Degradation and Mineralization of Benzohydroxamic Acid by ...

International Journal of

Environmental Research and Public Health Article

Degradation and Mineralization of Benzohydroxamic Acid by Synthesized Mesoporous La/TiO2 Xianping Luo 1,2,3, *, Junyu Wang 1,3 , Chunying Wang 1,2,3 , Sipin Zhu 1,3 , Zhihui Li 1,3 , Xuekun Tang 4 and Min Wu 2 1

2 3 4

*

Faculty of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China; [email protected] (J.W.); [email protected] (C.W.); [email protected] (S.Z.); [email protected] (Z.L.) Post-Doctoral Scientific Research Workstation of Western Mining Co. Ltd., Xining 810001, China; [email protected] Jiangxi Key Laboratory of Mining & Metallurgy Environmental Pollution Control, Ganzhou 341000, China School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China; [email protected] Correspondence: [email protected]; Tel.: +86-797-831-2706

Academic Editor: Miklas Scholz Received: 12 July 2016; Accepted: 26 September 2016; Published: 10 October 2016

Abstract: Rare earth element La-doped TiO2 (La/TiO2 ) was synthesized by the sol-gel method. Benzohydroxamic acid was used as the objective pollutant to investigate the photocatalytic activity of La/TiO2 . The physicochemical properties of the prepared materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, specific surface area and porosity, scanning electron microscopy and transmission electron microscopy. As a result, the doping of La could inhibit the crystal growth of TiO2 , increase its specific surface area and expand its response to visible light, thus improving its photocatalytic activity. La/TiO2 with the doping ratio of 0.75% calcined at 500 ◦ C, showing the highest photocatalytic activity to degrade benzohydroxamic acid under the irradiation of 300 W mercury lamp. About 94.1% of benzohydroxamic acid with the original concentration at 30 mg·L−1 was removed after 120 min in a solution of pH 4.4 with an La/TiO2 amount of 0.5 g·L−1 . Furthermore, 88.5% of the total organic carbon was eliminated after 120 min irradiation. In addition, after four recycling runs, La/TiO2 still kept high photocatalytic activity on the photodegradation of benzohydroxamic acid. The interfacial charge transfer processes were also hypothesized. Keywords: rare earth doped; La/TiO2 ; characterization; benzohydroxamic acid; photocatalytic degradation; mineralization

1. Introduction In the Gannan area of China, ion-type rare earth ore is widely distributed. Benzohydroxamic acid is a high-effect chelating collector of oxidized ore [1], which is widely used in the flotation of lead oxide ore, copper oxide and rare earth ores [2–4]. It belongs to the aromatic alkyl hydroxamic acid, due to the existence of the π-π conjugated effect, which enhances the stability of the chelate compound by increasing the electron cloud density of atomic oxygen. Benzohydroxamic acid has physiological toxicity due to its benzene ring structure, which increases the COD (Chemical oxygen demand) content of mineral processing wastewater [5]. At the same time, N element content is also increased, which would cause the eutrophication when accumulated in the water. Besides, benzohydroxamic acid is difficult to decompose, and it would remain for a few years or an even longer time once released into the water environment. Eventually, it would bring harm to the water environment and would need to be removed. Int. J. Environ. Res. Public Health 2016, 13, 997; doi:10.3390/ijerph13100997

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Int. J. Environ. Res. Public Health 2016, 13, 997

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Photocatalytic technology has a great application potential in sewage treatments [6]. Among the various semiconductor photocatalyst materials, TiO2 has been the most promising and widely used photocatalyst due to its unique optical property, strong oxidizing power, nontoxicity, low cost, and chemical stability [7]. However, the wide technological use of TiO2 is impaired by its wide band-gap (3.2 eV), which only uses about 5% of the solar light [8,9], and the recombination of photogenerated electron-hole pairs is easy to happen. These drawbacks largely restrict its photocatalytic efficiency [10,11]. Thus, the improvement of photo-catalytic efficiency is still a major challenge in the photocatalysis research field. Doping with ions in TiO2 lattice has been proven to be an efficient route to enhance photocatalytic activity [12]. The rare earth element has the special 4f electronic structure, which is able to produce a multi-electron configuration. Its oxide has many characteristics, such as crystal form, strong adsorption selectivity, electronic conductivity and thermal stability [13,14]. Some research has shown that the doping of rare earth ions could effectively improve the photocatalytic activity of TiO2 [15–17]. La-doped anatase TiO2 (101) surface tended to engender oxygen vacancies. The photoelectric conversion efficiency of dye-sensitized solar cells fabricated from 1 mol% La-doped TiO2 reached 6.72%, which improved efficiency by 13.5% compared with that of the cells fabricated from pure TiO2 [18]. Grujic-Brojcin et al. reported that La-doped TiO2 had shown a higher rate of degradation than pure TiO2 , with a maximum rate for 0.65 mol% La loading [19]. Li et al. studied the catalytic degradation of 4-chloroophenol with La/TiO2 . The results showed that the 10 wt% La/TiO2 has the highest percentage of 4-CP degradation (99.0%) [20]. The enhanced photocatalytic activity of La-doped TiO2 might be mainly due to the smaller particle size, larger specific surface area and total pore volume, as well as higher pore structure complexity. Therefore, numerous studies have been focused on the photocatalytic activities of rare earth doped TiO2 in recent years [21–23], but there are few studies in the treatment of flotation reagents. With the development of mining, mineral processing waste water has become the main source of pollution in the areas of the mine and its surroundings, especially the residual organic flotation reagent, which has high toxicity and high pollution. As far as we know, there is no similar report on the photodegradation of benzohydroxamic acid by the photocatlaytic oxidation technique. In this study, La doped TiO2 nanoparticles were prepared by the sol-gel method, and their intrinsic characteristics were analyzed by using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). The photocatalytic activity of La-doped TiO2 samples were evaluated by the degradation rate of benzohydroxamic acid. At the same time, the effects of doping ratio of La, calcination temperature, dosage of La/TiO2 , pH value, light intensity and other factors on the photocatalytic activity of La/TiO2 were investigated. 2. Materials and Methods 2.1. Materials The reagents used in this study were analytical grade: lanthanum nitrate (La(NO3 )3 ·6H2 O) and tetrabutyl titanate (Ti(OC4 H9 )4 ) were purchased from National Medicine Group Chemical Reagent Co. Ltd., Shanghai, China. Anhydrous ethanol (CH3 CH2 OH) and acetic acid (CH3 COOH) were purchased from Tianjin Damao Chemical Reagent Factory, Tianjin, China. Benzohydroxamic acid (C7 H7 NO2 ) was purchased from Shanghai EKEAR Bio@Tech Co. Ltd., Shanghai, China (the structure was listed as Figure S1 in the Supplementary Materials). All the experimental solutions were prepared with deionized water. 2.2. Preparation and Characterization In accordance with previous literatures and studies [24–26], the La/TiO2 catalyst was synthesized by the sol-gel process with the following procedure. A mixture solution of 10 mL tetrabutyl titanate dissolved in 15 mL anhydrous ethanol, with stirring for 30 min, was noted as solution


