Benefit of hydrophilicity for adsorption of methyl orange and electro

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Benefit of hydrophilicity for adsorption of methyl orange and electroFenton regeneration of activated carbon-polytetrafluoroethylene electrodes Ye Xiao, and Josephine M. Hill Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03409 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 15, 2018

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Environmental Science & Technology

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Benefit of hydrophilicity for adsorption of methyl orange and electro-Fenton

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regeneration of activated carbon-polytetrafluoroethylene electrodes

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Ye Xiao, Josephine M. Hill*

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Department of Chemical & Petroleum Engineering, Schulich School of Engineering, University

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of Calgary, 2500 University Dr NW, Calgary, AB, Canada, T2N 1N4

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*Corresponding Author Tel: +1 403 210 9488; fax: 1 403 284 4852; e-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract: Activated carbon (AC)-polytetrafluoroethylene (PTFE) electrodes were prepared and

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applied for methyl orange (MO) adsorption and electro-Fenton regeneration. The addition of

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PTFE to AC significantly decreased the hydrophilicity, which in turn, decreased both the amount

14

of MO adsorbed and the regeneration efficiency. With the minimum amount of binder (a 7:1

15

mass ratio of AC to binder), the MO adsorption was 176 mg g-1. The amount adsorbed decreased

16

to 23 mg g-1 for the electrode with a 1:1 mass ratio of AC to binder. For these ratios, the

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regeneration efficiencies were 81% and 49%, respectively. The adsorption kinetics were well fit

18

by a Webber-Morris model. The diffusion rate constants obtained from this model were linearly

19

related to the hydrophilicity of the electrode, i.e., the higher the hydrophilicity the higher the

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adsorption rate. Based on the results, an adsorption capacity >50 mg g-1 in 8 h with a

21

regeneration efficiency of >70% at cathodic potential of -0.8 V (vs Ag/AgCl) can be obtained if

22

the contact angle of water on the electrodes is lower than 90°.

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Key words: activated carbon-PTFE electrode; adsorption; electro-Fenton regeneration;

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hydrophilicity; methyl orange


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1 Introduction

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Carbon adsorption is one of the most effective and efficient methods to remove organic

27

pollutants from water. A high adsorption capacity is possible for carbon materials because of

28

their tunable pore structure and surface functional groups. After adsorption, regeneration of the

29

carbon material may reduce costs and possible secondary pollution. Numerous investigations

30

have studied the regeneration of carbon materials by using technologies including thermal

31

regeneration,1-3 solvent extraction and desorption,4, 5 biological regeneration,6 and

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electrochemical regeneration.7-11

33

As an advanced oxidation process (AOP), electro-Fenton (EF) oxidation regenerates the

34

adsorbent and decomposes the adsorbed pollutants simultaneously12 and has been widely

35

investigated for the treatment of various organic pollutants in water.13-18 Trellu et al. investigated

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the EF regeneration of phenol saturated activated carbon (AC) fiber with a regeneration

37

efficiency of >70%.19 Roth et al. used a carbon nanotube-based electrode for the adsorption of

38

Acid Red 14, and the electrode was then regenerated in an EF process with regeneration

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efficiencies of 97-100%.20 AC saturated with toluene and Orange II was also successfully

40

regenerated (> 95%) with EF oxidation.12, 21 In general, higher regeneration efficiencies were

41

reported when carbon materials had better contact with the electrodes,19, 22-29 even with other

42

types of cathodic regeneration methods, including cathodic polarization7 and electro-peroxone.29

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Carbon materials are frequently used as the cathode for EF oxidation because they have high

44

activity for the two-electron oxygen reduction reaction to produce H2O2 and low catalytic

45

activity for the hydrogen evolution and H2O2 decomposition reactions.30 Specifically,

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graphite/carbon felt and gas diffusion electrodes made from carbon materials and

47

polytetrafluoroethylene (PTFE) have been used as cathodes.13, 15, 31, 32 As indicated in some 3 ACS Paragon Plus Environment


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investigations, however, most of the conventional cathodes were not suitable for adsorption of

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organic pollutants because of their low adsorption capacities (< 10 mg g-1)33, 34 with exception of

50

the AC felt cathode.19

51

Recently, AC-PTFE cathodes were used for oxygen reduction reactions in microbial fuel cells.35,

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36

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black.36 In addition, AC based cathodes for the EF oxidation of organic pollutants in water was

54

also reported with greater than 90% removal in 1 h.37, 38 The application of AC-PTFE electrode

55

for adsorption and EF regeneration has not been reported in the literature, possibly because the

56

incorporation of PTFE in an electrode will reduce the hydrophilicity,39 impacting diffusion

57

within the electrode.40

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In this study, the application of AC-PTFE electrodes as both the adsorbent during adsorption and

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the cathode during EF regeneration is proposed and studied. More specifically, the impact of the

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electrode hydrophilicity on the adsorption and regeneration process was investigated. AC-PTFE

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electrodes with different AC to PTFE ratios were prepared, and their hydrophilicities were

62

measured. The electrodes were then tested for methyl orange (MO) adsorption followed by EF

63

regeneration. The relationships between the electrode hydrophilicity and MO adsorption kinetics

64

and regeneration efficiencies were then analyzed to determine the appropriate hydrophilicity.

