Environ. Sci. Technol. 2002, 36, 778-783
Novel Electrode System for Electroflotation of Wastewater XUEMING CHEN, GUOHUA CHEN,* AND PO LOCK YUE Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Electroflotation (EF) is an attractive method in wastewater treatment. The heart of EF is the dimensionally stable oxygen evolution anode that is usually expensive. In this paper, we present a stable anode by coating IrOx-Sb2O5SnO2 onto titanium. Accelerated life test showed that the electrochemical stability of the Ti/IrOx-Sb2O5-SnO2 anode containing only 2.5 mol % of IrOx nominally in the activated coating was even higher than that of the conventional Ti/IrOx anode. Its service life for electroflotation application is predicted to be about 20 yr. Voltammetric investigation demonstrated that the Ti/IrOx-Sb2O5-SnO2 anode could provide fast electron transfer. Moreover, the present anode was designed to be fork-like and arranged interlockingly at the same level as the cathode with a similar shape. Such an innovation in electrode configuration and arrangement allows bubbles produced at both electrodes to be dispersed into wastewater flow quickly and, therefore, enhances the effective contact between bubbles and particles, favorable for high flotation efficiency. In addition, the novel electrode system reduces the interelectrode gap to 2 mm, a spacing that is technically difficult for a conventional electrode system. This small gap results in a significant energy saving. Easy maintenance is found to be another advantage of this novel electrode system.
Introduction In water and wastewater treatment, flotation is the most effective process for the separation of oil and low-density suspended solids (1-5). Although dissolved air flotation (DAF) is predominant in use today, electroflotation (EF) has been an attractive alternative because of its important features of high separating efficiency, simple operation, and few accessories required. It can be used to treat palm oil mill effluent (6), oil-water emulsion (7), mining wastewater (8), groundwater (9), food processing wastewater (10), restaurant wastewater (11), and many other water and wastewaters (1215). EF was first proposed by Elmore (16) in 1904 for flotation of valuable minerals from ores. This process differs from DAF mainly in the mechanism of bubble generation. The electrode system is the most important part and is thus considered to be the heart of an EF unit. Despite the long history of EF, the lack of ideal anodes with long service lives, low O2 evolution overpotentials, and acceptable costs is still a big problem that needs to be solved imminently. Although iron, aluminum, and stainless steel are cheap, readily available, and able to fulfill the simultaneous electrocoagu* Corresponding author phone: (852)2358-7138; fax: (852)23580054; e-mail:
[email protected]. 778
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lation (EC) and EF, they are anodically soluble (8, 12, 14, 17). To make matters worse, the bubbles generated at partially dissolved electrodes usually have large sizes due to the coarse electrodes surfaces (11). Graphite and lead oxide are among the most common insoluble anodes used in EF (6, 13, 14). They are also cheap and easily available, but both show high O2 evolution overpotential and low durability. In addition, for the PbO2 anodes, there exists a possibility to generate highly toxic Pb2+, leading to severe secondary pollution. A few researchers reported the use of Pt or Pt-plated meshes as anodes (9, 18). They are much more stable than graphite and lead oxide. However, the known high cost makes largescale industrial applications impracticable. It is well-known that TiO2-RuO2 types of dimensionally stable anodes (DSA) discovered by Beer (19) possess a high quality for chlorine evolution in the chlor-alkali industry. However, their service lives are insufficient for oxygen evolution (20). In the past decade, IrOx-based DSA have received much attention. IrOx presents a service life about 20 times longer than that of the equivalent RuO2 (21). In general, Ta2O5, TiO2, and ZrO2 are used as stabilizing or dispersing agents to save cost and/or to improve the coating property (22-25). Occasionally, a third component such as CeO2 is also added (21, 26). It should be noted that although incorporation of Ta2O5, TiO2, and ZrO2 can save IrOx loading, the requirement of molar percentage