Environ. Sci. Technol. 2004, 38, 638-642
Electrochemical Hydrodehalogenation of 2,4-Dibromophenolin Paraffin Oil Using a Solid Polymer Electrolyte Reactor H. CHENG, K. SCOTT,* AND P. A. CHRISTENSEN School of Chemical Engineering & Advanced Materials, University of Newcastle upon Tyne, Marz Court, Newcastle upon Tyne NE1 7RU, U.K
A new technology for remediation of halogenated organics-oil systems, which can cause serious environmental problems, has been demonstrated using the electrochemical hydrodehalogenation of 2,4-dibromophenol (DBP) in paraffin oil in a solid polymer electrolyte reactor. The reactor has been evaluated in terms of cathode materials and structure and the ratio of the cathode surface area to the solution volume. A cathode of titanium minimesh with a palladium electrocatalyst produced by electrodeposition was particularly effective. Current efficiencies of up to 85% and percentage of DBP removal of up to 62%, spacetime yields of up to 7.6 kg DBP m-3 h-1, and energy consumption as low as 1.6 kW h (kg of DBP)-1 were achieved. The reactor showed stable operation for periods of up to 170 h. The results demonstrated that electroreduction could be an alternative technology to electrooxidation for the treatment of wastes and toxic halogenated compounds, making the process simpler in comparison to electrooxidation.
Introduction Halogenated organic compounds are toxic to biological systems and are distributed in the environment. Disposal of such compounds to landfill is now virtually precluded by environmental legislation. Incineration involves high costs, produces harmful substances, e.g., dioxins, and causes adverse public reaction. Hence, bioremediation (1) and chemical and electrochemical dehalogenation (1-3) have been investigated as alternatives. Bioremediation involves the treatment of halogenated compounds using the metabolism of microorganisms (1). The effectiveness of bioremediation greatly depends on the ability of microorganisms to survive in the environment containing the halogenated compounds. An issue with bioremediation is that the products are often toxic and, in some cases, may be more harmful to human health than the parent compounds (3). Microorganisms can evolve relatively quickly to develop biochemical traits, but, in some cases, long-term operation is necessary, e.g., several months for the bioremediation of polychlorobiphenyls (PCBs) (4). Chemical hydrodehalogenation (HDH) using chemicals such as LiAlH4 or NaBH4 is considered too expensive for * Corresponding author phone: +440191 222 8771; fax: + 44 0191 222 5292; e-mail:
[email protected]. 638
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treatment of wastes and are used only for preparative synthesis (5). Chemical HDH using zerovalent metals, such as iron, zinc, and tin, faces two major problems, i.e., slow reaction kinetics and poor effectiveness for HDH of aromatic halogenated compounds under ambient conditions (1, 2, 4). Catalytic HDH can provide high reaction rates, but it typically requires conditions of high temperature (above 400 °C in most cases) and high pressure. Catalytic HDH also suffers from a rapid deactivation of the catalyst (6). Recently, electrochemical HDH has been used for treatment of halogenated organic wastes (7-11). A typical example is the HDH of 1,2,3,5-tetrachlorobenzene (TCB) and chlorobenzene (CB) in methanol or dimethyl sulfoxide and acetonitrile (with 0.25 M tetraethylammonium bromide) at a cathode potential of -3.3 V versus Ag/AgCl (8). Greater than 95% conversion of CB at an initial concentration of 12 mM was achieved, with a current efficiency of 15-20% using carbon cloth or Pb cathodes. On the other hand, Pt, Ti, and Ni cathodes gave low current efficiencies of ca. 5% at lower conversions. The principle of the solid polymer electrolyte reactor has been successfully used in the electrochemical HDH of some halogenated organic compounds in aqueous media (1215). The method of HDH offers a possible means of recycling organic substances or a means of treating halogenated wastes prior to bioremediation. To use the electrochemical HDH technology at an industrial scale requires an appropriate design of reactor and, particularly, selection of a suitable cathode. The most desirable features of an industrial HDH reactor are high efficiency and engineering simplicity. Many industrial halogenated organic compounds exist in oil or as concentrated organic solutions, which are not suitable for conventional remediation by bioremediation (16, 17). For such compounds, conventional electrolysis cannot be directly used due to the very high resistance of the media. Hence, an HDH technology suitable for nonaqueous media is necessary. Our initial experiments demonstrated that HDH in oil media could not be carried out in a solid polymer electrolyte reactor under reported conditions (14, 15), although it showed effective performance in aqueous solutions (12-15). As a continuation of our current work, 2,4-dibromophenol (DBP) in paraffin oil was chosen as a model system to show that the HDH technology is appropriate in nonconducting solvents only under modified conditions. The overall cathode reactions can be written as
Br2C6H3OH (DBP) + 2e- + H+ )
BrC6H4OH (BP) + Br- (1)
BrC6H4OH (BP) + 2e- + H+ )
C6H5OH (phenol) + Br- (2)
The key to the HDH technology is to use dilute acid anolytes, which makes it possible to carry out HDH in a nonconductive medium, e.g., paraffin oil. The work expanded the HDH process from aqueous solutions to nonaqueous media. This expansion has significant environmental implication because treatment of halogenated organics as oil solutions or as concentrated organic solutions now becomes possible. The cathode design in terms of substrate, structure, and ratio of the cathode surface area to waste volume were evaluated during the HDH of DBP in paraffin oil under various conditions, and the related results are reported in this paper.
