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BiVO4 composites
Journal of Water Process Engineering 32 (2019) 100918
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Enhanced degradation of tetracycline hydrochloride using photocatalysis and sulfate radical-based oxidation processes by Co/BiVO4 composites
T
⁎
Xin Chena,b, Jiabin Zhoua,b, , Tianlei Zhangb, Lidan Dingb a b
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfate radical-based oxidation processes Photocatalysis Tetracycline hydrochloride Co/BiVO4 composites
The Co/BiVO4 composites were prepared by wet impregnation method and used to combine photocatalysis and sulfate radical-based oxidation processes for degradation of tetracycline hydrochloride. The effect factors including the mass percentage of cobalt and peroxymonosulfate (PMS) concentration were investigated in detail. The tetracycline hydrochloride solution with an initial concentration of 40 mg/L could be completely degraded within only 25 min in Co/BiVO4+Vis + PMS system. By cyclic degradation experiment, as-prepared Co/BiVO4 composites had good reusability and stability. Free radicals quenching experiment indicated that SO4−% and %OH played an important role for degradation of tetracycline hydrochloride. We confirmed the redox cycle of Co2+/ Co3+ during the PMS activation process by X-ray photoelectron spectroscopy. Based on the above results, a possible enhanced reaction mechanism was proposed.
1. Introduction In the past decades, with the rapid development of industries, environmental pollution has attracted widely attention [1–3]. Especially, some pollutants are difficult to remove owing to the stable chemical structure, which have caused serious environmental pollution and threatened human health [4–6]. A host of technologies have been developed to solve these problems [7–11]. Recently, Fenton-like processes involving hydrogen peroxide (H2O2) and Fe2+ to generate hydroxyl radicals (%OH) have been an effective approach to remove organic pollutants containing wastewater. Unfortunately, the half-life of %OH is too short to ensure react with pollutants. Besides, the generation of %OH is limited by the pH value (2.5-3.5). Some researchers have found that sulfate radicals SO4−%can be an alternative radical. This may be because it has a longer half-life, wider pH range (2–8) and higher redox potential than %OH [12,13]. In addition, SO4−% can be produced by activating peroxymonosulfate (PMS) [14]. Compared liquid H2O2, the solid PMS is safer and more convenient to be used, transported and stored. PMS react slowly with target pollutants at ambient temperature. But it is easily activated by using UV irradiation, heat, carbon and transition metal catalysts [15–18]. Among these methods, transition metal catalysis is regarded as the most promising technology owing to its strong activation ability and low energy consumption. Cobalt (Co2+) is one of the most effective metals during PMS activation processes
⁎
[19]. Compared with Fenton-like processes, the combination of Co2+/ PMS has some advantages, such as without production of ferric hydroxide sludge and pH adjustment [20]. Co3O4 have been studied extensively as an effective catalyst for PMS activation by the redox cycle of Co2+/Co3+ [21,22]. Notably, PMS can be also activated by photogenerated electrons from photocatalyst [23]. Recently, a large variety of Bi-based semiconductor photocatalysts such as Bi2WO6 [24], BiVO4 [25] and BiOX (X = Cl [26], I [27], Br [28]) have been reported. Especially, monoclinic scheelite BiVO4 has received considerable interest owing to its non-toxic, high stability and excellent optical properties [29]. However, BiVO4 still has some weaknesses, such as narrow response range of visible light, weak light absorption capacity and rapid recombination of photogenerated electron/hole pairs [30]. Many efforts have been made to overcome these shortcomings [31–33]. Among these methods, semiconductor coupling can greatly enhance visible light absorption and accelerate the separation and transportation of electrons/holes [34,35]. It is reported that Co3O4 exhibit obviously visible light activity, but has the inevitable problem of electrons/holes recombination. So it is usually used as cocatalysts in the photocatalytic reaction [36–38]. It is efficient to develop Co3O4/BiVO4 composites because Co3O4 and BiVO4 have the matched of valence and conduction band levels for facilitating electrons/holes separation. As another ideal method to enhance photocatalytic activity, external electrons acceptor were added into
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China. E-mail address: [email protected] (J. Zhou).
https://doi.org/10.1016/j.jwpe.2019.100918 Received 19 June 2019; Received in revised form 4 August 2019; Accepted 12 August 2019 2214-7144/ © 2019 Published by Elsevier Ltd.
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excitation light at 280 nm.
photocatalytic system [39]. More recently, PMS can trap photogenerated electrons to generate SO4−% and remarkably inhibit recombination of photogenerated electron/hole pairs [23]. Tetracycline hydrochloride (TC-HCl) is a kind of stable and refractory organic pollutants, and existing catalysts are difficult to degrade high concentration TC-HCl wastewater. Based on above discussion, we designed Co/BiVO4 composites used for the combining processes of photocatalysis and sulfate radical-based oxidation processes to degrade high concentration of TC-HCl. Subsequently, Co/ BiVO4 composites catalytic performance was evaluated by degradation of TC-HCl in presence of PMS under visible light. A possible reaction mechanism was comprehensively studied.
2.4. Degradation experiments of TC-HCl Photocatalytic activity of samples was investigated by the degradation of TC-HCl. For reaction, photocatalyst sample (20 mg) was dispersed in an aqueous solution of TC-HCl (100 ml, 40 mg/L). Adsorption-desorption equilibrium experiments were achieved by magnetically stirring the suspensions in the dark for 30 min. Afterward, PMS was added into the solution and the lamp was turned on. At certain time intervals, 3 mL of samples were withdrawn and methanol were added into samples to quench the radicals. Then, samples were centrifuged to remove the particles. Visible light source was provided by Xenon lamp with a 420 nm UV-cutoff filter (420–780 nm). The light source is located above the solution, and the distance from the light source to solution was around 12 cm. The concentration of TC-HCl was analyzed by measuring the absorbance at maximum absorption wavelength (357 nm for tetracycline hydrochloride) using a UV–vis spectrophotometer. To investigate the influence of PMS concentration, 0.5, 2, 5, 8 mM PMS were added into the reaction solution, respectively. In order to detect the free radicals, radical scavengers were added into the reactor. Tert-butyl alcohol (TBA) was used for scavenging the %OH and methanol (MeOH) was used for scavenging both %OH and SO4−%radicals. For the stability and reusability experiments of Co/BiVO4, the samples were collected after reaction and washed by distilled water and pure ethanol for several times respectively and dried at 60 ℃.
