., 2007). When the solid-liquid ratio increased from 0.2 g/L to 1.0 g/L, the degradation efficiency of CR increased from 72% to 99%. And when the light intensity increased from 20 W to 80 W, the degradation efficiency of MB increased from 24% to 99%. In contrast, when the carmine concentration increased, the photocatalytic effect became worse. When the concentration of CR was 200 mg/L, the degradation efficiency was only 36%, while when the concentration of CR was reduced to 30 mg/L, 50 mg/L, and 100 mg/L, the degradation efficiency was above 99%. These are some common rules. Interestingly, the degradation efficiency of CR was highest at pH of 6 (Sahel et al., 2010). When pH was 4 or 8, CR degradation efficiency remained high, but when pH was 3 or 9, CR degradation efficiency was significantly lower. This may be due to the maximum positive potential of calcium tungstate solution at pH of 6, reaching 2 mV. In this case, positively charged ions will attract photogenerated electrons, reducing the recombination of electron hole pairs, thus promoting the effect of photocatalysis. When the pH is too low or too high, especially at pH of 3 or 9, the potential of the CaWO4 solution is very negative, at -26 mV and -15 mV, respectively (Fig. S9). At this time, negatively charged ions will repel photogenerated electrons and accelerate the recombination of electron hole pairs, thus reducing the photocatalytic effect.
3.3.3 Identification of reactive species
To further study the participation of various active species in CR degradation, e-, h+, •OH, and O2•- quenching experiments were employed in this study. Because these 4 kinds of active species are the main active substances produced in the photocatalytic process (Barka et al., 2008). Effects of these quenching experiments on photocatalytic degradation of CR were depicted in Fig. 5a-d. First, photogenerated electrons are created in the presence of a photoexcited catalyst. However, electrons do not play a role in the photocatalytic degradation of CR. Because the degradation effect of CR was not affected when NaF was used as quench agent. The degradation of CR was not affected when 0.5 mM, 1.0 mM, and 5.0 mM NaF were added to the reaction system. Secondly, the electrons create the holes after their transition. EDTA-2Na was used to quench holes, which slightly influenced the degradation of CR. When the concentration of EDTA-2Na was 0.5 mM and 1.0 mM, CR degradation was almost unaffected. When the concentration of EDTA-2Na was increased to 5.0 mM, the degradation efficiency of CR was reduced by 28%. It can be seen that holes are involved in photocatalytic reactions but are not major participants. In addition, hydroxyl radicals are produced when water is oxidized. The investigation of experiment of quenching hydroxyl radical by TBA, shows a significant effect on the experimental results. When 0.5 mM TBA was used as quencher, the photocatalytic degradation efficiency of CR was reduced by 36%. When the concentration of TBA was increased to 1.0 mM and 5.0 mM, the degradation of CR was inhibited more obviously, and the degradation efficiency of CR was reduced by 42% and 55%. This indicates that hydroxyl radical is the main participant of CR degradation reaction. The superoxide anions produced by oxygen reduction are also important active species. It is because the addition of Tiron to the reaction system to quench superoxide ions has a significant inhibition effect on the degradation of CR. 0.5 mM and 1.0 mM Tiron reduced the degradation efficiency of CR by 35% and 36%. And Tiron of 5.0 mM reduced the CR degradation efficiency by 56%.
As a consequence, hydroxyl radical and superoxide anion dominated the photocatalytic degradation of CR, and holes were also involved in partly the reaction, while electrons were almost not involved in the reaction.
3.3.4 UV–vis spectra analysis
To further explore the degradation of CR, the UV–vis spectra of CR aqueous solutions in the course of photocatalytic degradation was carried out (Thiam et al., 2015). As shown in Fig. S10, the spectrum of dye initial solution shows a maximum absorption peak at 515 nm, the characteristic absorption peak corresponding to the N-C bond in CR. In addition, an absorption peak at 331 is nm related to the naphthalene rings and another peak at 250 nm is assigned to aromatic and polycyclic aromatic hydrocarbons (Li et al., 2014). The intensity of these characteristic peaks gradually decreases. It shows that CR is degraded gradually with the photocatalytic reaction.
