A reactive electrochemical filter system with an excellent penetration flux porous Ti/SnO2-Sb filter for efficient contaminant removal from water

Article abstract of DOI:10.1039/c8ra00603b

Tubular porous Ti/SnO₂-Sb filters with excellent penetration flux (～61.94 m³ m^-2 h^-1 bar^-1) and electrochemical activity were prepared by a sol-gel method using low-cost porous titanium filters as the

Full text of DOI:10.1039/c8ra00603b

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A reactive electrochemical filter system with an
excellent penetration flux porous Ti/SnO –Sb filter
2
Cite this: RSC Adv., 2018, 8, 13933
for efficient contaminant removal from water†
ab
b
c
d
b
b
Kui Yang, Hui Lin, * Shangtao Liang, Ruzhen Xie, Sihao Lv, Junfeng Niu,
b
ae
Jie Chen and Yongyou Hu
*
3
ꢁ2
ꢁ1
ꢁ1
Tubular porous Ti/SnO
electrochemical activity were prepared by a sol–gel method using low-cost porous titanium filters as the
substrates. The porous Ti/SnO –Sb filters were used as anodic reactive electrochemical membranes to
develop reactive electrochemical filter systems, by combining membrane filtration technology with the
2
–Sb filters with excellent penetration flux (ꢀ61.94 m
m
h
bar ) and
2
ꢁ4
ꢁ1
electrooxidation process, for water treatment. A convection-enhanced rate constant of 4.35 ꢂ 10 m s
4ꢁ
was achieved for Fe(CN)
6
oxidation, which approached the kinetic limit and is the highest reported in an
electrochemical system. The electrooxidative performance of the reactive electrochemical filter system
ꢁ1
was evaluated with 50 mg L rhodamine B (RhB). The results showed that the reactive electrochemical
filter system in flow-through mode resulted in an 8.6-fold enhancement in RhB oxidation as compared to
those in flow-by mode under the same experimental conditions. A normalized rate constant of 5.76 ꢂ
ꢁ4
ꢁ1
10
m s for RhB oxidation was observed at an anode potential of 3.04 V vs. SCE, which is much higher
than that observed in a reactive electrochemical filter system with carbon nanotubes and/or Ti
4
O
7
(1.7 ꢂ
ꢁ5
ꢁ4
ꢁ1
10
–1.4 ꢂ 10 m s ). The electrical energy per order degradation (EE/O) for RhB was as low as 0.28
Received 20th January 2018
Accepted 26th March 2018
ꢁ3
kW h m
in flow-through mode, with a relatively short residence time of 9.8 min. The overall
mineralization current efficiency (MCE) was calculated to be 83.6% with ꢀ99% RhB removal and ꢀ51%
TOC removal. These results illustrate that this reactive electrochemical filter system is expected to be
a promising method for water treatment.
DOI: 10.1039/c8ra00603b
rsc.li/rsc-advances
2
including paper production wastewater, textile waste-
1
. Introduction
3
–5
6,7
water,
The electrooxidation process has emerged as a promising compounds.
technique for the destruction of various toxic refractory
However, the relatively long retention time and low current
landll leachate
and refractory peruorinated
8
,9
organic pollutants in aqueous solutions due to its mild efficiency limit its large-scale application. This is because
reaction conditions, strong oxidation ability, easy operation electrooxidation reactions are specic to heterogeneous
1
and environmental compatibility. Recently, it has been catalysis, and the mass transfer rate of the traditional elec-
successfully applied in treating industrial wastewater trooxidation process limits the overall efficiency of the reac-
1
0
tion system. Traditional electrooxidation reactors typically
use parallel plate electrodes, in which wastewater is pumped
School of Environment and Energy, South China University of Technology, Guangzhou through the narrow ow channel (in the range of mm to cm)
a
5
10006, P. R. China. E-mail: 201520132411@mail.scut.edu.cn; ppyyhu@scut.edu.cn; between the anode and cathode either in one-pass or recir-
Fax: +86-20-3938-0508; Tel: +86-136-0274-6125
culation mode. This is dened as ow-by mode. In this uid
dynamic state, a thin stagnant boundary layer is formed near
the surface of the electrode. In addition, hydroxyl radicals
b
School of Environment and Civil Engineering, Dongguan University of Technology,
Dongguan 523808, P. R. China. E-mail: linhui@bnu.edu.cn; 23817001@qq.com;
junfengn@bnu.edu.cn; 1875898657@qq.com; Fax: +86-769-3901-8706; Tel: +86-
(
cOH) formed via the oxidation of water on the anode surface
7
69-3901-8706
c
AECOM Inc., Environment, Atlanta, Georgia 30309, USA. E-mail: sliang@uga.edu
can react unselectively with a wide range of organics at an
d
10
ꢁ1 ꢁ1 11,12
College of Architecture and Environment, Sichuan University, Chengdu 610065, P. R. extremely high reaction rate (ꢀ10 L mol
s
).
Experi-
China. E-mail: xieruzhen@scu.edu.cn
mental studies and modeling data suggest that the reaction
e
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,
between organics and cOH only occurs in a narrow zone,
within 1 mm around the anode surface.
boundary layer of the electrode surface is thicker than 100 mm
in typical ow-by modes, which is much thicker than the
Ministry of Education, South China University of Technology, Guangzhou Higher
Education Mega Centre, Guangzhou, 510006, PR China
1
1,12
However the
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra00603b
1
0
1
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thickness of the reaction zone between organics and cOH.
Under this circumstance, the diffusion of organics to the
reaction zone is extremely difficult, resulting in a low
ꢁ
6
normalized mass transfer rate constant of 10
to
ꢁ
5
ꢁ1 10,13,14
1
0
m s .
Therefore the reaction rate is limited by
the mass transfer rate, which lowers the overall current
efficiency.