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A. Another solution containing 10 mL ethanol, 2 mL acetic acid, 2 mL deionized water, and metal salts (La(NO3 )3 ·6H2 O) in the required stoichiometry was noted as solution B. Solution B was added dropwise to solution A under acute agitation to form a transparent, homogeneous sol. The wet gel of La/TiO2 was obtained by continuous stirring. After ageing at room temperature for 2 days and then drying at 90 ◦ C, the xerogel was formed. The xerogel was crushed to get fine powder and further calcined in a furnace under air atmospheres. Heating rates were 5 ◦ C·min−1 for all samples with 6 h holding time at different temperatures (400 ◦ C, 450 ◦ C, 500 ◦ C) and a cooling rate of 10 ◦ C·min−1 to achieve the La/TiO2 nano-photocatalyst. The rare earth La-doping ratio was set to 0%, 0.25%, 0.50%, 0.75% and 1% according to the mass fraction of La to TiO2 . La/TiO2 was characterized with an X-ray diffractometer (XRD) (RigakU Miniflex, Tokyo, Japan) by using monochromatized Cu Kα radiation (λ = 0.15418 nm) under 50 kV and 80 mA with the 2θ altering from 10◦ to 80◦ . The X-ray photoelectron spectroscopy (XPS) analyses of the samples were performed on a ESCALAB 250XI Thermo type multi-function imaging electronic spectrometer (Thermo Scientific, Surrey, UK), monochromatic Al Kα (hv = 1486.6eV), 150 W power, 500 µm beam spot. UV-vis diffuse reflectance spectra were achieved by using a UV-vis spectrophotometer (UV-2550, Shimadzu Co. Ltd., Kyoto, Japan), the absorption spectra were referenced to BaSO4 , and the scanning range was 200–700 nm. The specific surface area and average pore size of La/TiO2 were measured by a low temperature N2 physical adsorption apparatus (Micromeritics ASAP 2460, Micromeritics Instrument Corp., Norcross, GA, USA). Scanning electron microscopy (SEM) images were obtained by using a TES-CAN VEGA TS-5130SB (Tescan Company, Brno, Czech Republic), the light source of electron-beam was tungsten lamp, and the voltage was 10–30 kV. The morphology of the catalyst was analyzed by a G2-20 Tecnai transmission electron microscope (FEI, Hillsboro, TX, USA). The total organic carbon (TOC) was detected by the total organic carbon analyzer (Vario TOC, German Elementar Company, Hanau, Germany). The FTIR spectra of the composite samples (as KBr pellets) were recorded in transmittance mode using a Nicolet iS5 type Fourier transform infrared spectrometer (Nicolet Instrument Corporation, Madison, WI, USA). The scanning range was 400–4000 cm−1 . 2.3. Photocatalytic Degradation of Benzohydroxamic Acid The photocatalytic degradation reaction was performed in the light-chemical reaction apparatus (Xujiang Electro-mechanical Plant, Nanjing, China). At first, the suspension was magnetically stirred for 30 min in the dark to ensure the adsorption-desorption equilibrium of benzohydroxamic acid on the catalysts. Then, the irradiation was provided by lamps of different intensity and different ranges of radiation (100 W mercury lamp, 300 W mercury lamp, and 500 W xenon lamp). Approximately 5 mL of reaction solution was taken at given time intervals and centrifuged. At last, UV-vis spectrophotometry was used to measure the absorbance of benzohydroxamic acid under wavelength of 229 nm. The removal efficiency (R) was calculated by Equation (1): R (%) =

C0 − Ct × 100% C0

(1)

where C0 is the initial concentration of benzohydroxamic acid, in mg·L−1 , and Ct is the concentration at reaction time t (min), in mg·L−1 . All the experiments were performed twice at least to control the errors of the experiments. 3. Results and Discussion 3.1. Characterization 3.1.1. XRD The XRD patterns of different doping amounts of La (a) and different calcination temperatures (b) of La/TiO2 are shown in Figure 1. The major peaks at 2θ values of 25.21◦ , 36.75◦ , 37.53◦ , 38.40◦ ,

3.1. Characterization 3.1.1. XRD The XRD patterns of2016, different Int. J. Environ. Res. Public Health 13, 997 doping

amounts of La (a) and different calcination temperatures 4 of 14 (b) of La/TiO2 are shown in Figure 1. The major peaks at 2θ values of 25.21°, 36.75°, 37.53°, 38.40°, 47.87°, 53.53°, 54.86°, 62.37°, 68.30°, 70.03° and 74.63° corresponded to diffractions of the (101), (103), ◦ 53.53◦ , 54.86◦ , 62.37◦ , 68.30◦ , 70.03◦ and 74.63◦ corresponded to diffractions of the (101), (103), 47.87 (004),, (112), (200), (105), (211), (204), (116), (220), (215) and (301) planes of anatase TiO2, respectively (004), (112), (200), (105), (211), (204), (220), that (215)the anddiffraction (301) planes of anatase 2 ,2 respectively (JCPDS card NO.21-1272) [27]. It is(116), observed peak of pureTiO TiO is relatively (JCPDS NO.21-1272) It is observed that theintensity diffraction peak of pure is relativelyofnarrow, narrow,card while it became[27]. broader and the relative decreased withTiO the2 increasing the La while it became broader and the relative intensity decreased with the increasing of the La doping doping amount, indicating a systematic decrease in grain sizes and the increase of specific surface amount, a systematic decrease in grain sizes than and the of specific surface area. area. Theindicating ionic radius of La3+ is 0.106 nm, which is larger thatincrease of Ti4+ (which is 0.068 nm), so the 3+ 4+ The ionic radius of La is 0.106 nm, which is larger than that of Ti (which is 0.068 nm), so the doping doping ions would be difficult to get into the TiO2 lattice. The crystallite sizes of the samples were ions would by be the difficult to get into theEquation TiO2 lattice. calculated Scherrer formula, (2): The crystallite sizes of the samples were calculated by the Scherrer formula, Equation (2): k kλ (2) DD= (2)  cos βcosθ

0.75 % 0.50 % 0.25 %

30

40

50

60

2 Theta / degree

70

(215) (301)

500 ℃

450 ℃ 400 ℃

0.00 % 20

(116) (220)

(204)

(105) (211)

(200)

(103) (004) (112)

Intensity / a.u.

Intensity / a.u.

1.00 %

(101)

(215) (301)

(116) (220)

(204)

(105) (211)

(b) (200)

(103) (004) (112)

(a)

(101)

where whereDDisiscrystallite crystallitesize; size;kkisisScherrer Scherrerconstant, constant,kk==0.89; 0.89;λλisiswavelength wavelengthof ofX-ray; X-ray;ββisisfull fullwidth widthatat half halfmaximum maximumofofthe thepeak peak(in (inradians); radians);and andθθisisangle angleofofdiffraction diffraction(in (indegrees). degrees).

80

20

30

40

50

60

70

80

2 Theta / degree

Figure1.1.X-ray X-raydiffractometer diffractometer(XRD) (XRD)patterns patternsofofthe thesamples: samples:(a) (a)La La withdifferent differentdopant dopantamounts amounts 3+3+with Figure ◦ 2 at different calcined temperatures. calcined at 500 °C; (b) 0.75% La/TiO calcined at 500 C; (b) 0.75% La/TiO2 at different calcined temperatures.