of the precious Ir component is still very high. The optimal IrOx contents are 80 mol % for IrOx-ZrO2, 70 mol % for IrOx-Ta2O5, and 40 mol % for IrOx-TiO2, below which electrode service lives decrease sharply (23). Among the various electrocatalysts mentioned above, IrOx-Ta2O5 has the highest electrochemical stability. The IrOx-Ta2O5-coated titanium electrodes have been successfully used as anodes of EF (11, 27). Nevertheless, due to consumption of large amounts of the IrOx, Ti/IrOxTa2O5 electrodes are very expensive, limiting their wide application. There are also some problems with the arrangement of electrodes. Usually, an anode is installed at the bottom, while a stainless steel screen cathode is fixed at 10-50 mm above the anode (9, 12, 13). Such an electrode arrangement cannot ensure quick dispersion of the oxygen bubbles generated at the bottom anode into wastewater flow, affecting flotation efficiency. Moreover, if the conductivity of wastewater is low, energy consumption will be unacceptably high due to the large interelectrode spacing required for preventing the short circuit between the upper flexible screen cathode and the bottom anode. This makes EF unfavorable to compete with DAF economically. The objective of this paper is to demonstrate the superior performance of a novel EF electrode system with an open configuration using stable and cost-effective Ti/IrOx-Sb2O5SnO2 as an anode. As indicated in our precious works (28, 29), Ti/IrOx-Sb2O5-SnO2 anodes have extremely high electrochemical stability and good electrocatalytic activity for oxygen evolution. A Ti/IrOx-Sb2O5-SnO2 electrode containing only 10 mol % IrOx nominally in the oxide coating could be used for 1600 h in an accelerated life test and was predicted to have a service life over 9 yr in strong acidic solution at a current density of 1000 A/m2. Considering the much lower current density used and nearly neutral operating environments in EF, the IrOx content in the coating layer was further reduced to 2.5 mol % in this work to lower the electrode cost. The results obtained show that the Ti/IrOxSb2O5-SnO2 anode using reduced IrOx still remains sufficient electrochemical stability and good activity and is thus believed to be very suitable for application in EF. Meanwhile, 10.1021/es011003u CCC: $22.00
2002 American Chemical Society Published on Web 01/17/2002
FIGURE 1. Schematic diagram of the experimental setup. 1, Magnetic stirrer; 2, tank; 3, pump; 4, CE cell; 5, EF cell; 6, sludge chamber; 7, DC power supply; 8, separation chamber. the open configuration has been proven quite effective in flotation of oil and suspended solids (SS). Significant electrolysis energy saving has also been obtained due to the small interelectrode gap used in the novel electrode system.
Experimental Section Chemicals. All chemicals, including SnCl4‚5H2O (98+%, Acros, NJ), SbCl3 (99+%, Acros, NJ), iridium(IV) chloride hydrate (53.89% Ir, Strem Chemicals, MA), K2SO4 (99+%, Aldrich, WI), potassium ferrocyanide (99%, Sigma, MO), potassium ferricyanide (99%, Sigma, MO), 2-propanol (99.7%, Lab-scan, Bangkok, Thailand), hydrochloric acid (37%, Riedel-deHaen, German), and concentrated sulfuric acid (Acros, NJ) were used as received. Electrolyte solutions were prepared using 18 MΩ‚cm resistivity in deionized water. Anodes Preparation. The electrodes were prepared by a thermal decomposition method. Prior to coating, the titanium substrates underwent sandblasting, tap water washing, 10 min of ultrasonic cleaning in deionized water, 2 min of etching in boiling 37% hydrochloric acid, and another 10 min of ultrasonic cleaning in deionized water. After pretreatment, the titanium substrates were first brushed at room temperature with the precursor, which had a molar ratio of Ir:Sb:Sn ) 2.5:10:87.5 using the mixture of 2-propanol and concentrated hydrochloric acid as solvents, dried at 80 °C for 5 min to allow the solvents to vaporize, and then baked at 550 °C for 5 min. This procedure was repeated for 15-18 times. Finally, the electrodes were annealed at 550 °C for 1 h. The total oxide loading of the prepared electrodes was about 15 g/m2. More details of the preparation procedure can be found elsewhere (28). The cylinder electrodes with a dimension of 5.3 mm long and 12 mm in diameter and an effective area of 2 cm2 were used for morphology analysis and electrochemical characterization, while a fork-like electrode was used for EF. Morphology Analysis. The surface morphology of the electrode prepared was analyzed by scanning electron microscopy (SEM; JSM-6300F, JEOL, Japan). Accelerated Life Test. A good electrode should be able to work effectively for several years. It is thus time-consuming to test electrode stability under normal conditions. To reduce the test time, the accelerated life test after Hutchings et al. (30) was used to assess the electrode stability. The electrolyte
