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The reactivity of well-dispersed zerovalent iron

nanoparticles toward pentachlorophenol in water

Chih-ping Tso, Yang-hsin Shih* 

Department of Agricultural Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106,

Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 5 August 2014

Received in revised form

26 November 2014

Accepted 23 December 2014

Available online 30 December 2014

Keywords:

Pentachlorophenol (PCP)

Ultrasonication

Well-suspension

Pd/Fe nanoparticles

Anions

a b s t r a c t

In order to prevent the aggregation of nanoparticles (NPs), surface modification or the

addition of a stabilizer are used for stabilization. However, the real reactivity of NPs is still

unclear because of the surface coating. For different physical dispersion methods, the

particle stabilization for nanoscale zerovalent iron (NZVI) particles and their reactivity are

studied. The particle properties of different preparations and their reactivity toward one

polychlorinated aromatic compound, pentachlorophenol (PCP), with different electrolytes

are also evaluated. Ultrasonication (US) with magnetic stirring disperses NZVI and Pd/Fe

NPs well in water and does not affect the surface redox property a lot under the operating 

conditions in this study. The well-suspended NZVI cannot dechlorinate PCP but adsorption

removal is observed. Compared to shaking, which gives limited removal of PCP (about 43%),

Pd/Fe NPs remove 81% and 93% of PCP from water in the US and the US/stirring systems,

respectively, which demonstrates that a greater surface area is exposed because of effec-

tive dispersion of Pd/Fe NPs. As the Pd doping increases, the dechlorination kinetics of PCPis improved, which shows that a catalyst is needed. With US/stirring, chloride ions do not

significantly affect the removal kinetics of PCP, but the removal efficiency increases in the

presence of nitrate ions because PCP anions were adsorbed and coagulated by the greater

amount of iron (hydro)oxides that are generated from the reduction of nitrate on Pd/Fe.

However, bicarbonate ions significantly block the adsorption and reaction sites on the Pd/

Fe NP surface with US/stirring. The US/stirring method can be used to evaluate the actual

activity of NPs near the nanoscale. The use of Pd/Fe NPs with US/stirring removes PCP from

water effectively, even in the presence of common anions expect a high concentration of 

bicarbonate.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Pentachlorophenol (PCP) is widely used throughout the world

in disinfectants, biocides, wood preservatives and pesticides.

Therefore, PCP can be found in the air, surface water, soil, and

groundwater aquifers because of its environmental stability

(Goerlitz et al., 1985; Mannisto et al., 2001). An old shutdown

PCP factory in An-shun area of Taiwan is a seriously PCP-

contaminated site with a high PCP concentration ranging 

from 0.312 mg kg 1 to 110 mg kg 1 in the soil (Thuan and

Chang, 2012). PCP is a priority pollutant because of its

*   Corresponding author. Tel./fax:  þ886 2 33669443.E-mail address: yhs@ntu.edu.tw (Y.-h. Shih).

 Available online at www.sciencedirect.com

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carcinogenicity and toxicity (Morales et al., 2014; Duan et al.,

2008; Zhu and Shan, 2009). However, the remediation

methods for PCP have been found to be inefficient mainly due

to its adsorption characteristics (Lindsey and Tarr, 2000).

Some of the intermediate products during the decomposition

of PCP, such as polychlorinated dibenzo- p-dioxins and poly-

chlorinated dibenzofurans in the environment, may be more

toxic than the original PCP (Harnly et al., 2000). In the pastdecade, abiotic dechlorination using granular zerovalent iron

(ZVI) particles has garnered much interest (Su et al., 2012), but

the transformation of chlorinated organic compounds using 

granular ZVI is slow and may not be complete (Morales et al.,

2002).

