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Electrochemically-driven dosing of iron (II) for autonomous electro-Fenton processes with in situ generation of H2O2
Journal Pre-proof Electrochemically-driven dosing of iron (II) for autonomous electro-Fenton processes with in situ generation of H2O2 James I. Colades, Chin-Pao Huang, Joseph D. Retumban, Sergi Garcia-Segura, Mark Daniel G. de Luna PII:
S1572-6657(19)30907-5
DOI:
https://doi.org/10.1016/j.jelechem.2019.113639
Reference:
JEAC 113639
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 16 October 2019 Revised Date:
7 November 2019
Accepted Date: 8 November 2019
Please cite this article as: J.I. Colades, C.-P. Huang, J.D. Retumban, S. Garcia-Segura, M.D.G. de Luna, Electrochemically-driven dosing of iron (II) for autonomous electro-Fenton processes with in situ generation of H2O2, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/ j.jelechem.2019.113639. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Electrochemically-driven dosing of iron (II) for autonomous electroFenton processes with in situ generation of H2O2.
1 2 3 4 5
James I. Coladesa, Chin-Pao Huangb, Joseph D. Retumbana, c, Sergi Garcia-Segurad,*, Mark Daniel G. de Lunaa,e,*
6 a
7 8
Environmental Engineering Program, National Graduate School of Engineering, University of Philippines, Diliman, Quezon City 1101, Philippines
9
b
Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA
10
c
College of Engineering, National University, Manila 1008, Philippines
11 12 13
d
14
e
Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 85287-3005, United States Department of Chemical Engineering, University of Philippines, Diliman, Quezon City 1101, Philippines
15 16 17 18 19
Article submitted to be published in Journal of Electroanalytical Chemistry
20 21 22 23 24 25
*Corresponding author
26
Email: [email protected] (S. Garcia-Segura)
27
Email: [email protected] (M.D.G. de Luna)
1
28
ABSTRACT
29
Reliance of Fenton processes to hazardous chemicals diminishes the range of niche
30
applications of this highly efficient advanced oxidation process due to risks associated to
31
transport, storage, and handling of chemicals. In this work, an alternative approach towards
32
independent Fenton systems integrating (1) per demand in situ production of H2O2 from oxygen
33
cathodic reduction and (2) electrochemically-driven iron (II) dosing system is explored as a
34
novel strategy. For this purpose, a dual-cell system was designed to fulfill individual current
35
needs of both processes while avoiding excessive iron sludge production observed in
36
peroxicoagulation treatments. Experimental results indicate high reproducibility and resilience of
37
the proposed dual-cell electro-Fenton system, which attained complete organic methylene blue
38
dye decolorization in 80 min of treatment and over 80% mineralization in only 120 min of
39
electro-Fenton treatment. These results showcase a new approach that opens alternative
40
pathways for possible implementation of low-physical footprint electro-Fenton systems as point-
41
of-entry treatments or even to treat effluents of small and mid-sized industries.
42 43 44 45 46 47 48
Keywords: Electrochemical Advanced Oxidation Processes; electrochemical iron dosing;
49
electro-Fenton dual-cell; organic dyes; wastewater treatment
50
2
51
1. Introduction
52
Electrochemical advanced oxidation processes (EAOPs) are emerging water treatment
53
technologies that efficiently remove persistent organic pollutants [1,2]. These processes rely on
54
the
55
electrochemically-driven reactions [3,4]. Produced OH are strong oxidants (Eº = 2.80 V vs SHE
56
at acid pH) that react non-selectively with organics until their complete mineralization [5,6].
57
Water oxidation on anodic materials (M) with high over-potential of oxygen evolution yields
58
adsorbed OH according to reaction (1) [2]. The high capital cost of these novel anodic materials,
59
such as boron-doped diamond, hinders the translation of electrochemical oxidation technologies
60
for market applications [7,8]. Hence, alternative materials or approaches to electrogenerate OH
61
are required.
continuous
generation
of
highly
M + H2O → M(•OH) + H+ + e−
oxidizing
hydroxyl
radicals
(OH)
through
(1)
62 63
In this frame, electrochemical processes based on Fenton’s reaction such as electro-
64
Fenton (EF) process are promising alternatives. These indirect electrochemical oxidation
65
processes yield great amounts of OH in the bulk of the solution following Fenton’s reaction (2)
66
[1,3]. Note that OH are not generated by direct charge transfer processes on the electrode
67
surfaces but indirectly from the reaction of cathode-electrogenerated H2O2 from reaction (3) and
68
iron (II) in solution [9,10]. Cheap carbonaceous materials present excellent electrocatalytic
69
properties for H2O2 production. Efficient electrogeneration of H2O2 has been reported using
70
graphite rods [11], carbon felt [12], carbon polytetrafluoroethylene gas diffusion electrodes [13],
71
graphene [14], or carbon fiber brush [15].
3
H2O2 + H+ + Fe2+ → •OH + H2O + Fe3+
(2)
2H+ + O2 + 2e−→ H2O2
(3)
72 73
Electrochemical reduction of Fe3+ by reaction (4) allows faster regeneration of Fenton’s
74
catalyst than conventional Fenton-like reaction (5) [16,17]. This results in a higher production of
75
•
OH and reliability of electrochemical processes when compared to the conventional chemical
76
Fenton process [1,18]. Moreover, the in situ electrogeneration of required amounts of H2O2
77
minimizes risks of transportation and stock of hazardous chemicals [13,19]. Fe3+ → Fe2+ + e−
(4)
H2O2 + H+ + Fe3+ → HO2• + H2O + Fe2+
(5)
78 79
Experimental studies reported promising results on the abatement of recalcitrant
80
pollutants such as pharmaceuticals [20,21], pesticides [22,23], and dyes [6,24]. However, one of
81
the major challenges regarding the EF process is the design of strategies for their completely
82
autonomous operation [25,26]. Electrochemically-driven dosing of iron (II) would allow the
83
design of continuous flow operation modules that may be operated by non-technical trained
84
consumers [1,7]. Here, the challenge resides in the integration of easy to operate systems that
85
rely on similar electrochemical principles. An alternative that has been considered is the coupling
86
of H2O2 electrogeneration with sacrificial iron electrodes, but the high currents lead to an
87
excessive electrodissolution of iron [27,28]. Under these conditions the main removal
88
mechanism is associated to a physical phase separation by electrogocaulation with enhanced
89
efficiency due to H2O2 assistance, also so-called peroxicoagulation [29]. Peroxicoagulation is not
90
user-friendly due to the production of sludges that require further management after treatment. 4
91
Then, peroxicoagulation cannot be easily implemented as point-of-use technology. Herein we
92
present a feasible alternative that uses a dual-power supply system that independently feeds two
93
integrated electrochemical cells: (i) an H2O2 electrogeneration system, and (ii) an iron catalyst
94
dosing through electrodissolution of iron electrodes. Note that the controlled dosing can provide
95
low concentrations of iron catalyst for Fenton reaction without resulting in a coagulation process,
96
which will avoid sludge production. Herein it is presented a proof of concept on the capabilities
97
of a dual electrochemical cell to control iron dose, which may suppose a game changer for the
98
design of continuous flow electro-Fenton treatment systems. These systems will implement a
99
consumable interchangeable iron cartridge anode to control dosing of iron.
100
2. Experimental methods
101
2.1 Electrochemical reactor set-up
102
Figure 1 depicts the electrochemical batch reactor designed for simultaneous electro-
103
Fenton treatment with iron-dosing control. The reactor imbeds two independently integrated
104
electrochemical cells controlled by two power sources (DC). First, the conventional electro-
105
Fenton cell consisted of a packed-bed electrode with an O2 bubbling system to ensure
106
supersaturation conditions close to the cathodic surface. The main body of the packed-bed
107
configuration was a 4.0 cm diameter meshed plastic cylinder which is open on both ends. A
108
ceramic sparger was placed at the bottom end of this electrochemical cell casing. A flow control
109
valve with a flowmeter (Matheson Instruments) was used to control the gas flowrate through the
110
packed-bed. A 3.18 mm diameter graphite rod was placed inside which served as the cathode
111
connection to the DC power supply. A 7 cm x 7 cm activated carbon fiber (American Kynol,
112
Inc., spec. area of 1500 m2 g-1) was folded within the cathode bed to maximize the surface area
113
per volume of the electrocatalytic cathode for O2 reduction according reaction (3). The anode 5
114
used was a platinum wire 0.25 m long and 0.5 mm in diameter was used. Meanwhile, the second
115
electrochemical cell-controlled iron-dosing to the cell using a sacrificial iron anode and a
116
titanium mesh as cathode. Small interelectrode gap distance to minimize the potential drop
117
across the circuit was ensured by a plastic strip spacer that avoided short-circuiting.
