Oxidation kinetics of phenolic and indolic compounds by ozone ...

Water Research 36 (2002) 1513–1526

Oxidation kinetics of phenolic and indolic compounds by ozone: applications to synthetic and real swine manure slurry Jerry J. Wu1, Susan J. Masten* Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI 48824, USA Received 1 July 2000; received in revised form 1 April 2001; accepted 1 April 2001

Abstract In this study, an oxidation model combining the mass transfer of ozone and ozonation kinetics was developed to predict the degradation of several phenolic and indolic compounds in a semi-batch reactor. The mass transfer and partition coefficients were calculated at various physical and chemical conditions. In addition, the reaction rate constants of ozone with phenolic and indolic compounds were also estimated independently using the method of competition kinetics and relative reaction-rate constants. Incorporating mass transfer and chemical reaction concepts, an oxidation model that considers side reactions between ozone and byproducts has been established using non-linear simultaneous differential equations. Thus, numerical computation is capable of simulating the degradation of phenolic and indolic compounds both in synthetic and real manure. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ozone; Swine manure slurry; Malodorous metabolites; Mass transfer; Oxidation kinetics

1. Introduction Phenolic (phenol, p-cresol, and p-ethylphenol) and indolic (indole and skatole) compounds are usually regarded as amongst the most odorous malodorous compounds found in swine manure slurry. Although many researchers have investigated the kinetics of the reaction of ozone with phenol and p-cresol [1–6], no effort has been made to investigate the kinetics of the reaction of these compounds in a complex matrix such as wastewater. While in full-scale contactors ozonecontaining gas is bubbled into the liquid being treated, few researchers have determined the mass transfer and kinetics of the reactions involving ozone in such heterogeneous systems. In a few cases, empirical kinetic models of ozonation have been proposed for heterogeneous systems in order to determine an apparent rate *Corresponding author. Tel.: +1-517-353-8539; fax: +1517-355-0250. E-mail address: [email protected] (S.J. Masten). 1 Present address: Department of Environmental Engineering and Science, Feng-Chia University, Taichung, Taiwan.

constant for the overall reaction as a function of both flow and chemical parameters [7], but these constants are limited to the specific reactor type used. The objectives of this study were threefold. Firstly, we sought to describe the kinetics of the oxidation of phenolic and indolic compounds by ozone in a semibatch reactor. The effect of such system parameters, such as flowrate, temperature, pH, and solution composition on the removal of these malodorous compounds was determined. Lastly, a mathematical model, which combines the mass transfer of ozone and ozonation kinetics, to predict the degradation of those compounds in synthetic and real swine manure was developed.

2. Materials and methods 2.1. Determination of stoichiometric factors and reaction rate constants The stoichiometric factors for the reactions were obtained by mixing solutions of known concentrations

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 3 5 2 - 9

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J.J. Wu, S.J. Masten / Water Research 36 (2002) 1513–1526

Nomenclature Cgin Cgout E Kphenol kL a

kd

ki ks

kA kB

gaseous concentration of inlet ozone, mol/L gaseous concentration of outlet ozone, mol/L enhancement factor by chemical reaction equilibrium constant between phenol and pheolate ion in solution, mol/L overall mass transfer coefficient of ozone in solution in the absence of ozone-reactive compounds, 1/s self-decomposition rate constant of dissolved ozone, 1/s for m equal to 1 and L/mol s for m equal to 2 reaction rate constant for malodorous compound i with dissolved ozone, L/mol s rate constant for the removal of malodorous compound, i; by stripping or oxygenation, 1/s reaction rate constant between phenol and ozone, L/mol s reaction rate constant between phenolate ion and ozone, L/mol s

