Ind. Eng. Chem. Res. 2001, 40, 1083-1089
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Promoted Wet Oxidation of the Azo Dye Orange II under Mild Conditions Ivan I. Raffainer and Philipp Rudolf von Rohr* Institute of Process Engineering, Swiss Federal Institute of Technology (ETH), Sonneggstrasse 3, CH-8092 Zurich, Switzerland
Wet oxidation (WO) reactions of the azo dye Orange II were carried out under moderate conditions (T ) 130-190 °C, pH ) 2, poxygen ) 1.0 MPa) with addition of a promoter and FeIISO4. The promoted wet oxidation (PWO) leads to a faster decay of the dye compared with the unpromoted WO experiments. The decomposition of the dye can be described by first-order kinetics over the temperature range. The temperature dependency can be described by an Arrhenius relationship. A reduction in TOC of 70% was achieved at 160 and 190 °C. The influence of the promoter was established at 160 °C. The results indicate a strong dependency on the dye decomposition rates and show a changing reaction order. Whereas the nonpromoted decay can be described with zero-order kinetics, the addition of the promoter changes the decay to first-order kinetics. A combined rate law was adapted for the experimental results to describe the influence of the promoter on the dye decomposition. The influence of the initial promoter concentration was found to be of first order. The combination of the pH, the ferrous ions, and the promoter allows for degradation rates of the azo dye comparable with those of heterogeneously catalyzed wet oxidation. The amount of added promoter is, therefore, an important parameter in the promoted wet oxidation. Introduction Environmental constraints and an unfavorable public opinion require special attention by manufacturing industries to their waste streams. Usually, recalcitrant substances or high loads of organic pollutants can not be economically treated in a biological wastewater treatment plant. An attractive approach is to combine a pretreatment step with a sewage treatment plant. By converting the persistent or toxic pollutants to biodegradable substances, the biological treatment step removes all organic contaminants.1 The presented approach uses pure oxygen gas as an oxidizing agent so that the total pressure is low in comparison with processes using air. Because of the salinity (mainly chlorides) and high pressures and temperatures, the lining of the reactor is usually made of titanium. To decrease the temperature and therefore the system pressure, the conventional wet oxidation has to be enhanced by an appropriate homogeneous or heterogeneous catalyst. Other approaches to enhancing the capabilities have been discussed. For example, cooxidation with easily oxidizable substances increases the degradation of recalcitrant substances, as reported by Boock and Klein.2 Debellefontaine and co-workers3 adapted hydrogen-peroxide-promoted WO to treat olive mill wastewater. Horak4 and Vogel5 have shown that the addition of a promoter enhances the decomposition of refractory substances. The proposed system, which includes mild temperatures with a promoter and Fe ions, was successfully adapted to the wet oxidation of phenol.5 The semi-industrial application of the proposed * Author to whom correspondence should be addressed. Tel.: 0041 1 632 2488. Fax: 0041 1 632 11 41. E-mail:
[email protected].
system is shown by the work of Harf et al.6 To enlarge the range of examined model pollutants for this system, we have chosen an azo dye as a possible contaminant in dye-house effluents. Azo dyes are known to be refractory pollutants. Azo dyes are slowly biodegraded, even with carefully selected microorganisms and under favorable conditions.7 Therefore, years of research have been aimed at the decolorization of azo dye effluent. To decolorize these effluents, several physical and chemical methods or their combinations have been proposed, e.g., precipitation,8 ozonation,9 etc. Such approaches either are expensive for larger effluent streams or provide only a relocation of the disposal problems. Promoted wet oxidation seems to be a promising alternative for dyehouse and dye production wastewaters. To examine promoted wet oxidation, Orange II is used as a model pollutant. The azo dye Orange II (or Acid Orange 7, CI ) 15 510) is a tropaeolin dye and is well examined not only by advanced oxidation processes (AOP’s) (e.g., Bandara et al.10) but also by WO with or without a heterogeneous catalyst.11 So far, there have been a number of efforts on the oxidative degradation of azo dyes as a model pollutant, and this contribution adds some facts to this field. The degradation of organic compounds by wet oxidation under subcritical conditions does not lead to total mineralization but to carboxylic acids, as reported by Copa.12 These carboxylic acids and biodegradable intermediates are the desired products of the PWO. The formation of carbon dioxide by WO/PWO is not favorable because of the lower cost of mineralization by biological wastewater treatment. An approximate mineralization stoichiometry (TOD ) 1690 mg of O2/g of Orange II, measured COD ) 1706 ( 31 mg of O2/g of Orange II) of
10.1021/ie000629a CCC: $20.00 © 2001 American Chemical Society Published on Web 01/25/2001
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Figure 1. Schematic drawing of the bench-scale system used in this work.
