Synergistic Mechanism of Rare-Earth Modiﬁcation TiO 2 and Photodegradation on Benzohydroxamic Acid

: Rare earth elements are plentiful in Gannan area, China, and there is a large amount of wastewater from all kinds of mines. In this paper, rare-earth modiﬁcation TiO 2 composites (RE/TiO 2 , RE = La, Ce, Gd, Yb) was studied by theory computation and experimental performance. The prepared RE/TiO 2 was investigated for the degradation of benzohydroxamic acid (BHA) as a typical residual reagent in wastewater from beneﬁciation. The crystallinity, morphology, speciﬁc surface area, light absorption, and composition of compound were investigated by various techniques. As a result of computation and experimentation, four different electron conﬁgurations of rare earth all retained the anatase phase of TiO 2 and reduced the band gap of TiO 2 to some degree compared with pure TiO 2 . Different rare-earth elements and calcination temperatures resulted in different removal effects on BHA. The optimum doping contents were 0.75% (500 ◦ C), 0.20% (500 ◦ C), 0.70% (500 ◦ C) and 0.50% (450 ◦ C) for La, Ce, Gd, Yb respectively. All the RE/TiO 2 composites studied in this research still possessed good photoactivity after four runs, which supports the theoretical and practical basement for the photocatalytic treatment of mining and metallurgy wastewater.


Introduction
Many organic reagents were used in the froth flotation during the ore-dressing. Some of them discharged with the wastewater and are harmful to aquatic organisms and humans. The reported research indicated that some deformed cells of algae (Scenedesmus obliquus) were found in cultures exposed to 0145 for 48 h [1]; most of the fishes would die while the concentration of xanthate was over 5.0 mg·L −1 [2]; Acrylonitrile, as the synthetic raw material of Ester-105, is possibly carcinogenic to humans (Group 2B) [3]. Besides, the residual beneficiation reagents might induce the release of heavy metals from the tailings pond, resulting in secondary pollution [4].
Many methods have been developed to treat organic beneficiation reagents or other organic compounds such as physical adsorption [5], biodegradation [6] and chemical oxidation [7,8], and so on. For example, there needs to be 5 or more days to get to the removal efficiency of 85% for benzohydroxamic acid (BHA, the most widely-used collector in the flotation industry at present) with the method of biodegradation [9]. However, our previous studies indicated that La/TiO 2 and Yb/TiO 2 possessed high photodegradation activity to BHA (the removal efficiency was up to 90% in 2 h) [10]. TiO 2 has been considered as the most promising photocatalyst among various semiconductor photocatalyst materials due to its benign physicochemical property and low cost [11]. In recent years,

Preparation of RE/TiO 2
RE/TiO 2 were prepared by the sol-gel method according to previous research [10]. Rare-earth nitrate in the required stoichiometry was mixed with 2 mL deionized water, 2 mL acetic acid and 10 mL absolute ethanol, and the mixture solution after stirring was noted as solution A. Solution B was prepared with 10 mL tetrabutyl titanate and 15 mL absolute ethanol. Homogeneous and transparent sol was obtained after solution A was added dropwise to solution B under agitation. The continuous stirring of RE/TiO 2 produced the wet gel, and the xerogel was formed after aging at atmospheric temperature for 48 h and then drying at 90 • C. The synthetic product was crushed to get fine powder by agate mortar and calcined in a furnace (400 • C-600 • C). Finally, the RE/TiO 2 composites were achieved. The doped rare earth was set to 0~1.0% according to TiO 2 in mass.

Characterization of RE/TiO 2
Power X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (RigakU Miniflex, Japan) at 40 kV and 40 mA using monochromatized Cu Kα radiation. Element constitutions and values of RE/TiO 2 were analyzed by the X-ray photoelectron spectroscopy (XPS) with a Thermo type multi-function imaging electronic spectrometer (ESCALAB 250XI, Thermo Fisher, Waltham, USA). A UV-vis spectrophotometer (UV-2550, Shimadzu, Tokyo, Japan) was used to determine the light absorption of RE/TiO 2 . The Brunauer-Emmett-Teller (BET) surface areas and Nitrogen adsorption-desorption isotherms were measured by adsorption apparatus (ASAP 2460, Micromeritics, Norcross, GA, USA) at liquid nitrogen temperature (−196 • C).

