Chemosphere 91 (2013) 165–171

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Reactivity of Pd/Fe bimetallic nanotubes in dechlorination of coplanarpolychlorinated biphenyls

0045-6535/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2012.12.037

⇑ Corresponding author. Tel.: +1 305 284 4021; fax: +1 305 284 5637.E-mail address: bachas@miami.edu (L.G. Bachas).

1 Elsayed M. Zahran is on academic leave: Department of Applied OrganicChemistry, National Research Center, Egypt.

Elsayed M. Zahran a,1, Dibakar Bhattacharyya b, Leonidas G. Bachas a,⇑a Department of Chemistry, University of Miami, Coral Gables, FL 33146, USAb Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA

h i g h l i g h t s

" Design of novel iron nanotubematerial decorated with palladiumnanoparticles.

" Pd/Fe nanotubes demonstrate highefficiency in dechlorination of PCBs.

" The size of the tube, Pd content, andtemperature affect thedechlorination efficiency.

" The Pd/Fe nanotubes are stable formultiple dechlorination cycles.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 August 2012Accepted 16 December 2012Available online 17 January 2013

Keywords:Polychlorinated biphenylsBimetallic nanotubesDechlorinationPalladium/iron

a b s t r a c t

A new class of bimetallic materials based on palladium-decorated iron nanotubes is described that dem-onstrates high reactivity in dechlorination reactions. This high dechlorination efficiency was attributed tothe high surface area to volume ratio of the hollow nanotubes structure. Herein, we evaluated the effectof different conditions, such as the nanotube size, and the palladium loading on the efficiency of thedechlorination of PCB 77, a model coplanar polychlorinated biphenyl (PCB), by the Pd/Fe bimetallic nano-tubes system. The efficiency of the dechlorination was lowered by decreasing the tube diameter from 200to 100 nm. In addition, the interior surface as well as the exterior surface of the as-synthesized Pd/Febimetallic nanotubes was found to contribute to the high efficiency of the dechlorination of PCB 77.The dechlorination of PCB 77 by Pd/Fe bimetallic nanotubes demonstrated small activation energy indi-cating diffusion controlled reaction. The as-prepared Pd/Fe bimetallic nanotubes showed extended life-time activity when used in multiple dechlorination cycles.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Polychlorinated biphenyls (PCBs) are a well-defined class ofenvironmental contaminants that have adverse effects on humanhealth as well as the ecosystem (Ross et al., 2000; Aliyu et al.,

2010). Consequently, much attention has been given to the designof methods that lead to the degradation of PCBs (Venkatachalamet al., 2008; Gorbunova et al., 2010). Several approaches, includingtraditional, biological, and chemical methods, have been followedfor the remediation of PCBs (Chang et al., 2006; Beyer and Biziuk,2009; Van Aken et al., 2010). Among the chemical methods, thereductive dechlorination by palladium/iron (Pd/Fe) bimetallicnanoparticles has been proven an effective remediation technology(Zhang, 2003; Choi et al., 2008). This high efficiency of dechlorina-tion is attributed to the surface area to volume ratio of the nano-

166 E.M. Zahran et al. / Chemosphere 91 (2013) 165–171

scale particles. However, owing to their high reactivity and mag-netic properties, nanoparticles tend to aggregate and lose their sur-face area, which impact the rate of the dechlorination reaction(Zhang, 2003; Nurmi et al., 2005). Consequently, Pd/Fe nanoparti-cles are usually produced in the presence of surfactant or embed-ded in solid supports to achieve high dechlorination efficiency(Bhattacharyya, 2007; Joo and Zhao, 2008). We addressed thischallenge from a different direction based on the hypothesis thatthe nanotube architecture exhibits higher surface to volume ratiothan nanoparticles even when aggregates are formed. We devel-oped a new class of nanostructured material based on bimetallicnanotubes of Fe decorated with Pd nanoparticles that had high sur-face to volume ratio (Zahran et al., 2011). The Pd/Fe bimetallicnanotubes demonstrated higher reactivity in dechlorination ofPCBs than the comparable nanoparticles.