The crystallite size of pure TiO2 is 22 nm, and that of 0.75% La/TiO2 is 13 nm. As a result, the The crystallite size of pure TiO2 is 22 nm, and that of 0.75% La/TiO2 is 13 nm. As a result, doping of La could inhibit the crystal growth of TiO2. Compared with pure TiO2, there is no new the doping of La could inhibit the crystal growth of TiO2 . Compared with pure TiO2 , there is no phase for the doped catalyst, indicating that La3+ might be in the form of small oxide clusters highly new phase for the doped catalyst, indicating that La3+ might be in the form of small oxide clusters dispersed on the surface of TiO2, which would hinder the growth of TiO2 particles. The oxide also highly dispersed on the surface of TiO2 , which would hinder the growth of TiO2 particles. The oxide easily becomes light interception sub-trapping centers and ultimately affects the photocatalytic also easily becomes light interception sub-trapping centers and ultimately affects the photocatalytic activity [28]. activity [28]. 3.1.2. XPS 3.1.2. XPS The survey spectrum Figure 2a shows the predominant peaks corresponding to Ti, O and C. The The survey spectrum Figure 2a shows the predominant peaks corresponding to Ti, O and C. strength of La is not obvious due to the low doping amount. A carbon element may come from the The strength of La is not obvious due to the low doping amount. A carbon element may come from pollution of the X-ray photoelectron energy spectrum instrument. As seen from Figure 2b, after La the pollution of the X-ray photoelectron energy spectrum instrument. As seen from Figure 2b, after La doped, the main peaks at 853.92 and 836.79 eV are well in accordance with the standard XPS peaks doped, the main peaks at 853.92 and 836.79 eV are well in accordance with the standard XPS peaks of La3+. Figure 2c,d shows the Ti2p XPS spectra of pure TiO2 and 0.75% La/TiO2. The XPS spectrum of of La3+ . Figure 2c,d shows the Ti2p XPS spectra of pure TiO2 and 0.75% La/TiO2 . The XPS spectrum Ti2p for La/TiO2 could be fitted as two peaks that are composed by Ti2p1/2 and Ti2p3/2, which of Ti2p for La/TiO2 could be fitted as two peaks that are composed by Ti2p1/2 and Ti2p3/2, which implied that almost all of the Ti atoms exist in the form of +4 valences. The peaks of pure TiO2 are at implied that almost all of the Ti atoms exist in the form of +4 valences. The peaks of pure TiO2 are at 464.43 eV and 458.69 eV, but the peaks are at 464.41 eV and 458.65 eV after doping. As seen from Figure 2e,f, different peaks appear after the fitting of O1s, which indicates that the samples of oxygen exist in different forms. The fitted strong peak of O1s located at 529.91 eV was the lattice oxygen in TiO2 . The weak peak at 531.48 eV corresponded to the peak of adsorbed oxygen on the surface of TiO2 . After doped, the peaks of the O1s spectrum shifted slightly to 529.92 eV and 531.58 eV, while the binding energy of Ti2p was smaller than that of pure TiO2 . This might be due to the combination of La

464.43 eV and 458.69 eV, but the peaks are at 464.41 eV and 458.65 eV after doping. As seen from Figure 2e,f, different peaks appear after the fitting of O1s, which indicates that the samples of oxygen exist in different forms. The fitted strong peak of O1s located at 529.91 eV was the lattice oxygen in TiO2. The weak peak at 531.48 eV corresponded to the peak of adsorbed oxygen on the surface of TiOJ.2.Environ. After Res. doped, peaks Int. Publicthe Health 2016,of 13,the 997 O1s spectrum shifted slightly to 529.92 eV and 531.58 eV, while 5 of 14 the binding energy of Ti2p was smaller than that of pure TiO2. This might be due to the combination of La and Ti, which forms new chemical bonds of Ti-La, resulting in the electronic transfer from 4+ electronic and Ti, which new chemical bonds of Ti-La, resulting in Ti the transferreduced from titanium titanium atomforms to lanthanum atoms. The valence state of ions is slightly when 4+ ions is slightly reduced when incorporated into atom to lanthanum atoms. The valence state of Ti incorporated into the TiO2 lattice [29]. In addition, the doping of La combined with oxygen to form 3+ is the TiO [29]. In theSince doping La combined with oxygen form Laof surface 2 lattice 2O La2O 3 on the surface of addition, the sample. theofionic radius of La larger to than that Ti3 4+on , itthe is difficult 3+ is larger than that of Ti4+ , it is4+difficult for La3+ to replace 3+ 4+ of the sample. Since the ionic radius of La for La to replace Ti to form a stable solid solution [30]. However, Ti could enter the lattice of 4+ 4+ could enter the lattice of Ti form a stable solution [30]. of However, leading to the La2Oto 3, leading to thesolid charge imbalance the TiO2 Ti lattice and production of Ti3+ La [31]. well known 2 OIt 3 , is 3+ charge of the TiO2 lattice andimportant production Ti [31]. the It isexcited well known thattothe adsorption that theimbalance adsorption of oxygen is very foroftrapping electron suppress the of oxygen is very important for trapping the excited electron to suppress the recombination with hole. recombination with hole. Thus, the doped catalyst has higher activity than the non-doped Thus, the doped catalyst has higher activity than the non-doped counterpart. counterpart. (a)

853.92eV

Intensity / a.u.

La3d

Intensity / a.u.

836.79eV

La/TiO2

Ti2p

C1s 0.75%La/TiO2

Ti2p

(b)

In ten sity / a.u .

O1s

pure TiO2

458.69eV

(c)

464.43eV

TiO2

1000

800

600

400

200

0

870

458.65eV

464.41eV

(d)

850

O1s Intensity / a.u.

Intensity / a.u.

Ti2p

860

840

830

470 468 466 464 462 460 458 456 454 452 450

820

Binding Energy / eV

Binding Energy / eV

Binding energy / eV

529.91eV

531.48eV

(e)

O1s

Intensity / a.u.

1200

529.91eV

(f)

531.58eV

470 468 466 464 462 460 458 456 454 452 450

536 535 534 533 532 531 530 529 528 527 526

536 535 534 533 532 531 530 529 528 527 526

Binding Energy / eV

Binding Energy / eV

Binding Energy / eV

Figure 2. 2. X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS): (XPS): (a) survey spectra; of pure pure TiO TiO2;; Figure (a) survey spectra; (b) (b) La3d; La3d; (c) (c) Ti2p Ti2p of 2 (d) Ti2p Ti2p of of La/TiO La/TiO2;; (e) O1s of pure TiO2; (f) O1s of La/TiO2. (d) 2 (e) O1s of pure TiO2 ; (f) O1s of La/TiO2 .

3.1.3. UV-vis DRS 3.1.3. UV-vis DRS Figure 3 shows the UV-vis DRS of pure TiO2 and 0.75% La/TiO2 catalysts in the range of 200–700 Figure 3 shows the UV-vis DRS of pure TiO2 and 0.75% La/TiO2 catalysts in the range of nm. The data plots of absorption square versus energy in the absorption edge region are further 200–700 nm. The data plots of absorption square versus energy in the absorption edge region are further estimated in the inset of Figure 3. The spectra of La/TiO2 indicate a little red shift in the band-gap estimated in the inset of Figure 3. The spectra of La/TiO2 indicate a little red shift in the band-gap transition compared with pure TiO2. A red shift of this type can be attributed to the charge-transfer transition compared with pure TiO2 . A red shift of this type can be attributed to the charge-transfer transition between rare earth ions of 4f electrons and the TiO2 conduction or valence band. The light transition between rare earth ions of 4f electrons and the TiO2 conduction or valence band. The light response range extends to the visible light, and the electron-hole pairs increased by the enhancement response range extends to the visible light, and the electron-hole pairs increased by the enhancement of light absorption ability. So, the photocatalytic activity might be improved [32,33]. The square of of light absorption ability. So, the photocatalytic activity might be improved [32,33]. The square of the the absorption coefficient was linear with energy in the absorption edge region. Band-gap energies absorption coefficient was linear with energy in the absorption edge region. Band-gap energies were were deduced via extrapolating a straight line to the abscissa axis. The band-gap energy can be deduced via extrapolating a straight line to the abscissa axis. The band-gap energy can be calculated calculated by Equation (3) [34]. by Equation (3) [34]. n ahv = A(hv − Eg)n22 (3) (3) ahv  A(hv  Eg ) where a, h, v, A and Eg represent the absorption coefficient, Planck’s constant, light frequency, where a, h, v, A and Eg represent the absorption coefficient, Planck’s constant, light frequency, a a constant, and band-gap energy, respectively. The value of n is determined by the type of optical constant, and band-gap energy, respectively. The value of n is determined by the type of optical transition of the semiconductor (n = 1) for direct transition, and n = 4 for indirect transition. transition of the semiconductor (n = 1) for direct transition, and n = 4 for indirect transition. The bandThe band-gap energies of pure TiO2 and 0.75% La/TiO2 were estimated to be 3.09 eV and 3.34 eV, respectively. This showed that the doping of La could narrow the band-gap of TiO2 and reduce the band-gap energy, which is important to slow down the recombination rate of the electron-hole pairs and ultimately enhance the photocatalytic activity [35].

