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Ind. Eng. Chem. Res. 1997, 36, 3480-3486
Oxidation of an Azo Dye in Subcritical Aqueous Solutions Jelka -Donlagic´ † and Janez Levec* Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 30, SI-1001 Ljubljana, Slovenia, and Department of Chemical Engineering, University of Ljubljana, P.O. Box 53, SI-1001 Ljubljana, Slovenia
Oxidation of aqueous solutions of a model azo dye pollutant (Orange II) was studied in a semibatch reactor operated at temperatures between 180 and 240 °C and oxygen partial pressures from 10 to 30 bar. The dye concentrations were in a range (100 and 1000 mg L-1) that one may encounter in an industrial wastewater stream. Orange II oxidation undergoes a parallelconsecutive reaction pathway in which it first decomposes thermally and oxidatively to aromatic intermediates and then via organic acids to the final productscarbon dioxide. The thermal and oxidative disappearance rates of Orange II were found to be first-order reactions with respect to the mother compound, while the oxidation rate of intermediates was found to be second order when lumped by means of total organic carbon. The rate of organic carbon disappearance in solution can be predicted by adding up the rate at which organic carbon in Orange II disappears and the rate of carbon disappearance in lumped intermediates. Both oxidation rates obey the first-order dependence with respect to partial pressure of oxygen. The activation energies of all three steps, i.e. thermal and oxidative decompositions of Orange II and lump oxidation, are found to be 90, 104, and 57 kJ mol-1, respectively. The last activation energy suggests that some intermediates from the lump are oxidized directly to carbon dioxide. The results show the biodegradability of solutions increases with treatment time, but complete biodegradation with nonacclimated seed was not reached. Maximum biodegradability was reached in an experiment carried out at 200 °C. Introduction Synthetic dyes often receive considerable attention from researchers interested in textile wastewater treatment processes. Initial environmental efforts with dyes dealt with color pollution, which has a strong psychological effect. More recently interest has shifted to the potential toxicity of dyes, dye precursors (e.g. certain aromatic amines used in the production of azo dyes), and their degradation products, especially the suspected carcinogenicity of potential intermediate products. As toxicity standards are becoming more common and stringent, the development of technological systems for minimizing the concentration of dyes and their breakdown products in wastewater is nowadays necessary. Several studies have been performed on the physical, chemical, aerobic biological, and anaerobic-aerobic biological treatment of dye-containing wastewater. It should be pointed out that dyes are intentionally designed to be recalcitrant under typical usage conditions and resistant to microbial attack. However, it is generally accepted that the most inadequate is aerobic biological degradation, because of the low degradation rate and also due to a nonbiodegradable fraction that remains in wastewater following the treatment. Chemical treatment including oxidizing and reducing agents has yielded encouraging results of color and organics removal, but the required dosages are often too high to be economically feasible. Furthermore, the azo dyes that undergo reductive cleavage through anaerobic biological treatment potentially generate toxic aromatic amines. An attractive alternative to the above treatments might be the use of wet oxidation and advanced * To whom correspondence should be addressed at the University of Ljubljana. E-mail:
[email protected]. † Present address: Faculty of Mechanical Engineering, University of Maribor, P.O. Box 224, SI-2007 Maribor, Slovenia. S0888-5885(97)00034-1 CCC: $14.00