118
Electrocatalytic experiments were carried out under vigorous stirring at 200 rpm to ensure
119
transport of reactants from/toward electrodes in the described system. Degradative performance
120
(1) was evaluated from the treatment of 50 mg L(1)-1 solutions of Methylene Blue dye equivalent to
121
33.8 ppm of total organic carbon (TOC). –+
DC Power Supply
DC Power Supply
– +
122 Air or O2
(3)
123
(4)
(9) (2) (5)
124
(6)
2.50 cm
1.25 cm
125 126 127
(7)
(2)
(3)
(4)
128 (10)
(8)
129 130 131 132 133 134 135
Figure 1. Electro-Fenton dual-cell reactor with simultaneous electrochemically-driven iron (II) dosing control. The reactor consisted in two independent electrolytic cells fed by two (1) power supplies. Iron dosing cell consisted of a (2) sacrificial iron anode, (3) a plastic spacer, and (4) titanium mesh cathode. The electro-Fenton cell consisted of a plastic casing with a packed bed 6
136 137 138 139
electrode system containing (5) graphite rod connector to the cathode electrocatalytic material, (6) Pt wire anode, (7) activated carbon fiber cathode. Oxygen supersaturation was ensured by bubbling air or oxygen through a (8) ceramic sparger at the bottom part of the electrode set-up, with the flow controlled by an (9) air flow-meter.
140
2.2 Chemicals and analytical procedures
141
All chemicals were analytical grade and purchased from Millipore-Sigma. All solutions
142
were prepared with nano-pure water obtained from a Millipore Milli-Q system with resistivity
143
>18.2 MΩ cm at 25 ºC. Sodium perchlorate was used as supporting electrolye due to its inert
144
characteristics that allowed excluding degradation associated to alternative oxidant species such
145
as active chlorine species or sulfate radical [30,31]. All electrolytic experiments were conducted
146
using 0.05 M of Na2SO4 as supporting electrolyte. Solution pH was adjusted using 1.0 M HCl
147
and 1.0 M NaOH solutions.
148
Linear sweep voltammetry (LSV) analyses were conducted at scan rate 100 mV s-1 using
149
a three-electrode system controlled by a potentiostat/galvanostat Pine Instrument AFRDE4 and a
150
data logger DATAQ DI-710. The carbonaceous electrode was the working electrode in cathodic
151
scans, Pt wire was the counter electrode, and a saturated calomel electrode (SCE) within a
152
Luggin capillary was employed as reference electrode.
153
Color abatement and absorbance was determined using an UV-vis spectrophotometer
154
Hach DR 2000. TOC was measured with a TOC analyzer Apollo 9000HS from Teledyne-
155
Tekmar. From TOC abatement, mineralization current efficiency (MCE) was calculated from
156
equation (6) where n is the number of electrons required to attain complete mineralization of
157
methylene blue according to equation (7), F is the Faraday constant (96487 C mol-1), V is the
158
volume of solution in L, ∆(TOC) is the solution TOC decay (mg L-1), 4.32 x 107 is a conversion
159
factor to homogenize units (3600 s h-1 x 12000 mg mol-1), m is the number of carbon atoms of
160
methylene blue (16 C atoms), and I is the applied current (A). 7
161
MCE =
nFV ∆(TOC) x 100 4.32 x 107 mIt
(6)
C16H18N3S+ +45 H2O → 16 CO2 + 3 NO3- + SO42- +108 H+ + 102 e-
(7)
162 163
Chemical oxygen demand (COD) was quantified using low-range (LR) COD vials from
164
Hach. Concentration of H2O2 electrogenerated was followed with a colorimetric method based
165
on the formation of a yellow colored Ti-complex using K2TiO(C2O4)2·2H2O (Alfa-Aesar) and
166
measuring absorbance at 400 nm. Ferrous (Fe2+) and total iron were analyzed after forming a
167
colored complex with 1,10-phenanthroline using a Hach kit. Effective current (ieff) defined by
168
equation (8) was determined from the concentration of electrogenerated reagents ([R]). Then,
169
current efficiency (η) of electrochemical processes was quantified from equation (9).
nF[R]V t
(8)
ieff x 100 It
(9)
ieff =
η=
170
8
171
where n is the number of electrons consumed in the reaction, F is the Faraday constant (96487 C
172
mol-1), [R] is the concentration of reagent yielded in mol L-1, V is the volume of solution in L, t is
173
the electrolysis time in s, and I the total current applied in A.
174
3. Results and discussion
175
3.1 Understanding impact of operational variables on H2O2 electrogeneration
176
Degradation of organic pollutants by EAOPs is related to the capabilities of the
177
electrochemical system to efficiently generate OH [3,26]. Indirect electrochemical generation
178
of OH through Fenton’s reaction (2) during EF treatments appoints electrogeneration of H2O2
179
as the governing electrochemical process that defines performance [1, 32]. Electroanalytic tests
180
demonstrate excellent electrocatalytic properties of activated carbon fiber on oxygen reduction
181
towards H2O2. Figure 2 depicts the linear voltammograms during cathodic scan in N2, O2, or air
182
saturated solutions. A noticeable current increase can be observed with the increasing level of O2
183
saturation due to sparging of air and O2, which suggests an excellent direct charge transfer
184
performance of the carbonaceous cathode on O2 reduction processes.
185
0
186
-10
188
I / mA
187
0
-20
190
I / mA
189
-30
-20 -40 -60 -80
191
-1.5
-1
-0.5
0
0.5
E / V vs SCE
192 193
-40 -1.5
-1
-0.5
0
0.5
E / V vs SCE
9
194 195 196 197 198
Figure 2. Oxygen reduction current (iAir/O2 – iN2) of activated carbon fiber for (solid line) oxygen, and (dashed line) air. Inset panel shows linear sweep voltammetry under scan rate 33.3 mV s-1 recorded in 0.05 M NaClO4 at pH = 2.0 under solution saturated with (solid line) pure oxygen, (dashed line) air, and (dotted line) nitrogen.
199
Activated carbon fiber cathode presents high electrocatalytic response towards O2
200
reduction; however, H2O2 production should be evaluated under continuous operation mode
201
when aiming application for EF treatment. This section studies the impact of operational
202
variables of influence on O2 reduction efficiency of activated carbon fiber cathodes and H2O2
203
accumulation during continuous operation.
204
Fenton’s chemistry is applied in a narrow pH range defined by the low solubility of Fe3+
205
that precipitates as Fe(OH)3 above pH 4.0 according to solubility diagrams [29]. Therefore,
206
electrogeneration of H2O2 was evaluated within this operational range of pH that ensures
207
solubility and reactivity of Fenton’s catalyst (Fe2+/Fe3+) [1,33]. Figures 3a and b illustrate the
208
effect of initial pH on the electrogeneration of H2O2 and η, respectively. Higher accumulation of
209
H2O2 (0.8 mM) was observed at acidic pH of 1.5, which decreased subsequently with increasing
210
pH values. This trend is consistent with literature and may be explained by the larger availability
211
of H+ in solution, which initiates the two-electron reduction reaction (3) [34,35]. Lower
212
electrogeneration of H2O2 under identical experimental conditions of applied current diminishes
213
the η from ⁓25% at pH 1.5-2.0 down to ⁓15% at 3.5. Thus, optimum electrogeneration of H2O2
214
was observed at pH 2.0.
215
Applied current defines the number of electrons delivered per second which definitely
216
controls the electrokinetics of reactions on the cathode surface [1]. A noticeable increase in H2O2
217
production from 0.4 to 1.0 mM can be seen in Fig. 3c with increasing current from 50 to 250 mA.
