Ind. Eng. Chem. Res. 2007, 46, 2423-2427
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Decolorization of Textile Dyes by Wet Oxidation Using Activated Carbon as Catalyst Aurora Santos,* Pedro Yustos, Sergio Rodrı´guez, Fe´ lix Garcia-Ochoa, and Miguel de Gracia Departamento Ingenierı´a Quı´mica, Facultad Quı´micas, UniVersidad Complutense Madrid, 28040 Madrid, Spain
A commercial activated carbon, Industrial React FE01606A, without impregnation of any metal, was used as a catalyst in the wet oxidation of three dyes commonly found in textile wastewaters, Orange G, Methylene Blue, and Brilliant Green. Runs were carried out in a three phase fixed-bed reactor by feeding concurrently an aqueous phase containing 1000 mg/L of the dye and an oxygen gas flow rate of 90 mL/min. Temperature was set to 160 °C, and the pressure in the reactor was fixed to 16 bar. The catalyst showed high catalytic activity in dye conversion and color removal. The catalyst kept stable during the time tested on stream (200 h). Total decolorization is obtained at short residence times, but some refractory organic intermediates are obtained (mineralization achieved an asymptotic value about 40-60% depending on the dye). The toxicity of the inlet and outlet effluent was measured by the Microtox bioassay, and the oxidation intermediates identified and quantified explained the obtained toxicity evolution. Introduction Synthetic dyes are commonly found in several industrial wastewaters as they are extensively found in the textile industry,1 in paper production,2 in printing industries,3 and in dye houses.4,5 There are more than 100 000 commercially available dyes, and most of them are synthetic and soluble compounds.6 About ten million killigrams per year of these dyes are annually produced in the world, and approximately one million of them are released into the environment.7-9 Synthetic dyes exhibit great structural diversity, and the chemical classes of dyes more frequently employed on the industrial scale have in their structure azo, anthraquinone, sulfur, triphenylmethyl, and phatalocianine groups. In general dyes are toxic and even carcinogenic, being stable compounds that lead to highly colored effluents. Therefore the removal of textile dyes has become an economical and environmental problem to solve. These effluents cannot be treated through conventional biological processes such as activated sludges10 because of their poor biodegradability. Some other methods have been investigated and developed for the removal of synthetic dyes from waters and wastewaters to decrease their impact on the environment, such as adsorption,11 filtration,12 and chemical coagulation.13 Among the chemical methods, the advanced oxidation processes (AOP) based on the oxidizing effect of the OH• radical, are commonly used for decolorization.14,15 Fenton reagent, which is a typical AOP technology, has the disadvantage of the iron sludge generation,16 and ozonization could be too expensive for high wastewater flow rates or dye concentration.17,18 The wet oxidation technology, with molecular oxygen as the oxidant, looks like a promising method for removal of dyes in wastewater.19 Noncatalytic wet oxidation processes require high pressures and temperatures;20-22 it is only applicable for wastewater with high chemical oxygen demand values (above 10 000 mg/L). However, the use of catalysts permits the temperature to diminish with a decrease in the material and operation costs, and it makes the treatment of more diluted pollutants competitive by this technology.18,22 If a homogeneous * To whom correspondence should be addressed. E-mail: aursan@ quim.ucm.es.
Figure 1. Chemical formula of the acid dye OG (azoic structure).
catalyst, as a transition metal salt, is used, it should be recovered from the effluent.23-25 On the other hand, to implement heterogeneous catalysts23 they must be stable under long operation times. Common problems found are leaching of the active compounds26-28 or deactivation by fouling of the active centers.29,30 Activated carbon is a cheap and stable support that has been already used as catalyst with noble metals31 in the decolorization of textile wastewaters. Most of these studies have been carried out in batch, and the stability has been tested at short operation times. Recently, activated carbon without impregnation of metals has been succesfully used as catalyst in the catalytic wet oxidation (CWO) of several phenols. Its catalytic properties are due to its oxygen surface complexes;32,33 moreover, it is stable in acidic and alkaline media, cheaper than the other catalyst cited, and found to be stable for hundreds of hours.34 The aim of this work is to study the abatement of three synthetic dyes, Orange G (OG), Brilliant Green (BG), and Methylene Blue (MB), commonly used in textile industry wastewater by means of the CWO technology. An activated carbon previously selected elsewhere35 is used as catalyst. While these pollutants are not directly oxidized to CO2 though some organic oxidation intermediates are produced, the intermediate distribution from OG, BG, and MB has been studied, under wide intervals of catalyst concentration values. The knowledge of the corresponding oxidation routes for each dye would be useful for designing a process that allows an aqueous effluent of lower toxicity than the original aqueous stream to be obtained.36 The toxicity of the initial solution fed to the reactor and the effluent from the reactor will be quantified by means of the standard Microtox assay.
