Regeneration of Activated Carbon Fiber by the Electro-Fenton Process

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Remediation and Control Technologies

Regeneration of Activated Carbon Fiber by Electro-Fenton Process Clément Trellu, Nihal Oturan, Fanta Kaba Keita, Chloé Fourdrin, Yoan Péchaud, and Mehmet A. Oturan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01554 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Environmental Science & Technology

Graphical abstract

ACS Paragon Plus Environment


Regeneration of Activated Carbon Fiber by Electro-Fenton Process

Clément Trellu1, Nihal Oturan1, Fanta Kaba Keita1, Chloé Fourdrin1, Yoan Pechaud1, Mehmet A. Oturan1,*

1

Université Paris-Est, Laboratoire Géomatériaux et Environnement (EA 4508), UPEM, 77454 Marne-la-Vallée, France.

Manuscript submitted to Environmental Science and Technology for consideration

* Corresponding Author: Email: [email protected] (Mehmet A. Oturan) Phone: +33 149 32 90 65


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1

Abstract

2

An electro-Fenton (EF) based technology using activated carbon (AC) fiber as cathode

3

and BDD anode has been investigated for both regeneration of AC and mineralization of

4

organic pollutants. The large specific surface area and low intra-particle diffusion

5

resistance of AC tissue resulted in high maximum adsorption capacity of phenol (PH)

6

(3.7 mmol g-1) and fast adsorption kinetics. Spent AC tissue was subsequently used as

7

cathode during the EF process. After 6 h of treatment at 300 mA, 70% of PH was

8

removed from the AC surface. The effectiveness of the process is ascribed to (i) direct

9

oxidation of adsorbed PH by generated hydroxyl radicals, (ii) continuous shift of

10

adsorption equilibrium due to oxidation of organic compounds in the bulk, and (iii) local

11

pH change leading to electrostatic repulsive interactions. Moreover, 91% of PH removed

12

from AC was completely mineralized, thus avoiding adsorption of degradation by-

13

products and accumulation of toxic compounds such as benzoquinone. Morphological

14

and chemical characteristics of AC were not affected due to the effect of cathodic

15

polarization protection. AC tissue was successfully reused during 10 cycles of

16

adsorption/regeneration with regeneration efficiency ranging from 65 to 78%, in

17

accordance with the amount of PH removed from the AC surface.

18

Keywords

19

Electro-Fenton ; Activated carbon ; Regeneration ; Mineralization ; Phenol

20



21

Graphical abstract

22 23 24


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1. Introduction

26

The development of sustainable water treatment systems for the removal of

27

micropollutants is an important challenge for environmental engineering. Activated

28

carbon (AC) is currently widely used in water treatment plants because it has been

29

proven to be an effective adsorbent for removal of organic compounds from water.1–3 AC

30

is a microporous material with a large specific surface area and a large amount of

31

various surface functional groups.4 However, this process is only a separation step;

32

organic pollutants are not degraded and spent AC is a waste that needs to be treated.

33

The treatment must lead to both AC regeneration/reuse (in order to improve the

34

sustainability and cost-effectiveness of the adsorption process) and degradation of

35

organic pollutants (in order to avoid any environmental contamination). While the

36

effectiveness and mechanisms of adsorption of a large range of organic compounds onto

37

various AC materials have been already widely reported in the literature,3,5 there is still

38

a need to develop innovative and efficient processes for the regeneration of spent AC.

39

Thermal regeneration is the most widely used process. The efficiency tightly depends on

40

the nature of adsorbed organic compounds and nature of interactions with the AC

41

surface. Thermal treatment under non-oxidizing conditions often lead to low recovery of

42

AC adsorption capacity due to insufficient removal of chemisorbed compounds.6

43

Moreover, additional treatment is required for the degradation of desorbed pollutants.

