Environ. Sci. Technol. 1999, 33, 3438-3442
Electrochemical Continuous Decomposition of Chloroform and Other Volatile Chlorinated Hydrocarbons in Water Using a Column Type Metal Impregnated Carbon Fiber Electrode NORIYUKI SONOYAMA* AND TADAYOSHI SAKATA Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Trihalomethane and other chlorinated hydrocarbons are known to be toxic to human health. However, removal of these compounds from water is not easy. We attempted continuous electrochemical decomposition of chloroform that is the main compound of trihalomethanes and some toxic chlorinated hydrocarbons in water using a metalimpregnated carbon fiber electrode (CFE). At Ag- and Znimpregnated CFE, concentration of chloroform in 0.5 M K2SO4 (the supporting electrolyte) solution was decreased from 0.25 m mol/L to below the limit of detection of our analysis system (1 ppm) at a flow rate of 1 mL/min. The main product of electrolysis was methane. This high efficiency, determined by the chemical yield, hardly changed at a flow rate of 20 mL/min at a Ag-impregnated CFE. At a flow rate of 1 mL/min, chloroform was degraded with a decomposition efficiency of almost 100% even in the solution without the supporting electrolyte, whereas at a higher flow rate, the efficiency for the decomposition of chloroform decreased with a decrease in the concentration of the supporting electrolyte. Tetrachloroethylene, 1,1,1trichloroethane, and 1,1,2-trichloroethane were also decomposed at a Ag-impregnated CFE with an efficiency of almost 100%.
Introduction Pollution of drinking water by toxic substances is a serious subject among environmental problems, because our health will undergo a long-term effect even with very low concentrations of pollutants. Chlorinated compounds, represented by chloroform, tetrachloroethylene, and trichloroethylene, are the most widespread pollutants of drinking water. Chloroform is the major trihalomethane that is produced in the disinfection process of tap water (1, 2). Tetrachloroethylene and trichloroethylene have been detected in the groundwater in many places in the world (3, 4). It is wellknown that these compounds are carcinogenic and/or toxic (5-8). Concerning trihalomethane, Waller et al. recently reported an increased incidence of abortion by pregnant women who drank water containing trihalomethane at a * Corresponding author phone: +81-45-924-5400; fax: +81-45924-5489; e-mail:
[email protected]. 3438
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concentration below 0.1 mg/L (9), the regulatory standard of trihalomethane in tap water (10). The presently used methods of removal of these chlorinated compounds are air wiping or adsorption onto activated carbon. The former method removes chlorinated compounds from the liquid to the gas phase and therefore requires secondary treatment. In the latter method, the useful lifetime of the activated carbon is significantly limited in water by its adsorption capacity, and treatment of the used activated carbon is necessary. On the laboratory scale, decomposition of chlorinated compounds has also been carried out photocatalytically (11, 12) and biologically (13-15). However, these methods are not appropriate for the treatment of flowing water that contains only low concentrations of chlorinated compounds. The advantages of the electrochemical method are as follows. 1. The activity for decomposition hardly depends on the environment. The electrochemical reaction proceeds as long as a current is supplied to the electrode. 2. Secondary pollution hardly occurs, because reactive chemical agents are not used in the treatment. 3. Flowing water can be treated easily using a column type electrode. Despite these advantages of the electrochemical method, there have been few studies of electrochemical decomposition of chlorinated compounds in water (16-18). In the previous paper, we carried out electrochemical decomposition of chloroform dissolved in water using metal electrodes (16). At Cu, Ag, and Zn electrodes, decomposition of chloroform proceeded without production of H2 (the actual faradaic efficiency is nearly 100%), and methane and dichloromethane were produced. In this paper, we examined the advantages and disadvantages of the electrochemical method by electrochemical decomposition of chloroform in water using a Zn wire electrode, the most active metal in our previous study (16), under various conditions. To overcome the disadvantages of the electrochemical method elucidated by electrolyses at a Zn wire electrode, we attempted the continuous treatment of chloroform in water by using a column type metal-impregnated carbon fiber electrodes (CFEs). The activity for electrochemical decomposition of chlorinated hydrocarbons, at the concentration easy to estimate the electrocatalytic activity, and the effect of some factors of electrolyses were investigated.