oxidation processes. Most of them are designed to oxidize organic pollutants into intermediate products more amenable to biological treatment, which is generally considered as the most environmentally friendly and the cheapest method of wastewater treatment. Considering the beneficial results obtained by wet oxidation with various organic compounds, among them anthraquinone and phthalocyanine class reactive dyes (Shende and Mahajani, 1994), this process seems to have a great potential as a pretreatment method for azo dye containing wastewater streams. Wet oxidation, however, is a liquid-phase process which takes place at elevated temperatures (200-320 °C) and pressures (20200 bar) by means of active oxygen species. Orange II, a simple acid monoazo monosulfonated dye, a typical nonbiodegradable pollutant (Bandara et al., 1996; Kulla et al., 1983) from a dyehouse, commonly used in pharmaceutical, food, and cosmetic industries, has been employed by many investigators to evaluate the effectiveness of some oxidation processes. For example, Pulgarin and Kiwi (1996) reported results of photocatalytic degradation through chelation between Fe3+ and Orange II. Vinodgopal et al. (1995, 1996) studied photocatalytic degradation using TiO2 particles and visible light. Ozonation as a pretreatment step to biological oxidation was studied by Takahashi et al. (1994) and Liakou and Lyberatos (1996a). They found the pretreatment effluent nonbiodegradable. From the kinetic point of view, the reported results are meager. The kinetics of wet oxidation of many organic compounds was discussed in detail by Li et al. (1991) and also by Mishra et al. (1995). Li et al. (1991) proposed a generalized model based on a simple reaction scheme which accounts for acetic acid oxidation as the rate-controlling step. Recently the group of Gloyna (Li et al., 1996), and Krajnc and Levec (1996) approached this issue by lumping kinetics that has been traditionally used in treating the cracking processes. Li et al. (1996), however, have shown by many examples that © 1997 American Chemical Society
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the lumping technique is a powerful tool for handling experimental wet oxidation data as well as for modeling and efficient oxidation reactor design. In this work Orange II was employed to study wet oxidation as a pretreatment step to the conventional biological oxidation for purifying dyehouse wastewaters. Particular attention was paid to the ultimate fate of Orange II and to the reaction intermediates formed at different operating conditions, since they determine the biodegradability of the pretreatment process effluent. This study also aimed at the development of rate equations for predicting the model pollutant disappearance rate as well as the rate of TOC and/or COD reduction during the wet oxidation process. Experimental Section Apparatus and Experimental Procedure. Wet oxidation experiments were performed in a 2-L stainless steel autoclave reactor (Parr Instrument Company, Moline, IL) equipped with a magnetically driven turbine type impeller and temperature and pressure control units. The temperature of the reaction mixture was maintained within (1 K of the set value. The total pressure in the system was kept constant by a back pressure controller (Brooks); the deviation did not exceed (0.5% of the set pressure. The oxygen flow through the reactor, introduced into the vessel below the impeller, was controlled by an electronic mass flow controller (Brooks). The experimental setup is described in detail elsewhere (Pintar and Levec, 1992). The experiments were carried out in a temperature range of 180-240 °C, oxygen partial pressure of 10-30 bar, and mass concentration range of 0.1-1.0 g L-1 (i.e. 54.8-548.5 ppm of organic carbon). The total pressure is the sum of the oxygen partial pressure and the pressure of water vapor at a given temperature. To test whether the oxidation of Orange II was mass transfer limited, preliminary oxidation experiments were performed at 1000 and 1500 rpm and for different oxygen flow rates. Since the oxidation rates were found independent of both these variables, it was concluded that the transport of oxygen across the gas-liquid interface did not retard the rates. Thus, it was assumed the rates obtained at 1000 rpm and oxygen flow rate of 1.0 L min-1 were already in the kinetic regime (Pintar and Levec, 1992). In a typical run, a certain amount of recrystallized (Giles and Greczek, 1962) Orange II (Aldrich) was loaded in a glass ampule fixed near the reactor impeller. By placing Orange II in the ampule we prevent thermal degradation of the dye which would otherwise occur in the solution during its heating up to the reaction temperature. Thermal stability of dry, solid Orange II up to 400 °C was confirmed by a TGA analyzer. The reactor was filled with 1.5 L of distilled water, and during heating up to 70 °C, nitrogen was sparged continuously through it. Once the water in the reactor reached the desired temperature, a constant stream of oxygen (1.0 L min-1) was introduced into the reactor. A few minutes later the impeller was turned on; it broke the ampule. In a separate experiment we found that 0.5 g L-1 of Orange II dissolves within 2 min in the same amount of water at 180 °C; in comparison with the oxidation experiments, this time is very short and can be neglected. However, the time of turning on the impeller was considered as the starting point of an experiment (t ) 0). Approximately 10 mL of representative samples of the aqueous solution were withdrawn