the Na salt of Orange II at acidic pH can be suggested by
C16H11N2O4SNa + 18.5O2 f 16CO2 + 5H2O + N2 + Na+ + HSO4- (1) Assuming the formation of acetic acid as final carbon product, the amount of necessary oxygen is lower (228 mg of O2/g of Orange II) and the stoichiometry might look like
C16H11N2O4SNa + 2.5O2 + 11H2O f 8CH3COOH + N2 + Na+ + HSO4- (2) This stoichiometry should show the possible decrease of the amount of spent oxygen gas during the wet oxidation process. By converting the recalcitrant molecules to biodegradable substances, the amount of oxidant is decreased in the WO unit, but it is higher in the connected wastewater treatment plant. The overall stoichiometry is not known a priori, and experimental investigations must be performed to gain results for the promoted wet oxidation. Experiments Materials and Apparatus. Figure 1 shows the bench-scale system. An autoclave HPM-P-4 (Premex Reactor AG, Lengnau, Switzerland) is used as a batch reactor. The vessel is made of titanium, and the reactor volume is 4 L. The vessel can be lowered or lifted by a hydraulic system for filling and cleaning. The reactor lid is made of Hastelloy C-22; all of the fittings and the piping are fixed at this lid. The stirrer and baffles are made of titanium. A circulating oil heating/cooling system is used to control the temperature in the autoclave. The oxidant, i.e., oxygen gas, was taken directly from the pressurized bottle. The system is designed for a maximum pressure of 4 MPa and a maximum temperature of 250 °C. The used experimental setup enables a heat-up time of around 50 min to reach a temperature of 190 °C. For the wet oxidation experiments, the azo dye Orange II (2 g) was dissolved in 40 mL of deionized water, and the solution was inserted into a container. This container was connected to the reactor, and the contents could be pressurized with argon. To inject the temperature-sensitive dye, the concentrated solution of the dye could be inserted into the container (volume of 45 mL). The reactor was filled with 1.96 L of deionized and acidified water (pH ) 2, H2SO4). After 2 min of
evacuation, the solution was heated. Having reached the desired temperature, the contents of the container were injected. After the injection, the temperature of the solution dropped by no more than 4 K. After 2 min of mixing, the desired reaction temperature was reached again, and a sample was drawn. The stirrer was shut down, and the oxygen pressure was adjusted. When the stirrer was restarted, oxygen was forced through the solution. The time at which that occurs is defined as the start of reaction. For the promoted wet oxidation experiments, the dye and the promoter were mixed and inserted into the reactor. These experiments were carried out using Fe(II) ions and pretreated gallic acid as the promoter. The promoter was chosen from a range of possible precursor substances. This range of precursor substances and the preparation of the promoter are based on a work by Vogel.13 The promoter is produced by mixing 0.8 g of gallic acid monohydrate (as the precursor) and 1.0 g of NaOH at ambient temperature. This mixture is stirred for 15 min and afterward acidified with H2SO4. Deionized water (1.8 L) and azo dye Orange II (2 g) were mixed and acidified with a few drops of sulfuric acid. Then, 0.21 g of iron(II) sulfate heptahydrate was added. We observed formation of a precipitate, as reported by other authors.14,15 The acidified azo dye solution and a given part of the precursor solution were merged, and the final solution was adjusted to a volume of 2 L and a pH of 2.0 ( 0.1 with sulfuric acid. A sample was taken, and the rest of the prepared solution was poured into the vessel. Afterward, the experiments were performed as the wet oxidation experiments. The reason for the different preparations for wet oxidation and promoted wet oxidation is explained in the section Preliminary Experiments. Samples were taken from the solution at specific time intervals during the reaction time. The