In order to overcome the mass transfer limitation in the
traditional electrooxidation process, researchers have utilized
Ti O
membranes
(n ¼ 4 to 6) conductive ultraltration ceramic
n
2(nꢁ1)
2
. Experimental
13,15–17 14,18–20
or carbon nanotube (CNT) networks
as
2.1. Materials
anodes to build anodic reactive electrochemical lter systems,
by combining membrane ltration technology with the elec- Commercially available sintered powder porous tubular tita-
trooxidation process, in ow-through mode for water treat- nium lters (Baoji Yinggao Metal Material Co., Ltd. China) with
ment. The thickness of the boundary layer is a function of the inner and outer diameters of 26 and 30 mm, respectively, and
cross-ow velocity and/or the turbulence of the ow. In ow- a length of 100 mm were used as substrates. Sodium hydroxide
through mode, water moves adjectively through the pores on (NaOH), oxalic acid (C H O ), anhydrous sodium sulfate
2
2 4
the electrode materials. This process minimizes the thickness (Na SO ), ethylene glycol (C H O ), citric acid (C H O ), potas-
2
4
2
6
2
6
8 7
of the boundary layer, thus greatly improving the mass sium ferricyanide (K Fe(CN) ) and potassium ferrocyanide
3
6
transfer rate of pollutants moving towards the anode surface (K Fe(CN) ) were obtained from Tianjin Damao Chemical
4
6
n
zone. However, Ti O2(nꢁ1) conductive ceramic ultraltration Reagent Co., Ltd. Tin chloride pentahydrate (SnCl $5H O),
4
2
membranes with nanopores may be easily contaminated antimony trichloride (SbCl ) and rhodamine B (RhB) were ob-
3
because industrial effluents tend to be of poor water quality tained from Aladdin Reagent Co., Ltd. All of the chemicals were
and adding a pre-treatment increases the total cost. CNTs are reagent grade or higher.
not active for cOH production, and their high cost further
limits the large-scale application of CNT networks. Conse-
quently, it’s urgent to seek inexpensive, porous electrode
materials with macropores with excellent permeability for
2
2
.2. Porous Ti/SnO –Sb lter preparation
The substrate tubular porous titanium lters were rstly
immersed in NaOH solution (10%, m/m) at 90 C for 1 h to
remove grease, and then etched in boiling oxalic acid (10%, m/
m) for 1 h to form a gray surface with uniform roughness.
Tubular porous Ti/SnO –Sb lters were prepared by the sol–gel
method using a dip coater (SYDC-200, Shanghai Sanyan Tech-
nology Co., Ltd. China). The preparation process involved the
following steps: citric acid and ethylene glycol were mixed and
agitated at 60 C until fully dissolved, and then the solution was
heated to 90 C under continuous stirring. A mixture of SnCl
ꢃ
2
1
water treatment. Recently, Li et al. prepared a porous Ti/
SnO –Sb anode in ow-through mode from porous titanium
2
tubes with an average pore size of 2.5 mm to degrade pyridine.
Compared to the traditional ow-by mode, the ow-through
mode increased the pyridine degradation rate by 43.9%.
However, the mass transfer efficiency of this electrochemical
system was not mentioned.
As typical “non-active” electrodes, Sb-doped SnO2 anodes
have many advantages such as cost-effectiveness, being easy to
prepare, high oxygen evolution potential and good catalytic
2
ꢃ
ꢃ
4
-
$
5H
2
O and SbCl
3
was added to the solution at the molar ratio of
(ethylene glycol : citric acid : SnCl $5H -
1
40 : 30 : 9 : 1
21–24
4
2
activity.
mechanism of electrooxidation by studying the mass transfer
rate of a ow-through system using tubular Ti/SnO –Sb lters,
Thus, in this study, we focus on understanding the
ꢃ
O : SbCl ) at 90 C and the resulting mixture was maintained at
3
ꢃ
90 C for 1 h in order to obtain a sol–gel. Finally, the sol–gel
2
solution was used to coat the porous titanium lters through
dipping at a rate of 200 mm s . Aer that, the porous titanium
with an average pore size of about 30 mm, as reactive electro-
ꢁ1
chemical membranes. The standard probing molecule
ꢃ

lters were dried at 140 C for 20 min in an oven and sintered to
4
ꢁ
Fe(CN)6 was used to assess the mass transfer performance of
ꢃ
induce the thermal decomposition of the coating at 550 C for
the reactive electrochemical lter system for direct oxidation
2
1
0 min in a muffle furnace. The above operation was repeated
2 times and the last baking was annealed for 2 h at 550 C to
(outer-sphere charge transfer). Rhodamine B (RhB, the molec-
ꢃ
ular structure is shown below) was chosen as a model refractory
organic pollutant because RhB is a common dye in the triphe-
nylmethane family and has been used extensively in the textile,
obtain the tubular porous Ti/SnO –Sb lters. More information
concerning the Ti/SnO –Sb electrode preparation has been
given elsewhere.
2
2
27
25,26
printing, food and cosmetic industries.
The effects of the
operating mode and the applied anode potential on the
performance of the reactive electrochemical lter system were
2.3. Experimental setup
investigated. To further verify the feasibility of the reactive The experimental setup of the reactive electrochemical lter
electrochemical lter system for water treatment, mineraliza- system is shown in Fig. 1. The tubular porous Ti/SnO –Sb lter
2
tion current efficiency and energy consumption were also was used as the anode and a stainless steel perforated tube with
assessed.
an outside diameter of 6 mm and a length of 150 mm was used
as the inner cathode, which was also used as a water distributor.
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Fig. 1 A schematic diagram of the experimental setup: (1) the design of the reactive electrochemical filter system consisting of two perforated
stainless steel pipes acting as water distributors, one of which is also used as the inner cathode, and a stainless steel pipe acting as the outer
cathode and a tubular porous Ti/SnO –Sb filter acting as the anode; (2) a photo of the reactive electrochemical filter system.
2
ꢁ1
A stainless steel tube mold with an inside diameter of 50 mm constant (k
m
, m s ) for the pollutant diffusion to the electrode
and a length of 100 mm was used as the outer cathode. The surface in ow-through mode was characterized using the
4
ꢁ
anode and the cathodes were 10 mm apart. The electrochemical electrooxidation rate constant of Fe(CN)
6
, in which oxidation
workstation (PARSTAT 2273, USA) provided a stable current for only occurred on the anode surface through direct electron
the reaction system. In ow-by mode, pump 2 was turned on transfer (for details, see Text S1†). In this experiment, 2 L of
and pump 1 was turned off; the solution owed only by the 0.1 M KH PO with 5 mM K Fe(CN) and 1 mM K Fe(CN)6
2
4
4
6
3
surface of the electrode and then out of the reactor. In ow- solution was continuously owed and recirculated in ow-
through mode, pump 1 was on and pump 2 was off; the solu- through mode.
tion owed from the inside of the electrode to the outside of the
reactor aer passing through the electrode.
