Decolourization of direct blue 15 by Fenton/ultrasonic process using a zero-valent iron aggregate catalyst

Abstract

Decolourization of direct azo dye, direct blue 15 (DB15), by an advanced Fenton process coupled with ultrasonic irradiation (Fenton/US) was investigated. Zero-valent iron (ZVI) aggregates were used as the catalyst. A positive synergistic effect occurred when Fenton's reagent was combined with ultrasonic irradiation. Experimental results showed that the optimum conditions for decolourization were pH 3.0, Fe(0) 1g/L, H(2)O(2) 5.15×10(-3)mol/L with ultrasound density of 120W/L at 60kHz. These conditions yielded 99% decolouration of 4.7×10(-5) M DB15 (4130 ADMI) solution within 10min. DB15 decolouration follows the first-order decolouration kinetics. Although the solutions containing H(2)CO(3), Cl(-), ClO(4)(-), NO(3)(-) and SO(4)(2-) ions did not have a significant effect on the decolouration, the H(2)PO(4)(-) ion did decrease the decolouration rate. High ultrasonic input power accelerated the reaction and increased decolourization efficiency. The cost effectiveness of this process at high ultrasound density could be controlled despite the high electricity costs incurred by the process. ZVI aggregates were reusable; however, an increase in the number of times ZVI was recycled decreased the decolourization rate. This study demonstrates that a Fenton/US process can effectively decolour the direct azo dye DB15 in wastewater.

Full text

Decolourization of direct blue 15 by Fenton/ultrasonic process using
a zero-valent iron aggregate catalyst
Chih-Huang Weng
a,
⇑
, Yao-Tung Lin
b
, Cheng-Kuan Chang
a
, Na Liu
c
a
Department of Civil and Ecological Eng., I-Shou University, Kaohsiung 84001, Taiwan
b
Department of Soil and Environmental Sci., National Chung Hsing University, Taichung 402, Taiwan
c
Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130021, China
article info
Article history:
Received 24 July 2012
Received in revised form 28 September 2012
Accepted 28 September 2012
Available online 7 November 2012
Keywords:
Direct azo dye
Fenton
Ultrasound
Zero-valent iron aggregate
abstract
Decolourization of direct azo dye, direct blue 15 (DB15), by an advanced Fenton process coupled with
ultrasonic irradiation (Fenton/US) was investigated. Zero-valent iron (ZVI) aggregates were used as the
catalyst. A positive synergistic effect occurred when Fenton’s reagent was combined with ultrasonic irra-
diation. Experimental results showed that the optimum conditions for decolourization were pH 3.0, Fe(0)
1 g/L, H
2
O
2
5.15 10
3
mol/L with ultrasound density of 120 W/L at 60 kHz. These conditions yielded
99% decolouration of 4.7 10
5
M DB15 (4130 ADMI) solution within 10 min. DB15 decolouration fol-
lows the first-order decolouration kinetics. Although the solutions containing H
2
CO
3
,Cl

, ClO

4
,NO

3
and SO
2
4
ions did not have a significant effect on the decolouration, the H
2
PO

4
ion did decrease the decol-
ouration rate. High ultrasonic input power accelerated the reaction and increased decolourization effi-
ciency. The cost effectiveness of this process at high ultrasound density could be controlled despite the
high electricity costs incurred by the process. ZVI aggregates were reusable; however, an increase in
the number of times ZVI was recycled decreased the decolourization rate. This study demonstrates that
a Fenton/US process can effectively decolour the direct azo dye DB15 in wastewater.
Ó2012 Elsevier B.V. All rights reserved.
1. Introduction
Effluents discharged from textile and paper printing industries
normally contain many structural varieties of highly coloured
dye. Azo dyes, which represent approximately 70% of the world’s
dye production, are the major colourants used by these industries.
Azo dyes derived from benzidine have proven to be carcinogenic to
humans and have adverse effects on aquatic life. Therefore, they
need to be properly treated before being discharged. Most azo dyes
are refractory to biodegradation. Moreover, auxiliary textile chem-
icals such as polyvinyl alcohol used in the sizing process cannot be
degraded in an activated sludge process [1]. Thus, conventional
biological treatment may no longer be considered a satisfactory
decolourization process. Physico-chemical decolourization method
such as membrane separation, electrocoagulation and adsorption
are currently available; however, chemical sludge generation,
adsorbent regeneration and maintenance of fouled membranes
may raise serious concerns. A variety of advanced oxidation pro-
cesses (AOPs) have been developed to decolourize textile and
printing industry wastewater [2,3].
Fenton’s reaction and sonolysis are two of the most promising
AOPs for treating organic substances. In general, the Fenton re-
agent can oxidize a variety of organic contaminants. To improve
the effectiveness of Fenton’s reaction, the addition of zero-valent
iron (ZVI) has been explored for the degradation of hazardous or-
ganic compounds [4–6]. Recently, interest in Fenton’s reaction in-
creased when it was determined that combining Fenton’s process
with hydrodynamic cavitation, electrolysis, photocatalysis or son-
olysis resulted in greater versatility for removal of pollutants
[3,7–12]. The Fenton process coupled with ultrasonic irradiation
process (Fenton/US) to destroy recalcitrant organic substances is
of interest because it enables further degradation and is consider-
ably more efficient than Fenton’s reaction alone [13–17]. The
mechanisms of the Fenton/US process are described as follows
[18]:
H
2
O
2
!
ÞÞÞ
2OH

