Nanoscale Fe0particles for pentachlorophenol removal from aqueous solution: Temperature effect and particles transformation

Article abstract of DOI:10.1166/jnn.2014.8936

Pentachlorophenol (PCP), as an important contaminant which was toxic and intractable, has received extensive attention. In this paper, the temperature effect during the transformation of PCP using nanoscale Fe⁰particles was studied, and the transformation processes of PCP and iron particles was explained. The results revealed that the removal processes of PCP followed pseudo first-order kinetics. The scale of dechlorination to the transformation of PCP increased with the increase of temperature, though the transformation rate decreased after reacting for 2 h under the experimental condition. However, the initial apparent transformation rate constants were calculated to be 0.312-0.536 h^-1at the temperature of 20-50°C, which showed an increase of transformation rate along with the increase of temperature. And the surface-area-normalized rate constants were calculated to be 9.50 × 10^-3- 1.63 × 10^-2L·h^-1· m^-2. The experimental activation energy was calculated to be 15.0 kJ · mol^-1from these rate constants using Arrhenius equation. A phenomenon observed at 50°C indicated that more than one chlorine atom was removed from PCP and suggested β-elimination might be the major pathway for transformation. Sorption experiments showed that the sorption process on the surface of particles could be ignored in the kinetics and thermodynamics models. The changes of morphologies of nanoparticles before and after reaction indicated the transformation process of iron particles, and could be used to explain the changes of activity of nanoparticles. Magnetite (Fe₃O₄) and/or maghemite (Fe₂O₃) and lepidocrocite (γ-FeOOH) were corrosion products of iron. And along with the increase of temperature, the increased intensity of XRD peaks revealed the related a better crystallizing. Copyright

Full text of DOI:10.1166/jnn.2014.8936

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Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 6941–6949, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
Nanoscale Fe0 Particles for Pentachlorophenol
Removal from Aqueous Solution: Temperature
Effect and Particles Transformation
Rong Cheng1, Xiang Zheng1, Peng Liu1, and Jian-Long Wang2ꢀ∗
1School of Environment and Natural Resources, Renmin University of China, Beijing 100872, P. R. China
2Laboratory of Environmental Technology, INET, Tsinghua University, Beijing 100084, China
Pentachlorophenol (PCP), as an important contaminant which was toxic and intractable, has
received extensive attention. In this paper, the temperature effect during the transformation of PCP
using nanoscale Fe0 particles was studied, and the transformation processes of PCP and iron par-
ticles was explained. The results revealed that the removal processes of PCP followed pseudo
first-order kinetics. The scale of dechlorination to the transformation of PCP increased with the
increase of temperature, though the transformation rate decreased after reacting for 2 h under the
experimental condition. However, the initial apparent transformation rate constants were calculated
to be 0.312–0.536 h−1 at the temperature of 20–50 ꢀC, which showed an increase of transformation
rate along with the increase of temperature. And the surface-area-normalized rate constants were
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calculated to be 9.50×10−3–1.63×10−2 L·h−1 ·m−2. The experimental activation energy was cal-
IP: 5.101.221.73 On: Fri, 10 Jun 2016 07:58:14
culated to be 15.0 kJ · mol−1 from these rate constants using Arrhenius equation. A phenomenon
Copyright: American Scientific Publishers
ꢀ
observed at 50 C indicated that more than one chlorine atom was removed from PCP and sug-
gested ꢀ-elimination might be the major pathway for transformation. Sorption experiments showed
that the sorption process on the surface of particles could be ignored in the kinetics and thermody-
namics models. The changes of morphologies of nanoparticles before and after reaction indicated
the transformation process of iron particles, and could be used to explain the changes of activity
of nanoparticles. Magnetite (Fe3O4ꢁ and/or maghemite (Fe2O3ꢁ and lepidocrocite (ꢂ-FeOOH) were
corrosion products of iron. And along with the increase of temperature, the increased intensity of
XRD peaks revealed the related a better crystallizing.
Keywords: Temperature, Kinetics, Thermodynamics, Corrosion Products, Nanoparticles.