44

Higher removal rates are achieved during thermal treatment under oxidizing conditions

45

but the microporous structure of AC is strongly affected by AC burning.6,7 Chemical

46

oxidation regeneration using for example ozone or Fenton’s reagent limits AC burning

47

but can also strongly affect chemical and textural characteristics of the surface.7 Besides,



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48

low regeneration efficiency is often observed for microporous AC and chemical

49

regeneration can be often applied only to mesoporous or non-porous materials.8–10

50

Recently, electro-Fenton (EF) process appeared as a promising solution for AC

51

regeneration. The continuous electro-generation of H2O2 from the 2-electron reduction

52

of O2 at the AC surface combined with the supply of a catalytic amount of iron (II)

53

continuously regenerated at the cathode allows for the formation of hydroxyl radicals

54

(•OH) (eq 1).11–13 A wide range of organic pollutants have been observed to be

55

completely mineralized using EF-based processes.11,13,14 It has also been demonstrated

56

that •OH are able to readily oxidize organic compounds adsorbed onto granular AC and

57

thus, to participate to AC regeneration and pollutant degradation.15 Moreover, Bañuelos

58

et al. (2015) observed that cathodic polarization of granular AC during EF protects the

59

surface from oxidation and can avoid the loss of adsorption capacity.7 However, further

60

development and scale-up of AC regeneration by EF is still impaired by engineering

61

aspects when using granular AC packed bed as cathode,7,15 because of ohmic drops

62

leading to highly heterogeneous potential distribution within these cathodes.

Fe + H O → Fe + ∙OH + OH

(k = 63 M-1 s-1)

(1)

63

As an alternative, the use of AC fiber (under the form of tissue or felt) instead of AC

64

packed bed might be more suitable and could greatly improve regeneration process.

65

This material presents unique characteristics compared with granular or powder AC.16

66

The thin-fiber shape reduces intra-particle diffusion resistance and gives to this material

67

mechanical and geometrical characteristics adapted for the design of electrochemical

68

reactors. Moreover, AC fiber is an effective material for H2O2 generation and adsorption

69

of organic pollutants during water treatment processes.17,18 Besides, we also proposed

70

to combine the EF process with anodic oxidation by using boron-doped diamond (BDD)


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71

as anode. Anodic oxidation is expected to promote the oxidation of adsorbed compounds

72

by mediated oxidation (generation of ozone, persulfate, sulfate radical)13,19 and to

73

enhance the mineralization of desorbed pollutants and degradation by-products by

74

oxidation with •OH generated at the surface of the BDD anode from water discharge (eq

75

2).19

M + H O → M ∙ OH + H + e

(2)

76

Thus, this study aimed at the assessment of the regeneration efficiency of AC fiber

77

(tissue or felt) during the EF process using BDD anode and the spent AC fiber as cathode.

78

By choosing phenol (PH) as model organic pollutant, the objectives of this study were to

79

assess (i) the adsorption capacity and kinetics of PH and main aromatic oxidation by-

80

products (hydroquinone (HQ), benzoquinone (BQ), catechol (CAT)) on AC fiber (ii) the

81

removal of PH from the surface of the spent AC fiber by the EF process (iii) the release in

82

the solution of PH and degradation by-products and their subsequent mineralization (iv)

83

the adsorption capacity and the characteristics of the regenerated material after 1 and

84

10 cycles of adsorption/regeneration. From the results obtained, the main mechanisms

85

involved in regeneration of AC fiber and mineralization of organic compounds have been

86

discussed.

87 88

2. Materials and methods

89

2.1. Chemicals. All chemicals were of reagent grade purchased from Acros Organics (PH

90

and iron (II) sulphate heptahydrated), Sigma Aldrich (HQ, BQ, CAT), methanol, sodium

91

sulphate) or Fluka (sulphuric acid). All solutions were prepared using ultrapure water

92

(resistivity>18.2 MΩ cm) from a Millipore Milli-Q system (Molsheim, France).



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2.2 Adsorption. Microporous AC tissue (Dacarb, France), prepared from phenolic resin,

94

was used as adsorption material. N2 adsorption isotherms were performed for

95

determination of BET surface area, total pore volume and pore size distribution (using

96

the two-dimensional nonlocal density functional theory method). Main characteristics of

97

the material are presented in Table 1. Some experiments were also performed using AC

98

felt prepared from phenolic resin (Dacarb, France) with different morphological

99

characteristics but similar surface area and microporosity.

Thickness (mm)

Average pore size (nm)

BET surface 2 -1 area (m g )

Table 1 – Main characteristics of activated carbon tissue used during experiments

Weight -2 (g m )

100

90

0.5

0.82

1,306

Porous volume (cm3 g-1) - Pore size distribution Microporous Microporous Mesoporous Macroporous (20 nm) 65%

33%

1.7%

0.2%

Total 0.54

101 102

AC was first washed several times in deionized water and dried at 70 °C. The pH was set

103

at 3 for all adsorption experiments, according to the pH required for the EF regeneration

104

step. Control tests without AC showed that less than 3% of PH was lost by volatilization

105

or adsorption on glass after 24 h.

