The Photocatalytic Activity of CaTiO3 Derived from the Microwave-Melting Heating Process of Blast Furnace Slag

The extraction of titanium-bearing components in the form of CaTiO3 is an efficient utilization of blast furnace slag. The photocatalytic performance of this obtained CaTiO3 (MM-CaTiO3) as a catalyst for methylene blue (MB) degradation was evaluated in this study. The analyses indicated that the MM-CaTiO3 had a completed structure with a special length–diameter ratio. Furthermore, the oxygen vacancy was easier to generate on a MM-CaTiO3(110) plane during the photocatalytic process, contributing to improving photocatalytic activity. Compared with traditional catalysts, MM-CaTiO3 has a narrower optical band gap and visible-light responsive performance. The degradation experiments further confirmed that the photocatalytic degradation efficiency of pollutants by using MM-CaTiO3 was 3.2 times that of pristine CaTiO3 in optimized conditions. Combined with molecular simulation, the degradation mechanism clarified that acridine of MB molecular was stepwise destroyed by using MM-CaTiO3 in short times, which is different from demethylation and methylenedioxy ring degradation by using TiO2. This study provided a promising routine for using solid waste to obtain catalysts with excellent photocatalytic activity and was found to be in keeping with sustainable environmental development.


Introduction
Titanium-bearing blast furnace slag is a kind of alkaline solid waste formed in the pyrometallurgy process from titanomagnetite and has almost 100 million tons of total quality in the world [1]. The comprehensive utilization of blast furnace slag has become one key sustainability issue confronting the titanium industry in recent years [2]. Generally, the utilization of blast furnace slag is always based on the extraction titanium, which can be divided into acid leaching [3,4], molten salt decomposition [5,6] and carbonization chlorination [7][8][9]. It is further found that the Ca-bearing components are associated with Tibearing minerals in blast furnace slag, leading to the difficulty in separation extraction [10]. As perovskite is easily formed in the pyrometallurgical process, it is worth considering the extraction of titanium-bearing components in the form of CaTiO 3 from the slag.
It has been reported that CaTiO 3 is a good industrial catalyst alternative for TiO 2 in photocatalytic degradation pollutants in the environmental remediation field [11,12]. As is well-known, photocatalytic degradation is an effective method to eliminate organic pollutants [13] and exhibits considerable advantages over other physiochemical techniques such as chemical precipitation, adsorption and coagulation. In detail, it can directly degrade pollutants into small molecules represented by water and carbon dioxide to prevent secondary

Material Preparation
The blast furnace slag of vanadium titanium magnetite used in the experiment came from Panzhihua City in Sichuan Province, China. And contents of TiO 2 and SiO 2 were 20.75 wt.% and 25.36 wt.%, respectively. Based on our study, microwave-melting heating followed by gravity separation has been carried out to extract CaTiO 3 product from blast furnace slag (melting temperature at 1300 • C for pretreatment and microwave heating temperature at 1200 • C for 20 min). The CaTiO 3 component was enriched and precipitated at the bottom of the heating process. The obtained CaTiO 3 products were collected and dried in an oven at 105 • C for 4 h, which was named MM-CaTiO 3 .

Experimental Design of Photocatalysis
The photocatalytic reaction was carried out in a circulating reactor equipped with quartz immersion wells, and the photocatalytic reaction device is shown in Figure 1. Experiments were performed using an adjustable monochrome light source system (PL-KT300D, UV region (<390 nm) output power: 2.6 W, Beijing Precise Technology, Beijing, China) to ensure that the photocatalytic process was performed at a known fixed wavelength. The quartz immersion well was experimented with in a camera obscura (Beijing Precise Technology, Beijing, China) to rule out visible light interference. The peristaltic pump (100 rpm, Longer Precision Pump Co. Ltd., Baoding, China) was used to ensure that the photocatalyst powder was in suspension during the experiment. After the photocatalytic degradation experiment, the aqueous mixture was collected and centrifuged to remove the solid catalyst. The MB degradation efficiency (ε) and degradation quantity (Q) were calculated by Equations (1) and (2) [29]. Specifically, the volume of solution used in the experiment is 10 mL; The illumination wavelength was 200 nm to 550 nm, the initial concentration of MB was 2 mg/L to 10 mg/L, and the MM-CaTiO 3 dosage was 1 g/L to 10 g/L. MB calibration curve formulas such as Equation (3) show that its correlation coefficient (R 2 ) is 0.9995 and ranges from 2 mg/L to 10 mg/L, where C 0 and C t are initial and residue MB concentrations, respectively; t is time; Q 0 and Q t are initial and residue MB quantity; y and x represent the absorbance and concentration of MB, respectively. Experiments were performed using an adjustable monochrome light source system (PL-KT300D, UV region (<390 nm) output power: 2.6 W, Beijing Precise Technology, Beijing, China) to ensure that the photocatalytic process was performed at a known xed wavelength. The quar immersion well was experimented with in a camera obscura (Beijing Precise Technology, Beijing, China) to rule out visible light interference. The peristaltic pump (100 rpm, Longer Precision Pump Co. Ltd., Baoding, China) was used to ensure that the photocatalyst powder was in suspension during the experiment. After the photocatalytic degradation experiment, the aqueous mixture was collected and centrifuged to remove the solid catalyst. The MB degradation e ciency ( ) and degradation quantity (Q) were calculated by Equations (1) and (2) [29]. Speci cally, the volume of solution used in the experiment is 10 mL; The illumination wavelength was 200 nm to 550 nm, the initial concentration of MB was 2 mg/L to 10 mg/L, and the MM-CaTiO3 dosage was 1 g/L to 10 g/L. MB calibration curve formulas such as Equation (3) show that its correlation coecient (R 2 ) is 0.9995 and ranges from 2 mg/L to 10 mg/L, where C0 and Ct are initial and residue MB concentrations, respectively; t is time; Q0 and Qt are initial and residue MB quantity; y and x represent the absorbance and concentration of MB, respectively.

