Removal of methyl orange by heterogeneous fenton process using iron dispersed on alumina pillared bentonite pellet

Science & Technology Development Journal, 23(2):555-563 Open Access Full Text Article Research Article 1Department of Environmental Engineering, International University, Vietnam National University Quarter 6, Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Viet Nam 2Faculty of Environment, University of Science, Vietnam National University 227, Nguyen Van Cu Street, 4th Ward, District 5, Ho Chi Minh City, Viet Nam Correspondence Ngo Thi Thuan, Department of Environmental Engineering, International University, Vietnam National University Quarter 6, Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Viet Nam Email: ntthuan@hcmiu.edu.vn History  Received: 2020-04-16  Accepted: 2020-06-12  Published: 2020-06-30 DOI : 10.32508/stdj.v23i2.2139 Copyright © VNU-HCM Press. This is an open- access article distributed under the terms of the Creative Commons Attribution 4.0 International license. Removal of methyl orange by heterogeneous fenton process using iron dispersed on alumina pillared bentonite pellet Ngo Thi Thuan1,*, Tran Tien Khoi1, Nguyen Thi My Chi2, Nguyen Ngoc Vinh1 Use your smartphone to scan this QR code and download this article ABSTRACT Introduction: Heterogeneous Fenton is one of the Advanced Oxidation Processes (AOPs) and has been proven to be effective on azo dye degradation. However, a low-cost catalyst and factors af- fecting the processes of this system were further investigated. Methods: In this study, pellets of iron alumina pillared bentonite (PFeAPB) were prepared by dispersing iron ions on alumina pil- lared bentonite pellet. Catalyst activity and lifetime were investigated via efficiencies of Methyl Orange (MO) decolorization and Chemical Oxygen Demand (COD) removal, a typical dye type of textile wastewater. Characteristics of the PFeAPB catalyst were examined by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, and X-ray fluorescence (XRF).Results: Results of batch experiments showed that specific surface area of the PFeAPB catalyst was 111.22m2/g higher than its precursor by 2 times (57.79 m2/g). Goethite, Hematite and Maghemite phases with approxi- mately 11.5% of iron elements containing in the catalyst were detected via XRD and XRF. Experi- mental conditions of pH, initial MO solution, Hydrogen Peroxide concentration, reaction time and catalyst loading were 2.0 0.1, 12.7 mmol/L, 150 min and 20 g/L, respectively, to achieve 88.68 5.69% of MO decolorization and 50.27 6.05% of COD removal while dissolved iron in this hetero- geneous Fenton process was below standard limit (2 ppm). Catalyst activity decreased by 5.22% in decolorization efficiency after the two first reusages. Conclusion: These primary results showed the potential of applying PFeAPB catalyst in heterogeneous Fenton process with low iron leaching into water. Key words: Heterogeneous Fenton Catalyst, Alumina Pillared Bentonite, Pellet, Methyl Orange, Textile Wastewater INTRODUCTION Textile is a main export industry in Vietnam and its wastewater has been listed as difficult-to-degrade wastewater. Among several physical, chemical and biological processes, adsorption has been proven as a widely used, effectivemethod to decolorize dye in tex- tile effluent. However, pollutants in dye wastewater are adsor ed on the adsorbent and concentrated into a smaller volume but not degraded. Advanced Oxida- tion Processes (AOPs) have been proven worldwide as efficient methods in dye wastewater treatment due to high oxidation of active radicals andmineralization capability to persistent organic pollutants (such as azo dyes into CO2 and H2O). Thus, homogeneous Fen- ton processes are commonly applied to treat textile wastewater. However, these processes still have some disadvantages of iron treatment, sludge, and strict op- eration under acidic condition. Heterogeneous Fenton processes have been intro- duced to overcome disadvantages of homogeneous Fenton processes. These processes apply iron catalysts which are immobilized on the surface of the adsor- bent and combined with hydrogen peroxide (H2O2) to generate hydroxyl radicals (