ð1Þ
OH

þOH

!H
2
O
2
ð2Þ
Fe
0
!
ÞÞÞ
Fe
2þ
þ2e

ð3Þ
Fe
2þ
þH
2
O
2
!Fe
3þ
þOH

þOH

ð4Þ
1350-4177/$ - see front matter Ó2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2012.09.014
⇑
Corresponding author. Tel.: +886 929552662; fax: +886 76577461.
E-mail addresses: chweng@isu.edu.tw (C.-H. Weng), yaotung@nchu.edu.tw
(Y.-T. Lin), liuna@jlu.edu.cn (N. Liu).
Ultrasonics Sonochemistry 20 (2013) 970–977
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson

Fe
3þ
þH
2
O
2
!Fe-OOH
2þ
þH
þ
ð5Þ
Fe-OOH
2þ
!Fe
2þ
þOOH

ð6Þ
Fe
3þ
þOOH

!Fe
2þ
þO
2
þH
þ
ð7Þ
Fe
0
þ2Fe
3þ
!3Fe
2þ
ð8Þ
where ‘‘)))’’ represents ultrasound wave.
Ultrasonic irradiation of organics in water results in the gener-
ation of hydroxyl radicals that can react with and destroy a wide
range of organic contaminants. This effect is greatly enhanced by
the addition of hydrogen peroxide in the presence of iron particles
[13]. Although the partial recombination of OH

radicals could self-
produce H
2
O
2
from the sonolysis of water (Eq. (2)), the main source
of H
2
O
2
is external. The use of ZVI as a catalyst in Fenton’s reaction
could be a cost-effective method to treat organic waste [19]. In the
Fenton/US system, ultrasonic irradiation can enhance the corrosion
of ZVI, releasing massive amounts of Fe(II) continuously. Sufficient
amounts of Fe(II) and H
2
O
2
are required to trigger a series of Fen-
ton’s reactions (Eqs. (3)–(8)). Ultrasonic irradiation dramatically
enhances Fenton’s reaction.
In this study, the decolourization of direct blue 15 (DB15) by
Fenton/US was evaluated. Although studies have shown that
DB15 could be decolourized via photocatalysis of Fe(III)–oxalate
complexes/H
2
O
2
[20] and Fenton’s reagent [2], the use of ZVI
aggregates as the catalyst for decolourization of DB15 with the
Fenton/US oxidation is considered to be important and needs to
be assessed further. To evaluate the effectiveness of treating
DB15 with Fenton/US process, the following four objectives were
determined: (1) compare the effectiveness of US/H
2
O
2
, Fenton
and Fenton/US processes; (2) characterise the key parameters
(pH, ZVI dose, H
2
O
2
dose, salts and ultrasonic power) affecting
the Fenton/US process and identify suitable decolourization condi-
tions, (3) evaluate the cost-effectiveness of the Fenton/US process
and (4) test the availability of reusable ZVI in the Fenton/US
process.
2. Materials and methods
2.1. Reagents
DB15 (C
34
H
24
N
6
Na
4
O
16
S
4
, CAS No. 2429–74–5) (Fig. 1) was pur-
chased from Sigma-Aldrich. DB15 concentration was analysed by
measuring the adsorption at 595 nm using a spectrophotometer
(DR2800™, Hach, USA). ZVI aggregates (Fig. 2) with particle size
0.297–2.380 mm and specific weight 2240–2560 kg/m
3
were ob-
tained from Connelly-GPM Inc., USA. H
2
O
2
(analytical grade, 35%
w/w) was purchased from J.T. Baker (USA). Fe(II) and total Fe were
analysed by measuring absorption at 510 nm using Hach FerroVer
Ò
iron reagent [21]. Fe(III) was determined from the difference be-
tween total Fe and Fe(II).
2.2. Fenton/US experiments
Sonication of the dye solution was performed in air atmosphere
with a fixed frequency of 60 kHz generated by an ultrasonic
generator (S-450A, Brason, USA) equipped with a titanium probe
transducer. The tip of the probe was 1.2 cm in diameter; 6 cm of
the tip’s total length was submersed in the dye solution. The
sonication was administered in pulses with a 60% duty cycle. The
ultrasonic generator provides direct sonication, which will not
cause energy loss because the reaction matrix is in direct contact
with the mechanical vibration. Fenton/US experiments were con-
ducted for a 1000 mL DB15 dye placed in a glass beaker. The pH
was adjusted to a predetermined value using a solution of either
HNO
3
or NaOH. The experimental solution was placed in a beaker
and irradiated with an ultrasonic horn (Fig. 3) and appropriate
amounts of ZVI and H
2
O
2
were added. At desired time intervals,
a 5 mL sample was removed from the beaker and immediately fil-
tered using a 0.45
l
m fibre glass filter to collect the supernatant,
which was then analysed to determine the residual concentration.
3. Results and discussion
3.1. Enhancement of sonication in the Fenton process
Fig. 4a shows DB15 depletion as a result of treatment with dif-
ferent processes. Due to a positive synergistic effect, the coupled
Fenton/US process was much more effective than Fenton’s reaction
or sonolysis alone. As shown in the figure, after 20 min treatment,
the decolourization efficiency with either sonolysis or Fenton’s
process was only 15%. However, in the same period, nearly 100%
colour removal was achieved with Fenton/US. The synergistic
mechanism of the Fenton/US process is generally attributed to
the increase in OH