Various factors affect the efficiency of dechlorination of PCBs byPd/Fe bimetallic nanostructures (Liu and Lowry, 2006; Xu and Bhat-tacharyya, 2008). For instance, decreasing the size of Pd/Fe particlesincreases the surface to volume ratio, which in turn enhances theefficiency of dechlorination (Wu et al., 2005). Nanoscale Pd/Fe parti-cles have demonstrated much higher reactivity in dechlorination ofPCBs than macroscale particles (Wang and Zhang, 1997). Further,increasing the palladium loading of the Pd/Fe nanostructures in-creases the amount of active sites available for catalysis, therebyincreasing the dechlorination reaction rate (Chun et al., 2010). Onthe other hand, the presence of excessive (more than one monolayercoating) amount of palladium in Pd/Fe nanostructured materialpassivates the iron surface for further reaction in aqueous mediumto produce hydrogen, which leads to low dechlorination efficiency.Thus, when designing new classes of material for catalytic dechlori-nation reactions, these factors should be evaluated experimentally.The objectives of the work described herein are (i) to evaluate the ef-fect of chlorine position in PCB 77 dechlorination by Pd/Fe bimetallicnanotubes, (ii) to study the effect of nanotube size on the efficiencyof dechlorination of PCB 77, (iii) to determine the effect of the hollowinterior of the Pd/Fe nanotubes on the efficiency of dechlorination ofPCB 77, (iv) to investigate the effect of palladium content on thereactivity of Pd/Fe nanotubes, and (v) to calculate the activation en-ergy of the dechlorination reaction. The stability of the as-preparedPd/Fe bimetallic nanotubes for multiple dechlorination cycles wasalso evaluated.

2. Experimental

2.1. Synthesis of Pd/Fe nanotubes

Pd/Fe bimetallic nanotubes with various outer diameters wereprepared according to a procedure described by us previously(Zahran et al., 2011). Briefly, polycarbonate membranes (PC) of47 mm diameter and pore sizes of 100, 200 and 400 nm (Millipore,Billerica, MA) were placed between the two compartments of acustom made U-tube reaction cell. In this design, the nanoporousmembrane separates the two precursors of the electroless reduc-tion; 1 � 10�3 M ferrous sulfate stabilized with 1 � 10�2 M ascor-bic acid and 1 � 10�2 M sodium borohydride. Upon the diffusionof these reaction precursors into the membrane pores, Fe(0) isdeposited on the membrane pore walls as a result of a localizedelectroless reduction reaction. The as-prepared Fe nanotubes wereextracted by dissolving the PC membrane in 10 mL methylenechloride, and washing 2 times with methylene chloride and2 times with ethanol. Palladium nanoparticles were deposited onthe surface of the Fe nanotubes by adding 1 mg of palladium ace-tate to 10 mL Fe nanotubes colloidal solution.

Pd/Fe bimetallic nanorods of 200 nm in outer diameter wereprepared by running the U-tube reaction for 10 h. Solid sodium

borohydride was added into the reductant side every 2 h to replacethe loss of borohydride because of its decomposition. This platformensured ultimate nucleation of the Fe(0) to fill the membranepores.

In situ Pd/Fe bimetallic nanotubes were prepared inside the PCmembrane. Polycarbonate membranes with 200 nm outer diame-ter Fe nanotubes were prepared according to the aforementionedprocedure. Subsequently, the membrane was washed 3 times withoxygen-free deionized water and 3 times with ethanol. Palladiumnanoparticles were deposited on the internal surface of the Fenanotubes by filling the U-tube reaction cell with ethanolic solu-tion of palladium acetate. After washing, these membranes wereused as is for dechlorination of PCB 77.