Instrumental Characterization
X-ray di raction (XRD, Shimadzu, XD5A, Kyoto, Japan) was used to determine the chemical composition of the catalysts. The morphology and microstructure of catalysts were characterized by scanning electron microscopy (SEM, Fei Company, Nova 400, Hillsboro, OR, USA) and combined with energy dispersive spectroscopy (EDS, Oxford corporation, Penta FET X-3, Abington, UK) analysis to determine the chemical composition. The transmission electron microscope (TEM, JEOL, JEM-2100 UHR STEM/EDS, Kyoto, Japan) was used to determine the orientation of the crystal planes and interplanar spacing of catalysts. The light absorption characteristics and the forbidden band width of catalysts were studied by ultraviolet-visible di use re ectance spectroscopy (UV-vis DRS, Shimadzu, UV-2600, Kyoto, Japan). Photoluminescence (PL, Edinburgh Instruments, FLS1000, Edinburgh, UK) spectroscopy with an excitation wavelength of 335 nm was used to study the photoluminescence properties of di erent catalysts. Photoluminescence quantum yield (PLQY, Edinburgh Instruments, FLS980, Edinburgh, UK) was used to study the utilization e ciency of catalysts for photons. The MB concentration was monitored by measuring the absorbance at 664 nm using the ultraviolet-visible spectrometer (UV-Vis, Shanghai Yidian, Shanghai, China, N4). In situ infrared spectroscopy (In situ IR,

Instrumental Characterization
X-ray diffraction (XRD, Shimadzu, XD5A, Kyoto, Japan) was used to determine the chemical composition of the catalysts. The morphology and microstructure of catalysts were characterized by scanning electron microscopy (SEM, Fei Company, Nova 400, Hillsboro, OR, USA) and combined with energy dispersive spectroscopy (EDS, Oxford corporation, Penta FET X-3, Abington, UK) analysis to determine the chemical composition. The transmission electron microscope (TEM, JEOL, JEM-2100 UHR STEM/EDS, Kyoto, Japan) was used to determine the orientation of the crystal planes and interplanar spacing of catalysts. The light absorption characteristics and the forbidden band width of catalysts were studied by ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu, UV-2600, Kyoto, Japan). Photoluminescence (PL, Edinburgh Instruments, FLS1000, Edinburgh, UK) spectroscopy with an excitation wavelength of 335 nm was used to study the photoluminescence properties of different catalysts. Photoluminescence quantum yield (PLQY, Edinburgh Instruments, FLS980, Edinburgh, UK) was used to study the utilization efficiency of catalysts for photons. The MB concentration was monitored by measuring the absorbance at 664 nm using the ultraviolet-visible spectrometer (UV-Vis, Shanghai Yidian, Shanghai, China, N4). In situ infrared spectroscopy (In situ IR, Thermo Scientific, Nicolet iS50, Waltham, MA, USA) uses a light wavelength of 300 nm to observe dynamic functional groups in MB solutions. An ion chromatograph (IC, Thermofisher, ICS-6000, Waltham, MA, Nanomaterials 2023, 13, 1412 4 of 16 USA) was used to measure the final product of photocatalytic degradation of methylene blue. GaussView 6.0 was used to calculate the Gibbs free energy of elementary reaction on the degradation process.

First-Principles Theoretical Analysis
The photocatalytic process depends on the reaction in which the catalyst absorbs photons to generate high-energy electrons and holes, then reduction and oxidation reactions are initiated [30]. To reveal the effect of the crystal structure of CaTiO 3 on the photocatalytic activity, the formation of oxygen vacancy (E ov ) on different lattice planes was determined using DFT calculation as implemented in the Vienna ab initio Simulation Package (VASP). The E ov values were calculated by Equation (4) [31,32], where E ov-1 and E pure represent the gross energy with and without the loss of one oxygen atom, respectively; E O2 represents the energy of a single oxygen molecule in the gas phase.

