Fe bimetallic nanoparticles: Effects of copper, nickel, and ferric cations

Applied Catalysis B: Environmental 105 (2011) 24–29

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Pentachlorophenol reduction by Pd/Fe bimetallic nanoparticles: Effects of copper, nickel, and ferric cations Yang-hsin Shih a,∗ , Meng-Yi Chen a,b , Yuh-Fan Su a a b

Department of Agricultural Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung 402, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 18 December 2010 Received in revised form 24 February 2011 Accepted 17 March 2011 Available online 30 March 2011 Keywords: Pentachlorophenol Pd/Fe nanoparticles Degradation Cations Sulfate

a b s t r a c t Bimetallic nanoparticles have been used for effective reduction of chlorinated compounds; however, the study of cation effect on degradation is limited. This study examined the effect of three selected cations normally co-present in soil and groundwater contamination sites on the degradation kinetics and removal efficiency of pentachlorophenol (PCP) by Pd/Fe nanoparticles. Degradation of PCP by Pd/Fe nanoparticles was carried out in aqueous solutions containing different cations in sulfate form, Na2 SO4 , CuSO4 , NiSO4 , and Fe2 (SO4 )3 , respectively. The observed inhibitory effect of Na2 SO4 on degradation of PCP was contributed to the existence of SO4 2− ions. Overcoming the inhibitory effect of SO4 2− ions, Cu2+ , Ni2+ , and Fe3+ could facilitate the degradation kinetics and efficiencies of PCP by Pd/Fe nanoparticles. XANES absorption spectra were performed to characterize their valences. The enhancement effect of Cu2+ and Ni2+ ions result from the presence of reduced forms of copper and nickel on Pd/Fe surfaces. The presence of reduced forms of copper and nickel on Pd/Fe nanoparticles were confirmed by ICP–MS analysis. The addition of Fe3+ ions caused a decrease in pH and can reasonably account for the enhancement seen in the PCP degradation process. These observations lead to a better understanding of PCP degradation with Pd/Fe nanoparticles and can facilitate the remediation design and prediction of treatment efficiency of PCP at remediation sites. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Pentachlorophenol (PCP) is a manufactured chlorinated organic compound which has been widely used as a herbicide, pesticide, wood preservative and in a variety of other industrial applications. Broad use and environmental stability of PCP have led to the extensive contamination of soil, surface water, and groundwater aquifers [1,2]. It has been reported that adverse effects of PCP on the environment and humans might last for a long time [3,4]. Chronic exposure to PCP can damage the liver, kidney, blood, and nervous systems [5]. Furthermore, extremely toxic organic compounds such as PCDD/F may be generated from PCP by photochemical reactions [6]. Owing to its high toxic risk for humans and the environment, PCP is listed as one of the priority pollutants by the US Environmental Protection Agency, European Union [7], and Taiwan. Although PCP has been severely restricted since 1984, PCP still remains in the environment because of improper disposal of industrial wastes and its persistence in the environment. Especially, one heavily PCP contaminated site in an old shutdown PCP factory was found in Taiwan. Therefore, it is

∗ Corresponding author. Tel.: +886 2 33669442; fax: +886 2 33669443. E-mail address: [email protected] (Y.-h. Shih). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.03.024

essential to find ways to remove PCP from the environment effectively. Among the current methods for removal of chlorinated organic compounds, the chemical reduction method has attracted a lot of attention due to the short treatment time required [8]. Zerovalent metal serves as a donor of electrons (reducing agent) that is capable of promoting the reductive dechlorination of chlorinated organic compounds, as expressed by the following reaction [9,10]: M0 + RCl + H+ → M2+ + RH + Cl−

(1)

In recent years, zerovalent metals have been widely applied to the treatment of halogenated organic pollutants such as polychlorinated biphenyls, chlorophenols, trichloroethylene, and polybrominated diphenyl ethers [11,12]. However, the dechlorination of PCP by zerovalent metals was not effective due to slow dechlorination rates, high adsorption proportion, and incomplete dechlorination [13–15]. Various methods were developed to improve the efficiency of dechlorination of PCP by zerovalent metals. For example, the dechlorination of PCP by Fe0 coupled with hydrogen peroxide [16] and microwave [17], Ag/Fe0 coupled with ultrasound [18], as well as Ag/Fe0 or Pd/Mg0 coupled with subcritical water [19] and supercritical carbon dioxide [20,21] has been reported.

