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Optimizing P25-rGO composites for pesticides degradation: Elucidation of photo-mechanism
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Optimizing P25-rGO composites for pesticides degradation: Elucidation of photo-mechanism ⁎
G. Luna-Sanguinoa, A. Tolosana-Moranchelb, C. Duran-Vallec, M. Faraldosa, , A. Bahamondea,
⁎
a
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, Marie Curie 2, 28049 Madrid, Spain Departamento de Ingeniería Química, Facultad de Ciencias, Francisco Tomás y Valiente 7, Universidad Autónoma de Madrid, 28049 Madrid, Spain c Departamento de Química Orgánica e Inorgánica and IACYS, Universidad de Extremadura, Av. Elvas S/N, 06006 Badajoz, Spain b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2P25-reduced graphene oxide composites Pesticides Scavengers Photo-mechanism
The junction of graphene oxide with TiO2 particles can help develop more efficient photocatalysts capable to harvest radiation in a wider range of the electromagnetic spectrum for real photocatalytic applications. The synthesis procedure of TiO2 P25-rGO composites was optimized to photodegrade a selected mixture of pesticides classified by EU as priority pollutants (alachlor, diuron, atrazine and isoproturon). The influence of temperature and time of hydrothermal method, as well as the effect of graphene oxide (GO) percentage added in the synthesis, was studied to obtain the nanocomposite that showed the highest photoactivity. Long time and moderate temperature have offered the best interaction between TiO2 P25 and rGO. GO was quantitatively reduced to rGO during the hydrothermal treatment, but maintains a higher level of disorder. The optimal GO loading was found around 0.25 wt. %, which allowed the photocatalyst achieve high photocatalytic performance both in phenol and pesticides photodegradation. Finally, in order to try to elucidate the photocatalytic mechanism of the selected mixture of pesticides three scavengers were employed: methanol to scavenge hydroxyl radicals, formic acid for the photogenerated holes, and copper (II) nitrate to quench the electrons of the conduction band. In conclusion, all these pesticides were mostly photodegraded by the hydroxyl radicals (HO%) produced from the photo-induced holes (h+); given that the oxidant species generated from electrons or mediated by direct mechanism were not relevant.
1. Introduction At the beginning of the 21st century, mankind has had to face the problem of the availability of water as an important threat, due to the loss of the balance between the quantity and quality of available water and its demand [1]. In the last century the exponential increase of intensive agriculture around the world, and in particular in the Mediterranean area, has meant a rapid increase of pollution regarding our water resources as a consequence of excess use of pesticides [2]. Nowadays, one of the key ideas for the new water management approach has begun to take root by considering the problem as a whole called the “integral water cycle” [3]. For that, the development of more efficient technologies, such as AOPs, allows the decontamination of wastewater [4–6]. In this context, the heterogeneous photocatalysis is a very relevant “Advanced Oxidation Catalytic Technology”, being an affordable technology to reduce organic contaminants in wastewater, becoming energetically and cost efficient when solar radiation is used [7,8].
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Although TiO2 is one of the most demanded photocatalysts, it has some limitations. Because of its high band gap value (3.0–3.2 eV), TiO2 is only able to absorb radiation with wavelengths below 390 nm [9]. Therefore, the design of new photocatalysts able to absorb photons over a wider range of wavelengths is still required to improve solar photocatalytic applications [10–12]. In this sense, many strategies are being developed to increase the overall photoefficiency such as micro/nano sized, doped-catalyst, composites, surface engineering, among others [13]. Nowadays, the synthesis of composites based on TiO2 and graphene oxide (GO), which is subsequently reduced, attracts a great deal of attention due to its beneficial properties; among them, excellent absorption in the visible region, charges mobility, large surface area and good stability can be emphasized [14,15]. Therefore, the presence of reduced graphene oxide (rGO) in TiO2-rGO nanocomposites improves charge separation and decreases recombination of the photogenerated electron-hole pairs [16–18]. Additionally, other mechanisms of GO and rGO to enhance photocatalytic activity goes through the capacity to absorb Visible radiation, shortening of band gap [19] or
Corresponding authors. E-mail addresses: [email protected] (M. Faraldos), [email protected] (A. Bahamonde).
https://doi.org/10.1016/j.cattod.2019.01.025 Received 6 July 2018; Received in revised form 21 December 2018; Accepted 7 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Luna-Sanguino, G., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.025
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Anatase and rutile crystalline phases ratios were determined by Spurr and Myers method [30] and the average crystallite sizes were calculated using Scherrer equation [31]. UV–vis studies were performed with a Cary 5000 Agilent spectrophotometer equipped with an integrating sphere to obtain the absorption spectra and the band gap of composites was calculated by Tauc plots [32]. BET specific surface area and mesoporosity was determined from nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2420 apparatus) on samples previously outgassed overnight at 140 °C. The porosity studies were completed by mercury intrusion porosimetry (MIP) using Micromeritics AutoPore IV 9510. Total pore volume was evaluated combining both techniques. Raman spectroscopy analyses were carried out in a Raman Renishaw microscope to verify the presence of both TiO2 and rGO. X-ray photoelectron spectra (XPS) were obtained with a K-Alpha Thermo Scientific spectrometer in order to obtain qualitative information about surface composition and atomic oxidation state, and EDX with a Quanta 3D FEG of FEI company, to know quantitatively the surface composition, respectively; the corresponding XPS and EDX atomic ratios were calculated.
