Ozone-Enhanced Oxidation of Oxalic Acid in Water with Cobalt

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Ind. Eng. Chem. Res. 2003, 42, 3218-3224

Ozone-Enhanced Oxidation of Oxalic Acid in Water with Cobalt Catalysts. 2. Heterogeneous Catalytic Ozonation Fernando J. Beltra´ n,* Francisco J. Rivas, and Ramo´ n Montero-de-Espinosa Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain

Ozone and a Co3O4/Al2O3 catalyst have been used to remove oxalic acid from water at acidic pH. The influence of different variables, including the initial oxalic acid and ozone gas concentrations, the catalyst mass, and the temperature, has been investigated under conditions of chemical control. The ozone efficiency was found to reach values of up to 40%, which are higher than those found in homogeneous catalysis (see also the first part of this work, which immediately precedes this paper in this issue), and the mean stoichiometry was determined to be 0.8 mol of ozone consumed per mole of oxalic acid consumed. Metal leaching was also followed to check the stability of the catalyst. As a result, ozonation of oxalic acid was found to be due to both heterogeneous and homogeneous catalytic ozonation. The average contribution of heterogeneous catalytic ozonation for the removal of oxalic acid was found to be 75%. A kinetic study taking into account both catalytic contributions is also presented. The experimental heterogeneous catalysis was found to be first-, one-half-, and zeroth-order with respect to catalyst, dissolved ozone, and oxalic acid, respectively. A mechanism is also proposed to account for the kinetic results. 1. Introduction In the first part of this work,1 Co(II) was confirmed as an appropriate catalyst for enhancing the oxidation of oxalic acid in water under acidic pH conditions. However, in the treatment of water, homogeneous catalysis with heavy metals is not recommended because of their toxic nature. Nonetheless, metal catalysts, once their capacity to aid ozonation has been established, can be used, if supported on solid materials such as alumina or silica, to improve mineralization. Heterogeneous catalytic ozonation is, in fact, a process dating back to the 1970’s,2 although it was not until recently (past 10 years) that this subject attracted the interest of researchers.3 Thus, solid catalysts have been used to remove different pollutants from water, such as phenols, hydrocarbons, carboxylic acids, etc.4-6 In most cases, the catalysts were prepared by impregnation methods, and the results were expressed in terms of the effects of different variables on the ozone consumption and pollutant removal, without any data reported on possible metal leaching. Also, the issue of the kinetics of the process was absent from the discussion of results, and only a few cases proposed possible mechanisms involving surface reactions.3 Cobalt-type catalysts are of particular importance in oxidation processes. As solid oxides, these catalysts are p-type oxides characterized by their capacity to adsorb oxygen and yield electron-rich adsorbed species such as -O- and -O2-.7 Cobalt oxide catalysts have been tested in the catalysis of the decomposition of ozone in the gas phase. For example, Imamura et al.8 used a variety of oxide catalysts to increase the gaseous ozone decomposition rate and observed that p-type oxides such as Ag2O, NiO, and Co3O4 showed the highest catalytic activities. In another work, Rakitskaya et al.9 studied the action * To whom correspondence should be addressed. Tel.: 34924-289387. Fax: 34-924-271304. E-mail: [email protected].

of a Co(II)/SiO2 catalyst and also observed an improvement in the ozone gas decomposition rate. In contrast to the abundance of literature on catalytic ozonation in the gas phase, to our knowledge, only one work on aqueous heterogeneous catalytic ozonation with cobalt oxide catalysts has so far been reported. This study dealt with the removal of formic acid with different catalysts supported on activated carbon or SiO2.10 In this part of a two-part work, a Co catalyst supported on alumina has been prepared by the impregnation method to test the ozonation of oxalic acid in water with three main aims. These are the estimation of the actual importance of heterogeneous catalysis, the observation of any possible leaching of the metal, and the study of the kinetics of the process with the aid of results obtained in the first part of the work.1 2. Experimental Part Oxalic acid and ozone were obtained as indicated in the first part of this work.1 The catalyst was prepared from γ-Al2O3 pellets (Alcoa) and a Co(NO3)2 (Merck) salt by the impregnation method.5 The impregnation method involved the addition of known amounts of γ-Al2O3 to a saturated solution of cobalt(II) nitrate, followed by agitation of the system for 24 h. Then, after it had been filtered and dried at 105 °C, the impregnated alumina was calcined for 5 h at 800 °C. The final catalysts contained approximately 10 wt % cobalt. The prepared catalyst was characterized by N2 adsorption with a Quantachrome autosorb 1 automated gas adsorption system for determination of the BET specific surface area, which was found to be 128 m2 g-1. The porosity and apparent and true densities were determined through helium and mercuriy porosimetry, respectively, with a Steropicnometer and an Autoscan-60 Quantachrome apparatus. These parameters were determined to be 0.6, 1.3 g cm-3, and 3.3 g cm-3, respectively. According to the literature,11 given the preparation