2. Experimental section 2.1. Chemicals Bismuth nitrate pentahydrate (Bi(NO3)3%5H2O), ammonium metavanadate (NH4VO3), Cobalt nitrate hexahydrate (Co(NO3)2%6H2O), Ethanol (C2H6O), Ethylene glycol (C2H6O2), Methanol (CH4O), Tertbutylalcohol (C4H10O) were purchased from Chengdu Kelong chemical Co. Ltd (Chengdu, China); Tetracycline hydrochloride were purchased from Shanghai Macklin Biochemical Co. Ltd (Shanghai, China); Potassium monopersulfate triple salt (KHSO5%0.5KHSO4%0.5K2SO4) were purchased from Aladdin Industrial Corporation(Shanghai, China). All reagents used received from commercial suppliers without any further purification. 2.2. Sample synthesis
3. Results and discussion
2.2.1. The synthesis of BiVO4 The BiVO4 powder was synthesized by a hydrothermal method. The fabrication processes were as followed: 2 mmol Bi(NO3)3%5H2O were dissolved in 20 mL ethylene glycol to obtain solution A, and 2 mmol NH4VO3 were dissolved in 20 mL deionized water to form solution B. Then, the solution B was added drop-wise into solution A under magnetic stirring to get a stable mixture. After 30 min of stirring, the suspension was transferred to a 100-mL Teflon-lined autoclave and heated at 100 ℃ for 15 h. The yellowish precipitate from the reaction was washed with distilled water and pure ethanol for several times respectively. Finally, the products were dried at 80 ℃ to remove residual organic solution.
3.1. Characterization of the Co/BiVO4 composites The XRD patterns of the Co3O4, BiVO4 and Co/BiVO4 composites with different ratio of Co3O4 are shown in Fig. 1. For pristine Co3O4, it can be seen that entire characteristic peaks can be well indexed into the standard cards of Co3O4 (PDF-#43-1003). For pure BiVO4 the characteristic diffraction patterns at 18.8°, 28.9°, 30.5°, 34.8°, 39.9°, 42.3°, 45.8°, 47.0°, 50.1°, 53.2°, 58.8°, 59.8° are observed, corresponding to the (011), (121), (040), (002), (211), (051), (231), (240), (202), (161), (321) and (123) crystalline planes of BiVO4 (PDF-#75-2481), respectively. The narrow and sharp diffraction peaks indicates that BiVO4 has good crystalline nature and large particle size [40]. Interestingly, the XRD patterns of Co/BiVO4 are similar to that of BiVO4. With the increasing Co3O4 content, the diffraction peaks of BiVO4 become weak. While no characteristic peaks ascribed to Co3O4 are observed, which
2.2.2. The synthesis of Co/BiVO4 composites The 200 mg above-prepared BiVO4 powder was dispersed in 10 mL aqueous solution. The desired amounts (9.9, 49.5, 99 mg) of Co (NO3)2%6H2O were added into the solution, and slowly heated to 60 ℃ with slow stirring to evaporate water till the solvent was completely removed. The mixture was taken into a muffle furnace subsequently, and it was heated with a rate of 5 ℃/min from ambient temperature to 400 ℃ and maintained for 5 h under air. The final samples was denoted as x-Co/BiVO4 (x = 1, 5, 10 wt%), where the x represents the mass percentage of Co/BiVO4. 2.3. Characterization The crystal structure of the samples were characterized by an Bruker D8 Advance X-ray diffraction (XRD) using Cu Ka irradiation, and scanning rang was 5° to 90°. The chemical composition and chemical states of the samples were performed with Thermo Fisher Scientific ESCALAB 250 Xi X-ray photoelectron spectroscopy measurement (XPS). The morphologies of the samples were conducted on ZEISS SIGMA HD field emission scanning electron microscopy (FESEM). The UV–vis absorbance and diffuse-reflectance spectra (UV–vis DRS) were analyzed by USA PE Lambda 750 S. Photoluminescence (PL) spectra were recorded with an Edinburgh FLS 1000 fluorescence spectrometer with the
Fig. 1. XRD patterns: (a) pure Co3O4, (b) pure BiVO4, (c) 1%Co/BiVO4 composite, (d) 5%Co/BiVO4 composite and (e) 10%Co/BiVO4 composite. 2
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Fig. 2. XPS spectra of 5%Co/BiVO4 composite: (a) survey, (b) Bi 4f, (c) V 2p, (d) O 1s, (e) Co 2p of before reaction, and (f) Co 2p of after reaction.
[45]. The morphology of BiVO4 and 5%Co/BiVO4 were characterized by field emission scanning electron microscopy (FESEM). It can be clearly seen that the pure BiVO4 is peanut-shaped micron particles formed by the aggregation of nanoparticles with smooth surface (Fig. 3a-b). After the loaded of Co3O4, the BiVO4 still retains peanut-shaped, but its surface becomes obviously rough and has a large number of closely connected small nanoparticles (Fig. 3c-d). In order to further describe the element property, composition and distribution of Co3O4 species, elemental mapping analysis (Fig. 4a-e) and the EDS spectra (Fig. 4f) of 5%Co/BiVO4 were performed, respectively. It is proved the existence of Bi, V, O and Co elements and the mapping image matches well with the SEM image. Moreover, the mapping image of Co element verifies the uniform distribution of Co3O4 nanoparticles on the surface of the BiVO4. The UV–vis diffuse reflection spectra (DRS) of BiVO4 and 5%Co/ BiVO4 composite was carried out and shown in Fig. 5a. Consistent with the reference, BiVO4 has an absorption edge at about 540 nm. After the loading of Co3O4, the absorption edge of 5%Co/BiVO4 shows significant
may be ascribed that good dispersion and low content in the composites. The survey spectrum and high-resolution XPS spectra of 5%Co/ BiVO4 are shown in Fig. 2. It can be seen from Fig. 2a, 5%Co/BiVO4 composite mainly consists of Bi, V, O, Co elements. Fig. 2b presents Bi 4f peaks. The two peaks at 158.58 eV and 163.88 eV can be assigned to Bi 4f7/2 and Bi 4f5/2, indicating that Bi element are Bi3+ state [41]. The V 2p is shown in Fig. 2c. The two peaks at 516.28 eV and 523.78 eV can be assigned to V 2p3/2 and V 2p1/2, indicating that V element are V5+ state [42]. The O 1s spectra divided into three peaks at 528.85 eV, 529.41 eV and 531.24 eV in Fig. 2d, which may be assigned to the oxygen species of lattice oxygen of Co3O4, BiVO4 and the adsorbed oxygen, respectively [43,44]. The Co 2p is shown in Fig. 2e. The peaks at 793.38 eV and 795.28 eV are attributed to Co3+ and Co2+ of Co 2p3/ 3+ and 2, and the peaks at 778.78 eV and 780.48 eV are attributed to Co 2+ Co of Co 2p1/2, respectively. Besides, the peaks at around 805 eV and 788 eV are attributed to satellite peaks of Co 2p3/2 and Co 2p1/2 respectively. Notably, the binding energy distance of Co 2p3/2 and Co 2p1/2 is about 15 eV, which indicates the existence of the Co3O4 phase 3
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Fig. 3. FESEM images of (a–b) pure BiVO4 and (c–d) 5%Co/BiVO4 composite.
to dark green by enhancing the Co3O4 content suggesting that Co/ BiVO4 can make better use of visible light. As a crystalline semiconductor, the band gap value can be measured by the following equation (Eq. (1)):
red shift, and the continuous absorption of UV and visible light region have become more intense, which could be owing to a charge-transfer between the metal oxide and BiVO4. Notably, 5%Co/BiVO4 composite has two absorption bands that can be attributed to the two transitions of Co3O4, corresponding respectively to edges of O2−→Co3+ excitation and O2−→Co2+ charge transfer [34]. This result clearly also confirms the presence of Co3O4 in 5%Co/BiVO4 composite. Apart from that, the color of the Co/BiVO4 composites powder also change from yellowish
αhν = A(hν－Eg)n/2
(1)
where, hν, α, Eg and A represent the photon energy, optical absorption coefficient, band gap, and proportionality constant, respectively.