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Int. J. Environ. Res. Public Health 2016, 13, 997 Int. J. Environ. Res. Public Health 2016, 13, 997

0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 -0.1 -0.1

2 2 / (eV) (ahv) (ahv) / (eV)2 2

Absorbance / a.u. Absorbance / a.u.

gap energies of pure TiO2 and 0.75% La/TiO2 were estimated to be 3.09 eV and 3.34 eV, respectively. gap energies of pure TiO2 and 0.75% La/TiO2 were estimated to be 3.09 eV and 3.34 eV, respectively. This showed that the doping of La could narrow the band-gap of TiO2 and reduce the band-gap This showed that the doping of La could narrow the band-gap of TiO2 and reduce the band-gap energy, which is important Int. J. Environ. Res. Public Health 2016,to13,slow 997 down the recombination rate of the electron-hole pairs6 and of 14 energy, which is important to slow down the recombination rate of the electron-hole pairs and ultimately enhance the photocatalytic activity [35]. ultimately enhance the photocatalytic activity [35]. 3 3

0.75% 0.75% 0.00% 0.00%

2 2 1 1

0 2.96 3.04 3.12 3.20 3.28 3.36 0 2.96 3.04 3.12 3.20 3.28 3.36

Energy / eV Energy / eV

200 200

300 300

400 400

500 500

600 600

700 700

800 800

Wavelength / nm Wavelength / nm Figure 3. UV-vis diffuse reflectance spectroscopy (DRS)of of pureTiO TiO2 and 0.75% 0.75% La/TiO2.. Figure Figure 3. 3. UV-vis UV-vis diffuse diffuse reflectance reflectance spectroscopy spectroscopy(DRS) (DRS) ofpure pure TiO22and and 0.75%La/TiO La/TiO22.

0.0 0.0

0.2 0.2

0.4 0.6 0.8 0.4 0.6 0.8 Relative Pressure (P/P0) Relative Pressure (P/P0)

1.0 1.0

(b) (b)

-3

40 40 30 30

3 -1

-1

-1

3

3

Adsorption Adsorption Desorption Desorption

4 4 0 0

(a) (a)

3 -1 -1 dV/dw Volume nm-3) 10 ) dV/dw Pore Pore Volume / (cm/ g(cm nmg 10

8 8

0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 0.05 0.05 0.00 0 2 4 6 8 10 12 14 16 0.00 0 2 4Pore 6 width 8 10/ nm 12 14 16 Pore width / nm

Quantity Adsorbed (cm /g STP) Quantity Adsorbed (cm /g STP)

12 12

-3 -1

16 16

-3 -1 -1 dV/dw Volume nm-3)10 dV/dw Pore Pore Volume / (cm/ (gcmnmg 10

3

3

Quantity Adsorbed (cm /g STP) Quantity Adsorbed (cm /g STP)

-3

)

3.1.4. Specific Surface Area and Porosity Analysis 3.1.4. Specific Specific Surface Surface Area Area and 3.1.4. and Porosity Porosity Analysis Analysis As seen from Figure 4, the N2 adsorption-desorption isotherms are characteristic of the typical As seen isotherms are are characteristic characteristic of of the the typical As seen from from Figure Figure 4, 4, the the N N22 adsorption-desorption adsorption-desorption isotherms typical Langmuir IV isotherm with hysteresis loop [36,37], which indicate that the synthesized samples have Langmuir IV isotherm with hysteresis loop [36,37], which indicate that the synthesized samples Langmuir IV isotherm with hysteresis loop [36,37], which indicate that the synthesized samples have a mesoporous structure. The N2 adsorption-desorption isotherms of pure TiO2 and 0.75% La/TiO2 a mesoporous structure. N2 adsorption-desorption isotherms pure TiO 0.75%2 2 and ahave mesoporous structure. The N2The adsorption-desorption isotherms of pure of TiO 2 and 0.75% La/TiO show the H2-type hysteresis loop with uniform particle accumulation in the hole. Generally speaking, La/TiO show the H2-type hysteresis loop with uniform particle accumulation in the hole. Generally 2 show the H2-type hysteresis loop with uniform particle accumulation in the hole. Generally speaking, it is considered as an inkbottle shaped channel with a small mouth and large cavity [38]. The BJH speaking, it is considered as an shaped inkbottle shapedwith channel with a small mouth large cavity [38]. it is considered as an inkbottle channel a small mouth and large and cavity [38]. The BJH (Barrett-Joyner-Halenda) curve showed that the samples were with relatively narrow pore size The BJH (Barrett-Joyner-Halenda) curve showed the samples wererelatively with relatively narrow (Barrett-Joyner-Halenda) curve showed that thethat samples were with narrow pore pore size distribution, and the mesoporous ranges from 2–10 nm. The most probable pore size of pure TiO2 size distribution, and the mesoporous ranges from 2–10 nm. The most probable pore size of pure TiO distribution, and the mesoporous ranges from 2–10 nm. The most probable pore size of pure TiO22 was 6.99 nm, and the specific surface area was 10 m222·g−−1 . However, the most probable pore size of was g −11.. However, However, the the most most probable probable pore pore size size of was 6.99 6.99 nm, nm, and and the the specific specific surface surface area areawas was10 10mm··g of 0.75% La/TiO2 was 6.21 nm, and the specific surface area was 49 m222·g−−1 . The specific surface area of 1 0.75% La/TiO was 6.21 6.21 nm, nm, and and the the specific g −1. .The Thespecific specific surface surface area area of of 0.75% La/TiO22 was specific surface surface area area was was49 49m m ··g 3+ 0.75% La/TiO2 was significantly larger than that of pure TiO2. The relatively high surface area of La3+ 0.75% La/TiO was significantly larger than that of pure TiO . The relatively high surface area of La 0.75% La/TiO22was significantly larger than that of pure TiO22. The relatively high surface area of La3+ doped samples confirmed that the frameworks of TiO2 have better adsorption ability. This may be doped samples of of TiO better adsorption ability. ThisThis maymay be due 2 have doped samplesconfirmed confirmedthat thatthe theframeworks frameworks TiO 2 have better adsorption ability. be due to the linkage between the rare earth ions and titanium by oxygen bridge, which effectively to thetolinkage between the rare and titanium by oxygen effectively enhances due the linkage between theearth rareions earth ions and titanium by bridge, oxygenwhich bridge, which effectively enhances the specific surface area of TiO2 [39]. The larger surface area, the more surface reaction sites, the specific surface area of TiO The2 [39]. largerThe surface the area, more the surface which is 2 [39]. enhances the specific surface area of TiO largerarea, surface morereaction surface sites, reaction sites, which is beneficial to improve the photocatalytic activity. beneficial to improve photocatalytic activity. activity. which is beneficial to the improve the photocatalytic 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.00 0

4 8 12 16 4Pore8width 12/ nm16

20 20

Pore width / nm

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Adsorption Adsorption Desorption Desorption

10 10 0.0 0.0

0.2 0.2

0.4 0.6 0.8 0.4 0.6 0.8 Relative Pressure (P/P0) Relative Pressure (P/P0)

1.0 1.0

Figure 4. N2 adsorption-desorption isotherms and pore size distributions (inset) of pure TiO2 (a) and Figure 4. N isotherms TiO2 (a) and FigureLa/TiO 4. N22 adsorption-desorption adsorption-desorption isotherms and and pore pore size size distributions distributions (inset) (inset) of of pure pure TiO 2 (b). 0.75% 2 (a) and 2 (b). 0.75% La/TiO 0.75% La/TiO2 (b).

3.1.5. Microstructure Analysis Figure 5a,b shows the particulate morphology of pure TiO2 and the 0.75% La/TiO2 . They display an irregular structure and contain a mixture of shaped particles [40]. The particle size of La/TiO2


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Figure 5a,b shows the particulate morphology of pure TiO2 and the 0.75% La/TiO2. They display an irregular structure and contain a mixture of shaped particles [40]. The particle size of La/TiO2 was significantly smaller thanthan that that of pure TiO2TiO , and was better than than that of pure TiO2.TiO The2 . was significantly smaller of pure anddispersion the dispersion was better that of pure 2 , the doping of Laofmay have have decreased the particle size, increased the surface area and dispersion, which The doping La may decreased the particle size, increased the surface area and dispersion, is in accordance with the results of XRD. which is in accordance with the results of XRD.