10.1021/ie020999u CCC: $25.00 © 2003 American Chemical Society Published on Web 06/04/2003

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3219 Table 1. Oxalic Acid Removal Rate Reached during Heterogeneous Catalytic Ozonation with Co3O4/Al2O3 at Different Gas Flow Rates, Agitation Speeds, and Particle Sizesa gas flow rate (L h-1)

agitation speed (min-1)

particle size (mm)

oxalic acid rate (×105 M min-1)

24 24 24 24 12 36 24 24 24

100 200 300 400 300 300 300 300 300

1.6-2.0 1.6-2.0 1.6-2.0 1.6-2.0 1.6-2.0 1.6-2.0 2.0-2.3 1.0-1.6 powder form

7.2 7.9 9.2 8.9 8.6 9.0 8.0 8.8 8.4

a Other experimental conditions: initial oxalic acid concentration ) 8 × 10-3 M, concentration of ozone in the gas ) 30 mg L-1, catalyst concentration ) 1.25 g L-1, temperature ) 20 °C, pH ) 2.5.

conditions, the cobalt in the final catalyst was likely to be present as Co3O4 on the alumina support. Experiments were carried out in the 1000-mL jacketed glass vessel indicated in the first part of the work.1 For the heterogeneous catalytic experiments, however, the reactor was provided with an agitation bar that included a stainless steel basket (3 × 3 × 3 cm) at its lower edge to contain the catalyst. As in the first part of this work,1 800 mL of an aqueous oxalic acid concentration was charged to the reactor for each run. Also, the temperature inside the reactor was kept constant by circulating water from the thermostatic batch in which the reactor was submerged. The equilibrium isotherm for the adsorption of oxalic acid from aqueous solution onto the Co3O4/Al2O3 catalyst was determined at 20 °C using the batch bottle-point method. Different amounts of catalyst were weighed and suspended into glass bottles containing 50 mL of oxalic acid aqueous solution. The bottles were sealed and placed in an orbital shaker where samples were kept at a temperature that was constant to within (0.1 °C. The oxalic acid concentration in water, the ozone concentrations in gas and water, and the total organic carbon were determined as reported previously.1 Finally, the total cobalt concentration in water was quantified by mass spectrometry with a 6500 PerkinElmer ICP apparatus. 3. Results and Discussion Experiments on the ozonation of oxalic acid at pH 2.5 in the presence of the Co3O4/Al2O3 catalyst were carried out at different concentrations of oxalic acid [(1.6 × 10-2)-(8 × 10-3) M], ozone gas (15-45 mg L-1), and catalyst mass (1.25-3.75 mg L-1). Also, the temperature was varied between 10 and 40 °C. Previously, a series of experiments at different gas flow rates, agitation speed, and particle sizes was completed to establish the conditions under which the system was in the chemical control regime. In all cases, zeroth-order kinetics appropriately fitted the experimental results. After 90 min of reaction, these experiments led to oxalic acid conversions varying between 77 and 95%. However, as can be seen in Table 1, oxalic acid removal rates were very similar, regardless of the experimental conditions applied. Thus, according to these data, external and internal mass-transfer resistances can be considered negligible for experiments conducted at gas flow rates and agitation speeds equal

Figure 1. Evolution of the dimensionless concentration of oxalic acid with time for different ozonation experiments and concentrations of cobalt leached with time for the heterogeneous catalytic ozonation experiment. Conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm, T ) 20 °C, pH ) 2.5, initial oxalic acid concentration ) 8 × 10-3 M, ozone gas concentration ) 30 mg L-1. Experiment: ([) noncatalytic ozonation of oxalic acid, (2) oxalic acid adsorption on catalyst surface, (]) homogeneous catalytic ozonation [Co(II) catalyst concentration ) 0.8 mg L-1], heterogeneous catalytic ozonation (catalyst particle size ) 1.6-2 mm, Co3O4/Al2O3 catalyst mass concentration ) 1.25 g L-1) (9) without tert-butyl alcohol, (0) with 0.01 M tert-butyl alcohol, (b) metal leached during the heterogeneous catalytic ozonation.