The higher surface area of AC improved the activity compared to electrodes made from carbon

65 66

2 Materials and Methods

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2.1 Preparation of AC-PTFE electrodes

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As-received ColorSorb G5 (denoted as G5) from Jacobi Carbon company was sieved, and

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particles < 90 µm were collected for the AC-PTFE electrode preparation. G5 carbon was chosen 4 ACS Paragon Plus Environment

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because of its high mesoporosity (Table 1), which was beneficial for the regeneration process.41

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An appropriate amount of PTFE preparation solution (60 wt% dispersion in water, Sigma-

72

Aldrich) was added into a glass vial, and then ~5 mL of isopropanol (>99.5%, Sigma-Aldrich)

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and ~0.1 g of G5 carbon were added to the vial, which was then sonicated for 20 min. The

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obtained slurry was put in a water bath at 80 °C to evaporate the isopropanol until a dough-like

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material formed. After that, the material was rolled into two sheets (1 cm in width and 4 cm in

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length), and they were then pressed together with a stainless-steel mesh (40 mesh, 1 cm in width)

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in the middle under 2000 N for 10 min. The obtained assembly was then transferred into a muffle

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furnace and calcined at 350 °C for 1 h. The electrodes with different mass ratios of G5: PTFE

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were prepared, and the electrodes with G5: PTFE ratios of 7: 1, 5: 1, 3: 1, 2: 1, and 1: 1 were

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named as G5-PTFE-7-1, G5-PTFE-5-1, G5-PTFE-3-1, G5-PTFE-2-1, and G5-PTFE-1-1,

81

respectively. The highest G5: PTFE ratio was chosen as 7: 1 because this was the highest ratio

82

that the prepared electrode can bind together using the current preparation method. The as-

83

prepared electrodes had a carbon layer of ~0.2 mm on each side of the stainless-steel mesh as

84

measured with a Vernier caliper (resolution of 0.01 mm).

85

2.2 Characterization of AC-PTFE electrodes

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The nitrogen adsorption-desorption isotherms of the electrode materials were measured at -

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196 °C (TriStar II Plus, Micromeritics Tristar Instrument). Before analysis, the materials were

88

degassed at 150 °C under a vacuum of < 10 Pa overnight (>12 h). The Brunauer-Emmett-Teller

89

(BET) surface area and total pore volume of the electrode materials were obtained based on the

90

nitrogen adsorption isotherms within the partial pressure ranges of 0.05-0.3 and 0.97-0.99,

91

respectively. The pore size distribution and density functional theory (DFT) surface area were

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derived from the entire nitrogen adsorption-desorption isotherm using a two-dimensional 5 ACS Paragon Plus Environment


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nonlocal density functional theory (2D-NLDFT) method. Contact angles were measured using

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the sessile drop method by dropping 7 µL of deionized water on the electrodes. The contact

95

angles were analyzed by using ImageJ software. The surface morphology of the electrode

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material was analyzed by a scanning electron microscope (SEM, Phenom Pro, PhenomWorld) at

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an acceleration voltage of 15 kV. The surface elemental distribution was mapped by using

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energy-dispersive X-ray spectroscopy (EDX). The functional groups of the G5-PTFE electrode

99

material were determined with a Fourier-transform infrared spectroscopy (FTIR, Nicolet iS50,

100

Thermo Fisher Scientific,). For the FTIR analysis, the electrode material was diluted in

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potassium bromide (KBr) to ~2 wt% and the mixture was ground in a mortar pestle before

102

analysis. The FTIR spectrum was obtained using an attenuated total reflection (ATR) accessory

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from 4000 cm-1 to 400 cm-1.

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2.3 Adsorption and regeneration

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The fresh and regenerated G5-PTFE electrodes were used for MO (C14H14N3NaO3S, Ricca

106

Chemical Company) adsorption. MO was chosen as the adsorbate because it is a typical

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refractory organic pollutant likely to be irreversibly adsorbed on the carbon material and easily

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detected by ultraviolet/visible (UV/Vis) spectrophotometry.41-44 In a typical experiment, the

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electrode was put in 100 mL of ~300 mg L-1 MO solution at pH 7 and oscillated in a shaker

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(VWR Symphony 5000I Shaker, Henry Troemner LLC) at 25 °C and 250 rpm for 2 days. During

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the adsorption process, ~0.5 mL of MO solution was taken out from the system, diluted to an

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appropriate concentration, and then analyzed by a UV-Vis spectrometer (EVOLUTION 220,

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Thermo scientific). The concentration of MO solution at different times was determined to obtain

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the adsorption kinetics of the electrodes. The amount of adsorbed MO on the electrodes at


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different times was calculated based on the mass of G5 in the electrode, rather than the total

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electrode mass (no adsorption occurred on the PTFE binder).