radical concentration, resulting from the influ-
ence of ultrasound [17].
Fig. 1. Chemical structure of DB15.
Fig. 2. ZVI aggregates.
Fig. 3. Fenton/ultrasonic apparatus.
C.-H. Weng et al. / Ultrasonics Sonochemistry 20 (2013) 970–977 971

Fig. 4b shows the effect of the presence of Fe
2+
on decolouriza-
tion of DB15. In Fenton’s process, although Fe
2+
was generated con-
tinuously over a 30 min period, the concentration of Fe
2+
remained
low compared with that in the other two systems, i.e. Fenton/US
and Fe(0)/US. Particularly, in case of the Fenton/US system, the
release of Fe
2+
was significantly enhanced over the entire duration
of the experiment. For Fenton/US, the concentration of Fe
2+
was
even higher than that for Fe(0)/US within 10 min, which could be
attributed to H
2
O
2
as a result of Fe(0) corrosion. In Fenton(0)/US,
the concentration of released Fe(II) decreased after 20 min as a
consequence of Fe
2+
reacting with H
2
O
2
(Eq. (4)). In general, the
quantity of Fe
2+
released from ZVI was largely enhanced by ultra-
sound and influenced by the addition of H
2
O
2
.
3.2. UV–visible spectra changes during the reaction
The graph inset in Fig. 5a shows that the concentration of DB15
decreased dramatically as the Fenton/US reaction time increased.
Complete decolourization of DB15 was achieved within 15 min.
To clarify the changes in molecular and structural characteristics
of DB15 that resulted from oxidation via the Fenton/US process,
representative variations of UV–visible spectra were observed,
and the corresponding spectra are displayed in Fig. 5a. DB15 was
characterised by one main band in the visible region with the
absorbance peak at approximately 600 nm and by another band
in the ultraviolet region with the absorbance peak at 319 nm.
The peak at 600 nm was attributed to the chromophore-containing
azo linkage of the DB15 molecule. The peaks at 319 nm were asso-
ciated with benzene ring structures in the DB15 molecule [15]. The
characteristic peak decreased remarkably within the initial 5 min,
and after 15 min the peaks reduced to the minimum. This implies
that the conjugate azo structure of DB15 was destroyed and the
colour disappeared quickly. In addition to the rapid decolouriza-
tion, the quick decay of the absorbance at 319 nm was the evidence
of degradation of an aromatic fragment in the DB15 molecule and
its intermediates.
The photos in Fig. 5b show that the solution changed from dark
blue to pink after 2 min treatment and then gradually changed to
light brown. The solution became transparent after 15 min. The
American Dye Manufacturers Institute (ADMI) tristimulus filter
method [21] is the standard analytical method for true colour mea-
surements of wastewater samples. In Taiwan, environmental regu-
lations prescribe a discharge limit of 550 ADMI for the textile
industry. As indicated (Fig. 5b), the solution meets the discharge
effluent criteria after 10 min treatment. Overall, the coupled Fen-
ton/US process could decolour DB15 effectively and efficiently.
3.3. Effect of solution pH
The pH of the solution plays a key role in the decolouration effi-
ciency of the Fenton process, because it affects the solubility of
Fe(II)/Fe(III), and ultimately controls the production of OH