periodically by means of a specially designed tubing system equipped with a tube-and-shell type heat exchanger. The samples obtained were analyzed for the residual content of Orange II, total organic carbon, and intermediates. Since larger volumes were needed to determine the chemical and biochemical oxygen demand, separate experiments were performed for this purpose under identical conditions. Analysis. The quantity of residual Orange II in solution was determined by reversed phase HPLC using a Hewlett Packard 1100/DAD system equipped with a Rheodyne 7725i injection valve (20 µL sample loop). The analyses were made using a 250 × 4.6 mm i.d. Spherisorb ODS-2 (5µm) at 40 °C. The mobile phase consisted of 0.1 M ammonium acetate (A) and acetonitrile mixed with 0.1 M ammonium acetate (B) (v/v; 80:20) at a flow rate of 1.0 mL min-1. To achieve satisfactory separation of the breakdown products from the parent compound, a gradient elution from 100% A to 100% B in 30 min was used. The detector was set at the wavelengths of maximum absorption (483 and 254 nm) for Orange II. The residual organic carbon concentrations in the samples were measured by an advanced HTCO Rosemount/Dohrmann DC-190 TOC analyzer equipped with a nondispersive infrared (NDIR) CO2 detector. The identification of aromatic intermediate products was performed on a Hewlett-Packard GC/MSD system running in both SCAN and SIM modes of operation. A GC was equipped with an HP-1 (Ultra-1) high-resolution capillary column (25 m × 0.32 mm × 0.52 µm; HP) and interfaced directly to a quadrupole mass spectrometer (HP 5970B) as a detector. The GC was operated in the temperature-programming mode with an initial column temperature of 70 °C for 2 min, then increased linearly to 250 °C at a rate of 10 °C min-1, and held at the upper temperature for 10 min. The GC/MSD interface was maintained at 260 °C. A helium carrier gas of ultrahigh purity was used with a flow rate of 1.0 mL min-1. To extract analytes from aqueous samples, solid-phase microextraction using a SPME syringe assembly (Supelco) was used. Intermediates were identified by comparing the mass spectrum of a compound with spectra of compounds stored in the NBS library. Low molecular mass organic acids were identified and evaluated by means of ionic chromatography (Dionex 4000I) using a conductivity detector. Isocratic elution of organic acid anions was optimized on the IonPac ICEAS6 analytical column. Heptafluorobutyric acid (0.4 mM) was used as a mobile phase at a flow rate of 1 mL min-1. As a suppressor an Anion-ICE MicroMembrane supressor with a regenerant (5 mM tetrabutylammonium hydroxide) at a flow rate of 5 mL min-1 was utilized. The sample loop volume was 20 µL. The chemical oxygen demand (COD) was determined by the closed reflux dichromate method; its concentration was measured colorimetrically and titrimetrically. The biochemical oxygen demand (BOD) was assessed by the manometric and standard dilution methods using a commercially available special blend of bacterial cultures Bioseed (Interbio) as a seed source. Acclimation of the seed was not performed. Both COD and BOD tests were performed according to the Standard Methods for the Examination of Water and Wastewater (Greenberg, 1992) using WTW equipment. Results and Discussion Thermal Decomposition. Thermal decomposition of Orange II in aqueous solution was studied in an
3482 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 1. Normalized carbon concentration in Orange II and total organic carbon in solution as a function of time in thermal experiments.
Figure 2. Normalized carbon concentration in Orange II and total organic carbon in solution as a function of time in oxidation experiments for an oxygen partial pressure of 10 bar.