218
However, a sudden drop in H2O2 concentration down to 0.8 mM was recorded with applied
10
219
current beyond 250 mA. Indeed, an in-depth analysis of current efficiency reveals the continuous
220
decrease of η for applied current values above 100 mA, as seen in Fig. 3d. This trend highlights
221
the enhancement of concomitant parasitic reactions that compete with the desired bi-electronic
222
reduction of oxygen according to reaction (3) such as oxygen reduction to water following
223
reaction (10), H2O2 reduction by reaction (11), or hydrogen evolution reaction from reaction (12),
224
respectively [35]. Under undivided electrochemical set-up, the acceleration of H2O2 oxidation at
225
the anode surface following reaction (13) cannot be disregarded [1,24]. Note that electrochemical
226
generation of H2O2 at 250 mA clearly illustrates the attaining of a plateau of concentration,
227
which evidences the maximum concentration achievable due to equilibrium between cathodic
228
generation and anodic oxidation. 4H+ + O2 + 4e−→ 2 H2O
(10)
2H+ + H2O2 + 2e−→ 2 H2O
(11)
2H+ + 2e−→ H2
(12)
H2O2 → 2H+ + O2 + 2e−
(13)
229 230
1
231
η%
20 15
0.4 10 0.2
235 236
0 0
5
10
20
237
30
40
50
60
0 1
70
time / min
238
1.2
0.8 0.6
2
2.5
3
3.5
4
30 50 mA 100 mA 150 mA 200 mA 300 mA 350 mA
c)
d) 25 20 15
2
2
H O / mM
1
1.5
pH
η%
234
0.6
b) 25
2 2
233
1.5 2.0 2.5 3.0 3.5
0.8 H O / mM
232
30
a)
0.4
10
0.2
5
0
0
11
239 240 241 242 243 244 245 246
1
247
30 -1
0.8
-1
0.25 L min
252 253
2
20
0.50 L min
0.6
-1
η%
0.75 L min
-1
2
251
25
-1
H O / mM
250
f)
0.00 L min
248 249
e)
1.00 L min
15
0.4 10 0.2 0 0
5
10
20
30
40
time / min
50
60
70
0 -0.25
0
0.25
0.5
0.75
1
1.25
-1
Air flow / L min
254 255 256 257 258
Figure 3. Influence of operational variables on H2O2 electrogeneration evaluated from (a,c,e) H2O2 accumulation and (b,d,f) current efficiency: (a-b) Effect of solution pH at 100 mA and 0.05 L min-1 of air,(c-d) effect of applied current at pH 2.0 and 0.05 L min-1 of air, and (e-f) effect of air flow rate at 100 mA and pH 2.0. Electrolyses were conducted in 0.05 mM NaClO4.
259
Figure 3e depicts the effect of the flow rate of air delivered in solution through bubbling
260
using the ceramic sparging. It is important to notice that in absence of sparging a discrete H2O2
261
production of 0.17 mM was observed due to the cathodic reduction of oxygen dissolved in
262
solution and the O2 produced from water oxidation reaction (14) at the anode, but with a low
263
efficiency of ⁓5% due to the diffusive control of O2 under this experimental condition (see Fig.
264
3f). Continuous delivery of air through bubbling increased the availability of O2 molecules in the
265
proximity of the electrode which positively affected H2O2 electrogeneration. As shown,
266
maximum H2O2 electrogeneration at 100 mA was attained with 0.5 L min-1 of air. The η 12
1.5
267
improved from 5% in the absence of bubbling and 8% at 0.25 L min-1 to ⁓23% at higher air feed
268
flows. This sudden increase in η is attributed to better convective mixing in the ACF bed thereby
269
promoting effective mass transport of dissolved O2 towards the cathode surface. 2 H2O → 4H+ + O2 + 4e−
(14)
270 271 272
It was also observed in Fig. 3e that much higher air fed flows did not result in higher
273
concentrations of electrogenerated H2O2. The slight decrease in H2O2 accumulation at higher air
274
flows may be explained by bubble resistance which caused lower η [35]. In order to eliminate
275
unnecessary energy expense on air pumping to reach identical performance, an optimum air flow
276
rate of 0.5 L min-1 was defined for continuous O2 delivery. It is important to remark that
277
experimental results of Fig. 4 demonstrate that the electrogenerated H2O2 allows attaining high
278
degree of mineralization through EF treatment. Indeed, excessive concentration may result in a
279
decrease of efficiency due to the scavenging reaction of produced OH in the bulk with excess of
280
H2O2.
281 282
3.2 Evaluating the role of adsorption and electrochemically-driven processes
283
The holistic understanding of a complex treatment system requires the elucidation of the
284
individual contribution of different elements to water treatment performance. Figure 4 shows
285
almost no decolorization of MB solution in the absence of applied current. Thus, it can be
286
inferred that the organic dye is barely adsorbed on the surface of the activated carbon fiber. In
287
addition, the dye solution remained unaltered even after gas sparging irrespective of the gas
288
delivered (N2, O2, or air) which confirmed that gas dissolution did not contribute to MB
13
289
degradation. In order to evaluate the role of H2O2 on dye decolorization, H2O2 was added to the
290
solution yet no color removal was observed. This result precludes the involvement of H2O2 alone
291
in the decolorization of MB solutions. On the other hand, 10% MB decolorization was observed
292
when current was supplied to the H2O2 generation electrochemical cell (electrochemically-driven
293
dosing of Fe2+ off). However, mineralization of organic load was not observed under these
294
conditions (see Fig. 4b). This is the characteristic behavior of active anodes such as Pt that
295
promote electrochemical conversion but not incineration towards CO2 [2,7]. This is in agreement
296
with previous reports that observed solution decolorization without mineralization, which
297
suggests the yield of colorless by-products but not the aromatic ring opening [36,37]. Figure 4
298
denotes the greater performance of the EF treatment when using the dual system with
299
electrochemical dosing of Fe2+ that attained complete decolorization after 90 min of treatment.
300
Faster decolorization was attained due to the production of homogeneous OH in the bulk of the
301
solution through Fenton’s reaction (2). The oxidizing character of
302
mineralization as deduced from the 83% abatement of TOC in 120 min of treatment. Remaining
303
TOC is associated to the formation of stable iron-carboxylate complexes [1, 16]. These low
304
molecular weight carboxylic acids are known to be harmless, photodegradable, and
305
biodegradable [38,39]. Further experiments will discuss the optimization of the dual-power
306
supply composite electrochemical reactor configuration for simultaneous Fenton’s catalyst (Fe2+)
307
dosing and H2O2 electro-generation.
OH radical enabled
308 309 310 311
14
312 313 314 315 316 317 318 319 320 a) 321
1.0
322
324
0
MB/MB
323
0.8 0.6 0.4
325 326
0.2
327
0 b)
328
1
330 331
TOC/TOC
0
329
0.8 0.6
332
0.4
333
0.2
334
0 0
20
40
60
80
100
120
140
time / min 15
335 336 337 338 339
Figure 4. (a) Decolorization and (c) TOC abatement during the treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 under different processes: () adsorption, () electrogeneration of H2O2 and Pt anode, () electro-Fenton treatment using the dual cell.
340 341 342
3.3 Optimizing electrochemically-driven dosing of iron (II) for electro-Fenton treatment
343
Advanced oxidation processes based on Fenton chemistry rely on the use of Fe2+ as a
344
catalyst to yield OH from the catalytic decomposition of H2O2 by Fenton reaction (2) [13, 40].
345
This implies that conventional Fenton treatment requires transport and stock of hazardous H2O2
346
as well as iron (II) salts. The electro-Fenton process overcomes the safety issues associated with
347
H2O2 by electrogenerating in situ the exact amount needed for an efficient production of
348
[4,33]. However, iron dosing is still required. This limits automatization of electro-Fenton
349
technology and their translation to continuous flow treatment designs. The dual-cell electro-
350
Fenton system (see Fig. 1) allows introducing an electrochemically-driven system to dose the
351
iron required [28]. This first proof of concept shows a promising opportunity for electro-Fenton
352
technologies introducing a novel strategy that can incentivize technology transfer towards point-
353
of-entry treatment systems as well as low-physical footprint units for middle-sized industry.
OH
354
Figure 5 depicts the influence of the applied current in the iron-dosing cell (IFe) while
355
simultaneously operating the H2O2 electrogeneration unit of the reactor cell. It can be observed
356
that in the absence of IFe, slight decolorization is observed but with no mineralization. The
357
analysis of iron in solution shows no yield of iron ions under IFe = 0 mA. Note that under such
358
conditions electrodissolution of the sacrificial anode is not expected, whereas the only source of
16
359
iron would be associated exclusively to chemical dissolution (which was not observed). On the
360
other hand, application of IFe is followed by iron release that is used as catalyst in the EF reaction.
361
A small current such as 7.5 mA resulted in the accumulation of a total iron concentration of 0.8
362
mM, which is in the range of conventional optimum values (between 0.5 and 1.4 mM) for EF
363
treatment [1, 10]. Under such experimental conditions complete decolorization was attained after
364
80 min of dual electrolytic treatment. Additional increment of applied IFe did not accelerate
365
decolorization kinetics, but only accumulated higher concentration of iron in solution. Note that
366
the excess of iron in solution diminished mineralization percentage attained due to the major
367
complexation of by-product as well as the scavenging reaction (15) of Fe2+ with produced OH
368
[7,21]. Note that undesirable accumulation of iron may result in sludge production that would
369
eventually require a secondary treatment as well as solid waste management [29]. Therefore,
370
from these results an optimum IFe of 7.5 mA was identified for the operation of the dual-cell EF
371
system. Fe2++ OH → Fe3++ OH −
(15)
372 373
3.4 Evaluating influence of applied current (IH2O2) on electro-Fenton treatment
374
Applied current (IH2O2) is the operational parameter that defines amount and rate of
375
produced H2O2 (see Fig. 3b) and the electroregeneration of iron(II) from reaction (4). Thus, it
376
controls the overall electrokinetics and performance of the EF process [1,41]. Figure 6 describes
377
the impact of IH2O2 on decolorization and mineralization. As shown, increase of applied IH2O2
378
results in a faster decolorization rate due to the concomitant increase in OH production [9,42].
