Environ. Sci. Technol. 2007, 41, 7503-7508
Electrocatalytic Hydrodechlorination of 2,4,5-Trichlorobiphenyl on a Palladium-Modified Nickel Foam Cathode BO YANG, GANG YU,* AND JUN HUANG Department of Environmental Science and Engineering, POPs Research Centre, Tsinghua University, Beijing 100084, China
Palladium-modified materials have been found to be effective electrodes for the reductive degradation of chlorinated compounds in aqueous solution. This study investigated the electrocatalytic hydrodechlorination (ECH) of polychlorinated biphenyls (PCBs) in solvent/surfactantaided solutions in a palladium-modified nickel foam electrode using a divided flow-through cell. The reaction pathways of 2,4,5-PCB hydrodechlorination were proposed due to the analysis of intermediates by GC/MS. The mechanism of electrocatalytic reaction on the Pd/Ni foam cathode was examined by studying the effect of surfactant type, sorption behavior of PCBs on the electrode, and current densities on the ECH efficiency of PCBs. The conversion of PCBs was controlled by the micelle structures of the surfactants instead of the charged species. According to the analysis of hydrogen transformation processes on the electrode surface, we propose that the ECH process was initiated by the transfer of highly active hydrogen atoms [H] from the prior polarized Pd particles to the less polarized Pd particles by spillover on the Pd/Ni foam cathode. Therefore, the total available surface was larger than the originally polarized surface, and [H] could smoothly react with PCBs that were adsorbed on the surface. As a result, a high ECH efficiency can be achieved with the Pd/ Ni foam electrode.
Introduction Polychlorinated biphenyls (PCBs) are typical persistent organic pollutants (POPs), which are listed in the Stockholm Convention as priority chemicals for eventual elimination all over the world by 2025. Among a variety of promising chemical methods for the detoxification of PCBs, the reductive dechlorination methods attract more research attention than oxidative methods due to their environmental compatibility, selectivity, and cost effectiveness (1). Generally, the completely dechlorinated products of PCBs would not possess the characteristics of POPs and could be degraded easily by other cost-effective (e.g., biologic) treatment processes. The most common ways of reductive dechlorination for chloroorganics are the methods based on zerovalent iron (ZVI) particles, which are extremely chemically reactive toward toxic chlorinated solvents (2). However, the method suffers from very slow reaction rates for the more refractory PCBs under ambient temperature and pressure, for example, nanosized ZVI dechlorinated PCBs with congener half-lives * Corresponding author phone: (+86-10)62796960; fax: (+8610)62794006; e-mail:
[email protected]. 10.1021/es071168o CCC: $37.00 Published on Web 10/03/2007
2007 American Chemical Society
ranging from 40 days to 77 years (3). The bimetallic microsized particles of palladized iron (Pd/Fe) can promote the dechlorination rate of PCBs due to their reaction with highly reductive atomic hydrogen [H] adsorbed on a Pd catalyst, which is produced via the oxidation of H+ or H2O by ZVI (4). Wang et al. and Korte et al. (4, 5) found that, in an aqueous solution containing 10-40% of methanol, ethanol, and/or acetone, approximately 10-20 h was required for the complete dechlorination of PCBs when the initial concentration ranged from 5 to 100 ppm. However, Jovanovic et al. (6) reported appreciable deactivation after the extended use of a Pd/Fe bimetallic catalyst in aqueous solution, typically after several hours of continuous dechlorination, which is due to the dissolution loss of an Fe base, iron hydroxide precipitation, and extensive H2 formation. Recently, electrocatalytic hydrodechlorination (ECH) with a Pd-modified cathode has successfully been applied to treat a great variety of chloroorganics in water containing chlorophenols or chlorobenzenes (7, 8). However, few studies explored the degradation of more refractory PCBs and other POPs in aqueous solution. Bonin et al. (9) proposed that ECH has advantages over ZVI and bimetallic catalysis because it involves the electrochemical reduction of H+ or H2O to produce continuously adsorbed nascent [H]. In addition, the cathode only provides the reaction surface without involving oxidative corrosion, thereby improving the control ability of the hydrogen evolution reaction (HER) and extending the catalyst’s longevity. Recently, a