Adsorption and Electrocatalytic Dechlorination of Pentachlor

Separation and Purification Technology 47 (2005) 73–79

Adsorption and electrocatalytic dechlorination of pentachlorophenolon palladium-loaded activated carbon fibers

Chunyue Cui, Xie Quan ∗, Shuo Chen, Huimin ZhaoSchool of Environmental and Biological Science & Technology, Linggong Road, Dalian University of Technology, Dalian 116024, China

Received 22 November 2004; received in revised form 26 May 2005; accepted 2 June 2005

Abstract

The palladium-loaded activated carbon fiber (Pd/ACF) adsorption/electroreduction process was employed for electrocatalytic dechlorinationof pentachlorophenol (PCP) solution. PCP in water was adsorbed onto Pd (4.2 wt.%)/ACF first and then the PCP was degraded at a constantcurrent. At 25 mA/cm2, the degradation of PCP was 94.8% after electrolysis of 200 min and phenol was the main product with a yield of86.1% and a current efficiency of 17.4%. After being reused three times, the Pd (4.2 wt.%)/ACF cathode still had acceptable electrocatalyticactivity.© 2005 Published by Elsevier B.V.

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eywords: Electrocatalytic dechlorination; Activated carbon fiber; Palladium; Pentachlorophenol

. Introduction

Contamination of water and soil by chlorophenolic com-ounds now represents a significant environmental burdenue to their widespread industrial use, inherent toxicity andersistence in the environment [1]. Chlorophenols are usuallysed as the intermediates for organic synthesis, wood preser-atives and pesticides. The toxicity of these compounds isonnected with the chlorine content, and their biodegrad-bility is strongly affected by chlorine substitution in theolecules. Most chlorophenols are listed as priority pollu-

ants by the US EPA [2].Highly chlorinated aromatic compounds are recalcitrant

owards disposal. Biological treatment of low concentrationf such materials in aqueous waste streams often leaves thehlorinated compounds untreated or required a long time, e.g.everal days [3,4]. The chemical hydrodehalogenation meth-ds have been developed based on zero-valent metals, suchs iron, known as dissolving metal reductions [5–8]. Iron haseen used to dechlorinate compound containing one or twoarbon atoms [5,6]. Treatment of halogenated aromatic com-ounds has been suffered slow reaction rate [7,8]. Therefore,

it is desirable to develop efficient and safe alternative tech-nology for the dechlorination of chloroaromatic compounds.

Recently, the electrochemical reductive approach hasbeen suggested as a promising method for detoxificationof chlorine-containing aromatic hydrocarbons [9–14]. Thismethod ensured the selective removal of chlorine atoms undermild experimental conditions without using the highly reac-tive reducing agents. As a result, the much less toxic dechlo-rinated products could be treated further by more convenientand economic means, for example, biodegradation. In par-ticular, electrochemical processes have been developed forthe destruction of carbon–halogen bonds. However, in manyeases, the effective performance required the use of non-aqueous reaction media mainly aprotic solvents [15–19] orenvironmentally unpleasant cathode materials, such as mer-cury or lead [9,10,15], which made such methods unattrac-tive. Recently, the dehalogenation of aromatic compoundsin aqueous medium by electrochemical hydrodehalogena-tion using Pd as catalyst has been reported [11–14]. Dabo etal. reported 99% conversion of pentachlorophenol to phenolover Pd (5 wt.%)/Al2O3 entrapped in a reticulated vitreouscarbon electrode at 75 ◦C [12]. In general, electrochemi-

∗ Corresponding author. Tel.: +86 411 84706140; fax: +86 411 84708084.E-mail address: [email protected] (X. Quan).

cal hydrodehalogenation of low concentration compounds inaqueous solution observed low current efficiency and highenergy consumption. These features render the approach

383-5866/$ – see front matter © 2005 Published by Elsevier B.V.oi:10.1016/j.seppur.2005.06.005

74 C. Cui et al. / Separation and Purification Technology 47 (2005) 73–79

unattractive with respect to the treatment of large volumesof organic wastewater with low concentrations.

Activated carbon fiber (ACF) is widely used for waterpurification due to its high adsorption capacity and fastadsorption rates for organics [20]. And Pd is usuallyemployed as catalyst loaded on inert supports like carbonfor the electrochemical reactions [21]. In present work, wereported an adsorption/electroreduction process using Pdloaded ACF for the treatment of pentachlorophenol (PCP).Special attentions were paid to the adsorption capacity ofPd/ACF, the catalytic activity and the influence of currentdensity on the efficiency of PCP dechlorination.