The electrochemical degradation of RhB was conducted
ꢁ
1
using a 1 L solution containing 50 mM Na
2 4
SO and 50 mg L
RhB in the reactive electrochemical lter system at an inlet ow
ꢁ
1
rate of 1.8 L min . Both the feed and permeate streams were
00% recycled for 10 min to purge the system at zero current,
2
.4. Electrode characterization
1
The surface morphology of the electrode was characterized by
scanning electron microscopy (SEM, Hitachi S-4800, Japan). The
crystalline phases of the electrode were measured by X-ray
diffraction (XRD, Rigaku D/max-3C, Japan) with Cu Ka radia-
tion at a scanning rate of 2 min in 2q mode from 20 to 80 .
The surface composition of the electrode was characterized by
X-ray photoelectron spectroscopy spectra (XPS, ESCALAB 250Xi,
USA) with a monochromatic Al Ka source. The pore size
distributions and porosity of the electrode were characterized
by mercury porosimetry (AutoPore IV 9500, Micromeritics). The
pressure–membrane ux relation was calculated according to
the different inlet ow and corresponding pressure values in
then an input current between 0.25 A and 1.5 A was applied to
the reactive electrochemical lter system. Triplicate samples of
mL each were taken at different time intervals. During each
sampling, the electrolysis was stopped and the solution was
kept looping to ensure the solution was homogeneous. All of the
electrolysis experiments were triplicated and carried out at
5
ꢃ
ꢁ1
ꢃ
ꢃ
ꢃ
room temperature (25 ꢄ 1 C). The corresponding results were
averaged with a standard deviation below 10% of the means.
2
.6. Analytical methods

ow-through mode. The linear sweep voltammetry (LSV) scan-
3
ꢁ
The concentrations of RhB and Fe(CN)
6
were determined by
ning and cyclic voltammetry (CV) scanning were carried out on
the system in ow-through mode and were driven by an elec-
trochemical workstation (PARSTAT 2273, USA), with a tubular
porous Ti/SnO –Sb lter as the working electrode, two stainless
2
steel tubes as the counter electrode and a saturated calomel
electrode (SCE) as the reference electrode.
UV-vis spectrophotometry (UV-5100B, Shanghai Yuanxi Analysis
Instrument Co., Ltd. China) at the wavelengths of 553 and
20 nm, respectively. The total organic carbon (TOC) was
measured by a TOC analyzer (TOC-L CPH, Shimadzu Co., Ltd.
Japan).
The RhB degradation rate (h, %) was calculated according to
the following equation:
4
2.5. Electrochemical experiments
C
0
ꢁ C
t
h ¼
ꢂ 100%
(1)
The electrochemical experiments were performed using the
setup shown in Fig. 1. The normalized mass transfer rate
C
0
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ꢁ
1
ꢁ1
where C
0 t 2
(mg L ) and C (mg L ) are the concentrations at recorded using the fabricated porous Ti/SnO –Sb lter.
time zero and time t, respectively. The RhB oxidation ux Diffraction peaks for Sb and its oxide were not observed,
ꢁ
1
ꢁ2
(
OF, mol h
m
) was calculated according to the following however Fig. 2(d) shows the presence of Sb in the porous Ti/
28
equation:
SnO –Sb lter. Therefore, we infer that Sb is well-dispersed in
2
30
the SnO
Compared with the standard card (JCPDS card no: 41-1445), the
Sb-doped SnO coating lm is identied as a rutile-type crystal.
Additionally, no crystallized TiO diffraction peak was found,
2 2
lattice and forms a Sb-doped SnO solid solution.
C
0
ꢁ C
t
OF ¼
V
(2)
At
2
2
3
where A (m ) is the geometric surface area of the anode, V (m ) is
the reaction volume and t (h) is the reaction time. The reaction
time (t, h) was calculated according to the following equation:
2
which strongly suggested that the titanium support substrate is
not oxidized during the Sb-doped SnO2 coating lm
preparation.
The pore structure and pore size distribution were charac-
terized more thoroughly using Hg intrusion porosimetry as is
shown in Fig. 2(e). The intrusion pore volumes of the substrate
1
C
0
t ¼ k
ln
C
(3)
RhB
t
where kRhB is the pseudo-rst-order kinetic constant of RhB
oxidation. The overall mineralization current efficiency (MCE,
) was calculated according to the following equation:
and the Ti/SnO
attributed to the macropores of 6–60 mm. The median pore
diameters (based on volume) of the substrate and the Ti/SnO
Sb lter were 31.5 mm and 30.2 mm, respectively. The median
pore diameter (based on volume) of the Ti/SnO –Sb lter was
2
–Sb lter were almost unchanged, which was
%
n DTOC
2
–
MCEð%Þ ¼ 28 12IDt FV ꢂ 100%
(4)
ꢁ
1
where DTOC is the concentration of TOC (g L ) removed
during a given electrolysis time (Dt, s), I is the input current (A),
2
3
1.5 mm, slightly larger than that of the substrate (30.2 mm). This
ꢁ1
is mainly caused by oxalic acid corrosion during the substrate
pre-treatment.
F is the Faraday constant (96 485C mol ), V is the treatment
solution volume (L), and n ¼ 156 is the electron number
required to completely mineralize one molecule of RhB, and n ¼
Penetration ux is an important indicator for the perfor-
mance of porous membrane-like materials, and it was used to
characterize the permeability of the porous Ti/SnO –Sb lter.
2
The pressure–membrane ux relationship is shown in Fig. 2(f).
Due to the similar pore diameters of the substrate and the
2
8 and n ¼ 12 are the number of carbon atoms contained in
each RhB molecule and the atomic mass of carbon, respectively.