of a single aqueous target compound (phenol, p-cresol, p-ethylphenol, indole, or skatole) and ozone. In order to avoid, as much as possible, the interference of other reactions (i.e., between ozone and byproducts), the initial ratios of concentrations of the target compounds [M]o to ozone [O3]o were chosen to be between 5 and 10 mol/mol, thus allowing ozone to be consumed predominately by its reaction with the parent compound(s). Experiments were conducted at two different concentrations of the target organic chemical: 100 and 200 mg/L. An appropriate volume of the stock ozone solution (10.2 mg/L for pH=2.1 and 9.2 mg/L for pH=6.7) was added to the reaction vessel and allowed to react with the target compound. These experiments were conducted in 30 mL glass reaction vessels into which were placed mini-stir bars to keep the solution completely mixed. After the aqueous ozone was consumed, the residual concentrations of the compounds were determined by high-performance liquid chromatography (HPLC). The stoichiometric factor was then computed as Z ¼ D[O3]/D[M]. All experiments were conducted in triplicate. The rate constants for the reaction of the target compounds and ozone were estimated using the method of competition kinetics [8] using relative reaction rate constants [3]. A pair of organic compounds (M1 and M2) in some aqueous solution will compete for ozone; the kinetics of these competition reactions can be described mathematically as

m mi mo [M]i NO 3 [O3] [O*3] R

V Xj XExperiment a Zi

self-decomposition order of dissolved ozone molar flux of ozone at the reactor inlet, mol/s molar flux of ozone at the reactor outlet, mol/s concentration of malodorous compound, mol/L actual ozone absorption rate defined by equation, mol/L s dissolved ozone concentration, mol/L equilibrium dissolved ozone concentration at the water–gas interface, mol/L minimum residual for the observed and predicted data of target compounds in the tested solution liquid volume of the reactor, L predicted concentration for the parent compounds in the reactor, mol/L observed concentration during the experiment, mol/L partition coefficient of ozone in solution stoichiometric factor for malodorous compound, i; with ozone

follows: d½M1 ¼ k1;O3 ½M1 ½O3 ; dt

ð1Þ

d½M2 ¼ k2;O3 ½M2 ½O3 : dt

ð2Þ

Thus, d½M1 k1;O3 ½M1 ¼ ; d½M2 k2;O3 ½M2 k2;O3 ¼ k1;O3

lnð½M2 t =½M2 0 Þ : lnð½M1 t =½M1 0 Þ

ð3Þ

ð4Þ

p-Cresol was chosen as the reference compound since the reaction rate constant of ozone with p-cresol is published in the peer-reviewed literature [6]. From this reaction rate constant, the reaction rate constant of the other target compounds can be calculated. 2.2. Ozonation of synthetic and real swine manure in a semi-batch reactor In order to better understand the reaction kinetics between ozone and the target malodorous components, a formulation of synthetic swine manure was prepared referring to the concentrations of major malodorous composition reported by Yasuhara [9]. The synthesis swine manure contains phenol (28.1 mg/L), p-cresol (210 mg/L), p-ethylphenol (3.5 mg/L), indole (5.1 mg/ L), and skatole (12.8 mg/L) [10]. In these experiments,


the temperature (141C, 221C, 301C) and flowrate (100, 300, 500 mL/min) were varied to investigate their effect on the rate of ozonolysis of the target malodorous compounds. The pH of the synthetic manure was controlled using a phosphate buffer system (pH=6.7). The ionic strength of the synthetic manure was 0.1 M. A semi-batch stirred reactor (Fig. 1) made of pyrex glass with a capacity of 1.5 L was utilized to investigate the degradation of target compounds in the synthetic solution. Ozone was generated from pure oxygen by corona discharge using an ozone generator (Model T-408, Polymetrics Inc., San Jose, CA). The oxygen stream was dried using a molecular sieve trap (S/P Brand #G5301-21) prior to ozone generation. In this reactor, a fritted glass diffuser that is able to generate fine bubbles (bubble diameter o1 mm) was located near the bottom of the reactor and a magnetic stirrer bar was placed under the diffuser to achieve complete mixing throughout the liquid. In addition, a water jacket around the reactor maintained the desired temperature. The concentrations of gaseous ozone were monitored using a UV spectrophotometer (UV 1201, Shimadzu, Japan) by passing ozone gas, from both the inlet and outlet, through two 2-mm flow cells. The absorbance of the ozone was monitored at the wavelength 258 nm. An extinction coefficient of 3000 M1 cm1 was used to convert absorbances to concentration units [6]. A ozone