Nanoscale ZVI (NZVI) particles can remove several toxic

chemicals more promptly as compared to microscale ZVI

(MZVI) because they have a higher surface area than MZVI

(Tratnyek and Johnson, 2006; Shih and Tai, 2010). Compared

with NZVI, the bimetallic Fe nanoparticles (NPs) have exhibi-

ted higher potential for the dehalogenation of many haloge-

nated aromatic compounds (Shih et al., 2009; Wang et al.,

2013; Zhao et al., 2014), especially Pd/Fe NPs can degrade PCP(Shih et al., 2011). However, it is difficult to maintain the sta-

bility of Fe NPs in their original size during the reactions.

When Fe NPs aggregate and precipitate, their mobility and

reactivity in the environment are significantly reduced

(Glavee et al., 1995; He and Zhao, 2007). The aggregation of Fe

NPs depends upon their physical and chemical properties,

such as the size, the dosage and their surface characteristics,

as well as on the other chemical species in water (e.g., salinity,

solution composition, and the solution pH) (Mylon et al., 2004;

Yang et al., 2007; Hu et al., 2010). Therefore, various chemical

and physical dispersion methods have been proposed to pre-

vent the aggregation of metallic nanoparticles (Wang et al.,

2013; Zhao et al., 2014; Dong and Lo, 2013; Kaifas et al., 2014;Xie et al., 2014; Yang et al., 2014; Soukupova et al., 2015).

Chemical dispersion methods involve the use of solution

additives and surface modifiers such as adding carboxymethyl

cellulose, poly(methyl methacrylate), poly(ethylene glycol),

Tween 80, and chitosan (He and Zhao, 2007; Kaifas et al., 2014;

Soukupova et al., 2015; Parshetti and Doong, 2009; Wang and

Zhou, 2010; Kustov et al., 2011). These stabilizer molecules

provide inter-particle electrostatic forces and steric repulsion,

which are strong enough to overcome the interfacial forces

(He and Zhao, 2007; Harendra and Vipulanandan, 2008; He and

Zhao, 2005; Sakulchaicharoen et al., 2010). Although the

amendments can enhance the colloidal stability, they alter

the surface properties of NPs and affect their interaction withcontaminants (Kim et al., 2009; Phenrat et al., 2007; Tiraferri

et al., 2008). Modifiers can also affect the sorption and

desorption of contaminants so they limit the reaction rates of 

the reactions that occur at the iron surface (Phenrat et al.,

2009).

However, different physical techniques, including high-

energy ball milling, magnetic stirring, shaking and ultra-

sonication (US), are also used to disperse metal NPs ( Xing,

2004; Imamura et al., 2005; Mondragon et al., 2012; Lowry

and Reinhard, 2001; Dickson et al., 2012; Lai et al., 2013). Of 

these methods, US efficiently breaks down the parti-

cleeparticle interactions of nano-sized Fe NPs without the

need for synthetic modifiers (Dickson et al., 2012; Liang et al.,

2008; Rasheed et al., 2011). Moreover, using sonication irradi-

ation, nanoparticles with hydrodynamic diameters smaller

than 150 nm remain in suspension for more than 7 days

(Dickson et al., 2012). However, the collapse of cavitation

bubbles may also allow the formation of oxidizing species,

superoxide radicals, and hydrogen peroxide during ultrasonic

irradiation (Chen et al., 2011; Hou et al., 2012). US could not

only disperse nanoparticles, but also could alter the surfacecharacteristics of Fe NPs, which might further affect the re-

action efficiency. Therefore, the mechanisms of NZVI with

ultrasound could be complicated in the degradation of organic

compounds.

