Green Recovery of Titanium and Effective Regeneration of TiO2

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Green recovery of titanium and effective regeneration of TiO2 photocatalysts from spent SCR catalysts Qijun Zhang, Yufeng Wu, and Tieyong Zuo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03038 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Sustainable Chemistry & Engineering

Green recovery of titanium and effective regeneration of TiO2 photocatalysts from spent SCR catalysts

Qijun Zhang 1, Yufeng Wu 1, * and Tieyong Zuo 1

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Institute of Circular Economy, Beijing University of Technology, No.100, Pingleyuan Street, Chaoyang District, Beijing, 100124, P. R. China. * Corresponding Author: Fax: +86-10-67396234; Tel: +86-10-67396234; E-mail: [email protected].

Abstract The extensive use of selective catalytic reduction (SCR) catalysts will afford many spent SCR catalysts. The mass fraction of the titanium component is over 80% in spent SCR catalysts, but currently, it is usually thrown away without proper recycling. This work aims to develop a clean, green, and economical approach to recovering titanium and regenerating TiO2 photocatalysts from spent SCR catalysts based on the conversion of the titanium component. This titanium component is converted into metastable α-Na2TiO3 with high efficiency (> 98%) using a NaOH molten salt method, and the optimal conditions were found to be a roasting temperature of 550 °C, a NaOH-to-spent-SCR-catalysts mass ratio of 1.8:1, a roasting time of 10 min, and a NaOH concentration of 60-80 wt.%. And a possible chemical reaction mechanism is proposed. A subsequent hydrothermal treatment of α-Na2TiO3 regenerates TiO2 photocatalysts with high purity (> 99.0%) that can satisfy commercial requirements. In addition, the present iron element contained in spent SCR catalysts is doped into regenerated TiO2 photocatalysts, resulting in providing visible-light-driven photocatalytic activities. The regenerated TiO2 photocatalysts possess superior photocatalytic degradation capacities for dye pollutants and can be used to efficiently treat wastewater. This work introduces a promising technology for the cyclical regeneration of titanium from spent SCR catalysts.

Keywords: Spent SCR catalysts; Titanium; Recovery; Molten salts; Hydrothermal; TiO2 photocatalysts

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Introduction China is the world's largest coal-consuming country, and coal combustion produces a lot of nitrogen oxides (NOx), a major air pollutant that has a severe impact on local air pollution and poses regional environmental risks. Therefore, NOx must be converted into environmentally friendly substances before being discharged into the atmosphere. In order to achieve this purpose, selective catalytic reduction (SCR) catalysts are widely used in coal-fired power plants [1]. However, the efficiency of NOx removal and the operating life-span of SCR catalysts usually suffer from the high-temperature sintering caused by active components volatile and poisoning of sulfates, alkali metals and alkaline-earth metals [2]. As a result, China is projected to face its large amount of SCR catalysts waste starting in 2014 when SCR catalysts reach their designed life-span, and the production of spent SCR catalysts will stabilize at 210,000 to 240,000 cubic metres per year after 2020 [3]. The Chinese government has established regulations to categorize spent SCR catalysts as hazardous waste due to they contain various heavy metals contain such as Ti4+, V5+, Fe3+, W6+, and Ca2+, which may be detrimental to the surrounding environment and human health if they are not correctly treated [4]. However, spent SCR catalysts have also been viewed as a potential secondary resource for valuable elements titanium, tungsten and vanadium recovery in order to protect resources and alleviate insufficient domestic resources [5, 6]. Thus, the green and effective recovery of titanium from spent SCR catalysts is urgently required. TiO2 has attracted considerable attention as a photocatalyst to degrade organic pollutants in aqueous waste owing to its resistance to photocorrosion, chemical inertness, nontoxicity as well as ideally producing CO2 and H2O as the end-products [7-12]. However, there was still some shortcomings exist in practical applications of TiO2 photocatalysts: (1) low utilization of solar energy due to their wide band gap (3.2 eV for anatase and 3.0 eV for rutile); (2) the low quantum yield caused by their high electron-hole pair recombination rate; and (3) their high cost because they are mainly prepared with expensive chemical reagents, such as TiCl4 [13, 14], TiOSO4 [15, 16] and titanium isopropoxide [17]. To solve these problems, this work proposes the use of spent SCR catalysts as an ideal raw material. Spent SCR catalysts are hazardous waste with a high content of titanium components, and thus, their reuse is urgently required to protect titanium resources, reduce the cost of TiO2 production, and relieve the environmental burden. Importantly, the transition metals present in the spent catalysts will function as dopants for TiO2 to improve its solar energy utilization and to enhance photogenerated electron-hole pair separation, which has been proven to be an effective way to improve the efficiency of these photocatalysts. Therefore, based on the principles of protect resources, low preparation cost and environment burden, spent SCR catalysts are used to recovery titanium and synthesize TiO2 photocatalysis materials. This work aims to present a clean and effective process to recover titanium component from spent SCR catalysts and to produce sustainable TiO2 photocatalytic materials. The optimal process conditions for the green and highly efficient recovery 2


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of titanium are identified. In addition, the reaction mechanisms and elements behavior are briefly discussed. The physical and chemical characteristics of waste, recycled components, and regenerated TiO2 products are investigated in detail. Finally, the photocatalytic properties of regenerated TiO2 products are studied.