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After adsorption, the MO contaminated G5-PTFE electrode was installed in the EF reactor

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(Figure S1, Supplementary Information) as a cathode. Platinum (Pt) wire (10 cm in length and

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0.25 mm in diameter, Sigma-Aldrich) and an Ag/AgCl electrode (gel reference electrode, +199

120

mV vs NHE, Pine Research Instrumentation Inc.) acted as anode and reference electrode,

121

respectively. The regeneration experiment was conducted in 100 mL of 0.05 M Na2SO4 (≥99.0%,

122

Sigma-Aldrich) solution at an initial pH of 3, which was maintained at 2.8-3.2 after regeneration

123

as measured by a pH meter (Lab 850, SCHOTT Instruments). Approximately 1 mM of FeSO4 (≥

124

99.0%, Anachemia) was added into the electrolyte as the catalyst for the EF reaction. The

125

experiments were run under potentiostatic mode at a cathodic potential of -0.8 V (vs Ag/AgCl),

126

controlled by a potentiostat (SI 1287, Solartron), for 8 h with continuous stirring and ~70 mL

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min-1 air flow. After regeneration, the G5-PTFE electrodes were used for a second adsorption

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experiment under the same conditions as the first adsorption.

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The regeneration efficiency was defined using the following equation:

regeneration efficiency % =

× 100%

(1)

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where is the adsorbed amount of regenerated carbon, and is the adsorbed amount of fresh

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carbon under the same conditions as regenerated carbon adsorption. Although there are

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alternatives,45 this equation is the most widely applied in the literature.

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In addition to the above experiments, a set of electrochemical oxidation experiments was carried

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out with as prepared cathodes and aqueous solutions of MO. Specifically, the as prepared

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electrodes, Pt wire, and Ag/AgCl electrode were served as cathode, anode, and reference 7 ACS Paragon Plus Environment


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electrode, respectively. ~25 mg L-1 of MO was put in 100 mL of 0.05 M Na2SO4 solution with 1

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mM of FeSO4 as catalyst, and the solution pH was adjusted to 3. A constant potential of -0.8 V

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(vs Ag/AgCl) was applied on the cathode during EF oxidation of MO. ~3 mL of solution was

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taken out at different times and analyzed by the UV-Vis spectrometer, and then transferred back

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to the reactor.

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2.4 Regeneration cycles

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The G5-PTFE-7-1 was applied for several adsorption-regeneration cycles with or without

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changing the electrolyte in each cycle. For the first set of cycles, the adsorption experiments

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continued for 2 days, and the regeneration was conducted by supplying fresh electrolyte for each

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cycle. For second set of cycles, the adsorption experiments lasted for 8 h, and the electrolyte was

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stored and then reused for the next regeneration cycle.

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3 Results and Discussions

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3.1 Characterization of AC-PTFE electrodes

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The pore structures of the G5-PTFE electrode materials were characterized by the nitrogen

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adsorption and desorption isotherms (Figure 1a). All the G5-PTFE electrode materials had

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similarly shaped nitrogen adsorption-desorption isotherms, but the nitrogen adsorption capacity

153

decreased with increasing PTFE content. Correspondingly, the pore size distributions of these

154

materials (Figure 1b) were similar with micropores centered at ~1.0 nm and some mesopores at

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~3.5 nm, ~10 nm, and ~15 nm. The peak at ~1.0 nm shifted to smaller pore widths as the PTFE

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content increased, suggesting partial blockage of some of the smallest pores.


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Table 1 contains the surface area, pore volume and contact angles for the G5-PTFE electrode

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materials using both total mass and G5 mass as a basis. The BET surface area, DFT surface area,

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micropore volume, and total pore volume decreased as the amount of PTFE in the electrode

160

increased. For example, the BET surface areas (based on total mass) decreased from 770 m2 g-1

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for sample G5-PTFE-7-1 to 393 m2 g-1 for sample G5-PTFE-1-1, while the total pore volumes

162

changed from 0.49 cm3 g-1 to 0.25 cm3 g-1, respectively. The values based on the G5 mass,

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changed less between samples (Table 1), indicating that the PTFE was non-porous. The pore

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volume decreased by 12% from sample G5-PTFE-7-1 to sample G5-PTFE-1-1, consistent with

165

the blockage of some pores.

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Table 1 also includes the contact angles measured by the sessile drop method (Figure S2). The

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contact angle of water on sample G5-PTFE-1-1 was similar to the value (120-126°) of pure

168

PTFE material as reported in the literature.46 This electrode as well as sample G5-PTFE-2-1 were

169

hydrophobic (i.e., contact angles > 90°).47 The contact angles decreased as the PTFE content

170

decreased, which was consistent with PTFE lowering the affinity of the electrodes for water. The

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contact angle of sample G5-PTFE-7-1 was 47°, which was similar to the values reported for

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AC48 and AC-PTFE (10:1 weight ratio).39 The contact angles were higher than those reported by

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Dong et al., who calcined their materials at lower temperatures for shorter times.39

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Figure S3 shows the SEM and EDX images of the G5-PTFE electrode materials. The SEM

175

images show angular particles roughly ~10-40 µm in length with aggregates attached to some of

176

the particles. The EDX analysis identified the aggregates as PTFE and that PTFE was well

177

distributed on the G5.

