Towards understanding of heterogeneous Fenton reaction using carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment

Accepted Manuscript Towards understanding of heterogeneous Fenton reaction using carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment Ana I. Zárate-Guzmán, Linda V. González-Gutiérrez, Luis A. Godínez, Alejandro Medel-Reyes, Francisco Carrasco-Marín, Luis A. Romero-Cano PII:

S0045-6535(19)30325-X

DOI:

https://doi.org/10.1016/j.chemosphere.2019.02.101

Reference:

CHEM 23213

To appear in:

ECSN

Received Date: 29 November 2018 Revised Date:

14 February 2019

Accepted Date: 15 February 2019

Please cite this article as: Zárate-Guzmán, A.I., González-Gutiérrez, L.V., Godínez, L.A., MedelReyes, A., Carrasco-Marín, F., Romero-Cano, L.A., Towards understanding of heterogeneous Fenton reaction using carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.02.101. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Towards understanding of heterogeneous Fenton reaction using Carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment

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Ana I. Zárate-Guzmán a, Linda V. González-Gutiérrez a*, Luis A. Godínez a, Alejandro Medel-Reyes a, Francisco Carrasco-Marín b, Luis A. Romero-Cano a, b, 1

Centro de Investigación y Desarrollo Tecnológico en Electroquímica (CIDETEQ), Parque

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a

Tecnológico Sanfandila, Pedro Escobedo, Querétaro, 76703, MÉXICO. Grupo de Investigación en Materiales de Carbón, Facultad de Ciencias, Universidad de

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b

Granada. Av. Fuente Nueva, s/n. Granada, 18010. ESPAÑA 1

Present address: Facultad de Ciencias Químicas. Universidad Autónoma de Guadalajara. Av. Patria 1201, Zapopan, Jalisco, 45129, MÉXICO

[email protected]

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Corresponding author: Linda V. González-Gutiérrez, [email protected]

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Manuscript number: CHEM58636

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Title page -

ACCEPTED MANUSCRIPT Towards understanding of heterogeneous Fenton reaction using Carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment

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Ana I. Zárate-Guzmán a, Linda V. González-Gutiérrez a*, Luis A. Godínez a, Alejandro Medel-Reyes a, Francisco Carrasco-Marín b, Luis A. Romero-Cano a, b, 1

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Grupo de Investigación en Materiales de Carbón, Facultad de Ciencias, Universidad de Granada. Av. Fuente Nueva, s/n. Granada, 18010. ESPAÑA 1

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Present address: Facultad de Ciencias Químicas. Universidad Autónoma de Guadalajara. Av. Patria 1201, Zapopan, Jalisco, 45129, MÉXICO

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b

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Centro de Investigación y Desarrollo Tecnológico en Electroquímica (CIDETEQ), Parque Tecnológico Sanfandila, Pedro Escobedo, Querétaro, 76703, MÉXICO.

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*

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[email protected]

Abstract

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Iron-supported catalyst on granular activated carbon was prepared for its use in

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heterogeneous Fenton reaction coupled to an in situ H2O2 electro-generation. For this

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process, an electrolysis cell was employed, using carbon felt as cathode and graphite as

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anode. A solution of H2O2 (electrogenerated at a rate of 30 mg L-1 h-1) was obtained

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using a current intensity of 12 mA. In order to promote the decomposition of H2O2 to

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•

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impregnation using FeSO4 as precursor salt to obtain samples with 9% wt of iron.

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Samples were characterized by EDX, FTIR and XPS spectroscopy before and after

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wastewater treatment using phenol as model molecule. Two iron oxidation states on the

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samples were found, Fe2+ and Fe3+. The ratio between Fe2+/Fe3+ was 1.29 which was

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later reduced to 0.92 after Fenton process; this might be associated with the metal

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oxidation (Fe2+ to Fe+3) occurring during Fenton-reaction, thus indicating that H2O2

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decomposition was carried out by Fe2+ on carbon surface. Detection and quantification

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of hydroxyl radical were carried out by fluorescence spectroscopy, obtaining a radical

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OH, a Carbon-Fe catalyst was used. This catalyst was prepared by incipient wet

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ACCEPTED MANUSCRIPT concentration of 3.5 µM in solution. Iron in solution were determined, showing a

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concentration of 0.1 mg L-1, making evident that the supported metal is stable and the

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reaction is carried out in a heterogeneous phase. Results showed an environmentally

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friendly process that can generate reagents in situ, with high efficiencies in the

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degradation of pollutants and minimizing the formation of toxic byproducts, which are

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common in conventional treatments.

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Keywords: Hydroxyl radical •OH, Heterogeneous Fenton, Iron – activated carbon

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catalyst, H2O2 electrogeneration.