Fig. 4. (a) FESEM images, (b) mapping of Bi element, (c) mapping of V element, (d) mapping of O element, (e) mapping of Co element, (f) EDS analysis of 5%Co/ BiVO4 composite. 4
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Fig. 6. PL spectra of BiVO4 and 5%Co/BiVO4 composite.
5%Co/BiVO4 has high photocatalytic activity. Moreover, the phenomenon clearly reveals that the Co3O4 and BiVO4 form internal contact instead of just cover on the surface of BiVO4. 3.2. TC-HCl degradation 3.2.1. Photocatalytic activity The photocatalytic activity of BiVO4 and the Co/BiVO4 composites were determined by the degradation of TC-HCl under visible light irradiation. A plot of C/C0 versus t is set up, where C0 is concentration after adsorption and C is the actual concentration of TC-HCl. It can be clearly seen in Fig. 7a, only 30% of TC-HCl was degraded after 80 min reaction with pure BiVO4 under visible light. The photocatalytic activity was improved after modification by Co3O4 and 5%Co/BiVO4 exhibited the highest photocatalytic activity among all samples. However, when the contents of Co3O4 species was increased to 10%, photocatalyst exhibited lower photocatalytic activity. This may be due to the agglomeration and lower dispersibility of Co3O4 species, resulting in the low migration and separation rate of the photogenerated charge. Therefore, an appropriate loading level and dispersion of Co3O4 in the Co/BiVO4 composites are critical. After the reaction for 80 min, it was found the degradation date were well consistent with a pseudo-first-order kinetic in Fig. 7b, according to Eq. (4).
Fig. 5. (a) UV–vis DRS spectra, (b) (αhν)2 vs. (hν) of BiVO4 and 5%Co/BiVO4 composite.
Additionally, n = 1 and 4 means that the material is direct semiconductor and indirect semiconductor, respectively. As shown in Fig. 5b, the calculation of direct energies of pure BiVO4 and 5%Co/ BiVO4 composite estimated from a plot of (αhν)2 versus (hν) were 2.28 eV and 2.16 eV respectively according to the K-M model [46]. Consistent with the reference, the band gap of BiVO4 is 2.28 eV. 5%Co/ BiVO4 in our experiment is 2.16 eV indicating the Co3O4 loaded can shorten the band gap of BiVO4 to enhance light harvesting and visible light utilization. The band edge positions of BiVO4 and Co3O4 are estimated in this study based on the electronegativity. The conduction band (CB) and valence band (VB) potentials of BiVO4 and Co3O4 can be evaluated by the formula as follows (Eqs. ((2) and (3))): ECB = X－Ee－0.5Eg
(2)
EVB = ECB + Eg
(3)
ln(
C ) = kt C0
(4)
where, k is the apparent rate constant for the TC-HCl degradation (min−1), which were obtained from the slope of plot ln(C/C0) versus t [48]. The k value increased gradually when the contents of Co3O4 increased from 1% to 5%. Further increasing the amounts of Co3O4 from 5% to 10%, the k value of TC-HCl decreased. It indicated that the 5%Co/BiVO4 sample had the highest photocatalytic activity and was selected for further studies.
Where, EVB and ECB are the VB and CB potentials, X is the absolute electronegativity of the semiconductor (X value for BiVO4 is 6.036), Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor. According to the UV–vis diffuse reflectance absorption spectra, the band gap energy of BiVO4 are 2.28 eV, hence, the calculated EVB and ECB of BiVO4 is 2.67 eV and 0.39 eV, respectively. The PL spectra is an efficient technique to reveal the recombination rate of photogenerated electron/hole pairs [47]. The PL spectra of BiVO4 and 5%Co/BiVO4 are shown in Fig. 6. The 5%Co/BiVO4 sample shows weaker intensity than BiVO4, indicating that Co3O4 loading can inhibit the recombination of photogenerated electron/hole pairs. So
3.2.2. Enhanced degradation of TC-HCl As shown in Fig. 8a, performance comparison between different systems was conducted by the degradation of TC-HCl. The degradation rate of TC-HCl was lightly increased in Co/BiVO4+Vis system, indicating that the photocatalytic degradation ability of Co/BiVO4 for TCHCl was unsatisfactory. If only PMS was introduced into system, the TCHCl solution could hardly be degraded under visible light. The degradation rate of TC-HCl was increased in Co/BiVO4+PMS system, indicating that Co/BiVO4 have potential ability to activate PMS. TC-HCl was more degraded in BiVO4+Vis + PMS system, indicating photogenerated electron can help PMS to produce SO4−% radicals. Significantly, TC-HCl was completely and rapidly degraded in the Co/ 5
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Fig. 8. Images of TC-HCl degradation with different systems (a): [TCHCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L, [PMS]0 = 5 mM, and influence of PMS concentration on the 5%Co/BiVO4+Vis + PMS system (b): [TCHCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L.
Fig. 7. Photocatalytic activity of BiVO4 and Co/BiVO4 composites, reaction conditions: [TC-HCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L, Vis irradiation.
BiVO4+Vis + PMS system. The influence of PMS concentration on TC-HCl degradation is exhibited in Fig. 8b. When 0.5 mM PMS was added, the degradation rate of TC-HCl increased obviously. TC-HCl was completely degraded when the PMS concentration increased from 0.5 mM to 5.0 mM. Obviously, the higher PMS concentration would generate more SO4− % radicals. When PMS concentration was increased from 5.0 mM to 8.0 mM, only a slight improvement of the degradation rate of TC-HCl was observed. This was because too many SO4− % radicals could be scavenged as shown in Eq. (12-13). It is worth noting that the S2O8 is less reactive than SO4s. (12 and 13). It is worth noting that the S2O8− is less reactive than SO4− is less reactive than SO4− thus the degradation rate had only a slight improvement. Therefore, 5.0 mM PMS concentration was selected as the most appropriate concentration. 3.3. Proposed enhanced degradation mechanism Fig. 9. Effect of radical scavengers on the degradation of TC-HCl. Reaction conditions: [TC-HCl]0 = 40 mg/L, [5%Co/BiVO4]0 = 0.2 g/L, [PMS]0 = 5 mM, Vis irradiation.