10.1021/ie0614576 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
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Figure 2. Chemical formula of the basic dye MB (thiazinic structure).
Figure 3. Chemical formula of the basic dye BG (triaryletane structure).
Experimental Section In this work, the CWO of 1000 mg/L aqueous solutions OG, MB, and BG has been carried out by using a commercial activated carbon Industrial React FE01606A, kindly supplied by Chemviron Carbon, as catalyst. The chemical structures of the three employed dyes are shown in Figures 1-3. OG is a monoazo-conjugated aromatic compound with acid ionization, BG has a thiazinic structure that presents basic ionization, and MB is a triarylethane compound with basic ionization. The physical properties of the catalyst are shown in Table 1. Runs were carried out in an integral fixed-bed reactor (FBR) with co-current up-flow of gas and liquid phases at a temperature of 160 °C and 16 bar of pressure. The gas flow rate (oxygen) was 90 mL min-1 (STP conditions). The FBR reactor is made of a stainless steal tube 0.75 cm in internal diameter and 25 cm in length, with a reactor volume VR ) 11.04 cm3. A weight of 3.5 g was placed in the reactor. The liquid flow rate QL was varied among 12 and 200 mL/h; therefore, the catalyst weight to liquid flow rate ratio (W/QL) was changed from 1.05 to 17.5 g‚min/mL. The residence time (ratio reactor volume to liquid flow rate) was changed from 3.31 to 55.19 min. A total of 15 catalytic runs were carried out (five for each dye: OG, MB, and BG). A scheme of the experimental setup is given elsewhere.33 Some blank runs (four for each dye: OG, MB, and BG) were also accomplished by replacing the catalyst bed with glass spheres of the same diameter (0.8-1 mm). Under the operational conditions employed, the steady state for the outlet composition of the reactor was achieved in the first 20 h of operation. Liquid samples were periodically drawn and analyzed. Dyes were quantified by UV spectrophotometry (Shimadzu, model UV-1603). The wavelengths used for OG, MB, and BG were 478 nm, 662 nm, and 623 nm, respectively. The color of the liquid phase was measured in a colorimeter (Hach, model DR-890 series) with the APHA colorimetric method, by direct comparison with a Platinum-Cobalt Units Color Standard Solution. Some organic intermediates have been analyzed by GC/MS (Hewlett-Packard 6890 GC-MS, with a detector MSD 5973). A HP-INNOWAX (cross-linked poly(ethylene glycol)) 30 m × 0.25 mm × 0.25 µm column has
been used for this intermediate determination at the following conditions: carrier gas, helium, 1.8 mL/min, Tinjector ) 230 °C, Tdetector ) 250 °C, oven temperature T0 ) 90 °C, 3 min, rate 20 °C/min, T1 ) 190 °C, 20 min. Organic acids and anions (nitrites, nitrates) were analyzed by ionic chromatography (Metrohm, model 761 Compact IC) using a conductivity detector; a column of anion suppression Metrosep ASUPP5 (25 cm long, 4 mm diameter) was used as the stationary phase, and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 was used as the mobile phase, at a constant flow rate of 0.7 mL min-1. Total organic carbon (TOC) values in the liquid phase were determined with a Shimadzu TOC-V CSH analyzer by oxidative combustion at 680 °C, using an infrared detector. Microtox Toxicity Text. The toxicity of the liquid samples at the reactor outlet obtained at different oxidation conversions of the pollutants was determined by means of a bioassay following the standard Microtox test procedure (ISO 11348-3, 1998; based on the decrease of light emission by Photobacterium phosphoreum resulting from its exposure to a toxicant), using a Microtox M500 analyzer (Azur Environmental). The inhibition of the light emitted by the bacteria was measured after 15 min of the contact time. The EC50 is defined as the effective nominal concentration of toxicant (mg/L) that reduces the intensity of light emission by 50%. The parameter IC50 is defined as the percental ratio of the initial volume of sample to the one yielding, after the required dilution, a 50% reduction of the light emitted by the micro-organisms. The toxicity units of the wastewater are calculated as
