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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of the Oxidation of p-Hydroxybenzoic Acid by the H2O2/UV System Jesus Beltran-Heredia,*,† Joaquin Torregrosa,† Joaquin R. Dominguez,† and Jose A. Peres‡ Departamento de Ingenieria Quimica y Energetica, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Quimica, Universidade de Tra´ s-os-Montes e Alto Douro, 5001 Vila Real, Portugal
The decomposition of p-hydroxybenzoic acid in aqueous solution by UV radiation and by H2O2/ UV radiation has been studied. The experimental results indicated that the kinetics of both oxidation processes fit well by pseudo-first-order kinetics. In the second oxidation process, the overall kinetic rate constant was split into two components: direct oxidation by UV irradiation (photolysis) and oxidation by free radicals (mainly OH•) generated in the decomposition of H2O2. The effect of pH (2, 5, 7, and 9), temperature (10, 20, 30, and 40 °C), and initial hydrogen peroxide concentration (1, 2.5, and 5 mM) on the conversion of p-hydroxybenzoic acid was established. The importance of these two reaction paths for each specific value of pH, temperature, and initial hydrogen peroxide concentration was quantified. Last, a general expression is proposed for the reaction rate which takes into account both pathways and is a function of known operating variables. 1. Introduction Industries of olive oil extraction, table olive production, and alcohol distillation from different wine fractions give rise to highly contaminant wastewaters. This is a major environmental problem in Mediterranean countries in general and particularly in certain areas of Spain and Portugal where there are many small plants. Most of the pollutant properties of these wastewaters have been imputed to phenolic compounds, because of their toxicity1 and power to inhibit biological treatments.2 As a consequence of this and the more stringent regulations concerning effluents released into public rivers and streams, new technologies have been developed to reduce these refractory contaminants. Among them, techniques of chemical oxidation provoked by UV radiation3,4 and ozone5 are increasingly being used to reduce organic contaminants present in a variety of wastewaters from different industries. However, decomposition using such single treatments may sometimes be ineffective if the pollutants are present at low concentrations or if they are especially refractory to the oxidants. For such situations, it has been necessary to develop more effective processes for the destruction of the contaminants. For example, systems based on the generation of very reactive oxidizing free radicals, especially hydroxyl radicals, have generated increasing interest because of their high oxidant power (+2.8 V). These systems are * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: +34-924-289385. Fax: +34-924271304. † Universidad de Extremadura. ‡ Universidade de Tra ´ s-os-Montes e Alto Douro.
commonly termed advanced oxidation processes (AOPs). Hydroxyl radicals can be produced by combinations of hydrogen peroxide and UV radiation.6-8 In this work, p-hydroxybenzoic acid, a phenolic model compound found in wastewater from food processing factories, has been oxidized in aqueous solutions by means of UV radiation and by the combination of hydrogen peroxide and UV radiation. Previously, this compound has been studied by ozone,9-11 Fenton’s reagent,12 and several advanced oxidation processes.13 Some intermediates formed were identified by highperformance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC/MS). Among them aldehydes,10 such as glyoxal, formaldehyde, methylglyoxal, and acetaldehyde, carboxylic acids,12 such as maleic and oxalic acids, catechol, and hydroquinone12 were analyzed throughout the experiments. In a previous work,13 12 oxidation technologies based on the combination of ozone, UV radiation, Fenton’s reagent, hydrogen peroxide, and photocatalysis with titanium dioxide were applied to the degradation of p-hydroxybenzoic acid. The objectives of the present study are to provide data on the degree of removal, to determine the kinetic rate constants for the overall processes, to compare the efficiency of the two oxidation methods, and to determine the increase in the degradation levels due to the presence of free hydroxyl radicals in the H2O2/UV combined process compared to the single UV process. 2. Experimental Section Analytical-grade p-hydroxybenzoic acid was obtained from Sigma and hydrogen peroxide (33% w/v) from Merck. The radiation source was a Heraeus TQ-150
10.1021/ie001069i CCC: $20.00 © 2001 American Chemical Society Published on Web 06/14/2001
Ind. Eng. Chem. Res., Vol. 40, No. 14, 2001 3105 Table 1. Experiments of p-Hydroxybenzoic Acid Photodegradation expt UV-1 UV-2 UV-3 UV-4 UV-5 UV-6 UV-7
Figure 1. Influence of pH on the degradation of the p-hydroxybenzoic acid concentration in UV radiation experiments.