Experimental Section Electrolyses in a Batch Type Cell. The details of experiments in a batch type cell were already described in a previous paper (16). The area of a Zn wire electrode (cathode) was 16 cm2. As a supporting electrolyte, 0.5 M K2SO4 (GR Wako Pure Chemical) was added. Purified N2 gas was bubbled into the solution for at least 20 min to remove dissolved oxygen. Chloroform (0.1 mL, trihalomethane measurement grade; Wako) was introduced into the electrolyte solution (200 mL) using a syringe. The initial concentration of chloroform was 6.20 m mol/L. Electrolyses were carried out galvanostatically (passage of 50 coulombs,unless otherwise noted) with the aid of a potentiostat-galvanostat (Hokuto model HA-501) connected in a series with a coulomb-ampere-h meter (Hokuto model HF-201). The sampled gas was analyzed by gas chromatography; an Ohkura GC-802 instrument equipped with an activated carbon column (4 mm × 2 m) and a thermal conductivity detector (TCD) for H2, an Ohkura GC-202 instrument equipped with a VZ-10 column (4 mm × 2 m) and a flame ionization detector (FID) for hydrocarbons, and an Ohkura GC-103 instrument equipped with a Porapak QS column (4 mm × 2 m) and an FID for chloroform and other chlorohydrocarbons. For the analysis of chloroform in the 10.1021/es980903g CCC: $18.00
1999 American Chemical Society Published on Web 08/28/1999
NP-KX-120). Electrolyses were carried out potentiostatically at -1.2 V vs Ag/AgCl, unless otherwise noted, using a potentiostat-galvanostat (Hokuto model HABF-501). The schematic description of the electrolysis system is shown in Figure 1. Dichloromethane (Wako GR grade), tetrachloroethylene (Wako GR grade), trichloroethylene (Wako GR grade), 1,2-dichloroethane (Wako GR grade), 1,1,1-trichloroethane (Wako GR grade), and 1,1,2-trichloroethane (Wako EP grade) were used without further purification. The initial concentrations of the aqueous solution of these compounds were 22.6, 33.1, 28.8, 25.6, 26.2, and 25.6 mg/L, respectively.
Results and Discussion
FIGURE 1. Schematic description of the flow electrolysis system and the structure of the flow cell. solutions, 1-10 µL of solution was introduced into the gas chromatograph directly using a microsyringe. The limit of detection of chlorinated hydrocarbons in water is about 1 ppm. The faradaic efficiencies for the products were estimated from the amounts of products determined by gas chromatograph. The concentration of Cl- ion was determined by an ion chromatograph (Toha Denpa model ICA-3000) equipped with an anion exchange column PCI-201S (Toha Denpa) and a electrical conductivity detector, using a phthalic acid eluent (2.5 × 10-3 M) and tris-buffer. Distilled water (Wako) and chloroform were used without further purification. Electrolyses in a Flow Type Cell. As a flow cell system, we used a Hokuto model HX-201 (19). The structure of the flow cell is shown in Figure 1. This flow cell consists of a “working electrode compartment” and a “counter electrode compartment”, and they are separated by a Vycor glass tube (Corning Vycor-7930). The diameter and the length of the tube were 8 and 50 mm, and the thickness of Vycor glass was 1 mm. Ions can permeate through this glass tube, whereas the solution cannot; i.e., a current can flow between the working and the counter electrodes without mixing water in the working electrode compartment and that in the counter electrode compartment. In the working electrode compartment, 300 carbon strings (about 0.5 mm in diameter) (Nihon Carbon GF-20-P7) are packed as a working column electrode (cathode). A graphite carbon rod is inserted into the column for the electrical contact. Water entering from a water inlet flows through the column electrode and drains through a water outlet. The counter electrode compartment is filled with 0.5 M K2SO4 solution (this solution does not come in direct contact with flowing water), and the counter electrode (Pt spiral wire) and the reference electrode (Ag/AgCl) are located near the Vycor tube. Microparticles of metal are supported on the carbon fibers by an impregnation method (20). The pre-electrolysis of the solution and N2 gas bubbling were carried out. The initial concentration of chloroform was 0.25 m mol/L (29.8 mg/L). The flow rate was controlled by a minichemical pump (Nihon Seimitsu Kagaku model