3.4 Study on photocatalytic mechanism
It is well known that photocatalysis transfers electrons to jump from the valence band to the conduction band under ultraviolet light to create electron hole pairs (K et al., 2022). The electron hole pair has the ability of redox, which degrades the pollutant. Therefore, the production of photogenerated electrons is an important part of the photocatalytic reaction. In order to explore the photocatalytic degradation mechanism of MB and CR by CaWO4, the photoelectric response performance of CaWO4 material was detected. The i-t curve of CaWO4 was measured by an electrical workstation and the results were shown in Fig. 6a. It is obvious that the generation of current is related to the irradiation of the light source. When the light is turned on and off at the same time interval, the generation and disappearance of current also change regularly with the light source being turned on and off. These results indicate that CaWO4 is a preferable photocatalyst material with superior photoelectric response ability. Under ultraviolet light, CaWO4 will rapidly produce a large number of electron transitions, which means it will form a large number of electron hole pairs, and then participate in the photocatalytic degradation of pollutants.
As depicted in Fig. 6b, XRD diffraction reflection before and after CaWO4 reaction do not change significantly, indicating that the catalyst adsorption of pollutants is in negligible amounts. The main reaction process is the degradation of pollutants. A similar situation can be seen on the FT-IR spectrum. As is shown in Fig. 6c, the functional groups of the samples before and after the reaction were investigated using FT-IR spectroscopy. In the FT-IR spectra of CaWO4 original sample, CaWO4 sample after reaction with MB, and CaWO4 sample after reaction with CR, a broad absorption peak appeared at around 3500 cm-1 contributed to the stretching vibration of the physically absorbed water molecule (Zhai et al., 2022). And the bands at around 2900 cm-1, 1500 cm-1, 760 cm-1, and 490 cm-1 were assigned to the stretching vibrations of O-H, H-O-H, O-W-O, and W-O bonds, respectively (Syed et al., 2021). In addition, it is not difficult to see that the characteristic absorption peaks of samples do not change significantly before and after the reaction. The characteristic absorption peaks of MB and CR were not obvious in the samples after reaction. That is to say, the characteristic absorption peaks of MB and CR do not appear in the samples after reaction. It indicates that the adsorption capacity of CaWO4 on MB and CR is in negligible amounts through the photocatalytic reaction process. The degradation of MB and CR is the main process of photocatalysis.
3.5 Reusability of CaWO4
The reusability of the activators is vital for actual applications (Zhu et al., 2022). Thus, cycle experiments were conducted to investigate the stability of CaWO4. As seen in Fig. 7a and Fig 7b, the reusability of CaWO4 in photocatalytic degradation of MB and CR was verified. As for MB degradation, after five cycles, the material did not exhibit obvious deactivation, and the MB degradation efficiency are still more than 80%. It suggests that CaWO4 can be reused in the photocatalytic degradation of MB. However, in the case of carmine, after the same 5 cycles, the activity of the CaWO4 decreased significantly, and the CR degradation efficiency only reached 48%. It suggests that CaWO4 is not recommended to reuse more than three times in the photocatalytic degradation of CR. The different reusability exhibition in the degradation experiments of MB and CR is attributed to the different properties of MB wastewater and CR wastewater. MB is a cationic dye that dissolves in water and ionizes to produce positively charged ions. The positive ion and photocatalytic electrons attract each other, which inhibits the recombination of electron hole pairs and promotes the photocatalytic reaction. In contrast, CR is an anionic dye, and its solution is acidic when dissolved in water. The dye wastewater is relatively unstable, and the color of the wastewater changes with pH vibration. In addition, anionic dyes dissociate into negatively charged ions in water, which repel photogenerated electrons and promote the recombination of electron hole pairs, thus inhibiting the photocatalytic reaction.