1. INTRODUCTION
Especially in the 1990s, iron has been used as a reagent
to dehalogenate halogenated hydrocarbons containing one
to two carbon atoms broadly.6–8 Then, Fe0 permeable reac-
tive barrier technology attracted broad attention. Prelimi-
nary data suggest that such barriers may function for many
years before replacement is required.9 However, there is
still much uncertainty in predicting long-term effectiveness
of the reactive Fe0. The formation of precipitates is a con-
cern in this field.10 Precipitates in Fe0 reactive barriers may
decrease surface reactivity, thus influencing the effective-
ness and lifespan of the barrier.11ꢀ12 Recently, nanoscale
Fe0 particles have been synthesized to accelerate reac-
tion rate.13–16 Compared to their micro-scale counterparts,
nanoscale particles offer higher reactivity because of their
high specific surface area and high surface reactivity.17ꢀ18
Permeable reactive barrier (PRB) technology, which was an
effective technology for the remediation of soil and ground-
water contaminated by chlorinated organic compounds and
toxic metals, received much interest because that it repre-
sents an economical alternative to traditional remediation
strategies such as “pump-and-treat.” 1ꢀ2 A PRB consisted
of a zone of reactive material such as reduced metal or
electron donor-releasing compounds, and installed in the
path of a plume of contaminated groundwater.3 Among the
various reactive materials, zero-valent iron (Fe0) and iron-
containing minerals attracted a great deal of attention since
1980s for they were easy to get and of high-activity.4ꢀ5
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9
1533-4880/2014/14/6941/009
doi:10.1166/jnn.2014.8936
6941

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Nanoscale Fe0 Particles for PCP Removal from Aqueous Solution: Temperature Effect and Particles Transformation
Cheng et al.
Chlorophenols, which are listed as priority pollutants
by the US EPA, have been used extensively throughout
the world as rice herbicides, wood preservatives and gen-
eral biocides.19 It is toxic, carcinogenic, and intractable.20
Moreover, it could persist in the environment for a long
time, and do harm to human health by bioaccumulation.21
In an EPA study, pentachlorophenol (PCP) was detected
in 80% of human urine specimens.22 Chlorophenols are
readily reduced rather than oxidated since they are elec-
tronegative. And along with the increase of the chlorine
substituents on the aromatic ring, the chlorophenols are
more prone to be reduced.23
FeSO4 · 7H2O aqueous solution at ambient temperature.
The process was performed in Ar atmosphere. Ferric iron
was reduced by borohydrate according to the following
reaction:23
FeꢂH2Oꢁ26+ +2BH4− → Fe0 ↓ +2BꢂOHꢁ3 +7H2 ↑ (1)
Synthesized Fe0 particles were washed twice with
deionized water and then vacuum dried.
The morphology of synthesized Fe0 particles were
viewed with a scanning electron microscope (SEM, JSM-
6301F, JEOL) equipped with energy dispersive spec-
troscopy (EDS). It was observed from the SEM images
that diameters of the nanoscale particles were lower than
100 nm. The EDS results illustrated that the nanoscale
particles were Fe. The crystal structure of the as-prepared
iron sample was characterized by X-ray powder diffraction
(XRD) using a Rigaku D/max-RB X-ray diffractometer
with Cu Kꢃ radiation (ꢄ = 0ꢅ1542 nm). Peaks identified
on the XRD patterns of the synthesized iron particles
were zero-valent iron. The peaks of oxide phases were
not detected in the XRD pattern, indicating insignificant
oxidation of the synthesized iron particles. BET analysis
gave specific surface area of 32.84 m2 · g−1 (Nova3200e
nitrogen adsorption surface area analyzer, Quantachrome,
America).
To date, the published literature on application of
nanoscale Fe0 in pollution abatement is focused on
remediation of chlorinated organic compounds,24ꢀ25 heavy
metals26ꢀ27 and nitrate.28ꢀ29 Among the chlorinated organic
compounds, chlorinated aliphatic hydrocarbons especially
those containing one or two carbon atoms have received
extensive attention. The research on degradation of chlo-
rinated aromatic compounds such as PCP using nanoscale
Fe0 is still limited. Moreover, the ability to predict perfor-
mance of nanoscale Fe0 over a range of hydrogeochemical
conditions remains weak. Environmental factors such as
temperature and common ions present in groundwater may
affect the reaction rates and pathways, leading to incom-
plete treatment of contaminants.30 On the other hand,
characterization of nanoscale Fe0 before use has been
achieved using varies analysis tools such as X-ray diffrac-
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2.3. Experimental Procedure
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Batch experiments for PCP transformation were conducted
tion (XRD), transmission electron microscopy (TEM)
Copyright: American Scientific Publishers
in 18-ml bottles. Fe0 nanoscale particles (15 mg) were
and scanning electron microscopy (SEM).17ꢀ29 While the
description of the particles after reaction remains rather
limited.
loaded into bottles containing 15 ml of an aqueous solution
of PCP. The initial concentration of PCP was 50 mg·L−1
.