radicals.
Fig. 6 shows oxidation of DB15 as a function of pH and time. In gen-
eral, depletion of DB15 increased with decreasing pH. A first-order
equation was used to analyse the decolouration rate:
dC=dt¼kt ð9Þ
C¼C
0
expðktÞð10Þ
where C
0
= initial dye concentration (M); k= rate constant (min
1
);
t= reaction time (min). The obtained kas a function of pH is shown
0 100
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
0 5 10 15 20 25 30
Fenton/US
Fe(0)/US
H2O2/US
Fenton
US
Direct Blue 15 (mol/L)
Time (min)
C0 4.7x10-5 M
Fe(0) 0.5 g/L
H2O2 5.15x10-3 M
pH 3.0
US 120W/L (60kHz)
(a)
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Fenton/US
Fenton
Fe(0)/US
Fe(II) mg/L
Time (min)
Fenton
Fenton/US
Fe(0)/US
(b)
Fig. 4. (a) Depletion of DB15 treated with different processes; (b) concentration of
Fe
2+
during the treatment of different processes.
0
0.5
1
1.5
2
2.5
Abs
0 min
2 min
5 min
10 min
15 min
20 min
25 min
30 min
200 400 600 800 1000
Wave length (nm)
DB 15 4.7x10-5 M
Fe(0) 1.0 g/L
H2O2 1.03x10-2 M
pH 3.0
US 120 W/L
0
1
2
3
4
5
0 5 10 15 20 25 30
Direct Bl ue 15 (x10 -5 mol/L)
Time (min)
(a)
0 min 2 5 10 15 20
(b)
Fig. 5. (a) Changes in the UV–visible spectra of DB15 during Fenton/US treatment.
(b). Colour changes during the treatment. Values of ADMI at 0, 2, 5, 10, 15 and
20 min were 4130, 1640, 495, 144, 48, 34 and 13, respectively.
972 C.-H. Weng et al. / Ultrasonics Sonochemistry 20 (2013) 970–977

in the graph inset in Fig. 6. The high regression coefficient (r
2
> 0.99)
indicates that decolouration of DB15 follows first-order kinetics. As
the pH decreased from 5.0 to 2.0, the oxidation rate increased from
0.095 to 0.264 min
1
. It is evident that pH values <3.0 lead to in-
crease in the extent of DB15 oxidation.
Although the production of the OH

radical could be inhibited by
the formation of the complex species [Fe(II)(H
2
O)
6
]
2+
, which reacts
with H
2
O
2
to form [Fe(II)(OH)(H
2
O)
5
]
+
[22–24], in this study, we
demonstrated that the oxidation was not retarded at pH below
3.0, and in fact, the most effective oxidation occurred in such an
acidic medium. The maintenance of effective oxidation at such
low pH could be explained by the use of ZVI aggregates as the cat-
alyst. ZVI aggregates provide a source of Fe(II) sufficient to ensure a
chain oxidation. As expected, the oxidation decreased substantially
at pH > 4 because of the formation of Fe precipitates (Fe(OH)
2(s)
and Fe(OH)
3(s)
), which suppressed the regeneration of OH

radicals
and lowered catalytic activity during the decomposition of H
2
O
2
.
The decolouration did not improve when pH was lowered from
3.0 to 2.5 or 2.0. Indeed, a sudden drop in the rate of decolouration
was observed at pH > 3.0 (Fig. 6). In terms of cost-effectiveness for
practical applications, the optimum operational condition is pH 3.
The results could be explained by the considerable effect of pH on
sonolysis and the Fenton reaction. Relevant studies of dye decol-
ouration have reported that the optimum pH for Fenton or Fen-
ton/US reaction was 3.0 [16,17,25].
3.4. Effects of ZVI dose
Fig. 7 shows that the depletion of DB15 during the Fenton/US
process was affected by the ZVI dose and the corresponding first-
order rate constants. The decolouration of DB15 appears to be
appreciably influenced by the dosage of ZVI. Within 10 min of
the treatment, when the ZVI dose was increased from 0.25 to
4.0 g/L, the decolouration efficiency increased from 80% to 98%,
respectively. In this study, since ZVI aggregates were used, the cor-
rosion of ZVI surfaces in aqueous solution was the only source of
Fe
2+
. Under the influence of ultrasound, higher doses of ZVI lead
to the release of greater amounts of Fe
2+
(Eq. (3)). In the Fenton
process, Fe
2+
acts as a catalyst, which initiates the decomposition
of H
2
O
2
to produce the reactive OH

