Degradation and Mineralization of Carbamazepine Using an Electro

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Remediation and Control Technologies

Degradation and Mineralization of Carbamazepine Using an Electro-Fenton Reaction Catalyzed by Magnetite Nanoparticles Fixed on an Electrocatalytic Carbon Fiber Textile Cathode Kai Liu, Joseph Che-Chin Yu, Heng Dong, Jeffrey C. S. Wu, and Michael R Hoffmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03916 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Environmental Science & Technology

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Degradation and Mineralization of Carbamazepine Using an Electro-Fenton

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Reaction Catalyzed by Magnetite Nanoparticles Fixed on an Electrocatalytic

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Carbon Fiber Textile Cathode

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Kai Liu,† Joseph Che-Chin Yu,†,‡ Heng Dong,† Jeffrey C. S. Wu,‡ Michael R. Hoffmann†,*

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†Department

of Environmental Science and Engineering, California Institute of Technology,

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Pasadena, California 91126, United States ‡ Department

of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

# Author Contributions:

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K. Liu and C. Yu contributed equally to the work. K. Liu conceived the idea and designed the

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experiment, K. Liu and C. Yu carried out the experiment. K. Liu wrote the manuscript.

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*Corresponding

Author: Michael R. Hoffmann

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Tel: +1 626 395 4391. Fax: +1 626 395 4391. E-mail: [email protected]

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Table of Contents Art

O2 Reduction

E-Fenton

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ACS Paragon Plus Environment


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Abstract

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Pharmaceutical wastes are considered to be important pollutants even at low concentrations.

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In this regard, carbamazepine has received significant attention due to its negative effect on both

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ecosystem and human health. However, the need for acidic conditions severely hinders the use of

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conventional Fenton reagent reactions for the control and elimination of carbamazepine in

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wastewater effluents and drinking water influents. Herein, we report of the synthesis and use of

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flexible bifunctional nano-electrocatalytic textile materials, Fe3O4-NP@CNF, for the effective

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degradation and complete mineralization of carbamazepine in water. The non-woven porous

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structure of the composite binder-free Fe3O4-NP@CNF textile is used to generate H2O2 on the

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carbon nanofiber (CNF) substrate by O2 reduction. In addition, ·OH radical is generated on the

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surface of the bonded Fe3O4 nanoparticles (NPs) at low applied potentials (-0.345 V). The Fe3O4-

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NPs are covalently bonded to the CNF textile support with a high degree of dispersion throughout

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the fiber matrix. The dispersion of the nano-sized catalysts results in a higher catalytic reactivity

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than existing electro-Fenton systems. For example, the newly synthesized Fe3O4-NPs system uses

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an Fe loading that is 2 orders of magnitudes less than existing electro-Fenton systems, coupled

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with a current efficiency that is higher than electrolysis using a boron-doped diamond electrode.

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Our test results show that this process can remove carbamazepine with high pseudo first-order rate

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constants (e.g., 6.85 h-1) and minimal energy consumption (0.239 kW·h/g carbamazepine). This

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combination leads to an efficient and sustainable electro-Fenton process.

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Introduction

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Personal care and pharmaceutical products (PPCPs) are becoming ubiquitous contaminants in

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aquatic and marine environments.1 PPCPs are often regarded as pseudo-persistent chemical

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contaminants due to their high consumption rates, partial degrees of biological transformation in

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conventional wastewater treatment plants, and their eventual discharge into natural waters.2 For

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example, carbamazepine has been identified as one of the future emerging priority pollutant

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candidates by the European Union Water Framework Directive (WFD). Carbamazepine

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bioaccumulates in the living organism with subsequent endocrine disrupting effects, neurotoxicity,

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and developmental toxicity effects.3-5 In order to eliminate carbamazepine from water, advanced

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oxidation processes (AOP) are needed.6 Electrochemical oxidation is a unique class within the

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array of available AOPs, by which an oxidant can be generated onsite. To this end, we have

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previously investigated carbamazepine removal by electrochemically-generated chloramine, and

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achieved and high level of removal with a low energy consumption (13 kWh/log/m3); however

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halogenated byproducts were detected.7, 8 Therefore, a strong oxidant such as hydroxyl radical

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(·OH) is needed to ensure complete mineralization. However, electrochemical oxidation is energy

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intensive when employing dimensionally-stable electrodes made from platinum group elements

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(PGE) as catalytic ohmic contacts. Platinum group metals, noble metal oxides, and boron-doped

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diamond electrodes are typically used for in situ reactive oxygen species generation.9 Numerous

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attempts have been made to achieve comparable degradation efficiencies with inexpensive, earth

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abundant materials. We have recently reported on the use of SbSn/CoTi/Ir multilayer

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heterojunction anodes, which reduced the use of PGM.10 Nevertheless, PGM-free electrodes

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represent a significant step towards green and sustainable use of AOPs. It has been demonstrated

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that carbon-based electrode material can be coupled in an electro-Fenton reaction to completely

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degrade PFOA.11 However, a major drawback of this strategy is that large amount of FeSO4 needs

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to be dissolved in the electrolyte in order to facilitate the Fenton’s reagent reaction. Therefore, it

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is urgent to develop green and sustainable electrochemical AOPs that can work with minimal but

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stable non-precious metal catalysts.