design heterojunctions [20]. The studied water-soluble pesticides are considered among priority substances (PS) included in the European Water Framework Directive WFD, Directive 2000/60/EC and Directive 2013/39/EU; and classified in the Watch List of Decision 2015/495/EU, because of their toxicity even at very low concentration, persistence in the environment and bioaccumulation [21]. Besides, biological treatment of wastewatercontaining these pesticides, is not recommended and cause damage tor microorganisms [22]. To better understand the overall photocatalytic process, the analysis of the photo-mechanisms of pollutants photodegradation is a crucial point to try to know the optimal performance under photoreaction conditions. Despite the role of active species leading to the initial photoreaction process has been widely investigated, it is still very controversial [23]. It is known there are three ways: i) via the direct electron transfer between substrate and positive holes; ii) via the ROS (Reactive Oxygen species) formed from the conduction band (CB) electrons or iii) via the hydroxyl radical mediated pathway; but our major doubt is how the photo-oxidation exactly takes place [24–26]. In order to resolve this controversy, inhibitors or scavengers are usually employed to elucidate the importance of each mechanism during the photocatalytic process [16,17,27,28]. As a result, the main objective of this work was the optimization of the procedure to synthesize TiO2 P25rGO composites for their application in the photocatalytic degradation of a selected mixture of pesticides classified by EU as priority pollutants (alachlor, diuron, atrazine and isoproturon) [29]. The influence of temperature and time on hydrothermal method, as well as the effect of GO loading was studied in phenol photodegradation runs to define an optimal ratio of GO to TiO2 in the initial synthesis gel. Finally, the photocatalytic mechanism of a selected mixture of pesticides using this optimized hybrid photocatalyst was elucidated through the use of three scavengers: methanol to scavenge the hydroxyl radicals, formic acid for the photogenerated holes, and copper (II) nitrate to quench the electrons of the conduction band (CB).
2.3. Photocatalytic activity studies
An aqueous suspension, prepared with 2 g of commercial TiO2 P25 (Evonik) and the corresponding amount of commercial 4 mg mL−1 GO suspension (Graphenea), previously stirred for 3 h, was poured in a pressure PTFE-stainless steel jacket reactor for hydrothermal treatment. During this step the mix was continually stirred to keep the dispersion stable and avoid using other substances, such as dispersants. The resulted suspension was centrifuged for 1 h at 12,000 rpm and the obtained solid was dried overnight at 50 °C. The optimization of the synthesis procedure has involved the study of two temperatures (120 °C and 180 °C) at different times: 6–18 h, and some GO/TiO2 ratios: 0.0, 0.1, 0.25, 0.50 and 1.0 wt. % GO. Therefore, composites are nominated P25-rGO-X, where X refers to the GO added in the synthesis procedure, in wt. %.
All photocatalytic runs were carried out in a discontinuous slurry type photoreactor, previously described in detail [33]; surrounded by 10 fluorescent lamps: six UV lamps (Narva LT 15 W/073 Black-Light Blue, λmax = 366 nm) and four Vis lamps (Narva LT 15 W/865 Cool Day-Light, λ = 400–700 nm). The provided irradiance was: UV 44 ± 4 W·m−2 and Vis 71 ± 7 W·m−2, measured by a Delta Ohm HD 2302.02 LightMeter radiometer equipped with LP 471 UVA (315–400 nm) and LP 471 RAD (400–1050 nm) probes. To optimize the variables of the hydrothermal synthesis, phenol photodegradation runs were carried out under a 1 L reactant mixture consisting in 50 mg L−1 of phenol at natural pH (≈ 6) and 250 mg L−1 of suspended hybrid photocatalyst premixed with 75 Ncm3 min−1 of continuous air flow in dark conditions during 30 min to guarantee homogeneity in the photo-reactor. After this period, a sample was withdrawn to measure the initial concentration of phenol. Then, photocatalytic runs were started by turning on all the lamps. 50 mg L−1 phenol concentration was chosen to obtain and quantify intermediates at sufficient concentration to define clearly the optimal photocatalyst. On the other hand, the performance of TiO2-rGO-0.25 composite, which provided the highest photoactivity, was evaluated in photocatalytic runs of 1 L selected mixture of pesticides using 5.0 mg L−1 of each pesticide (diuron, atrazine, alachlor and isoproturon), at natural pH (≈6), under the same operating conditions described above. Photolysis runs in absence of photocatalysts, and blank runs in dark conditions were also carried out under the mentioned operating conditions. Negligible degradation was observed in any case (phenol conversions < 5%). Total Organic Carbon (TOC) concentration was measured by a TOCVCSH/CSN Shimadzu analyzer, whereas pesticides and phenol were monitored by HPLC chromatography (Azura by Knauer).
2.2. Characterization studies
3. Results and discussion
2. Experimental 2.1. Composites
Powder XRD patterns were performed on a PANalytical X’Pert PRO.
Table 1 summarizes the main physico-chemical properties of the
Table 1 Main physico-chemical properties and photocatalytic activity of composites synthesized at different temperature and time with 0.5 wt. % of GO. Temperature (ºC)
time (h)
danatase (nm)
drutile (nm)
Anatase (%)
Rutile (%)
Band-gap (eV)
XTOC (%)
XPhenol (%)
120 120 180 180
6 18 6 18
19 19 18 21
30 27 26 31
83 82 76 82
17 18 24 18
3.2 3.2 3.1 3.1
55 70 62 58
80 86 83 79
2
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obtained composites from the study of temperature and time under the hydrothermal method. All these hybrid catalysts were prepared with a 0.5 wt. % of GO to select the most adequate synthesis parameters. First, XRD results showed that differences among anatase and rutile percentages in the diverse composites prepared were of the same order than statistical errors. Therefore, it could be concluded that crystalline phases composition of the synthetized P25-rGO composites was invariable, 78 ± 4% anatase and 22 ± 4% rutile, no relevant differences were noted except for a slight increase in anatase and rutile crystallite diameters when temperature and time under the hydrothermal method were increased, in the P25-rGO composite prepared at 180 °C/18 h. XRD results of P25-rGO composites have showed the absence of the characteristic peak of GO at 2θ 11° (see Supporting Information Figures SI.1), that is pointing out the reduction of graphene oxide during the hydrothermal treatment. This supposition is corroborated with XRD pattern of P25-rGO-100 (equivalent to rGO), obtained from GO in the same hydrothermal conditions than composites, where there is no presence of the above-mentioned peak. Moreover, the presence of rGO in the composite although has provoked a slight absorption in the visible region of UV–vis spectra, hardly differences were observed in comparing band gap values when temperature and time were increased (see Table 1 and Supporting Information Figures SI.2). A comparative study in the photodegradation of phenol was carried out to select the most adequate synthesis parameters given that is a model pollutant widely studied in photocatalytic applications [34]. The best photoactivity, both in phenol and TOC conversions at 300 min of irradiation time, was obtained with the hybrid catalyst hydrothermally prepared at 120 °C for 18 h under continuous stirring. After 5 h of irradiation time, around 86 and 70% of phenol and TOC conversions were achieved, respectively (see Table 1). Opposite effects can be observed in composites obtained at both temperatures for longer synthesis time, as it is shown in Supporting Information Figure SI.3. Optimal GO content was studied in the range of low concentrations according to scientific studies regarding aqueous photocatalytic applications [35]. For that, various nanocomposites were prepared by adding different amounts of GO (from 0 to 1 wt. % GO content) in the P25-rGO synthesis, prepared under optimal hydrothermal conditions, at 120 °C for 18 h and continuous stirring. Their main physico-chemical properties are summarized in Table 2. XRD results of P25-rGO composites showed that differences among anatase and rutile percentages in the diverse composites prepared were of the same order than statistical errors. Therefore, crystalline phases of the synthetized P25-rGO composites keep at invariable composition, 78 ± 4% anatase and 22 ± 4% rutile. Textural properties of these hybrid photocatalysts (see Table 2), presented a bimodal pore size distribution in the range of meso and macroporosity without relevant differences except for a slight increase in BET surface area owing to an increase of rGO content in the composite. N2 adsorption-desorption isotherms for all composites (see Supporting Information Figure SI.4), were type II isotherms, with a H2 hysteresis loop characteristic of mesoporous materials [36]. Regarding electronic properties, band gap values resulted scarcely modified with the increase of rGO content in the composite.