Characterization
Explicating the main phase and morphology structure of MM-CaTiO 3 is helpful in explaining the difference in their distinguished photocatalytic performance with synthesized CaTiO 3 . The phase composition of blast furnace slag and MM-CaTiO 3 are shown in Figure 2. The results show that the main Ti-bearing components in the slag are directional transfer to CaTiO 3 on a microwave-melting heating process. Compared with the predominant orientation of CaTiO 3 (121) in slag, the MM-CaTiO 3 has a different plane (112) and a (110) crystal plane. The detailed lattice parameters of CaTiO 3 are shown in Table 1. The relevant literature reported that the CaTiO 3 (121) in the space group of Pnma could be transformed into CaTiO 3 (112) in the space group of Pbnm by rotating 90 • , which has the same crystal parameters [33,34]. Hence, CaTiO 3 (110) is the newly-generated dominant crystal plane in MM-CaTiO 3 . The SEM-EDS analysis of MM-CaTiO 3 showed that the main component of MM-CaTiO 3 was CaTiO 3 , as shown in Figure 3a. The micromorphology further indicated that the CaTiO 3 particle gathers and grows with dense crystal branches and a special length-diameter ratio. In addition, a clear boundary was formed between CaTiO 3 particles and diopside particles. The results of SEM-EDS analysis with larger magnification (Figure 3b) show that the size of CaTiO 3 grains is about 5 µm. Thermo Scientific, Nicolet iS50, Waltham, MA, USA) uses a light wavelength of 300 nm to observe dynamic functional groups in MB solutions. An ion chromatograph (IC, Thermofisher, ICS-6000, Waltham, MA, USA) was used to measure the final product of photocatalytic degradation of methylene blue. GaussView 6.0 was used to calculate the Gibbs free energy of elementary reaction on the degradation process.

First-Principles Theoretical Analysis
The photocatalytic process depends on the reaction in which the catalyst absorbs photons to generate high-energy electrons and holes, then reduction and oxidation reactions are initiated [30]. To reveal the effect of the crystal structure of CaTiO3 on the photocatalytic activity, the formation of oxygen vacancy (Eov) on different lattice planes was determined using DFT calculation as implemented in the Vienna ab initio Simulation Package (VASP). The Eov values were calculated by Equation (4) [31,32], where Eov-1 and Epure represent the gross energy with and without the loss of one oxygen atom, respectively; EO2 represents the energy of a single oxygen molecule in the gas phase.