Experimental Section Materials and Chemicals. The following materials and chemicals were used as received: Ti minimesh (99.6%; open 10.1021/es034775u CCC: $27.50
2004 American Chemical Society Published on Web 12/03/2003
FIGURE 1. Flow circuit of the solid polymer electrolyte reactor rig: (1) anolyte feed (dilute aqueous H2SO4 solution); (2) solid polymer electrolyte reactor; (3) power supply; (4) condenser; (5) catholyte feed (DBP in paraffin oil); (6) heating mantle; (7) Pt/Ti minimesh anode; (8) two-head pump; (9) Nafion 117 membrane; (10) Pd/Ti minimesh cathode. area, 37%; wire diameter, 0.2 mm; Goodfellow), carbon cloth (GC-14, E-Tek Inc.), 2,4-dibromophenol (DBP, 99%, Aldrich), 2- or 4-bromophenol (BP, 99%, Aldrich), phenol (99.9%, Aldrich), PdCl2 (99%, Aldrich), H2PtCl6 (99%, Johnson Matthey), and H2SO4 (98%, AnalaR, BDH). All oil solutions were prepared using light paraffin oil (Aldrich), and all aqueous solutions were prepared using Millipore-Q water (18.2 MΩ cm). Solid Polymer Electrolyte Reactor Rig. The rig used for electrochemical HDH of DBP is shown schematically in Figure 1. The solid polymer electrolyte reactor, which consisted of an electrode membrane (Nafion 117) assembly placed between stainless steel blocks with machined flow channels, is inserted into a circulation loop consisting of a two-head peristaltic pump (Cole-Parmer) and reservoirs (0.1, 1, or 2 dm3) placed in two heating mantles (Electrothermal Flask/ Funnel, Cole-Parmer). The rig is operated in a batch recirculation mode and used with both oil and aqueous media. Glass condensers were mounted on top of the catholyte reservoir to condense organic vapor from the gases exiting the reservoir before passing them through a concentrated aqueous NaOH solution. Noncondensable gases were vented to the atmosphere. In operation, the catholyte and the anolyte, each with a volume of 0.05, 0.1, 0.2, 1, or 2 dm3, were pumped through the cell and then returned to the reservoirs for recycle by the pumps. Flow rate, electrolyte temperature, and applied currents were controlled using the control unit. Electrodes. Palladized cathodes (2 mg of Pd cm-2, 20 cm2 in the geometric area) and platinized anodes (2 mg of Pt cm-2, 20 cm2 in the geometric area) were prepared by electrodeposition using either titanium minimesh or carbon cloth as substrates. The deposition methods, including pretreatment of substrates and after-treatment of electrodes, are detailed elsewhere (18). Batch Electrochemical HDH. Batch electrolyses were performed in the solid polymer electrolyte reactor using a FARNELL LS60-5 power supply. All electrolyses were carried out at constant current density, ranging from 5 to 100 mA cm-2, for periods between 2 h and 170 h. The longer-term testing was performed to investigate the durability of the reactor components. There were several short shutdown periods during the long-term operation to ensure continued operation of the experiments by changing the anolyte feeds. The concentrations of DBP, BPs, and phenol were monitored using HPLC during the electrolysis. Product Analysis. High-performance liquid chromatography (HPLC) was performed in a DIONEX HPLC system, which consisted of a P 580 pump and a Softron 2000 UVD
170S/340S UV/vis detector with an Econosphere C8 column (5 µm particle size and 25 × 0.46 cm, Alltech Associates, Inc.). The wavelengths used in HPLC measurements were determined using UV-vis spectroscopy (UV-160A UV-visible recording spectrophotometer, Shimadzu, Japan). Normally, the UV detector was set to 270 nm for phenol and 290 nm for DBP, 2-BP, and 4-BP. The mobile phase was an acetonitrile/water mixture (52/48 by volume) with a flow rate of 1.0 mL min-1. The peaks for phenol (retention time tr ) 1.92 min) and 2-BP and 4-BP (tr ) 2.20-2.30 min; it was not possible to discriminate between 2- and 4-BP under the analytical conditions), and DBP (tr ) 3.15 min) were characterized by using standard paraffin oil solutions. Quantification of the product distribution during the electrolysis was accomplished by the use of calibration curves with the authentic samples. A sample volume of 20 µL was generally employed. We have tried various operating conditions in terms of compositions of the mobile phase and flow rate, etc., using available equipment including