Table 3 Results of photocatalytic activiy, (O/Ti) and (C/Ti) ratios of composites synthesized at different GO loading. Catalyst
(O/Ti)EDX
(O/Ti)XPS
(C/Ti)EDX
(C/Ti)XPS
XTOC (%)
XPhenol (%)
P25-rGO-0 P25-rGO-0.1 P25-rGO-0.25 P25-rGO-0.5 P25-rGO-1.0
2.00 2.00 2.07 2.07 2.19
2.38 2.35 2.39 2.37 2.38
– 0.173 0.176 0.180 0.198
– 0.647 0.671 0.672 0.762
60 66 71 70 42
80 83 87 86 76
On the other hand, Raman spectroscopy (see Supporting Information Figure SI.5) showed the characteristic peaks of both TiO2 and reduced graphene oxide. The TiO2 Eg peak reached intensity greatly higher than A1g, related to the lack of {001} exposed facets. From Raman ratios (see Table 2), ID/IG ratio associated to the proportion of Csp3 (1360 cm−1) hybridized versus Csp2 (1590 cm−1), it can be assumed that the order was not recovered during hydrothermal treatment; and the resulted composites have a high number of defects, verified by IS1/IS2 ratios, which compares 2D (S1) peak at approximately 2700 cm−1 versus D + D‘(S2) peak (2900 cm−1). Therefore, considering Raman ratios it can be assumed that rGO maintains a higher level of disorder, even when GO become quantitatively reduced to rGO during the hydrothermal synthesis. From a comparative analysis of O/Ti atomic ratios obtained from EDX and XPS (see Table 3), can be observed that both O/Ti ratios are not dependent on rGO loading in the P25-rGO composites, consequently must be linked to TiO2 fraction. Contrarily a tendency has been found in EDX and XPS C/Ti ratios, where the higher the rGO loading the greater ratio observed. Always a large XPS C/Ti was observed in comparison with EDX C/Ti, which suggested that C seems to be predominately located in the outer surface of the hybrid catalyst particles. TOC and phenol conversions at 300 min of irradiation time are also given in Table 3. It is known that phenol oxidation proceeds through a complex reaction scheme giving rise to some intermediate compounds before mineralization was reached [37]. In all cases, catechol, p-benzoquinone and hydroquinone were always the primary oxidation intermediates as a result of phenol hydroxylation, and they underwent further oxidation yielding short-chain organic acids, such as maleic, oxalic and formic, which were responsible of the final TOC detected in the reaction medium. It can be observed in Supporting Information Figures SI.6 that the phenol photo-oxidation was improved with a loading as low as 0.1 wt. % of GO (P25-rGO-0.1). However, the best phenol and TOC conversion were obtained by increasing the rGO content up to 0.25 wt. % (P25rGO-0.25). With a higher GO content, such as 1 wt. % (P25-rGO-1.0), lower phenol and TOC conversions were obtained, probably as a consequence of the poor radiation reaching TiO2, given that C seems to be deposited in the most outer surface of the hybrid catalysts, as was found before in the comparative analysis of EDX and XPS C/Ti ratios. Therefore, the P25-rGO-0.25 composite, synthesized at 120 °C for 18 h of continual stirring during the hydrothermal method, was selected to photodegradate a selected mixture of pesticides.
Table 2 Main physico-chemical properties of composites synthesized at different GO loading. Composite
danatase (nm)
drutile (nm)
Anatase (%)
Rutile (%)
SBET (m2·g−1)
Vmeso (cm3·g−1)
VMacro (cm3·g−1)
VTOTAL (cm3·g−1)
Band-gap (eV)
ID/IG
IS1/IS2
P25-rGO-0.0 P25-rGO-0.1 P25-rGO-0.25 P25-rGO-0.5 P25-rGO-1.0 GO
19 19 19 19 19 –
30 29 27 27 27 –
82 84 82 83 81 –
18 16 18 17 19
55 56 57 56 59 –
0.38 0.37 0.39 0.35 0.38 –
0.50 0.50 0.48 0.55 0.48 –
0.88 0.87 0.87 0.89 0.86 –
3.0 3.1 3.1 3.2 3.2 –
– 0.96 0.98 0.99 1.00 0.96
– 0.94 0.85 0.85 0.86 0.94
3
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the reaction, isoproturon and diuron photo-oxidation were slightly achieved. Given that the presence of Cu2+ ions in the reaction medium has given place to a very low contribution, this could be related to the reduction of Cu2+ ions by the conduction band electrons [20,21]. In order to better understand the effect of the different scavengers studied and the importance or relevance of each mechanism during the photodegradation process with the above mixture of pesticides, the following kinetic ratios have been calculated:
• k = (k − k 1
HCOOH
− kCH3 OH )/ k
represents the oxidation by the HO% radicals produced from the photo-induced holes (h+).