Electrolytes that are dissolved in the environment are an

important influencing factor on the reactivity of Fe aggregates

(Phenrat et al., 2007; Giasuddin et al., 2007; Shih et al., 2010;

Wang and Zhang, 1997) because they interact with ferrous

and ferric ions to form iron oxides or hydroxides on the iron

surface. The formation of passive layers on the surface of ZVI

causes a rapid decrease in the activity because it inhibits ac-

cess for target compounds to the active sites (Agrawal et al.,

2002; Kober et al., 2002). There have been many studies of the effect of anions on the removal reactions by Fe NPs, but

there has been little study of the performance of well-

suspended Fe NPs in the presence of anions. The aims of 

this study are to investigate the feasibility of removing PCP

using well suspended Fe NPs under different physical

dispersion methods, to understand the reaction mechanisms

of Fe NPs assisted with ultrasonication, and to evaluate the

effect of common anions (chloride, nitrate and bicarbonate)

on the Fe NPs at close to nanoscale.

2. Materials and methods

2.1. Chemicals

Pentachlorophenol (PCP) was purchased from Sigma. Ferrous

sulfate heptahydrate, sodium chloride, sodium nitrate, so-

dium hydroxide, methanol,   n-hexane and hydrochloric acid

were obtained from J. T. Baker. Sodium borohydride was

purchased from Shiyaku Kogyo Co. Ltd., Japan. Potassium

hexachloropalladate (IV) was provided by Acros. Sodium bi-

carbonate was purchased from Riedel-deHa€en. Chlorophenol

standard (DIN EN 12673 chlorophenols) including 19 chlor-

ophenols was purchased from Supelco (USA). All organic  sol-

vents used  for the study  were  of   analytical grade. All solutions

were made in water that was purified using a Milli-Q system(18.2 MU /cm).

2.2. The synthesis of Fe bimetallic nanoparticles

NZVI particles and Pd/Fe bimetallic (Pd/Fe) nanoparticles were

chemically synthesized in an anaerobic chamber (Coy Lab)

and used immediately. An aqueous solution of ferrous sulfate

was reduced, using sodium borohydride at a molar ratio for

BH4 /Fe2þ of 2.0, with vigorous stirring (14,000 rpm) at ambient

temperature. The obtained black nanoparticle suspension

was filtered through a 0.45   mm membrane and then washed

with pH 4 sulfuric acid and deionized water several times to

remove the residual reagents and salts. Pd/Fe NPs were

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synthesized by injecting potassium hexachloropalladate (IV)

solution into fresh NZVI suspension under the anaerobic

condition. After synthesis, the concentration of Pd ions in

solution decreased from 16.4 mg/L to 0.06 mg/L, that is, over

99% of Pd ions were deposited and reduced on NZVI surface.

The Pd/Fe NPs were then rinsed several times with deionized

and deoxygenated ultrapure water to remove chloride ions.

The concentrations of Fe and Pd were quantified using aninductively coupled plasma spectrophotometer (ICP). The

hydrodynamic particle size of the suspended nanoparticles

was measured by dynamic light scattering   (DLS) (Nano-ZS,

Malvern). The atomic and molecular structures of the crystals

of the Pd/Fe NPs were determined using X-ray diffraction

(XRD, National Synchrotron Radiation Research Center,

Taiwan). During XRD analysis, these nanoparticle samples

were stored in plastic bags with their original solutions.

2.3. Dispersion techniques

The PCP stock solution was prepared by dissolving PCP in

methanol/water (1:200, v/v) as the stock solution and this was

stored at 4   C. In order to compare the different dispersion

techniques, a shaker (LM-530R, YIH DER), a magnetic stirrer

(PC-420D, Corning), an US probe (Ultrasonic 250, HOYU), as

well as a combination of a magnetic stirrer and an US probe

(US/stir) were used to disperse Fe nanoparticles.

