Ind. Eng. Chem. Res. 1 9 9 6 , 3 4 ,
1607-1615
1607
Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 2. UV Radiation and Ozonation in the Presence of UV Radiation Fernando J. BeltrBn,*rt Gabriel Ovejero,*Juan F. Garcla-Araya,?and Javier Rivast Departamento de Ingenieria Quimica y Energktica, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Ingenieria Quimica, Universidad Complutense, 28040 Madrid, Spain
Direct photolysis with W radiation (254 nm) and oxidation with ozone combined with W radiation of three polynuclear aromatic hydrocarbons, fluorene, phenanthrene, and acenaphthene, has been studied. Quantum yields of the direct photolysis of the PAHs determined were 7.5 x 6.9 x and 52 x mol(photon)-l for fluorene, phenanthrene, and acenaphthene, respectively. Contributions of direct ozonation, direct photolysis, and radical oxidation have also been estimated for the oxidation with ozone combined with W radiation. Fluorene is oxidized by direct photolysis and radical reactions, phenanthrene through direct mechanisms, ozonation, and photolysis, and acenaphthene mainly by direct ozonation.
Introduction In the preceding paper (Beltran et al., 1995), the importance and impact of ozonation on the degradation of polynuclear aromatic hydrocarbons, PAHs, in water was extensively studied and documented. In addition to ozonation, removal of PAHs, like other water contaminants, can be carried out through advanced oxidation processes (AOP)(Glaze et al., 1987). The common factor of these AOP is the generation of hydroxyl radicals. From the mechanistic point of view, hydroxyl radicals and ozone react with PAHs in a similar way, through electrophilic aromatic substitution reactions (Bailey, 1982;HoignB, 1982). However, rate constants for the reactions between hydroxyl radicals and organics are generally much higher than those of the direct ozonation reactions (HoignB and Bader, 1983;Buxton et al., 1988). Despite the potential importance of AOP for the water treatment of PAHs there are few studies published on this matter (Glaze et al., 1992; Tuhkanen, 1994). Among the AOP the combination between ozone and W radiation deserves special consideration because of its possibilities. Thus, in this system up to three possible ways of removal can develop: direct photolysis, direct ozonation, and radical oxidation. Regarding the kinetics, in the case of W radiation, the quantum yield defined as the ratio of molecules of PAH decomposed photochemically per photon absorbed is the main kinetic parameter to determine. So far, there is only data obtained from sunlight photolysis of PAHs in water mainly a t the specific wavelengths of 313 and 366 nm. Thus, Mill et al. (1981)reported quantum yields of eight PAHs and heteroatom analogs corresponding to the conditions indicated. The compounds studied were, among others, benz[alanthracene, benzo[a]pyrene, and dibenzothiophene. Quantum yields were of similar order of magnitude (between 10-3 and mol(photon)-l) and varied strongly in sunlight depending on the season of the year. In any case, the low values of quantum yields indicated the importance of competitive reactions that deactivate the excited state intermediates. However, in the case of W radiation
* To whom all correspondence should be addressed. F a x number: 3424271304. Electronic mail:
[email protected]. t Universidad de Extremadura. t Universidad Complutense. Fax number: 34-1-3944114. Electronic mail:
[email protected]. 0888-588519512634-1607$09.00/0
Figure 1. Experimental annular photochemical tank: 1,sampling point; 2,porous plate; 3, temperature measuring device; 4, quartz well; 5,low-pressure Hg vapor lamp; 6, gas outlet; 7,cooling water flow inlet; 8, cooling water flow outlet.
of 254 nm wavelength, often used in laboratory experiments of photochemical degradation, no quantum yield data is found in the literature, except that for naphthalene (0.019mol(photon)-l) reported by Tuhkanen
(1994). Finally, as far as the combined o z o n e W radiation process is concerned, it is interesting to know the improvement of the oxidation rate with respect t o direct photolysis and ozonation, if any, and the importance of the oxidation pathways involved. This paper is the second part of a work aimed to study the oxidation of PAHs in water. The main objectives are the study of W radiation both alone and combined with ozone to enhance their synergistic effects and establish their kinetics. For so doing, kinetic parameters of the direct ozonation of PAHs (see previous paper, Beltran et al., 1995)and direct photolysis (quantum yields) determined in this work are considered as shown below.
Experimental Part The experiments were carried out in a 1000 cm3 glass annular jacketed photochemical reactor (see Figure 1).