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In the FTIR spectra of the G5-PTFE electrode materials (Figure S4), no peaks were observed

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from wavenumbers of 4000 cm-1 to 2250 cm-1 for all materials, indicating a lack of hydrogen 9 ACS Paragon Plus Environment


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related functional groups such as C-H, O-H, and N-H.49 The peaks at 2150 cm-1 ~2000 cm-1, and

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1560 cm-1 may be assigned to C≡C, C=C, and benzene ring C=C stretching,49, 50 separately,

182

which are the basic structures of the G5 AC. Consequently, the intensity of these peaks

183

decreased with an increase in PTFE content. The broad peak within 960-1270 cm-1 might be

184

ascribed to the C-C bond.49 The peak at 1150 cm-1 corresponded to the C-F stretching vibration

185

from the PTFE molecules (Figure S5).49, 51 The absence of peaks from 1600-1900 cm-1 and

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sharp peaks at ~1000 cm-1 indicated a lack of oxygen functional groups, C=O and C-O,49, 52 on

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G5 and the prepared G5-PTFE materials. The FTIR spectra implied that the electrode preparation

188

method did not add functional groups to the surface of the G5 AC even though the mixture was

189

calcined in air at 350 °C for 1 h.

190 191

3.2 Hydrogen peroxide generation

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The kinetics of electrochemical generation of H2O2 by different G5-PTFE cathodes were

193

measured (Figure 2). The curves reached or were approaching plateau values, which varied

194

between ~14 mg L-1 for G5-PTFE-7-1 and >75 mg L-1 for G5-PTFE-1-1. At longer times,

195

decomposition of H2O2 on the anode may occur.53 The H2O2 concentration at 2 h decreased with

196

the decrease in PTFE content and had similar values when the G5: PTFE ratios were over 3. The

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highest H2O2 concentration of 79 mg L-1 was obtained with a G5: PTFE ratio of 1:1 after 2 h

198

reaction at -0.8 V cathodic potential. This value was similar to the accumulated H2O2

199

concentration (~75 mg L-1) produced by a commercialized gas diffusion electrode under similar

200

conditions.54 As shown in Figure 2b, a lower concentration of H2O2 (< 20 mg L-1) was generated

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when the electrodes became hydrophilic. The low H2O2 concentration produced by cathodes with

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higher carbon-PTFE ratio was also observed by Zhou et al. for a graphite-PTFE cathode.55 The 10 ACS Paragon Plus Environment

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authors ascribed cathode flooding and low gas diffusion as the reasons for lower H2O2

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production on hydrophilic cathodes.55

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3.3 Electro-Fenton oxidation of methyl orange in solution

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The G5-PTFE electrodes were used as cathodes for the EF oxidation of MO in aqueous solutions

208

(not adsorbed on the electrode) at a cathodic potential of -0.8 V (vs Ag/AgCl) to confirm that

209

these electrodes had activity for the decomposition of MO. The change in MO concentration

210

over time is shown in Figure 3. With each of the cathodes, > 95% of the initial MO was

211

removed within 2 h. Similar to other EF oxidation studies,56, 57 the kinetics data were well fit by

212

the pseudo first-order model (Text S1) with rate constants between 0.025 min-1 and 0.098 min-1

213

(Table S1). Sample G5-PTFE-2-1 had the highest rate constant, which was similar to those in

214

the literature under similar conditions for PTFE based cathodes,55, 58 while sample G5-PTFE-7-1

215

had the lowest rate constant, of similar value to that of an air diffusion AC packed electrode at an

216

applied current of 50 mA.37

217

These experiments demonstrated that the electrodes were active for EF degradation of MO in

218

solution. Thus, any MO desorbed during regeneration of contaminated electrodes (Sections 3.4

219

and 3.5) could be decomposed in solution. For EF oxidation in solution, the less hydrophilic

220

electrodes performed better (higher rate constants). The inconsistent trend of pseudo first-order

221

rate constants with G5: PTFE ratios (Table S1) may be due to the different H2O2 generation,

222

Fe2+ regeneration, and adsorption rates on the cathodes. Therefore, optimization of the entire

223

system will be important.

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3.4 Methyl orange adsorption before and after electro-Fenton regeneration

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The adsorption kinetics of the G5-PTFE electrodes before and after EF regeneration are shown in

227

Figure 4. With increasing G5 to PTFE ratio, the adsorption rate and amount adsorbed increased

228

for both fresh and regenerated G5-PTFE cathodes. After 2 days, sample G5-PTFE-7-1 had the

229

highest amount of adsorbed MO - 176 mg g-1 (normalized by the carbon mass in the cathode) -,

230

which was 76% of the saturation adsorption capacity (240 mg g-1) of a pure G5 sample (Text S2).

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Sample G5-PTFE-1-1 had the lowest amount adsorbed - 23 mg g-1 after 2 days. The low MO

232

adsorbed of samples G5-PTFE-1-1 and G5-PTFE-2-1 indicated that they were not suitable as

233

adsorbents for MO removal from water. Reasonable amounts of MO were adsorbed (> ~100 mg

234

g-1) after two days with the hydrophilic (Table 1) electrodes (i.e., those with G5: PTFE ratios

235

larger than 3).