samples were filtered (0.45 µm, PTFE syringe filters) and analyzed. Analysis of the total organic carbon (TOC) was carried out using a TOC analyzer (Dohrmann, DC-190). The chemical oxygen demand (COD) was quantified by using a test tube analytical system (DIN 38409-H41-1) from Macherey-Nagel AG, Oensingen, Switzerland. The concentration of the dye was quantified using an HPLC system (Alliance, Waters) with a Nucleosil 100-5 C18 HD, 250 × 4 mm column. A precolumn having the same characteristics was used. The absorption was measured at a wavelength of 230 nm. The eluent was a mixture of 50% methanol and 50% ammonium hydrogen phosphate solution (10 mM). Carboxylic acids were quantified using a GromGelAcid-1 instrument. The eluent was acidic water (pH ) 2, H2SO4). The chemicals were used as received from the manufacturers. The dye Orange II and other organic substances were from Fluka Chemie AG, Buchs, Switzerland. Iron(II) sulfate heptahydrate (FeSO4‚7H2O) was from Siegfried Handel AG, Zofingen, Switzerland. Oxygen (g99.5 vol %) was obtained from PanGas, Lucerne, Switzerland. The purity of the dye was checked by TOC and HPLC measurements. Because of the water content of the product, the concentration of the dye was adjusted according to the TOC measurements. All reactants and HPLC solvents were prepared in deionized water from a water purification system (MilliRO 10 plus and Milli-Q 185 Plus from Millipore AG, Volketswil, Switzerland).
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Identical experiments resulted in coefficients of variation (CV) of 5% for the HPLC measurements and 4% for the COD and TOC analyses over the reaction time. Results and Discussion Preliminary Experiments. A dye solution with pH ) 2 was degassed and heated to 190 °C. The samples before and after the heating were compared. The TOC of the dye solution showed no significant difference. However, the azo dye concentration dropped by around 15% at 190 °C. This decrease in dye concentration is due to the thermal instability of Orange II, which was also reported by Donlagic and Levec.16 It is known that azo dyes are not stable at low pH values. To overcome the problem of this partial decomposition during heating, two solutions are possible: The first way to prevent the azo dye from thermal decomposition is to inject the azo dye after the heating of the acidified solution. This approach was used for our wet oxidation experiments. The second way is to inhibit the decomposition during heating by adding an inhibiting substance. By addition of the promoter solution, heating showed no significant influence on the decomposition. The reason for this inhibition can be found in the nature of the precursor of the promoter, i.e., gallic acid. Gallic acid is not only the precursor for the promoter used but also a source for a variety of antioxidants. We assume that the partial decomposition of the azo dye is a radical reaction initiated by the slow decomposition of the dye itself. Therefore, the decomposition of the azo dye can be minimized by addition of antioxidants. The advantages of this procedure are easier handling and the verified initial concentrations through the sample analyzed after the preparation. To prove the successful inhibition by the second method, one has to compare the concentrations of the dye before and after heating. However, the addition of Fe(II) results in precipitation, and the true amount of the dye in solution is not accessible by a single analytical tool. This precipitation is well-known for the complex Fe(III)‚‚‚Orange II and is described by Bandara, Nadtochenko, and Kiwi.15,17 By the addition of an EDTA source to the samples, the precipitate is redissolved and the dye is accessible through HPLC analysis. By using this sample treatment, the true concentration of the dye in solution is measured. It was experimentally demonstrated that the dye does not significantly decompose during heating by comparing the samples after preparation and after heating. Therefore, the addition