radical (Eqs. (4) and (6)). More
OH

radicals were produced from the Fenton’s reaction when the
dosage of ZVI was increased, thereby increasing the decolouration
efficiency. From the results of first-order kinetic analysis, a rela-
tionship of k and the ZVI dose was observed for the ZVI dose be-
tween 0.1 and 4 g/L. Since ZVI aggregates can be reused, 1 g/L of
ZVI was selected as the optimum dosage for the decolouration of
4.7 10
5
M DB15 by the Fenton/US process.
3.5. Effects of H
2
O
2
dose
Fig. 8 shows the influence of H
2
O
2
concentrations on the decol-
ouration of DB15 and the corresponding first-order rate constants.
The findings indicate that the decolouration of DB15 was affected
by the H
2
O
2
dosage. The decolouration efficiency increased from
86% to 98% at 10 min as a consequence of increasing the H
2
O
2
dos-
age from 2.58 10
3
to 5.15 10
3
M, respectively (Fig. 8).
Increasing the H
2
O
2
dosage from 5.15 10
3
to 1.03 10
2
to
2.06 10
2
M had an adverse effect on decolouration.
In the Fenton/US system, the main sources of OH

radicals are
majorly generated via two reactions: (1) H
2
O
2
under the influence
of ultrasound (Eq. (1)) and (2) H
2
O
2
reacting with ferrous irons (Eq.
0 10 0
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
0 5 10 15 20 25 30 35
pH 2.0
pH 2.5
pH 3.0
pH 3.5
pH 4.0
pH 5.0
Direct Bl ue 15 (mol/L)
Time (min)
0
0.1
0.2
0.3
2345
k (1/min)
pH
Fig. 6. Effect of pH on the decolouration of DB15. Solid lines are the best fit of first-
order kinetic equation. Conditions: ZVI 0.5 g/L, H
2
O
2
5.15 10
3
M, ultrasound
density 120 W/L.
0 100
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
0 5 10 15 20 25 30 35
0.1 g/L
0.25 g/L
0.5 g/L
1 g/L
2 g/L
4 g/L
Direct Blue 15 (mol/L)
Time (min)
ZVI dose
0
0.2
0.4
0.6
0.8
012345
k (1/min)
Fe(0) dose (g/L)
Fig. 7. Effect of ZVI addition on the decolouration of DB15. Solid lines are the best fit
of first-order kinetic equation. Conditions: pH 3.0, H
2
O
2
5.15 10
3
M, ultrasound
density 120 W/L.
0 100
1 10 -5
2 10 -5
3 10 -5
4 10 -5
5 10 -5
0 5 10 15 20 25 30 35
H2O2 2.58x10-3 M
H2O2 5.15x10-3 M
H2O2 1.03x10-2 M
H2O2 2.06x10-2 M
Direct Blue 15 (mol/L)
Time (min)
0
0.2
0.4
0.6
0.8
0 0.005 0.01 0.015 0.02 0.025
k (1/min)
H2O2 dose (mol/L)
Fig. 8. Effect of H
2
O
2
dose on the decolouration of DB15. Conditions: ZVI 1 g/L, pH
3.0, ultrasound density 120 W/L.
C.-H. Weng et al. / Ultrasonics Sonochemistry 20 (2013) 970–977 973

(4)). Theoretically larger amount of H
2
O
2
should result in the gen-
eration of more active OH

radicals if ferrous irons are not a limiting
factor in the system. However, in this study, such a phenomenon
does not seem to occur. In fact, a high concentration of H
2
O
2
has
an adverse effect on Fenton’s reaction. This result has been re-
ported elsewhere [2,26]. In the Fenton system, when the available
H
2
O
2
exceeds a critical concentration, OH