Y.-h. Shih et al. / Applied Catalysis B: Environmental 105 (2011) 24–29

2. Materials and methods 2.1. Chemicals and standards Pentachlorophenol was purchased from Sigma. Methanol and n-hexane were obtained from J. T. Baker. Ferrous sulfate, sodium borohydride, hydrochloride acid, sodium hydroxide, sodium sulfate, cupric sulfate, nickel sulfate and ferric sulfate were purchased from Riedel-deHaën. All aqueous solutions were made in water purified with a Milli-Q system (18.2 M/cm). 2.2. Preparation of nanoscale Pd/Fe particles NZVI particles were produced by adding a NaBH4 aqueous solution to a flask containing FeSO4 7 H2 O aqueous solution at ambient temperature. The zerovalent Fe particles were washed with HCl solution and with deionized water twice. Ferrous ion was reduced

100 80

intensity (%)

In the last decade, bimetallic particles have been exploited to dechlorinate chlorinated organic compounds due to their high efficiencies compared with the primary metal alone. It has been suggested that supplying a second catalytic metal such as Pd, Pt, Ag, Ni, or Cu on primary zerovalent metal could prevent toxic byproduct formation by dechlorinating chlorinated pollutants via hydrogen reduction rather than via electron transfer [22,23]. The dechlorination of PCP by microscale bimetallic particles including Pd/Fe, Pt/Fe, Ni/Fe and Cu/Fe [24], Pd/Mg and Pd/Fe [14], and Ag/Mg and Pd/Mg [25] have been reported. Furthermore, researchers found that nanoscale zerovalent iron (NZVI), which has high specific surface area, showed much more reactivity for the transformation of halogenated organic compounds than commercial iron powder [9,26–28]. On the other hand, nanoscale bimetallic technology offers new potential in treatment of organic and inorganic pollutants in the environment. Nanoscale palladium/iron particles are highly reactive remediation agents for chemical reduction of halogenated compounds. The use of nanoscale palladium/iron on groundwater remediation of chlorinated solvents such as trichloroethylene has been well studied [29]. An efficient degradation of hexachlorobenzene [30] and PCP [31] by Pd/Fe bimetallic nanoparticles has been demonstrated in our previous work. However, studies on the effect of cations on degradation of PCP, a polychlorinated compound, by nanoscale metals are scarce. Common cations in water, soil and groundwater will affect the degradation of halogenated compounds by metals. And the mixed contamination of heavy metal ions and chlorinated compounds has been found in many sites. The cation Cu2+ can enhance the dechlorination of carbon tetrachloride by iron nanoparticles and led to the production of more benign products such as CH4 [32]. The tetrachloride reduction rates with green rust (GR) were greatly increased for systems amended with Cu+ , Au3+ , and Ag+ relative to GR alone [33]. In order to assess the applicability of Pd/Fe nanoparticles for remediation of PCP contaminated soil and groundwater, this study examined the effect of three selected cations normally present in soil and groundwater on the removal kinetics and efficiency of PCP by Pd/Fe nanoparticles. Salts are generally used to introduce the heavy metal cations. They usually accompany anions such as SO4 2− , Cl− , HCO3 − , HPO4 2− , and NO3 − . The effect of the anions on the reaction of the NZVI has been investigated in literature. Fan et al. [34] reported that SO4 2− had a negative effect on the decolorization of azo dye methyl orange with NZVI. Lim and Zhu [35] concluded that SO4 2− anions have no influence on dechlorination of trichlorobenzene by Pd/Fe. A huge controversy over the effect of anions still exists. In this work, sulfate was added in the solutions that introduced cations such as Cu2+ , Ni2+ and Fe3+ .

