Preparation of Titanium Dioxide
Electrocoagulated Sludge using
Sacrificial Titanium Electrodes
H . K . S H O N , *, †S . P H U N T S H O ,†
S . V I G N E S W A R A N ,†J . K A N D A S A M Y ,†
L . D . N G H I E M ,‡G . J . K I M ,§J . B . K I M ,| , ⊥
A N D J . - H . K I M| , ⊥
Faculty of Engineering, University of Technology,
Sydney (UTS), P.O. Box 123, Broadway, NSW 2007,
Australia, School of Civil Mining and Environmental
Engineering, The University of Wollongong, Wollongong,
NSW 2522, Australia, Department of Chemical Engineering,
253 Yonghyun-dong, Nam-gu, Inha University, Incheon,
402-751, Korea, Photo & Environmental Technology Co.
Ltd., Gwangju 500-460, and Korea, School of Applied
Chemical Engineering & The Institute for Catalysis
Research, Chonnam National University, Gwangju 500-7 57,
Korea, Chonnam National University,
Gwangju 500-757, Korea
Received January 29, 2010. Revised manuscript received
May 29, 2010. Accepted June 4, 2010.
A comprehensive investigation of electrocoagulation using
sacrificial titanium (Ti) electrodes in wastewater was carried
out. The effects of specific process variables, such as
Ti-based sludge. The sludge was incinerated at 600 °C to
revealed that TiO2produced at optimum electrocoagulation
conditions was mostly anatase structure. The specific surface
area of the synthesized TiO2photocatalyst was higher than
that of the commercially available and widely used Degussa
P-25 TiO2. Furthermore, energy dispersive X-ray and X-ray
to titanium and oxygen, this photocatalyst is also composed
of carbon and phosphorus. These elements were mainly
doped as a substitute site for the oxygen atom. Transmission
electron microscopy images exhibited sharply edged
of gaseous acetaldehyde by this photocatalyst was also
conducted under UV and visible light irradiation to study the
photocatalytic properties of the doped TiO2photocatalyst.
While no photocatalytic activity was observed under visible
light irradiation, this doped TiO2photocatalyst exhibited high
photocatalytic activity under UV light.
As fresh water resources become scarcer, wastewater reuse
and desalination are increasingly being utilized as part of
drinking water standards for protecting public health are
are being investigated. Electrocoagulation, for example, has
been widely considered for its simplicity in design, high
separating efficiency, ease of operation and automation (1).
Electrocoagulation can be utilized to treat wastewater from
oil-water emulsion, mining, dye and textile, and food
wastewater treatment applications (9).
Removal mechanisms in an electrocoagulation process
(1, 10-12). To optimize its operating parameters, the effects
electrolyte concentration, solution temperature, and pol-
lutant compound and concentration must be carefully
considered (1, 13-16). Electrodes, current density, and pH
have been identified as key operational parameters influ-
encing pollutant removal mechanism.
Dedicated efforts have been made on the development
of novel electrodes, which is the most important part of an
electrocoagulation unit, to improve performance (13). The
authors of refs 1 and 13 compared different anodes, such as
graphite, Pt, PbO2, IrO2, SnO2-Sb2O5, IrOx, RuO2, and TiO2
in their reviews. However, most current electrocoagulation
applications still use the conventional aluminum or iron
O2evolution overpotential and are anodically soluble with
low durability (17). Reports on novel electrodes materials
remain very scarce in the literature.
Similar to chemical coagulation, electrocoagulation also
produces a large amount of chemical sludge that requires
could cause secondary contamination. To reduce the en-
vironmental impact from sludge disposal, Shon et al. (18)
developed a novel method to produce alumina (Al2O3),
hematite (Fe2O3), and titanium oxide (TiO2) nanoparticles
from Al-, Fe-, and Ti-coagulated sludge respectively. Of all,
of cosmetics, paints, electronic paper, solar cells, and other
nanotubes are also attracting great interest because of their
Among the several methods for preparing TiO2nanotubes,
titanium electrochemical anodization is regarded as a
nanotube structure. The preparation method of TiO2nano-
based solution was first reported by Gong et al. (22). The
The use of TiO2in a wide range of applications means
that the demand for this metal oxide is increasing rapidly.
