638
Ind. Eng. Chem. Res. 1997, 36, 638-644
Ozonation Kinetics of Phenolic Acids Present in Wastewaters from Olive Oil Mills F. Javier Benı´tez,* Jesus Beltra´ n-Heredia, Juan L. Acero, and Maria L. Pinilla Departamento de Ingenieria Quimica y Energetica, Universidad de Extremadura, 06071 Badajoz, Spain
A kinetic study of the degradation by ozone of eight phenolic acids present in wastewaters from olive oil mills has been performed by using a competition kinetic method. The selected phenolic acids are: caffeic, p-coumaric, syringic, vanillic, 3,4,5-trimethoxybenzoic, veratric, p-hydroxybenzoic, and protocatechuic. The influence of the operating variables (temperature, pH, and ozone partial pressure in the gas stream) is established, and the stoichiometric ratios for the individual direct reactions between ozone and each acid are determined. Once the reaction rate constants are evaluated, they are correlated as a function of temperature and pH into kinetic expressions which are provided for every phenolic acid. The global process occurs in the fast and pseudo-first-order kinetic regime of absorption, a condition required by the competition model to be used. Introduction The olive oil extraction industries in the Mediterranean countries generate each year an increasing volume of wastewaters with a great pollutant power. It is mainly due to the high organic fraction present which includes sugars, tannins, acids, pectins, lipids, and especially phenols and polyphenols (Balice et al., 1990; Hamdi, 1993a). It is estimated that this olive mill wastewaters (OMW) production passes beyond 30 million m3/year (Fiestas and Borja, 1992). As it represents a serious problem of environmental contamination, great efforts are carried out in the last years to develop new technologies for reducing that pollutant content (Hamdi, 1993b; Gonzalez et al., 1994) and to propose different alternatives to the usual ways of elimination, which are in many cases disposal into evaporation ponds (Annesini and Gironi, 1991; Saez et al., 1992) or use as fertilizer in agricultural soils (Gonzalez et al., 1992). In recent times, most of the studies about OMW treatments are focused on aerobic biodegradation (Hamdi and Ellouz, 1992; Velioglu et al., 1992) or anaerobic digestion of that material (Gharsallah, 1994; Martin et al., 1994). However, many problems concerning the high toxicity and the biodegradability of the effluents have been encountered during these anaerobic treatments (Gonzalez et al., 1990; Hamdi, 1992), and the experimental results are not satisfactory: they must be conducted on a highly dilute substrate, because the aromatic and phenolic compounds are toxic for methanogenic bacteria (Boari et al., 1984; Hamdi, 1991). So, other strategies must be carried out, as some appropriate pretreatments, in order to remove the organic compounds and to reduce the methanogenic bacteria inhibition of the OMW. This goal can be obtained by biological (Hamdi et al., 1992; Borja et al., 1995) or chemical (Chakchouk et al., 1994; Wlassics and Visentin, 1994) pretreatment methods which degrade those organics, produce OMW less toxic for methanogenic bacteria, and facilitate the anaerobic digestion. Among them, the oxidation by ozone has shown to be effective for the abatement of phenolic compounds (Gurol and Vatistas, 1987; Bondioli * Author to whom correspondence should be addressed. E-mail address:
[email protected]. Fax number: 34-24271304. S0888-5885(96)00025-5 CCC: $14.00
et al., 1992; Wang, 1992), of which removal is of particular interest because many substituted phenols have also been found to be harmful to fish and other life forms present in waters (Blum and Speece, 1990; Capasso et al., 1992). With this general aim in mind, a competitive kinetic method is used in this research to study the ozonation kinetics of several phenolic acids which are present in the OMW. This dynamic approach has proved to be a simple and reliable technique for measuring the rates of fast reactions of ozone in aqueous solutions with phenols (Gurol and Nekouinaini, 1984) and herbicides (Xiong and Graham, 1992). The selected acids, major pollutants in OMW (Balice and Cera, 1984; Cichelli and Solinas, 1984), were the following: caffeic (3,4-dihydroxycinnamic), p-coumaric (trans-4-hydroxycinnamic), syringic (4-hydroxy-3,5dimethoxybenzoic), vanillic (4-hydroxy-3-methoxybenzoic), 3,4,5-trimethoxybenzoic, veratric (3,4-dimethoxybenzoic), p-hydroxybenzoic, and