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1. Introduction

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One of the advanced oxidation processes that recently has gained attention is the one

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that involves Fenton reaction in heterogeneous phase, mainly because this process has

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proven to be effective in degradation processes of recalcitrant organic compounds in

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water, and it also bears great advantages when compared to conventional or

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homogeneous Fenton diminishment of undesired byproducts (residual sludge) and the

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use of a catalyst during repeated degradation cycles (Rahim Pouran et al., 2014;

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Yamaguchi et al., 2018). In heterogeneous Fenton process, the •OH generation takes

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place from the catalytic decomposition of H2O2 mainly in acidic medium (pH 3), using

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as catalyst iron supported on a porous material (He et al., 2016) this reaction usually

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takes place in wide range of pH, how. In this sense, different materials have been

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reported as catalyst supports, such as silicates (clay and zeolites) (Chou and Huang,

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1999; Cheng et al., 2006;

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ACCEPTED MANUSCRIPT Gokulakrishnan et al., 2009; Luo et al., 2009) ionic exchange resins (Zeng and Lemley,

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2009; Herney-Ramirez et al., 2011) and, carbon-based materials (Duarte et al., 2009,

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2012). From the supports above mentioned, activated carbon is the most interesting one

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due to its multiple advantages, such as: low cost, great surface area, textural, chemical

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and morphological controllable properties (Mesquita et al., 2012). Until now, there is

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little information available in the literature that addresses the study of the mechanisms

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(kinetic and reaction mechanisms) involved in the heterogeneous Fenton reaction (Lin

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and Gurol, 1998; Andreozzi et al., 2002) . However, the mechanisms present in the

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heterogeneous Fenton reaction using iron suported catalyst are still under discussion

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because there is few information regarding how the process is taking place. Published

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research have focused on the preparation and evaluation of the catalyst performance

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during the degradation of different pollutants (Ramírez et al., 2010; Mesquita et al.,

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2012; Banuelos et al., 2015); this can be seen in Table SM-1, which presents a

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compilation and comparison of the use of carbon-Fe catalysts for the Fenton reaction in

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heterogeneous phase, according to this information, the papers presented so far, report

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the catalytic activity of the Carbon-Fe catalysts indirectly, through the degradation of a

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model compound, being therefore necessary to provide fundamental information about

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the process by studying the generation of •OH radicals by a direct method (Duarte et al.,

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2009; Liao et al., 2009; Hu et al., 2011; Chun et al., 2012; Duarte et al., 2012; Duan et

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al., 2014). By having a global understanding of the mechanisms that take place during

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this process, specific catalyst could be designed in order to promote the desired reaction;

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also, clean technology could be optimized for its use in wastewater treatment processes

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at industrial level.

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In this work, a heterogeneous Fenton reaction promoted by the use of Carbon-Fe

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catalyst is studied; also, H2O2 in situ electro-generation was evaluated. With the aim of

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ACCEPTED MANUSCRIPT contributing with information that will help in the elucidation of the heterogeneous

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Fenton reaction, •OH radicals were identified and quantified. These latter were

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employed in the degradation of a model molecule (phenol) during repeated cycles of

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reuse. Finally, characterization studies on the catalyst, prior and after degradation

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processes, were carried out by employing conventional spectroscopic techniques (FTIR,

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EDX, XPS). Obtained information related to the degradation efficiencies of the model

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molecules. The information obtained was related to the degradation efficiencies of the

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model molecule and allowed to elucidate the role of iron during the heterogeneous

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Fenton reaction.

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2. Experimental.

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2.1 Activated carbon modification as support for the iron catalyst. Clarimex PR-200 commercial granular activated carbon provided by Clarimex S.A. de

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C.V. (sample C), was used as a support material of iron and as a catalyst in Fenton

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process. With this purpose, as a first step, material was functionalized with oxygen

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groups, which function as metal binding sites. This functionalization was performed by

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means of an acidic treatment that consists in rinsing the material with 0.1M HCl

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solution, in order to remove impurities from the material. Then, a treatment using a 68

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% HNO3 solution was carried out, in a 1:10 relation (g of carbon: mL of HNO3) and a

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constant stirring at 180 rpm at room temperature for 15 h (Shi et al., 2011). Finally, the

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sample was filtered and thoroughly rinsed with distilled water until rinsed water

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reached a constant pH value (~ pH 6). The obtained material was then dried at 105ºC

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for 24 h (C-AC sample). The iron catalyst was prepared by the incipient wet

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impregnation method employing

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FeSO4·7H2O was dissolved in a small amount of water (which corresponds to a

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sample C-AC as support; for which, 1.74g of

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ACCEPTED MANUSCRIPT modification for obtaining 9% wt of iron in the carbon). This solution was slowly

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added until 5 g of C-AC was moistened homogenously. This sample was kept wet for

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12 h with the aim of enhancing diffusion of salt along carbon texture. Then, solvent

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was eliminated by means of evaporation in a conventional oven at 105ºC. Finally, dried

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sample was placed in a tubular oven at 400 ºC in inert atmosphere; for this procedure, a

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heating ramp of 10 ºC min-1 was employed and the desired temperature was kept for 2

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h, this sample was denominated as C-AC-Fe. (Mesquita et al., 2012)

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2.2 Characterization of the materials prepared.

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The textural properties of the carbon materials were studied from the physisorption of

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N2 at -196 °C, for which a QUADRASORB SI (Quantachrome Inc.) equipment was

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used. From the obtained isotherms, the specific surface areas and the size distribution of

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meso- microporosity were calculated using the BET equation and the DFT model.

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Morphology of the materials was studied by electronic scan microscopy coupled with

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energy-dispersive

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instrument, where the presence and homogeneity of the supported catalyst was

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evaluated. In addition, with the aim of obtaining information on the crystallographic

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phases of iron that are present on the carbon surface, X-ray diffraction (XRD)

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experiments were carried out. These experiments were performed at room temperature

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with an XRD Bruker D8 advance diffractometer at 0.17º min-1, from 10 to 70º (2θ).