To further identify the mechanism of TC-HCl degradation, free radicals trapping experiments were conducted (Fig. 9). TBA was used as an effective radical scavenger for %OH while MeOH was applied as an effective radical scavenger for both %OH and SO4−%. A slightly decrease of TC-HCl degradation efficiency was observed after TBA scavenger was added in the Co/BiVO4+Vis + PMS system, which indicated that the % OH was a minor factor contributing to TC-HCl degradation. By contrast, TC-HCl degradation efficiency was remarkable reduced when MeOH was added indicating that SO4−% plays an important role in the degradation of TC-HCl. It is well known that BiVO4 and Co3O4 are typical semiconductor materials. According to the above calculation results, EVB and ECB of BiVO4 are 2.67 eV and 0.39 eV, respectively. According to previous
articles, the EVB and ECB of Co3O4 are 2.44 eV and 0.37 eV, respectively [34]. Hence, the potential difference between the two semiconductors created a feasible path which favoring the enrichment of electrons on the surface of BiVO4 while the accumulation of holes in Co3O4. When the BiVO4 is coupled with the Co3O4, a heterojunction is formed as demonstrated by the DRS and PL results. The XPS spectra of Co 2p on 5%Co/BiVO4 surface before and after reaction were further studied to clarify reaction mechanism (Fig. 2e-f). The peaks with binding energy positioned at 793.38 eV and 778.78 eV 6
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Fig. 10. Schematic illustration for the possible mechanism in the 5%Co/BiVO4+Vis + PMS system.
SO4−% / %OH/ h+ + TC-HCl → products
can be attributed to Co3+, and the peaks with binding energy positioned at 795.28 eV and 780.48 eV can be attributed to Co2+. The relative area of the four peaks is 43%, 17%, 27%, and 12%, respectively. For the XPS spectra of Co 2p after reaction, the peaks with binding energy positioned at 794.38 eV and 779.38 eV can be attributed to Co3+, and the peaks with binding energy positioned at 796.18 eV and 781.68 eV can be attributed to Co2+. The relative area of the peaks as mentioned above are 52%, 18%, 18% and 10%, respectively. This variation indicates that the degradation reaction initializes on the surface of Co/BiVO4, whereby the Co2+ possibly reacted with HSO5− and further caused an increase of Co3+. To maintain the electrons/charge balance on the surface of catalyst, the Co3+ would react with HSO5− and afterward turned back to Co2+. This explanation implies a redox cycle of the Co3O4 (Co2+/Co3+) during the PMS activation process [49]. According to above discussions, a synergistic mechanism of advanced oxidation processes coupling photocatalysis degradation for TCHCl was illustrated in Fig. 10. Firstly, after the loaded with Co3O4, the ability of visible light absorption of photocatalysts was enhanced and more electron/hole pairs could be produced. Secondly, photogenerated electrons was trapped by PMS to generate SO4−% and electron/hole pairs recombination was suppressed (Eq. (6)). Thirdly, Co3O4 from Co/ BiVO4 composites could activate PMS to generate SO4−% and SO5−%radicals (Eq. (8–9)). The above XPS results clarified the redox cycle of the Co3O4 (Co2+/Co3+) during the PMS activation process. Lastly, the VB potential of Co3O4 is higher than the standard redox potential of OH−/%OH (2.4 eV vs. NHE), so holes on the surface of catalysts were able to react with OH− to produce %OH or oxidize directly pollutant [50]. The SO4−% and %OH species generated from above mentioned approach has strong oxidation properties, which can oxidize effectively the target organic pollutant TC-HCl. Reaction process is as follows: Co/BiVO4 + hν → e− + h+ HSO5− HSO5− Co
2+
Co
3+
+e
−
+h
+
→
+ OH
−%
+
→ SO5
+
HSO5−
→ Co
+
HSO5−
→ Co
+H
3+ 2+
+ SO4 + SO5
−%
−
+ OH
(8)
+
(9)
+H
SO4−% + H2O → HSO4− + %OH
(11)
2SO4−%
(12)
SO4 OH
−
+
S2O82−
S2O82− %
→SO4
+ h → OH +
2−
4. Conclusions In this work, the high-efficiency Co/BiVO4 composites were successfully prepared by wet impregnation method and used for degradation of tetracycline hydrochloride (TC-HCl). As we can saw that 100 mL 40 mg/L TC-HCl was completely degraded within only 25 min when combining of photocatalysis and sulfate radical-based oxidation processes. Based on free radicals trapping experiments, it was found that SO4−% was main free radicals for the degradation of TC-HCl, %OH played the minor role on it. The results indicated that the introduction of PMS and Co3O4 suppressed the recombination of photogenerated electron/hole pairs. Moreover, the PMS not only can be activated by photogenerated electrons but also by Co3O4 to generate SO4−%, which leaded to the efficient degradation of TC-HCl. Besides, the Co/BiVO4 catalysts show good stability after 4 cycles. This work may provide a new insight into the design of catalyst used to combine photocatalysis and sulfate radical-based oxidation processes for refractory organic pollutants degradation.
(7) −%
(10)
→
It is worth noting that the reusability and stability of photocatalyst has a significant influence on the actual application. The result of cyclic experiments of Co/BiVO4 composites is shown in Fig. 11. After four cyclic experiments, there was no distinct reduction of TC-HCl degradation rate. The little reduction might be result from the loss of catalyst during recycling.
(6)
2SO5−% → 2SO4−% + O2
−%
3.4. Reusability of Co/BiVO4
(5) −
SO4−%
+
S2O8−%
(15)
(13) (14) Fig. 11. Cyclic experiments of 5%Co/BiVO4 for TC-HCl degradation. 7
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Acknowledgement
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Durrant, Ultra-thin Al2O3 coatings on BiVO4 photoanodes: impact on performance and charge carrier dynamics, Catal. Today 321 (2019) 59–66. [42] M. Wang, Q.M. Wang, P.Z. Guo, Z.B. Jiao, In situ fabrication of nanoporous BiVO4/ Bi2S3 nanosheets for enhanced photoelectrochemical water splitting, J. Colloid Interfaces Sci 534 (2019) 338–342. [43] N.N. Liu, Z.H. Li, Bimetal-organic frameworks derived carbon doped ZnO/Co3O4 heterojunction as visible-light stabilized photocatalysts, Mater. Sci. Semicon. Proc. 79 (2018) 24–31. [44] Z.H. Ye, X.Y. Xiao, J.Y. Chen, Y. Wang, Fabrication of BiVO4/BiOBr composite with enhanced photocatalytic activity by a CTAB-assisted polyol method, J. Photochem. Photobiol. A 368 (2019) 153–161. [45] J.H. Zheng, L. Zhang, Incorporation of CoO nanoparticles in 3D marigold flowerlike hierarchical architecture MnCo2O4 for highly boosting solar light photo-oxidation and reduction ability, Appl. Catal. B: Environ. 237 (2018) 1–8. [46] Z.H. Wei, Y.F. Wang, Y.Y. Li, L. Zhang, H.C. Yao, Z.J. Li, Enhanced photocatalytic CO2 reduction activity of Z-scheme CdS/BiVO4 nanocomposite with thinner BiVO4 nanosheets, J. CO2 Util. 28 (2018) 15–25. [47] H.J. Wu, C.M. Li, H.N. Che, H. Hu, W. Hu, C.B. Liu, J.Z. Ai, H.J. Dong, Decoration of mesoporous Co3O4 nanospheres assembled by monocrystal nanodots on g-C3N4 to construct Z-scheme system for improving photocatalytic performance, Appl. Surf. Sci. 440 (2018) 308–319. [48] Y. Zhang, J.B. Zhou, X. Chen, Q.Q. Feng, W.Q. Cai, MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light, J. Alloys Compd. 