medium-pressure mercury vapor lamp which emits polychromatic radiation in the range from 185 to 436 nm. The emittivity of the UV lamp was 3.30 × 10-5 einstein/s.13 The reactor consisted of a 500 mL glass cylinder provided with the necessary elements (inlets for sampling, bubbling of the oxygen feed, venting, and measuring of the temperature) for the experiments. It was kept at the desired temperature to within (0.5 °C by an external water jacket surrounding the reactor. The radiation source was located axially and held in a quartz sleeve. The reactor was loaded with 350 mL of solution in all cases. Samples were withdrawn at regular times to determine the phenolic acid remaining concentration. The assay of p-hydroxybenzoic acid was performed by HPLC using a Waters chromatograph equipped with a 996 photodiode array detector and a Nova-Pack C-18 column. Detection was at 254 nm with a mobile phase composed of a methanol/water/acetic acid mixture (10:88:2 by volume) at a flow rate of 1 mL/min. 3. Results and Discussion To demonstrate the greater oxidizing power of the combination of oxidants and to quantify the additional levels of degradation attained, we performed experiments on the oxidation of p-hydroxybenzoic acid by UV radiation and by the H2O2/UV combination. The final goal will be to quantify the additional oxidation pathway due to the hydroxyl radicals generated by the combined system. 3.1. UV-Radiation-Mediated Oxidation. Several experiments of individual p-hydroxybenzoic acid photodecomposition were performed. In this case the oxidizing agent is only the polychromatic UV radiation emitted by the radiation source described in the Experimental Section. 3.1.1. Influence of the Operating Variables. Photodegradation experiments using UV radiation modifying temperature (10, 20, 30, and 40 °C) and pH (2, 5, 7, and 9) are shown in Table 1. The operating conditions of the experiments and the conversions achieved in the p-hydroxybenzoic acid degradation at two reference times (30 and 60 min) can be seen. Figure 1 shows the conversion curves versus irradiation time in experiments in which the reaction pH was modified. One sees that this variable has a positive effect on the photodecomposition rate. For a given time,
T (°C) pH X30 (%) X60 (%) k′UV × 103 (s-1) Φ (L/einstein) 10 20 30 40 20 20 20
5 5 5 5 2 7 9
59 65 65 66 24 83 83
83 86 87 88 43 97 98
0.49 0.53 0.56 0.60 0.16 0.94 1.08
39.8 42.7 45.4 48.4 9.95 81.4 80.2
the degree of conversion increases with increasing pH. This may be because there is a greater facility in the generation of free radicals R• as the pH rises. The difference is significant up to a pH of 7. An additional rise in pH is not reflected in an increase of the photodegradation rate. The temperature also has a positive effect on the conversion level, as can be observed in Table 1. The effect, however, is only mild. This is natural given that radical reactions usually possess low activation energies due to the instability of the initial reactants themselves. 3.1.2. Kinetic Study. From an observation of the degradation curves of p-hydroxybenzoic acid by UV radiation shown in Figure 1, it can be deduced that the elimination of the said acid fulfills the conditions established for a first-order homogeneous reaction kinetic model. Such a simplified model has been used previously by various researches for the present system of UV radiation.14,15 By means of the present model, one can carry out a simplified kinetic study of the process and calculate the first-order kinetic rate constants. The objective of its application is to be able to determine the said reaction rate constants and compare them with those obtained in other more complex oxidation systems such as the H2O2/UV combination. Thus, comparing the values of the first-order constants obtained in the two processes, one will be able to compare their efficacy and quantify the additional component of oxidation due to the hydroxyl radicals (generated in the photolysis of hydrogen peroxide). Applying the above model to the simple photodegradation system, one obtains the following kinetic equation:
-
dCB ) k′UVcB dt
(1)
According to this equation, a plot of ln(CB0/CB) against the reaction time should be a straight line of slope k′UV. Table 1 gives the values of the slopes found by leastsquares fitting each of the experiments. The slopes clearly increase with pH up to 7, after which a further increase in this variable is not reflected in any considerable increase in the slope. For p-hydroxybenzoic acid, the values of the quantum yield at different values of pH and temperature have been calculated by Benitez et al.16 In this case, the linear source spherical emission model (LSSEM) proposed by Jacob and Dranoff17 was used, because it seems to be the most suitable given the physical and geometric characteristics of the reactor and lamp. The values obtained for this parameter are listed in Table 1. From the observations made and taking into account the values of the quantum yield, one finds a linear relationship between the pseudo-first-order constant and the quantum yield:
k′UV ) kUVΦ ) 1.24 × 10-5Φ, s-1
(2)
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Figure 2. Influence of pH on the degradation of the p-hydroxybenzoic acid concentration in the H2O2/UV radiation system.