Figure 2. Isothermal data on the adsorption of oxalic acid on Co3O4/Al2O3 catalyst in water at pH 2.5 and 20 °C.

or higher than 24 L h-1 and 300 rpm, respectively, and particle sizes of 1.6-2.0 mm or lower. Also, comparative experiments on single ozonation, homogeneous catalytic ozonation,1 single adsorption, and heterogeneous catalytic ozonation were carried out to check the relative importance of these methods in removing oxalic acid from water. In Figure 1, an example of this comparison is shown. As can be seen, ozonation without catalyst hardly removes oxalic acid from water, whereas single adsorption leads to about 11% elimination after 90 min, which represents 7.04 × 10-4 mol of oxalic acid adsorbed per gram of catalyst. This value is close to the equilibrium value, as can be deduced from Figure 2, where isothermal data on oxalic acid adsorption on the Co3O4/ Al2O3 catalyst at pH 2.5 and 20 °C are plotted. Thus, from Figure 2, it can be seen that an adsorbed oxalic acid concentration of about 7 × 10-4 mol per gram of catalyst is in equilibrium with about 7 × 10-3 M oxalic acid in water. From Figure 1, on the other hand, it is also seen that, at 90 min of adsorption, the concentration of oxalic acid remaining in water is nearly the same, about 7 × 10-3 M. This means that equilibrium is reached in this short time. Thus, adsorption of oxalic acid on the catalyst surface seems to be a rapid process, especially compared to other adsorption processes of a similar nature.12

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Table 2. Concentration and Percentage of Metal Leacheda metal leached Co3O4/Al2O3 (g L-1)

T (°C)

concentration (mg L-1)

percent (%)

1.25 1.25 1.25 2.50 3.75

10 20 40 20 20

0.65 0.79 1.00 1.34 1.68

0.52 0.63 0.80 0.54 0.45

a Other experimental conditions: initial oxalic acid concentration ) 8 × 10-3 M, ozone gas concentration ) 30 mg L-1, gas flow rate ) 24 L h-1, pH ) 2.5, particle size ) 1.6-2 mm, agitation speed ) 300 rpm.

Regarding catalytic ozonations, the presence of a solid Co3O4/Al2O3 catalyst allows for the total removal of oxalic acid, but some leaching of metal occurs. Under the experimental conditions of Figure 1, 0.78 mg L-1 of Co leached from the catalyst surface to the water in the same period of time. This dissolved Co likely contributed to the oxalic acid ozonation. Figure 1 also shows the results from a homogeneous catalytic ozonation, which was also reported in the first part of this work,1 where the starting Co(II) concentration was 0.8 mg L-1, that is, similar to the concentration leached at the end of the heterogeneous catalytic ozonation experiment. Comparing the two catalysis types, in 90 min, when 0.8 mg L-1 of Co(II) is initially present in solution, about 35% of the oxalic acid is eliminated, whereas the total elimination of this acid is achieved in the presence of the heterogeneous catalyst. In fact, the contribution of homogeneous catalysis during the heterogeneous ozonation must be lower than 35% as the metal slowly leaches from the solid surface so that only after 90 min does the concentration reach about 0.8 mg L-1 (see Figure 1). The literature also reports that Co3O4 oxides can partly dissolved in acids to give Co(II).13 Thus, if it is assumed that the leached cobalt is present in cobalt(II)oxalic acid complexes (see also our previous paper1), from the experimental rate data (see the Kinetics and Mechanism section later in this paper), it can be deduced that heterogeneous catalysis made the highest contribution to oxalic acid removal. Therefore, the Co3O4/Al2O3 solid acts as an efficient heterogeneous catalyst in the conversion of oxalic acid. Finally, Figure 1 also shows that the presence of tertbutyl alcohol, a hydroxyl free-radical scavenger, does not affect the oxalic acid removal rate, a situation that was also observed during the homogeneous catalytic ozonation of oxalic acid.1 As a consequence, cobalt-catalyzed (homogeneous and heterogeneous) ozonation of oxalic acid is not a hydroxyl free-radical process. On the other hand, the amount of cobalt leached from the catalyst surface after 90 min of ozonation represented only 0.63% of the total metal catalyst deposited on the alumina support. In any case, the highest percentage of leached metal after 90 min reaction was 0.8%, as observed in an experiment at 40 °C. As can be seen from Table 2, the percentage of metal in solution due to leaching increased with increasing temperature and decreased with increasing total solid catalyst concentration. No or a negligible influence of other variables such as the ozone gas concentration was noticed (not shown). These results suggest that the stability of the catalyst needs improvement, a situation that can likely be achieved by altering the preparation

Figure 3. TOC from carbon analyzer (TOC/TOC0a, vertical axis) versus TOC from remaining oxalic acid concentration (TOC/TOC0b, horizontal axis).