106

Equilibrium adsorption experiments were performed at room temperature (20 °C) with

107

single compounds in 500 mL glass bottles continuously agitated during 24 h in a rotary

108

shaker set at 20 rpm. For isotherm experiments, 250 mL of PH (1 mM), BQ (0.5 mM),

109

CAT (0.8 mM) or HQ (0.9 mM) were mixed with various AC concentrations from 0.08 to

110

1 g L-1. The most widely used models, Langmuir (eq 3) and Freundlich (eq 4), were

111

applied for data analysis.


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112


=

(3)

1+

113

where qe is the amount of solute adsorbed per unit weight of AC at equilibrium, qm is the

114

maximum adsorption capactity (mmol g-1), KL is a constant related to the free energy of

115

adsorption (L mmol-1) and Ce is the concentration of solute in the bulk solution at

116

equilibrium (mmol L-1).

117

= 1/

(4)

118

where KF and n are constants related to adsorption capacity and intensity, respectively.

119

Spent AC used for EF regeneration experiments were obtained by mixing 250 mL of 11

120

mM PH with 500 mg of AC (2 g L-1).

121

Dynamic adsorption experiments were performed with single compounds and using

122

similar configuration than during electrochemical regeneration in order to ensure the

123

same hydrodynamic conditions. Initial concentrations of AC (2 g L-1) and organic

124

compounds ([PH] = 1 mM; [HQ] = [CAT] = 0.1 mM; [BQ] = 0.05 mM) were chosen in

125

accordance to the experimental conditions observed during the EF regeneration step.

126

Data were analyzed using both pseudo-first-order (eq 5) and pseudo-second-order

127

model (eq 6)

128

ln − = ln −

!"

(5)

129

where qt is the amount of solute adsorbed per unit weight of AC at time t and k1 is the

130

first-order rate constant.

131

#$

=

! %& #'&

+

(6)

#'



132

where k2 is the second-order rate constant.

133 134

2.3. EF regeneration of AC. Electrochemical regeneration of spent AC fiber was

135

performed in batch mode using an open, cylindrical and undivided electrochemical cell,

136

similarly to the setup previously described by Trellu et al. (2016).20 500 mg of spent AC

137

(55 cm2 x 0.5 mm) was used as cathode. The AC was not treated in any way between

138

adsorption and regeneration steps. The anode was made of a thin film of BDD deposited

139

on a Nb substrate (24 cm2 × 0.2 cm, Condias Gmbh, Itzehoe, Germany). Electrodes were

140

set up face to face with a gap of 3 cm between the anode and the cathode. AC cathode

141

was fixed in the electrochemical cell by using a teflon grid. Oxygen supply for hydrogen

142

peroxide production was ensured by continuous air bubbling through a glass frit.

143

Following optimal experimental conditions for the EF process determined by other

144

authors from our research group, 0.05 M Na2SO4 (electrolyte) was dissolved in milli-Q

145

water, pH was adjusted at 3.0 with 1 M H2SO4 and 0.1 mM Fe2+ (catalyst) was added to

146

the solution.21 Continuous agitation was ensured by magnetic agitation and air bubbling.

147

Constant current supply was provided by a power supply (HAMEG, model 7042-5,

148

Germany) set at 300 mA as soon as the spent AC cathode was immersed in the

149

electrolyte. This corresponds to 12.5 mA cm-2 as current density by considering BDD as

150

working electrode. Current density was determined for optimizing H2O2 and OH

151

generation and minimizing secondary reactions such as oxygen and hydrogen

152

evolution.11

153


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154

2.4. Analytical methods. Chemical analyses were performed in order to follow the

155

evolution of the concentration of PH and degradation by-products in the bulk and

156

adsorbed on AC fiber. Aqueous samples (1 mL) were periodically collected from the bulk

157

during the treatment, while the analysis of organic compounds adsorbed on AC required

158

to stop the experiment in order to perform a desorption step. The AC fiber used as

159

cathode was completely immersed in a solution of 90% ethanol - 10% 1M NaOH and set

160

during 30 min in an ultrasound bath. After mixing under magnetic agitation during

161

additional 30 min, an aliquot was collected and analyzed. Different authors observed

162

that such conditions effectively remove adsorbed organic compounds from AC

163

surface.22,23 Preliminary experiments showed that >97% of adsorbed PH was desorbed

164

and recovered after repeating two times this procedure.