Experimental Materials All the chemical reagents employed in this study were of analytical grade and were purchased from the Chemical Reagent Company of Beijing. Deionized water (18.2 MΩ•cm) obtained from a Direct-Pure Up 10 water system (RephiLe Bioscience, Ltd.) was used in the experiments whenever needed Spent honeycomb monolith SCR catalysts were supplied by Zhejiang Tuna Environmental Science & Technology Co., Ltd. Spent honeycomb monolith SCR catalysts had been used for more than 30,000 h in a low grade coal fired power plant (Baoding, China). Spent honeycomb monolith SCR catalysts were crushed and ground using a planetary mill (48 h, 400 rpm), then dried at 105 °C until a constant weight was achieved and passed through a 0.074 mm sieve before use. Characterization The chemical composition of each solid sample was analysed using X-ray fluorescence (XRF, PW2403, PANalytical, Holland). The mineralogical characterization and identification of crystalline phases were investigated using X-ray diffraction (XRD) analysis with a Bruker AXS D8 Advance using Cu Kα radiation (Bruker, Germany). The morphology of the samples was examined by SU-8020 field-emission scanning electron microscopy (SEM) with an energy dispersive spectrometer (EDS) system (Hitachi, Japan). To perform the SEM measurements, the samples were suspended in ethanol and supported on Si substrates. Fourier transform infrared spectroscopy (FT-IR) spectrum was performed using a Bruker TENSOR II microscope (Germany). The concentrations of elements in each solution were analysed using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8000, PerkinElmer, Shelton, CT 06484, USA). The X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300WAl KR radiation (E = 1486.6 eV) in a base pressure of 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The photoluminescence (PL) spectra were recorded with a fluorescence spectrophotometer using a 300 nm line from a xenon lamp (F7000, Hitachi, Japan). The diffuse reflectance spectra (DRS) were recorded by an ultraviolet-visible (UV-vis) spectrophotometer (Cary 5000, Varian, USA). Proposed Process A clean and effective procedure for recovering titanium and producing sustainable TiO2 photocatalytic materials from spent SCR catalysts was proposed, based on combination a NaOH molten salts method with hydrothermal synthesis. A general flowchart of the titanium recovery and regeneration of TiO2 photocatalysts process is shown in Figure 1. NaOH molten salt decomposition 3



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A NaOH molten salt decomposition process was investigated for the spent SCR catalysts using a muffle furnace (SX-G07123, Tianjin Zhonghuan Lab Furnace Co., Ltd) and nickel crucibles. In order to ensure the reproducibility and accuracy of the results, each experiment was carried out in triplicate. In a typical process, NaOH and spent SCR catalysts with a mass ratio of 1.0-3.0 were mixed in a nickel crucible. Then, a small amount of deionized water was added to the mixture, which was stirred to make a slurry containing 60-80 wt.% NaOH solution with stirring. Finally, put the nickel crucibles into the muffle furnace when the temperature reached the pre-set value for different times, with free access of air. The extent of the titanium conversion was calculated by dissolving the alkali melting products in 4.7 vol.% HCl [22]. The unreacted titanium component could be separated from solution by filtration because it does not dissolve under such low acidity. Then, the concentration of titanium in the filtrate could be analysed by ICP-OES. The dissolution reaction could be described as follows [23]: Na 2 TiO3 + 4HCl = TiO 2+ + 4Cl − + 2Na + + 2H 2 O

(1)

Water leaching to separate Na2TiO3 and NaOH recycling Water leaching experiments were performed using a flask reactor in an oil bath. The filter cakes obtained from alkali conversion were washed with water at 55 °C. The resulting solutions were filtered to obtain enriched Na2TiO3. The filtrate was concentrated and purified, and then, NaOH was recycled using vacuum evaporation. Hydrothermal reaction to regenerate TiO2 photocatalysts In a typical hydrothermal reaction process, the as-obtained Na2TiO3 was first leached by 1.0 mol/L HCl or 0.5 mol/L H2SO4 with RL/S was controlled at 60:1 and filtered in order to remove the insoluble impurity elements. Then, appropriate amounts of the filtered solution were transferred to a stainless steel autoclave lined with polytetrafluoroethylene (Teflon) (200 mL). The hydrothermal synthesis was conducted at 180 °C for 1 h in an electric oven. After the synthesis, the autoclave was then removed from the oven and allowed to cool naturally to room temperature. The prepared TiO2 powders were removed by centrifugation, rinsed extensively with dilute hydrochloric acid and deionized water, and fully dried at 60 °C in an oven. The as-prepared TiO2 products obtained from H2SO4 and HCl system were denoted as samples A and B, respectively. Evaluation of photocatalytic activities. The photocatalytic abilities of the as-prepared regenerated TiO2 products were evaluated by measuring the degradation of Rhodamine B (RhB) and Methylene blue (MB) aqueous solutions. A 450 W xenon lamp (94023A, Newport, USA) was used as the visible-light source with UV cut-off filters (UVCUT420, Newport, USA). In a typical photocatalytic experiment, 0.2 g photocatalysts were dispersed in 150 mL of a dye aqueous solution (10 ppm). The suspension was magnetically stirred at 600 rpm before and during illumination. The suspensions were first soaked in the dark for 60 min to reach adsorption equilibrium on the semiconductor surface. During the irradiation, 3.0 mL of suspension was collected every 30 minutes using a pipetting gun with a maximum capacity of 5 mL. After removing the sample, the photocatalyst 4