25

60 40 20 0

0

20

40

60

80

100

120

size (nm) Fig. 1. The particle size analysis of Pd/Fe nanoparticles by DLS.

to zerovalent Fe particles according to the following reaction [9]: Fe(H2 O)6 2+ + 2BH4 − → Fe0 ↓ + 2B(OH)3 + 7H2 ↑

(2)

Bimetallic Pd/Fe nanoparticles were then synthesized by the reaction of the fresh zerovalent iron particles with the desired amount of potassium hexachloropalladate(IV) aqueous solution under a stirring condition according to the following equation [30]: PdCl6 2− + 2Fe0 → 2Fe2+ + Pd0 + 6Cl−

(3)

The bimetallic particles were then rinsed three times with deionized water to remove chloride ions. The final Pd loading was 0.54 mg of Pd per gram of iron (0.054 wt%) determined by ICP–MS. The freshly prepared Pd/Fe nanoparticles were then immediately used to carry out batch dechlorination experiments. 2.3. Characterization of Pd/Fe nanoparticles The morphology of Pd/Fe nanoparticles has been determined by transmission electron microscopy (TEM, JEM2010 microscope, JEOL, Japan) in our published paper [30,31]. The particle size distribution of Pd/Fe nanoparticles by dynamic light scattering (DLS, Zetasizer Nano, Malvern, UK) from 60 nm to 90 nm is shown in Fig. 1. The copper, nickel, palladium, and iron concentrations of nanoparticles were analyzed by inductively coupled plasma-mass spectrometer (ICP–MS, PerkinElmer, model SCIEX ELAN 5000, USA). X-ray absorption near edge structure (XANES) spectra was collected at Wiggler beamline BL 17C1 at the National Synchrotron Radiation Research Center (NSRRC) of Taiwan. 2.4. Batch dechlorination experiments The batch experiments were conducted in bottles capped with Teflon coating septa and aluminum caps. To each bottle containing the desired concentration of cations and 5 mg/L PCP in methanol–water (1: 200, v/v), freshly prepared Pd/Fe nanoparticles (12.5 g/L) were added. The concentrations of cations (1.67–10 mM) used in this study cover the wide concentration range reported in literature for the remediation of contaminated sites [36,37]. The reactors were shaken continuously at 150 rpm for the duration of experiments at 25 ◦ C. For each bottle, the samples were extracted using liquid–liquid extraction with n-hexane after adding concentrated HCl for gas chromatography (GC) analysis. The addition of concentrated HCl around 0.5 mL resulted in a low pH to promote the dissolution of ZVI surfaces and protonation of chlorophenolates, thereby releasing adsorbed PCP for extraction into n-hexane [24,38]. Control vials were prepared identically except for the addition of Pd/Fe nanoparticles. The concentrations of PCP were quantified using an Agilent 6890 gas chromatograph with ␮electron capture detector (␮-ECD).

Y.-h. Shih et al. / Applied Catalysis B: Environmental 105 (2011) 24–29

1.2

a 1.2

1

1

0.8

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C/Co .

C/Co .

26

0.6

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0.4

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0 0

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time (min) Blank

without salts

5 mM sodium sulfate

10 mM sodium sulfate

2.5 mM sodium sulfate

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Blank without salts

b

Fig. 2. Effect of different concentrations of Na2 SO4 on the degradation kinetics of 5 mg/L PCP by 12.5 g/L Pd/Fe nanoparticles.

time (min)

The pseudo-first-order model was employed to describe the reduction of PCP: −kobs t

(4)

where C is the concentration at any time (mg/L), C0 is the initial concentration (mg/L), kobs is the observed reaction rate constant (min−1 ), and t is the reaction time (min).

100

5 mM copper sulfate 5 mM sodium sulfate

150 10 mM copper sulfate 10 mM sodium sulfate

1.2 1

C/Co .

2.5. Chemical reduction rate constants

C = C0 e

0

160

0.8 0.6 0.4 0.2 0

0

50

100

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time (min) Blank without salts

5 mM nickel sulfate 5 mM sodium sulfate

10 mM nickel sulfate 10 mM sodium sulfate

Fig. 3. Effect of different concentrations of (a) CuSO4 ; (b) NiSO4 on the degradation kinetics of 5 mg/L PCP by 12.5 g/L Pd/Fe nanoparticles.

