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Ind. Eng. Chem. Res. 2008, 47, 1058-1065
Kinetics of Activated Carbon Promoted Ozonation of Polyphenol Mixtures in Water Fernando J. Beltra´ n,* Ine´ s Gira´ ldez, and Juan F. Garcı´a-Araya Departamento de Ingenierı´a Quı´mica y Energe´ tica. UniVersidad de Extremadura. AVenida de ElVas S/N, 06071 Badajoz, Spain
The kinetics of the activated carbon ozonation of mixtures of three polyphenol compounds (gallic acid, tyrosol, and syringic acid) in water has been studied from the application of a double mechanism of reactions to experimental data. These mechanisms involve direct ozone reactions with starting polyphenols and hydroxyl free radical reactions with intermediates and end reaction products, mainly saturated carboxylic acids (the concentration being represented with the chemical oxygen demand, COD). The direct ozone reaction mechanism develops during the initial period (15-60 min) where polyphenols are removed. The kinetic regime of the process was fast of pseudo first order, and at 25 °C and pH 5 the direct rate constants of the reactions between ozone and gallic acid, syringic acid, and tyrosol were determined from different kinetic methods. Once polyphenols disappear and saturated carboxylic acids (end reaction products) are present, the removal of these compounds goes through hydroxyl free radical oxidation. This second mechanism involves the reaction of ozone and the organic matter, through hydrogen peroxide formation, both in bulk water and on the carbon surface, as the initiating step of hydroxyl radical formation. In the absence of activated carbon, the activation energy of the bulk water ozone-organic matter reaction was found to be 36.3 ( 4.5 kcal‚mol-1. In the presence of activated carbon, at the conditions here applied in addition to bulk water chemical reaction, the process depends on mass-transfer resistances, and the values of the volumetric liquid-side and individual liquid-solid external mass-transfer coefficients were found to be 0.71 ( 0.05 min-1 and 1.19 ( 0.10 × 10-3 m‚min-1, respectively. 1. Introduction
3. Results and Discussion
In a previous article,1 the simultaneous use of activated carbon and ozone was found to be highly satisfactory to mineralize aqueous mixtures of polyphenol compounds compared to the ozone single process in the absence of activated carbon. The starting polyphenols (gallic acid, syringic acid, and tyrosol) in the synthetic wastewater were completely removed during the first 15-60 min, depending on experimental conditions, with little or negligible influence of activated carbon. While polyphenols disappear, hydrogen peroxide was formed and accumulated in water. At further oxidation times, the beneficial effects of activated carbon were noticed, with significant increases of intermediate removal rates and mineralization compared to those observed with ozone alone. So far, ozone-activated carbon processes to remove contaminants from water have been the subject of several works where the influence of some variables, activated carbon properties, and ozone consumption was presented.2-4 With the exception of the previous article,1 the literature reports few results of the ozone-activated carbon treatment of real or synthetic wastewater.5,6 In addition, kinetic studies of these systems involving neither chemical or masstransfer phenomena are reported. This is the second part of a work aimed to study the ozoneactivated carbon process of a polyphenol wastewater, the specific aim of this second part being the kinetic study of the process.
The results obtained in the previous work1 suggest that the ozonation of aqueous mixtures of polyphenols, both in the presence or absence of activated carbon, develops through two possible mechanisms. During the first 15-60 min, a molecular reaction mechanism develops and polyphenols directly react with ozone with no or negligible influence of activated carbon adsorption. During this period, hydrogen peroxide and saturated carboxylic acids are accumulated in water. Once polyphenols have disappeared, a free hydroxyl radical mechanism develops. This mechanism accounts for the mineralization of intermediate and/or end products (saturated carboxylic acids), and hydroxyl free radicals are mainly generated from the decomposition of ozone through different routes depending on the presence and absence of activated carbon (hydrogen peroxide initiation reaction, ozone adsorption, and surface decomposition, etc.).1 According to these results, the kinetic study of this ozonation system has been divided into two parts related to each of the mechanisms developed. Kinetics of the Molecular Mechanism. As shown in the previous article,1 during the first period where polyphenols are removed, no dissolved ozone was present, which suggests that fast gas-liquid ozone direct reactions develop in the water.7 These reactions are likely due the ozone attack on the aromatic ring (electrophilic substitution and 1,3 cycloaddition reactions).8 The key parameters of this mechanism are the rate constant of the reactions between ozone and each of the polyphenols studied. Determination of the rate constants has been carried out through different methods. The Absolute Method. This method implies the direct determination of the rate constant and was applied to results of the ozonations of individual polyphenols. The appropriate kinetic regime of these reactions to determine the rate constant is called
2. Experimental Section All compounds used, experimental setup, procedures, and analysis are presented in the previous work.1 * To whom correspondence should be addressed. Tel.: 34924289387. E-mail:
[email protected]. Fax: 34924289385.
10.1021/ie071285s CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1059
fast of pseudo first order. Because ozone is a scarcely soluble gas in water, gas transfer resistance can be neglected and the reaction rate of a given polyphenol, Pi, reduces to9
rPi ) zPiNO3 ) zPi
P O3 a k C D He x Pi Pi O3
(1)
where NO3 is the ozone absorption rate, zPi, kPi, and CPi are the stoichiometric ratio and rate constant of the reaction between ozone and the polyphenol and the concentration of the latter, respectively, PO3 and DO3 are the ozone partial pressure and ozone diffusivity in water, respectively, and a and He are the specific interfacial gas-liquid area and the Henry constant of the ozone-water system, respectively. The above reaction rate corresponds to a second-order irreversible gas-liquid reaction as is the case in aqueous ozone chemical kinetics.10 On the other hand, the kinetic regime can be confirmed with the determination of the Hatta number, Ha, that relates the importance of chemical and mass-transfer rates, defined as follows:
Ha )
xkP CP DO i
i
(2)
where kL is the individual liquid-side mass-transfer coefficient. For the kinetic regime to be fast of pseudo first order, the following condition must be fulfilled9
Ei 2
(3)
with Ei being the instantaneous reaction factor:
Ei ) 1 +
DPiCPiHe
(4)
zPiDO3PO3
where DPi is the diffusivity of polyphenol, Pi, in water. Because ozone reactions were carried out in a semibatch well agitated reactor,1 with the water phase charged and the gas passing continuously, a perfect mixture was considered for both the gas and water phases. Thus, for any polyphenol compound the mass balance in this kind of reactor is
dCPi
) -rPi
dt
(5)
Substitution of eq 1 in eq 5, separation of variables, and integration leads to
xCP ) xCP 0 -zP 2HexkP DO ∫0 PO dt a
i
i
i
t
i
Table 1. Stoichiometric Ratio and Rate Constants of the Direct Reaction between Ozone and Polyphenols Studieda polyphenol
3
kL
3 < Ha
