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Ind. Eng. Chem. Res. 2000, 39, 2214-2220
Sodium Dodecylbenzenesulfonate Removal from Water and Wastewater. 1. Kinetics of Decomposition by Ozonation Fernando J. Beltra´ n,* Juan F. Garcı´a-Araya, and Pedro M. A Ä lvarez Departamento de Ingenierı´a Quı´mica y Energe´ tica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain
Ozonation experiments were conducted to investigate the sodium dodecylbenzenesulfonate (NaDBS) removal from aqueous solution and domestic wastewater. The influence of pH and tert-butyl alcohol concentration, a known scavenger of hydroxyl radicals, on the ozonation rate was observed. The degradation rate was especially fast at pH 10 in the absence of hydroxyl radical scavengers. The rate constants of the reactions NaDBS-ozone and NaDBS-hydroxyl radical were found to be 3.68 and 1.16 × 1010 M-1‚s-1, respectively. Organic matter in wastewater competes with NaDBS for both dissolved ozone and hydroxyl radical, resulting in a decrease in the overall removal rate of the surfactant. COD and TOC were not completely removed from the wastewater due to an ozone-resistant fraction which remained after the treatment. Nevertheless, the ability of ozone to cause alteration in the molecular structures of dissolved compounds resulted in an increase of the wastewater biodegradability, demonstrated by the increase of the BOD5/COD ratio. The contributions of the direct and free radical reactions to the oxidation of NaDBS were evaluated in percentages of pollutant removal. Introduction Alkylbenzenesulfonates (ABSs) constitute by far the largest group of surfactants in detergent and cleaning formulations. Linear alkylbenzenesulfonates (LASs) were introduced in the mid 1960s to substitute the poorly biodegradable tetrapropylbenzenesulfonate and today constitute the most widely used anionic surfactants.1,2 LASs are consumed in the home and disposed of in wastewater, where they typically receive treatment. Their presence in sewage works is variable depending on their use in industrial processing in addition to domestic activities. A LAS average concentration of 1-10 mg‚L-1 can be found in plants treating only domestic wastewater.3,4 This range is markedly increased when municipal sewers are also receiving industrial wastes from washing processes.5,6 Much work has been reported to assess the aerobic biodegradability and environmental safety of LASs and their degradation products.7-10 As aerobic biodegradability has been proven, LAS removal from wastewater has generally been restricted to conventional biological treatments. Although the percent of LAS removed in biological systems is usually over 95% by activated sludge and 70% by biological filter sewage treatment systems,11-12 however, some difficulties have been found in the treatment of wastewater with high LAS concentrations: (i) Foaming occurs and negatively affects the oxygen mass transfer in the aeration basin and the sludge settling characteristics. (ii) For LAS concentrations over 20 mg‚L-1, the pH self-regulation capacity of the wastewater decreases and, as a consequence, neutralization is required.13 (iii) A long retention time is needed to accomplish complete removal.12 Thus, for wastewater with a high LAS concentration, it is necessary to employ less conventional techniques, such as physical-chemical treatments or chemical oxidation to aid biological * To whom correspondence should be addressed. E-mail:
[email protected].