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particles in the suspension were filtered through a 0.45 µm filter (HVLP, Millipore), and the supernatant solutions were analysed using a Hitachi U-3900H spectrophotometer. The Lambert-Beer rule was applied at the characteristic absorbance bands of dyes, namely, 553 nm for RhB and 664 nm for MB, to determine their concentration changes.

Results and Discussion Waste characterization The chemical compositions of spent SCR catalysts were analysed by XRF, as shown in Table 1. Numerous samples were studied, thus confirming their uniform composition. The main components of spent SCR catalysts were TiO2 (68.5 wt.%), SiO2 (10.3 wt.%), SO3 (6.45 wt.%), Al2O3 (5.47 wt.%), and WO3 (4.67 wt.%). The high TiO2 contents of the spent SCR catalysts make them valuable for recovering titanium. Other minor components included Fe2O3 (1.59 wt.%), CaO (1.463 wt.%), V2O5 (0.427 wt.%), K2O (0.405 wt.%), Na2O (0.191 wt.%), and MgO (0.188 wt.%), as well as small amounts of P2O5 (0.088 wt.%), Nb2O5 (0.0666 wt.%), ZrO2 (0.0621 wt.%), As2O3 (0.0611 wt.%), and SrO (0.0195 wt.%). Figure 2 shows the crystal structure of the spent SCR catalysts. The XRD patterns of the spent samples show sharp peaks assigned to anatase TiO2 (JCPDS No. 21-1272), with some small peaks assigned to Al2SiO5 (JCPDS No. 74-1976). Figure S1 is a typical SEM image of the spent SCR catalysts. It shows that the spent SCR catalysts are composed of agglomerated quasi-spherical nanoparticles. Decomposition of spent SCR catalysts Effect of reaction temperature The effects of the reaction temperature on the conversion of titanium were examined in the range of 350-750 °C with a NaOH-to-spent SCR catalysts mass ratio of 1.8:1, reaction time of 10 min and NaOH concentration of 60 wt. %. The results shown in Figure 3a indicate that the conversion of titanium can be significantly influenced by reaction temperature. The conversion of titanium reached 92.6% at 350 °C for 10 min. When the reaction temperature was increased to 550 °C, the conversion of titanium slightly increased to 98.1%. However, at 750 °C, the conversion of titanium decreased to only 51.0%. While Hill et al. [24] found that different crystal phases formed when TiO2 reacted with molten Na2CO3 salt at different temperatures. They reported that three crystal phases, namely, α-Na2TiO3, β-Na2TiO3, γ-Na2TiO3, formed at high temperature. Unfortunately, β-Na2TiO3 and γ-Na2TiO3 are more stable than α-Na2TiO3. Thus, at roasting temperatures over 550 °C, more β-Na2TiO3 and γ-Na2TiO3 were produced, resulting in a lower conversion of titanium. Thus, the optimal roasting temperature for titanium conversion was 550 °C. As shown in Figure 3b, XRD patterns were recorded to analyse the phase change of the alkali melting products in the temperature range of 350-750 °C. (Before XRD analysis, the alkali melting products were ground and washed with ethanol to remove excess NaOH.) Considerable anatase TiO2 decomposed at 350 °C, mainly forming the α-Na2TiO3 phase with some γ-Na2TiO3 phase. In the roasting temperature 5