could be depleted
through the scavenging of OH

by excessive H
2
O
2
and the recombi-
nation of OH

(Eq. (2)). The scavenging effect resulting from an
overdose of H
2
O
2
will decrease the decolouration rate. Compared
with the value of rate constants shown in Fig. 8, the rate constant
for 5.15 10
3
MH
2
O
2
is 0.527 min
1
, which is the highest in the
tested dose range. Therefore, this concentration of H
2
O
2
was
selected as an optimum dose for the decolouration of DB15 by Fen-
ton/US.
3.6. Effects of addition of salts
Textile effluents normally contain a variety of inorganic salts. A
number of studies have shown that the presence of salts could af-
fect the decolouration of dyes in the Fenton process. Malik and
Saha [27] have reported that chloride salts can decrease the rate
of decolouration. However, the effect of SO
2
4
on the decolouration
of some direct dyes is insignificant. In sono-Fenton studies, Li and
Song [28] showed that the presence of SO
2
4
,Cl

and H
2
PO

4
could
retard the oxidation of acid red 97; however, Minero et al. [29] re-
ported that the effects of SO
2
4
and Cl

on the decolouration of acid
blue 40 and methylene blue were insignificant. Apparently, the ef-
fects of various anions on the decolouration in the Fenton or sono-
Fenton processes depend on pollutant characteristics. Thus, it is
necessary to assess the impact of salts on the rate of DB15 decol-
ouration with the Fenton/US process.
A series of experiments were conducted by adding various salts
to the DB15 solution while keeping the remaining variables con-
stant at the optimised decolouration conditions (ZVI 0.5 g/L, pH
3.0, H
2
O
2
5.15 10
3
M). The target salts, NaCl, Na
2
ClO
4
,Na
2
SO
4
,
NaHCO
3
,Na
2
HPO
4
and NaNO
3
, were selected because they are
likely to be present in wastewater from textile and printing indus-
tries. Since the pH of the water has a significant effect on the chem-
ical species in the solution, at pH 3.0 and in an air atmosphere, the
dominant ion species would be Cl

, ClO

4
,NO

3
and SO
2
4
. In the case
of Na
2
HPO
4
and NaHCO
3
salt additions, the dominant species
would be H
2
PO

4
and H
2
CO
3,
respectively. The results (Fig. 9) reveal
that the decolouration of DB15 was not suppressed by the presence
of Cl

, ClO

4
,SO
2
4
,H
2
CO
3
and NO

3
species. However, the extent of
decolouration was not favourable in the presence of H
2
PO

4
. A pos-
sible reason for the retardation of DB15 oxidation is that H
2
PO

4
formed complexes with Fe(III) ion, such as ferric dihydrophosphate
(Fe(H
2
PO
4
)
3
), which are more stable than the complexes formed by
other ions (Cl

, ClO

4
,SO
2
4
and NO

3
). Such complexation formation
greatly affects the distribution and reactivity of Fe species with
H
2
O
2
[30].Fig. 10 shows the effect of the presence of 0.01 M
Na
2
HPO
4
on Fe(II) and Fe(III) concentrations over time. Because
of the precipitation of Fe(H
2
PO
4
)
3(s)
that occurs in the Fenton/US
process with the addition of Na
2
HPO
4
, the concentration of Fe(II)
and Fe(III) is expected to be much lower than the process without
Na
2
HPO
4
. The H
2
PO

4
ion could accept an OH

radical [31] to form
the less reactive (H
2
PO
4
)

radical (Eq. (11)), thereby leading to the
shortage of oxidants in the system; however, H
2
PO

4
is extremely
slow in reacting with OH

, and its scavenging effect can usually
be neglected [32] in the Fenton/US process.
H
2
PO