processes.14-16 Conventional oxidants such as hydrogen peroxide and potassium permanganate have little effect on the removal of LAS, and when chlorine is used low yields of chloroform, a known carcinogenic compound, are formed.17 Also, the rate of photodegradation of LAS is highly reduced in the presence of organic matter.18 On the other hand, ozone is an alternative agent that is becoming more used in water and wastewater treatment due to its high oxidizing power.19 Moreover, ozone is readily generated, is soluble in water, and decomposes to oxygen and water without formation of byproducts that need further removal. Therefore, ozonation offers an innovative approach to solve the problems outlined above. Ozonation of several alkylbenzenesulfonates has been previously reported.16 Although data on the ozone dose and the reaction time required to achieve a given ABS removal have been published, little information is available on the ozonation kinetics of LAS. In the present work we have focused on the ozonation of a LAS model compound, sodium dodecylbenzenesulfonate (NaDBS). The investigation deals with the kinetics and mechanism of the ozonation of NaDBS and the fate this surfactant presents during synthetic and real domestic wastewater ozonation. In the following paper the impact of ozonation on the biodegradability of wastewater containing NaDBS is presented.20 Experimental Section Materials. The specific LAS compound studied, sodium dodecylbenzenesulfonate (C12H25C6H4SO3Na) was purchased from Aldrich. NaDBS aqueous solutions were prepared in organic-free water produced in a Milli Q system (Milli-Water System, resistivity 18 MΩ‚cm-1, organic carbon content < 50 µg‚L-1). If required, the pH was adjusted at the beginning of the experiment to achieve acidic or basic conditions by the addition of phosphoric acid or sodium hydroxide. Ozone was produced from air by means of a Fisher 500 ozone genera-
10.1021/ie990721a CCC: $19.00 © 2000 American Chemical Society Published on Web 06/13/2000
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2215 Table 1. Main Characteristics of Domestic Wastewater Used in This Work parameter
unit mg‚L-1 mg‚L-1 mg‚L-1 mg‚L-1 mg‚L-1 mg‚L-1 mg‚L-1 mg‚L-1 mS‚cm-1
COD BOD5 TKN NH3-N TC IC TS TSS conductivity pH dissolved oxygen alkalinity
mg‚L-1 mg‚L-1 (as CaCO3)
value 274 ( 25 158 ( 13 35 ( 6 29 ( 5 137 ( 18 31 ( 7 212 ( 41 137 ( 18 1.3 ( 0.2 7.6 ( 0.4 2.0 ( 0.4 250 ( 28
Table 2. Synthetic Wastewater Composition constituent
conc, mg‚L-1
constituent
conc, mg‚L-1
glucose glutamic acid NaDBS K2HPO4 KH2PO4 NH4Cl
200 100 15 138 69 52
MgSO4‚7 H2O FeSO4‚7 H2O ZnSO4‚7 H2O MnSO4‚7 H2O CaCl2 NaHCO3
26 0.75 0.75 0.75 3.5 80
tor. Phenol, hydrogen peroxide, and tert-butyl alcohol, used for the kinetic study, were all obtained from Merck. Samples of domestic wastewater were collected from a sewage work at Badajoz (Spain) and used after solids separation. The main features of this wastewater before solid removal are summarized in Table 1. A synthetic wastewater was prepared in distilled water with the composition listed in Table 2. Experimental Setup and Procedure. Chemical oxidation experiments were conducted in a 2.5 L glass bubble column (i.d. 9 cm; length 45 cm) that operated batchwise, as detailed in previous works.21 The airozone gas mixture was fed to the reactor through a diffuser plate (pore diameter 16-40 µm) situated at its bottom. For UV/H2O2 oxidation runs, completed to perform part of the kinetic study, the reactor was supplied with a central wall made of quartz. A 15 W TNN Hannau low-pressure mercury vapor lamp was used in these cases. The reactor was always charged with 1.5 L of aqueous solution, and this volume was kept constant through each experiment. A peristaltic pump was used to recirculate the wastewater at a rate of 20 L‚h-1, providing appropriate mixing conditions. Experiments were conducted at 20 °C, and samples were withdrawn periodically for analysis. The unreacted and residual gases leaving the column were directed toward an Anseros Ozomat GM-109 ozone analyzer. Analytical Methods. Concentrations of dissolved NaDBS and the total amount of methylene blue active substances (MBASs) in domestic wastewater, including NaDBS, were measured using the methylene blue test.22 It is worth noting that this method is not specific for LASs but for all types of anionic surfactants. Phenol was analyzed by high-performance liquid chromatography using a Waters NovaPak C18 column and a UV detector (Waters 486), the mobile phase being a mixture of water and acetonitrile (6:4 v/v), at a flow rate of 1.0 mL‚min-1. Ozone in water was analyzed by the Indigo method23 while in the gas phase it was measured by means of an Anseros Ozomat GM109 analyzer. Hydrogen peroxide was determined iodometrically. Chemical characterization of wastewater was performed by determining the parameters of Table 1. All these analyses were carried out in accordance with standard methods.22
Figure 1. Influence of pH on the ozonation of NaDBS. Variation of concentration of NaDBS with ozonation time. Conditions: T ) 20 °C; CNaDBS0 ) 15 mg‚L-1; F ) 30 L‚h-1; CO3g0 ) 10 mg‚L-1. Symbols: 0, pH ) 2; O, pH ) 7; 4, pH ) 10.