165

PH and aromatic by-products were identified and quantified by reverse phase HPLC,

166

while carboxylic acids were identified and quantified by ion-exclusion chromatography.

167

Conditions of analysis were similar to the study of Pimentel et al. (2008).24 The

168

mineralization rate of PH was followed by measuring the total organic carbon (TOC)

169

with a Shimadzu TOC-V analyzer.

170 171

2.5. Material characterization. A scanning electron microscope (Phenom XL,

172

PhenomWorld, The Netherlands) was used for analyzing the surface morphology of the

173

AC tissue. Since AC is conductive, none surface treatment was necessary prior analysis.

174

Raman measurements were carried out on a Renishaw INVIA spectrometer equipped

175

with a microscope and a CCD detector (LGE, France). Details are given in SI (Text SI 1,

176

Figures SI 6 and 7).



177 178

3. Results and discussion

179 180

3.1. Sorption of phenol and main aromatic oxidation by-products onto AC tissue

181 182

The first step of this study was to determine the adsorption behavior of PH and main

183

aromatic oxidation by-products on AC tissue. Adsorption isotherms of PH, BQ and CAT

184

are presented in Table 2. As reported by previous studies, a classical L shape adsorption

185

isotherm was obtained for all compounds (Figure SI 1).2,25,26 Both Langmuir and

186

Freundlich equations are applicable but slightly higher correlation coefficients are

187

obtained using Langmuir equation for the three compounds, indicating that assumptions

188

on which the Langmuir model is based are the most suitable for this material

189

(adsorption of a monolayer of solutes onto a homogeneous adsorbent surface with

190

uniform energies of adsorption).25,26 Maximum adsorption capacity of PH (3.73 mmol g-

191

1)

192

due to the higher BET surface area (1,326 vs 929 m2 g-1) as well as microporous

193

structure of AC fiber since adsorption energy is enhanced within small size pores.

194

Besides, according to the literature, average pore size must be above 1.2 times27 or 1.7

195

times28 of the second widest dimension of the adsorbate molecule (molecular size of PH:

196

0.57 x 0.42 nm) in order to allow for effective adsorption.29 Therefore, low steric

197

hindrance effect on ultimate uptake of PH is expected in this study because this ratio is

198

2.0.27–29 Compared to PH, aromatic oxidation by-products (BQ and CAT) had much lower

199

adsorption capacity on AC tissue (lower qm and KF values). This is consistent with the

200

lower hydrophobicity of hydroxylated by-products, which have lower tendency to be

was higher than previous results reported using granular AC (2.32 mmol g-1). This is


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201

adsorbed on the carbon surface. Physical adsorption has been reported to play the most

202

important role for adsorption of PH on AC fiber, particularly π-π interactions.29

203

Kinetic study showed that a large amount of PH can be rapidly adsorbed on AC tissue.

204

Similarly to what has been demonstrated by several previous studies, the rate-

205

determining step in the adsorption process of PH on AC is the intra-particle diffusion

206

(linear relation between qt and t1/2).29,30 The great advantage of AC tissue is that intra-

207

particle diffusion resistance is strongly reduced compared to granular AC thanks to the

208

open pore structure.29 In fact, AC tissue is made of thousands of thin fibers, which

209

strongly increase the external surface area. Much better correlation coefficients were

210

obtained by using the pseudo-second order model, compared to the pseudo-first order

211

model. Such behavior is often observed for the adsorption of low molecular weight

212

compounds on small adsorbent particles (i.e adsorbent with large external surface).31

213

Adsorption processes also obeys the pseudo-second order model when the initial

214

concentration of solute is sufficiently low.32 Experiments were performed by using

215

concentrations of PH (1 mM), CAT (0.1 mM) and BQ (0.05 mM) consistent with the

216

maximum concentrations observed during the regeneration step. Therefore, kinetic

217

parameters could not be directly compared since pseudo-first order and pseudo-second

218

order kinetic constants are complex functions of the initial concentration of solute.32

219

However, Wu et al.31 showed that k2qe (eq 6) is the inverse of the half-life of adsorption