0 1995 American Chemical Society
1608 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1.0
0.0
I
0
2
4
6 Time, min.
8
IO
0
12
Figure 2. U V radiation of PAHs. Experimental conditions: l o = 3.8 x einstein L-l s-l, pH 7,20 "C. Fluorene: 0,Cpmo = 6.2 x low6M. Phenanthrene; e, CPMO= 1.7 x M. AcenaphM. thene: M, CPWO= 1.7 x
The reactor was equipped with a central inlet at its top to place a quartz well. Inside this well a TNN15/32 Hanau low-pressure mercury vapor lamp was immersed. Another porous plate was placed at the reactor bottom below the quartz well to provide some agitation through bubbling during the photolysis experiments. In most of the cases, oxygen was used a t 25 L h-' for this purpose (results of actinometry experiments are given in the Appendix). Water from a thermostatic bath was circulated through the reactor jacket to ensure a constant temperature inside the reactor. The external surface of the reactor was wrapped with a black paper to avoid any leak of radiation from inside. The reactor was operated in the semibatch mode in the O m oxidation experiments. During the experiments, samples were withdrawn regularly from the reactors for analysis. The products used, the procedure, and the analytical methods for PAHs and ozone are given in the preceding paper (Beltran et al., 1995). Hydrogen peroxide, for actinometry experiments (see Appendix), was analyzed iodometrically or with a modified fluorescence method (Lazrus et al., 1985) for concentrations higher than 0.1 M and lower than M, respectively. Extinction coefficients and excitation and emission wavelengths for hydrogen peroxide were 19 M-l cm-l (Nicole et al., 1990) and 320 and 400 nm, respectively. Extinction coefficients (in M-' cm-') for PAHs at 254 nm determined experimentally with a U-2000 Hitachi spectrophotometer were 16 654,40 540,and 1333 for fluorene, phenanthrene, and acenaphthene, respectively.
Results and Discussion The variables studied were pH (2-12) and hydroxyl radical inhibitor concentration (bicarbonate ion: 0-0.01 M). Ozone partial pressure ( O W oxidation), temperature, and incident flux of radiation (see Appendix) were einstein L-l always at 500 Pa, 20 "C,and 3.8 x s-l, respectively. U V Photolysis Aqueous solutions of PAHs were exposed to 254 nm UV radiation. At pH 7 and 20 "C, reaction times needed for total removal of PAHs varied from 20 to 7 min for acenaphthene and phenanthrene, respectively, although photolysis rates are strongly dependent on the initial concentration of irradiated organics (Beltran et al.,
2
4
6
Time, min.
Figure 3. O m oxidation of PAHs. Experimental conditions: l o = 3.8 x einstein L-' s-l, P O 3 = 577 Pa (average value), 20 "C, pH 7. Fluorene: 0, Cpmo = 5.6 x M. Phenanthrene: CPAHO = 1.8 x M. Acenaphthene: B, CPAHO = 1.5 x M.
*,
1993) (see Figure 2). The influence of radical inhibitors and pH on the PAH degradation rate was then analyzed. The aqueous solution was again agitated by bubbling oxygen. Since the presence of oxygen can lead to the formation of singlet oxygen that can act as a photosensitizer (Haag and Hoign6, 1985) and activate the decomposition of PAHs, in some experiments agitation was provided with nitrogen. However, the rate of PAH degradation was not dependent on the type of feeding gas. Influence of Radical Inhibitors. Absorption of energy by organic compounds leads to excited states that can decompose into free radicals and trigger reaction mechanisms. The result is the destruction of the organics through a combination between radiation and radical oxidation. The presence of radical reactions involving the hydroxyl free radical was checked through analysis of W radiation experiments of PAHs in the absence and presence of radical inhibitors. In this case, bicarbonate ion, which is one of the most representative natural hydroxyl radical scavengers was used. However, concentrations of bicarbonate ion equal to or lower than M (a concentration even higher than those found in natural water environments) do not affect the degradation rate of fluorene. Similar results were obtained with the rest of the PAHs investigated. Therefore, it can be concluded that UV radiation of PAHs does not involve a hydroxyl free radical mechanism. Influence of pH. As happens with hydroxyl radical inhibitors, variation of pH (from 2 to 12) does not exert any influence on the photolysis rate of PAHs under the conditions applied here. This was expected because the PAHs studied showed no change of extinction coefficient with pH.