range of 450-550 °C, the anatase TiO2 phase disappeared, and the α-Na2TiO3 phase peaks became stronger. However, a considerable amount of the γ-Na2TiO3 phase was observed at 750 °C. Notably, the Na2Ti2Si2O9 (JCPDS No. 84-1207) phase also formed at 750 °C, which might be another reason for the low titanium conversion efficiency at this temperature. Na2Ti2Si2O9 is stable in water, and thus, it formed a large amount of gel in the subsequent TiO2 production process, resulting in a significant loss of titanium components. Thus, the formation of γ-Na2TiO3 and Na2Ti2Si2O9 should be avoided in the molten-salt conversion of titanium. Effect of NaOH-to-spent SCR catalysts mass ratio The effects of NaOH-to-spent SCR catalysts mass ratio on titanium conversion yield were examined in the range of 1:1-3:1 at the optimized roasting temperature of 550 °C, a reaction time of 10 min, and a NaOH concentration of 60 wt. %. According to the chemical analysis results of spent SCR catalysts, the theoretical mass ratio for the complete is 0.946:1. As shown in Figure 4a, the titanium conversion increased from 83.5% to 98.1%, when the NaOH-to-spent SCR catalysts mass ratio increased from 1:1 to 1.8:1, respectively. The XRD patterns in Figure 4b indicate that γ-Na2TiO3 phase peaks appeared when the NaOH-to-spent SCR catalysts mass ratio was over 1.8:1. This found means that the titanium conversion unexpected decreased when too much NaOH was present in the molten salt reaction system. For example, when the NaOH-to-spent SCR catalysts mass ratio was 3:1, the conversion of titanium decreased to 93.5%. More importantly, at a higher NaOH-to-spent SCR catalysts mass ratio, more NaOH remained unreacted, resulting in bind the product particles and undergo serious caking. As a consequence, 1.8:1 was the recommended NaOH-to-spent SCR catalysts mass ratio. Effect of reaction time The effect of the reaction time on the conversion yield of titanium at 550 °C with a NaOH-to-spent SCR catalysts mass ratio of 1.8:1 was investigated, as shown in Figure 5a. The titanium conversion yield increased as the reaction time was prolonged, especially in the initial stages of the reaction. When the roasting time increased from 0 to 2 min, the titanium conversion yield improved from 4.9% to 83.3%. Interestingly, roasting times longer than 10 min negatively affected the titanium conversion yield. The XRD patterns of products obtained after different reaction times are shown in Figure 5b, wherein γ-Na2TiO3 and Na2Ti2Si2O9 phase peaks were obtained at 550 °C after 60 min. The results indicate that 10 min was the optimum roasting time. Effect of H2O In order to investigate the effect of H2O on titanium conversion, the NaOH concentrations of 40, 60, 80 and 100 wt.% were tested with a NaOH-to-spent SCR catalysts of 1.8:1 at 550 °C for 10 min. The results in Figure 6a show that the titanium conversion yield could be improved when a suitable amount of H2O was added. When there was no H2O in the molten salt system, titanium conversion efficiency was only 79.8%. After adding a small amount of H2O into the system (a NaOH concentration of 60-80 wt.%), the titanium conversion efficiency rapidly reached over 98%. However, when too much H2O was added into the system, some 6


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splashing resulted, accompanied by a decrease in the titanium conversion efficiency. SEM images of the products with and without H2O are shown in Figure S2. The results reveal that the product with H2O has a loose and porous structure with a lot of belt-shaped structures formed (Figure S2a). In contrast to without H2O, the product has a compact and smooth surface (Figure S2b). The XRD patterns shown in Figure 6b indicate that more γ-Na2TiO3 formed in the reaction system without H2O than in the system with added H2O. Titanium conversion with H2O added to the roasting system offers three advantages: (1) it ensures that NaOH and the titanium component particles mixed homogeneously, (2) it improves the mobility of the system at the low temperature, and (3) it prevents the formation of excessive γ-Na2TiO3. Chemical reaction mechanism In this work, we also studied the chemical reaction mechanism of the titanium components. Figure S3 shows FT-IR spectra of the as-prepared products obtained from pure TiO2 reacted with NaOH and H2O under different experimental conditions. The bands observed in the range of 800-400 cm-1 owe to Ti-O and Ti-O-Ti groups [25] (Figure S3a). As shown in Figure S3b, the band at 870.64 cm-1 is attributed to Ti-O-Na [26], indicating that Ti-O-Ti bonds were broken. The bands observed at 2922.95, 2851.49, 2366.89 and 1645.01 cm-1 can be attributed to the strong interaction between -OH groups and Ti4+ [27]. However, the narrow vibration at 2922.95, 2851.49 and 2366.89 cm-1 disappeared when pure TiO2 reacted with NaOH and H2O for 10 min (Figure S3c). These results demonstrate that NaOH with H2O show a synergistic enhancement effect on titanium conversion. To study the mechanisms of titanium component decomposition in the NaOH-H2O system, the effect of roasting time on the concentration of Na+ ions was investigated using pure TiO2 as a raw material at a temperature of 550 °C, a NaOH-to-pure TiO2 mass ratio of 1.8:1, and a NaOH concentration of 60 wt.%. As shown in Figure 7a, when the reaction time was 1 min, 39.4% of the titanium was converted, while the Na+ ion contents in the system remained almost constant. This means that the reaction mainly consumed H2O. However, the H2O in the system completely evaporated when the roasting time was prolonged to 2 min (Figure 7b), and the Na+ concentration showed a decrease in anticipation, indicating that the reaction consumed NaOH after H2O was completely evaporated. According to the experimental and analysis results, NaOH molten salt reaction mechanisms with and without H2O are proposed in Figure 8. The reaction of anatase TiO2 component with molten NaOH salt resulted in active and exposed broken Ti-O-Ti bonds [28]. Then, these broken Ti-O bonds could be attacked by O2- and Na+, forming Ti-O-Na and Ti-OH as expressed in Equation (2): Ti-O-Ti + NaOH → Ti-O-Na + Ti-OH (2) Moreover, in the initial process, due to the presence of H2O, Ti-O-Na bonds reacted with H2O to form more Ti-OH bonds [28], which can be corroborated by the FT-IR results.

