Enhanced dechlorination performance of 2,4-dichlorophenol by vermiculite supported iron nanoparticles doped with palladium

Article abstract of DOI:10.1039/c4ra02618g

In this study, environment-friendly vermiculite (VMT) was used to support nanoscale zero-valent iron (nZVI) and nZVI doped with palladium (abbreviated as Fe-VMT and Pd/Fe-VMT, respectively). The physicochemical properties of the products obtained were analyzed by X-ray diffraction (XRD), specific surface area (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the BET surface areas of Fe-VMT and Pd/Fe-VMT were 39.5 m² g^-1 and 59.1 m² g^-1, and 18.9 m² g^-1 for unsupported nZVI nanoparticles. The presence of vermiculite led to a decrease in the aggregation of nZVI and Pd/Fe as observed by SEM and TEM. Batch experiments were conducted to investigate the catalytic performance of nZVI, Pd/Fe, VMT, Fe-VMT and Pd/Fe-VMT via the dechlorination reaction of 2,4-dichlorophenol (2,4-DCP). The dechlorination rates of 2,4-DCP by Pd/Fe-VMT (by adding Pd) were greater than that achieved by Fe-VMT. Additionally, the dechlorination of 2,4-DCP by Pd/Fe-VMT would be influenced by temperature, initial pH values, Pd loading, initial concentration of 2,4-DCP and the dosage of materials. It was confirmed that the ultimate reduction product of 2,4-DCP was phenol. Overall, Pd/Fe-VMT is a promising material for the dechlorination of 2,4-DCP. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02618g

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PAPER
Enhanced dechlorination performance of 2,4-
dichlorophenol by vermiculite supported iron
nanoparticles doped with palladium†
Cite this: RSC Adv., 2014, 4, 25580
Pingxiao Wu,*abc Chongmin Liu,abc Zhujian Huangabc and Wanmu Wangabc
In this study, environment-friendly vermiculite (VMT) was used to support nanoscale zero-valent iron (nZVI)
and nZVI doped with palladium (abbreviated as Fe-VMT and Pd/Fe-VMT, respectively). The physicochemical
properties of the products obtained were analyzed by X-ray diffraction (XRD), specific surface area (BET),
scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron
spectroscopy (XPS). The results showed that the BET surface areas of Fe-VMT and Pd/Fe-VMT were
2
ꢀ1
2
ꢀ1
2
ꢀ1
3
9.5 m
g
and 59.1 m
g
, and 18.9 m
g for unsupported nZVI nanoparticles. The presence of
vermiculite led to a decrease in the aggregation of nZVI and Pd/Fe as observed by SEM and TEM. Batch
experiments were conducted to investigate the catalytic performance of nZVI, Pd/Fe, VMT, Fe-VMT and
Pd/Fe-VMT via the dechlorination reaction of 2,4-dichlorophenol (2,4-DCP). The dechlorination rates of
2
,4-DCP by Pd/Fe-VMT (by adding Pd) were greater than that achieved by Fe-VMT. Additionally, the
Received 25th March 2014
Accepted 22nd May 2014
dechlorination of 2,4-DCP by Pd/Fe-VMT would be influenced by temperature, initial pH values, Pd
loading, initial concentration of 2,4-DCP and the dosage of materials. It was confirmed that the ultimate
reduction product of 2,4-DCP was phenol. Overall, Pd/Fe-VMT is a promising material for the
dechlorination of 2,4-DCP.
DOI: 10.1039/c4ra02618g
www.rsc.org/advances
4
been used. Notably, abiotic reductive dehalogenation plays an
important role in the removal of 2,4-DCP from water bodies by
1
. Introduction
The wide use of chlorinated phenols in the production of decomposing or transforming this class of compounds into
5
pesticides, wood preservative, and bactericide, has resulted in non-toxic or low-toxic products.
chlorinated phenols being detected in all major environmental
Recently, a lot of attention have been paid to the application
1
media. The presence of chlorinated phenols in the environ- of nanoscale zero-valent iron nanoparticles (nZVI) on cleaning
ment has received much attention because most of them are up chlorinated phenols from contaminated water and soil
2
6,7
toxic, biorefractory, mutagenic, and probably carcinogenic.
owing to their magnetic, nanoscale and catalytic properties.