379
Similarly, greater mineralization percentage was attained under identical treatment times for
380
higher applied currents. However, the lower enhancements observed when increasing IH2O2 from 17
381
250 mA up to 350 mA suggest a lower mineralization efficiency of the process due to the
382
competitive consumption of electrons in parasitic reactions that do not lead to organic dye
383
mineralization, such as oxygen evolution by reaction (14) or OH dimerization following
384
reaction (16). This trend agrees with the lower MCE values observed for increasing applied
385
currents. 2 OH → H2O2
(16)
386 387 388
a)
390
0.8 MB/MB
391
0
389
1.0
0.6 0.4
392
0.2
393
0
394
b) 1.0
396 397
TOC/TOC
0
395 0.8 0.6 0.4
398 0.2
399
0
c)
400
1.5
/ mM
403
1
total
402
Fe
401
0.5
18 0 0
20
40
60
80
time / min
100
120
140
404 405 406 407 408 409 410
Figure 5. (a) Decolorization, (b) TOC abatement, and (c) total iron accumulation during the dual-cell electro-Fenton treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 at IH2O2=150 mA under different applied current for iron delivery: () IFe= 0 mA, () IFe= 7.5 mA, and () IFe= 15.0 mA.
411 412 413
In this frame, it can be concluded that an excess of current would increase energy
414
consumption without effectively accelerating the mineralization process. It may be inferred that
415
the optimum IH2O2 is 150 mA since it allows attaining faster decolorization and higher
416
mineralization
percentage with the minimal energy requirement.
417
a)
419
0.8
15 % MCE
1.0
420 421
MB/MB
0
418
20
10
0.6
5
0.4
0 0
20
422
424
0
425
1.0
b)
TOC/TOC
0
426
428
60 80 100 120 140 time / min
0.2
423
427
40
0.8 0.6 0.4 19
0.2 0 0
20
40
60
80
time / min
100
120
140
429 430 431 432 433 434 435 436 437
Figure 6. (a) Decolorization, and (b) TOC abatement during the dual-cell electro-Fenton treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 at IFe= 7.5 mA mA under different applied current for iron delivery: () IH2O2=50 mA, () IH2O2=150 mA, () IH2O2= 250 mA, and () IH2O2= 350 mA. Inset panel depicts the mineralization current efficiency.
438
3.5 Understanding the effect of pH on the dual-cell electro-Fenton system
439
Fenton-based technologies are sensitive to the initial pH of the wastewater. Figure 7
440
describes the influence of initial pH of the dye solution on MB abatement and the efficient
441
mineralization of the organic load. Interestingly, the controlled dose of iron into the reactor
442
contributes to minimize the negative impacts of higher pH usually reported in Fenton reaction.
443
The narrow window of operational pH is defined by the solubility of iron(II)/iron(III) species in
444
solution [29]. Controlled electrodissolution of sacrificial iron electrode provides a continuous
445
supply of Fe2+ for the Fenton’s reaction, which ensures continuous availability of Fe2+ in solution
446
to efficiently react with H2O2 by reaction (2). This explains the slight difference on dye
447
decolorization observed in Fig. 7a. However, TOC analysis shows a slower mineralization rate at
448
highly acidic pH. This trend may be explained by the lower amount of OH produced by
449
Fenton’s reaction from H3O2+ [1]. It is important to remark that mineralization attained at pH 4.0
450
is higher than usually observed for conventional electro-Fenton systems. The results suggest that
451
the continuous supply of Fe2+ due to a controlled dosage unit can contribute to overcome pH
452
limitations due to iron hydroxides precipitation. Therefore, iron dosing strategies is an approach
453
worthy of further research to understand future applicability in automized electro-Fenton devices.
20
454 455 456
1.0
457
459
0
0.8 MB/MB
458
460
0.6 0.4
461
0.2
462
0
463 464
1
467 468
TOC/TOC
466
0
465
0.8 0.6 0.4
469 470 471 472
0.2 0 0
20
40
60
80
100
120
140
time / min 473 474 475 476
Figure 7. (a) Decolorization and (c) TOC abatement during the treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 with the dual cell under applied current of IFe= 7.5 mA and IH2O2= 150 mA under different pH: () 1.5, () 2.0, () 3.0, and () 4.0.
477
21
478
3.6 Assessing electro-Fenton process reliability through electrode stability
479
Water treatment technologies must be reliable under long operation conditions. Research
480
focused on the study and development of novel electrocatalysts that have low stability. Electrode
481
deactivation can occur due to the deposition of polymeric films due to reduction or oxidation of
482
organics on electrodes surface under low operational potentials [7]. In order to demonstrate the
483
stability of the proposed dual electrochemical reactor system for autonomous EF,
484
electrochemical cell performance under successive treatment cycles was tested. Figure 8 shows
485
high stability of electrodes that achieve consistently identical results on MB decolorization and
486
TOC abatement. It is important to remark that some experimental results suggest that certain
487
catalysts and electrocatalysts must be reactivated after each cycle to ensure catalytic activity and
488
avoid inhibition [1,35]. The carbon fiber electrode maintains effectiveness without pre-treatment
489
or reactivation requirements, even though further cycles or continuous operation must be further
490
evaluated to ensure long term performance and electrode stability.
491 492
st
1.0
493
496 497
nd
2 cycle
rd
3 cycle
th
4 cycle
th
5 cycle
0.8
0
495
TOC/TOC 0 MB/MB
494
1 cycle
0.6 0.4
498 499 500 501
0.2 0 0
40
80
0
40
80
0
40
80
0
40
80
0
40
80 120
time / min
22
502 503 504
Figure 8. () Decolorization and ()TOC abatement during the treatment of 700 mL of 50 mg L1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 with the dual cell under applied current of IFe= 7.5 mA and IH2O2= 150 mA after succesive cycles.
505
4. Conclusions
506
Electrogeneration of H2O2 makes electro-Fenton technologies reliable and more
507
competitive due to their independence from hazardous H2O2 transport and storage. Applied
508
current can control the amount of available H2O2 in the electrochemical reactor. However,
509
electro-Fenton process is still dependent on iron(II) addition as Fenton’s catalyst, usually added
510
as iron sulfate salt. In this work, the implementation of a dual-cell system demonstrates the next
511
technological step towards “reagent free” wastewater treatment. The compact electrode system
512
of carbon fiber cathode/platinum anode produces high concentrations of H2O2 from the
513
electrochemical reduction of oxygen while the iron dosing cell consisting of a sacrificial iron
514
anode and a titanium mesh as cathode effectively controls iron delivery in solution by the
515
application of small currents. Results show the efficient electrogeneration of 0.8 mM of H2O2
516
under optimum conditions of pH 3.0 and applied current of 150 mA. In the absence of iron
517
dosing, slight decolorization was observed from the electrochemical oxidation of platinum.
518
However, complete dye decolorization accompanied by an 80% of organic load mineralization
519
was attained with iron dosing. Higher treatment performance was attributed to the indirect
520
electrogeneration of OH produced by Fenton’s reaction. Optimization of operational variables
521
indicate an optimum IFe of 7.5 mA to deliver required iron dose while avoiding sludge formation.
522
In contrast, an optimum IH2O2 of 150 mA attained high degree of mineralization. Overall, the
523
system demonstrated high reliability under continuous operation while consistently maintaining
524
similar decolorization and mineralization performance after several consecutive cycles. These
23
525
results open promising opportunities for dual-cell systems for the autonomous operation of
526
electro-Fenton systems using consumable cartridges consisting of a sacrificial iron electrode.
527
528
Declarations of interest
529
The authors declare that they have no known competing financial interests or personal
530
relationships that could have appeared to influence the work reported in this paper.
531 532
Acknowledgements
533
The authors would like to thank the Department of Science and Technology (DOST),
534
Philippines and University of Delaware, United States for the financial support. The authors are
535
grateful to Rovshan Mahmudov, Ph.D. and Michael Davidson for their assistance.
536
537
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27
Highlights Dual cell system can control iron dosing Electrochemically-driven dosing of iron diminishes reliance on chemical addition Iron dosing strategies can minimize sludge formation Electro-Fenton in dual cell enhances performance attaining dye degradation
Conflict of interest Authors declare no conflict of interest.
