Reductive dechlorination of 2,4-dichlorophenol by Pd/Fe nanoparticles prepared in the presence of ultrasonic irradiation

Article abstract of DOI:10.1016/j.ultsonch.2012.11.015

Palladium/Iron (Pd/Fe) nanoparticles were prepared by using ultrasound strengthened liquid phase reductive method to enhance dispersion and avoid agglomeration. The dechlorination of 2,4-dichlorophenol (2,4-DCP) by Pd/Fe nanoparticles was investigated to understand its feasibility for an in situ remediation of contaminated groundwater. Results showed that 2,4-DCP was first adsorbed by Pd/Fe nanoparticles, then quickly reduced to o-chlorophenol (o-CP), p-chlorophenol (p-CP), and finally to phenol (P). The induction of ultrasound during the preparation of Pd/Fe nanoparticles further enhanced the removal efficiency of 2,4-DCP, as a result, the phenol production rates increased from 65% (in the absence of ultrasonic irradiation) to 91% (in the presence of ultrasonic irradiation) within 2 h. Our data suggested that the dechlorination rate was dependent on various factors including Pd loading percentage over Fe⁰, Pd/Fe nanoparticles availability, temperature, mechanical stirring speed, and initial pH values. Up to 99.2% of 2,4-DCP was removed after 300 min reaction with these conditions: Pd loading percentage over Fe ⁰ 0.3 wt.%, initial 2,4-DCP concentration 20 mg L^-1, Pd/Fe dosage 3 g L^-1, initial pH value 3.0, and reaction temperature 25 °C. The degradation of 2,4-DCP followed pseudo-first-order kinetics reaction and the apparent pseudo-first-order kinetics constant was 0.0468 min ^-1.

Full text of DOI:10.1016/j.ultsonch.2012.11.015

Ultrasonics Sonochemistry 20 (2013) 864–871
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Reductive dechlorination of 2,4-dichlorophenol by Pd/Fe nanoparticles prepared
in the presence of ultrasonic irradiation
Deming Zhao a, Min Li a,c, Dexing Zhang a, Shams Ali Baig b, Xinhua Xu b,
⇑
a College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, PR China
b College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, PR China
c Ningbo Research and Design Institute of Chemical Industry, Ningbo 315040, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 August 2012
Received in revised form 29 October 2012
Accepted 25 November 2012
Available online 3 December 2012
Palladium/Iron (Pd/Fe) nanoparticles were prepared by using ultrasound strengthened liquid phase
reductive method to enhance dispersion and avoid agglomeration. The dechlorination of 2,4-dichlorophe-
nol (2,4-DCP) by Pd/Fe nanoparticles was investigated to understand its feasibility for an in situ remedi-
ation of contaminated groundwater. Results showed that 2,4-DCP was first adsorbed by Pd/Fe
nanoparticles, then quickly reduced to o-chlorophenol (o-CP), p-chlorophenol (p-CP), and finally to phe-
nol (P). The induction of ultrasound during the preparation of Pd/Fe nanoparticles further enhanced the
removal efficiency of 2,4-DCP, as a result, the phenol production rates increased from 65% (in the absence
of ultrasonic irradiation) to 91% (in the presence of ultrasonic irradiation) within 2 h. Our data suggested
that the dechlorination rate was dependent on various factors including Pd loading percentage over Fe0,
Pd/Fe nanoparticles availability, temperature, mechanical stirring speed, and initial pH values. Up to
99.2% of 2,4-DCP was removed after 300 min reaction with these conditions: Pd loading percentage over
Fe0 0.3 wt.%, initial 2,4-DCP concentration 20 mg Lꢀ1, Pd/Fe dosage 3 g Lꢀ1, initial pH value 3.0, and reac-
tion temperature 25 °C. The degradation of 2,4-DCP followed pseudo-first-order kinetics reaction and the
Keywords:
2,4-DCP
Reductive dechlorination
Pd/Fe nanoparticles
Ultrasound
Kinetics
apparent pseudo-first-order kinetics constant was 0.0468 minꢀ1
.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
utilize the produced H2 from Fe0 corrosion and accelerate the rates
of dechlorination reaction [7,8]. However, the presence of Pd not
only reduces the accumulation of toxic byproducts, but also inhib-
its particle oxidation in air [9]. Pd/Fe bimetallic nanoparticles,
when compared to the conventional large particles have some
advantages with possessing large specific surface area and high
surface reactivity [10].
In order to obtain the stabilized and high reactive Pd/Fe bime-
tallic nanoparticles and decrease their agglomeration and accumu-
lation in the effluent, ultrasound is applied to the preparation of
Pd/Fe bimetallic nanoparticles. Sono-chemistry arises from acous-
tic cavitation, the formation, growth, and implosive collapse of
bubbles in a liquid. Acoustic cavitation can increase the surface
area of the reactive solids by causing particles to rupture [10,11].