4
þOH

!ðH
2
PO
4
Þ

þOH

ð11Þ
3.7. Effects of ultrasound density
In a Fenton/US system, ultrasound power can affect the oxida-
tion rate of DB15 to some extent. In a liquid–solid system, ultra-
sonic cavitation produces a series of related phenomena. Cavity
collapse near an extended solid surface becomes non-spherical,
which drives high-speed jets of liquid into the surface and creates
shockwave damage to the surface [33]. Thus, increasing ultrasound
intensity (power) would enlarge the effects of acoustic cavitation,
i.e. increasing the force of bubble collapse. As such, the cavitation
collapse of bubbles on the ZVI surface increases the surface area
and reactivity of ZVI which speedens up the release of Fe(II). Con-
sequently, the production of hydroxyl radicals and the oxidation
rate increase. Fig. 11 shows the effects of ultrasound power on
the decolouration of DB15. The results clearly indicate that oxida-
tion is prominent at a higher acoustic power level. Fitting results of
first-order kinetics show that the oxidation rate accelerated from
7.14 10
2
to 3.90 10
1
min
1
as power increased from 80 to
140 W/L, respectively. If the goal of the treatment is a removal effi-
ciency of 99%, the reaction time for achieving this goal is only
10 min at a power of 140 W/L, while the processing time becomes
three times with a power of 80 W/L. An increase of ultrasound
power leads to an enhancement of the Fenton/US performance
by accelerating the decolouration rate within a short period of
time. Thus, to shorten treatment time and ultimately lower energy
costs, the successful application of this process is achieved by a
higher level of ultrasound power.
3.8. Cost-effectiveness analysis of Fenton/US system
Table 1 provides an economic analysis of the Fenton/US process.
Note that the analysis is preliminary because it only includes the
costs of operating parameters at bench scale, including energy
expenditure, addition of H
2
O
2
and ZVI. When the system is applied
in the field, capital cost, equipment maintenance and depreciation
costs should be included. For a 98% oxidation efficiency in the
treatment of 4.7 10
5
M DB15 (ADMI 4130), energy requirement
ranged from 23.3 to 41.7 kW h/m
3
, which is equivalent to 1.87 to
3.33 USD/m
3
based on the current cost of electricity in Taiwan.
Based on a unit price of 500 USD/tonne, the cost of ZVI aggregates
would be 0.25 USD/m
3
. Assuming that 0.5 g/L ZVI is consumed,
when purchased in large quantities, the cost of ZVI is approxi-
mately 1 USD/kg. As indicated in Table 1, the cost of H
2
O
2
is
0 100
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
0 10203040506070
No salt
NaCl 1x10-2 M
NaClO4 1x10-2 M
Na2SO4 1x10-2 M
NaHCO3 1x10-2 M
Na2HPO4 1x10-2 M
NaNO3 1x10-2 M
Direct Blue 15 (mol/L)
Time (min)
Fe(0) 0.5 g/L
H2O2 5.15x10 -3 M
pH 3.0
US 120W (60kHz)
Fig. 9. Effect of salt addition on the decolouration of DB15. Solid lines are the best
fit of first-order kinetic equation.
974 C.-H. Weng et al. / Ultrasonics Sonochemistry 20 (2013) 970–977

negligible. The addition of ZVI constitutes approximately 10% of
the total cost, a relatively small expenditure compared to the cost
of power, which constitutes 90% of the total cost. The cost of oper-
ating the process at the higher rate of acoustic power (140 W/L) is
only 2.12 USD/m
3
, which is 60% less than the lower rate (100 W/L).
The results also show that the lowest power density (80 W/L) can
only degrade 80% of 4130 ADMI within 30 min, which does not
meet the effluent discharge limit of 550 ADMI. Although increasing
the treatment time can potentially degrade the dye further, cost of
power will increase proportionally. In general, power consumption
is a major concern. Our results show that, if the acoustic power is
properly controlled, using the Fenton process with ultrasound does
not incur a substantial increase in power consumption.
3.9. Effects of recycled ZVI
Performance using recycled ZVI in the Fenton/US process was
monitored for three consecutive runs. The results for decolouration
and the first-order rate constants are shown in Fig. 12. The exper-
imental data implies that a negative effect results from using recy-
cled ZVI. Unlike the rapid decolouration observed when fresh ZVI
was used, the use of recycled ZVI retarded the decolouration pro-
cess significantly. The value of kis inversely proportional to the
number of times ZVI is recycled. If ZVI is recycled twice over three
runs, the value of kdrops from approximately 0.260 L/min for the
first run to 0.164 and 0.104 L/min for the second and third runs,
respectively. Although the rate of decolouration decreases, a decol-
ouration efficiency of 99% for DB15 could be achieved after 25 min
treatment for the second run using recycled ZVI. The dwindling of
decolouration efficiency evident when recycled ZVI was used could
be partly because of the gradual loss of ZVI activity and Fe content.
In addition, the experimental observations could partly be
explained by the shading effects on the release of Fe(II). An
increase in the number of times ZVI is recycled leads to an increase
0
1
2
3
4
5
0 5 10 15 20 25 30
Fenton/US: No salt
Fenton/US: With 0.01 M Na2HPO4
Fe(II) mg/L
Time (min)
With Na 2HPO4
No salt
(a)
0
1
2
3
4
5
0 5 10 15 20 25 30
Fenton/US
Fenton/US/Na2HPO4
Fe(III) mg/L
Time (min)
With Na2HPO4
No salt
(b)
Fig. 10. Effect of Na
2
HPO
4
on the concentration of (a) Fe(II) and (b) Fe(III) in Fenton/
US process. Condition: Na
2
HPO
4
110
2
M, H
2
O
2
5.15 10
3
M, ZVI 0.5 g/L, pH
3.0, US power density 120 W/L.
0 100
1 10-5
2 10-5
3 10-5
4 10-5
5 10-5
0 5 10 15 20 25 30 35
US 80 W/L
US 100 W/L
US 120 W/L
US 140 W/L
Direct Blue 15(mol/L)
Time (min)
0
0.1
0.2
0.3
0.4
0.5
60 80 100 120 14 0
k (1/min)
US density (W/L)
Fig. 11. Effect of ultrasound density on the decolouration of DB15. Solid lines are
the best fit of first-order kinetic equation. Conditions: ZVI 0.5 g/L, pH 3.0, H
2
O
2
5.15 10
3
M.
Table 1
Cost analysis of Fenton/US process.
Power intensity
(W/L)
Decolourization
efficiency (%)
Reaction time (min) Power consumption
(kW h/m
3
)
Electricity
(USD/m
3
)
Fe(0)
(USD/m
3
)
H
2
O
2
(USD/m
3
)
Total cost
(USD/m
3
)
80 80 30 40.0 3.20 0.25 1 10
2
3.45
100 98 25 41.7 3.33 0.25 1 10
2
3.58
120 98 15 30.0 2.40 0.25 1 10
2
2.65
140 98 10 23.3 1.87 0.25 1 10
2
2.12
Note: (1). The current electricity rate in Taiwan is 0.08 USD/kW h. (2). Power consumption (kW h/m
3
) = Power density (W/L) 1000 W/kW time (min) 60 min/
h1000 L/m
3
; Electricity fee (USD/m
3
) = Power consumption (kW h/m
3
)0.08 USD; Fe(0) cost (USD/m
3
) = 0.5 g/L 0.001 kg/g 1000 L/m
3
500 USD/ton 0.001 ton/kg;
H
2
O
2
cost (USD/m
3
) = 0.0103 mol/L 18 g/mol 0.001 kg/g 1 USD/kg 1000 L/m
3
.
C.-H. Weng et al. / Ultrasonics Sonochemistry 20 (2013) 970–977 975

