Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 3. UV

Jan 15, 1996 - the main way of degradation at acid pH (76% at pH 2 with 10-3 M hydrogen peroxide concentration, for fluorene oxidation). Rate constant...
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Ind. Eng. Chem. Res. 1996, 35, 883-890

883

Oxidation of Polynuclear Aromatic Hydrocarbons in Water. 3. UV Radiation Combined with Hydrogen Peroxide Fernando J. Beltra´ n,*,† Gabriel Ovejero,‡ and Javier Rivas†,§ Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Ingenierı´a Quı´mica, Universidad Complutense, 28040 Madrid, Spain

Oxidation in water of three polynuclear aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and acenaphthene, with UV radiation combined with hydrogen peroxide has been studied. The effect of hydrogen peroxide concentration, pH, and bicarbonate ion has been investigated. Disappearance rates of PAHs are substantially increased with respect to those from UV radiation alone if proper conditions of hydrogen peroxide concentration and pH are established. Direct photolysis contribution decreases with the increasing hydrogen peroxide concentration and is the main way of degradation at acid pH (76% at pH 2 with 10-3 M hydrogen peroxide concentration, for fluorene oxidation). Rate constants of reactions between the hydroxyl radical and PAHs were found to be 9.9 × 109, 8.8 × 109, and 13.4 × 109 M-1 s-1, for fluorene, acenaphthene, and phenanthrene, respectively. Both UV radiation and UV/H2O2 oxidation of PAHs yield numerous intermediate compounds. Most of these compounds disappear as oxidation time is increased. Introduction Polynuclear aromatic hydrocarbons, PAHs, constitute a well-known water priority pollutant family due to their toxicity and potential health hazard (Andelman and Snodgrass, 1974). Advanced oxidation processes can be among the most appropriate technologies to remove these organics from water, but some aspects related to reactivity, mechanism, kinetics and intermediates formed still remain unknown. In previous papers (Beltra´n et al., 1995a,b) two advanced oxidation systems, ozonation and UV radiation combined with ozone, were studied to treat three PAHs: fluorene, acenaphthene, and phenanthrene. Other potentially powerful oxidation systems involving the addition of hydrogen peroxide to generate hydroxyl radicals also deserve specific studies. Thus, combinations of hydrogen peroxide with UV radiation (UV/H2O2) or ozone (O3/H2O2) can be more practical from the economic point of view since electric energy requirements are lower. In fact, some processes based on the UV/H2O2 and O3/H2O2 systems have already been commercialized with the names of perox-pure (Foelich, 1992), Ultrox (Lewis et al., 1990), and Perozone (Wable et al., 1993). In this paper, the UV/H2O2 oxidation system applied to remove the three above mentioned PAHs is studied, while in a following work (Beltra´n et al., 1995c), the O3/ H2O2 system is also treated. The use of the UV/H2O2 system to eliminate pollutants from water has already been the subject of many works. Thus, Malaiyandi et al. (1980) studied the removal of organics from model solutions prepared in distilled and tap water observing TOC reductions between 88 and 98%. Later, Sundstrom et al. (1989) reported on the effectiveness of this process in the treatment of typical mononuclear aromatics like benzene, toluene, phenol, chlorophenols, and alkyl phthalates. They also reported that extended treatment * To whom correspondence should be addressed. † Universidad de Extremadura. ‡ Universidad Complutense. § Fax number: 34-1-3944114. Electronic mail: CALLES@ QUIM.UCM.ES. Fax number: 34-24-271304. Electronic mail: [email protected].