Degradation of BHA by RE/TiO 2
The photocatalytic experiments were operated in a photochemical reactor. RE/TiO 2 was dispersed into a 50 mL BHA (30 mg/L) solution. In order to make sure of the adsorption-desorption equilibrium of BHA on RE/TiO 2 , the reaction solution was stirred continuously without irradiation for 30 min. Then, the 300 W mercury light was turned on. Approximately 5 mL of suspension solution was sampled and centrifuged (10,000 r/min) at set time intervals and then was determined by UV-vis spectrophotometry at 229 nm. The removal rate (R) was determined as follows: where C 0 (mg/L) is the initial concentration of BHA and C t (mg/L) is the concentration at reaction time t (min).

Recycle Test of RE/TiO 2
Batch experiments were conducted to invest the reusability of RE/TiO 2 . In five quartz test tubes (No 1, 2, 3 and 4), 0.0500 g RE/TiO 2 was added followed by the addition of 50 mL of BHA aqueous solution at 20 mg/L. Before irradiation, the suspension was stirred in the dark for 30 min to establish adsorption-desorption equilibrium of BHA on the catalysts. Approximately 4 mL of aqueous sample was withdrawn from the tube of No 1 at certain time intervals of irradiation for analysis, and the reaction lasted for 120 min. For the other three test tubes, the reaction solution was centrifuged after 120 min of irradiation. The supernatant was removed and 50 mL of fresh BHA aqueous solution at 20 mg/L was added. Another 120 min of irradiation was continued. For the tubes of No 2, 3 and 4, the photodegradation was repeated for two, three and four runs, respectively.

Texture Properties
As seen from Figure 2, XRD patterns of prepared pure TiO2 and RE/TiO2 were all in accordance with JCPDS Card No.21-1272 of anatase TiO2 [27,28]. The characteristics peak of all RE/TiO2 became broader and lower than pure TiO2, suggesting a decrease in crystallite sizes (Table 1). The higher the rare-earth doping amount, the broader the XRD pattern peaks and the lower the intensity [29]. In other words, the doping of rare-earth elements slowed down the growth of TiO2 nanoparticles, changed the surface structure of TiO2 particles, and improved the diffusion barrier of TiO2 grain.

Texture Properties
As seen from Figure 2, XRD patterns of prepared pure TiO 2 and RE/TiO 2 were all in accordance with JCPDS Card No.21-1272 of anatase TiO 2 [27,28]. The characteristics peak of all RE/TiO 2 became broader and lower than pure TiO 2 , suggesting a decrease in crystallite sizes ( Table 1). The higher the rare-earth doping amount, the broader the XRD pattern peaks and the lower the intensity [29]. In other words, the doping of rare-earth elements slowed down the growth of TiO 2 nanoparticles, changed the surface structure of TiO 2 particles, and improved the diffusion barrier of TiO 2 grain.  10.4 There are no new phases except anatase TiO 2 by XRD analysis for the doped catalyst. So, RE 3+ might highly disperse on the surface or enter into the lattice of TiO 2 . Therefore, the growth of TiO 2 particles was hindered by doped rare-earth elements. Besides, the rare-earth compounds might become light interception sub-trapping centers, which is important to retard the recombination of the electron-hole pairs and to affect the photodegradation activity [30].
Appl. Sci. 2019, 9, 339 5 of 14 There are no new phases except anatase TiO2 by XRD analysis for the doped catalyst. So, RE 3+ might highly disperse on the surface or enter into the lattice of TiO2. Therefore, the growth of TiO2 particles was hindered by doped rare-earth elements. Besides, the rare-earth compounds might become light interception sub-trapping centers, which is important to retard the recombination of the electron-hole pairs and to affect the photodegradation activity [30].

