Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 4. Ozone

Ind. Eng. Chem. Res. 1996, 35, 891-898

891

Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 4. Ozone Combined with Hydrogen Peroxide Fernando J. Beltra´ n,*,† Gabriel Ovejero,‡ and Javier Rivas†,§ Departamento de Ingenierıá Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Ingenierıá Quı´mica, Universidad Complutense, 28040 Madrid, Spain

Three polynuclear aromatic hydrocarbons, fluorene, phenanthrene, and acenaphthene, have been treated in water with ozone combined with hydrogen peroxide. The effect of hydrogen peroxide concentration, pH, and bicarbonate ions has been investigated. The process goes through direct and radical reactions in the case of fluorene and phenanthrene oxidation, while acenapthene is removed exclusively by direct ozonation. At concentrations of hydrogen peroxide higher than 10-5 M, ozone mass transfer controls the process rate, regardless of pH. In any case, however, the presence of hydrogen peroxide does not improve the oxidation rate compared to ozonation alone due to the importance of the direct reactions. Intermediate compounds identified during oxidation with ozone alone and combined with UV radiation or hydrogen peroxide are similar and justify the high consumption of ozone in these processes. Introduction As shown in previous works (Beltrań et al., 1995ac), the hydroxyl radical mediated oxidations, also called advanced oxidation technologies, are widely used in the treatment of micropollutants in water. Specifically, polynuclear aromatic hydrocarbons, PAHs, present high reactivity with these short-life species, since the rate constants of their reactions vary between 5 × 109 and 5 × 1010 M-1 s-1 (Buxton et al., 1988; Beltrań et al., 1995c). In addition to the use of ozone and UV radiation combined with ozone or hydrogen peroxide, another important oxidation system is the combination of these two oxidants (Glaze et al., 1987). This O3/H2O2 process has also been commercialized with the name of perozone (Wable et al., 1993) and used to remove different organics from synthetic water solutions and surface or groundwaters (Brunet et al., 1984; Glaze and Kang, 1989a,b). Brunet et al. (1984) were among the first who studied the effectiveness of this system. They reported on high oxidation rates of oxalic acid, a compound often formed during ozonation of aromatic molecules in water. Duguet et al. (1984) also reported the beneficial effect of the perozone system to remove THM precursors from surface waters. Paillard et al. (1988) established that 0.5 mol of hydrogen peroxide/mol of ozone is the optimum oxidation ratio to remove oxalic acid in a batch reactor at pH 7.5. Similar results were achieved by Glaze and Kang (1989a,b) with the oxidation of tetrachloroethylene in a semibatch reactor. These authors present a complete mechanism and kinetic study of this oxidation system applied to organochlorine volatile compounds in water. Pesticides are also compounds very treated with the O3/H2O2 process. Thus, Allemane (1994) presents a rigorous study of the oxidation of several herbicides like atrazine, simazine, aldicarbe, mecoprope, and isoproturon with the main emphasis on identification byproducts. In a recent work, Beltrań et al. (1994) applied the mechanism reported by Glaze and Kang (1989a) to determine the contribution of radical * To whom correspondence should be addressed. Fax number: 34-24-271304. Electronic mail: [email protected]. † Universidad de Extremadura. ‡ Universidad Complutense. § Fax number: 34-1-3944114. Electronic mail: CALLES@ QUIM.UCM.ES.

0888-5885/96/2635-0891$12.00/0

reactions in the oxidation of mecoprope and the rate constant of its reaction with the hydroxyl radical. As far as our knowledge is concerned, there is only one work reported in the literature dealing with the oxidation of PAHs with O3/H2O2. Thus, Trapido et al. (1994) presented some comparative studies of the removal rates of phenanthrene with ozone alone and combined with UV radiation or hydrogen peroxide. From their results, it is concluded that phenanthrene is eliminated mainly through direct reactions with ozone, which agrees with our previous results on PAH ozonation and O3/UV oxidation (Beltrań et al., 1995a,b). Since the usefulness of the O3/H2O2 oxidation is well accepted, it was thought convenient to apply this system to the oxidation of fluorene, phenanthrene, and acenaphthene, PAHs treated in previous parts of this work, with the following objectives: analysis of the influence of variables, identification of byproducts, study of the kinetics of oxidation, and, finally, comparison with the other reported oxidation technologies. Experimental Part The oxidation runs were carried out in a 5000 cm3 standard agitated tank already described in detail (Beltrań et al., 1995a). Reagents, procedure of oxidation, and analytical methods are also reported in previous parts of this work (Beltrań et al., 1995a-c). Results and Discussion Influence of Variables. In the line followed in previous parts of this series, the effect of three variables was first investigated: hydrogen peroxide concentration (0-0.17 M), pH (2-12), and total bicarbonate ion concentration (0-10-2 M). Influence of Hydrogen Peroxide Concentration. Figures 1 and 2 show the variation of the remaining fraction of PAH concentration with time for experiments of ozonation and O3/H2O2 oxidation. It can be observed that concentrations of hydrogen peroxide lower than 0.01 M do not yield significant variations of PAH degradation rate compared to those obtained from ozonation alone and regardless of the nature of PAH. However, when the oxidation is carried out in the presence of higher hydrogen peroxide concentrations, the process rate is inhibited. The inhibition is especially © 1996 American Chemical Society

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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996

Figure 1. O3/H2O2 oxidation of phenanthrene and acenaphthene. Comparison with ozonation alone. Conditions: T ) 20 °C, pH ) 7, PO3 ) 500 Pa (average value). Phenanthrene oxidation: (9) CPH0 ) 1.8 × 10-6 M; CH2O2 ) 0; (0) CPH0 ) 5.1 × 10-6 M; CH2O2 ) 10-3 M. Acenaphthene oxidation: ([) CA0 ) 1.4 × 10-5 M; CH2O2 ) 0; (]) CA0 ) 1.7 × 10-5 M; CH2O2 ) 10-3 M.