777 (2019) 109–118. [49] S.J. Yang, X.J. Qiu, P.K. Jin, M. Dzakpasu, X.C.C. Wang, Q.H. Zhang, L. Zhang, L. Yang, D.H. Ding, W.D. Wang, K. Wu, MOF-templated synthesis of CoFe2O4 nanocrystals and its coupling with peroxymonosulfate for degradation of bisphenol A, Chem. Eng. J. 353 (2018) 329–339. [50] C. Lai, M.M. Zhang, B.S. Li, D.L. Huang, G.M. Zeng, L. Qin, X.G. Liu, H. Yi, M. 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This work was supported by the National Natural Science Foundation of China (No. 21277108). References [1] B. Amanulla, S. Sannasi, A.K.M. Abubakker, S.K. Ramaraj, A magnetically recoverable bimetallic Au-FeNPs decorated on g-C3N4 for efficient photocatalytic degradation of organic contaminants, J. Mol. Liq. 249 (2018) 754–763. [2] C. Liang, C.G. Niu, H. Guo, D.W. Huang, X.J. Wen, S.F. Yang, G.M. Zeng, Combination of efficient charge separation with the assistance of novel dual Zscheme system: self-assembly photocatalyst Ag@AgI/BiOI modified oxygen-doped carbon nitride nanosheet with enhanced photocatalytic performance, Catal. Sci. Technol. 8 (2018) 1161–1175. [3] J.Y. Li, X.A. Dong, G. Zhang, W. Cui, W.L. Cen, Z.B. Wu, S.C. Lee, F. Dong, Probing ring-opening pathways for efficient photocatalytic toluene decomposition, J. Mater. Chem. A 7 (2019) 3366–3374. [4] Z. Behzadifard, Z. Shariatinia, M. 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Enhanced degradation of tetracycline hydrochloride using photocatalysis and sulfate radical-based oxidation processes by Co/BiVO4 composites
T
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Xin Chena,b, Jiabin Zhoua,b, , Tianlei Zhangb, Lidan Dingb a b
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfate radical-based oxidation processes Photocatalysis Tetracycline hydrochloride Co/BiVO4 composites
The Co/BiVO4 composites were prepared by wet impregnation method and used to combine photocatalysis and sulfate radical-based oxidation processes for degradation of tetracycline hydrochloride. The effect factors including the mass percentage of cobalt and peroxymonosulfate (PMS) concentration were investigated in detail. The tetracycline hydrochloride solution with an initial concentration of 40 mg/L could be completely degraded within only 25 min in Co/BiVO4+Vis + PMS system. By cyclic degradation experiment, as-prepared Co/BiVO4 composites had good reusability and stability. Free radicals quenching experiment indicated that SO4−% and %OH played an important role for degradation of tetracycline hydrochloride. We confirmed the redox cycle of Co2+/ Co3+ during the PMS activation process by X-ray photoelectron spectroscopy. Based on the above results, a possible enhanced reaction mechanism was proposed.
1. Introduction In the past decades, with the rapid development of industries, environmental pollution has attracted widely attention [1–3]. Especially, some pollutants are difficult to remove owing to the stable chemical structure, which have caused serious environmental pollution and threatened human health [4–6]. A host of technologies have been developed to solve these problems [7–11]. Recently, Fenton-like processes involving hydrogen peroxide (H2O2) and Fe2+ to generate hydroxyl radicals (%OH) have been an effective approach to remove organic pollutants containing wastewater. Unfortunately, the half-life of %OH is too short to ensure react with pollutants. Besides, the generation of %OH is limited by the pH value (2.5-3.5). Some researchers have found that sulfate radicals SO4−%can be an alternative radical. This may be because it has a longer half-life, wider pH range (2–8) and higher redox potential than %OH [12,13]. In addition, SO4−% can be produced by activating peroxymonosulfate (PMS) [14]. Compared liquid H2O2, the solid PMS is safer and more convenient to be used, transported and stored. PMS react slowly with target pollutants at ambient temperature. But it is easily activated by using UV irradiation, heat, carbon and transition metal catalysts [15–18]. Among these methods, transition metal catalysis is regarded as the most promising technology owing to its strong activation ability and low energy consumption. Cobalt (Co2+) is one of the most effective metals during PMS activation processes
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[19]. Compared with Fenton-like processes, the combination of Co2+/ PMS has some advantages, such as without production of ferric hydroxide sludge and pH adjustment [20]. Co3O4 have been studied extensively as an effective catalyst for PMS activation by the redox cycle of Co2+/Co3+ [21,22]. Notably, PMS can be also activated by photogenerated electrons from photocatalyst [23]. Recently, a large variety of Bi-based semiconductor photocatalysts such as Bi2WO6 [24], BiVO4 [25] and BiOX (X = Cl [26], I [27], Br [28]) have been reported. Especially, monoclinic scheelite BiVO4 has received considerable interest owing to its non-toxic, high stability and excellent optical properties [29]. However, BiVO4 still has some weaknesses, such as narrow response range of visible light, weak light absorption capacity and rapid recombination of photogenerated electron/hole pairs [30]. Many efforts have been made to overcome these shortcomings [31–33]. Among these methods, semiconductor coupling can greatly enhance visible light absorption and accelerate the separation and transportation of electrons/holes [34,35]. It is reported that Co3O4 exhibit obviously visible light activity, but has the inevitable problem of electrons/holes recombination. So it is usually used as cocatalysts in the photocatalytic reaction [36–38]. It is efficient to develop Co3O4/BiVO4 composites because Co3O4 and BiVO4 have the matched of valence and conduction band levels for facilitating electrons/holes separation. As another ideal method to enhance photocatalytic activity, external electrons acceptor were added into
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China. E-mail address: [email protected] (J. Zhou).
https://doi.org/10.1016/j.jwpe.2019.100918 Received 19 June 2019; Received in revised form 4 August 2019; Accepted 12 August 2019 2214-7144/ © 2019 Published by Elsevier Ltd.
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excitation light at 280 nm.
photocatalytic system [39]. More recently, PMS can trap photogenerated electrons to generate SO4−% and remarkably inhibit recombination of photogenerated electron/hole pairs [23]. Tetracycline hydrochloride (TC-HCl) is a kind of stable and refractory organic pollutants, and existing catalysts are difficult to degrade high concentration TC-HCl wastewater. Based on above discussion, we designed Co/BiVO4 composites used for the combining processes of photocatalysis and sulfate radical-based oxidation processes to degrade high concentration of TC-HCl. Subsequently, Co/ BiVO4 composites catalytic performance was evaluated by degradation of TC-HCl in presence of PMS under visible light. A possible reaction mechanism was comprehensively studied.