Pd-modified metal electrode was first applied to dechlorinate PCBs in our work, and a high conversion efficiency was achieved (10). However, the reaction mechanism, including the PCB degradation pathway and the electrocatalytic reaction process with a Pd-modified metal electrode, has not been investigated. Pd-modified electrode materials play a key role in ECH of chloroorganics. Various carbon materials are frequently used as cathode substrates supporting the Pd catalyst because of their chemical inertness and strong adsorption capability, such as activated carbon fiber/cloth/felt, reticulated vitreous carbon, and carbon nanotubes (7, 11, 12). Some metallic mesh materials are also used as cathode substrates. In the study on ECH of 2,4-dichlorophenol, Cheng et al. (13) found that Ti mini-mesh increased 20% of the current efficiency and decreased approximately 50% of the energy consumption than activated carbon cloth. The different ECH capability for various Pd-modified materials implies the complexity of the ECH mechanism involved. However, the most frequently cited mechanism of interface reaction in ECH is based on the carbon substrates, which included chloroorganic adsorption on the carbon surface and then ECH with [H] at the Pd/carbon interface, proposed by Cheng et al. (7). It is unclear as to whether the previous ECH mechanism is still applicable to the electrocatalytic system using palladized metal materials. This paper investigates the interface reaction mechanism of the ECH of PCBs in aqueous medium on a Pd-modified Ni foam electrode. The ECH pathways of PCBs on the Pd/Ni foam cathode were inferred by detecting the degradation byproducts of PCBs. The electrocatalytic process of the Pd/ Ni foam cathode was proposed by studying the effect of surfactant type, sorption behavior of PCBs on the electrode, and current densities on the ECH efficiency of PCBs. Solvent/ surfactant-aided solutions were used to increase the apparent aqueous solubility of PCBs. In Tsyganok’s work, cationic surfactants such as cetyltrimethylammonium bromide (CTAB) were used in ECH (11). In addition, nonionic surfactants, anionic surfactants, and hydroxy-β-cyclodextrin (HPCD) are VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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often used to dissolve hydrophobic organic pollutants in industrial applications such as soil washing due to their low toxicity and/or reagent residuals in situ (14, 15). Four solvent/ surfactant-aided solutions, a nonionic surfactant (polyoxyethyene-4-lauryl ether, Brij 30), an anionic surfactant (sodium dodecyl sulfate, SDS), a cationic surfactant (CTAB), and HPCD in a methanol/water solution (50 vol %), were evaluated by testing the ECH efficiency of PCBs on the Pd/Ni foam electrode.
Experimental Procedures Reagents. Polychlorinated biphenyls including 2-chlorobiphenyl (2-CB), 2,5-dichlorobiphenyl (2,5-DCB), and 2,4,5trichlorobiphenyl (2,4,5-PCB) as well as biphenyl were purchased from Acros Organics. Ni foam and PdCl2 were the same as those presented in ref 10. Activated carbon felt (ACF, SBET: 1200 ( 50 m2/g, pore size distribution: 2-400 Å) was purchased from Sutong Carbon Fiber Co. Brij 30 and CTAB (99%) were obtained from Acros Organics. SDS was from Beijing Chemical Reagents Co. HPCD (99%) was supplied by Shandong Xinda Fine Chemical Industrial Co. The cationexchange membrane was Nafion-324 (DuPont). Methanol (HPLC grade), acetonitrile, isooctane, and acetone were from Fisher Chemical Co. Batch Experiments. The methods for the preparation of the Pd/Ni foam electrode and the ECH system of PCBs were described in our previous paper (10). The electrode of the Pd/Ni foam (14 mm × 48 mm × 4 mm) was used to reduce 100 mg/L (or 1 mM) 2,4,5-PCB (or 2-CB) in 50 mL of catholyte. During electrolysis, 0.5 mL samples of catholyte were periodically withdrawn from the reservoir for product analysis. Analytical Methods. A HPLC system (Shimadzu) was used to analyze 2-CB, 2,4,5-PCB, and biphenyl in the catholyte after ECH, as described previously (10). GC/MS (Finnigan Trace DSQ) was used for the detection of the intermediates according to EPA SW-846 method 8270C for semivolatile organic compounds such as PCBs. For GC/MS analysis of PCBs in aqueous solution containing a surfactant (e.g., Brij 30), the sample was diluted into a solution containing 95 vol % methanol to ensure a sufficient increase in the cmc of the surfactants as a result of which any micelles present in the sample