The electrical energy consumption, EE/O, was calculated to
evaluate the energy efficiency of the reactive electrochemical
porous Ti/SnO –Sb lter, the pure water membrane ux of the

lter system. The EE/O is dened as the energy required to
2
substrate and the porous Ti/SnO
2
–Sb lter is almost the same
reduce the concentration of the target pollutant by one order of
ꢁ
3
(see Fig. 2(e)). The pure water penetration ux of the porous Ti/
magnitude. The EE/O (kW h m ) was calculated according to
3
ꢁ2 ꢁ1
ꢁ1
29
SnO
2
–Sb lter is as high as 61.94 m m
h
bar , thus both
the following equation:
the permeability and water treatment capacity are superior
(5) compared with the reactive electrochemical membranes re-
cell
U I
EE=O ¼
t
90%
V
ported in the literature, including Ti O
conductive ceramic
n
2(nꢁ1)
3
ꢁ2
ꢁ1
ꢁ1
3
ꢁ2
ꢁ1
where Ucell is the average cell voltage applied during the treat-
ment (V), I is the input current (A), t90% is the time required for
0% RhB degradation (h), and V is the treatment solution
membranes (0.05 m
m
h
bar
to 3.21 m
m
h
ꢁ1
13,27,31
3
ꢁ2 ꢁ1
3
bar
)
and CNT-networks (0.017 m m
h
to 0.34 m
ꢁ
2
ꢁ1 14
9
m
h
). As a result, the reactive electrochemical lter system
volume (L).
in this work can maintain a high water ux under low-pressure
conditions, which makes it easy to operate during water
treatment.
3
. Results and discussion
3
.1. Physical characterization
The photos and SEM images of the substrate, sintered powder
porous tubular titanium lter, and the fabricated porous Ti/ The linear polarization curves of the porous Ti/SnO
SnO –Sb lter are shown in Fig. 2. As shown in Fig. 2(a), the are displayed in Fig. 3(a) and the measured oxygen evolution
3.2. Electrochemical characterization
2
–Sb lter
2
surface of the substrate aer cleaning and corrosion treatment potentials (OEP) are 1.74 V and 1.87 V (vs. SCE) in neutral and
appears very rough, which contributes to the coating by the sol acidic conditions, respectively. The high oxygen evolution over-
precursor solution. Aer the sol–gel preparation process, the Ti/ potential indicated a low side reaction of oxygen formation,
SnO –Sb lter maintained its highly porous structure, and the which would be benecial for promoting the oxidation effi-
2
size of the aperture did not appear to change signicantly (see ciency of pollutants during electrocatalytic oxidation processes.
Fig. 2). From the enlarged view in Fig. 2(b), we can observe that Therefore, the electrode has a better electrochemical perfor-
the Sb-doped SnO
SnO –Sb lter is compacted and crack-free, which helps prolong
the service life of the porous Ti/SnO –Sb lter.
2
coating on the surface of the porous Ti/ mance in acidic conditions.
The cyclic voltammetry (CV) curves of the porous Ti/SnO
lter in 0.5 M Na SO solution and 0.5 M Na SO solution with
2
2
–Sb
28
2
2
4
2
4
ꢁ
1
Fig. 2(c) shows the wide-angle XRD analysis of the substrate 10 mg L RhB were generated to investigate the reaction type of
and the fabricated porous Ti/SnO –Sb lter. A series of diffrac- RhB degradation in the reactive electrochemical lter system in
tion peaks for the titanium metal and SnO
2
2
structures were this study. As shown in Fig. 3(b), no additional peaks were
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Fig. 2 SEM images: (a) the substrate and tubular sintered powder porous titanium filter; (b) the fabricated porous Ti/SnO
2
–Sb filter; (c) XRD
patterns; (d) XPS analysis; (e) the results of the Hg intrusion porosimetry analysis of the pore size distribution; (f) the pure water penetration flux of
the substrate and porous Ti/SnO –Sb filter.
2
found in Na
recorded in Na
oxidation of RhB over the porous Ti/SnO
2
SO
4
with RhB solution compared to the CV curve plays a key role in the RhB electrooxidation process in ow-
solution, indicating that the direct electro- through mode.
–Sb lter did not The electrochemical stability of the tubular porous Ti/SnO
occur. Obviously, we can conclude that an indirect cOH attack Sb lter was also evaluated. Fig. S1† shows the results of the
2
SO
4
2
2
–
21
ꢁ1
Fig. 3 (a) The linear polarization curves and (b) cyclic voltammetry curves of the porous Ti/SnO
2
–Sb filter at a scan rate of 50 mV s . Penetration
3
ꢁ2 ꢁ1
flux: 12.3 m m
h .
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accelerated service life test on the porous Ti/SnO
2
–Sb lter at
ꢁ2
ꢃ
a current density of 0.5 A cm in 0.5 M H SO solution at 30 C.
2
4
It was observed that the accelerated service life time was about
0.23 h when the cell voltage increased from 5 V to 10 V. The
actual service life can be calculated by the following equation:
1
ꢀ
ꢁ
2
I
I
2
1
s
1
¼
s
2
(6)
ꢁ
2
ꢁ2
where I
1
(A m ) and I
2
(A m ) are the actual current density
and the accelerated current density, respectively. s
h) are the actual service life time and the accelerated service life
time, respectively. The actual service life of the porous Ti/SnO –
1 2
(h) and s
(
2
ꢁ2
Sb lter was 2.92 years under a current density of 10 mA cm
Compared with previous studies (as shown in Table S1†), the
service life of the porous Ti/SnO
–Sb lter in this study was Fig. 4 A plot of the normalized observed rate constant (kobs, m s ) for
.
ꢁ
1
2
4
ꢁ
Fe(CN) oxidation in the reactive electrochemical filter system
operated in flow-through mode as a function of penetration fluxes
J, m s ). The solid orange line represents eqn (7), the model fit, the
red dashed line is the normalized kinetic rate constant (k, m s
6
much longer. The improved service life was probably attributed
to the use of porous materials, which are conductive and could
tightly combine the Sb-doped SnO
ꢁ1
(
2
coating layer with the
ꢁ1
)
3
2
substrate.