2.2. Characterizations of Pd/Fe nanotubes

A Philips XL30 field-emission environmental scanning electronmicroscope (Philips XL30 ESEM-FEG) running at 20 kV andequipped with an Oxford energy-dispersive X-ray detector wasused for characterization of the morphology of the as-preparedFe nanotubes. Transmission electron microscopy (TEM) images ofFe nanotubes and Pd/Fe nanotubes were obtained using a JEOL2010F field emission electron microscope equipped with an en-ergy-dispersive X-ray detector and running at an accelerating volt-age of 200 kV. A Varian Vista-Pro CCD Simultaneous ICP-OES wasused to determine the metallic composition of the Fe and Pd/Fenanoparticles and nanotubes.

2.3. Dechlorination experiment

The dechlorination of 3,30,4,40-tetrachlorobiphenyl (PCB 77), atetrachlorinated PCB congener, was performed in 65-mL or 20-mL glass vials with PTFE septa caps in which 60 mL or 18 mL of25 lM 50/50 (v/v) ethanol/water solution of PCB 77 was added.A calculated amount of Pd/Fe bimetallic nanotubes produced fromtwo PC membranes was added into each vial in order to have con-sistent metal loading of �0.25 mg mL�1. In the case of nanorods,two membranes provided a metal loading of 0.8 mg mL�1. Thehead space in all vials was purged with ultra-high purity nitrogen.All vials were placed on a VX-2500 multi-tube vortexer (VWR, Rad-nor, PA) at maximum agitation speed for the duration of the reac-tion. At specific time intervals, the glass vial was placed on a strongmagnet to collect the Pd/Fe nanotubes or nanorods at the bottomof the vial, while a sample of 1-mL was withdrawn using a glasssyringe. The sample was then combined with 1 mL hexanes in a5-mL glass vial in order to extract PCB 77 and dechlorination prod-ucts. The analysis of the samples was achieved using GC–MS (Agi-lent, Santa Clara, CA); dodecane was used as internal standard. Thereuse of the material for multiple dechlorination cycles was con-ducted by replacing the reaction solution with a fresh 25 lM 50/50 (v/v) ethanol/water solution of PCB 77 when ultimate dechlori-nation had been achieved.

3. Results and discussion

The U-tube template based method was used to prepare Fe(0)nanotubes with different outer diameters. Palladium particleswere post-deposited on the Fe nanotubes from an ethanolic solu-tion of palladium acetate. Following this procedure, we were ableto produce 100, 200, and 400 nm Pd/Fe bimetallic nanotubes(Fig. 1). These nanotubes were found to exhibit body-centered cu-bic crystalline structure with high degree of metallic properties(Zahran et al., 2011).

It is well established that the reductive dechlorination of PCBswith metallic iron based particles follows a stepwise mechanism,

A B

C D E

50 nm500 nm 500 nm

200 nm

Fig. 1. Electron microscopy images of Fe metallic and Pd/Fe bimetallic nanotubes: (A) SEM image of partially dissolved PC membrane with 200 nm Fe nanotubes, (B) TEMimage of 200 nm Fe nanotubes, (C) TEM image of 400 nm Pd/Fe nanotubes, (D) TEM image of 100 nm Fe nanotubes, and (E) high magnification image of 200 nm Pd/Fenanotube.

E.M. Zahran et al. / Chemosphere 91 (2013) 165–171 167

in which a chlorine atom is replaced by hydrogen at each step ofthe reaction (Yak et al., 2000; Agarwal et al., 2009). PCB 77 is a tet-rachloro coplanar PCB single congener with one chlorine at themeta and one chlorine at the para positions on each of the two ben-zene rings. Accordingly, the stepwise dechlorination mechanism ofPCB 77 can be represented by removing one chlorine atom at atime (see Fig. 1SI in Supporting Information) (Yak et al., 2000).All daughter dechlorination products of PCB 77 were detected,and their amounts were determined during the course of thedechlorination reaction using Pd/Fe bimetallic nanotubes. It hasbeen noted previously in the literature that the selectivity of thedechlorination reaction depends on the position of the chlorineatom, and it follows the order para > meta � ortho (Greaveset al., 1994; Miyoshi et al., 2000). Accordingly, the ratios of all