atmosphere of pure nitrogen. A typical Orange II concentration, expressed in terms of carbon content, as well as the concentration of organic carbon in the solution, as a function of time, are depicted in Figure 1. While Orange II concentration decreases with time, the organic carbon in the solution remains constant even at higher temperatures. This implies that Orange II breaks thermally into fragments: according to March (1992) most probably into benzenesulfonic acid and naphthol. Benzenesulfonic acid was found to be hardly biodegradable (Liakou and Lyberatos, 1996b); thus, an aqueous solution of benzenesulfonic acid and naphthol is also expected to be less biodegradable. Wet Oxidation. The dependence of carbon-in-dye concentration in the presence of oxygen is illustrated in Figure 2 for experiments carried out at different temperatures. It can be seen that the dye concentration decreases quite rapidly; at 200 °C it disappears completely after 80 min, while at 240 °C only 15 min is needed. The experimental points of the 200 °C experiment at low conversion suggest the induction period is involved. This phenomenon, characteristic of wetoxidation reactions, was found even more pronounced at 180 °C. However, comparing the dye decays represented in Figures 1 and 2, one can see the dye disappears much faster in the presence of oxygen. Since thermal decomposition cannot be a reversible reaction, it is obvious that there must be another route of Orange II disappearance apart from thermal decomposition. Therefore, the dye-decay curves in Figure 2 consist of
Figure 3. Normalized carbon concentration in Orange II, total organic carbon in solution, and total organic carbon in intermediates as a function of time at 200 °C and 10 bar of oxygen partial pressure.
two contributions, namely, thermal and oxidative decomposition. The reason the dye concentration does not take a close-to-zero value lies in the HPLC analysis: at high dye conversions large amounts of the intermediates made the separation more difficult. The reductions of organic carbon concentration in solution which accompanied the experiments with oxygen are also shown in Figure 2. Inspecting these results more closely, one can see that the data points at lower temperature constitute a S-shape curve which, in its inverse form, is characteristic of the final product concentration change with time in an irreversible consecutive reaction scheme. This behavior diminishes at higher temperature since Orange II disappears quite rapidly. It should be noted here that the difference between the initial carbon content and the one in the solution is equal to the concentration of carbon dioxide, the final product of wet oxidation. The difference between the carbon found in Orange II and the one in the solution obviously accounts for the intermediate products formed during the course of oxidation. A typical accumulation of intermediate products is presented in Figure 3, where the concentration of organic carbon in the solution and in Orange II is also shown. The shape of the intermediate product concentration curve is, again, characteristic of a consecutive reaction. As illustrated in Figure 3, complete oxidation of all intermediate products is accomplished a long time after the mother compound has disappeared. This calls, however, for a rate equation which can be used for predicting the conversion of all organic species in the solution rather than the disappearance of the mother compound alone. Simplified Reaction Scheme. By means of the GC/ MSD analysis the following aromatic intermediates were identified in the course of Orange II oxidation: benzenesulfonic acid, naphthol, 1,2-benzenedicarboxylic acid, 4-hydroxybenzenesulfonic acid, 2-(hydroxymethyl)benzoic acid, and 1,3-isobenzofurandione. The pH value of the solutions dropped within the first 40 min of oxidation (from 6.43 to 3.4-2.8) and then remained almost constant. This implies that organic acids are formed at a later stage of oxidation and that the sum of their concentrations also remains constant. By means of ionic chromatography the following acids were identified: acetic acid, formic acid, oxalic acid, and glycolic acid. Their occurrence is shown on the right coordinate in Figures 4 and 5. At 190 °C formic acid
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Figure 4. Distribution of organic acids in the experiment carried out at 190 °C and 10 bar of oxygen partial pressure.
Figure 5. Distribution of organic acids in the experiment carried out at 230 °C and 10 bar of oxygen partial pressure.