Figure 4. Evolution of the dimensionless concentration of oxalic acid with time for catalytic ozonation experiments at different ozone gas concentrations. Conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm, catalyst particle size ) 1.6-2 mm, T ) 20 °C, pH ) 2.5, catalyst mass concentration ) 1.25 g L-1, initial oxalic acid concentration ) 8 × 10-3 M, ozone gas concentration (mg L-1) ) (9) 15, ([) 30, (2) 45.

method, an aspect that will be the subject of future work. Mineralization of Oxalic Acid. Unlike the case with homogeneous catalysis,1 in the presence of Co3O4/ Al2O3 catalyst, complete oxalic acid mineralization was achieved, as indicated in Figure 3 where the TOC determined using the carbon analyzer practically coincides, at any conditions, with that calculated from the remaining oxalic acid concentration. It is evident that mineralization was mainly due to the contribution of heterogeneous catalysis, as the homogeneous process did not allow total removal of carbon (i.e., at low oxalic acid concentrations; see the first part of the work1). This conclusion is supported by the fact that the highest level of oxalic acid removal was also due to the heterogeneous process (see Table 2). Thus, the good results obtained for the heterogeneous catalytic contribution prompted us to complete a study on the influence of variables and ozonation kinetics. Influence of Variables. In Figures 4 and 5 are presented plots of the dimensionless remaining concentration of oxalic acid with time for experiments at different ozone gas concentrations and masses of catalyst, respectively. As can be seen from these figures, increases in each variable led to significant increases in the oxalic acid conversion for a given reaction time. For example, after 90 min of reaction with 45 mg L-1 of ozone (1.25 g L-1 of catalyst), complete oxalic acid conversion was reached. This compared favorably for the heterogeneous catalytic ozonation with respect to the homogeneous catalytic process. Also, higher catalyst concentrations allowed total oxalic acid conversion to

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Figure 5. Evolution of the dimensionless concentration of oxalic acid with time for catalytic ozonation experiments at different catalyst mass concentrations. Conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm, catalyst particle size ) 1.6-2 mm, T ) 20 °C, pH ) 2.5, inlet ozone gas concentration ) 30 mg L-1, initial oxalic acid concentration ) 8 × 10-3 M, catalyst mass concentration (g L-1) ) (9) 1.25, ([) 2.50, (2) 3.75.

Figure 6. Evolution of the dimensionless concentration of oxalic acid with time for catalytic ozonation experiments at different initial oxalic acid concentrations. Conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm, catalyst particle size ) 1.6-2 mm, T ) 20 °C, pH ) 2.5, catalyst mass concentration ) 1.25 g L-1, inlet ozone gas concentration ) 30 mg L-1, initial oxalic acid concentration (×103 M) ) (9) 8, ([) 12, (2) 16.

Figure 7. Evolution of the dimensionless concentration of oxalic acid with time for catalytic ozonation experiments at different temperatures. Conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm, catalyst particle size ) 1.6-2 mm, pH ) 2.5, catalyst mass concentration ) 1.25 g L-1, initial oxalic acid concentration ) 8 × 10-3 M, inlet ozone gas concentration ) 30 mg L-1, temperature (°C) ) (9) 10, ([) 20, (2) 40.

be reached in much lower reaction times, as indicated in Figure 4. For example, with 3 g L-1 of catalyst, total oxalic acid conversion was reached in 50 min. In Figure 6, the effect of the initial oxalic acid concentration is shown. The results indicate the expected trend, that is, the higher the initial oxalic acid concentration, the lower the conversion reached at a given time. Figure 7, on the other hand, shows the effect of temperature. As can be seen, temperature slightly affects the oxidation rate of oxalic acid, a situation that would suggest that the process is controlled by masstransfer resistances. However, the experimental data shown in Figure 7 correspond to the chemical control