In the present work, an attempt has been made to prepare Pd/Fe
bimetallic nanoparticles with the help of sonication for better dis-
persion and also avoiding the agglomeration. The performance of
bimetallic systems in the remediation of contaminated groundwa-
ter under ambient condition, the dechlorination of 2,4-DCP by
nanoscale Pd–Fe particles was investigated. In addition, other fac-
tors contributing to 2,4-DCP reduction, such as Pd/Fe nanoparticles
dosage, Pd loading percentage over Fe0, initial 2,4-DCP concentra-
tion, mechanical stirring speed and initial pH values, were also
examined.
Chlorinated phenols (CPs) have been widely used in the produc-
tion of wood preservers, pesticides, dyestuffs, lubricants, dielectric
and biocides, that is why found in nearly all major environmental
components [1]. Most of them are toxic, carcinogenic, and bio-
refractory [2,3]. Moreover, they could persist in the environment
for a long time, and do harm to human health by bioaccumulation
[4]. Due to the aryl structure and the presence of chlorine atom,
chloroaromatics are exceptionally recalcitrant towards chemical
reactions aimed at their destruction [2]. This raises an urgent need
for efficient reductive dechlorination methods to eliminate chloro-
aromatics from both concentrated industrial effluents and diluted
polluted groundwater.
Zero-valent iron (Fe0, ZVI) is a mild reducing agent with reduc-
tion potential of ꢀ0.440 V. Treatment of recalcitrant chemicals
using Fe0 has been a main focus of researches in recent years [1–
3,5]. A rapid and complete reductive dechlorination technology
of chloroaromatics involving the use of bimetallic Pd/Fe nanoparti-
cles that led to the formation of non-chlorinated hydrocarbons
have also been reported [1,3,6]. As one of the noble metal, Pd can
⇑
Corresponding author. Tel./fax: +86 571 88982031.
E-mail address: xuxinhua@zju.edu.cn (X. Xu).
1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2012.11.015

D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
865
HITACHI Instruments Corp., JP). X-ray diffraction (XRD) analysis
was performed by using X’Pert Pro advanced X-ray diffractometer
(k = 1.5418 Å).
2. Experimental
2.1. Chemicals
Organic compounds such as 2,4-DCP, p-CP, o-CP and phenol
were analyzed by SHIMADZU High Performance Liquid Chroma-
tography. Agilent C18 Column, 150 ꢂ 4.6 mm, mobile phase:
MeOH/H2O (60/40, v/v), flow rate: 1.0 mL minꢀ1, detector: UV at
Iron sulfate heptahydrate (AR grade), 2,4-DCP (chemically pure,
or AP grade), o-CP (CP grade), p-CP (CP grade), phenol (CP grade)
were purchased from the Sinopharm Group Chemical Reagent
Co., Ltd., China, Potassium hexachloropalladate (K2PdCl6, 99%)
obtained from Aladdin-Reagent. Sodium borohydride (AR grade)
purchased from Tianjin Chemical Reagent Research Institute. All
chemicals were used as received without further purification.
2,4-DCP is dissolved in deionized water and stored at 4 °C. Pd/Fe
nanoparticles were synthesized immediately before use.
280 nm, sample size: 20
lL.
3. Results and discussion
3.1. Characterization of Pd/Fe nanoparticles
The BET specific surface area of the synthesized Pd/Fe nanopar-
ticles in the presence and absence of 20 kHz ultrasonic irradiation
were 37.77 and 22.39 m2 gꢀ1, respectively. Fig. 1(a)–(c) shows TEM
images of the freshly prepared Pd/Fe nanoparticles in the presence
and absence of ultrasonic irradiation, and the reacted (after
300 min) Pd/Fe nanoparticles prepared in the presence of ultra-
sound. The freshly prepared Pd/Fe nanoparticles in the absence
of ultrasound, were spherical in shape with particle size ranging
from 20 to 100 nm, and appeared to aggregate together
(Fig. 1(a)). However, Pd/Fe nanoparticles prepared in the presence
of ultrasound were also spherical, but the particle size ranges from
10 to 100 nm, and appeared to be smaller particles diameter and
better dispersion (Fig. 1(b)). Spherical particles aggregate to form
dendrites due to geomagnetic forces between nanoscale particles
and small particles, and also their surface tension interactions
[12,13]. A mucous layer was adhered onto the surface of Pd/Fe
nanoparticles prepared in the absence of ultrasound, reflects the
possibility of lowering the dechlorination efficiency. A thickly mu-
cous layer was showed on the surface of the Pd/Fe nanoparticles
prepared in the presence of ultrasound after 300 min of reaction
(Fig. 1(c)). More organic components such as phenol and 2,4-
DCP, as well as metal hydroxides and carbonate passivating layers
on the nanoparticles’ surface inhibited the particles’ active sites,
likely leading to lower dechlorination efficiency. The SEM images
of the fresh Pd/Fe nanoparticles prepared in the presence of ultra-
sound, and the reacted (after 300 min) Pd/Fe nanoparticles pre-
pared in presence of ultrasound are shown in Fig. 1(d) and (e).