3. Results and discussion 3.1. Effect of sodium sulfate on the degradation kinetics of PCP by nanoscale Pd/Fe particles The degradation kinetics of PCP by Pd/Fe nanoparticles without electrolytes was studied as the basis of PCP degradation and for later comparison (Fig. 2). In the absence of salts, the rate constant and degradation efficiency within 100 min of PCP by Pd/Fe nanoparticles were 0.083 min−1 and 97%, respectively (Table 1). The effect of different concentrations of Na2 SO4 on the degradation kinetics of PCP by Pd/Fe nanoparticles is also shown in Fig. 2. As the Na2 SO4 concentration increased from 2.5 to 10 mM, the rate constant decreased from 0.067 to 0.048 min−1 , and the degradation efficiency decreased from 60% to 33% (Table 1). The higher the Na2 SO4 concentration, the lower the rate constant and degradation efficiency were. The presence of Na2 SO4 inhibited PCP degradation by Pd/Fe nanoparticles as compared with that without salts. However, the standard reduction potential of Na+ ions (−2.71 V) is below Table 1 Dechlorination rate constants and efficiencies of PCP by Pd/Fe nanoparticles containing various cations in sulfate form. Salts

Concentration (mM)

Rate constants (min−1 )

Degradation efficiencya (%)

Without salts

0 2.5 5 10

0.083 0.067 0.060 0.048

97 60 53 33

CuSO4

5 10

0.069 0.082

67 86

NiSO4

5 10

0.190 0.223

99 99

Fe2 (SO4 )3

1.67 5

0.082 0.081

96 77

Na2 SO4

a

The degradation efficiency was determined at 100 min.

that of Fe2+ ions (−0.44 V) therefore, Na+ ions have no effect on PCP degradation by Pd/Fe nanoparticles due to its high reduction potential [39]. Previous study has shown that Na+ ions showed no effect on the degradation of organic compounds with NZVI while Na2 SO4 had a negative effect on the degradation process [34]. Therefore, the inhibition of PCP with Pd/Fe nanoparticles was attributed mainly to the SO4 2− ions. The blockage of reactive sites on the surface of Fe0 and its corrosion products by specific adsorption of the sulfate forming inner-sphere complex may be responsible for the inhibitory effect of the presence of SO4 2− ions [40]. In the following experiments, the degradation kinetics in the presence of Na2 SO4 was considered as the reference kinetics because of the negative impact of SO4 2− ions on degradation. 3.2. Effect of copper sulfate on the degradation kinetics of PCP by nanoscale Pd/Fe particles Fig. 3(a) shows the effect of different concentrations of CuSO4 on the degradation kinetics of PCP by Pd/Fe nanoparticles. In order to distinguish the effect of copper ions from copper sulfate, the degradation kinetics in the presence of Na2 SO4 is also shown in Fig. 3(a). As compared with degradation of PCP at the same concentrations of Na2 SO4 , the presence of Cu2+ ions enhanced both rate constants and degradation efficiencies, increasing dramatically as the CuSO4 concentration increased. The rate constant and degradation efficiency within 100 min at 5 mM CuSO4 of 0.069 min−1 and 67%, respectively, were slightly higher than that at 5 mM Na2 SO4 of 0.060 min−1 and 53% (Table 1). The rate constant and degradation efficiency at 10 mM CuSO4 of 0.082 min−1 and 86%, respectively, were significantly higher than 0.048 min−1 and 33% at 10 mM Na2 SO4 . At 10 mM CuSO4 , the enhancement effect of Cu2+ ions counteracted the inhibitory effect of the SO4 2− ions since

Y.-h. Shih et al. / Applied Catalysis B: Environmental 105 (2011) 24–29

Intensity (a.u.)

a

Cu foil (Cu0) Cu2O CuO CuSO4 (Pd/Fe)

8960

8980

9000

9020

Energy (eV)