220

process and is a key parameter for comparison of adsorption kinetics. Thus, from k2qe

221

values, it can be concluded that adsorption gets faster in the following order: PH > BQ >

222

CAT. PH adsorption kinetic on AC tissue was slower than a previously reported study,29

223

most probably because of the lower average pore size diameter affecting intra-particle

224

diffusion. Steric hindrance should not affect ultimate PH uptake but could reduce

225

adsorption kinetics. ACS Paragon Plus Environment


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During the adsorption of HQ on AC tissue, the release of BQ in the bulk was

227

simultaneously observed; then, BQ was also adsorbed according to the equilibrium

228

sorbent-sorbate (Figure SI 2). This can be explained by the oxidation of HQ by molecular

229

O2 bound to graphite edges.2 When increasing concentrations of AC tissue during

230

equilibrium experiments, a linear correlation was observed between the ratio

231

[BQ]eq/[HQ]eq and the concentration of AC (Figure SI 2), indicating that oxidation of HQ

232

at the surface of the AC tissue is governed by a stoichiometric ratio. Moreover, none

233

oxidation of HQ was observed in absence of AC. This confirms that AC tissue acted as a

234

mediator for the oxidation of HQ into BQ.

235 236

Table 2 – Langmuir and Freundlich parameters of adsorption isotherms as well as

237

pseudo-first order and pseudo-second order kinetic constants for the adsorption of

238

phenol (PH), benzoquinone (BQ) and catechol (CAT) on activated carbon tissue at

239

25°C. The kinetic studies were performed with 2 g L-1 of activated carbon tissue and

240

the following initial concentrations: [PH] = 1.0 mM ; [BQ] = 0.05 mM ; [CAT] = 0.1 mM.

Langmuir Isotherms Freundlich

Pseudo-first order Kinetics Pseudo-second order

PH

BQ

CAT

R2

0.994

0.995

0.997

qm (mmol g-1)

3.73

1.41

1.88

KL (L mmol-1)

19.1

44.3

29.6

R2

0.991

0.994

0.992

n

3.30

3.47

4.37

KF ((mmol g-1)(L mmol-1/n))

4.24

1.97

2.14

R2

0.911

0.943

0.982

qe (mmol g-1)

0.31

0.016

0.042

k1 (min-1)

0.089

0.092

0.066

R2

1.00

0.999

0.999

qe (mmol g-1)

0.62

0.026

0.054

k2 (g mmol-1 min-1)

0.78

14.0

2.65

k2qe (min-1)

0.48

0.37

0.14


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241 242

3.2. Phenol removal from AC fiber and mineralization of organics during EF

243

regeneration

244 245

Phenol-loaded AC tissue was regenerated by using the EF process with BDD as anode

246

and the spent AC tissue as cathode. Preliminary experiments showed that AC tissue was

247

able to produce a higher amount of H2O2 than a conventional carbon felt usually used for

248

the EF process (Figure SI 3). This is probably a beneficial effect of the AC microporous

249

structure, which leads to a larger electro-active surface area.

250

After the adsorption step, the amount of phenol adsorbed on AC was 3.2 mmol g-1; this

251

corresponds to a concentration in the electrochemical cell of 6.4 mM of PH ([PH]0) and a

252

TOC of 461 mg L-1 (TOC0). After 6 h of treatment at 300 mA, 70% of initial adsorbed PH

253

was removed from the surface of the AC tissue (Figure 1). By comparison, 12.5% of PH

254

was desorbed from the AC tissue during the control experiment without current supply.

255

This was only due to a desorption process in accordance with the sorption equilibrium.

256

Various phenomena can contribute to the removal of PH from the AC surface. First, it

257

was observed a stronger increase of the aqueous concentration of PH during the first

258

minutes of electro-oxidation at 300 mA, compared to the control experiment without

259

current supply (Figure SI 4). This phenomenon is ascribed to a local pH increase in the

260

vicinity of the electrode due to OH- generation, which leads to repulsive interactions

261

between the anionic form of PH and the AC surface. Conventional cathodic regeneration

262

is based on this mechanism.33 Unfortunately, pH-induced desorption is often not

263

sufficient to achieve high regeneration efficiency of spent AC, particularly in case of

264

chemical (irreversible) sorption of pollutants.34,35 The advantage of the EF process is to



265

promote simultaneously the oxidation of organic compounds both in the bulk and

266

adsorbed on AC tissue.