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concentration of 64.8 mg/L in the oxygen gas stream was used in these experiments. The dissolved ozone concentration was measured continuously by circulating water from the reactor through the flow chamber of the spectrophotometer. Before initiating each experiment, a three-way valve was used to pass the ozone gas into a 2% potassium iodine solution (trap) until it was observed, spectrophotometrically, that the inlet concentration was constant. When the desired concentration of inlet ozone was obtained, the experiment was commenced by rotating this valve and admitting the ozoneenriched oxygen gas into the reactor. A control experiment, in which the manure slurry was sparged with oxygen using the same flowrate as that used in the ozone experiments, was conducted to investigate the effect of stripping and oxygenation. Phenolic and indolic compounds in the synthetic and real manure were extracted by methylene chloride (1 : 1). Gas chromatography then was employed to separate and quantify these compounds [11]. Swine manure was collected from the storage pits under one of the swine houses at MSU Swine Teaching and Research Facility. After collection, the manure was placed in a 4 L glass bottle for two weeks to allow fermentation to occur. After storage, oxygenation and ozonation experiments were conducted at a temperature of 221C and a flowrate of 300 mL/min. Samples were taken at ozone applied dosages from 0 to 2 g/L at

Fig. 1. System setup for the semi-batch ozonation process.

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increments of 0.25 g/L. At the ozone gas-phase concentration used, 19.3 min was needed to obtain an incremental increase of 0.25 g/L ozone. 2.3. Model development A model combining mass transfer and chemical oxidation kinetics was developed to predict the accumulation of dissolved ozone concentration and the decrease in the concentrations of malodorous compounds in the synthetic and real swine manure during lab-scale ozonation. The parameters regarding the transport of ozone in pure water had been determined using a mass transfer model of ozone developed previously [12]. Our study showed the transport of gas through the reactor could be described as plug flow, supporting the work of Nishikawa et al. [13] and Zhou and Smith [14] describing the transport of gas in a reactor. Incorporated into this model were the concepts of enhancement factor, defined as the ratio between the actual and maximum physical absorption rates [15], and chemical kinetics. The following basic assumptions were made: 1. the liquid phase is completely mixed; 2. the flow pattern of gaseous ozone through the system is plug flow; 3. constant temperature and pressure are maintained during the experimental period; 4. ozone is consumed mainly by the malodorous parent compounds (phenol, p-cresol, p-ethylphenol, indole, and skatole); and 5. the enhancement factor is constant during the ozonation process. Assuming plug flow conditions in the gas phase, the average driving force [16] can be described either as the arithmetic-mean driving force, [O3]*=aðCin þ Cout Þ=2; or as the logarithmic-mean driving force, [O3]*= aðCin Cout Þ=lnðCin =Cout Þ: Since the ozone concentration of outlet gas ðCout Þ is essentially zero during reaction with our target compounds, the logarithmicmean driving force could not be applied because of the mathematical error in logarithm that would have occurred. Model I describes mathematically the mass transfer and the ozonation reactions by the following equations: ! ! Cgin þ Cgout d½O3 ¼ EkL a a ½O3 dt 2 X ð5Þ Zi ðki Þ½O3 ½Mi kd ½O3 m ; d½Mi ¼ ks ½Mi þ ðki Þ½O3 ½Mi ; dt