b

27

the dechlorination of PCP by NZVI has not been previously reported. Kim and Carraway [24] found that copper amended iron powder showed slower removal rates of PCP as compared to unamended iron powder. However, it has been reported that the presence of a Cu coating enhanced the reaction rates of the dehalogenation of some chlorinated aliphatics and aromatics. Choi et al. [42] reported that copper-coated iron enhanced reactivity on the dechlorination of 2,4,6-trichlorophenol as compared to iron alone. Shih et al. [43] also found that the deposition of Cu on NZVI resulted in a significant increase in dechlorination of hexachlorobenzene by NZVI in presence of Cu2+ ions. Maithreepala and Doong [44] indicated that the removal of carbon tetrachloride, tetrachloroethene, and trichloroethene by chloride green rust was enhanced by the reduction of Cu2+ ions to both Cu+ and Cu0 . Enhanced degradation of halogenated compounds by copper coatings on iron particles has been attributed to the ability of copper to facilitate electron transfer and promote corrosion of the Fe0 . The initial pH of PCP solution is 2.55 after the addition of 10 mM CuSO4 . After adding Pd/Fe nanoparticles, the solution pH immediately increases and reaches a stable value around 7.8. The chemical reduction of chlorinated compounds by nanoscale ZVI is favorable in acid conditions [45,46]. A lower pH is another reason for the enhancement of CuSO4 on the rate constant of degradation kinetics of PCP.

Intensity (a.u.)

3.3. Effect of nickel sulfate on the degradation kinetics of PCP by nanoscale Pd/Fe particles

Ni foil (Ni0) NiO Ni(OH)2 NiSO4 (Pd/Fe)

8320

8340

8360

8380

Energy (eV) Fig. 4. (a) Cu K-edge XANES spectra of zerovalent Cu, Cu2 O, CuO and Pd/Fe nanoparticles after the PCP degradation experiments in the presence of CuSO4 ; (b) Ni K-edge XANES spectra of zerovalent Ni, NiO, NiOH2 and Pd/Fe nanoparticles after the PCP degradation experiments in the presence of NiSO4 .

the rate constants with and without CuSO4 were almost the same, about 0.083 min−1 . The standard reduction potential of Cu2+ ions (+0.34 V) is well above that of Fe2+ ions (−0.44 V) and the reduction of Cu2+ ions would be expected to take place primarily via a redox reaction [41]. The ICP–MS analysis of the copper content on Pd/Fe nanoparticles after the PCP degradation experiments in the presence of CuSO4 indicated that there were about 26.6 mg and 54.1 mg of Cu per gram of iron for 5 mM and 10 mM CuSO4 , respectively. Fig. 4(a) showed the normalized XANES spectra of zerovalent Cu, Cu2 O, CuO and our experimental Pd/Fe nanoparticle sample in the presence of CuSO4 . The spectrum of our experimental sample was near the reference spectrum of Cu0 , indicating that some portion of Cu2+ ions were reduced to Cu0 and present on the Pd/Fe particles. The enhancement effect of the presence of Cu2+ ions on the rate constant of degradation kinetics of PCP was attributed to the Cu coating on Pd/Fe nanoparticles. The enhancement effect of Cu on

The influence of various concentrations of NiSO4 on the degradation kinetics of PCP by Pd/Fe nanoparticles is shown in Fig. 3(b). The degradation kinetics in the presence of Na2 SO4 is also presented in Fig. 3(b) in order to evaluate the differences between the effect of nickel ions and nickel sulfate. When compared with degradation of PCP without any salts and with the same concentrations of SO4 2− , Ni2+ ions enhanced both rate constants and degradation efficiencies significantly; even exceeding the inhibitory effect of the presence of SO4 2− ions. The enhancement effect on the rate constant of degradation kinetics increased with increasing NiSO4 concentration. The rate constants were 0.190 min−1 at 5 mM NiSO4 and 0.223 min−1 at 10 mM NiSO4 , which were significantly higher than that of 0.083 min−1 in the absence of salts and those of 0.060 min−1 and 0.048 min−1 in the presence of Na2 SO4 (Table 1). PCP can be rapidly and completely degraded by Pd/Fe nanoparticles in the presence of NiSO4 at concentration of 5 and 10 mM. The pH value after adding 10 mM NiSO4 is 5.7, which is close to that in the presence of 10 mM Na2 SO4 . The standard reduction potential of Ni2+ ions (−0.257 V) is above that of Fe2+ ions (−0.44 V) and the reduction of Ni2+ ions would be expected to take place primarily via a redox reaction [39]. ICP–MS analysis on Pd/Fe nanoparticles after PCP degradation experiments in the presence of Ni2+ ions indicated that there were about 25.2 mg and 50.2 mg of Ni per gram of iron for 5 mM and 10 mM NiSO4 , respectively. Fig. 4(b) showed the normalized XANES spectra of zerovalent Ni, NiO, NiOH2 and our experimental Pd/Fe nanoparticle sample in the presence of NiSO4 . After the reduction of an element, the lower oxidation state of an element generally shifts to lower energy in the position of the XANES absorption edge [47,48]. Therefore, the spectrum of our experimental sample which was located between the reference spectra of zerovalent Ni and NiO indicated some portion of a reduced form of nickel atoms present on the particles. The enhancement effect of Ni2+ ions on the degradation rates and removal efficiencies of PCP was attributed to the catalytic effect of the reduced form of nickel atoms on Pd/Fe nanoparticles. Nickel and palladium are known as hydrodehalogenation and hydrogenation catalysts, and the catalytic hydrodechlorina-

