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Fenton oxidation of gallic and p-coumaric acids in water assisted by an activated carbon cloth María A. Fontecha-Cámara, Miguel A. Álvarez, Victoria López-Ramón and Carlos Moreno-Castilla

ABSTRACT The objective of this study was to investigate the effect of the presence of an activated carbon cloth (ACC) during the degradation and removal of gallic acid (GA) and p-coumaric acid (pCA) by Fenton oxidation using H2O2 and FeSO4 as catalyst. Removal of GA or pCA by Fenton oxidation was much higher than that of total organic carbon (TOC), indicating that a large proportion of GA or pCA degradation products was not mineralized. The presence of ACC increased the concentration of hydroxyl radicals generated in the FeSO4 þ H2O2 system. The presence of ACC during Fenton oxidation largely increased TOC and GA removal, attributable to the adsorption of GA and its degradation products and the increased generation of OH• radicals that mineralize them. In the

María A. Fontecha-Cámara Miguel A. Álvarez Victoria López-Ramón (corresponding author) Departamento de Química Inorgánica y Orgánica, Universidad de Jaén, 23071 Jaén, Spain E-mail: [email protected] Carlos Moreno-Castilla Departamento de Química Inorgánica, Universidad de Granada, 18071 Granada, Spain

Fenton oxidation of pCA, the presence of ACC produced the same effects as for GA, but now the increased removal of pCA was due to adsorption on the activated carbon and not to the increased generation of hydroxyl radicals, due to the greater affinity of pCA for the carbon surface and its more difficult mineralization in comparison to GA. Key words

| activated carbon cloth, Fenton oxidation, gallic acid, p-coumaric acid

INTRODUCTION Phenolic acids such as gallic acid (GA) and p-coumaric acid (pCA) are present in natural waters from the decay of vegetation and are abundant in the wastewater effluents of different agro-industries, including those related to winemaking and olive-oil extraction, which are of major importance in many Mediterranean regions (Gernjak et al. ; Cañizares et al. ; El-Gohary et al. ). Phenolic acids act as microbial inhibitors due to their antibacterial effects; therefore, they cannot be eliminated from effluents by conventional aerobic biological treatments. An alternative option is to utilize advanced oxidation processes (AOPs), characterized by the production of OH• radicals, 1 superoxide radical anion (O•– 2 ), and singlet oxygen ( O2). Fenton oxidation is one of the most attractive AOPs because it is simple to apply and economic (Du et al. ; Lofrano et al. ). The generation of OH• radicals is described by Equation (1) Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH doi: 10.2166/wst.2015.034

(1)

Ferric ions produced in Equation (1) can be reduced again by H2O2 according to Equation (2), which is designated a Fenton-like reaction Fe3þ þ H2 O2 ! Fe2þ þ HO2 þ Hþ

(2)

The reaction rate is much slower for Equation (2) than for Equation (1); therefore, Fe2þ ions are rapidly consumed but slowly regenerated (Du et al. ). There are also two main side reactions competing with those given by Equations (1) and (2). One is the oxidation of Fe2þ ions by the dissolved oxygen in water, and the other is the formation of ion complexes between the iron ions and some degradation products, e.g., oxalate ions (Caudo et al. ), whose high stability leaves few iron ions in solution. The presence of activated carbons during Fenton oxidation can play an important dual role in this process, as previously demonstrated (Fontecha-Cámara et al. ). On one hand, they can adsorb the phenolic acids and their

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degradation products and, on the other, carbon basic surface sites can promote H2O2 decomposition into hydroxyl radicals (Kimura & Miyamoto ; Khalil et al. ; Santos et al. ), thereby enhancing the oxidation of organic compounds in aqueous media. Surface basic sites can be delocalized π-electrons from the graphene layers (C-π), which transfer electrons according to the mechanism given by Reactions (3) and (4) and proposed by Kimura & Miyamoto (), which is similar to the Fenton reaction and Haber-Weisz mechanism C-π þ H2 O2 ! C-π þ þ OH þ OH

(3)

C-π þ þ H2 O2 ! C-π þ HO2 þ Hþ

(4)

AOPs removal of some phenolic acids has been previously studied (Gernjak et al. ; Beltrán et al. ; Cañizares et al. ; El-Gohary et al. ), but the influence of the presence of activated carbons during GA and pCA removal by Fenton oxidation has not been investigated; therefore, the aim of this study was to explore this influence by using a commercial activated carbon cloth (ACC).

