Fe2+-Catalyzed Wet Oxidation of Phenolic Acids ... - ACS Publications

Ind. Eng. Chem. Res. 2010, 49, 12405–12413

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Fe2+-Catalyzed Wet Oxidation of Phenolic Acids under Different pH Values Sergio Collado, David Quero, Adriana Laca, and Mario Diaz* Department of Chemical Engineering and EnVironmental Technology, UniVersity of OViedo, E-33071, OViedo, Spain

Catalytic oxidation in the aqueous phase of phenol and phenol-derivates present in pharmaceutical wastewaters has been investigated using FeSO4 · 7H2O as a homogeneous catalyst at 413 K and 1.0 MPa. Different initial pH values and catalyst concentrations have been tested, and the results obtained have been discussed taking into account the different roles that the iron plays depending on the pH value and the phenolic compound studied. The degradation rate of each pollutant assayed and the degree of mineralization achieved during the wet oxidation process were greatly affected by pH. The catalytic effect of iron(II) during the wet oxidation of phenol was only observed at pH values ranging between 2 and 3, due to the establishment of a Fe(III)/ Fe(II) redox cycle. However, the best results for the catalytic oxidation of salicylic acid were obtained for pH values below 2, and they were related to the formation of an Fe(II)-salicylic acid complex. On the other hand, the presence of iron(II) had a prejudicial effect on the degradation of p-hydroxybenzoic and 5-hydroxyisophthalic acids, provoking the apparition of induction periods in both cases, due to the competition between the autoxidation of Fe(II) and the hydroxylation of the phenolic compound by the initial OH•. 1. Introduction High quantities of phenolic compounds are yearly spilled, forming part of industrial wastewaters. Major sources of phenolic effluents include petroleum refineries; coke ovens; synfuels production facilities; wood preserving plants; and manufacturers of plastics, resins, dyes, pesticides, and pharmaceuticals, among others.1,2 Phenolic compounds occupy a prominent position on the U.S. EPA priority pollutants list. Their dangerousness lies in the effect that they have on the nervous system of living beings. Additionally, in drinking water, phenols can combine with chlorine and give rise to chlorophenols, compounds that are even more toxic. Phenols also contribute to an increase in the chemical oxygen demand of wastewaters. The biological degradation of phenolic compounds is possible at low concentrations, but these pollutants are harmful for microorganisms at relatively high concentrations. For this reason, stringent limits (100 ppm) have been imposed on the discharge of phenols into municipal sewage treatment plants.3 The introduction of alternative technologies to degrade phenolic molecules before the biological treatment has become imperative for the treatment of some industrial wastewaters. Today, the trend goes toward advanced oxidation processes (AOP) as ozonation,4 Fenton5,6 and electrochemical oxidation,7,8 or photocatalysis9-11 in order to remove phenolic residues in the wastewater. However, the high concentrations of phenolic compounds in wastewaters originated from the synthesis of salicylic acid and its derivates, involving a high consumption of oxidant and/or energy, increasing operational costs. Wet air oxidation, a useful method for small quantities of wastewater with a high pollutant concentration, has shown its effectiveness in the treatment of effluents containing phenols.12 Frequently, the incorporation of a catalyst has been considered to reduce the required temperature and pressure, and/or to treat phenolic pollutants that cannot be effectively destroyed by noncatalytic processes.12,13 The efficacy of the treatment also depends on the chemical characteristics of the medium. In particular, the pH is a key factor; pH affects the types of freeradical reactions that occur during the wet oxidation of phenolic * To whom correspondence should be addressed. Tel.: 34985103439. Fax: 3498103434. E-mail: [email protected].

compounds, the chemical structure of phenolic compounds (formation of phenolates at high pH), the resistance to oxidation of the reaction products such as acetic acid and oxalic acid, and the activity of some catalysts.12,14-16 There is a large amount of published literature about the wet oxidation of phenol, usually taken as a model compound for advanced wastewater treatment studies.1,12,13 However, there are other phenolic compounds present in industrial wastewaters whose degradation by this technique has been little studied. This is the case of salicylic acid, p-hydroxybenzoic acid, and 5-hydroxyisophthalic acid, which originate mainly from the synthesis of salicylic acid and its derivates, mainly acetylsalicylic acid, during the carboxylation of phenol with carbon dioxide (Figure 1). In addition to pharmaceutical wastewaters, the presence of these kinds of phenolic compounds is also frequent in wastewaters coming from cosmetic manufacturers and the olive industry.17,18 Table 1 collects the main published studies about the wet oxidation of salicylic acid and p-hydroxybenzoic acid during recent years. Degradation of 5-hydroxyisophthalic acid has not been studied previously. The aim of this work was to study the effect of pH, a key parameter, on the homogeneous catalytic wet oxidation of phenol and three other phenolic derivates scarcely studied and whose presence is frequent in wastewater coming from the pharmaceutical industry.