Fig. 1(d) shows many Pd particles loaded on the surface of Pd/Fe
nanoparticles. After 300 min of reaction, Pd particles loaded on
the surface of Pd/Fe nanoparticles decreased and the white plate-
let-shaped crystals appeared (Fig. 1(e)), suggests the formation of
iron oxides resulting from iron corrosion in water. These minerals
2.2. Synthesis
In the presence or absence of 20 kHz and 150 W KQ-3200DB
ultrasonic elutriation apparatus with effective volume of 2000 mL
purchased from Kunshan ultrasonic apparatus Co., Ltd., China,
Pd/Fe bimetallic nanoparticles were prepared in a 500 mL three-
necked flask under nitrogen gas. Fe0 nanoparticles were synthe-
sized by drop wise addition of stoichiometric amounts of NaBH4
(0.50 mol Lꢀ1) aqueous solution into a flask containing FeSO4ꢁ7H2O
(0.25 mol Lꢀ1) aqueous solution simultaneously with mechanical
stirring at 25 °C by external circulation at low temperature water
cooling system. The ferrous iron was reduced to zero-valent iron
according to the following reaction:
2þ
FeðH2OÞ þ 2BHꢀ4 ! Fe # þ2BðOHÞ þ 7H2 "
ð1Þ
6
3
The Fe0 nanoparticles were then rinsed several times with deox-
ygenated deionized water. Subsequently, Pd/Fe nanoparticles were
prepared by reacting with the wet Fe0 nanoparticles in an aqueous
solution of potassium hexachloropalladate (6.13 mmol Lꢀ1) under
mechanical stirring according to the following equation:
PdCl26ꢀ þ 2Fe ! 2Fe2þ þ Pd # þ6Clꢀ
ð2Þ
2.3. Batch experimental procedures
Batch experiments of 2,4-DCP dechlorination were performed
in the same three-necked flask into which Pd/Fe nanoparticles
were added. 2,4-DCP stock solutions and a certain amount of deox-
ygenated deionized water were added into the flask containing
freshly prepared Pd/Fe nanoparticles into 500 mL of total reaction
volume. The reaction solution was stirred under nitrogen flow to
simulate anaerobic environment at 25 °C. Aliquots of samples were
periodically collected with glass syringes and the reaction was
were likely composed of goethite (a-FeOOH) or lepidocrocite (c-
FeOOH) [3,14,15]. Huang and Zhang [15] also suggested that a
stratified ZVI corrosion coating would form in water, for which
the outer and middle layers comprised both FeOOH and Fe3O4,
while the inner layer mainly consisted of Fe3O4. This is generally
consistent with our observation in XRD patterns. Fig. 1(f) shows
the XRD patterns of the fresh and the 300 min aged Pd/Fe nanopar-
ticles prepared in the presence of ultrasound. The XRD pattern for
the fresh sample presents a strong peak 44.66° which corresponds
to the body-centered cubic N–Fe0 at the (110) plane. The peak in
the XRD pattern of the aged sample shows evidence of iron oxides,
possibly Fe3O4 (magnetite) or Fe2O3 (maghemite), or their mixture.
This agrees with the fact that Fe0/Fe3O4 couple is more thermody-
namically favorable at pH above 6.1 [18].
quenched by passing through 0.22
membrane filters.
lm polyether sulphone (PES)
2.4. Methods of analysis
Prior to the characterization, all freshly synthesized Pd/Fe nano-
particles (with a Pd bulk loading of 0.3%) were immersed in
absolute ethyl alcohol and dispersed by an ultrasonator.