The bottles were sealed with rubber plugs and placed on
a rotary shaker (TZ-2EH, Beijing Wode Company) during
the entire experiment period. The rotate speed of rotary
shaker was set to be 150 rpm. Parallel control experiments
were performed without the nanoscale particles. All sam-
ples were filtrated with 0.45-ꢆm filter film to be analyzed.
To investigate the adsorption of PCP on the surface of
nanoscale particles, 5 ml deionized water was used to wash
the residual iron particles after solutions were extracted.
Then the lotions were extracted and filtrated to be ana-
lyzed. The particles were vacuum-dried and investigated
with an SEM.
In this study, PCP was chosen as the target compound.
Nano-scale Fe0 particles were synthesized with kalium
borohydride (KBH4ꢁ reduction method.31 Their reactivity
in degrading PCP at various surrounding temperatures was
contrasted. Specially, the particle morphology was inves-
tigated during reaction, which indicated transformation of
iron particles.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Pentachlorophenol (PCP, ≥ 95%, reagent) was purchased
from Changping Shiying Chemical Plant, Beijing. Kalium
borohydride (KBH4, 96%) was supplied by Nankai Fine
Chemical Factory. Ferrous sulfate (FeSO4 ·7H2O, 99.0%–
101.0%) was supplied by Shenyang Reagent Factory. The
chemicals used were as received without further purifi-
cation. Argon gas (Ar, 99.99%) was supplied by Beijing
Aolin Gas Company.
2.4. Analysis
PCP samples were quantified with an Agilent 1200 HPLC
(Agilent Lt.D) equipped with an XDB-C18 Analytical
HPLC column 4ꢅ6 ∗ 150 and a 1200 Diode-array detec-
tor. The mobile phase for PCP consisted of 85% methanol
and 15% water distilled for three times. The flow rate
was 1 ml/min. The detector wavelength was 320 nm for
PCP. Chlorine ion was quantified with a DX-100 ion chro-
matogram (IC, DIONEX Company, Germany). The opera-
tional conditions were: eluent at 3.5 mM Na2CO3/1.0 mM
NaHCO3; eluent flow at 1.2 ml/min, sample loop volume
2.2. Preparation of Nanoparticles
Synthesis of nanoscale iron particles was achieved by
adding 100-ml 0.2 M KBH4 aqueous solution drop-
wise to a four-necked flask containing 100-ml 0.04 M
6942
J. Nanosci. Nanotechnol. 14, 6941–6949, 2014

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Cheng et al.
Nanoscale Fe0 Particles for PCP Removal from Aqueous Solution: Temperature Effect and Particles Transformation
0.8
0.6
0.4
0.2
0
at 250 ꢆl, and run time at 5 min. Deionized water was used
as blank. The intermediates and products of PCP were ana-
lyzed with a mass spectrum (MS, P. E API3000) and the
HPLC referred above.
3. RESULTS AND DISCUSSION
3.1. PCP Transformation and Cl− Release
ꢀ
The temperature of rotary shaker was seꢀt to be 20 C,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
25 C, 30 C, 35 C, 40 C, 45 C and 50 C respectively.
The concentration of PCP was 50 mg · L−1. As shown in
Figure 1, the removal efficiency of PCP increased with
the increase of surrounding temperature in the first 1 h.