Figure 5. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy Figure 5. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of samples: (a) SEM image of pure TiO2; (b) SEM image of 0.75% La/TiO2; (c) (HRTEM) images of samples: (a) SEM image of pure TiO2 ; (b) SEM image of 0.75% La/TiO2 ; (c) HRTEM HRTEM micrograph of pure TiO2; (d) HRTEM micrograph of 0.75% La/TiO2. micrograph of pure TiO2 ; (d) HRTEM micrograph of 0.75% La/TiO2 .

Figure 5c,d shows a high-resolution transmission electron microscopy (HRTEM) diagram of Figure 5c,d shows a high-resolution transmission electron microscopy (HRTEM) diagram of pure pure TiO2 and the 0.75% La/TiO2. In Figure 5c, the lattice fringe spacing of pure TiO2 is 0.349 nm and TiO2 and the 0.75% La/TiO2 . In Figure 5c, the lattice fringe spacing of pure TiO2 is 0.349 nm and 0.209 nm, corresponding to the lattice planes (101) and (004) of anatase phase, respectively. In Figure 0.209 nm, corresponding to the lattice planes (101) and (004) of anatase phase, respectively. In Figure 5d, 5d, the lattice fringe spacing of 0.75% La/TiO2 is mainly 0.352 nm and 0.237 nm. The average grain the lattice fringe spacing of 0.75% La/TiO2 is mainly 0.352 nm and 0.237 nm. The average grain size size of TiO2 was about 13–22 nm, which is consistent with the results of XRD analysis. The clear crystal of TiO was about 13–22 nm, which is consistent with the results of XRD analysis. The clear crystal lattice 2fringe suggests that the sample has good crystallinity. The fast Fourier transform (FFT) image lattice fringe suggests that the sample has good crystallinity. The fast Fourier transform (FFT) image in the inset of Figure 5d indicates that the sample is in a well-organized mesophase [41,42]. The FFT in the inset of Figure 5d indicates that the sample is in a well-organized mesophase [41,42]. The FFT pattern also suggests the single crystal diffraction point, obviously. This suggests that the prepared pattern also suggests the single crystal diffraction point, obviously. This suggests that the prepared mesostructure is a cubic phase oriented along the (101) and (004) directions, respectively [43,44]. mesostructure is a cubic phase oriented along the (101) and (004) directions, respectively [43,44]. 3.2. Photocatalytic Degradation of Benzohydroxamic Acid 3.2. Photocatalytic Degradation of Benzohydroxamic Acid 3+ 3.2.1. 3.2.1. Effect Effect of of La La3+Doping DopingAmount Amount 3+ doping amounts on photocatalytic degradation of benzohydroxamic The of different La3+ The effects effects of different La doping amounts on photocatalytic degradation of benzohydroxamic acid are as shown in Figure 6. acid are as shown in Figure 6. As ofof benzohydroxamic acid on on La/TiO 2 is negligible. The As observed observed in inFigure Figure6a, 6a,the theadsorption adsorption benzohydroxamic acid La/TiO 2 is negligible. photocatalytic activity of La/TiO 2 is higher than that of pure TiO2. The degradation efficiency of the The photocatalytic activity of La/TiO2 is higher than that of pure TiO2 . The degradation efficiency target first increased and then decreased as the doping La from to 1.00%, of the pollutant target pollutant first increased and then decreased as theamount doping of amount of0.00% La from 0.00% and 0.75% La/TiO 2 indicated the highest photocatalytic activity. The increasing of doped La3+ amount to 1.00%, and 0.75% La/TiO indicated the highest photocatalytic activity. The increasing of doped

2

La3+ amount would lead to an expansion of TiO2 lattice, which might produce crystal defects and distortion. Then, energy band structure would change and the recombination rate of electron-hole

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would would lead lead to to an an expansion expansion of of TiO TiO22 lattice, lattice, which which might might produce produce crystal crystal defects defects and and distortion. distortion. Then, Then, energy band structure would change and the recombination rate of electron-hole pairs would energy band structure would change and the recombination rate of electron-hole pairs would 3+ pairs would decrease. However, would too many due todistortion the latticewhen distortion when decrease. However, there would be many defects due to excess La decrease. However, there would there be too too manybe defects due defects to the the lattice lattice distortion when excess La3+ 3+ excess La was doped, and the photocatalytic activity would be inhibited. The degradation results was and activity be The degradation results the was doped, doped, and the the photocatalytic photocatalytic activity would would be inhibited. inhibited. The degradation results give give that that the give that the optimal doping ratio is 0.75%, which corresponded to the results of UV-vis DRS. In brief, optimal doping ratio is 0.75%, which corresponded to the results of UV-vis DRS. In brief, the optimal doping ratio is 0.75%, which corresponded to the results of UV-vis DRS. In brief, the the appropriate doping amount only avoids thewaste wasteof rareearth earthelements, elements,but but also also improves appropriate La doping amount not only avoids the appropriate La La doping amount notnot only avoids the waste ofofrare rare earth elements, but also improves the photocatalytic activity. In addition, the mineralization of benzohydroxamic acid was up to to the photocatalytic photocatalytic activity. activity. In In addition, addition, the the mineralization mineralization of of benzohydroxamic benzohydroxamic acid acid was was up up to 88.5% 88.5% 88.5% by 0.75% La/TiO , as depicted in Figure Zhou studiedthe the biodegradation biodegradation of by La/TiO 2, as 2 depicted in Figure 6b. 6b. Zhou andand HuHustudied by 0.75% 0.75% La/TiO 2, as depicted in Figure 6b. Zhou and Hu studied the biodegradation of benzonhydroxamic days to degrade more than 85% acid. They needed five or more benzonhydroxamic acid. They needed five or more days to degrade more than 85% of of the the pollutant pollutant under the conditions of additional nutritions [45,46]. So, the degradation of benzohydroxamic acid by the conditions of additional nutritions [45,46]. So, the degradation of benzohydroxamic acid under the conditions of additional nutritions [45,46]. So, the degradation of benzohydroxamic acid photocatalytic oxidation with TiO -based catalysts has a significant advantage. by photocatalytic oxidation with TiO 2-based catalysts has a significant advantage. 2 by photocatalytic oxidation with TiO2-based catalysts has a significant advantage.

(a) (a)

100 100

80 80 60 60

Blank Blank 0.00% La/TiO2 0.00% La/TiO2 0.25% La/TiO2 0.25% La/TiO2 0.50% La/TiO2 0.50% La/TiO2 0.75% La/TiO2 0.75% La/TiO2 1.00% La/TiO2 1.00% La/TiO2 with 0.75% La/TiO2 no irradiation with 0.75% La/TiO2 no irradiation

40 40 20 20 00 00

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Degradationefficiency efficiency/ /%% Degradation

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0.75% 0.75% La/TiO La/TiO22

20 20 00 00

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Time Time // min min

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Figure 6. 6. (a) La doping Figure doping ratio ratio on on the the photodegradation photodegradation of of benzohydroxamic acid by by (a) Effect Effect of of different different La benzohydroxamic acid La/TiO 2, pure TiO2 and blank. Reaction conditions: C La/TiO (b)TOC TOCmineralization mineralizationrates ratesof of0.75% 0.75%La/TiO La/TiO , pure TiO and blank. Reaction conditions: La/TiO222;; ;(b) (b) TOC mineralization rates of 0.75% La/TiO 2, 2pure TiO 2 and blank. Reaction conditions: C 2 −1− −1 1 − 1 (benzohydroxamic acid) = 30 mg·L , C (catalyst dosage) = 0.3 g·L , 300 W mercury lamp, calcined −1 −1 C (benzohydroxamicacid) acid)==30 30mg·L mg·L , C, C(catalyst (catalystdosage) dosage)==0.3 0.3 g·L g·L , 300 , 300W Wmercury mercury lamp, lamp, calcined (benzohydroxamic temperature at at 500 500 ◦°C. temperature C. °C.