protocatechuic (3,4dihydroxybenzoic). The literature provides a few investigations about the ozonation of some of those acids, but these works have focused their interest in the identification of degradation products and reaction mechanisms (Calvosa et al., 1991; Andreozzi et al., 1995), and there are no kinetic data available for these reactions. So, the objective of this work is the determination of those kinetic parameters. Previously, the influence of operating variables is observed, the stoichiometric ratios are evaluated, and the kinetic regime of absorption is established. Once the direct degradation rate constants by ozone are deduced, they are correlated as a function of temperature and pH. Those resulting kinetic expressions for the ozonation reactions are useful for the successful design and operation of ozone reactors in water and wastewaters treatment plants. Experimental Section The experiments were conducted in the semicontinuous reactor described in more detail in a previous investigation (Benı´tez et al., 1994a). Basically, it consists of a glass vessel submerged in a thermostatic bath and provided with inlets for bubbling the gas feed, sampling, venting, and measuring of temperature. Ozone was produced in an ozone generator from commercial oxygen, and the resulting mixture can be © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 639
sent either to the reactor or to a flask for analysis. Its concentration in the gas phase (in both the inlet and outlet streams) was determined iodometrically (Kolthoff and Belcher, 1957), while its concentration in the aqueous solutions was measured colorimetrically by the indigo method (Bader and Hoigne´, 1981). The reactor was always charged with 350 cm3 of aqueous solutions. They were prepared by dissolving the amount of each acid needed to obtain a concentration of 100 ppm in buffered organic-free water, corresponding to the following initial acid concentrations: 5.55 × 10-4 M for caffeic, 6.09 × 10-4 M for p-coumaric, 5.94 × 10-4 M for syringic, 5.95 × 10-4 M for vanillic, 4.71 × 10-4 M for 3,4,5-trimethoxybenzoic, 4.90 × 10-4 M for veratric, 7.24 × 10-4 M for p-hydroxybenzoic, and 6.49 × 10-4 M for protocatechuic. Once every experiment had started, liquid samples were taken out at different ozonation times, and the remaining acids concentrations were analyzed by HPLC, using a Waters chromatograph with a UV detector Model 441 and with a 15 cm Novapak C18 column. As the mobile phase a mixture of methanol and water was used (30/70 v/v), with a flow rate of 1 cm3‚min-1. As later will be needed, the equilibrium concentration of ozone CA* was deduced by Henry’s law; data of Henry’s law constants for ozone in water were obtained from Sotelo et al. (1989). On the other hand, the diffusivities of ozone in water DA were obtained from Matrozov et al. (1976), while the diffusivities of the different acids DB were calculated by means of the Wilke-Chang equation (Reid et al., 1977). Finally, the liquid mass-transfer coefficients kL and interfacial areas at different temperatures were evaluated according to “Danckwerts’ plot method” (Charpentier, 1981) and were provided in a previously mentioned work (Benı´tez et al., 1994a).
Nekouinaini (1984), ozonation can be conducted in a mixture of solutes where a reference compound having a known rate constant is present. According to eq 2, the rate expression for this reference compound R will also be written in the form:
dCR ) kR[O3]CR dt
-
Dividing both equations, the following is obtained:
dCB kB CB ) dCR kR CR
The direct reaction between an organic compound and ozone can be represented by the general expression:
B + zO3 f P
(1)
where z is the stoichiometric ratio defined as the moles of ozone consumed per mole of organic degraded. According to the literature, second-order kinetics for the reactions between ozone and organic compounds can be assumed. Thus, as several authors pointed out (Hoigne´ and Bader, 1983a,b; Gurol and Nekouinaini, 1984; Hoigne´ et al., 1985; Yao and Haag, 1991; Xiong and Graham, 1992, etc.), the second-order reaction is a common situation in the ozonation of different organic substances, especially in the case of phenolic derivatives. In addition, this kinetics was also obtained in a previous research (Benı´tez et al., 1994b), where the individual ozonation of vanillic acid, one of the phenolic acids present in this reaction system, was studied. So, if it can be assumed that the reaction is first order with respect to both ozone and the organic B, the rate of disappearance of the solute is