Figure 2. O3/H2O2 oxidation of fluorene. Influence of hydrogen peroxide concentration. Conditions: CF0 ) 5.5 × 10-5 M (average value); T ) 20 °C, pH ) 7, PO3 ) 500 Pa (average value); CH2O2, M: (0) 0; (2) 10-5; (*) 10-4; (4) 10-3; (9) 10-2; ([) 0.17.

strong at 0.17 M hydrogen peroxide concentration as observed in Figure 2. These results are similar to those found when other micropollutants are oxidized with O3/ H2O2 as in the case of mecoprope (Beltrań et al., 1994). In the first part of this series we showed that at pH 7 the ozonation of fluorene mainly develops through radical reactions possibly initiated by the hydroxyl ion catalyzed decomposition of ozone. Therefore, the oxidation of fluorene with O3/H2O2 should also be due to radicals since the main free-radical initiation reaction, reaction between ozone and the ionic form of hydrogen peroxide, is faster than the initiation reaction of ozonation alone (the ratio of these two initiation rates at pH 7 varies between 6285.7 and 62857 for a 10-3 and 10-2 M hydrogen peroxide concentration, respectively). As a consequence one should expect an increase of fluorene disappearance rate when hydrogen peroxide is applied combined with ozone. A possible explanation of the results of Figure 2 could be due to the high consumption of hydroxyl radicals by hydrogen peroxide, avoiding in this way the oxidation of fluorene. However, the reaction between hydrogen peroxide and hydroxyl radicals gives rise to the superoxide ion radical, O2-, which in the presence of ozone eventually regenerates the hydroxyl radical (Staehelin and Hoigne´, 1985; Glaze and Kang, 1989a). Thus, hydrogen peroxide does not act as a scavenger of these radicals during the O3/H2O2 oxidation system. Paillard et al. (1988) and Glaze and

Figure 3. O3/H2O2 oxidation of fluorene. Influence of pH. Conditions: CF0 ) 7.7 × 10-6 M (average value); T ) 20 °C, pH ) 7, PO3 ) 500 Pa (average value); CH2O2 ) 10-3 (when added); pH and type of oxidation: pH 2 and ([) O3; (9) O3/H2O2; pH 7 and (O) O3; (]) O3/H2O2; pH 12 and (b) O3; (0) O3/H2O2.

Figure 4. O3/H2O2 oxidation of fluorene. Influence of bicarbonate ion concentration. Conditions: CF0 ) 8.7 × 10-6 M (average value); T ) 20 °C, pH ) 7, PO3 ) 500 Pa (average value); CH2O2 ) 10-3; CHCO3-, M: (0) 0; (9) 10-3; ([) 10-2.

Kang (1989a) showed that there is an optimum hydrogen peroxide concentration above which the rate of micropollutant disappearance decreases with the increase of hydrogen peroxide concentration which is due to some competition reactions involving ozone, hydrogen peroxide, and the micropollutant. On the other hand, the oxidation rate of micropollutant is also strongly affected by the value of the rate constant of its direct reaction with ozone. Thus, during the oxidation of tetrachloroethylene (Glaze and Kang, 1989a,b), which does not react with ozone directly, low concentrations of hydrogen peroxide (up to the optimum value) lead to improvements of the oxidation rate, while in the case of mecoprope, which does react with ozone directly (the rate constant at pH 7 and 20 °C is 101 M-1 s-1, Beltrań et al., 1994), the presence of low concentrations of hydrogen peroxide does not yield any difference with respect to that of ozonation alone as also happens with the O3/H2O2 oxidation of PAHs (see Figures 1 and 2). Note that the direct rate constants of the reactions between ozone and fluorene, phenanthrene, and acenaphthene are 29, 2413, and 1.08 × 105 M-1 s-1, respectively (Beltrań et al., 1995a). In the case of acenaphthene and phenanthrene oxidations, the null effect of hydrogen peroxide is also more expected since their direct reactions with ozone are faster.

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 893 Table 1. Products Found or Identified by GC/MS in the Ozonation and O3/UV and O3/H2O2 Oxidations of PAHs C#a

compound

Rtb

fluorene

phenanthrene

acenaphthene

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

toluene oxalic acid, dimethyl ester (1-methylethyl)benzene nonanal nonanoic acid, methyl estera 3,4-dihydro-2H-1-benzopyran-2-one 1(3H)-isobenzofuranone benzofuran acenaphthene dibenzofuran unknown fluorene unknownc unknownc 2-methyl-1,1′-biphenyl unknownc unknown c 4-ethoxy 3-methoxyphenol unknown 9,10-dihydroanthracene unknownc 9,10-dihydrophenanthrene unknownc 4-methyl-1-naphthalenol 9-fluoren-9-one 9-fluoren-9-ol unknownc unknown 2-methyl-1-naphthalenol 9-methylene-9-fluorene phenanthrene o-hydroxybiphenyl unknown unknown unknownc unknown unknown unknown 2-dibenzofuranol unknown unknown 1,1’-biphenyl 4-carboxaldehyde unknown unknown unknown 1H,3H-naphtho(1,8-c,d)pyran-1-one unknown unknown 4-methyldibenzofuran xanthone 2-hydroxy-9-fluorenone unknown 9,10-anthracenedione 9-phenanthrenol unknownc unknownc