was 3 M H2SO4, and the cell temperature was approximately 35 °C. A dc power supply (PD110-5AD, Kenwood, Japan) was used to provide a constant anodic current density of 10 000 A/m2. The potential of the working electrode was periodically monitored using saturated Ag/AgCl,KCl as a reference electrode and quoted with respect to the normal hydrogen electrode (NHE). Because of generation of large amounts of bubbles, use of a Luggin capillary was impossible. The reference electrode was placed as close as possible to the working electrode however. Because the ohmic drop from the solution was not compensated, the true potential could be a bit smaller than that measured. Cyclic Voltammetric (CV) Investigation. CV was performed in both 0.5 M H2SO4 and solution of 10 mM [Fe(CN)6]3-/[Fe(CN)6]4- and 0.2 M K2SO4 with a standard three-electrode cell (RDE0018, EG&G) and a potentiostat/ galvanostat (PGSTAT 100, Autolab, The Netherlands). Pt wire was used as a counter-electrode, and saturated Ag/AgCl,KCl with a Luggin capillary served as a reference electrode. The resistances between a working electrode and the Luggin capillary were measured using the frequency response analyzer of the potentiostat/galvanostat. The ohmic drops in the solutions have been compensated. Bubble Size Measurement. The bubble sizes were determined using a phase-Doppler anemometer (31). The principle is based on a simple scattering theory where the received light was scattered by either refraction or reflection. A scheme recently developed in another laboratory of HKUST was used to optimize the off-axis angle to minimize the measurement uncertainty. A combined refraction-reflection model and Mie scattering theory were applied in the new scheme. The resolution of the state-of-the-art configuration can be as small as 1 µm (32). Wastewater Treatment. The experimental setup is schematically shown in Figure 1. EF was coupled with electrocoagulation (EC). In this combined process, EC mainly plays the role of destabilizing and aggregating the fine particles, while EF is responsible for floating the flocs formed in the EC cell. The effective dimensions of the apparatus are 250 mm × 18 mm × 52 mm for the EC cell, 170 mm × 40 mm × 52 mm for the EF cell, and 250 mm × 78 mm × 52 mm for the separation chamber. The total effective volume is 1.6 L. Three aluminum plates with a dimension of 100 mm VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Potential change during accelerated life test in 3 M H2SO4 solution under 1 A/cm2 at 35 °C.
FIGURE 2. Surface morphology of Ti/IrOx-Sb2O5-SnO2: (a) fresh electrode; (b) after accelerated life test. × 55 mm × 3 mm were used as electrodes of EC, while forklike Ti/IrOx-Sb2O5-SnO2 and Ti that had also undergone a sandblast pretreatment were used as the anode and the cathode of EF, respectively. The effective electrode areas of EF are 6 cm2 for the anode and 9 cm2 for the cathode. The gap between the fork-like electrodes is 2 mm. EF electrodes shared a dc power supply with EC electrodes. The investigated wastewaters included the industrial wastewater collected from a used oil re-refinery and the synthetic wastewater containing fluoride. Because some of the pollutants, especially SS, might settle during an experimental run, the feeding tank was stirred continuously to maintain a consistent influent to the treatment system. SS were analyzed by the standard methods (33). Conductivity and pH were measured with a conductivity meter (4200, Jenway, U.K.) and a pH meter (420A, Orion, MA), respectively. Chemical oxygen demand (COD) was examined using a COD reactor and a direct reading spectrophotometer (DR/2000, Hach). Oil was first extracted with petroleum ether at pH < 2 and then measured with an UV spectrophotometer (UV-1206, Shimadzu, Japan) at 210 nm using lubricant oil as the reference standard. Fluoride concentrations were analyzed using an ion-selective electrode (9609BN, Orion, MA).