OH)1,2, thus possibly minimizing iron leaching into water and operating under less acidic condition, enabling the catalyst to be reused and recycled. Bentonite clay has been used as an adsorbent and has gained much attention in envi- ronmental remediation due to iron content in clay, its low cost, its abundance, and ion-exchange capability, while still having low specific surface area3,4. Specific surface area of bentonite clay can be increased by in- tercalating inorganic/organic cations into expandable clay layers (so-called cation pillared bentonite). They are fabricated by cation exchange with polyoxycations of silica-alumina layers, then calcinated 5,6. Among several cations to be pillared into clay, polycations of aluminum are preferred due to the well-known struc- ture, synthesis conditions and stabilities7. In addi- tion, alumina pillared clays give much higher surface Lewis acidity than their precursor6,8. Hence, alumina pillared bentonite may be used as a good supporter to be impregnated with irons which are active sites of heterogenous Fenton catalysts. Cite this article : Thuan N T, Khoi T T, Chi N T M, Vinh N N. Removal of methyl orange by heterogeneous fenton process using iron dispersed on alumina pillared bentonite pellet. Sci. Tech. Dev. J.; 23(2):555-563. 555 Science & Technology Development Journal, 23(2):555-563 So far there are very few investigations of iron dis- persed on alumina pillared bentonite used as hetero- geneous Fenton catalyst for environmental remedia- tion. Some studies focus on iron pillared bentonite as Fenton catalysts for degradation of cinnamic acid 9, or dyestuff with UV light assistance10. However, re- searchers focus on powder type which cannot be used in a continuous system due to system clogging iron leaching into water of this catalyst as well as dye re- moval have not been investigated. Iron phases of this Fenton catalyst can leach into water possibly due to a poor supporter, especially in relatively acidic con- ditions. Therefore, a heterogeneous Fenton catalyst based on alumina pillared bentonite pellet was exam- ined. The objective of this study was to investigate the reactivity of the PFeAPB catalyst during hetero- geneous Fenton process for Methyl Orange (MO) re- moval from water. MATERIALS-METHODS Materials A commercial clay product, bentonite powder, was purchased from a local company in Vietnam (Minh Ha Bentonite Mineral Joint Stock Company, Phan Thiet Province, Viet Nam); iron catalyst was prepared from ferric nitrate nonahydrate (Fe(NO3)3.9H2O; alumina pillared in clay was prepared from aluminum nitrate nonahydrate (Al(NO3)3.9H2O); sodium hy- droxidewas purchased fromSigma-AldrichChemical Co. (St. Louis, MO, USA), hydrogen peroxide (>30 wt.%) and hydrochloric acid (37%) were purchased from Fisher Scientific (UK). Catalyst preparation Inorganic pillaring technique has been reported pre- viously5,6,11,12 and was adapted as follows: the ben- tonite powder was sieved with 2 mm to remove all big contaminants, then added into Al3+ solution to be- come alum-bentonite slurry. The slurry was stirred vigorously in 1 hour and left to age for 24 hours un- der ambient conditions. The alum-bentonite after ag- ing was centrifuged and compacted into pellet shape (3 mm of diameter, 2-3 cm of length). This pellet was dried at 105oC for 12 hours and calcinated at 600oC to form the so-called pellet of alumina pillared bentonite (PAPB). Fe(NO3)3.9H2O 1Mwith 10% of HNO3 so- lution was impregnated on the surface of PAPB for 4 hours and dried at 105oC for 15 hours, and finally baked at 350oC for 4 hours. The final product after iron impregnation was referred to as the pellet of iron alumina pillared bentonite (PFeAPB) (Figure 1). Experimental procedure The Fenton reactor was a 250 mL beaker filled with 100 mL of MO solution and placed in a magnetic stir- ring machine. The initial pH, H2O2 concentration, MO concentration and catalyst loading were as fol- lows: 3.00.1, 12.7 mmol/L, 100 ppm and 20 g/L, re- spectively; the reactionmixture was constantly stirred at 200 rpm for 120 min. Samples of the reaction mix- ture were taken