MATERIALS AND METHODS Materials ACC from Kynol Europe had a Brunauer-Emmett-Teller (BET) surface area from N2 adsorption at 196 C of 2,092 m2 g1, and a micropore volume and a mean micropore width, from Dubinin-Radushkevich equation, of 0.798 cm3 g1 and 1.25 nm, respectively. The mesopore volume was 0.129 cm3 g1. The pH at the point of zero charge of ACC was 8.4, and its total surface basicity and acidity were 0.54 and 0.28 meq g1, respectively. GA (3,4,5-trihydroxybenzoic acid) and pCA ((E)-3-(4hydroxyphenyl)-2-propenoic acid) were supplied by Sigma– Aldrich with high-pressure liquid chromatography (HPLC) grade purity. Water solubility at 25 C for GA (11.9 g L1) is greater than for pCA (0.7 g L1). W

W

Oxidation studies

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solution, adjusting the pH to 3, and adding the H2O2 solution. These experiments were run in the absence and presence of ACC in the form of 6-mm diameter circles (∼3 mg). Blank experiments were run to determine the percentage of GA or pCA removed by adsorption on ACC. Analytical method GA and pCA concentrations were measured by HPLC (Thermo-Fisher) equipped with a UV8000 photodiode detector, using a Hypersil Gold (250 × 4.6 mm) chromatographic column. The mobile phase was a mixture of 10% HPLC grade methanol and 90% ultrapure water (0.1% formic acid) for GA and a mixture of 35% HPLC grade methanol and 65% ultrapure water (0.1% formic acid) for pCA, in isocratic mode at a flow of 1 mL min1. The detector wavelength was 260 nm for GA and 309 nm for pCA. In addition, GA and pCA mineralization was followed by measuring the total organic carbon (TOC) with a TOC5000A model Shimadzu analyzer. The formation kinetics of OH• radicals generated in the Fenton processes were determined by using a method to measure them in AOPs (Tai et al. ). Thus, dimethyl sulfoxide was added at different reaction times to trap the OH• radicals to quantitatively produce formaldehyde, which then reacted with 2,4-dinitrophenylhydrazine to form the corresponding hydrazone that was analyzed by HPLC. According to Tai et al., 2.17 mol of hydroxyl radicals react with dimethyl sulfoxide to give 1 mol of formaldehyde. Concentration variations of oxalic and formic acid (obtained as degradation products) as a function of reaction time were followed by ionic chromatography using a Dionex DX-120 equipped with an AS9-HC column and a conductivity detector with suppressor device. The mobile phase was 3.8 mM NaHCO3 and 3.0 mM Na2CO3 at a flow rate of 1 mL min1 (Zhang et al. ). Aromatic degradation products were qualitatively identified by gas chromatography-mass spectrometry (GC-MS) analysis in electron ionization mode (Zhang et al. ) with an HP-5 capillary column (15 m × 0.25 mm × 0.25 μm). Before the GC-MS analysis, samples were extracted three times with dichloromethane. The extracted solution was dehydrated with anhydrous sodium sulfate and concentrated by rotary evaporation. Next, the sample was trimethylsilylated at 80 C for 30 min using hexamethyldisilazane and anhydrous ammonium sulfate. The initial temperature of the column oven was held at 50 C for 2 min and then increased up to 250 C at a heating rate of 10 C min1. MS was operated in the 70 eV with ionization current of W

Oxidation experiments were carried out at pH 3 using 0.2 L of 0.60 mM GA or pCA solutions in thermostated flasks at 25 C shaken at 300 rpm. The Fenton process was performed by dissolving FeSO4 · 7H2O in the phenolic acid

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250 μA and ion source temperature of 250 C. Mass spectra were recorded in full scan mode (m/z 50–1000). W

RESULTS AND DISCUSSION Fenton oxidation of GA Fenton oxidation was carried out at pH 3, the optimum pH for the generation of OH• radicals (Ramirez et al. ). The initial GA concentration was 0.60 mM, while 7 mM H2O2 was required for its complete mineralization according to Reaction (5)