3.2.2. Effect of Light Light Intensity Intensity 3.2.2. Effect of The effects effects of different light intensity photocatalytic degradation of The photocatalytic degradation of benzohydroxamic acidacid are effectsof ofdifferent differentlight lightintensity intensityonon on photocatalytic degradation of benzohydroxamic benzohydroxamic acid are as shown in Figure 7. as shown in Figure 7. are as shown in Figure 7.

Degradationefficiency efficiency/ /%% Degradation

100 100 80 80 60 60 300W 300W mercury mercury lamp lamp 100W 100W mercury mercury lamp lamp 500W xenon lamp 500W xenon lamp

40 40 20 20 00 00

20 20

40 40

60 80 60 80 Time / min Time / min

100 100

120 120

Figure 7. Effect of light intensity on photocatalytic degradation of benzohydroxamic acid. Reaction Figure 7. 7. Effect Effect of of light light intensity intensity on on photocatalytic photocatalytic degradation degradation of of benzohydroxamic benzohydroxamic acid. acid. Reaction Reaction Figure −1 −1 conditions: C (benzohydroxamic acid) == 30 mg·L (catalyst dosage) = 0.3 g·L La/TiO −1,, C −1,, 0.75% conditions: C (benzohydroxamic acid) 30 mg·L C (catalyst dosage) = 0.3 g·L 0.75% La/TiO22,,, conditions: C (benzohydroxamic acid) = 30 mg·L−1 , C (catalyst dosage) = 0.3 g·L−1 , 0.75% La/TiO 2 calcined temperature at 500 °C. calcined temperature temperature at at 500 500 ◦°C. calcined C.

As As can can be be seen seen from from Figure Figure 7, 7, the the degradation degradation rate rate of of benzohydroxamic benzohydroxamic acid acid under under the the As can of beaseen from Figure 7, better the degradation rate of benzohydroxamic acid under the irradiation irradiation mercury lamp is than under a xenon lamp. The light intensity of irradiation of a mercury lamp is better than under a xenon lamp. The light intensity of the the 300 300 W W of a mercury lamp is better than under a xenon lamp. The light intensity of the 300 W mercury lamp

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mercury lamp and the 100 W mercury lamp were 5.6 mW·cm−2 and 0.46 mW·cm−2, respectively, while 2 , respectively, −2. ·The the light intensity of the lamp 500 Wwere xenon was−239.5 distribution the and the 100 W mercury 5.6lamp mW·cm andmW·cm 0.46 mW cm−wavelength while theoflight − 2 mercury of lamps and xenon is listed in Table and Figure S2 respectively in the intensity the 500 W the xenon lamplamp was 39.5 mW·cm . The S1 wavelength distribution of the mercury supplementary material. Theis photocatalytic the increase light intensity lamps and the xenon lamp listed in Tableactivity S1 and increased Figure S2 with respectively in theofSupplementary under theThe same light source.activity Becauseincreased the light-response range of 2 is mainly in the Material. photocatalytic with the increase of La/TiO light intensity under the ultraviolet same light or near Because ultraviolet the number increaseorofnear light intensity,band, and source. the band, light-response rangeofofphotons La/TiO2increased is mainly with in thethe ultraviolet ultraviolet holes increased accordingly. more ·OH.and So holes the increased degradation rate of the number of photons increasedThis withproduced the increasemuch of light intensity, accordingly. benzohydroxamic acid increased. This produced much more ·OH. So the degradation rate of benzohydroxamic acid increased. 3.2.3. Effect of Catalyst Dosage Figure 8 shows the variations variations in the ratio ratio of of degradation degradation at different different dosages dosages of photocatalyst photocatalyst 1 . It can be seen that the photocatalytic degradation efficiency increases −1− ranging from from 00to to0.7 0.7g·L g·L . It can be seen that the photocatalytic degradation efficiency increases with with an increasing amount of the 0.75% La/TiO and reachesthe thehighest highestvalue value when when the an increasing amount of the 0.75% La/TiO 2 photocatalyst and reaches 2 photocatalyst concentration is 0.5 ·L−−11. .There Thereare are three three reasons to explain this: 0.5 gg·L this: (1) the the smaller smaller dosage dosageof ofLa/TiO La/TiO22 generates less electron-hole pairs, which leads to the lower photocatalytic activity; (2) the availability of active sites increase dosage; (3) overload of the photocatalysts would increase with with the the increase increase of of La/TiO La/TiO22 dosage; decrease the light penetration and increase radiation scattering by the suspension catalyst and finally reduce the degradation rate.

Degradation efficiency / %

100 80 0.0g/L 0.1g/L 0.3g/L 0.5g/L 0.7g/L

60 40 20 0 0

20

40

60

80

100

120

Time / min

Figure 8. 8. Effect Effect of of La/TiO La/TiO2 dosage on photocatalytic degradation of benzohydroxamic acid. Reaction Figure 2 dosage on photocatalytic degradation of benzohydroxamic acid. Reaction −1 conditions: C (benzohydroxamic acid) 30mg mg·L W mercury mercury lamp, lamp, 0.75% 0.75%La/TiO La/TiO2,, calcined calcined conditions: C (benzohydroxamic acid) == 30 ·L−1,, 300 300 W 2 temperature at 500 °C. ◦ temperature at 500 C.

3.2.4. Effect of Initial pH Value of Solution 3.2.4. Effect of Initial pH Value of Solution As can be seen from Figure 9, the pH values have different effects on the degradation of As can be seen from Figure 9, the pH values have different effects on the degradation of benzohydroxamic acid when the pH value was adjusted by a different regulator. In general, the benzohydroxamic acid when the pH value was adjusted by a different regulator. In general, the highest highest photocatalytic activity can be reached when the pH of benzohydroxamic acid solution is 4.43 photocatalytic activity can be reached when the pH of benzohydroxamic acid solution is 4.43 (original (original pH value of 30 mg·L−1 benzohydroxamic acid solution). The basic solution adjusted by pH value of 30 mg·L−1 benzohydroxamic acid solution). The basic solution adjusted by NaOH almost NaOH almost shows no effect on the degradation of benzohydroxamic acid. However, there exists shows no effect on the degradation of benzohydroxamic acid. However, there exists an interesting an interesting phenomenon: the degradation of benzohydroxamic acid was suppressed at stronger phenomenon: the degradation of benzohydroxamic acid was suppressed at stronger acidic conditions acidic conditions with HNO3 as the regulator (Figure 9a), while there was almost no change with HCl with HNO3 as the regulator (Figure 9a), while there was almost no change with HCl as the regulator as the regulator (Figure 9b). In order to clarify the difference, another experiment using two anions (Figure 9b). In order to clarify the difference, another experiment using two anions including Cl− and including Cl− and NO3− in their sodium salt form was designed to investigate different effects on the NO3 − in their sodium salt form was designed to investigate different effects on the photodegradaton photodegradaton of benzohydroxamic acid by La/TiO2 (the relative figure is listed as Figure S3 in the of benzohydroxamic acid by La/TiO2 (the relative figure is listed as Figure S3 in the Supplementary Supplementary Materials). As the result, NO3− indicates obviously inhibitory effect compared with Materials). As the result, NO3 − indicates obviously inhibitory effect compared with Cl− . As seen from − Cl . As seen from the structure of benzohydroxamic acid (Figure S1), there exist N-containing the structure of benzohydroxamic acid (Figure S1), there exist N-containing functional groups in the functional groups in the molecular. NO3−, added from HNO3 or NaNO3, would compete the adsorptive sites on the surface of La/TiO2 with the N-containing functional groups from the target pollutant.