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Surface chemistry of the materials was studied by infrared spectroscopy with Fourier -

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transform infrared spectroscopy (FTIR) from 4000 to 400 cm-1 with a Shimadzu

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Iraffinity-1S spectrophotometer. In order to characterize the composition at the materials

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microscopy

(SEM-EDX),

using

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JSM-6510LVEDS

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X-ray

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ACCEPTED MANUSCRIPT surface, X-ray photoelectron spectroscopy (XPS) was employed. This analysis was

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carried out with an Intercovamex XPS 110 device; XP spectra were obtained with a

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monochromatic source of Al Ka X ray (1486.71 eV) at a pressure of 6 x 10-10 Torr in

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the analytic chamber. For wide-scan spectra, an energy interval from 0 to 1100 eV,

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energy step of 80 eV and a step size of 1 eV were employed. Once the signals were all

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examined, C1s, O1s and Fe2p regions were explored at high resolutions at an energy step

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of 40 eV and step size of 0.05 eV. Each region of interest was scanned several times in

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order to obtain reasonable signal-noise ratio. With the aim of obtaining the exact

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number of components, peak positions and their respective areas, resulting spectra from

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background signal correction were then adjusted to Lorentz and Gauss functions (Voigt

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profile).

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2.3 Electro-generation of H2O2

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With the aim of proposing an ecological water treatment technology, in situ electro-

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generation of H2O2 was studied in an undivided electrolysis cell (40 mL), using a 0.1 M

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Na2SO4 solution as supporting electrolyte at pH 3. Carbon felt (Alfa Aesar 14630,

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metal basis) and graphite (Alfa Aesar 10132) were used as cathode and anode,

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respectively; both with a geometric area of 3.6 cm2 and a separation of 2.5 cm between

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them, since they have been successfully applied in oxygen reduction to H2O2 (

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Pimentel et al., 2008; Martínez-Huitle and Brillas, 2009; Özcan et al., 2009). Studies

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were carried out by applying different values of current: 0.9 mA, 5 mA, 9 mA and 20

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mA. In each experiment, solution was saturated with air prior to the beginning of

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electrolysis using an air flow of 0.15 L min-1, this was maintained during all the

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process. H2O2 concentration was determined by UV-Vis spectroscopy using the

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TiOSO4 method (Ribeiro et al., 2009), with a Shimadzu UV 2600 spectrophotometer. 2.4 Identification and quantification OH radicals.

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Identification and quantification of radicals was achieved by means of fluorescence

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spectroscopy, using coumarin as probe molecule. The reaction that takes place when a

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coumarin molecule reacts with an •OH radical (Rxn 1) is the formation of 7-

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hydroxycoumarin, which is a stable compound with a well-known fluorescence

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spectrum (λexcitation 345 nm and λemission 455 nm) (Ishibashi et al., 2000).

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Fluorescence Spectra were obtained with an AGILENT Cary Eclipse spectrophotometer;

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both excitation and emission slit values were fixed to 10.0 nm for all measurements. 11

O

HO

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+ 2 OH

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+ H2O

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(1)

This method is effective for the determination of •OH radicals due to its high sensitivity

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(detection limit of 3 * 10-18 M) and selectivity (Ishibashi et al., 2000; Louit et al., 2005).

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2.5 Degradation of phenol as a probe molecule and reuse cycles

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In order to evaluate heterogeneous Fenton reaction, phenol was employed as model

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molecule. As a first step, adsorption studies were carried out to saturate the sample; in

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this way, it becomes possible to despise the effect of phenol removal due to adsorption

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phenomena. This procedure was performed with 5 g of sample and a 1000 mg L-1

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ACCEPTED MANUSCRIPT phenol solution, at a constant stirring of 180 rpm and room temperature (~ 25 °C) for 4

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d. Once the material was saturated, it was dried at 105 º for 12 h.

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As a second step, degradation of a 50 mg L-1 phenol solution was studied, using the

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electro-generated H2O2 and the saturated catalyst (C-AC-Fe). For this purpose, a batch-

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type oxidation reactor of 40 mL of capacity that contained 0.1 g of C-AC-Fe was used;

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the reactor was fed with a H2O2 generated solution in the electrochemical reactor in a

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4.6 molar ratio H2O2/phenol, so as to achieve complete mineralization of phenol and its

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byproducts (Espinosa et al., 2015). The whole degradation process was monitored for

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different periods of time by means of UV-Vis spectroscopy at a wavelength of 270 nm.

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Once this cycle was concluded, C-AC-Fe was recovered, saturated again with phenol

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and then dried at 105 ºC in order to study its performance in repeated cycles of reuse.

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Finally, with the aim of study the degree of mineralization of phenol molecule, total

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organic carbon (TOC) was determinate using a Shimadzu TOC-L series analyzer. In

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order to obtain the mineralization percentage of phenol to CO2, TOC data were

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processed according with Moctezuma et al., (Moctezuma et al., 2016).