of the promoter inhibits the decomposition of the dye. The precipitate is not visually observable in the first sample after heating. This complete redissolving was also shown by the EDTA treatment of the samples and the following HPLC analysis. An overview of the substances used is given in Figure 2. The ratio Cdye,0/CI,0 is used to describe the ratio of the initial loads of the promoter and of the azo dye. The corresponding concentrations are inserted as the partial TOC amount of these compounds. Therefore, the sum of Cdye,0 and CI,0 is equal to the initial TOC of the treated solution. Influence of Oxygen Pressure and Temperature. In WO, the influence of oxygen depends on the substrate. Therefore, no universal dependency can be assumed. Varieties of published results18 indicate a range of oxygen reaction order from 0 to 1. The WO treatment of Orange II, reported by Donlagic and
Figure 2. Azo dye Orange II (1) is converted in the presence of pretreated (indicated by *) gallic acid (2) and Fe(II) ions under mild conditions. Table 1. Promoter Addition to Evaluate Temperature Dependency Cdye,0/CI,0
Cdye,0 (mg of C/L)
CI,0 (mg of C/L)
∞ 2.9 ( 0.1
520 ( 20 520 ( 20
0 180 ( 2
Levec,16 led to first-order kinetics with respect to oxygen for TOC and dye degradation. Three experiments were conducted by varying the partial pressure of oxygen. The oxygen pressure was set to 0.5, 1.0, and 1.5 MPa at 160 °C. The ratio Cdye,0/CI,0 was set to 2.9 ( 0.1. No significant differences in concentration were obtained for the experiments at 1.0 and 1.5 MPa. The experiment at 0.5 MPa showed lower conversions in TOC, COD, and Cdye than the experiments with higher poxygen. We assume, therefore, a mass transfer limitation or a change of the rate-determining step below 1 MPa oxygen pressure and only slow influence of poxygen on the degradation under the observed conditions. The reaction is therefore of zero order with respect to oxygen when the oxygen pressure is higher than 1.0 MPa. The dependency on temperature was evaluated between 130 and 190 °C. The initial concentration of the dye was varied. The amount of added promoter was kept constant in experiments with promoter. Therefore, two different sets of Cdye,0/CI,0 ratios were examined. (See Table 1.) By treating the azo dye only at pH ) 2 (i.e., Cdye,0/CI,0 ) ∞), the experiments show, for 160 and 130 °C, zeroorder kinetics with respect to the azo dye. At 190 °C, first-order kinetics can be adapted to the nonpromoted decay of the dye. Figure 3 shows the dye conversions by variation of the temperature in the WO experiments. The experiments with the promoter (i.e., Cdye,0/CI,0 ) 2.9) were conducted with the same initial concentrations in dye, promoter, and Fe ions. The pH and poxygen values were equal as well. Using the promoter, the dye decomposition can be described as first-order decay over the chosen range of temperature. The parameters of the Arrhenius-like relationship (see eq 1) were obtained by least-squares evaluation of the obtained experimental results.
k(T) ) Ae-EA/RT
(1)
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Figure 3. Unpromoted wet oxidation: Dye decay at different temperatures and poxygen ) 1 MPa. Table 2. Rate Constants for Cdye,0/CI,0 ) 2.9 at poxygen ) 1 MPaa temperature (°C)
observed k(T) (s-1)
predicted k(T) (s-1)
130 160 190
1.7 × 10-3 3.2 × 10-3 10.2 × 10-3
1.5 × 10-3 3.9 × 10-3 8.9 × 10-3
Figure 5. Promoted wet oxidation: Temperature dependency of TOC in PWO (Cdye,0/CI,0 ) 2.9) and WO experiments (Cdye,0/CI,0 ) ∞) at poxygen ) 1 MPa. Table 3. Variation of Promoter Concentration and the Corresponding Ratio Cdye,0/CI,0a run a b c d e
Activation energy EA ) 46 kJ/mol (A ) 1370 s-1). Average CV for rate constants ) 0.2 (see Figure 4). a
a
Figure 4. Promoted wet oxidation: Dye decay at different temperatures. The curves are the corresponding integration of the first-order rates; the parameters of the corresponding Arrhenius relationship were used (see Table 2). poxygen ) 1.0 MPa, Cdye,0/CI,0 ) 2.9.