0888-5885/96/2635-0883$12.00/0

times can destroy the intermediates formed. A study of the UV/H2O2 system is also reported by Glaze and Lay (1989) and more extensively by Lay (1989). In their work, a priority pollutant 1,2-dibromo-3-chloropropane is treated with UV/H2O2, O3/H2O2, and UV/O3 systems, and a kinetic model is proposed for the former process based on chemical and photochemical principles. Different chloroethanes and s-triazine herbicides have also been treated with the UV/H2O2 system by Tace (1992). In a recent work, Beltra´n et al. (1993) also report on results of the chemical and photochemical degradation of atrazine with UV/H2O2 oxidation. In all cases, this oxidation technology seems to be very attractive if optimum conditions (hydrogen peroxide concentration, flux of radiation, etc.) are applied. As far as PAH oxidation in water is concerned, only naphthalene is reported to be extensively treated with the UV/H2O2 system. Thus, Tuhkanen (1994) in the line of works of Glaze et al. (Glaze and Kang, 1989; Glaze and Lay, 1989, Glaze et al., 1992) has studied the oxidation of naphthalene showing its high reactivity toward hydroxyl radicals, the contribution of direct photolysis (UV radiation) in the disappearance rate, and identification of intermediates. More recently, Trapido et al. (1994) have studied the advanced oxidation of phenanthrene in water although their work is limited to present comparisons among the phenanthrene removal rates from different advanced oxidation systems. The present work is divided into sections to show the influence of variables, identification of intermediates, and kinetics. Experimental Part Experiments were carried out in a photochemical reactor described in a previous paper (Beltra´n et al., 1995b). Agitation of the water solution was achieved by bubbling oxygen at a rate of 25 L h-1. Hydrogen peroxide was obtained from a 33% w/v solution obtained from Merck. PAHs, UV lamp, procedure for oxidation runs, and analysis methods were those followed when ozonation and UV/O3 oxidation systems were applied (Beltra´n et al., 1995a,b). Two series of experiments were made for product identification. In the first one, the reaction was stopped © 1996 American Chemical Society

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Figure 1. UV/H2O2 oxidation of phenanthrene and acenaphthene. Comparison with UV radiation alone. Conditions: T ) 20 °C, pH ) 7, I0 ) 3.8 × 10-6 einstein L-1 s-1. Phenanthrene oxidation: (9) CPAHo ) 1.7 × 10-6 M, (CH2O2)0 ) 0; ([) CPAHo ) 3.9 × 10-6 M, (CH2O2)0 ) 10-3 M. Acenaphthene oxidation: (0) CPAHo ) 1.7 × 10-5 M, (CH2O2)0 ) 0; (]) CPAHo ) 1.6 × 10-5 M, (CH2O2)0 ) 10-3 M.

when approximately 90% conversion of PAH was achieved, while in the second one, reaction time was extended at least 30 min after total disappearance of the initial PAH. In both cases, treated aqueous solutions of PAHs (approximately 1 L) were first acidified with HCl until pH < 2 and extracted two times with 100 mL of methylene chloride; the organic phase was dried with Na2SO4 and concentrated to less than 2 mL. Then an aliquot of the sample was directly analyzed by GC/MS and another one derivatized with diazomethane and also analyzed by GC/MS. In each case, 5 µL of sample was injected into a Hewlett-Packard HP 5890 chromatograph coupled to a Hewlett-Packard 5972 mass selective detector operated in scan mode (1.9 scans‚s-1) with a mass range between m/z 20 and 400. A NBS75K mass spectral library was used to identify compounds. A 30 m × 0.25 mm × 0.25 µm film thickness HP-5MS, cross-linked 5% phenyl methyl silicone column was used with the following temperature program: 50 °C hold for 2 min, ramp at 8°/min to 210 °C, and 210 °C hold for 15 min. The injector port was at 250 °C, and the carrier gas was helium. Results and Discussion Experiments were first carried out to analyze the influence of hydrogen peroxide concentration (0-0.4 M), pH (2, 7, and 12), and bicarbonate ion concentration (00.01 M). PAHs were found to be refractory to oxidation with hydrogen peroxide applied alone. Influence of Variables. Figures 1 and 2 show the variation of the remaining fraction of PAHs with time corresponding to experiments of UV radiation alone and UV/H2O2 oxidation. As observed the presence of hydrogen peroxide (10-3 M) increases significantly the disappearance rate of PAHs compared to UV radiation alone. In Figure 2 it can also be seen that the increase of hydrogen peroxide concentration up to 10-2 M leads to an increase of the disappearance rate of fluorene, but a further increase of hydrogen peroxide concentration up to 0.4 M reduces the oxidation rate, becoming similar to that of UV radiation alone. Similar results were obtained with the other two PAHs. These opposing effects suggest a double role of hydrogen peroxide as initiator and inhibitor of oxidation that has already been observed in the UV/H2O2 oxidation of other compounds like atrazine (Beltra´n et al., 1993) and naphthalene

Figure 2. UV/H2O2 oxidation of fluorene. Influence of hydrogen peroxide concentration. Conditions: CF0 ) 5.4 × 10-6 M (average value); T ) 20 °C, pH ) 7, I0 ) 3.8 × 10-6 einstein L-1 s-1. (CH2O2)0, M: (b) 0; (4) 10-4; (2) 10-3; (]) 10-2; (9) 0.2; (0) 0.4.