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3. Results and discussion

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3.1 Characterization of the prepared materials

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N2 adsorption/desorption isotherms as well as, pore size distribution studies of the

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materials are showed in Table SM-2 and Fig. SM-1. As observed, SBET areas of 769 and

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575 m2g-1 for C-AC and C-AC-Fe respectively was obtained, this implies a decrease in

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the SBET surface area as well as in the pore volume, this is associated with the entry of

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iron into the porous structure blocking the porosity and therefore decreasing the

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ACCEPTED MANUSCRIPT adsorption capacity of the N2 (Duarte et al., 2009). Functional groups that were

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introduced to the carbon after different treatments were further studied by infrared

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spectroscopy. Fig.1a shows FT-IR spectra that corresponds to samples C, C-AC and C-

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AC-Fe, in which five bands were identified. The first at 3410 cm-1 is associated with

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stretching vibrations of O-H bonds due to phenolic, hydroxyl and carboxylic groups

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(Khelifi et al., 2010; Rodríguez et al., 2011). The second band at 2900 cm-1 is due to

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symmetric and asymmetric stretching vibrations of the C-H groups from aliphatic

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structures (Khelifi et al., 2010). The third band of interest is found at 1640 cm-1 and is

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attributed to C=O vibrations, both in carboxylic and carbonyl groups that are highly

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conjugated (Khelifi et al., 2010; Gokce and Aktas, 2014). The fourth signal, only

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present in the C-AC sample, is found in the fingerprint region (approx. at 1100 cm-1)

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and is associated to an oxidation process of carbon, and to the increase number of

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hydroxyl and ethers group in the sample (Swiatkowski et al., 2004); this is due to the

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strong oxidant character of nitric acid, which is capable of oxidizing carbon atoms and

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thus, it causes that the material surface loses electrons; as a result, carbon surface

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acquires a positive charge, promoting adsorption of oxygenated anions in solution and

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hence, the number of oxygenated functional groups considerably increases (Khelifi et

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al., 2010). Finally, the fifth signal of interest, close to 570 cm-1, is only appreciated in

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sample C-AC-Fe and is related to Fe-O bonds [3].

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X-ray diffraction studies showed amorphous structures for every case; however, for C-

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AC-Fe sample (Fig. 1b), two crystallographic phases were identified, being hematite the

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main one (γFe2O3), with a diffraction angle of 29º. The second signal at 35º might be

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associated to similar structures to magnetite (Fe3O4) (Mesquita et al., 2012). Then, it is

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expected that both Fe2+ and Fe3+ species are present in the carbon structure.

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ACCEPTED MANUSCRIPT The morphology of C-AC-Fe was studied by scanning electron microscopy, the SEM

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images obtained are shown in Fig. SM-2a, in which the metal particles supported in the

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carbon are observed, with the aim of identifying metal distribution along carbon surface,

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SEM-EDX micrographs were obtained. Fig.1c shows a SEM-EDX micrograph that

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corresponds to iron mapping of sample C, where the absence of iron is evident whereas

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in Fig.1d, which is obtained from sample C-AC-Fe, the presence of iron is clearly

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appreciated (highlighted in red) in the carbon surface. Quantification of iron was

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performed by EDX spectra within different sample zones; from the obtained

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measurements it can be concluded that metal impregnation occurred in average in a 9%

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weight surface proportion.

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Quantitative chemical analysis performed by XP spectroscopy is presented in Table

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SM-3; the following sensitivity factors were used: 0.205 for C1s, 0.66 for O1s y 3.8 for

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Fe2p.

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Oxygen content increases after functionalization with HNO3 from 10.6% to 16.4%; then

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it finally diminishes to 13.8% at the moment when iron binds to these active sites.

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Moreover, it can be observed that sample impregnated with FeSO4 presents a superficial

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iron content of 8.1%; therefore, most of the catalyst locates available in the most

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exposed superficial part of the material, since XP spectroscopy is an analysis performed

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on the outermost layer, 3-5 nm. Fig. 2 show XP high resolution spectra of the Fe2p and

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C1s, O1s, regions, respectively. C1s region (Fig. 2a) is deconvoluted into 6 peaks in all

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materials, in a similar way to other activated carbons from the same origin (Rey et al.,

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2016). A signal whose binding energy is close to 284.7 eV is attributed to C-C bonds

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located in the graphitic layer; 286.0 eV signal is due to C-H bonds, whereas the peak at

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287.2 eV is related to C-OH bonds in phenolic functional groups. Signals at 288.9 and

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290.6 eV are associated with C=O and -COOH, respectively, which are carbonyl,

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ACCEPTED MANUSCRIPT carboxyl and ether functional groups. Finally, the signal at 292.2 eV can be associated

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to π−π transitions between aromatic rings at the graphitic layers. As expected, the

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presence of the functional groups described above are confirmed, when the spectra near

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region O1s are decomposed; their main feature are broad peaks that indicate the

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presence of oxygen in its different chemical states, such as organic oxygen (carboxyl,

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carbonyl and ether functional groups) and inorganic oxygen (oxygen as iron oxides).

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The broad form of the spectra in the O1s region (Fig. 2b) indicates the presence of

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oxygen in different chemical states, for this reason the spectra were deconvoluted into 2

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peaks centered at 531.6 and 533.0 eV; for the C and C-AC samples can be attributed to

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species C=O and C-O (Alvarez-Merino et al., 2000; Fontecha-Cámara et al., 2011), due

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to functional groups carboxyl, carbonyl and ester, which have been identified by FTIR

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spectroscopy evidencing the functionalization of the material. The fact that the spectrum

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of the C-AC-Fe sample was inclined at lower binding energies is attributable to the

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incorporation of the metal in the oxygenated functional groups, since signals centered at

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531.5 ± 0.5 eV correspond to species FeO(OH) with different stoichiometry and signal

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at 529.8 eV correspond to species Fe2O3 and FeO (Grosvenor et al., 2004),

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corroborating the identification made by XRD.