•k =k 2
CH3 OH / k
denotes the direct photodegradation by holes (h+) because the reactive radicals formed from the photogenerated holes (HO% radicals) or electrons (O2%−, HO2%, HO%) were trapped
Fig. 1. Pesticides and TOC evolution with the selected P25-rGO-0.25 composite.
• k =k 3
Although all the pesticides were totally eliminated, a TOC concentration was still found at the end of the process, as it can be seen in Fig. 1, where the evolution of pesticides and TOC concentrations for the P25-rGO-0.25 composite is shown. While three of the studied pesticides (diuron, alachlor and isoproturon) were completely eliminated after one hour and a half, atrazine needed longer irradiation time (3 h). In this sense, some identified organic by-products and short-chain organic acids [38] could well be the responsible for the detected residual organic matter. In order to know the role of holes (h+), hydroxyl radicals (HO%) and electrons (e−) during the corresponding photodegradation mechanism, some inhibitors or scavengers have been employed to try to elucidate the photocatalytic mechanism of the photodegradation of pesticides with the selected photocatalyst: P25-rGO-0.25. Quenching experiments with methanol (MeOH), a strong competitor for reactions with hydroxyl radicals in the liquid phase [17,20] have been used to analyze the efficiency of HO% production. Formic acid was chosen as a photogenerated hole scavenger [16,20,21], and finally copper (II) nitrate was used to quench the electrons of the conduction band (CB) [20,21,39,40]. From previous studies, the employed concentrations of methanol, formic acid and Cu(NO3)2 were 250, 5 and 10 mM, respectively. This study has been always carried out with the selected mixture of pesticides, both in absence or presence of the three scavengers studied. The evolution of normalized concentrations of each pesticide, in the absence or the presence of the three scavengers can be observed in Fig. 2, where the corresponding results of each pesticide in the mixture are given. In this figure, the photodegradation results of each pesticide are given to ease and better understand the whole photodegradation process of the studied mixture of pesticides. All the experiments have shown that pesticides were only slightly photo-oxidized when formic acid was added to the reaction. Furthermore, almost negligible photodegradation of every pesticide was observed in the presence of methanol. These results were observed because hydroxyl radicals were not produced or available in the presence of these used scavengers or inhibitors. Therefore, taking into account that null or very slight photodegradation conversions of each pesticide were obtained when methanol or formic acid were added to the reaction, it can be said these pesticides could not be hydroxylated. In this line, it is known that aromatic compounds such as pesticides are mainly photodegraded by HO% radicals when UV light is used as irradiation source [20]. However, some photo-oxidation, especially in the case of isoproturon, was observed when copper (II) nitrate was used. When no HO% scavenger or when Cu(NO3)2 (electron scavenger) was added to
HCOOH/k
represents the oxidation by the reactive oxygen species produced from electrons (O2%−, HO2%, HO%), as holes (h+) were quenched by formic acid (HCOOH). kCH3 OH , kHCOOH, kCu (NO3 )2 are the pseudo-first order kinetic rate constants calculated by the reactions carried out in the presence of methanol, formic acid and copper (II) nitrate, respectively. Finally, k represents the total oxidation both by the mechanism mediated by the holes and by the mechanism mediated by the electrons. Therefore, k is the kinetic constant obtained in the case of the fastest photo-oxidation process either in the presence or in the absence of scavengers. In this case, all pesticides, as it can be seen in Fig. 2, have reached the fastest photodegradation rates in the absence of the three scavengers used at the operating conditions studied here. A comparative study of the relative significance of these defined kinetic ratios, calculated in percentages, is shown in Fig. 3. From these kinetic ratios the importance of each photo-mechanism during the photodegradation process with the above mixture of pesticides can be stablished. In all cases k1 was the most relevant ratio which greatly exceeds k2 and k3 ratios, with values higher than 94–95 %. Therefore, it can be said that all these pesticides were mostly photodegraded by the hydroxyl radicals (HO%) produced from the photo-induced holes (h+); given that the oxidant species produced from electrons or mediated by direct mechanism were not relevant. 4. Conclusions P25-rGO composites have been prepared by a hydrothermal procedure where long time and moderate temperature have led to the best interaction between TiO2 and rGO. The presence of reduced graphene oxide in the composite has provoked a slight increase of absorption in the visible region of UV–vis spectra, but band gap values resulted scarcely modified with the increase of rGO content in the composite. Given that no characteristic peak of GO was observed in the composites XRD patterns, even in the P25-rGO-100 composite, graphene oxide could be considered quantitatively reduced to rGO during the hydrothermal treatment. However, a high level of disorder was still observed in Raman spectra of rGO. The optimal GO loading in the initial synthesis gel was found to be around 0.25 wt. %, achieving high photocatalytic performance both in phenol and pesticides photodegradation. Mixing reduced graphene oxide with titania in a composite gives rise to an improvement of the photo-oxidation process, limited by the radiation shielding effect as a consequence of reduced graphene oxide wrapping TiO2 particles that worsened at higher rGO loadings 4
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Fig. 2. Pesticides evolution in the absence and the presence of scavengers (methanol, formic acid and copper (II) nitrate), with the selected P25-rGO-0.25 composite.
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Fig. 3. Relative comparison significance of the defined kinetic ratios.
Finally, the photodegradation mechanism of this selected mixture of pesticides was analyzed. They were mostly photodegraded by the hydroxyl radicals (HO%) produced by the photo-generated holes, since the oxidant species generated from electrons (O2%−, HO2%, HO%) or direct photodegradation by holes were not relevant.