Results and Discussion Surface Morphology. Figure 2 shows typical SEM images of Ti/IrOx-Sb2O5-SnO2. As observed from the electrode con780
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taining 10 mol % IrOx in the IrOx-Sb2O5-SnO2 coating, the present electrode (Figure 2a) also had a smooth surface, with no cracks or pores observed. In addition, after undergoing an accelerated life test, the titanium substrate was still covered well with IrOx-Sb2O5-SnO2 coating as shown in Figure 2b, indicating good bonding between the substrate surface and the coating. Moreover, no cracks and pores were exposed after the accelerated life test, suggesting a compact structure of the whole coating layer. Electrochemical Activity and Stability. Figure 3 displays the potential change during the accelerated life test. The initial potential obtained was 2.26 V vs NHE, comparable with that of the conventional Ti/IrOx electrode at the same condition (28). This reveals that the Ti/IrOx-Sb2O5-SnO2 electrode containing only 2.5 mol % IrOx in the coating layer also has good electrocatalytic activity for oxygen evolution. After being maintained almost unchanged for about 200 h, the potential increased gradually. Sharp potential increase was observed during the last few hours, indicating the failure of the electrode. Electrode deactivation can be the consequence of several mechanisms, including metal base passivation, coating consumption, coating detachment, and mechanical damage (34). Since 83% of coating was still retained and the surface of titanium substrate was covered well with the coating at the end of the accelerated life test, it is concluded that the failure of the Ti/IrOx-Sb2O5-SnO2 electrode was attributed to passivation, i.e., the formation of an insulating TiO2 layer at the interface between the titanium substrate and the coating. Actually, this is the most frequent deactivation mechanism associated with operations at high current densities (34). The service life of an electrode is defined here as the operation time at which the anodic potential increases rapidly by 5 V vs NHE. Therefore, it can be estimated from Figure 3 that the Ti/IrOx-Sb2O5-SnO2 electrode containing 2.5 mol % IrOx in the activated layer has a service life 437 h under the accelerated life test condition. In contrast, the service life of Ti/IrOx is 355 h under the same condition (28). This means that the Ti/IrOx-Sb2O5-SnO2 electrode not only saves 97.5% of the precious Ir content but also exceeds the conventional Ti/IrOx in electrochemical stability. The significant saving of IrOx is attributed to good conductivity of Sb2O5-SnO2, formation of a solid solution, bonding improvement, and compact structure of the IrOx-Sb2O5-SnO2 film (28). The electrode service life is strongly dependent on the current density used. Hine et al. (20) reported that RuO2TiO2-coated titanium electrodes could be used for only 18 h at 20 000 A/m2 in 2 M HClO4 plus 0.2 M NaCl solution, but when the current density was lowered to 5000 A/m2, the
FIGURE 4. Cyclic voltammograms obtained in 0.5 M H2SO4 solution at scan rate of 100 mV/s. lifetime increased to about 200 h. Malkin (35) reported that the lifetime of DSA was about 25 times higher at 1600 A/m2 than at 16 000 A/m2. A simple model relating the service life (SL) to the current density (i) has been obtained (28):
1 SL ∝ n i
(1)
FIGURE 5. Cyclic voltammograms obtained in 10 mM [Fe(CN)6]3-/ [Fe(CN)6]4- + 0.2 M K2SO4 solution at different scan rate: (a) 20, (b) 50, (c) 100, (d) 200, and (e) 400 mV/s.
TABLE 1. Characteristics of Fresh Industrial Wastewater oil SS COD pH conductivity
710 mg/L 330 mg/L 2,120 mg/L 6.78 705 µs/cm
where n ranges from 1.4 to 2.0. According to eq 1, the service life of Ti/IrOx-Sb2O5-SnO2 for EF application (SLEF) at a current density (iEF) can be estimated:
SLEF ) SLa
() ia iEF
n
(2)
where SLa and ia are the service life and the current density under the accelerated life test conditions. Usually, the current density used in EF is below 300 A/m2. Assume an average n of 1.7 and a current density of 300 A/m2, then SLEF ) SLa(ia/iEF)n ) 437h × (10 000 A/m2/300 A/m2)1.7 ) 170 000 h ≈ 20 yr. Since Ti/IrOx-Sb2O5-SnO2 electrodes are easy to fabricate, it is possible to enlarge the electrodes from a laboratory scale to an industrial scale without many problems. However, it is understandable that the real electrode lifetime in industrial application may be shorter than that estimated here due to the possible mechanical wear of the activated coating film. Voltammetric Behavior in 0.5 M H2SO4 Solution. Figure 4 shows cyclic voltammograms obtained on Ti/IrOx-Sb2O5SnO2 in 0.5 M H2SO4 solution. In the first a few scan cycles, the voltammogram changed dramatically as observed on Ti/ SnO2-Sb2O5 (36) and Ti/IrOx (37) probably as a consequence of hydration of the coating surface (38). The