0

20

40

60

80

100

PC

B r

atio

0 h0.5 h1 h2 h3 h5 h24 h

Fig. 2. Dechlorination profile of PCB 77 with Pd/Fe nanotubes. The Z-axis valuesrepresent the % of PCB 77 and other dechlorination products relative to the carbonbalance.

daughters along with PCB 77 and biphenyl produced from thedechlorination of 25 lM PCB 77 using 0.25 mg mL�1 Pd/Fe bimetal-lic nanotubes were calculated relative to the carbon balance. Theseresults are depicted in Fig. 2. It is evident from the data thatdechlorination takes place preferentially at the para substitutedchlorines over the meta ones. For instance, the measured amountof PCB 35 with two meta-substituted chlorines is about 15-foldhigher than that of PCB 37, which has two para substituted chlo-rines. Similar meta/para substituted chlorines preference was no-ticed among the di- and mono-chloro-biphenyl products. Thistrend is consistent with previous reports that used zerovalent ironin subcritical water at 250 �C and 10 MPa to dechlorinate PCBs (Yaket al., 2000). However, the meta/para-substituted chlorines selec-tivity noticed here is 10-fold higher than the reported value, indi-cating a high reactivity of the Pd/Fe bimetallic nanotubes towardmeta-substituted chlorines.

3.1. Effect of nanotube size

As shown in Fig. 1, we synthesized Pd/Fe bimetallic nanotubesthat had outer diameters of 400, 200, and 100 nm. The synthesizednanotubes had a length, L, of 3–7 lm. Assuming an outer nanotubediameter of 2R and an inner diameter of 2r, the surface area ANT of ananotube can be calculated from the following equation:

ANT ¼ 2pLðRþ rÞ þ 2pðR2 � r2Þ

Because L� R, 2pLðRþ rÞ � 2pðR2 � r2Þ, and therefore

ANT � 2pLðRþ rÞ

The volume of this tube can be calculated from the followingequation:

30

168 E.M. Zahran et al. / Chemosphere 91 (2013) 165–171

VNT ¼ pLðR2 � r2Þ

Thereby, the surface-to-volume ratio, ANTVNT

, is equal to 2R�r. In other

words, the surface-to-volume ratio does not change with L as longas L � R of the tube, yet depends on the thickness (R � r) of thenanotube wall. Therefore, if there is no other factor affecting thedechlorination reaction, nanotubes of different diameters but hav-ing the same wall thickness should demonstrate similar dechlori-nation efficiencies. To investigate this hypothesis, thedechlorination of a 25 lM solution of PCB 77 was carried out usingPd/Fe bimetallic nanotubes having outer diameter of 400, 200 and100 nm. As shown in Fig. 3, complete dechlorination of PCB 77 wasachieved within 10 or 24 h of the reaction using 400 or 200 and100 nm Pd/Fe bimetallic nanotubes, respectively. The plots of�ln(C/C0) vs. time were linear for the first 3 h, indicating a pseu-do-first order kinetic behavior.

lnCPCB

C0PCB

¼ �kobst

Mass-normalized rate constants, k1, were calculated for differ-ent Pd/Fe bimetallic nanotubes based on the observed rate con-stant, kobs, and the metal loading according to the followingequation:

lnCPCB

C0PCB

¼ �kobst ¼ �k1qmt

In this approach, the values of k1 reflect the difference in thesurface area to volume ratio between different nanotubes (Fangand Al-Abed, 2008). It was found that there was no significant dif-ference between the rates of the dechlorination reactions whenusing 400 or 200 nm nanotubes with rate constants, k1, of 3.96and 4.12 L g�1 h�1, respectively (Fig. 3 inset). On the other hand,the 100 nm Pd/Fe nanotubes demonstrated a slower dechlorina-tion rate with k1 of 2.84 L g�1 h�1. This decline in the efficiencyof the dechlorination is attributed to an increase of the nanotubewall thickness. It might be also attributed to limited diffusion ofthe reactants into the internal surface of the nanotube as the tubeinner diameter becomes smaller.