regime, as discussed above (see Table 1), although it is also possible that the experiment at 40 °C presents some problems related to the mass-transfer effect (see the Kinetics and Mechanism section later in this paper). Also, it should be mentioned that the a priori anomalous results of Figure 7 are likely due to the double effect of temperature in ozonation processes. Thus, the expected increase in the ozonation rate is hindered by the negative effect of solubility, which, for the same ozone partial pressure applied, decreases with increasing temperature. Hence, the ozone available for reaction decreases as well. These opposite effects have been observed in different ozonation reactions.14,15 Ozone Efficiency and Stoichiometry. For the heterogeneous catalytic ozonation experiments, the ozone efficiency and stoichiometry (see definitions in the previous paper1) were also calculated. For the experimental conditions most usually applied in this work, the ozone efficiency remained higher than in the homogeneous catalytic ozonation, about 25-30%, although some higher values were also obtained at the highest oxalic acid concentration investigated (50% for an initial oxalic acid concentration of 0.016 M). The average stoichiometric value, however, was between 0.6 and 1.2 mol of ozone consumed per mole of oxalic acid consumed. The higher values of around 1.2 were determined specifically at the highest oxalic acid and ozone gas concentrations applied, 0.016 M and 45 mg of O3 L-1, respectively, and at the highest temperature, 40 °C. Kinetics and Mechanism. As deduced from the above comments, the importance of the homogeneous catalytic ozonation, due to leaching effects, will depend on the concentration of Co(II) in solution. Here, an attempt is made to evaluate the contributions of both homogeneous and heterogeneous catalysis to oxalic acid removal by assuming that all cobalt in solution is present as Co(II) and, more specifically, as Co(II)-oxalic acid complexes. In addition, some aspects on the kinetics and mechanism are also discussed. It should be highlighted that most of data (runs at 20 °C) used to discuss the kinetics and mechanism of the process correspond to experiments carried out at conditions of gas flow rate, agitation speed, and catalyst particle size for which mass-transfer limitations have been eliminated (see Table 1). Hence, the rate of the process is assumed to depend exclusively on the chemical reaction steps. For the system studied, both homogeneous (in bulk liquid) and heterogeneous (on the internal surface of the catalysts) catalytic ozonation reactions contribute to the oxalic acid removal rate. A least-squares analysis of the experimental results shown in Figures 4-7 indicates that oxalic acid is removed mainly following zeroth-order kinetics (correlation coefficients of 0.995 or higher, except in run 7 of Table 4). Only for one run carried out at the highest catalyst concentration used (3.75 g L-1) did first-order kinetics provide a better fit to the results. In summary, the catalytic ozonation of oxalic acid seems to follow preferentially zeroth-order kinetics that change to firstorder in some cases. This suggests that the catalytic ozonation of oxalic acid occurs through a LangmuirHinshelwood (LH) mechanism where the rate of ozonation is proportional to an expression of the type kCB/(1 + KCB), where CB is the bulk concentration of oxalic acid and k and K are constants related to the chemical surface reaction and adsorption equilibrium, respectively (see later aspects on the mechanism).

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Table 3. Contribution of Heterogeneous Catalytic Ozonation of Oxalic Acid during One Co3O4/Al2O3 Catalytic Experimenta time (min)

rhomb (×105 M min-1)

rhetc (×105 M min-1)

contribution of heterogeneous process (%)

15 30 45 60 75 90

1.9 2.6 3.4 4.0 4.4 4.7

19.1 18.4 17.6 17.0 16.6 16.3

82.8 79.8 78.8 81.4 87.7 93.1

a Experimental conditions: agitation speed ) 300 rpm, gas flow rate ) 24 L h-1, catalyst particle size ) 1.6-2 mm, T ) 20 °C, CB0 ) 1.8 × 10-3 M, inlet ozone gas concentration ) 30 mg L-1, catalyst mass concentration ) 1.25 g L-1. b Calculated as rhom ) 4.76 × 1015exp(-7207/T)CBCCo1.1CO30.4 as reported in the first part of this work.1 c Calculated from eq 2 as rT - rhom.

Table 4. Mean Values of the Heterogeneous Catalytic Ozonation Rate of Oxalic Acid and Percentage Contribution to the Removal Ratea C O3 (mg run L-1) 1 2 3 4 5 6 7 8 9

30 30 30 15 45 30 30 30 30

CB0 (M) 8.0 × 10-3 1.2 × 10-2 1.6 × 10-2 8.0 × 10-3 8.0 × 10-3 8.0 × 10-3 8.0 × 10-3 8.0 × 10-3 8.0 × 10-3

contribution of rhetb (×105 heterogeneous Ccat T process (%) (g L-1) (°C) M min-1) 1.25 1.25 1.25 1.25 1.25 2.50 3.75 1.25 1.25

20 20 20 20 20 20 20 10 40

6.53 6.40 6.16 3.59 8.31 6.92 15.75 6.58 3.18

83.9 77.3 74.1 87.9 38.3 83.4 96.1 92.5 39.0

Ozone Adsorption k3

Other experimental conditions: gas flow rate ) 24 L h-1, agitation speed ) 300 rpm. b Determined from the mean value of the difference rT - rhom in each run. a