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The application of nanostructured catalysts is wide spread12,13,14 However, many of these

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catalytic materials are used in colloidal suspension, which significantly limits their use in practical

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applications. Therefore, supported metal nanoparticle (NP) have recently gained attention due to

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low energy consumption and ease of preparation.15 Typically, supported metal NPs can be

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prepared by thermal or chemical decomposition of metal precursors on heterogeneous supports.16,

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17

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the topology, size, distribution of metal NPs, as well as the chemical composition of the structural

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support.18 To date, however, metal NPs with heterogeneous support are rarely explored for

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electrochemical AOPs due to the difficulty in (1) achieving a uniform dispersion of metal NPs on

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a conductive substrate, (2) formation of an appropriate bond between metal a NP and the substrate

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to facilitate interfacial electron transfer, and (3) provide structural stability. Indeed, well dispersed

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nanostructures on supporting substrates have been demonstrated to have superior mechanical and

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electrical properties compared to random dispersed counterparts.19, 20

In addition, the reactivity of such heterogeneous catalytic systems can be tuned by controlling

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Herein, we describe the application of metal (M) and nitrogen (N) co-doped graphite (C)

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MNC flexible electrodes which can act as a multi-phasic, bifunctional electro-Fenton system for

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the degradation of pharmaceutical byproducts such as carbamazepine. The functionalized flexible

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MNC electrodes consist of Fe3O4 nanoparticles (NP) bonded to a carbon nanofiber (CNF) matrix,

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subsequently referred to as Fe3O4-NP@CN. This composite electrode is synthesized via pyrolysis

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of electrospun nanofiber (NF) which allows for the precision control of the Fe3O4-NP dispersions

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even at low concentration (< 1 wt% loading). The degradation of carbamazepine on Fe3O4-

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NP@CNF electrodes is investigated in order to minimize the Fe3O4-NP loading, while maximizing

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·OH production via the electro-Fenton reaction.

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Materials and Methods

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Materials

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All reagent-grade chemicals were obtained from Sigma-Aldrich and were used without any

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further purification. Solvents used were HPLC grade unless otherwise stated.

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Preparation of CNF and Fe3O4 NP@CNF

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CNF was prepared by carbonization of polyacrylonitrile (PAN) NF, which was prepared

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using an electrospinning method.21 In a typical electrospinning synthesis, 1.5 g of PAN (molecular

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weight = 150,000 g/mol) was dissolved in 15.6 ml of N,N-dimethylformamide (DMF) and mixed

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thoroughly for 12 h. The solution was fed through a stainless needle using a syringe pump (New

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Era pump system) at a rate of 0.6 mL/h. An aluminum foil collector was placed at a distance of 10

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cm from the needle. A voltage of 10 kV was applied (EDSEMC high-voltage power supply) to

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produce a non-woven mat of PAN nanofibers. The prepared PAN-NF mats were separated from

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the collectors and heated at 240 ºC for 2 h to promote crosslinking in order to enhance mechanical

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strength. In a final step, the crosslinked PAN-NF mats were carbonized at 900 ºC for 0.5 h under

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a N2 atmosphere in order to produce flexible CNF mats.

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Fe3O4-NP@CNF composites were prepared using a similar process. DMF solutions with

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an appropriate amounts of iron acetylacetonate (Fe(acac)3) and PAN were used to produce non-

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woven, yellow Fe-doped PAN mats. Further treatment was employed to promote crosslinking via

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pyrolysis at 900 ºC for 0.5 h under a N2 atmosphere to produce the flexible Fe3O4-NP@CNF mats.

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Fe3O4-NP@CNF samples were prepared from solutions with the ratio of 0.05, 0.1, 0.3, 0.5, 1.0, 5.0

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wt% of Fe(acac)3 to PAN; they were denoted as FeCNF0.05, FeCNF0.1, FeCNF0.3, FeCNF0.5,

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FeCNF1, and FeCNF5, respectively.