2.4. Degradation experiments of TC-HCl Photocatalytic activity of samples was investigated by the degradation of TC-HCl. For reaction, photocatalyst sample (20 mg) was dispersed in an aqueous solution of TC-HCl (100 ml, 40 mg/L). Adsorption-desorption equilibrium experiments were achieved by magnetically stirring the suspensions in the dark for 30 min. Afterward, PMS was added into the solution and the lamp was turned on. At certain time intervals, 3 mL of samples were withdrawn and methanol were added into samples to quench the radicals. Then, samples were centrifuged to remove the particles. Visible light source was provided by Xenon lamp with a 420 nm UV-cutoff filter (420–780 nm). The light source is located above the solution, and the distance from the light source to solution was around 12 cm. The concentration of TC-HCl was analyzed by measuring the absorbance at maximum absorption wavelength (357 nm for tetracycline hydrochloride) using a UV–vis spectrophotometer. To investigate the influence of PMS concentration, 0.5, 2, 5, 8 mM PMS were added into the reaction solution, respectively. In order to detect the free radicals, radical scavengers were added into the reactor. Tert-butyl alcohol (TBA) was used for scavenging the %OH and methanol (MeOH) was used for scavenging both %OH and SO4−%radicals. For the stability and reusability experiments of Co/BiVO4, the samples were collected after reaction and washed by distilled water and pure ethanol for several times respectively and dried at 60 ℃.
2. Experimental section 2.1. Chemicals Bismuth nitrate pentahydrate (Bi(NO3)3%5H2O), ammonium metavanadate (NH4VO3), Cobalt nitrate hexahydrate (Co(NO3)2%6H2O), Ethanol (C2H6O), Ethylene glycol (C2H6O2), Methanol (CH4O), Tertbutylalcohol (C4H10O) were purchased from Chengdu Kelong chemical Co. Ltd (Chengdu, China); Tetracycline hydrochloride were purchased from Shanghai Macklin Biochemical Co. Ltd (Shanghai, China); Potassium monopersulfate triple salt (KHSO5%0.5KHSO4%0.5K2SO4) were purchased from Aladdin Industrial Corporation(Shanghai, China). All reagents used received from commercial suppliers without any further purification. 2.2. Sample synthesis
3. Results and discussion
2.2.1. The synthesis of BiVO4 The BiVO4 powder was synthesized by a hydrothermal method. The fabrication processes were as followed: 2 mmol Bi(NO3)3%5H2O were dissolved in 20 mL ethylene glycol to obtain solution A, and 2 mmol NH4VO3 were dissolved in 20 mL deionized water to form solution B. Then, the solution B was added drop-wise into solution A under magnetic stirring to get a stable mixture. After 30 min of stirring, the suspension was transferred to a 100-mL Teflon-lined autoclave and heated at 100 ℃ for 15 h. The yellowish precipitate from the reaction was washed with distilled water and pure ethanol for several times respectively. Finally, the products were dried at 80 ℃ to remove residual organic solution.
3.1. Characterization of the Co/BiVO4 composites The XRD patterns of the Co3O4, BiVO4 and Co/BiVO4 composites with different ratio of Co3O4 are shown in Fig. 1. For pristine Co3O4, it can be seen that entire characteristic peaks can be well indexed into the standard cards of Co3O4 (PDF-#43-1003). For pure BiVO4 the characteristic diffraction patterns at 18.8°, 28.9°, 30.5°, 34.8°, 39.9°, 42.3°, 45.8°, 47.0°, 50.1°, 53.2°, 58.8°, 59.8° are observed, corresponding to the (011), (121), (040), (002), (211), (051), (231), (240), (202), (161), (321) and (123) crystalline planes of BiVO4 (PDF-#75-2481), respectively. The narrow and sharp diffraction peaks indicates that BiVO4 has good crystalline nature and large particle size [40]. Interestingly, the XRD patterns of Co/BiVO4 are similar to that of BiVO4. With the increasing Co3O4 content, the diffraction peaks of BiVO4 become weak. While no characteristic peaks ascribed to Co3O4 are observed, which
2.2.2. The synthesis of Co/BiVO4 composites The 200 mg above-prepared BiVO4 powder was dispersed in 10 mL aqueous solution. The desired amounts (9.9, 49.5, 99 mg) of Co (NO3)2%6H2O were added into the solution, and slowly heated to 60 ℃ with slow stirring to evaporate water till the solvent was completely removed. The mixture was taken into a muffle furnace subsequently, and it was heated with a rate of 5 ℃/min from ambient temperature to 400 ℃ and maintained for 5 h under air. The final samples was denoted as x-Co/BiVO4 (x = 1, 5, 10 wt%), where the x represents the mass percentage of Co/BiVO4. 2.3. Characterization The crystal structure of the samples were characterized by an Bruker D8 Advance X-ray diffraction (XRD) using Cu Ka irradiation, and scanning rang was 5° to 90°. The chemical composition and chemical states of the samples were performed with Thermo Fisher Scientific ESCALAB 250 Xi X-ray photoelectron spectroscopy measurement (XPS). The morphologies of the samples were conducted on ZEISS SIGMA HD field emission scanning electron microscopy (FESEM). The UV–vis absorbance and diffuse-reflectance spectra (UV–vis DRS) were analyzed by USA PE Lambda 750 S. Photoluminescence (PL) spectra were recorded with an Edinburgh FLS 1000 fluorescence spectrometer with the
Fig. 1. XRD patterns: (a) pure Co3O4, (b) pure BiVO4, (c) 1%Co/BiVO4 composite, (d) 5%Co/BiVO4 composite and (e) 10%Co/BiVO4 composite. 2
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Fig. 2. XPS spectra of 5%Co/BiVO4 composite: (a) survey, (b) Bi 4f, (c) V 2p, (d) O 1s, (e) Co 2p of before reaction, and (f) Co 2p of after reaction.
[45]. The morphology of BiVO4 and 5%Co/BiVO4 were characterized by field emission scanning electron microscopy (FESEM). It can be clearly seen that the pure BiVO4 is peanut-shaped micron particles formed by the aggregation of nanoparticles with smooth surface (Fig. 3a-b). After the loaded of Co3O4, the BiVO4 still retains peanut-shaped, but its surface becomes obviously rough and has a large number of closely connected small nanoparticles (Fig. 3c-d). In order to further describe the element property, composition and distribution of Co3O4 species, elemental mapping analysis (Fig. 4a-e) and the EDS spectra (Fig. 4f) of 5%Co/BiVO4 were performed, respectively. It is proved the existence of Bi, V, O and Co elements and the mapping image matches well with the SEM image. Moreover, the mapping image of Co element verifies the uniform distribution of Co3O4 nanoparticles on the surface of the BiVO4. The UV–vis diffuse reflection spectra (DRS) of BiVO4 and 5%Co/ BiVO4 composite was carried out and shown in Fig. 5a. Consistent with the reference, BiVO4 has an absorption edge at about 540 nm. After the loading of Co3O4, the absorption edge of 5%Co/BiVO4 shows significant
may be ascribed that good dispersion and low content in the composites. The survey spectrum and high-resolution XPS spectra of 5%Co/ BiVO4 are shown in Fig. 2. It can be seen from Fig. 2a, 5%Co/BiVO4 composite mainly consists of Bi, V, O, Co elements. Fig. 2b presents Bi 4f peaks. The two peaks at 158.58 eV and 163.88 eV can be assigned to Bi 4f7/2 and Bi 4f5/2, indicating that Bi element are Bi3+ state [41]. The V 2p is shown in Fig. 2c. The two peaks at 516.28 eV and 523.78 eV can be assigned to V 2p3/2 and V 2p1/2, indicating that V element are V5+ state [42]. The O 1s spectra divided into three peaks at 528.85 eV, 529.41 eV and 531.24 eV in Fig. 2d, which may be assigned to the oxygen species of lattice oxygen of Co3O4, BiVO4 and the adsorbed oxygen, respectively [43,44]. The Co 2p is shown in Fig. 2e. The peaks at 793.38 eV and 795.28 eV are attributed to Co3+ and Co2+ of Co 2p3/ 3+ and 2, and the peaks at 778.78 eV and 780.48 eV are attributed to Co 2+ Co of Co 2p1/2, respectively. Besides, the peaks at around 805 eV and 788 eV are attributed to satellite peaks of Co 2p3/2 and Co 2p1/2 respectively. Notably, the binding energy distance of Co 2p3/2 and Co 2p1/2 is about 15 eV, which indicates the existence of the Co3O4 phase 3
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Fig. 3. FESEM images of (a–b) pure BiVO4 and (c–d) 5%Co/BiVO4 composite.