Moreover, this class of compounds is easily accumulated in However, there are still some challenging issues in the appli-
biological tissues, which poses a great threat to agriculture, cation of nZVI which need to be solved. For example, nZVI tends
3
food chain and human health. 2,4-Dichlorophenol (2,4-DCP) to aggregate, resulting in adverse effects on the removal effi-
has been identied as a primary pollutant by United States ciency. Therefore, some researchers have studied materials
8
9
Environmental Protection Agency (USEPA) and is considered a such as activated carbon and membranes as supporting
representative contaminant of chlorinated phenols. Therefore, materials to overcome the above limitations of nZVI and
it is essential that efficient methods are in place for 2,4-DCP enhance their reactivity and stability. A better alternative is
elimination from the environment. For now, many excellent using clay minerals which are abundant, environmental-
treatment technologies, including reverse osmosis, ion friendly and at much lower cost than activated carbon and
exchange, electro-dialysis, electrolysis and adsorption, have membranes as the supporting materials for iron nano-
10,11
particles.
Vermiculite (VMT) is a typical clay mineral with a
a
layered structure composed of two tetrahedral sheets bonded
College of Environment and Energy, South China University of Technology,
12,13
Guangzhou 510006, P. R. China. E-mail: pppxwu@scut.edu.cn
b
together by a central octahedral sheet.
For decades,
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, vermiculite has been widely applied on the treatment of heavy
Ministry of Education, Guangzhou 510006, P. R. China
14
15
16
metal ions, organics and ammonium because of its high
cation exchange capacities and excellent adsorption abilities.
Therefore, the properties and wide applications of vermiculite
make it an ideal supporting material to reduce the aggregation
of iron nanoparticles.
c
The Key Laboratory of Environmental Protection, Eco-Remediation of Guangdong
Regular Higher Education Institutions, P. R. China
†
Electronic supplementary information (ESI) available: The retention time of the
standard substance of 2,4-DCP, o-CP, p-CP and Phenol. See DOI:
0.1039/c4ra02618g
1
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Although vermiculite could reduce the aggregation of iron into 250 mL round bottom ask. 2.0 g of natural VMT was
ꢀ
1
nanoparticles and enhance their stability, its low reductive introduced by the dropwise addition of 1.0 mol L NaBH4
ability limits its application on the dechlorination of chlor- (75 mL) into the ask with magnetic stirring under an argon
ophenols. Recently, efforts were made to further enhance the atmosphere. Aer the adequate response, the nZVI were washed
performance of nZVI via the doping of Pd, Pt, or Ni since these with deoxygenated, deionized water three times. The sample
ꢁ
new bimetallic systems could bring about a rapid, efficient and was dried at 45 C for 20 h in a vacuum oven. Vermiculite-
complete reductive dechlorination process and non-chlorinated supported iron nanoparticles and Pd/Fe nanoparticles were
5,17–19
hydrocarbons end-products.
Among all the catalyst metals, denoted as Fe-VMT and Pd/Fe-VMT, respectively, while the
Pd exhibited an extremely high catalytic activity for dechlori- unsupported iron nanoparticles and Pd/Fe nanoparticles were
nation, which strongly improved iron corrosion and generated denoted as nZVI and Pd/Fe, respectively. The reactions are as
more activated hydrogen atom to accelerate the dechlorination follows:
1,5,20
process.
2
Fe + 2BH + 6H
+
4ꢀ
0
In our previous study, the physicochemical properties and
the reduction performance of organo-montmorillonite-sup-
2
O / Fe Y + 2B(OH)
3
+ 7H
2
[
(1)
(2)
2
6
ꢀ
0
2+
ꢀ
10,21
PdCl
+ 2Fe / 2Fe + Pd + 6Cl
ported nZVI
were investigated. However, further investiga-
tions need to be conducted to improve the chemical application
of nZVI. In this study, nZVI doped with precious metal (palla-
dium) was supported onto vermiculite for enhancing its
reductive property. The samples were characterized terms of
XRD for structural feature, BET for specic surface area, SEM
for morphology, TEM for size and XPS for chemical state. The
reduction experiments of 2,4-DCP by vermiculite-supported Pd/
Fe bimetal was conducted, and the effects of some important solution with a concentration of 20 mg L (pH ¼ 5.0) at 30 C
reaction parameters on dechlorination efficiencies, such as with magnetic stirring. Aer a certain time interval (3, 5, 10, 20,
temperature, initial pH value, Pd loading, concentration of 2,4- 30, 40, 60, 90, 120 min), 5 mL of the solution containing 2,4-DCP
2
.3. The experiments of 2,4-DCP dechlorination
The tests were conducted using 0.05 g of VMT, nZVI, Pd/Fe
nanoparticles and 0.100 g of Fe-VMT nanoparticles and Pd/Fe
nanoparticles in a comparative study. The experiments were
conducted in 250 mL asks containing 100 mL of 2,4-DCP
ꢀ
1
ꢁ
DCP and dosage, were investigated. The goal of this paper is to
determine the optimal reaction conditions on the dechlorina-
tion of 2,4-DCP and further evaluate the reducing property of
vermiculite-supported Pd/Fe nanoparticles.
were collected by glass syringes and ltered via a 0.45 mm poly-
tetrauoroethylene membrane. The ltered solutions were
analyzed for the concentration of 2,4-DCP by high performance
liquid chromatography (HPLC, Hitachi, L-2000).