Results and Discussion The destruction of organics via ozonation can be represented as a gas-liquid process with simultaneous mass transfer and a series of complex reactions that can involve free radical reactions in addition to direct ozonolysis.24 Free radicals, such as hydroxyl radicals HO•, are generated from the ozone decomposition in water. Depending on the experimental conditions and type of reactor used, ozonation develops in the slow, fast, instantaneous, or in between regimes of absorption.25 Therefore, the first step in an ozonation kinetic study is to elucidate the kinetic regime of ozone absorption and the influence of both reaction methods: direct attack of molecular ozone to NaDBS and intermediates generated and free radical oxidation of compounds. Preliminary Study: Influence of pH and Presence of Hydroxyl Radical Scavengers. Due to the strong effect of hydroxide ions on the ozone decomposition, especially when pH is raised above 8, pH is usually considered the most important parameter for ozonation kinetics.24 Thus, a series of ozonations of NaDBS in aqueous solution was first carried out by varying the pH between 2 and 10 while other experimental variables (initial LAS concentration, temperature, gas flow rate, and ozone concentration in the gas entering the reactor) were kept constant. Figure 1 shows the remaining concentration of surfactant as a function of the ozonation time in these experiments. From this figure it can be deduced that the NaDBS removal rate was significantly enhanced at neutral and especially at basic conditions. In Figures 2 and 3 the ozone concentrations in water and in the gas leaving the reactor, respectively, have been plotted against the reaction time. As observed, at a given time both ozone concentrations decreased with the increase of pH. These results are a logical consequence of the increase of ozone decomposition when pH is raised. At low pH, ozone is mainly consumed through direct ozone-NaDBS and ozoneintermediate reactions. In addition to these reactions, at elevated pHs, the consumption of ozone increases because it decomposes into free radicals. Also, the
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Figure 2. Influence of pH on the ozonation of NaDBS. Variation of dissolved ozone concentration with time. Conditions: T ) 20 °C; CNaDBS0 ) 15 mg‚L-1; F ) 30 L‚h-1; CO3g0 ) 10 mg‚L-1. Symbols: 0, pH ) 2; O, pH ) 7; 4, pH ) 10.
Figure 4. Influence of pH and tert-butyl alcohol on the ozonation of NaDBS. Variation of concentration of NaDBS with ozonation time. Conditions: T ) 20 °C; CNaDBS0 ) 15 mg‚L-1; F ) 30 L‚h-1; CO3g0 ) 10 mg‚L-1. Symbols: 0, pH ) 2 and CtBu ) 0; 9, pH ) 2 and CtBu ) 0.01 M; O, pH ) 7 and CtBu ) 0; b, pH ) 7 and CtBu ) 0.01 M.
Kinetic Study. The results obtained confirmed that, depending on the experimental conditions, ozonation of NaDBS in aqueous solution involves a free radical mechanism, in addition to direct reactions of molecular ozone. A general mechanism of the ozonation of NaDBS can be summed up in the following manner:
Direct reactions NaDBS + zO3 f intermedates f final products (CO2, SO42-, H2O) (1) Free radical reactions NaDBS + HO• f intermediates f final products (CO2, SO42-, H2O) (2)
Figure 3. Influence of pH on the ozonation of NaDBS. Variation of concentration of ozone in the gas leaving the reactor with time. T ) 20 °C; CNaDBS0 ) 15 mg‚L-1; F ) 30 L‚h-1; CO3g0 ) 10 mg‚L-1. Symbols: 0, pH ) 2; O, pH ) 7; 4, pH ) 10.
increase of the percentage of NaDBS removed when the pH is raised was likely due to the oxidizing power of the hydroxyl radicals formed. To ascertain the contribution of direct and free radical reactions to remove NaDBS, a second series of experiments in the presence of tert-butyl alcohol, a known scavenger of hydroxyl radicals,26 was carried out at pH 2 and 7. The results are shown in Figure 4. At pH 2 the residual concentration of NaDBS at any given reaction time was similar regardless of the presence of tert-butyl alcohol. Therefore, it should be assumed that only the direct reaction NaDBS-ozone develops at pH 2 to remove NaDBS. However, when pH was increased up to 7, the hydroxyl radicals had an important role in the oxidation of NaDBS, as can be deduced from Figure 4.