sionless), kL a the mass transfer coefficient of ozone in solution in the absence of ozone-reactive compounds (1/s), a the partition coefficient of ozone (dimensionless), Cgin the gaseous concentration of inlet ozone (mol/L), Cgout the gaseous concentration of outlet ozone (mol/L), Zi the stoichiometric factor for malodorous compound i (mol/mol), ki the reaction rate constant for malodorous compound i with dissolved ozone (L/mol s), [M]i the concentration of malodorous compound (mol/L), kd the self-decomposition rate constant of dissolved ozone (1/s for m equal to 1; L/mol s for m equal to 2), m the selfdecomposition order of dissolved ozone (dimensionless), ks the coefficient for the removal of malodorous compound i; by stripping or oxygenation (1/s). The enhancement factor can be expressed mathematically as [17,15] E¼

NO 3 ¼

where [O3] is the dissolved ozone concentration (mg/L), E the enhancement factor by chemical reaction (dimen-

mi mo ; V

ð7Þ ð8Þ

where NO3 (mol/L s) is determined from the difference between the molar flux of ozone at the reactor inlet and outlet, mi and mo (mol/s), kL a is the liquid phase volumetric mass transfer coefficient (1/s), [O*3] is the equilibrium dissolved ozone concentration at the water– gas interface (mol/L), and V is the liquid volume in the reactor (L). The enhancement factor was determined in the solution containing all target compounds. Another important feature to be considered is the occurrence of competition reactions involving ozone and the intermediates formed during the ozonation. Ozonation of phenolic compounds yields dihydroxybenzenes as the initial byproducts. These compounds are still as reactive with ozone as the parent phenols [1,5]. These circumstances make kinetic studies more difficult to carry out especially for an initial reaction system with more than one parent compound. Therefore, assumption 4 in Model I should be discarded for this situation (Model II). To overcome the difficulty in using the mathematical model, a first-order reaction rate constant, kf ; was used to describe the consumption of ozone by all intermediates or impurities. This reaction rate constant ðkf Þ is incorporated into Model II, which can be expressed mathematically as ! ! Cgin þ Cgout d½O3 ½O3 ¼ EkL a a 2 dt X ð9Þ Zi ðki Þ½O3 ½Mt kf ½O3 kd ½O3 m ;

ð6Þ

NO3 ; kL a½O3*

d½Mt ¼ ks ½Mt þ ðki Þ½O3 ½Mi ; dt

ð10Þ

where kf is the first-order reaction rate constant for the reaction of ozone with the byproducts or other compounds (1/s).

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The overall reaction coefficient was estimated by fitting our experimental data to find a minimum residual ðRÞ; which is defined as below: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 j¼1 ðXj XExperiment Þ R¼ ; ð11Þ N where Xj is the predicted concentration of the parent compounds in reactor, XExperiment is the observed concentration during the experiment, and N is the number of total observations.

the stoichiometric factor at pH 6.7 was less than 1 (but close to 1). To evaluate the effects of stripping and oxygenation on the removal of the phenolic and indolic compounds, oxygen was used to purge the solution. The maximum flowrate, 500 mL/min, was employed. Almost no change occurred in the concentration of any of the target compounds during a period of 80 min. 3.2. Determination of the enhancement factor and reaction rate constant ðkf Þ

3. Results 3.1. Stoichiometric factors and rate constants for ozonation reactions The stoichiometric factors and rate constants for the reaction of ozone with phenolic and indolic compounds are summarized in Table 1. Results show that the stoichiometric factors for phenol are 1.8170.16 at pH 2.1 and 1.8670.16 at pH 6.7. For p-cresol, the stoichiometric factors were found to be 1.0070.07 at pH 2.1 and 1.3870.11 at pH 6.7. The value reported by Zheng et al. [6] for cresol isomers was 3; the value documented by Beltran et al. [18] for o-cresol was 2. Using the stoichiometric factor for p-cresol determined in this work, it appears that approximately 1 mol of ozone is consumed by 1 mol of p-cresol. Due to the lack of references relating to the ozonation kinetics of pethylphenol, indole, and skatole, no comparison could be made for the stoichiometric factors of these compounds. In our work, the stoichiometric factor of p-ethylphenol was found to be approximately 1.2 over the pH range used. For indolic compounds, however,