In order to estimate the dechlorination efficiency of 2,4-DCP
by Pd/Fe-VMT nanoparticles, reduction experiments were
carried out under constant conditions with Pd loading (ranging
from 0.50%, 0.75%, 1.0%, 1.25% and 1.5%), the dosage of Pd/Fe
nanoparticles (0.025 g, 0.05 g, 0.10 g and 0.15 g), the initial
2
. Materials and methods
2.1. Materials
Chemicals including FeSO $7H O, NaOH, HCl, KCl, NH Cl and
absolute ethyl alcohol were of analytical reagent grades and
were purchased from the Guangzhou Chemical Reagent Factory
4
2
4
ꢀ1
ꢀ1
ꢀ1
concentration of 2,4-DCP (10 mg L , 20 mg L , 30 mg L and
ꢀ
1
4
0 mg L ), the solution pH value (3.0, 4.0, 5.0, 7.0, 9.0, 11.0)
ꢁ
ꢁ
ꢁ
ꢁ
and the reaction temperature (25 C, 30 C, 35 C and 45 C).
(
(
(
Guangzhou, P. R. China). Potassium hexachloropalladate
PdCl analytical reagent grade), sodium borohydride
NaBH
, analytical reagent grade), 2,4-dichlorophenol (2,4-DCP, 2.4. Methods of product test
98%), o-dichlorophenol (o-CP, $98%), p-dichlorophenol (p-
K
2
6
,
4
$
The chemical concentrations of 2,4-DCP and its dechlorination
products, o-chlorophenol (o-CP), p-chlorophenol (p-CP) and
phenol were determined quantitatively by a HPLC consisted of a
UV/vis detector (L-2420) and a reversed-phase column of Luna 5
CP, $98%) and phenol ($98%) were obtained from Aladdin
Reagent Database Inc. (Shanghai, P. R. China). Deionized water
was used throughout the experiment. Natural vermiculite was
collected from Feilaifeng Nonmetallic Mineral Materials
Company of Sihui city, Guangdong Province, China. It had a
cation exchange capacity (CEC) of 85.5 mmol/100 g according to
u C18. The relevant parameters were: 80 : 20 for mobile phase
ꢀ1
(
V) methanol : water, 1.0 mL min for the ow rate of mobile
ꢁ
phase, 20 mL for the injection volume, 36 C for column
temperature and the detection wavelengths for 2,4-DCP and its
degradation products o-chlorophenol (o-CP), p-chlorophenol (p-
CP) and phenol were 285 nm, 274 nm, 280 nm and 269 nm,
respectively.
21
our previous study.
2.2. Preparation of vermiculite-supported Pd/Fe
nanoparticles
The supported nZVI were formed by the reduction of ferrous salt
2.5. Characterization
with NaBH . Next, nZVI doped with palladium (Pd/Fe) was
4
prepared by dispersing the fresh nZVI in an aqueous solution of The samples of nZVI, Fe-VMT, Pd/Fe and Pd/Fe-VMT were
K PdCl . The synthesized procedure of palladium and iron measured by X-ray diffraction (XRD) to determine existing
2
6
nanoparticles is detailed as follows: 9.93 g of FeSO
4
$7H
2
O with forms of iron under the condition of Cu Ka radiation ¼ 0.15406
ꢁ
30 mL of ethanol solution (ethanol–water ¼ 2 : 1) were dissolved nm at 40 kV/40 mA, a step size of 0.02 , a rate of 17.7 s per step,
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ꢁ
3 4 2 3
and a 2q range of 10–60 . The samples were measured by testing the peaks of iron oxides were likely to be Fe O or Fe O , or their
23
the adsorption volume at various pressures and at 77 K in the mixture. In addition, it was found that the prepared Pd/Fe-
nitrogen medium aer degassing under vacuum at 473 K for 4 VMT burned easily when it was exposed to air, indicating that it
h. Then the BET equation was used to obtain the specic has a relatively high activity.