2. Experimental

2.1. Chemicals and materials

The following chemicals were used as received: pen-tachlorophenol (PCP, >99%, Fanghua Agent Company,China); tetramethyl-ammonium chloride (TMAC1, >99%,Kemiou Agent Company, China); methanol, acetone,dichloromethane and acetonitrile (HPLC-grade, Tedia AgentCompany, USA); PdCl2 (>99%, Tianjin Chemical ReagentCompany, China).

ACF made from polyacrylonitrile resin was supplied byA

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Fig. 1. Two-compartment cell used for the electrolyses. (A) Pt anode; (B)cation exchange membrane; (C) Pd/ACF cathode; (D) magnetic bar; (E)reference electrode; (F) potentiostat.

counter-electrode was a platinum plate with the dimension of10 mm × 10 mm × 0.2 mm. Anodic compartment was filledwith 1 M NaOH supporting electrolyte. A saturated calomelelectrode (SCE) was used as the reference electrode. Theelectrolysis was performed at a constant current and 40 ◦C. Apotentiostat (DJS-292, Shanghai Rex Instrument Co., China)was used as power source.

2.2.3. Analytical methodPCP concentrations in solution and on the Pd/ACF elec-

trode were measured at different intervals during electrolysisseparately. Pd/ACF electrode was extracted with 25 mL mix-ture of acetone and dichloromethane (1:1, v/v) plus 0.2 mL30% HC1 (shaking at 250 rpm for 4 h). The above proce-dure was repeated six times. The solvent fractions werecombined and dried on a rotary evaporator (Shanghai Bio-chemistry Instrument Co., China). The resulted sample wasdissolved in acetonitrile and analyzed by HPLC (PU-1580,UV-1575, Jasco, Japan) at 220 nm with a Kromasil ODScolumn (250 mm × 4.6 mm, 5 �m). The mobile phase wasacetonitrile:water (0.1% acetic acid) = 3:2 (v/v) at a flow rateof 1 mL/min. The volume of the injected sample was 20 �L.

Identification of intermediates was carried out with an HP6890 gas chromatography (GC) coupled with 5973N massselective detector (MSD) with a capillary column (HP-5 MS,30 m × 0.25 mm × 0.25 �m, Agilent, USA). The flow rate ofcai1st7

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nshan Activated Carbon Fiber Company, China.

.2. Apparatus and procedures

.2.1. Preparation of Pd/ACF electrodeAfter cleaned with deionized water, the ACF was boiled

n 1 M HNO3 for 3 h, then rinsed with deionized water andried for 24 h at 100 ◦C. The surface area of the ACF wasetermined by the BET method.

PdCl2 with various concentrations in 1 M HCl solutionsere deposited into the ACF by incipient-wetness method.he Pd-impregnated ACF was then dried at 80 ◦C for 4 hnd was electroreduced in 0.3 M H2SO4 solution. Surfaceharacterization of the Pd/ACF was performed by Scanninglectron Microscope (SEM, JSM-5600L, UK) and X-rayiffractometer (XRD, LabxXRD-6000, Shimadzu, Japan)sing Cu K� as radiation. Contents of Pd were determinedy ICP-AES (Advantage, IRIS, USA.).

.2.2. Electrocatalytic reactorElectrocatalytic dechlorination of PCP was carried out

n a Teflon-made H-cell divided into two compartments byation-exchange membrane (Nafion N324, Dupont Com-any, USA), as shown schematically in Fig. 1. The volumef each electrolyte compartment was 50 mL. The workinglectrode was a piece of Pd/ACF (15 mm × 15 mm × 3mmnd 70 mg in weight), which was fixed on tile titaniumesh. The working electrode was placed in the cathodic

ompartment. The catholyte was a 40% methanol aqueousolution with 0.25 M TMAC1 supporting electrolyte. The

arrier gas (He) was 1.0 mL/min. A solvent delay at 8 min,nd full scan mode was used. The oven temperature wasncreased from 40 to 260 ◦C (hold for 20 min) at a rate of5 ◦C/min. The injector temperature was 260 ◦C, and the ionource temperature was 230 ◦C. The MSD was operated withhe electron impact (El) mode with the electron energy being0 eV.

. Results and discussion

.1. Characterization of ACF

The structural parameters of ACF were calculated fromhe nitrogen isotherms. Surface area (SBET) was 1335.4 m2/g,

C. Cui et al. / Separation and Purification Technology 47 (2005) 73–79 75

Fig. 2. SEM micrographs of (A) virgin ACF and (B) Pd (4.2 wt.%).

Fig. 3. XRD patterns of (A) virgin ACF and (B) Pd (4.2 wt.%)/ACF.

micropore volume was 0.5857 cm3/g and average pore diam-eter was 2.13 nm.

