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Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids
Journal Pre-proofs Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Maria J. Rivero, Paula Ribao, Beatriz Gomez-Ruiz, Ane Urtiaga, Inmaculada Ortiz PII: DOI: Reference:
S1383-5866(19)34896-8 https://doi.org/10.1016/j.seppur.2020.116637 SEPPUR 116637
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
24 October 2019 27 January 2020 27 January 2020
Please cite this article as: M.J. Rivero, P. Ribao, B. Gomez-Ruiz, A. Urtiaga, I. Ortiz, Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116637
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© 2020 Published by Elsevier B.V.
Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero, Paula Ribao, Beatriz Gomez-Ruiz, Ane Urtiaga, Inmaculada Ortiz*
Department of Chemical and Biomolecular Engineering. University of Cantabria. Av. de Los Castros s/n. 39005 Santander. Spain. * Corresponding author: [email protected]
1
ABSTRACT Halogenated organic compounds are frequently characterized by their bioaccumulative nature, and long-term effects on human health. Their low biodegradability and their difficult removal by conventional technologies lead to their undesirable persistence in the environment. Titanium dioxide and reduced graphene oxide TiO2-rGO composites have shown promising results with enhanced photocatalytic degradation rates compared to bare TiO2 in the degradation of a good number of persistent pollutants. In this context, this work deepens on the influence of the type and number of halogen substitution on the photocatalytic degradation of halogenated organic compounds (HOCs). For the experimental
analysis
two
HOCs,
dichloroacetic
acid
(DCA)
and
perfluorooctanoic acid (PFOA), have been selected as both molecules differ in the type of halogen atoms, chlorine and fluorine, the number of halogen atoms, and in the length of the alkyl chain. The results showed that TiO2-rGO catalysts achieved a similar kinetic performance for the removal of the primary contaminant in terms of its degradation rate. Besides, TiO2-rGO composite successfully induced the release of all chlorine atoms from the DCA molecule, achieving its total mineralization. However, PFOA defluorination and mineralization rates were remarkably lower than its degradation rate. Although both contaminants released two halogen atoms step-by-step for the degradation of each molecule, PFOA dehalogenation was slowed down due to the generation of secondary products that retain fluorine. Therefore, we conclude that the composite photocatalysts showed a similar performance in terms of degradation rate of the primary contaminant independent on the length of the alkyl chain and on the number of halogen atoms, however, the mineralization and dehalogenation rates were 2
strongly dependent on the number of halogen substituents and the length of the alkyl chain.
KEYWORDS Halogenated organic compounds; perfluorooctanoic acid; dichloroacetic acid; Photocatalysis; reduced graphene oxide
1. Introduction Water pollution is one of the main challenges that society has to face to ensure the quality and availability of freshwater resources due to the inevitable prospects of water scarcity (Ortiz et al., 2015). Indeed, the quality of water resources is continuously threatened due to the increased release of anthropogenic pollutants from industrial, agricultural and human activities. Among water contaminants, halogenated organic compounds (HOCs) are persistent, bio-cumulative and potentially toxic (Fujii et al., 2007; Luo et al., 2015). Due to these properties, international action to control the use of HOCs and their impact on the environment started in the 1970s, when North America, Western Europe and Japan banned dieldrin, dichlorodiphenyltrichloroethane (DDT) and restricted polychlorinated biphenyls (PCBs). Since then, many HOCs have been subjected to stricter regulations (UNEP, 2001). Within the HOCs group, dichloroacetic acid (DCA) is an haloacetic acid (HAA) analogous to acetic acid, where 2 of the 3 hydrogen atoms of the methyl group have been replaced by chlorine atoms (Fig. 1A). HAAs constitute the second most important group of disinfection by-products (DBPs) found in drinking water 3
after chlorine disinfection (Esclapez et al., 2012). DBPs have been widely related to adverse effects on human health, including cancer, growth retardation, congenital cardiac defects, and spontaneous abortion (Zhang et al., 2010). Although, the European Drinking Water Directive 98/83/EC (DWD) does not currently stablish any guideline for HAAs in drinking water, in the US EPA classification, DCA is considered as a potential human carcinogen, and accordingly the US EPA has set a maximum level of 60 µg L-1 for HAAs in drinking water (USEPA, 1998). Moreover, Australia and New Zealand regulate a limit of 100 µg L-1 of DCA in drinking water (Zhang et al., 2010) and Japan’s drinking water standards for DCA is 40 µg L-1 (Sakai et al., 2013). Perfluorooctanoic acid (PFOA) is another hazardous compound ubiquitously detected in the natural environment (Fig. 1B). PFOA belongs to the family of perfluoroalkyl substances (PFAS) and consists of a fully fluorinated alkyl chain of eight carbons with a carboxylic acid end-group. This molecular structure offers unique physical-chemical properties, such as the amphiphilic character, and the extraordinary thermal and chemical stability, that have led to its use in pesticides, cosmetics, greases, paints, adhesives or aqueous film forming foams used for firefighting (Kissa, 2001; Prevedouros et al., 2006). As a result, PFOA has been found in drinking and ground waters, in industrial effluents and landfill leachates (Fuertes et al., 2017; B. Gomez-Ruiz et al., 2018; Hu et al., 2016), as well as in human blood, serum, plasma and tissues (Bartolomé et al., 2017). Many studies have reported that human PFAS exposure has been linked to elevated cholesterol, obesity, immune suppression, endocrine disruption and cancer (Gilliland and Mandel, 1993; Nelson et al., 2010). The USEPA has also established health advisory levels for PFOA and PFOS in drinking water at 0.07 4
µg L-1 (USEPA, 2016). Since June 2017, the European Union has also published actions to ban the PFOA production, and its use and placement in the market (The European Commission, 2017). Some technologies have already been applied but its fully removal is still an issue (Banks et al., 2020). B
A
128.94 g mol-1
414.07 g mol-1
Figure 1 Chemical structures of DCA (A) and PFOA (B). The recalcitrant nature of HOCs towards most conventional wastewater treatments was typically attributed to the strong C−X bonds (where X is the halogen atom) (Kissa, 2001). The energy bond increases in this order: C−F > C−Cl > C−Br > C−I. Nevertheless, over the last few decades, advanced oxidation processes (AOPs) have appeared as non-conventional techniques that are able to enhance the oxidation ability for the degradation of recalcitrant compounds. AOPs stand out thanks to their versatility, non-selectivity to the target compound and the production of the highly reactive oxygen species (ROS), mainly the hydroxyl radicals (·OH), which are able to degrade organic compounds (Comninellis et al., 2008; Miklos et al., 2018). Among them, research on heterogeneous photocatalysis is increasing. Although the positive features of TiO2 (Pelaez et al., 2012), it still presents limitations to real photocatalytic applications (Spasiano et al., 2015). It is only activated under ultraviolet light, which represents about 5% of the solar spectrum, and exhibits 5
high electron-hole pair recombination. To overcome these drawbacks of the TiO2 catalyst, new strategies based on the combination of TiO2 with other elements, such as noble metals, and carbon-based materials have been recently addressed (Dahl et al., 2014). Among them, graphene oxide (GO) shows outstanding properties because it can be easily reduced to reduced graphene oxide (rGO) during coupling with TiO2 nanoparticles via hydrothermal synthesis, providing a significant opportunity for TiO2-rGO materials. These composites can provide several advantages: (i) rGO can act as an electron trap, avoiding the undesirable electron-hole recombination; (ii) the excitation wavelength can be broaden to the visible region; (iii) the composite can be easily recovered because of the large surface area of the rGO nanosheets, and (iv) promotion of the adsorption of reactants thanks to additional active sites (Leary and Westwood, 2011). The effective photocatalytic activity of the composites based on GO and TiO2 nanoparticles has become of increasing interest in the past years in the literature (Adamu et al., 2016; Kumordzi et al., 2016; Lavanya et al., 2016; PastranaMartínez et al., 2012; Wang et al., 2013). The present work aims to generate further knowledge on the photocatalytic performance of TiO2-rGO composites by comparatively analyzing the degradation and mineralization rates of two different recalcitrant halogenated organic compounds, DCA and PFOA. Both contaminants differ in the halogen type, chloride and fluoride, in the number of halogens that are bonded to carbon atoms, and in the length of the alkyl chain thus, offering a broader scope to the study. 2. Experimental
6
2.1.