Figure 2

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W

Removal kinetics of GA (open symbols) and TOC (closed symbols) at 25 C with different systems: ( ) FeSO4 þ H2O2, ( ) FeSO4 þ H2O2 þ ACC (25 mg), and ( ) FeSO4 þ H2O2 þ ACC (100 mg). Removal kinetics of GA at 25 C for sysW

tems: (×) ACC (25 mg) and (

) H2O2 þ ACC (25 mg). V ¼ 0.2 L, CFe2þ ¼ 3.6 ×

102 mM, C0 ¼ 0.60 mM, CH2O2 ¼ 7 mM, pH ¼ 3. Bars indicate standard deviation from three replicate experiments.

C7 H6 O5 þ 12H2 O2 ! 7CO2 þ 15H2 O

(5)

GA degradation is negligible with H2O2 alone at the above proportions (less than 3% in 1 h), which is due to the low oxidation potential of H2O2 (1.77 V) in comparison to that of the OH• radicals (2.84 V) generated by the presence of Fe2þ and the carbon surface basic sites. The effect of Fe2þ concentration between 5 × 10–3 and 0.27 mM on the GA removal rate is depicted in Figure 1. The GA removal rate increases with higher Fe2þ concentrations, reaching 100% in less than 1 min with an Fe2þ concentration of 0.27 mM, attributable to the increase in OH• radicals at higher Fe2þ concentrations. An Fe2þ concentration of 3.6 × 102 mM, which removes 90% of GA in around 30 min, was selected for this study. Figure 2 depicts GA and TOC removal kinetics for some selected systems, yielding the data exhibited in Table 1 after 30 min reaction. Results show that although the Fenton oxidation of GA removes 90 ± 4% of this compound in 30 min, only 12 ± 1% of the TOC is removed in this time, i.e., a large proportion of GA degradation products is not mineralized.

Figure 1

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Removal kinetics of GA at 25 C with different Fe2þ concentrations: ( ) 5 × 103 mM; ( ) 3.6 × 102 mM; ( ) 0.27 mM. Volume V ¼ 0.2 L, initial concentration W

C0 ¼ 0.60 mM GA, CH2O2 ¼ 7 mM, pH ¼ 3. Bars indicate standard deviation from three replicate experiments.

When the above Fenton reaction is carried out in the presence of ACC, the removal rates of GA and TOC are increased at higher ACC doses, and a greater removal of TOC than of GA is observed. The residual TOC is due to the presence of different aromatic degradation products (Figures S1 and S2 in the Supplementary Material, available online at http://www.iwaponline.com/wst/071/034.pdf) and non-aromatic degradation products such as formic and oxalic acids, whose concentrations are plotted against oxidation time in Figure 3. This figure also shows that the concentration of these acids is lower when Fenton oxidation is carried out in the presence versus absence of ACC. These degradation products are very difficult to mineralize (Zazo et al. ; Beltrán et al. ; Carbajo et al. ; Boye et al. ). Furthermore, oxalic acid can form a highly stable trischelate complex with iron ions withdrawing them from the solution and thereby reducing the Fenton oxidation activity (Boye et al. ). The concentrations of OH• radicals generated in the different systems in the absence of GA, [OH•], are plotted against time in Figure S3 in the Supplementary Material (available online at http://www.iwaponline.com/wst/071/ 034.pdf), and Table 1 exhibits their concentration generated after 30 min. These results demonstrate that higher ACC doses during Fenton oxidation increase the concentration of OH• radicals generated. This finding indicates that ACC surface basic sites catalyze the generation of OH• radicals (Kimura & Miyamoto ; Khalil et al. ; Santos et al. ). Thus, the amount of GA removed by the H2O2 þ ACC system is greater than the amount adsorbed on ACC (Table 1). The concentration of OH• radicals generated in the presence of GA, [OH•]GA, is lower than that obtained in the absence of GA. This is because in the presence of GA,

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Table 1

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GA, pCA, and TOC removal at 25 C with different Fenton systems after 30 min of reaction in the absence and presence of ACC, and time to remove 90% of GA or pCA, t90%. W



