Environ. Sci. Technol. 2010, 44, 8629–8635
Wet Oxidation of Salicylic Acid Solutions SERGIO COLLADO, LAURA GARRIDO, ADRIANA LACA, AND MARIO DIAZ* Department of Chemical Engineering and Environmental Technology, University of Oviedo, E-33071 Oviedo, Spain
Received July 2, 2010. Revised manuscript received October 18, 2010. Accepted October 18, 2010.
Salicylic acid is a frequent pollutant in several industrial wastewaters. Uncatalyzed wet air oxidation, which is a promising technique for the treatment of phenolic effluents, has not been analyzed yet for the removal of salicylic acid. The effect of different conditions of pH (1.3-12.3), pressure (1.0-4.1 MPa), temperature (413-443 K), and initial concentrations (1.45-14.50 mM) on the wet oxidation of salicylate/salicylic acid solutions have here been investigated. The pH value of the reaction media was found to be a key parameter for the rate of the oxidation process with an optimum at pH 3.1, when the concentrations of salicylic acid and salicylate were similar. The oxidation reaction followed pseudofirst-order kinetics with respect to salicylic acid and 0.82 order with respect to dissolved oxygen. Additionally, the evolution of the color during the wet oxidation was analyzed and discussed in relation with the formation of intermediate compounds. Then, a reaction pathway for the noncatalytic wet oxidation of the salicylic acid was proposed.
1. Introduction Salicylic acid is a key additive in many skin-care ointments, creams, gels, and transdermal patches and also a common chemical in dyes. It is also used as a raw material for the synthesis of acetylsalicylic acid and its derivates. Consequently, salicylic acid is frequently present in wastewaters originated in pharmaceutical and cosmetic industries. This contaminant, apart from contributing to wastewater COD, has toxic effects on ecosystems and human health (1), and it must be eliminated from wastewater to avoid its spreading into the environment. A variety of techniques have been proposed for the treatment of wastewaters containing salicylic acid including electrochemical oxidation (1), Fenton process (2, 3), adsorption (4), precipitation (5), heterogeneous photocatalysis (6), or biological digestion (7). To allow biological treatment, industrial waters must sometimes be pretreated to reduce the content of nonbiodegradable or toxic organic pollutants. A possible alternative is wet air oxidation (WAO), which is already employed in some industries to treat highly toxic wastewaters. Usually, a WAO treatment results in the production of more easily biodegradable compounds, but it is necessary to take into consideration that for short reaction times (where the process of degradation is not completed), very lowconcentrated, but highly biotoxic intermediates can be formed, thus increasing the harmful effects of the wastewater. * Corresponding author phone: 34985103439; fax: 3498103434; e-mail:
[email protected]. 10.1021/es1021944
2010 American Chemical Society
Published on Web 10/27/2010
During the wet oxidation process, different kinds of benzoquinones and quinhydrones are formed. These highly biotoxic intermediates may significantly increase the toxicity of the wastewater during the early stages of phenolic compound oxidation, even when they are at low concentrations (see Table S1 in the Supporting Information (SI)) (8, 9). These intermediate compounds are also associated with a change in the color of the reaction mixture from clear to a brownish color. Therefore, the appearance of this color can be a good indicator of the presence of these toxic compounds in the treated wastewater (10). As far as we know, studies on the noncatalytic wet air oxidation of salicylic acid do not exist. Only a few references focused on the catalytic wet oxidation of this compound have been found in the literature. Tukac and Hanika (11) reported low conversions (around 40%) during the wet oxidation of salicylic acid in a trickle bed reactor using activated carbon as catalyst. In a more recent work, Yang et al. (12) found that salicylic acid can be effectively removed by WAO employing a perovskite-type oxide LaFeO3 as catalyst. The study of noncatalytic wet oxidation of salicylic acid is very important to establish the kinetic and mechanism as a step toward to know the best conditions for the degradation and, at the same time, to know their participation in systems with different designed catalyst. In view of these considerations, the aim of the present work was to investigate the noncatalytic oxidation kinetics of salicylic acid, paying special attention to the effect of the operating conditions. Moreover, color progression during the wet oxidation process was also studied as an indicator of the toxicity of the effluent and the presence of intermediates, in order to propose a suitable reaction mechanism.