Chemicals
PFOA (C7F15COOH, 96% purity), perfluoroheptanoic acid (PFHpA, C6F13COOH, 99% purity), perfluorohexanoic acid (PFHxA, C5F11COOH, 96% purity), perfluoropentanoic acid (PFPeA, C4F9COOH, 97% purity) and DCA (99% purity) from Sigma Aldrich Chemicals were used. All solutions were prepared with QPOD Millipore water with conductance of 18.2 mΩ cm at 25 °C and 3 ppb of dissolved organic carbon (DOC). TiO2 P25 (Evonik) was selected to prepare the composite material. 2.2. Synthesis and characterization of TiO2-rGO photocatalyst Graphene oxide (GO) based on hard oxidation of graphite powder and subsequent exfoliation for GO nanosheets following modified Hummers method (Hummers and Offeman, 1958) was carried out. TiO2-rGO composite was synthetized via the hydrothermal method with GO content of 5 wt% (Ribao et al., 2017). The material was characterized via several techniques: transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method for specific surface area calculation, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), thermogravimetric analysis (TGA), and UV/Vis diffuse reflectance spectroscopy (Gomez-Ruiz et al., 2018; Ribao et al., 2017; Ribao et al., 2018). During the synthesis, GO was reduced and TiO2 nanoparticles were dispersed on the graphene nanoplatelets (Ribao et al., 2017). Moreover, Table 1 provides an overview of the main TiO2-rGO features. The specific surface area of the nanocomposite was evaluated from nitrogen adsorption isotherms using the BET equation. The number of graphene layers 7
was obtained from AFM; atomic composition was calculated by XPS; rGO real content was derived from TGA analysis heating the sample from 25ºC to 910 °C at 20 °C min−1 under 50 mL min-1 of nitrogen flow in an alumina cell and the band gap was calculated from UV-vis diffuse reflectance spectroscopy using the Kubelka-Munk function described elsewhere (Ribao et al., 2018).
TiO2 (Degussa)
TiO2-rGO 5 wt.%
50.0
62.2
-
2
Ti
26.6
28.1
O
73.4
58.1
C
-
13.8
-
7.27
3.25
2.94
Specific area (m2 g-1) Number of overlapping layers Atomic composition (at.%)
rGO content (wt.%) Band gap (eV)
Table 1. Summary of the main characterization results of TiO2 and TiO2-rGO catalysts. 2.3. Photocatalytic activity Photocatalytic experiments were carried out in a Heraeus Laboratory UV Reactor under continuous magnetic stirring and temperature was kept at 25 °C thanks to a water/ethylene glycol cooling bath (PolyScience). The total reactor volume was 1 L. A 150 W medium-pressure mercury lamp (Heraeus Noblelight TQ z1) with 8
an emission spectrum between 200 and 600 nm was used as irradiation source. Aliquots of the reacting mixture were filtered through 0.45 µm polypropylene filters. The irradiation received on the outer wall of the reactor was 145 ± 41 W m-2, 82 ± 13 W m-2, 1.7 ± 0.1 W m-2 and 729 ± 72 W m-2 for UV-A, UV-B, UV-C and visible light ranges, respectively, measured by a HD2102.1 photo/radiometer (Delta OHM). The initial concentrations of DCA and PFOA aqueous solutions used as feed in the experiments are higher than that in environmental matrices for the ease and accuracy of the analysis. The photocatalytic experiments were carried out with 0.8 L of 7.75 mmol L-1 DCA solution (pH 2.2) with 0.3 g L-1 of photocatalyst. On the other hand, the initial concentration of the PFOA solution was set at 0.24 mmol L-1, (initial pH 3.8). The TiO2-rGO composite dose was 0.1 g L-1 Every photocatalytic experiment was performed in duplicate and pH was not controlled during the process (Gomez-Ruiz et al., 2018). Several control tests were also performed. Adsorption of DCA and PFOA onto the catalysts was tested. After 24 h without light irradiation, there was no significant adsorption of DCA either on the TiO2 or on the TiO2-rGO. However, it was observed slight PFOA adsorption of 5.2±0.1% and 5.3±1.2% on TiO2 and TiO2-rGO surfaces, respectively, at a catalyst concentration of 0.1 g L-1. Additionally, experiments using dispersed GO nanosheets under light irradiation did not lead to any significant difference in DCA and PFOA concentration, indicating the absence of photocatalytic activity of GO. 2.4.
Analytical methodology
PFOA, PFHpA, PFHxA, PFPeA, were analyzed with a HPLC-DAD (Water 2695) 9
system with an X Bridge C18 column (5 µm, 250 mm x 4.6 mm, Waters). The column was placed in an oven at 40 ºC. A mixture of methanol (65%) and dihydrogen phosphate (35%) was used as mobile phase in isocratic mode with a flow rate of 0.5 mL min-1. The detector’s wavelength was fixed at 204 nm. The DCA and halogen ions (Cl- and F-) concentrations were measured in an ICS-1100 (Dionex) ion chromatograph equipped with an AS9-HC column in isocratic mode using Na2CO3 9 mM as the eluent at a flow rate of 1 mL min−1 and a pressure of approximately 2000 psi, based on Standard Methods 4110B (APHA (American Public Health Association), 1998). A TOC-V CPH (Shimadzu Corp. Japan) was used to quantify dissolved organic carbon (DOC).
3. Results and discussion The photocatalytic activity of the TiO2 and synthesized TiO2-rGO composite was studied through the degradation of two different halogenated organic compounds, DCA and PFOA. Moreover, the mineralization extent was monitored by both the release of halogen ions (chloride and fluoride) and the progress of DOC along the photocatalytic degradation time. 3.1. DCA and PFOA degradation over TiO2 catalyst Fig. 2A provides the degradation yield of DCA and PFOA after 8 hours when using bare TiO2 photocatalyst. The removal of DCA and PFOA was 35% and 10%, respectively. These limited degradation percentages likely resulted from the rapid recombination of electron/hole pairs formed by the TiO2 catalyst that restricts further degradation of the contaminants. Detailed data of the compounds degradation is included in the Supplementary material (Figure S1).
10
The experimental data of DCA and PFOA degradation by TiO2 catalyst were fitted to a first order kinetic model (Equation 1). d [HOC] = ―𝑘𝐻𝑂𝐶[HOC] dt
(Eq. 1)
The first order kinetic constants (kHOC) for DCA and PFOA photocatalytic degradation by means of TiO2 were 0.055 h-1 (R2 = 0.989) and 0.018 h-1 (R2 = 0.845) respectively. Therefore, the kinetic constant of DCA degradation by TiO2 photocatalysis is 3 times higher than in the case of PFOA. Fig. 2B shows the chloride and fluoride ions release into the reaction medium, during the degradation of DCA and PFOA, respectively. Results are expressed as percentage of the total halogen content in the initial contaminant. Chloride and fluoride release arose from the cleavage of the highly energetic C-Cl and C-F bonds. As a result, 31% and 2% of the total content of Cl- and F- in the initial feed solutions were released, respectively. Therefore, the percentage of subtracted chloride was similar to the DCA degradation (35%, Fig. 2A), meaning that TiO2 successfully induced the release of the chlorine atoms from the DCA molecule. However, in the case of PFOA treatment, although the obtained PFOA degradation raised up to 10% (Fig. 2A), only 2% of its fluorine content was released into the solution, confirming that the photoactivity of the TiO2 catalyst for PFOA dehalogenation was significantly constrained. Hence, PFOA decay during TiO2-mediated photocatalysis can be partially associated to the adsorption on the catalyst surface (~5%). In addition, only 0.012 mmol L-1 of perfluoropentanoic acid (PFHpA) was observed in the analytical survey. PFHpA is expected to be formed in the first step of PFOA degradation route; the observed PFHpA
11
generation matches with the 5% degradation of the initial PFOA (value obtained subtracting the concentration of PFOA adsorbed on the catalyst).
Figure 2 DCA and PFOA degradation after 8 hours (A), dehalogenation (B), and mineralization after 8 hours (C) of photocatalytic treatment using TiO2.
12
In order to get further insight on the TiO2 performance over halogenated compounds degradation, Fig. 2C shows DOC removal during the photocatalytic treatment of DCA and PFOA solutions. After 8 hours of irradiation, the values of mineralization were 32% and 4% for DCA and PFOA, respectively, consistent with their dehalogenation ratios. Detailed data of the compounds mineralization is included in the Supplementary material (Figure S2). 3.2. DCA and PFOA degradation over TiO2-rGO catalyst Fig. 3A shows the photocatalytic degradation yield of DCA and PFOA after 8 h of irradiation using the TiO2-rGO composite, 87% and 86%, respectively. Although the molecular structures of these contaminants exhibit different complexities, in terms of halogen atom nature, number of halogen substituents, and alkyl-chain length, the TiO2-rGO composite provided similar rates of removal for DCA and PFOA degradation, being the values of the first order kinetic constant 0.211 h-1 (R2 = 0.924). Detailed data of the kinetic fitting is included in the Supplementary material (Figure S3).These results pointed out a significant enhancement for both DCA and PFOA degradation rates compared to bare TiO2. In addition, it is remarkable that although TiO2 exhibited different photoactivity in the degradation of DCA and PFOA, the TiO2-rGO composite degraded both compounds to the same extent. Hence, the combination of TiO2 nanoparticles with rGO nanosheets overcomes the limited performance of TiO2 catalysts. Since the specific surface area of the composite increased by 24% with respect to bare TiO2, this property alone could not explain the outstanding performance of the TiO2-rGO composite. Nevertheless, two advantages in terms of photocatalytic activity of the composite could be pointed out: (i) rGO can contribute as a charge carrier contributing to a reduction in the recombination of the electron-hole pairs and, (ii) band-gap tuning 13
(Leary and Westwood, 2011; Liang et al., 2014). Raman spectroscopy results demonstrated a systematic variation of D and G bands to lower wavelengths and an increase in the full width at half maximum in TiO2-rGO spectrum with respect to GO (Ribao et al., 2018), which could indicate electron transfer processes in rGO nanosheets. Moreover, UV/Vis reflectance measurements provided an indirect band gap value for TiO2-rGO (5 wt. % of rGO) of 2.94 eV (Table 1). This would imply a band gap extension leading to a better use of the light irradiation.