(Tuhkanen, 1994). Both effects confirm the development of the following reactions: (a) Formation of hydroxyl radicals due to direct photolysis of hydrogen peroxide (Baxendale and Wilson, 1957): hν

H2O2 98 2OH•

(1)

(b) Oxidation of PAH through their reaction with hydroxyl radicals: kOHP

PAH + OH• 98 products

(2)

(c) Competitive reactions of hydrogen peroxide to consume hydroxyl radicals (Christensen et al., 1982): k3 ) 2.7 × 107 M-1 s-1

H2O2 + OH• 98 HO2• + H2O

(3)

and k4 ) 7.5 × 109 M-1 s-1

HO2- + OH• 98 HO2• + OH-

(4)

The direct photolysis of hydrogen peroxide is the initiation step of the free-radical mechanism of PAH oxidation. In addition to reactions (1)-(4), there are other important reactions to be considered such as direct photolysis of PAHs (Beltra´n et al., 1995b) hν

PAH 98 products

(5)

and reactions (direct photolysis and hydroxyl radical reactions) of intermediates formed. It seems clear that at low hydrogen peroxide concentration hydroxyl radicals formed (which depends on the competition between direct photolysis reactions) are mainly consumed by the initial PAH. However, when hydrogen peroxide concentration is much higher (>0.01 M), reactions (3) and (4) become more important than reaction (2) and the PAH rate is reduced. Figure 3 presents the effect of pH on the UV/H2O2 oxidation of fluorene. It can be observed that the increase of pH from 2 to 7 leads to an increase of PAH disappearance rate, but a higher increase of pH, up to 12, definitively reduces the oxidation rate. This was also observed in the oxidation of acenaphthene and phenanthrene. Since PAHs are nondissociating com-

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 885

Figure 3. UV/H2O2 oxidation of fluorene. Influence of pH. Conditions: CF0 ) 4.9 × 10-6 M (average value); T ) 20 °C, (CH2O2)0 ) 10-3 M, I0 ) 3.8 × 10-6 einstein L-1 s-1. pH: (9) 2; (0) 7; ([) 12.

pounds in water with constant extinction coefficient and quantum yield, variation of oxidation rate should also be due to competition between reactions (1)-(5). In fact, the increase of pH also yields two opposing effects: on the one hand, there is an improvement of hydroxyl radical formation through reaction (1) since at 254 nm the extinction coefficient of the ionic form of hydrogen peroxide, the hydroperoxide ion (major peroxide species at pH 12), 240 M-1 cm-1 (Tace, 1992), is higher than that of the nonionic form, 19 M-1 cm-1. On the other hand, the consumption rate of hydroxyl radicals by hydrogen peroxide due to reactions (3) and (4) also increases because the hydroperoxide ion reacts faster than the nonionic form. In summary, when pH is increased (especially above 11.8, pKa of hydrogen peroxide dissociation equilibrium), both the fraction of radiation that hydrogen peroxide absorbs and its hydroxyl radical scavenging effect become stronger and the oxidation rate of PAH diminishes compared to that obtained at acid or neutral pH. The effect of bicarbonate-carbonate ions was also investigated. These ions are considered scavengers of hydroxyl radicals and, in natural waters, as one of the main inconveniences to benefit the use of advanced oxidation processes. They mainly act through the following reactions (Weeks and Rabani, 1966): k6 ) 1.5 × 107 M-1 s-1

HCO3- + OH• 98 CO3•- + H2O (6) and k7 ) 4.2 × 108 M-1 s-1

CO32- + OH• 98 CO3•- + OH- (7) In this work some experiments of UV/H2O2 oxidation were carried out in the presence of these ions (see Figure 4). Surprisingly, as can be seen from Figure 4, bicarbonate ions do not retard the oxidation rate of fluorene (similar results were found with the other PAHs), but the process rate remains unaltered compared to the blank experiment carried out in the absence of bicarbonate ions. There are some possible explanations for these results: (a) Consumption of hydroxyl radicals through reactions (6) and (7) is still negligible, even with 0.01 M total bicarbonate ion concentration, compared to hydroxyl radical reactions with the initial PAH and intermediates (this, however, does not seem to be a plausible reason due to the high concentration of bicarbonate ion present). (b) Reactions (6) and (7) lead to the carbonate ion radical, CO3‚-, which is known to

Figure 4. UV/H2O2 oxidation of fluorene. Influence of total bicarbonate ion concentration. Conditions: CF0 ) 4.7 × 10-6 M (average value); T ) 20 °C, pH ) 7, I0 ) 3.8 × 10-6 einstein L-1 s-1. (CH2O2)0 ) 10-3 M; CHCO3-, M: (0) 0; (9) 10-3; ([) 10-2.