2. Experimental Section 2.1. Apparatus and Procedure. Experiments were completed in a 1-L capacity reactor (Parr T316SS) equipped with two agitators (identified as “1” in the SI Figure S1). The reactor was preceded by a 2-L stainless steel water reservoir (“2” in figure). The loaded volume in each vessel was about 70% of the total in order to ensure safe operating conditions. The equipment, charged only with water with the pH adjusted by addition of H2SO4 or KOH, was pressurized and preheated to the desired working conditions, while the stirrer speed was adjusted to 500 rpm for all the experiments. The pH of the water was adjusted to a suitable value, so that after the addition of the salicylic acid solution the medium had the desired pH value. The operating pressure was provided by bottled compressed oxygen (“3” in the figure), with the oxygen flow rate adjusted to 2.33 × 10-5 m3 s-1, and controlled by an electronic mass flow controller (Brooks) (“4” and “6” in the figure). The pressure was kept constant by means of a back pressure controller located at the end of the gas line (“5” in the figure). The oxygen was bubbled through the water reservoir to become saturated with water vapor, and then it was introduced into the reaction vessel. Once the equipment was pressurized and preheated up to the desired conditions, 2 mL of a concentrated salicylic acid solution was injected into the reactor (“7” in the figure). The concentration of this solution was calculated to give the desired final concentration of salicylic acid inside the reactor (in most cases 7.25 mM). During the preparation of this solution, it was needed the addition of an amount of KOH in order to dissolve the poorly soluble salicylic acid (2 g/L at 293 K). The injection time was taken as the zero-time for the reaction. A valve and a coil fitted to the top of the vessel allowed the withdrawal of samples during the reaction (“8” in the figure). Reaction VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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temperature and pressure were maintained during the course of each experiment. In addition to these experiments, a series of two consecutive runs was performed without removal of the salicylic acid degradation products under the following conditions: 413 K, 1.0 MPa, pH 2.0, and 6.52 mM of salicylic acid as the initial concentration for both runs. The experiment involved carrying out a second salicylic acid injection once the conversion from the preceding run reached a 90% conversion level, without stopping the oxygen flow at any point. Oxidations reported in this work were performed under kinetic control. Verification was done by calculating the Hatta number (13). 2.2. Analytical Methods. Salicylic acid concentration was analyzed using the Trinder reaction (14). The chemical oxygen demand (COD) was determined according to the Standard Methods for the Examination of Water and Wastewater (15). Color was determined by direct measurement (without colorimetric additives) of the absorbance at 455 nm (10). For the identification of the intermediates, a liquid chromatograph (Agilent 1200) equipped with a C18 column using an absorbance detector at 280 nm was used. The mobile phase used was a mixture of methanol and water solution (30/70 v/v) using a flow rate of 0.4 mL/min. The concentration of acetic acid was monitored by means of an ion exchange chromatograph (Dionex DX-120 Ion Chromatograph) and a suppressed conductivity detector ((ASRS-ULTRA Autosuppression Recycle Mode). The eluent employed was a solution 1.8 mM Na2CO3/1.7 mM NaHCO3, the flow rate 1.46 mL/ min, the precolumn IonPac AG4A-SC (4 × 50 mm), and the column IonPac AS4A-SC (4 × 250 mm).