After reacting for 2 h, the removal efficiency went an
opposite trend, that is, the removal efficiency decreased
with the increase of temperature. Meanwhile, the rela-
tive concentrations of Cl− release during the process were
obtained through paralleling with the initial concentration
of PCP. Different from the removal efficiency of PCP, rel-
ative concentrations of Cl− increased obviously with the
increase of temperature. Figure 2 illustrated the removal
efficiencies of PCP and relative concentrations of Cl− at
various temperatures after reacting for 8 h. It observed
that the concentration of Cl− increased evidently with the
increase of temperatꢀure especially when the temperature
was higher than 40 C, though the removal efficiency of
PCP decreased gradually. It indicated that the scale of
20 ºC 25 ºC 30 ºC 35 ºC 40 ºC 45 ºC 50 ºC
Removal efficiency of PCP Cl-release
Figure 2. Removal of PCP and Cl− release at various temperatures after
reacting for 8 h. Where C0 is the initial concentration of PCP, and C is
concentration of PCP/Cl− in solution.
Considering the reductive activity of Fe2+ and H2, which
was produced from the above reactions, the following reac-
tions may also take place theoretically:
2Fe2+ +RCl+H2O → 2Fe3+ +RH+Cl− +OH− (5)
H2 +RCl → RH+H+ +Cl−
(6)
In neutral or alkaline solutions,
Fe2+ +2OH− → FeꢂOHꢁ2
(7)
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dechlorination to the transformation of PCP increased with
Unstable Fe(OH) could be transformed to Fe(OH)3
rapidly. So the reaction (5) would hardly occur.32 How-
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the increase of temperature.
Copyright: American Scientific Publishers
As for this system, there are three main species (Fe0,
PCP, H2O) and a small quantity of O2 in the initial solu-
tions. And the following reactions could take place accord-
ing to electrochemistry theories:
ever, it’s not all the PCP that would be dechlorinated.
And some PCP would transform to oxidation compounds,
which could be implied from Figure 2. In our experiment,
it was obsꢀerved that the plugꢀs of bottles at higher temper-
ꢀ
ature (40 C, 45 C and 50 C) dashed out after reacting
Fe0 +RCl+H2O → Fe2+ +RH+Cl− +OH− (2)
over 16 h. It indicated that there was some gas accumu-
lated in this system. Considering the reactions mentioned
above, the gas produced was H2. The phenomena sug-
gested that there was more H2 produced at higher temper-
ature, which meant that the reaction (4) was more prone to
occur at higher temperature. As a result, the reaction (6)
would be more capable of going. Moreover, the presence
of H2 would block the oxidation of PCP. Upon that, the
scale of dechlorination increased while that of oxidation
decreased.
There was another phenomenon that should be noticꢀed.
That is the removal of PCP and Cl− release at 50 C.
As shown in Figure 3, the concentration of Cl− release
after reacting for 12 h was apparently higher than that
of PCP removed. It indicated that more than one chlo-
rine atom was removed from PCP. Arnold and Roberts33
studied pathways and kinetics though which chlori-
nated ethylenes and their daughter products react with
Fe0 particles, and indicated that reductive ꢇ-elimination
accounted for a major percent of dechlorination pro-
cess for tetrachloroethylene (PCE), trichloethylene (TCE),
cis-dichloroethylene (cis-DCE) and trans-dichloroethylene
2Fe0 +O2 +2H2O → 4OH− +2Fe2+
Fe0 +2H2O → H2 +2OH− +Fe2+
(3)
(4)
1.0
0.8
0.6
0.4
0.2
0
2
4
6
8
10
12
Reaction time (h)
20 ºC
25 ºC
30 ºC
35 ºC
40 ºC
45 ºC
50 ºC
Figure 1. Transformation of PCP by nanoscale Fe0 at different temper-
atures. Where C0 is the initial concentration of PCP, and C is concentra-
tion of PCP in solution.
J. Nanosci. Nanotechnol. 14, 6941–6949, 2014
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Nanoscale Fe0 Particles for PCP Removal from Aqueous Solution: Temperature Effect and Particles Transformation
Cheng et al.
the effect of oxidation of Fe0. Actually, the process after
2 h had deviated from the pseudo-first-order kinetic model.