The corresponding values are listed in Table 2; the comparison of the experimental results and the discussed Arrhenius-like relationship is given in Figure 4. The fate of the intermediates is discussed in terms of the decay of lumped parameters, i.e., of the TOC values. As seen in Figure 5, the experiment at 130 °C shows only a slow decay in TOC. At higher temperatures, the decay is more pronounced. In the case of 190 °C, a slower decay after 20 min can be observed. At the 160 °C reaction temperature, such a fast drop is not recognized but is still significantly higher than that at 130 °C. Reaction temperatures higher than 130 °C provide complete conversion of the dye as long as enough promoter is added. From an economical point of view, the addition of the precursor and the reaction temperature have to be minimized. The work of Donlagic and Levec16 reveals that the TOC reduction achieves more
CI,0 (mg of C/L)
Cdye,0/CI,0
0.0 4.5 22.3 89.4 178.8
∞ 122 25 6 3
See Figure 6a,b.
than 80% at temperatures above 220 °C. The promoted wet oxidation results in a TOC reduction of about 70% above 160 °C. We decided to evaluate the influence of the addition of promoter at 160 °C because of the promising results in TOC reduction at this reaction temperature. Influence of Promoter Addition. The influence of the addition of promoter was established at 160 °C and 1 MPa of oxygen partial pressure. In addition, the amount of FeIISO4 was kept constant in these experiments. The only varied parameter, therefore, was the amount of added precursor solution, expressed as CI,0. The quantity CI,0 was varied between 0 and 180 mg of C/L. Table 3 gives an overview of the values of added precursor solution. It was observed that the dye decay seems to be zeroorder without the precursor. This behavior was not influenced by added Fe ions. At small amounts of precursor, only marginal acceleration is observed. With increasing initial amount of promoter, the azo dye decomposes rapidly. Therefore, the decay of the dye can be fitted by a first-order rate law. For the description, the following model was adapted to the data. The combined rate law consists of a zero-order decay of the azo dye
-
dCdye ) k1 dt
(2)
and a first-order decay rate of the dye by a correction with the precursor adding.
-
dCdye ) k2Cm I,0Cdye dt
(3)
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1087 Table 4. Obtained Values for Equations 3 and 4 at T ) 160 °C and poxygen ) 1.0 MPa parameter
value
m k1 k2
1.0 0.12 s-1 1.4 × 10-5 L s-1 (mg of C)-1
The combined rate law looks as follows:
-
dCdye ) k1 + k2Cm I,0Cdye dt
(4)
By integrating, the quantity Cdye,t results as a function of the initial concentrations of the dye and of the promoter
Cdye )
k1 + k2Cm k1 I,0Cdye,0 exp(-tk2Cm , I,0) m k2CI,0 k2Cm I,0 0 e Cdye (5)
This combined rate law (eqs 3 and 4) implies a change of the rate-determining step during the reaction. For high amounts of dye in solution, the first-order kinetics dominates the overall kinetics. At low dye concentrations, the zero-order kinetics will be rate-determining. However, the term CI,0 controls this change in the ratedetermining reaction. At low concentrations of promoter, zero order kinetics will be predicted, and at higher concentrations of promoter, first-order kinetics will be predicted. These predictions are consistent with the observed results. The promoter is not analytically accessible, and therefore, we assume that, e.g., by doubling the amount of the precursor solution, the amount of added promoter will be doubled. We used the initial TOC of the precursor addition as the parameter CI,0 to describe the influence of the promoter. This description, expressed in eqs 4 and 5, is only true for the case of the change from zero-order to first-order kinetics by variation of the promoter amount, i.e., at 160 °C reaction temperature. At 190 °C, the azo dye decay can be described by first-order kinetics for promoted and nonpromoted WO. Therefore, the description has to be adapted for both first-order kinetics. The results of adapting the experiments at T ) 160 °C and poxygen ) 1.0 MPa are summarized in Table 4. Noteworthy is the comparison between