Characterization
Explicating the main phase and morphology structure of MM-CaTiO3 is helpful in explaining the difference in their distinguished photocatalytic performance with synthesized CaTiO3. The phase composition of blast furnace slag and MM-CaTiO3 are shown in Figure 2. The results show that the main Ti-bearing components in the slag are directional transfer to CaTiO3 on a microwave-melting heating process. Compared with the predominant orientation of CaTiO3 (121) in slag, the MM-CaTiO3 has a different plane (112) and a (110) crystal plane. The detailed lattice parameters of CaTiO3 are shown in Table 1. The relevant literature reported that the CaTiO3 (121) in the space group of Pnma could be transformed into CaTiO3 (112) in the space group of Pbnm by rotating 90°, which has the same crystal parameters [33,34]. Hence, CaTiO3 (110) is the newly-generated dominant crystal plane in MM-CaTiO3. The SEM-EDS analysis of MM-CaTiO3 showed that the main component of MM-CaTiO3 was CaTiO3, as shown in Figure 3a. The micromorphology further indicated that the CaTiO3 particle gathers and grows with dense crystal branches and a special length-diameter ratio. In addition, a clear boundary was formed between CaTiO3 particles and diopside particles. The results of SEM-EDS analysis with larger magnification ( Figure 3b) show that the size of CaTiO3 grains is about 5 µm.     According to the XRD analyses of MM-CaTiO3, the interplanar spacing(s) of various CaTiO3 crystal orientations were calculated based on the Braggs equation [35,36], as shown in Table 1. Meanwhile, the TEM analyses were used to further determine the newly-generated crystal orientation of MM-CaTiO3, as shown in Figure 4. The regional spectrum analyses showed that the main component of the catalyst was CaTiO3. The typical oblique downward (110) crystal plane and the almost vertical (112) crystal plane of CaTiO3 were founded in the HRTEM image, as shown in Figure 4b,c. The interplanar spacing of (110) and (112) crystal planes were 0.266 nm and 0.272 nm, respectively, which agreed well with the XRD analyses. According to the XRD analyses of MM-CaTiO 3 , the interplanar spacing(s) of various CaTiO 3 crystal orientations were calculated based on the Braggs equation [35,36], as shown in Table 1. Meanwhile, the TEM analyses were used to further determine the newlygenerated crystal orientation of MM-CaTiO 3 , as shown in Figure 4. The regional spectrum analyses showed that the main component of the catalyst was CaTiO 3 . The typical oblique downward (110) crystal plane and the almost vertical (112) crystal plane of CaTiO 3 were founded in the HRTEM image, as shown in Figure 4b,c. The interplanar spacing of (110) and (112) crystal planes were 0.266 nm and 0.272 nm, respectively, which agreed well with the XRD analyses.
The photocatalytic property depends on the ability to generate photogenerated electrons and the recombination rate of electron-hole pair, which can be characterized by UV-vis DRS spectrum and PL spectroscopy, as shown in Figures 5 and 6. The UV-vis DRS spectrum analyses show that the threshold wavelength of MM-CaTiO 3 reaches 551 nm (Figure 5a), indicating that MM-CaTiO 3 has a wider response frequency band than that of pristine CaTiO 3 (354 nm) [37] and TiO 2 (400 nm) [38]. In addition, the threshold wavelength of 551 nm is within the wavelength range of visible light (400 to 760 nm), which indicates that MM-CaTiO 3 can be used as a visible light-responsive photocatalyst. The band gap (E g ) is further calculated by the Tauc plot method [39], as shown in Figure 5b. The results show that the E g of MM-CaTiO 3 is 2.25 eV, which is significantly lower than that of pristine CaTiO 3 (3.5 eV) [20] and TiO 2 (3.1 eV) [26]. The relationship between the lattice structure and the band gap is further investigated based on molecular simulation and theoretical calculation. According to the preponderant orientation of MM-CaTiO 3 and pristine CaTiO 3 , the oxygen vacancy formation energy (E ov ) on different lattice planes was determined using DFT calculation, as shown in Table 2. The theoretical calculation analyses indicate that the oxygen vacancy is easier to generate on the CaTiO 3 (110) plane.  The photocatalytic property depends on the ability to generate photogenerated electrons and the recombination rate of electron-hole pair, which can be characterized by UVvis DRS spectrum and PL spectroscopy, as shown in Figures 5 and 6. The UV-vis DRS spectrum analyses show that the threshold wavelength of MM-CaTiO3 reaches 551 nm (Figure 5a), indicating that MM-CaTiO3 has a wider response frequency band than that of pristine CaTiO3 (354 nm) [37] and TiO2 (400 nm) [38]. In addition, the threshold wavelength of 551 nm is within the wavelength range of visible light (400 to 760 nm), which indicates that MM-CaTiO3 can be used as a visible light-responsive photocatalyst. The band gap (Eg) is further calculated by the Tauc plot method [39], as shown in Figure 5b. The results show that the Eg of MM-CaTiO3 is 2.25 eV, which is significantly lower than that of pristine CaTiO3 (3.5 eV) [20] and TiO2 (3.1 eV) [26]. The relationship between the lattice structure and the band gap is further investigated based on molecular simulation and theoretical calculation. According to the preponderant orientation of MM-CaTiO3 and pristine CaTiO3, the oxygen vacancy formation energy (Eov) on different lattice planes was determined using DFT calculation, as shown in Table 2. The theoretical calculation analyses indicate that the oxygen vacancy is easier to generate on the CaTiO3 (110) plane.  