As mentioned previously, a model combining mass transfer and oxidation kinetics was developed to predict the concentration profiles of phenolic and indolic compounds in water under different pH values and flowrates. The mass transfer coefficients, partition coefficients (Table 2), stoichiometric factors, and reaction rate constants were determined previously [12]. The enhancement factor was determined at a temperature of 221C and is presented in Table 3. According to Eqs. (7) and (8), the enhancement factor can be regarded as a constant since the gaseous ozone concentration in the outlet stream was not detectable until the phenolic or indolic compounds were removed from the reactor. Phenolic mixtures (phenol, p-cresol, p-ethylphenol) and indolic mixtures (indole and skatole) were ozonated, respectively, under the following conditions: pH 2.1, temperature of 221C, flowrate of 300 mL/min, and ionic strength of 0.1 M. The reaction coefficients that resulted in minimum residuals obtained using Model II are 0.8 and 1.0 for the phenolic compounds and indolic compounds, respectively. At pH 6.7, the reaction rate constants ðkf Þ are much larger than those found at pH

Table 1 Stoichiometric factors and rate constants for the reaction of ozone with phenolic and indolic compounds under different pH conditions Stoichiometric factor

Reaction rate constant (M1 s1)

Conc. at t0 (mM)

Final conc. (mM)

460710 5200a 16507280 45,00072900 10,20072200

2.13 1.89 1.69 1.71 1.55

2.01 1.68 1.52 1.49 1.31

pH=6.7, I ¼ 0:1; T ¼ 221C, initial DO3 ¼ 0:192 mM Phenol 1.8670.16 750,000733,000 p-Cresol 1.3870.11 850,000a p-Ethylphenol 1.2970.11 990,0007110,000 Indole 0.9870.07 5,000,00073,600,000 Skatole 0.8970.04 4,500,0007780,000

2.12 1.89 1.69 1.70 1.54

2.01 1.75 1.54 1.50 1.32

pH=2.1, I ¼ 0:1; T ¼ 221C, initial DO3 ¼ 0:213 mM Phenol 1.8170.16 p-Cresol 1.0070.07 p-Ethylphenol 1.1370.05 Indole 0.9870.07 Skatole 0.9070.03

a

Value was determined by Zheng et al. [6] and CL were not reported. Errors are reported at the 95% confidence interval.

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Table 2 Fitting parameters obtained from the mass transfer equations Inlet flowrate (mL/min)

100 300 500

pH=2.1

pH=6.7

a

KL a (min1)

a

KL a (min1)

0.219 0.222 0.238

0.1574 0.2357 0.3188

0.232 0.222 0.240

0.1479 0.2719 0.3866

Table 3 Enhancement factors calculated under different flowrates and pH values Inlet flowrate, Q (mL/min)

pH=2.1

pH=6.7

100 300 500

1.9 3.8 4.4

2.0 3.2 3.8

2.1. The fitted values for the reaction rate constants at pH=6.7 are summarized in Table 4.

3.3. Oxidation of synthetic manure and stored swine manure by ozone Synthetic manure and stored ‘‘real’’ swine manure were ozonated to investigate the oxidation of phenolic and indolic compounds in a complex matrix. In the synthetic manure, at ozone dosages greater than 0.1 g/L, p-ethylphenol, indole, and skatole were removed to below detection limits, whereas ozone dosages of greater than 0.5 g/L were necessary to remove phenol and p-cresol to below detection limits. Stripping and oxygenation were ineffective at removing any of the phenolic or indolic compounds at the same hydrodynamic conditions as those used with ozone-enriched oxygen gas [12]. As such, the losses of phenolic and indolic compounds from the synthetic manure appear to be due entirely to ozonation reactions. The oxidation model was used to predict and compare the decreases in concentrations of the target compounds. Simulations using Model I resulted in a much more rapid reduction in concentrations than those observed experimentally. As such, it is necessary to incorporate the reaction rate constant ðkf Þ into the model to account for competing reactions. The reaction rate constants are summarized in Table 5. The fitted values of the reaction rate constants are in the same range as those obtained when the individual phenolic or indolic compounds were ozonated in the deionized water.