-
dCB ) kB[O3]CB dt
(2)
where kB is the rate constant for the direct degradation of B by ozone. In order to determine kB for several organic compounds simultaneously, as proposed by Gurol and
(4)
which is integrated between t ) 0 and t ) t, yielding
ln
CB0 kB CR0 ) ln CB kR CR
(5)
Thus, a plot of ln(CB0/CB) against ln(CR0/CR) yields a straight line whose slope is the ratio of rate constants. As kR is known, kB can be determined for each compound. This dynamic approach and eq 5 are applicable for situations in which the concentration of the organic compounds remain virtually undepleted in the liquid film. According to the film theory (Danckwerts, 1970; Charpentier, 1981), two absorption regimes satisfy this condition: the very slow or diffusional kinetic regime with reaction taking place in the bulk liquid and the fast and pseudo-first-order regime with reaction occurring in the liquid film. The film theory proposes that those situations are possible when the following criteria are fulfilled:
For very slow regime: Ha < 0.02
Reaction Model
(3)
(6)
For fast and pseudo-first-order regime: Ei/2 > Ha > 3
(7)
with Ha and Ei being the Hatta number and the instantaneous enhancement factor, respectively. These parameters for a single reaction between the gas dissolved (ozone) and an organic compound in the liquid phase following the assumed second-order kinetics, are also defined by the film theory in the form:
Ha )
xzkBDACB
Ei ) 1 +
kL DB zCB DA CA*
(8)
(9)
Results and Discussion Influence of Variables. Ozonation experiments of mixtures of the eight phenolic acids selected were conducted by varying the temperature (10-40 °C), pH (2-9), and ozone partial pressure in the gas stream feeding the reactor (0.25-0.79 kPa). Table 1 specifies the values of those variables in a set of experiments with the corresponding CA* values after application of Henry’s law. The influence of temperature was investigated at pH ) 5 and 0.47 kPa. Figure 1 shows the results obtained for the oxidation of p-hydroxybenzoic acid taken as an
640 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 1. Values of Operating Variables and Experimental Parameters Obtained expt
T, °C
pH
PO3, kPa
CA* × 105, M
XO3, %
NAa × 106,a M‚s-1
Eexpa
1 2 3 4 5 6 7 8 9
20 20 20 20 30 10 40 20 20
7 7 7 5 5 5 5 2 9
0.25 0.43 0.79 0.47 0.48 0.47 0.44 0.46 0.44
2.07 3.72 6.61 3.93 3.10 5.07 2.48 4.27 3.52
100 97.9 70.4 65.1 71.9 54.0 83.6 25.7 98.0
3.26 5.49 7.25 3.99 4.50 3.31 4.80 1.55 5.62
21.0 19.6 14.6 13.5 13.3 10.3 13.6 4.8 21.2
a
After 30 min of reaction.
Figure 2. pH influence on the p-hydroxybenzoic acid conversion. Experimental conditions: T ) 20 °C; PO3 ) 0.47 kPa.
Figure 1. Temperature influence on the p-hydroxybenzoic acid conversion. Experimental conditions: pH ) 5; PO3 ) 0.47 kPa.
example: it can be seen that there is a positive effect on the degradation, due to the increase of the reaction rate constants with temperature. Similar results are obtained for the rest of the acids. On the other hand, the effect of the pH was studied at 20 °C and 0.47 kPa. As can be observed in Figure 2 also for p-hydroxybenzoic acid as an example, the increase of this variable leads to an increment of the conversion and, subsequently, in the oxidation rate. The explanation of this effect is attributed to the anion resulting in the dissociation of the acids, which are more reactive than the undissociated forms (Hoigne´ and Bader, 1983b). Those anions are favored at higher pHs. Referring to the ozone partial pressure, its influence on the ozonation rate is also positive. Figure 3 shows the results obtained again for p-hydroxybenzoic acid as an example, for partial pressures of 0.25, 0.43, and 0.79 kPa at 20 °C and pH ) 7, equivalent to equilibrium concentrations of ozone at the gas-liquid interface of 2.07, 3.72, and 6.61 × 10-5 M, respectively. It is clear that the ozone absorbed, and consequently the oxidation rates, is directly affected by the ozone concentration in the gas mixture: both increase when this concentration is increased. Finally, in order to settle the different levels of oxidation rates among the studied acids in the ozonation process, Figure 4 shows the degradation curves in an experiment as an example. Although difficult to show
Figure 3. Ozone partial pressure influence on the p-hydroxybenzoic acid conversion. Experimental conditions: T ) 20 °C; pH ) 7.
