Ind. Eng. Chem. Res. 1999, 38, 1341-1349
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Chemical Decomposition of 2,4,6-Trichlorophenol by Ozone, Fenton’s Reagent, and UV Radiation F. Javier Benitez,* Jesus Beltran-Heredia, Juan L. Acero, and F. Javier Rubio Departamento de Ingenieria Quimica y Energetica, Universidad de Extremadura, 06071 Badajoz, Spain
The kinetics of the decomposition of 2,4,6-trichlorophenol by ozonation, by Fenton’s reagent reaction, and by a polychromatic UV radiation is investigated from experiments performed in a batch reactor. In each oxidation system, the degree of removal of the organic compound from water is evaluated and the influence of the operating variables is established. The ozonation process is conducted at pH ) 2 and in the presence of radical scavengers: In these conditions the kinetic constants and reaction orders for the direct reaction between ozone and that organic compound are deduced by using a model based in the film theory. The oxidation by Fenton’s reagent (Fe2+ ion and H2O2), a generating system of hydroxyl radicals, leads to the evaluation of pseudo-first-order rate constants for the global reaction and to the determination of the kinetic constant for the direct radical reaction. Finally, the quantum yields in the photodecomposition process are determined from the rate equation, with the radiation flow rates absorbed previously calculated by means of a radiation source emission model. Introduction Chlorophenols have been widely used in many industrial processes, as synthesis intermediates or as raw materials in the manufacturing of pesticides, insecticides, wood preservatives, and so forth. They are also formed as byproducts in the bleaching of pulp with chlorine, in the chlorination of drinking water, and in incomplete incineration. Because of the great diversity of their origins, they have a great ubiquity and can be found not only in industrial wastewaters but also in soils and surface waters and groundwaters, as a consequence of their release in industrial effluents or improper waste disposal practices and accidental leakages. Several of these organic compounds have been listed among the 65 priority pollutants by the U.S. EPA,1 and they have obtained notoriety as hazardous substances because many of them are toxic and only partially biodegradable and constitute a threat to human health. Therefore, it is necessary to develop effective methods for their removal from water, which is highly recommended, either to less harmful intermediates or to complete mineralization. Single-chemical oxidation processes, like ozonation, UV radiation, hydrogen peroxide, and Fenton’s reagent (Fe2+ ions + H2O2), and advanced oxidation processes (such as UV/H2O2, O3/H2O2, and O3/UV, which generate very reactive and oxidizing free radicals) have been shown to be effective technologies to remove from water specially toxic and hazardous pollutants, such as pesticides and herbicides, phenols and polyphenols, volatile organochlorine compounds, and so forth;2-7 and also, to eliminate organo-chlorinated compounds in general, and particularly chlorophenols. Thus, the ozonation of chlorophenols has been studied by Trapido et al.8 and Kuo and Huang9 among others; their destruction by the Fenton’s reagent was investigated by Barbeni et al.10 and Kawaguchi and Inagaki.11 Similarly, their photodegradation by UV radiation, alone or combined with H2O2, has been reported by several authors.12-16 * To whom correspondence should be addressed. E-mail address:
[email protected]. Fax number: 34-924-271304.
Frequently, these investigations are focused in the establishment of the degree of degradation of those pollutants or the identification of decomposition products formed, although in these studies specific aromatic degradation products are not provided: thus, for example, Abe and Tanaka16 specifically remarked that no aromatic intermediates were identified for 2,4,6-trichlorophenol ozonation, while Trapido et al.8 during the ozonation of several chlorophenols measured increasing amounts of inorganic chlorine and quinones but did not determine particular compounds. Also, and in some cases, approximate kinetic constants for their destruction are reported, but there are only a few studies about the specific decomposition kinetics of these organics in water, with the evaluation of kinetic rate constants and quantum yields. Among the chlorophenols, 2,4,6-trichlorophenol is one of the most representative compounds of this group: it is present in several wastewaters and specially abundant in effluents from the pulp bleaching process, and it is soluble enough at ambient temperatures to exceed EPA standards. Because of its importance, this compound was selected in this work and a study on its chemical decomposition by ozone, Fenton’s reagent, and UV radiation has been conducted with the aim to provide levels of destruction with the different oxidation treatments, and to determine the kinetic parameters of its removal in water: kinetic constants for the ozonation and Fenton’s reagent oxidation and quantum yields for the photodecomposition. This knowledge will be useful in the design of reactors and contactors in which these processes must take place in the wastewater treatment plants. Experimental Section The reactor used in all the experiments of the present work was described in detail in previous investigations.2,3 It consisted of a 500 cm3 cylindrical glass reactor provided with the necessary elements for the development of the three processess, ozonation, Fenton’s reagent oxidation, and photodecomposition, and always operated in batch mode.