267

As observed in previous studies with different molecules such as toluene or humic acids,

268

adsorbed organic compounds can directly react with oxidant species such as •OH and

269

other electrochemically generated redox reagents (H2O2, O3, S2O82-, SO4•-), thus leading

270

to the release of oxidation by-products in the bulk.15,36 By performing classical Fenton

271

oxidation, very low regeneration efficiency of microporous AC has been reported due to

272

the limited availability of molecules adsorbed in micropores towards oxidant species.8

273

During EF process, H2O2 is generated at the surface of AC pores, therefore, •OH can be

274

produced close to target pollutants adsorbed on AC surface according to the

275

electrochemically supported Fenton reaction (eq 1). Therefore, the availability of

276

adsorbed pollutants for oxidation is increased and higher regeneration effectiveness of

277

microporous AC can be achieved by EF compared to classical Fenton oxidation. Besides,

278

high degradation rate of PH in the bulk involves a shift of the sorption equilibrium and

279

the continuous release of PH from the AC tissue to the bulk.

280

Similar experiments were also performed by using AC felt instead of AC tissue (Figure SI

281

5). A higher amount of PH was removed from the AC felt surface (88% after 6 h). This

282

might be ascribed to a lower intra-particle diffusion resistance within the felt, which

283

favors desorption kinetics and mass transport of oxidant species from the bulk to AC

284

pores. Unfortunately, this material presents insufficient mechanical properties and was

285

not suitable for water treatment purposes.

286

Using both AC felt and tissue, PH was mainly removed from the cathode during the first

287

3 hours, then, the efficiency of the process strongly decreased. This might be related to

288

the presence of both physisorbed and chemisorbed pollutants and slower removal of


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289

chemisorbed PH. Moreover, a lower availability towards oxidant species of PH

290

molecules adsorbed in the smallest pores of AC fibers could also reduce the efficiency

291

after the first 3 h of treatment.8

292 293

Figure 1 – Evolution of the concentration of phenol adsorbed on activated carbon

294

(AC) tissue (Phenol ads), phenol in the solution + adsorbed on AC tissue (Phenol

295

total) and total organic carbon in the solution + adsorbed on AC tissue (TOC total)

296

during electro-Fenton regeneration of the spent AC. Control experiment was

297

performed without current supply (I = 0). Concentrations are expressed as percent of

298

the total initial concentration ([PH]0 or TOC0) in the electrochemical cell, which

299

corresponds to the initial amount of phenol adsorbed on AC tissue.

300 301

The great advantage of this process is to avoid the accumulation of organic compounds

302

in the bulk. Only 6% of the initial adsorbed TOC was in the solution after 6 h of

303

treatment (Figure 2). This means that 91% of PH removed from the AC tissue was

304

completely mineralized to CO2 and H2O. PH and TOC concentration in the bulk rapidly



305

increased during the first 20 min due to the rapid desorption of a part of adsorbed PH.

306

Subsequent evolution of PH and TOC concentration in the bulk depends on: (i)

307

desorption and degradation kinetic of PH adsorbed on AC, (ii) degradation kinetic of PH

308

in the bulk and shift of the adsorption equilibrium leading to PH desorption, and (iii)

309

mineralization kinetic of oxidation by-products in the bulk. Higher accumulation of TOC

310

was observed in the bulk, compared to PH. In fact, TOC in the solution originates from

311

both PH desorption and by-products release from oxidation of adsorbed and dissolved

312

PH. However, a rapid decrease of the TOC in the solution was observed due to the high

313

production rate of •OH both in the bulk (eq 1) and at the surface of the BDD anode (eq 2).

314

Besides, there was not any degradation by-product detected as adsorbed on the AC

315

tissue. Adsorption of oxidation by-products on the AC tissue is avoided because

316

degradation kinetics are faster than adsorption kinetics. Formation of more hydrophilic

317

by-products, occupation of adsorption sites by residual PH and water molecules as well

318

as electrostatic interactions due to the high local pH at the surface of the AC tissue also

319

participate to prevent adsorption of degradation by-products.

320 321


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322

Figure 2 – Evolution of the normalized concentration of phenol and normalized

323

concentration of TOC in the solution during regeneration of spent AC tissue by the

324

electro-Fenton process. Error bars represent standard deviations obtained from

325

triplicate experiments.