The hollow interior of the nanotube architecture provides withhigh surface-to-volume ratio for the catalytic reaction. In order toevaluate the effect of the internal surface of the nanotube on theefficiency of the dechlorination reaction, Pd/Fe bimetallic nano-tubes were prepared inside the PC membrane. In such architecture,the external surface of the Fe nanotube is obstructed by the PC

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Time, h

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4

-ln(

C/C

0 )

Time, h

PC

B 7

7, µ

M

Fig. 3. Dechlorination of 25 lM PCB 77 using Pd/Fe nanotubes of different outerdiameter sizes with 0.25 mg mL�1 metal loading: (N) 400 nm Pd/Fe nanotubes, (�)200 nm Pd/Fe nanotubes, and (d) 100 nm Pd/Fe nanotubes. Inset: linear fit kineticsof the dechlorination reaction, where C0 is the initial concentration of PCB 77, and Cis the concentration at the corresponding time.

membrane template, and only the internal surface is available forpalladium deposition and dechlorination reaction. On the otherhand, in order to evaluate the effect of the external surface of thePd/Fe bimetallic nanotubes on the efficiency of the dechlorinationreaction, Pd/Fe nanorods of 200 nm diameter were also synthe-sized. The dechlorination results of 25 lM PCB 77 using 200 nmPd/Fe bimetallic nanotubes, 200 nm Pd/Fe bimetallic nanotubesin membrane, and 200 nm Pd/Fe bimetallic nanorods are shownin Fig. 2SI (Supporting Information). The free Pd/Fe bimetallicnanotubes displayed high dechlorination rate with 95% degrada-tion of PCB 77 in the first 5 h of the batch reaction. On the otherhand, the Pd/Fe bimetallic nanorods and the Pd/Fe bimetallic nano-tubes in membrane (i.e., those that had Pd only in the interior sur-face) exhibited much slower rates, with about 90% and 75%degradation in 24 h for nanorods and nanotubes in membrane,respectively. Rate constants k1 were calculated from the linear bestfit line in the first 2 h of the reactions to be 4.8, 0.68, and1.72 L g�1 h�1 for free nanotubes, nanorods, and nanotubes inmembrane, respectively. These values indicate that both the inter-nal and the external surface of the hollow tube contribute to thehigh efficiency of dechlorination of the synthesized Pd/Fe bimetal-lic nanotubes.

3.2. Effect of palladium loading

The amount of the catalyst plays a vital role in the dechlorina-tion reaction. Therefore, the optimum amount of the catalystshould be identified experimentally. As described previously (Zah-ran et al., 2011), palladium particles were deposited as a uniformlydistributed monolayer on the surface of Fe nanotubes. This archi-tecture provides high concentration of active sites for the catalytichydrodechlorination reaction between PCB 77 molecules and ac-tive hydrogen. To study the effect of palladium content on the reac-tivity of the as-synthesized Pd/Fe bimetallic nanotubes, fourbatches of nanotubes were synthesized with different palladiumcontent. The Pd wt.% in these batches was determined by ICP-AES analysis as 0.9, 1.9, 3.2, and 5.1 wt.%. Four batch dechlorinationreactions of 25 lM PCB 77 solutions were conducted using consis-tent metal loading of 0.25 mg mL�1 of Pd/Fe bimetallic nanotubeswith different contents (Fig. 4). kobs values of 0.58, 1.22, 1.84, and2.25 h�1 were determined for Pd/Fe nanotubes with 0.9, 1.9, 3.2,

0

5

10

15

20

25

0 5 10 15 20 25 30

Time, h

0

0.5

1

1.5

2

2.5

0 2 4 6

k obs

,h-1

Pd loading, wt/wt%

PC

B 7

7, µ

M

Fig. 4. Dechlorination of 25 lM PCB 77 using (metal loading of 0.25 mg mL�1) Pd/Febimetallic nanotubes with 200 nm external diameter and different wt.% ofpalladium: (N) 0.9 wt.% palladium, (j) 1.9 wt.% palladium, (�) 3.2 wt.% palladiumand (d) 5.1 wt.% palladium. Inset: Effect of palladium loading on the rate ofdechlorination of PCB 77 by Pd/Fe bimetallic nanotubes with 200 nm externaldiameter.