Acknowledgements This work has been supported by the Spanish Plan Nacional de I+D +i through the project CTM2015-64895-R and Junta de Extremadura (GR15123). Gema Luna-Sanguino and Alvaro Tolosana-Moranchel thank the Spanish “Ministerio de Economia, Industria and Competitividad” and the “Ministerio de Educacion, Cultura y Deporte”, for funding their predoctoral contracts BES-2016-077191 and FPU14/01605, respectively. 5
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Optimizing P25-rGO composites for pesticides degradation: Elucidation of photo-mechanism ⁎
G. Luna-Sanguinoa, A. Tolosana-Moranchelb, C. Duran-Vallec, M. Faraldosa, , A. Bahamondea,
⁎
a
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, Marie Curie 2, 28049 Madrid, Spain Departamento de Ingeniería Química, Facultad de Ciencias, Francisco Tomás y Valiente 7, Universidad Autónoma de Madrid, 28049 Madrid, Spain c Departamento de Química Orgánica e Inorgánica and IACYS, Universidad de Extremadura, Av. Elvas S/N, 06006 Badajoz, Spain b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2P25-reduced graphene oxide composites Pesticides Scavengers Photo-mechanism
The junction of graphene oxide with TiO2 particles can help develop more efficient photocatalysts capable to harvest radiation in a wider range of the electromagnetic spectrum for real photocatalytic applications. The synthesis procedure of TiO2 P25-rGO composites was optimized to photodegrade a selected mixture of pesticides classified by EU as priority pollutants (alachlor, diuron, atrazine and isoproturon). The influence of temperature and time of hydrothermal method, as well as the effect of graphene oxide (GO) percentage added in the synthesis, was studied to obtain the nanocomposite that showed the highest photoactivity. Long time and moderate temperature have offered the best interaction between TiO2 P25 and rGO. GO was quantitatively reduced to rGO during the hydrothermal treatment, but maintains a higher level of disorder. The optimal GO loading was found around 0.25 wt. %, which allowed the photocatalyst achieve high photocatalytic performance both in phenol and pesticides photodegradation. Finally, in order to try to elucidate the photocatalytic mechanism of the selected mixture of pesticides three scavengers were employed: methanol to scavenge hydroxyl radicals, formic acid for the photogenerated holes, and copper (II) nitrate to quench the electrons of the conduction band. In conclusion, all these pesticides were mostly photodegraded by the hydroxyl radicals (HO%) produced from the photo-induced holes (h+); given that the oxidant species generated from electrons or mediated by direct mechanism were not relevant.
1. Introduction At the beginning of the 21st century, mankind has had to face the problem of the availability of water as an important threat, due to the loss of the balance between the quantity and quality of available water and its demand [1]. In the last century the exponential increase of intensive agriculture around the world, and in particular in the Mediterranean area, has meant a rapid increase of pollution regarding our water resources as a consequence of excess use of pesticides [2]. Nowadays, one of the key ideas for the new water management approach has begun to take root by considering the problem as a whole called the “integral water cycle” [3]. For that, the development of more efficient technologies, such as AOPs, allows the decontamination of wastewater [4–6]. In this context, the heterogeneous photocatalysis is a very relevant “Advanced Oxidation Catalytic Technology”, being an affordable technology to reduce organic contaminants in wastewater, becoming energetically and cost efficient when solar radiation is used [7,8].
⁎
Although TiO2 is one of the most demanded photocatalysts, it has some limitations. Because of its high band gap value (3.0–3.2 eV), TiO2 is only able to absorb radiation with wavelengths below 390 nm [9]. Therefore, the design of new photocatalysts able to absorb photons over a wider range of wavelengths is still required to improve solar photocatalytic applications [10–12]. In this sense, many strategies are being developed to increase the overall photoefficiency such as micro/nano sized, doped-catalyst, composites, surface engineering, among others [13]. Nowadays, the synthesis of composites based on TiO2 and graphene oxide (GO), which is subsequently reduced, attracts a great deal of attention due to its beneficial properties; among them, excellent absorption in the visible region, charges mobility, large surface area and good stability can be emphasized [14,15]. Therefore, the presence of reduced graphene oxide (rGO) in TiO2-rGO nanocomposites improves charge separation and decreases recombination of the photogenerated electron-hole pairs [16–18]. Additionally, other mechanisms of GO and rGO to enhance photocatalytic activity goes through the capacity to absorb Visible radiation, shortening of band gap [19] or
Corresponding authors. E-mail addresses: [email protected] (M. Faraldos), [email protected] (A. Bahamonde).
https://doi.org/10.1016/j.cattod.2019.01.025 Received 6 July 2018; Received in revised form 21 December 2018; Accepted 7 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Luna-Sanguino, G., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.025
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Anatase and rutile crystalline phases ratios were determined by Spurr and Myers method [30] and the average crystallite sizes were calculated using Scherrer equation [31]. UV–vis studies were performed with a Cary 5000 Agilent spectrophotometer equipped with an integrating sphere to obtain the absorption spectra and the band gap of composites was calculated by Tauc plots [32]. BET specific surface area and mesoporosity was determined from nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2420 apparatus) on samples previously outgassed overnight at 140 °C. The porosity studies were completed by mercury intrusion porosimetry (MIP) using Micromeritics AutoPore IV 9510. Total pore volume was evaluated combining both techniques. Raman spectroscopy analyses were carried out in a Raman Renishaw microscope to verify the presence of both TiO2 and rGO. X-ray photoelectron spectra (XPS) were obtained with a K-Alpha Thermo Scientific spectrometer in order to obtain qualitative information about surface composition and atomic oxidation state, and EDX with a Quanta 3D FEG of FEI company, to know quantitatively the surface composition, respectively; the corresponding XPS and EDX atomic ratios were calculated.