with syringe at selected time intervals and then increased to pH~10 with NaOH 2N, and fi- nally filtered through a 0.45mmmembrane for analy- sis. Each experiment was repeated 3 times. The mean value and standard deviation ( SD) of three repli- cated results in each experiment were calculated and presented. The used PFeAPB catalyst was washed with distilled water and dried at 105oC in an oven for 5 h. These regenerated pellets were used to investigate catalyst reusability and stability. Analytical methods The UV-VIS spectra of MO were recorded from 200 to 700 nm using a UV-VIS spectrophotometer with a spectrophotometric quartz cell and its concentration wasmeasured at themaximumwavelength. Chemical Oxygen Demand (COD) and total ferrous ions were determined by bichromate and 1,10-phenanthroline methods, respectively, according to the Standard Methods for the examination of water and wastewa- ter13. Degradation of MO was investigated via MO decol- orization (MO removal) andCOD removal efficiency: MO(%) =  C0Ct C0  100 Where Co (ppm) is the MO initial concentration and initial COD, and Ct (ppm) is the MO concentration and COD at time of withdrawal. The catalysts were characterized by X-ray diffrac- tion spectroscopy (XRD), X-ray fluorescence (XRF) and nitrogen adsorption/desorption isotherm by Brunauer–Emmett–Teller (BET) surface area. RESULTS Active phases of Fenton catalysts were examined with XRD and XFR analysis. Figure 2 shows X-ray diffrac- tion spectra of 5% iron dispersed on the alumina pillared bentonite catalyst; there were Goethite (a- FeOOH), Hematite (a-Fe2O3) and Maghemite (g- Fe3O4) found in the PFeAPB catalyst. Element com- positions of the catalyst and its precursor were ob- tained by XRF analysis and are shown in Table 1. No- tably, Si, Al, Fe, Ca and Mn were five elements found 556 Science & Technology Development Journal, 23(2):555-563 Figure 1: Configuration of (a) PAPB and (b) PFeAPB catalysts. in both PFeAPB catalyst and its precursor in the range of 0.09 to 51.0%. Si, Al and Fe contents in the PFeAPB catalyst accounted for 49.1%, 21.1% and 11.5% of the total content, while their presence in its catalyst pre- cursor accounted for 51.0%, 12.2% and 6.95%, respec- tively. Performance of PFeAPB catalyst in the heterogeneous Fenton system was evaluated by comparing removal efficiencies among H2O2, PAPB, PAPB + H2O2, and PFeAPB + H2O2 systems with time; the data are pre- sented in Figure 3. The results showed that increas- ing time from 15 to 180 minutes led to enhanced decolorization efficiency of MO from 2.090.66% to 75.235.35%, respectively, and in a sequence of H2O2<PAPB<PAPB+H2O2<PFeAPB+H2O2. Iron element was likely a main contribution to the heterogeneous Fenton processes. The effects of the iron contents impregnated in the catalyst were eval- uated and recorded in Figure 4. An increase from 5 to 20% of iron impregnated in the PFeAPB catalyst enhanced MO decolorization efficiency from 68.50 6.59% to 82.10  7.02%, respectively. The dissolved iron started to leach into the water and its level was over the standard limit (2 ppm)when 15% of iron ions were dispersed in the catalyst. The pH effect on leaching of iron ions and
OH pro- duction on MO decolorization was depicted in Fig- ure 5. The results in Figure 5 indicate that the high decolorization efficiencies ofMOwere achieved at pH =2 (80.32  5.26%) and pH=3 (69.10  4.26%). The other pH values yielded around 47.20 - 52.05% of de- colorization efficiencies. TheH2O2 concentrationswere examined in a range of 0 – 50.8 mmol/L.The results of the effect of H2O2 are shown in Figure 6 a, b. When H2O2 concentration was increased from 0 to 12.7 mmol/L, the decoloriza- tion efficiencies of MO increased to 75.69  4.35%. This efficiency decreased when H2O2 concentration was over 12.7 mmol/L. The effect of reaction time on MO decolorization, COD removal, and iron leaching in water are shown in Figure 7. MO decolourization and COD re- moval efficiencies were achieved from 60.456.26% to 88.685.69% and 30.323.69% to 50.276.05%, respectively, in the range of 15-150 min. Reusability and stability of the PFeAPB catalyst were investigated