Figure 3. Influence of the initial hydrogen peroxide concentration on the degradation of the p-hydroxybenzoic acid concentration in the H2O2/UV radiation system.
Table 2. Experiments of p-Hydroxybenzoic Acid Degradation by Means of the H2O2/UV Combination expt
T (°C)
pH
[H2O2]0 × 103 (mol/L)
X10 (%)
X20 (%)
UH-1 UH-2 UH-3 UH-4 UH-5 UH-6 UH-7 UH-8 UH-9
20 20 20 10 30 40 20 20 20
5 5 5 5 5 5 2 7 9
1 2.5 5 2.5 2.5 2.5 2.5 2.5 2.5
64 82 93 87 85 76 37 89 73
87 97 100 99 98 94 64 99 94
where the value of the constant kUV is 1.24 × 10-5 einstein L-1 s-1 and Φ is the quantum yield of the product in L/einstein for given pH and temperature conditions. 3.2. Oxidation by the H2O2/UV Radiation System. 3.2.1. Influence of the Operating Variables. Experiments at different temperatures, pH, and initial hydrogen peroxide concentrations were performed. Table 2 summarizes the values taken by the said operating variables in each experiment and the conversions obtained for two arbitrary reaction times (10 and 20 min). With respect to the influence of temperature, the effect observed was not very clear or very marked (experiments UH-4, UH-2, UH-5, and UH-6): the differences in the experiments performed at 10, 20, and 30 °C were minimal, while the experiment at 40 °C evolved more slowly. This phenomenon may be because, as the temperature rises at these levels, there exist processes of self-decomposition of the hydrogen peroxide to produce oxygen and water (eq 3). With respect to pH, this had a direct positive influence in the range from 2 to 7. At pH 9, however, the degradation rate underwent a considerable decline (see Figure 2). These effects may also be observed in the conversion levels listed in Table 2 (experiments UH-7, UH-2, UH-8, and UH-9). This finding may also be due to the phenomena of self-decomposition of hydrogen peroxide when the reaction pH rises to these levels. In summary, high values of pH and temperature promote the following reaction:
2H2O2 f O2 + 2H2O
∆G° ) -56.9 kcal/mol (3)
The influence of the initial hydrogen peroxide concentration (experiments UH-1, UH-2, and UH-3) is positive, as can be seen from Figure 3, which shows the
Figure 4. Determination of the apparent kinetic rate constant, kT, at different pH values in the H2O2/UV radiation system.