Figure 1. Formation of phenolic pollutants during the process of acetylsalicylic acid production.

10.1021/ie101497s  2010 American Chemical Society Published on Web 10/28/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

Table 1. Main Studies about the Wet Oxidation of Salicylic Acid and p-Hydroxybenzoic Acid Published during Recent Years compound 19

Yang et al. Tukac and Hanika20 Triki et al.21 Creanga et al.22 Pham Minh et al.23 Caudo et al.24 Nikolopoulos et al.25

Salicylic acid Salicylic acid p-hydroxybenzoic p-hydroxybenzoic p-hydroxybenzoic p-hydroxybenzoic p-hydroxybenzoic

acid acid acid acid acid

T (K)

P (MPa)

catalyst

% efficacy

413 393-433 413 413 413 343 293

2.5 2.0-5.1 5.1 0.2 5.0 0.1 0.1

LaFeO3 AC Ru/Ce Ti AC Ru and Pt WHPO Cu-PILC Fe-PILC US+WHPO Al-Fe

84% COD removal (3 h) 100% conversion (LHSV: 10 h-1) 60-100% conversion (5 h) 70% conversion and 80% COD removal (3.3 h) 100% conversion and 70% COD removal (7-8 h) 100% conversion (0.5 h) 90% conversion (1 h)

2. Experimental Section 2.1. Experimental Setup. The experiments were carried out in a 1 L stainless steel high-pressure Parr autoclave (model 4520, Parr Instrument, Inc.) equipped with an electrical heating jacket, a temperature controller, and a magnetically driven six-blade turbine type impeller. The autoclave was operated batchwise with a continuous flow of oxygen. A schematic diagram of the apparatus used was presented in Collado et al.26 The oxygen, before being sparged into the reaction vessel, was bubbled through a water reservoir in order to become saturated in humidity. 2.2. Procedure. In each run, the autoclave was loaded with 700 mL of distilled water and a known amount of FeSO4 · 7H2O. The pH of the water was adjusted to a suitable value using H2SO4, so that after the addition of the phenolic compound solution the medium had the desirable value of pH. The autoclave was then sealed, pressurized with oxygen, and heated to the desired temperature. Once the desired conditions were reached, 2 mL of the phenolic compound concentrated solution was injected into the reactor. This moment was taken as the starting time of the reaction. The phenolic compounds tested were phenol, salicylic acid, p-hydroxybenzoic acid, and 5-hydroxyisophthalic acid. In the cases of salicylic, p- hydroxybenzoic, and 5-hydroxyisopthlic acids, the initial solutions were concentrated in KOH in order to dissolve these poorly soluble compounds. In all cases, the concentrations of the solutions were calculated to give the desired concentration inside the reactor. The initial pH value (after the injection of the phenolic compound) ranged between 1.8 and 3.5 (real pharmaceutical wastewaters from aspirin synthesis have a pH value around 2). In all cases, the concentration of protons increased slightly during the wet oxidation processes (around 0.2 pH units). At preset reaction times, aliquots of the solution were withdrawn and analyzed. Initial concentrations of phenolic compounds employed are similar to the concentrations found in real wastewaters. Conditions of temperature and pressure were selected according to the typical values found in industrial wet air oxidation processes.12