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−Public Health 2016, 13, 997 Int. J. Environ. Res. 10 of 14 molecular. NO 3 , added from HNO3 or NaNO3 , would compete the adsorptive sites on the surface of La/TiO2 with the N-containing functional groups from the target pollutant. 100

(b)

80100

Degradation efficiency / % Degradation efficiency / %

80100

Degradation efficiency / % Degradation efficiency / %

100

(a) (a)

(b)

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Figure 9. Effect of initial pH value on photocatalytic degradation of benzohydroxamic acid: (a) the Figure 9. Effect of initial by pHHNO value3 on photocatalytic degradation benzohydroxamic acid: (a)NaOH). the pH and (b) the pH valueofwas by HCl and pH value adjusted Figure 9.was Effect of initial pH value onNaOH; photocatalytic degradation ofadjusted benzohydroxamic acid: (a) the −1 −1 value wasconditions: adjusted byCHNO and NaOH; (b) the pH value was adjusted by HCl and NaOH). Reaction Reaction (benzohydroxamic acid) = 30 mg·L , C (catalyst dosage) = 0.5 g·L , 300 W 3 pH value was adjusted by and NaOH). pH value was adjusted by HNO3 and NaOH; (b) −1 , the −1 ,HCl conditions: C (benzohydroxamic acid) = 30 mg · L C (catalyst dosage) = 0.5 g · L 300 W mercury mercury lamp, 0.75% La/TiO 2, calcined temperature at 500 °C. Reaction conditions: C (benzohydroxamic acid) = 30 mg·L−1, C (catalyst dosage) = 0.5 g·L−1, 300 W lamp, 0.75% La/TiO2 , calcined temperature at 500 ◦ C. mercury lamp, 0.75% La/TiO2, calcined temperature at 500 °C.

3.2.5. The Reusability of Photocatalyst 3.2.5. The ofofPhotocatalyst 3.2.5. TheReusability Reusability Photocatalyst Except for the activity of photocatalysts, the reusability is meaningful to investigate for their Except for the activity of the isismeaningful for practical application. Therefore, four successive recycling tests for tothe degradation of Except for the activity ofphotocatalysts, photocatalysts, thereusability reusability meaningful toinvestigate investigate fortheir their practical application. Therefore, four successive recycling tests for the degradation of benzohydroxamic benzohydroxamic acid by Therefore, La/TiO2 were performed. As shown in Figure the the removal efficienciesof practical application. four successive recycling tests10,for degradation acid by La/TiO were performed. As shown in Figure 10, the removal efficiencies were 93.9%, 93.6%, were 93.9%, 93.6%, 90.7%, and 92.7% for the first to the fourth runs, respectively. The degradation 2 benzohydroxamic acid by La/TiO2 were performed. As shown in Figure 10, the removal efficiencies 90.7%, and 92.7% for the first to the fourth runs, respectively. The degradation efficiency decreased efficiency decreased about 3.2% after four cycles. A gradually decreasing trend can be found from were 93.9%, 93.6%, 90.7%, and 92.7% for the first to the fourth runs, respectively. The degradation about 3.2% after four cycles. A3.2% gradually decreasing trend can be found from the results degradation the results of degradation efficiency, thecycles. differences among the fourth runs were not obvious, efficiency decreased about afterbut four A gradually decreasing trend canofbe found from efficiency, but the differences the fourth runs were not obvious, which indicates that which indicates that La/TiO 2 among possesses a good stability and reusability in the photodegradation of2 the results of degradation efficiency, but the differences among the fourth runs were notLa/TiO obvious, possesses a good stability and reusability in the photodegradation of benzohydroxamic acid. benzohydroxamic acid.La/TiO2 possesses a good stability and reusability in the photodegradation of which indicates that benzohydroxamic acid.

1.0 1.0 0.8 0.8 0.6

C/C C/C0 0

0.6 0.4

1st 1st

2nd

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0.00 40 93.9% 80 120 80 120 0 4092.7% 80 120 0 4090.7% 80 120 0 4093.6%

Time / min

0 40 80 120 0 40 80 120 0 40 80 120 0 40 80 120

Figure 10. Recycle the photocatalytic photocatalytic degradation degradation of benzohydroxamic benzohydroxamic acid acid by by La/TiO La/TiO2.. Time / min of Figure 10. Recycle runs runs in in the 2 −11, C (catalyst dosage) = 0.5 g·L− −11, 300 W Reaction conditions: C (benzohydroxamic acid) = 30 mg·L − Reaction conditions: (benzohydroxamic acid) =degradation 30 mg·L , Cof(catalyst dosage) = 0.5 g·Lby ,La/TiO 300 W 2. Figure 10. Recycle Cruns in the photocatalytic benzohydroxamic acid ◦ C. 2, calcined temperature at 500 °C. mercury lamp, 0.75% La/TiO La/TiO −1, C (catalyst dosage) = 0.5 g·L−1, 300 W mercury lamp, 0.75% , calcined temperature at 500 Reaction conditions: C (benzohydroxamic acid) = 30 mg·L 2

mercury lamp, 0.75% La/TiO2, calcined temperature at 500 °C.

3.2.6. Fourier Transform Infrared Spectroscopy (FTIR) Analysis and Interfacial Charge Transfer 3.2.6. Fourier Transform Infrared Spectroscopy (FTIR) Analysis and Interfacial Charge Processes Transfer Processes 3.2.6. Fourier Transform Infrared Spectroscopy (FTIR) Analysis and Interfacial Charge Transfer Processes Figure 11 shows shows the the FTIR FTIR spectra spectra of of 0.75% 0.75% La/TiO La/TiO22 (Curve hours’ photocatalytic photocatalytic Figure 11 (Curve a) a) and and after after 22 hours’ degradation of benzohydroxamic acid by 0.75% La/TiO 2 (Curve b). The following information could degradation acid byof 0.75% La/TiO b).a)The information could Figureof 11benzohydroxamic shows the FTIR spectra 0.75% La/TiO 2 (Curve andfollowing after 2 hours’ photocatalytic 2 (Curve −1 −1 , be given analysis of FTIR spectra [47,48]: the characteristic peak of TiO 2 is 400–800 cm , caused bedegradation givenby bythe the analysis of FTIR spectra [47,48]: the characteristic peak of TiO is 400–800 cm of benzohydroxamic acid by 0.75% La/TiO2 (Curve b). The following 2 information could bybethe stretching vibration and bending vibration of Ti-O-Ti and Ti-O bond; the absorption located −1 given by the analysis of FTIR spectra [47,48]: the characteristic peak of TiO2 is 400–800 cm , caused atby3435 cm−1 characterizes the bending hydroxylvibration groups of of Ti-O-Ti Ti-OH and at weak surface sites, located with the stretching vibration and Ti-O bond; theactive absorption at 3435 cm−1 characterizes the hydroxyl groups of Ti-OH at weak surface active sites, with


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11 of 14 and Ti-O bond; the absorption 11 of 14 weak surface active sites, with physisorbedwater watermolecules moleculesbound boundby byweak weakhydrogen hydrogenbonds bondswith withOH OH–– –groups groupsof TiO22 2surfaces; surfaces;the the physisorbed water molecules bound by weak hydrogen groups ofofTiO TiO surfaces; the physisorbed bonds with OH −1 − 1 characteristic peaks of 1117, 1269 and 1722 cm corresponding to the Ti-O-C, the C-O, and C=O in characteristic peaks peaks of of 1117, 1117, 1269 1269 and correspondingto tothe theTi-O-C, Ti-O-C, the the C-O, C-O, and and C=O C=O in in characteristic and 1722 1722 cm cm−1 corresponding −1 − 1 may Curvebbbare arestronger strongerthan thanthose thosein Curvea; 730cm cm maybe beaaalong longcarbon carbonchain chainof ofCH CH222in inCurve Curveb; b; −1 may Curve are stronger be long carbon chain of CH in Curve b; Curve than those ininCurve Curve a;a;730 730 cm −1 and 2961 cm −1 − 1 − 1 stretching vibration of CH 3 groups is around 2926 cm . All the information above stretching vibration vibration of of CH and 2961 All the the information information above above stretching CH3 groups groups is is around around 2926 2926cm cm−1 and 2961 cm cm−1. . All could suggest that the benzene ring in benzohydroxamic acid had been fractured, and some alkanes could suggest that the benzene ring in benzohydroxamic acid had been fractured, and some alkanes or could suggest that the benzene ring in benzohydroxamic acid had been fractured, and some alkanes or ester compounds had been generated by the photocatalytic reaction. ester compounds had been generated byby thethe photocatalytic reaction. or ester compounds had been generated photocatalytic reaction.