The photocatalytic property depends on the ability to generate photogenerated electrons and the recombination rate of electron-hole pair, which can be characterized by UVvis DRS spectrum and PL spectroscopy, as shown in Figures 5 and 6. The UV-vis DRS spectrum analyses show that the threshold wavelength of MM-CaTiO3 reaches 551 nm (Figure 5a), indicating that MM-CaTiO3 has a wider response frequency band than that of pristine CaTiO3 (354 nm) [37] and TiO2 (400 nm) [38]. In addition, the threshold wavelength of 551 nm is within the wavelength range of visible light (400 to 760 nm), which indicates that MM-CaTiO3 can be used as a visible light-responsive photocatalyst. The band gap (Eg) is further calculated by the Tauc plot method [39], as shown in Figure 5b. The results show that the Eg of MM-CaTiO3 is 2.25 eV, which is significantly lower than that of pristine CaTiO3 (3.5 eV) [20] and TiO2 (3.1 eV) [26]. The relationship between the lattice structure and the band gap is further investigated based on molecular simulation and theoretical calculation. According to the preponderant orientation of MM-CaTiO3 and pristine CaTiO3, the oxygen vacancy formation energy (Eov) on different lattice planes was determined using DFT calculation, as shown in Table 2. The theoretical calculation analyses indicate that the oxygen vacancy is easier to generate on the CaTiO3 (110) plane.  Besides the band gap, the recombination of the photogenerated electron-hole pair is also an essential issue impacting the photocatalytic ability of the catalyst. The analyses of photogenerated electron-hole pairs recombination by PL spectroscopy are shown in Figure 6. The MM-CaTiO 3 and pristine CaTiO 3 have characteristic peaks at the same wavelength (614 nm), while the characteristic peak wavelength of pristine TiO 2 at 830 nm. As confirmed in Figure 6a, the MM-CaTiO 3 has a lower recombination rate of photogenerated electron-hole pairs than both pristine CaTiO 3 and TiO 2 , which would contribute to its higher photocatalytic activity. Here, the possible mechanism for the recombination rate of the MM-CaTiO 3 is discussed. As MM-CaTiO 3 still contains small amounts of diopside, the photogenerated electrons are transferred at the interface of different components that suppress the recombination rate of photogenerated electrons and holes [40]. photogenerated electrons are transferred at the interface of different components that suppress the recombination rate of photogenerated electrons and holes [40].
On the other hand, the SiO2 tetrahedron structure of the diopside may promote the separation of photogenerated electron-hole pairs [41,42]. The utilization efficiency of photons is further characterized by quantum yield, as shown in Figure 6b. The results show that the quantum yield of MM-CaTiO3 is 0.11%, which is much higher than that of pristine CaTiO3, indicating a higher photon utilization efficiency, although quantum yields of TiO2 reach 0.2%, which is higher than both of MM-CaTiO3 and pristine CaTiO3. MM-CaTiO3 still shows a good catalytic performance considering its lower band gap and recombination rate of photogenerated electron-hole pairs.   Besides the band gap, the recombination of the photogenerated electron-hole pair is also an essential issue impacting the photocatalytic ability of the catalyst. The analyses of photogenerated electron-hole pairs recombination by PL spectroscopy are shown in Figure 6. The MM-CaTiO3 and pristine CaTiO3 have characteristic peaks at the same wavelength (614 nm), while the characteristic peak wavelength of pristine TiO2 at 830 nm. As confirmed in Figure 6a, the MM-CaTiO3 has a lower recombination rate of photogenerated electron-hole pairs than both pristine CaTiO3 and TiO2, which would contribute to its higher photocatalytic activity. Here, the possible mechanism for the recombination rate of the MM-CaTiO3 is discussed. As MM-CaTiO3 still contains small amounts of diopside, the photogenerated electrons are transferred at the interface of different components that suppress the recombination rate of photogenerated electrons and holes [40].
On the other hand, the SiO2 tetrahedron structure of the diopside may promote the separation of photogenerated electron-hole pairs [41,42]. The utilization efficiency of photons is further characterized by quantum yield, as shown in Figure 6b. The results show that the quantum yield of MM-CaTiO3 is 0.11%, which is much higher than that of pristine CaTiO3, indicating a higher photon utilization efficiency, although quantum yields of TiO2 reach 0.2%, which is higher than both of MM-CaTiO3 and pristine CaTiO3. MM-CaTiO3 still shows a good catalytic performance considering its lower band gap and recombina-  Besides the band gap, the recombination of the photogenerated electron-hole pair is also an essential issue impacting the photocatalytic ability of the catalyst. The analyses of photogenerated electron-hole pairs recombination by PL spectroscopy are shown in Figure 6. The MM-CaTiO3 and pristine CaTiO3 have characteristic peaks at the same wavelength (614 nm), while the characteristic peak wavelength of pristine TiO2 at 830 nm. As confirmed in Figure 6a, the MM-CaTiO3 has a lower recombination rate of photogenerated electron-hole pairs than both pristine CaTiO3 and TiO2, which would contribute to its higher photocatalytic activity. Here, the possible mechanism for the recombination rate of the MM-CaTiO3 is discussed. As MM-CaTiO3 still contains small amounts of diopside, the photogenerated electrons are transferred at the interface of different components that suppress the recombination rate of photogenerated electrons and holes [40].
On the other hand, the SiO2 tetrahedron structure of the diopside may promote the separation of photogenerated electron-hole pairs [41,42]. The utilization efficiency of photons is further characterized by quantum yield, as shown in Figure 6b. The results show that the quantum yield of MM-CaTiO3 is 0.11%, which is much higher than that of pristine CaTiO3, indicating a higher photon utilization efficiency, although quantum yields of TiO2 reach 0.2%, which is higher than both of MM-CaTiO3 and pristine CaTiO3. MM-CaTiO3 still shows a good catalytic performance considering its lower band gap and recombination rate of photogenerated electron-hole pairs. On the other hand, the SiO 2 tetrahedron structure of the diopside may promote the separation of photogenerated electron-hole pairs [41,42]. The utilization efficiency of photons is further characterized by quantum yield, as shown in Figure 6b. The results show that the quantum yield of MM-CaTiO 3 is 0.11%, which is much higher than that of pristine CaTiO 3 , indicating a higher photon utilization efficiency, although quantum yields of TiO 2 reach 0.2%, which is higher than both of MM-CaTiO 3 and pristine CaTiO 3 . MM-CaTiO 3 still shows a good catalytic performance considering its lower band gap and recombination rate of photogenerated electron-hole pairs.