For shaking, the batch reactions were conducted in 5 mL

amber vials, sealed with Teflon-lined septa. Each vial con-

tained2 mL of PCP stocksolution, 2.5mL of Fe NPs suspension,

and around 0.5 mL deoxygenated ultrapure water to eliminate

headspace, which was prepared in the anaerobic chamber. An

orbital shaker operating at 150 rpm was used. For the US

(23 kHz and a rated output power of 250 W) and US/stirring 

systems, all of the time-dependent experiments were per-

formed in the anaerobic chamber. The removal rates of PCP

increased with an increase in output intensity of US from 0 to

75 W, accompanied with a decrease in hydrodynamic particle

size of Pd/Fe NPs (Figure S1). However, the increase of US

output intensity from 75 W to 250 W did not significantly

enhance the removal but increased the particle size. The

output intensity of the US probe about 30% of 250 W (i.e. 75 W)

was chosen in this study. The 10 mL of PCP stock solution and

12.5 mL Fe NPs suspension were placed in a 100 mL beaker,

which was covered with aluminum foil in a water bath and

maintained at a constant temperature of 25   C during soni-

cation. The initial concentration of the PCP in all reactors was5.0 mg/L. The Fe dosage for all the experiments quantified

using ICP was 4.5 g L1. The setting for the US/stirring system

is shown in Scheme 1. The magnetic stirring rate was 120rpm.

The solution pH and the reduction potential were monitored

during these reactions.

The effectof nanoparticle dosage, Pd loading andanionson

the removal of PCPby well-suspended Fe NPs wasdetermined.

Individual solutes of three common anions were used in so-

dium form (i.e. chloride, nitrate and bicarbonate) without a

buffer.

2.4. Analytic methods

The concentration of PCP was quantified using high perfor-

mance liquid chromatography (HPLC, Agilent 1200) with a UV/

Vis detector and gas chromatography (GC, Agilent 6890) with a

micro-electron capture detector or mass spectrometer (Agi-

lent 5975B). The mobile phase was a mixture of methanol and

1% H3PO4  (80:20 v/v) at a flow rate of 1.0 mL min1 and the

wavelength was set at 254 nm for PCP analysis. The GC was

equipped with an HP-5 column (30m 0.25 mm, 0.25 mm film).

The carrier gas was ultra-high-purity N2  of 99.999% and the

flow rate was 60 cm/s. The injector temperature was 250  C,

the oven temperature was maintained at 80  C for 2 min and

then increased at 7   C/min to 280   C, at which temperature

was maintained for 5 min. The detector temperature was

320  C. For byproduct analysis, chlorophenols were separated

using a GC program with an initial temperature of 60   C for

2 min, ramping at 20  C/min to 140   C with 0.5 min holding,

1   C/min to 150   C with 0.5 min holding, and 15   C/min to

300  C with 3 min final holding time. The flow rate was 30 cm/

s. The anion concentrations (chloride and nitrate) in the so-

lutions were monitored using ion chromatography (IC, Met-

rohm, Switzerland) after filtering through a 0.22   mm filter

(Minisart, non-pyrogenic).

2.5. Kinetic experiments

At selected time intervals, aliquots were withdrawn using a

magnet to separate the Fe nanoparticles. The ratio of the final

aqueous PCP concentration to its initial concentration is

defined as the   residual fraction. The removal rate constant is

described by a pseudo-first order reaction.

Ct  ¼ C0ekt (1)

where C0 and Ct are the initial concentration (mg L1) and the

concentration at reaction time t (min), in theaqueous phase of 

the reacted vial, and k  is the measured rate constant (min1).

The Pd/Fe particles were dried and extracted with hexane

and concentrated HCl, until there was no PCP residue in the

extract. This was then used to determine whether PCP and its

Scheme 1  e  Schematic of the main experimental setup

employed in the present study.

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derivatives had been adsorbed on the Fe surface. The ratio of 

the measured adsorbed PCP concentration to its initial con-

centration is defined as the adsorption fraction. The degradation

efficiency is calculated by subtracting the initial PCP from the

residual and the adsorption PCP fractions.

Removal efficiencyð%Þ ¼

C0 C final

C0 100 (2)

Degradation efficiencyð%Þ ¼

C0 adsorption fraction C final

C0 100

(3)

where   C final   is the final PCP concentration (mg L1) in the

aqueous phase.