Table 4 Reaction rate constant kf for phenols and indoles under different flowrates at pH 6.7 Flowrate, Q (mL/min)

Phenols (s1)

Indoles (s1)

100 300 500

350 400 350

450 400 400

Table 5 Reaction rate constant kf for synthetic manure determined at different flowrates Flowrate, Q (mL/min)

Reaction rate constant kf (s1)

100 300 500

300 400 600

Swine manure slurry was ozonated using the lab-scale reactor at a flowrate of 300 mL/min and temperature of 221C. During ozonation, all of the target malodorous compounds were oxidized within 2 h, i.e., at an ozone dosage of 1.5 g/L. The pH of the manure slurry increased from 7.5 to 8.2 after 2.5 h of ozonation. As the initial pH of the slurry was 7.5, and Hoigne! and Bader [3,4] showed that the ozonation rate constants of phenolic compounds increase by a factor of 10 for each pH unit increase, we can assume that the reaction rate constants were 10 times greater than those obtained for a pH of 6.7. Thus, the rate constants used in the oxidation model were 107 M1 s1 for the phenolic compounds and 5 107 M1 s1 for skatole. A reaction rate constant ðkf Þ of 40,000 s1 was found to yield the minimum residual in fitting the experimental data.

4. Discussion Hoign!e and Bader [3,4] indicated that for the ozonation of phenol-like compounds, 2.5 mol of ozone is needed to consume 1 mol of phenol; Li et al. [19] also reported a stoichiometric factor of 2. However, Eisenhauer [20], Gould and Weber [1], and Roth et al. [7] found that for the total destruction of phenol, between 4 and 6 mol of ozone is required per mole of phenol. These values are higher than that found in this work because they did not distinguish between the amount of ozone that reacts with phenol and that which reacts with the ozonation byproducts. Since the reaction conditions


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Fig. 2. Comparison of phenol, p-cresol, and p-ethylphenol concentrations in the mixture of phenolic compounds (phenol+pcresol+p-ethylphenol) between experimental data and model prediction at pH 2.1, flowrate 300 mL/min, and temperature 221C.

used in this study favored the reaction of ozone with the target chemical over side reactions, the stoichiometric factors found in this work are lower than that found by others and appear to be reasonable. The reaction rate constants for the phenolic and indolic compounds were found to increase significantly with increasing pH. This increase for phenolic compounds is thought to be due to the greater reactivity of the phenolate ion as compared to that of phenol. Augugliaro and Rizzuti [2] and Hoign!e and Bader [3,4] both showed that the phenolate ion has a much larger rate constant towards ozone than that

of phenol (109 M1 s1 for phenolate ion and 500 M1 s1 for phenol). If using an overall reaction rate constant to represent the reaction of phenol and ozone, the following mathematical expressions can be introduced: ½C6 H5 OHtot ¼ ½C6 H5 OH þ ½C6 H5 O ;

Kphenol ¼

½Hþ ½C6 H5 O ; ½C6 H5 OH

ð12Þ

ð13Þ

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Fig. 3. Comparison of indole and skatole concentrations in the mixture of indolic compounds (indole+skatole) between experimental data and model prediction at pH 2.1, flowrate 300 mL/min, and temperature 221C.