Brunauer–Emmett–Teller (BET) specific surface area of all synthe-
sized Pd/Fe nanoparticles were measured using nitrogen adsorp-
tion method with a surface analyzer (ASAR2020M+, Micromeritics
Instrument Corp., US). Before the analysis, the particles were dried
in vacuum at 25 °C for 24 h and then hydrogen flow at 260 °C for
further 4 h. Transmission electron microscope (TEM) images of
the particles were obtained with a JEOL JEM 200CX microscope
(JEOL Electronics Co., JP) performed at a voltage of 160 kV for mor-
phological measurements. Scanning electron microscope (SEM)
images were obtained through a microscope (HITACHI S-4800
3.2. Comparisons on 2,4-DCP reductive dechlorination by Pd/Fe
nanoparticles prepared in the presence and absence of ultrasonic
irradiation
Effects of different Pd/Fe nanoparticles synthesized methods on
2,4-DCP reductive dechlorination were investigated at Pd/Fe

866
D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
a
b
e
c
d
f
0
magnetite/maghemite
fresh
aged
30
40
50
60
70
80
90
100
2
(degree)
Fig. 1. (a) TEM image of fresh Pd/Fe nanoparticles prepared in the absence of ultrasonic irradiation, (b) TEM image of fresh Pd/Fe nanoparticles prepared in the presence of
ultrasonic irradiation, (c) TEM image of aged Pd/Fe nanoparticles prepared in the presence of ultrasonic irradiation, (d) SEM image of fresh Pd/Fe nanoparticles prepared in the
presence of ultrasonic irradiation, (e) SEM image of aged Pd/Fe nanoparticles prepared in the presence of ultrasonic irradiation (300 min) and (f) XRD patterns of fresh and
aged Pd/Fe nanoparticles prepared in the presence of ultrasonic irradiation.
nanoparticles dosage of 3 g Lꢀ1, the Pd loading percentage over Fe0
of 0.3 wt.%, initial 2,4-DCP concentration of 20 mg Lꢀ1, an initial pH
value of 3.0, reaction temperature of 25 °C and mechanical stirring
speed of 400 rpm. Fig. 2 shows reductive dechlorinations of 2,4-
DCP by Pd/Fe nanoparticles synthesized in the presence and ab-
sence of ultrasonic irradiation.
2,4-DCP was first adsorbed by Pd/Fe nanoparticles, then re-
duced to o-CP and p-CP, and later converted to phenol, as a sole fi-
nal organic product. No other chlorinated intermediates or final
organic products were detected. The concentration of 2,4-DCP de-
creased rapidly and the removal percentage reached 94% in
180 min, then further improved to nearly 98% in 300 min for Pd/
Fe nanoparticles synthesized in the presence of ultrasound. In con-
trast, only about 70% and 85% of the removal percentage were ob-
tained for Pd/Fe nanoparticles synthesized in the absence of
ultrasound during the same reaction periods, respectively. Phenol
and inorganic chlorine were detected as the final products in the
dechlorination reaction. Compared to initial concentration
(20 mg Lꢀ1 2,4-DCP), so approximately 17–27% carbon mass losses
were observed. This indicates that a fraction of organic compounds
was absorbed or covered by surface passivating layers due to the
precipitation of metal hydroxides on the surface of iron and Pd/
Fe particles. This is also evidenced by the fact that the 2,4-DCP con-
centration dropped rapidly in the first 15 min, but the phenol being
generated was much less than the maximum attainable. The non-
detected fraction of intermediates may be associated with Pd/Fe
nanoparticles, which appear to serve as non-reactive sorption sites
for intermediates [16,17].
Fig. 2(c) and (d) presents the generation and further degrada-
tion process of p- and o-CP during the same reaction period with
different Pd/Fe nanoparticles prepared in the presence and absence
of ultrasound. The experimental results demonstrated that the
generation of o-CP was more than that of p-CP, although in the fur-
ther reductive degradation process, o-CP was easily reduced to
phenol appreciably than p-CP. So the concentration of o-CP is much
higher than the concentration of p-CP during the reductive dechlo-
rination of 2,4-DCP. The maximum concentrations of o-CP were
4.13 and 4.36 mg Lꢀ1 with different Pd/Fe nanoparticles prepared

D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
867
20
15
10
5
12
(a)
(b)
Pd/Fe nanoparticles prepared in the presence of Ultrasound
Pd/Fe nanoparticles prepared in the absence of Ultrasound
Pd/Fe nanoparticles prepared in the presence of Ultrasound
Pd/Fe nanoparticles prepared in the absence of Ultrasound
10
8
6
C
4
C
2
0
0
0
60
120
180
240
300
0
60
120
180
240
300
Time , min
Time , min
1.5
5
4
3
2
1
0
(c)
(d)
Pd/Fe nanoparticles prepared in the presence of Ultrasound
Pd/Fe nanoparticles prepared in the absence of Ultrasound
1.0
0.5
0.0
o
p
C
C
Pd/Fe nanoparticles prepared in the presence of Ultrasound
Pd/Fe nanoparticles prepared in the absence of Ultrasound
0
60
120
180
240
300
0
60
120
180
240
300
Time , min
Time , min
Fig. 2. Dechlorination of 2,4-DCP by Pd/Fe nanoparticles prepared in the presence and absence of ultrasonic irradiation (T = 25 °C, pHin = 3.0, C2,4-DCP = 20 mg Lꢀ1, CPd/Fe
=
3 g Lꢀ1, mechanical stirring speed at 400 rpm, the Pd loading percentage over Fe0 was 0.3 wt.%).