236

The Weber-Morris diffusion model59

= . +

(2)

237

where qt is the adsorbed amount of MO at adsorption time t, kW is the Webber-Morris diffusion

238

rate constant, and W is the constant, was applied to the adsorption kinetics. As indicated in

239

Figure 4, the MO adsorption kinetics data of G5-PTFE-7-1 was not well fit by the one-step

240

Weber-Morris model (Figures 4a and b) but was better fit by the two-step Weber-Morris model

241

(Figures 4c and d). This result indicated that the MO adsorption was controlled by interparticle

242

diffusion initially (0-8 h) and then by intraparticle diffusion at longer times (8-48 h) for the G5-

243

PTFE-7-1 electrode.60 The MO adsorption on the other electrodes, however, remained within the

244

interparticle diffusion control region. The Weber-Morris interparticle diffusion rate constant

245

increased with decreasing PTFE content in the electrode. As indicated in the SEM images


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(Figure S3), PTFE was located on the external surfaces of the G5 particles and would affect the

247

transportation of water and water soluble species between the G5 particles.39

248

The interparticle diffusion rate constants for the different electrodes increased linearly with the

249

hydrophilicity of the electrodes (represented by the cosine of the contact angle) as shown in

250

Figure 5. The MO molecules were dissolved in the solution and therefore travelled within the

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pores with water molecules. The penetration of water into a packed powder sample is controlled

252

by capillary forces and has a linear relationship with the square root of time (√) as indicated by

253

the Washburn equation,40

! = "√cos $

(3)

254

where ! is the mass of a penetrating liquid into the packed powder sample at time , cos $

255

represents the wettability of the packed powder sample, and " is a constant related to packing

256

parameters of the powder sample. Thus, the penetration, or diffusion, of water into the electrodes

257

is likely determined by the electrode hydrophilicity and controlled the MO adsorption rate (per

258

gram of G5). Water cannot penetrate a hydrophobic material according to the Washburn

259

equation,40 but it has been reported that a hydrophobic surface can still adsorb water because of

260

the defects at the atomic/nanometer scales.61 Nevertheless, the hydrophilicity of the electrodes

261

did affect the MO adsorption rate as shown in Figure 5.

262

The hydrophilicity also affected the EF regeneration as shown in Figure 5. After regeneration,

263

the adsorbed MO decreased (Figure 4), but the regeneration efficiency increased with increasing

264

electrode hydrophilicity. The highest regeneration efficiency of 81% was obtained with the G5-

265

PTFE-7-1 electrode, while the G5-PTFE-1-1 electrode had the lowest regeneration efficiency of

266

49%. From this minimum, the regeneration efficiency increased by 21% to 70% with a decrease



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in the contact angle from 121 ± 7° to 88 ± 3°. After the electrode became hydrophilic, however,

268

the regeneration efficiency only increased by 11% from 70% to 81% when the contact angle

269

decreased from 88 ± 3° to 47 ± 2°.

270

The regeneration efficiency might also be affected by the electrical conductivity of the G5-PTFE

271

electrodes due to different G5: PTFE ratios. According to the electro-desorption mechanism, less

272

potential drop would be applied for the desorption of ions if the electrode was less conductive.62,

273

63

274

activated carbon-PTFE electrodes were similar for PTFE contents of 20-40% for palm shell AC

275

62

276

G5-PTFE-1-1, were within these ranges. Thus, the differences in regeneration efficiency were

277

likely not because of changes in electrode conductivity.

278

As indicated by the results, although G5-PTFE-7-1 had low H2O2 production rate (Figure 2) and

279

low removal rate for MO in the electrolyte (Figure 3), the highest regeneration efficiency was

280

obtained by this electrode (Figure 5). In many cathodic regeneration processes, including

281

cathodic polarization, electro-peroxone, and EF regeneration, electrodesorption, from high local

282

pH or surface polarization, was proposed to be the main mechanism for regeneration,7, 19, 29 and

283

also demonstrated by our recent study on EF regeneration. As a homogeneous electro-catalytic

284

process by using the hydroxyl radicals produced in the bulk,13 the oxidation of organic pollutants

285

mainly happened in the electrolyte during EF oxidation. Thus, the regeneration of G5-PTFE-7-1

286

was not limited by the H2O2 generation and EF oxidation. Instead, the diffusion of MO within

287

the electrodes controlled the regeneration process, which was consistent with the

288

electrodesorption mechanism of cathodic regeneration.

According to the impedance spectra reported in the literature, however, the conductivities of

and 8-25% for a commercial AC.35 The PTFE contents of the electrodes in this study, except

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3.5 Regeneration cycles

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As indicated above, the hydrophilicity, MO adsorption rate, and regeneration efficiency of the

292

G5-PTFE-7-1 electrode were the highest among all the electrodes. Therefore, this electrode was

293

employed for several adsorption-regeneration cycles with adsorption over 2 days followed by EF

294

regeneration. As shown in Figure 6a, the amount adsorbed after 2 days decreased from 176 mg

295

g-1 in the first cycle to 143 mg g-1 in the second cycle and to 112 mg g-1 in the fifth cycle. The

296

calculated regeneration efficiencies were 81%, 74%, 71%, and 64%, respectively.