TUs )
100 IC50
(1)
Before measuring the toxicity, the pH values of all the samples were re-adjusted to between 6 and 7, to prevent the pH effect. All the chemicals used were purchased from SigmaAldrich, and the micro-organisms were Microtox Acute Reagent supplied by I.O. Analytical. Results and Discussion Figure 4 shows conversion and mineralization values of the three dyes, OG, MB, and BG, versus the time on stream for a fixed value of the ratio catalyst weight (W) to liquid flow rate (Q), 1.05 g‚min/mL, used in the CWO reactor. As can be seen, about 20 h are necessary to achieve the steady state, and when it is reached, the catalyst keeps its activity and stability for at least 200 h, which has been the studied range. The catalyst weight lost was found to be less than 5% in this period of time. As can be deduced from Figure 4 at initial values of the time on stream the removal of the dye is taking place by both adsorption and oxidation at the catalyst surface. Moreover, this catalyst surface is also changing in this period while the
Table 1. Physical Properties of the Activated Carbon Industrial React FE01606A Employed as Catalyst Vmesopore (cm3/g) activated carbon (Chemviron Carbon)
SBET (m2/g)
Sg (m2/g)
Vt (cm3/g)
Vmicropore (cm3/g)
D < 80 Å
D > 80 Å
Vmacropore (cm3/g)
Ind React FE0160A
745
63
0.598
0.322
0.025
0.095
0.156
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Figure 6. Toxicity units at the reactor effluent for the CWO of OG, MB, and BG. PO2 ) 16 bar; T ) 160 °C. C0,dye ) 1000 mg/L. Figure 4. Catalyst stability. Dye and TOC conversion vs time on stream. T ) 160 °C, P ) 16 bar. W/Q ) 1.05 g‚min/mL. C0 ) 1000 mg/L dye.
Figure 5. Dye, color, and TOC conversion for the CWO of (a) OG, (b) MB, and (c) BG. PO2 ) 16 bar; T ) 160 °C. C0,dye ) 1000 mg/L.
micropores are lost and the surface available by the oxidation reaction corresponds to mesopores. When the steady state is achieved (i.e., the normal operation in a continuous reactor as used here), the removal of the dye from the liquid phase occurs by oxidation: the amount of adsorbed dye is not increasing with the time on stream, the rates of adsorption and oxidation at the catalyst surface being similar. Negligible conversion and mineralization of the dyes were obtained without catalyst at the operation conditions tested. The results obtained in the CWO experiments for dye conversion, mineralization, and color removal versus the ratio W/Q are plotted in Figure 5 for OG (a), MB (b), and BG (c). As can be deduced, almost complete pollutant conversion and color removal is achieved for the three textile dyes tested at W/Q values lower than 4 g‚mL/min (that correspond to VR/QL values lower than 15 min). However, asymptotic TOC conversion values of 40% for OG (Figure 5a) and 60% for MB and BG (Figure 5b,c) are also reached at short reaction times. That means that the initial pollutants OG, MB, or BG are not completely oxidized to CO2 and much larger quantities of oxidation intermediate compounds are generated. Figure 6 shows the toxicity units of the liquid-phase samples
at the reactor outlet obtained in the CWO of OG, MB, and BG. As it can be seen in Figure 6 at the initial stage of the BG oxidation, a remarkable increase of toxicity occurs while only a slight increment or a decrease of the toxicity units is produced in the CWO of MB and OG, respectively. Only small amounts of acetic acid identified by ionic chromatography and traces of aromatic intermediates (naphthol and nitronaphthol) detected by GC-MS analysis have been identified and quantified, but as it can be seen in Figure 6, even though there are non-identified oxidation intermediates the oxidation route of the OG leads to a nontoxic effluent. No sulfonic aromatic intermediates were detected in the OG oxidation pathway; therefore, the sulfonic