14
Figure 3 DCA and PFOA degradation after 8 hours (A), dehalogenation (B), and mineralization after 8 hours (C) of photocatalysis with TiO2-rGO. The dehalogenation progress of DCA and PFOA using the TiO2-rGO composite is depicted in Fig.3B. The release of halogen anions to the aqueous solution increased up to 80% and 30% for chlorine and fluorine, respectively. Similar DCA degradation (87%, Fig 3A) and chloride release (80%, Fig 3B) was attained. In contrast, the fluoride release ratio (30%) was significantly lower than the total PFOA degradation ratio that was 86% (Fig 3A) after 8h of irradiation. However, PFOA dehalogenation was drastically enhanced using the TiO2-rGO composite compared to bare TiO2, which provided only the release of 2% of the fluorine content. Fig. 3C shows DOC removal during the photocatalytic treatment of DCA and PFOA solutions using the TiO2-rGO composite. It can be observed that the mineralization degree was 81% and 43% for DCA and PFOA, respectively. In the case of PFOA photocatalytic decomposition, lower values of DOC removal compared to its degradation rate could be attributed to the formation of intermediate perfluorocarboxylic acids (PFCAs). Shorter-chain PFCAs were monitored in experiments using TiO2-rGO catalysts (Fig. 4). The first PFOA 15
degradation product was PFHpA which was further decomposed to shorter-chain intermediates, PFHxA and PFPeA. The next shorter-chain PFCA in the degradation pathway would be volatile perfluorobutanoic acid (PFBA), which could not be determined by the analytical technique used in the present work.
Figure 4 Evolution of PFHpA, PFHxA and PFPeA concentrations (mmol L-1) with the irradiation time during the photocatalytic treatment of PFOA employing TiO2rGO catalyst. Moreover, theoretical values of DOC were calculated for each contaminant (Fig 3C). DOC reduction revealed that the measured values approximately matched remaining DCA (black dots in Fig. 3C). This could be attributed to the lack of organic intermediates formed during DCA degradation. DOC calculated from PFOA and intermediate products concentrations (black dots in Fig. 3C) was slightly higher than the measured DOC values, which can be likely assigned to undetected shorter-chain intermediate products. In view of the results, it is concluded that despite the fact that the TiO2-rGO catalyst managed to degrade both organohalogenated compounds, DCA and PFOA, following the same kinetic performance for the degradation of the primary 16
contaminant, the dehalogenation and mineralization ratios differed due to the different molecular structure of the contaminants (Fig. 1). The clear differences in PFOA and DCA defluorination and mineralization herein shown can be explained by their reaction pathways that are summarized in Figure 5. The initial step of HOCs degradation consists of radical reactions to form halogenated alkyl radicals (Eq.2-3). The initiation of DCA decomposition took place by the hydroxyl radical attack, whereas PFOA transformation can result not only from direct reaction of PFOA with the photogenerated holes of the photocatalyst surface but also through indirect reaction with hydroxyl radical; or even by the combination of both mechanisms. The formed halogenated alkyl radicals were further oxidized or hydrolyzed to give either phosgene (COCl2) (Eq. 4a) or perfluorinated alcohol (C7F15OH) (Eq. 4b). These unstable compounds undergo further dehalogenation in a second step, in which two halogen atoms are released for both contaminants (Eq. 5a and Eq. 5b)). As a result of the release of one -CX2- unit (where X is Cl or F), DCA was degraded and mineralized to the same extent since the molecule contains only one –CCl2- unit with two chlorine atoms.
In
contrast,
PFOA
forms
a
shorter-chain
perfluorocarboxylate
intermediate, that needs to proceed through the same degradation pathway in consecutive cycles until the complete mineralization is reached.
17
Figure 5 Reaction pathways for DCA and PFOA degradation In order to further confirm the validity of the common reaction pathway, the relationship between the release of halogen ions and the degradation of contaminants along the photocatalytic treatment is shown in Fig. 6. As it can be observed in Fig.6A, the ratio between the degraded DCA and the chloride release was found to be around two moles of chloride per each mole of DCA removed, according to the proposed mechanism. Moreover, Fig. 6B shows that each mole of PFOA initially involved the generation of 2 moles of fluoride, that is consistent with the reaction 5b leading to the formation of PFHpA as the first secondary product in the initial degradation stages. However, as it was remarked previously, the total PFOA dehalogenation process implies further degradation cycles of intermediate PFCAs, until the total dehalogenation would produce 15 moles of F-. For longer reaction times, Fig. 6B shows that for each mole of PFOA, only 5 moles of fluoride were detected. This result represents one third of the total fluorine contained in the initial PFOA according to the dehalogenation degree (30%, Fig 3B), the incomplete PFOA degradation and the presence of perfluorinated intermediates. 18
Figure 6 Linear relationship between the release of halides and the degradation of the mother contaminants along the photocatalytic treatment using TiO2-rGO composite. A: chloride release vs. DCA removal; B: fluoride release vs. PFOA removal. Taking into account the similar degradation degree of the primary HOCs and different mineralization and dehalogenation ratios obtained with the TiO2-rGO catalyst, it can be assumed that the photocatalytic performance mainly depends on the number of halogen substituents and the length of the alkyl-chain instead of the C-X bond strength of the HOCs molecules.
4. Conclusions This study focuses on the comparative photodegradation performance of TiO2rGO catalyst for two persistent organohalogenated
carboxylic acids,
dichloroacetic and perfluorooctanoic acids, DCA and PFOA, which differ in the type of halogen atoms, chloride and fluoride, in the number of halogen atoms that are bonded to carbon atoms, and in the length of the alkyl chain. TiO2, which was used as benchmark catalyst, exhibited low degradation rates in the treatment of both halogenated organic compounds. However, the TiO2-rGO catalyst 19
overcame the limited performance of bare TiO2 achieving successful degradation of DCA and PFOA, regardless of the molecule complexity. Moreover, similar first order kinetic constants were calculated for the photocatalytic degradation of both contaminants. rGO could efficiently contribute as charge carrier limiting the electron/hole pair recombination. Besides, a desirable modification of the band gap of the composite was achieved, which could allow its utilization in a wider range of the UV-Vis spectra. Conversely, although TiO2-rGO catalyst reported high performance for both contaminants in terms of the degradation rate of the primary contaminant after 8 hours of irradiation time (87% and 86% for DCA and PFOA, respectively), the dehalogenation and mineralization rates for PFOA were only 30% and 43%, respectively. During the process, only one third of the total fluorine contained in the parent molecules was released into the solution due to the generation of shorter chain perfluorinated intermediates that retain fluorine atoms in their molecules. In contrast, DCA was degraded and mineralized to the same extent releasing the two chloride ions. It is concluded that the higher number of halogen substituents and the greater complexity of the longer alkylchain length of PFOA molecule compared to DCA molecule involved more reaction steps and needed longer times to achieve the complete mineralization and dehalogenation of the HOC.