The electrochemically active surface area is where active sites the convective mass transfer limit calculated by kobs ¼ J. Experimental
solution: 2 L of 0.1 M KH PO with 5 mM K Fe(CN) and 1 mM
estimated by model fit to experimental data, and the blue dotted line is
are accessible to the electrolyte when the electrochemical
2
4
4
6
3
3
K
3
Fe(CN)
6
solution.
reaction occurs. Voltammetric charge (q*), which is closely
related to the real surface area and the number of electro-active
sites on an electrode (especially for a porous electrode), is
a major factor for determining the electrochemical performance
ꢁ
4
ꢁ1
4
.2 ꢂ 10 m s and at a penetration ux greater than 3.01 ꢂ
ꢁ
3
ꢁ1
34–36
10 m s , the kobs values only increased by 4.5% when the
penetration uxes were enhanced by 1.72 times from 3.01 ꢂ
of an electrode.
SnO –Sb lter electrode, performed within the potential range
of 0.4 V to 1.2 V vs. SCE in 0.5 M Na SO solution in ow-
The CV curves of the tubular porous Ti/
2
ꢁ3
ꢁ1
ꢁ3
ꢁ1
3
ꢁ2 ꢁ1
1
0
m s to 5.17 ꢂ 10 m s (or 10.8–18.6 m m
h
).
2
4
4
6
ꢁ
These results strongly suggest that the Fe(CN)
oxidation
through mode at different sweep rates as shown in Fig. S2(a),†
are used to calculate the q* by integrating the CV over the whole
potential range to further calculate the electrochemical porosity
process at very high penetration uxes is controlled by kinetic
limitation rather than mass transfer. To the best of our
ꢁ4
ꢁ1
knowledge, the measured kobs value (4.35 ꢂ 10 m s ) in this
study is the highest in an electrochemical ow-through reactor
compared with those reported in the literature. The measured
kobs values were 3.1-fold and 4.4-fold higher than the highest
value reported for Ti O -based reactive electrochemical
and roughness factor (R
S2(c)†).
f
) (for details, see Text S2, Fig. S2(b) and
The values of the charges, electrochemical porosity, and R
are calculated and listed in Table S2.† As shown in Table S2,†
the electrochemical porosity and R of the porous Ti/SnO –Sb
lter are 73.3% and 1428.2 ꢄ 37.2, respectively. These values for
the electrochemical porosity and roughness factor indicate that
the porous Ti/SnO –Sb lter is three-dimensional in nature, and
hence could provide large numbers of active sites for electro-
catalytic oxidation.
f
4
4
7
f
2
ꢁ
ꢁ1 13
membranes (1.4 ꢂ 10
m s
)
and CNT network based

ꢁ
4
ꢁ1 37
reactive electrochemical lters (1.0 ꢂ 10
m s ) . More
contrastive details are shown in Table S3.†
2
Fitting the measured kobs values with eqn (S4)† yielded
a value of k ¼ (4.7 ꢄ 0.08) ꢂ 10 m s , which is the assumed
kinetic limit. Our k value is 2.8 times higher than the value re-
ꢁ
4
ꢁ1
ꢁ
4
ꢁ1
ported by Chaplin (1.7 ꢂ 10 m s ) who used a Ti O -based
4
3
7
1
3
.3. The mass transfer performance of the reactive
reactive electrochemical ultraltration membrane . This is
electrochemical lter system
mainly attributable to the great roughness factor (R ¼ 1428.2 ꢄ
f
3
2
7.2) of the Sb-doped SnO coating layer, which was much
To investigate the mass transfer performance of the reactive
electrochemical lter system in ow-through mode, the kinetics
4 7
higher than that of the Ti O -based reactive electrochemical
4
ꢁ
ultraltration membrane (R ¼ 246.3 ꢄ 11.4), which provided
of Fe(CN)
penetration ux (J, m s ). It should be noted that the pene-
tration ux of the porous Ti/SnO –Sb lter is extremely high, i.e.
bar . Such a high ux means that the
permeability of the porous Ti/SnO –Sb lter is pretty good and
even gravity ow can be achieved. The penetration uxes tested
6
oxidation (kobs) were measured as a function of
f
ꢁ1
much more electrochemical reactive sites. A conservative esti-
mation of the boundary diffusion length (d) was calculated by
2
ꢁ
10
2
ꢁ1
3
ꢁ2 ꢁ1
ꢁ1
d ¼ D/k, where D ¼ 7.6 ꢂ 10
m s is the water diffusion
61.94 m
m
h
3ꢁ
ꢁ4
ꢁ1
coefficient for Fe(CN) . Based on k ¼ 4.7 ꢄ 0.08 ꢂ 10 m s
6
2
a value of d ¼ 1.6 mm was estimated, which is much lower than
the average pore radius (about 15 mm). These results support the
hypothesis that the reactive electrochemical lter system with
ꢁ
3
ꢁ1
in this study are also very high (up to 5.17 ꢂ 10 m s or 18.6
3
ꢁ2 ꢁ1
m m
ported in the previous literature.
very interesting that the kobs values were plateauing at around
h ), tens to hundreds of times higher than that re-
1
3,14,27,30
the porous Ti/SnO –Sb lter, with excellent water permeability,
As shown in Fig. 4, it is
2
developed in this study could greatly improve the mass transfer
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of the electrochemical process by reducing the thickness of the outstanding oxidation efficiency when operating in ow-
boundary diffusion layer. Additionally, these results also through mode by improving the mass transfer rates of pollut-
suggest that for further improvement of the intrinsic kinetic ants from the bulk solution to the lter surface. This treatment
reactivity of the reactive electrochemical lter system, it is system has great potential to bring down the high cost of the
necessary to design new electrode materials with higher specic traditional electrooxidation water treatment technology, in
electroactive surface area or roughness.
investment and operation.