The reactions described in this work were performed under kinetic control. Verification was done by calculating the Hatta number,26 which relates diffusion and reaction rates: Ha ) β-1 1/kL[2/(β + 1)krDO2CSAT,O CR], kL being the mass transfer 2 i0 coefficient for oxygen in the liquid phase, DO2 the oxygen diffusivity in water, kr the reaction constant, and R and β reaction orders (-ri- ) krCRi COβ 2; generally R ) β ) 1).12 During all runs, Ha was lower than 10-3, which ensures the absence of mass transfer limitations and the existence of a kinetic control. 2.3. Analytical Methods. The concentration of phenol was measured using the 4-aminoantipyrine colorimetric method.27 The concentration of salicylic acid was measured using the ferric nitrate colorimetric method.28 The concentrations of p-hydroxybenzoic acid and 5-hydroxyisophthalic acid were measured by high performance liquid chromatography (Agilent Technologies 1200 Series) using a UV detector and a C18 reverse phase liquid chromatography column. The mobile phases and detection wavelengths were methanol/water (40:60) and 210 nm for the p-hydroxybenzoic acid and methanol/water (30:70) and 240 nm for the 5-hydroxyisophthalic acid. In both cases, a flow rate of 0.5 mL/min and an operation temperature of 298 K were selected. The chemical oxygen demand (COD) was determined according to the Standard Methods for the Examination of Water and Wastewater.27 3. Results and Discussion The effect of the pH on the catalytic wet oxidation of four pharmaceutical wastewaters contaminants is discussed next. The experiments were carried out employing Fe(II) as a homogeneous catalyst. 3.1. Phenol. To study the influence of the initial pH on the degradation of phenol, several reactions were carried out with solutions containing 10.6 mM phenol (1000 ppm) and 1.79 mM (100 ppm) Fe(II). The concentration of phenol in pharmaceutical wastewater usually ranges from 700 to 1300 ppm. Temperature and pressure were fixed at 413 K and 1.0 MPa, respectively. Results for different pH values are shown in Figure 2.

Figure 2. Evolution of phenol concentration and COD during the catalytic wet oxidation at different pH values: 1.9 (]); 2.4 (0); 3.8 (∆). Black symbols denote phenol concentrations, and gray symbols denote COD. In all cases, T ) 413 K, P ) 1.0 MPa, the concentration of Fe(II) ) 1.79 mM, and the initial concentration of phenol ) 10.6 mM. Solid lines denote model curves according to kinetic parameters shown in Table 2.


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Figure 3. Evolution of phenol concentration and COD during the catalytic wet oxidation process in the presence of different catalyst concentrations: 0 mM (∆), 1.43 mM (0), and 14.3 mM (]) of Fe(II) at pH ) 1.9 (a) and pH ) 2.4 (b). Black symbols denote concentrations; gray symbols denote COD. In all cases, T ) 413 K, P ) 1.0 MPa, and the initial concentration of phenol ) 10.6 mM. Solid lines denote the model according to kinetic parameters shown in Table 2.

Observing Figure 2, it is obvious that the shape of the degradation curve of phenol was clearly affected by the initial pH of the media. When the initial pH was 1.9 or 3.8, the phenol degradation (and COD removal) took place in a single stage, whereas the reaction at pH 2.4 took place in two different steps: an initial induction period, with a duration of approximately 100 min, where the phenol degradation was slow, and a second one, where phenol degradation rate was higher, obtaining a complete elimination of the compound after 270 min of reaction. A similar behavior was observed for COD removal. These results can be explained according to the catalytic mechanism of the iron during the wet oxidation process. Wet oxidation of phenol is based on a radicalary mechanism.14,29 Initially, phenol is attacked by radicals formed from the dissolved oxygen, giving rise to hydroquinones. These compounds generate radicals (eqs 1 and 2) and act as cocatalysts with the Fe(II), which generates new radicals during its oxidation to Fe(III) (eq 3) and the consequent reduction to Fe(II) (eq 4). Thus, the establishment of a Fe(III)/Fe(II) redox system increases the oxidation rate. The redox potential of the Fe(III)/ Fe(II) system abruptly drops for pH values above 2 but is relatively high for pH values below 2.30 This means that under highly acidic conditions, Fe(III) is unstable, and the catalytic activity of the pair Fe(III)/Fe(II) is not possible; this fact explains the low degradation rate observed when the initial pH value was fitted to 1.9. In addition, when the pH is higher than 3, the stabilization of Fe3+ in the form of hydroxide occurs, and the redox cycle is again broken.30 This fact was corroborated after carrying out a set of experiments at a pH of 1.9 with different concentrations of iron(II) (Figure 3a). Similar reaction times

were required to degrade the initial phenol, and a similar degree of mineralization was obtained independently of the iron concentration.