3. Results and Discussion 3.1. Effect of pH. When the wet oxidation of salicylic acid dissolved in distilled water at pH 5.4 (without initial pH adjustment of the water in the reactor) was studied employing a temperature of 413 K and a pressure of 1.0 MPa, an initial induction period was not observed and the degradation took place apparently in a single step (Figure 1). Low percentage conversions were reached during the experiment, and after one hour only a 10% level of conversion was achieved. During the experiment, the pH of the media decreased slightly, from 5.4 to 4.7. In order to investigate the effect of the pH on the degradation of the salicylic acid, a set of experiments employing the same operating conditions was carried out at different initial pH values, ranging between 1.3 and 12.3. In all cases, the increase in the proton concentration during the wet oxidation process was very small (between 0.2 and 0.7 pH units); obviously, the decrease in pH was more marked when the initial pH was almost neutral. In Figure 1 it can be observed that similar reaction times were required to degrade 7.25 mM of salicylic acid when the pH of the media was basic or slightly acid (pH values higher than 5). However, pH values lower than 5 favored the degradation rate of the salicylic acid, with a maximum reaction rate at pH 3.1 (90% conversion after 140 min). For pH 2.0 and pH 4.0, the reaction rate were slightly lower than for pH 3.1 and when the pH was 1.3, the reaction rate was very similar to those observed for pH values higher than 5. The COD elimination showed similar behavior, the degradation rate increasing when the pH was between 2.0 and 4.0. All the experiments were successfully fitted to a pseudofirst order kinetic model with a good degree of concordance. Kinetic constants are reported in Table 1. Cybulski and Trawczynski (16) reported that the reaction orders for the oxidation of phenol and other phenolic compounds (such as p-chlorophenol and p-nitrophenol) are predominantly one, the same behavior as observed for salicylic acid. 8630
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FIGURE 1. Evolution of salicylic acid concentration (a), COD (b), and absorbance at 455 nm (c) during wet oxidation conducted at different pH values: 1.3 (°), 2.0 ((), 3.1 (2), 4.0 (•), 5.4 (×), and 12.3 (0). In all cases: initial concentration of salicylic acid ) 7.25 mM; T ) 413 K; P ) 1.0 MPa. Solid lines denote model data according to Table 1. Figure 2 shows the kinetic constants kSA and kCOD (the asymptotic COD value is not considered) and the distribution of salicylic acid/salicylate at different pHs. It can be observed that when the pH of the media is higher than 5, the salicylate ion is the predominant species and the reaction rate is approximately constant. However, the reaction rate increased when the concentration of salicylic acid began to be appreciable (pH lower than 4) and decreased after achieving a maximum value at pH 3.1. This indicates that the coexistence of the two species, salicylic acid/salicylate, has a beneficial effect on the degradation and mineralization rates of this compound. It is interesting to note that the highest degradation rate was reached when the concentrations of salicylic acid/salicylate were similar. This could be attributed to complementary roles of the salicylic acid and salicylate during the wet oxidation process. It is possible that salicylic acid is a better generator of free radicals than the salicylate ion but a worse radical scavenger, therefore giving maximum oxidation rates when the concentrations of both compounds are similar. The small degradation observed for pH < 2 was also observed during the wet oxidation of phenol at 448 K and 0.4 MPa (17). In this work, a maximum of rate was reported at pH 4, and practically no conversion of phenol was observed between neutrality and 10. These authors attributed the effect of the pH on the degradation rate to the
TABLE 1. Kinetic Data Results for the Wet Oxidation of Salicylic Acid at Different Operating Conditionsa T (K)
413
P (MPa) Co (mM) pH CO2 (mM) kSA (s-1) r2 Ha kCOD (s-1) r2 T (K) P (MPa) Co (mM) pH CO2 (mM) kSA (s-1) r2 Ha kCOD (s-1) r2 T (K) P (MPa) Co (mM) pH CO2 (mM) kSA (s-1) r2 Ha kCOD (s-1) r2 T (K) P (MPa) Co (mM) pH CO2 (mM) kSA (s-1) r2 Ha kCOD (s-1) r2
1.0 7.25 1.3
2.0
3.85 × 10-5 0.99 5.8 × 10-4 2.13 × 10-5 0.97
2.70 × 10-4 0.992 1.4 × 10-3 1.48 × 10-4 0.99
3.1 2.88 × 10-4 0.996 1.6 × 10-3 1.63 × 10-4 0.993
1.0 5.04 2.70 × 10-4 0.992 1.4 × 10-3 1.70 × 10-4 0.99 413
5.04 2.70 × 10-4 0.992 1.4 × 10-3 1.70 × 10-4 0.99 1.45 9.23 × 10-4 0.99 1.3 × 10-3 1.43 × 10-3 0.97
5.04
4.2
5.4
12.3
2.73 × 10-4 0.98 1.5 × 10-3 1.33 × 10-4 0.99
2.92 × 10-5 0.98 1.7 × 10-3 2.03 × 10-5 0.97
1.88 × 10-5 0.97 1.4 × 10-3 1.18 × 10-5 0.96
413 2.5 7.25 2.0 16.8 6.50 × 10-4 0.998 1.3 × 10-3 5.53 × 10-4 0.98 428 1.0 7.25 2.0 3.98 4.08 × 10-4 0.98 2.0 × 10-3 2.77 × 10-4 0.98 413 1.0 7.25 2.0 5.04 2.70 × 10-4 0.992 1.4 × 10-3 1.70 × 10-4 0.99
4.1 28.5 1.18 × 10-3 0.98 1.5 × 10-3 9.53 × 10-4 0.99 443
2.10
5.43 × 10-4 0.98 3.9 × 10-3 3.88 × 10-4 0.97 14.50 2.17 × 10-4 0.98 2.1 × 10-3 9.58 × 10-5 0.99
a T is temperature, P is pressure, Co is initial concentration of salicylic acid, CO2 is oxygen concentration, kSA and kCOD are pseudofirst order kinetic constants for the elimination of salicylic acid and COD, respectively, Ha is the Hatta number, and r is the regression coefficient.