Optical Absorption Property
The UV-Vis DRS spectra of pure TiO2 and RE/TiO2 are presented in Figure 3. The band gap energy (Eg) was calculated by Formula (2) as follows [31]: where α, h, v, and A represent the absorption coefficient, Planck's constant, light frequency and a constant respectively. The value of n is 4 as indirect transition for TiO2. By calculation, 0.75%La/TiO2 (500 °C), 0.20%Ce/TiO2 (500 °C), 0.70%Gd/TiO2 (500 °C) and 0.50%Yb/TiO2 (450 °C) have the most red-shift compared with pure TiO2 in their own doped range, indicating a decrease in the band gap; the Eg were 3.17 eV, 2.93 eV, 3.10 eV and 3.16 eV respectively, while the Eg of pure TiO2 was 3.21 eV and 3.20 eV for 500 °C and 450 °C, respectively. The red-shift could be attributed to the charge-transfer transition between 4f or 5d electrons of rare-earth elements and the conduction or valence band of TiO2 due to the hybridization between the O 2p and RE 4f/5d orbitals after the RE element is substituted or interstitially incorporated into the TiO2 lattice [32]. As discussed above, the doping of rare-earth elements reduced the band gap compared with pure TiO2, and ultimately augments the photodegradation ability of RE/TiO2 composites [33]. There is further 20

Optical Absorption Property
The UV-Vis DRS spectra of pure TiO 2 and RE/TiO 2 are presented in Figure 3. The band gap energy (Eg) was calculated by Formula (2) as follows [31]: where α, h, v, and A represent the absorption coefficient, Planck's constant, light frequency and a constant respectively. The value of n is 4 as indirect transition for TiO 2 . By calculation, 0.75%La/TiO 2 (500 • C), 0.20%Ce/TiO 2 (500 • C), 0.70%Gd/TiO 2 (500 • C) and 0.50%Yb/TiO 2 (450 • C) have the most red-shift compared with pure TiO 2 in their own doped range, indicating a decrease in the band gap; the Eg were 3.17 eV, 2.93 eV, 3.10 eV and 3.16 eV respectively, while the Eg of pure TiO 2 was 3.21 eV and 3.20 eV for 500 • C and 450 • C, respectively. The red-shift could be attributed to the charge-transfer transition between 4f or 5d electrons of rare-earth elements and the conduction or valence band of TiO 2 due to the hybridization between the O 2p and RE 4f/5d orbitals after the RE element is substituted or interstitially incorporated into the TiO 2 lattice [32]. As discussed above, the doping of rare-earth elements reduced the band gap compared with pure TiO 2 , and ultimately augments the photodegradation ability of RE/TiO 2 composites [33]. There is further explanation about the effect of rare area earth elements on the band structure and hybridization of anatase TiO 2 in the computation section.
Appl. Sci. 2019, 9,339 6 of 14 explanation about the effect of rare area earth elements on the band structure and hybridization of anatase TiO2 in the computation section.