3.1 4.1 5.8 9.5 11.5 14.5 14.5 16.5 16.7 17.2 18.1 18.3 18.4 18.6 18.7 19.0 19.2 19.5 19.6 19.7 19.8 19.9 20.4 20.5 20.6 20.8 20.8 20.9 21.0 21.2 21.3 21.6 21.9 22.0 22.2 22.3 22.4 22.5 22.6 22.7 22.7 22.9 23.0 23.1 23.1 23.2 23.2 23.4 23.6 23.9 23.9 24.1 24.2 27.8 28.2 28.6

O3, O3/H2O2 O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2

O3/H2O2 O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2

O3/H2O2 O3/UV O3, O3/UV, O3/H2O2 O3, O3/H2O2 O3/UV O3/UV

a

O3/H2O2 O3, O3/UV, O3/H2O2 O3/UV, O3/H2O210 O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2

O3, O3/H2O2 O3/UV O3/H2O2

O3, O3/UV, O3/H2O2 O3, O3/UV O3/H2O2 O3/UV O3/UV O3, O3/H2O2 O3, O3/H2O2 O3, O3/UV, O3/H2O2 O3/UV O3/UV O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2

O3, O3/UV O3/UV O3, O3/UV, O3/H2O2 O3/UV

O3, O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2 O3/UV O3/UV O3/H2O2 O3/UV O3/UV O3/UV O3 O3, O3/H2O2

O3, O3/UV, O3/H2O2 O3/H2O2 O3/UV

O3/UV O3/UV O3/UV O3/UV, O3/H2O2 O3/UV O3, O3/UV, O3/H2O2 O3/UV, O3/H2O2 O3, O3/UV, O3/H2O2 O3/UV O3 O3 O3, O3/UV, O3/H2O2 O3 O3/UV, O3/H2O2 O3/UV

Compound number. b Retention time, min. c From derivatization.

Influence of pH. Figure 3 presents the influence of pH on the O3/H2O2 oxidation of fluorene. As can be observed, the increase of pH exerts different effects on the rate of oxidation. Thus, the effect is positive (leading to higher oxidation rates) when pH increases from 2 to 7, while it is negative (leading to lower oxidation rates) when pH is further increased up to 12. Also, in Figure 3 one can observe that oxidation rates are similar in the presence and absence of hydrogen peroxide at pHs 2 and 7, but at pH 12 the presence of hydrogen peroxide strongly inhibits the oxidation rate. The results corresponding to neutral and basic pH values can be interpreted on the basis of the mechanism reported elsewhere (Glaze and Kang, 1989a,b; Beltrań et al., 1994) and are presented in the appendix. However, at acid pH the mechanism of oxidation likely

involves other initiation steps for radical formation, as suggested by Sehensted et al. (1991), which could lead to different kinetics. It should be noted that the effect of pH shown in Figure 3 was also observed when hydrogen peroxide was used in combination with UV radiation (Beltrań et al., 1995c). Influence of Hydroxyl Radical Inhibitors. The main feature of advanced oxidation technologies is the formation of hydroxyl radical to enhance the oxidation rate of pollutants. Thus, a few experiments of O3/H2O2 oxidation were carried out in the presence of bicarbonate ion, a strong hydroxyl radical inhibitor. Figure 4 presents the variation of the remaining fraction of fluorene with time corresponding to O3/H2O2 oxidation experiments in the absence and presence of bicarbonate ions. It can be seen from Figure 4 that a 10-3 M total

894


bicarbonate ion concentration does not affect the oxidation rate, but when the concentration is increased 1 order of magnitude, the process rate is inhibited to some extent. According to this, it could be deduced that the O3/H2O2 oxidation of fluorene is possibly a hydroxyl radical process, at least at pH g 7, although more data are needed to confirm this. This effect was also observed during ozonation alone (with tert-butanol, another hydroxyl radical scavenger, Beltrań et al., 1995a) and during the O3/UV oxidation (Beltrań et al., 1995b). However, in the presence of UV radiation and hydrogen peroxide, the effect of bicarbonate ion was negligible (Beltrań et al.,1995c). Identification of Intermediates. Qualitative analysis of intermediate compounds of PAH oxidation with ozone alone and combined with hydrogen peroxide or UV radiation was carried out. Results are shown in Table 1 for compounds identified or simply found and in Table 2 for their corresponding mass spectrum data and molecular weight. Compounds identified are similar to those presented in the preceding paper (Beltrań et al., 1995c). Some of these compounds are presumably very reactive toward ozone and hydroxyl radicals due to the presence of nucleophilic positions in their molecules. This can justify the high consumption of ozone and that these products eventually disappear from aqueous solution. As also observed with UV radiation and UV/H2O2 oxidation, final products are likely carboxylic acids (however, after 30 min of oxidation only oxalic acid was identified among them) due to the acid pH of the final solution (about 4) and the presence of many unidentified peaks in the GC analysis of derivatized samples. Stoichiometry and Kinetic Regime of PAH Ozonation with the O3/H2O2 System. Table 3 gives the results of stoichiometry, zt, total and local efficiency of ozonation, ηt and ηl, respectively, and total and local reaction factors, Et and El, respectively, parameters defined in a previous paper (Beltrań et al., 1995a) and also calculated during the oxidation of PAHs with ozone (Beltrań et al., 1995a) and the O3/UV radiation system (Beltrań et al., 1995b). It is observed from Table 3 that the oxidation of PAHs with the O3/H2O2 system develops with consumptions of ozone higher than those of ozonation alone and similar to those found with the O3/ UV radiation system (see also Tables 3 and 1 of preceding papers, Beltrań et al., 1995a and b, respectively). Since the presence of hydrogen peroxide does not improve the oxidation rate of PAHs, the consumption of ozone should be due to competitive reactions with hydrogen peroxide and intermediates. The high and low total and local ozone efficiencies, respectively, show the complexity of this reacting system and allow us to suspect the formation of ozone consuming intermediates (see Table 1). Thus, all ozone fed into the system is nearly consumed (ηt > 90%) but that exclusively reacted with the PAH is very low (ηl < 10%, except when a 10-2 M bicarbonate ion concentration is present). Another exception to these figures results from the acenaphthene ozonation with an ozone local efficiency of about 60%. The ozone absorption rate, on the other hand, is at least accompanied by two parallel chemical reactions in water since the start of oxidation (reactions with PAH and hydrogen peroxide). This process seems to follow a slow kinetic regime, which coincides with the results of ozonation alone but differs from those of O3/UV radiation. It should be noted, however, that experi-