Dechlorination efficiency can be expressed as the ratio of 

the measured chloride concentration (MCl , mM) to the theo-

retical chloride concentration (MCl

0, mM) from the complete

dechlorination of PCP.

Dechlorination efficiencyð%Þ ¼ MCl

5 MCl  0 100 (4)

3. Results and discussion

3.1. The removal of PCP using different iron NPs for

different dispersion systems

The range of the particle sizes for the synthesized NZVI and

Pd/Fe NPs was 50e100 nm shown in the TEM images

(Figure S2); however, the hydrodynamic particle sizes of these

two nanoparticles in aqueous solutions were more than 10 mm

(about 34  mme58  mm) measured by DLS. Because of the mag-

netic attractive force and the van der Waals forces between

these iron NPs, they tend to from aggregates in solution

(Giasuddin et al., 2007; Shih et al., 2010; Wang and Zhang,

1997). Four different dispersion methods were used to sus-

pend particles in solution, but the size remained on the

microscale except for the ultrasonic method. The average

particle size of the Pd/Fe NPs decreased from the microscale

(34  mm) to the nanoscale (123 nm) after US for 60 min ( Fig. 1).

The same trend was observed for NZVI. Since Pd/Fe NPs were

generated from NZVI, more aggregates of Pd/Fe NPs could

form during their synthesis, which might result in the larger

particle size of Pd/Fe NPs than NZVI after ultrasonication. An

ultrasonic probe has also been used to disperse other metal

nanoparticle aggregates to a small particle size (Tso et al.,

2010).

In order to determine the effect of well-suspended Fe NPson the removal of PCP, comparative experiments were per-

formed for the four different dispersion methods: the

shaking, the stirring, the US and the US/stirring system. No

obvious PCP removal was noted for NZVI in a shaking system.

PCP was removed using NZVI in the US system, but no

chloride ions were measured, which showed that no

dechlorination occurred, as shown in   Figure S3   (discussed

later). Obviously, PCP is not removed in the US system, which

shows that the US power used in this study does not elimi-

nate PCP.  Fig. 2a shows the results for the removal experi-

ments that were conducted under the same operating 

conditions of a temperature of 25   C, an initial pH value of 

about 6.7, an iron dosage of 4.5 g L1 and a Pd loading of 0.1%(w/w). Using US dispersion in both the US and the US/stirring 

systems, Pd/Fe NPs were well dispersed in aqueous solutions

(Figure S4) and the reactant molecules were not removed.

The average particle size is 295  ±  176 nm during the irradi-

ation time.

In the presence of Pd/Fe NPs, the PCP removal efficiency

was only around 43% and 74% for the shaking system and the

stirring system after 1 h but 81% and 93% for the US and the

US/stirring systems, respectively (Fig. 2a). The removal rates

for PCP for the shaking, thestirring, the US and the US/stirring 

systems were 0.01, 0.05, 0.06 and 0.13 min1, respectively. The

adsorption fractions of PCP for the US, thestirring,and the US/

stirring systems were about 70%, 64%, and 73%, which weremuch higher than that for the shaking system (12%) (Fig. 2b).

When large NP aggregates become smaller, there is more

surface area for the reaction. Using the US probe, Pd/Fe NPs

revert to a nanoscale size so there are more active sites than

nano-aggregates. The increased removal of PCP in US systems

Fig. 1 e

 DLS results of NZVI and Pd/Fe nanoparticles under US probe irradiation.

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is due to an increase in PCP adsorption on the Pd/Fe NP

surface.

Several dechlorination products (2,3,4,5-tetrachlorophenol

(TeCP), 2,3,4,6-TeCP, 2,3,4-trichlorophenol (TCP), and 3,4,5-

TCP) were identified in the US/stirring system. Three

byproducts (2,3,4,5-TeCP, 2,4,5-TCP, and 3,4,5-TCP) were

detected in the stirring system; however, only 2,3,4,5-TeCP

were found in the shaking system, which indicated furtherdechlorination occurred in other systems but not in the

shaking system although it had more degradation of parent

compound, PCP.