in the presence and absence of ultrasound in 60 and 180 min,
respectively. The maximum concentrations of o-CP appears to be
delayed with Pd/Fe nanoparticles prepared in the absence of ultra-
sound, but for p-CP, the maximum concentrations were just 0.90
and 1.11 mg Lꢀ1 in 15 and 180 min, respectively, much less than
those of o-CP. Obviously, the catalytic degradation process quickly
proceeded with Pd/Fe nanoparticles prepared in the presence of
ultrasound than with Pd/Fe nanoparticles prepared in the absence
of ultrasound. During the catalytic degradation process, most 2,4-
DCP were first transformed into o-CP and p-CP then reduced rap-
idly to phenol. This was evident from the concentration changing
during the reduction of 2,4-DCP, while more and more phenol
was produced. However, the concentration of o-CP and p-CP were
initially increased in the prophase reaction, followed by a slow de-
crease. Therefore, Pd/Fe nanoparticles were prepared by using
ultrasound strengthened liquid phase reductive method in the fol-
lowing experiments.
Fig. 3 shows the efficiencies of dechlorination and phenol forma-
tions were significantly increased as Pd loading percentage over
Fe0 increased from 0.20 to 0.30 wt.%. The removal percentage of
2,4-DCP reached from 64 to 92% within 240 min. The increase in
the Pd loading percentage over Fe0 from 0.3 to 0.4 wt.% only caused
slight improvement to the 2,4-DCP dechlorination efficiency. The
most likely explanation for this is that the maximum Pd coverage
is less than one layer, and in this way, the loss of available catalytic
reactive sites due to the overlapping between Pd atoms can be ex-
cluded. Nonetheless, the amount of hydrogen gas absorbed by Pd
atoms increased with the increasing Pd loading percentage over
Fe0 [19,20], and so make it possible to improve the efficiencies of
dechlorination and phenol formations. The slight improvement of
the removal efficiency at Pd loading percentage over Fe0 > 0.30 -
wt.% is that the accumulation of excessive hydrogen gas hinders
the contact between target pollutant and metal particles, and re-
duces the surface area available for 2,4-DCP dechlorination. This
phenomenon is in agreement with previously reported studies
[1,17,18]. Pd loading percentage over Fe0 in Pd/Fe nanoparticles
was selected at about 0.30 wt.% for efficient reductive dechlorina-
tion and yet minimal palladium usage.
3.3. Effects of Pd loading percentage over Fe0 on 2,4-DCP
dechlorination
It has been elucidated that zero-valent iron can promote a
hydrogenolysis reaction where a chlorine atom in the organic chlo-
rinated compounds is replaced by a hydrogen atom. Palladium is a
well-known catalyst for the hydrogenolysis. Co-existence of palla-
dium and iron in the particles has been proved to be very effective
to accelerate the dechlorination reaction. Therefore, the Pd loading
percentage over Fe0 on Pd/Fe nanoparticles may be one of the
important influential factors on the dechlorination efficiency. As
3.4. Effects of Pd/Fe nanoparticles dosage on 2,4-DCP dechlorination
Since the catalytic reductive dechlorination by Pd/Fe nanoparti-
cles takes place on the surface of nanoparticles, therefore Pd/Fe
nanoparticles to 2,4-DCP ratio (g Pd/Fe nanoparticles /mg 2,4-
DCP) is also a significant influential parameter. The quantity of
available surface area is among the most significant experimental

868
D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
20
16
12
8
20
Pd loading percentage over Fe0 in Pd/Fe nanoparticles
Pd/Fe nanoparticles dosage
0.20 wt%
0.25 wt%
0.30 wt%
0.35 wt%
0.40 wt%
16
12
8
-1
2 g L
-1
3 g L
-1
4 g L
-1
5 g L
-1
6 g L
4
4
0
8
0
8
6
4
2
0
6
4
2
0
0
60
120
180
240
300
Time , min
0
60
120
180
240
300
Fig. 4. Effects of Pd/Fe nanoparticles dosage on 2,4-DCP dechlorination (T = 25 °C,
C2,4-DCP = 20 mg Lꢀ1, mechanical stirring speed at 400 rpm, pHin = 5.0, the Pd loading
percentage over Fe0 was 0.3 wt.%).
Time , min
Fig. 3. Effects of Pd loading percentage over Fe0 in Pd/Fe nanoparticles on 2,4-DCP
dechlorination by Pd/Fe nanoparticles prepared in the presence of ultrasonic
irradiation (T = 25 °C, pHin = 5.0, C2,4-DCP = 20 mg Lꢀ1 CPd/Fe = 3 g Lꢀ1
, , mechanical
stirring speed at 400 rpm).