Ti-O-Na+H 2 O → Ti-OH + NaOH

(3)

Meanwhile, an acid-base equilibrium existed due to the presence of water in 7



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NaOH sub-molten salt environment [29]: 2OH − → H 2 O + O 2 −

(4)

Then, O2- product could also react with O2 in the air according to the following reaction [30]: O 2 + 2O 2 − → 2O 22 −

(5)

The resulting highly oxidizing species could effectively break Ti-O-Ti bonds, thus accelerating the conversion of titanium. However, H2O easily evaporated at 550 °C in the air. Then, many Ti-OH groups reacted with the molten NaOH salt, which may have formed metastable α-Na2TiO3 phase more easily than Ti-O-Na. In this sense, a certain amount of H2O should be added into the NaOH molten salt system to obtain effectively convert titanium.

Ti-OH + 2NaOH → α -Na 2 TiO3 + H 2 O( gas)

(6)

On the other hand, there was a small amount of SiO2 existed in spent SCR catalyst. SiO2 can react with NaOH, forming Na2Si2O5. Then, Na2TiO3 formed in the reaction may integrate with Na2Si2O5 to form Na2Ti2Si2O9, according to the XRD results.

2SiO 2 + 2NaOH → Na 2Si 2 O5 + H 2 O Si 2 O52 − + 2H 2 O + 2Na + + 2TiO32- → Na 2 Ti 2Si 2 O 9 ( s ) + 4OH −

(7) (8)

Compare with the bond length of Ti-O in α-Na2TiO3 (2.250 Å), Na2Ti2Si2O9 has a shorter bond length (1.949-1.980 Å). This indicates that Na2Ti2Si2O9 unexpectedly obtained because it was more stable than α-Na2TiO3. Water leaching of alkali melting products and NaOH recycling Based on the above results, the optimized reaction conditions for titanium conversion of spent SCR catalyst in NaOH roasting were a temperature of 550 °C, a reaction time of 10 min, a NaOH-to-spent SCR catalysts mass ratio of 1.8:1 and a NaOH concentration of 60-80 wt.%. Under these optimized parameters, more than 98% of the titanium was converted. At the optimized reaction conditions, the other main component of spent SCR catalysts such as SiO2, WO3, Al2O3, SO3 and V2O5, respectively, were converted to Na2Si2O5, Na2WO4, NaAlO2, Na2SO4 and NaVO3 in their water-soluble states, according to the following mechanism:

2SiO 2 + 2NaOH → Na 2Si 2 O5 + H 2 O

(7)

WO3 + 2NaOH → Na 2 WO 4 + H 2 O

(9)

Al 2 O3 + 2NaOH → 2NaAlO 2 + H 2O

(10)

SO3 + 2NaOH → Na 2SO 4 + H 2 O

(11)

8


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V2 O5 + 2NaOH → 2NaVO3 + H 2O

(12)

To selectively recover titanium and recycle the NaOH, we applied a water-leaching treatment to the alkali melting products at 55 °C for three times. The XRF results of water washed products are shown in Table 2, which reveals that most of the impurity elements were removed after the water-leaching treatment. The XRD patterns of water leaching product are shown in Figure 9. It is obviously revealed that water washed product presented as an amorphous phase with very weak FeTi2O5 (JCPDS No. 76-1158) and NiO (JCPDS No. 78-0423). The inset photograph in Figure 9 shows that Na2TiO3 could be effectively separated from water-soluble impurities by water leaching. The Na2TiO3, Na2Si2O5, Na2WO4, and NaAlO2 peaks all disappeared, thus indicating their excellent ion-exchange ability in water. The ion-exchange reaction of α-Na2TiO3 could be expressed as follows [31]:

α -Na 2 TiO3 + xH 2O → Na 2-x HTiO3 + xNaOH

(13)

Regenerated TiO2 photocatalysts The other aim of this work is to synthesize low-cost TiO2 photocatalysts using recycled α-Na2TiO3 as a raw material. We therefore further purified treated as-obtained α-Na2TiO3 with HCl or H2SO4 and then performed a hydrothermal treatment with the resulting solution. Finally, the regenerated TiO2 products can be applied in environmental pollutant, and acid wastewater from hydrothermal treatment process can also be reused in the acid leaching alkali slag process. Morphologies and structures Figure 10 displays the XRD patterns of samples obtained with different acid species. As depicted in Figure 10a, the as-synthesized sample A showed peaks at 2θ values of 25.3, 38.4, 48.7, 54.8, 55.9, 63.7, 70.1, 71.5 and 76.5 °, corresponding to the anatase TiO2 (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes (JCPDS No. 21-1272), respectively. By contrast, sample B shows sharp peaks assigned rutile (JCPDS No. 73-1232) and some small anatase peaks (Figure 10b). These results indicate that single anatase type TiO2 could be obtained in H2SO4 solution in addition the anatase phase has better photocatalytic properties. The morphology and structure of samples obtained with different acid species were characterized by SEM, as shown in Figure 11. Figure 11a shows that the morphology of sample A consists of numerous regular-spherical nanoparticles and a small amount of nanoparticles agglomerated with the average sizes range from 20 to 300 nm. The main elements O, Ti, and Fe were detected with mass ratios of 73.96, 25.69 and 0.35 wt.%, respectively, as shown in the EDS spectrum of sample A (Figure 11b). Meanwhile, sample B exhibited numerous rod-shaped structures and a few massive structures (Figure 11c). Sample B was also mainly composed of O, Ti, and Fe, as shown in Figure 11d. However, these elements were more uniformly distributed in sample A than those in sample B, as shown by the EDS maps. XPS analysis was also performed in order to investigate its elements composition and chemical state, as shown in Figure S4. The binding energies of Ti 2p3/2 and Ti 2p1/2 are around 456.7 and 464.3 eV, respectively, in agreement with values of Ti4+ [32]. In 9