FIGURE 2. Salicylic acid (s) and salicylate (---) concentrations and calculated kinetic constants for the degradation of salicylic acid (•) and COD (O) at different pHs. different reactions that occur for some key free radical species at different pH values and to the formation of the protonated form of the compound. In Figure 1b, it can be observed that during the time that the experiments lasted a complete mineralization of the salicylic acid was not attained in any case. The highest degree of mineralization was achieved for pH 2, and even in this case 14% of the initial COD remained in the medium after 340 min. Comparing its effectiveness with other techniques,
the noncatalytic wet oxidation at acid pH is an attractive treatment for the elimination of salicylic acid. For example, overall COD removal efficiency employing Fenton treatment comprised nearly 90% under optimal treatment conditions applied (3). Moreover, it is necessary to take into consideration that the selection of the wet oxidation avoids subsequent steps for recovering the catalyst from the medium and the utilization of expensive chemical oxidants as H2O2 or O3. The concentration of acetic acid was measured in the final media, and, comparing these values with the COD of the media, it was calculated that around of the 95% of the final COD was due to the presence of acetic acid, which was also indicated by many authors as the final product during the wet oxidation of other phenolic compounds (18). Although the best reaction rates were obtained at pH 3.1, the subsequent experiments in this work were carried out at pH 2.0 because this is the typical value found in real pharmaceutical wastewaters arising from the aspirin synthesis process, which is the main application of salicylic acid. 3.2. Effect of Pressure. The effect of oxygen pressure was studied in the range from 1.0-4.1 MPa; at 413 K, pH 2, and for initial salicylic acid concentrations of 7.25 mM (see SI Figure S2). In the range of pressures considered, the behavior of the system correctly fitted to a pseudofirst order mechVOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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anism with respect to the pollutant. The kinetic constants obtained are reported in Table 1. An increase in the oxygen pressure allowed the complete elimination of the salicylic acid in few hours. As expected, the reaction rate depended on the pressure conditions (Table 1). So, a 98% conversion was achieved at 4.1 MPa in one hour, whereas 140 min were necessary to reach the same conversion when the working pressure was 2.5 MPa. The COD concentration also decreased faster at higher pressures (Table 1). In no case was a complete depletion of the initial COD achieved, and a final COD reduction of approximately 85% was seen in all cases. The acetic acid formed during the oxidation process was not destroyed by increasing the operating pressure. The main role of the oxygen in the wet oxidation pathway is to participate in the radical initiation process according to reactions 1-5 (19). A higher concentration of dissolved oxygen implies an increase in the amount of radicals generated and also in the oxidation rate, as was proved experimentally O2 f 2O*
(1)
RH + O2 f R* + HO*2
(2)
H2O + O2 f HO*2 + OH*
(3)
RH + O2 + RH f 2R* + H2O2
(4)
R* + O2 f RO*2
(5)
The oxygen reaction order was determined by correlating the oxygen concentration in the liquid medium (considered saturated) and the reaction rate constants obtained for different working pressures. The value obtained for the oxygen reaction order was 0.82 (r2 ) 0.99). The oxygen reaction orders obtained in other studies for wet oxidation of a variety of compounds ranged from 0 to 1.5 (20, 21). In the case of the COD evolution, the initial data were also successfully fitted to a pseudofirst order model, taking into account the asymptotic values of the COD (- rCOD ) kCOD(CCOD - CCOD,final) where CCOD,final is the asymptotic value of the COD). A value of 0.98 (r2 ) 0.99) for the oxygen order was obtained beginning from the reaction rate constants (kCOD). Martino and Savage (22) reported values between 0.23 and 1.26 for the oxygen kinetic order in TOC destruction during the supercritical wet oxidation of monosubstituted phenols. 3.3. Effect of Temperature. The evolution of salicylic acid and COD concentration was studied at different operating temperatures (413-443 K) (see SI Figure S3). Again, the salicylic acid degradation rate was successfully fitted to a first order kinetic model (Table 1). As expected, the degradation of salicylic acid was faster at higher temperatures, at 443 K a total conversion of salicylic acid was achieved in 90 min, whereas at 413 K only an 80% conversion was achieved for the same reaction time. The COD elimination rate was also higher when the temperature increased. Another point to mention is that at the end of the experiments similar COD concentrations were measured for all the assayed temperatures (around 15% of the initial COD remained in the bulk media). This fact means that when temperature increases the mineralization rate increases but not the final degree of mineralization. Assuming an Arrhenius-type dependence, a value of 84.8 kJ/mol was obtained for the activation energy of salicylic acid wet oxidation. Values of the activation energy for the noncatalytic wet oxidation of phenol and substituted phenols were found to be in the very broad range 12-200 kJ/mol at 8632