297

The continuous decrease in amount adsorbed and regeneration efficiency could indicate that

298

some organic pollutants remained on the electrode after each regeneration or that the electrode

299

structure was changing. The first possibility was tested by a saturation experiment (as described

300

in SI text S2). Before the fourth regeneration, the adsorption experiment was extended to 6 days

301

to allow the electrode to reach saturation. An additional 54 mg g-1 was adsorbed with a total

302

adsorption of 178 mg g-1. In the fifth adsorption, the adsorption capacity was only 166 mg g-1

303

after 6 days, consistent with only partial regeneration. On the other hand, the electrode structure

304

was not changed both in macroscopic and microscopic range (Figure S6), which was also

305

demonstrated in other EF regeneration research.19, 21 The results indicated that the incomplete

306

regeneration of the electrodes was likely due to the remaining adsorbates.

307

The adsorption kinetics (23-26 mg g-1 h-0.5) and amount adsorbed (65-72 mg g-1) on the

308

regenerated electrodes from the 4 regeneration cycles, however, were similar to each other

309

within the first 8 h. After this time, the curves deviated, which the fraction of sites occupied in

310

the first 8 h being easily accessible and regenerated. After these adsorption and regeneration

311

cycles, the total amount of MO removed from solution was 740 mg g-1 based on the G5 mass,

312

which was 3.1 times the saturation adsorption capacity of G5. During these processes, only 1829 15 ACS Paragon Plus Environment


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C of applied charge was consumed (Table S2), which was much lower than reported in the

314

literature.29, 64

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The electrolyte was reused for the G5-PTFE-7-1 electrode (Figure 6b) with adsorption time of 8

316

h (rather than 2 days) because the initial MO loading had a marginal impact on regeneration

317

(Text S3 and Figure S10). As shown in Figure 6b, the MO adsorbed decreased gradually from

318

82 mg g-1 in the second adsorption cycle to 66 mg g-1 in the fifth adsorption cycle. The

319

corresponding regeneration efficiencies were 84%, 77%, 72%, and 68%, respectively, for the

320

first four cycles. These values were similar to those obtained when fresh electrolyte was supplied

321

for each regeneration.

322

The regeneration conditions were varied to recover more adsorption sites. As indicated in Figure

323

6b, the MO adsorbed did not increase by increasing the cathodic potential to -1.6 V and

324

regenerating for 4 h in the fifth and sixth regeneration cycles. On the contrary, the MO adsorbed

325

decreased significantly to 42 mg g-1. After these regeneration cycles, red deposits were visible on

326

the cathode surface (Figure S7), which may be the precipitation of Fe(OH)3 due to the high local

327

pH at high cathodic potential.13, 54, 65, 66 These iron species deposits could block the pores and

328

result in a lower adsorption. The iron hydroxide precipitate had a low azo dye adsorption

329

capacity.66

330

The MO adsorbed was recovered to 64 mg g-1 after the seventh regeneration cycle, however,

331

under the typical conditions (i.e. -0.8 V for 8 h). The MO adsorbed decreased again after the

332

eighth regeneration cycle under a cathodic potential of -3.0 V for 0.5 h because of iron

333

precipitation (Figure S7). Further regeneration under the cathodic potential of -0.8 V did not

334

recover the adsorption of the G5-PTFE-7-1 electrodes in the ninth, tenth, and eleventh

335

regeneration cycles, even with longer regeneration time. The UV-Vis spectra of the electrolyte 16 ACS Paragon Plus Environment

Page 17 of 31


336

after each regeneration cycle are shown in Figure S8. The UV-Vis absorbance at 200 – 350 nm

337

increased continuously in the first four regeneration cycles, indicating the incomplete

338

degradation of MO and accumulation of desorbed organic pollutants. Even though the

339

regeneration efficiency was not improved, an increase in the cathodic potential and regeneration

340

time reduced the remaining organic pollutants in the solution (Figure S8b, d). Nonetheless, the

341

amount adsorbed was greater than 40 mg g-1 after 12 adsorption-regeneration cycles without

342

changing the electrolyte.

343

An equivalent amount of MO was removed by one use of G5 powdered carbon or 3 adsorption

344

cycles with the G5-PTFE-7-1 electrode. After 11 cycles with the same batch of electrolyte, this

345

electrode has removed 688 mg g-1, requiring 5673 C of charge (Table S3), which was similar to

346

the results reported in the literature for just one cathodic regeneration cycle with a regeneration

347

efficiency of ~75%.28, 64

348 349

3.6 Environmental implications

350

As indicated in many electrochemical processes for the treatment of contaminated water, the

351

modular design of the electrochemical reactor was one of the benefits to apply these

352

technologies.67 In this study, an adsorption and EF regeneration system, which could be part of a

353

modular design, was proposed for the removal of organic pollutants from water. The results were

354

all based on batch adsorption experiments, which had limitations of incomplete adsorption and

355

small treatment capacity, but this technology may be applied in a flow-through adsorption

356

system with proper electrode design after further investigation. Unlike most conventional

357

cathode materials for EF oxidation that had low adsorption capacity for organic pollutants, the



Page 18 of 31

358

prepared AC-PTFE electrodes had relatively high adsorption capacities and were prepared from

359

inexpensive materials. After adsorption, the electrodes can be easily removed from the water

360

without any complicated separation process, and then put in an electrochemical reactor for EF

361

regeneration. High adsorption (176 mg g-1) and EF regeneration efficiency (81%) could be

362

obtained for hydrophilic electrodes (Figures 4 and 5). The electrodes can be regenerated for

363

several cycles (Figure 6) with low applied charge consumption (< 2000 C for 3 times fresh

364

carbon adsorption capacity) (Tables S2), and the electrolyte could be reused. Thus, through the

365

use of an inexpensive AC-PTFE electrode, the organic pollutants in contaminated water can be

366

concentrated and then decomposed in an electrochemical reactor.