surface area of the tested samples using the nitrogen adsorption
method with a Micromeritics ASAP 2010 Chemisorption Surface to estimate the reactivity of nanoparticles, that is, the dechlo-
3.1.2. BET analysis. In general, specic surface area is used
24
Area Analyzer. The scanning electron microscopy (SEM) images rination reactivity is related to the specic surface area. The
of nZVI, Fe-VMT, Pd/Fe and Pd/Fe-VMT were collected with an BET surface area analysis of VMT, nZVI and the supported
accelerating voltage of 10 kV, a beam current of 80 mA and a samples are listed in Table 1. The specic surface area of nZVI
2
ꢀ1
magnication of 100 000 times. The samples were obtained by was very low (only 18.9 m g ), whereas the specic surface
2
using ultrasonic dispersion with the ethanol. And then the area of Fe-VMT and Pd/Fe-VMT increased to 39.5 and 59.1 m
samples were xed on the surface of a conductive adhesive,
ꢀ1
g , respectively. Fe supported by VMT resulted in the increase
blown using the pipette ller coated with gold. The samples in the specic surface area of Fe-VMT by 109.0% compared to
were observed under SEM aer all the above-mentioned steps nZVI, and a 76.4% increase for Pd/Fe-VMT compared to Pd/Fe.
above were conducted. In this study, the particle size and In line with this, both the specic surface area and external
2
ꢀ1
distribution of nZVI, Fe-VMT, Pd/Fe and Pd/Fe-VMT were surface area of Pd/Fe-VTM increased to 41.11 m g and 17.99
2
ꢀ1
measured by a Phillips CM300 transmission electron micro- m g , respectively. Therefore, the increasing surface area of
scope (TEM) and an ANELVA AES-430S X-ray photoelectron Pd/Fe-VMT was largely dependent on the increase of external
spectrometer (XPS).
surface area. This phenomenon can be reduced to the fact that
nZVI dispersed on the surface or the pores of VMT were of huge
21
surface area.
.1.3. SEM analysis. The particle sizes and morphological
changes of VMT, nZVI, Pd/Fe and Pd/Fe-VMT are shown in the
.1.1. XRD analysis. The XRD patterns of various nanoscale SEM images (Fig. 2). Obviously, in the absence of Pd/Fe, the
materials are illustrated in Fig. 1. The characteristic peak at 2q surface morphology of pristine VMT with a typical layered
3
. Results and discussion
3
3.1. Physicochemical properties of materials
3
ꢁ
0
¼
44.8 declared the existence of a-Fe and the crystallinity of structure was smooth (Fig. 2a). Due to the strong surface effects
iron particles in Fe-VMT and Pd/Fe-VMT (Fig. 1b and d). Obvi- of van der Waals forces and magnetic forces, nZVI tends to
ously, the characteristic peaks of ferric oxide were unobserv- agglomerate into a block (Fig. 2b). The doped Pd deposited on
able, as demonstrated by the non-oxidation of Fe . Since the the surface of nZVI formed small protrusions, which brought
0
ꢁ
ꢁ
ꢁ
amount of Pd (2q ¼ 39.7 /46.2 /67.7 ) in the samples of Pd/Fe, Pd/Fe nanoparticles above a relative rough surface (Fig. 2c).
Pd/Fe-VMT and Pd/Fe-VMT was too low to reach the detection Aer the Pd/Fe nanoparticles were supported by VMT, it can be
limit of XRD (5.0 wt%), no characteristic peaks of Pd appeared found that the aggregation of Pd/Fe nanoparticles was signi-
here. Although the compounds of Pd and Fe did not show any cantly reduced and remained well-dispersed, which resulted in
diffraction peaks in the XRD patterns, Pd particles doped on the Pd/Fe-VMT with a different surface appearance. As shown in
surface of nZVI ought to be in the form of nanoparticles. The Fig. 2d, Pd/Fe nanoparticles were randomly distributed onto the
XRD patterns (Fig. 1e) exhibited the signicant difference in Pd/ surface of VMT, which formed a columnar structure of dendrite-
Fe-VMT before and aer the reaction. The peak of Fe dis- like due to their geomagnetic forces and surface tension inter-
22
1
appeared aer the reaction, and turned to be the typical peak of actions between the nanoscale particles and small particles.