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To complete the analysis, the XP spectrum in high resolution for the Fe2p region was

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obtained, which is only perceptible for the C-AC-Fe sample. The deconvolution of the

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obtained spectrum is shown in Fig. 2c, in which a first peak is observed at a binding

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energy close to 707.8 eV, which can be attributed to binding energies of Fe-S

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compounds (Nesbitt, 1998; Mullet et al., 2002) , corresponding to ferrous sulphate used

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as a precursor salt for the introduction of iron to the carbon porous structure. At higher

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binding energies, the characteristic doublet of iron is shown, showing two predominant

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peaks centered at 711.6 and 725.4 eV, corresponding to the 2p3/2 and 2p1/2 orbitals,

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ACCEPTED MANUSCRIPT respectively (Borda et al., 2003). The position and separation of energy are very similar

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to those observed for FeO(OH) structures (Allen et al., 1974), so it is possible to

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conclude that iron is incorporated into the carbonaceous matrix through the oxygenated

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groups introduced during functionalization with HNO3. At 720 eV a signal is also

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observed, which is attributed to a satellite peak characteristic of Fe3+ species in the

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Fe2p3/2 region (Kónya et al., 2001). The presence of these components indicates the

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presence of iron with different oxidation states (Ramirez et al., 2007), in such a way, the

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decomposition of the spectrum was carried out in two peaks, the first of them centered

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at 711.4 eV, associated with Fe2+ and the second centered at 713.2 eV correspond to the

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Fe3+ species (Sobti et al., 2014; Idczak et al., 2016). These results show that the iron

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supported in the carbon is present in the oxidation states II and III. A parameter that can

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be used to identify the distribution of these species in the sample is the ratio of peak

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areas of both species, for the case of the sample C-AC-Fe a ratio of Fe2+/Fe3+ = 1.29 is

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observed.

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3.2 Electrogeneration of H2O2

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With the aim of evaluating the use of an eco-friendly technology, in situ H2O2 electro-

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generation was evaluated. Fig. 3a shows the kinetics of H2O2 electro-generation using

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four current intensity values for a period of 250 min; from this plot, an exponential

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behavior during the first 150 min is observed, where the H2O2 start to gradually

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diminish until reaching a constant value at 200 min. From experimental data, a response

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surface plot was built (Fig. 3b), where the variation of H2O2 production in a range of

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concentrations from 0 to 20 mA is presented. It can be observed that for current

24

intensity values under 6 mA, H2O2 production is inefficient due to the energy supplied

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ACCEPTED MANUSCRIPT to the system is lower than the required for promoting the desired reaction. From 6 mA,

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concentration starts to increase considerably, reaching a maximum production at 12

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mA; hence, according with the experimental design proposed, 12 mA is the optimal

4

current requires to produce H2O2 in acidic medium (Rxn 2)

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 + 2  + 2  → 


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At this condition, the maximum concentration of H2O2 is 100 mg L-1 (30 mg L-1 h-1).

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This high rate of generation is higher than the one obtained for other materials, such as

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graphite (2.45 mg L-1 h-1) (Huang et al., 2013), and is due to the great specific area of

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carbon felt (which is generally higher than 1000 m2 g-1) and nitrogen functions that are

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present (Xia et al., 2015). It should be noted that although the production rate presented

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in this research is high, other research groups such as Perez et al., (2017) have reported

12

higher production rates, which have been attributed to changes in operating parameters,

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such as the air pressure supplied to the system. At current intensity values higher than

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12 mA, H2O2 production starts to diminish drastically because the cell potential is

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higher; this effect promotes undesired cathodic reactions, such as hydrogen evolution

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(Rxn 3), oxygen reaction via 4e- (Rxn 4), and the reduction of H2O2 to H2O (Rxn 5),

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(Brillas and Casado, 2002; Luo et al., 2015). Another factor that can affect H2O2

18

generation at these current intensity values is that generated H2O2 decomposes

19

immediately at the anode, generating hydroxyperoxyl radical (Rxn 6) and (Rxn 7)

20

(Martínez-Huitle and Brillas, 2009; Babuponnusami and Muthukumar, 2012), or its

21

direct decomposition in medium Rxn 8 (Qiang et al., 2002).

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(2)

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2  + 2  → 

(3)


 + 4  + 4  → 2


(4)

24 25

13

ACCEPTED MANUSCRIPT 1 2


 + 2  + 2  → 2


(5)


 → 
• +   +  

(6)

3 4

6

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• →
 +   +  

(7)

7


 → 
+ 


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(8)

Once H2O2 is electro-generated, it is recovered and used for the •OH radical generation

11

using C-AC-Fe sample, with the aim of studying Fenton reaction in a heterogeneous

12

Fenton process.

13 14

3.3 Identification and quantification of •OH radicals in phenol degradation process

15

One of the critical factors to define if an advanced oxidation process is taking place is

16

the presence of highly oxidant species in the system. Until today, there is little

17

information in the literature regarding Fenton process in heterogeneous phase. Studies

18

indirectly show how this process occurs, because they relate a diminishment in the

19

concentration of a model compound with the efficiency of an advanced oxidation

20

process (Ramírez et al., 2010; Banuelos et al., 2015); these reports do not provide

21

enough information for a clear elucidation of the reaction mechanism in heterogeneous

22

phase.