and shown in Figure 8. The repeatabil- ity of catalyst reactivities for the first two runs was achieved below 5.22%. Decolorization efficiencies were dropped to 16.39% and 39.08% in the 3rd and 4th run of the experiments. DISCUSSION Catalyst characterization Iron active phases observed at around 21.23o, 33.1o and 35.6o were Goethite (a-FeOOH), Hematite (a-Fe2O3) and Maghemite (g-Fe3O4), respectively, with increasing intensity (Figure 2). Hematite and Maghemite phases are proposed to be the active sites of heterogenous Fenton catalysts14,15. The bentonite is a natural clay with 2:1 silicate layer which is featured with hydroxide aluminium phyl- losilicates. Aluminum (Al) and Silicon (Si) elements are dominant in bentonite clay. Change of PFeAPB composition was indicated by successful intercalation and impregnation of aluminum and iron ions into Bentonite clay (Table 1). The specific surface area of the pillared catalysts increased dramatically, from 57.79 m2/g to 111.22 m2/g, compared to the precur- sor clay. The alumina pillared into the Bentonite lay- ers increased the specific surface area and adsorp- tion capabilities due to the wider basal space among the 2 silicate layers. These results support many pre- vious observations that the specific surface area in- creases when the precursor clay is pillared with cation 557 Science & Technology Development Journal, 23(2):555-563 Figure 2: XRD pattern of PFeAPB catalyst with 5% of iron loading. ions8–10,16,17. Iron content in the PFeAPB catalyst increased from 6.95% to 11.5% due to introducing 5% iron into the catalyst. Loss of 0.45% iron may come from preparation and vaporization during cal- cination. Evaluation on performance of heteroge- nous Fenton process The results in Figure 3 were attributed to two char- acteristics including clay adsorption of PAPB and ox- idation of H2O2, as well as hydroxyl radical (
OH). Decolorization efficiencies of H2O2 oxidation and PAPB adsorption were 6.040.78% and 18.121.49% at 120 min, respectively. Synergetic effect of PAPB and H2O2 yielded 40.363.27% of MO conversion. Decolorization efficiency of MO was increased from 40.363.27% to 71.505.69% when the iron catalyst was added into PAPB. This was explained by increas- ing iron content in the catalyst (~4.55%) and byH2O2 which releases
OH in the system; these steps may oc- cur in Equation (1) and (2). FeOOH(s)+2H++ 12 H2O2!Fe2++ 12 O2 +2H2O (1) Fe2O3(s) + 6H+! 2Fe2+ + 3H2O (1-1) Fe3O4(s)+ 8H+! 3/2Fe3+ + 3/2Fe2+ + 4H2O. (1-2) Fe2+ + H2O2! Fe3+ + OH + OH (2) Effect of iron content impregnated into the PFeAPB catalyst Decolorization efficiency increased from 68.506.59% to 82.107.02% when the impregnated iron amount was enhanced from 5% to 20% because iron phases are supposedly the main active catalysts of PFeAPB materials and can provide highly reactive radicals (
OH, HO2
) for MO decolorization via Fenton processes, according to Equations (2) to (8) : Fe2+ + H2O2! Fe3+ + OH + OH(2) Fe2+ + OH! Fe3+ + + OH(3) Fe3+ + H2O2! Fe2+ + HO2 + H+(4) Fe2+ + HO2! Fe3+ + HO2(5) Fe3+ + HO2! Fe2+ + O2 + H+(6) C14H13N3NaO3SH +  OH!H2O + R(7) R + OH!H2O +. by products (8) However, a small change of MO decolorization effi- ciencies (80.205.69% & 82.107.02%) at 15% and 20% of impregnated irons was possibly due to the scavenging effect of HO2
(Equation 5 and 6). In addi- tion, dissolved irons of 2.010.48 ppmwere observed at 15% of impregnated irons. The amount of impreg- nated irons in the catalyst should be below 10% to avoid iron leaching and to achieve the national stan- dard limit (2 mg/L). However, change of iron con- tents in the catalysts need to be further investigated with XRD spectra to confirm reactivity of iron active phases during the heterogeneous Fenton processes. Effect of pH on decolorization efficiency The active phases of iron oxide, such as goethite (a- FeOOH), hematite (a-Fe2O3) and maghemite (g- Fe2O3) of the PFeAPB, tend to be dissolved in acidic condition to become ferrous and ferric ions, accord- ing to Equations 1, 9 and 10. These ferrous and ferric ions promoted OH radicals to discolor MO. Acidic 558 Science & Technology Development Journal, 23(2):555-563 Figure 3: Decolorization efficiency (SD) of MO under various systems. Experimental conditions: pH=3.0  0.1, [MO]=100 mg/L, [H2O2]=12.7mM, catalyst loading: 20 g/L. Figure 4: Effect of iron contents impregnated onMOdecolorization efficiency (SD) and dissolved iron. Ex- perimentalconditions: pH=3.00.1, [MO]=100mg/L, [H2O2]=12.7mM, catalyst loading: 20g/L, reaction time=120 min. pH values also shift the equilibrium of Equation 2 to the right which promotes