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Preparation of Electrodes

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Even though CNF and Fe3O4-NP@CNF mats are conductive, maintaining a constant

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anode-cathode distance within a batch reactor is a difficult challenge due to their flexibility and

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deformability. In order to overcome this problem, rigid carbon paper was coated with the active

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catalyst to function as the working electrode. The electroactive coating was prepared by

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suspending ground CNFs or Fe3O4-NP@CNF powders (10 mg) in to 2.2 ml of milli-Q water, 0.75

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mL of isopropanol, and 50 μL of Nafion solution. The mixture was then spray-coated onto carbon

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paper (AvCarb MGL 190) at a catalyst loading of 0.5 mg/cm2. Both sides of carbon paper were

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uniformly coated (total geometric surface area of 8 cm2) to avoid electrolyte exposure of the

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supporting carbon paper substrate.

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Characterization of the Electrocatalysts

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The surface morphology and elemental composition of the catalytic composites were

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examined using a ZEISS 1550 VP field-emission scanning electron microscope (FE-SEM)

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equipped with an Oxford X-Max SDD energy dispersive X-ray spectrometer (EDS). TEM and

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TEM-EDS measurements were obtained on a FEI Tecnai F30ST (300 kV) transmission electron

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microscope (TEM) equipped with Oxford ultra-thin window EDS detector. X-ray powder

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diffraction patterns were collected from a Panalytical X’pert Pro diffractometer with Cu Kα

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radiation (λ=1.5418 Å). The elemental composition of the electrocatalysts was characterized by

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X-ray photoelectron spectroscopy (XPS) using a Surface Science Instruments M-Probe ESCA

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surface spectrometer. Monochromatic Al Kα radiation (1486.6 eV) was used.

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Electrochemical Experiments

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Electrochemical experiments were performed using a BioLogic VSP potentiostat with a

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three-electrode reactor cell at room temperature. A platinum (Pt) wire and saturated calomel

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electrode (SCE) were used as a counter electrode and a reference electrode, respectively. The

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recorded potentials were converted to the reversible hydrogen electrode (RHE) scale (RHE = SCE

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+ 0.059 × pH + 0.241 V). Cyclic voltammetry (CV) was carried out in an Ar- or O2- saturated

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electrolyte (0.1 M Na2SO4) at a scan rate of 10 mV/s. Constant potentials were applied to

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investigate H2O2 production on CNF and the electro-Fenton degradation of carbamazepine using

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Fe3O4-NP@CNF. An O2- saturated electrolyte (0.1 M Na2SO4) was used, while the solution pH

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was adjusted over the range of 4-10 using a combination of sulfuric acid and sodium hydroxide.

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H2O2 production yields were determined in triplicate. Electro-Fenton experiments were carried out

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in duplicates with average values reported.

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Analytical Methods

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The concentration of H2O2 in samples collected at given time intervals were analyzed by

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titanium oxalate spectrophotometric method.22 In this procedure, the collected reaction sample was

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acidified with 3 M sulfuric acid solution and then mixed with a potassium titanium oxalate solution

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(0.5 wt% in water) to produce the yellow pertitanic acid complex. The absorbance of the solution

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was measuring using a NanoDrop spectrophotometer at 400 nm. H2O2 solutions with known

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concentrations were used to construct a calibration curve.

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During the electro-Fenton treatment process, the concentrations of carbamazepine and

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terephthalic acid (TPA) were analyzed by an ultrahigh performance liquid chromatography

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(Waters Acquity UPLC) system coupled to a UV detector (Acquity PDA) and a time-of-flight

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mass spectrometer (Waters XEVO GS-2 TOF). Acquity BEH C18 column (2.1x50 mm; 1.7 µm

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particles) was used. (A) 0.1% formic acid and (B) acetonitrile were used as mobile phase with flow

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rate of 0.5 mL min-1. The mass spectrometer was operated with source temperature of 120 °C, a

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solvent removal temperature of 400 °C, and capillary voltage of 0.2 kV.

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RESULTS AND DISCUSSION

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Design Approach

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Carbon-based electrodes (graphite, graphene, carbon nanotubes (CNTs), etc.) are utilized

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due to their ability to produce H2O2 via the selective oxygen reduction reaction (ORR). They serve

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as effective and economic alternatives for the state-of-art platinum group metals for the ORR.