3.2. Distribution of palladium on the ACF

The SEM micrographs of the ACF and Pd (4.2 wt.%)/ACFare shown in Fig. 2. The ACF (Fig. 2A) appeared as flatfibers without any visible pores. The uniformly dispersed Pdparticles were seen on ACF (Fig. 2B).

Fig. 3 shows the XRD patterns of the Pd (4.2 wt.%)/ACFwhere peaks for Pd particles were observed [22] while, asexpected, no Pd signal for the ACF.

3.3. Adsorption kinetics and isotherms of PCP on ACFand Pd/ACF

The adsorption kinetics curves of PCP on ACF and Pd(4.2 wt.%)/ACF are plotted using 100 mL of 200 mg/L PCPsolution in a series of 250 mL flasks under the conditionsof 20 ◦C and 250 rpm. Experimental data was applied to theLagergren rate equation [23]:

qt = qe(1 − e−kLt) (1)

where qe and qt (both in mg/g) are the amounts of PCPadsorbed at equilibrium and at any given time t (h), respec-tively, and kL (h−1) is the Lagergren rate constant. Fig. 4shows the adsorption kinetic curves of PCP on the ACF andPd (4.2 wt.%)/ACF. The parameters were calculated by non-linear regression (Table 1). From Fig. 4 and Table 1, it could

Fig. 4. Adsorption kinetics of PCP on ACF and Pd (4.2 wt.%)/ACF at 20 ◦C.


Table 1The Lagergren parameters for PCP on ACF and Pd (4.2 wt.%)/ACF at 20 ◦C

qe (mg/g) kL (h−1) r2

ACF 273.7 0.22 0.996Pd (4.2 wt.%)/ACF 267.9 0.19 0.998

Fig. 5. Adsorption isotherm of PCP on ACF and Pd (4.2 wt.%)/ACF at 20 ◦C.

be seen that the load of 4.2% of Pd did not lead to significantchanges in the adsorption rate.

The adsorption isotherms of PCP on ACF and Pd(4.2 wt.%)/ACF at the temperature of 20 ◦C and pH 7.50 weremeasured according to the Mollah method [24] (Fig. 5). Theadsorption isotherms can be correlated well by the Freundlichmodel:

Q = kFC1/n (2)

where Q is adsorption capacity (mg/g), C the equilibrium con-centration in solution (mg/L), n (dimensionless) the empiricalparameter that represents the heterogeneity of the site ener-gies and kF (mg/g) (L/mg)1/n the unit capacity factor. kF andn are indicators of adsorption capacity and adsorption inten-sity, respectively. Fig. 5 shows the adsorption capacity wasslightly affected by the impregnation with Pd (4.2 wt.%), andTable 2 displays the corresponding parameters calculated forthe adsorption isotherms. The results implied that ACF has anexcellent adsorption capacity for PCP in solution; for exam-ple, a 1 mg/L PCP solution produces an equilibrium loadingof about 174 mg PCP/g of Pd (4.2 wt.%)/ACF at 20 ◦C. Suchhigh adsorption capacity is crucial considering the removalof PCP from the aqueous solution.

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3.4. Electrocatalytic dechlorination of PCP on Pd/ACFat various current densities

PCP was adsorbed on the Pd (4.2 wt.%)/ACF before elec-trolysis by mixing 100 mL PCP solution (200 mg/L, pH 7.50)in a 250 mL flask. The flask was sealed and placed in a ther-mostatic shaker (250 rpm, 20 ◦C) for 25 h.

The degradation of PCP and yield of phenol were calcu-lated by the Eqs. (3) and (4):

degradation of PCP = mads − (msol + mACF)

mads× 100% (3)

where the mads was the initial amount of PCP adsorbed onthe Pd (4.2 wt.%)/ACF before electrolysis; msol and mACFwere the amounts of PCP residue in the catholyte and on thePd/ACF after electrolysis, respectively,

yield of phenol = Mphenol

MPCP× 100% (4)

where the MPCP was the initial amount (in moles) of adsorbedPCP and Mphenol were the amount of phenol in the catholyteand on the Pd/ACF after eletrolysis.