ence of each individual intermediate identified in the effluent solution simply because of the lack of their commercial availability. In general, even though the BOD5/COD values for each intermediate involved are known, the prediction of biodegradability of the effluent solution would still remain uncertain. However, one may conclude that the distribution of intermediates is responsible for the biodegradability of the reactor effluent. The complete biodegradability of liquid samples has not been reached (BOD5/COD < 0.5), but one must keep in mind that the biodegradability tests were performed without any previous seed acclimation. On the basis of the concentration-time behavior and the intermediate products identified, the oxidation of Orange II undergoes a parallel-consecutive reaction scheme. For the purpose of modeling the organic carbon disappearance rate, a simplified reaction scheme is given in Figure 7. The first step, the two routes of Orange II disappearance, is relatively fast and leads to many aromatic intermediates. The disappearance of global aromaticity, which brings intermediates into saturated aliphatic substances, is also considered to be a rapid step, but further oxidation to carbon dioxide is assumed to be the rate-controlling step. The formation of the intermediates undoubtedly involves serial radical processes. The presence of water might cause additional ionic processes. However, the appearance of hydroxylated aromatic intermediates mentioned above supports the conclusion that the radical mechanism is probably the prevailing process, but it is beyond the scope of this work to further speculate on this issue. Disappearance Rate of Organic Carbon. On the basis of the reaction pathway (Figure 7), the overall oxidation of Orange II may be kinetically treated as a parallel-consecutive reaction. It is obvious that Orange II disappears by two reactions; namely, it breaks thermally and oxidatively via azoxy compounds (March, 1992; Patai, 1975; Mitsuhashi et al., 1970) into intermediates discussed in the previous section. However, there is no reason not to treat them as first-order reactions with respect to Orange II. Thus, the total disappearance rate of Orange II written in terms of organic carbon concentration is given by
-
Figure 6. Biodegradability of Orange II solution treated at two different temperatures and 10 bar of oxygen partial pressure.
was found to be the dominant organic acid while acetic acid was dominant when the experiment was carried out at 230 °C. In the former case, the total amount of acids made up 15% of the organic carbon in the solution, whereas in the latter case this ratio increased up to 55%. This finding allows us to further speculate that acetic acid is one of the last intermediates before the appearance of the final productscarbon dioxide. It is interesting to note that formic and oxalic acids were found when Orange II was oxidized by ozone (Liakou and Lyberatos, 1996a). The biodegradability of the liquid samples taken at different time, assessed by the BOD5/COD ratio, is illustrated in Figure 6. As can be seen, the rate of COD reduction increases with temperature, but the biodegradability is lower at a higher temperature. It was impossible to determine the biodegradability influ-
dCOR ) k1COR + k2CORf[p(O2)] ) (k1 + k* 2)COR (1) dt
where k1 and k2 stand for the thermal decomposition rate constant and the oxidation rate constant, respectively, while the constant k*2 is a merger of k2 and f[p(O2)]. Since the solubility of oxygen in the solution changed with temperature, oxygen concentration term is instead written in terms of partial pressure. However, numerical values of the constant k1(T) can be obtained by fitting the integrated form of the equation
-
dCOR ) k1COR dt
(2)
to the carbon concentration of Orange II vs time curves (Figure 1) of the thermal experiments carried out at different temperatures. These results are summarized in Table 1 and graphically presented in Figure 8. The solid curves in Figure 1 present the dependence predicted by eq 2. The sum of both constants results when confronting the integrated form of eq 1 with the carbon concentration of Orange II vs time curves provided by the
3484 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 7. Simplified reaction pathway of Orange II oxidation in subcritical aqueous solution.
Figure 9. Global oxidation rate constants as a function of oxygen partial pressure. Figure 8. Temperature dependence of the rate constants. Table 1. Rate Constants and Activation Energies T, °C 200 220 230 240
k1 × 102, min-1
k2 × 103, (min bar)-1
k3 × 105, L(min bar mol)-1
1.0 ( 0.05 2.7 ( 0.01 4.4 ( 0.2 6.1 ( 0.3
5.0 ( 0.3 12.2 ( 1.6 22.0 ( 0.9 40.9 ( 3.7
1.6 ( 0.07 2.7 ( 0.05 2.8 ( 0.8 5.8 ( 1.1
Ea ) 90 kJ mol-1
Ea ) 104 kJ mol-1
Ea ) 57 kJ mol-1
experiments with oxygen (Figure 2). Since k*2 plotted against the partial pressure of oxygen yields a linear dependence (Figure 9), one can conclude that the oxidative decomposition of Orange II is the first-order reaction with respect to oxygen. Such dependency has been found with many organic compounds upon oxidation in aqueous media (Li et al., 1991). However, k2 alone is given in Table 1 and shown in Figure 8 as the Arrhenius plot. Eq 1 is confronted with experimental data in Figure 2. We believe that the disagreement exhibited is mainly due to the induction period not being accounted for in the modeling. If one bears in mind that the data points at low conversions are also encumbered with an error due to the dye dissolution delay, the agreement seems reasonable.