Concentration of OH radicals generated after 30 min in the absence of GA or pCA. V ¼ 0.2 L, CFe2þ ¼ 3.6 × 102 mM, pH ¼ 3. Initial concentration of GA or pCA, C0 ¼ 0.60 mM. CH2O2 ¼ 7 mM for GA removal and CH2O2 ¼ 11.5 mM for pCA removal p-Coumaric acid

Gallic acid System

GA %

TOC %

t90% min



pCA %

[OH ]mM

TOC %

t90% min



[OH ] mM

FeSO4 þ H2O2

90 ± 4

12 ± 1

30

0.41 ± 0.002

81 ± 4

7±1

55

0.75 ± 0.003

FeSO4 þ H2O2 þ ACC (25mg)

93 ± 3

30 ± 2

19

0.45 ± 0.003

92 ± 2

25 ± 3

30

0.80 ± 0.009

FeSO4 þ H2O2 þ ACC (50mg)

95 ± 5

47 ± 4

15

0.49 ± 0.006

94 ± 1

40 ± 4

23

0.84 ± 0.008

FeSO4 þ H2O2 þ ACC (100mg)

96 ± 2

72 ± 6

10

0.54 ± 0.002

96 ± 2

65 ± 3

15

0.87 ± 0.005

ACC (25mg)

17 ± 2

18 ± 3





21 ± 2

20 ± 2





ACC (50mg)

28 ± 1

26 ± 2





34 ± 4

35 ± 3





ACC (100mg)

44 ± 1

40 ± 1





55 ± 3

54 ± 2





H2O2 þ ACC (25mg)

23 ± 3

17 ± 2





30 ± 3







Figure 4

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Variation of the difference between the concentration of OH radicals in the W

absence and in the presence of GA as a function of time at 25 C in different systems: ( ) H2O2 þ ACC (25 mg); (

) FeSO4 þ H2O2; ( ) FeSO4 þ H2O2 þ ACC

(25 mg); ( ) FeSO4 þ H2O2 þ ACC (100 mg). V ¼ 0.2 L, CFe2þ ¼ 3.6 × 102 mM, C0 ¼ 0.60 mM, CH2O2 ¼ 7 mM, pH ¼ 3. Bars indicate standard deviation from three replicate experiments.

Figure 3

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Removal kinetics of GA (triangles) and formation kinetics of formic acid (a) and oxalic acid (b), in the absence of ACC (open symbols) and in the presence of 100 mg ACC (closed symbols) at 25 C. V ¼ 0.2 L, CFe2þ ¼ 3.6 × 102 mM, C0 ¼ W

0.60 mM, CH2O2 ¼ 7 mM, pH ¼ 3. Bars indicate standard deviation from three replicate experiments.

some OH• radicals are used in its degradation and mineralization and, in addition, they are less abundantly generated because GA adsorption on the ACC surface covers surface basic sites that catalyze the generation of OH• radicals. The variation in the difference [OH•][OH•]GA as a function of reaction time is depicted in Figure 4 and shows also an increase with higher ACC dose. The increase in GA and TOC removal with higher ACC dose in the Fenton oxidation systems can be attributed to an increased generation of OH• radicals by the ACC and/or to

the removal of GA and TOC by adsorption on the ACC. Thus, 25 mg of ACC alone removes around 18 ± 3% of both GA and TOC, while the FeSO4 þ H2O2 system removes 12 ± 1%, making a total of 30 ± 2%, i.e., the same percentage TOC removal obtained with the FeSO4 þ H2O2 þ ACC (25 mg) system. However, when 50 and 100 mg of ACC are used alone, 26 ± 2% and 40 ± 1% of TOC are adsorbed, respectively. Addition of these percentages to the 12 ± 1% of TOC removed by the FeSO4 þ H2O2 system would give 38 ± 2% and 52 ± 1% TOC removal for the Fenton reaction in the presence of 50 and 100 mg of ACC, respectively. These values are lower than those observed with the FeSO4 þ H2O2 þ ACC (50 mg) and FeSO4 þ H2O2 þ ACC (100 mg) systems (47 ± 4% and 72 ± 6%, respectively). These results indicate that the large increase in TOC removal obtained with the Fenton reaction in the presence of 50 and 100 mg