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Although the inherent chemical reactivity of H2O2 is insufficient to achieve complete

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mineralization of aqueous chemical contaminants such as PPCPs and perfluorinated compounds

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(PFCs). However, the O-O single bond strength (e.g., 142 kJ mol-1) is relatively weak and thus in

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H2O2 the O-O bond is easily broken to produce hydroxyl radical. Incorporation of electroactive

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metals into carbon substrates provides a convenient method to generate ·OH of H2O2. The electro-

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Fenton process using Fe(III)-doped CNT has been shown to be able assist in the degradation of

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recalcitrant organic compounds such as trifluoroacetic acid or trichloroacetic acid.23 However,

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weak electrostatic interactions between metal ion dopant and carbon substrate results in a

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significant metal loss by leaching; in addition, regeneration of the active electrocatalyst is

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required.23 To overcome this problem, we have designed the Fe3O4-NP@CNF electrocatalyst

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based on the MNC approach. We believe that the ideal MNC electrocatalyst must meet certain

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criteria: (1) the metal-based catalyst must be well dispersed on the support substrate to allow

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sufficient contact of the electrolyte with both the catalyst and the substrate, (2) the metal-based

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catalyst (e.g., Fe3O4) must form a bond with the substrate, and (3) the electrocatalyst should exhibit

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flexibility. The first criterion allows an optimal utilization of the metal-based nano-catalytic

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material, which tends to aggregate into clusters in the absence of the substrate since the surface

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free energy is increased due to the decreasing particle size. The third criterion allows the

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construction of roll-type or foldable electrodes with associated form-factor advantages, which can

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maximize the electrode surface to electrolyte volume ratio.

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To develop MNC based on these criteria, the formation of a fibrous scaffold was chosen.

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Electrospun PAN nanofibers were chosen for this purpose due to the abundance of nitrile (-C≡N)

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functional groups. Magnetite (Fe3O4) incorporation was achieved using a gelation-free co-electro-

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spraying method. This method involves the mixing the nanofibers and iron acetylacetonate

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(Fe(acac)3), which serves as the Fe3O4 precursor. The electrospun Fe(acac)3@PAN nanofiber mat

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was heating to 240°C for 30 min to enhance the physical strength of the mat. Two conversion

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process occurred simultaneously: 1) the thermal decomposition of Fe(acac)3 into Fe3O4

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nanoparticles (NP) and 2) the formation of a thermoset ladder-structure of the thermoplastic PAN

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nanofibers via cyclization.24 The initial product produced, Fe3O4-NP@PAN, had a low

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conductivity; however, after calcination at 800°C under a nitrogen atmosphere a conductive Fe3O4

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NP@CNF was produced. Figure 1 shows that a macroscopic non-woven “nano-electrocatalytic

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textile” is produced (vide supra).

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Morphology and Structure

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SEM micrographs show that both CNF and Fe3O4-NP@CNF are composed of interwoven

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and porous networks (Fig. 2) that are fully accessible to electrolytes and dissolved gas molecules

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in an aqueous electrolyte solution. These open networks should allow for effective mass transfer

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and gas dispersion during electrolysis. Due to the low concentrations of Fe3O4-NPs the underlying

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CNF substrate is unaffected by the presence of the magnetite nanoparticles. Both CNF and Fe3O4-

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NP@CNF have an average diameter of 180 nm. Due to the simplicity of self-assembling Fe3O4-

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NPs within the electrospun PAN-NF, control over the Fe3O4-NP loading level can be achieved. As

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show in Figure S1, a series of Fe3O4-NP@CNF assemblages were obtained using different

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Fe(acac)3 precursor concentrations (0.1-5.0 wt%). The actual Fe loading in the Fe3O4-NP@CNF

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arrays was determined using SEM-EDS elemental analysis; these results are summarized in Table

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S2. EDS mapping provided direct evidence that the Fe3O4-NPs were evenly dispersed in CNF

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support substrate (Fig. S1). Smooth nanofibers were observed except for the FeCNF5 sample; the

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SEM images indicate that the Fe3O4-NPs aggregate at high loadings (i.e., ≥ 5 wt%). To identify

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the crystal shapes of Fe3O4 NP doped onto CNF, TEM measurements of FeCNF1 were conducted

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(Fig. S2). However, due to low loading levels (1 wt%), Fe3O4-NPs were indistinguishable from

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the CNT substrates. However, TEM-EDS characterization (Fig. S3) confirms a uniform dispersion

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of Fe3O4-NPs.

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The structure and phase composition of the CNFs and Fe3O4-NP@CNFs were

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characterized using powder X-day diffraction (PXRD). Due to the low detection limit of PXRD

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(3%), Fe3O4-NP@CNF at the highest magnetite loading (FeCNF10) was used for PXRD analyses.

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As shown in Figure S4, the metal nanocatalyst in Fe3O4-NP@CNF is indeed magnetite, Fe3O4.