Photocatalytic Performance of MM-CaTiO 3
The experiments on the photocatalytic degradation of MB by using MM-CaTiO 3 are systemically investigated, as shown in Figures 7-9. The effect of illumination wavelength on the photocatalysis of MM-CaTiO 3 was investigated on the conditions of initial MB concentration of 6 mg/L and MM-CaTiO 3 dosage of 6 g/L for illumination time of 1000 s in the light range of 200 nm to 550 nm. Figure 7 shows that the degradation efficiency of MB reached the peak value at 300 nm, which is related to the fixed energy required for photogenerated electrons in MM-CaTiO 3 to achieve the energy level transition. In addition, it also generated an inflection point at 500 nm in the visible light range [43]. Combined with the previous analysis of photocatalytic property, the newly generated CaTiO 3 (110) lattice plane was helpful for the response in the visible light range to achieve partial MB degradation, agreed well with the threshold wavelength analysis of MM-CaTiO 3 (λ = 551 nm) in Figure 5a.

Photocatalytic Performance of MM-CaTiO3
The experiments on the photocatalytic degradation of MB by using MM-CaTiO3 are systemically investigated, as shown in Figures 7-9. The effect of illumination wavelength on the photocatalysis of MM-CaTiO3 was investigated on the conditions of initial MB concentration of 6 mg/L and MM-CaTiO3 dosage of 6 g/L for illumination time of 1000 s in the light range of 200 nm to 550 nm. Figure 7 shows that the degradation efficiency of MB reached the peak value at 300 nm, which is related to the fixed energy required for photogenerated electrons in MM-CaTiO3 to achieve the energy level transition. In addition, it also generated an inflection point at 500 nm in the visible light range [43]. Combined with the previous analysis of photocatalytic property, the newly generated CaTiO3 (110) lattice plane was helpful for the response in the visible light range to achieve partial MB degradation, agreed well with the threshold wavelength analysis of MM-CaTiO3 (λ = 551 nm) in Figure 5a. The effect of MB initial concentration was further investigated with a MM-CaTiO3 dosage of 6 g/L for an illumination time of 1000 s illumination wavelength of 300 nm. Within the MB initial concentration range of 2 mg/L to 10 mg/L, the degradation efficiency of MB decreased with the increase in the MB initial concentration. The initial excess concentration of MB reduced the transparency of the solution and hindered the UV light absorption of MM-CaTiO3, whereas the degradation amount of MB obviously increased as the increasing of MB initial concentration. Based on this, the initial MB concentration of 4 mg/L was appropriate. The e ect of the MM-CaTiO3 dosage on photocatalysis was investigated at the illumination wavelength of 300 nm with an initial MB concentration of 4 mg/L for an illumination time of 1000 s. The degradation e ciency of MB gradually increased and then decreased as the MM-CaTiO3 dosage increased. The degradation e ciency reached a peak value of 67% with the dosage of 8 g/L. As further increasing the photocatalyst dosage, the degradation e ciency obviously decreased. In addition, the degradation amount of MB showed a similar trend with degradation e ciency. As an excessive dosage of MM- nation time of 1000 s. The degradation e ciency of MB gradually increased and then decreased as the MM-CaTiO3 dosage increased. The degradation e ciency reached a peak value of 67% with the dosage of 8 g/L. As further increasing the photocatalyst dosage, the degradation e ciency obviously decreased. In addition, the degradation amount of MB showed a similar trend with degradation e ciency. As an excessive dosage of MM-CaTiO3 would a ect the UV light absorption, the MM-CaTiO3 dosage of 8 g/L was suitable for MB photocatalytic degradation.  The effect of the MM-CaTiO 3 dosage on photocatalysis was investigated at the illumination wavelength of 300 nm with an initial MB concentration of 4 mg/L for an illumination time of 1000 s. The degradation efficiency of MB gradually increased and then decreased as the MM-CaTiO 3 dosage increased. The degradation efficiency reached a peak value of 67% with the dosage of 8 g/L. As further increasing the photocatalyst dosage, the degradation efficiency obviously decreased. In addition, the degradation amount of MB showed a similar trend with degradation efficiency. As an excessive dosage of MM-CaTiO 3 would affect the UV light absorption, the MM-CaTiO 3 dosage of 8 g/L was suitable for MB photocatalytic degradation.
The results of the kinetic study on the photocatalytic degradation of the MB solution are shown in Figure 10 and Table 3. As shown in Figure 10a, the MB degradation efficiency was about 20% when the photocatalytic time was 0. This phenomenon indicated that seldom MB molecular was absorbed on the surface of MM-CaTiO 3 . When the initial MB concentration was 4 mg/L and MM-CaTiO 3 dosage was 8 g/L, the photocatalytic degradation efficiency of MB increased significantly as the extension of the illumination time. When the photocatalytic time exceeded 3 min, the photocatalytic degradation efficiency of MB increased slowly and then became almost stable at 10 min. In order to clarify the degradation behavior of MB, the typical kinetic models were applied to test the fitness of experimental data. The corresponding equations and parameters are shown in Figure 10. The first-order reaction kinetics of ln(C t /C 0 ) vs. t resulted in a straight line with higher R 2 values, which indicated the better applicability of other models. According to the first-order reaction ln(C t /C 0 ) = −0.2358 t − 0.2364, the rate constant of the photocatalytic degradation reaction was −0.2358 in this study.
increased slowly and then became almost stable at 10 min. In order to clarify the degrada-tion behavior of MB, the typical kinetic models were applied to test the fitness of experimental data. The corresponding equations and parameters are shown in Figure 10. The first-order reaction kinetics of ln(Ct/C0) vs. t resulted in a straight line with higher R 2 values, which indicated the better applicability of other models. According to the first-order reaction ln(Ct/C0) = −0.2358 t − 0.2364, the rate constant of the photocatalytic degradation reaction was −0.2358 in this study.  According to the above analyses of the photocatalytic process, the MB degradation efficiency was further compared under the optimized condition by using different  Zero-order reaction kinetics C t = −0.5220 t + 3.0770 0.98422 First-order reaction kinetics ln(C t /C 0 ) = −0.2358 t − 0.2364 0.99902 Second-order reaction kinetics 1/C t = 0.0998 t + 0.3101 0.98969 According to the above analyses of the photocatalytic process, the MB degradation efficiency was further compared under the optimized condition by using different photocatalysts, as shown in Figure 11. The photocatalytic degradation efficiency of MB in the presence of MM-CaTiO 3 was much higher, which was 3.2 times than that of pristine CaTiO 3 at an illumination wavelength of 300 nm with an initial MB concentration of 4 mg/L and catalyst dosage of 8 g/L for illumination for 1000 s. The degradation efficiency of MB by pristine TiO 2 is less than 30%. Based on the crystal structure and photocatalytic property analyses, the MM-CaTiO 3 with different preponderant lattice orientation contributed to the improved photocatalytic degradation efficiency. In addition, the lower recombination rate of photogenerated electron-hole pairs delays the annihilation of photogenerated electrons and further promotes the degradation of pollutants.
CaTiO3 at an illumination wavelength of 300 nm with an initial MB concentration of 4 mg/L and catalyst dosage of 8 g/L for illumination for 1000 s. The degradation efficiency of MB by pristine TiO2 is less than 30%. Based on the crystal structure and photocatalytic property analyses, the MM-CaTiO3 with different preponderant lattice orientation contributed to the improved photocatalytic degradation efficiency. In addition, the lower recombination rate of photogenerated electron-hole pairs delays the annihilation of photogenerated electrons and further promotes the degradation of pollutants. Due to the inconsistency of photocatalytic experimental conditions, especially wavelength and light source power, researchers cannot measure the photocatalytic ability of catalysts by comparing the degradation efficiencies of pollutants. At present, the commonly used measurement method is to compare the band gap of different catalysts. The synthesis process, including raw materials and method of MM-CaTiO3, are compared with the previous literature, as shown in Table 4. The results show that the photocatalyst obtained in this study was derived from secondary resources with a narrow band gap by using a simple synthesis process. Calcium carbonate and titanium dioxide are firstly mixed and then calcined at 1400 °C for 2 h.
Ag-doped TiO2 NPs were synthesized by sol-gel technology.
Cu(NO3)2 is added to the ultrasonically dispersed TiO2 NPs aqueous suspension and magnetically stirred. Subsequently, add NaBH4 solution and continue stirring for 24 h. Finally, centrifuge, wash, and dry.
Melting pretreatment at 1300 °C for 1 h and then roasted in a microwave field at 1200 °C with a heating time of 20 min.