different HPLC columns. The peaks for 2-bromophenol and 4-bromophenol (the authentic samples from Aldrich) were still not completely separated. Considering that complete separation is not urgent for the project and our final aim is to realize complete removal of the bromide ions from the bromophenols, no further trials for complete separation of the peaks for 2- or 4-bromophenol were carried out, although accurate quantitative analysis of monobromophenol isomers using GC or other techniques in a further work would be beneficial. Parameter Definitions. Percentage of DBP removal (PR), space-time yield (STY), current efficiency (CE), and energy consumption (ECN) were used to evaluate the process performance and efficiency. The percentage of DBP removal (PR) is expressed as
PR )
C 0 - Ct × 100% C0
(3)
where C0 and Ct are DBP concentrations at start and at electrolysis time t, respectively. The extent of HDH is defined, in terms of the space-time yield (kg m-3 h-1), using the following formula (19):
STY )
3600ajCEMFW nF
(4)
where a is a specific area (m-1), defined as a ratio of the electrode area to the waste volume in the reactor, j is the current density (A m-2), CE is the current efficiency, n is the number of electrons in the concerned reaction, F is the Faraday constant (96 500 C mol-1), and MFW is the molar mass (kg mol-1). The current efficiency was calculated as that part of the current (or charge) passed to convert the starting DBP to intermediates and products, and the total current efficiency is the sum of the individual current efficiencies for forming 2- or 4-BP and phenol according to eqs 1 and 2. Energy consumption for HDH processes was calculated according to the following equation (17):
ECN ) nFEcell/CEMFW
(5)
where Ecell is the cell voltage. The instant Ecell values were used in ECN calculations. The total energy consumption included the energy used for the HDH processes, heaters, and pumps, etc.
Results and Discussion As reported previously, the cathode plays a decisive role in the electrochemical HDH of DBP in aqueous media and VOL. 38, NO. 2, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of cathode substrate on the percentage of DBP removal and the space-time yield during the electrochemical HDH of DBP in paraffin oil media using a Nafion 117 membrane reactor: (b) destruction percentage, Ti minimesh; (2) destruction percentage, carbon cloth; (9) space-time yield, Ti minimesh; ([) space-time yield, carbon cloth. Cathode: Pd/Ti minimesh or carbon cloth (20 cm2, 2 mg of Pd cm-2). Anode: Pt/Ti minimesh or carbon cloth (20 cm2, 2 mg of Pt cm-2). Controlled current density: 10 mA cm-2. Catholyte: 200 mM DBP in paraffin oil (50 cm3). Anolyte: 0.5 M H2SO4 aqueous solution (50 cm3). Flow rate: 200 mL min-1. Temperature: 20.0 ( 0.5 °C. palladized cathodes have been demonstrated as effective materials (15). The evaluation of an HDH reactor thus focused on palladized cathodes with different substrates and structures. Effect of Electrode Substrate. Two promising electrode substrates, Ti minimesh and carbon cloth, were examined in a solid polymer electrolyte reactor during the HDH of DBP. To make the data comparable, the catalyst loading of these electrodes was fixed at 2 mg of Pd cm-2. Typical results collected in the paraffin oil solutions are discussed below. Figure 2 shows the influence of the cathode substrate on the percentage of DBP removal and the space-time yield for 200 mM DBP in paraffin oil using a Nafion 117 membrane reactor. The percentages of DBP removal achieved with carbon cloth and Ti minimesh were 26.9 or 49.3%, respectively, after 2 h. The space-time yields were correspondingly higher by 0.3-0.5 kg of DBP m-3 h-1 at the Ti minimesh than at the carbon cloth cathodes. The product distributions for the two cathodes were different. The decrease in the DBP concentration and increase in the phenol formation were greater with the Ti minimesh than with the carbon cloth. After 2 h HDH, the DBP concentration fell from 200 mM to 101.5 or 146.2 mM and the phenol concentrations were 48.8 or 37.9 mM with the Ti minimesh or the carbon cloth cathodes, respectively. The aim of the project is to show that the halogenated organic substances can be converted to phenol. It then may be possible to destroy phenol using