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The XRD pattern is also consistent with cubic Fe3O4.

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XPS analysis was used to determine other features of the surface chemistry of CNF and

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Fe3O4-NP@CNF (Fig. S5). Due to the sensitivity of the XPS instrument used, the loadings of

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magnetite on FeCNF0.05 and FeCNF0.1 were too low to obtain suitable Fe2p spectra, however,

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the FeCNF1 sample gave a resolved XPS spectrum. The high-resolution XPS spectra confirmed

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the presence of N, O, C, and Fe in FeCNF1 sample (Fig. S6), while Fe is clearly absent in the CNF

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spectrum (Fig. S6). The N1s peak of CNF can be fitted to pyridinic N, pyrrolic N, graphitic N, and

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oxidized nitrogen at 398.0, 399.2, 400.6, 403.9 eV, respectively (Fig. S6). When compared with

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the N1s of CNF spectrum, the FeCNF1 spectrum showed similar peaks to CNF (Fig. S7), but an

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additional peak at 398.8 eV was also detected. We thus assigned the speak at 398.8 eV to Fe-N. A

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recent report suggested that Fe-N sites can serve as active sites for inducing the Fenton reaction25.

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The relative abundance of Fe-N sites in FeCNF1 suggest a moderate to high reactivity in terms of

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the Fenton reaction. In order to confirm our hypothesis, we utilized a method for identifying Fe

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species in a high-resolution XPS spectrum of Fe 2p.26 It is evident that CNF does not contain Fe

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(Fig. S6). In contrast, characteristic Fe 2p3/2 and Fe 2p1/2 peaks at 711.7 and 724.9 eV confirmed

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that both Fe2+ and Fe3+ are present in FeCNF1 (Fig. S7). The O1s spectrum of CNF consisted of 3

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multiplets including O C, O–C, and oxygen atoms adsorbed onto the CNF surface at 531.0, 532.3, and

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535.1 eV, respectively. In contrast, additional peak at 530.2 eV from FeCNF1 can be attributed to

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presence of lattice oxygen in Fe3O4. Table S1 shows that the oxygen levels are significantly higher

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in FeCNF1 than in CNF, thus providing additional conformation for the presence of magnetite

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nanoparticles, Fe3O4-NP.

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Electrolytic Production of H2O2 on CNF

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The electrochemical activity of the CNF substrate for the oxygen reduction reaction (ORR)

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was determined in part by using cyclic voltammetry (CV) to follow the oxygen reduction versus

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potential. As shown in Figure 3, the CNF electrode shows a pronounced reduction peak in an O2-

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saturated electrolyte solution (0.1 M Na2SO4), while the CV curve for CNF in an Ar-saturated 0.1

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M Na2SO4 is featureless. The onset potential for CNF at pH 4, 7, and 10 was 0.314, 0.570, and

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0.766 V vs. RHE, respectively. The solution pH did not detectably change after the ORR reaction.

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In addition, the ORR peak of CNF electrode at pH 4 had the highest current density at 1.055

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mA/cm2, while the current densities at pH 10 (0.5807 mA/cm2) and pH 7 (0.5775 mA/cm2) are

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similar. These results suggest that the CNF electrode can electrochemically catalyze ORR over a

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wide range of pH.

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It has been demonstrated that the selectivity of the oxygen reduction reactions (ORR) can

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be altered by controlling the calcination temperature of the carbon precursor.27 Literature reports

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suggest that the ORR current efficiency on carbon-based electrodes varies between 60 to 95%.27-

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29

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carbon materials are highly selective for the two-electron oxygen reduction reaction to form H2O2,

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with selectivity above 90%.27, 30 To determine the dominant reaction pathway for the ORR on the

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CNF electrode, we quantified the amount of H2O2 produced in O2 saturated electrolyte solutions.

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The H2O2 generation rate was found to be the highest at pH 7 (39.65 mmol/h/g), followed by pH

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4 (32.35 mmol/h/g), and pH 10 (18.07 mmol/h/g) (Fig. 4). Even though CNF materials are

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considered to be nonporous with low reactive surface areas, the reported H2O2 production rates are

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greater than those obtained using porous carbon electrodes under similar conditions.31 DFT

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calculations and experimental evidence suggest that carbon nitride and oxidized carbon favors the

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2e- transfer pathway for O2 reduction leading to H2O2 formation.32, 33 Our high resolution XPS data

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for CNF indicates the presence of both oxygen functional groups and carbon nitride on the CNF

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surface (Fig. S6).