Given the fact that, under the conditions applied, mass-transfer limitations can be neglected (see Table 1) and chemical reactions control the rate of the process, for the semibatch well-agitated reactor used, the balance of oxalic acid during ozonation in the presence of solid catalyst is

dCB ) rT ) rhom + rhet dt

dCB ) rT ) kT ) khomCB + rhet dt

O3 + S {\ } O3-S k ′ 3

(1)

(2)

Using the rate equation deduced in the first part of this work1 for the catalytic homogeneous ozonation of oxalic acid, the contribution of the heterogeneous catalytic process can also be estimated from eq 2. Thus, Table 3 shows, as an example, the contribution of the heterogeneous catalytic ozonation reaction with time during one ozonation experiment in the presence of the solid Co3O4/Al2O3 catalyst. It can be seen that the importance of the heterogeneous catalysis is always greater than 75%. In Table 4, on the other hand, mean values of the heterogeneous catalytic ozonation rate, determined from eq 2, and the percentage contribution to the total oxalic

(3)

Oxalic Acid Adsorption k5

B + S {\ } B-S k ′ 5

(4)

Reaction of Adsorbed Oxalic Acid with Adsorbed Ozone k6

} 2P-S O3-S + B-S {\ k ′ 6

where rT, rhom, and rhet represent the total catalytic rate of oxalic acid ozonation and the contributions of the homogeneous and heterogeneous processes, respectively. Because the homogeneous ozonation (see first part of this work1) was found to follow first-order kinetics with respect to oxalic acid, eq 1 becomes

-

acid removal rate for different experimental conditions are given. As can be seen, except for two experiments at 40 °C and with the highest ozone concentration applied (45 mg L-1), the contribution of the heterogeneous catalytic ozonation was always greater than 70%, with a mean value of 85%, which confirms the efficiency of the solid Co3O4/Al2O3 catalyst in the removal of oxalic acid from water. Aspects of the Reaction Mechanism. The kinetic orders (between zeroth and first order) deduced above for oxalic acid catalytic ozonation suggest that adsorption or any reaction involving oxalic acid on the catalyst surface, following a LH-type mechanism, can be the rate-controlling step of the heterogeneous catalytic process (see Table 5). On the other hand, reports in the literature suggest that, in the gas phase, ozone is adsorbed on heterogeneous surfaces and then decomposes into other adsorbed oxygen species.7 However, in this work, the decompositions of ozone in organic-free water at pH 2.5 in the absence and presence of the Co3O4/Al2O3 catalyst were similar. This means that ozone does not decompose on the surface of the catalyst used, although it possibly adsorbs. According to this information, it is unlikely that adsorbed ozone decomposes into active adsorbed oxygen species as reported for catalytic decomposition studies of ozone in the gas phase.7 Therefore, the mechanism of the heterogeneous process might be as follows:

(5)

Desorption of Adsorbed Products k7

2P-S {\ } 2S + CO2 + H2O k ′ 7

(6)

In this mechanism, S represents any active center on the catalyst surface. Another possibility is that ozone does not adsorb on the catalyst surface but reacts directly with adsorbed oxalic acid species, i.e. k8

} P-S O3 + B-S {\ k ′ 8

(7)

The mechanism can also be simplified if reaction steps 5 and 7 directly yield desorbed products (CO2, H2O, and free active centers S). If this assumption is made, there could be two possible mechanisms consisting of steps 3-5 or steps 4 and 7. According to these mechanisms, a theoretical stoichiometry of 1 mol of ozone consumed per mole of oxalic acid consumed is deduced, which agrees with the experimental results. From the two mechanisms, five possible reaction rate equations can be derived depending on the controlling step. In Table 5, these possibilities are listed, together with the corresponding rate equations deduced from

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3223 Table 5. Rate-Controlling Steps and Kinetic Equations Derived from the Proposed Mechanism of Co3O4/Al2O3 Solid Catalytic Ozonation of Oxalic Acid in Water at Acidic pHa mechanism steps and controlling step

reaction orders oxalic acid ozone

rate equation

steps 3-5 ozone adsorption

1 -rhet ) kaCO3 KaCB + Kb + Cp

steps 3-5 oxalic acid adsorption

C O3 -rhet ) kbCB Cp + KcCO3

steps 3-5 surface reaction 5

CO3 -rhet ) kcCB (1 + K3CO3 + K4CB)2

steps 4 and 7 oxalic acid adsorption

CO3 -rhet ) kdCB Cp + KdCO3

steps 4 and 7 surface reaction 7

C O3 -rhet ) keCB 1 + KeCB

(8)

0 to -1

1

1

0 to 1

1 to -1

1 to -1

1

0 to 1

0 to 1

1

(9)

(10)

(11)

(12)

a Equations deduced with the assumption that reaction products are directly formed from surface reactions. K -K are different constants a e made up of combinations of rate constants of the mechanism steps. Constants ka-ke are also functions of the total number of catalyst active centers. Cp ) concentration of products.