FeO
x
. The form of iron before and aer the reaction indicated
3.1.4. TEM analysis. Vermiculite is a 2 : 1 mica-type silicate
the existence of redox reactions between the nZVI and the VMT mineral with a layered structure. TEM image of VMT shows
in the experiment where the nZVI acted as a reductant. The the typical vermiculite particles with a layered structure
results were similar to the those reported by Zhou, who thought (Fig. 3a). Unsupported iron particles showed a structure of
chain-like aggregates leading to the difficulty of combining with
25
20
the target pollutants (Fig. 3b). This result was consistent with
26
the study by Wang et al. The nZVI are generally spherical in
shape with the majority in the size ranging from 20 nm to
Table 1 Specific surface area of samples
2
ꢀ1
2
ꢀ1
2
ꢀ1
3
ꢀ1
Samples
S
BET (m g
)
S
ext (m g
)
S
micro (m g
t
) V (cm g )
VMT
nZIV
Fe-VMT
Pd/Fe
67.09
18.90
39.50
33.50
46.73
12.80
25.90
23.24
41.11
20.36
6.10
13.60
10.16
17.99
0.0074
0.04
0.012
0.011
0.05
Fig. 1 The XRD patterns of various nanoscale materials: (a) nZVI; (b) Pd/Fe-VMT 59.1
Fe-VMT; (c) Pd/Fe; (d) Pd/Fe-VMT; (e) Pd/Fe-VMT after reaction.
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3.1.5. XPS analysis. XPS spectra of Pd/Fe-VMT nano-
particles are presented in the Fig. 4a. Pd/Fe-VMT nanoparticles
surface mainly consisted of iron, oxygen and carbon. The exis-
tence of oxygen indicated that nZVI might have oxidised, and
the carbon element on the surface of Pd/Fe-VMT nanoparticles
probably came from carbon dioxide in air and on the glassware
during sample preparation. Silicon was from the sample itself,
and the detected Pd showed a successfully supported Pd on the
surface of VMT. Both silicon and Pd were in trace amounts.
Simultaneously, the high resolution XPS lines of Fe 2p3/2 and
2
p
1/2 for Pd/Fe-VMT nanoparticles are shown in Fig. 4b, which
were separated into three binding energy peaks recorded at
24.0 eV, 710.9 eV, and 706.7 eV, respectively. The peak at
binding energy at 724.0 eV obtained here was from Fe 2p1/2, and
7
Fig. 2 SEM patterns of the samples: (a) VMT; (b) nZVI; (c) Pd/Fe; (d) Pd/ the peak at 710.7 eV was well corresponded to the binding
Fe-VMT.
energies of 2p3/2 of ferric oxide. In addition, a smaller energy
peak at 706.7 eV proved the existence of zero-valent iron. As a
result, the coexistence of iron oxides and ZVI proves that nZVI
consists of a core–shell structure with a core of iron and the
shell of iron (hydro)oxide because iron particles react with water
and oxygen to form iron oxyhydroxides in aqueous solution:
2
8–30
0
Fe(s) + 2H O (aq) ¼ FeOOH + 1.5H (g).
Besides, Fe was fully
2
2
oxidized during the dechlorination reduction which indicated
an oxidation–reduction system during the dechlorination
31
process.
3.2. The removal of 2,4-DCP by different iron nanoparticle
systems
Fig. 5 shows the comparative study on different systems for
2,4-DCP removal. It can be concluded that Pd/Fe-VMT was the
optimal system for the removal of 2,4-DCP and was superior to
Pd/Fe. Compared with the use of single nZVI (the removal rate:
4.0%) and Fe-VMT (the removal rate: 20.5%), the removal effi-
ciencies of 2,4-DCP achieved up to nearly 99.3% and 94.8% for
Pd/Fe-VMT and Pd/Fe in 120 min, respectively. Generally, there
are two important steps for the 2,4-DCP hydrodehalogenation
including hydrogenolysis and dehalogenation (Fig. 6). First,
2
iron (as an electron donor) and H O (as a proton donor) produce
the active hydrogen in the hydrogenolysis reaction; second, the
active hydrogen replaces the chlorine atom of 2,4-DCP in the
32–34
reductive dehalogenation.
Compared with the use of a
single nZVI, Pd obviously acted as a catalyst that effectively
promote the activities of hydrogen and the abilities of electron
35
transformation in the hydrodehalogenation reactions.
Fig. 3 TEM patterns of the samples: (a) VMT; (b) nZVI; (c) Pd/Fe-VMT.
100 nm which could connect in chains. In contrast, iron
nanoparticles were dispersed on the surface of VMT with less
aggregation, which suggested that Pd/Fe-VMT could combine
with the contaminants effectively (Fig. 3c). Some of the spher-
ical Pd/Fe nanoparticles supported by VMT (Fig. 1b) were
arranged in chains or attached by the chains, and some others
embedded in the VMT which revealed that Pd/Fe nanoparticles
Fig. 4 XPS spectra of the samples: (a) Pd/Fe-VMT; (b) Fe 2p3/2 and Fe
1/2 core levels of iron nanoparticles.
27
could be xed on VMT.