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Electro-Fenton Degradation of Carbamazepine

In addition, simulations and experimental results indicate that graphite and nitrogen-doped

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Carbamazepine was chosen as a probe compound to demonstrate the feasibility of utilizing

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the heterogeneous electro-Fenton reaction on Fe3O4-NP@CNF for the treatment of dilute

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pharmaceutical wastes in water. In order to distinguish between loss by sorption alone and

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hydroxyl radical oxidation of carbamazepine, the concentration versus time profiles for

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electrosorption on CNF and oxidation via H2O2 formation are compared in Figure 5. The initial

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carbamazepine concentration was 1 ppm in 50 mL electrolytes. All processes follow pseudo first-

265

order kinetics, but the removal rate of each differs significantly. Complete removal of

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carbamazepine was achieved within 30 min for the electro-Fenton process using Fe3O4-NP@CNF

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electrode. On the contrary, carbamazepine removal efficiency was only 21.1 and 8.8% for CNF

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electrosorption and H2O2 oxidation, respectively. Degradation kinetics are shown in Figure S8,

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where electro-Fenton process has a much larger first-order rate constant compared to loss by

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sorption or molecular oxidation by hydrogen peroxide.

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Fe3O4-NP@CNF is a bi-functional electrochemical catalyst. The electro-Fenton reaction is

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catalyzed by the Fe3O4-NPs, while H2O2 is generated in situ by O2 reduction catalyzed by the CNF

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substrate. Therefore, the effect of Fe3O4-NP loading level on the CNF was investigated to optimize

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·OH radical production. The synthesized series of Fe3O4-NP@CNFs at loading levels of 0.05 to

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1.00 wt% were compared for carbamazepine removal at pH 7 as shown in Figure 6. The

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carbamazepine removal efficiency increased as Fe3O4-NP loading level was reduced from 1.0 to

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0.1 wt%, while further reduction in the Fe3O4-NP loading resulted in a reduction in the

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carbamazepine removal efficiency. All of the Fe3O4-NP@CNF electro-Fenton followed pseudo

279

first-order kinetics (Fig. S9). The kinetic constants for carbamazepine degradation were

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determined to be 1.79, 6.85, 2.43, 1.35, and 0.52 h-1 for FeCNF0.05, FeCNF0.1, FeCNF0.3,

281

FeCNF0.5, and FeCNF1, respectively (Table S3). As shown in Figure S10, the carbamazepine

282

removal efficiency is also a function of the solution pH. The kinetic constants for carbamazepine

283

removal are 4.78, 6.85, and 3.30 h-1 for FeCNF0.1 electrode at pH 4, 7, 10, respectively (Fig. S11,

284

Table S3). Our data suggests that the electro-Fenton process catalyzed using the bi-functional

285

Fe3O4-NP@CNF electrode can be performed over a wide range of solution pH. The moderately

286

enhanced removal efficiency at pH 7 can be attributed to an increased H2O2 production at circum-

287

neutral pH (Fig. 4), which in turn increases ·OH production. The CVs of the CNF substrate show

288

that the current density of the CNF electrode is directly proportional to the applied potential. This,

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in turn, should impact the H2O2 production rate (Fig. 3). Figure S12 shows that in fact the

290

carbamazepine removal efficiency was significantly improved as the applied potential was

291

increased from -0.145 to -0.545 V. The kinetic constants for carbamazepine removal are 4.81,

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6.85, 9.00 h-1 for FeCNF0.1 electrode at -0.145, -0.345, and -0.545 V, respectively (Fig. S13,

293

Table S3). Figure 7 shows that complete mineralization of carbamazepine is achieved within 3 h

294

s the electro-Fenton reaction as catalyzed by FeCNF0.1 at pH 7. In contrast, H2O2 oxidation only

295

removes 8.8% of carbamazepine under similar conditions.

296

Stability of the electrocatalysts is important for practical applications. In this regard, we

297

have examined the electro-Fenton degradation of carbamazepine using the recycled FeCNF0.1

298

electrode. Our results (Fig. S15) show that the catalytic activity (>90%) is retained even after

299

repeated usage.

300

Mechanism of Carbamazepine Mineralization

301

The high degree of carbamazepine mineralization efficiency obtained using a Fe3O4-

302

NP@CNF cathode (Fig. 7) is clearly attributed to the production of ·OH via electro-Fenton

303

reaction. CVs of Fe3O4-NP@CNF electrodes suggest the onset potential of oxygen reduction is

304

not significantly affected by the Fe loading concentration (Fig. S16).