The objective is to determine the rate equation which predicts the organic carbon disappearance rate in the solution; it can be determined by fitting appropriate experimental data (Figure 2) to an equation that accounts for the organic carbon as well as oxygen concentration and employing a nonlinear regression technique. In a way, this is suggested by the generalized kinetic model proposed by Li et al. (1991). For the sake of essentiality, in this work the carbon concentration in the solution is obtained by adding up the carbon in Orange II and the one found in the intermediate products, thus
COC ) CIN + COR
(3)
where the organic carbon concentration in Orange II is given by eq 1. For this purpose, the intermediates within brackets in Figure 7 are accounted for in a lump with the oxidation driving force measured by means of the total organic carbon concentration. As shown in Figure 3, the carbon concentration of the lumped intermediates undergoes a maximum typical of a consecutive reaction. Therefore, its time dependence, written in terms of total organic carbon concentration, is governed by the following relation
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3485
dCIN n ) (k1 + k* 2)COR - k3CIN f[p(O2)] ) dt n (k1 + k* 2)COR - k* 3 CIN (4) where k3 represents the rate constant for the disappearance of the lump. Due to lumping, it is assumed here that essentially a first-order oxidation reaction of each individual intermediate with oxygen may be correlated by power law kinetics. The value of k*3 can be obtained, in principle, by fitting the integrated form of eq 4. Since the rate order (n) is not known, we approach the problem by trial and error with the help of Mathematica (Wolfram, 1991). A variation of coefficients in the model differential equation was used and the mean square discrepancy calculated for different values of n. For each pair of coefficients (n and k) the numerical solution of the differential equation was calculated and compared to the experimental data using the mean square discrepancy criteria. The global minimum in discrepancy between the experimental and calculated data was found by iterative procedure when coefficients n and k were varied. The optimization criteria employed in fitting the experimental data has yielded a value of 2 for the coefficient n (solid curves in Figure 3). It is interesting to note that a similar dependence was found by Krajnc and Levec (1996) when lumping phenol intermediates in supercritical water phenol oxidation. The apparent rate constant k* 3 was also found to be proportional to the partial pressure of oxygen (Figure 9). The values of k3 are listed in Table 1 and illustrated in Figure 8. A comparison of experimental and predicted (eq 3) time dependence of the carbon in the solution is shown in Figure 3. As one can see, the source of disagreement arise mainly from eq 4. One should be aware that lumping the compounds with considerably different reactivity is a departure from the theory of continuous mixture. For example, acids are known to be very refractory. The first-order dependence of oxygen partial pressure on both disappearance rates illustrated in Figure 9 seems to be reasonable and is in good agreement with the findings of other investigators for different organic compounds (Li et al., 1991). While the line for k* 2 in Figure 9 passes through the origin, the line for k* 3 obviously does not. From this one can conclude that the linear dependence of k*3 is unlikely to be exhibited in a wide range of oxygen partial pressures but in the range of pressures investigated here. A smaller dependence of k*3 on the oxygen partial pressure when compared to k* 2 seems to be reasonable because the lump itself consists of highly oxygenated compounds. Although the straight lines in the Arrhenius plot (Figure 8) may be considered as a good test for the modeling approach, the agreement was tested further. A comparison of experimental and predicted (eq 1) time dependence of the carbon concentration in Orange II with different initial carbon concentrations is presented in Figure 10. These data were obtained in the experiments where the dye was not placed in the ampule and were not used in the previously discussed calculations. A parity plot of the normalized organic carbon concentrations in solution predicted by eq 3 and those measured experimentally is shown in Figure 11. Although a slight trend is seen, the majority of data points are found within a (20% error band. From the values of rate constants (Table 1) one can conclude that the thermal decomposition of Orange II is a slightly faster reaction compared to the oxidation
Figure 10. Comparison of measured and predicted concentration profiles for an oxygen partial pressure of 10 bar.