.4.2 The effect of anode potential (E) in ow-through
3
mode on RhB oxidation. The applied anode potential is a key
parameter, which affects the capability of cOH generation on the
anode surface in the electrooxidation process. Meanwhile, the
3
.4. Electrooxidation performance of the reactive
electrochemical lter system
3
.4.1 The effect of ow-by and ow-through modes on RhB correct anode potential has a positive impact on energy saving.
oxidation. To determine the potential of the reactive electro- Therefore, the effect of anode potentials on the degradation of
chemical lter system developed in this study (see Fig. 1) for RhB was investigated to further evaluate the electrooxidation
water treatment applications, a series of experiments for RhB performance in ow-through mode. The reactivity of the reac-
oxidation were performed in both ow-by and ow-through tive electrochemical lter system for a 1 L solution containing
ꢁ
1
modes using 1 L solution containing 50 mM Na
2
SO
4
2 4
and 50 mM Na SO and 50 mg L RhB was characterized at various
ꢁ
1
5
0 mg L RhB at 0.5 A constant input current with the same applied anode potentials (1.94 V to 3.04 V vs. SCE). The system
inlet ow of 1.8 L min . The solutions were 100% recycled to was operated in ow-through mode at a penetration ux of 12.3
ꢁ1
3
2
ꢁ1
the feed reservoir in order to characterize the RhB decay kinetics
m m h , and solutions were 100% recycled to the feed
and TOC removal. As presented in Fig. 5, the reactive electro- reservoir. The RhB decay rate and TOC removal results of the
chemical lter exhibited a remarkable oxidation efficiency in individual experiments are shown in Fig. 6, and the corre-

ow-through mode compared with that in ow-by mode, e.g., sponding kinetics parameters of the pseudo-kinetic order are
>
2
5
99% RhB decay rate and 51% TOC removal were achieved at summarized in Table 1. The oxidation efficiency of RhB was
0 min in ow-through mode. While in ow-by mode, only positively correlated with the applied anode potential. For
ꢁ
1
8.8% RhB decay rate and 8.8% TOC removal aer 30 min example, the RhB decay rate (kRhB, min ) approximately
treatment were observed. The change in RhB concentration in doubled upon raising the potential from 1.94 to 2.18 V vs. SCE,
ꢁ
1
both ow-through mode and ow-by mode can be well tted by with kRhB values of 0.135 min
at 1.94 V vs. SCE and
ꢁ
1
a pseudo-rst-order model, based on which the half-life (t1/2
values (see Table 1) were calculated as 2.9 min and 25.3 min for by only 30% to 0.304 min when the anode potential was
ow-through mode and ow-by mode, respectively. As expected, raised from 2.18 V to 3.04 V vs. SCE, which indicated that the
the pseudo-rst-order kinetic constant (kRhB) of ow-through rate of the kRhB increment will no longer be signicant with the
)
0.235 min at 2.18 V vs. SCE. However, the kRhB value increased
ꢁ
1

ꢁ
1
mode is 0.235 min , which is 8.6 times higher than that of further improvement of the anode potential. This phenomenon
ow-by mode (0.0274 min ), clearly showing that the mass is predictable because the reactive electrochemical lter system
transfer of RhB molecules from the bulk solution to the anode in ow-through mode at a very high penetration ux, 12.3 m
surface was much faster in ow-through mode. Furthermore, m h , might be under kinetic limitation as revealed in Section
there was very little TOC decay in ow-by mode. However 3.3. In addition, the yield of cOH also increased linearly with the
continuous rapid decay of TOC in ow-through mode was anode potential at high anode potentials, being much higher
observed with a TOC removal value of 52.5% aer 30 min of than the oxygen evolution potential of the electrode. More-
treatment, indicating that signicant mineralization of RhB over, an excessively high electrode potential led to a water
occurred. These results strongly conrm that the reactive elec- discharge occurring violently with the generation of a larger
trochemical lter system constructed in this study achieves an amount of oxygen and hydrogen microbubbles, which impeded
ꢁ
1

3
2
ꢁ1
38
Fig. 5 Comparison of the electroxidation efficiency of RhB in flow-by mode and flow-through mode under the same conditions (RhB
ꢁ
1
ꢁ1
3
concentration: 50 mg L ; input current: 0.5 A; pH ¼ 7; supporting electrolyte: 50 mM Na
2 4
SO ; flow rate/penetration flux: 1.8 L min or 12.3 m
2
ꢁ1
m h ): (a) decay kinetics, and (b) TOC removal.
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the oxidation of RhB. A similar decay trend for the TOC removal
was also observed, and the TOC removal values were 47.5%,
52.5%, and 52.7% for 1.94 V, 2.18 V, and 3.04 V vs. SCE,
respectively, aer 30 min of treatment. In addition, it is worth
noting that RhB oxidation in ow-through mode reached an
ꢁ
1
ꢁ4
ꢁ1
observed rate constant (kobs, RhB, m s ) of 5.76 ꢂ 10 m s at
ꢁ
2
an anode potential of 3.04 V vs. SCE (ꢀ8.5 mA cm ) (see Table
1
), which is much higher than the highest rate constants
observed in the reactive electrochemical lter systems in ow-
through mode with carbon nanotube and/or Ti O (1.7 ꢂ
14,39–41
4
7
ꢁ5
ꢁ4
ꢁ1
10
–1.4 ꢂ 10 m s ) reported in the literature to date.
Once again, these results demonstrate the benet of the reactive
electrochemical lter system in ow-through mode using
2
a porous Ti/SnO –Sb lter with an excellence penetration ux.
3.5. The RhB oxidation ux, mineralization current
efficiency and mechanism
To better understand the oxidative capacity of the system with
the porous Ti/SnO
2
–Sb lter and to better facilitate the design of
reactive electrochemical lter water treatment systems, the RhB
ꢁ
1
ꢁ2
oxidation ux (OF, mol h
m ) was calculated using eqn (2),
similar to the membrane ux. The RhB oxidation ux was
examined as a function of the operating mode and anode
potential, as displayed in Fig. 7(a). As expected, it was observed
that the RhB oxidation ux in ow-through mode was much
higher than that in ow-by mode. For a 99% RhB oxidation, an
ꢁ
2
ꢁ1
oxidation ux value of 0.033 mol m
h
was achieved in ow-
through mode at an anode potential of 2.18 V vs. SCE, approx-
imately 8.5 times higher than that in ow-by mode (0.0039 mol
ꢁ2
ꢁ1
m
h ) under the same experimental conditions. This is
mainly due to the excellent mass transfer performance in ow-
through mode, resulting in the rapid diffusion of RhB mole-
cules towards the anode surface reaction zone. RhB molecules
are thus able to react with the cOH generated by the porous Ti/
SnO –Sb lter. Additionally, the oxidation ux increased with
2
the increasing anode potential in ow-through mode. At an
anode potential of 3.04 V vs. SCE, about 99% of 0.10 mM
ꢁ
1
(
0
50 mg L ) RhB was oxidized with an oxidation ux of
ꢁ
2
ꢁ1
.044 mol m
h , which was about 1.8 times higher than the
ꢁ
1
value for tetracycline (0.2 mM) reported by Liu (0.024 mol h
ꢁ2
m
) who used a CNT reactive electrochemical lter in ow-
through mode. More contrastive information is shown in
Table S4.†
28
As shown in Fig. 7(b) and Table 1, the overall mineralization
current efficiencies (MCEs) for RhB over the entire electrolysis
process of 30 min were 104.1%, 57.5%, and 19.2% in ow-
through mode at anode potentials of 1.94, 2.18, and 3.04 V vs.