design heterojunctions [20]. The studied water-soluble pesticides are considered among priority substances (PS) included in the European Water Framework Directive WFD, Directive 2000/60/EC and Directive 2013/39/EU; and classified in the Watch List of Decision 2015/495/EU, because of their toxicity even at very low concentration, persistence in the environment and bioaccumulation [21]. Besides, biological treatment of wastewatercontaining these pesticides, is not recommended and cause damage tor microorganisms [22]. To better understand the overall photocatalytic process, the analysis of the photo-mechanisms of pollutants photodegradation is a crucial point to try to know the optimal performance under photoreaction conditions. Despite the role of active species leading to the initial photoreaction process has been widely investigated, it is still very controversial [23]. It is known there are three ways: i) via the direct electron transfer between substrate and positive holes; ii) via the ROS (Reactive Oxygen species) formed from the conduction band (CB) electrons or iii) via the hydroxyl radical mediated pathway; but our major doubt is how the photo-oxidation exactly takes place [24–26]. In order to resolve this controversy, inhibitors or scavengers are usually employed to elucidate the importance of each mechanism during the photocatalytic process [16,17,27,28]. As a result, the main objective of this work was the optimization of the procedure to synthesize TiO2 P25rGO composites for their application in the photocatalytic degradation of a selected mixture of pesticides classified by EU as priority pollutants (alachlor, diuron, atrazine and isoproturon) [29]. The influence of temperature and time on hydrothermal method, as well as the effect of GO loading was studied in phenol photodegradation runs to define an optimal ratio of GO to TiO2 in the initial synthesis gel. Finally, the photocatalytic mechanism of a selected mixture of pesticides using this optimized hybrid photocatalyst was elucidated through the use of three scavengers: methanol to scavenge the hydroxyl radicals, formic acid for the photogenerated holes, and copper (II) nitrate to quench the electrons of the conduction band (CB).
2.3. Photocatalytic activity studies
An aqueous suspension, prepared with 2 g of commercial TiO2 P25 (Evonik) and the corresponding amount of commercial 4 mg mL−1 GO suspension (Graphenea), previously stirred for 3 h, was poured in a pressure PTFE-stainless steel jacket reactor for hydrothermal treatment. During this step the mix was continually stirred to keep the dispersion stable and avoid using other substances, such as dispersants. The resulted suspension was centrifuged for 1 h at 12,000 rpm and the obtained solid was dried overnight at 50 °C. The optimization of the synthesis procedure has involved the study of two temperatures (120 °C and 180 °C) at different times: 6–18 h, and some GO/TiO2 ratios: 0.0, 0.1, 0.25, 0.50 and 1.0 wt. % GO. Therefore, composites are nominated P25-rGO-X, where X refers to the GO added in the synthesis procedure, in wt. %.
All photocatalytic runs were carried out in a discontinuous slurry type photoreactor, previously described in detail [33]; surrounded by 10 fluorescent lamps: six UV lamps (Narva LT 15 W/073 Black-Light Blue, λmax = 366 nm) and four Vis lamps (Narva LT 15 W/865 Cool Day-Light, λ = 400–700 nm). The provided irradiance was: UV 44 ± 4 W·m−2 and Vis 71 ± 7 W·m−2, measured by a Delta Ohm HD 2302.02 LightMeter radiometer equipped with LP 471 UVA (315–400 nm) and LP 471 RAD (400–1050 nm) probes. To optimize the variables of the hydrothermal synthesis, phenol photodegradation runs were carried out under a 1 L reactant mixture consisting in 50 mg L−1 of phenol at natural pH (≈ 6) and 250 mg L−1 of suspended hybrid photocatalyst premixed with 75 Ncm3 min−1 of continuous air flow in dark conditions during 30 min to guarantee homogeneity in the photo-reactor. After this period, a sample was withdrawn to measure the initial concentration of phenol. Then, photocatalytic runs were started by turning on all the lamps. 50 mg L−1 phenol concentration was chosen to obtain and quantify intermediates at sufficient concentration to define clearly the optimal photocatalyst. On the other hand, the performance of TiO2-rGO-0.25 composite, which provided the highest photoactivity, was evaluated in photocatalytic runs of 1 L selected mixture of pesticides using 5.0 mg L−1 of each pesticide (diuron, atrazine, alachlor and isoproturon), at natural pH (≈6), under the same operating conditions described above. Photolysis runs in absence of photocatalysts, and blank runs in dark conditions were also carried out under the mentioned operating conditions. Negligible degradation was observed in any case (phenol conversions < 5%). Total Organic Carbon (TOC) concentration was measured by a TOCVCSH/CSN Shimadzu analyzer, whereas pesticides and phenol were monitored by HPLC chromatography (Azura by Knauer).
2.2. Characterization studies
3. Results and discussion
2. Experimental 2.1. Composites
Powder XRD patterns were performed on a PANalytical X’Pert PRO.
Table 1 summarizes the main physico-chemical properties of the
Table 1 Main physico-chemical properties and photocatalytic activity of composites synthesized at different temperature and time with 0.5 wt. % of GO. Temperature (ºC)
time (h)
danatase (nm)
drutile (nm)
Anatase (%)
Rutile (%)
Band-gap (eV)
XTOC (%)
XPhenol (%)
120 120 180 180
6 18 6 18
19 19 18 21
30 27 26 31
83 82 76 82
17 18 24 18
3.2 3.2 3.1 3.1
55 70 62 58
80 86 83 79
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obtained composites from the study of temperature and time under the hydrothermal method. All these hybrid catalysts were prepared with a 0.5 wt. % of GO to select the most adequate synthesis parameters. First, XRD results showed that differences among anatase and rutile percentages in the diverse composites prepared were of the same order than statistical errors. Therefore, it could be concluded that crystalline phases composition of the synthetized P25-rGO composites was invariable, 78 ± 4% anatase and 22 ± 4% rutile, no relevant differences were noted except for a slight increase in anatase and rutile crystallite diameters when temperature and time under the hydrothermal method were increased, in the P25-rGO composite prepared at 180 °C/18 h. XRD results of P25-rGO composites have showed the absence of the characteristic peak of GO at 2θ 11° (see Supporting Information Figures SI.1), that is pointing out the reduction of graphene oxide during the hydrothermal treatment. This supposition is corroborated with XRD pattern of P25-rGO-100 (equivalent to rGO), obtained from GO in the same hydrothermal conditions than composites, where there is no presence of the above-mentioned peak. Moreover, the presence of rGO in the composite although has provoked a slight absorption in the visible region of UV–vis spectra, hardly differences were observed in comparing band gap values when temperature and time were increased (see Table 1 and Supporting Information Figures SI.2). A comparative study in the photodegradation of phenol was carried out to select the most adequate synthesis parameters given that is a model pollutant widely studied in photocatalytic applications [34]. The best photoactivity, both in phenol and TOC conversions at 300 min of irradiation time, was obtained with the hybrid catalyst hydrothermally prepared at 120 °C for 18 h under continuous stirring. After 5 h of irradiation time, around 86 and 70% of phenol and TOC conversions were achieved, respectively (see Table 1). Opposite effects can be observed in composites obtained at both temperatures for longer synthesis time, as it is shown in Supporting Information Figure SI.3. Optimal GO content was studied in the range of low concentrations according to scientific studies regarding aqueous photocatalytic applications [35]. For that, various nanocomposites were prepared by adding different amounts of GO (from 0 to 1 wt. % GO content) in the P25-rGO synthesis, prepared under optimal hydrothermal conditions, at 120 °C for 18 h and continuous stirring. Their main physico-chemical properties are summarized in Table 2. XRD results of P25-rGO composites showed that differences among anatase and rutile percentages in the diverse composites prepared were of the same order than statistical errors. Therefore, crystalline phases of the synthetized P25-rGO composites keep at invariable composition, 78 ± 4% anatase and 22 ± 4% rutile. Textural properties of these hybrid photocatalysts (see Table 2), presented a bimodal pore size distribution in the range of meso and macroporosity without relevant differences except for a slight increase in BET surface area owing to an increase of rGO content in the composite. N2 adsorption-desorption isotherms for all composites (see Supporting Information Figure SI.4), were type II isotherms, with a H2 hysteresis loop characteristic of mesoporous materials [36]. Regarding electronic properties, band gap values resulted scarcely modified with the increase of rGO content in the composite.