Ozone Combined with U V Radiation Our previous paper (Beltran et al., 1995) and others have shown that both ozonation and UV radiation independently lead to high degradation rates of PAHs (higher when ozone is used). Another possibility for oxidation is the simultaneous application of ozone and UV radiation to generate hydroxyl radicals (Glaze et al., 1987) and increase the rate of degradation. This advanced oxidation pathway was also investigated in this work. Figure 3 presents the variation of dimensionless remaining concentrations of the three PAHs
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1609 Table 1. Total Stoichiometric Ratio, Total and Local Reaction Factors, and Ozone Efficiencies during the Oxidation of PAHs Using Ozone Combined with UV Radiationa ~
0.6 -
0.4
. -
0.2
-
Ci'IrSLO
0.0
0
PH
p03, Pa
~t
7b 7 7c 12 2 7d 7e
496 496 496 542 544 517 511 511
29.9 34.0 39.7 48.3 46.8 42.7 69.5 13.0
7f
2
4
6
Et 8.6 8.5 8.4 8.1 8.2 7.5 6.0 7.0
E1 0.37 0.37 0.36 0.35 0.35 0.35 0.09 1.05
~~
qt, %
11,
98.7 97.4 96.7 93.7 93.7 86.2 63.0 80.6
4.2 4.2 3.9 4.0 4.0 4.0 1.0 12.4
%
a t = 2 min, oxidation of fluorene unless indicated. t = 1 min. t = 2.5 min. Cid = 0.01 M (total bicarbonate ion). e Phenanthrene. f Acenaphthene.
Time, min.
Figure 4. O m oxidation of fluorene: influence of hydroxyl radical inhibitors. Experimental conditions: inhibitor, bicarbonate ion; lo = 3.8 x einstein L-' s-l; CPAHO = 5.6 x low6M;PO3 = 502 Pa (average values); 20 "C; pH 7. Chh, M; 0,0;W, e, 10-2. 1.o
0.8
0.6
LY ICY8 0.4
0.2
0.0
0
2
4
6
Time, min.
Figure 5. O m oxidation of fluorene: influence of pH. Experimental conditions: lo = 3.8 x einstein L-' s-', Cpmo = 5.5 x M,PO3 = 574 Pa (average values), 20 "C. p H W, 2;A, 7; 0,12.
studied with time corresponding to experiments of ozonation combined with U V radiation. As can be seen, the degradation rates are important, being slightly higher than those of ozonation alone. With this type of oxidation, disappearance of PAHs is accomplished in less than 6 min. Another important point to highlight is the absence of dissolved ozone during these experiments, which is probably due to the contribution of the direct photolysis of ozone (Peyton and Glaze, 1988). Influence of Radical Inhibitors. The influence of this variable on the degradation rate of fluorene is shown in Figure 4. As can be seen, the oxidation of fluorene is decreased in the presence of high bicarbonate ion concentration (0.01 M). This suggests a strong competition of hydroxyl radical reactions since a M concentration of bicarbonate ion does not affect the oxidation rate of fluorene. Regarding the oxidation of phenanthrene and acenaphthene, the presence of bicarbonate ion does not affect their oxidation rates. Therefore, the contribution of radical reactions to the degradation of PAHs is only important during the oxidation of fluorene. For the rest of the PAHs, the radical pathway is negligible compared to the action of direct, photolysis and ozonation, reactions. Influence of pH. Figure 5 shows the variation of the dimensionless remaining concentration of fluorene with time corresponding to experiments of ozonefUV
radiation carried out at different pH values. It can be observed that an increase of pH from 2 to 7 leads to an increase of the oxidation rate of fluorene. A further increase of pH, up to 12, reduces the oxidation rate to the levels obtained at pH 2. It should be noted that the increase of pH could yield opposing effects as far as the production of hydroxyl radicals is concerned. Thus, the increase of pH leads, on one hand, to an increase of the hydroxyl ion catalyzed decomposition of ozone into hydroxyl radicals (Staehelin and HoignB, 1985) and, on the other hand, to a decrease of the ozone available to undergo direct photolysis and product hydrogen peroxide and eventually more hydroxyl radicals, which is particularly important at pH 12. It is likely that a combination of both phenomena is responsible of the effect of pH. Given the importance of this variable more studies are needed to clarify its effect, especially under basic conditions.