OH radical formation; the high decolorization efficiencies of MO were achieved at pH =2 (80.32 5.26%) and pH=3 (69.10 4.26%). However, if the soluble constant of Fe(OH)3 is the- oretically 2.79*1039, precipitation of Fe(OH)3 starts to occur at approximate pH=4.0, thus resulting in de- creasing decolorization efficiency (52.055.96%) as expected, thus preventing the Fenton process (Equa- tions 1&2). When hydrogen ions in the system are too high (pH=1), these ions may become scavengers of  OH radicals (Equation 11), inducing MO decol- orization capability (50.105.23%). These results are relatively comparable to other studies1,2,9 and indi- cate a wider range of pH in the heterogeneous Fenton system. A pH=2 was chosen to achieve the highest efficiency for further investigation in this study. FeOOH(s)+2H++ 12 H2O2! Fe2++ 12 O2 +2H2O (1) Fe2O3(s)+ 6H+! 2Fe3+ + 3H2O (9) Fe3O4(s) + 8H+! 3/2Fe3+ + 3/2Fe2+ + 4H2O (10) H++ OH + e!H2O (11) 559 Science & Technology Development Journal, 23(2):555-563 Figure 5: Effect of pH on MOdecolorization efficiency (SD) and dissolved iron. Experimental conditions: [MO]=100 mg/L, [H2O2]=12.7 mM, catalyst loading: 20 g/L, reaction time=120 min. Effect of H2O2 concentration Effect of H2O2 concentration on MO decolorization efficiencies are contributed by highly oxidation capa- bility of H2O2 species as well as the release of OH and HO2
radicals during reactions between ferrous and ferric ions and H2O2 (Equations 2 & 4). How- ever, when the H2O2 amount is increased over 12.7 mmol/L, the decolorization efficiencies were dropped by 20% due to the scavenging of  OH by H2O2, as shown in Equations 12 &13.  OH + O2! HO2 + H2O (12) HO2+OH! H2O+O2 (13) UV-VIS spectroscopy of MO before and after het- erogenous Fenton reaction under different H2O2 concentrations are shown in Figure 6 b. The strongest absorbance of MO molecules was at 464 nm. After 120 min of heterogenous Fenton reaction, this peak disappeared totally when H2O2 concentration was 12.7 mmol/L. A new peak appeared from 232 to 239 nm, possibly because of p!p* transition in aromatic compounds. The results indicate that decolorization of MO molecules may occur at azo site (-N=N-) and that the peaks at 232 – 239 nm are possibly benzy- lamine compounds such as sulfanilic acid. This result is comparable to standard spectroscopy of pure sul- fanilic acid 15,18. The final products of the heteroge- nous Fenton processes need to be further analyzed by GC-MS. Effect of reaction time The relatively low decolorization efficiency (60.45  3.26%) and COD removal (30.32  3.69%) of MO at the beginning of the heterogeneous Fenton processes (15 min) were possible due to MOmolecule diffusion and adsorption on the surface of the catalysts. Once MO molecules were adsorbed on the active sites of irons, heterogenous Fenton process occurred in the presence of H2O2, thus increasing MO decoloriza- tion with time. However, this process was slowed af- ter 120 min because the low catalytic activity, possi- bly relating to the formation of intermediate oxidation products which inhibited OH released at iron active sites19. Efficient differences between MO decolorization and COD removal efficiencies were from 30 to 40%. The maximumCOD removal efficiency was 50.276.05% at 150 min while the decolorization efficiency was 88.685.69% (Figure 7). These results indicate that heterogeneous Fenton processes using the PFeAPB catalysts may oxidize MO molecules into smaller or- ganic molecules and partly mineralize into CO2 and H2O. Mineralization of azo dyes occurred slowly compared to the decolorization processes. The COD of the MO solution cannot be removed completely, only 50.276.05% in 150 min, possibly