d½C6 H5 OHtot ¼ kA ½O3 ½C6 H5 OH dt þ kB ½O3 ½C6 H5 O

¼

kA ½Hþ þ kB Kphenol ½O3 ½C6 H5 OHtot ; Kphenol þ ½Hþ

ð14Þ ð15Þ

where [C6H5OH]tot is the total molar concentration of phenol and phenolate ion, Kphenol is the equilibrium constant (109.9 M), kA is the reaction rate constant between phenol and ozone (500 M1 s1), and kB is the reaction rate constant between phenolate ion and ozone (109 M1 s1). Therefore, the overall reaction rate constant at pH=6.7 for phenol is determined to be 6.3 105 M1 s1, which is close to the one we found in our experiment (7.5 105 M1 s1). Hoign!e and Bader [4] also demonstrated that the reaction rate constants for phenolic compounds increase over a wide range of pH values by a factor of 10 per pH unit, corresponding to the incremental increase in the degree of dissociation. Although another explanation is that hydroxyl radicals cause the increase of the reaction rate with pH, Beltran

et al. [21] negated this hypothesis when they found that the rate constants for the ozonation of o-cresol in aqueous solution with and without tert-butanol, a radical scavenger, were nearly identical. In our experiment, we used the phosphate buffer system to control the pH at 6.7. Masten et al. [22] demonstrated that, at this pH, hydrogen phosphate (HPO2 4 ) and dihydrogen phosphate (H2PO 4 ) are the predominant OH radical scavengers in oxidizing trichlorobenzene by advanced oxidation process. Thus, the direct reactions between ozone and the target compounds predominate because any free radicals generated from the decomposition of ozone are likely to be scavenged by the phosphate species present in the buffer system we used. The reaction rate constants determined at pH 2.1 were ranked, indole>skatole>p-cresol>p-ethylphenol> phenol. The electrophilic reaction is thought to be the major mechanism by which aromatic compounds react with ozone. This is especially true for aromatic compounds that have electron donating groups substituted at the ortho and para positions because of the high electron densities that result from this substitution.


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Fig. 4. Comparison of phenol, p-cresol, and p-ethylphenol concentrations in the mixture of phenolic compounds (phenol+ p-cresol+p-ethylphenol) between experimental data and model prediction at pH 6.7, flowrate 300 mL/min, and temperature 221C.

As a result of the high electron density, these compounds are very reactive with ozone [23]. As pcresol contains two electron donating groups, –OH and –CH3 [24], the reactivity of p-cresol by the initial attack of the ozone molecule is expected to be greater than that of phenol (with only a –OH group). The ethyl group (–CH2CH3) on p-ethylphenol is also an electron donat-

ing group [25]. The reactivity of p-ethylphenol is, therefore, expected to be greater than that of phenol. p-Cresol has a greater reaction rate constant than p-ethylphenol because the methyl group is a better electron donating group than is the ethyl group. In addition, it was reported by Geissman [24] that indole and skatole are very reactive nucleophiles, and undergo

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electrophilic substitution reactions with ease. Nevertheless, no explanation can be offered at this point for why the reaction rate constants of indolic compounds are much faster than those of phenolic compounds based upon structural differences in the two groups of compounds. At pH 6.7, the reaction rate constants for the three phenolic compounds are almost of the same magnitude (close to 1 106 M1 s1) and the rate constants of indole and skatole are 5 106 M1 s1. When the reactions between intermediates and ozone were ignored, the predicted concentration profiles of the target compounds decreased much faster than those determined experimentally (Figs. 2 and 5). In Figs. 2–5, the experimental data and the predicted values determined using Models I and II are shown. It should be noted that competing reactions between the several target parent compounds and side reactions between the reactive intermediates (generated) have the most significant effect on the compound having the lowest reaction rate constant in our system, i.e., phenol. On the contrary, the reaction of the compound with the greater reaction rate constant, p-cresol in our system, can be