SCE, respectively, while it was only 9.6% in ow-by mode at an
anode potential of 2.18 V vs. SCE, which is similar to the value
(
about 10%) reported by others using traditional parallel plate
42
electrode systems. At an anode potential of 2.18 V vs. SCE, the
overall mineralization current efficiency was calculated to be
8
3.6% with 99% RhB oxidation and ꢀ51% TOC removal. It is
noteworthy that the mineralization current efficiency in ow-
through mode was greater than 100% over the rst several
minutes of operation, e.g., the MCEs were 49.1%, 146.3%, and
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ꢁ1
Fig. 6 The effect of anode potential on the electrooxidation efficiency of RhB in flow-through mode (RhB concentration: 50 mg L ; pH ¼ 7;
3
2
ꢁ1
2 4
supporting electrolyte: 50 mM Na SO ; penetration flux: 12.3 m m h ): (a) decay kinetics, and (b) TOC removal.
Fig. 7 (a) The RhB oxidation flux and (b) the overall mineralization current efficiency (MCE) vary with treatment time. The experimental
conditions are the same as in Fig. 5 and 6.
2
62.5%, respectively, in the rst 10 min of electrolysis at anode (iso/tere)-phthalic acid and benzoic acid, which have been
43–47
potentials of 3.04, 2.18, and 1.94 V vs. SCE, respectively. Due to detected in the previous literature.
These compounds were
the fact that the reactive electrochemical lter system in ow- further degraded into smaller aliphatic acids and were eventu-
through mode was suffering from severe kinetic limitation, ally completely mineralized to CO , which can be proved by the
the cOH concentration was not sufficient for RhB to be results of the TOC measurement. Moreover, some of the
completely mineralized to CO at a low anodic potential. Thus, aromatic phenol intermediates such as o-phenylphenol and 2,3-
2
2
the MCE being over 100% may be mainly attributable to part of
the aromatic phenol intermediates produced by RhB oxidation
being electro-polymerized to form polymers by direct electron
transfer, which further aggregate to form macromolecules and
are trapped or adsorbed onto the lter anode surface or
precipitated. The observation of an anode potential of 1.94 V vs.
SCE also supports the hypothesis that a layer of very sparse
yellow substance was attracted to the surface of the porous Ti/
SnO –Sb lter aer electrooxidation.
2
The oxidation mechanism of RhB by cOH has been studied in
43,44
detail previously.
It is recognized that two competitive
processes occur simultaneously during the cOH attraction: the
N-deethylation and destruction of the dye chromophore struc-
ture (a conjugated xanthene ring). In our case, there was no
obvious shi in the characteristic absorption bands but a sharp
reduction was observed as shown in Fig. 8, which proved that
the decomposition of the conjugated xanthene ring was the
dominant oxidation pathway for RhB. The open-ring reaction of
the conjugated xanthene will generate a series of aromatic
phenol intermediates, such as o-phenylphenol, 2,3-dihydrox-
ybenzoic acid, 2,5-hydroxybenzoic acid, 3-hydroxybenzoic acid, trolyte: 50 mM Na
Fig. 8 The UV-vis spectrum of the RhB solution at various time
intervals for the reactive electrochemical filter system in flow-through
mode with an applied anode potential of 2.18 V vs. SCE. Experimental
ꢁ1
conditions: RhB concentration: 50 mg L ; pH ¼ 7; supporting elec-
3
2
ꢁ1
2
4
SO ; penetration flux: 12.3 m m h .
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Fig. 9 A schematic diagram illustrating the RhB mineralization mechanism.
ꢁ
3
dihydroxybenzoic acid are capable of forming polymers over the removal. Alternatively, the value of 0.28 kW h m is compara-
anode surface at a low anode potential such as 1.94 V vs. SCE. tive to the state-of-the-art electrochemical oxidation processes
ꢁ
3
28
On the basis of all of the above experimental results and the with energy consumptions in the range of 0.1–40 kW h m
previous studies, a simple schematic to describe the RhB
.
oxidation process in the reactive electrochemical lter system in
this study is given in Fig. 9.
4. Conclusions
The major conclusions drawn from this study are summarized
as follows.
3.6. Energy consumption
Energy consumption is an important factor to consider when
(1) Tubular porous Ti/SnO –Sb lters were prepared by a sol–
2
evaluating the application prospects of the reactive electro- gel method using low-cost powder sintered porous titanium
chemical lter system with the porous Ti/SnO –Sb lter devel- lters with an average pore size of about 30 mm as the
oped in the present study for wastewater treatment. The substrates. These porous Ti/SnO –Sb lters exhibited excellent
2
2
ꢁ
3
3
ꢁ2 ꢁ1
ꢁ1
electrical efficiency per log order degradation (EE/O, kW h m
)
penetration uxes (up to 61.94 m m
h
bar ) and good
of RhB was calculated to evaluate the economic feasibility of the electrochemical properties, such as high oxygen evolution
reactive electrochemical lter system and is shown in Table 1. potentials, electrochemical stability and high electro-active
The energy demand of the reactive electrochemical lter system surfaces.