disintegrated due to the dilution (16); then, 2 mL of isooctane was used to extract the intermediate products from the solution. The extraction was repeated twice, and the extracted liquid was concentrated to 1 mL by pure N2 before GC/MS analysis. The recovery efficiency for the extraction of PCBs and biphenyl was close to 95% by this method. The GC/MS instrument used a capillary column (DB-5 MS, 30 m × 0.25 mm × 0.25 µm). Standard solutions of PCBs in hexane were used for the calibration of the instrument. Voltammetry. All linear sweep voltammetry experiments were performed in a conventional H-cell using the CHI 636B electrochemical workstation. The working electrode was Pd/ Ni foam (3.5 mg Pd /cm2, 2 cm2) or Ni foam (2 cm2), and the counter electrode was platinum foil (1 cm × 4.5 cm × 0.011 cm) with platinum black. The reference electrode was an external saturated calomel electrode (SCE, Russell). All of the solutions studied were thoroughly degassed using pure N2. Sorption Experiments. Batch sorption experiments were conducted in a series of 50 mL flasks with 25 mL of a 100 mg/L 2,4,5-PCB (or biphenyl) solution prepared using methanol/H2O (1:1, v/v) and surfactant (0.1 M Brij 30, 0.5 M SDS, 0.1 M CTAB, or 0.05 M HPCD). A 14 mm × 48 mm × 4 mm piece of Pd/Ni foam (or Ni foam) was added to a flask and then shaken at 120 rpm in a thermostatic shaker at 25 °C for 5 h. ACF with the same size was added into a new flask using the same process as the control experiment. The sorption behavior of 2,4,5-PCB (or biphenyl) on the previous 7504
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FIGURE 1. Typical reaction profile for ECH of 2,4,5-PCB on a Pd/Ni foam cathode in a methanol/H2O (50 vol %) solution containing 0.1 M Brij 30 (smaller amounts of 2,4-DCB and 3-CB also could be detected in electrolysis samples but are not shown in the figure for clarity). C0 ) 100 mg/L, V ) 50 mL, I ) 15 mA, T ) 30 ( 0.5 °C, and flow rate ) 50 mL/min. materials was described by detecting their concentrations in solution at various time intervals.
Results and Discussion Degradation Pathway. Rapid degradation of 2,4,5-PCB was found on the Pd/Ni foam electrode. After 90 min of electrolysis, the 2,4,5-PCB concentration decreased from 100 to 2.8 mg/L. Biphenyl was the main product, and a small amount of partially dechlorinated intermediates was detected including 2,5-DCB, 2,4-DCB, 2-CB, and 3-CB. It was shown that biphenyl increased first, reached a maximum at 94.6% of 2,4,5-PCB conversion, and then decreased (Figure 1). The decrease could be due to the further hydrogenation of biphenyl to cyclohexylbenzene, which was first detected by GC/MS in approximately 10 mA h electrolysis. However, the conversion of biphenyl to cyclohexylbenzene was very slow according to the results of extended 75 mA h electrolysis, and only 7% of biphenyl disappeared in the extended hydrogenation. Other intermediates exhibited a similar trend but earlier, which resulted from the mechanism of consecutive hydrodechlorination of 2,4,5-PCB. The degradation pathways of PCBs with photolysis, bimetallic reduction, or ionizing radiation have been reported (5, 17, 18); however, the pathways by ECH are still not reported. On the basis of the previous results, the effect of the chlorine position in the aromatic ring on ECH products was utilized to infer ECH pathways of 2,4,5-PCB (Figure 2). 2,4- and 2,5-DCB were the products of the first hydrodechlorination step of 2,4,5-PCB in electrolytes by GC/MS analysis. This suggests that the reactivity of Cl at the 2-position of 2,4,5-PCB is lower than that of Cl at the 3- or 4-positions due to the steric hindrance of the neighboring phenyl. In addition, 2,5-DCB was detected by GC/MS prior to 2,4-DCB in electrolytes, and 2,5-DCB was the major intermediate. These results indicate that the chlorine in the 4-position is more facile to be eliminated than that in the 3-position. This suggests that the electronic effects of chlorine at the 2-position can assist the departure of chlorine at the 4-position rather than at the 3-position. Similar results were found in sequential hydrodechlorination because only 4-CB was not detected by GC/MS analysis. Hence, the principal pathway for 2,4,5-PCB ECH may follow the pathway that is shown by boldfaced arrows in Figure 2. This mechanism is different from that of PCBs in photolysis where dechlorination at the ortho-position is more favorable (17), which indicates that the photolysis of PCBs is not a process of contact reaction.