eV
This study Figure 11. The photocatalytic degradation performance of catalysts.
Due to the inconsistency of photocatalytic experimental conditions, especially wavelength and light source power, researchers cannot measure the photocatalytic ability of catalysts by comparing the degradation efficiencies of pollutants. At present, the commonly used measurement method is to compare the band gap of different catalysts. The synthesis process, including raw materials and method of MM-CaTiO 3 , are compared with the previous literature, as shown in Table 4. The results show that the photocatalyst obtained in this study was derived from secondary resources with a narrow band gap by using a simple synthesis process. Titanium-bearing blast furnace slag.
Melting pretreatment at 1300 • C for 1 h and then roasted in a microwave field at 1200 • C with a heating time of 20 min.

Degradation Mechanism of MB Using MM-CaTiO 3
Furthermore, the mechanism of the photocatalytic degradation of MB using MM-CaTiO 3 was explored. The in situ IR was used to observe the dynamic behaviors of the functional group on the degradation process, as shown in Figure 12. The MB solution exhibited bands for the C-H asymmetric stretching of CH 3 at 2921 cm −1 , the C-H symmetric stretching of CH 3 at 2850 cm −1 , the C=N central ring stretching at 1645 cm −1 , the C=C side ring stretching at 1537 cm −1 , the acridines stretching at 1461 cm −1 , the N-C stretching at 1041, 868 and 746 cm −1 and the single ring stretching 539 and 466 cm −1 . The acridines stretching at 1461 cm −1 disappeared immediately in the presence of MM-CaTiO 3 under UV light illumination. As the extension of illumination time, the intensity of C-H asymmetric stretching of CH 3 at 2921 cm −1 , the C-H symmetric stretching of CH 3 at 2850 cm −1 and the C=C side ring stretching at 1537 cm −1 gradually weakened and then disappeared. However, the N-C stretching at 1041, 868, and 746 cm −1 and the single ring stretching at 539 and 466 cm −1 were significantly enhanced [46]. This phenomenon indicated that the acridine was directly destroyed by using MM-CaTiO 3 in a short amount of time. Then, the destroyed acridines further broke the C=C band and demethylated, which then generated small molecules.