Table 3 Results of photocatalytic activiy, (O/Ti) and (C/Ti) ratios of composites synthesized at different GO loading. Catalyst
(O/Ti)EDX
(O/Ti)XPS
(C/Ti)EDX
(C/Ti)XPS
XTOC (%)
XPhenol (%)
P25-rGO-0 P25-rGO-0.1 P25-rGO-0.25 P25-rGO-0.5 P25-rGO-1.0
2.00 2.00 2.07 2.07 2.19
2.38 2.35 2.39 2.37 2.38
– 0.173 0.176 0.180 0.198
– 0.647 0.671 0.672 0.762
60 66 71 70 42
80 83 87 86 76
On the other hand, Raman spectroscopy (see Supporting Information Figure SI.5) showed the characteristic peaks of both TiO2 and reduced graphene oxide. The TiO2 Eg peak reached intensity greatly higher than A1g, related to the lack of {001} exposed facets. From Raman ratios (see Table 2), ID/IG ratio associated to the proportion of Csp3 (1360 cm−1) hybridized versus Csp2 (1590 cm−1), it can be assumed that the order was not recovered during hydrothermal treatment; and the resulted composites have a high number of defects, verified by IS1/IS2 ratios, which compares 2D (S1) peak at approximately 2700 cm−1 versus D + D‘(S2) peak (2900 cm−1). Therefore, considering Raman ratios it can be assumed that rGO maintains a higher level of disorder, even when GO become quantitatively reduced to rGO during the hydrothermal synthesis. From a comparative analysis of O/Ti atomic ratios obtained from EDX and XPS (see Table 3), can be observed that both O/Ti ratios are not dependent on rGO loading in the P25-rGO composites, consequently must be linked to TiO2 fraction. Contrarily a tendency has been found in EDX and XPS C/Ti ratios, where the higher the rGO loading the greater ratio observed. Always a large XPS C/Ti was observed in comparison with EDX C/Ti, which suggested that C seems to be predominately located in the outer surface of the hybrid catalyst particles. TOC and phenol conversions at 300 min of irradiation time are also given in Table 3. It is known that phenol oxidation proceeds through a complex reaction scheme giving rise to some intermediate compounds before mineralization was reached [37]. In all cases, catechol, p-benzoquinone and hydroquinone were always the primary oxidation intermediates as a result of phenol hydroxylation, and they underwent further oxidation yielding short-chain organic acids, such as maleic, oxalic and formic, which were responsible of the final TOC detected in the reaction medium. It can be observed in Supporting Information Figures SI.6 that the phenol photo-oxidation was improved with a loading as low as 0.1 wt. % of GO (P25-rGO-0.1). However, the best phenol and TOC conversion were obtained by increasing the rGO content up to 0.25 wt. % (P25rGO-0.25). With a higher GO content, such as 1 wt. % (P25-rGO-1.0), lower phenol and TOC conversions were obtained, probably as a consequence of the poor radiation reaching TiO2, given that C seems to be deposited in the most outer surface of the hybrid catalysts, as was found before in the comparative analysis of EDX and XPS C/Ti ratios. Therefore, the P25-rGO-0.25 composite, synthesized at 120 °C for 18 h of continual stirring during the hydrothermal method, was selected to photodegradate a selected mixture of pesticides.
Table 2 Main physico-chemical properties of composites synthesized at different GO loading. Composite
danatase (nm)
drutile (nm)
Anatase (%)
Rutile (%)
SBET (m2·g−1)
Vmeso (cm3·g−1)
VMacro (cm3·g−1)
VTOTAL (cm3·g−1)
Band-gap (eV)
ID/IG
IS1/IS2
P25-rGO-0.0 P25-rGO-0.1 P25-rGO-0.25 P25-rGO-0.5 P25-rGO-1.0 GO
19 19 19 19 19 –
30 29 27 27 27 –
82 84 82 83 81 –
18 16 18 17 19
55 56 57 56 59 –
0.38 0.37 0.39 0.35 0.38 –
0.50 0.50 0.48 0.55 0.48 –
0.88 0.87 0.87 0.89 0.86 –
3.0 3.1 3.1 3.2 3.2 –
– 0.96 0.98 0.99 1.00 0.96
– 0.94 0.85 0.85 0.86 0.94
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the reaction, isoproturon and diuron photo-oxidation were slightly achieved. Given that the presence of Cu2+ ions in the reaction medium has given place to a very low contribution, this could be related to the reduction of Cu2+ ions by the conduction band electrons [20,21]. In order to better understand the effect of the different scavengers studied and the importance or relevance of each mechanism during the photodegradation process with the above mixture of pesticides, the following kinetic ratios have been calculated:
• k = (k − k 1
HCOOH
− kCH3 OH )/ k
represents the oxidation by the HO% radicals produced from the photo-induced holes (h+).