evolution of the p-hydroxybenzoic acid concentration versus reaction time in the experiments where the said variable was modified. It can be observed that an increase in this variable leads to a parallel increase in the conversion. These results clearly reflect the role of hydrogen peroxide in the H2O2/UV combination process: the hydroxyl radicals generated in its photolysis provide an additional contribution to the overall oxidation process (the production of a hydroxyl radical requires 1/2 peroxide molecule and 1/2 photon). This contribution rises with increasing hydrogen peroxide concentration, reflecting the greater production of these free radicals according to the following reaction: hν
H2O2 98 2OH•
(4)
3.2.2. Kinetic Study. The degradation of p-hydroxybenzoic acid by the H2O2/UV combination follows firstorder kinetics.6,8,18 The values of the first-order constants were calculated for each experiment in order to compare the said kinetic rate constants with those obtained in the simple treatment with UV radiation. Figure 4 shows the results of this determination of the pseudo-first-order constant for the experiments in which the pH was varied. The goodness of the straight line fit confirms the assumption of the kinetic regime. We determined the values of the first-order constants kT, which are listed in Table 3. These values are greater than those obtained in the individual treatment with
Ind. Eng. Chem. Res., Vol. 40, No. 14, 2001 3107 Table 3. Values of the Apparent Kinetic Rate Constants for Each Reaction Pathway expt
T (°C)
pH
[H2O2]0 × 103 (mol/L)
kT × 103 (s-1)
k′UV × 103 (s-1)
k′R × 103 (s-1)
Φ (L/einstein)
R (%)
UH-1 UH-2 UH-3 UH-4 UH-5 UH-6 UH-7 UH-8 UH-9
20 20 20 10 30 40 20 20 20
5 5 5 5 5 5 2 7 9
1 2.5 5 2.5 2.5 2.5 2.5 2.5 2.5
1.75 3.01 4.71 3.57 3.74 2.55 0.96 4.08 2.44
0.53 0.53 0.53 0.49 0.56 0.60 0.16 0.94 1.08
1.22 2.48 4.18 3.08 3.18 1.95 0.80 3.14 1.36
42.7 42.7 42.7 39.8 45.4 48.4 9.95 81.4 80.2
70 82 89 86 85 76 83 77 56
UV radiation k′UV, thereby confirming the positive influence of the presence of hydrogen peroxide on the reaction rate. The rigorous reaction mechanism of these combined systems is highly complex, with numerous individual reactions involved. To a first approximation, however, one may assume for the H2O2/UV combination that the reaction mechanism consists of the following individual stages: (1) direct reaction of the organic compound with the UV radiation emitted by the polychromatic source and (2) reaction between the organic compound and the hydroxyl radicals generated by the photolysis of the hydrogen peroxide. The contribution of the direct reaction of the organic compound with hydrogen peroxide was found to be null, because experiments carried out with hydrogen peroxide alone showed no degradation whatsoever. In accordance with this simple mechanism, therefore, one can assume that the overall rate of disappearance of the organic compound, rT, is due to the sum of the contributions of the different individual oxidation stages:
-rT )
-dCB ) -(rUV + rR) ) kTCB ) (k′UV + k′R)CB dt (5)
where rUV and rR represent the photochemical and hydroxyl radical reaction rates, respectively. This last expression can be expressed as functions of known variables:
-rT ) kTCB ) (k′UV + k′R)CB ) (kUVΦ + kR[H2O2]0n)CB (6) where Φ is the quantum yield of the photolytic reaction in L/einstein as deduced previously for determined conditions of pH and temperature, [H2O2]0 is the initial hydrogen peroxide concentration in mol/L, n is an apparent reaction order with respect to the said concentration, and kUV and kR are the respective apparent kinetic rate constants. According to the above equation, the contribution of the radical reaction to the overall reaction rate may be quantified as the difference between the rates of the total reaction rT and the photochemical reaction rUV. Also, the value of the first-order constant k′R of the radical reaction may be easily calculated by subtracting the previously determined value of k′UV from the value of the overall constant determined for the combined process kT. The said values of k′R are shown in Table 3. The parameter R of this table indicates the importance of the hydroxyl radical oxidation route in the overall oxidation process for each of the experiments carried out. As can be seen, the overall reaction constant kT rises with pH in the interval 2-7 but falls in the interval