The relationship between the current density and faradaic efficiencies for decomposition of chloroform and product formation in the electrolyses of chloroform at a Zn wire electrode is shown in Figure 2 (a), where the faradaic efficiency is defined as the ratio of the charge used for decomposition of chloroform or for the formation of a product to the total charge passed during electrolysis. Electrolysis at each current density was carried out at least three times with an error of at most 10% in faradaic efficiency. In the region of 1-10 mA cm-2, the faradaic efficiency for decomposition of chloroform was almost 100%, and methane was produced with the faradaic efficiency of 90%. With an increase in the current density, the faradaic efficiency for decomposition of chloroform decreased steeply, and mainly hydrogen production began to occur. This decrease in the faradaic efficiency would be caused by exhaustion of chloroform at the surface of the electrode at high current density. The faradaic efficiency for dichloromethane formation was almost independent of current density. Figure 2(b) is the relationship between the initial concentration of chloroform and the faradaic efficiencies for decomposition of chloroform and product formation. Electrolysis at each point was carried out at least three times with an error of at most 10% in faradaic efficiency. Above 2 m mol/L, the faradaic efficiency for decomposition of chloroform was almost 100%, and methane was produced with a faradaic efficiency of about 90%. In more dilute solutions, the faradaic efficiency for decomposition of chloroform was decreased with a decrease in the concentration of chloroform. The decrease in the faradaic efficiency in the more dilute solution seems to be caused by the excess current to chloroform at the surface of the electrode. These results indicate that electrochemical decomposition of chloroform at the metal electrode does not proceed efficiently under the condition of high current density and in dilute solution. The continuous decomposition of chloroform was attempted using this batch type reaction system. The initial concentration of chloroform was 6.20 m mol/L, and the current density was 1 mA cm-2. At first, decomposition of chloroform proceeded with a faradaic efficiency of 100%. After the total passed charge reached 300 C, the faradaic efficiency for decomposition of chloroform decreased with an increase in the charge passed. This decrease in the faradaic efficiency would be caused by the decrease in the concentration of chloroform due to the continuous decomposition. It took about 19 h (the total charge: 1100 C) to decompose chloroform until the concentration of chloroform came below the detection limit of our analysis system. The concentration of Cl- ion in the electrolyte solution after the continuous electrolysis was measured by an ion chromatograph. After the electrolysis, the concentration of Cl- ion was 15 m mol/ L, whereas that of the electrolyte solution left for 19 h without electrolysis was 3.8 m mol/L. This result indicates that by the electrochemical treatment, chloroform is decomposed to methane, or dichloromethane, and Cl- ion. The result of the continuous electrolysis at a Zn wire electrode demonstrates that it is not efficient to treat water containing chloroform electrochemically at a metal wire VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. (a) Relationship between current density and (b) relationship between concentration of chloroform and faradaic efficiencies for decomposition of chloroform and products formation at a Zn wire electrode: b, decomposition of chloroform; O, production of methane; 4, production of dichloromethane; and ], production of hydrogen.
TABLE 1. The Efficiency for Decomposition of Chloroform by Electrochemical Decomposition at Various Metal-Impregnated CFEs in the Flow Cella metal Zn Ag Cu no metal
effic for decomposition convsn to of chloroform/% current/mA dichloromethane/% 100 100 100 44.4
0.8 1.0 0.6 0.5
0 0 33.0 0
a Flow rate: 0.2 mL/min, electrolyte: 50 mL of 0.5 M K SO , imposed 2 4 potential: -1.2 V vs Ag/AgCl, concentration of chloroform: 0.25 µmol/L.
electrode in a batch type reactor. For the treatment of a dilute aqueous solution of chloroform, the electrode that has a much larger surface area than plate or wire electrodes and a constant supply of chloroform to the electrode would be necessary; using the electrode with a very large area, a certain amount of chloroform can be treated efficiently at low current density, and the supply of a constant amount of chloroform to the electrode will enable constant decomposition of chloroform at high efficiency, because an appropriate condition of electrolysis can be determined. We adopted a column type metal impregnated carbon fiber electrode (CFE) as a working electrode with a flow type electrolysis cell for the continuous electrochemical decomposition of a dilute chloroform solution. This electrode has about 2000 cm2 of surface area. Water that contains chloroform is supplied constantly to the column type electrode by a chemical pump and is treated electrochemically while it passes through the column and then is drained from the outlet. In subsequent experiments, the efficiency for decomposition was defined by eq 1