Table 2. Mass Spectra of Products Identified or Found by GC/MS in the Ozonation and O3/UV and O3/H2O2 Oxidations of PAHs C#a

mol wt

m/z(relative abundance)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

92 118 120 142 172 148 134 118 154 168

92(100), 65(25.8), 51(16.9), 39(26.6) 118(2.2), 59(100), 45(34.3), 29(63.6) 120(26.9), 105(100), 97(5.8), 84(6.6) 114(5.2), 98(25.1), 82(27.3), 57(100) 141(5.5), 129(8.5), 87(39.8), 74(100) 148(100), 120(40.0), 91(39.1), 65(18.6) 134(35.3), 105(100), 77(48.2), 50(13.1) 118(89.3), 90(100), 63(25.4), 50(9.3) 154(100), 126(4.7), 76(19.1), 63(9.1) 168(100), 139(32.5), 113(4.0), 87(2.8) 132(100), 104(58.2), 78(15.6), 51(21.2) 166(100), 163(15.6), 139(7.8), 82(11.7) 191(22.7), 177(21.2), 159(100), 105(45.2) 188(18.2), 157(100), 132(20.6), 115(17.2) 168(100), 152(26.5), 139(11.5), 115(10.6) 174(56.0), 159(100), 146(12.3), 131(25.9) 174(48.4), 159(100), 146(42.2), 131(27.4) 168(99.3), 140(100), 113(9.1), 70(19.2) 170, 152, 141, 115 180(70.2), 179(100), 178(58.0), 89(29.8) 190(48.5), 175(25.4), 159(100), 132(40.6) 180(100), 179(76.8), 178(50.2), 152(10.6) 206(29.8), 190(5.5), 174(100)159(12.1) 158(100), 129(27.6), 115(46.3), 103(12.1) 180(100), 152(38.4), 126(6.6), 76(16.2) 181(100), 165(16.6), 152(57.0), 76(26.7) 192(19.6), 161(100), 134(20.1), 89(23.2) 184(100), 152(9.0), 139(15.5), 113(1.9) 158(73.5), 141(9.8),128(100), 115(47.0) 178(100), 152(9.2), 89(7.7), 76(8.5) 178(100), 152(9.3), 126(1.5), 76(7.7) 170(100), 160(15.0), 141(27.3), 115(29.0) 157(100), 128(20.2), 115(7.1), 102(4.5) 188(4.8), 157(100)128(20.1), 115(6.9) 216(21.6), 184(46.7), 156(100), 128(99.5) 226(9.3), 198(16.4), 181(100), 152(24.6) 186(35.8), 157(100), 144(26.7), 128(27.1) 210(3.3), 181(100), 152(36.8), 126(3.7) 184(100), 155(9.3), 128(36.5), 102(23.1) 188(0.9), 157(100), 128(18.9), 115(6.0) 172(6.1), 157(100), 128(18.3), 115(8.8) 182(100), 152(39.1), 76(21.2), 57(13.6) 218(100), 187(96.7), 160(87.3), 129(41.8) 192(24.5), 174(100), 146(12.1), 118(23.0) 212(12.2), 181(87.2), 115(100), 127(72.0) 184(45.2), 178(14.1), 155(100), 127(68.0) 182(100), 165(17.3), 152(33.5), 76(19.8) 132(100), 104(60.2), 77(23.1), 51(15.6) 182(100), 152(28.2), 91(6.9), 76(2.8) 196(100), 168(40.5), 139(40.6), 113(5.6) 196(100), 165(18.1), 152(33.0), 113(53.2) 216(42.9), 181(100), 152(26.2), 76(20.2) 208(100), 180(61.1), 152(65.4), 126(29.1) 194(100), 178(10.7), 165(95.5), 139(10.1) 222(100), 194(19.0), 163(41.8), 111(15.0) 258(2.8), 209(100), 181(27.0), 152(22.7)

a

166 168 168 180 180 158 180

158 178 178 170

184 182

184 182 196 196 208 194

Compound numbers as in Table 1.

ments of O3/UV oxidation were carried out in a photochemical reactor whose mass-transfer coefficient was about 1 order of magnitude lower (see Table 1 of Beltrań et al., 1995a). Ozone/Hydrogen Peroxide Oxidation Kinetics. The oxidation rate of PAHs (or a given compound) in water when treated with the system O3/H2O2 has two contributions due to the direct reactions with ozone and hydroxyl radicals (the direct reaction with hydrogen peroxide is generally negligible). This is expressed by eq 1:

rPAH ) -

dCPAH ) kDCO3CPAH + krCPAH dt

(1)