S1572-6657(19)30907-5
DOI:
https://doi.org/10.1016/j.jelechem.2019.113639
Reference:
JEAC 113639
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 16 October 2019 Revised Date:
7 November 2019
Accepted Date: 8 November 2019
Please cite this article as: J.I. Colades, C.-P. Huang, J.D. Retumban, S. Garcia-Segura, M.D.G. de Luna, Electrochemically-driven dosing of iron (II) for autonomous electro-Fenton processes with in situ generation of H2O2, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/ j.jelechem.2019.113639. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Electrochemically-driven dosing of iron (II) for autonomous electroFenton processes with in situ generation of H2O2.
1 2 3 4 5
James I. Coladesa, Chin-Pao Huangb, Joseph D. Retumbana, c, Sergi Garcia-Segurad,*, Mark Daniel G. de Lunaa,e,*
6 a
7 8
Environmental Engineering Program, National Graduate School of Engineering, University of Philippines, Diliman, Quezon City 1101, Philippines
9
b
Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA
10
c
College of Engineering, National University, Manila 1008, Philippines
11 12 13
d
14
e
Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 85287-3005, United States Department of Chemical Engineering, University of Philippines, Diliman, Quezon City 1101, Philippines
15 16 17 18 19
Article submitted to be published in Journal of Electroanalytical Chemistry
20 21 22 23 24 25
*Corresponding author
26
Email: [email protected] (S. Garcia-Segura)
27
Email: [email protected] (M.D.G. de Luna)
1
28
ABSTRACT
29
Reliance of Fenton processes to hazardous chemicals diminishes the range of niche
30
applications of this highly efficient advanced oxidation process due to risks associated to
31
transport, storage, and handling of chemicals. In this work, an alternative approach towards
32
independent Fenton systems integrating (1) per demand in situ production of H2O2 from oxygen
33
cathodic reduction and (2) electrochemically-driven iron (II) dosing system is explored as a
34
novel strategy. For this purpose, a dual-cell system was designed to fulfill individual current
35
needs of both processes while avoiding excessive iron sludge production observed in
36
peroxicoagulation treatments. Experimental results indicate high reproducibility and resilience of
37
the proposed dual-cell electro-Fenton system, which attained complete organic methylene blue
38
dye decolorization in 80 min of treatment and over 80% mineralization in only 120 min of
39
electro-Fenton treatment. These results showcase a new approach that opens alternative
40
pathways for possible implementation of low-physical footprint electro-Fenton systems as point-
41
of-entry treatments or even to treat effluents of small and mid-sized industries.
42 43 44 45 46 47 48
Keywords: Electrochemical Advanced Oxidation Processes; electrochemical iron dosing;
49
electro-Fenton dual-cell; organic dyes; wastewater treatment
50
2
51
1. Introduction
52
Electrochemical advanced oxidation processes (EAOPs) are emerging water treatment
53
technologies that efficiently remove persistent organic pollutants [1,2]. These processes rely on
54
the
55
electrochemically-driven reactions [3,4]. Produced OH are strong oxidants (Eº = 2.80 V vs SHE
56
at acid pH) that react non-selectively with organics until their complete mineralization [5,6].
57
Water oxidation on anodic materials (M) with high over-potential of oxygen evolution yields
58
adsorbed OH according to reaction (1) [2]. The high capital cost of these novel anodic materials,
59
such as boron-doped diamond, hinders the translation of electrochemical oxidation technologies
60
for market applications [7,8]. Hence, alternative materials or approaches to electrogenerate OH
61
are required.
continuous
generation
of
highly
M + H2O → M(•OH) + H+ + e−
oxidizing
hydroxyl
radicals
(OH)
through
(1)
62 63
In this frame, electrochemical processes based on Fenton’s reaction such as electro-
64
Fenton (EF) process are promising alternatives. These indirect electrochemical oxidation
65
processes yield great amounts of OH in the bulk of the solution following Fenton’s reaction (2)
66
[1,3]. Note that OH are not generated by direct charge transfer processes on the electrode
67
surfaces but indirectly from the reaction of cathode-electrogenerated H2O2 from reaction (3) and
68
iron (II) in solution [9,10]. Cheap carbonaceous materials present excellent electrocatalytic
69
properties for H2O2 production. Efficient electrogeneration of H2O2 has been reported using
70
graphite rods [11], carbon felt [12], carbon polytetrafluoroethylene gas diffusion electrodes [13],
71
graphene [14], or carbon fiber brush [15].
3
H2O2 + H+ + Fe2+ → •OH + H2O + Fe3+
(2)
2H+ + O2 + 2e−→ H2O2
(3)
72 73
Electrochemical reduction of Fe3+ by reaction (4) allows faster regeneration of Fenton’s
74
catalyst than conventional Fenton-like reaction (5) [16,17]. This results in a higher production of
75
•
OH and reliability of electrochemical processes when compared to the conventional chemical
76
Fenton process [1,18]. Moreover, the in situ electrogeneration of required amounts of H2O2
77
minimizes risks of transportation and stock of hazardous chemicals [13,19]. Fe3+ → Fe2+ + e−
(4)
H2O2 + H+ + Fe3+ → HO2• + H2O + Fe2+
(5)
78 79
Experimental studies reported promising results on the abatement of recalcitrant
80
pollutants such as pharmaceuticals [20,21], pesticides [22,23], and dyes [6,24]. However, one of
81
the major challenges regarding the EF process is the design of strategies for their completely
82
autonomous operation [25,26]. Electrochemically-driven dosing of iron (II) would allow the
83
design of continuous flow operation modules that may be operated by non-technical trained
84
consumers [1,7]. Here, the challenge resides in the integration of easy to operate systems that
85
rely on similar electrochemical principles. An alternative that has been considered is the coupling
86
of H2O2 electrogeneration with sacrificial iron electrodes, but the high currents lead to an
87
excessive electrodissolution of iron [27,28]. Under these conditions the main removal
88
mechanism is associated to a physical phase separation by electrogocaulation with enhanced
89
efficiency due to H2O2 assistance, also so-called peroxicoagulation [29]. Peroxicoagulation is not
90
user-friendly due to the production of sludges that require further management after treatment. 4
91
Then, peroxicoagulation cannot be easily implemented as point-of-use technology. Herein we
92
present a feasible alternative that uses a dual-power supply system that independently feeds two
93
integrated electrochemical cells: (i) an H2O2 electrogeneration system, and (ii) an iron catalyst
94
dosing through electrodissolution of iron electrodes. Note that the controlled dosing can provide
95
low concentrations of iron catalyst for Fenton reaction without resulting in a coagulation process,
96
which will avoid sludge production. Herein it is presented a proof of concept on the capabilities
97
of a dual electrochemical cell to control iron dose, which may suppose a game changer for the
98
design of continuous flow electro-Fenton treatment systems. These systems will implement a
99
consumable interchangeable iron cartridge anode to control dosing of iron.
100
2. Experimental methods
101
2.1 Electrochemical reactor set-up
102
Figure 1 depicts the electrochemical batch reactor designed for simultaneous electro-
103
Fenton treatment with iron-dosing control. The reactor imbeds two independently integrated
104
electrochemical cells controlled by two power sources (DC). First, the conventional electro-
105
Fenton cell consisted of a packed-bed electrode with an O2 bubbling system to ensure
106
supersaturation conditions close to the cathodic surface. The main body of the packed-bed
107
configuration was a 4.0 cm diameter meshed plastic cylinder which is open on both ends. A
108
ceramic sparger was placed at the bottom end of this electrochemical cell casing. A flow control
109
valve with a flowmeter (Matheson Instruments) was used to control the gas flowrate through the
110
packed-bed. A 3.18 mm diameter graphite rod was placed inside which served as the cathode
111
connection to the DC power supply. A 7 cm x 7 cm activated carbon fiber (American Kynol,
112
Inc., spec. area of 1500 m2 g-1) was folded within the cathode bed to maximize the surface area
113
per volume of the electrocatalytic cathode for O2 reduction according reaction (3). The anode 5
114
used was a platinum wire 0.25 m long and 0.5 mm in diameter was used. Meanwhile, the second
115
electrochemical cell-controlled iron-dosing to the cell using a sacrificial iron anode and a
116
titanium mesh as cathode. Small interelectrode gap distance to minimize the potential drop
117
across the circuit was ensured by a plastic strip spacer that avoided short-circuiting.