23

With the purpose of obtaining information about this process, the experimental system

24

described in the section of materials and methods. If the Fenton reaction takes place,

25

•

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OH radicals must be generated; these latter are able to react with coumarin molecule, 14

ACCEPTED MANUSCRIPT 1

generating a fluorescent compound (7-hydroxycoumarin) whose identification and

2

quantification is proportional to the •OH radical formation, according to Eq. 9

3

(Tokumura et al., 2011).

4

  = 

5

From Eq. 9, it would be necessary to know the concentration of 7-hydroxycoumarin in

6

order to estimate the desired value. From standard solutions of this compound at

7

different concentrations, fluorescence spectra were obtained (Fig. SM-3a), and a

8

calibration plot was built for its determination. Maximum fluorescence intensity occurs

9

at 455 nm, which corresponds to the wavelength of the emission spectra of this

10

molecule. Values of fluorescence intensity versus 7-hydroxycoumarin concentrations

11

were plotted; from the calibration plot presented in Fig. SM-3b, a lineal behavior is

12

observed, along with a reasonable correlation coefficient (R2 = 0.9937). Lineal

13

regression of the experimental data allows the determination of an equation for 7-

14

hydroxycoumarin quantification (Eq. 10):

15

  = 0.0006 ∗ . .!""#$ % + 0.0104

16

From the combination of Eq. 9 and 10, a mathematical function for the quantification of

17

•

18

  = 

19

The first experiment consisted of placing C-AC sample in a cell containing electro-

20

generated H2O2 and coumarin as probe molecule. The process was monitored for 360

21

min and the characteristic signal of 7-hydroxycoumarin observed was insignificant,

22

since there is no catalyst that promotes the formation of •OH radicals as shown Fig. SM-



 ∗  

(9)

(10)

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OH radicals by means of fluorescence spectroscopy is obtained (Eq. 11): 

 ∗ 0.0006 ∗ . .!""#$ % + 0.0104

.

15

(11)

ACCEPTED MANUSCRIPT 4a and SM-4b. However, with C-AC-Fe sample, the result is different, as shown in Fig.

2

SM 4c; after 1 min, the characteristic signal of 7-hydroxycoumarin is observed, thus

3

indicating the presence of •OH radicals in this experimental system. This concentration

4

starts to increase lineally with time for the first 150 min. From that moment, an

5

asymptotic behavior is observed, reaching a constant value of 3.5 µM (Fig. 4a, black

6

plot). The kinetic of 7-hydroxycoumarin formation, that is proportional to •OH radicals,

7

can be described into two stages: first, •OH radicals are generated following a zero-order

8

kinetics '

9

proportion for promoting the conversion reaction to 7-hydroxycoumarin; once that

10

coumarin becomes the limiting reagent, the generation of the radical starts to seem

11

constant and then it gradually diminishes because of the lack of the necessary species to

12

form the fluorescent product. From experimental data, it is concluded that during the

13

first 150 min it is possible to obtain the kinetics constant of the radical formation (Fig.

14

SM-4d), which is equal to 0.0197 min-1. Finally, with the purpose of corroborating that

15

the Fenton reaction is taking place heterogeneously between iron supported on carbon

16

and H2O2 in solution, the concentration of iron due to its possible lixiviation was

17

monitored (Table 1). At the end of the experiment, Fe2+ concentration was 0.1 mg L-1;

18

hence, the supported catalyst remained stable in the carbon surface and that reaction is

19

occurring in the heterogeneous phase.

20

The results obtained in the quantification of the •OH radicals were contrasted with those

21

obtained in the degradation of phenol to corroborate that the process of degradation of

22

the contaminant is due to the Fenton process in heterogeneous phase. The degradation

23

of phenol was monitored in the same time interval as the kinetics of radical formation

24

(Fig. 4a, blue graph); the kinetic degradation of the model molecule is inversely

25

proportional to the kinetics of the •OH radical formation, thus proving that its

= )* during 150 min, due that coumarin molecule exists in an adequate

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 (

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16

ACCEPTED MANUSCRIPT degradation is due to oxidation with the radicals generated during the Fenton reaction in

2

heterogeneous phase. Therefore, •OH radicals initiate an electrophilic attack on the

3

phenol molecule, leading to a hydroxylation of the aromatic ring (Diaz-Uribe et al.,

4

2015), and therefore the mineralization of the molecule through their respective reaction

5

intermediates, such as catechol, hydroquinone, benzoquinone and carboxylic acids

6

(Ramos et al.,2009; Espinosa et al., 2015; Jarrah and Mu’azu, 2016; Mousset et al.,

7

2016; Amado-Piña et al., 2017).

8

In order to evaluate the capacity of reuse of the catalyst, several cycles of reuse for the

9

degradation of phenol were carried out. In each of them, the kinetics of phenol

10

degradation (Fig. 5a) and •OH radical formation were determined (Fig. 5b). According

11

to the experimental data obtained, it is concluded that the degradation efficiency

12

decreases in relation to the cycles of reuse (Table 1). It is appreciated that during the

13

first cycle of use of the catalyst there is a high percentage of mineralization (99.1%)

14

compared to a conventional Fenton process, this effect is attributed to the low amount of

15

iron present in solution, this being one of the main advantages of the Fenton process in

16

heterogeneous phase. It is also important to mention that after the fourth cycle a

17

percentage of mineralization of 88.6 % was obtained showing the efficiency of the

18

process. These results can be related to the kinetics of formation of •OH radicals

19

determined for each cycle of reuse of the catalyst, observing that the loss in efficiency is

20

due to the fact that the catalyst begins to deplete as the generation of •OH radicals begins

21

to decrease.