In Figure 2, it can also be noted that the degradation rate during the first stage of the wet oxidation at pH 2.4 was similar to the rate achieved at pH 1.9 or 3.8. So, conversions obtained in the first 110 min were very similar in the three cases. This fact can be explained taking into account that the concentration of hydroquinones during the first minutes of reaction was too small to exhibit any catalytic effect. However, when the concentration of hydroquinones began to be appreciable, for pH 2.4, which enables the establishment of a Fe(III)/Fe(II) redox system, the degradation was accelerated (Figure 2). Two steps were considered to fit the experimental data; an induction period and an oxidation phase. The degradation of phenol during the oxidation phase was fitted to a pseudo-firstorder kinetic order (CPh ) CPh,o e- k(t-ti)). This fitting allowed for obtaining the kinetic constant (k) and the duration of the

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Table 2. Relevant Kinetic Data for the Catalytic Wet Oxidation of Phenol, Salicylic Acid, p-Hydroxybenzoic Acid, and 5-Hydroxyisophthalic Acid under Different Operational Conditionsa phenol Co (mM) CFe2+ (mM) pH ti (min) k (s-1) × 105 r2 Ha

1.43 1.9 0 4.63 0.990 8.3 × 10-4

0

2.4 112 32.8 0.996 2.9 × 10-3

3.8 0 4.65 .992 7.9 × 10-4

0 4.48 0.993 7.7 × 10-4

10.6 1.43 1.9 0 4.63 0.993 7.8 × 10-4

14.3

0

0 4.98 0.994 8.1 × 10-4

0 4.67 0.992 7.9 × 10-4

1.43 2.4 112 32.8 0.996 2.1 × 10-3

14.3 146 47.2 0.998 2.5 × 10-3

salicylic acid Co (mM) CFe2+ (mM) pH ti (min) k (s-1) × 105 r2 Ha

14.3 1.8 0 95.8 0.991 3.9 × 10-3

2.6 31 72.0 0.998 3.4 × 10-3

0 3.5 80 20.5 0.994 1.8 × 10-3

13.0 4.47 1.8 0 0 58.7 83.8 0.991 0.990 3.1 × 10-3 3.7 × 10-3 1.43

0 19.3 0.993 1.8 × 10-3

14.3

0

0 95.8 0.991 3.9 × 10-3

0 3.67 0.987 7.7 × 10-4

1.43 3.5 110 6.17 0.996 1.0 × 10-3

14.3 88 20.5 0.994 1.8 × 10-3

p-hydroxybenzoic acid Co (mM) CFe2+ (mM) pH ti (min) k (s-1) × 105 r2 Ha

2.17 14.3 1.9 166 5.83 0.999 4.0 × 10-4

0

1.43 2.6 161 49.2 0.990 1.2 × 10-3

0 45.3 0.994 1.1 × 10-3

14.3 193 37.3 0.994 1.0 × 10-3

5-hydroxyisophthalic acid Co (mM) CFe2+ (mM) pH ti (min) k (s-1) × 105 r2 Ha a

1.65 14.3 1.9 50 3.50 0.990 2.7 × 10-4

0 0 10.5 0.989 4.7 × 10-4

1.43 2.6 309 1.11 0.946 4.8 × 10-4

14.3 130 7.92 0.991 4.0 × 10-4

In all cases, T ) 413 K, P ) 1.0 MPa, CO2 ) 5.1 mM.

induction period (ti). Note that experimental data showed a gradual transition from the induction to the rapid reaction phases, whereas the proposed model assumes an abrupt transition and, therefore, does not match the data in this area. The calculated values of ti and k for each of the runs are summarized in Table 2. One-step reactions imply a ti equal to zero. In order to study the catalytic effect of the catalyst, different iron concentrations during the wet oxidation of phenol at pH 2.4 were tested (Figure 3b). It is interesting to observe that the noncatalytic oxidation of phenol proceeded in an single step, whereas an induction period was clearly visible when Fe(II) was present in the media. This induction period is the time necessary to form hydroquinones that catalyze the degradation. It is necessary to take into account that in the catalytic process, there are two competing reactions for the OH• generated from the dissolved oxygen: the auto-oxidation of Fe(II) (eq 5) and the hydroxylation of the phenolic compound. In addition, there is an equilibrium between iron and quinones according to eq 6, so that high initial Fe(II) concentrations may decreases the formation rates of benzoquinones (eq 1).31 Fe(II) + OH• f Fe(III) + OH-

(5)