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temperatures in the range 423-453 K (23), in agreement with the value determined in this work. Taking into account the asymptotic behavior of the COD at the end of the reaction, an activation energy for the COD reduction of 42.0 kJ/mol was also calculated. Li et al. (21) reported values that ranged from 32 to 95 kJ/mol for the oxidation of the chlorophenols and from 12 to 35 kJ/mol for the oxidation of cresols. Values of the activation energy for the TOC reduction from 17 to 158 kJ/mol were obtained by Lopes et al. (24) during the wet oxidation of six model phenolic acids present in wastewater from olive oil mills. 3.4. Effect of the Initial Concentration of Salicylic Acid. Several oxidations at 413 K and 1.0 MPa with different concentrations of salicylic acid were performed in order to evaluate the effect of this variable on the degradation of the compound. The concentrations assayed were 1.45, 7.25, and 14.50 mM (200, 1000, and 2000 ppm, respectively) (see SI Figure S4). According to the results obtained, it can be affirmed that high initial concentrations of salicylic acid had a negative effect on the pseudofirst kinetic constant. So, a 45% conversion was achieved in 50 min starting with 14.50 mM of salicylic acid in comparison to the 95% achieved when the initial concentration was 1.45 mM. It might be possible that low initial concentrations of salicylic acid favored the formation of certain radicals rather than other less reactive ones, which could explain the high degradation rate observed after the induction period at low initial concentrations. Around 15% of the initial COD again remained in the media at the end of the experiments, independently of the initial concentration of salicylic acid selected (see SI Figure S4b). The initial concentration of salicylic acid certainly had an effect on the mineralization rates (see SI Figure S4b). So, degrees of mineralization of 80%, 30%, and 10% were observed after 30 min in the experiments with 1.45 mM, 7.25 mM, and 14.50 mM of salicylic acid, respectively. According to the pseudofirst kinetic constants shown in Table 1, a value of -0.65 (r2 ) 0.98) was calculated for the exponential fitting of the initial concentration during the wet oxidation of salicylic acid. In the case of the COD reduction and considering the asymptotic values of the COD, a value of -1.12 (r2 ) 0.993) was obtained for the exponential fitting of the initial salicylic acid concentration. Pintar and Levec (25) also observed an inhibition effect of initial concentration on the rate constant during the wet oxidation of phenol and nitrophenol over a catalyst comprising CuO, ZnO, and alumina. 3.5. Effect of the Presence of Intermediate Compounds. Two consecutive oxidations of salicylic acid were conducted in order to study the effect of the reaction intermediates, mainly quinone-like compounds (8), on the oxidation process. The second injection of salicylic acid was made after 150 min of reaction without stopping the oxygen flow and without removing the products of the previous oxidation. In both cases the initial concentration of salicylic acid was 6.25 mM (900 ppm). As can be seen in Figure 3a, the degradation rate in the second run was higher than in the first one. A 50% conversion was reached in 60 min in the first injection, whereas only 20 min were needed to obtain the same conversion after the second injection. The two runs were fitted to a pseudofirst kinetic model successfully, obtaining the value of kSA ) 2.0 × 10-4 s-1 (r2 ) 0.98) for the first run and kSA ) 3.8 × 10-4 s-1 (r2 ) 0.99) for the subsequent oxidation. The reaction rate increased from run to run probably as a result of the accumulation of intermediate compounds whose role in radical chain degradation has been shown with other compounds (13). This is of interest with respect to a future decision of whether to select continuous reactors. The quinone-like compounds in the reaction media act as stronger