predicted using Model I, without considering side reactions. The importance of these side reactions is also substantiated when considering the results obtained upon the ozonation of the real and synthetic manure. When ozonating the synthetic manure (Fig. 6.), an ozone dosage of 0.5 g/L resulted in complete removal of all of the malodorous parent components. One would expect that in real manure (COD=30,000 mg/L) the extent of ozone depletion by side reactions would be significantly greater than that required in the synthetic manure (COD=3200 mg/L), and this is exactly what is observed. If Model II is used to fit our experimental data for the ozonation of the real swine manure, a reaction rate constant of 40,000 s1 is obtained (Fig. 7). This value, which is much greater than that obtained for synthetic manure, is likely to be larger because of greater numbers and concentrations of impurities in real swine manure that can react with ozone. Any model used to predict the removal of odorous compounds in swine manure would be more effective if the reaction coefficient could be determined from a ‘‘gross parameter’’ such as COD. As

Fig. 5. Comparison of indole and skatole concentrations in the mixture of indolic compounds (indole+skatole) between experimental data and model prediction at pH 6.7, flowrate 300 mL/min, and temperature 221C.


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Fig. 6. Comparison of phenolic and indolic concentrations in the synthetic manure between experimental data and model prediction at pH 6.7, flowrate 300 mL/min, and temperature 221C.

such, the relationship between kf and COD was considered. Assuming that a mathematical relationship between COD and kf exists, kf can be expressed as kf ¼ k0f (COD). The intrinsic reaction rate constant, k0f ; is calculated to be 1.33 for the real manure (COD=30,000 mg/L) we tested. Similarly, k0f is calculated as 0.125 for the synthetic manure, in which the

COD is 3200 mg/L. It should be noted that the pH of real manure is about a unit larger than that of synthetic manure, possibly explaining why the intrinsic reaction rate constant ðk0f Þ for the real manure is approximately 10 times greater than that obtained for the synthetic manure. At the higher pH value the hydroxide ion concentration would be approximately 10 times that in

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Fig. 7. Comparison of phenolic and indolic concentrations in the real manure between experimental data and model prediction at pH 7.5, flowrate 300 mL/min, and temperature 221C.

the synthetic manure and the reaction rates of ozone with byproducts or impurities would also increase by a factor of approximately 10 [4]. Thus, the future work

should be carried out to establish such a mathematical expression between COD and reaction rate constant ðkf Þ:


5. Conclusions The combination of a mass transfer and kinetic study has led to the development of a model to successfully predict the degradation of malodorous, phenolic and indolic compounds in a semi-batch ozonation reactor. In this study, several important conclusions can be drawn: 1. The reaction rate constants of the phenolic and indolic compounds increase with increasing pH. 2. No dissolved ozone and outlet gaseous ozone can be observed prior to the complete removal of the phenolic and indolic compounds. Almost 100% utilization of ozone was obtained under our experimental conditions. Therefore, ozone dosage becomes a very important and reliable parameter for the design of an ozone reactor in removing malodorous substances from the liquid manure slurry. 3. The effect of pH on the oxidation of phenolic and indolic compounds is determined predominately by their reaction rate constants and competition kinetics with ozone. For the solution containing phenolic compounds at lower pH values, the degradation of phenol is the slowest of all target compounds as its reaction rate is the smallest of the target compounds; the degradation of p-cresol is the fastest as it has the greatest reaction rate. As these reaction rates of phenolic compounds are identical at a pH of 6.7, the degradation of these compounds follows similar patterns. 4. A model combining mass transfer and oxidation kinetics of ozone should include the effect of side reactions involving ozone and byproducts. Using Model I, which ignores the effect of side reactions, the predicted rates of the oxidation of these malodorous compounds are much greater than those observed experimentally. The reaction coefficients for artificial and real swine manure were estimated as 400 (s1) and 40,000 (s1), respectively. It is apparent that the presence of ozonated reactive impurities results in a significant increase in the ozone dosage required to oxidize the target compounds.

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