Acknowledgments This work was supported by the Spanish Ministry of Science, Innovation and Universities [grants number RTI2018-099407-B-I00 (MCIU/AEI/FEDER,UE) and CTM2016-75509-R (MINECO/FEDER, UE)]
20
References Adamu, H., Dubey, P., Anderson, J.A., 2016. Probing the role of thermally reduced graphene oxide in enhancing performance of TiO2 in photocatalytic phenol removal from aqueous environments. Chem. Eng. J. 284, 380–388. APHA (American Public Health Association), 1998. Standard Methods for the Examination of Water and Wastewater, 20th Ed. ed, Standard Methods. Washington DC. Banks,D., Jun, B.M., Heo, J., Her, N., Yoon, Y., 2020. Selected advanced water treatment technologies for perfluoroalkyl and polyfluoroalkyl substances: A review. Sep. Purif. Technol.,231, 115929-115942. Bartolomé, M., Gallego-Picó, A., Cutanda, F., Huetos, O., Esteban, M., PérezGómez, B., Castaño, A., 2017. Perfluorinated alkyl substances in Spanish adults: Geographical distribution and determinants of exposure. Sci. Total Environ. 603–604, 352–360. Comninellis, C., Kapalka, A., Malato, S., Parsons, S.A., Poulios, I., Mantzavinos, D., 2008. Advanced oxidation processes for water treatment: Advances and trends for R&D. J. Chem. Technol. Biotechnol. 83, 769–776. Dahl, M., Liu, Y., Yin, Y., 2014. Composite titanium dioxide nanomaterials. Chem. Rev. 114, 9853–9889. Esclapez, M.D., Tudela, I., Díez-García, M.I., Sáez, V., Rehorek, A., Bonete, P., González-García, J., 2012. Towards the complete dechlorination of chloroacetic acids in water by sonoelectrochemical methods: Effect of the anodic material on the degradation of trichloroacetic acid and its by-products. 21
Chem. Eng. J. 197, 231–241. Fernández-Castro, P., Vallejo, M., San Román, M.F., Ortiz, I., 2015. Insight on the fundamentals of advanced oxidation processes: Role and review of the determination methods of reactive oxygen species. J. Chem. Technol. Biotechnol. 90, 796–820. Fuertes, I., Gómez-Lavín, S., Elizalde, M.P., Urtiaga, A., 2017. Perfluorinated alkyl substances (PFASs) in northern Spain municipal solid waste landfill leachates. Chemosphere 168, 399–407. Fujii, S., Polprasert, C., Tanaka, S., Lien, N.P.H., Qiu, Y., 2007. New POPs in the water environment: Distribution, bioaccumulation and treatment of perfluorinated compounds - A review paper. J. Water Supply Res. Technol. - AQUA 56, 313–326. Gilliland, F.D., Mandel, J.S., 1993. Mortality among employees of a perfluorooctanoic acid production plant. J Occup Med 35, 950–954. Gomez-Ruiz, B., Gómez-Lavín, S., Diban, N., Boiteux, V., Colin, A., Dauchy, X., Urtiaga, A., 2017. Efficient electrochemical degradation of poly- and perfluoroalkyl substances (PFASs) from the effluents of an industrial wastewater treatment plant. Chem. Eng. J. 322, 196–204. Gomez-Ruiz, B., Ribao, P., Diban, N., Rivero, M.J., Ortiz, I., Urtiaga, A., 2018. Photocatalytic degradation and mineralization of perfluorooctanoic acid (PFOA) using a composite TiO2−rGO catalyst. J. Hazard. Mater. 344, 950– 957. Hu, X.C., Andrews, D.Q., Lindstrom, A.B., Bruton, T.A., Schaider, L.A., Grandjean, P., Lohmann, R., Carignan, C.C., Blum, A., Balan, S.A., Higgins, 22
C.P., Sunderland, E.M., 2016. Detection of Poly- and Perfluoroalkyl Substances (PFASs) in U.S. Drinking Water Linked to Industrial Sites, Military Fire Training Areas, and Wastewater Treatment Plants. Environ. Sci. Technol. Lett. 3, 344–350. Hummers, W.S., Offeman, R.E., 1958. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 80, 1339–1339. Kissa, E., 2001. Fluorinated surfactants and repellents, 2nd Edn. R. ed. Marcel Dekker, New York. Kumordzi, G., Malekshoar, G., Yanful, E.K., Ray, A.K. , 2016. Solar photocatalytic degradation of Zn2+ using graphene based TiO2. Sep. Purif. Technol. 168,294-301. Lavanya, T., Dutta, M., Satheesh, K., 2016. Graphene wrapped porous tubular rutile TiO2 nanofibers with superior interfacial contact for highly efficient photocatalytic performance for water treatment. Sep. Purif. Technol. 168, 284-293. Leary, R., Westwood, A., 2011. Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis. Carbon N. Y. 49, 741–772. Liang, D., Cui, C., Hub, H., Wang, Y., Xu, S., Ying, B., Li, P., Lu, B., Shen, H., 2014. One-step hydrothermal synthesis of anatase TiO2/reduced graphene oxide nanocomposites with enhanced photocatalytic activity. J. Alloys Compd. 582, 236–240. Luo, J., Hu, J., Wei, X., Fu, L., Li, L., 2015. Dehalogenation of persistent halogenated organic compounds: A review of computational studies and quantitative structure–property relationships. Chemosphere 131, 17–33. 23
Miklos, D.B., Remy, C., Jekel, M., Linden, K.G., Drewes, J.E., Hübner, U., 2018. Evaluation of advanced oxidation processes for water and wastewater treatment – A critical review. Water Res. 139, 118–131. Nelson, J.W., Hatch, E.E., Webster, T.F., 2010. Exposure to polyfluoroalkyl chemicals and cholesterol, body weight, and insulin resistance in the general U.S. population. Environ. Health Perspect. 118, 197–202. Ortiz, I., Mosquera-Corral, A., Lema, J.M., Esplugas, S., 2015. Advanced technologies for water treatment and reuse. AIChE J. 61, 3146–3158. Pastrana-Martínez, L.M., Morales-Torres, S., Likodimos, V., Figueiredo, J.L., Faria, J.L., Falaras, P., Silva, A.M.T., 2012. Advanced nanostructured photocatalysts based on reduced graphene oxide-TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye. Appl. Catal. B Environ. 123–124, 241–256. Pelaez, M., Nolan, N.T., Pillai, S.C., Seery, M.K., Falaras, P., Kontos, A.G., Dunlop, P.S.M., Hamilton, J.W.J., Byrne, J.A., O’Shea, K., Entezari, M.H., Dionysiou, D.D., 2012. A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal. B Environ. 125, 331–349. Prevedouros, K., Cousins, I.T., Buck, R.C., Korzeniowski, S.H., 2006. Critical Review Sources , Fate and Transport of Fate. Environ. Sci. Technol. 40, 32– 44. Ribao, P., Rivero, M.J., Ortiz, I., 2017. TiO2 structures doped with noble metals and/or graphene oxide to improve the photocatalytic degradation of dichloroacetic acid. Environ. Sci. Pollut. Res. 24, 12628-12637. 24
Ribao, P., Rivero, M.J., Ortiz, I., 2018. Enhanced photocatalytic activity using GO/TiO2 catalyst for the removal of DCA solutions. Environ. Sci. Pollut. Res. 25, 34893-34902. Sakai, H., Autin, O., Parsons, S., 2013. Change in haloacetic acid formation potential during UV and UV/H2O2 treatment of model organic compounds. Chemosphere 92, 647–651. Spasiano, D., Marotta, R., Malato, S., Fernandez-Ibañez, P., Di Somma, I., 2015. Solar photocatalysis: Materials, reactors, some commercial, and preindustrialized applications. A comprehensive approach. Appl. Catal. B Environ. 170–171, 90–123. The European Commission, 2017. Commission Regulation (EU) 2017/1000 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals as regards perfluorooctanoic acid (PFOA), its salts and PFOArelated substances, Official Journal of the European Union. UNEP, 2001. Final Act of the Conference of Plenipotentiaries on The Stockholm ConventionOnPersistent Organic Pollutants. United Nations Environ. Progr. Geneva, Switz. 44. USEPA, 2016. Drinking Water Health Advisory for Perfluorooctanoic Acid (PFOA). USEPA, 1998. Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules. Wang, P., Wang, J., Wang, X., Yu, H., Yu, J., Lei, M., Wang, Y., 2013. One-step synthesis of easy-recycling TiO2-rGO nanocomposite photocatalysts with enhanced photocatalytic activity. Appl. Catal. B Environ. 132–133, 452–459. 25
Zhang, Y., Collins, C., Graham, N., Templeton, M.R., Huang, J., Nieuwenhuijsen, M., 2010. Speciation and variation in the occurrence of haloacetic acids in three water supply systems in England. Water Environ. J. 24, 237–245.
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Supplementary material Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero, Paula Ribao, Beatriz Gomez-Ruiz, Ane Urtiaga, Inmaculada Ortiz* Department of Chemical and Biomolecular Engineering. University of Cantabria. Av. de Los Castros s/n. 39005 Santander. Spain. * Corresponding author: [email protected]
A
B
Figure S1. DCA and FPOA removal during the photocatalytic treatment with TiO2 (A) and TiO2-rGO (B) as catalysts respectively.
A
B
Figure S2. DCA and FPOA mineralization during the photocatalytic treatment with TiO2 (A) and TiO2-rGO (B) as catalysts respectively.