22

With the aim of obtaining information on the catalyst spent process in the Fenton

23

reaction in heterogeneous phase, C-AC-Fe-spent were characterized. The SEM images

24

of the material are shown in Fig. SM-2b, in which the metallic particles adhered to the

25

support can be seen; it is observed that the texture of the carbon undergoes a slight

AC C

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17

ACCEPTED MANUSCRIPT chemical attack due to its exposure to the oxidation process, however these changes are

2

barely perceptible to the 4th cycle of use. The FTIR spectroscopy studies are shown in

3

Fig. 1a (Green), in which it is observed that there is no change in the functional groups

4

of the material, thus, the possible adsorption of by-products due to the oxidation of

5

phenol can be discarded since no new absorption bands are appreciated. The XP

6

spectroscopy studies are presented in Fig. 2; the spectrum shows significant differences

7

with respect to the material prior to its use in the advanced oxidation process, once the

8

supported catalyst begins to be consumed, its XP spectrum its similar to the spectrum of

9

Fe3O4 (Mullet et al., 2002), which can be considered as a ferric oxide molecule bonded

10

to a ferrous oxide molecule, that is, a molecule in which there are two atoms of Fe(III)

11

and one atom of Fe(II), showing the change in the proportion of these species in

12

comparison with the original material. An increase in the signal attributed to the

13

characteristic satellite peak of Fe3+ species (720 eV) is observed. The mathematical

14

analysis after the deconvolution of the spectra corresponding to spent sample presents a

15

percentage of oxygen higher than the original sample (23.6% vs. 13.8%), which is due

16

to the fact that during the process the generated •OH radical does not carry out the

17

selective oxidation of the phenol, since it also attacks the activated carbon used as

18

catalyst support. This chemical attack on the surface of the carbon brings about changes

19

in its textural properties because the walls of the pores are easily destroyed by the

20

oxidizing agent (Mesquita et al., 2012). The quantitative analysis performed for the

21

spent catalyst showed a surface iron amount of 7.8% vs. 8.1% for the initial sample,

22

such a low metal loss after 4 cycles of use, shows the stability of the catalyst supported

23

in the carbon structure.

24

After the deconvolution of the spectrum, the data obtained showed that after 4 cycles of

25

use, the Fe2+ / Fe3+ ratio is 0.92, that is, 28% less than that found for the C-AC-Fe

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18

ACCEPTED MANUSCRIPT sample. From the information obtained, the graph presented in Fig.4b was constructed,

2

in which the process of catalyst depletion is clearly seen. After four cycles of use of the

3

supported catalyst, the efficiency in the degradation of phenol decreases (Fig. 5a),

4

which is attributed to the decrease in the production of •OH radicals (Fig. 5b), this

5

decrease was of 20%. This phenomenon can be attributed to the change of the iron

6

species present in the material, as was observed in the XP spectra, so that during the

7

advanced oxidation process the Fe2 + supported in the carbon catalyzes the

8

decomposition of H2O2 to •OH, having as consequence its change of oxidation state to

9

Fe3+, in a similar way as it happens in the reaction of oxidation in homogeneous phase.

SC

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1

In such a way that the catalyst will be completely depleted when all the Fe2+ species

11

change their oxidation state due to the reaction.

12

It is important to note that when using a heterogeneous catalyst during the Fenton

13

reaction, formation of undesirable precipitates is avoided (sludge due to the presence of

14

iron in the effluent, See Fig. SM-5); therefore, leached iron was determined in the

15

treated solution. It is observed that after the first cycle of use of the catalyst, the amount

16

of total iron in the effluent is 0.8 mg L-1 (0.1 mg L-1 of Fe2+), which slowly increases for

17

the subsequent cycles, reaching a maximum of 1.2 mg L-1 (1.0 mg L-1 of Fe2+) after the

18

fourth cycle. Finally, when comparing the amounts of iron leached in the process

19

against similar studies previously reported by Bayat et al., (2012), 6 mg L-1, Ramirez et

20

al., (Ramirez et al., 2007), 2 mg L-1, it can be concluded that such a low degree of

21

release makes the stability of the catalyst in the carbon structure evident. Also, the

22

concentration of the metal is below the maximum permissible limits for treated water

23

effluents (2 mg L-1 according to current European legislation) (Mesquita et al., 2012).

24

So far, there are no studies reported in the literature on what is the contribution of this

25

iron leached in the overall efficiency of the process, since the metal in solution would

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19

ACCEPTED MANUSCRIPT carry out the Fenton reaction in homogeneous phase. Therefore, in order to determine

2

the contribution of leached Fe2+, quantification of •OH radicals generated by the

3

homogenous Fenton process was performed (Fig. 6). From this plot, concentration of

4

OH radicals for the process was estimated as 0.29 µM, whereas for the heterogeneous

5

Fenton process a concentration of 3.25 µM was obtained at the same experimental

6

conditions. This would represent that 92% of the generation of •OH radicals is carried

7

out by Fenton reaction in heterogeneous phase, while 8% of the generation of those •OH

8

radicals is carried out by the reaction in homogeneous phase. Finally, TOC data

9

obtained were converted to phenol mineralized to CO2. Table 1, shows the conversion

10

percentage of phenol to CO2 for each reuse cycle. For the first cycle a mineralization of

11

99.1% was obtained, thus indicating that almost all of by-products have been converted

12

to CO2. This mineralization percentage decreased with each reuse cycle, and was

13

congruent with •OH radicals generation decrease obtained for each cycle too.