305

In order to explain the observed differences in the carbamazepine degradation efficiency

306

as a function of the loading level of magnetite on the Fe3O4-NP@CNF electrodes, the amount of

307

·OH generated for each loading level was probed using terephthalic acid (TPA) as a hydroxyl

308

radical trap. TPA has been widely used as a chemical probe to detect ·OH, since the rate constant

309

between TPA and ·OH (4.4×109 M-1S-1) are orders of magnitudes higher than other reactive

310

oxygen species that might be involved in this study, such as H2O2 (3.0×107 M-1S-1) or singlet

311

oxygen (5.0×104 M-1S-1).34-36 In addition, TPA (pKa1 = 3.5 and pKa2 = 4.4), which predominately

14


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312

exists in doubly deprotonated carboxylate form at pH 7, is electrostatically repelled from the

313

negatively charged Fe3O4@CNF cathode, and thus TPA is an excellent radical trap for probing

314

solution phase production of ·OH. Because TPA reacts with ·OH to produce 2-hydroxyterephthalic

315

acid (HTA), the steady-state concentration of ·OH can be determined by monitoring the TPA

316

concentration in the electrolyte. Figure S14 shows the that amount of TPA adsorbed onto the

317

platinum needle anode was negligible. This is due to the low surface area of the platinum needle

318

anode in addition to the formation of oxygen gas bubbles generated via water oxidation on the Pt

319

surface. As shown in Figure 8, the Fe3O4-NP loading level impact on ·OH production has a similar

320

trend in activity as observed for carbamazepine degradation (Fig. 6). Again, the FeCNF0.1

321

composite electrode produced the most ·OH, followed by FeCNF0.05 > FeCNF0.5 > FeCNF1. In

322

energy conversion, there an interest reducing the size of the electrocatalyst in order to maximum

323

its utilization efficiency by providing the greatest number of accessible active sites.37 Indeed, the

324

Fenton reaction on crystalline particles of Fe3O4, which are nonporous, normally prevents non-

325

surface accessible Fe(II) to participate in the reaction. Thus, doping of the CNF substrate with a

326

minimal amount of Fe3O4-NP decreases the formation of bulk-phase crystalline particles. This is

327

evident from the PXRD spectrum of Fe3O4-NP@CNF with low Fe content (Fig. S4). Therefore,

328

Fe(II) from FeCNF0.1 embedded in the graphitic molecular support has approximately identical

329

local coordination environments, which, in turn, maximize the active sites available for the electro-

330

Fenton reaction. In the case of Fe3O4-NP@CNF with a lower Fe content (FeCNF0.05), in situ

331

generated H2O2 is found in excess due to an insufficient amount of Fe(II) to generate hydroxyl

332

radical.

333

The electro-Fenton promoted degradation of carbamazepine is similar to the ·OH oxidation

334

of carbamazepine.38 Based on the intermediates detected in herein (Fig. S17) and on other reports

15



335

of carbamazepine degradation38, 39, we propose a mechanistic pathway leading to carbamazepine

336

mineralization by the electro-Fenton process as shown in Fig. S18.

337

Energy Efficiency for Carbamazepine Degradation

338

During the Fe3O4-NP@CNF catalyzed electro-Fenton reactions, the applied potential was

339

-0.345 V with a corresponding current density of 3.46 mA/cm2 at pH 7; this value can be compared

340

to a previously reported current density of 190 mA/cm2 at a similar applied potential during

341

electrochemical degradation of carbamazepine.40 Given these conditions, the energy consumption

342

required by the electro-Fenton process to degrade carbamazepine is 0.239 kW·h/g carbamazepine.

343

When compared with other advanced oxidation processes such as electrolysis with a boron doped

344

diamond electrode (4.0 kW·h/g carbamazepine), electrolysis with PbO2 (4.4 kW·h/g

345

carbamazepine), photodegradation using UV/peroxymonosulfate (933 kW·h/g carbamazepine),

346

photodegradation using UV/H2O2 (396 kW·h/g carbamazepine), electro-Fenton oxidation using

347

Fe3O4-NP@CNF cathodes requires significantly less energy.41, 42 It should be noted that the the

348

previously reported results were obtained under different operational parameters, which could

349

have impact the apparent power consumption levels and treatment efficiencies. An in-depth study

350

is necessary to provide a comprehensive understanding of the energy requirement of these

351

alternative AOPs.