Vaidya and Mahajani16 also reported the existence of an induction period during the wet oxidation of phenol in the presence of ferrous sulfate at 448 K and 0.69 MPa oxygen partial

pressure, but these authors did not observe an induction period at 473 K. Under the operating conditions employed in this work, after the induction period, a positive effect of the catalyst was clearly observed. For example, it can be appreciated that in the reaction with 1.43 mM Fe(II) (80 ppm; see Figure 3b), 95% of the initial phenol concentration and 65% of the initial COD were degraded after 240 min, whereas only 45% conversion and 35% mineralization were reached in the absence of ferrous ions during the same time period. In a previous work, Vicente et al.32 also observed the catalytic effect of Fe during the fast step of the wet oxidation of phenol carried out at 423 K, 5.1 MPa, and an initial pH of 6.5. However, they observed that the induction period was shorter employing an iron salt than without iron. This induction period was not observed in works that employed Cu(II) as a catalyst, temperatures between 313 and 448 K, and pressures between 0.6 and 1.9 MPa.16,34 This could be due to the higher activity of the copper with respect to iron and to the direct reaction between the copper and the organic compound.2,35 A different behavior has been reported when the phenol oxidation was carried out employing Fe(II) together with hydrogen peroxide, as a radical promoter. In this case, the reaction proceeded in two steps, but the first step was the fastest step, associated with hydrogen peroxide consumption, and the second slower step with a rate comparable to that of conventional wet air oxidation.36 Initial induction periods were observed on the COD profiles at pH 1.9 (see Figure 3a) and 3.8 (see triangular symbols in Figure 2), independently of the presence or absence of iron. This was attributed to the accumulation of small amounts of hydroquinones and benzoquinones, as previously discussed. The


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Figure 4. Evolution of the salicylic acid concentration and COD during the catalytic wet oxidation process at different initial pH’s: 1.8 (]), 2.6 (0), 3.5 (∆). Black symbols denote concentrations; gray symbols denote COD. In all of the cases: T ) 413 K, P ) 1.0 MPa, the concentration of Fe(II) ) 14.3 mM, and the initial concentration of salicylic acid ) 13.04 mM. Solid lines denote models according to kinetic parameters shown in Table 2.

concentration of Fe(II) and pH also had an effect on the formation of intermediates, as can be observed comparing the COD and concentration profiles: the higher difference between (COD/CODo) and (Ci/Ci,o), the higher the amount of partially oxidized products in the reaction media. In all one-step reactions (oxidations at pH 1.9, 3.8, or without catalyst), the intermediates’ concentration increased during the first minutes of the reaction and then remained approximately constant during the rest of the experiment. However, the evolution of the intermediates’ concentration in the experiments with Fe(II) at pH 2.4 (twostep reactions, see Figure 3b) was different; the difference between (COD/CODo) and (Ci/Ci,o) was small during the induction period, but it increased along the fast step of the reaction. In these experiments, the higher intermediate concentrations were obtained operating with 1.43 mM Fe(II). So, for a phenol conversion of 80%, the remaining COD was 50% of the initial COD for a Fe(II) concentration of 1.43 mM and 41% of the initial COD for 14.3 mM Fe(II). It can be affirmed that the Fe(II) increases not only the degradation rate of the phenol but also the mineralization rate of the intermediates. 3.2. Salicylic Acid. To study the influence of the initial pH on the degradation of salicylic acid during the wet oxidation process, several reactions were carried out. In these experiments, the initial concentrations of salicylic acid and Fe(II) were 13.0 mM (1800 ppm salicylic acid) and 14.3 mM (800 ppm Fe(II)), respectively. Temperature and pressure were 413 K and 1.0 MPa. Samples were taken at different times to analyze COD and salicylic acid concentration. Results are shown in Figure 4. It can be observed that the fastest degradation took place at the lowest pH assayed (1.8). When the wet oxidation reaction was carried out at pH 3.5, after 250 min, only 80% of the initial salicylic acid had been degraded. For the lower pH values assayed, a complete removal of the contaminant was reached after 150 min of reaction. The degree of mineralization achieved during the oxidation process is also shown in Figure 4 as COD evolution. The mineralization of the salicylic acid was high for the three pH’s assayed, quite in parallel with salicylic acid degradation. For a pH of 1.8, a total mineralization was achieved. As can be seen in Figure 4, the shape of the degradation profile was also affected by the initial pH of the media. When the initial pH of the media was 3.5, the oxidation of salicylic acid took place in two steps with different degradation rates. The initial slow step was followed by a faster oxidation phase. Lower pH implied a reduction in the duration of the first step