Stoichiometry and Absorption Kinetic Regime Parameters like the stoichiometricratio, the total and local efficiency of ozonation, and the total and local reaction factors (see preceding paper, Beltran et al., 1995) were also calculated for the experiments using ozone/LTVradiation. Table 1shows some of the results obtained. It can be seen that consumption of ozone per mole of PAH consumed is now much higher than during ozonation at similar conditions (see Table 3 of preceding paper, Beltran et al., 19951, which suggests an important consumption of ozone with intermediate compounds or by direct photolysis (Peyton and Glaze, 1988). This is supported by the higher total efficiencies of ozonation during the ozone/UV radiation process compared to those of ozonation alone (see Tables 1, this paper, and 3, preceding paper, Beltran et al., 1995). Finally, the total reaction factors are higher than those obtained from ozonation alone. This is undoubtedly due to the complexity of this advanced oxidation system in which ozone, while being absorbed, is at least decomposed by direct photolysis and direct reactions with PAHs, intermediates, and hydrogen peroxide formed during its own photolysis (Peyton and Glaze, 1988). Values of Et and E1 indicate a fast and moderate kinetic regime for the total absorption of ozone and its local gas-liquid reaction with PAHs, respectively. W Radiation Kinetics. When a compound, like some PAH,absorbs radiation, it passes to a higher state of energy, an excited state, PAH", where it decomposes through different routes (fluorescence,internal conversion, intersystem crossing, photochemical reaction, etc.)
1610 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995
(Owen, 1971). This can be simplified with the following mechanism:
PAHkPAH"
(1)
PAH"
(2)
-
-
PAH
PAH* products
(3)
According to this mechanism, in an elementary volume of reaction, dV, the disappearance rate of PAH due to photolysis, (rpAH)dV, is given by the following equation: (rPAH)dV
= ItlFPAH1a
- Iz2cPAH*
(4) 0
2
4
6
X
10
12
where I a is the total flow of absorbed radiation and FPAH Time, min is the fraction of radiation that the PAH absorbs, Figure 6. Verification of eq 11. Determination of the quantum defined as follows: yield of fluorene, phenanthrene, and acenaphthene corresponding to their U V radiation at 254 nm. Conditions: l o = 3.8 x einstein L-' e.-1, pH 7,20"C, 0,CFO= 6.2 x M,H, CPHO= 1.7 x 10-6 M, A, cAo = 1.7 x 10-5 M.
kl and
are the rate constants for steps 1 and 2, respectively, the latter involving all pathways but the photochemical reaction, CPAH* is the concentration of the PAH in the excited state, and ci and Ci are the extinction coefficient and concentration of any species i that absorbs radiation. By applying the stationary state situation to CPAH* eq 4 becomes k2
(rPAH)dV
= @PAHFPAH1a
= kl
k3
(7)
k3 being the rate
constant of the photochemical reaction (3). In the photochemical reactor used, the disappearance rate of the PAH due to W radiation will be
W radiation experiments on PAHs were carried out in the presence of bubbles of oxygen or nitrogen to provide agitation which made the system heterogeneous. Due to possible effects of refraction, photochemical reactor models that express the flow of absorbed radiation as a function of the geometry of the reactor and the absorbance of the solution, CriCi, cannot be applied (Jacob and Dranoff, 1970) unless some correction factor is introduced. Therefore, in this work the flow of absorbed radiation was expressed as a function of the Lambert-Beer law as follows:
where IO is the intensity of the incident radiation and L is the effective path of the radiation through the reactor (see Appendix). Finally, according to eqs 6-9 the W photolysis rate of PAH becomes % ' AH
Cpm0 - C,
(6)
where (PPAHis the quantum yield of the PAH,that is, the number of moles of PAH consumed per photon absorbed, defined as follows: QPAH
This equation is extensively used in photochemical kinetic studies of environmental processes (Leifer, 1988). By assuming the initial PAH t o be the main W radiation absorber, that is, FPAH= 1, eq 10 can be integrated analytically to yield
- QpAHFpAHIo[l - exp(-2.303L&Ci)l (10)
+
1 - exp(-aC,,) = I0QpAHt 1 - exp(-aCpAH) (11)
where
a = 2.303Lr,
(12)