Composition and Chemical State Analysis
XPS was applied to analyse the composition and chemical state of pure TiO2 and RE/TiO2. Figure  4a indicates the survey spectra; there are obvious O and Ti peaks that come from synthesized pure TiO2 or RE/TiO2. Figure 4b indicates the spectra of doped La, and the characteristic peaks of La were detected near 853.92 eV and 836.79 eV, which suggests that rare-earth element La was doped successfully. However, the other three elements Ce, Gd and Yb were not detected by XPS. In order to prove the existence of Ce, Gd and Yb in the prepared RE/TiO2, EDS was used to analyze the content. The result indicates that there was 0.016% of Ce in 0.20%Ce/TiO2, 0.20% of Yb in 0.5% Yb/TiO2, and 0.40% of Gd in 0.70% Gd/TiO2.
The peaks of Ti2p and O1s for pure TiO2 and Gd/TiO2 were analyzed as shown in Figure 4c-f. Ti peaks of pure TiO2 are located at 464.45 eV and 458.74 eV while Ti peaks of Gd/TiO2 are located at 464.48 eV and 458.76 eV. The peak of O1s was fitted into two peaks: crystal lattice oxygen and hydroxyl oxygen [11]. The fitted peak of O1s at 529.83 eV corresponded to the lattice oxygen while the peak at 530.31 eV corresponded to hydroxyl oxygen for pure TiO2. As seen from Figure 4c-f, after doping with Gd, the binding energy of Ti2p increased while the binding energy of O1s decreased for Gd/TiO2 compared with pure TiO2. The other three RE/TiO2 composites (La/TiO2, Ce/TiO2 and Yb/TiO2) all present a similar situation, which might be due to the interaction of RE with Ti and O in RE/TiO2 [34].

Composition and Chemical State Analysis
XPS was applied to analyse the composition and chemical state of pure TiO 2 and RE/TiO 2 . Figure 4a indicates the survey spectra; there are obvious O and Ti peaks that come from synthesized pure TiO 2 or RE/TiO 2 . Figure 4b indicates the spectra of doped La, and the characteristic peaks of La were detected near 853.92 eV and 836.79 eV, which suggests that rare-earth element La was doped successfully. However, the other three elements Ce, Gd and Yb were not detected by XPS. In order to prove the existence of Ce, Gd and Yb in the prepared RE/TiO 2 , EDS was used to analyze the content. The result indicates that there was 0.016% of Ce in 0.20%Ce/TiO 2 , 0.20% of Yb in 0.5% Yb/TiO 2 , and 0.40% of Gd in 0.70% Gd/TiO 2 .
The peaks of Ti2p and O1s for pure TiO 2 and Gd/TiO 2 were analyzed as shown in Figure 4c-f. Ti peaks of pure TiO 2 are located at 464.45 eV and 458.74 eV while Ti peaks of Gd/TiO 2 are located at 464.48 eV and 458.76 eV. The peak of O1s was fitted into two peaks: crystal lattice oxygen and hydroxyl oxygen [11]. The fitted peak of O1s at 529.83 eV corresponded to the lattice oxygen while the peak at 530.31 eV corresponded to hydroxyl oxygen for pure TiO 2 . As seen from Figure 4c-f, after doping with Gd, the binding energy of Ti2p increased while the binding energy of O1s decreased for Gd/TiO 2 compared with pure TiO 2 . The other three RE/TiO 2 composites (La/TiO 2 , Ce/TiO 2 and Yb/TiO 2 ) all present a similar situation, which might be due to the interaction of RE with Ti and O in RE/TiO 2 [34].

Analysis of SBET and Pore Size Distribution
As seen from Figure S1 of the N2 adsorption-desorption isotherms, all prepared RE/TiO2 present the mesoporous structure [35,36]. The hysteresis loops of pure TiO2 and RE/TiO2 are consistent with type H2 that has uniform particle accumulation on the hole. The shape of the holes is considered to be a bottle-shaped channel of small mouth and large cavity, like an ink [14]. Besides, the distribution of pore size ranges from 2 nm to 10 nm, which belongs to the mesoporous pore. Intensity / a.u.
Binding Energy / eV