the O (1s) spectrum peaks appearing at 529.8 and 531.8 eV were attributed to the lattice oxygen and the hydroxyl groups, respectively [32]. The Fe 2p XPS peaks appearing at 710.8 and 723.7 eV were also emerged, which belong to the binding energy of Fe 2p3/2 and Fe 2p1/2 for the samples, respectively. These results suggest that dopant of Fe in TiO2 samples both show a chemical state of Fe3+ [33].After the hydrothermal treatment, the regenerated products were centrifuged and washed extensively with dilute hydrochloric acid and deionized water, resulting in a 97.5% yield of recycled TiO2. The XRF results of the regenerated TiO2 products are shown in Table 3. The products both contain more than 99.0% TiO2, meaning that the samples prepared in our work satisfy commercial requirements. Moreover, the main impurity component of the samples was Fe2O3, and the calculated mass fraction of Fe2O3 in the samples was in close agreement with the amount of Fe detected by EDS. The UV-vis DRS of our samples and pure P25 TiO2 are presented in Figure 12. The absorption onset of pure P25 TiO2 was about 396 nm. However, samples A and B exhibited a significant red shift, and their absorption onsets were drastically extended to around 447 and 458 nm, respectively. The results indicated that the regenerated TiO2 products exhibit an enhanced absorbance in the broad UV-vis region comparing with the pure P25. The band gap energy can be estimated from the plot of (Ahν)1/2 versus photo energy (hν), as shown in the inset in Figure 12. The indirect transitions in P25 TiO2 powders showed a value of 2.88 eV, which is agreement with the results of recent work by Wang et al. [34]. In strong contrast, band gap values of samples A and B were 2.28 and 2.15 eV, respectively. These results show that Fe2O3 doping narrowed the band gap and enhanced visible light absorption of TiO2. In order to investigate the separation efficiency of electron-hole pairs in regenerated TiO2 products doped with Fe3+, PL spectra were recorded for all samples in the range of 320 to 500 nm at an excitation wavelength of 300 nm and a photomultiplier tube operating voltage of 700 V. As shown in Figure 13, all the samples exhibited obvious PL signals with similarly shaped curves, suggesting that such a small amount of Fe3+ was not enough to generate a new PL signal. However, compared to pure P25 TiO2, samples doped with Fe3+ revealed less intense emission peaks in the same position, indicating that Fe3+ doping led to more efficient transfer and separation of photogenerated electrons and holes. This may be because the Fe3+ ions could act as an electron-trapping agent [35]. Evaluation of photocatalytic activities Dye effluents from the textile industry are becoming a serious environmental problem as they are stable in light and resistant to chemical and biological degradation. Photocatalysis in the presence of nanostructured TiO2 has been proven to effectively remove of various harmful pollutants. Therefore, the photocatalytic efficiency of the as-prepared nano TiO2 samples and P25 TiO2 for degrading dyes (RhB and MB) was evaluated in air under visible-light irradiation, as shown in Figure 14. Sample A, sample B, and P25 TiO2 photodegraded 66.9, 44.9 and 41.1% of RhB under visible-light irradiation for 180 min, respectively (Figure 14a). Over 99% of MB was photodegraded in the presence of sample A under visible-light irradiation for 120 min, compared with 74.3 and 37.8% of MB degraded by sample B and P25 TiO2 10


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under visible-light irradiation for 180 min, respectively (Figure 14c). The initial concentrations of RhB (10 ppm) and MB (10 ppm) were both small; therefore, the apparent rate constant (κapp) for the photocatalytic oxidation of RhB and MB with different samples was evaluated using a pseudo-first-order model [32], as shown in Figure 14b and d and expressed as follows:

ln(C0 / Ct ) = κ app t

(14)

where κapp is the apparent rate constant, Ct is the solution-phase concentration of dyes, and C0 is the initial concentration at t = 0. The apparent rate constant κapp values are listed in Table 4. Clearly, the samples doped with Fe2O3 exhibited superior photocatalytic performance. For the kinetics of degrading RhB, the slopes of samples A and B are 2.23 and 1.35 times that of P25 TiO2, respectively, meaning that the Fe2O3-doped samples more efficiently photodegrade RhB. Meanwhile, the calculated rate constant for the photodegradation of MB with P25 TiO2 is 2.8×10-3 min-1. In contrast, the slopes of sample A (35.9×10-3 min-1) and sample B (6.9×10-3 min-1) are 12.82 and 2.46 times that of P25 TiO2 for the photodegradation of MB under visible-light irradiation, respectively. The photodegradation results indicate that the existence of Fe2O3 is responsible for the superior photocatalytic performance. Figure S5 demonstrates the reusability of the best catalyst (sample A). Its loss of photocatalytic activity was negligible during three cycles of use, indicating its outstanding reusability. According to the results of UV-vis and PL spectra, the benefits of Fe2O3 doping could be summarized as hindering the recombination of photoexcited electrons and holes. The detailed photocatalytic reaction mechanism of samples can be expressed as follows [36]: TiO 2 + hν → h + + e −