ED ) (Win - Wout)/Win
(1)
where ED, Win, and Wout mean the efficiency for decomposition and the concentration of chloroform in water before and after the electrochemical treatment, respectively. The values of ED were reproduced with an error of about 5% at most except for the case of ED ) 100%. At the points of ED ) 100%, chloroform could not be detected in all electrolyses. The efficiencies for decomposition of chloroform at Zn-, Ag-, and Cu-impregnated CFEs and a CFE without metal impregnation are summarized in Table 1. At Ag-, Cu-, and Zn-impregnated CFEs, chloroform was not detected by our analysis system from the water sampled from water drained after the electrochemical treatment, whereas at a CFE not 3440
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impregnated with metal, the efficiency for decomposition of chloroform was 44.4%. When a potential was not imposed on the electrode, more than 50% chloroform remained in the drained water from each metal-impregnated CFE. This does not mean the loss of 50% of chloroform but the delay of effusion of chloroform by its weak adsorption on the surface of metal impregnated CFE. The residual chloroform was drained by flowing additional water. When potential was imposed on the metal-impregnated CFE, chloroform was not detected by flowing additional water. These results indicate that the metals impregnated in the CFE catalyze decomposition of chloroform electrochemically even at the concentration of chloroform, where the activity at a Zn wire electrode was very low (Figure 2(b)). The selectivity of products was dependent on the metal impregnated in the CFE. In the gas sampled from the gas phase of electrochemically treated water, methane and hydrogen were detected in all of the metal-impregnated CFE systems. In the liquid phase, dichloromethane was produced at a Cuimpregnated CFE at the conversion of 33%, whereas the concentration of dichloromethane was hardly changed between before and after the electrochemical treatment at other electrodes. Dichloromethane is a less toxic substance than chloroform (21). However, it is still toxic and should not be produced in the treatment of drinking water. Therefore, it is desirable that Ag- and Zn-impregnated CFEs are used for electrochemical decomposition of chloroform in water. Figure 3 is the relationship between the flow rate of the solution and the efficiency for decomposition of chloroform in water. At a Zn-impregnated CFE, the efficiency for decomposition of chloroform decreased steeply with an increase in the flow rate, and at 20 mL/min, it became about 60%. At a Ag-impregnated CFE, the efficiency for decomposition of chloroform was almost 100% irrespective of the flow rate in the region of 0.2-5.0 mL/min. In the region over 5.0 mL/min, the efficiency for decomposition gradually decreased with an increase in the flow rate. The efficiency for decomposition of chloroform remained 90% even at 20.0 mL/min at a Ag-impregnated CFE. This result suggests that the chloroform is adsorbed more rapidly on the surface of Ag than Zn and then is decomposed electrochemically. These results demonstrate a high electrocatalytic activity for decomposition of chloroform at a Ag-impregnated CFE in the flow system. Therefore, we used a Ag-impregnated CFE in the subsequent experiments. In the experiments mentioned above, we added 0.5 M K2SO4 to the solution as a supporting electrolyte for efficient
TABLE 2. The Efficiency for Decomposition of Chlorohydrocarbons by Electrochemical Decomposition at a Ag-Impregnated CFE in the Flow Cella compds tetrachloroethylene trichloroethylene 1,1,1-trichloroethane 1,1,2-trichloroethane dichloromethane 1,2-dichloroethane
potential vs effic for Ag/AgCl/V current/mA decomposition/% -1.2 -1.2 -1.6 -2.0 -1.2 -1.2 -1.2 -1.6 -2.0 -1.2 -1.6 -2.0
2 3 32 80 2 4 4 48 125 5 37 71
100 77.8 95.1 95.9 100 100 39.8 48.1 52.1 29.8 25.1 44.3
a Flow rate: 1.0 mL/min, electrolyte: 50 mL of 0.5 M K SO , the initial 2 4 concentration, tetrachloroethylene: 33.1 mg/L, trichloroethylene: 28.8 mg/L, 1,1,1-trichloroethane: 26.2 mg/L, 1,1,2-trichloroethane: 28.8 mg/ L, dichloromethane: 26.6 mg/L, 1,2-dichloroethane: 25.6 mg/L.
FIGURE 3. Relationship between the flow rate and the efficiency for decomposition of chloroform at metal-impregnated CFEs: b, Zn; O, Ag; potential, -1.2 V vs Ag/AgCl; and electrolyte, 0.5 M K2SO4.