react with hydrogen peroxide (Behar et al., 1970): k8 ) 8 × 105 M-1 s-1

CO3- + H2O2 98 products

(8)

Peyton and Glaze (1988) in their studies of aqueous ozone photolysis suggest that products of reaction (7) are HCO3-, which then can be regenerated, and HO2‚, which finally could also regenerate hydrogen peroxide through the following reactions (Behar et al., 1970):

HO2• + HO2• f O2 + H2O2 pK ) 4.8

(9)

HO2• {\} O2•- + H+

(10)

HO2• + O2•- f O2 + HO2-

(11)

‚-,

(c) The carbonate ion radical, CO3 is not an inert species but shows different degrees of reactivity with organic compounds in water as reported by Chen et al. (1975). This reactivity is highly dependent on the nature of the organics contrary to the unselective character of the hydroxyl radical. In support of this latter explanation, von Gunten and Hoigne´ (1994) has recently reported a carbonate ion radical as a secondary oxidant during ozonation of bromide ion to form bromate ion. Identification of Intermediates. Products found or identified from UV radiation alone and UV/H2O2 oxidation of PAHs and their corresponding mass spectrum are listed in Tables 1 and 2, respectively. Results from low reaction times (approximately 90% disappearance of PAH) present the following features: (a) most of the identified products like 9-fluoren-9-one (case of fluorene), 9,10-phenanthrenedione (case of phenanthrene), or 4-ethoxy-3-methoxyphenol (case of acenaphthene) appear from both processes: UV radiation alone and UV/H2O2 oxidation; (b) other products identified such as some dibenzofuran, 9-fluoren-9-one, and 9,10phenanthrenedione have also been found by different authors (Helleur et al., 1979; Bailey, 1982; Legube et al., 1986; Gaul et al., 1987; Tuhkanen, 1994) during the ozonation or UV/H2O2 oxidation of fluorene, naphthalene, and other PAHs; (c) some of the compounds identified at low reaction times, like dibenzofuran or 2-methyl-1,1′-biphenyl, are potentially hazardous but they disappear at higher reaction times; (d) although only oxalic acid was identified, other carboxylic acids

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Table 1. Products Found or Identified by GC/MS in the UV Radiation and UV/H2O2 Oxidation of PAHs C#a

product

Rtb

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 57

toluene oxalic acid, dimethyl ester ethylbenzene (1-methylethyl)benzene 3,3-dimethyl-2-hexanol nonanal nonanoic acid, methyl esterc nonanoic acid 1H-indene-1,3(2H)-dione 1(3H)-isobenzofuranone 3,4-dihydro-2(H)-1-benzopyran-2-one acenaphthylene acenaphthene dibenzofuran 2-hydroxy-1,4-naphthalenedione unknown unknown fluorene 2-hydroxy-1-naphthalenecarboxaldehyde 2-methyl-1,1’-biphenyl 4-ethoxy-3-methoxyphenol 9,10-dihydroanthracene unknownc dodecanoic acid, methyl esterc unknownc unknownc 9-fluoren-9-one 9-fluoren-9-ol unknownc 9-methylene-9H-fluorene phenanthrene unknown o-hydroxybiphenyl unknown 4-hydroxy-9-fluorenone unknownc unknown unknownc unknown 1,1’-biphenyl-4-carboxaldehyde unknownc unknown unknown 1H,3H-naphtho(1,8-c,d)pyran-1-one unknown unknown unknownc unknown 4-methyldibenzofuran xanthone 2-hydroxy-9-fluorenone 9,10-anthracenedione 1,4-anthracenedione unknownc 9-phenanthrenol unknownc 9,10-phenanthrenedione

3.1 4.1 4.6 5.8 6.2 9.5 11.9 12.9 14.5 14.5 14.5 16.0 16.7 17.2 17.9 18.1 18.2 18.3 18.8 18.7 19.5 19.7 19.8 20.2 20.3 20.5 20.6 20.8 20.8 21.2 21.3 21.4 21.6 21.9 21.9 22.4 22.5 22.6 22.7 22.9 23.0 23.1 23.1 23.2 23.2 23.4 23.5 23.6 23.6 23.9 23.9 24.2 24.3 25.3 27.8 28.2 29.6