other techniques. We have not reached the target yet, but we can completely hydrodehalogenate DBP to phenol without residual monobromophenols present in the best case. In the cases of incomplete HDH, separation of the products and the unconverted DBP from the paraffin oil is necessary and extra cost will be a penalty, which should be avoided. The products and the unconverted DBP may possibly have negative effect on the microorganisms and make bioremediation difficult, which requires investigation. Figure 3 shows current efficiencies and energy consumptions for the HDH of DBP in paraffin oil. Using the Ti minimesh as a cathode substrate, current efficiencies reached above 66%, which were 10% higher than 640
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FIGURE 3. Effect of cathode substrate on the current efficiency and the energy consumption during the electrochemical HDH of DBP in paraffin oil media using a Nafion 117 membrane reactor: (b) current efficiency, Ti minimesh; (2) current efficiency, carbon cloth; (9) energy consumption, Ti minimesh; ([) energy consumption, carbon cloth. Conditions: the same as those in Figure 2. those obtained with the carbon cloth substrate. The energy consumptions were 1.6-2.0 and 2.4-3.2 kW h (kg of DBP)-1 using the Ti minimesh and the carbon cloth, respectively. This is a consequence of the higher current efficiencies and the lower electrical resistance of the Ti minimesh, compared with the carbon cloth. Better mass transport of DBP at the Ti minimesh than at the carbon cloth can be expected because of its very open structure, which may have contributed to better HDH performance, compared with the carbon cloth. In addition, black (Pd) particles were observed in the catholyte during the HDH of DBP using the carbon cloth cathode, suggesting that the palladized Ti minimesh cathode was more stable than the palladized carbon cloth cathode. Electrode Structure (Single- or Three-Layer Electrodes). The electrochemical HDH of DBP was carried out using multilayer meshes to achieve high HDH rates and high process efficiency. The results were compared with those achieved at a single-layer electrode. Figure 4 shows data for the percentage of DBP removal and the space-time yield achieved during the HDH of 200 mM DBP in paraffin oil. Increases of around 3% in the percentage of DBP removal were observed with the threelayer electrodes, compared with those with the single-layer electrodes. The space-time yields increased by 0.4 kg of DBP m-3 h-1 with the three-layer electrodes, compared to the single-layer cathode. The current efficiencies for HDH, shown in Figure 5, are 10% higher with the three-layer electrodes than with the single-layer electrodes. The energy consumption decreased by approximately 0.1-0.4 kW h (kg of DBP)-1 using the threelayer electrodes rather than the single-layer electrodes. The decrease was mainly due to the increase in current efficiency; the cell voltages in both reactors (with the three- or singlelayer electrode) were less than 50 mV. The above improvements in the HDH of DBP using the three-layer electrode rather than the single-layer electrode were achieved at a price of using three times as much catalyst as with the single-layer electrode. An optimization of the multilayer electrode is necessary from views of increased electrode activity and economic competitiveness. Ratio of Surface Area to Waste Volume. One of the methods to achieve a high HDH rate in a solid polymer electrolyte reactor is reaching a high ratio of the electrode surface area to the waste volume (RAV). There are several
FIGURE 4. Effect of cathode structure on the percentage of DBP removal and the space-time yield during the electrochemical HDH of DBP in paraffin oil media using a Nafion 117 membrane reactor: (b) percentage of DBP removal, single layer; (2) percentage of DBP removal, three layers; (9) space-time yield, single layer; ([) space-time yield, three layers. Cathode: single layer or threelayer Pd/Ti minimesh (20 cm2, 2 mg of Pd cm-2). Anode: single layer or three-layer Pt/Ti minimesh (20 cm2, 2 mg of Pt cm-2). Controlled current density: 10 mA cm-2. Catholyte: 200 mM DBP in paraffin oil (100 cm3). Anolyte: 0.5 M H2SO4 aqueous solution (100 cm3). Flow rate: 100 mL min-1. Temperature: 20 ( 0.5 °C.