27
Figure S3. DCA and FPOA removal fitting to first order kinetic model for TiO2-rGO
28
Highlights
Photo-activity of TiO2-rGO was tested as a function of the organic pollutant nature
TiO2-rGO showed similar kinetic performance in terms of PFOA and DCA degradation rates
DCA was degraded and mineralized to the same extent using TiO2-rGO composite
PFOA defluorination and mineralization were remarkably lower than its degradation
Mineralization strongly depended on the number of halogen substituents and chain length
29
CRediT author statement Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero: Conceptualization, Methodology, Writing - Review & Editing, Project administration, Funding acquisition. Paula Ribao: Investigation, Validation, Formal analysis, Writing - Original Draft. Beatriz Gomez-Ruiz: Investigation, Validation, Formal analysis, Writing - Original Draft. Ane Urtiaga: Conceptualization, Methodology, Project administration, Funding acquisition. Inmaculada Ortiz: Conceptualization, Supervision, Project administration
30
Dichloroacetic acid (DCA)
TiO2-rGO
TiO2
100
Removal (%)
Perfluorooctanoic acid (PFOA)
80
DCA
PFOA
60 40 20 0
TiO₂
TiO₂-rGO
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
31
S1383-5866(19)34896-8 https://doi.org/10.1016/j.seppur.2020.116637 SEPPUR 116637
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
24 October 2019 27 January 2020 27 January 2020
Please cite this article as: M.J. Rivero, P. Ribao, B. Gomez-Ruiz, A. Urtiaga, I. Ortiz, Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116637
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Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero, Paula Ribao, Beatriz Gomez-Ruiz, Ane Urtiaga, Inmaculada Ortiz*
Department of Chemical and Biomolecular Engineering. University of Cantabria. Av. de Los Castros s/n. 39005 Santander. Spain. * Corresponding author: [email protected]
1
ABSTRACT Halogenated organic compounds are frequently characterized by their bioaccumulative nature, and long-term effects on human health. Their low biodegradability and their difficult removal by conventional technologies lead to their undesirable persistence in the environment. Titanium dioxide and reduced graphene oxide TiO2-rGO composites have shown promising results with enhanced photocatalytic degradation rates compared to bare TiO2 in the degradation of a good number of persistent pollutants. In this context, this work deepens on the influence of the type and number of halogen substitution on the photocatalytic degradation of halogenated organic compounds (HOCs). For the experimental
analysis
two
HOCs,
dichloroacetic
acid
(DCA)
and
perfluorooctanoic acid (PFOA), have been selected as both molecules differ in the type of halogen atoms, chlorine and fluorine, the number of halogen atoms, and in the length of the alkyl chain. The results showed that TiO2-rGO catalysts achieved a similar kinetic performance for the removal of the primary contaminant in terms of its degradation rate. Besides, TiO2-rGO composite successfully induced the release of all chlorine atoms from the DCA molecule, achieving its total mineralization. However, PFOA defluorination and mineralization rates were remarkably lower than its degradation rate. Although both contaminants released two halogen atoms step-by-step for the degradation of each molecule, PFOA dehalogenation was slowed down due to the generation of secondary products that retain fluorine. Therefore, we conclude that the composite photocatalysts showed a similar performance in terms of degradation rate of the primary contaminant independent on the length of the alkyl chain and on the number of halogen atoms, however, the mineralization and dehalogenation rates were 2
strongly dependent on the number of halogen substituents and the length of the alkyl chain.
KEYWORDS Halogenated organic compounds; perfluorooctanoic acid; dichloroacetic acid; Photocatalysis; reduced graphene oxide
1. Introduction Water pollution is one of the main challenges that society has to face to ensure the quality and availability of freshwater resources due to the inevitable prospects of water scarcity (Ortiz et al., 2015). Indeed, the quality of water resources is continuously threatened due to the increased release of anthropogenic pollutants from industrial, agricultural and human activities. Among water contaminants, halogenated organic compounds (HOCs) are persistent, bio-cumulative and potentially toxic (Fujii et al., 2007; Luo et al., 2015). Due to these properties, international action to control the use of HOCs and their impact on the environment started in the 1970s, when North America, Western Europe and Japan banned dieldrin, dichlorodiphenyltrichloroethane (DDT) and restricted polychlorinated biphenyls (PCBs). Since then, many HOCs have been subjected to stricter regulations (UNEP, 2001). Within the HOCs group, dichloroacetic acid (DCA) is an haloacetic acid (HAA) analogous to acetic acid, where 2 of the 3 hydrogen atoms of the methyl group have been replaced by chlorine atoms (Fig. 1A). HAAs constitute the second most important group of disinfection by-products (DBPs) found in drinking water 3
after chlorine disinfection (Esclapez et al., 2012). DBPs have been widely related to adverse effects on human health, including cancer, growth retardation, congenital cardiac defects, and spontaneous abortion (Zhang et al., 2010). Although, the European Drinking Water Directive 98/83/EC (DWD) does not currently stablish any guideline for HAAs in drinking water, in the US EPA classification, DCA is considered as a potential human carcinogen, and accordingly the US EPA has set a maximum level of 60 µg L-1 for HAAs in drinking water (USEPA, 1998). Moreover, Australia and New Zealand regulate a limit of 100 µg L-1 of DCA in drinking water (Zhang et al., 2010) and Japan’s drinking water standards for DCA is 40 µg L-1 (Sakai et al., 2013). Perfluorooctanoic acid (PFOA) is another hazardous compound ubiquitously detected in the natural environment (Fig. 1B). PFOA belongs to the family of perfluoroalkyl substances (PFAS) and consists of a fully fluorinated alkyl chain of eight carbons with a carboxylic acid end-group. This molecular structure offers unique physical-chemical properties, such as the amphiphilic character, and the extraordinary thermal and chemical stability, that have led to its use in pesticides, cosmetics, greases, paints, adhesives or aqueous film forming foams used for firefighting (Kissa, 2001; Prevedouros et al., 2006). As a result, PFOA has been found in drinking and ground waters, in industrial effluents and landfill leachates (Fuertes et al., 2017; B. Gomez-Ruiz et al., 2018; Hu et al., 2016), as well as in human blood, serum, plasma and tissues (Bartolomé et al., 2017). Many studies have reported that human PFAS exposure has been linked to elevated cholesterol, obesity, immune suppression, endocrine disruption and cancer (Gilliland and Mandel, 1993; Nelson et al., 2010). The USEPA has also established health advisory levels for PFOA and PFOS in drinking water at 0.07 4
µg L-1 (USEPA, 2016). Since June 2017, the European Union has also published actions to ban the PFOA production, and its use and placement in the market (The European Commission, 2017). Some technologies have already been applied but its fully removal is still an issue (Banks et al., 2020). B
A
128.94 g mol-1
414.07 g mol-1
Figure 1 Chemical structures of DCA (A) and PFOA (B). The recalcitrant nature of HOCs towards most conventional wastewater treatments was typically attributed to the strong C−X bonds (where X is the halogen atom) (Kissa, 2001). The energy bond increases in this order: C−F > C−Cl > C−Br > C−I. Nevertheless, over the last few decades, advanced oxidation processes (AOPs) have appeared as non-conventional techniques that are able to enhance the oxidation ability for the degradation of recalcitrant compounds. AOPs stand out thanks to their versatility, non-selectivity to the target compound and the production of the highly reactive oxygen species (ROS), mainly the hydroxyl radicals (·OH), which are able to degrade organic compounds (Comninellis et al., 2008; Miklos et al., 2018). Among them, research on heterogeneous photocatalysis is increasing. Although the positive features of TiO2 (Pelaez et al., 2012), it still presents limitations to real photocatalytic applications (Spasiano et al., 2015). It is only activated under ultraviolet light, which represents about 5% of the solar spectrum, and exhibits 5
high electron-hole pair recombination. To overcome these drawbacks of the TiO2 catalyst, new strategies based on the combination of TiO2 with other elements, such as noble metals, and carbon-based materials have been recently addressed (Dahl et al., 2014). Among them, graphene oxide (GO) shows outstanding properties because it can be easily reduced to reduced graphene oxide (rGO) during coupling with TiO2 nanoparticles via hydrothermal synthesis, providing a significant opportunity for TiO2-rGO materials. These composites can provide several advantages: (i) rGO can act as an electron trap, avoiding the undesirable electron-hole recombination; (ii) the excitation wavelength can be broaden to the visible region; (iii) the composite can be easily recovered because of the large surface area of the rGO nanosheets, and (iv) promotion of the adsorption of reactants thanks to additional active sites (Leary and Westwood, 2011). The effective photocatalytic activity of the composites based on GO and TiO2 nanoparticles has become of increasing interest in the past years in the literature (Adamu et al., 2016; Kumordzi et al., 2016; Lavanya et al., 2016; PastranaMartínez et al., 2012; Wang et al., 2013). The present work aims to generate further knowledge on the photocatalytic performance of TiO2-rGO composites by comparatively analyzing the degradation and mineralization rates of two different recalcitrant halogenated organic compounds, DCA and PFOA. Both contaminants differ in the halogen type, chloride and fluoride, in the number of halogens that are bonded to carbon atoms, and in the length of the alkyl chain thus, offering a broader scope to the study. 2. Experimental
6
2.1.