118
Electrocatalytic experiments were carried out under vigorous stirring at 200 rpm to ensure
119
transport of reactants from/toward electrodes in the described system. Degradative performance
120
(1) was evaluated from the treatment of 50 mg L(1)-1 solutions of Methylene Blue dye equivalent to
121
33.8 ppm of total organic carbon (TOC). –+
DC Power Supply
DC Power Supply
– +
122 Air or O2
(3)
123
(4)
(9) (2) (5)
124
(6)
2.50 cm
1.25 cm
125 126 127
(7)
(2)
(3)
(4)
128 (10)
(8)
129 130 131 132 133 134 135
Figure 1. Electro-Fenton dual-cell reactor with simultaneous electrochemically-driven iron (II) dosing control. The reactor consisted in two independent electrolytic cells fed by two (1) power supplies. Iron dosing cell consisted of a (2) sacrificial iron anode, (3) a plastic spacer, and (4) titanium mesh cathode. The electro-Fenton cell consisted of a plastic casing with a packed bed 6
136 137 138 139
electrode system containing (5) graphite rod connector to the cathode electrocatalytic material, (6) Pt wire anode, (7) activated carbon fiber cathode. Oxygen supersaturation was ensured by bubbling air or oxygen through a (8) ceramic sparger at the bottom part of the electrode set-up, with the flow controlled by an (9) air flow-meter.
140
2.2 Chemicals and analytical procedures
141
All chemicals were analytical grade and purchased from Millipore-Sigma. All solutions
142
were prepared with nano-pure water obtained from a Millipore Milli-Q system with resistivity
143
>18.2 MΩ cm at 25 ºC. Sodium perchlorate was used as supporting electrolye due to its inert
144
characteristics that allowed excluding degradation associated to alternative oxidant species such
145
as active chlorine species or sulfate radical [30,31]. All electrolytic experiments were conducted
146
using 0.05 M of Na2SO4 as supporting electrolyte. Solution pH was adjusted using 1.0 M HCl
147
and 1.0 M NaOH solutions.
148
Linear sweep voltammetry (LSV) analyses were conducted at scan rate 100 mV s-1 using
149
a three-electrode system controlled by a potentiostat/galvanostat Pine Instrument AFRDE4 and a
150
data logger DATAQ DI-710. The carbonaceous electrode was the working electrode in cathodic
151
scans, Pt wire was the counter electrode, and a saturated calomel electrode (SCE) within a
152
Luggin capillary was employed as reference electrode.
153
Color abatement and absorbance was determined using an UV-vis spectrophotometer
154
Hach DR 2000. TOC was measured with a TOC analyzer Apollo 9000HS from Teledyne-
155
Tekmar. From TOC abatement, mineralization current efficiency (MCE) was calculated from
156
equation (6) where n is the number of electrons required to attain complete mineralization of
157
methylene blue according to equation (7), F is the Faraday constant (96487 C mol-1), V is the
158
volume of solution in L, ∆(TOC) is the solution TOC decay (mg L-1), 4.32 x 107 is a conversion
159
factor to homogenize units (3600 s h-1 x 12000 mg mol-1), m is the number of carbon atoms of
160
methylene blue (16 C atoms), and I is the applied current (A). 7
161
MCE =
nFV ∆(TOC) x 100 4.32 x 107 mIt
(6)
C16H18N3S+ +45 H2O → 16 CO2 + 3 NO3- + SO42- +108 H+ + 102 e-
(7)
162 163
Chemical oxygen demand (COD) was quantified using low-range (LR) COD vials from
164
Hach. Concentration of H2O2 electrogenerated was followed with a colorimetric method based
165
on the formation of a yellow colored Ti-complex using K2TiO(C2O4)2·2H2O (Alfa-Aesar) and
166
measuring absorbance at 400 nm. Ferrous (Fe2+) and total iron were analyzed after forming a
167
colored complex with 1,10-phenanthroline using a Hach kit. Effective current (ieff) defined by
168
equation (8) was determined from the concentration of electrogenerated reagents ([R]). Then,
169
current efficiency (η) of electrochemical processes was quantified from equation (9).
nF[R]V t
(8)
ieff x 100 It
(9)
ieff =
η=
170
8
171
where n is the number of electrons consumed in the reaction, F is the Faraday constant (96487 C
172
mol-1), [R] is the concentration of reagent yielded in mol L-1, V is the volume of solution in L, t is
173
the electrolysis time in s, and I the total current applied in A.
174
3. Results and discussion
175
3.1 Understanding impact of operational variables on H2O2 electrogeneration
176
Degradation of organic pollutants by EAOPs is related to the capabilities of the
177
electrochemical system to efficiently generate OH [3,26]. Indirect electrochemical generation
178
of OH through Fenton’s reaction (2) during EF treatments appoints electrogeneration of H2O2
179
as the governing electrochemical process that defines performance [1, 32]. Electroanalytic tests
180
demonstrate excellent electrocatalytic properties of activated carbon fiber on oxygen reduction
181
towards H2O2. Figure 2 depicts the linear voltammograms during cathodic scan in N2, O2, or air
182
saturated solutions. A noticeable current increase can be observed with the increasing level of O2
183
saturation due to sparging of air and O2, which suggests an excellent direct charge transfer
184
performance of the carbonaceous cathode on O2 reduction processes.
185
0
186
-10
188
I / mA
187
0
-20
190
I / mA
189
-30
-20 -40 -60 -80
191
-1.5
-1
-0.5
0
0.5
E / V vs SCE
192 193
-40 -1.5
-1
-0.5
0
0.5
E / V vs SCE
9
194 195 196 197 198
Figure 2. Oxygen reduction current (iAir/O2 – iN2) of activated carbon fiber for (solid line) oxygen, and (dashed line) air. Inset panel shows linear sweep voltammetry under scan rate 33.3 mV s-1 recorded in 0.05 M NaClO4 at pH = 2.0 under solution saturated with (solid line) pure oxygen, (dashed line) air, and (dotted line) nitrogen.
199
Activated carbon fiber cathode presents high electrocatalytic response towards O2
200
reduction; however, H2O2 production should be evaluated under continuous operation mode
201
when aiming application for EF treatment. This section studies the impact of operational
202
variables of influence on O2 reduction efficiency of activated carbon fiber cathodes and H2O2
203
accumulation during continuous operation.
204
Fenton’s chemistry is applied in a narrow pH range defined by the low solubility of Fe3+
205
that precipitates as Fe(OH)3 above pH 4.0 according to solubility diagrams [29]. Therefore,
206
electrogeneration of H2O2 was evaluated within this operational range of pH that ensures
207
solubility and reactivity of Fenton’s catalyst (Fe2+/Fe3+) [1,33]. Figures 3a and b illustrate the
208
effect of initial pH on the electrogeneration of H2O2 and η, respectively. Higher accumulation of
209
H2O2 (0.8 mM) was observed at acidic pH of 1.5, which decreased subsequently with increasing
210
pH values. This trend is consistent with literature and may be explained by the larger availability
211
of H+ in solution, which initiates the two-electron reduction reaction (3) [34,35]. Lower
212
electrogeneration of H2O2 under identical experimental conditions of applied current diminishes
213
the η from ⁓25% at pH 1.5-2.0 down to ⁓15% at 3.5. Thus, optimum electrogeneration of H2O2
214
was observed at pH 2.0.
215
Applied current defines the number of electrons delivered per second which definitely
216
controls the electrokinetics of reactions on the cathode surface [1]. A noticeable increase in H2O2
217
production from 0.4 to 1.0 mM can be seen in Fig. 3c with increasing current from 50 to 250 mA.