shape of voltammogram obtained afterward became consistent quickly. After 100 cycles, the voltammogram kept almost identical, demonstrating the high electrochemical stability of Ti/IrOxSb2O5-SnO2 again. Voltammetric Behavior for [Fe(CN)6]3-/[Fe(CN)6]4Redox Couple. To examine the electron-transfer rate on Ti/IrOx-Sb2O5-SnO2, the voltammetric behavior of the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple was studied. Cyclic voltammograms obtained in 10 mM solution at steady state under different scan rates are shown in Figure 5. Each voltammogram shows a well-defined anodic peak at 0.50 V and a corresponding cathodic peak at 0.44 V. The peak separation value is close to 60 mV and not significantly subjected to the scan rate. These features indicate that the redox reaction of the [Fe(CN)6]3-/[Fe(CN)6]4- couple at
FIGURE 6. Results of treating industrial wastewater under different EF charge loadings at a total retention time of 0.5 h. Ti/IrOx-Sb2O5-SnO2 is reversible and that the Ti/IrOxSb2O5-SnO2 electrode can provide fast electron transfer. Wastewater Treatment. The characteristics of the fresh industrial wastewater tested are listed in Table 1. The major organic pollutants are from used lubricant oils that contain base oils, antioxidant, and antiwear chemicals (39). The major SS contents were the polymerized hydrocarbons, inorganic additives, particles acquired during the service of automobiles, and metal wears (40). Figure 6 shows pollutant removal efficiencies under different charge loadings. Oil separation is very effective. Even at a charge loading as low as 0.5 faraday/m3, the oil was reduced from 710 mg/L to around 10 mg/L with a removal efficiency over 98%. As the charge loading increased to 1.5 faraday/m3, the oil content in the effluent was only 4 mg/L. In contrast, a charge loading of 3 faraday/m3 was necessary to reduce oil from initial 650 mg/L to about 6 mg/L when a conventional electrode system was used with a stainless steel screen being a cathode over an anode (11). Obviously, the EF using the present electrode system has higher separation efficiency. SS separation was also effective. The concentration VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Results of treating industrial wastewater under different retention times at an EC charge loading of 2.98 faraday/m3 and an EF charge loading of 1.49 faraday/m3.
TABLE 2. F- Removal under Different Conditions initial F(mg/L)
residual F- in effluent (mg/L)
EF charge loading (faraday/m3)
10 10 15 15 20 20
1.30 0.99 1.80 1.25 3.05 1.33
1.87 2.49 1.87 2.49 2.49 3.11
of SS can be reduced from 330 to 74 mg/L at a charge loading of 0.5 faraday/m3 and to only 17 mg/L at a charge loading of 3.0 faraday/m3. As seen in the figure, COD in the effluent decreased significantly as the charge loading increased. This is because the Al(OH)3 flocs formed in the EC cell are capable of adsorbing soluble organic pollutants. As the charge loading increased, the amount of Al(OH)3 flocs increased, and thus more soluble organic pollutants were removed. Figure 7 demonstrates the effect of retention time on removal efficiency. It is found that removal efficiency is almost independent of retention time in the large investigated range. Ten minutes is enough to reach effective pollutant removal. This is a very attractive feature for densely populated areas where the compactness of the treatment facility is particularly concerned. The novel electrode system has also been used to remove F- effectively from synthetic wastewater by combining an electrochemical reactor using aluminum plates as electrodes. F- removal is based on the adsorption of F- ions on Al(OH)3 flocs that are generated by electrolysis. The Al(OH)3 flocs having adsorbed F- ions are then separated from wastewater by EF. The experimental results are shown in Table 2. The residual fluorides in the effluents were only 0.99-3.05 mg/L, depending on the initial concentration and the charge loading used, again indicating high separation efficiency of the EF equipment using the novel electrode system. The effective flotation obtained is first attributed to the generation of uniform and tiny bubbles. It is well-known that the separation efficiency of a flotation process depends strongly on bubble sizes. Usually the smaller the bubbles, the better the separation efficiency. This is because smaller bubbles provide larger surface area for particle attachment. The sizes of the bubbles generated in the present system were found to be nearly randomly distributed with over 90% of the bubbles in the range of 15-45 µm. In contrast, typical bubble sizes range from 50 to 70 µm for DAF (41) and from 22 to 50 µm for the conventional electrode system (18). 782