in some cases due to very similar ozonation rates, the following sequence of degradations can be established: caffeic > p-coumaric ) syringic > vanillic > 3,4,5trimethoxybenzoic ) veratric > p-hydroxybenzoic . protocatechuic. This sequence is confirmed by the degradation curves in the rest of the experiments. Stoichiometric Ratio. As was seen in the reaction model, the stoichiometric ratio is an important parameter also needed for the kinetic study. In order to determine this ratio, previous experiments were carried out by mixing individual aqueous solutions of each acid and ozone of known concentrations, with the acid concentration being at a higher value (between 2 and 8 times) to assure the total consumption of ozone practically at an instantaneous rate. Those ozone solutions
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 641
is no ozone dissolved in the solutions, that ozone absorbed is equivalent to the ozone reacted. Table 1 depicts the XO3 values obtained in the experiments described there after 30 min of reaction: from them it can be concluded that most of the ozone fed to the reactor is absorbed and consumed by the numerous reactions taking place in the reactor medium, except for the experiment at pH ) 2 where the ozone absorbed and reacted is low. In a following step, an approach to the kinetic regime of ozonation can be made from the experimental results, in order to determine if the conditions established by the reaction model in relation to the kinetic regime of reaction are fulfilled in the present system. Thus, in a general process of absorption with chemical reaction where no ozone dissolved is detected in the liquid phase, the gas absorption rate is given by (Charpentier, 1981):
NAa ) kLaCA*E
Figure 4. Comparison of oxidation rates of the phenolic acids studied. Experimental conditions: T ) 30 °C; pH ) 5; PO3 ) 0.48 kPa. Table 2. Stoichiometric Ratios acid
z
acid
z
caffeic p-coumaric syringic vanillic
1.5 1 2 1.5
3,4,5-trimethoxybenzoic veratric p-hydroxybenzoic protocatechuic
1.5 1.5 1.5 2
were prepared by bubbling an ozone-oxygen stream into high-purity water until saturation was reached. The concentrations of ozone and acids in the initial aqueous solutions and the remaining acid concentrations in the final solutions were measured as described in the Experimental Section. The experimental stoichiometric ratio was calculated by the expression:
[O3]0 z) CB0 - CB
(10)
By means of eq 10, z is evaluated for every experiment conducted with different initial concentration ratios, and Table 2 shows the average value of z proposed for each acid. As can be seen, in all cases the values obtained are between 2 and 1 mol of ozone consumed/mol of acid degraded. These different stoichiometric ratios are related with the nature of the molecules and their more reactive positions which can be attacked directly by ozone. Ozonation Ratios and Absorption Regime. There is a parameter that gives information about the oxidation process extent. This is the ozonation ratio, defined in the form:
XO3 )
ne - ns × 100 ne
(11)
where ne and ns are the inlet and outlet concentrations of ozone in the gas stream, the difference between both being the ozone absorbed by the liquid phase. As there
(12)
E is the enhancement factor defined as the ratio of the absorption rate in the presence of a chemical reaction to the maximum rate of pure physical absorption, and kLa is the volumetric mass-transfer coefficient in the liquid phase. On the other hand, this absorption rate can be determined from the ozone absorbed and reacted, by the expression:
N Aa )
(ne - ns)Fg V
(13)
where Fg is the volumetric gas flow rate (40 L‚h-1 at room conditions in these experiments) and V the reactor volume (350 cm3). From both eqs 12 and 13, the experimental E can be calculated by:
Eexp )
(ne - ns)Fg kLaCA*V
(14)