and an increase in the rate of degradation. In fact, at a pH of 1.8, the degradation took place in a single step. It can be noted that the effect of the pH on the degradations of salicylic acid and phenol was different. An optimum pH between 2 and 3 was found for the wet oxidation of phenol, whereas the higher degradation rate of salicylic acid was observed at the lowest pH, when the Fe2+ was a stable cation in the solution. A possible explanation for this behavior could be the formation of a complex between the Fe(II) and the salicylic acid, more easily oxidizable than the uncomplexed acid. This is in accord with the fact that the induction period on the COD profile observed during the oxidation at a pH of 1.8 disappeared when Fe(II) was present in the media (Figure 5a). In order to obtain a deeper analysis of the pH effect on the catalytic degradation of the salicylic acid, two sets of experiments with different concentrations of Fe(II) were carried out at two different pH values: 1.8 and 3.5. In Figure 5a, it can be observed that the degradation rate of salicylic acid at pH 1.8 increased for higher concentrations of Fe(II). So, the time needed to reach a conversion over 80% was around 2 h without a catalyst and 40 min with 14.3 mM Fe(II). It can also be observed that at a pH of 1.8 in the absence of Fe(II), around 20% of the initial COD remained in the media at the end of the oxidation, whereas the salicylic acid has been almost completely eliminated. This noncomplete oxidation indicates the presence of some intermediate compounds that were not further oxidized. Analyses carried out in the final samples proved that this remaining COD was mainly due to the presence of acetic acid. However, the presence of Fe(II) allowed a total elimination of COD (even for the lowest concentration of Fe(II)). It was observed that the lowest differences between the salicylic acid conversion and the degree of mineralization at a pH of 1.8 (see Figure 5a) were obtained when the concentration of Fe(II) was 1.43 mM. Thus, an excessive amount of iron had a detrimental effect on the mineralization of the intermediates. This fact could be due to interactions of the Fe(II) with one or several intermediates, increasing their refractoriness to oxidation. Mijangos et al.37 observed the formation of Fe(III) complexes with organic intermediates (catechol and carboxylic acids) during the Fenton treatment of phenol. In Figure 5b, it can be observed that at pH 3.5 the presence of iron had an important effect on the shape of the degradation profiles. Whereas the oxidation of salicylic acid without a catalyst took place in a single step, two different steps were observed in the presence of iron(II). During the first step or

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Figure 5. Evolution of salicylic acid concentration and COD during the catalytic wet oxidation process in the presence of different catalyst concentrations: 0 mM (O), 1.43 mM (∆), 4.47 mM (0), and 14.3 mM (]) Fe(II) at pH ) 1.8 (a) and pH 3.5 (b). Black symbols denote concentrations; gray symbols denote COD. In all cases, T ) 413 K, P ) 1.0 MPa, and the initial concentration of salicylic acid ) 13.0 mM. Solid lines denote the model according to kinetic parameters shown in Table 2.

induction period, the salicylic concentration remained approximately constant, independent of the concentration employed. This fact was attributed to the formation of an Fe(III)-salicylic acid complex, which reduces the formation of the initial radicals. This complex has proved to be more refractory to the oxidation than the salicylic acid during the Fenton treatment.17 In the subsequent period, the oxidation itself took place, and the catalyst appreciably increased the degradation rate despite the pH of the media that provoked the stabilization of the Fe(III) as hydroxide. The presence of a part of the catalyst as iron(II) at the beginning of the reaction would explain the catalytic effect observed at this pH. In fact, Morgan and Lahav38 reported that the rate of oxidation of Fe(II) to Fe(III) at room temperature is constant and small until the pH becomes larger than 3.5, and then it starts to increase quickly. The kinetic constants obtained were smaller than the calculated ones for the phenol degradation at a pH of 2.4, where Fe(III) did not precipitate as hydroxide. Each of the experimental runs was fitted to a pseudo-firstorder kinetic model with an induction period. Calculated values of ti and k are summarized in Table 2. The assumed first-order kinetic in the salicylic acid concentration allowed a proper fitting of the exponential data during the oxidation phase. 3.3. p-Hydroxybenzoic Acid. In order to study the influence of the initial pH on the degradation of the p-hydroxybenzoic acid, two pH values were assayed: 1.9 and 2.6. These values were selected because they gave the best results for the degradation of salicylic acid and phenol, respectively. The initial concentration of p-hydroxybenzoic acid was 2.17 mM, and the