Regardless of the pH, the presence of dissolved ozone suggested that NaDBS ozonation developed through slow or moderate gas-liquid reactions.25 If the slow regime of ozone absorption is assumed, the overall degradation rate of NaDBS can be expressed as follows:
-rNaDBS ) -
dCNaDBS kd ) CO3CNaDBS + dt z kHO•CHO•CNaDBS (3)
where kd and kHO• are the rate constants of NaDBS decomposition through reactions 1 and 2, respectively. CO3, CHO•, and CNaDBS are the molar concentrations of dissolved ozone, hydroxyl radical, and NaDBS, respectively, and z is the stoichiommetric ratio of reaction 1. Determination of the Rate Constant of Reaction 1, kd. Results of semibatch ozonation experiments carried out in the presence of tert-butyl alcohol (0.01 M) showed a negligible contribution of radical reactions to the ozonation rate. Hence, direct molecular ozonolysis was the main method of oxidation. Accordingly, eq 3 can
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2217
be simplified as follows:
-rNaDBS ) -
dCNaDBS kd ) CO3CNaDBS dt z
(4)
Moreover, as the concentration of dissolved ozone, CO3, became constant after an initial period of time (see Figure 2 as an example), a slow pseudo-first-order kinetic regime was assumed to hold. Thus, eq 4 can be integrated after variable separation to yield
kd CNaDBS ) - CO3(t - t0) ln CNaDBS0 z
(5)
where the subscript 0 refers to the time at which the ozone concentration reaches a stationary value in Figure 2. Following eq 5, the stoichiometric ratio of the direct reaction ozone-NaDBS, z, is needed for the later rate constant determination. The proposed mechanism suggests that some intermediate products were accumulated in the liquid phase during the oxidation. As these intermediates consume ozone, the stoichiometric factor, z, should not be calculated from semibatch experiments. For this purpose, homogeneous ozonations of NaDBS were completed at constant pH in the presence of tertbutyl alcohol to scavenge the hydroxyl radical oxidation pathway. The procedure consisted of mixing separately aqueous solutions of ozone and the surfactant of known concentrations. To avoid the interference of reactions between ozone and intermediates and, therefore, ensure that ozone was practically consumed by its reaction with NaDBS, the initial concentration of NaDBS in the mixed solution was 1 to 10 times higher than that of ozone. Thus, the stoichiometric ratio, z, was calculated by eq 6, since at concentrations of NaDBS much higher than those of ozone, the reaction ozone-NaDBS is the only ozone-consuming reaction in the system:
CO3i z) CNaDBSi - CNaDBSf
(6)
where CO3i is the initial molar concentration of ozone within the mixed solution while CNaDBSi and CNaDBSf are the concentrations of NaDBS just after the mixing and at the end of the reaction, respectively. It was observed that NaDBS-ozone initial ratios > 5 led to a constant value of z equal to 1 mole of ozone per mole of NaDBS. This value was considered as the actual stoichiometric coefficient of reaction 1. Once z was determined, kd was obtained from the slope of the straight lines resulting when the left-hand side of eq 5 was plotted against CO3(t - t0)/z (not shown). With this procedure, the average value of the rate constant was found to be 3.68 M-1‚s-1 through several repeated experiments (R2 > 0.99). Finally, the condition of the slow regime of ozone absorption, Ha < 0.3, must be checked for each experiment used for the rate constant determination.25 The Hatta number, Ha, represents the ratio between the chemical reaction rate and the physical absorption rate of ozone. For the irreversible second-order reaction here considered, Ha is defined as follows:
Ha )
xkdCNaDBSDO3 kL
(7)
The ozone diffusivity in water, DO3, was taken as 1.76 × 10-9 m2‚s-1 according to the work of Johnson and Davis27 while the liquid-phase mass-transfer coefficient, kL, was calculated to be 3.71 × 10-4 m‚s-1 by applying Calderbrank’s equation.28 As the value of Ha corresponding to different reaction times in experiments directed to kd determination was always 98%. Then, eq 8 was simplified to yield
-rM ) -
dCM ) kHO•CHO•CM dt
(9)
By applying eq 9 to both NaDBS and phenol, and after dividing the resulting equations, the following was obtained:
dCNaDBS kNaDBSCNaDBS ) dCP kPCP
(10)