E.M. Zahran et al. / Chemosphere 91 (2013) 165–171 169

5.1 Pd wt.%, respectively. A batch reaction with iron nanotubes(0 wt.% Pd) showed insignificant dechlorination of PCB 77. Plottingthe values of the rate constant, kobs, against the Pd wt.% (Fig. 4, in-set) indicated linear relationship between kobs and Pd content inthe Pd/Fe bimetallic nanotubes. The deviation from linearity after3 wt.% Pd might be attributed to the number of catalytic activesites becoming more than that required for the reaction. In addi-tion, high palladium coverage on the Fe(0) surface of the nanotubeblocks Fe(0) from reacting with water to produce the hydrogen re-quired for the dechlorination.

3.3. Effect of temperature

For most chemical processes, temperature is a factor that influ-ences the rate of the reaction. For instance, the dechlorination ofPCBs with zerovalent iron particles has demonstrated very slowreaction rates. Therefore, severe conditions of pressure and tem-perature (400 �C) were required to achieve complete dechlorina-tion (Chuang et al., 1995). Generally, a catalyst is used todecrease the activation energy, Ea, of a chemical process. Dopingthe zerovalent iron with catalytic metal particles such as palladiumor nickel made the dechlorination of PCBs accessible at ambientconditions (Schreier and Reinhard, 1995; Siantar et al., 1996). Eval-uation of the activation energy of the reaction provides insight onthe mechanism of the reaction (Su and Puls, 1999; Xu and Bhatta-charyya, 2007). It is generally accepted that reactions that involve aphysical process as the rate determining step exhibit small activa-tion energies (Ea < 30 kJ mol�1) (Laidler, 1987). On the other hand,high activation energy indicates that the reaction is controlled byreaction at the metal-water interface (Laidler, 1987). For instance,Xu and Bhattacharyya reported Ea = 24.5 kJ mol�1 for the dechlori-nation of dichlorobiphenyl by membrane-supported Pd/Fe nano-particles suggesting a diffusion-controlled reaction (Xu andBhattacharyya, 2007). On the other hand, Tratnyek and co-workers

Iron oxides/hydroxide

Iron metal

Fig. 5. TEM images of the Pd/Fe nanotubes after being used for eight dechlorination cycbox denotes a reference area to track the spatial drift. (For interpretation of the referenarticle.)

found that the dechlorination of CCl4 by zerovalent iron particles isa chemical process controlled reaction (Scherer et al., 1997). Theeffect of temperature on the bimetallic nanotubes system de-scribed here was investigated by conducting two batch dechlorina-tion reactions of PCB 77 at 25 �C and 40 �C. Values of kobs of 0.8 and1.4 h�1 were calculated from the liner fit line of the dechlorinationreactions at 25 �C and 40 �C, respectively. Subsequently, the appar-ent activation energy was calculated according to Arrhenius equa-tion to be 29 kJ mol�1. This value indicates that the diffusion of PCB77 through the solution to the palladium surface is the rate-deter-mining step of the dechlorination reaction with Pd/Fe bimetallicnanotubes.