group may have been oxidized to sulfate and sulfuric acid, explaining the pH evolution to acid values in the absence of large quantities of short chain acids. No ammonia, nitrites, or nitrates were detected, so the initial azo group may be converted to molecular nitrogen (N2) or remain close to an aromatic ring as organic nitrogenous intermediates. Short chain organic acids such as acetic, formic, malonic, maleic, and oxalic acid and traces of 3-carbamide-thioxazole, acetamide, oxamide, and 2-hydroxypropanamide were identified and quantified as oxidation intermediates of MB. The formation of the thioxazole intermediate detected may be explained because one of the oxidative pathways of MB occurs by direct oxidation of both dimethylaminophenolic structures of the MB, and this is due to the sulfur atom in MB involved in a cation with double-aromatic and single-aromatic bonds, decreasing its reactivity. The atomic nitrogen in MB is converted into ammonia and short chain amide compounds, and no nitrites or nitrates were detected. As it can be seen in Figure 6, complete detoxification of MB effluents was not achieved, maybe as a result of a non-identified intermediate compound more toxic than MB. The primary products of the catalytic BG oxidation are 3-benzyl-N-ethylaminobenzene, 3-benzyl-N,N-diethylaminobenzene, traces of 2-methylquinoline, aniline, N-ethyl aniline, and N,N-diethyl aniline, these compounds being further oxidized at a later stage to benzoic acid, benzamide, dihydroxyethyl methylamine, aliphatic amides, and short chain acids. This arylmethane dye has a conjugated bond system with an electronacceptor type substitute such as a quaternary amino group and an electron-donor type substitute such as the diethyl amino group, at the molecule endpoints; in addition, the third phenyl group to the carbon central atom enriches with π electrons to the central carbon atom. This extended p-conjugated system is easily oxidized by attack on the methylene group to obtain phenyl derivative intermediates, or the attack of the oxidant agent is produced on the aromatic ring close to the quaternary amino group. No ammonia, nitrites, or nitrates were detected
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Figure 7. Proposed oxidation pathway for the CWO of BG by using activated carbon as catalyst.
from the nitrogen atoms of BG. A maximum in the toxicity of the BG is shown in Figure 6, corresponding to the highest concentration of the toxic oxidation intermediates containing amino groups, and therefore the obtained aromatic amines are more toxic than the original pollutant. As the ratio W/Q increases, these kinds of intermediates are further oxidized to benzoic acid and short chain acids, decreasing the toxicity of the effluent. The proposed oxidation pathway for the CWO of BG shown in Figure 7 explains the obtained toxicity. From the results in Figure 7, it can be surmised that BG is oxidized through three different routes. One of these is the oxidation via ring opening reaction of the phenyl system that contains a quaternary amino group to produce a non-cationic biphenyl methylene intermediate and subsequent attack to oxidize the methylene group which leads to the formation of aromatic molecules with lower molecular weight like anilines and benzoic acid; on the other hand, BG is oxidized directly to aromatic functionalized intermediates such as aniline, N-aliphatic anilines, and benzoic acid. The third oxidation route occurs via transposition to obtain methyl quinoline being further oxidized to the corresponding benzoic acid and short chain acids and