367 368

Supporting information: Pseudo first-order kinetics model, equilibrium adsorption experiments,

369

effect of MO amount adsorbed on regeneration, scheme of the EF reactor, characterization of

370

G5-PTFE electrodes (contact angle, FTIR, SEM-EDX), UV-Vis spectra of electrolytes during EF

371

oxidation and EF regeneration, picture of iron species deposition, and applied charge of different

372

regeneration cycles.

373 374

Acknowledgement

375

The authors gratefully acknowledge funding from the Canada Research Chairs program for

376

equipment and operating funds, and the Department of Chemical & Petroleum Engineering,

377

University of Calgary for scholarships (Y.X.).

378


Page 19 of 31


379

Reference

380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423

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33. Le, T. X. H.; Esmilaire, R.; Drobek, M.; Bechelany, M.; Vallicari, C.; Cerneaux, S.; Julbe, A.; Cretin, M., Nitrogen-doped graphitized carbon electrodes for biorefractory pollutant removal. J. Phys. Chem. C 2017, 121, (28), 15188-15197. 34. Wang, A.; Qu, J.; Ru, J.; Liu, H.; Ge, J., Mineralization of an azo dye Acid Red 14 by electro-Fenton's reagent using an activated carbon fiber cathode. Dyes Pigm. 2005, 65, (3), 227233. 35. Dong, H.; Yu, H.; Wang, X.; Zhou, Q.; Feng, J., A novel structure of scalable air-cathode without Nafion and Pt by rolling activated carbon and PTFE as catalyst layer in microbial fuel cells. Water Res. 2012, 46, (17), 5777-5787. 36. Dong, H.; Yu, H.; Wang, X., Catalysis kinetics and porous analysis of rolling activated carbon-PTFE air-cathode in microbial fuel cells. Environ. Sci. Technol. 2012, 46, (23), 1300913015. 37. Bañuelos, J. A.; El-Ghenymy, A.; Rodríguez, F. J.; Manríquez, J.; Bustos, E.; Rodríguez, A.; Brillas, E.; Godínez, L. A., Study of an air diffusion activated carbon packed electrode for an electro-Fenton wastewater treatment. Electrochim. Acta 2014, 140, 412-418. 38. Zarei, M.; Salari, D.; Niaei, A.; Khataee, A., Peroxi-coagulation degradation of C.I. Basic Yellow 2 based on carbon-PTFE and carbon nanotube-PTFE electrodes as cathode. Electrochim. Acta 2009, 54, (26), 6651-6660. 39. Dong, H.; Yu, H.; Yu, H.; Gao, N.; Wang, X., Enhanced performance of activated carbon–polytetrafluoroethylene air-cathode by avoidance of sintering on catalyst layer in microbial fuel cells. J. Power Sources 2013, 232, 132-138. 40. Alghunaim, A.; Kirdponpattara, S.; Newby, B.-m. Z., Techniques for determining contact angle and wettability of powders. Powder Technology 2016, 287, 201-215. 41. Xiao, Y.; Hill, J. M., Impact of pore size on Fenton oxidation of methyl orange adsorbed on magnetic carbon materials: Trade-off between capacity and regenerability. Environ. Sci. Technol. 2017, 51, (8), 4567-4575. 42. Darmograi, G.; Prelot, B.; Geneste, A.; De Menorval, L.-C.; Zajac, J., Removal of three anionic orange-type dyes and Cr(VI) oxyanion from aqueous solutions onto strongly basic anionexchange resin. The effect of single-component and competitive adsorption. Colloids Surf., A 2016, 508, 240-250. 43. Yao, Y.; Bing, H.; Feifei, X.; Xiaofeng, C., Equilibrium and kinetic studies of methyl orange adsorption on multiwalled carbon nanotubes. Chem. Eng. J. 2011, 170, (1), 82-89. 44. Mohammadi, N.; Khani, H.; Gupta, V. K.; Amereh, E.; Agarwal, S., Adsorption process of methyl orange dye onto mesoporous carbon material–kinetic and thermodynamic studies. J. Colloid Interface Sci. 2011, 362, (2), 457-462. 45. Narbaitz, R. M.; Cen, J., Alternative methods for determining the percentage regeneration of activated carbon. Water Res. 1997, 31, (10), 2532-2542. 46. Horsthemke, A.; Schröder, J. J., The wettability of industrial surfaces: Contact angle measurements and thermodynamic analysis. Chemical Engineering and Processing: Process Intensification 1985, 19, (5), 277-285. 47. Law, K.-Y., Definitions for hydrophilicity, hydrophobicity, and superhydrophobicity: Getting the basics right. The Journal of Physical Chemistry Letters 2014, 5, (4), 686-688. 48. García, A. B.; Martínez-Alonso, A.; Leon y Leon, C. A.; Tascón, J. M. D., Modification of the surface properties of an activated carbon by oxygen plasma treatment. Fuel 1998, 77, (6), 613-624.