3.2. The removal mechanism for PCP using Pd/Fe NPs in

US dispersion systems

The XRD spectra for the Pd/Fe NPs, before and after irradiation

with US, are shown in Fig. 3. A weak peak at 44, representing 

the crystal structure of zerovalent iron, and some peaks for

iron (hydro)oxides were observed for our synthesized Pd/Fe

nanoparticles. It might be because the zerovalent iron wasamorphous, oxidized by the reduction process of Pd ions by

NZVI surface, or oxidized during the specimen preparation for

XRD. No significant increase in oxidized metals was observed

in a comparison of the XRD spectra for Pd/Fe before and after

US irradiation. This demonstrates that the reactive oxygen

species(ROS) that are generated by cavitation under US do not

affect the character of the Pd/Fe surface. In general, an in-

crease in the power and duration of US results in the gener-

ation of more ROS, due to cavitation (Hou et al., 2012).

However, under the operating conditions in this study, cavi-tation does not destroy PCP (Fig. 2a) and seems not to modify

the Pd/Fe nanoparticle surface.

Using US, the removal rates for PCP are accelerated by

adsorption because there is good dispersion of Pd/Fe for both

the US and the US/stirring systems shown in  Scheme S1. Fe

particles in natural water are almost charged because there

are Fe (hydro)oxides on the iron surface that can be ionized.

This surface charge is opposite in sign to that of the existing 

ionized PCP (pka ¼ 4.75), in common neutral pH, so there is an

electrostatic attraction between the PCP anions in solution

and the generally positive Fe hydroxides on the Pd/Fe nano-

particle surface (Scheme S2). The same interactive force at-

tracts PCP anions and chlorides that are released from PCPnear a positively charged Fe surface in water. The adsorption

of PCP on the Pd/Fe NPs surface also increases when the

particle size decreases, for systems using US.   Kim and

Carraway (2000) also noted that PCP losses occur because of 

strong sorption on ZVI and bimetallic ZVI, rather than by

dechlorination.

3.3. The effect of Pd doping on the removal of PCP using

well-dispersed Fe NPs

The addition of Pd onto iron nanoparticles increased the

removal rate and the degradation of PCP as compared to NZVI

(Fig. 4a). For the US/stirring system, when the Pd loading 

increased from0 % to 0.2 %, the removal rate constants for PCP

generally increased from 0.038 to 0.14 min1. For the US/stir-

ring system, more than 90% PCP was removed after 40 min

with different amounts of Pd doping. Most of the PCP was

adsorbed on the Pd/Fe surface and only 12% of PCP was

degraded in the presence of 0.05% Pd (Fig. 4b). When the Pd

doping increased to 0.2%, the degradation faction of PCP

Fig. 2 e (a) Removal reactions of PCP with Pd/Fe NPs under

 various suspended system and (b) the degradation,

adsorption, and residual fractions of PCP reacted with Pd/ 

Fe. PCP initial concentration was 5 mg/L, Pd/Fe dosage was

4.5 g L¡1 and Pd content was 0.1% (w/w).

Fig. 3 e XRD spectra of Pd/Fe nanoparticles and those after

reactions under various conditions.

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increased to around 30% but the adsorption fraction

decreased to 75%. Obviously, the increase of Pd doping can

enhance the degradation.