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 895 Table 3. Total Stoichiometric Ratio, zt, Total and Local Reaction Factor, Et and El, and Total and Local Ozone Efficiencies, ηt and ηl, during the Oxidation of PAHs Using Ozone Combined with Hydrogen Peroxidea run

CPAH0 × 106, M

1 2 3 4 5 6 7 8 9b 10c 11d 12e

6.3 6.4 6.6 5.4 5.4 6.6 6.8 7.7 7.7 10 5.1 17

pH

CH2O2, M

zt

Et

El

ηt, %

η l, %

7 7 7 7 7 2 4 12 7 7 7 7

10-5

15.6 15.7 18.4 31.0 52.1 41.8 20.8 32.4 12.0 6.8 6.9 2.2

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.4 0.3

0.005 0.005 0.004 0.005 0.005 0.004 0.006 0.007 0.014 0.027 0.012 0.08

94.9 93.9 94.6 94.9 98.2 92.8 89.3 98.6 77.9 92.6 93.4 96.8

3.9 3.6 3.2 3.9 0.34 3.2 5.0 4.3 10.2 20.4 9.1 56.7

10-4 10-3 10-2 0.17 10-3 10-3 10-3 10-3 10-3 10-3 10-3

a Results correspond to fluorene oxidation at 3 min reaction time unless indicated. b CHCO3- ) 10-2 M, t ) 2 min. c CHCO3- ) 10-3 M, t ) 2. d Phenanthrene oxidation, t ) 0.75 min. e Acenaphthene oxidation, t ) 0.75 min.

where

kr ) kOHPCOH

(2)

kD and kOHP being the rate constants of the reactions between ozone and hydroxyl radicals with the PAH, respectively, and CO3 and CPAH the ozone and PAH concentrations, respectively. According to the mechanism presented in previous papers (Glaze and Kang, 1989; Beltrań et al., 1994), COH can be expressed as follows:

COH )

(2ki1COH- + 2ki2CHO2-)CO3 kT

(3)

where the numerator represents the rate of free-radical initiation, composed by two terms, due to the action of hydroxide and hydroperoxide ions on ozone (reactions (IV) and (VI) of the appendix) and the denominator is the inhibiting character of the water:

kT )

∑kOHiCi

(4)

where i represents any species present in water that reacts with hydroxyl radicals and does not release the superoxide ion or hydroperoxide radical, O2•- or HO2•, respectively. In this work PAHs are supposed to be included among these species, but not hydrogen peroxide (see reactions (XIII) and (XIV) of the appendix). Since rate constants involved in eq 2 are known (Beltrań et al., 1995c), it was tried to determine the contribution of both ways, direct and radical, to the oxidation of PAHs studied and the concentration of hydroxyl radicals obtained at different experimental conditions. In order to simplify the procedure, eq 1 was applied at the start of oxidation once eqs 2 and 3 have been accounted for, with kT ) kOHPCPAH:

-rPAH0 ) -

|

dCPAH dt

t)0

) kDCPAH(CO3)t)0 +

(2ki2CHO2- + 2ki1COH-)(CO3)t)0 (5)

Equation 5 can further be simplified by eliminating the term 2ki1COH-CO3, negligible against 2ki2CHO2-CO3 if the total hydrogen peroxide concentration is higher than 10-5 M and pH is lower than 13. Thus, from the simplified eq 5 a value of ozone concentration at the

Table 4. Concentration of Hydroxyl Radicals and Contribution of Direct Ozonation to the PAH Oxidation Rate with Ozone Combined with Hydrogen Peroxidea run 1 2 3 4 5 6 7 8 9b 10c 11d 12e

rPAH0 × 108, M s-1

(CO3)t)0,f M

COH × 1013, M

γO3,g %

4.6 4.4 3.7 1.8 0.2 1.3 2.6 1.5 3.5 5.2 6.8 18.0

4.3 × 4.9 × 10-6 4.2 × 10-7 2 × 10-8 1.5 × 10-10 7.0 × 10-5 9.4 × 10-5 4.5 × 10-12 4 × 10-7 5.8 × 10-7 6.5 × 10-7 9.1 × 10-8

6.2 6.8 5.9 3.4 0.4 0.009 1.3 2.0 4.6 5.2 8.5 0.5

16.4 1.9 0.2 0.0 0.0 99.5 68.1 0.0 0.3 0.3 14.7 97.7

10-5

a For run conditions see Table 3. Results correspond to the start of oxidation unless indicated. b-e As in Table 3. f Calculated from eq 5. g Calculated from eq 6.

start of oxidation, (CO3)t)0, can be obtained and hence the concentration of hydroxyl radicals. Table 4 shows that, except at pH 2 or in the presence of a 0.17 M hydrogen peroxide concentration or in the ozonation of acenaphthene, COH is higher than 10-13 M. Also notice that the presence of bicarbonate ions (experiments of Figure 4) is also considered in Table 4 (runs 9 and 10) and the corresponding inhibiting effect (kt2CHCO3- + kt3CCO32-; see reactions (XVI) and (XVII) of the appendix) has also been determined considering that the carbonate ion radical generated has no effect on the process rate. As can be seen from Table 4, for CH2O2 ) 10-3 M, the presence of a 10-2 M total bicarbonate ion still leads to a low dissolved ozone concentration. This means that, for the conditions applied in this work, ozone mass transfer likely controls the rate of oxidation since hydrogen peroxide is in excess. Also, notice that for the above conditions dissolved ozone concentration is at least 1 order of magnitude lower than the solubility value, CO3* (which is about 5 × 10-5 M). Contribution of direct ozonation, γO3, was also determined as follows:

γO3 )

kDCO3CPAH

× 100

(6)

-rPAH0

As can also be seen from Table 4, for fluorene oxidation, γO3 is nearly 100% at pH 2, decreases to about 60% at pH 4, and is near zero at pH g 7 regardless of hydrogen peroxide concentration. It should be highlighted that these figures also apply in the presence of bicarbonate ions (γO3 is only 0.3% with CHCO3- ) 10-2 M). This indicates the importance of radical reactions in the process. However, it should also be noted that PAH oxidation rate does decrease with the increasing hydrogen peroxide concentration. Regarding the oxidation of phenanthrene and acenaphthene, contribution of direct ozonation is higher, especially for the latter, due to the higher values of the direct rate constant, kD. Thus, for a 10-3 M hydrogen peroxide concentration, the oxidation of phenanthrene and acenaphethene due to the direct way is 15% and near 100%, respectively. As previously reported (Paillard et al., 1988), the stoichiometric point of the O3/H2O2 oxidation is achieved when the ratio of oxidant concentrations is 2 mol of ozone/mol of hydrogen peroxide. In our experiments, given the values of calculated CO3 and CH2O2 at the start of oxidation, the ratio of oxidants is kept lower than 1 so the ozone mass transfer rate likely controls the

896


oxidation rate. The final equation rate for PAH oxidation can then be obtained by combining eq 1 with an ozone mass balance in water (see the appendix for reactions):

kLa(CO3* - CO3) ) dCO3

∑

+ (kDCPAH + kDiCi + 3ki1COH- + dt 2ki2CHO2-)CO3 + (kH(CH2O2)t + 2kP4CO3)COH (7)

where

kH(CH2O2)t ) kP5CH2O2 + kP6CHO2-

(8)

At the start of oxidation and under the aboveindicated experimental conditions, eq 7 reduces to eq 9:

kLaCO3 ) (3ki1COH- + 2ki2CHO2- + kDCPAH)CO3 + kH(CH2O2)tCOH (9) Combination of eqs 9 and 3 yields the following equation for the concentration of hydroxyl radicals:

COH )

kLaCO3* - kDCPAH(CO3)t)0 - ki1COH-(CO3)t)0 kOHPCPAH + kH(CH2O2)t (10)

that substituted in eq 1, once eq 2 has been accounted for, leads to eq 11:

-rPAH0 ) kD(CO3)t)0CPAH + kOHPCPAH × kLaCO3* - kDCPAH(CO3)t)0 - ki1COH-(CO3)t)0 kOHPCPAH + kH(CH2O2)t

(11)

Equation 11 can be generalized for any reaction time, including the competitive effects of intermediates and other substances like bicarbonate ions for hydroxyl radicals and ozone. For this general case the oxidation rate becomes:

-rPAH ) kDCO3CPAH + kOHPCPAH × 3

2

3). At these conditions, the concentration of hydrogen peroxide applied, 10-3 M, is still higher than that corresponding to the stoichiometric point (even considering an ozone concentration equal to its solubility, 5 × 10-5 M). Glaze and Kang (1989b) also noticed that the kinetics of O3/H2O2 oxidation of low molecular organochlorine compounds at pH 5.5 did not follow the same pattern observed at higher pH. A possible explanation could be due to reactions proposed by Sehensted et al. (1991), who reported that the primary step of ozone decomposition in water at acid medium is the dissociation reaction:

O3 a O + O2

-

3

(12)

2

Both eqs 11 and 12 are valid if the concentration of hydrogen peroxide is in excess to that corresponding to the stoichiometric point, that is, when the process rate is controlled by the rate of ozone mass transfer. Also, notice that eq 12 reduces to that deduced by Glaze and Kang (1989) for the case of oxidation of compounds with negligible oxidation through direct ozonation. Equation 12 explains, at least qualitativaly, the results presented in this work on the O3/H2O2 oxidation of PAHs when both hydrogen peroxide concentration is changed and pH is increased from 7 to 12. At pH 2 the rate of oxidation could be exclusively due to the direct reaction ozone-PAH (see Table 4) because ozone does not react with hydrogen peroxide at these conditions (Staehelin and Hoigne´, 1982). However, eq 12 does not support the increase observed in the oxidation rate when pH is also increased from pH 2 to 7 (see Figure

(13)

where the oxygen atom is the precursor of hydroxyl radical formation:

O + H2O f 2OH•

∑kDiCi)CO - ki1COH CO kOHPCPAH + kH(CH O )t + ∑kOHiCi

kLaCO3* - (kDCPAH +

Figure 5. Comparison of advanced oxidation technologies of PAHs. PAH conversion versus type of oxidation. Results correspond to 2 min reaction time. Results correspond to experiments carried out in a photochemical reactor (see Beltrań et al., 1995b). For experiments involving UV radiation: Io ) 3.8 × 10-6 Einstein L-1 s-1. For experiments involving ozonation: PO3 ) 500 Pa (average value). For experiments involving hydrogen peroxide: CH2O2 ) 10-3 M. kLa values: 3.5 × 10-3 s-1. Average initial concentrations of PAHs: CF0 ) 5.3 × 10-6 M; CPH0 ) 2.7 × 10-6 M; CA0 ) 1.5 × 10-5 M. Symbols: ([) fluorene, (0) phenanthrene, (9) acenaphthene.