a

fluorene

phenanthrene

UV/H2O2 UV/H2O2

UV/H2O2

UV, UV/H2O2

UV

acenaphthene UV/H2O2

UV UV UV

UV/H2O2 UV/H2O2 UV/H2O2 UV/H2O2

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

UV, UV/H2O2 UV/H2O2 UV

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

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

not identified were also found as observed in the chromatograms of derivatized samples (see Table 2 for mass spectrum data). This was especially important during the UV/H2O2 oxidation of fluorene. It should also be noted that the final pH was always between 4 and 5 and that from the mass spectrum of some unknown products (see compound numbers 26 or 38 in Table 2) the presence of peaks with 31 or 32 ion mass lower than that of the parent peak was observed, which suggests loss of a CH3O or CH3OH group, evidence of the formation of an aromatic methyl ester (Silverstein and Bassler, 1967). Kinetics. Assuming reactions (1)-(5) as the main steps of the UV/H2O2 oxidation mechanism of PAHs in organic-free water, the rate of disappearance of these

compounds in a batch photochemical reactor would be given by eq 12:

rPAH ) -

dCPAH ) rUV + kOHPCOHCPAH dt

(12)

where the first and second terms on the right-hand side represent the contribution of direct photolysis and hydroxyl radical attack, respectively. Concretely, the direct photolysis rate would be as follows (Beltra´n et al., 1995b):

∑iCi))

rUV ) ΦPAHI0FPAH(1 - exp(-2.303L

(13)

with ΦPAH, I0, and L as the PAH quantum yield, flow of

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 887 Table 2. Mass Spectra of Products Found or Identified by GC/MS in the UV Radiation and UV/H2O2 Oxidation of PAHsa C# 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 57 a

mol wt 92 118 106 120 130 142 172 158 146 134 148 152 154 168 174 132 166 172 168 168 180 214 180 178 178 170 196

182

184

182 196 196 208 208 194 208

Table 3. Initial Rates of PAH Oxidation and Contribution of Direct Photolysis to the UV/H2O2 Oxidation of PAHsa

m/z(relative abundance) 92(100), 65(25.8), 51(16.9), 39(26.6) 118(2.2), 59(100), 45(34.3), 29(63.6) 106(28.8), 91(100), 77(9.3), 51(12.5) 120(26.9), 105(100), 97(5.8), 84(6.6) 70(81.6), 55(37.4), 43(100), 29(18.1) 114(6.1), 98(24.2), 82(25.7), 57(100) 141(7.0), 129(8.6), 87(42.9), 74(100) 129(16.9), 115(20.2), 73(79.3), 60(100) 46(100), 134(26.2), 105(96.4), 90(31.8) 134(34.1), 105(100), 77(46.6), 51(16.6) 148(100), 120(40.0), 91(39.1), 65(18.6) 152(100), 76(12.1), 63(5.2) 154(100), 126(4.7), 76(19.1), 63(9.1) 168(100), 139(32.5), 113(4.0), 87(2.8) 174(100), 146(17.6), 105(93.6), 77(32.0) 161(3.0), 160(25.7), 132(81.5), 104(100), 172(100), 143(11.4), 115(44.0), 63(11.1) 166(100), 163(15.6), 139(7.8), 82(11.7) 172(100), 144(38.4), 115(55.1), 89(10.6) 168(100), 152(26.5), 139(11.5), 115(10.6) 168(99.3), 140(100), 113(9.1), 70(19.2) 180(70.2), 179(100), 178(58.0), 89(29.8) 190(48.5), 175(25.4), 159(100), 132(40.6) 171(10.1), 143(13.9), 87(55.7), 74(100) 200(54.0), 184(12.1), 168(100), 140(67.1) 206(29.5), 174(100), 157(26.6), 118(20.9) 180(100), 152(38.4), 126(6.6), 76(16.2) 181(100), 165(16.6), 152(57.0), 76(26.7) 192(18.1), 161(100), 134(19.0), 89(25.6) 178(100), 152(9.2), 89(7.7), 76(8.5) 178(100), 152(9.3), 126(1.5), 76(7.7) 172(83.3), 170(95.3), 157(14.1), 115(100) 170(100), 160(15.0), 141(27.3), 115(29.0) 157(100), 128(20.2), 115(7.1), 102(4.5) 196(100), 168(42.6), 139(33.2), 113(52.4) 186(35.8), 157(100), 144(26.7), 128(27.1) 210(3.3), 181(100), 152(36.8), 126(3.7) 218(100), 187(76.2), 173(34.8), 129(44.5) 172(6.1), 157(100), 128(18.3), 115(8.8) 182(100), 152(38.7), 76(21.1), 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), 155(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(58.7), 77(20.0), 51(19.0) 224(5.1), 209(19.0), 181(46.5), 155(100) 226(5.9), 197(100), 178(13.8), 149(21.0) 182(100), 152(28.2), 91(6.9), 76(2.8) 196(100), 168(40.5), 139(40.6), 113(5.6) 196(100), 168(7.4), 139(77.5), 113(6.6) 208(100), 180(61.1), 152(65.4), 126(29.1) 208(100), 180(33.4), 152(57.3), 126(43.4) 185(7.3), 157(100), 128(17.6), 115(6.5) 194(100), 178(10.7), 165(95.5), 139(10.1) 222(100), 194(19.0), 163(41.8), 111(15.0) 208(25.2), 180(100), 152(45.3), 127(36.0)