FIGURE 5. Effect of cathode structure on the current efficiency and energy consumption during the electrochemical HDH of DBP in paraffin oil media using a Nafion 117 membrane reactor: (b) current efficiency, single layer; (2) current efficiency, three layers; (9) energy consumption, single layer; ([) energy consumption, three layers. Conditions: the same as those in Figure 4. ways to realize the target, such as increasing the electrode surface areas of a single reactor or using a multiple reactor stack, etc. For convenience, the present work investigated the effect of the RAV through changing the solution volume in a reactor with constant electrode surface areas. It is our intention to achieve high HDH rates via increasing the RAV. The measure is effective, as shown by the data in space-time yield collected during the HDH of 50-1000 cm3 of 200 mM DBP with different volumes using a Nafion 117 membrane reactor with fixed electrode surface areas, as shown in Figure 6. The space-time yield increased from 0.3 to 5.6 kg of DBP m-3 h-1 when the RAV increased from 2 to 40 after 3 h
FIGURE 6. Effect of ratio of electrode area to catholyte volume (RAV, (m2/m3)) on the space-time yield during the electrochemical HDH of DBP in paraffin oil media using a Nafion 117 membrane reactor: (9) RAV ) 40; (2) RAV ) 20; (b) RAV ) 10; ([) RAV ) 2. Cathode: Three-layer Pd/Ti minimesh (100 cm2, 2 mg of Pd cm-2). Anode: Three-layer Pt/Ti minimesh (100 cm2, 2 mg of Pt cm-2). Controlled current density: 10 mA cm-2. Catholyte: 200 mM DBP in paraffin oil (50-1000 cm3). Anolyte: 0.5 M H2SO4 aqueous solution (50-1000 cm3). Flow rate: 100 mL min-1. Temperature: 19.7 ( 0.5 °C.
FIGURE 7. Long-term evaluation on the percentage of DBP removal and current efficiency during the electrochemical HDH of 200 mM DBP in paraffin oil media using a Nafion 117 membrane reactor: (b) percentage of DBP removal; (9) current efficiency. Cathode: Pd/Ti minimesh (20 cm2, 2 mg of Pd cm-2). Anode: Pt/Ti minimesh (20 cm2, 2 mg of Pt cm-2). Catholyte: 200 mM DBP in paraffin oil (1000 cm3). Anolyte: 0.5 M H2SO4 aqueous solution (1000 cm3). Controlled current density: 10 mA cm-2. Flow rate: 100 mL min-1. Temperature: 17.5 ( 1.0 °C. electrolysis, suggesting that increasing RAV can greatly enhance the HDH rates. So the benefit of one of the initial design ideas, i.e., to achieve high RAVs, has been demonstrated. No effect of RAV on the current efficiency and the energy consumption was observed. So practical reactors should be designed using as high RAV values as possible in order to achieve a fast HDH process. Evaluation of the Cathode during a Long-Term HDH. The reactor design was tested in a long-term HDH with particular attention paid to the cathode stability. Preliminary VOL. 38, NO. 2, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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long-term HDH of DBP, up to 170 h, was carried out in paraffin oil (1000 dm3 volume) and at a current density of 10 mA cm-2. Figure 7 shows data for percentage of DBP removal and current efficiency during the HDH of 200 mM DBP in the paraffin oil. The percentages of DBP removal initially increased rapidly for the first 30 h of electrolysis and then slowed as the concentration of DBP decreased. The space-time yield was 0.85 kg of DBP m-3 h-1 after 170 h HDH of 200 mM DBP in the paraffin oil. The current efficiency decreased from 86% (2 h HDH) to 36% after 170 h. The energy consumption increased from 2.7 (2 h HDH) to 3.0 kW h (kg of DBP)-1 after the 170 h HDH, as the DBP concentration decreased during the HDH, which led to the slow reaction rate. In addition, the cell voltages increased from 2.70 at the start to 3.05 V, which corresponds to an increase in cell resistance of 13.515.2 Ω, after the 170 h HDH. No obvious damage of the electrodes was found after the tests. The above data demonstrate that the designed solid polymer electrolyte reactor, particularly its electrodes, can be subjected to a relatively long HDH operation. To satisfy the requirements of a commercial HDH process, the electrode must run for thousands of hours without loss in performance. So effective procedures for preparation of electrodes, including pretreatment, preparation, and posttreatment, have to be explored, to ensure this operation; these, however, are outside the scope of this work.
Acknowledgments The authors thank the United Kingdom Engineering and Physical Sciences Research Council (EPSRC) for funding. The work was performed in research facilities provided through an EPSRC/HEFCE Joint Infrastructure Fund Award (No. JIF4NESCEQ).
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Received for review July 16, 2003. Revised manuscript received October 21, 2003. Accepted November 1, 2003. ES034775U