Low pH favors more iron surface available for reaction with the
chlorinated molecules or at least promote the corrosion rate, lead-
ing to release of chloride ions. However, when oxygen was present
in the reaction solution, the corrosion product, OHꢀ, was gener-
ated. Therefore, the solution pH should increase even if the initial
pH is low. At higher pH values, carbonate and hydroxide coatings
undoubtedly develop, which inhibit further decomposition of iron
surface and hinder access to the Fe0 surface. As a result, the cata-
lytic activity decreases. Fig. 5 shows the effect of different initial
pH values on the reductive dechlorination of 2,4-DCP by Pd/Fe
nanoparticles. Prior to initializing reaction all reactant solutions
were adjusted to different pH values by dilution with sulfuric acid
(H2SO4) and sodium hydroxide (NaOH), and during the reaction pH
values were not adjusted. When the initial pH values increased
from 3, 5, 7 to 9, the removal percentages of 2,4-DCP dropped from
nearly 97%, 96%, 93% to 75%, respectively in 240 min. The increase
of initial pH values of the reactant solutions from 7 to 9, leads to
the reactant solution change from acidic to alkaline condition, con-
sequently the removal percentages of 2,4-DCP dropped obviously.
It indicates that the presence of H+ largely enhances the 2,4-DCP
reductive dechlorination efficiency. The possible reasons may be
that (1) the surface of Pd/Fe nanoparticles would be oxidized inev-
itably during the preparation and storage, which had been demon-
strated by the earlier published literatures [21]. However, at lower
pH values, the oxides on the particles surface were dissolved, and
the active sites of the particle surface were exposed; (2) at lower
pH values, the iron corrosion could be accelerated, producing en-
ough hydrogen (or hydrogen atoms), which were in favor of hydro-
genation reaction [22,23]; (3) iron corrosion in solution of pH
higher than 7 tends to form a passive film of iron oxides and
hydroxide on the iron surface, which inhibits further reaction. In
the previous study [17], detection of the solution pH during the
entire period of the reaction, and ferrous ions/total iron ions pro-
duced in the reaction could further support our assumptions.
variables affecting 2,4-DCP reductive dechlorination reaction. With
increasing the concentration of Pd/Fe nanoparticles in the solution,
it is insignificant to the final removal efficiency in excess of Pd/Fe
nanoparticles dosage. However, it will accelerate the initial reac-
tion rate and provide more active sites of Pd/Fe nanoparticles for
collision with 2,4-DCP during the reductive process. Effects of dif-
ferent Pd/Fe nanoparticles dosage on 2,4-DCP dechlorination were
explored as shown in Fig. 4. With the elevation of the Pd/Fe nano-
particles dosage from 2 to 6 g Lꢀ1, obvious differences were ob-
served, the removal percentage of 2,4-DCP increased from 62% to
94% after 120 min of the reaction. Increasing the dosage of Pd/Fe
nanoparticles means the larger Pd/Fe nanoparticles surface area.
The higher the Pd/Fe nanoparticles surface area concentration is,
the faster the reaction velocity. The removal percentage of 2,4-
DCP is similar for a dosage of Pd/Fe nanoparticles of 3 or 4 g Lꢀ1
after 300 min of the reaction. Hence, the appropriate dosage of
Pd/Fe nanoparticles in the level of 3 g Lꢀ1 is chosen for 2,4-DCP
dechlorination.
3.5. Effects of the initial pH value on 2,4-DCP dechlorination
Initial pH value in aqueous solution is an important influential
factor in reductive dechlorination of chlorinated organic com-
pounds using ZVI. Different corrosion reactions of iron in acidic
and weak acidic or neutral solutions were given below:
Acidic condition:
Fe0 þ 2Hþ ! Fe2þ þ H2 "
ð3Þ
Weak acidic or neutral condition:
Fe0 þ 2H2O ! Fe2þ þ 2OHꢀ þ H2 "
ð4Þ

D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
869
20
15
10
5
20
pH=3
pH=5
pH=7
pH=9
mechanical stirring speed
200 rpm
15
400 rpm
600 rpm
10
5
0
10
0
8
8
6
4
2
0
6
4
2
0
0
60
120
180
240
300
0
60
120
180
240
300
Time , min
Time , min
Fig. 6. Effects of mechanical stirring speed on 2,4-DCP dechlorination (T = 25 °C,
C2,4-DCP = 20 mg Lꢀ1, pHin = 3.0, the Pd loading percentage over Fe0 was 0.3 wt.%,
CPd/Fe = 3 g Lꢀ1).
Fig. 5. Effects of solution initial pH on 2,4-DCP dechlorination (T = 25 °C, C2,4-DCP
=
20 mg Lꢀ1
, mechanical stirring speed at 400 rpm, pHin = 3.0, the Pd loading
percentage over Fe0 was 0.3 wt.%, CPd/Fe = 3 g Lꢀ1).