The degradation of PCP and the yield of phenol fromelectrocatalytic dechlorination over the cathode of Pd(4.2 wt.%)/ACF at different current densities are presentedin Fig. 6. As seen from Fig. 6, the higher current density,

Fig. 6. Dechlorination of PCP at Pd (4.2 wt.%)/ACF (PCP adsorption240 mg/g) cathode at different current densities: catholyte volume 50 mL;0.25 M TMACl; MeOH 40% (v/v); temperature 40 ◦C.

able 2he Freundlich isotherm parameters for PCP on ACF and Pd (4.2 wt.%)/ACFt 20 ◦C

kF (mg/g) (L/mg)1/n n r2

CF 191.7 6.36 0.998d (4.2 wt.%)/ACF 170.1 6.00 0.996


Table 3The electrochemical reduction of PCP at various current densities

Current density(mA/cm2)

Cathode potential(V)

pH Current efficiencyof phenolproduction (%)

10 −0.90 to −1.06 7.50–7.88 22.115 −1.13 to −1.28 7.50–8.91 21.020 −1.32 to −1.44 7.50–9.25 19.825 −1.47 to −1.59 7.50–9.87 17.4

the higher degradation efficiency and yield of phenol. 94.8%of degradation was obtained after 200 min of electrolysis at25 mA/cm2, while the value was 63.8% at 10 mA/cm2. Andthe yield of phenol was 86.1% at 25 mA/cm2 being about twotimes of that at 10 mA/cm2.

Table 3 shows that the current efficiency of completedechlorination to phenol was significantly affected by theapplied current density. The highest current density led tothe lowest current efficiency. High current density need morenegative potential to be applied. This fact may be related tothe competition of side reactions, mainly hydrogen evolu-tion, which was not favorable for the contact of PCP withthe electrode [13]. These kinds of side reactions will result inlower efficiencies at higher current density and more nega-tive potential. In present experiment, the production of H2 atpotential of −1.20 V was observed visually. The pH of solu-tion increased gradually with the increase of current density,suggesting that the rate of water electrolysis increase with thecurrent density.

3.5. Catalytic activity of the catalyst

The Pd (4.2 wt.%)/ACF working electrode was used threetimes at the current density of 25 mA/cm2 to test the cat-alytic activity. After each run, the used Pd/ACF was shaken at2(rpicwnumenitTtaafci

Fig. 7. Relation between degradation of PCP (adsorption 200 mg/g) andcatalyst reused times for (a) fresh catalyst; (b) once used catalyst; (c) two-time used catalyst at Pd (4.2 wt.%)/ACF cathode at 25 mA/cm2; 0.25 MTMACl; MeOH 40% (v/v); temperature 40 ◦C.

3.6. Identification of intermediate products by GC/MS

The extracts from the Pd/ACF electrode and electrolytesolution were analyzed by GC/MS and main results areshown in Fig. 9. After 40 min of electrolysis, five kindsof main dechlorination products were detected includ-ing tetrachlorophenols (TetraCP), trichlorophenols (TriCP),

Fig. 8. SEM micrograph of three times used Pd/ACF.

50 rpm for 4 h in a mixture of acetone and dichloromethane1:1, v/v) plus 0.2 mL 30% HCl. The above procedure wasepeated six times to remove the adsorbed organic com-ounds. The analysis shows that no phenols were detectedn the sixth extract. Fig. 7A shows that the degradation effi-iency decreased from 94.8 to 65.1% when the electrodeas used three times. Fig. 7B shows that the yield of phe-ol decreased from 86.1 to 37.3% after the electrode wassed three times. The three times used Pd/ACF still had auch higher electrocatalytic activity than bare ACF consid-

ring that PCP degradation efficiency was only 36.5% ando phenol was detected with the bare ACF acting as work-ng electrode. The results of this experiment indicate thathe Pd plays an important role in the dechlorination of PCP.he SEM micrograph of used Pd/ACF is given in Fig. 8, and

here is no evident difference between the images of usednd fresh Pd/ACF (Figs. 8 and 2B). The results of ICP-AESnalysis show that 10.5% of the initial amount of Pd leachedrom Pd/ACF after being used three times. The decrease ofatalytic activity may be attributed to the loss of Pd or thenactivation of catalyst.


Fig. 9. GC/MS analysis of electrocatalytic dechlorination of PCP on Pd (4.2 wt.%)/ACF cathodes at 25 mA/cm2: (A) 0 min; (B) 40 min; (C) 100 min; (D)200 min.

Fig. 10. Degradation pathways of PCP on Pd (4.2 wt.%)/ACF Electrode.

dichlorophenols (DiCP), chlorophenols (CPL) and phenol(PNL). After 200 min, only phenol and small amount ofchlorophenols were detected. On the basis of the aboveresults, the dechlorination pathways of PCP in Pd/ACF cath-ode are proposed as Fig. 10.

4. Conclusions

Pd/activated carbon fiber showed high adsorption capac-ity for PCP (kF = 170.1 (mg/g) (L/mg)1/6, n = 6.00) and highdechlorination efficiencies of up to 94.8% with phenol as the


main products (86.1% in yield). A further improvement instability of the Pd/ACF is required.

Acknowledgments

This work was financially supported by Nature Sci-ence Foundation of China (20337020) and National BasicResearch Program of China (2003CB415006).

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Adsorption and Electrocatalytic Dechlorination of Pentachlor