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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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Graphical abstract
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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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Abstract
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An electro-Fenton (EF) based technology using activated carbon (AC) fiber as cathode
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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.
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Keywords
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Electro-Fenton ; Activated carbon ; Regeneration ; Mineralization ; Phenol
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Graphical abstract
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1. Introduction
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The development of sustainable water treatment systems for the removal of
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micropollutants is an important challenge for environmental engineering. Activated
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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.
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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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low regeneration efficiency is often observed for microporous AC and chemical
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regeneration can be often applied only to mesoporous or non-porous materials.8–10
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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
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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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as anode. Anodic oxidation is expected to promote the oxidation of adsorbed compounds
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by mediated oxidation (generation of ozone, persulfate, sulfate radical)13,19 and to
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enhance the mineralization of desorbed pollutants and degradation by-products by
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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
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(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
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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
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the solution of PH and degradation by-products and their subsequent mineralization (iv)
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the adsorption capacity and the characteristics of the regenerated material after 1 and
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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
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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,
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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
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at 3 for all adsorption experiments, according to the pH required for the EF regeneration
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step. Control tests without AC showed that less than 3% of PH was lost by volatilization
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or adsorption on glass after 24 h.
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Equilibrium adsorption experiments were performed at room temperature (20 °C) with
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single compounds in 500 mL glass bottles continuously agitated during 24 h in a rotary
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shaker set at 20 rpm. For isotherm experiments, 250 mL of PH (1 mM), BQ (0.5 mM),
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CAT (0.8 mM) or HQ (0.9 mM) were mixed with various AC concentrations from 0.08 to
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1 g L-1. The most widely used models, Langmuir (eq 3) and Freundlich (eq 4), were
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applied for data analysis.
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=
(3)
1+
113
where qe is the amount of solute adsorbed per unit weight of AC at equilibrium, qm is the
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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)
#'
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where k2 is the second-order rate constant.
133 134
2.3. EF regeneration of AC. Electrochemical regeneration of spent AC fiber was
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performed in batch mode using an open, cylindrical and undivided electrochemical cell,
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similarly to the setup previously described by Trellu et al. (2016).20 500 mg of spent AC
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(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.
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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
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the solution.21 Continuous agitation was ensured by magnetic agitation and air bubbling.
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Constant current supply was provided by a power supply (HAMEG, model 7042-5,
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Germany) set at 300 mA as soon as the spent AC cathode was immersed in the
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electrolyte. This corresponds to 12.5 mA cm-2 as current density by considering BDD as
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working electrode. Current density was determined for optimizing H2O2 and OH
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generation and minimizing secondary reactions such as oxygen and hydrogen
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evolution.11
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2.4. Analytical methods. Chemical analyses were performed in order to follow the
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evolution of the concentration of PH and degradation by-products in the bulk and
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adsorbed on AC fiber. Aqueous samples (1 mL) were periodically collected from the bulk
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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
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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.
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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.
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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).
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3. Results and discussion
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3.1. Sorption of phenol and main aromatic oxidation by-products onto AC tissue
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The first step of this study was to determine the adsorption behavior of PH and main
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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
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(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-
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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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adsorbed on the carbon surface. Physical adsorption has been reported to play the most
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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.
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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 >
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CAT. PH adsorption kinetic on AC tissue was slower than a previously reported study,29
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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
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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
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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
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promote simultaneously the oxidation of organic compounds both in the bulk and
266
adsorbed on AC tissue.
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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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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
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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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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-
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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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treatment purposes. Several cycles of adsorption followed by EF regeneration were
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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
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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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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
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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
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