From the measurement of chloride ions, the dechlorination

rate constants and dechlorination efficiencies for PCP for Pd/

Fe NPs with different Pd contents are shown in  Fig. 5. When

the Pd loading increased from 0.05 % to 0.2 %, the dechlori-

nation rate constants for PCP increased from 0.0024 to0.0064 min1 (Fig. 5a), which were calculated from the gener-

ation of chloride and the pseudo-first order kinetics. Since no

significant dechlorination of PCP is observed for zerovalent

iron NPs, even with US irradiation (Fig. 5b), the well-dispersed

Fe NPs remove PCP by adsorption onto the iron surface. There

is also a strong linear relationship between Pd doping and the

dechlorination efficiency (R2 is 0.968) of well dispersed Pd/Fe

NPs. The dechlorination of PCP increased from 12 to 30 %

when Pd loading increased from 0.05 % to 0.2 %. When the Pd

loading increases, more hydrogen, which is generated during 

the corrosion of iron, is adsorbed onto the Pd and increases

the reactivity (Cheng et al., 1997). The process of iron oxidation

is also accelerated by the galvanic contact between Pd andiron (Cheng et al., 1997; Yan et al., 2010; Cwiertny et al., 2006).

Both of these effects result in the increased dechlorination of 

PCP.

3.4. The effect of anions on the removal of PCP using Pd/ 

Fe NPs

The effect of anions on the removal of PCP, using well-

dispersed Pd/Fe NPs, is determined using three common

electrolytes  e  NaCl, NaNO3   and NaHCO3  e   in a US/stirring 

dispersion system (Fig. 6a). In the presence of chloride, the

removal rate for PCP increased slightly from 0.12 min1 to0.16 min1 when the chloride concentration increased from

5 mMto 10 mM, which comparedto 0.099min1 forpure water

(Fig. 6), but the dechlorination efficiency did not increase,

compared to that for pure water (Fig. 7). In general, chloride

promotes corrosion and increases the reactivity of granular

iron toward chlorinated compounds (Devlin and Allin, 2005;

Gotpagar et al., 1999; Johnson et al., 1998). However, in this

dispersion system, chloride does not significantly affect the

reaction rate or the degradation efficiency.

For nitrate, the removal rate constants increased from 0.26

to 0.54 min1, when the concentration of nitrate was raised

from 5 mM to 10 mM, and most of the PCP was removed in the

presence of nitrate (Fig. 6). Compared to pure water and otheranion systems, the presence of nitrate accelerated the ki-

netics. However, nitrate generally inhibits the reduction

power of zerovalent iron nanoparticles, because it competes

Fig. 4 e   (a) Removal reactions of PCP with Pd/Fe NPs with

 various Pd dosage in the US/stirring system and (b) the

degradation, adsorption, and residual fractions of PCP

reacted with Pd/Fe. PCP initial concentration was 5 mg L¡1

and Fe dosage was 4.5 g L¡1.

Fig. 5  e  Effect of Pd dosage on (a) the dechlorination

reaction rates and (b) dechlorination efficiencies of PCP by

Pd/Fe nanoparticles under US/stirring system. PCP initial

concentration was 5 mg L¡1 and Fe dosage was 4.5 g L¡1.

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with chemicals that have a high oxidation state, such as

halogenated compounds. Since nitrate obtains electrons from

the iron surface and reduces them to nitrite and ammonia,

using iron powder (Alowitz and Scherer, 2002) and NZVI (Shih

et al., 2009), the concentrations of nitrate, nitrite and ammo-

nium were measured. Over 99% of the nitrate was trans-

formed by Pd/Fe NPs and about 72% and 63% of these

transformed nitrate ions were further reduced to ammonium

the presence of 5 mM and 10 mM nitrate, respectively. This

result demonstrates that nitrate is reduced by Pd/Fe NPs,

during the degradation of PCP. The reduction of nitrate by iron

is also a corrosive process, which causes more iron (hydro)

oxides to form a film on the iron surface. In the XRD analysis

(Fig. 3), the specific peaks for zerovalent metals almost dis-

appeared and more peaks for iron oxides were found than for

other treatments. In particular, magnetite (Feþ2Fe2þ

3O4) was

observed on the Pd/Fe nanoparticle surface in the presence of nitrate. Fig. 7 shows that the adsorption fraction for PCP in the

presence of nitrate (about 89%) is the highest among these

anions. This result is consistent with the nitrate trans-

formation and the XRD observation and only around 10% is

degraded by Pd/Fe NPs. Since the electrochemical reduction of 

PCP is mediated in the range, 1.6 to 2.3 V (Ross et al., 1997;

Lin and Tseng, 1999), which is higher than the reduction po-

tential of nitrate (0.88 V), the Pd/Fe surface reacts with the

nitrate first and generates iron (hydro)oxides. The fast

removal kinetics is mainly due to the adsorption or coagula-

tion of PCP on these iron oxides.