(14)

If reactions (13) and (14) develop, the oxidation rate would be different from that predicted by eq 12. Comparison between Oxidation Technologies Applied. Advanced oxidation technologies used in this work have in common a free-radical mechanism involving hydroxyl radical reactions with PAHs, with differences due to initiation steps and the nature of direct reactions (direct photolysis and/or direct ozonation). The appendix resumes the whole mechanism followed to study the advanced oxidation of PAHs (see also previous works: Beltrań et al., 1995a-c). As far as a comparison of oxidation results is concerned, in Figure 5 conversion of the three PAHs studied, achieved in 2 min, with the different oxidation and radiation technologies is presented. It can be seen that any oxidation system significantly increases the conversion with respect to UV radiation alone. However, this is not the case if oxidation systems are compared with ozonation. Thus, only the combination between ozone and UV radiation slightly improves conversions in the case of fluorene and phenanthrene.

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 897 Table 5. Contribution of Direct Ways (UV Radiation and/or Ozonation) in the Advanced Oxidation of PAHs: Comparative Resultsa PAH

UV/H2O2: γUV, %

fluorene phenanthrene acenaphthene

48.8 38.8 30.3

a

UV/O3 γUV, % γO3, % 31.9 48.0 19.2

0.8 52.0 80.8

O3/H2O2: γ O 3, %

O3: γ O 3, %

0.3 12.5 100

4.1 32.6 100

Conditions as in Figure 5.

In Table 5, on the other hand, the contribution of direct ways, photolysis and/or ozonation, in all the oxidation systems studied is given. At the concentrations applied in this work PAHs absorb radiation and react with ozone through slow-moderate gas-liquid reactions (see Table 3). Fluorene seems to be the most refractory PAH studied toward direct ways, while acenaphthene reacts exclusively through direct ozonation regardless of the presence of hydrogen peroxide or the simultaneous application of UV radiation. Conclusions The main conclusion about the oxidation of PAHs in water with O3/H2O2 is the negligible and even negative effect of hydrogen peroxide to improve the oxidation rate with respect to that from ozonation alone. From the mechanism and kinetics point of view, direct reactions of ozone with PAHs are so important that the contribution of hydroxyl radical oxidation (due to the presence of hydrogen peroxide) has no effect on the rate of PAH oxidation (see the numerator of the second term on the right-side of eq 12). The O3/H2O2 system is neither important when hydrogen peroxide concentration is very low (that is, when 2CO3 > CH2O2) as in run 5 of Table 3. Neutral pH is the most appropriate to carry out the oxidation. This is conveniently explained by eq 12, valid for pH g 7. At lower pH, the oxidation rate decreases and eq 12 does not hold. At these conditions, in addition to reactions shown in the appendix, other contributions like reactions (13) and (14) can develop. Regarding intermediate compounds, there is no much difference between oxidation systems involving ozone, hydrogen peroxide, and UV radiation as observed from Table 1 and Table 1 of the preceding paper (Beltrań et al., 1995c). Intermediate compounds are numerous as a consequence of ozone and hydroxyl radical reactivity (see ηt and ηl in Table 3), but high oxidation times (more than 30 min) suffice to destroy most of them. Some potentially hazardous compounds that appear at low reaction times are eventually destroyed if oxidation time is extended. Among oxidation systems, ozone combined with UV radiation allows the highest oxidation rates,, although differences with respect to ozonation alone are so small that in a practical case it is likely that the cost associated with the use of UV radiation makes ozonation alone the most convenient technology to remove PAHs from water. Acknowledgment We thank C.I.C.Y.T. of Spain for its economic support through Grant AMB93/654.

Et ) total reaction factor, dimensionless k ) reaction rate constant, M-1 s-1 or s-1 kLa ) liquid phase volumetric mass-transfer coefficient, s-1 r ) reaction rate, M s-1 t ) reaction time, s V ) reaction volume, m3 zt ) total stoichiometric factor, dimensionless Greek Letters γ ) percentage contribution of direct ozonation reaction of PAHs, defined by eq 6, dimensionless ηl ) local ozone efficiency, dimensionless ηt ) total ozone efficiency, dimensionless Superindex * ) gas-water interface Subindexes A ) acenaphthene F ) fluorene F0 ) fluorene at initial conditions H ) hydrogen peroxide H2O2 ) nonionic form of hydrogen peroxide HO2- ) ionic form of hydrogen peroxide i ) any species present in solution o ) start of oxidation O3 ) ozone OHi ) the reaction between hydroxyl radical and species i P or PAH ) any polynuclear aromatic hydrocarbon PH ) phenanthrene PH0 ) phenanthrene at initial conditions r ) radical reactions of ozone

Appendix The mechanism followed to study the kinetics of advanced oxidation of PAHs (see also Staehelin and Hoigne´ (1985) and Glaze and Kang (1989)) is shown below (for rate constant values see this paper and Beltrań et al., 1995a-c):

Mass transfer of ozone: k1a

O3(g) 98 O3 Direct reactions: With ozone: kD

zO3 + PAH 98 products

C ) concentration of a given substance or species, M El ) local reaction factor, dimensionless

(II)

With UV radiation: ΦPAH, hν

PAH 98 products

(III)

Radical reactions: Initiation reactions: Ozone systems: ki1

O3 + OH- 98 O2•- + HO2•

(IV)

Ozone/hydrogen peroxide system: pK ) 11.8

Nomenclature

(I)

H2O2 {\} HO2- + H• ki2

HO2- + O3 98 HO2• + O3•-

(V) (VI)

898


and reaction (IV).