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of ACC is attributable not only to the adsorption on ACC but also to the higher concentration of OH• radicals generated that mineralized GA and its degradation products. Fenton oxidation of pCA The degradation and removal of pCA was carried out using initial pCA and H2O2 concentrations of 0.60 and 11.5 mM, respectively, which were calculated from Reaction (6) C9 H8 O3 þ 19H2 O2 ! 9CO2 þ 23H2 O

(6)

The pCA removed with H2O2 alone is negligible, less than 3% after 40 min reaction. Kinetics of pCA and TOC removal are depicted in Figure S4 in the Supplementary Material (available online at http://www.iwaponline.com/ wst/071/034.pdf), while the variation in the [OH•] in the absence of pCA is plotted against time in Figure S5 in the Supplementary Material (online at http://www.iwaponline. com/wst/071/034.pdf). Results obtained from these figures are compiled in Table 1. In the FeSO4 þ H2O2 system, the pCA removal is much higher than the TOC removal, indicating that much of the pCA removed is not mineralized and remains as degradation products, as observed with GA. Figures S6 and S7 in the Supplementary Material (available online at http://www. iwaponline.com/wst/071/034.pdf) show some of the aromatic degradation products and Figure S8 in the Supplementary Material (online at http://www.iwaponline. com/wst/071/034.pdf) depicts the variation in concentration of formic and oxalic acids versus reaction time. As in the case of GA, higher doses of ACC in the FeSO4 þ H2O2 system enhance the removal of pCA, TOC, formic acid, and oxalic acid and increase the difference [OH•] [OH•]pCA as a function of reaction time (Figure S9 in the Supplementary Material, available online at http://www. iwaponline.com/wst/071/034.pdf). In all cases, the increase in pCA and TOC removal with higher ACC dose in the FeSO4 þ H2O2 system is due to the adsorption of pCA and its degradation products on ACC; this can be observed in Table 1, when comparing TOC removal in the absence and presence of ACC and by ACC adsorption. This behavior differs from that observed with GA and can be explained by the following: (i) pCA has a greater affinity for the ACC surface in comparison to GA due to its lower solubility, which increases the pCA uptake, covering to a larger extent the active centers that generate OH• radicals, and (ii) the greater difficulty of mineralizing pCA than GA under these experimental conditions.

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In general, the degradation of aromatic compounds is greater and faster with the presence of electron-donating substituents in the aromatic rings (Parra et al. ; Peres et al. ), and given that GA has more electron-donating substituents than pCA it would be more easily mineralized.

CONCLUSIONS The main conclusions that can be drawn from the results obtained are as follows: An increase in the ACC dose added to the FeSO4 þ H2O2 system of between 25 and 100 mg enhances the concentration of OH• radicals generated and their formation kinetics, attributable to the presence of basic surface sites on ACC that catalyze the generation of OH• radicals. (ii) The large increase in GA and TOC removal with Fenton oxidation in the presence of a higher ACC dose is attributable not only to their adsorption on ACC but also to the generation of a higher concentration of OH• radicals that degrade and mineralize the GA. (iii) The major increase in pCA and TOC removal with Fenton oxidation in the presence of increasing ACC doses is due to the adsorption of pCA and its degradation products on ACC. This behavior is different from that observed with GA, attributable to the greater affinity of pCA than of GA for the ACC surface and the greater difficulty of mineralizing pCA in comparison to GA. (i)

ACKNOWLEDGEMENTS The authors are grateful to the Ministerio de Ciencia e Innovación and FEDER project CTQ2011-29035-C02-01 for financial support.

REFERENCES Beltrán, F. J., Gimeno, O., Rivas, F. J. & Carbajo, M.  Photocatalytic ozonation of gallic acid in water. Journal of Chemical Technology and Biotechnology 81 (11), 1787– 1796. Boye, B., Brillas, E., Buso, A., Farnia, A., Flox, C., Giomo, M. & Sandon, G.  Electrochemical removal of gallic acid from aqueous solutions. Electrochimimica Acta 52 (1), 256–262. Cañizares, P., Lobato, J., Paz, R., Rodrigo, M. A. & Sáez, C.  Advanced oxidation processes for the treatment of olive-oil mills wastewater. Chemosphere 67 (4), 832–838.