Analysis of S BET and Pore Size Distribution
As seen from Figure S1 of the N 2 adsorption-desorption isotherms, all prepared RE/TiO 2 present the mesoporous structure [35,36]. The hysteresis loops of pure TiO 2 and RE/TiO 2 are consistent with type H2 that has uniform particle accumulation on the hole. The shape of the holes is considered to be a bottle-shaped channel of small mouth and large cavity, like an ink [14]. Besides, the distribution of pore size ranges from 2 nm to 10 nm, which belongs to the mesoporous pore. The S BET was 39.39, 41.19, 40.42, 91.84, 86.78, and 113.89 m 2 /g for pure TiO 2 (500 • C), 0.75%La/TiO 2 (500 • C), 0.20%Ce/TiO 2 (500 • C), 0.70%Gd/TiO 2 (500 • C), pure TiO 2 (450 • C), and 0.50%Yb/TiO 2 (450 • C) respectively. Obviously, all the S BET of RE/TiO 2 composites are larger than that of pure TiO 2 calcined at the same temperature, especially for Gd and Yb doped TiO 2 , which is in accordance with analysis result of XRD. The result may be owing to the linkage between the rare-earth ions and titanium by the oxygen bridge, which could effectively increase the specific surface area of TiO 2 [37]. Table 2 shows the lattice parameters of pure TiO 2 and RE/TiO 2 . Parameters a and b almost have no changes compared to pure TiO 2 (2 × 2 × 1 supercell), while the c is longer than pure TiO 2 , resulting in the increase of cell volume (the radius of La 3+ , Ce 3+ , Gd 3+ , and Yb 3+ is larger than Ti 4+ ). The result is inconsistent with the reference's calculation [23,24]. The calculated result indicates the indirect band gap of 2.105 eV for anatase TiO 2 (unit cell), which is significantly lower than the laboratory data (approximately 3.2 eV). However, the electronic structure is essentially the same [24]. The band gaps of calculated doped TiO 2 are 1.951 eV, 2.059 eV, 0.475 eV, and 2.029 eV for La/TiO 2 , Ce/TiO 2 , Gd/TiO 2 , and Yb/TiO 2 ( Figure 5), respectively, all less than pure anatase TiO 2 , which is in agreement with the laboratory result in the study. Gd/TiO 2 has the biggest changes on band gap according to the poor stability of Gd's electrons. Accodring to Bian's statement, the energy state forms of RE dopants inside the CB and VB are divided into three parts including deep impurity states (electron transfer from RE energy state to CB), shallow impurity states (electron transfer from VB to RE energy state) and both states [25]. It could be concluded that element Yb belongs to deep impurity states, La and Ce belong to shallow impurity states, and Gd belongs to both.

Computation and Analysis
Similar to the pure anatase TiO 2 , there also exists the hybridization of O 2p and Ti 3d states in the valence band region for RE/TiO 2 observed from the calculated PDOS ( Figure S2). As seen from Figure 6, there exists impurity energy localized at about 0.743 eV above the Fermi level for Gd/TiO 2 (2 × 2 × 1 supercell), which indicates that some of the Gd 4f states are partially occupied. Simultaneously, the formation of Gd-O bonds leads to the partial hybridization of the Gd 4f orbitals with O 2p states below the Fermi level (Figure 6b). Therefore, the introduced Gd 4f orbital mainly contributes to extend the impurities' energy level into the forbidden band and produce a wider valence band in comparison with the pure TiO 2 . Besides, the top of the valence band shifts obviously while the bottom of the conduction band shifts slightly with respect to the Fermi level, which results in the narrower band gap and the consequent redshift response of the optical absorption edge [38].

Effect of Doped Amount of Rare Earth
Different rare-earth elements and doped amounts affected the photocatalytic activity of RE/TiO2 on BHA. As shown in Figure 7, the removal efficiency of BHA by direct photolysis was very low and the adsorption ability of RE/TiO2 to BHA was negligible. Compared with pure TiO2, La and Ce indicate significant effects on BHA photodegradation while Gd and Yb only have tiny promoted effects on the photodegradation. Along with the overall trend, the photocatalytic activity of RE/TiO2 on BHA enhanced with the increased amount of doped rare-earth element and then was weakened with the further increased amount, especially for La and Ce doped composites. In a word, the photodegradation ability of 0.75%La/TiO2, 0.20%Ce/TiO2, 0.70%gd/TiO2, and 0.50%Yb/TiO2 are up to the highest compared with pure TiO2 or other kinds of the same doped materials, which are in consistence with the analysis of UV-Vis DRS.