In the presence of bicarbonate, the removal rate for PCP

was significantly reduced in the same dispersion system asthat used for the anions (Fig. 6). When the bicarbonate con-

centration increased from 0 to 10 mM, the reaction rate con-

stant decreased from 0.099 to 0.023 min1 and the removal

efficiency after 40 min decreased from 96 to 46 % (Fig. 6a). The

degradation and adsorption fractions were less than those for

other anions and that for pure water (Fig. 7), which demon-

strated that bicarbonate blocks the reactive and attachment

sites for PCP on the Pd/Fe surface. However, no significant

peaks for metal oxides on Pd/Fe surface were observed (Fig.3).

These inneresphere complexes form passivation layers that

inhibit the adsorption and degradation of PCP by Pd/Fe

nanoparticles (Phenrat et al., 2007; Agrawal et al., 2002; Kober

et al., 2002). The solution pH also increased from 5.4 to 8.9 inthe presence of 10 mM bicarbonate. Because the chemical

reduction of polychlorinated aromatic compounds by NZVI

favors acidic conditions (Shih et al., 2009), this high solution

pH, buffered by hydrogen carbonate, significantly decreases

the rate of degradation PCP by Pd/Fe NPs.

4. Conclusions

The effect of physically mixing zero-valent iron nanoparticles,

with or without Pd, on the removal of PCP by Pd/Fe NPs is

studied. The US/stirring system, suspends NZVI andPd/Fe NPs

most effectively of anyof the dispersion methods, at about thenanoscale. Since NP aggregates become smaller, most of the

PCP is removed by adsorption onto the iron surface because

there is a larger surface area. Under the operating conditions,

US does not degrade PCP, even with NZVI, and does not affect

the surface characteristics of Pd/Fe. When the Pd doping on

the Pd/Fe NPs increases, the dechlorination kinetics for PCP

increases in the US/stirring system. The actual activity of well-

suspended Pd/Fe NPs is evaluated for different electrolytes

that are commonly found in treatment systems. In the pres-

ence of common anions, nitrate ions are reduced by well-

suspended Pd/Fe NPs and more (hydro)oxides are generated,

which increases the adsorption or coagulation of PCP from

water, while chloride ions only cause a slight increase in the

Fig. 6 e  (a) Effect of anions on the removal of PCP by Pd/Fe

under US/stirring. (b) The relation between the reaction

rate constants and anions. PCP initial concentration was

5.0 mg/L, iron dosage was 4.5 g/L, and Pd content was 0.1%.

Fig. 7 e The degradation, adsorption, and residual fractions

of PCP reacted with Pd/Fe in the presence of anions under

US/stirring system after 40 min.

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removal kinetics. However, bicarbonate significantly inhibits

the removal of PCP because bicarbonate could form com-

plexes with metal cations from Pd/Fe and then precipitate on

the iron surface, which blocks the adsorption and reaction

sites. These better understandings of the real activity and

removal mechanisms of well-dispersed Pd/Fe as well as the

common ion effect can facilitate the application of NPs in

water treatments and the environmental remediation.

Acknowledgments

The authors gratefully acknowledge the financial support of 

the Ministry of Science and Technology of Taiwan, R. O. C.

(Contract 101-2313-B-002-035-MY3).

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://dx.doi.org/10.1016/j.watres.2014.12.038.

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