218
However, a sudden drop in H2O2 concentration down to 0.8 mM was recorded with applied
10
219
current beyond 250 mA. Indeed, an in-depth analysis of current efficiency reveals the continuous
220
decrease of η for applied current values above 100 mA, as seen in Fig. 3d. This trend highlights
221
the enhancement of concomitant parasitic reactions that compete with the desired bi-electronic
222
reduction of oxygen according to reaction (3) such as oxygen reduction to water following
223
reaction (10), H2O2 reduction by reaction (11), or hydrogen evolution reaction from reaction (12),
224
respectively [35]. Under undivided electrochemical set-up, the acceleration of H2O2 oxidation at
225
the anode surface following reaction (13) cannot be disregarded [1,24]. Note that electrochemical
226
generation of H2O2 at 250 mA clearly illustrates the attaining of a plateau of concentration,
227
which evidences the maximum concentration achievable due to equilibrium between cathodic
228
generation and anodic oxidation. 4H+ + O2 + 4e−→ 2 H2O
(10)
2H+ + H2O2 + 2e−→ 2 H2O
(11)
2H+ + 2e−→ H2
(12)
H2O2 → 2H+ + O2 + 2e−
(13)
229 230
1
231
η%
20 15
0.4 10 0.2
235 236
0 0
5
10
20
237
30
40
50
60
0 1
70
time / min
238
1.2
0.8 0.6
2
2.5
3
3.5
4
30 50 mA 100 mA 150 mA 200 mA 300 mA 350 mA
c)
d) 25 20 15
2
2
H O / mM
1
1.5
pH
η%
234
0.6
b) 25
2 2
233
1.5 2.0 2.5 3.0 3.5
0.8 H O / mM
232
30
a)
0.4
10
0.2
5
0
0
11
239 240 241 242 243 244 245 246
1
247
30 -1
0.8
-1
0.25 L min
252 253
2
20
0.50 L min
0.6
-1
η%
0.75 L min
-1
2
251
25
-1
H O / mM
250
f)
0.00 L min
248 249
e)
1.00 L min
15
0.4 10 0.2 0 0
5
10
20
30
40
time / min
50
60
70
0 -0.25
0
0.25
0.5
0.75
1
1.25
-1
Air flow / L min
254 255 256 257 258
Figure 3. Influence of operational variables on H2O2 electrogeneration evaluated from (a,c,e) H2O2 accumulation and (b,d,f) current efficiency: (a-b) Effect of solution pH at 100 mA and 0.05 L min-1 of air,(c-d) effect of applied current at pH 2.0 and 0.05 L min-1 of air, and (e-f) effect of air flow rate at 100 mA and pH 2.0. Electrolyses were conducted in 0.05 mM NaClO4.
259
Figure 3e depicts the effect of the flow rate of air delivered in solution through bubbling
260
using the ceramic sparging. It is important to notice that in absence of sparging a discrete H2O2
261
production of 0.17 mM was observed due to the cathodic reduction of oxygen dissolved in
262
solution and the O2 produced from water oxidation reaction (14) at the anode, but with a low
263
efficiency of ⁓5% due to the diffusive control of O2 under this experimental condition (see Fig.
264
3f). Continuous delivery of air through bubbling increased the availability of O2 molecules in the
265
proximity of the electrode which positively affected H2O2 electrogeneration. As shown,
266
maximum H2O2 electrogeneration at 100 mA was attained with 0.5 L min-1 of air. The η 12
1.5
267
improved from 5% in the absence of bubbling and 8% at 0.25 L min-1 to ⁓23% at higher air feed
268
flows. This sudden increase in η is attributed to better convective mixing in the ACF bed thereby
269
promoting effective mass transport of dissolved O2 towards the cathode surface. 2 H2O → 4H+ + O2 + 4e−
(14)
270 271 272
It was also observed in Fig. 3e that much higher air fed flows did not result in higher
273
concentrations of electrogenerated H2O2. The slight decrease in H2O2 accumulation at higher air
274
flows may be explained by bubble resistance which caused lower η [35]. In order to eliminate
275
unnecessary energy expense on air pumping to reach identical performance, an optimum air flow
276
rate of 0.5 L min-1 was defined for continuous O2 delivery. It is important to remark that
277
experimental results of Fig. 4 demonstrate that the electrogenerated H2O2 allows attaining high
278
degree of mineralization through EF treatment. Indeed, excessive concentration may result in a
279
decrease of efficiency due to the scavenging reaction of produced OH in the bulk with excess of
280
H2O2.
281 282
3.2 Evaluating the role of adsorption and electrochemically-driven processes
283
The holistic understanding of a complex treatment system requires the elucidation of the
284
individual contribution of different elements to water treatment performance. Figure 4 shows
285
almost no decolorization of MB solution in the absence of applied current. Thus, it can be
286
inferred that the organic dye is barely adsorbed on the surface of the activated carbon fiber. In
287
addition, the dye solution remained unaltered even after gas sparging irrespective of the gas
288
delivered (N2, O2, or air) which confirmed that gas dissolution did not contribute to MB
13
289
degradation. In order to evaluate the role of H2O2 on dye decolorization, H2O2 was added to the
290
solution yet no color removal was observed. This result precludes the involvement of H2O2 alone
291
in the decolorization of MB solutions. On the other hand, 10% MB decolorization was observed
292
when current was supplied to the H2O2 generation electrochemical cell (electrochemically-driven
293
dosing of Fe2+ off). However, mineralization of organic load was not observed under these
294
conditions (see Fig. 4b). This is the characteristic behavior of active anodes such as Pt that
295
promote electrochemical conversion but not incineration towards CO2 [2,7]. This is in agreement
296
with previous reports that observed solution decolorization without mineralization, which
297
suggests the yield of colorless by-products but not the aromatic ring opening [36,37]. Figure 4
298
denotes the greater performance of the EF treatment when using the dual system with
299
electrochemical dosing of Fe2+ that attained complete decolorization after 90 min of treatment.
300
Faster decolorization was attained due to the production of homogeneous OH in the bulk of the
301
solution through Fenton’s reaction (2). The oxidizing character of
302
mineralization as deduced from the 83% abatement of TOC in 120 min of treatment. Remaining
303
TOC is associated to the formation of stable iron-carboxylate complexes [1, 16]. These low
304
molecular weight carboxylic acids are known to be harmless, photodegradable, and
305
biodegradable [38,39]. Further experiments will discuss the optimization of the dual-power
306
supply composite electrochemical reactor configuration for simultaneous Fenton’s catalyst (Fe2+)
307
dosing and H2O2 electro-generation.
OH radical enabled
308 309 310 311
14
312 313 314 315 316 317 318 319 320 a) 321
1.0
322
324
0
MB/MB
323
0.8 0.6 0.4
325 326
0.2
327
0 b)
328
1
330 331
TOC/TOC
0
329
0.8 0.6
332
0.4
333
0.2
334
0 0
20
40
60
80
100
120
140
time / min 15
335 336 337 338 339
Figure 4. (a) Decolorization and (c) TOC abatement during the treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 under different processes: () adsorption, () electrogeneration of H2O2 and Pt anode, () electro-Fenton treatment using the dual cell.
340 341 342
3.3 Optimizing electrochemically-driven dosing of iron (II) for electro-Fenton treatment
343
Advanced oxidation processes based on Fenton chemistry rely on the use of Fe2+ as a
344
catalyst to yield OH from the catalytic decomposition of H2O2 by Fenton reaction (2) [13, 40].
345
This implies that conventional Fenton treatment requires transport and stock of hazardous H2O2
346
as well as iron (II) salts. The electro-Fenton process overcomes the safety issues associated with
347
H2O2 by electrogenerating in situ the exact amount needed for an efficient production of
348
[4,33]. However, iron dosing is still required. This limits automatization of electro-Fenton
349
technology and their translation to continuous flow treatment designs. The dual-cell electro-
350
Fenton system (see Fig. 1) allows introducing an electrochemically-driven system to dose the
351
iron required [28]. This first proof of concept shows a promising opportunity for electro-Fenton
352
technologies introducing a novel strategy that can incentivize technology transfer towards point-
353
of-entry treatment systems as well as low-physical footprint units for middle-sized industry.
OH
354
Figure 5 depicts the influence of the applied current in the iron-dosing cell (IFe) while
355
simultaneously operating the H2O2 electrogeneration unit of the reactor cell. It can be observed
356
that in the absence of IFe, slight decolorization is observed but with no mineralization. The
357
analysis of iron in solution shows no yield of iron ions under IFe = 0 mA. Note that under such
358
conditions electrodissolution of the sacrificial anode is not expected, whereas the only source of
16
359
iron would be associated exclusively to chemical dissolution (which was not observed). On the
360
other hand, application of IFe is followed by iron release that is used as catalyst in the EF reaction.
361
A small current such as 7.5 mA resulted in the accumulation of a total iron concentration of 0.8
362
mM, which is in the range of conventional optimum values (between 0.5 and 1.4 mM) for EF
363
treatment [1, 10]. Under such experimental conditions complete decolorization was attained after
364
80 min of dual electrolytic treatment. Additional increment of applied IFe did not accelerate
365
decolorization kinetics, but only accumulated higher concentration of iron in solution. Note that
366
the excess of iron in solution diminished mineralization percentage attained due to the major
367
complexation of by-product as well as the scavenging reaction (15) of Fe2+ with produced OH
368
[7,21]. Note that undesirable accumulation of iron may result in sludge production that would
369
eventually require a secondary treatment as well as solid waste management [29]. Therefore,
370
from these results an optimum IFe of 7.5 mA was identified for the operation of the dual-cell EF
371
system. Fe2++ OH → Fe3++ OH −
(15)
372 373
3.4 Evaluating influence of applied current (IH2O2) on electro-Fenton treatment
374
Applied current (IH2O2) is the operational parameter that defines amount and rate of
375
produced H2O2 (see Fig. 3b) and the electroregeneration of iron(II) from reaction (4). Thus, it
376
controls the overall electrokinetics and performance of the EF process [1,41]. Figure 6 describes
377
the impact of IH2O2 on decolorization and mineralization. As shown, increase of applied IH2O2
378
results in a faster decolorization rate due to the concomitant increase in OH production [9,42].