3.4. Stability and catalyst reuse

Long stability of the Pd/Fe nanostructures is very important forin situ remediation. However, owing to the high reactivity of thesenanostructures in aqueous solution, their dechlorination efficiencyis diminished as a result of surface oxidation and deposition of ironoxides/hydroxide (Sarathy et al., 2008; Choi et al., 2009; Yan et al.,2010). To study the long term reactivity of the as-synthesized Pd/Fe bimetallic nanotubes, eight dechlorination cycles of PCB 77were conducted with the same amount of Pd/Fe bimetallic nano-tubes. Complete dechlorination was achieved within the first12 h of the reaction for the first 4 cycles, while 90% dechlorinationwas noticed within 24 h for the last 4 cycles (Fig. 3SI SupportingInformation). In order to investigate the cause of this deactivation,fresh amount of Fe nanotubes and palladium nanoparticles wereindividually added to two vials with the Pd/Fe bimetallic nano-tubes after eight dechlorination cycles. Subsequently, an amountof 50 mL of 25 lM PCB 77 was added to each vial and the batchdechlorination reaction was run as described in Section 2. The vialwith fresh Fe nanotubes showed 75% dechlorination in 2 d of thereaction. On the other hand, no dechlorination was noticed for

Fe map

O2 map

Pd map

les. The green box denotes the area scanned with the electron beam and the yellowces to color in this figure legend, the reader is referred to the web version of this

170 E.M. Zahran et al. / Chemosphere 91 (2013) 165–171

the vial with fresh palladium nanoparticles. This indicates that thedecline in the dechlorination efficiency is attributed to the deposi-tion of iron oxides/hydroxide on the nanotubes surface that limitsfurther reaction. Further, TEM images were acquired for Pd/Fenanotubes after dechlorination. We found that some short lengthnanotubes were still present after being used in eight dechlorina-tion cycles, and most of the nanotubes had opened forming nano-sheets (Fig. 5). However, such transformation (nanotube/nanosheet) maintains high exposed surface area, which explainsthe long lifetime dechlorination reactivity of these nanotubes.HRTEM imaging shows core/shell architecture with different crys-talline structures, which are attributed to a core of iron metal cov-ered with a shell of iron oxides/hydroxide. This was further provenby EDS analysis, which showed a broad peak at 0.5 keV. However,quantification of the oxygen percentage could not be achievedfrom the EDS spectrum because of the overlap between the oxygenpeak and the Fe La peak. Therefore, to investigate the ensemble ofthe iron, palladium, and iron oxides/hydroxide in the material afterdechlorination, EDS mapping in dark field STEM mode was ac-quired. In this case, the iron Ka peak, which is very distal fromthe oxygen peaks, was collected along with the Pd La and oxygenKa while scanning a small area of the material with the 1-nm elec-tron probe. As evident from Fig. 5, the palladium map indicates thepresence of Pd in the nanotubes after eight dechlorination cycles,which is consistent with the aforementioned dechlorination exper-iments. The composed mapping image in Fig. 5 indicates the pres-ence of a surface layer of iron oxide on a core of metallic iron. Thislayer limits the interaction of the Pd/Fe nanomaterial with thereactants.

4. Conclusions

Pd/Fe bimetallic nanotubes demonstrated high efficiency indechlorination of the coplanar 3,30,4,40-tetrachlorobiphenyl(PCB77). This efficiency was reflected on the stepwise mechanismof PCB 77 dechlorination by remarkably high selectivity for para-substituted PCBs over meta-substituted ones. Pd/Fe bimetallicnanotubes of 100 nm outer diameter showed reduced dechlorina-tion efficiency in comparison with the 200 and 400 nm ones. Thehollow interior of the as-synthesized Pd/Fe bimetallic nanotubesalso contributes to their reactivity. Increasing the palladium con-tent of the bimetallic nanotubes increases the rate of the dechlori-nation reaction in a linear fashion up to a content of 3 wt.%, whilefurther increasing the Pd ratio had lesser effect. The activation en-ergy of the dechlorination reaction of PCB 77 by Pd/Fe bimetallicnanotubes was found to be 29 kJ mol�1, suggesting that the diffu-sion of PCB 77 through the solution to the palladium surface isthe rate-determining step. The as-synthesized Pd/Fe bimetallicnanotubes displayed extended lifetime reactivity in dechlorinationreactions.

Acknowledgments

The authors acknowledge support from the NIEHS-SRP program(P42ES007380). Partial funding from the University of Miami isalso acknowledged. E.M.Z acknowledges the National ResearchCenter, Egypt for granting an academic leave.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2012.12.037.

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