Compound numbers as in Table 1.

incident radiation, and effective path of radiation in the photochemical reactor, respectively, and FPAH as the fraction of incident radiation that PAH absorbs, defined as follows:

FPAH )

PAHCPAH

∑iCi

(14)

Ci and i being the concentration and extinction coefficient of any species i present in solution (that is, PAH, intermediates, and hydrogen peroxide) that absorbs radiation. Also, in eq 12, COH, CPAH, and kOHP represent the hydroxyl radical and PAH concentrations and the rate constant of their reaction, respectively.

run no.

CPAHo × 106, M

(CH2O2)t, M

rPAHo × 108, M s-1

rUVo × 108, M s-1

FH2O2

γUV, %

1 2 3 4 5 6 7b 8c 9d 10e 11f 12f 13f 14g 15g 16g

4.3 4.1 3.8 4.8 9.4 5.9 4.9 5.8 5.1 4.8 3.9 5.6 5.6 16.0 24.1 25.0

10-4 10-3 2.6 × 10-3 10-2 0.2 0.4 10-3 10-3 10-3 10-3 10-3 0.2 0.4 10-3 0.2 0.4

2.24 2.73 3.78 4.82 4.39 1.63 1.97 1.64 3.40 3.20 5.03 4.00 2.10 11.8 11.6 6.40

1.49 1.33 1.10 0.80 0.11 0.04 1.50 0.98 1.55 1.49 1.96 0.12 0.08 3.58 0.16 0.08

0.026 0.218 0.439 0.703 0.960 0.998 0.189 0.579 0.182 0.191 0.108 0.940 0.970 0.468 0.991 0.995

66.4 48.8 29.2 16.6 2.6 2.2 76.2 59.9 45.5 46.5 38.9 3.7 3.6 30.3 1.4 1.3

a Results correspond to the start of oxidation. T ) 20 °C, I ) 0 3.8 × 10-6 einstein L-1 s-1, pH ) 7, PAH ) fluorene unless indicated. b pH ) 2. c pH ) 12. d CHCO3 ) 10-3 M. e CHCO3 ) 10-2 M. f PAH ) phenanthrene. g PAH ) acenaphthene.

On the other hand, COH can be expressed as the ratio of the rates of initiation and termination steps assuming a steady-state situation holds (Glaze et al., 1992):

COH )

∑iCi)) + ∑kOHICI

2ΦHI0FH(1 - exp(-2.303L kOHPCPAH + k3CH2O2 + k4CHO2-

(15) where ΦH and FH are the quantum yield and fraction of radiation absorbed by hydrogen peroxide, respectively. In eq 15 ∑kOHICOHI is the contribution of intermediate compounds to trap hydroxyl radicals through termination reactions similar to reaction (2). It should be noted that hydroxyl radical reactions are considered here first-order termination reactions since it is assumed that they do not yield radicals that propagate the main chain. In an attempt to determine the rate constant kOHP, eq 12 was first applied at the start of oxidation to neglect the contribution of intermediates. Thus, the initial rate of PAH disappearance, rPAHo, was calculated from polynomial least-squares analysis of CPAH-t data (correlation coefficients were always higher than 0.98) and contribution of direct photolysis, rUV, from data (I0, ΦPAH, etc.) reported on in the preceding paper (Beltra´n et al., 1995b). Table 3 gives the results obtained. In a second step, the percentage contribution of direct photolysis, γUV, was calculated as follows:

γUV )

rUV × 100 rPAH

(16)

The results are also shown in Table 3, where it can be observed that the increase of hydrogen peroxide concentration leads to a significant decrease of γUV. Thus, for the case of fluorene oxidation, this parameter decreases from 66.4% to 2.2% when hydrogen peroxide concentration varies from 10-4 to 0.4 M. Obviously, this is the result of the increased fraction of radiation that hydrogen peroxide absorbs (FH2O2 increases from 0.026 to practically unity for the above-mentioned hydrogen peroxide concentrations, respectively). Regarding the pH effect, it is also observed from Table 3 that contribu-