4000 4000

1019 1019

1462 1462

1722 1722 1631 1631 1382 1382 1269 1269 1117 1117

2957 2926 2957 2929 2926 2929 2861 2861

3435 3435

Transmittance / % Transmittance / %

a a b b

730 730

Int. J. Environ. Res.stretching Public Healthvibration 2016, 13, 997 caused by the and bending vibration of Ti-O-Ti Int. J. Environ. Res. Public Health 2016, 13, 997 located at 3435 cm−1 characterizes the hydroxyl groups of Ti-OH at

3000 3000

2000 2000

-1

1000 1000

Wavenumber / cm -1 Wavenumber / cm

Figure11. 11. Fourier Fourier Transform Infrared Spectroscopy (FTIR) spectra of samples: (a) 0.75% La/TiO 2; (b) Figure TransformInfrared InfraredSpectroscopy Spectroscopy(FTIR) (FTIR) spectra samples: 0.75% La/TiO 2; Figure 11. Fourier Transform spectra ofof samples: (a) (a) 0.75% La/TiO 2; (b) photocatalytic degradation of benzohydroxamic acid by 0.75% La/TiO 2 after 2 h. (b) photocatalytic degradation of benzohydroxamic by 0.75% La/TiO 2 after photocatalytic degradation of benzohydroxamic acidacid by 0.75% La/TiO 2 after 2 h. 2 h.

Figure 12illustrates illustratesthe the possibleinterfacial interfacial chargetransfer transferprocesses. processes.Element Element Laexists exists inthe the Figure Figure 12 12 illustrates the possible possible interfacial charge charge transfer processes. Element La La exists in in the 3+ stable formofofLa La3+asasdescribed describedininthe the sectionofofXRD XRD andXPS, XPS, whichmight might trapthe the photoexcited stable stable form form of La3+ as described in the section section of XRD and and XPS, which which might trap trap the photoexcited photoexcited 2+. However, La2+ is relatively unstable: the electrons can be easily detrapped electrons to produce La 2+. However, La2+ 2+ 2+ electrons to produce La is relatively unstable: the electrons can be easily electrons to produce La . However, La is relatively unstable: the electrons can be easily detrapped detrapped and transfer to O 2 adsorbed on the surface of TiO2 to produce the ·O−2− [49]. ·O−2− and the photoexcited − and to O O22 adsorbed adsorbed on onthe thesurface surfaceofofTiO TiO 2to to produce the ·O 2 [49]. [49]. ·O 2 −and the photoexcited and transfer transfer to produce the · O · O and the photoexcited 2 2 − or H2O2 to produce holes wouldreact reactwith withHH2O, 2O, OH ·OH.Then, Then,2benzohydroxamic benzohydroxamic acid wouldbe be −, ,or holes OH− acid holes would would react with H2 O, OH , orH H22OO2 2totoproduce produce·OH. ·OH. Then, benzohydroxamic acid would would be 3+ could increase the electron transfer from oxided to products including CO 2 and H2O. The doped La 3+could oxided CO22 and oxided to to products products including including CO and H H22O. O. The The doped doped La La3+ couldincrease increasethe theelectron electrontransfer transfer from from the surface of the catalyst and decrease the recombination of photogenerated electrons andholes. holes. the of photogenerated photogenerated electrons electrons and and the surface surface of of the the catalyst catalyst and and decrease decrease the the recombination recombination of holes.

hv hv

CB eeCB

TiO2 TiO 2

OO2 2

La2+2+ La

-

OH2O, ,OH HH2O

ee-

La3+3+ La

+ VB hh+ VB

· O-2- , H2O2 ·O 2 , H2O2

OH · ·OH · OH + · OH + ++ Benzohydroxamic acid Benzohydroxamic acid Products Products Figure Figure12. 12.Schematic Schematicrepresentation representationofofthe theinterfacial interfacialcharge chargetransfer transferprocesses processesininthe theLa/TiO La/TiO22.. Figure 12. Schematic representation of the interfacial charge transfer processes in the La/TiO2.

Conclusions 4.4.Conclusions La-dopedTiO TiO2 2photocatalysts photocatalystswere weresynthesized synthesizedby bythe thesol-gel sol-gelmethod. method.XRD XRDdiffraction diffractionpeaks peaksofof La-doped doped TiO 2 were broader and the relative intensity was weaker than pure TiO2. It might improve the doped TiO2 were broader and the relative intensity was weaker than pure TiO2. It might improve the thermalstability stabilityofofthe theanatase anatasephase phaseofofTiO TiO2,2,suppress suppressparticle particleaggregation, aggregation,grain graingrowth growthofofTiO TiO2 2 thermal and increase the specific area of TiO 2. The red shift of La/TiO2 in the band-gap transition could reduce and increase the specific area of TiO2. The red shift of La/TiO2 in the band-gap transition could reduce


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4. Conclusions La-doped TiO2 photocatalysts were synthesized by the sol-gel method. XRD diffraction peaks of doped TiO2 were broader and the relative intensity was weaker than pure TiO2 . It might improve the thermal stability of the anatase phase of TiO2 , suppress particle aggregation, grain growth of TiO2 and increase the specific area of TiO2 . The red shift of La/TiO2 in the band-gap transition could reduce gap energy of TiO2 and improve the response strength and threshold value of TiO2 for visible light and extend its absorption side band. La/TiO2 also had a larger specific surface area and a more regular shape in morphology. Furthermore, 0.75% La/TiO2 (500 ◦ C) indicated the highest photocatalytic degradation ability to benzohydroxamic acid: the removal rate and mineralization efficiency of benzohydroxamic acid reached 94.1% and 88.5%, respectively, at the conditions of pH 4.43, 30 mg·L−1 of benzohydroxamic acid, 0.5 g·L −1 of catalyst, and the irradiation of 300 W mercury lamp. The doping of La3+ could reduce the recombination of photoexited electrons and holes, resulting in the improvement of removal efficiency of benzohydroxamic acid by La/TiO2 . Supplementary Materials: The following are available online at www.mdpi.com/1660-4601/13/10/997/s1, Table S1: The emission range of 100 W and 300 W Mercury, Figure S1: Structure of benzohydroxamic acid, Figure S2: The emission range of the 500 W xenon lamp, Figure S3: Effect of inorganic anions Cl− and NO3 − on photocatalytic degradation of benzohydroxamic acid. Acknowledgments: The authors gratefully acknowledge the financial support of “Twelfth five-year” national science and technology support programme (2012 BAC11b07); The ministry of education in the new century excellent talents to support plan (NCET-10-0183); National natural science foundation of China (51408277); “Jiangxi province talent project 555” Talents training plan; Jiangxi province natural science fund project (20122 BAB203027, 20142BAB213019) and China’s Postdoctoral Science Fund (2015M582776XE, 2016T90967). Author Contributions: Xianping Luo and Chunying Wang presented the original idea for the study. Junyu Wang, Sipin Zhu and Zhihui Li carried out the experiment, analyzed the data and drafted the manuscript. Chunying Wang, Xuekun Tang and Min Wu conducted the discussion, interpreted the data and modified the manuscript. All authors have read and agreed the submission. Conflicts of Interest: The authors declare no conflict of interest.

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