(15)

Fe3+ + e − → Fe 2+

(16)

Fe 2 + + O 2 → Fe 3+ + O⋅−2

(17)

Fe3+ + h + → Fe 4 +

(18)

Fe 4 + + OH − → ⋅OH + Fe3+

(19)

First, an electron-hole pair is created in Fe2O3/TiO2 under visible-light irradiation. Subsequently, Fe3+ ions can trap photogenerated electron, thus forming Fe2+ ions. In addition, Fe2+ ions react with adsorbed O2, thus forming highly reactive superoxide ions (O2·-). On the other hand, Fe3+ ions can also react with holes, forming Fe4+ ions. The unstable and very reactive Fe4+ ions can oxidize -OH to form hydroxyl groups (·OH). Finally, these highly oxidizing species can strongly degrade dyes to produce CO2 and H2O.

Conclusions 11



This work addresses the development of a clean, green, and economical approach to recovering titanium and regenerated TiO2 photocatalysts from spent SCR catalysts, and it optimizes its most relevant steps. The proposed integrated process comprises four steps, starting with a NaOH molten salt treatment of spent SCR catalysts (step 1); followed by enriching the titanium-containing products via water-leaching and filtration (step 2); then acid dissolving the titanium-enriched products and hydrothermally treating the as-obtained titanium-containing solution (step 3); and finally, recycling and reusing NaOH and the waste acid solution (step 4). The results are listed below. (i) A NaOH molten salt process is proposed and proven feasible in our work. Using this molten salt process, the valuable industrial metal Ti can be quickly and efficiently recovered to re-enter the mainstream material flow. The temperature, NaOH-to-spent SCR catalysts mass ratio, roasting time, and H2O content are significantly influenced titanium conversion. Under the optimized reaction conditions, i.e. a temperature of 550 °C, a NaOH-to-spent-SCR-catalyst mass ratio of 1.8:1, a roasting time of 10 min and a NaOH concentration of 60-80 wt.%, > 98% of titanium was converted. The XRD results indicated that metastable a-Na2TiO3 was the main phase. (ii) A possible molten salt reaction mechanism was postulated. The reaction of anatase TiO2 component with molten NaOH salt yielded dissolved Ti4+. When H2O was added into the molten salt system, more metastable α-Na2TiO3 formed. Analyzing the reaction mechanism indicated that H2O not only supplies hydroxyl groups and hydrogen ions but also acts as an auxiliary accelerator reagent to decompose the titanium component during the molten salt decomposition process. (iii) The regenerated TiO2 products exhibit high purity (> 99.0%) and are thus fully compliant with commercial requirements. The morphologies and structures of TiO2 products can be easily tuned by controlling the experimental parameters. Importantly, the Fe elements present in spent SCR catalysts was doped into the regenerated TiO2 products, thus narrowing the band gap to enable visible-light driven photocatalysis. In addition, the Fe-doped TiO2 photocatalysts exhibited higher photocatalytic efficiency than P25 TiO2 to oxidize both MB and RhB under visible-light irradiation. (iv) Excess NaOH agents can be regenerated and reused to reduce the cost of this method. The acid-containing wastewater from the hydrothermal treatment can also be reused in the acid leaching process. Moreover, no hazardous chemicals were used, making the process clean, green, and economic.

Acknowledgments. This research was financially supported by Beijing Natural Science Foundation (2174067, 2174065, 2182009), Beijing Nova Program (Z1511000003150141) and the Fundamental Research Fund Project of Beijing University of Technology (033000546317501).

Supporting Information 12


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The SEM images, FT-IR spectrum, XPS spectrum and photocatalytic stability results of the samples. This information is available free of charge via the Internet at http://pubs.acs.org/.