The demand could partially be met by TiO2produced from
Ti-coagulated sludge which has been found to be superior
to the commercially available TiO2(P-25) in both photo-
* Corresponding author phone: +61295142629; fax: +61295142633);
†University of Technology, Sydney (UTS).
‡The University of Wollongong.
|Photo & Environmental Technology Co. Ltd.
⊥Chonnam National University.
Environ. Sci. Technol. 2010, 44, 5553–5557
Published on Web 06/18/2010
2010 American Chemical SocietyVOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY95553
catalytic activity and surface area (24). The same concept
can be applied to Ti-electrocoagulation not only to reduce
the sludge production but also to generate reusable sludge
offering a novel solution to many environmental and
economic issues associated with sludge handling.
No previous studies have attempted to comprehensively
Therefore, this study aims to provide a holistic assessment
of titanium as a novel electrode material. Specific process
variables of electrocoagulation with Ti electrodes were
systematically optimized. These process variables included
and ionic/electrolyte strength. The optimum operating
conditions were applied to produce a large amount of
recyclable sludge. The sludge was incinerated at 600 °C to
produce a TiO2photocatalyst. Subsequently, photocatalytic
properties of the synthesized TiO2were examined in detail.
Synthetic Wastewater. Synthetic wastewater (SWW) was
prepared with a wide range of organic molecular sizes,
representing effluent organic matter generally found in
are provided elsewhere (25, 26).
Electrocoagulation. The basic premise of electrocoagu-
lation is the production of a coagulant via electrolysis. By
applying electric current to titanium electrode plates, tita-
solution. Hydrogen gas is released at the cathode which
causes the flotation. The electrode reactions using titanium
are as follows:
The Ti4+ions formed are hydrolyzed and subsequently
generate titanium hydroxides and polyhydroxides (27). In
the process, water is also electrolyzed in a parallel reaction,
These gases destabilize the contaminants, such as colloids,
suspended solids, organic matter, heavy metals, microor-
ganisms, and phosphorus. The aggregation of destabilized
result in the purification of wastewater.
of a 6 L pyrex glass beaker with a capacity of 5 L, 4 titanium
electrodes (23 cm ×9 cm ×0.3 cm) in a monopolar config-
uration, and a DC power converter (Q1770, Dick Smith
Electronics, Australia). Prior to each test, 5 L of synthetic
wastewater was introduced to the electrocoagulation cell.
was set constant at 30 V. When not in use, the titanium
electrodes were immersed in acid bath (1 M HCl) and prior
to each experiment, they were carefully cleaned using steel
Analysis of Water Sample. The sampling during elec-
trocoagulation was carried out at regular time intervals.
Organic carbon was measured using a Dohrmann Phoenix
8000 UV-persulphate TOC analyzer equipped with an auto
prior to organic measurement. The molecular weight (MW)
size exclusion chromatography (HPSEC, Shimadzu Corp.,
Japan) with a SEC column (Proteinpak 125, Waters Milford,
U.S.A.). Polystyrene sulfonates (PSS 210, 1800, 4600, 8000,
and 18000 Da) were used as standards to calibrate the
Characterization of TiO2. XRD images (Rigaku, Japan)
and rutile TiO2photocatalysts. All the XRD patterns were
analyzed with MDI Jade 5.0 (Materials Data Inc., U.S.A.).
UV-vis-NIR spectrophotometer (Cary 500 Scan, Varian,
U.S.A.) was used to identify the absorbance range. XPS
measurements were performed with a Leybold LHS10
spectrometer, equipped with a twin anode (Mg KR/Al KR)
nonmonochromatized source (operated at 280 W) and a
hemispherical electron analyzer. SEM and TEM (Rigaku,
Japan) images were used to investigate the microscopic
(U.S.A.) with automatic surface area analyzer was used for
steel (Top-face quartz, volume 3.8 L) airtight reactor was
used to study the adsorption and photocatalytic oxidation
F10T8BL, Japan) and had three rubber openings for the
injection of acetaldehyde, air mixing inside the reactor and
for withdrawal of samples. The last opening was connected
(hp5890 series II, Wilmington, U.S.A.) for measuring varia-
and 60 min photodecomposition). Photocatalysts were
uniformly sprinkled on a glass Petri-dish (9 cm of diameter)
and placed at a distance of 10 cm from UVA lamps in the
center of the reactor. The reaction began after closing the
reactor door and injecting acetaldehyde (Stem Supply, SA,
Australia) through the injection cavity using an airtight
syringe. The concentration of acetaldehyde was recorded at
fixed intervals for adsorption and photocatalysis. A detailed
protocol of acetaldehyde oxidation using TiO2can be found
Results and Discussion
current density, and the initial organic loading were first
optimized to produce recyclable Ti-based sludge. The
optimum values of each process variables shown in Table 1
The details related to study on the optimum operating
parameters are available in the Supporting Information
The optimum current density of 8.3 mA/cm2was in fact
four times higher than that of Al or Fe electrodes, which
require approximately 2 mA/cm2for a long period of
operation (1). This is probably because of less dissolution of
metal ions from the Ti sacrificial metal anode.