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Carbajo, M., Beltrán, F. J., Medina, F., Gimeno, O. & Rivas, F. J.  Catalytic ozonation of phenolic compounds. The case of gallic acid. Applied Catalysis B: Environmental 67 (3–4), 177–186. Caudo, S., Centi, G., Genovese, C. & Perathoner, S.  Homogeneous versus heterogeneous catalytic reactions to eliminate organics from waste water using H2O2. Topics in Catalysis 40 (1–4), 207–219. Du, Y., Zhou, M. & Lei, L.  Role of the intermediates in the degradation of phenolic compounds by Fenton-like process. Journal of Hazardous Materials 136 (3), 859–865. El-Gohary, F. A., Badawy, M. I. & El-Khateeb, M. A.  Integrated treatment of olive mill wastewater (OMW) by the combination of Fenton’s reaction and anaerobic treatment. Journal of Hazardous Materials 162 (2–3), 1536–1541. Fontecha-Cámara, M. A., Álvarez-Merino, M. A., Carrasco-Marín, F., López-Ramón, M. V. & Moreno-Castilla, C.  Heterogeneous and homogeneous Fenton processes using activated carbon for the removal of the herbicide amitrole from water. Applied Catalysis B: Environmental 101 (3–4), 425–430. Gernjak, W., Krutzler, T., Glaser, A., Malato, S., Cáceres, J., Bauer, R. & Fernández-Alba, A. R.  Photo-Fenton treatment of water containing natural phenolic pollutants. Chemosphere 50 (1), 71–78. Khalil, L. B., Girgis, B. S. & Tawfik, T. A. M.  Decomposition of H2O2 on activated carbon obtained from olive stones. Journal of Chemical Technology and Biotechnology 76 (11), 1132–1140. Kimura, M. & Miyamoto, I.  Discovery of the activatedcarbon radical ACþ and novel oxidation-reactions comprising the AC/ACþ cycle as a catalyst in an aqueous

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solution. Bulletin of the Chemical Society of Japan 67 (9), 2357–2360. Lofrano, G., Meriça, S., Belgiornoa, V. & Napoli, R. M. A.  Fenton’s oxidation of various-based tanning materials. Desalination 211 (1–3), 10–21. Parra, S., Olivero, J., Pacheco, L. & Pulgarin, C.  Structural properties and photoreactivity relationships of substituted phenols in TiO2 suspensions. Applied Catalysis B: Environmental 43 (3), 293–301. Peres, J. A., Domínguez, J. R. & Beltran-Heredia, J.  Reaction of phenolic acids with Fenton-generated hydroxyl radicals: Hammett correlation. Desalination 252 (1–3), 167–171. Ramirez, J. H., Maldonado-Hódar, F. J., Pérez-Cadenas, A. F., Moreno-Castilla, C., Costa, C. A. & Madeira, L. M.  Azo-dye orange II degradation by heterogeneous Fenton-like reaction using carbon-Fe catalysts. Applied Catalysis B: Environmental 75 (3–4), 312–323. Santos, V. P., Pereira, M. F. R., Faria, P. C. C. & Órfao, J. J. M.  Decolourisation of dye solutions by oxidation with H2O2 in the presence of modified activated carbons. Journal of Hazardous Materials 162 (2–3), 736–742. Tai, C., Peng, J. F., Liu, J. F., Jiang, G. B. & Zou, H.  Determination of hydroxyl radicals in advanced oxidation processes with dimethyl sulfoxide trapping and liquid chromatography. Analytica Chimica Acta 527 (1), 73–80. Zazo, J. A., Casas, J. A., Mohedano, A. F., Gilarranz, M. A. & Rodríguez, J. J.  On the chemical pathway and kinetics of phenol oxidation by Fenton’s reagent. Environmental Science & Technology 39 (23), 9295–9302. Zhang, X., Ding, Y., Tang, H., Han, X., Zhu, L. & Wang, N.  Degradation of bisphenol A by hydrogen peroxide activated with CuFeO2 microparticles as a heterogeneous Fenton-like catalyst: efficiency, stability and mechanism. Chemical Engineering Journal 236, 251–262.

First received 20 October 2014; accepted in revised form 13 January 2015. Available online 27 January 2015

Fenton oxidation of gallic and p-coumaric acids in water assisted by an activated carbon cloth.

The objective of this study was to investigate the effect of the presence of an activated carbon cloth (ACC) during the degradation and removal of gal...
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