3.6. Effects of mechanical stirring speed on 2,4-DCP dechlorination
of 2,4-DCP was close to each other, the absolute removal amount
increased with increasing initial 2,4-DCP concentrations.
Previous studies have reported that the reductive dechlorina-
tion of 2,4-DCP by Pd/Fe nanoparticles involves three main steps
[1–3,5–10]: (1) 2,4-DCP in aqueous solution diffuse to the Pd/Fe
nanoparticles surface and adsorb in the surface of Pd/Fe nanopar-
ticles; (2) the adsorbed 2,4-DCP takes reduction reaction under
the effect of Pd/Fe nanoparticles; (3) the products of reaction des-
orbs from the Pd/Fe nanoparticles and diffuses to the bulk solution.
The reductive dechlorination rate is controlled by mass transfer if
Pd/Fe nanoparticles dosage is excessive in the reaction. Therefore,
the mechanical stirring speed will significantly affect the reductive
dechlorination efficiency. Fig. 6 shows the effect of different
mechanical stirring speed on the dechlorination of 2,4-DCP by
Pd/Fe nanoparticles. When the mechanical stirring speed increased
from 200, 400 to 600 rpm, the removal percentages of 2,4-DCP in-
creased from nearly 78, 96 to 99%, respectively in 240 min. With
increasing the mechanical stirring speed, it is significant to im-
prove the final removal efficiency because of coupling with the
greater mass transfer rate. Hence, the appropriate mechanical stir-
ring speed in the level of 600 rpm is chosen for 2,4-DCP
dechlorination.
3.8. Kinetic modeling of 2,4-DCP reductive dechlorination by Pd/Fe
nanoparticles
Fig. 2 shows the hypothetical transformation pathways of 2,4-
DCP. The products consist of phenol, p-CP and o-CP, so it is
assumed that 2,4-DCP was hydro-dechlorinated according to the
following sequence of steps. In the following equations, 2,4-DCP,
CP and P are the abbreviated forms of 2,4-dichlorophenol, chloro-
phenol, and phenol, respectively:
K1
K2
2; 4-DCP ! CP þ Clꢀ ! P þ 2Clꢀ
ð5Þ
where CP represents the total molecules of o-CP and p-CP in the
reaction. It is well established that the pseudo-first-order kinetics
could be applied in the reductive dechlorination of chlorinated
organics by the bimetallics nanoparticles if the bimetallics nanopar-
ticles dosage is excessive in the reaction [1,3,7]. Therefore, the pseu-
do-first order reaction kinetics was adopted to model the 2,4-DCP
dechlorination reaction by Pd/Fe nanoparticles prepared in the
presence of ultrasonic irradiation. The corresponding reaction rate
equations for the disappearance of 2,4-DCP, the transient formation
of CP (including o-CP and p-CP) intermediates, and the accumula-
tion of P in the batch system are shown as follows:
3.7. Effects of initial 2,4-DCP concentration on 2,4-DCP dechlorination
Five different initial 2,4-DCP concentrations (10, 20, 30, 40 and
50 mg Lꢀ1) were employed. Fig. 7 shows the 2,4-DCP reductive pro-
files with different initial 2,4-DCP concentrations. The removal
rates of 2,4-DCP reached 82%, 89%, 74%, 64%, and 71% each after
60 min, and then reached 98%, 99%, 99%, 98%, and 97%, respectively
after 300 min of the reaction. Though the final removal percentage
ꢀdC2;4-DCP
¼ k1C2;4-DCP
ð6Þ
ð7Þ
dt
dCCP
dt
¼ k1C2;4-DCP ꢀ k2CCP

870
D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
1t
a2;4-DCP ¼ eꢀk
ð9Þ
ð10Þ
ð11Þ
50
40
30
20
10
k1
1t
2t
aCP
¼
ðeꢀk ꢀ eꢀk
Þ
k2 ꢀ k1
initial 2,4-DCP concentration
10 mg L-1
20 mg L-1
30 mg L-1
aP ¼ 1 ꢀ a2;4-DCP
where
ꢀ
aCP
40 mg L-1
50 mg L-1
a
represents the molar fraction of the subscript compound to
the initial concentration of the parent compound (i.e., 2,4-DCP).