the results of the Arrhenius relationship and the proposed model. For T ) 160 °C and Cdye,0/CI,0 ) 2.9, the Arrhenius relationship results in k ) 3.9 × 10-3 s-1, and the model yields k2CI,0 ) 2.5 × 10-3 s-1. Figure 6a and b shows the comparison between the experimental results and the predicted values. Parameter Presentation. The added carbon, i.e., the addition of promoter, increases the initial TOC of the solution. The question arises whether or not the addition of promoter increases the final TOC. It is observed that, after 15 min of reaction time, the solution releases more carbon than was added. Neither coating on the reactor wall nor a significant difference between DOC and TOC was found. Therefore, the formation of CO2 is assumed. Figure 7 shows how the promoter overproportionally releases CO2 out of the solution after 30 and 60 min of reaction time. Further, the released CO2 has to be formed out of the azo dye and the promoter.
Figure 6. (a) Comparison between experimental values (see Table 3) and results according eq 5 (solid curves) and Arrhenius relationship (dashed curve, see Table 2) at poxygen ) 1 MPa and T ) 160 °C. (b) Comparison between experimental values (see Table 3) and results according to eq 5 at poxygen ) 1 MPa and T ) 160 °C. Dotted lines: (20%.
Figure 7. Comparison of added carbon (i.e., promoter ) CI,0) and released carbon at T ) 160 °C and poxygen ) 1.0 MPa after t ) 30 min and t ) 60 min.
The effect of the addition of promoter on the intermediates (by observation of the TOC decay) is shown in Figure 8. The experiments at Cdye,0/CI,0 ) 2.9 show a final TOC of about 30% of the initial value. This value
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the azo linkage.10 Reported wet oxidation experiments at higher temperatures indicate that the first observable reaction of the azo dye is the breaking of the azo linkage by the release of molecular nitrogen.16 The analysis of our experiments did not indicate the presence of ammonia or nitrate. In addition, we assume the release of nitrogen gas during the wet oxidation of the azo dye. Conclusions
Figure 8. TOC decay as a function of the promoter addition at T ) 160 °C and poxygen ) 1.0 MPa (see Table 3 for conditions).
of 70% TOC reduction also results at lower values of Cdye,0/CI,0 (i.e., lower dye concentration and CI,0 ) 180 mg of C/L). The COD reduction is in the same range as the TOC reduction. For the observed range of promoter addition, the experiments for Cdye,0/CI,0 ) 2.9 and 6 show no significant difference in TOC decay. Figure 8 clearly reveals the influence of the promoter not only on the decay of the dye but also on the conversion of the intermediates. The high amount of nitrogen in an azo dye molecule and the sulfo group do not allow the proper use of an average oxidation state of carbon. The TOC measurements of the solutions show a characteristic quality of wet oxidation processes: The pollutant decomposes first toward intermediates that convert toward stable products, e.g., acetic acid, formic acid, and CO2. Because of the acidic solution, carbon dioxide is volatile, and the released CO2 reduces the TOC. Although the azo dye is no longer detectable in solution, the TOC values show that reactions (expressed as reduction of TOC and formation of CO2) are still going on. The same behavior is indicated by the COD measurements. Intermediates. Low-molecular-weight carboxylic acids were observed by HPLC using a size-exclusion column. Additionally, the concentration of acetic acid was verified by GC measurements. Acetic acid, formic acid, oxalic acid, maleic acid, and fumaric acid were identified and quantified. The course of appearance shows that the first observable intermediate is oxalic acid. Later in the course of the reaction, fumaric and maleic acids in low concentrations are observed. Acetic and formic acids increase during the reaction time. HPLC/RI measurements indicate only acetic and formic acid as intermediates in significant concentrations. The examination of the obtained samples toward the occurrence of nitrogen-containing ions (NH4+, NO3-) revealed no evidence of their production in amounts more than 7 mg of N/L. The estimation of the produced sulfo group was not established because of the sulfuric acidic solution. Measurement of the concentration or proof of the existence of aromatic substances was not possible by the analytical means used. A further question was the fate of the azo linkage. It is reported that AOPs potentially generate ammonia and nitrate from