to dark green by enhancing the Co3O4 content suggesting that Co/ BiVO4 can make better use of visible light. As a crystalline semiconductor, the band gap value can be measured by the following equation (Eq. (1)):
red shift, and the continuous absorption of UV and visible light region have become more intense, which could be owing to a charge-transfer between the metal oxide and BiVO4. Notably, 5%Co/BiVO4 composite has two absorption bands that can be attributed to the two transitions of Co3O4, corresponding respectively to edges of O2−→Co3+ excitation and O2−→Co2+ charge transfer [34]. This result clearly also confirms the presence of Co3O4 in 5%Co/BiVO4 composite. Apart from that, the color of the Co/BiVO4 composites powder also change from yellowish
αhν = A(hν－Eg)n/2
(1)
where, hν, α, Eg and A represent the photon energy, optical absorption coefficient, band gap, and proportionality constant, respectively.
Fig. 4. (a) FESEM images, (b) mapping of Bi element, (c) mapping of V element, (d) mapping of O element, (e) mapping of Co element, (f) EDS analysis of 5%Co/ BiVO4 composite. 4
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Fig. 6. PL spectra of BiVO4 and 5%Co/BiVO4 composite.
5%Co/BiVO4 has high photocatalytic activity. Moreover, the phenomenon clearly reveals that the Co3O4 and BiVO4 form internal contact instead of just cover on the surface of BiVO4. 3.2. TC-HCl degradation 3.2.1. Photocatalytic activity The photocatalytic activity of BiVO4 and the Co/BiVO4 composites were determined by the degradation of TC-HCl under visible light irradiation. A plot of C/C0 versus t is set up, where C0 is concentration after adsorption and C is the actual concentration of TC-HCl. It can be clearly seen in Fig. 7a, only 30% of TC-HCl was degraded after 80 min reaction with pure BiVO4 under visible light. The photocatalytic activity was improved after modification by Co3O4 and 5%Co/BiVO4 exhibited the highest photocatalytic activity among all samples. However, when the contents of Co3O4 species was increased to 10%, photocatalyst exhibited lower photocatalytic activity. This may be due to the agglomeration and lower dispersibility of Co3O4 species, resulting in the low migration and separation rate of the photogenerated charge. Therefore, an appropriate loading level and dispersion of Co3O4 in the Co/BiVO4 composites are critical. After the reaction for 80 min, it was found the degradation date were well consistent with a pseudo-first-order kinetic in Fig. 7b, according to Eq. (4).
Fig. 5. (a) UV–vis DRS spectra, (b) (αhν)2 vs. (hν) of BiVO4 and 5%Co/BiVO4 composite.
Additionally, n = 1 and 4 means that the material is direct semiconductor and indirect semiconductor, respectively. As shown in Fig. 5b, the calculation of direct energies of pure BiVO4 and 5%Co/ BiVO4 composite estimated from a plot of (αhν)2 versus (hν) were 2.28 eV and 2.16 eV respectively according to the K-M model [46]. Consistent with the reference, the band gap of BiVO4 is 2.28 eV. 5%Co/ BiVO4 in our experiment is 2.16 eV indicating the Co3O4 loaded can shorten the band gap of BiVO4 to enhance light harvesting and visible light utilization. The band edge positions of BiVO4 and Co3O4 are estimated in this study based on the electronegativity. The conduction band (CB) and valence band (VB) potentials of BiVO4 and Co3O4 can be evaluated by the formula as follows (Eqs. ((2) and (3))): ECB = X－Ee－0.5Eg
(2)
EVB = ECB + Eg
(3)
ln(
C ) = kt C0
(4)
where, k is the apparent rate constant for the TC-HCl degradation (min−1), which were obtained from the slope of plot ln(C/C0) versus t [48]. The k value increased gradually when the contents of Co3O4 increased from 1% to 5%. Further increasing the amounts of Co3O4 from 5% to 10%, the k value of TC-HCl decreased. It indicated that the 5%Co/BiVO4 sample had the highest photocatalytic activity and was selected for further studies.
Where, EVB and ECB are the VB and CB potentials, X is the absolute electronegativity of the semiconductor (X value for BiVO4 is 6.036), Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor. According to the UV–vis diffuse reflectance absorption spectra, the band gap energy of BiVO4 are 2.28 eV, hence, the calculated EVB and ECB of BiVO4 is 2.67 eV and 0.39 eV, respectively. The PL spectra is an efficient technique to reveal the recombination rate of photogenerated electron/hole pairs [47]. The PL spectra of BiVO4 and 5%Co/BiVO4 are shown in Fig. 6. The 5%Co/BiVO4 sample shows weaker intensity than BiVO4, indicating that Co3O4 loading can inhibit the recombination of photogenerated electron/hole pairs. So
3.2.2. Enhanced degradation of TC-HCl As shown in Fig. 8a, performance comparison between different systems was conducted by the degradation of TC-HCl. The degradation rate of TC-HCl was lightly increased in Co/BiVO4+Vis system, indicating that the photocatalytic degradation ability of Co/BiVO4 for TCHCl was unsatisfactory. If only PMS was introduced into system, the TCHCl solution could hardly be degraded under visible light. The degradation rate of TC-HCl was increased in Co/BiVO4+PMS system, indicating that Co/BiVO4 have potential ability to activate PMS. TC-HCl was more degraded in BiVO4+Vis + PMS system, indicating photogenerated electron can help PMS to produce SO4−% radicals. Significantly, TC-HCl was completely and rapidly degraded in the Co/ 5
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Fig. 8. Images of TC-HCl degradation with different systems (a): [TCHCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L, [PMS]0 = 5 mM, and influence of PMS concentration on the 5%Co/BiVO4+Vis + PMS system (b): [TCHCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L.