due to for- mation of some persistent by-products and short life- time of radicals. Dissolved irons were increased with reaction time; however, the concentration (max. 1.130.068 ppm) was still below the national stan- dard limit in water (2 ppm). Resistance to iron leaching into solution is possibly due to metal com- plexes between acid organic compounds, such as 2- carboxyphenylacetic acid, phthalic acid, and oxalic acid, which are released as by-products and iron sites on the PFeAPB surface15. In addition, iron ions were properly immobilized within the interlayer space and associated with alumina pillars, thus becoming highly resistant to iron leaching20,21. Catalyst reusability and stability The pillared process between intercalation of alu- minum and iron greatly increases accessibility of re- actants and reduces their intermolecular collision 560 Science & Technology Development Journal, 23(2):555-563 Figure 6: (a) Effect of H2O2 on decolorizationefficiency (SD); (b) UV-VIS spectroscopy of MO before and after heterogenous Fenton reaction. Experimental conditions:[MO]=100mg/L, pH= 2.0 0.1,catalyst loading: 20 g/L, reaction time=120min. Figure 7: Effect of reaction time on performance of PFeAPB catalyst in heterogenous Fenton process (meanSD). Experimental conditions: [MO]=100mg/L, [H2O2]=12.7mM, catalyst loading:20 g/L, pH=2 0.1. and competition at the catalyst sites22,23. Thus, the PFeAPB could be reused two times and yielded 5.22% repeatability (Figure 8). The PFeAPB catalyst ac- tivity decreased after 2 runs of reusage (16.39% and 39.08%)- at the 3rd and 4th runs- which may be at- tributed to iron leaching and intermediate products adsorbed on the active sites. CONCLUSIONS This study primarily developed a pellet type of Iron dispersed on Alumina Pillared Bentonite and investi- gated its capability ofMO treatment fromwater. XRD results showed that there were a-FeOOH, a-Fe2O3 and g-Fe3O4 which were supposedly catalyst phases of the heterogeneous Fenton reaction. Because of suc- cessful intercalation of Alumina into Bentonite lay- ers, the specific surface area was increased from 57.79 m2/g to 111.22 m2/g, according to the BET results. The PFeAPB catalysts achieved 88.68 5.69% of MO decolorization and 50.27  6.05% of COD removal when experimental conditions of pH, H2O2 concen- tration, catalyst loading, reaction time and initial MO concentration were 2 0.1, 12.7 mmol/L, 20 g/L, 150 min and 100 ppm, respectively. The PFeAPB catalyst can be resistant to iron leaching with 11.5% of iron content in the catalyst and can be reused 2 times with 5.22% of repeatability, compared to the new catalyst. The study results indicate that PFeAPB can be a po- tential catalyst for the heterogeneous Fenton process and applied in textile wastewater with low loss of iron leaching into water. This catalyst should be further investigated in a continuous system. 561 Science & Technology Development Journal, 23(2):555-563 Figure 8: Reusability of PFeAPB catalyst (mean SD). Experimental conditions: [MO]=100 mg/L, [H2O2]= 12.7 mM, catalyst loading: 20 g/L, pH = 2 0.1, reaction time = 150 min. LIST OF ABBREVIATIONS COD: Chemical Oxidation Demand BET: Nitrogen adsorption/desorption isotherm MO:Methyl Orange XRD: X-ray diffractometer XRF: X-ray fluorescence PFeAPB: Pellet of iron alumina pillared bentonite UV-VIS:Ultraviolet visible AUTHORS’ CONTRIBUTIONS The author Ngo Thi Thuan discussed the results and wrote the manuscript. The author Tran Tien Khoi edited and revised the final manuscript. The author Nguyen Thi My Chi did the experiment. The au- thor Nguyen Ngoc Vinh trained the instrument op- erations in laboratory. All authors approved the final manuscript. COMPETING INTERESTS The authors declare that they have no competing in- terests. ACKNOWLEDGEMENTS The authors would like to thank MSc. Le Thi Song Thao for her contribution on primarily investigation of the catalyst activity. REFERENCES 1. Feng J, Hu X, Yue PL. Discoloration and mineralization of Or- ange II by using a bentonitee clay-based Fe nanocomposite film as a heterogeneous photo-Fenton catalyst. 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