A plot of the left-hand side of eq 11versus time should give a straight line with a slope equal to IOQPAH. As IO and L are known from actinometry (see Appendix) the quantum yield can be calculated. Figure 6 presents such a plot corresponding to direct photolysis experiments of the PAHs studied: fluorene, phenanthrene, and acenaphthene. Quantum yield data in mol(photon)-' obtained from triplicate experiments were (7.5 f 0.3) x (6.9 f 0.5) x and (52 f 2) x for fluorene, phenanthrene, and acenaphthene, respectively. These parameters are used in the following section to calculate the contribution of direct photolysis in the oxidation of PAHs with the combination ozone/ W radiation. Ozone/W Advanced Oxidation Kinetics. The oxidation of the PAHs treated in this work by the combined effect of ozone and W radiation develops through two or three pathways depending on the nature of the hydrocarbon. Thus, fluorene is oxidized by direct photolysis, direct ozonation, and radical attack while phenanthrene and acenaphthene are eliminated only through direct ways, photolysis and ozonation (see preceding paper, B e l t r b et al., 1995). Consequently, the kinetics of these oxidations has been treated separately. Oxidation of Fluorene. The mechanisms of advanced oxidations involving ozone, hydrogen peroxide, andor W radiation are very similar (Glaze et al., 1992). In fact these mechanisms hardly differ in some reactions able to initiate radical chains. The combination of ozone and W radiation yields the most complex mechanism among this type of advanced oxidation since hydrogen
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1611 peroxide is formed and, hence, its reactions with UV radiation and ozone have to be considered. Thus, initiation reactions in this system ( U V / o 3 oxidation) are Direct photolysis of ozone (Peyton and Glaze, 1988)
O3
+ H,O -!% H,O, + 0,
(13)
were under the detection limit (in the case of ozone, c 0 3 I M). However, at least the concentration of ozone could be estimated from an ozone mass balance applied to the start of ozonation. As shown later, this is enough to determine KRF and COH. The ozone mass balance is given by
direct photolysis of hydrogen peroxide (Baxendale and Wilson, 1957)
H,O, -!% 20H'
(14)
direct action between ozone and the ionic form of hydrogen peroxide (Staehelin and Hoign6, 1982)
O3
+ H0,-
k,, = 2.8
x 106 M-15-1
HO,'
+ 0;-
(15)
and direction action between ozone and the hydroxyl ion (Staehelin and Hoigne, 1985)
O3
+ OH-
k, = 70 M-15-1
HO,'
+ 0;-
(16)
Hydroperoxide, ozonide, and superoxide ion radicals, HOz', 03*,and O$-, respectively, trigger a radical mechanism which has been described in detail in previous works (Beltran et al., 1994). The rate of oxidation of fluorene according to this mechanism and reactions 13-16 is given by the following equation:
which expresses the three possible contributions to the F elimination rate of fluorene. The parameter ~ R represents the rate constant of the radical pathway given by eq 17 of the preceding paper (Beltran et al., 1995) with the concentration of hydroxyl radicals expressed now as follows:
where the terms on the right side represent the rates of accumulation and consumption of ozone with fluorene and intermediates, by direct photolysis (reaction 131, initiation reactions (reactions 15 and 161, and with radicals. According to these reactions and the radical mechanism shown in a previous paper (Beltran et al., 1994) the latter term, m,becomes
~i1c03CH0z-
+ 2ki2C03C0HkT
where k 0 ~ 0 3is the rate constant of the reaction between ozone and hydroxyl radicals (2.9 x lo9 M-' s-l, according to Staehelin and HoignB, 1985), C H 2 0 2 t is the total concentration of hydrogen peroxide, and k~ is defined as follows:
where (PH is the primary quantum yield of photolysis of hydrogen peroxide (reaction 141, 0.5 mol(photon)-l (Baxendale and Wilson, 1957), and FHis the fraction of radiation that hydrogen peroxide absorbs (see eq 5). In eq 18 the terms in its numerator represent the hydroxyl radical initiation rates due to reactions 15,16, and 14, respectively, whose importance varies depending on the pH and the intensity of the incident radiation, among other factors. The denominator of eq 18, on the other hand, represents the scavenging term of hydroxyl radicals, defined as follows:
where 2.7 x lo7 and 7.5 x lo9 are the rate constants of the reactions between the hydroxyl radical and the nonionic and ionic forms of hydrogen peroxide, respectively, in M-' s-' (Christensen et al., 1982) and PKH (=11.8) corresponds to its equilibrium in water. Equation 20 can be simplified at the start of ozonation. Thus, for this case the accumulation and consumption rates due to intermediates (first and third terms of the righthand side of eq 20 can be suppressed). In addition, other terms, depending on the pH value, can be considered negligible. In order to estimate the photolysis rate term of ozone, @&'oda, the quantum yield of ozone was experimentally determined. Thus, an ozone aqueous solution was irradiated and the remaining concentration of ozone followed with time. The photolysis rate of ozone is given by an equation similar to eq 10. As the exponential term, 2.303L~o3C03is higher than 2.0 (extinction coefficient of ozone, € 0 3 , is 3300 M-' cm-l, Gordon et al., 1988) this equation reduces to the following one:
where kow and Ci are the rate constant of the reaction between the hydroxyl radical and any species i present in solution and its concentration, respectively. In this case, eq 19 can be approximated to ~OHFCFO as explained in a previous work (Beltran et al., 1994). In order to obtain kw and COH,the concentrations of ozone and hydrogen peroxide should be known. Unfortunately, the experimental concentrations of these species could not be determined because of the presence of some interferences (in the case of hydrogen peroxide) or because they
This is a zero-order kinetic equation, so that a plot of C o 3 l C 0 3 0 versus time should give a straight line, the intercept and slope being 1and @ 0 d d c 0 3 0 , respectively. Figure 7, prepared from results of direct photolysis of ozone confirms eq 23 for the first 40 s of reaction (ozone is consumed in less than 1 min). It is obvious that for times higher than 40 s the exponential term in eq 10 becomes lower than 2.0 and eq 23 is no longer appropriate. From the slope of the line plotted in Figure 7, (PO3
(18)
1612 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
Note that in the presence of bicarbonate ion, the scavenging term, eq 19, contains the contribution of this scavenger:
4.2 x 10sC,032- (27)
\
0.4
0.2
\
,
0.0
2s
0
L y - 4
-
50
75
Time, s.
Figure 7. Determination of the ozone quantum yield at 254 nm in water. Verification of zero-order kinetics for the ozone photolysis rate, eq 23. Conditions: 10= 3.8 x einstein L-l s-l, M. pH 7, 20 "C, c030 = 1.15 x
was found t o be 0.64 mol(photon)-l, a value very close to 0.62 reported by Taube (1957). The contribution from the photolysis rate of ozone seems to be the main term for the consumption of ozone. For example, by assuming a concentration of ozone and M and a 5 x M hydrogen peroxide of 1 x fluorene concentration, the main term of the right side of eq 20 corresponds t o the ozone photolysis rate, at pH 2 and 7. At pH 12, however, all terms except that of direct ozonation can have similar importance. Regarding the photolysis rate, it should be noted that the main absorber is fluorene (FF= 0.962 for the conditions given above) so that the photolysis rate of ozone can be simplified to E03C03
@O$O$a
= @03=
10[1
- exp(-2.303kFCF)] (24)
Consequently, at pH 2 and 7 the concentration of ozone can be expressed as follows:
At pH 12, on the other hand, the concentration of hydrogen peroxide is also required. This can be deduced together with the ozone concentration from eq 20 and the hydrogen peroxide mass balance given below: dCH,02t -- @O$O$a
dt
- @H*H1a
+ ki1CH0,-C03 +
where the subscript H refers to hydrogen peroxide. This equation is simplified a t high pH since the accumulation rate of hydrogen peroxide (dC~202Jdt)is negligible and hence the formation and decomposition rates coincide. Once the concentration of ozone (and hydrogen peroxide at pH 12) is known from eq 25 (and eq 26 for hydrogen peroxide), the mass balance equation of fluorene, eq 17, allows the determination of kRF. Table 2 presents the results obtained. On the other hand, (see preceding paper, assuming kom is 5 x lo9 M-l Beltran et al., 1995) the concentration of hydroxyl radicals can be estimated (see eq 17 of preceding paper).
where 1.5 x lo7 and 4.2 x lo8 are the rate constants of the reactions between the hydroxyl radical and bicarbonate and carbonate ions, respectively, in M-l (Weeks and Rabani, 1966). As observed from Table 2, values of kRF and COHare somewhat lower than those calculated for the case of ozonation alone (see Table 5 of preceding paper) except a t pH 12 which can be interpreted as an increasing importance of the radical pathway with pH when photolysis is also present. However, the contribution of direct photolysis is similar at pH 7 and 12. In the presence of 0.01 M total bicarbonate ion both parameters, km and COH,have the lowest values. This is expected because of the inhibition of radical reactions. Finally, the contributions of direct ozonation and direct photolysis have also been estimated by using the following equations: YO3
=
kFc03cF rF
x 100
and Y w = T x 100
As also seen from Table 2, the contribution of direct photolysis varies from about 25% (pH 7) to 55-60% (pH 2 and 12) while direct ozonation is practically negligible (