Since a fraction of organic compounds were absorbed on the larger
surface of Pd/Fe nanoparticles, the actual concentration of the or-
ganic compound in aqueous phase has to be revised. As the produc-
tion of P was step wise, equilibrium time for organic compounds
absorbed onto Pd/Fe nanoparticles was little, and the total molar
fraction of the organic compounds did not change, the proportion
of P in the aqueous phase can be seen as invariable, as a result,
Eq. (11) can be revised as follows:
C
0
initial 2,4-DCP concentration
10 mg L-1
16
20 mg L-1
30 mg L-1
40 mg L-1
ap0
¼
ap ꢂ ð1 ꢀ aÞ
ð12Þ
12
8
50 mg L-1
where a0 represents the actual molar fraction of the subscript com-
pound to the initial concentration of the parent compound, and a
represents the molar fraction of the organic compounds which were
adsorbed onto Pd/Fe nanoparticles to the initial concentration of the
parent compound (i.e., 2,4-DCP). Then k values were derived from
fitting the experimental data into Eq. (12) according to the non-lin-
ear least-square regression.
C
4
0
0
60
120
180
240
300
k values in different reaction conditions were listed in Table 1. It
Time , min
shows that
k values increased obviously from 0.000786 to
0.0413 minꢀ1 under Pd/Fe nanoparticles prepared in the absence
and presence of ultrasonic irradiation, respectively. And they in-
creased from 0.00425, 0.00486, 0.00826, 0.00940 to 0.01074 minꢀ1
as the Pd loading percentage over Fe0 in Pd/Fe nanoparticles was
increased from 0.20, 0.25, 0.30, 0.35 to 0.40 wt.%. In short, k values
increased with the increasing Pd/Fe nanoparticles dosage and
mechanical stirring speed, with the decrease of pH values, and k
values have nothing to do with the initial 2,4-DCP concentration.
Fig. 7. Effects of initial 2,4-DCP concentration on 2,4-DCP dechlorination (T = 25 °C,
pHin = 3.0, mechanical stirring speed at 600 rpm, the Pd loading percentage over Fe0
was 0.3 wt.%, CPd/Fe = 3 g Lꢀ1).
d½Pꢃ
¼ k2CCP
ð8Þ
dt
The above simultaneous rate equations are integrated, leading
to the following molar fractions:
Table 1
k values in different experimental conditions.
Reaction conditions
1 ꢀ a
k1/minꢀ1
R
k2/minꢀ1
R
Pd/Fe nanoparticles prepared method
In the absence of ultrasound
In the presence of ultrasound
0.948
0.991
0.968
0.934
0954
0.941
0.963
0.948
0.954
0.946
0.957
0.969
0.920
0.996
0.906
0.965
0.942
0.954
0.985
0.979
0.985
0.984
0.917
0.989
0.00786
0.04130
0.00425
0.00486
0.00826
0.00940
0.01074
0.00722
0.00826
0.01206
0.01676
0.02028
0.02101
0.02073
0.01196
0.00572
0.00277
0.02073
0.04678
0.05767
0.04678
0.05872
0.02036
0.02055
0.98
0.97
0.94
0.97
0.97
0.98
0.96
0.98
0.97
0.99
0.99
0.99
0.96
0.99
0.99
0.96
0.94
0.99
0.96
0.95
0.99
0.95
0.97
0.97
0.00613
0.00777
0.00106
0.00320
0.00430
0.00556
0.00405
0.00545
0.00430
0.00790
0.00960
0.09276
0.00726
0.00549
0.00341
0.00405
0.00277
0.00549
0.01338
0.01546
0.01321
0.01516
0.0353
0.97
0.96
0.97
0.99
0.99
0.99
0.97
0.99
0.99
0.94
0.99
0.98
0.99
0.98
0.97
0.96
0.99
0.98
0.98
0.98
0.98
0.97
0.99
0.98
Pd loading percentage over Fe0
0.20 wt.%
0.25 wt.%
0.30 wt.%
0.35 wt.%
0.40ꢀw1t.%
2 g L
Pd/Fe dosage
3 g Lꢀ1
4 g Lꢀ1
5 g Lꢀ1
6 g Lꢀ1
3
5
7
9
200 rpm
400 rpm
600 rpm
10 mg Lꢀ1
20 mg Lꢀ1
30 mg Lꢀ1
40 mg Lꢀ1
50 mg Lꢀ1
Initial pH values
Mechanical stirring speed
Initial 2,4-DCP concentration
0.00908
Note: 1 ꢀ a represents the molar fraction of the total organic compounds in solution; k1 and k2 refer the corresponding reaction rate for the disappearance of 2,4-DCP and CP,
respectively; R denotes the correlation coefficient between the experimental data can match the calculated number.

D. Zhao et al. / Ultrasonics Sonochemistry 20 (2013) 864–871
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and avoid the agglomeration. In the reductive dechlorination of
2,4-DCP, the dechlorination efficiency was dependent on a number
of factors including Pd/Fe availability, mechanical stirring speed,
and initial pH values. The degradation of 2,4-DCP followed pseu-
do-first-order kinetics reaction and the apparent pseudo-first-or-
der kinetics constants were obtained.
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