379
Similarly, greater mineralization percentage was attained under identical treatment times for
380
higher applied currents. However, the lower enhancements observed when increasing IH2O2 from 17
381
250 mA up to 350 mA suggest a lower mineralization efficiency of the process due to the
382
competitive consumption of electrons in parasitic reactions that do not lead to organic dye
383
mineralization, such as oxygen evolution by reaction (14) or OH dimerization following
384
reaction (16). This trend agrees with the lower MCE values observed for increasing applied
385
currents. 2 OH → H2O2
(16)
386 387 388
a)
390
0.8 MB/MB
391
0
389
1.0
0.6 0.4
392
0.2
393
0
394
b) 1.0
396 397
TOC/TOC
0
395 0.8 0.6 0.4
398 0.2
399
0
c)
400
1.5
/ mM
403
1
total
402
Fe
401
0.5
18 0 0
20
40
60
80
time / min
100
120
140
404 405 406 407 408 409 410
Figure 5. (a) Decolorization, (b) TOC abatement, and (c) total iron accumulation during the dual-cell electro-Fenton treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 at IH2O2=150 mA under different applied current for iron delivery: () IFe= 0 mA, () IFe= 7.5 mA, and () IFe= 15.0 mA.
411 412 413
In this frame, it can be concluded that an excess of current would increase energy
414
consumption without effectively accelerating the mineralization process. It may be inferred that
415
the optimum IH2O2 is 150 mA since it allows attaining faster decolorization and higher
416
mineralization
percentage with the minimal energy requirement.
417
a)
419
0.8
15 % MCE
1.0
420 421
MB/MB
0
418
20
10
0.6
5
0.4
0 0
20
422
424
0
425
1.0
b)
TOC/TOC
0
426
428
60 80 100 120 140 time / min
0.2
423
427
40
0.8 0.6 0.4 19
0.2 0 0
20
40
60
80
time / min
100
120
140
429 430 431 432 433 434 435 436 437
Figure 6. (a) Decolorization, and (b) TOC abatement during the dual-cell electro-Fenton treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 at IFe= 7.5 mA mA under different applied current for iron delivery: () IH2O2=50 mA, () IH2O2=150 mA, () IH2O2= 250 mA, and () IH2O2= 350 mA. Inset panel depicts the mineralization current efficiency.
438
3.5 Understanding the effect of pH on the dual-cell electro-Fenton system
439
Fenton-based technologies are sensitive to the initial pH of the wastewater. Figure 7
440
describes the influence of initial pH of the dye solution on MB abatement and the efficient
441
mineralization of the organic load. Interestingly, the controlled dose of iron into the reactor
442
contributes to minimize the negative impacts of higher pH usually reported in Fenton reaction.
443
The narrow window of operational pH is defined by the solubility of iron(II)/iron(III) species in
444
solution [29]. Controlled electrodissolution of sacrificial iron electrode provides a continuous
445
supply of Fe2+ for the Fenton’s reaction, which ensures continuous availability of Fe2+ in solution
446
to efficiently react with H2O2 by reaction (2). This explains the slight difference on dye
447
decolorization observed in Fig. 7a. However, TOC analysis shows a slower mineralization rate at
448
highly acidic pH. This trend may be explained by the lower amount of OH produced by
449
Fenton’s reaction from H3O2+ [1]. It is important to remark that mineralization attained at pH 4.0
450
is higher than usually observed for conventional electro-Fenton systems. The results suggest that
451
the continuous supply of Fe2+ due to a controlled dosage unit can contribute to overcome pH
452
limitations due to iron hydroxides precipitation. Therefore, iron dosing strategies is an approach
453
worthy of further research to understand future applicability in automized electro-Fenton devices.
20
454 455 456
1.0
457
459
0
0.8 MB/MB
458
460
0.6 0.4
461
0.2
462
0
463 464
1
467 468
TOC/TOC
466
0
465
0.8 0.6 0.4
469 470 471 472
0.2 0 0
20
40
60
80
100
120
140
time / min 473 474 475 476
Figure 7. (a) Decolorization and (c) TOC abatement during the treatment of 700 mL of 50 mg L-1 of Methylene Blue solution with 0.05 M of NaClO4 with the dual cell under applied current of IFe= 7.5 mA and IH2O2= 150 mA under different pH: () 1.5, () 2.0, () 3.0, and () 4.0.
477
21
478
3.6 Assessing electro-Fenton process reliability through electrode stability
479
Water treatment technologies must be reliable under long operation conditions. Research
480
focused on the study and development of novel electrocatalysts that have low stability. Electrode
481
deactivation can occur due to the deposition of polymeric films due to reduction or oxidation of
482
organics on electrodes surface under low operational potentials [7]. In order to demonstrate the
483
stability of the proposed dual electrochemical reactor system for autonomous EF,
484
electrochemical cell performance under successive treatment cycles was tested. Figure 8 shows
485
high stability of electrodes that achieve consistently identical results on MB decolorization and
486
TOC abatement. It is important to remark that some experimental results suggest that certain
487
catalysts and electrocatalysts must be reactivated after each cycle to ensure catalytic activity and
488
avoid inhibition [1,35]. The carbon fiber electrode maintains effectiveness without pre-treatment
489
or reactivation requirements, even though further cycles or continuous operation must be further
490
evaluated to ensure long term performance and electrode stability.
491 492
st
1.0
493
496 497
nd
2 cycle
rd
3 cycle
th
4 cycle
th
5 cycle
0.8
0
495
TOC/TOC 0 MB/MB
494
1 cycle
0.6 0.4
498 499 500 501
0.2 0 0
40
80
0
40
80
0
40
80
0
40
80
0
40
80 120
time / min
22
502 503 504
Figure 8. () Decolorization and ()TOC abatement during the treatment of 700 mL of 50 mg L1 of Methylene Blue solution with 0.05 M of NaClO4 at pH 3.0 with the dual cell under applied current of IFe= 7.5 mA and IH2O2= 150 mA after succesive cycles.
505
4. Conclusions
506
Electrogeneration of H2O2 makes electro-Fenton technologies reliable and more
507
competitive due to their independence from hazardous H2O2 transport and storage. Applied
508
current can control the amount of available H2O2 in the electrochemical reactor. However,
509
electro-Fenton process is still dependent on iron(II) addition as Fenton’s catalyst, usually added
510
as iron sulfate salt. In this work, the implementation of a dual-cell system demonstrates the next
511
technological step towards “reagent free” wastewater treatment. The compact electrode system
512
of carbon fiber cathode/platinum anode produces high concentrations of H2O2 from the
513
electrochemical reduction of oxygen while the iron dosing cell consisting of a sacrificial iron
514
anode and a titanium mesh as cathode effectively controls iron delivery in solution by the
515
application of small currents. Results show the efficient electrogeneration of 0.8 mM of H2O2
516
under optimum conditions of pH 3.0 and applied current of 150 mA. In the absence of iron
517
dosing, slight decolorization was observed from the electrochemical oxidation of platinum.
518
However, complete dye decolorization accompanied by an 80% of organic load mineralization
519
was attained with iron dosing. Higher treatment performance was attributed to the indirect
520
electrogeneration of OH produced by Fenton’s reaction. Optimization of operational variables
521
indicate an optimum IFe of 7.5 mA to deliver required iron dose while avoiding sludge formation.
522
In contrast, an optimum IH2O2 of 150 mA attained high degree of mineralization. Overall, the
523
system demonstrated high reliability under continuous operation while consistently maintaining
524
similar decolorization and mineralization performance after several consecutive cycles. These
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525
results open promising opportunities for dual-cell systems for the autonomous operation of
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electro-Fenton systems using consumable cartridges consisting of a sacrificial iron electrode.
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Declarations of interest
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The authors declare that they have no known competing financial interests or personal
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relationships that could have appeared to influence the work reported in this paper.
531 532
Acknowledgements
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The authors would like to thank the Department of Science and Technology (DOST),
534
Philippines and University of Delaware, United States for the financial support. The authors are
535
grateful to Rovshan Mahmudov, Ph.D. and Michael Davidson for their assistance.
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537
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Highlights Dual cell system can control iron dosing Electrochemically-driven dosing of iron diminishes reliance on chemical addition Iron dosing strategies can minimize sludge formation Electro-Fenton in dual cell enhances performance attaining dye degradation
Conflict of interest Authors declare no conflict of interest.