326 327

Total mineralization of pollutants avoids the accumulation of toxic by-products such as

328

BQ (Figure 3). Other aromatic intermediates identified were mainly CAT and HQ.

329

Resorcinol was detected only in very low amount since hydroxylation of phenol is

330

mainly favored in para (HQ) and ortho (CAT) positions.37 CAT reached rapidly a

331

maximum concentration at t = 30 min (2.2% of TOC0) because the production rate from

332

PH oxidation is the highest at the beginning of the experiment and then continuously

333

decrease because of the lower PH concentration. BQ concentration also rapidly reached

334

a maximum at t = 20 min (1.8% of TOC0) and then rapidly decreased below the detection

335

limit at t = 120 min. By comparison, HQ concentration reached a maximum value later (t

336

= 90 min, 2.8% of TOC0) and decreased much more slowly. Pimentel et al. (2008)

337

observed similar behavior during PH removal by EF with a classical carbon felt

338

cathode.24 As suggested by other authors, this might be explained by taking into account

339

the equilibrium of the redox couple HQ/BQ (E° = 0.70 V) and possible reduction of BQ to

340

HQ.37,38 Moreover, Mousset et al. reported that degradation kinetic constants of BQ

341

oxidation into muconic and maleic acids are around one order of magnitude higher than

342

those of HQ oxidation into the same degradation by-products.37

343

Then, all aromatic by-products undertook oxidative ring opening reactions to form

344

short-chain carboxylic acids.11 The main short chain carboxylic acids detected were

345

succinic, oxalic and formic acids and reached maximum concentration at 90, 120 and 90

346

min of electrolysis, respectively. The evolution of the concentrations of oxidation by-



347

products in the bulk depends on (i) the amount generated from degradation of PH in the

348

bulk or adsorbed on AC tissue and (ii) the degradation kinetics in the bulk and at the

349

anode surface. Thus, the concentration of short-chain carboxylic acids decreased more

350

slowly than that of aromatic by-products due to slower reaction kinetics with •OH.37,39

351 352 353

Figure 3 – Evolution of the concentration in the solution of main phenol degradation

354

by-products (Csol,t) during electro-Fenton regeneration of spent AC tissue.

355

Concentration of organic compounds are calculated in mg of carbon per liter and

356

expressed as percentage of the initial concentration of total organic carbon (TOC0) in

357

the electrochemical cell. Error bars represent standard deviations obtained from

358

triplicate experiments.

359 360

3.3 Reuse of regenerated AC fiber

361 362

Additional experiments were performed in order to assess the potential of this

363

technology for reuse of the regenerated AC. AC tissue was selected as the most

364

promising material, since AC felt presented insufficient mechanical properties for water


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365

treatment purposes. Several cycles of adsorption followed by EF regeneration were

366

implemented in order to follow the evolution of the regeneration efficiency (Figure 4).

367

Besides, the morphological texture and chemical structure of the AC surface was

368

characterized after one and ten cycles of adsorption/regeneration. The optimal

369

regeneration time by EF was set at 6 h because the efficiency of the process is reduced

370

between 6 h and 9 h of treatment (part 3.2).

371

The regeneration efficiency (RE) was calculated by comparing the amount of PH that can

372

be adsorbed on the regenerated AC (qreg) and the amount of PH that can be adsorbed on

373

the initial AC (qi) (eq 7)

374

() % =

#+', #-

× 100

(7)

375

RE was 78% after one cycle, while only 70% of adsorbed PH was removed from the AC

376

surface after 6 h of treatment. Thus, by taking into account both residual PH (30% of

377

initial adsorption capacity of the AC) and new PH adsorbed on AC (78%) after the first

378

regeneration cycle, the adsorption capacity of the regenerated AC is higher than the one

379

of the initial material.