FIGURE 4. Relationship between the concentration of K2SO4 and the efficiency for decomposition of chloroform at a Ag-impregnated CFE. At a flow rate of b, 1 mL/min; 2, 5 mL/min; and 9, 20 mL/min. electrical conduction. In actual tap water, such a high concentration of electrolyte is not dissolved. The relationship between the concentration of the supporting electrolyte (K2SO4) and the efficiency for decomposition of chloroform is shown in Figure 4. The efficiency for decomposition of chloroform depended largely on the flow rate in a solution of dilute supporting electrolyte. At a flow rate of 1 mL/min, the efficiency for decomposition of chloroform was almost independent of the concentration of K2SO4, and chloroform was not detected from electrochemically-treated water by our present analysis system. Even in the absence of the supporting electrolyte, chloroform was decomposed at an
efficiency of almost 100% at a flow rate of 1 mL/min. At a flow rate of 5 mL/min, the efficiency for decomposition of chloroform decreased gradually with a decrease in the concentration of K2SO4. At a concentration of 5.0 × 10-4 mol/ L, the efficiency for decomposition of chloroform was about 80%. At a flow rate of 20 mL/min, the efficiency for decomposition of chloroform decreased steeply with a decrease in the concentration of K2SO4. This steep decrease in the efficiency for decomposition of chloroform at a flow rate of 20 mL/min may be explained as follows. With a decrease in the concentration of the supporting electrolyte, the electrical resistance of the solution increases, and the potential would not be imposed on the entire CFE homogeneously. Therefore, the available area of the CFE for the treatment of chloroform decreased and was not sufficient for decomposition of chloroform at a high flow rate. In the solution with no K2SO4, chloroform was almost decomposed at a flow rate of 1 mL. Originally, no electrical current flows in pure water, because of its high resistivity. Therefore, the current in distilled water without K2SO4 shown in Figure 4 would be carried by a slight amount of ions contained as impurities in distilled water and/or a CFE. This result supports the possibility of the application for decomposition of chloroform contained in tap water using a Ag-impregnated CFE system at a low flow rate, because natural water contains certain concentration of CaCO3 and other ions and current would flow to a certain extent. Now we are improving the structure of the cell to impose a potential on the entire column electrode homogeneously. We also attempted electrochemical decomposition of other toxic chlorohydrocarbons, tetrachloroethylene, trichloroethylene 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,2dichloroethane, and dichloromethane, at a Ag-impregnated CFE. These chlorinated compounds are the main components that contaminate groundwater. The efficiencies for decomposition of these toxic chlorohydrocarbons at the potentials of -1.2, -1.6, and -2.0 V vs Ag/AgCl are summarized in Table 2. Tetrachloroethylene, 1,1,1-trichloroethane, and 1,1,2trichloroethane were decomposed at -1.2 V at an efficiency of almost 100%. The efficiency for decomposition of trichloroethylene at -1.2 V was 77.8%. The efficiency for decomposition of trichloroethylene increased with the increasing negative potential imposed on the cathode CFE: 95.1% at -1.6 V. When the cathode potential was made more negative, the efficiency for decomposition hardly changed and the production of H2 increased. 1,2-Dichloroethane and dichloromethane showed persistence for electrochemical decomposition. VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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For the actual treatment of tap water, it is important that the compounds produced in the process of decomposition of chloroform are harmless to human health and the cost of the treatment of water is not so high. The main products of the electrochemical treatment of chloroform in this system are methane and chloride ion. These compounds are the most harmless among the considerable products from decomposition of chloroform, because chloride ion is contained in natural water and most foods and methane is a very inactive gas. This electrochemical system can be attached to a tap water faucet. Therefore, modification of the public water supply and its disinfection facility is unnecessary. Electricity for decomposition of chloroform (not including the electricity for driving the potentiostat) is only 0.2 kWh/m3. It costs about 3 cents in Tokyo at the rate of 1$ ) ¥120. With further improvement of the material of the electrode and the structure of the flow cell, this system would be useful for the treatment of tap water to remove trihalomethane. The actual concentration of trihalomethane is about 10 ppb at most. Now we are developing a flow cell that is more suitable for decomposition of trihalomethane in tap water, and an attempt to decompose chloroform at ppb levels in tap water is in progress. Based on the results mentioned above, conclusions are as follows. Using metal-impregnated CFEs in the flow cell, dilute chloroform dissolved in water that contained 0.5 M K2SO4 was decomposed at an efficiency of almost 100%. A Ag-impregnated CFE also decomposed tetrachloroethylene, trichloroethylene, 1,1,1-trichloroethane, and 1,1,2-trichloroethane at high efficiency. At a Ag-impregnated CFE, chloroform was decomposed at an efficiency of 100% in water hardly containing a supporting electrolyte at a flow rate of 1 mL/min. With further improvement of the electrode and the structure of the cell, this system will be useful in the treatment of tap water.
Acknowledgments We thank the Hokuto Denko Corporation for their financial support and their provision of a flow cell, a potentiostatgalvanostat, and a chemical pump. We also thank Mr. Fushimi and Ms. Hosono of the Meidensha Corporation for analysis of Cl- ion in water.
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Received for review September 1, 1998. Revised manuscript received July 6, 1999. Accepted July 15, 1999. ES980903G