The influence of initial pH (range 4-12) indicated that
pH. Earlier studies have reported that electrocoagulation
performed normally at slightly acidic to neutral pH because
Ti(s)f Ti4++ 4e-
4H2O + 4e-f 2H2(g)+ 4OH-
TABLE 1. Optimum Parameters for Ti-Based Electrocoagulation
parametersrange tested optimum value
current density (mA/cm2)
agitation speed (rpm)
initial organic loading (mg/L)
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for the organic removal by Ti-electrocoagulation was ob-
served to be 4. However, in most cases, the solution pH
increased as electrocoagulation progressed because of
continuous OH-formation at the cathode as shown by
eq 2 (32). When the initial organic concentration of SWW
was high, the removal efficiency by electrocoagulation was
the removal efficiencies were similar regardless of the initial
of electrocoagulation in fresh water, brackish water, and
an increase in current density at the same cell voltage or the
was adjusted by adding NaCl. The organic removal efficiency
with the Ti electrodes was slightly enhanced at higher ionic
strength (Supporting Information, Figure S5).
The organic removal by electrocoagulation in terms of
DOC under optimum parameters was between 60 and 70%
range of molecular weight distribution (MWD) of organic
matter. The organic removal in terms of MWD suggests that
MW compounds in comparison to chemical coagulation-
(24) reported that the TiCl4coagulant removed some of the
smaller MW compounds (860-1000 Da), while the smallest
MW ranges of compounds of around 250 Da could not be
Electrocoagulation was performed at optimum operating
conditions (pH 4, initial organic loading (C0) ) 10 mg/L,
mixing rate )700 rpm, and current density )8.3 mA/cm2),
and the electrocoagulated sludge produced was collected.
Two types of electrocoagulated sludge were collected sepa-
Ti-salt floc that settled at the bottom (ECS). This sludge was
then incinerated at 600 °C to remove water content and
producing functional nanoparticle from sludge containing
water and organic matter and was adapted from Shon et al.
(24). The detail characteristics of the anatase TiO2produced
from ECS and ECF were studied using advanced analytical
techniques as described below.
XRD Image and Surface Area. XRD patterns were used
to identify the particle structures of the ECF and ECS, and
their structures were compared with the commercially
available P-25 TiO2(Figure 1). Both ECF and ECS exhibited
only anatase structure, whereas P-25 TiO2 showed both
to rutile at ambient pressure was found to be at about 550
°C probably because of the impurities in the TiO2produced
from the floated and settled floc. The crystallite size was
The crystallite sizes of the particles for ECF, ECS, and P-25
were 32, 21, and 24 nm, respectively (Table 2). The primary
crystallization for ECS was smaller than that of ECF. This
suggests that the ECS could have the smaller floc size and
less impurity, which could lead to the production of smaller
pore size or pore volume so it is probably because of
aggregated pore volume and pore size of the particles.
The BET specific surface areas of both ECF and ECS were
significantly higher than that of P-25 TiO2. This result is in
a good agreement with the crystallite size of ECF and ECS.
The average pore diameter of ECF and ECS was 7.4 and 5.6
and ECS photocatalysts. The SEM images show different size,
of aggregated particles were found to be less than 50 nm. The
aggregated size of ECF was larger than that of ECS.
electrocoagulation after incineration at 600 °C (A, anatase; R,
1. XRDimageofECF andECSproducedfrom
TABLE 2. Crystallite Size, BET Surface Area, Average Pore
Diameter, and Pore Volume of ECF and ECS Produced from
FIGURE 2. SEM images of ECF and ECS nanoparticles.