Effect of Doped Amount of Rare Earth
Different rare-earth elements and doped amounts affected the photocatalytic activity of RE/TiO2 on BHA. As shown in Figure 7, the removal efficiency of BHA by direct photolysis was very low and the adsorption ability of RE/TiO2 to BHA was negligible. Compared with pure TiO2, La and Ce indicate significant effects on BHA photodegradation while Gd and Yb only have tiny promoted effects on the photodegradation. Along with the overall trend, the photocatalytic activity of RE/TiO2 on BHA enhanced with the increased amount of doped rare-earth element and then was weakened with the further increased amount, especially for La and Ce doped composites. In a word, the photodegradation ability of 0.75%La/TiO2, 0.20%Ce/TiO2, 0.70%gd/TiO2, and 0.50%Yb/TiO2 are up to the highest compared with pure TiO2 or other kinds of the same doped materials, which are in consistence with the analysis of UV-Vis DRS.

Effect of Doped Amount of Rare Earth
Different rare-earth elements and doped amounts affected the photocatalytic activity of RE/TiO 2 on BHA. As shown in Figure 7, the removal efficiency of BHA by direct photolysis was very low and the adsorption ability of RE/TiO 2 to BHA was negligible. Compared with pure TiO 2 , La and Ce indicate significant effects on BHA photodegradation while Gd and Yb only have tiny promoted effects on the photodegradation. Along with the overall trend, the photocatalytic activity of RE/TiO 2 on BHA enhanced with the increased amount of doped rare-earth element and then was weakened with the further increased amount, especially for La and Ce doped composites. In a word, the photodegradation ability of 0.75%La/TiO 2 , 0.20%Ce/TiO 2 , 0.70%gd/TiO 2 , and 0.50%Yb/TiO 2 are up to the highest compared with pure TiO 2 or other kinds of the same doped materials, which are in consistence with the analysis of UV-Vis DRS.
The addition of rare earth would result in expanding the lattice of TiO 2 , which has been proved by computation. The expansion might lead to crystal defects and distortion, ultimately affecting the energy band structure and weakening the recombination of photogenerated electrons and holes. However, too many defects would become the recombination center if excess rare-earth element was doped. So, there always exists an optimum doped amount [39].
Appl. Sci. 2019, 9,339 10 of 14 The addition of rare earth would result in expanding the lattice of TiO2, which has been proved by computation. The expansion might lead to crystal defects and distortion, ultimately affecting the energy band structure and weakening the recombination of photogenerated electrons and holes. However, too many defects would become the recombination center if excess rare-earth element was doped. So, there always exists an optimum doped amount [39].

Effect of Calcination Temperature
Different calcination temperature would result in different morphology of prepared materials, ultimately affecting the photodegradation activity of catalysts. As seen from Figure 8, 500 °C was the appropriate temperature for 0.75%La/TiO2, 0.20%Ce/TiO2 and 0.70%Gd/TiO2 while 450 °C was the appropriate temperature for 0.50%Yb/TiO2. As we know, the material prepared at a low temperature would have a low crystallization degree and have a large amount of recombination centers for photogenerated electrons and holes. However, the excessive sinter would cause the agglomeration of catalyst particles or transform the phase of TiO2 from anatase to rutile, leading to the lower photocatalytic activity. Accodring to the published reports, the anatase form of TiO2 always had higher photocatalytic ability [14,15]. As a result, it could be concluded that rare-earth elements and calcination temperature both affect the photodegradation of BHA.