working conditions were 413 K and 1.0 MPa. The concentration of Fe(II) ranged from 0 mM to 14.3 mM (0-800 ppm). As can be seen in the Figure 6, the maximum degradation of the p-hydroxybenzoic acid was obtained without iron(II), achieving an almost complete elimination of the p-hydroxybenzoic acid in 2 h. The addition of Fe(II) provoked the appearance of an initial induction period, reducing the degradation of the p-hydroxybenzoic acid. In these cases, appreciable conversions were only obtained after 200 min of reaction. The reaction at a pH of 1.9 was the slowest, achieving only a 49% conversion after 6 h. In the case of the COD evolution, it is interesting to note the existence of an initial induction period in all of the cases, even in the experiment without Fe(II). In this experiment, it was also observed that around 20% of the initial COD remained in the media at the end of the reaction at a pH of 2.6, without a catalyst. Each of the experimental runs was fitted to a pseudo-firstorder kinetic model with an induction period when necessary. Calculated values of ti and k are summarized in Table 2. As can be observed in Figure 6, the length of the induction period increased for higher Fe(II) concentrations. Thus, the length of the induction period (ti) in the reaction with 14.3 mM Fe(II) and pH 2.6 (193 min) was higher than with 1.43 mM Fe(II) and the same pH (161 min). As in the case of the phenol, the presence of an induction period was attributed to a competition between the autoxidation of Fe(II) (eq 5) and the hydroxylation of the phenolic compound by the initial OH•. The Fe(II) acts as a radical scavenger and results in the concentration of phydroybenzoic remaining approximately constant, provoking the


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Figure 6. Evolution of the p-hydroxybenzoic acid concentration and COD during the catalytic wet oxidation process under different conditions: pH 1.9 and 14.3 mM Fe(II) (∆), pH 2.6 and 1.43 mM Fe(II) (]), pH 2.6 and 14.3 mM Fe(II) (0), pH 2.6 and without catalyst (O). Black symbols denote concentrations; gray symbols denote COD. In all cases, T ) 413 K, P ) 1.0 MPa, and the initial concentration of p-hydroxybenzoic acid ) 2.17 mM. Solid lines denote the model according to kinetic parameters shown in Table 2.

Figure 7. Evolution of the 5-hydroxyisophthalic acid concentration and COD during the catalytic wet oxidation process under different conditions: pH 1.9 and 14.3 mM Fe(II) (∆); pH 2.6 and 1.43 mM Fe(II) (]); pH 2.6 and 14.3 mM Fe(II) (0); pH 2.6 and without catalyst (O). Black symbols denote concentrations; gray symbols denote COD. In all cases, T ) 413 K, P ) 1.0 MPa, and the initial concentration of 5-hydroxyisophthalyc acid ) 1.65 mM. Solid lines denote the model according to kinetic parameters shown in Table 2.

appearance of the induction period. During this period, Fe(III) is formed, and when the amount of Fe(II) is small, the degradation rate of the phenolic compounds begins to be appreciable. In addition, Rivas et al.39 proposed a third reaction implied by the initiation of the oxidation in Fenton systems. In this reaction, p-hydroxybenzoic acid acts as a chelator of ferrous iron, accelerating the rate of attack of the p-hydroxybenzoic acid by the hydroxyl radical and reducing the length of the induction period. However, this did not occur in the experiments shown in Figure 6, probably because the extension of this reaction is lower than other routes under the operating conditions here employed. In contrast to phenol, the degradation rate of the p-hydroxybenzoic acid after the induction period was quite similar, independently of the concentration of the catalyst. The effect of the pH was more notorious, obtaining a lower k value at pH 1.8 than at pH 2.6. In conclusion, it seems that the presence of Fe(II) provokes the apparition of an induction period, but it has a slight effect on the degradation rate. 3.4. 5-Hydroxyisophthalic Acid. A series of experiments were carried out in order to determine the effect of pH and the Fe(II) concentration on the degradation of 5-hydroxyisophthalic acid (Figure 7). As can be seen in Figure 7, the highest degradation of 5-hydroxyisophthalyc acid was obtained operating at pH 2.6 without a catalyst (50% conversion after 2 h). The presence of