where the subscript P refers to phenol. From eq 10, after rearranging and integrating, the following results:
ln
kNaDBS CP CNaDBS ) ln CNaDBS0 kP CP0
(11)
According to eq 11, a plot of ln(CNaDBS/CNaDBS0) versus ln(CP/CP0) should yield a straight line of slope equal to kNaDBS/kP (not shown). Since kP is known from the literature (1.1 × 1010 M-1‚s-1 at 20 °C and pH 7),30 after repeated experiments (R2 g 0.99) kNaDBS ) kHO• was found to be 1.16 × 1010 M-1‚s-1. Wastewater Treatment. Since one of the principal routes of surfactant disposal is through municipal wastewater treatment, the ability of ozone to remove LAS from a synthetic and a real domestic wastewater was considered of interest. Table 3 summarizes the effect of ozonation of wastewater containing an initial NaDBS concentration of approximately 15 mg‚L-1, on COD, TOC, BOD5, and MBAS removals. At pH 2, free radical formation is negligible and the removal of MBAS in synthetic wastewater was found to be higher than that in real wastewater. This fact can be due to the presence of other substances in domestic wastewater that compete with NaDBS for the available ozone whereas the organic matter in synthetic waste-
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Table 3. Ozonation of Wastewater Containing Sodium Dodecylbenzene Sulfonate ozonation run
wastewater
1a
domestic
2a
synthetic
3b
domestic
4b
synthetic
5c
domestic
6c
synthetic
a
t, min
ozone dose,
0 15 30 90 0 15 30 90 0 15 30 90 0 15 30 60 0 15 30 90 0 15 30 60
mg‚L-1
removal, %
ozone consumed,
0 72.7 145.5 436.5 0 81 162 486 0 75.8 151.5 454.5 0 77.2 154.5 309 0 77.2 154.5 463.5 0 71.2 138 276
mg‚L-1
0 55.1 92.6 219.4 0 65.1 112.6 247.4 0 60 99.4 226.9 0 62.6 108 180 0 65.9 114.2 269.5 0 60.4 108 183.7
ratio
MBAS
COD
BOD5
TOC
BOD5/COD
COD/TOC
0 21.9 26.0 39.0 0 23.3 42.0 85.3 0 14.9 25.0 43.9 0 37.3 60.1 79.3 0 23.8 27.8 47.0 0 50.0 71.3 82.7
0 18 22 28 0 4 6 24 0 19 23 28 0 11 22 27 0 17 24 31 0 15 28 33
0
0
0.67
2.58
10.3 0
12 0
0.84 0.71
2.2 2.74
16.4 0
11 0
0.79 0.63
2.14 2.53
15.7 0
14 0
0.76 0.73
2.27 2.71
17.7 0
11 0
0.81 0.67
2.19 2.48
0
14 0
0.73
2.32 2.63
21.9
15
0.77
2.24
pH ) 2. pH ) 7. pH ) 10. b
c
water (i.e glucose and glutamic acid) reacts very slowly with ozone.31 On the other hand, at neutral and basic conditions free radical reactions become important. For a given ozone dose, the MBAS removal was significantly improved when pH was increased, as far as synthetic wastewater was concerned. However, in real wastewater, the MBAS removal rate was found to be pH independent. It is likely that hydroxyl radicals from the decomposition of ozone are also consumed by reacting with inorganic and organic compounds in wastewater different than MBAS. This especially applies to carbonate/bicarbonate ions,21 which were found to be highly concentrated in this type of wastewater (see alkalinity in Table 1). Regardless of the pH, after 1 h of ozonation with an ozone dose of approximately 300 mg per liter of wastewater, COD and TOC never decreased more than 33% and 16%, respectively. This suggests the presence of an ozone resistant fraction of organic matter after ozonation in both types of wastewater, resulting in residual COD and TOC. Nevertheless, ozonation also resulted in reduced COD/TOC and increased BOD5/COD ratios in all cases, as can be seen in Table 3. This effect was also clear when treating pure NaDBS aqueous solutions. This fact indicates that ozonation did not result in total mineralization but in molecular structure changes from initial organic matter to more biodegradable products. This is supported by literature data that report the transformation of organic matter to more readily biodegradable compounds.32 Contributions of Molecular and Radical Methods To the Decomposition of NaDBS. Of interest is the determination of the contributions of direct and free radical ozonation methods to the oxidation of a given pollutant. Assuming constant values for the ozone and hydroxyl radical concentrations in water, which is rather true after an initial lag period, t0, eq 3 becomes
-rNaDBS ) -
dCNaDBS ) kCNaDBS dt
(12)
Figure 5. Determination of pseudo-first-order rate constant k, for the oxidation of NaDBS. Conditions: T ) 20 °C; CNaDBS0 ) 15 mg‚L-1; F ) 30 L‚h-1; CO3g0 ) 10 mg‚L-1. Symbols: 0, aqueous solution, pH ) 2; O, aqueous solution, pH ) 10; 4, synthetic wastewater pH ) 7.