380

Raman analyses have been performed in order to assess the evolution of the chemical

381

composition of the AC tissue. Spectra have been analyzed by using the following

382

deconvolution procedure: a combination of three lorentzian-shaped bands at about

383

1600 cm-1 (G), 1340 cm-1 (D1) and 1185 cm-1 (D2) and a Gaussian-shaped band at 1545

384

cm-1 (D3) was used for curve fitting (an example is given in Figure SI 6). These bands

385

correspond to different vibration modes; G band and D bands are characteristics of

386

ordered materials and disordered materials, respectively.40,41 Overall, results show that

387

chemical composition of the AC tissue was not strongly modified after 10 cycles of



388

adsorption/EF regeneration (Figure SI 7). However, a slight decrease of 8% of the ratio

389

between the integrated intensity of the sum of D bands and the G band (IΣD/IG) was

390

observed after one regeneration cycle. Higher decrease of the ratio ID2/IG (21%) and

391

ID3/IG (15%) was observed compared to ID1/IG (6%). D1, D2 and D3 bands have been

392

reported to be characteristics of graphene layer edges, ionic impurities and amorphous

393

carbon, respectively.40,41 Therefore, these results indicate that the higher adsorption

394

capacity of the regenerated AC (1 cycle) might be attributed to a cleaning effect of the AC

395

surface by the EF process. Some impurities from the virgin AC tissue are removed during

396

the first EF regeneration. Therefore, an electrochemical cleaning step could be suitable

397

before the first adsorption cycle.

398

A slight decrease of RE was observed during cycle 2 (74%) and 3 (70%). Then, RE

399

reached a plateau, with a slight variation between 65% and 72%. The high RE obtained

400

all along the 10 regeneration cycles demonstrates the suitability of this treatment

401

strategy. Cathodic polarization of the AC surface avoids any damage of the AC surface.

402

While Raman analyses showed a cleaning effect of the AC tissue, comparison of SEM

403

images showed the absence of any change in the morphological texture of the AC tissue

404

between initial and regenerated (10 cycles) samples (Figure 5). AC tissue is made of

405

thousands of tightly interwoven fibers with a diameter around 10 µm. Both initial and

406

regenerated AC contained broken fibers. The morphological texture of fibers was very

407

similar in both samples, even in the vicinity of the breaking point of a fiber. Since EF

408

regeneration does not affect the morphological and chemical structure of AC tissue, fiber

409

breaking seems to arise only from a mechanical stress. Using AC tissue, the release of

410

fibers in water was not visible and not detectable by TOC analysis. On the contrary,

411

agitation conditions during the adsorption step led to the release in water of large


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Page 23 of 28


412

amounts of small fibers from AC felt (Figure SI 8). This is why AC tissue appeared to be

413

more suitable than AC felt for this kind of application.

414 415

Figure 4 – Evolution of the regeneration efficiency (RE) according to the number of

416

adsorption/regeneration cycles performed. Red dotted line corresponds to the

417

removal rate of adsorbed phenol after 6 h of regeneration by electro-Fenton. Error

418

bar on the point “cycle 1” (contained within data point) represent standard deviation

419

obtained from triplicate experiment.

420

421



422

Figure 5 – Scanning electron microscopy images of initial activated carbon tissue (A,

423

E) and after 10 regeneration cycles (B, F). Images C and D focus on breaking fibers

424

observed in the material after 10 regeneration cycles.

425 426

The main drawbacks of conventional regeneration methods are avoided by using the EF

427

process. By comparison to chemical oxidation, much higher regeneration effectiveness

428

of a microporous adsorbent can be achieved. This process can also mineralize organic

429

pollutants, while thermal treatment under non-oxidizing conditions only lead to

430

desorption of organic compounds. Moreover, the adsorption capacity of AC tissue is not

431

affected, on the contrary to chemical oxidation and thermal treatments under oxidizing

432

conditions. The selection of AC tissue as adsorbent is crucial for the effectiveness of the

433

process because this material present suitable features for both adsorption and

434

regeneration steps. Next studies should focus on scale-up of this process in order to

435

make a proper economical assessment. As an example, after 6 h of EF regeneration of

436

the spent AC tissue using the bench-scale setup of this study, the mineralization current

437

efficiency reached 35%, while the energy consumption was 0.2 kWh per gram of TOC

438

mineralized.11,42

439

One of the main drawback of electrochemical advanced oxidation processes for the

440

removal of low concentrations of organic compounds is the loss of efficiency caused by

441

mass transfer limitations.43–45 Therefore, combination with a pre-concentration step

442

using AC tissue is a promising way to improve the cost-efficiency of these processes and

443

to take advantage of their outstanding effectiveness for full mineralization of organic

444

pollutants.

445 446


Page 24 of 28

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447

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Regeneration of Activated Carbon Fiber by the Electro-Fenton Process

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