Degradation Mechanism of MB Using MM-CaTiO3
Furthermore, the mechanism of the photocatalytic degradation of MB using MM-CaTiO3 was explored. The in situ IR was used to observe the dynamic behaviors of the functional group on the degradation process, as shown in Figure 12. The MB solution exhibited bands for the C-H asymmetric stretching of CH3 at 2921 cm −1 , the C-H symmetric stretching of CH3 at 2850 cm −1 , the C=N central ring stretching at 1645 cm −1 , the C=C side ring stretching at 1537 cm −1 , the acridines stretching at 1461 cm −1 , the N-C stretching at 1041, 868 and 746 cm −1 and the single ring stretching 539 and 466 cm −1 . The acridines stretching at 1461 cm −1 disappeared immediately in the presence of MM-CaTiO3 under UV light illumination. As the extension of illumination time, the intensity of C-H asymmetric stretching of CH3 at 2921 cm −1 , the C-H symmetric stretching of CH3 at 2850 cm −1 and the C=C side ring stretching at 1537 cm −1 gradually weakened and then disappeared. However, the N-C stretching at 1041, 868, and 746 cm −1 and the single ring stretching at 539 and 466 cm −1 were significantly enhanced [46]. This phenomenon indicated that the acridine was directly destroyed by using MM-CaTiO3 in a short amount of time. Then, the destroyed acridines further broke the C=C band and demethylated, which then generated small molecules. The intermediates product and their Gibbs free energies of MB elementary degradation are presented in Table S1 and Figure 13. Obviously, the two hydroxyl radicals preferentially combine with carbon atoms located symmetrically, which has a lower Gibbs free energy reaction than that of a single hydroxyl radical (−75.8 eV). In addition, more hydroxyl radicals react in the symmetry of the benzene ring to promote the further destruction of acridine structure and chromophores cleavage. Furthermore, the enhancement of single-ring stretching foreshadowed the opening ring reactions. Combined with the in situ IR analysis, the demethylation and opening ring reaction almost react at the same time. Finally, the functional group further decomposed to form inorganic ions, H2O and CO2 [47]. The intermediates product and their Gibbs free energies of MB elementary degradation are presented in Table S1 and Figure 13. Obviously, the two hydroxyl radicals preferentially combine with carbon atoms located symmetrically, which has a lower Gibbs free energy reaction than that of a single hydroxyl radical (−75.8 eV). In addition, more hydroxyl radicals react in the symmetry of the benzene ring to promote the further destruction of acridine structure and chromophores cleavage. Furthermore, the enhancement of singlering stretching foreshadowed the opening ring reactions. Combined with the in situ IR analysis, the demethylation and opening ring reaction almost react at the same time. Finally, the functional group further decomposed to form inorganic ions, H 2 O and CO 2 [47]. Nanomaterials 2023, 13, x FOR PEER REVIEW 13 of 16 The relevant literature reported that the first step of the demethylation of the MB molecular used TiO2. Furthermore, the breaking of the central aromatic ring relied on the transfer of photogenerated electrons and H + , thus slowing down the conversion of MB to CO2, H2O, NH 4+ , and SO4 2− [48,49]. In contrast, high-activity species were produced in the presence of MM-CaTiO3 under UV light illumination, which directly reacted with acridines to achieve stepwise destruction and improve degradation efficiency.

Conclusions
The titanium-bearing components in the form of CaTiO3 derived from blast furnace slag by microwave-melting had excellent photocatalytic properties. This study showed that the MM-CaTiO3 obtained a complete crystal structure with a special length-diameter ratio and various predominant crystal orientations. Based on the DFT theoretical calculation, the new-generated CaTiO3 (110) plane was easier to form the oxygen vacancy and contributed to the lower band gap than that of pristine CaTiO3. The UV-vis DRS spectrum showed that the MM-CaTiO3 had a photocatalytic response under visible light, which was further confirmed by the light wavelength experiment of MB degradation. The photocatalytic experiments clarified that the degradation of MB reached 67% at the wavelength of 300 nm with an initial MB concentration of 4 mg/L and dosage of 8 g/L for an illumination time of 1000 s. On the optimized condition, the photocatalytic degradation efficiency by using MM-CaTiO3 was 3.2 times that of pristine CaTiO3 and 2.2 times that of pristine TiO2. The degradation mechanism also indicated that the hydroxyl radical was directly reacted with acridines to achieve stepwise destruction by using MM-CaTiO3 in short time, which was different from the gradual decomposition process of demethylation and the MB ring structure breaking by using TiO2. Generally, the degradation efficiency of MB using MM-CaTiO3 could be further increased by using light with more power. This study provided a promising routine for using solid waste to obtain catalysts with excellent photocatalytic activity and was thereby favorable to sustainable environmental development. The relevant literature reported that the first step of the demethylation of the MB molecular used TiO 2 . Furthermore, the breaking of the central aromatic ring relied on the transfer of photogenerated electrons and H + , thus slowing down the conversion of MB to CO 2 , H 2 O, NH 4+ , and SO 4 2− [48,49]. In contrast, high-activity species were produced in the presence of MM-CaTiO 3 under UV light illumination, which directly reacted with acridines to achieve stepwise destruction and improve degradation efficiency.

Conclusions
The titanium-bearing components in the form of CaTiO 3 derived from blast furnace slag by microwave-melting had excellent photocatalytic properties. This study showed that the MM-CaTiO 3 obtained a complete crystal structure with a special length-diameter ratio and various predominant crystal orientations. Based on the DFT theoretical calculation, the new-generated CaTiO 3 (110) plane was easier to form the oxygen vacancy and contributed to the lower band gap than that of pristine CaTiO 3 . The UV-vis DRS spectrum showed that the MM-CaTiO 3 had a photocatalytic response under visible light, which was further confirmed by the light wavelength experiment of MB degradation. The photocatalytic experiments clarified that the degradation of MB reached 67% at the wavelength of 300 nm with an initial MB concentration of 4 mg/L and dosage of 8 g/L for an illumination time of 1000 s. On the optimized condition, the photocatalytic degradation efficiency by using MM-CaTiO 3 was 3.2 times that of pristine CaTiO 3 and 2.2 times that of pristine TiO 2 . The degradation mechanism also indicated that the hydroxyl radical was directly reacted with acridines to achieve stepwise destruction by using MM-CaTiO 3 in short time, which was different from the gradual decomposition process of demethylation and the MB ring structure breaking by using TiO 2 . Generally, the degradation efficiency of MB using MM-CaTiO 3 could be further increased by using light with more power. This study provided a promising routine for using solid waste to obtain catalysts with excellent photocatalytic activity and was thereby favorable to sustainable environmental development.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano13081412/s1, Table S1: The Gibbs free energy of MB molecular and intermediate products.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.