•k =k 2
CH3 OH / k
denotes the direct photodegradation by holes (h+) because the reactive radicals formed from the photogenerated holes (HO% radicals) or electrons (O2%−, HO2%, HO%) were trapped
Fig. 1. Pesticides and TOC evolution with the selected P25-rGO-0.25 composite.
• k =k 3
Although all the pesticides were totally eliminated, a TOC concentration was still found at the end of the process, as it can be seen in Fig. 1, where the evolution of pesticides and TOC concentrations for the P25-rGO-0.25 composite is shown. While three of the studied pesticides (diuron, alachlor and isoproturon) were completely eliminated after one hour and a half, atrazine needed longer irradiation time (3 h). In this sense, some identified organic by-products and short-chain organic acids [38] could well be the responsible for the detected residual organic matter. In order to know the role of holes (h+), hydroxyl radicals (HO%) and electrons (e−) during the corresponding photodegradation mechanism, some inhibitors or scavengers have been employed to try to elucidate the photocatalytic mechanism of the photodegradation of pesticides with the selected photocatalyst: P25-rGO-0.25. Quenching experiments with methanol (MeOH), a strong competitor for reactions with hydroxyl radicals in the liquid phase [17,20] have been used to analyze the efficiency of HO% production. Formic acid was chosen as a photogenerated hole scavenger [16,20,21], and finally copper (II) nitrate was used to quench the electrons of the conduction band (CB) [20,21,39,40]. From previous studies, the employed concentrations of methanol, formic acid and Cu(NO3)2 were 250, 5 and 10 mM, respectively. This study has been always carried out with the selected mixture of pesticides, both in absence or presence of the three scavengers studied. The evolution of normalized concentrations of each pesticide, in the absence or the presence of the three scavengers can be observed in Fig. 2, where the corresponding results of each pesticide in the mixture are given. In this figure, the photodegradation results of each pesticide are given to ease and better understand the whole photodegradation process of the studied mixture of pesticides. All the experiments have shown that pesticides were only slightly photo-oxidized when formic acid was added to the reaction. Furthermore, almost negligible photodegradation of every pesticide was observed in the presence of methanol. These results were observed because hydroxyl radicals were not produced or available in the presence of these used scavengers or inhibitors. Therefore, taking into account that null or very slight photodegradation conversions of each pesticide were obtained when methanol or formic acid were added to the reaction, it can be said these pesticides could not be hydroxylated. In this line, it is known that aromatic compounds such as pesticides are mainly photodegraded by HO% radicals when UV light is used as irradiation source [20]. However, some photo-oxidation, especially in the case of isoproturon, was observed when copper (II) nitrate was used. When no HO% scavenger or when Cu(NO3)2 (electron scavenger) was added to
HCOOH/k
represents the oxidation by the reactive oxygen species produced from electrons (O2%−, HO2%, HO%), as holes (h+) were quenched by formic acid (HCOOH). kCH3 OH , kHCOOH, kCu (NO3 )2 are the pseudo-first order kinetic rate constants calculated by the reactions carried out in the presence of methanol, formic acid and copper (II) nitrate, respectively. Finally, k represents the total oxidation both by the mechanism mediated by the holes and by the mechanism mediated by the electrons. Therefore, k is the kinetic constant obtained in the case of the fastest photo-oxidation process either in the presence or in the absence of scavengers. In this case, all pesticides, as it can be seen in Fig. 2, have reached the fastest photodegradation rates in the absence of the three scavengers used at the operating conditions studied here. A comparative study of the relative significance of these defined kinetic ratios, calculated in percentages, is shown in Fig. 3. From these kinetic ratios the importance of each photo-mechanism during the photodegradation process with the above mixture of pesticides can be stablished. In all cases k1 was the most relevant ratio which greatly exceeds k2 and k3 ratios, with values higher than 94–95 %. Therefore, it can be said that all these pesticides were mostly photodegraded by the hydroxyl radicals (HO%) produced from the photo-induced holes (h+); given that the oxidant species produced from electrons or mediated by direct mechanism were not relevant. 4. Conclusions P25-rGO composites have been prepared by a hydrothermal procedure where long time and moderate temperature have led to the best interaction between TiO2 and rGO. The presence of reduced graphene oxide in the composite has provoked a slight increase of absorption in the visible region of UV–vis spectra, but band gap values resulted scarcely modified with the increase of rGO content in the composite. Given that no characteristic peak of GO was observed in the composites XRD patterns, even in the P25-rGO-100 composite, graphene oxide could be considered quantitatively reduced to rGO during the hydrothermal treatment. However, a high level of disorder was still observed in Raman spectra of rGO. The optimal GO loading in the initial synthesis gel was found to be around 0.25 wt. %, achieving high photocatalytic performance both in phenol and pesticides photodegradation. Mixing reduced graphene oxide with titania in a composite gives rise to an improvement of the photo-oxidation process, limited by the radiation shielding effect as a consequence of reduced graphene oxide wrapping TiO2 particles that worsened at higher rGO loadings 4
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Fig. 2. Pesticides evolution in the absence and the presence of scavengers (methanol, formic acid and copper (II) nitrate), with the selected P25-rGO-0.25 composite.
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Fig. 3. Relative comparison significance of the defined kinetic ratios.
Finally, the photodegradation mechanism of this selected mixture of pesticides was analyzed. They were mostly photodegraded by the hydroxyl radicals (HO%) produced by the photo-generated holes, since the oxidant species generated from electrons (O2%−, HO2%, HO%) or direct photodegradation by holes were not relevant.
Acknowledgements This work has been supported by the Spanish Plan Nacional de I+D +i through the project CTM2015-64895-R and Junta de Extremadura (GR15123). Gema Luna-Sanguino and Alvaro Tolosana-Moranchel thank the Spanish “Ministerio de Economia, Industria and Competitividad” and the “Ministerio de Educacion, Cultura y Deporte”, for funding their predoctoral contracts BES-2016-077191 and FPU14/01605, respectively. 5
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