13



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prepared by chemical processing. Adv. Mater., 1999, 11, 1307-1311. [29] Duan, S. Z.; Qiao, Z. Y. Molten salt chemistry-Principle and application. Beijing: Metallurgical industry press, 1990. [30] Sun, Z.; Zhang, Y.; Zheng, S. -L.; Zhang, Y. A new method of potassium chromate production from chromite and KOH-KNO3-H2O binary submolten salt system. AIChE J., 2009, 55, 2646-2656. [31] Liu, Y. H.; Zhao, W.; Wang, W. J.; Yang, X.; Chu, J. L.; Xue, T. Y.; Qi, T.; Wu, J. Y.; Wang, C. R. Study on the transformation from NaCl-type Na2TiO3 to layered titanate. J. Phys. Chem. Solids, 2012, 73, 402-406. [32] Zhang, Q. J.; Fu, Y.; Wu, Y. F.; Zhang, Y. -N.; Zuo, T. Y. Low-Cost Y-Doped TiO2 Nanosheets Film with Highly Reactive {001} Facets from CRT Waste and Enhanced Photocatalytic Removal of Cr(VI) and Methyl Orange. ACS Sustain. Chem. Eng., 2016, 4, 1794-1803. [33] Kong, L.; Wang, C.; Wan, F.; Zheng, H.; Zhang, X. Synergistic effect of surface self-doping and Fe species-grafting for enhanced photocatalytic activity of TiO2 under visible-light. Appl. Surf. Sci., 2017, 296, 26-35. [34] Oshani, F.; Marandi, R.; Rasouli, S.; Farhoud, M. K. Photocatalytic investigations of TiO2-P25 nanocomposite thin films prepared by peroxotitanic acid modified sol-gel method. Appl. Surf. Sci., 2014, 311, 308-313. [35] Li, J. -Q.; Wang, D. -F.; Guo, Z. -Y.; Zhu, Z. -F. Preparation, characterization and visible-light-driven photocatalytic activity of Fe-incorporated TiO2 microspheres photocatalysts. Appl. Surf. Sci., 2012, 263, 382-388. [36] Yu, J. G.; Xiang, Q. J.; Zhou, M. H. Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures. Appl. Catal. B: Environ., 2009, 90, 595-602.

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Figure Captions:

Figure 1. Brief flowchart of titanium recovery to produce sustainable TiO2 photocatalytic materials from spent SCR catalysts.

17



Figure 2. XRD pattern of spent SCR catalysts.

18


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Figure 3. (a) Effect of reaction temperature on the titanium conversion yield. (b) XRD patterns of products roasted at different temperatures.

19



Figure 4. (a) Effect of NaOH-to-spent-SCR-catalyst mass ratio on the titanium conversion yield. (b) XRD patterns of products at different NaOH-to-spent SCR catalyst mass ratios.

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Figure 5. (a) Effect of reaction time on conversion on the titanium conversion yield. (b) XRD patterns of products after different reaction times.

21



Figure 6. (a) Effect of H2O on conversion of titanium. (b) The XRD patterns of products with H2O and without H2O in molten salt system.

22


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Figure 7. (a) Effect of roasting time on the concentration of Na+. (b) Photographs of molten salt products after different roasting times.

23



Figure 8. Schematic diagram of a possible chemical reaction mechanisms.

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Figure 9. XRD pattern of water-leached product (the inset photograph is the water-leached solution).

25



Figure 10. XRD patterns of (a) sample A and (b) sample B.

26


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Figure 11. (a) SEM image of sample A and the element distribution mapping results for O, Ti and Fe; and (b) the corresponding EDS spectrum of sample A; (c) SEM image of sample Band the element distribution mapping results for O, Ti and Fe; and (d) the corresponding EDS spectrum of sample B.

27



Figure 12. UV-vis DRS results of sample A, sample B, and P25 TiO2.

28


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Figure 13. PL spectra of different samples excited at 300 nm.

29



Figure 14. Photodegradation of (a) RhB and (c) MB with all samples under visible-light irradiation; photocatalytic reaction kinetics of (b) RhB and (d) MB as a function of reaction time.

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Table Captions:

Table 1. Chemical compositions (wt.%) of spent SCR catalysts. (XRF analysis).

Spent

TiO2

SiO2

SO3

Al2O3

WO3

Fe2O3

CaO

V2O5

K2 O

Na2O

MgO

others

68.5

10.3

6.45

5.47

4.67

1.59

1.463

0.427

0.405

0.191

0.188

0.346

Table 2. Chemical compositions (wt.%) of water-leached products. (XRF analysis). TiO2

Na2O

Fe2O3

NiO

CaO

SiO2

MgO

Al2O3

WO3

Others

87.34

5.28

1.86

1.81

1.63

1.37

0.371

0.26

0.109

0.03

Table 3. Chemical compositions (wt.%) of TiO2 products (XRF analysis).

sample A sample B

TiO2

Fe2O3

NiO

CaO

SiO2

MgO

WO3

Na2O

others

99.38 99.25

0.323 0.407

0.041 0.048

0.035 0.054

0.026 0.042

0.012 0.011

0.011 0.024

0.010 0.013

0.162 0.151

Table 4. Reaction rate constant κapp (min-1) for the degradation of RhB and MB using various TiO2 samples. degradation of RhB

degradation of MB

κapp (min-1)

R2

κapp (min-1)

R2

Sample A

7.8×10-3

0.9839

35.9×10-3

0.9964

Sample B

3.5×10

-3

2.6×10

-3

P25 TiO2

0.9978 0.9964

31


6.9×10

-3

0.9975

2.8×10

-3

0.9985


“For Table of Contents Use Only.”

This work introduces a clean, green, and economical process for recovering the valuable and abundant titanium component of spent SCR catalysts and regenerating TiO2 photocatalysts for efficiently treat wastewater.

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Green Recovery of Titanium and Effective Regeneration of TiO2

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