in the amount of iron hydroxide precipitates that cover the ZVI sur-
face and mask the release of Fe(II), which slows down the forma-
tion of hydroxyl radicals.
4. Conclusions
The Fenton process with ultrasound irradiation using ZVI aggre-
gates as the catalyst is more effective for the decolouration of
direct blue 15 in aqueous solution compared with the use of either
processes alone. The use of ultrasound can enhance the corrosion
of ZVI, resulting in the continuous release of large quantities of
Fe(II), which in turn triggers a more effective Fenton’s reaction.
Compared to the use of the Fenton reagent alone, ultrasonic irradi-
ation enhanced Fenton’s reaction significantly; the production of
OH

radicals was markedly increased. A positive synergistic effect
was observed and the oxidation rate was substantially enhanced
by ultrasonic irradiation.
The decolouration was obviously affected by pH, ZVI dose, H
2
O
2
dose, ultrasound power density and addition of salts. The decolour-
ation kinetics of DB15 can be described by the first-order equation.
Under a constant ultrasonic irradiation frequency (60 kHz) and
power density (120 W/L), the optimum conditions for decolour-
ation were: 4.7 10
5
M DB15, pH 3.0, ZVI 1 g/L and H
2
O
2
5.15 10
3
M. Under these optimum conditions, a decolouration
efficiency of 99% was achieved within 10 min, which complies with
Taiwan’s effluent true colour regulation of 550 ADMI. The decol-
ouration was inhibited by the presence of H
2
PO

4
in the solution,
but it was not affected significantly by the presence of H
2
CO
3
,
Cl

, ClO

4
,NO

3
or SO
2
4
. An increase of ultrasonic power led to
the enhancement of Fenton/US performance and decreased the en-
ergy cost by shortening the treatment time. ZVI can be reused;
however, the rate of decolouration decreased with repeated use
of ZVI.
The operating cost, including chemicals and energy expendi-
ture, for the process with an acoustic power of 140 W/L was esti-
mated to be only 2.12 USD/m
3
for a 99% decolouration of 4130
ADMI within 10 min. This study shows that oxidation power is
remarkably enhanced by ultrasonic irradiation. Overall, it can be
concluded that the Fenton/US process using ZVI aggregates as the
catalyst is a cost-effective technology for treating DB15 provided
the operational conditions are properly controlled.
Acknowledgements
This research was supported by the National Science Council of
R.O.C. (NSC 99-2221-E-214-021-MY3). The authors are very much
thankful to the fellows of Connelly-GPM Inc., USA for providing the
ZVI aggregate samples.
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4 10-5
5 10-5
0 5 10 15 20 25 30 35
1st run
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