Fe(II) in the reaction media had a negative effect on the degradation rate, provoking the appearance of an induction period. Each of the experimental runs was fitted to a pseudo-firstorder kinetic model with an induction period when necessary. Calculated values of ti and k are summarized in Table 2. As can be observed in Figure 7, the longest induction period was observed at pH 2.6 when 1.43 mM Fe(II) was employed (309 min), whereas the duration of this period was appreciablely lower employing 14.3 mM Fe(II) (130 min). The appearance of this induction period may be due to the formation of an iron-5-hydroxyisophthalic complex. The initial pH of the media also had an effect on the induction period. Fitting the initial pH to 1.9, the length of the induction period can be reduced to 50 min. However, the selection of a more acidic medium implied a reduction in the degradation rate of the 5-hydroxyisophthalic acid during the second phase of the reaction. As can be observed in Table 2, the highest degradation rates were at pH 2.6 without a catalyst or with 1.43 mM Fe(II). The poor regression factor of the fitting under these conditions has to be taken into account. 3.5. Comparison of the Wet Oxidations of Phenolic Acids. Figure 8 collects the kinetic parameters (k and ti) calculated for all of the experiments in this work (see also Table 2). On the one hand, the presence of iron provoked the appearance of induction periods (see Figure 8b and c) when

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increased when higher Fe(II) concentrations were employed. The iron had a slight effect on the degradation rate during the fast steps, whereas the effect of the pH was more notorious, obtaining better results at pH 2.6 than at pH 1.9. In the case of the wet oxidation of 5-hydroxyisophthalic acid, the presence of Fe(II) also had a negative effect on the degradation rate, because of the appearance of induction periods. The selection of a more acidic medium implied a reduction in the length of the induction period, but the degradation rate during the fast step decreased. The unequal behaviors observed in the wet oxidation process of the phenolic compounds studied are due to the variety of roles that iron plays during the different oxidation processes, which includes the formation of phenolic-compound-iron complexes, autoxidation with OH•, interactions with reaction intermediates, or the presence of different iron species according to the pH of the media. Acknowledgment The work upon which this paper is based was financed by the Spanish Ministry of Education and Science (MEC-06-CTM08688). Literature Cited

Figure 8. Pseudo-first-order kinetic constants (symbols) and induction periods (brackets) calculated at different pH values for the wet oxidation of phenol ([), salicylic acid (9), p-hydroxybenzoic acid (2), and 5-hydroxyisophthalic acid (b) in the presence of different catalyst concentrations: without Fe(II) (a), with 1.43 mM Fe(II) (b), and with 14.3 mM Fe(II) (c). In all cases, T ) 413 K and P ) 1.0 MPa.

the pH of the media was between 2 and 3.5. This fact confirms the role of the redox pair Fe(II)/Fe(II) as a radical scavenger. On the other hand, Fe(II) and pH had an important effect on the fast step of the wet oxidation (k). Whereas the Fe(II) had a slight effect on the degradation rate of the p-hydroxybenzoic and 5-hydroxyisophthalic acids, catalytic activity was observed during the oxidation processes of phenol (at pH 2.6) and salicylic acid. The catalytic activity of Fe(II) can be attributed to reactions of quinone-Fe(II) and to the formation of complexes where the phenolic compound is more easily oxidizable. 4. Conclusions Experimental results showed that Fe(II) cations act as a catalyst during the wet oxidation of phenol and salicylic acid. In both cases, the pH of the media has a crucial role during the wet oxidation process. During the catalytic wet oxidation of phenol, catalytic activity was only observed at pH values between 2 and 3, which was explained according to the formation of a redox cycle of Fe(III)/Fe(II) in this interval of pH. Regarding salicylic acid, the maximum rate of degradation of salicylic acid was achieved at the lowest pH value assayed (1.8). This fact was attributed to the formation of the complex Fe(II)-salicylic acid, which favors the degradation process. In this case, an Fe(II) concentration between 1.43 and 14.3 increases the mineralization degree. The presence of Fe(II) in the reaction media had a negative effect on the wet oxidation of p-hydroxybenzoic acid, provoking the appearance of an induction period. The length of this period

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ReceiVed for reView July 13, 2010 ReVised manuscript receiVed September 10, 2010 Accepted October 6, 2010 IE101497S

Fe2+-Catalyzed Wet Oxidation of Phenolic Acids ... - ACS Publications

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