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EDX analysis was performed to determine the presence
of the different elements in ECF, ECS, and P-25 (Table 3).
The constitutive elements of ECF and ECS were mainly Ti,
electrocoagulation gets incorporated into the flocs predomi-
nantly in the heavier flocs, which settle down. The atomic
in Table 3. These results imply that in ECF and ECS, most
of the C and P atoms were mainly doped as a substitute site
for an O atom. The detailed valence states of Ti, O, C, and
P are shown in Supporting Information, Figure S7.
in Figure 3 show sharply edged nanorods, round nanopar-
ticles, and nanotubes. These structures were not uniform in
shape showing structural defects and interconnections at
certain sites. The nanorods had estimated dimensions of
nanoparticles consisted of close-packed and tube-type
nanocrystals with sizes ranging from 10-100 nm. The
like features included inner diameters of 20-80 nm, wall
size distribution of nanotubes may be the result of the high
voltage applied (30 V) in this study. Mor et al. (23) reported
that the nanotube structure at anodizing voltages greater
than 23 V was lost with a sponge-like randomly porous
structure. Compared to ECF, ECS photocatalyst had more
nanotubes with different and bigger nanosize distributions.
The nanorods and nanoparticles were probably from ag-
gregated floc with the Ti4+ions released from Ti electrodes,
arrays via anodic oxidation of the titanium electrode.
UV-Vis Spectrophotometer. Carbon (C) doping can be
used to generate visible light responsive TiO2(34, 35). The
localized C (2p) formed above the valence band is the origin
of visible light sensitivity, which leads to an inferior hydro-
philic property when irradiating with visible light compared
with UV light. The optical property of ECF and ECS
photocatalysts was thus examined using the ultraviolet-
visible-near-infrared spectrophotometer. It was observed
that P-25 photocatalyst absorbed the majority of UV light
(less than a 400 nm wavelength), while the ECF absorbed
both UV light and visible light (Figure 4).
Photocatalytic Activity. The photodecomposition rate of
and visible light irridiation was compared to that of the
commercially available P-25 photocatalyst (Figure 5). In the
capacity (up to 50% removal of acetaldehyde), while ECF and
P-25 adsorbed only about 15% of acetaldehyde. The high
the larger surface area increase the interaction between
P-25 under UV light irradiation after 200 min operation were
91.7% and 98.5%, respectively, whereas the photodecompo-
sition with ECF was only 27.7%. However, under visible light
(fluorescent light at 436 nm and a light power of 0.9 mW/cm2)
irradiation, none of the photocatalyst exhibited any photo-
light absorbance (Figure 5).
In summary, Ti-electrocoagulation of wastewater results
in recyclable sludge. This treatment option not only solves
the environmental issues associated with sludge disposal
but also enhances the economy of wastewater treatment by
producing a valuable photocatalyst as byproduct. Further
process optimization could result in higher electrocoagu-
lation efficiency. TiO2 nanotubes with a high surface to
TABLE 3. Atomic (%) Fraction of Different Elements in ECF and
ECS Powders after Incineration at 600 °C and P25
element ECF (atomic %)ECS (atomic %) P-25 (atomic %)
FIGURE 3. HR-TEM images of ECF and ECS nanoparticles.
FIGURE 4. Optical absorbance of ECF and ECS photocatalysts.
5556 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 14, 2010
volumeratioareobtainedfromtheelectrocoagulatedsludge Download full-text
at lower temperature. The photocatalytic efficiency of this
TiO2is comparable to the commercially available photo-
catalyst P-25. The simplicity in design and layout offered by
electrocoagulation is what makes this method attractive for
water and wastewater treatment.
This work was supported by an ARC grant and Priority
Research Centers Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (2009-0094057) and the
Center for Photonic Materials and Devices at Chonnam
Supporting Information Available
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FIGURE 5. Variation of acetaldehyde (CH3CHO) concentration
with irradiation time (initial concentration of CH3CHO, 2000 mg/
L; UV irradiation, black light of three lamps of 10 W each).
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