The wet oxidation at pH ) 2.0, poxygen ) 1.0 MPa, and T ) 190 °C showed a fast and complete conversion after 1 h of reaction time. By lowering the reaction temperature, the decay rate of the dye decreases, and the reaction kinetics changes from first- to zero-order. By addition of Fe(II) ions and promoter, the decay is accelerated and can be described by first-order kinetics over the chosen temperature range. The addition of the promoter initiates the degradation of the target compound. This is, therefore, not only a consequence of the pH sensitivity of the azo dye. The oxygen pressure shows no observable influence on the decay, but the reaction temperature does influence the decay. This can be satisfactorily described by an Arrhenius relationship. By varying the amount of promoter, the system shifts from self-initiated dye decay to promoted decay with co-oxidation of the added substances and the initial intermediates with the azo dye. The promoter itself can not be observed (except as additional TOC), but the description by the initial concentration of promoter allows the description of the influence. The initial promoter concentration has a firstorder influence on the obtained combined rate law. By using a defined preparation procedure and measuring the added carbon, a reproducible evaluation of industrial wastewater problems is given. The examined temperatures show only small conversion rates without the promoter. Increasing amount of promoter not only increases the decay of the dye but also changes the reaction kinetics. However, treatment of the azo dye Orange II by promoted wet oxidation has shown that the amount of promoter and the temperature are the important parameters influencing the overall performance of the proposed system. By addition of promoter, the final TOC/COD and rate of dye decay can be controlled. This allows for an existing WO plant to be run at fixed reaction temperature. By varying the promoter, changes or fluctuations in the wastewater (e.g., concentration of refractory compounds) can be easily controlled to establish constant effluent qualities. In addition, the promoted wet oxidation allows lower reaction temperatures in comparison with wet air oxidation. According to the treatment results based on the use of the azo dye Orange II as a model pollutant for dye-house effluents, the promoted wet oxidation seems to be a promising alternative to other disposal processes. Acknowledgment The authors acknowledge the support of this research provided by the Swiss Federal Institute of Technology (ETH) Zurich. We thank Bertrams Chemieanlagen, AG, Muttenz, Switzerland, for support. Notation A ) preexponential factor of Arrhenius-type relationship (s-1)
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1089 Cdye ) concentration of Orange II (mg of C L-1) Cdye,0 ) initial concentration of dye (mg of C L-1) CI,0 ) initial concentration of promoter (mg of C L-1) Cdye,0/CI,0 ) carbon ratio of dye and promoter CI ) color index COD ) chemical oxygen demand CV ) coefficient of variation, standard deviation/mean value EA ) parameter of Arrhenius-type relationship, activation energy HPLC ) high-performance liquid chromatography EDTA ) ethlyenediaminetetraacetic acid k(T) ) temperature-dependent rate constant (mg of C L-1 s-1) k1, k2 ) rate constant (mg of C L-1 s-1 or s-1) m ) exponent for promoter in combined rate law PWO ) promoted wet oxidation poxygen ) partial pressure of oxygen (MPa) RI ) refractive index t ) reaction time (min) T ) Temperature (°C) TOC ) total organic carbon (mg of C L-1) TOD ) theoretical or total oxygen demand (mg of O L-1) WO ) wet oxidation
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Received for review June 28, 2000 Revised manuscript received November 20, 2000 Accepted November 22, 2000 IE000629A