Figure 8. Determination of the ozone apparent reaction order for the catalytic ozonation of oxalic acid in water. Variation of the heterogeneous catalytic rate with the concentration of dissolved ozone in logarithmic coordinates. Data correspond to runs 1, 4, and 5 of Table 4.

Langmuir-Hinshelwood16

approaches. As can be seen, oxalic acid adsorption steps can be disregarded as the controlling steps, since rate eqs 9 and 11 suggest firstorder kinetics with respect to oxalic acid, which is in disagreement with experimental observations. Furthermore, the isothermal and adsorption kinetic data shown in Figures 2 and 1, respectively, confirm this conclusion. Thus, as was explained before, the adsorption of oxalic acid is a rapid process since equilibrium is reached in less than 100 min. Also, the fact that the catalyst surface is saturated clearly explains the finding that the rate of heterogeneous catalytic ozonation is independent of the concentration of oxalic acid in water, at least for experiments initiated with 0.0064 mol of oxalic acid per gram of catalyst (which represented the conditions typically used in this work). On the other hand, from results at different ozone concentrations in the gas fed to the reactor, it was observed that the corresponding heterogeneous rate follows a linear relationship with respect to the dissolved ozone concentration in logarithmic coordinates (see Figure 8, R2 ) 0.985) with a slope of 0.44. Thus, the kinetic order with respect to ozone is between 0 and 1. Therefore, eqs 8 and 12 can also be disregarded. As a result, eq 10 seems to be the rate equation that best supports the experimental observa-

tions for the oxalic acid and ozone kinetic orders. According to eq 10, the kinetic orders of oxalic acid and ozone can theoretically vary from -1 to 1, a situation that can be explained by the experimental results. In fact, the experimental data do suggest kinetic order variations for oxalic acid and ozone between 0 and 1, which, in turn, also suggests the development of a LH mechanism. The only situation in which these variable kinetic orders is achieved results from eq 10, that is, when the surface reaction between ozone and oxalic acid adsorbed species controls the rate of the process. Empirical Kinetics. For comparison with the homogeneous process,1 an empirical kinetic equation was proposed as follows

-rB ) khetCBnCO3mCCop

(13)

In this case, however, given the fact that the oxalic acid kinetic order has a variable behavior, a multivariable least-squares analysis was performed to determine the best-fit parameters for experimental results. The resulting equation was

-rB ) 100 exp(-1000/T)CO30.44CCo0.95 (M min-1) (14) which presents deviations lower than 15% in all cases but three (correlation coefficient ) 0.901). The experiments that gave the highest deviations were those carried out at concentrations of oxalic acid higher than 0.01 M and at 40 °C. It is likely that, under these conditions, mass-transfer resistances also play a role. For example, experiments carried out with initial oxalic acid concentrations of 0.012 and 0.016 M (runs 2 and 3 in Table 4) presented Hatta numbers (see procedure in the first part of this work1) close to 0.3, which means that a moderate reaction occurred in the water. Thus, in these cases, both chemical reaction and gas-liquid mass transfer would influence the rate of the process. On the other hand, at 40 °C, the increase in reaction rate could also led to a change in the kinetic regime, so that liquid-to-solid or internal mass transfer could influence the rate of the process. In support of this idea,