Chemicals
PFOA (C7F15COOH, 96% purity), perfluoroheptanoic acid (PFHpA, C6F13COOH, 99% purity), perfluorohexanoic acid (PFHxA, C5F11COOH, 96% purity), perfluoropentanoic acid (PFPeA, C4F9COOH, 97% purity) and DCA (99% purity) from Sigma Aldrich Chemicals were used. All solutions were prepared with QPOD Millipore water with conductance of 18.2 mΩ cm at 25 °C and 3 ppb of dissolved organic carbon (DOC). TiO2 P25 (Evonik) was selected to prepare the composite material. 2.2. Synthesis and characterization of TiO2-rGO photocatalyst Graphene oxide (GO) based on hard oxidation of graphite powder and subsequent exfoliation for GO nanosheets following modified Hummers method (Hummers and Offeman, 1958) was carried out. TiO2-rGO composite was synthetized via the hydrothermal method with GO content of 5 wt% (Ribao et al., 2017). The material was characterized via several techniques: transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method for specific surface area calculation, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), thermogravimetric analysis (TGA), and UV/Vis diffuse reflectance spectroscopy (Gomez-Ruiz et al., 2018; Ribao et al., 2017; Ribao et al., 2018). During the synthesis, GO was reduced and TiO2 nanoparticles were dispersed on the graphene nanoplatelets (Ribao et al., 2017). Moreover, Table 1 provides an overview of the main TiO2-rGO features. The specific surface area of the nanocomposite was evaluated from nitrogen adsorption isotherms using the BET equation. The number of graphene layers 7
was obtained from AFM; atomic composition was calculated by XPS; rGO real content was derived from TGA analysis heating the sample from 25ºC to 910 °C at 20 °C min−1 under 50 mL min-1 of nitrogen flow in an alumina cell and the band gap was calculated from UV-vis diffuse reflectance spectroscopy using the Kubelka-Munk function described elsewhere (Ribao et al., 2018).
TiO2 (Degussa)
TiO2-rGO 5 wt.%
50.0
62.2
-
2
Ti
26.6
28.1
O
73.4
58.1
C
-
13.8
-
7.27
3.25
2.94
Specific area (m2 g-1) Number of overlapping layers Atomic composition (at.%)
rGO content (wt.%) Band gap (eV)
Table 1. Summary of the main characterization results of TiO2 and TiO2-rGO catalysts. 2.3. Photocatalytic activity Photocatalytic experiments were carried out in a Heraeus Laboratory UV Reactor under continuous magnetic stirring and temperature was kept at 25 °C thanks to a water/ethylene glycol cooling bath (PolyScience). The total reactor volume was 1 L. A 150 W medium-pressure mercury lamp (Heraeus Noblelight TQ z1) with 8
an emission spectrum between 200 and 600 nm was used as irradiation source. Aliquots of the reacting mixture were filtered through 0.45 µm polypropylene filters. The irradiation received on the outer wall of the reactor was 145 ± 41 W m-2, 82 ± 13 W m-2, 1.7 ± 0.1 W m-2 and 729 ± 72 W m-2 for UV-A, UV-B, UV-C and visible light ranges, respectively, measured by a HD2102.1 photo/radiometer (Delta OHM). The initial concentrations of DCA and PFOA aqueous solutions used as feed in the experiments are higher than that in environmental matrices for the ease and accuracy of the analysis. The photocatalytic experiments were carried out with 0.8 L of 7.75 mmol L-1 DCA solution (pH 2.2) with 0.3 g L-1 of photocatalyst. On the other hand, the initial concentration of the PFOA solution was set at 0.24 mmol L-1, (initial pH 3.8). The TiO2-rGO composite dose was 0.1 g L-1 Every photocatalytic experiment was performed in duplicate and pH was not controlled during the process (Gomez-Ruiz et al., 2018). Several control tests were also performed. Adsorption of DCA and PFOA onto the catalysts was tested. After 24 h without light irradiation, there was no significant adsorption of DCA either on the TiO2 or on the TiO2-rGO. However, it was observed slight PFOA adsorption of 5.2±0.1% and 5.3±1.2% on TiO2 and TiO2-rGO surfaces, respectively, at a catalyst concentration of 0.1 g L-1. Additionally, experiments using dispersed GO nanosheets under light irradiation did not lead to any significant difference in DCA and PFOA concentration, indicating the absence of photocatalytic activity of GO. 2.4.
Analytical methodology
PFOA, PFHpA, PFHxA, PFPeA, were analyzed with a HPLC-DAD (Water 2695) 9
system with an X Bridge C18 column (5 µm, 250 mm x 4.6 mm, Waters). The column was placed in an oven at 40 ºC. A mixture of methanol (65%) and dihydrogen phosphate (35%) was used as mobile phase in isocratic mode with a flow rate of 0.5 mL min-1. The detector’s wavelength was fixed at 204 nm. The DCA and halogen ions (Cl- and F-) concentrations were measured in an ICS-1100 (Dionex) ion chromatograph equipped with an AS9-HC column in isocratic mode using Na2CO3 9 mM as the eluent at a flow rate of 1 mL min−1 and a pressure of approximately 2000 psi, based on Standard Methods 4110B (APHA (American Public Health Association), 1998). A TOC-V CPH (Shimadzu Corp. Japan) was used to quantify dissolved organic carbon (DOC).
3. Results and discussion The photocatalytic activity of the TiO2 and synthesized TiO2-rGO composite was studied through the degradation of two different halogenated organic compounds, DCA and PFOA. Moreover, the mineralization extent was monitored by both the release of halogen ions (chloride and fluoride) and the progress of DOC along the photocatalytic degradation time. 3.1. DCA and PFOA degradation over TiO2 catalyst Fig. 2A provides the degradation yield of DCA and PFOA after 8 hours when using bare TiO2 photocatalyst. The removal of DCA and PFOA was 35% and 10%, respectively. These limited degradation percentages likely resulted from the rapid recombination of electron/hole pairs formed by the TiO2 catalyst that restricts further degradation of the contaminants. Detailed data of the compounds degradation is included in the Supplementary material (Figure S1).
10
The experimental data of DCA and PFOA degradation by TiO2 catalyst were fitted to a first order kinetic model (Equation 1). d [HOC] = ―𝑘𝐻𝑂𝐶[HOC] dt
(Eq. 1)
The first order kinetic constants (kHOC) for DCA and PFOA photocatalytic degradation by means of TiO2 were 0.055 h-1 (R2 = 0.989) and 0.018 h-1 (R2 = 0.845) respectively. Therefore, the kinetic constant of DCA degradation by TiO2 photocatalysis is 3 times higher than in the case of PFOA. Fig. 2B shows the chloride and fluoride ions release into the reaction medium, during the degradation of DCA and PFOA, respectively. Results are expressed as percentage of the total halogen content in the initial contaminant. Chloride and fluoride release arose from the cleavage of the highly energetic C-Cl and C-F bonds. As a result, 31% and 2% of the total content of Cl- and F- in the initial feed solutions were released, respectively. Therefore, the percentage of subtracted chloride was similar to the DCA degradation (35%, Fig. 2A), meaning that TiO2 successfully induced the release of the chlorine atoms from the DCA molecule. However, in the case of PFOA treatment, although the obtained PFOA degradation raised up to 10% (Fig. 2A), only 2% of its fluorine content was released into the solution, confirming that the photoactivity of the TiO2 catalyst for PFOA dehalogenation was significantly constrained. Hence, PFOA decay during TiO2-mediated photocatalysis can be partially associated to the adsorption on the catalyst surface (~5%). In addition, only 0.012 mmol L-1 of perfluoropentanoic acid (PFHpA) was observed in the analytical survey. PFHpA is expected to be formed in the first step of PFOA degradation route; the observed PFHpA
11
generation matches with the 5% degradation of the initial PFOA (value obtained subtracting the concentration of PFOA adsorbed on the catalyst).
Figure 2 DCA and PFOA degradation after 8 hours (A), dehalogenation (B), and mineralization after 8 hours (C) of photocatalytic treatment using TiO2.
12
In order to get further insight on the TiO2 performance over halogenated compounds degradation, Fig. 2C shows DOC removal during the photocatalytic treatment of DCA and PFOA solutions. After 8 hours of irradiation, the values of mineralization were 32% and 4% for DCA and PFOA, respectively, consistent with their dehalogenation ratios. Detailed data of the compounds mineralization is included in the Supplementary material (Figure S2). 3.2. DCA and PFOA degradation over TiO2-rGO catalyst Fig. 3A shows the photocatalytic degradation yield of DCA and PFOA after 8 h of irradiation using the TiO2-rGO composite, 87% and 86%, respectively. Although the molecular structures of these contaminants exhibit different complexities, in terms of halogen atom nature, number of halogen substituents, and alkyl-chain length, the TiO2-rGO composite provided similar rates of removal for DCA and PFOA degradation, being the values of the first order kinetic constant 0.211 h-1 (R2 = 0.924). Detailed data of the kinetic fitting is included in the Supplementary material (Figure S3).These results pointed out a significant enhancement for both DCA and PFOA degradation rates compared to bare TiO2. In addition, it is remarkable that although TiO2 exhibited different photoactivity in the degradation of DCA and PFOA, the TiO2-rGO composite degraded both compounds to the same extent. Hence, the combination of TiO2 nanoparticles with rGO nanosheets overcomes the limited performance of TiO2 catalysts. Since the specific surface area of the composite increased by 24% with respect to bare TiO2, this property alone could not explain the outstanding performance of the TiO2-rGO composite. Nevertheless, two advantages in terms of photocatalytic activity of the composite could be pointed out: (i) rGO can contribute as a charge carrier contributing to a reduction in the recombination of the electron-hole pairs and, (ii) band-gap tuning 13
(Leary and Westwood, 2011; Liang et al., 2014). Raman spectroscopy results demonstrated a systematic variation of D and G bands to lower wavelengths and an increase in the full width at half maximum in TiO2-rGO spectrum with respect to GO (Ribao et al., 2018), which could indicate electron transfer processes in rGO nanosheets. Moreover, UV/Vis reflectance measurements provided an indirect band gap value for TiO2-rGO (5 wt. % of rGO) of 2.94 eV (Table 1). This would imply a band gap extension leading to a better use of the light irradiation.