Ozone/UV radiation system: ΦO , hν 3

O3 + H2O 98 H2O2 + O2

(VII)

and reactions (IV) and (V).

Propagations reactions: pK ) 4.8

HO2- {\} O2•- + H+ kp1

O2•- + O3 98 O3•kp2

O3•- + H+ 98 HO3• kp3

HO3• 98 OH• + O2 kp4

OH• + O3 98 HO2• + O2 kp5

OH• + H2O2 98 HO2• + H2O kp6

OH• + HO2- 98 HO2• + OH-

(VIII) (IX) (X) (XI) (XII) (XIII) (XIV)

Termination reactions: kOHP

OH• + PAH 98 products kt2

OH• + CO32- 98 CO3•- + OHkt3

OH• + HCO3- 98 CO3•- + H2O

(XV) (XVI) (XVII)

It is possible that the carbonate ion radical also propagates the chain mechanism in the presence of hydrogen peroxide. Reactions (XIII) and (XIV) are also termination reactions in the UV/H2O2 oxidation system. In addition to reactions (II), (III), and (XV), there are similar reactions that intermediates undergo. Equilibrium of hydroxyl radical and carbonate ion radical should also be considered. At acid pH in systems involving ozone, different initiation reactions, like reactions (13) and (14), can develop according to Sehensted et al. (1991). Literature Cited Allemane, H. Oxydation de quelques compose´s organiques en milieu aqueux par ozonation et ozonation catalytique. Ph.D. Thesis, University of Poitiers, France, 1994. Beltrań, F. J.; Gonza´lez, M.; Rivas, F. J.; Marıń, M. Oxidation of mecoprop in water with ozone and ozone combined with hydrogen peroxide. Ind. Eng. Chem. Res. 1994, 33, 125-136.

Beltrań, F. J.; Ovejero, G.; Garcıá-Araya, J. F.; Rivas, J. Oxidation of polynuclear aromatic hydrocarbons in water. 1. Ozonation. Ind. Eng. Chem. Res. 1995a, 34, 1596-1606. Beltrań, F. J.; Ovejero, G.; Encinar, J. M.; Rivas, J. Oxidation of polynuclear aromatic hydrocarbons in water. 2. UV radiation and ozonation with the presence of UV radiation. Ind. Eng. Chem. Res. 1995b, 34, 1607-1615. Beltrań, F. J.; Ovejero, G.; Rivas, J. Oxidation of polynuclear aromatic hydrocarbons in water. 3. UV radiation combined with hydrogen peroxide. Ind. Eng. Chem. Res. 1995c, 35, 883-890. Brunet, R.; Bourbigot, M. M.; Dore´, M. Oxidation of organic compounds through the combination ozone-hydrogen peroxide. Ozone Sci. Eng. 1984, 6, 163-183. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of data constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH/•O-) in aqueous solutions. J. Phys. Chem. Ref. Data 1988, 17, 513886. Duguet, J. P.; Brodard, E.; Dussert, B.; Mallevialle, J. Improvement in the effectiveness of ozonation of drinking water through the use of hydrogen peroxide. Ozone Sci. Eng. 1985, 7, 241257. Glaze, W. H.; Kang, J. W. Advanced oxidation processes. Description of a kinetic model for the oxidation of hazardous materials in aqueous media with ozone and hydrogen peroxide in a semibatch reactor. Ind. Eng. Chem. Res. 1989a, 28, 1573-1580. Glaze, W. H.; Kang, J. W. Advanced oxidation processes. Test of a kinetic model for the oxidation of organic compounds with ozone and hydrogen peroxide in a semibatch reactor. Ind. Eng. Chem. Res. 1989b, 28, 1580-1587. Glaze, W. H.; Kang, J. W.; Chapin, D. H. The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone Sci. Eng. 1987, 9, 335-342. Paillard, H.; Brunet, R.; Dore´, M. Optimal conditions for applying an ozone-hydrogen peroxide oxidizing system. Water Res. 1988, 22, 91-103. Sehensted, K.; Corfitzen, H.; Holcman, J.; Fischer, C. H.; Hart, E. J. The primary reaction in the decomposition of ozone in acidic aqueous solutions. Environ. Sci. Technol. 1991, 25, 1589-1596. Staehelin, J.; Hoigne´, J. Decomposition of ozone in water: Rate of initiation by hydroxide ion and hydrogen peroxide. Environ. Sci. Technol. 1982, 16, 676-681. Staehelin, J.; Hoigne´, J. Decomposition of ozone in water in the presence of organic solutes acting as promotors and inhibitors of radical chain reactions. Environ. Sci. Technol. 1985, 19, 1206-1213. Trapido, M.; Veressinina, J.; Munter, R. Ozonation and advanced oxidation process (AOP) treatment of phenanthrene in aqueous solutions. Ozone Sci. Eng. 1994, 16, 475-485. Wable, O.; Jousset, M.; Crisinel, P.; Ledon, H.; Duguet, J. P. Optimisation of perozoneTM process for water treatment: Choice of H2O2 injection point. Proc. Eleventh Ozone World Congr.: Ozone Water Wastewater 1993, 2, S-17-112 to S-17-118.

Received for review June 20, 1995 Accepted November 7, 1995X IE9503757

X Abstract published in Advance ACS Abstracts, January 15, 1996.

Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 4. Ozone

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