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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 Table 4. Hydroxyl Radical Concentration and Scavenging Terms in the UV/H2O2 Oxidation of PAHsa hydroxyl radical scavenging terms: kOHiCi

Figure 5. Verification of pseudo-first-order kinetics for UV/H2O2 oxidation of PAH at high hydrogen peroxide concentration. Determination of kOHP. Conditions: T ) 20 °C, pH ) 7, I0 ) 3.8 × 10-6 einstein L-1 s-1, (CH2O2)0 ) 0.4 M. PAH and initial concentration: (9) fluorene, CF0 ) 5.9 × 10-6 M; (4) phenanthrene, CPHo ) 5.6 × 10-6 M; (0) acenaphthene, CA0 ) 2.5 × 10-5 M.

tion of direct photolysis decreases when pH increases from 2 to 7 and then increases to be approximately 60% at pH 12. Finally, concentration of bicarbonate ion does not affect γUV which is about 45% at the conditions here investigated. Thus, bicarbonate ions should play a different role in the mechanism of oxidation in order to account for the results obtained. In the other advanced oxidations studied (Beltra´n et al., 1995b,c) bicarbonate ion retards the oxidation rate of PAHs. It seems that both the presence of an appreciable concentration of hydrogen peroxide (g10-3 M) and application of UV radiation are necessary to observe the effect of bicarbonate ions shown in Figure 4. As deduced from these results, PAH disappearance is exclusively due to hydroxyl radical attack when a high hydrogen peroxide concentration is present (if CH2O2 ) 0.4 M, FH2O2 is 1). Furthermore, at these conditions, it is evident that the main scavenging species of hydroxyl radicals is also hydrogen peroxide (k3CH2O2 + k4CHO2. kOHPCPAH). It is also reasonable to accept that even at any time this situation holds: k3CH2O2 + k4CHO2- . ∑kOHICI + kOHPCPAH, so that eq 12 reduces to eq 17:

rPAH ) -

dCPAH ) krCPAH dt

(17)

where

2ΦHI0 kr ) kOHP k3CH2O2 + k4CHO2-

(18)

Note that the numerator of eq 15 reduces to that of eq 18 when the exponential term is higher than 2. As a consequence, pseudo-first-order kinetics for PAH disappearance was tested in triplicate experiments of UV/ H2O2 oxidation with 0.4 M initial hydrogen peroxide concentration (Figure 5 shows some of the results obtained). PAH concentration (see Figure 2 for fluorene oxidation) was followed 10 min, during which hydrogen peroxide concentration was kept practically constant (reduction was less than 1%). After least-squares analysis of experimental data according to eq 17, once integrated, and Figure 5, the following average values of kOHP were found from three runs: (9.9 ( 0.8) × 109, (13.4 ( 1.4) × 109, and (8.8 ( 0.3) × 109 M-1 s-1 for the

run no.

COH × 1013, M

kOHPCPAH, s-1

k3CH2O2 + k4CHO2, s-1

1 2 3 4 5 6 7b 8c 9d 10e 11f 12f 13f 14g 15g 16g

1.8 3.5 7.2 8.4 4.6 2.8 0.9 1.1 3.7 3.6 5.9 5.2 2.7 5.8 5.2 2.9

42 570 40 491 37 422 47 718 93 159 58 029 48 312 57 321 50 688 47 718 51 858 74 772 75 040 142 884 208 152 219 821

2 712 27 118 70 508 271 184 5 423 687 10 847 375 27 000 4 608 971 27 118 27 118 27 118 5 423 687 10 847 375 27 118 5 423 687 10 847 375

a-g

k6CHCO3 + k7CCO32-, s-1

12 151 121 516

See Table 3.