10.1021/ie980441f CCC: $18.00 © 1999 American Chemical Society Published on Web 03/17/1999
1342 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999
Thus, in the ozonation experiments, ozone was produced in an ozone generator from commercial oxygen, and the resulting mixture was fed to the reactor through a porous plate gas sparger located at the bottom. The reactor was also provided with inlets for the temperature measurement and outlets for withdrawal of samples and exit of effluent gas. And for the photochemical experiments, the reactor is equipped with a radiation lamp located in the axial position and a quartz sleeve which houses the lamp. This radiation source is a Hanau TQ150 high-pressure mercury vapor lamp which emits a polychromatic radiation in the range from 185 to 436 nm. These experiments were carried out by bubbling an oxygen stream in order to operate at conditions similar to those in the ozonation process. It was checked that oxygen alone does not oxidize 2,4,6trichlorophenol at all. Another external jacket surrounded the reactor, and a water stream was pumped from a thermostatic bath in order to maintain the temperature at the selected value within (0.5 °C. Obviously, in the Fenton’s reagent experiments, ozone was not generated and the UV radiation lamp was not connected. Analytical grade 2,4,6-trichlorophenol was used from Sigma, and for every experiment conducted, the reactor was filled with 350 cm3 of 2,4,6-trichlorophenol aqueous solutions (plus the required amounts of tert-butyl alcohol in the ozonation experiments, and ferrous sulfate and hydrogen peroxide in the Fenton’s reagent oxidation experiments). The 2,4,6-trichlorophenol initial concentration was always the same, 5 × 10-4 M, and during the experiments in the three processes, samples were retired from the reactor at regular times. The 2,4,6-trichlorophenol concentration in these samples (and the p-chlorobenzoic acid in the experiments of Fenton’s reagent oxidation of mixtures, when it was used as a reference compound) was analyzed by HPLC using a Waters Chromatograph with a 996 photodiode array detector and a Nova-Pak C18 column. The detection was made at 290 nm with a mobile phase of a mixture of methanol-water-acetic acid (65/33/2 in volume) and with a flow rate of 1 cm3/ min. In addition, in the ozonation experiments, the ozone concentration was measured both in gas and water phases iodometrically and colorimetrically, respectively, as was indicated in a previous work.4 And the concentration of H2O2 in the Fenton’s reagent experiments was determined by the colorimetric method of Bader et al.17 For the discussion of the ozonation experiments, the equilibrium concentration of ozone C /A, the diffusivities of ozone in water DA, and the liquid mass-transfer coefficient and interfacial areas, kL and a, were provided in the mentioned previous work,4 while the diffusivity of 2,4,6-trichlorophenol in water DP was calculated with the Wilke-Chang equation.18 Also, for the discussion of photochemical experiments, previous experiments in the form described in the former paper3 and based in the uranyl sulfate photodecomposition of oxalic acid19 were performed to determine the radiation emitted by the lamp into the reactor, and the value obtained was 1.76 × 10-5 einstein‚s-1. Results and Discussion Decomposition by Ozone. Previous ozonation experiments were conducted in the way already described in a former work2 in order to determine the stoichio-
Table 1. Kinetic Parameters in the Ozonation of 2,4,6-Trichlorophenol P O3 expt. T (°C) (kPa) O-1 O-2 O-3 O-4 O-5 O-6
10 30 40 20 20 20
0.17 0.17 0.17 0.17 0.24 0.31
E
k′ × 105 (M0.5‚s-1)
k (M-1‚s-1)
Ha
Ei
6.75 5.73 6.35 6.79 6.86 9.75
1.50 1.27 1.36 1.53 1.78 2.53
1.53 × 106 2.11 × 106 3.19 × 106 1.96 × 106 1.86 × 106 1.92 × 106
8.17 7.39 7.85 9.08 8.61 8.92
28.50 31.97 33.63 28.23 23.19 18.98
Figure 1. Influence of ozone partial pressure in the direct ozonation of 2,4,6-trichlorophenol. Experiments O-4, O-5, and O-6.
metric ratio for the reaction between 2,4,6-trichlorophenol and ozone. The values obtained in those experiments were z ) 2 ( 0.1 mol of ozone consumed per mol of 2,4,6trichlorophenol reacted. In the 2,4,6-trichlorophenol decomposition by ozonation, several experiments summarized in Table 1 were carried out by varying the ozone partial pressure in the gas stream (from 0.17 to 0.30 kPa) and the temperature (10, 20, 30, and 40 °C). For the last objective of this first stage, the evaluation of the kinetic rate constants for the direct reaction between ozone and this organic substance, the experiments were conducted at pH ) 2 (the solutions were buffered by using phosphoric acid and sodium hydroxide) and in the presence of tert-butyl alcohol (1 × 10-2 M). As it is known, at this pH the autodecomposition of ozone, which is favored by the catalytic action of hydroxyl ions,20 is too low, and consequently the formation of free and oxidizing radicals is limited. In addition, tert-butyl alcohol is a well-known scavenger of these free radicals, specially hydroxyl radicals.20 Therefore, at those operating conditions it can be assured that the contribution of the radical reactions to the global ozonation is almost negligible. Regarding the influence of these operating variables on the ozonation, while the effect of the ozone partial pressure is clear (see Figure 1, when the ozone partial pressure increases, the 2,4,6-trichlorophenol conversion also increases), similar conversions are obtained for the same reaction time when the temperature is varied. It is due to a double effect: when the tempearture increases, the reaction rate constant also increases, but
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1343
at the same time, the ozone equilibrium concentration decreases, and subsequently the global influence on the ozonation process depends on the predominant effect. Therefore, from the conversions obtained it can be deduced that both effects have a similar importance in this case. The kinetic study is performed with the aim to determine the rate constants for the direct 2,4,6trichlorophenol-ozone decomposition reaction. Under a general point of view, the absorption of ozone into aqueous solutions of organics is a complex process involving several chemical reactions together with physical mass transfer. In such a process, and according to the film theory,21 when a gas is absorbed into a liquid following an irreversible m,n-order reaction with the liquid solute, the gas absorption rate can be expressed by the following equation:
NAa ) kLaC /A E
(1)
where E is the reaction factor. Also, in this particular case of the ozonation of 2,4,6trichlorophenol, the ozone absorption rate can also be expressed as a function of the 2,4,6-trichlorophenol decomposition rate:
(
NAa ) z -
)
dCP dt
(
)
xm 2+ 1 kD C
1 kL
A
3 < Ha