14
Figure 3 DCA and PFOA degradation after 8 hours (A), dehalogenation (B), and mineralization after 8 hours (C) of photocatalysis with TiO2-rGO. The dehalogenation progress of DCA and PFOA using the TiO2-rGO composite is depicted in Fig.3B. The release of halogen anions to the aqueous solution increased up to 80% and 30% for chlorine and fluorine, respectively. Similar DCA degradation (87%, Fig 3A) and chloride release (80%, Fig 3B) was attained. In contrast, the fluoride release ratio (30%) was significantly lower than the total PFOA degradation ratio that was 86% (Fig 3A) after 8h of irradiation. However, PFOA dehalogenation was drastically enhanced using the TiO2-rGO composite compared to bare TiO2, which provided only the release of 2% of the fluorine content. Fig. 3C shows DOC removal during the photocatalytic treatment of DCA and PFOA solutions using the TiO2-rGO composite. It can be observed that the mineralization degree was 81% and 43% for DCA and PFOA, respectively. In the case of PFOA photocatalytic decomposition, lower values of DOC removal compared to its degradation rate could be attributed to the formation of intermediate perfluorocarboxylic acids (PFCAs). Shorter-chain PFCAs were monitored in experiments using TiO2-rGO catalysts (Fig. 4). The first PFOA 15
degradation product was PFHpA which was further decomposed to shorter-chain intermediates, PFHxA and PFPeA. The next shorter-chain PFCA in the degradation pathway would be volatile perfluorobutanoic acid (PFBA), which could not be determined by the analytical technique used in the present work.
Figure 4 Evolution of PFHpA, PFHxA and PFPeA concentrations (mmol L-1) with the irradiation time during the photocatalytic treatment of PFOA employing TiO2rGO catalyst. Moreover, theoretical values of DOC were calculated for each contaminant (Fig 3C). DOC reduction revealed that the measured values approximately matched remaining DCA (black dots in Fig. 3C). This could be attributed to the lack of organic intermediates formed during DCA degradation. DOC calculated from PFOA and intermediate products concentrations (black dots in Fig. 3C) was slightly higher than the measured DOC values, which can be likely assigned to undetected shorter-chain intermediate products. In view of the results, it is concluded that despite the fact that the TiO2-rGO catalyst managed to degrade both organohalogenated compounds, DCA and PFOA, following the same kinetic performance for the degradation of the primary 16
contaminant, the dehalogenation and mineralization ratios differed due to the different molecular structure of the contaminants (Fig. 1). The clear differences in PFOA and DCA defluorination and mineralization herein shown can be explained by their reaction pathways that are summarized in Figure 5. The initial step of HOCs degradation consists of radical reactions to form halogenated alkyl radicals (Eq.2-3). The initiation of DCA decomposition took place by the hydroxyl radical attack, whereas PFOA transformation can result not only from direct reaction of PFOA with the photogenerated holes of the photocatalyst surface but also through indirect reaction with hydroxyl radical; or even by the combination of both mechanisms. The formed halogenated alkyl radicals were further oxidized or hydrolyzed to give either phosgene (COCl2) (Eq. 4a) or perfluorinated alcohol (C7F15OH) (Eq. 4b). These unstable compounds undergo further dehalogenation in a second step, in which two halogen atoms are released for both contaminants (Eq. 5a and Eq. 5b)). As a result of the release of one -CX2- unit (where X is Cl or F), DCA was degraded and mineralized to the same extent since the molecule contains only one –CCl2- unit with two chlorine atoms.
In
contrast,
PFOA
forms
a
shorter-chain
perfluorocarboxylate
intermediate, that needs to proceed through the same degradation pathway in consecutive cycles until the complete mineralization is reached.
17
Figure 5 Reaction pathways for DCA and PFOA degradation In order to further confirm the validity of the common reaction pathway, the relationship between the release of halogen ions and the degradation of contaminants along the photocatalytic treatment is shown in Fig. 6. As it can be observed in Fig.6A, the ratio between the degraded DCA and the chloride release was found to be around two moles of chloride per each mole of DCA removed, according to the proposed mechanism. Moreover, Fig. 6B shows that each mole of PFOA initially involved the generation of 2 moles of fluoride, that is consistent with the reaction 5b leading to the formation of PFHpA as the first secondary product in the initial degradation stages. However, as it was remarked previously, the total PFOA dehalogenation process implies further degradation cycles of intermediate PFCAs, until the total dehalogenation would produce 15 moles of F-. For longer reaction times, Fig. 6B shows that for each mole of PFOA, only 5 moles of fluoride were detected. This result represents one third of the total fluorine contained in the initial PFOA according to the dehalogenation degree (30%, Fig 3B), the incomplete PFOA degradation and the presence of perfluorinated intermediates. 18
Figure 6 Linear relationship between the release of halides and the degradation of the mother contaminants along the photocatalytic treatment using TiO2-rGO composite. A: chloride release vs. DCA removal; B: fluoride release vs. PFOA removal. Taking into account the similar degradation degree of the primary HOCs and different mineralization and dehalogenation ratios obtained with the TiO2-rGO catalyst, it can be assumed that the photocatalytic performance mainly depends on the number of halogen substituents and the length of the alkyl-chain instead of the C-X bond strength of the HOCs molecules.
4. Conclusions This study focuses on the comparative photodegradation performance of TiO2rGO catalyst for two persistent organohalogenated
carboxylic acids,
dichloroacetic and perfluorooctanoic acids, DCA and PFOA, which differ in the type of halogen atoms, chloride and fluoride, in the number of halogen atoms that are bonded to carbon atoms, and in the length of the alkyl chain. TiO2, which was used as benchmark catalyst, exhibited low degradation rates in the treatment of both halogenated organic compounds. However, the TiO2-rGO catalyst 19
overcame the limited performance of bare TiO2 achieving successful degradation of DCA and PFOA, regardless of the molecule complexity. Moreover, similar first order kinetic constants were calculated for the photocatalytic degradation of both contaminants. rGO could efficiently contribute as charge carrier limiting the electron/hole pair recombination. Besides, a desirable modification of the band gap of the composite was achieved, which could allow its utilization in a wider range of the UV-Vis spectra. Conversely, although TiO2-rGO catalyst reported high performance for both contaminants in terms of the degradation rate of the primary contaminant after 8 hours of irradiation time (87% and 86% for DCA and PFOA, respectively), the dehalogenation and mineralization rates for PFOA were only 30% and 43%, respectively. During the process, only one third of the total fluorine contained in the parent molecules was released into the solution due to the generation of shorter chain perfluorinated intermediates that retain fluorine atoms in their molecules. In contrast, DCA was degraded and mineralized to the same extent releasing the two chloride ions. It is concluded that the higher number of halogen substituents and the greater complexity of the longer alkylchain length of PFOA molecule compared to DCA molecule involved more reaction steps and needed longer times to achieve the complete mineralization and dehalogenation of the HOC.