To better compare the reaction rates observed with differ-
ent particle size, it is useful to normalize the reactivity per
unit metal surface area. Johnson et al.34 studied degrada-
tion of halogenated organic compounds in a batch system,
and summed up the following equation to describe the rate
of dechlorination:
2.5
PCP
Cl–
2.0
1.5
1.0
0.5
0
dC
= −kSAasꢈmC
(9)
dt
Where kSA is surface-area-normalized rate coefficient
(L·h−1 ·m−2ꢁ; as is specific surface area of metal (m2 ·g−1ꢁ;
ꢈm is mass concentration of metal (g · L−1ꢁ. Considering
Eq. (8), the following expressions can be got:
0
2
4
6
8
10
12
Reaction time (h)
Figure 3. Removal of PCP and Cl− release at 50 ꢀC Where C0 is
the initial concentration of PCP, and C is concentration of PCP/Cl− in
solution.
kobs = kSA ·as ·ꢈm
(10)
In this system, as=32.84 m2 · g−1; ꢈm = 1 g · L−1. The
value of kSA was got as shown in Table I. We can see
that the surface-area-normalized rate constant is almost
two orders of magnitude higher than that of common Fe0
(3.2( 0.6)×10−4 L·h−1 ·m−2, obtained from Ref. [35]).
It could be inferred that the difference between nanoscale
Fe0 and common Fe0 was mainly due to the higher density
of reactive sites provided by the smaller particle size as
well as the higher surface area.
(trans-DCE). While 1,1-DCE was consistent with a reduc-
tive ꢃ-emilination pathway. Consider the structure of PCP,
ꢇ-elimination might be prone to occur according to the
results mentioned above. However, further researches on
the pathways for transformation of PCP by nanoscale Fe0
are needed.
3.2. Intermediates and Products of PCP
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The experimental activation energy was calculated from
From the MS results of PCP solutions after reacting with
nZVI, we can see that PCP was still present in the solution,
IP: 5.101.221.73 On: Fri, 10 Jun 2016 07:58:14
these rate constants using Arrhenius equation:
Copyright: American Scientific Publishers
but it was not the most predominant matter yet (Fig. 4).
In the solution that reacted for 2 h, chlorinated phenols
with four, three, two or one chlorine atom were detected.
And chlorinated phenols with two chlorine atoms were the
most predominant matters. Since there are several isomers
for dichlorophenol, we can’t determine that which one was
the certain matter in the solution from the MS results,
or even a mixture of some isomers. However, the HPLC
results can confirm the presence of 2,4-dichlorophenol
(2,4-DCP) (Fig. 5). The second most predominant matters
were chlorophenols. Similarly, there might be a mixture of
some chlorophenol (CP) isomers. And the HPLC results
confirm the presence of 2-CP, 3-CP and 4-CP (Fig. 6).
Besides, phenol was also detected in the intermediates of
PCP (Fig. 6).
Ea
lnk = −
+B
(11)
RT
Where Ea is the activation energy (kJ·mol−1ꢁ, B is the
pre-exponential factor, k is the reaction rate constant (h−1ꢁ,
R is the universal gas constant (8.314 J·mol−1 ·K−1ꢁ, and
T is the reaction temperature (K).
The kobs values calculated at various temperatures were
correlated by Eq. (11), and estimated activation energy of
15.0 kJ·mol−1 was obtained. This relatively small activa-
tion energy indicated that the reaction was kinetically and
thermodynamically favorable on the nanoscale Fe0 surface.
3.4. Sorption Process
Casey and co-workers36 studied the fate and transport
of chlorinated solvents flowing through zerovalent met-
als with miscible-displacement experiments, and indicated
that sorption was an important process to be considered
when TCE was flowing through zerovalent metals. While
Morrison and co-workers37 indicated that only a small
amount of uranium could be adsorbed to ferric minerals
when they tested the feasibility of Fe0 to treat uranium
through a large-scale column experiment. To investigate
the adsorption of PCP on the surface of nanoscale parti-
cles, the residual iron particles were washed with deion-
ized water after solutions were extracted. As shown in
Figure 7, there was only a small amount of PCP washed
3.3. Kinetics and Thermodynamics
It was found that the transformation of various chlorinated
compounds with Fe0 or nanoscale Fe0 could be satisfacto-
rily described with the following pseudo-first-order kinetic
model:
dC
= −kobs
C
(8)
dt
Where C is concentration of the parent compound (mg·
L−1ꢁ, kobs is the pseudo-first-order rate constant (h−1ꢁ.
We calculated the transformation rate constants of PCP
at different temperature for the first 1 h (Table I), despite
6944
J. Nanosci. Nanotechnol. 14, 6941–6949, 2014