where
k)
kd C + kHO•CHO• z O3
(13)
According to the integrated form of eq 12, Figure 5 shows a plot of ln(CNaDBS/CNaDBS0) against (t - t0) corresponding to data of experiments carried out at different pH values and with different types of water. Experimental data fitted well straight lines, confirming the validity of the proposed kinetic equation. On the basis of the slope of these lines, the pseudo-first-order reaction rate constant, k, was calculated (see values obtained in Table 4). Once k was known, CHO• was
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2219 Table 4. Percentage Contribution of Free Radical Reactions To Remove NaDBS by Ozonation type of water
pH
pure water pure water pure water pure water pure water synthetic wastewater real wastewater
2 7 10 2 7 7 7
NaDBS free radical CtBu, 10-4k, M s-1 removal, % reactions, %
0.01 0.01
2.30 4.78 5.12 2.47 3.19 4.50 1.07
24.0 60.0 82.0 23.3 24.7 60.7 25.0
6.7 76.5 90.9 23.9 55.4 99.5
calculated from eq 13 and contributions of direct and radical reactions were evaluated from both terms on the right-hand side of eq 13. From data of Table 4, free radical reactions are confirmed to be the main pathway to remove NaDBS from water, especially at high pH and in real wastewater. Conclusions From this study it may be concluded that the pH and the organic load of the aqueous solution are crucial parameters that influence NaDBS removal by ozonation. Alkaline conditions favor the ozonation rate of the LAS compound, since hydroxyl radical reactions are the main route of NaDBS degradation. Thus, the rate constant of the reaction between hydroxyl radical and NaDBS was found to be high (kHO• ) 1.6 × 1010 M-1‚s-1) whereas the corresponding rate constant of the direct reaction ozone-NaDBS was not very significant (kd ) 3.68 M-1‚s-1). The ozonation process, even at optimal operating conditions, can only partially remove surfactant and COD from municipal wastewater. However, ozonation alters the molecular structures of the organic matter initially present in the wastewater, leading to readily biodegradable products. This suggests the combination of ozonation with a biological treatment to remove the remaining organic matter as an advisable integrated process, as will be shown in the following paper.20 Acknowledgment The authors thank the CICYT of Spain (Project AMB97/339). Nomenclature ABS ) alkylbenzenesulfonate BOD5 ) biochemical oxygen demand C ) concentration (mol‚L-1) COD ) chemical oxygen demand D ) diffusivity in water (m2‚s-1) F ) gas flow rate (L‚h-1) HO• ) hydroxyl radical IC ) inorganic carbon (mg‚L-1) k ) rate constant defined by eq 13 (M-1 s-1) kd ) rate constant of reaction 1 (M-1 s-1) kHO• ) rate constant of reaction 2 (M-1 s-1) kL ) individual mass-transfer coefficient (m‚s-1) LAS ) linear alkylbenzenesulfonate M ) general compound that reacts with HO• MBASs ) methylene blue active substances NaDBS ) sodium dodecylbenzenesulfonate NH3-N ) ammonia concentration (mg‚L-1) O3 ) ozone P ) phenol t ) time (min) TC ) total carbon (mg‚L-1) TKN ) total Kjeldahl nitrogen (mg‚L-1)
TS ) total solids (mg‚L-1) TSS ) total suspended solids (mg‚L-1) z ) stoichiometric factor of reaction 1 (mole of O3 per mole of NaDBS) Super- and Subscripts f ) final value g ) gas i ) initial value tBu ) tert-butyl alcohol 0 ) initial value
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Received for review September 30, 1999 Revised manuscript received March 30, 2000 Accepted April 19, 2000 IE990721A