2p
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Fig. 7 Effect of reaction temperature on the dechlorination of 2,4-
DCP by Pd/Fe-VMT (inset): (*) 120 min reaction (experimental
Fig. 5 Removal of 2,4-DCP by different iron nanoparticles (experi-
ꢁ
ꢀ1
mental conditions: T ¼ 30 C, pH ¼ 5.0, C2,4-DCP ¼ 20 mg L
,
ꢀ1
ꢀ1
conditions: pH in ¼ 5.0, C2,4-DCP ¼ 20 mg L , CPd/Fe-VMT ¼ 1.0 g L , Pd
ꢀ1
ꢀ1
C
VMT,Fe and Pd/Fe ¼ 0.5 g L , CFe-VMT and Pd/Fe-VMT ¼ 1.0 g L ).
loading ¼ 1.0 wt%).
temperature could accelerate the movement of molecules,
which improves the 2,4-DCP dechlorination possibly in two
ways: speed up 2,4-DCP movement from aqueous phase to Pd/
Fe-VMT or lessen the activation energy (E ) of 2,4-DCP dechlo-
a
5,37
rination. Hence, the temperature should be considered in the
dechlorination of 2,4-DCP by Pd/Fe-VMT. According to Fig. 7b
(inset), the order of dechlorination efficiency of Pd/Fe-VMT was
as follows: Pd/Fe-VMT40 z Pd/Fe-VMT35 z Pd/Fe-VMT30 $
Pd/Fe-VMT20. Therefore, the reaction temperature was prefer-
ꢁ
ably 30 C by considering the practical application.
3.3.2. Effects of Pd loading on the dechlorination of 2,4-
DCP. Some studies showed that Pd/Fe could greatly enhance the
31,38
effect on organic chlorides removal.
However, for economy
and efficiency, there should be an appropriate ratio between
nZVI and Pd. In order to investigate the effect of Pd loading on
the degradation efficiency of 2,4-DCP, the Pd loading rate of 0.5
wt% to 1.5 wt% (Pd : Fe, mass fractions) was investigated and
Fig. 6 Schematic diagram for 2,4-DCP dechlorination by Pd/Fe-VMT.
Noticeably, Pd in Pd/Fe-VMT enhanced the dechlorination the results are shown in Fig. 8. The dechlorination rates could
performance in three ways. First, the doped Pd always has a benet from the increased dosage of Pd loading at the range of
lower hydrogen over-potential which accelerates the dechlori- 0.5 wt% to 1.0 wt%. The dechlorination percentages of 2,4-DCP
nation process as a catalyst. Second, the reductive surface sites were slowly increased in Pd loading between 0.5 wt% and
of bimetallic iron are usually more than that of a single iron. 0.75 wt% with low-speed. When increasing Pd loading from
Third, the loaded precious metals act as an electron transfer 0.75 wt% to 1.0 wt%, the dechlorination rates increased rapidly.
medium which facilitates the hydrodehalogenation reac- It was found that the maximum dechlorination rate was
32,36
tions.
Therefore, Pd doping played an important role in the
enhancement of the reactivity of vermiculite supported iron
nanoparticles.
3.3. The dechlorination of 2,4-DCP by Pd/Fe-VMT
3.3.1. Effects of reaction temperature on the dechlorina-
tion of 2,4-DCP. The results of the dechlorination of 2,4-DCP at
ꢁ
the different temperature (25–45 C) are shown in Fig. 7. At the
ꢁ
low temperature (25 C), the dechlorination efficiency of 89.5%
aer a 120 min reaction was unsatisfactory. The dechlorination
efficiency was observed to increase with increasing temperature
which demonstrated that higher temperature is benecial for
ꢁ
2
,4-DCP dechlorination. At the temperature of 30 C, the
Fig. 8 Effect of Pd loading (Pd : Fe mass ratio) on the dechlorination
ꢁ
dechlorination rate of 2,4-DCP was up to 99.9%, 99.7% for 35 C
and 99.5% for 40 C aer 120 min. As we all know, high 5.0, C2,4-DCP ¼ 20 mg L , CPd/Fe-VMT ¼ 1.0 g L ).
ꢁ
of 2,4-DCP by Pd/Fe-VMT (experimental conditions: T ¼ 30 C, pH ¼
ꢁ
ꢀ1
ꢀ1
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obtained for the Pd : Fe mass ratio of 1.0 wt%. The dechlori-
nation efficiencies were observed to decline with the ratios of
1
.25 wt% and 1.5 wt%, but it was higher than 0.5 wt% and 0.75
wt%. This can partly be attributed to the accumulation of
excessive H , which hinders the contact areas between 2,4-DCP
and metal particles. The trend of dechlorination performance
2
39
9,31
was consistent with previous reports. As a result, Pd loading
percentage over Fe in Pd/Fe nanoparticles of 1.0 wt% was
0
chosen as the optimal Pd doping content.