7-9. To explain this finding, one has to analyze the individual contributions. The photolysis component k′UV rises gradually with pH because of the increase in the quantum yield with this variable. The radical component has the greatest specific weight over the whole pH range because of oxidation by hydroxyl radicals: it rises in the range of pH 2-7 and falls in the range 7-9. It is therefore the component which drives the behavior of the overall constant. In other words, the increase in pH is observed to favor the generation of hydroxyl radicals. Also, as was already shown in the experiments UH-1, UH-2, and UH-3, the said constant depends strongly on the hydrogen peroxide concentration. At higher pH values (pH 9), however, there appear to exist phenomena of self-decomposition of hydrogen peroxide which hinder the generation of the radicals. With respect to the influence of temperature, one observes in Table 3 that the overall constant kT falls from 10 to 20 °C and rises from 20 to 30 °C, respectively. It then falls more sharply from 30 to 40 °C. The photolytic component k′UV rises slightly over the whole 10-40 °C range because of the increase in the quantum yield Φ, whereas the component with greatest specific weight k′R oscillates in the same way as the overall constant. Between 10 and 30 °C, this oscillating behavior may be due to two opposed effects: on the one hand, as the temperature rises, the apparent radical reaction constant kR (eq 6) and the hydroxyl radical production reaction constant (eq 4) also rise, while, on the other hand, there is a countereffect from the self-decomposition of hydrogen peroxide into water and oxygen (eq 3). On passing from 30 to 40 °C, the hydrogen peroxide selfdecomposition has a greater specific weight, with a sharp decline observed in the overall and radical constants, kT and k′R. As can be expected, the initial hydrogen peroxide concentration has a positive effect on the overall kinetic constant kT. This fact is attributable to the major hydroxyl radical production (eq 4). One can see from Table 3 that, in the present combined process, the radical reaction has a far greater contribution than does the UV reaction. These partial contributions of the different reactions that form part of a combined process may be estimated by theoretical calculations applying the proposed simple mechanism. As can be seen from eq 6, the experimental pseudofirst-order constants k′UV and k′R, in turn, depend on certain operating variables. The photochemical component is directly proportional to the quantum yield Φ, in L/einstein, whose values are given in Table 1 for each experiment. The apparent kinetic constant kUV takes the value 1.24 × 10-5 einstein L-1 s-1. The apparent radical kinetic constant k′R is directly proportional to the initial hydrogen peroxide concentration, in mol/L, raised to a power of n. A plot of ln k′R against ln [H2O2]0 for the
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experiments UH-1, UH-2, and UH-3 should, therefore, give a straight line of slope n. In this case, the points lie perfectly along a straight line, and the value of n is 0.75. The apparent kinetic constant, kR, takes values of 7.19 × 10-2, 22.2 × 10-2, 28.1 × 10-2, and 12.2 × 10-2 L3/4 mol-3/4 s-1 for pH 2, 5, 7, and 9, respectively, at 20 °C. With respect to the influence of temperature on the constant kR, it is difficult to distinguish any trends because this constant reflects not only the positive effect of temperature on hydroxyl radical generation (eq 4) but also the self-decomposition of hydrogen peroxide as the temperature rises (eq 3). Nonetheless, one can propose a mean value for the temperatures 10, 20, and 30 °C of 25.3 × 10-2 L3/4 mol-3/4 s-1 and for 40 °C a value of 17.7 × 10-2 L3/4 mol-3/4 s-1. 4. Conclusions The experimental results indicated that the kinetics for both oxidation processes, UV radiation and H2O2/ UV radiation, fit pseudo-first-order kinetics well. In the second oxidation process, the overall kinetic rate constant was split into two components: direct oxidation by UV radiation (photolysis) and oxidation by free radicals (mainly OH•) generated in the photodecomposition of H2O2. In the combined process, the hydroxyl radical oxidation pathway represented between 59% and 89% of the overall oxidation process. We also studied the effect of pH, temperature, and initial hydrogen peroxide concentration on the two reaction pathways. We found positive effects on the kinetics in all cases, except when the pH was increased from 7 to 9 and the temperature from 30 to 40 °C, where there was a clear recession in the reaction rate, presumably because of effects of the hydrogen peroxide selfdecomposition at these high values of pH and temperature. Finally, a general expression for the reaction rate which quantifies the two reaction pathways and which is a function of the known operating variables was proposed. Acknowledgment This research has been supported by Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain, under Project AMB 97-0339, and by Junta de Extremadura, under Project IPR 98A014. J.R.D. thanks Ministerio de Educacion y Cultura for financial support by way of a Ph.D. grant.
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Received for review December 8, 2000 Revised manuscript received March 27, 2001 Accepted April 6, 2001 IE001069I