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559 560 561 562 563 564


66. Almeida, L. C.; Garcia-Segura, S.; Arias, C.; Bocchi, N.; Brillas, E., Electrochemical mineralization of the azo dye Acid Red 29 (Chromotrope 2R) by photoelectro-Fenton process. Chemosphere 2012, 89, (6), 751-758. 67. Radjenovic, J.; Sedlak, D. L., Challenges and opportunities for electrochemical processes as next-generation technologies for the treatment of contaminated water. Environ. Sci. Technol. 2015, 49, (19), 11292-11302.

565 566



Page 24 of 31

567 568

TOC Art


Page 25 of 31


(a)

(b)

569

Figure 1. The nitrogen adsorption and desorption isotherms (a) and pore size distributions (b) of

570

G5-PTFE electrode materials: G5 (◄), G5-PTFE-7-1 (♦), G5-PTFE-5-1 (▼), G5-PTFE-3-1(▲),

571

G5-PTFE-2-1 (●), and G5-PTFE-1-1 (■).



(a)

Page 26 of 31

(b)

572

Figure 2. The H2O2 production kinetics (a) of different AC-PTFE electrode at applied cathodic

573

potential of -0.8 V (vs Ag/AgCl) and (b) the relationship between H2O2 concentration at 2 h and

574

hydrophilicity (represented by cosθ). Experiments were carried out in 100 mL of 0.05 M Na2SO4

575

electrolyte at pH 3.0 under room temperature (~20 °C).

576


Page 27 of 31


577 578

Figure 3. Electro-Fenton oxidation kinetics of a MO aqueous solution with fresh (i.e., not

579

saturated or previously exposed to MO) G5-PTFE cathodes. Reaction conditions: initial pH of

580

3.0, 0.05 M Na2SO4 as electrolyte, 1 mM FeSO4 as catalyst, at cathodic potential of -0.8 V (vs

581

Ag/AgCl), air flow rate of ~70 mL min-1, at room temperature (~20 °C).



Page 28 of 31

(a)

(b)

(c)

(d)

582

Figure 4. MO adsorption kinetics for fresh (a) and regenerated (b) G5-PTFE electrodes with

583

corresponding fits to the one-step Webber-Morris model (lines), and for the G5-PTFE-7-1 (c)

584

fresh and (d) regenerated electrode fit to the two-step Webber-Morris model. Adsorption

585

conditions: 100 mL of ~300 mg L-1 MO solution, 25 °C, and 250 rpm, for maximum 2 days. The

586

adsorbed amount was normalized by the mass of carbon in the electrodes. Regeneration was

587

carried out in 100 mL 0.05 M Na2SO4 solution in presence of 0.1 mM Fe2+ for 8 h under room

588

temperature (~20 °C) by controlling the cathodic potential of -0.8 V (vs Ag/AgCl).


Page 29 of 31


589 590

Figure 5. The relationships of Webber-Morris intraparticle diffusion rate constant as well as

591

regeneration efficiency with the hydrophilicity (represented by cos(theta)) of the G5-PTFE

592

electrodes. Solid lines are linear fits of intraparticle diffusion rate constant.



(a)

Page 30 of 31

(b)

593

Figure 6. Adsorption-regeneration cycles of G5-PTFE-7-1 with (a) or without (b) supplying fresh

594

electrolyte. In (a), adsorption was carried out for 2 days, while in (b), adsorption was carried out

595

for 8 h; unless otherwise indicated, the regeneration was carried out at a cathodic potential of -

596

0.8 V for 8 h, and the electrolyte was reused for every regeneration cycle.


Page 31 of 31

597


Table 1. The pore structure properties of the G5-PTFE electrode materials. AC-PTFE

BET surface

DFT surface

Micropore

Total volume

Contact

electrodes

area (m2 g-1)

area (m2 g-1)

volume (cm3 g-1)

(cm3 g-1)

angle (°)

(a)*

(b)**

(a)

(b)

(a)

(b)

(a)

(b)

G5-PTFE-1-1

393

786

328

656

0.14

0.28

0.25

0.50

121 ± 7

G5-PTFE-2-1

565

852

474

715

0.20

0.30

0.36

0.54

118 ± 7

G5-PTFE-3-1

638

851

498

664

0.22

0.29

0.40

0.53

88 ± 3

G5-PTFE-5-1

708

851

564

678

0.25

0.30

0.45

0.54

86 ± 7

G5-PTFE-7-1

770

880

580

663

0.26

0.30

0.49

0.56

47 ± 2

0.59

-

G5

918

704

598

* based on the total mass of G5-PTFE

599

**based on the mass of G5 in the G5-PTFE

0.31


Benefit of hydrophilicity for adsorption of methyl orange and electro

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