minutes of reaction. Figure S5a,b shows that the peak of absorbance is achieved after 130 min of reaction, whereas the highest concentrations of phenol and catechol were reached after 80 min. In the 50 min between the peaks, probably benzoquinones were formed beginning from catechol and hydroquinone, allowing the appearance of quinhydrones and increasing the absorbance of the media. The nonidentified COD increased during the first minutes of the reaction, then reached a maximum (minute 110, 44% of the COD), and finally decreased, achieving a final constant value of around 15% of the initial COD, due to the formation of the acetic acid (see SI, Figures S3 and S5). In order to compare the color generated by the quinonelike compound, several single standards of catechol, hydroquinone, and p-benzoquinone were prepared at concentrations between 0 and 7.25 mM, and their absorbances were measured at 455 nm. Only the p-benzoquinone showed color (yellow), and the absorbance of the standard containing 7.25 mM of this compound was 0.12. This value was lower than absorbance monitored during wet oxidation, which suggests the presence of other colored compounds. It was proved that the mixture of p-benzoquinone (0.30 mM), catechol (0.30 mM), and hydroquinone (0.30 mM) solutions fits approximately the brown color observed in the reaction media (after two days at room temperature), giving an absorbance of 0.72, similar to the maximum values measured during the oxidation of salicylic acid. This fact was attributed to occasional intermolecular interactions between quinones and dihydroxylated rings. The charger-transfer complex obtained, called quinhydrone is a highly colored compound, even at low concentrations (eq 6) (27). This complex was also suggested by Mijangos et al. (10) to be the main cause of the color which is seen during the elimination of phenol by the Fenton reaction FIGURE 3. Evolution of salicylic acid concentration (a), COD (b), and absorbance at 455 nm (c) during wet oxidation conducted with a second injection of salicylic acid after 150 min of reaction. pH ) 2; T ) 413 K; P ) 1.0 MPa; initial concentration of salicylic acid ) 6.52 mM. Solid lines denote model data according to Table 1. initiators than salicylic acid, thus increasing the generation of radicals and the degradation rate. COD reduction was also accelerated during the second cycle (Figure 3b). It is remarkable that the COD measured at the end of the experiment was practically the same as the final COD at the end of the first cycle, instead of the expected value of two times higher. This fact could be explained if the high concentration of radicals achieved with a subsequent injection allowed a more effective attack on the acetic acid, thus obtaining a lower remaining COD at the end of the reaction. 3.6. Discussion of the Evolution of Color and Intermediates Formation. Phenol, catechol, hydroquinone, and p-benzoquinone were identified as intermediates by analyzing several samples taken during the wet oxidation of salicylic acid by HPLC. It was not possible to identify o-benzoquinone since its unstable specie it is not a commercial reference (26). Additionally, as previously commented, acetic acid was identified by ionic chromatography as the main cause of the remaining COD in the final media. The concentrations of salicylic acid, catechol, and phenol were quantified, and the absorbance at 455 nm was measured during the oxidation process of salicylic acid (see SI Figure S5). The maximum concentration of catechol detected in the media was 0.16 mM, whereas phenol was obtained at concentrations up to 0.64 mM. In both cases, the concentrations clearly exceed the EC50 of each compound (9) (see SI Table S1), corroborating the increase of the toxicity of the wastewater during the first
In order to discuss the effect of the operational conditions on the color (and therefore the toxicity of the water), the evolution of the absorbance during the previous experiments was also monitored (Figure 1c and 3c, Figures S2c, S3c and S4c in the Supporting Information). During the wet oxidations at different pH values, the color of the solution did not change at pH 12.3, 5.4, and 1.3, which indicates the absence of intermediate compounds and is in agreement with the low conversions obtained at these pH values. However, the change of color was evident at pH between 2.0 and 4.0, it being observed that the absorbance achieved a maximum value before decreasing. This behavior was as expected, agreeing with the role of the quinone-like compounds as intermediates in the reaction pathway of the oxidation of phenolic compounds (10). It is also worth noting that the evolution of the solution color, and, hence the concentration of quinone-like compounds and the toxicity of the effluent was similar for the runs at pH 3.1 and 4.0 (Figure 1c). Lower absorbances were obtained at pH 2.0 for each time assayed, which indicates a faster degradation of the quinone-like compounds when the pH was more acid. It can be also observed that the absorbance reached a constant value at the end of the experiments at pH values between 2 and 4. This value was lower when more acid pH values were employed, a fact that confirms instability of the quinone-like compounds at acid pH. During the