occurred more quickly than using pure Ni foam when the solution contained Brij 30, SDS, or HPCD except for CTAB. The electrical interaction between surfactants and cathode surfaces most likely plays a key role in the HER sequence. CTAB, being the cationic surfactant, could be adsorbed rapidly on the cathode due to static electric attraction. Moreover, according to the well-known electrocatalytic volcano plots, Pd is more active than Ni in the HER. Pd particles are the earlier polarized portion that initiates the HER and adsorbs CTAB than the Ni substrate in the electrode. The adsorbed CTAB could form a condensed layer at the electrode surface, especially on the Pd particles. As a result, the HER was restrained, and the reaction on the Ni substrate was faster than that on the Pd particle (Figure 3a). On the other hand, due to electronic effects, anionic/nonionic surfactants and HPCD are adsorbed with difficulty onto the Pd/Ni foam or the Ni foam cathode. Therefore, as shown in Figure 3b-d, Pd particles could normally generate hydrogen earlier than Ni substrate. In addition, there was a wave raised at E1/ 2 ≈ -0.55 V in the voltammogram of SDS (Figure 3c). This was possibly due to the hydrogenation of the sulfate ion of SDS on the cathode, which had been observed by electrochemical reduction of sulfate using a graphite cathode in Bilal and Tributsch’s work (19).
FIGURE 2. Proposed ECH mechanism of 2,4,5-PCB on the Pd/Ni foam cathode. (f) Major pathway and (f) minor pathway. Effect of Surfactant Type. Typical voltammograms (Figure 3) showed that the HER using Pd/Ni foam as the cathode
Subsequently, the ECH of 1 mM 2-CB was conducted to evaluate the effect of the previous surfactants on ECH. After 20 min of electrolysis, the 2-CB conversion rates were 82.3, 77.8, 73.6, and 68.1% for SDS, Brij 30, CTAB, and HPCD, respectively. The reported values were the average results of three repeated experiments. The results indicate that the carbon chain length of the surfactant has a significant effect on the ECH efficiency because the length determines the
FIGURE 3. Linear sweep voltammograms (scan rate 5 mV/s) in deaerated methanol/H2O (50 vol %) solutions of 0.1 M CTAB (a), 0.1 M Brij 30 (b), 0.5 M SDS (c), or 0.05 M HPCD (d) in the presence of supporting electrolyte (0.1 M NaAc) and pH buffer (0.1 M HAc). VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Profiles of sorption behavior of 2,4,5-PCB (or biphenyl) on Pd/Ni foam, Ni foam, or ACF in a methanol/H2O (50 vol %) solution containing a surfactant (0.1 M CTAB or 0.1 M Brij 30). transfer distance of the 2-CB molecule from the hydrophobic inner core to the outer surface of the surfactant micelle. For example, in the solution containing SDS, which has the shortest aliphatic chain (12 carbon atoms), the highest removal rate of 2-CB was achieved when compared with Brij 30 and CTAB. In addition, the branched chain of the surfactant molecule also delays the transfer of the 2-CB molecule in the micelle. Therefore, the removal rate of 2-CB with CTAB was lower than the rate of Brij 30 due to the effect of three methyl branched chains in CTAB. The relatively low ECH efficiency of 2-CB with HPCD is due to its compact cup-shaped structure, in which the 2-CB molecule is released with more diffuculty than in the common surfactant micelle (15). The previous effect of the molecule structure of surfactants on the 2-CB removal implies the diversity of the surfactant function in ECH, which is not significantly impacted by the cage effect described in Tsyganok’s work (11). CTAB used as the preferential surfactant in their work was mainly due to its cage effect, which could combine the hydrophobic chloroaromatic molecule into an ion-molecule pair and thus form a positively charged species to migrate rapidly to the cathode. The anionic (SDS) and nonionic (Brij 30) surfactants can achieve approximately equal ECH rates as the cationic surfactant (CTAB) in this work, which is mainly due to the fact that the mass transfer effects of the micelles in the flowthrough cell overcame the electronic effects of the surfactants. Therefore, the surfactant charge was not a significant factor for ECH efficiency, and Brij 30, SDS, and HPCD are also favorable surfactants for ECH. Sorption Effect. The sorption characteristics of PCBs on Pd/Ni foam were investigated to understand the ECH process of PCBs using metal materials. Figure 4 illustrates the sorption behavior of 2,4,5-PCB (or biphenyl) on the Pd/Ni foam (or Ni foam) in an aqueous solution. Similar characteristics were found when SDS or HPCD was used as the surfactant under the same conditions (not shown in the figure for clarity). After 5 h of contact time, only a very small change (