hydroxyl radical reaction of fluorene, phenanthrene, and acenaphthene, respectively. These rate constants are of the order of magnitude reported for different organics (Buxton et al., 1988). Regarding polynuclear aromatics, Evers et al. (1980) reported a value of 5 × 109 M-1 s-1 for the rate constant of the hydroxyl radical-naphthalene reaction. Once kOHP is known, concentration of hydroxyl radicals was calculated from eq 15 applied to the start of oxidation. The results are shown in Table 4. As observed from Table 4, the increase of pH (from 2 to 7) and hydrogen peroxide concentration (from 0 to 0.02 M) was found to significantly increase the concentration of hydroxyl radicals. However, higher concentration of hydrogen peroxide up to 0.4 M and pH up to 12 lead to an expected decrease of hydroxyl radical concentration. These observations are in agreement with contributions of hydroxyl radical termination steps (terms of the denominator of eq 15) also shown in Table 4. At this respect, only when the initial hydrogen peroxide concentration is 0.4 M is the contribution of kOHPCPAH negligible (the worst case is 5%, corresponding to the oxidation of acenaphthene). This validates the procedure followed to determine kOHP. In Table 4 the results corresponding to experiments in the presence of bicarbonate ion are also given. For these cases, it was assumed that bicarbonate-carbonate ions only act through reactions (6) and (7), that is, exclusively as inhibitors of hydroxyl radicals. Thus, its contributing term, k6CHCO3- + k7CCO32-, represents approximately 19 and 64% of the total scavenging term at the start of oxidation for total bicarbonate concentrations of 10-3 and 10-2 M, respectively. It is evident that the presence of bicarbonate ions should have retarded the oxidation rate of fluorene if these ions only participate through reactions (6) and (7). It is likely that bicarbonate ion, or more specifically its anion radical, participates in the process through other reactions as before suggested. Due to the importance of this inorganic compound in natural waters, there is much to be done to elucidate the role of bicarbonate ions in this process, which undoubtedly deserves more research attention. Conclusions The simultaneous presence of hydrogen peroxide at concentrations between 10-3 and 10-2 M and 254 nm UV radiation yields significant improvements in the rate

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 889

of PAH disappearance in water. Thus, total oxidation of the initial PAH is achieved in less than 7 min compared to 20 min in the UV radiation or direct photolysis process (case of acenaphthene). There is an optimum concentration of hydrogen peroxide below and above which PAH oxidation rate is reduced. This concentration, at the conditions of this work, turned out to be about 10-2 M. Neutral pH seems to be the most appropriate to carry out the UV/H2O2 oxidation, while bicarbonate ions do not affect the oxidation rate at the concentrations here applied (10-3-10-2 M). Due to the importance of these ions in natural waters, it is recommended to further study the mechanism through which bicarbonate ion or its anion radical, CO3‚-, reacts with PAHs. The UV radiation or UV/H2O2 oxidation of PAHs, on the other hand, gives rise to the appearance of a series of intermediate compounds whose competitive effect and potential hazard should also be assessed. Most compounds of complex structure initially formed are finally destroyed if oxidation time is prolonged. It seems that carboxylic acids are the main final products (although only oxalic acid was successfully identified in the oxidation of fluorene and phenanthrene). Contribution of direct photolysis in the UV/H2O2 oxidation also varies depending on the hydrogen peroxide concentration and pH. With 0.4 M hydrogen peroxide concentration, PAH oxidation rate is exclusively due to hydroxyl radical attack, but the competitive effect of peroxide leads to significant inhibition of degradation. The UV/H2O2 oxidation seems to follow a mechanism of free-radical and direct photolysis reactions, from which a kinetic equation can be deduced. The application of this kinetics at high hydrogen peroxide concentration allows the determination of the rate constant of the reaction between the hydroxyl radical and PAH. Rate constants obtained are on the order of 109 M-1 s-1, hence, the same order of magnitude of others reported in literature (Evers et al., 1980; Buxton et al., 1988). Acknowledgment We thank C.I.C.Y.T. of Spain for its economic support through Grant AMB93/654. Nomenclature C ) concentration in water, M F ) fraction of absorbed light, dimensionless I0 ) flow of incident radiation, einstein L-1 s-1 k ) reaction rate constant, M-1 s-1 or s-1 L ) effective path of radiation in a photochemical reactor, cm r ) reaction rate, M s-1 t ) reaction time, s Greek Letters γ ) percentage contribution of direct photolysis in the oxidation of PAHs, defined by eq 16, dimensionless Φ ) photochemical reaction quantum yield, mol (photon)-1  ) extinction coefficient, M-1 cm-1 Subindexes A ) acenaphthene A0 ) acenaphthene at initial conditions 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 that absorbs radiation o ) start of oxidation OHI ) reaction between hydroxyl radical and species I PAH ) any polynuclear aromatic hydrocarbon PH ) phenanthrene PH0 ) phenanthrene at initial conditions UV ) direct photolysis r ) radical reactions between PAH and hydroxyl radicals

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Received for review June 16, 1995 Revised manuscript received October 17, 1995 Accepted November 7, 1995X IE950363L

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