Table 1 also shows the values obtained at 30 min of reaction for NAa and Eexp by using eqs 13 and 14, respectively: as can be seen, the Eexp values are always much higher than 1, and according to the film theory (Charpentier, 1981), it suggests that the kinetic regime of the general ozonation process is fast and the reactions mainly take place in the liquid film. This conclusion partially satisfies condition 7 (fast reaction regime), although the requirement of pseudo-first-order will be confirmed later with more accuracy by determining the Ha numbers for each experiment when the rate constants will be known. Kinetic Study. Once the fast regime has been established in this process, the evaluation of the kB constants for the direct reaction between ozone and each phenolic acid can be performed by the competitive method previously described in the Reaction Model section and, more specifically, by using eq 5. As was explained there, it can be used when the absorption regime is fast and pseudo-first-order. The film theory proposes that the pseudo-first-order reaction takes place when the concentration of reactant CB in the liquid bulk is much greater than CA* and the reaction does not cause any significant depletion of component B through the liquid film. This is the situation that probably occurs in the present system due to the fact that the
642 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 3. Values of kB × 10-6, M-1‚s-1 expt acid
1
2
3
4
5
6
7
8
9
caffeic p-coumaric syringic vanillic 3,4,5-trimethoxybenzoic veratric p-hydroxybenzoic protocatechuic
4.95 4.22 4.56 3.94 2.53 2.44 2.51 1.57
4.83 4.06 4.52 3.94 2.36 2.21 2.36 1.77
3.78 3.85 3.98 3.94 2.36 2.23 2.40 1.67
2.54 2.22 2.22 0.92 0.57 0.52 0.56 0.44
3.87 4.09 3.47 1.64 1.10 1.04 2.54 0.69
1.50 1.23 1.23 0.49 0.29 0.28 3.87 0.26
6.77 5.94 5.96 2.82 2.19 2.03 1.85 1.35
1.57 0.69 0.97 0.10 0.05 0.04 0.03 0.15
17.20 14.86 17.99 16.96 10.48 9.89 12.29 8.61
Figure 5. Determination of kB/kR for caffeic, syringic, p-hydroxybenzoic, and protocatechuic acids. Experimental conditions: T ) 40 °C; pH ) 5; PO3 ) 0.44 kPa.
Figure 6. Determination of kB/kR for p-coumaric, 3,4,5-trimethoxybenzoic, and veratric acids. Experimental conditions: T ) 40 °C; pH ) 5; PO3 ) 0.44 kPa.
initial concentration of each acid is at least 10 times higher than CA* (see Table 1). For this objective, vanillic acid is taken as the reference compound: as was commented before, its direct reaction with ozone was studied individually in a previous investigation (Benı´tez et al., 1994b). In that work, an overall second order (first order with respect to ozone and the vanillic acid) was found, and the kinetic constants were deduced, being well correlated as a function of the temperature and pH by the following empirical expression:
Table 4. Parameter k0, Ea, and p for Each Acid
kR ) 2.84 × 1016 exp(-5160/T)[OH-]0.317 (L‚mol-1‚s-1) (15) Thus, according to eq 5, Figures 5 and 6 show as examples the plots of ln(CB0/CB) versus ln(CR0/CR) corresponding to an experiment. As can be seen, experimental points lie satisfactorily around straight lines, confirming the goodness of the model used. Similar plots were obtained for the rest of the experiments. After least-squares regression analysis in all cases, the slopes were obtained, and with kR values, kB were deduced for each phenolic acid. Table 3 depicts the values found for every experimental series. Later, assuming that these rate constants are functions of temperature and pH, the following general expression can be proposed for each acid which correlates the kB
acid
k0, M-1‚s-1
Ea, kJ‚mol-1
p
caffeic p-coumaric syringic 3,4,5-trimethoxybenzoic veratric p-hydroxybenzoic protocatechuic
3.81 × 4.65 × 1014 1.85 × 1014 6.69 × 1017 9.99 × 1017 4.07 × 1017 1.56 × 1014
32.37 37.35 35.13 51.23 51.76 48.66 34.49
0.148 0.174 0.179 0.329 0.343 0.360 0.256
1013
values:
kB ) k0 exp(-Ea/RT)[OH-]p
(16)
A regression analysis is performed, and Table 4 shows the values determined for k0, Ea, and p for every acid. As was explained in the Introduction, there are no available data about the kinetic constants for the reaction between ozone and these acids. So, the values obtained cannot be compared to other reported values. As an approximation, the constants reported for other similar organic compounds can be mentioned (Hoigne´ and Bader, 1983b): 18 × 106 M-1‚s-1 for phenol, 3.4 × 106 M-1‚s-1 for 4-chlorophenol, 14 × 106 M-1‚s-1 for 4-nitrophenol, and 0.3 × 106 M-1‚s-1 for resorcinol, all at pH 8. As can be observed, these constants are in the same range as those proposed in this work for the acids studied. Once the direct kinetic constants are known, the kinetic regime of absorption must be definitively estab-