acetic amide. The formation of 1-methylquinoline can be explained by reaction between aniline (formed in the oxidation process and detected by GC/MS) with a carboxy-ketonic compound containing at least one methylene group R to the carbonyl moiety, and after that, cyclization to the quinoline is produced such as it occurs in the Frienla¨nder quinoline synthesis. The carboxy-ketone must be obtained during the oxidation process, but there is no oxidation compound with this kind of structure that was detected. Conclusions The CWO of textile dyes by using activated carbon as the catalyst seems to be a promising technology for the removal of this kind of pollutant, although the efficiency on the detoxification achieved depends on the dye. Complete pollutant removal and noncolored effluents are obtained. Complete mineralization is not achieved as initial pollutant is not directly oxidized to CO2 but oxidation intermediates are formed. Toxic ones are obtained in CWO of MB while nontoxic effluents are obtained in CWO of OG. A maximum in the toxicity of the CWO of
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BG is achieved as a result of nitrogen containing aromatic intermediates. No deactivation or significant catalyst loss weight was observed in the range of time on stream studied (200 h). Steady state is achieved in 20 h. Acknowledgment The authors acknowledge financial support for this research from the Spanish MEC (Contract No. PPQ2003-01452) and CAM (S-0505-AMB-000395). The authors wish also to thank Chemviron Carbon for kindly supplying the catalyst used in this work. Literature Cited (1) Grupta, G. S.; Shukla, S. P.; Prasad, G.; Singh, V. N. China clay as an adsorbent for dye house wastewaters. EnViron. Technol. 1992, 13, 925. (2) Ivanov, K.; Gruber, E.; Schempp, W.; Kirov, D. Possibilities of using zeolite as filler and carrier for dyestuffs in paper. Papier (Bingen, Ger.) 1996, 50, 456. (3) Claus, H.; Faber, G.; Konig, H. Redox-Mediated decolorization of shyntethic dyes by fungal laccases. Appl. Microbiol. Biotechnol. 2002, 59, 672. (4) Arslan, I.; Balcioglu, I. A.; Bahnemann, D. W. Heterogeneous photocatalytic treatment of simulated dyehouse effluents using nobel TiO2 photocatalyst. Appl. Catal., B 2000, 26, 193. (5) Allegre, C.; Moulin, P.; Maisseu, M.; Charbit, F. Savings and reuse of salts and water present in dye house effluents. Desalination 2004, 162, 13. (6) Khehra, M. S.; Saini, H. S.; Sharma, D. K.; Chadha, B. S.; Chimni, S. S. Biodegradation of azo dye C.I. Acid Red 88 by an anoxic-aerobic sequential bioreactor. Dyes Pigm. 2006, 70, 1. (7) Wong, Y. C.; Szeto, Y. S.; Cheung, W. H.; McKay, G. Equilibrium studies for acid dye adsorption onto chitosan. Langmuir 2003, 19, 7888. (8) Kim, T. H. K.; Park, C.; Lee, J.; Shin, E. B.; Kim, S. Pilot scale treatment of textile wastewater by combined process. Fluidized biofilm process-chemical coagulation-electrochemical oxidation. Water Res. 2002, 36, 3979. (9) Sakalis, A.; Mpoulmpasakos, K.; Nickel, U.; Fytianos, K.; Voulgaropoulos, A. Evaluation of a Novel Electrochemical Pilot Plant Process for Azodyes Removal from Textile Wastewater. Chem. Eng. J. 2005, 111, 63. (10) Shaul, G. M.; Holdsworth, T. J.; Dempsey, C. R.; Dostal, K. A. Fate of water soluble azo dyes in the activated sludge process. Chemosphere 1991, 22, 107. (11) Centri, G.; Perarthoner, S. Oxidation Catalyst: New trends. Curr. Opin. Solid State Mater. 1999, 4, 74. (12) Capar, G.; Yetis, U.; Yilmaz, L. Membrana based strategies for the pre-treatment of acid dye bath wastewaters. J. Hazard. Mater. 2006, 135, 423. (13) Koprivanac, N.; Bosanac, G.; Grabaric, Z.; Papic, S. Treatment of wastewaters from dye industry. EnViron. Technol. Lett. 1993, 14, 385. (14) Slokar, Y. M.; Le Marechal, A. M. Methods of decoloration of textile wastewaters. Dyes Pigm. 1998, 37, 335. (15) Park, D.; Chang, W. Decolorizing dye wastewater with low temperature catalytic oxidation. Water Sci. Technol. 1999, 40, 115. (16) Robinson, T.; McMulland, G.; Marchant, R.; Nigam, P. Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresour. Technol. 2001, 77, 247. (17) Xu, Y.; Lebrun, R. E. Treatment of textile dye plant effluent by nanofiltration membrane. Separ. Sci. Technol. 1999, 34, 2501.
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ReceiVed for reView November 14, 2006 ReVised manuscript receiVed February 8, 2007 Accepted February 9, 2007 IE0614576