Y.-h. Shih et al. / Applied Catalysis B: Environmental 105 (2011) 24–29

tion of 4-chlorophenol (4-CP) and 2,6-dichlorophenol (2,6-DCP) in aqueous solution using nickel as a catalyst has been studied [49]. Zerovalent Ni can reduce dehalogenation of halogenated compounds such as aryl halide to arene [50]. Co-existence of nickel and iron in the particles has been proven to be very effective in accelerating dechlorination. Zhang et al. [18] found that the removal of PCP by Ni/Fe with the assistance of ultrasound increased from 33% to 98% within 20 min as nickel content increased from 0 to 10 wt.%. We first demonstrate the enhancement effect of nickel co-existence on dechlorination of PCP by NZVI in this study. The enhancement effect of iron coexisted with nickel on the degradation of various chlorophenols has also been widely investigated [42,51,52]. After deposition and reduction of nickel ions on a ZVI surface, NiFe bimetallic particles were also found to degrade toxaphene at a higher degradation rate than bare ZVI [53]. 3.4. Effect of ferric sulfate on the degradation kinetics of PCP by nanoscale Pd/Fe particles The effect of different concentrations of Fe2 (SO4 )3 on the degradation kinetics of PCP by Pd/Fe nanoparticles is examined as shown in Fig. 5. To discriminate the effect of ferric ions from ferric sulfate, the degradation kinetics in the presence of Na2 SO4 is also shown in Fig. 5. As compared with degradation of PCP in the presence of Na2 SO4 with the same concentrations of SO4 2− ions, Fe3+ ions seem to increase both rate constants and degradation efficiencies The rate constant and degradation efficiency within 100 min were 0.082 min−1 and 96%, respectively, at 1.67 mM Fe2 (SO4 )3 , which were higher than that of 0.060 min−1 and 53% at 5 mM Na2 SO4 (Table 1). Similar results were also found at a higher concentration Fe2 (SO4 )3 of 5 mM. The rate constants with and without 1.67 mM Fe2 (SO4 )3 were about the same, suggesting that the enhancement effect of Fe3+ ions could be comparable with the inhibitory effect of SO4 2− ions. The increase in the rate constant of degradation kinetics in the presence of Fe3+ ions could be attributed to the solution pH of this system being 2.55 after the addition of 5 mM Fe2 (SO4 )3 , which is lower than that in control experiments and general conditions. After adding Pd/Fe nanoparticles, the solution pH increases to 7.64

1.2 1 0.8

C/Co .

28

0.6 0.4 0.2 0

0

Blank without salts

50

time (min)

100

1.67 mM ferric sulfate 5 mM sodium sulfate

150 5 mM ferric sulfate 10 mM sodium sulfate

Fig. 5. Effect of different concentrations of Fe2 (SO4 )3 on the degradation kinetics of 5 mg/L PCP by 12.5 g/L Pd/Fe nanoparticles.

within 3 min. The pH then increases gradually with increasing the reaction time. It reaches a stable value of 7.88 after about 20 min. When a higher concentration of Fe2 (SO4 )3 was added, both lower initial and final pH in the solution was obtained. That is, Fe3+ ions can lower the pH in the nanoscale Pd/Fe system because the ferric ion is one of the small, highly charged exchangeable cations that could produce Br␸nsted acidity by promoting a reaction with water to release H+ ions [54]. The phenomenon can be described by the following reaction [55]: Fe3+ + H2 O → Fe(OH)2+ + H+

(5)

Zhang et al. [18] discovered that the removal efficiency of PCP by Ni/Fe nanoparticles increased with a decrease of pH value. Patel and Suresh [25] found that the rate constant and removal efficiency increased when pH values decreased for dechlorination of PCP by Pd/Mg. The enhancement effect of the acidic condition on the rate constant of degradation kinetics is attributed to two reasons. First, oxides naturally formed on the iron particle surface would be dissolved and the active sites of the Fe particle surface unlocked. Second, iron corrosion could be accelerated, pro-

Fig. 6. Schematic diagram illustrating the effect of cations on degradation of PCP by Pd/Fe nanoparticles.