3.3.3. Effects of initial pH on the dechlorination of 2,4-
DCP. The effects of initial pH values on the reductive dechlo-
rination of 2,4-DCP by Pd/Fe-VMT are shown in Fig. 9. All
dechlorination rates were up to 99.6% at pH 3, 5 and 7 aer a
Fig. 10 Effect of Pd/Fe-VMT dosage on the dechlorination of 2,4-DCP
ꢁ
ꢀ1
,
(experimental conditions: T ¼ 30 C, pH ¼ 5.0, C2,4-DCP ¼ 20 mg L
120 min reaction time. It was favourable to 2,4-DCP dechlori- Pd loading ¼ 1.0 wt%).
nation when initial pH was kept in acidic condition. It can be
showed that the optimal dechlorination efficiency for 2,4-DCP
was achieved at initial pH 3. However, the dechlorination effi-
ciencies declined from nearly 98.8% to 50.4% in 120 min with
the increasing initial pH from 9 to 11. The pH change in reac-
tant solution from acidic to alkaline condition could be one of
the most important factors on the dechlorination efficiency. Pd/
Fe-VMT could effectively dechlorinate 2,4-DCP in acidic and
weak alkaline solutions, which could be interpreted in three
ways: (1) iron corrodes at the surface of catalytic Pd where
protons in solution were reduced to atomic and molecular
hydrogen; (2) it could provide more atomic hydrogen on the Pd
surface at lower pH, which could attack chlorophenol and take
the place of the chlorine forming phenol and chlorine ion
effectively; (3) the surface of Pd/Fe could adsorb a portion of
chlorophenol where the hydrogen replaces the chlorine atom
dechlorination could be observed, and the dechlorination
percentage of 2,4-DCP increased to 96.4% and 99.5%, respec-
tively. It was quick in reaching the equilibrium in the two
dosages situation of 0.05 g and 0.10 g aer about a 30 min
reaction. However, the increase in the dosage to 0.15 g only
caused a slight improvement for 2,4-DCP dechlorination. The
most likely explanation for this phenomenon is that the further
increase in the dosages of Pd/Fe-VMT facilitated more active
sites of H evolution, where excessive H evolution impeded the
2
2
mass transfer, which eventually restricted 2,4-DCP dechlorina-
40
tion. The dechlorination percentage of 2,4-DCP is similar for
the dosage of Pd/Fe-VMT of 0.10 g and 0.15 g in a 60 min
reaction. Accordingly, the optimum dosage of Pd/Fe-VMT was
ꢀ1
0
.10 g (equivalent to 1.0 g L ).
6
because of broken C–Cl bond. Besides, it could reach equilib-
3.3.5. Effects of initial concentration of 2,4-DCP on the
rium faster in the both acidic and weak alkaline solutions.
dechlorination of 2,4-DCP. It is widely accepted that the quan-
tity of contaminant is one of the most signicant experimental
parameters. Fig. 11 illustrates the dechlorination of 2,4-DCP by
Pd/Fe-VMT at various concentrations of 2,4-DCP. The 2,4-DCP
3.3.4. Effects of Pd/Fe-VMT dosages on the dechlorination
of 2,4-DCP. Increasing the dosage of Pd/Fe-VMT will accelerate
the initial reactivity and result in more active sites of Pd/Fe
39
ꢀ
1
ꢀ1
ꢀ1
nanoparticles for 2,4-DCP dechlorination. Fig. 10 shows the
dechlorination efficiency of 2,4-DCP was the lowest (only 60.3%)
at the minimum amount of 0.025 g, and it did not reach the
equilibrium in a 120 min reaction. As the dosages of Pd/Fe-VMT
increased from 0.05 g to 0.1 g, obvious enhancement of
concentration was 10 mg L , 20 mg L , 40 mg L , 100 mg
ꢀ
1
L
, which brought about a decrease in 2,4-DCP dechlorination
percentages from almost 100%, 99.5%, 92.0% to 58.0% in
0 min, and the trend of the initial 2,4-DCP concentration and
4
dechlorination were of a negative correlation. The dechlorina-
tion efficiency of 2,4-DCP was 99.5% at the initial concentration
Fig. 9 Effects of initial pH on the dechlorination of 2,4-DCP by Pd/Fe- Fig. 11 Effects of initial concentration on the dechlorination of 2,4-
ꢁ
ꢀ1
ꢁ
VMT (experimental conditions: T ¼ 30 C, C2,4-DCP ¼ 20 mg L
,
DCP by Pd/Fe-VMT (experimental conditions: T ¼ 30 C, CPd/Fe-VMT
¼
ꢀ1
ꢀ1
C
Pd/Fe-VMT ¼ 1.0 g L , Pd loading ¼ 1.0 wt%).