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The effective flotation obtained is also attributed to quick dispersion of the small bubbles generated into the wastewater flow. Quick bubble dispersion is essentially as important as the generation of tiny bubbles. For a conventional electrode system, only the upper screen cathode faces the wastewater flow, while the bottom anode does not contact the flow directly. Therefore, the oxygen bubbles generated at the bottom anode cannot be dispersed immediately into the wastewater being treated. Consequently, some oxygen bubbles may coalesce to form useless large bubbles. This not only decreases the availability of the effective small bubbles but also increases the possibility of breaking the flocs formed previously, affecting the flotation efficiency. The situation for the present electrode system is different. The anode and the cathode are leveled. Such an open configuration allows both the cathode and the anode to contact the wastewater flow directly. Therefore, the bubbles generated at both electrodes can be dispersed into wastewater rapidly and attach onto the flocs formed in the EC cell effectively, ensuring high flotation efficiency. The electrolysis energy consumption of EF can be calculated according to
E ) UI/(1000Q)
(3)
where E is the energy consumption (kWh/m3 wastewater); U is the electrolysis voltage (V); I is the electrolysis current (A); and Q is the wastewater flow rate (m3/h). Equation 3 can be rewritten as
E)
96 500 3600UI 1000 × 3600 96 500Q
(
)
or
E ) 0.0268qU
(4)
where q ) 3600I/96 500Q, EF charge loading (faraday/m3). The electrolysis voltage between two electrodes is the summation of the equilibrium potential difference, anode overpotential, cathode overpotential, and ohmic potential drop of the solution resistance as shown below (42, 43):
d U ) Eeq + ηa + |ηc| + i κ
(5)
where Eeq is the equilibrium potential difference for water split, 1.23 V; ηa is the anode overpotential (V); ηc is the cathode overpotential (V); d is the interelectrode distance (m); κ is the conductivity (S/m); and i is the current density (A/m2). Usually, overpotentials include activation overpotentials and concentration potentials. They are related to many factors including the electrochemical properties of electrodes, the current density used, and the pH value of wastewater. Therefore, direct calculation of the electrolysis voltage using eq 5 is still difficult. However, it is useful to point out that the electrolysis voltage required in an EF process is mainly from the ohmic potential drop of the solution resistance, especially when the conductivity is low and the current density is high. Since the ohmic potential drop is proportional to the interelectrode distance, reducing this distance is of great importance for reducing the electrolysis energy consumption. For a conventional electrode system, due to the easy short circuit between the upper flexible screen electrode and the bottom electrode, use of a very small spacing is technically difficult. The typical value is 10-50 mm (9, 12, 13). But for the present electrode system, the interelectrode gap can be as small as 2 mm. Figure 8 shows the experimental result of U dependence on i at a given conductivity. The current density here refers to the average current density of the anode and the cathode.
FIGURE 8. Electrolysis voltage dependence on current density for the novel electrode system at a conductivity of 705 µs/cm. In the investigated range, U increased linearly with i. According to eq 4 and Figure 8, it is calculated that the electrolysis energy required by the EF using the novel electrode system for treating the industrial wastewater tested in this work is only 0.32 kWh/m3 under the typical operational conditions with a conductivity of 705 µs/cm, a charge loading of 1.5 faraday/m3, and a current density of 150 A/m2. In contrast, it is estimated that the energy consumption for a conventional electrode system with an interelectrode gap of 10-50 mm is as high as 0.9-4.0 kWh/m3 under the same operational conditions. Therefore, the use of the present electrode system can bring about a large energy saving. Easy maintenance is found to be an additional advantage of the present electrode system. For the conventional electrode system, the screen electrode is easy to be twined by undesirable deposits such as fabric and is difficult to clean once twined. Such a problem is solved by using the present electrode system.
Acknowledgments We are very grateful to Dr. Huihe Qiu for measuring the bubble sizes using his state-of-the-art phase-Doppler anemometer. We also thank Mr. K. F. Lo and Mr. S. C. C. Leung for their help in collecting and analyzing wastewater and in examining the electrode surface morphology.
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Received for review May 24, 2001. Revised manuscript received October 8, 2001. Accepted November 8, 2001. ES011003U
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