Y.-h. Shih et al. / Applied Catalysis B: Environmental 105 (2011) 24–29

ducing enough hydrogen (or hydrogen atoms), which favors the hydrogenation reaction. In the presence of Fe2 (SO4 )3 , pH decrease is likely to be the main contributor to the degradation enhancement, not Fe3+ ions. Cations Cu2+ , Ni2+ , and Fe3+ could facilitate the rate constant of degradation kinetics and removal efficiencies of PCP by Pd/Fe nanoparticles in a sequence of Ni2+ > Fe3+ > Cu2+ as compared to those under the same concentration of SO4 2− ions. A schematic diagram illustrating the effect of cations on the degradation of PCP by Pd/Fe nanoparticles is shown in Fig. 6. The dissociation of water molecules by NZVI leads to hydrogen evolution and the formation of atomic hydrogen on the Pd surface. Atomic hydrogen then degrades PCP through a surface-mediated process to produce less chlorinated phenols. Cu2+ , Ni2+ , and Fe3+ ions were introduced in sulfate form. SO4 2− ions may form inner-sphere complexes which adsorb on Pd/Fe surfaces and impede degradation of PCP on the surface. Na+ ions showed no influence on degradation of PCP because Na+ cannot be involved in the dechlorination of PCP by Pd/Fe [34,39]. The positive standard potentials of the reduction of Cu2+ , Ni2+ , and Fe3+ ions coupled with the oxidation of Fe suggested that these redox reactions are spontaneous processes. Some portion of Cu2+ ions were reduced to zerovalent copper and coated on Fe particles. Cu coatings facilitated electron transfer and local corrosion of Fe [56]. Similarly, some portion of reduced form of Ni2+ was also deposited on Fe particles. Like palladium, nickel is known as hydrodehalogenation and hydrogenation catalyst [49,50]. Ni coatings which may act as new reactive sites could significantly enhance the degradation of PCP due to their catalytic effect. Hydrolysis of the added Fe2 (SO4 )3 resulted in the production of H+ ions and decreased the pH which likely accounted for the enhancement in degradation of PCP. 4. Conclusions The removal of PCP by synthesized Pd/Fe nanoparticles in the presence of different types and concentrations of cations was examined. Rapid degradation kinetics of PCP by synthesized Pd/Fe nanoparticles was observed. Na2 SO4 was found to reduce degradation performance. Since Na+ ion cannot affect degradation kinetics and removal efficiencies of PCP due to its high reduction potential, SO4 2− ions are the primary contributors to the inhibition of degradation. As compared with the degradation kinetics of Na2 SO4 , Cu2+ , Ni2+ , and Fe3+ also facilitate the rate constant of degradation kinetics and removal efficiencies of PCP by Pd/Fe nanoparticles in a sequence of Ni2+ > Fe3+ > Cu2+ . XANES absorption spectra were obtained to characterize the valance states of the cation coatings on Pd/Fe nanoparticles. Enhanced degradation of PCP was observed in the presence of and with increasing concentrations of Cu2+ ions as a result of the formation of zerovalent Cu on Pd/Fe surfaces. Owing to the catalytic effect of the reduced form of Ni on the Fe surface, the presence of Ni2+ ions significantly enhanced both the rate constant of degradation kinetics and removal efficiencies of PCP. These benefits even exceeded the inhibitory effect of the presence of SO4 2− ions. The addition of Fe3+ ions caused a decrease in pH which can explain the enhancement in the PCP degradation process observed. Our experimental results contribute to a better understanding of PCP degradation and serve as a useful reference for remediation design and prediction of treatment efficiency of PCP with Pd/Fe nanoparticles. Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council of Taiwan, ROC (Contract NSC 97-2313-B002-048-MY3).

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