1.0 g L , Pd loading ¼ 1.0 wt%).
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ꢀ
1
ꢀ1
of 20 mg L and 92.0% for 40 mg L , which were very close.
Therefore, taking into account the aspects of removal efficiency
and material dosage, the concentration of 2,4-DCP here was
ꢀ
1
20 mg L
.
3.4. The analysis of reduction products from 2,4-DCP
dechlorination by Pd/Fe-VMT
3.4.1. Reduction products analysis. Total ion chromato-
gram of the reaction products of 2,4-DCP was carried out by
HPLC, and the results are shown in Fig. 12. In the rst 5 min,
2
,4-DCP adsorbed onto Pd/Fe-VMT nanoparticles, then gradu-
Fig. 13 Percentage of dechlorination products of 2,4-DCP after
ally reduced to o-CP, p-CP and phenol. Subsequently, 2,4-DCP
was decomposed into o-CP, p-CP and phenol at 10 and 30 min,
meanwhile some of o-CP and p-CP was transferred to phenol. In
the whole reaction, no other intermediate products were
detected except for the two chlorinated substances (o-CP and p-
CP). The results demonstrated that the generation of o-CP was
always more than that of p-CP, which proved the selective
ꢁ
treatment by Pd/Fe-VMT (experimental conditions: T ¼ 30 C, pHin
¼
ꢀ1
5
.0, CPd/Fe-VMT ¼ 1.0 g L , Pd loading ¼ 1.0 wt%).
unstable C–Cl at the ortho-position. Apparently, this conclusion
is corresponding with the above results. Otherwise, it was not
surprising to nd the total amount (o-CP, p-CP and phenol) was
not 100% (Table S1†), since the decrease of 2,4-DCP did not only
involve dechlorination, but also a little adsorption.
cleavage of the C–Cl bond at the ortho-position in the process of
2
2,4-DCP dechlorination. Therefore, the concentration of o-CP is
much higher than that of p-CP during 2,4-DCP dechlorination.
Finally, 2,4-DCP, o-CP and p-CP were completely reduced to
phenol in 60 min, and phenol was the sole nal organic
4. Conclusions
23
product, which were consistent with the study by Zhou et al.
In this study, Pd/Fe-VMT has been demonstrated to have
excellent reductive reactivity in the dechlorination of 2,4-DCP in
aqueous solution. Supported Pd/Fe on the surface of vermicu-
lite signicantly increased the specic surface areas of the Pd/Fe
nanoparticles. The characterization showed that vermiculite
was an effective dispersant and stabilizer in decreasing aggre-
gation of iron nanoparticles in the supported nZVI and Pd/Fe
systems. The dechlorination experiments suggested that the
doped Pd could be an efficient catalyst to be used as dechlori-
nation catalysts in promoting the corrosion of iron nano-
particles and the formation of [H]. Consequently, it could
largely enhance the dechlorination efficiency of 2,4-DCP. In
addition, reaction factors such as temperature, initial pH, Pd
loading, dosage of materials and initial concentration was
investigated to optimize the operation parameters to maximize
the 2,4-DCP reduction. Reduction products study showed that
The percentage of dechlorination products of 2,4-DCP was
obtained by HPLC, as shown in Fig. 13. The content of 2,4-DCP
was gradually decreased with the reaction which was constant
until 60 min (Fig. 13). On the contrary, the content of phenol
was increased up to a maximum of 95.89% aer 60 min. The
two intermediates (o-CP and p-PC) had a common trend: the
amount was increased in a short period of time, then declined
until an equilibrium was reached. The results demonstrated
that 2,4-DCP was more prone to transforming into o-CP (8.35%
at 3 min) than p-CP (6.20% at 3 min) which was attributed to the
2,4-DCP can be catalytically dechlorinated to reductive inter-
mediates e.g. o-CP, p-CP and the nal product phenol. This
study supports that Pd/Fe-VMT with dechlorination function is
a potential material for treating water or soil contaminated by
halogenated organic compounds.
Acknowledgements
The authors are grateful for nancial support from the National
Science Foundation of China (Grant no. 40973075, 41073058,
41273122), Research Fund for the Doctoral Program of Higher
Education of China (no. 20100172110028).
Fig. 12 Total ion chromatogram of the reactant and products of 2,4-
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ꢁ
1
C
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ꢀ1
ꢀ1
1
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25586 | RSC Adv., 2014, 4, 25580–25587
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