Effect of Calcination Temperature
Different calcination temperature would result in different morphology of prepared materials, ultimately affecting the photodegradation activity of catalysts. As seen from Figure 8, 500 • C was the appropriate temperature for 0.75%La/TiO 2 , 0.20%Ce/TiO 2 and 0.70%Gd/TiO 2 while 450 • C was the appropriate temperature for 0.50%Yb/TiO 2 . As we know, the material prepared at a low temperature would have a low crystallization degree and have a large amount of recombination centers for photogenerated electrons and holes. However, the excessive sinter would cause the agglomeration of catalyst particles or transform the phase of TiO 2 from anatase to rutile, leading to the lower photocatalytic activity. Accodring to the published reports, the anatase form of TiO 2 always had higher photocatalytic ability [14,15]. As a result, it could be concluded that rare-earth elements and calcination temperature both affect the photodegradation of BHA.

Stability of RE/TiO2
Reusability is an important indicator in evaluating the photocatalyst except for their photoactivity in the practical application. RE/TiO2 composites were used four times to test their reusability on the photodegradation of BHA. As seen in Figure 9, during the last three runs, the removal rate only decreased about 0.8%~3.4% compared with the first run, which indicates that RE/TiO2 continuously keeps the good photodegradation effect on BHA, and confirms the potential ability in engineering applications.

Stability of RE/TiO 2
Reusability is an important indicator in evaluating the photocatalyst except for their photoactivity in the practical application. RE/TiO 2 composites were used four times to test their reusability on the photodegradation of BHA. As seen in Figure 9, during the last three runs, the removal rate only decreased about 0.8%~3.4% compared with the first run, which indicates that RE/TiO 2 continuously keeps the good photodegradation effect on BHA, and confirms the potential ability in engineering applications.

Conclusions
RE/TiO2 composites were prepared successfully by a simple Sol-Gel method. Four rare-earth elements La, Ce, Gd, and Yb might highly disperse on the surface or enter into the lattice of TiO2 and hinder the growth of TiO2 particles. Due to the linkage between the rare-earth ions and titanium by the oxygen bridge, the specific surface area of RE/TiO2 composites was effectively increased. Besides, the rare-earth compounds might become light interception sub-trapping centers and reduce the band gap compared with pure TiO2, which is important to retard the recombination of the electron-hole pairs and ultimately affect the photodegradation activity of the composites. The photodegradation experiment indicates that the doped rare-earth elements could enhance the removal efficiency of BHA from beneficiation wastewater. Each rare-earth element had its own optimum doped amount, and the optimum doping contents were 0.75%La/TiO2, 0.20%Ce/TiO2, 0.70%gd/TiO2, and 0.50%Yb/TiO2 for La, Ce, Gd, and Yb doped composites respectively. The calculated result is basically in line with the experiment. However, there exists a big difference in the band gap between the experiment and computation because of the different ratio for Gd/Ti. Four types of RE/TiO2 all demonstrate the high photocatalytic activity and excellent reusability for BHA after four runs.

Conclusions
RE/TiO 2 composites were prepared successfully by a simple Sol-Gel method. Four rare-earth elements La, Ce, Gd, and Yb might highly disperse on the surface or enter into the lattice of TiO 2 and hinder the growth of TiO 2 particles. Due to the linkage between the rare-earth ions and titanium by the oxygen bridge, the specific surface area of RE/TiO 2 composites was effectively increased. Besides, the rare-earth compounds might become light interception sub-trapping centers and reduce the band gap compared with pure TiO 2 , which is important to retard the recombination of the electron-hole pairs and ultimately affect the photodegradation activity of the composites. The photodegradation experiment indicates that the doped rare-earth elements could enhance the removal efficiency of BHA from beneficiation wastewater. Each rare-earth element had its own optimum doped amount, and the optimum doping contents were 0.75%La/TiO 2 , 0.20%Ce/TiO 2 , 0.70%gd/TiO 2 , and 0.50%Yb/TiO 2 for La, Ce, Gd, and Yb doped composites respectively. The calculated result is basically in line with the experiment. However, there exists a big difference in the band gap between the experiment and computation because of the different ratio for Gd/Ti. Four types of RE/TiO 2 all demonstrate the high photocatalytic activity and excellent reusability for BHA after four runs.