352

The unique characteristic of Fe3O4-NP@CNF arrays integrates multifunctional catalysts

353

into a single stable material to maximize the catalytic efficiency using minimal amount of Fe3O4

354

as an iron(II)-containing catalyst. A loading level of 0.36 wt% Fe or 0.0012 mg Fe/cm2 of electrode

355

surface area (equivalent to 0.48 mg Fe/L under the applied experimental conditions) is needed for

356

Fe3O4-NP@CNF cathodes to maximize the ·OH production from H2O2. In contrast, 56 mg Fe/L

357

of an Fe(II) electrolyte is required for the electro-Fenton reaction to fully convert H2O2 produced

16


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358

by MOFs derived carbon electrode into ·OH under similar condition.11 Our current results indicate

359

that Fe3O4-NP@CNF can provide a cost-effective material to catalyze electro-Fenton reactions.

360

In summary, efficient degradation of carbamazepine has been achieved using the electro-

361

Fenton process based on a bifunctional Fe3O4-NP@CNF electro-catalyst. In this novel electro-

362

Fenton system, uniformly dispersed Fe3O4-NPs, which are bonded to the CNF cathode substrate,

363

are able to obtain a high electro-Fenton efficiency with minimal Fe-mass loading level (0.48 mg

364

Fe/L). Given the relatively low energy consumption of this system combined with its wide pH

365

range, the Fe3O4-NP@CNF composite electrode potentially provides a cost-effective treatment

366

method to achieve the total mineralization of carbamazepine is a relatively short period of time.

367

Furthermore, this electro-Fenton system can be applied for the removal of other pharmaceutical

368

and person care product from wastewater and drinking water.

369 370

Associated Contents

371

Supporting Information

372

Includes SEM, TEM images, XPS spectrum, and carbamazepine removal kinetic study.

373 374

Author Information

375

Corresponding Author

376

*(Michael R. Hoffmann) Tel: +1 626 395 4391. Fax: +1 626 395 4391. E-mail: [email protected]

377

Notes

378

The authors declare no competing financial interest.

379

17



380

Acknowledgments

381

This study was supported by the Bill and Melinda Gates Foundation (Grant No. OPP1149755),

382

Caltech Rosenburg Innovation Initiative, and grant 106-2917-I-002-008 from Taiwan Ministry of

383

Science and Technology.

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506

Figure 1. Digital photograph of folded non-woven (A) PAN, (B) Fe(acac)3@PAN, (C) Fe3O4

507

NP@PAN, and (D) Fe3O4 NP@CNF (FeCNF0.1) mats.

508

509 510 511

Figure 2. SEM images of (A) CNF and (B) Fe3O4 NP@CNF (FeCNF0.1).

512

513 514

25



515

Figure 3. CV curves of CNF substrate in O2 and Ar saturated electrolyte solution (0.1 M

516

Na2SO4) at pH 4 (A), pH 7 (B), and pH 10 (C) with a scan rate of 10 mV/s.

517 518 519 520 521

Figure 4. The concentration of H2O2 produced by CNF at -0.345 V at pH 4, pH 7, and pH 10.

H2O2 concentration (mmol/L)

522

523

pH 4 pH 7 pH 10

40 30 20 10 0

0

10

20 30 40 Time (min)

50

524 525

26


60

Page 26 of 28

Page 27 of 28


526

Figure 5. Carbamazepine removal by electro-Fenton (FeCNF0.1, -0.345 V), electrosorption

527

(FeCNF0.1, -0.345 V), electrosorption (CNF, -0.345 V), and H2O2 degradation (200 ppm).

528

Electrosorption and H2O2 degradation are performed in Ar saturated electrolyte at pH7.

1.0

C/C0

0.8 0.6

FeCNF0.1 Electro-Fenton FeCNF0.1 Electrosorption CNF H2 O 2

0.4 0.2 0.0

0

10

529

20 30 40 Time (min)

50

60

530 531

Figure 6. Effect of Fe3O4 loading level on electro-Fenton removal efficiency of carbamazepine

532

at pH 7.

533 FeCNF0.05 FeCNF0.1 FeCNF0.3 FeCNF0.5 FeCNF1

1.0

C/C0

0.8 0.6 0.4 0.2 0.0 534

0

10

20 30 40 Time (min)

27

50


60


Page 28 of 28

535

Figure 7. TOC removal efficiency of electro-Fenton removal of carbamazepine (FeCNF0.1, -

536

0.345 V, pH 7).

537

TOC Removal (%)

100 80 60 40 20 0

0

30

538

60 90 120 Time (min)

150

539 540 541

Figure 8. Effect of Fe3O4 loading level on ·OH steady state concentration (-0.345 V, pH 7).

542

OH concentration (mM)

0.06

.

543

FeCNF0.05 FeCNF0.1 FeCNF0.5 FeCNF1

0.04

0.02

0.00

0

10

20 30 40 Time (min)

28

50


60

Degradation and Mineralization of Carbamazepine Using an Electro

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