Fig. 7. Photocatalytic activity of BiVO4 and Co/BiVO4 composites, reaction conditions: [TC-HCl]0 = 40 mg/L, [catalyst]0 = 0.2 g/L, Vis irradiation.
BiVO4+Vis + PMS system. The influence of PMS concentration on TC-HCl degradation is exhibited in Fig. 8b. When 0.5 mM PMS was added, the degradation rate of TC-HCl increased obviously. TC-HCl was completely degraded when the PMS concentration increased from 0.5 mM to 5.0 mM. Obviously, the higher PMS concentration would generate more SO4− % radicals. When PMS concentration was increased from 5.0 mM to 8.0 mM, only a slight improvement of the degradation rate of TC-HCl was observed. This was because too many SO4− % radicals could be scavenged as shown in Eq. (12-13). It is worth noting that the S2O8 is less reactive than SO4s. (12 and 13). It is worth noting that the S2O8− is less reactive than SO4− is less reactive than SO4− thus the degradation rate had only a slight improvement. Therefore, 5.0 mM PMS concentration was selected as the most appropriate concentration. 3.3. Proposed enhanced degradation mechanism Fig. 9. Effect of radical scavengers on the degradation of TC-HCl. Reaction conditions: [TC-HCl]0 = 40 mg/L, [5%Co/BiVO4]0 = 0.2 g/L, [PMS]0 = 5 mM, Vis irradiation.
To further identify the mechanism of TC-HCl degradation, free radicals trapping experiments were conducted (Fig. 9). TBA was used as an effective radical scavenger for %OH while MeOH was applied as an effective radical scavenger for both %OH and SO4−%. A slightly decrease of TC-HCl degradation efficiency was observed after TBA scavenger was added in the Co/BiVO4+Vis + PMS system, which indicated that the % OH was a minor factor contributing to TC-HCl degradation. By contrast, TC-HCl degradation efficiency was remarkable reduced when MeOH was added indicating that SO4−% plays an important role in the degradation of TC-HCl. It is well known that BiVO4 and Co3O4 are typical semiconductor materials. According to the above calculation results, EVB and ECB of BiVO4 are 2.67 eV and 0.39 eV, respectively. According to previous
articles, the EVB and ECB of Co3O4 are 2.44 eV and 0.37 eV, respectively [34]. Hence, the potential difference between the two semiconductors created a feasible path which favoring the enrichment of electrons on the surface of BiVO4 while the accumulation of holes in Co3O4. When the BiVO4 is coupled with the Co3O4, a heterojunction is formed as demonstrated by the DRS and PL results. The XPS spectra of Co 2p on 5%Co/BiVO4 surface before and after reaction were further studied to clarify reaction mechanism (Fig. 2e-f). The peaks with binding energy positioned at 793.38 eV and 778.78 eV 6
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Fig. 10. Schematic illustration for the possible mechanism in the 5%Co/BiVO4+Vis + PMS system.
SO4−% / %OH/ h+ + TC-HCl → products
can be attributed to Co3+, and the peaks with binding energy positioned at 795.28 eV and 780.48 eV can be attributed to Co2+. The relative area of the four peaks is 43%, 17%, 27%, and 12%, respectively. For the XPS spectra of Co 2p after reaction, the peaks with binding energy positioned at 794.38 eV and 779.38 eV can be attributed to Co3+, and the peaks with binding energy positioned at 796.18 eV and 781.68 eV can be attributed to Co2+. The relative area of the peaks as mentioned above are 52%, 18%, 18% and 10%, respectively. This variation indicates that the degradation reaction initializes on the surface of Co/BiVO4, whereby the Co2+ possibly reacted with HSO5− and further caused an increase of Co3+. To maintain the electrons/charge balance on the surface of catalyst, the Co3+ would react with HSO5− and afterward turned back to Co2+. This explanation implies a redox cycle of the Co3O4 (Co2+/Co3+) during the PMS activation process [49]. According to above discussions, a synergistic mechanism of advanced oxidation processes coupling photocatalysis degradation for TCHCl was illustrated in Fig. 10. Firstly, after the loaded with Co3O4, the ability of visible light absorption of photocatalysts was enhanced and more electron/hole pairs could be produced. Secondly, photogenerated electrons was trapped by PMS to generate SO4−% and electron/hole pairs recombination was suppressed (Eq. (6)). Thirdly, Co3O4 from Co/ BiVO4 composites could activate PMS to generate SO4−% and SO5−%radicals (Eq. (8–9)). The above XPS results clarified the redox cycle of the Co3O4 (Co2+/Co3+) during the PMS activation process. Lastly, the VB potential of Co3O4 is higher than the standard redox potential of OH−/%OH (2.4 eV vs. NHE), so holes on the surface of catalysts were able to react with OH− to produce %OH or oxidize directly pollutant [50]. The SO4−% and %OH species generated from above mentioned approach has strong oxidation properties, which can oxidize effectively the target organic pollutant TC-HCl. Reaction process is as follows: Co/BiVO4 + hν → e− + h+ HSO5− HSO5− Co
2+
Co
3+
+e
−
+h
+
→
+ OH
−%
+
→ SO5
+
HSO5−
→ Co
+
HSO5−
→ Co
+H
3+ 2+
+ SO4 + SO5
−%
−
+ OH
(8)
+
(9)
+H
SO4−% + H2O → HSO4− + %OH
(11)
2SO4−%
(12)
SO4 OH
−
+
S2O82−
S2O82− %
→SO4
+ h → OH +
2−
4. Conclusions In this work, the high-efficiency Co/BiVO4 composites were successfully prepared by wet impregnation method and used for degradation of tetracycline hydrochloride (TC-HCl). As we can saw that 100 mL 40 mg/L TC-HCl was completely degraded within only 25 min when combining of photocatalysis and sulfate radical-based oxidation processes. Based on free radicals trapping experiments, it was found that SO4−% was main free radicals for the degradation of TC-HCl, %OH played the minor role on it. The results indicated that the introduction of PMS and Co3O4 suppressed the recombination of photogenerated electron/hole pairs. Moreover, the PMS not only can be activated by photogenerated electrons but also by Co3O4 to generate SO4−%, which leaded to the efficient degradation of TC-HCl. Besides, the Co/BiVO4 catalysts show good stability after 4 cycles. This work may provide a new insight into the design of catalyst used to combine photocatalysis and sulfate radical-based oxidation processes for refractory organic pollutants degradation.
(7) −%
(10)
→
It is worth noting that the reusability and stability of photocatalyst has a significant influence on the actual application. The result of cyclic experiments of Co/BiVO4 composites is shown in Fig. 11. After four cyclic experiments, there was no distinct reduction of TC-HCl degradation rate. The little reduction might be result from the loss of catalyst during recycling.
(6)
2SO5−% → 2SO4−% + O2
−%
3.4. Reusability of Co/BiVO4
(5) −
SO4−%
+
S2O8−%
(15)
(13) (14) Fig. 11. Cyclic experiments of 5%Co/BiVO4 for TC-HCl degradation. 7
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Acknowledgement
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