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 643
Acknowledgment
Table 5. Values Obtained for Ha and Ei expt
Ha
Ei
expt
Ha
Ei
1 2 3 4 5
44.0 43.7 41.7 26.6 26.9
188 105 58 99 125
6 7 8 9
20.3 30.5 16.0 90.0
77 155 91 111
lished in order to confirm if the previous condition required by the competitive method is fulfilled (eq 7). For this purpose, it is necessary to evaluate the Hatta number and the instantaneous reaction factor. However, in the present case in which the reaction medium is constituted by a mixture of solutes which are simultaneously ozonated, instead of eqs 8 and 9 for Ha and Ei in a simple reaction, it must be written according to Onda et al. (1970):
Ha )
xDA∑(zikBiCBi)
Ei ) 1 +
kL
∑(DBiziCBi) DACA*
(17)
(18)
which take into account the contributions of the stoichiometric ratios and rate constants of each individual ozone-acid reaction. Table 5 reports the values deduced for Ha and Ei with eqs 17 and 18 in the experimental series of Table 1 at the initial times of the ozonation process, when it is assumed that ozone only reacts with the initial acids, and its reactions with the intermediates products formed are nearly negligible. As can be observed, the condition for a fast kinetic regime of pseudo-first-order as eq 7 establishes is satisfied in most of the experiments. Conclusions The kinetics of the ozonation in aqueous solutions of eight phenolic acids, major pollutants in wastewaters coming from the olive oil production process, are studied simultaneously in a semicontinuous batch reactor. From the results obtained, a direct influence of temperature, pH, and ozone partial pressure in the gas mixture on the ozonation rate of each acid is shown, with the order of acid degradation being caffeic > p-coumaric ) syringic > vanillic > 3,4,5-trimethoxybenzoic ) veratric > p-hydroxybenzoic . protocatechuic. From homogeneous experiments, stoichiometric ratios varying from 1 to 2 are determined for the individual direct reactions between ozone and each acid. An approach to the kinetic regime of absorption in the ozonation process is made by determining the experimental enhancement factors. They suggest that the global reaction occurs in the fast pseudo-first-order regime, a condition required by the competitive kinetic model. The use of this competition method, taking the vanillic acid as the reference compound, allows one to determine the rate constants for every acid in all experiments. These constants are well correlated into individual expressions as a function of temperature and pH. The evaluation of the Hatta numbers and instantaneous enhancement factors confirms the kinetic regime initially supposed.
Authors wish to thank CICYT of Spain for financial support of this research through the Project AMB96/ 0815. Nomenclature a ) specific interfacial area, m-1 CA* ) ozone equilibrium concentration, M CB ) phenolic acid concentration, M CR ) reference acid concentration, M DA) ozone diffusivity in the liquid phase, m2‚s-1 DB ) phenolic acid diffusivity in the liquid phase, m2‚s-1 E ) enhancement factor Ea ) activation energy, kJ‚mol-1 Ei ) instantaneous enhancement factor Fg ) volumetric gas flow rate, L‚s-1 Ha ) Hatta number kB ) kinetic rate constant for the direct ozone-acid reaction, M-1‚s-1 kR ) kinetic rate constant for the direct ozone-reference acid reaction, M-1‚s-1 k0 ) preexponential factor kL ) liquid phase mass-transfer coefficient, m‚s-1 kLa ) volumetric mass transfer coefficient in liquid phase, s-1 ne ) inlet ozone concentration in the gas stream, M ns ) outlet ozone concentration in the gas stream, M NAa ) rate of absorption of ozone in the liquid phase, M‚s-1 [O3] ) ozone concentration in the solution, M p ) exponent of the [OH-] PO3 ) ozone partial pressure, kPa T ) temperature, K t ) reaction time, s V ) reactor volume, cm3 XB ) phenolic acid conversion XO3 ) ozonation ratio defined by eq 11 z ) stoichiometric ratio for the direct O3-acid reaction
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Received for review January 4, 1996 Revised manuscript received October 30, 1996 Accepted November 8, 1996X IE9600250
X Abstract published in Advance ACS Abstracts, January 15, 1997.