4
ꢁ
in ow-through mode increases with the increase in the applied
(2) A convection-enhanced rate constant for Fe(CN)
6
ꢁ
4
ꢁ1
anode potential, and the system in ow-through mode exhibi- oxidation of 4.35 ꢂ 10 m s was achieved at a high pene-
3
ꢁ2 ꢁ1
ted a much lower energy consumption than that in ow-by tration ux of 18.6 m m
h , which approached the kinetic
mode under the same conditions. The EE/O value was 0.28 limit and is the highest reported in a reactive electrochemical
ꢁ3
kW h m in ow-through mode at an anode potential of 2.18 V lter system. The results also suggested that for further
vs. SCE, which was only 11.43% of that in ow-by mode at an improvement of the intrinsic kinetic reactivity of the reactive
ꢁ3
anode potential of 2.25 V vs. SCE (2.45 kW h m ). The EE/O electrochemical lter system, it is necessary to design new
ꢁ
3
ꢁ3
values were 0.21 kW h m and 0.82 kW h m , respectively, electrode materials with higher specic electroactive surface
at anode potential values of 1.94 V and 3.04 V vs. SCE in ow- area or roughness.
through mode. Notably, the EE/O value was increased by 3.9
(3) The reactive electrochemical lter system in ow-through
times, while the kRhB value was only enhanced by 2.25 times mode resulted in a 8.6-fold enhancement in the observed rst-
ꢁ
1
when the anode potential increased from 1.94 V to 3.04 V vs. order rate constant for RhB oxidation (kRhB, min ) relative to
ꢁ
2
SCE, indicating that a higher anode potential led to a lower ow-by mode at an input current of 0.5 A (ꢀ2.8 mA cm ). RhB
energy efficiency. However, the evaluation of a system depends oxidation in ow-through mode reached an observed rate
ꢁ
1
ꢁ4
ꢁ1
not only on the energy consumption, but also on the space constant (kobs, RhB, m s ) of 5.76 ꢂ 10 m s at an anode
ꢁ2
efficiency. The residence time values (t90%) were 17.1 min, potential of 3.04 V vs. SCE (ꢀ8.5 mA cm ), which is much
9
3
.8 min and 7.8 min at the anode potentials of 1.94 V, 2.18 V and higher than the highest rate constants reported in the literature
ꢁ
5
ꢁ4
ꢁ1
.04 V vs. SCE, respectively. In view of the appreciable space to date (1.7 ꢂ 10 –1.4 ꢂ 10 m s ).
(4) The mineralization current efficiency in ow-through
.18 V vs. SCE was a good choice due to the low energy mode was more than one magnitude higher than that in ow-
consumption (0.28 kW h m , or 97.2 kW h per kg TOC at ꢀ44% by mode under the same experimental conditions. Under an
TOC removal) and the relatively short residence time (9.8 min). anode potential of 2.18 V vs. SCE, the electrical energy per order
efficiency and energy consumption, the anode potential of
2
ꢁ
3
ꢁ3
The value of 97.2 kW h per kg TOC (equal to 36.4 kW h per kg degradation (EE/O) for RhB was as low as 0.28 kW h m in ow-
COD in theory) was much lower than the value reported by through mode, with a relatively short residence time of 9.8 min.
48
ꢁ1
Yang (101.34 kW h kg COD ) who used a CNT ow-through This value is comparative to the state-of-the-art electrochemical
reactor for the degradation of reactive Brilliant red X-3B oxidation processes with energy consumptions in the range of
ꢁ1
ꢁ3
(
50 mg L ) with ꢀ82% oxidation removal and ꢀ46% COD 0.1–40 kW h m
.
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These anodic reactive electrochemical membranes, the 16 S. E. Elsherif, D. B. Bejan and N. J. B. J Bunce, Can. J. Chem.,
porous Ti/SnO –Sb lters, are low-cost and easy to mass- 2010, 88, 928.
2
produce. In this study, we developed a reactive electro- 17 D. Bejan and N. J. Bunce, Can. J. Chem., 2012, 90, 666.
chemical lter system that is practical and can be implemented 18 G. S. Ajmani, D. Goodwin, K. Marsh, D. H. Fairbrother,
for large-scale industrial applications and has a high penetra-
tion ux, a high mass transfer rate and a strong electrooxidation
K. J. Schwab, J. G. Jacangelo and H. Huang, Water Res.,
2012, 46, 5645.
performance. It is a promising technology for treating organics 19 C. D. Vecitis, M. H. Schnoor, M. S. Rahaman, J. D. Schiffman
in wastewater. Further investigations into the toxicity as well as and M. Elimelech, Environ. Sci. Technol., 2011, 45, 3672.
the effects of the matrix and operation parameters should be 20 G. Gao and C. D. Vecitis, Environ. Sci. Technol., 2011, 45,
carried out before applying this technology to wastewater
9726.
purication.
21 D. Li, J. Tang, X. Zhou, J. Li, X. Sun, J. Shen, L. Wang and
W. Han, Chemosphere, 2016, 149, 49.
2
2 L. Zang, L. Xu, J. He and J. Zhang, Electrochim. Acta, 2014,
Conflicts of interest
117, 192.
2
2
3 M. Panizza and G. Cerisola, Chem. Rev., 2009, 109, 6541.
4 B. Adams, M. Tian and A. Chen, Electrochim. Acta, 2009, 54,
The authors declare no competing nancial interest.
1
491.
25 C. J. Miller, H. Yu and T. D. Waite, Colloids Surf., A, 2013, 435,
47.
Acknowledgements
1
This study was nancially supported by the National Natural
Science Foundation of China (No. 21607006), the Special
Financial Grant from the China Postdoctoral Science Founda-
tion (No. 212400227), and the Guangdong Innovation Team
Project for Colleges and Universities (No. 2016KCXTD023). The
authors also acknowledge the support of the Sichuan Province
Science and Technology Department (No. 2017HH0065).
2
2
2
2
3
6 M. F. Hou, L. Liao, W. D. Zhang, X. Y. Tang, H. F. Wan and
G. C. Yin, Chemosphere, 2011, 83, 1279.
7 H. Lin, J. Niu, S. Ding and L. Zhang, Water Res., 2012, 46,
2281.
8 Y. Liu, H. Liu, Z. Zhou, T. Wang, C. N. Ong and C. D. Vecitis,
Environ. Sci. Technol., 2015, 49, 7974.
9 Y. Bessekhouad, R. Brahimi, F. Hamdini and M. Trari, J.
Photochem. Photobiol., A, 2012, 248, 15.
0 Z. Sun, H. Zhang, X. Wei, X. Ma and X. Hu, J. Solid State
Electrochem., 2015, 19, 2445.
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