experiments at pH 2 it was observed that the time before the maximum coloration was reached was less VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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when the conditions were more severe and higher absorbance peaks were seen (see SI Figures S2c and S3c). Supposing that the absorbance is proportional to the concentration of quinone-compounds, this behavior indicates that an increase in temperature or pressure accelerates the rate of formation of quinones in the reaction bulk. It was also observed that the highest absorbance was not directly proportional to the initial concentration (see SI Figure S4c). In the experiment with repeated injections, the accumulation of quinone-like compounds is clear from observation of the evolution of the color (Figure 3c). The high absorbance of the reaction media at the moment of the second injection confirms the beneficial effect of the quinonelike compounds on the degradation of the salicylic acid. It is interesting to note that the maximum absorbance was always reached when between 20 and 40% of the initial salicylic acid concentration remained in the medium, independently of the operating conditions. In like manner, the COD corresponding to the peak absorbance ranged from 30 to 50% of the initial COD in all the cases. Finally, it was observed that the time needed for the elimination of color was similar to, and in some cases slightly longer than, that required for the complete removal of salicylic acid. If the oxidation process is not properly completed, the final wastewater may be brown in color and have higher toxicity than the initial wastewater, two key aspects for the subsequent management of the treated water. 3.7. Proposition of a Reaction Pathway. The proposed reaction pathway is based on the pioneer work of Devlin and Harris (28) for the wet oxidation mechanism of the phenol. We suggest that salicylic acid proceeds through decarboxylation to phenol (see Figure 4), which is in agreement with the identification of phenol as an intermediate during the reaction. The decarboxylation was also considered by Minh et al. (29) to be the first step of the oxidation of phydroxybenzoic acid. The hydroxylation of the salicylic acid was considered negligible, since dihydroxybenzoic acids were not detected in the analyzed samples. The phenol that is formed is degraded to dihydroxylated rings (catechol and hydroquinone). The role of hydroquinone as an intermediate has been reported in many oxidations of phenolic compounds (18) but in this work could not be confirmed due its instability. These aromatic intermediates generate highly colored compounds such as o-benzoquinone and p-benzoquinone (10). The presence of p-benzoquinone was confirmed by analyzing the samples, which confirms the previous formation of hydroquinones. The subsequent breakdown of the aromatic ring resulting in the formation of a wide range of cleavage products (see Figure 4) was postulated as a common step in the wet oxidation of several phenolic compounds (9, 28) finally giving acetic acid (18-20). Acetic acid was experimentally identified as the main end product remaining in the liquid media, responsible for the majority of the remaining COD at the end of the oxidation. 3.8. Kinetic Model. Taking into account the experimental results mentioned above (Table 1), the following kinetic models can be proposed for the wet oxidation of salicylic acid (time, temperature, and concentrations are given in terms of s, K, and mM) -0.65 0.82 × CSA -rSA ) 5.80x108 × e-84.8/RT × CSA × CO 2 o
(7)
0.98 × -rCOD ) 6.85x106 × e-42.0/RT × CO 2 -1.12 CCOD x(CCOD - CCOD,final) 0
(8)
These kinetic models are valid for the following operating conditions ranges: 1.0-4.1 MPa, 413-443 K, 1.45 mM-14.50 mM, and pH 2. All the experiments at pH 2 presented in this 8634
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FIGURE 4. Proposed reaction mechanism for the noncatalytic wet oxidation of salicylic acid. work were simulated using eqs 7 or 8 with a good degree of accordance (r2SA ) 0.97 and r2COD ) 0.97).
Acknowledgments The work upon which this paper is based on was financed by the Spanish Ministry of Education and Science (MEC06-CTM-08688).
Appendix A Ci CCOD,final Ea k Ha P -ri R SA T
concentration of component i in the reaction mixture asymptotic value of the COD activation energy apparent reaction rate constant Hatta number pressure reaction rate for the component I gas constant salicylic acid temperature
Supporting Information Available A schematic diagram of experimental equipment, EC50 values of salicylic acid and typical intermediates and evolution of the salicylic acid concentration, COD, and absorbances at 455 nm during the wet oxidation processes at different
temperatures, pressures, and initial concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org.
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