Figure 11. Normalized concentration of organic carbon in solution: experimental measurements vs eq 3 calculations.
reactions. The activation energy for the lump oxidation is too low to be attributed only to the oxidation of acids. Since the activation energies for the oxidation of acids range between 120 and 170 kJ mol-1 (Li et al., 1991), one may further speculate that the lump (some compounds in it) also oxidizes directly to carbon dioxide. One can further lump acids and the rest of intermediates separately and obtain a deeper insight into the contribution of each group, but it was not the aim of this work. Nevertheless, kinetic lumping seems to be a simple and appropriate tool for handling experimental data of wet oxidation. Conclusions Orange II oxidation in subcritical aqueous solutions obeys a parallel-consecutive reaction scheme in which oxidation proceeds via aromatic intermediates and aliphatic acids to the final carbon dioxide. The conversion of organic carbon initially present in Orange II may be effectively predicted by employing a rate equation for parallel-consecutive reactions. The equation accounts for the parallel thermal and oxidative disappearance of Orange II, yielding intermediates which further produce, in a consecutive manner with oxygen, the final oxidation product. While the mother compound decomposes quite easily and rapidly, the oxidation of aliphatic acids seems to be a rate-controlling step. The experimental results also imply that some intermediate products are oxidized directly to carbon dioxide. The lumping of intermediates by means of total organic
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carbon concentration (TOC), which relates to the chemical oxygen demand (COD), has been demonstrated here as an efficient technique for the interpretation of the wet oxidation data. The biodegradability of Orange II solutions treated by the wet oxidation process increases with time at 200 °C but remains almost constant at 240 °C, despite the fact that COD decreases more rapidly in the latter case. This shows that the intermediate products formed at higher temperatures are less amenable to biological oxidation. It is believed, however, that the formation of acids is the most responsible for decreasing biodegradability at higher temperatures. One can overcome this problem by employing a catalyst to accelerate the oxidation of acids to carbon dioxide. Nevertheless, in order to minimize the harmful effects of intermediate products to microorganisms in the conventional biological oxidation process that follows, an optimal temperature for the wet oxidation pretreatment has to be chosen. Acknowledgment The authors acknowledge support from the EU/DGXII under Grant ERB CIPD CT94 0111 and support from the Slovenian Ministry of Science and Technology under Grant J2-6179. Nomenclature COR,0 ) initial carbon concentration in Orange II, mg L-1 COR ) carbon concentration in Orange II, mg L-1 CIN ) carbon concentration in intermediates, mg L-1 COC ) organic carbon concentration in solution, mg L-1 COR ) normalized concentration of carbon in Orange II (COR/COR,0) CIN ) normalized concentration of carbon in intermediates (CIN/COR,0) COC ) normalized concentration of organic carbon in solution (COC/COR,0) Ea ) apparent activation energy, kJ mol-1 f[p(O2)] ) undefined function of oxygen partial pressure, bar k1 ) rate constant of Orange II thermal decomposition, min-1 k2 ) rate constant of Orange II oxidation, (min bar)-1 k3 ) rate constant of lump (intermediates) oxidation, L (min bar mol)-1 k*2 ) apparent rate constant of Orange II oxidation, min-1 k*3 ) apparent rate constant of lump oxidation, L (min mol)-1 n ) reaction rate order T ) temperature, K BOD5 ) biochemical oxygen demand, mg L-1 COD ) chemical oxygen demand, mg L-1 TOC ) total organic carbon, mg L-1
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Received for review January 8, 1997 Revised manuscript received April 24, 1997 Accepted May 4, 1997X IE970034O
Literature Cited Bandara, J.; Nadtochenko, V.; Kiwi, J.; Pulgarin, C. Dynamics of Oxidant Addition as an Important Parameter in the Modellization of Dye Mineralization (Orange II) via Advanced Oxida-
X Abstract published in Advance ACS Abstracts, July 1, 1997.