Acknowledgments This work was supported by the Spanish Ministry of Science, Innovation and Universities [grants number RTI2018-099407-B-I00 (MCIU/AEI/FEDER,UE) and CTM2016-75509-R (MINECO/FEDER, UE)]
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References Adamu, H., Dubey, P., Anderson, J.A., 2016. Probing the role of thermally reduced graphene oxide in enhancing performance of TiO2 in photocatalytic phenol removal from aqueous environments. Chem. Eng. J. 284, 380–388. APHA (American Public Health Association), 1998. Standard Methods for the Examination of Water and Wastewater, 20th Ed. ed, Standard Methods. Washington DC. Banks,D., Jun, B.M., Heo, J., Her, N., Yoon, Y., 2020. Selected advanced water treatment technologies for perfluoroalkyl and polyfluoroalkyl substances: A review. Sep. Purif. Technol.,231, 115929-115942. Bartolomé, M., Gallego-Picó, A., Cutanda, F., Huetos, O., Esteban, M., PérezGómez, B., Castaño, A., 2017. Perfluorinated alkyl substances in Spanish adults: Geographical distribution and determinants of exposure. Sci. Total Environ. 603–604, 352–360. Comninellis, C., Kapalka, A., Malato, S., Parsons, S.A., Poulios, I., Mantzavinos, D., 2008. Advanced oxidation processes for water treatment: Advances and trends for R&D. J. Chem. Technol. Biotechnol. 83, 769–776. Dahl, M., Liu, Y., Yin, Y., 2014. Composite titanium dioxide nanomaterials. Chem. Rev. 114, 9853–9889. Esclapez, M.D., Tudela, I., Díez-García, M.I., Sáez, V., Rehorek, A., Bonete, P., González-García, J., 2012. Towards the complete dechlorination of chloroacetic acids in water by sonoelectrochemical methods: Effect of the anodic material on the degradation of trichloroacetic acid and its by-products. 21
Chem. Eng. J. 197, 231–241. Fernández-Castro, P., Vallejo, M., San Román, M.F., Ortiz, I., 2015. Insight on the fundamentals of advanced oxidation processes: Role and review of the determination methods of reactive oxygen species. J. Chem. Technol. Biotechnol. 90, 796–820. Fuertes, I., Gómez-Lavín, S., Elizalde, M.P., Urtiaga, A., 2017. Perfluorinated alkyl substances (PFASs) in northern Spain municipal solid waste landfill leachates. Chemosphere 168, 399–407. Fujii, S., Polprasert, C., Tanaka, S., Lien, N.P.H., Qiu, Y., 2007. New POPs in the water environment: Distribution, bioaccumulation and treatment of perfluorinated compounds - A review paper. J. Water Supply Res. Technol. - AQUA 56, 313–326. Gilliland, F.D., Mandel, J.S., 1993. Mortality among employees of a perfluorooctanoic acid production plant. J Occup Med 35, 950–954. Gomez-Ruiz, B., Gómez-Lavín, S., Diban, N., Boiteux, V., Colin, A., Dauchy, X., Urtiaga, A., 2017. Efficient electrochemical degradation of poly- and perfluoroalkyl substances (PFASs) from the effluents of an industrial wastewater treatment plant. Chem. Eng. J. 322, 196–204. Gomez-Ruiz, B., Ribao, P., Diban, N., Rivero, M.J., Ortiz, I., Urtiaga, A., 2018. Photocatalytic degradation and mineralization of perfluorooctanoic acid (PFOA) using a composite TiO2−rGO catalyst. J. Hazard. Mater. 344, 950– 957. Hu, X.C., Andrews, D.Q., Lindstrom, A.B., Bruton, T.A., Schaider, L.A., Grandjean, P., Lohmann, R., Carignan, C.C., Blum, A., Balan, S.A., Higgins, 22
C.P., Sunderland, E.M., 2016. Detection of Poly- and Perfluoroalkyl Substances (PFASs) in U.S. Drinking Water Linked to Industrial Sites, Military Fire Training Areas, and Wastewater Treatment Plants. Environ. Sci. Technol. Lett. 3, 344–350. Hummers, W.S., Offeman, R.E., 1958. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 80, 1339–1339. Kissa, E., 2001. Fluorinated surfactants and repellents, 2nd Edn. R. ed. Marcel Dekker, New York. Kumordzi, G., Malekshoar, G., Yanful, E.K., Ray, A.K. , 2016. Solar photocatalytic degradation of Zn2+ using graphene based TiO2. Sep. Purif. Technol. 168,294-301. Lavanya, T., Dutta, M., Satheesh, K., 2016. Graphene wrapped porous tubular rutile TiO2 nanofibers with superior interfacial contact for highly efficient photocatalytic performance for water treatment. Sep. Purif. Technol. 168, 284-293. Leary, R., Westwood, A., 2011. Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis. Carbon N. Y. 49, 741–772. Liang, D., Cui, C., Hub, H., Wang, Y., Xu, S., Ying, B., Li, P., Lu, B., Shen, H., 2014. One-step hydrothermal synthesis of anatase TiO2/reduced graphene oxide nanocomposites with enhanced photocatalytic activity. J. Alloys Compd. 582, 236–240. Luo, J., Hu, J., Wei, X., Fu, L., Li, L., 2015. Dehalogenation of persistent halogenated organic compounds: A review of computational studies and quantitative structure–property relationships. Chemosphere 131, 17–33. 23
Miklos, D.B., Remy, C., Jekel, M., Linden, K.G., Drewes, J.E., Hübner, U., 2018. Evaluation of advanced oxidation processes for water and wastewater treatment – A critical review. Water Res. 139, 118–131. Nelson, J.W., Hatch, E.E., Webster, T.F., 2010. Exposure to polyfluoroalkyl chemicals and cholesterol, body weight, and insulin resistance in the general U.S. population. Environ. Health Perspect. 118, 197–202. Ortiz, I., Mosquera-Corral, A., Lema, J.M., Esplugas, S., 2015. Advanced technologies for water treatment and reuse. AIChE J. 61, 3146–3158. Pastrana-Martínez, L.M., Morales-Torres, S., Likodimos, V., Figueiredo, J.L., Faria, J.L., Falaras, P., Silva, A.M.T., 2012. Advanced nanostructured photocatalysts based on reduced graphene oxide-TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye. Appl. Catal. B Environ. 123–124, 241–256. Pelaez, M., Nolan, N.T., Pillai, S.C., Seery, M.K., Falaras, P., Kontos, A.G., Dunlop, P.S.M., Hamilton, J.W.J., Byrne, J.A., O’Shea, K., Entezari, M.H., Dionysiou, D.D., 2012. A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal. B Environ. 125, 331–349. Prevedouros, K., Cousins, I.T., Buck, R.C., Korzeniowski, S.H., 2006. Critical Review Sources , Fate and Transport of Fate. Environ. Sci. Technol. 40, 32– 44. Ribao, P., Rivero, M.J., Ortiz, I., 2017. TiO2 structures doped with noble metals and/or graphene oxide to improve the photocatalytic degradation of dichloroacetic acid. Environ. Sci. Pollut. Res. 24, 12628-12637. 24
Ribao, P., Rivero, M.J., Ortiz, I., 2018. Enhanced photocatalytic activity using GO/TiO2 catalyst for the removal of DCA solutions. Environ. Sci. Pollut. Res. 25, 34893-34902. Sakai, H., Autin, O., Parsons, S., 2013. Change in haloacetic acid formation potential during UV and UV/H2O2 treatment of model organic compounds. Chemosphere 92, 647–651. Spasiano, D., Marotta, R., Malato, S., Fernandez-Ibañez, P., Di Somma, I., 2015. Solar photocatalysis: Materials, reactors, some commercial, and preindustrialized applications. A comprehensive approach. Appl. Catal. B Environ. 170–171, 90–123. The European Commission, 2017. Commission Regulation (EU) 2017/1000 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals as regards perfluorooctanoic acid (PFOA), its salts and PFOArelated substances, Official Journal of the European Union. UNEP, 2001. Final Act of the Conference of Plenipotentiaries on The Stockholm ConventionOnPersistent Organic Pollutants. United Nations Environ. Progr. Geneva, Switz. 44. USEPA, 2016. Drinking Water Health Advisory for Perfluorooctanoic Acid (PFOA). USEPA, 1998. Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules. Wang, P., Wang, J., Wang, X., Yu, H., Yu, J., Lei, M., Wang, Y., 2013. One-step synthesis of easy-recycling TiO2-rGO nanocomposite photocatalysts with enhanced photocatalytic activity. Appl. Catal. B Environ. 132–133, 452–459. 25
Zhang, Y., Collins, C., Graham, N., Templeton, M.R., Huang, J., Nieuwenhuijsen, M., 2010. Speciation and variation in the occurrence of haloacetic acids in three water supply systems in England. Water Environ. J. 24, 237–245.
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Supplementary material Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero, Paula Ribao, Beatriz Gomez-Ruiz, Ane Urtiaga, Inmaculada Ortiz* Department of Chemical and Biomolecular Engineering. University of Cantabria. Av. de Los Castros s/n. 39005 Santander. Spain. * Corresponding author: [email protected]
A
B
Figure S1. DCA and FPOA removal during the photocatalytic treatment with TiO2 (A) and TiO2-rGO (B) as catalysts respectively.
A
B
Figure S2. DCA and FPOA mineralization during the photocatalytic treatment with TiO2 (A) and TiO2-rGO (B) as catalysts respectively.
27
Figure S3. DCA and FPOA removal fitting to first order kinetic model for TiO2-rGO
28
Highlights
Photo-activity of TiO2-rGO was tested as a function of the organic pollutant nature
TiO2-rGO showed similar kinetic performance in terms of PFOA and DCA degradation rates
DCA was degraded and mineralized to the same extent using TiO2-rGO composite
PFOA defluorination and mineralization were remarkably lower than its degradation
Mineralization strongly depended on the number of halogen substituents and chain length
29
CRediT author statement Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids Authors: Maria J. Rivero: Conceptualization, Methodology, Writing - Review & Editing, Project administration, Funding acquisition. Paula Ribao: Investigation, Validation, Formal analysis, Writing - Original Draft. Beatriz Gomez-Ruiz: Investigation, Validation, Formal analysis, Writing - Original Draft. Ane Urtiaga: Conceptualization, Methodology, Project administration, Funding acquisition. Inmaculada Ortiz: Conceptualization, Supervision, Project administration
30
Dichloroacetic acid (DCA)
TiO2-rGO
TiO2
100
Removal (%)
Perfluorooctanoic acid (PFOA)
80
DCA
PFOA
60 40 20 0
TiO₂
TiO₂-rGO
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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