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from eq 14, an apparent activation energy of barely 2 kcal mol-1 can be deduced, which is too low for a process exclusively controlled by chemical reaction steps. As can also be seen, eq 14 predicts zeroth and nearly one-half kinetic orders for oxalic acid and ozone, respectively, in agreement with the experimental evidence. 4. Conclusions The use of a Co3O4/Al2O3 catalyst prepared by the impregnation method has been observed to significantly enhance the ozonation rate of oxalic acid in an acidic water environment. Furthermore, this catalytic ozonation leads to the total mineralization of oxalic acid. The catalyst, however, in its present form, leaches some cobalt into the water that likely contributes to the removal of oxalic acid according to the rates given in the first part of this work.1 Nonetheless, the contribution of the heterogeneous catalytic ozonation to the removal of oxalic acid was about 80% in most cases. At the highest temperature investigated (40 °C), the increase in the amount of metal leached makes the homogeneous contribution the main mode of ozonation. The kinetics of the heterogeneous catalysis was found to be zeroth-order with respect to oxalic acid, and on the basis of a proposed LH mechanism, the experimental results were checked at different ozone gas concentrations. According to this mechanism and experimental (kinetics and isothermal) data, the rate-controlling step is the oxalic acid-ozone surface reaction step. Despite the observed leaching problem, the main conclusion of this work is the positive effect of this type of catalyst in enhancing the ozonation rate. Because of the potential interest in this catalytic ozonation process and the excellent results obtained in terms of the removal of oxalic acid, work is in progress to increase the stability of the catalyst and avoid the leaching problems. Acknowledgment This work was supported by the CICYT of Spain and The European Region Development Funds of the European Commission (Project PPQ2000/0412). Literature Cited (1) Beltra´n, F. J.; Rivas, F. J.; Montero-de-Espinosa, R. Ozoneenhanced oxidation of oxalic acid in water with cobalt catalysts. 1. Homogeneous catalytic ozonation. Ind. Eng. Chem. Res. 2003, 42, 3210-3217.

(2) Chen, J. W.; Hui, C.; Keiler, T.; Smith, G. Catalytic ozonation in aqueous system. AIChE Symp. Ser. 1976, 73, 206212. (3) Legube, B.; Karpel Vel Leitner, N. Catalytic ozonation: A promising advanced oxidation technology for water treatment. Catal. Today 1999, 53, 61-72. (4) Al-Hayek, N.; Legube, B.; Dore´, M. Catalytic ozonation (Fe III/Al2O3) of phenol and its ozonation by-products. Environ. Lett. 1989, 10, 415-426. (5) Cooper, C.; Burch, R. An investigation of the catalytic ozonation for the oxidation of halocarbons in drinking water preparation. Water Res. 1999, 33, 3695-3700. (6) Gracia, R.; Corte´s, S.; Sarasa, J.; Ormad, P.; Ovelleiro, J. L. Heterogeneous catalytic ozonation with supported titanium dioxide in model and natural waters. Ozone Sci. Eng. 2000, 22, 461-471. (7) Dhandapani, B.; Oyama, S. T. Gas-phase ozone decomposition catalysts. Appl. Catal. B: Environ. 1997; 11: 129166. (8) Imamura, S.; Ikebata, M.; Ito, T.; Ogita, T. Decomposition of ozone on a silver catalyst. Ind. Eng. Chem. Res. 1991, 30, 217221. (9) Rakitskaya, T. L.; Ennan, A. A.; Granatyuk, I. V.; Bandurko, A. Y.; Balavoine, G. G. A.; Geletii, Y. V.; Paina, V. Y. Kinetics and mechanism of low-temperature ozone decomposition by Co-ions adsorbed on silica. Catal. Today 1999, 53, 715-723. (10) Lin, J.; Nakajima, T.; Jomoto, T.; Hiraiwa, K. Effective catalysts for the wet oxidation of formic acid by oxygen and ozone. Ozone Sci. Eng. 2000, 22, 241-247. (11) Cotton, F. A.; Wilkinson, G. Quı´mica Inorga´ nica Avanzada; Limusa: Me´xico, 1969 (Translation from the book Advanced Inorganic Chemistry; John Wiley & Sons: New York, 1966). (12) Beltra´n, F. J.; Rivas, J.; Ferna´ndez, L. A.; A Ä lvarez, P.; Montero-de-Espinosa, V. Kinetics of catalytic ozonation of oxalic acid in water with activated carbon. Ind. Eng. Chem. Res. 2002, 41, 6510-6517. (13) Burriel, F.; Lucena, F.; Arribas, S. Quı´mica Analı´tica Qualitativa, 9th ed.; Paraninfo: Madrid, Spain, 1974. (14) Beltra´n, F. J.; Garcı´a-Araya, J. F.; Acedo, B. Advanced oxidation of atrazine in water I. Ozonation. Water Res. 1994, 28, 2153-2164. (15) Beltra´n, F. J.; Ovejero, G.; Encinar, J. M.; Rivas, J. Oxidation of polynuclear hydrocarbons in water. 1. Ozonation. Ind. Eng. Chem. Res. 1995, 34, 1596-1606. (16) Fogler H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 1999.

Received for review December 6, 2002 Revised manuscript received April 7, 2003 Accepted May 1, 2003 IE020999U