14

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4. Conclusions

16

Fenton heterogeneous reaction is carried out on the surface of the carbon impregnated

17

with iron, where the supported metal performs the catalytic decomposition of H2O2

18

obtaining a production of •OH radicals of 3.5 µM using a mass of 0.1 g C-AC- Fe.

19

Studies of leached iron in solution showed a concentration of 0.1 mg L-1, thus

20

evidencing that the supported metal is stable, and the reaction is carried out in a

21

heterogeneous phase. The depletion of the catalyst is due to the fact that during the

22

oxidation process, the iron present in the surface changes its oxidation state from Fe2+ to

23

Fe3+, directly affecting the degradation efficiency of the pollutant, decreasing by 20% in

24

the fourth cycle of use with respect to the first.

AC C

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20

ACCEPTED MANUSCRIPT In-situ H2O2 production was obtained at high concentrations (30 mg L-1 h-1). By

2

coupling this experimental design with the use of catalyst (previously described) the

3

degradation of the model pollutant was achieved in repeated cycles, showing that the

4

proposed process is a clean technology for the treatment of wastewater, with a

5

minimum generation of by-products, avoiding the generation of sewage sludge and does

6

not require the handling and storage of reagents.

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Acknowledgments

9

The authors thank the National Council of science and technology (CONACYT,

10

México), for the project No. 256943 “Fondo de investigación científica básica 2015”.

11

A.I. Zarate-Guzmán thanks CONACyT for the support received with the scholarship

12

number 265212, and also thanks to Dr. Gerardo Torres Delgado for his valuable

13

contributions to this work.

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TOC – GRAPHICAL ABSTRACT

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Table 1.- Monitoring of the process in repeated cycles of use of the catalyst for the

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degradation of phenol.

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(%) 100.0

(%) 99.1

formation efficiency (%) 100.0

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87.8

97.9

90.0

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83.1

92.3

86.1

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Fe2+ leached

Fe2+/Fe 3+ Ratio

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OH radical

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Mineralization of phenol to CO2*

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Phenol degradation efficiency

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1.0

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Figure captions

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Fig.1. Characterization of the catalyst supported on activated carbon: a) FTIR spectra — C, — C-AC, — C-AC-Fe and — C-AC-Fe spent b) Diffractograms of samples C-AC-Fe (▼ γ Fe2O3 and ▽ Fe3O4), SEM-EDX mapping of samples c) C-AC and d) C-AC-Fe.

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Fig.2. XP high resolution spectra for the regions: a) C1s, b) O1s and c) Fe2p of the activated carbon and Carbon-Fe catalysts.

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Fig.3. a) Electro-generation of H2O2 at different current intensity:
i1= 0.9 mA,  i2=5 mA,
i3= 9 mA,
i4= 20 mA, using carbon felt as cathode and graphite as anode. b) Response surface plot of H2O2 production for the DOE proposal.

Fig.4. a) ■ Kinetics of •OH radicals production using 0.1 g of C-AC-Fe and ♦ phenol degradation, b) Graph of the depletion process of sample C-AC-Fe:  Formation of •OH radical (%), ○ Phenol degradation (%).

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Fig.5. Reuse cycle of C-AC-Fe as catalyst in Fenton reaction. a) Kinetics of phenol degradation ♦ cycle I,  cycle II, ▲ cycle III, and ■ cycle IV b) Kinetics of •OH radicals production: ◊ cycle I, ○� cycle II, ∆ cycle III and  cycle IV.

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Fig.6. Kinetics of generation of •OH radicals for ♦ heterogeneous Fenton, ▲ homogeneous Fenton and ■ global reaction (Homogeneous Fenton + Heterogeneous Fenton)

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Towards understanding of heterogeneous Fenton reaction using Carbon-Fe catalysts coupled to in-situ H2O2 electro-generation as clean technology for wastewater treatment

Centro de Investigación y Desarrollo Tecnológico en Electroquímica (CIDETEQ), Parque Tecnológico Sanfandila, Pedro Escobedo, Querétaro, 76703, MÉXICO.

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Grupo de Investigación en Materiales de Carbón, Facultad de Ciencias, Universidad de Granada. Av. Fuente Nueva, s/n. Granada, 18010. ESPAÑA

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Present address: Facultad de Ciencias Químicas. Universidad Autónoma de Guadalajara. Av. Patria 1201, Zapopan, Jalisco, 45129, MÉXICO

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Ana I. Zárate-Guzmán a, Linda V. González-Gutiérrez a*, Luis A. Godínez a, Alejandro Medel-Reyes a, Francisco Carrasco-Marín b, Luis A. Romero-Cano a, b, 1

Highlights

Carbon catalyst was obtained with a Fe2+/Fe3+ rate of 1.29.

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Heterogeneous Fenton reaction was proved by surface-Fe and •OH radicals analysis.

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No iron enriched sludge or by products are generated.

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In-situ H2O2 was produced at high concentrations up to 30 mg L-1 h-1.

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The complete mineralization of Phenol as a model contaminant was achieved.

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•