Phenol Chlorination and Photochlorination in the Presence of

May 26, 2005 - hydrogen peroxide and chloride in dark acidic solutions. In the former case phenol photochlorination is most likely due to the formatio...
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Environ. Sci. Technol. 2005, 39, 5066-5075

Phenol Chlorination and Photochlorination in the Presence of Chloride Ions in Homogeneous Aqueous Solution D A V I D E V I O N E , * ,† V A L T E R M A U R I N O , † CLAUDIO MINERO,† PAOLA CALZA,† AND EZIO PELIZZETTI† Dipartimento di Chimica Analitica, Universita` di Torino, Via P. Giuria 5, 10125 Torino, Italy

Phenol chlorination was studied in the presence of dissolved Fe(III) and chloride under irradiation and of hydrogen peroxide and chloride in dark acidic solutions. In the former case phenol photochlorination is most likely due to the formation of Cl2•- as a consequence of Fe(III) irradiation in the presence of chloride. The most efficient pathway is the photolysis of FeOH2+ producing hydroxyl, which oxidizes chloride to Cl•. The latter finally yields Cl2•- upon further reaction with chloride. The importance of the pathway involving FeOH2+ is higher at higher pH and moderately low chloride concentration. At pH 2.0 and [Cl-] > 0.03 M chlorophenol generation rate decreases with increasing [Cl-], due to the formation of the much less photoactive species FeCl2+/FeCl2+. The photolysis of FeCl2+/ FeCl2+ yielding Cl• is likely to play an important role at pH 0.5 and high chloride, but under such conditions chlorophenol formation rates are about an order of magnitude lower than at pH 2.0. Due to pH and kinetic constraints, under most environmental conditions the photochemistry of FeCl2+/FeCl2+ can be expected to play a minor role toward chlorination when compared with the one of FeOH2+, which leads to hydroxyl-mediated chloride oxidation. Hydrogen peroxide and chloride react in dark acidic solutions to yield HClO, involved in electrophilic chlorination processes. Chlorophenol formation rates under such conditions are directly proportional to [H+]. The described chlorination and photochlorination processes can take place in acidic aerosols of marine origin, naturally rich in chloride and Fe(III). Antarctic aerosol is also rich of hydrogen peroxide and often strongly acidic due to the presence of sulfuric acid of biogenic origin.

Introduction The occurrence and generation of organohalogens in the environment has become an important issue in the field of atmospheric chemistry since the discovery that man-made halogenated gases affect the rate of stratospheric ozone destruction. Apart from anthropogenic sources, a relevant amount of naturally occurring organohalogens has been found to originate as a consequence of environmental processes (1-4). For instance, oceans are very relevant sources of halogenated methanes such as CH3Cl, CH3Br, CH3I, * Corresponding author phone: +39-011-6707633; fax: +39-0116707615; e-mail: [email protected]. www.abcrg.it. † Research group Web address: www.environmentalchemistry.unito.it. 5066

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and CHBr3 (5, 6). The biogenic processes that can act as sources of these compounds have been extensively studied. The formation of CH3Cl is likely to take place upon methylation of chloride by the S-adenosylmethionine/halide ion methyltransferase enzyme (7). Halide methylation can also take place upon reaction with dimethylpropiothetin, produced by many marine algae and phytoplankton (8). On the contrary, the formation of bromoform and similar compounds seems to derive from the halogenation of natural organic matter by the enzyme bromoperoxidase in the presence of bromide and hydrogen peroxide (1). The reactions taking place in the presence of bromo- and chloroperoxidase, natural organic matter, halide ions, and hydrogen peroxide are also thought to be a major source of organohalogen compounds in soil (9-11). To date relatively little attention has been focused on abiotic, chemical processes for organohalogen production and relatively few examples are known. For instance, some of the environmental CH3Cl can derive from the reaction between the biogenic CH3I and chloride ions in seawater (1). Furthermore, active chlorine species can form in chloridecontaining aerosol in the presence of N2O5 and ozone (1216):

ClNO3(g) + NaCl(s) f Cl2(g) + NaNO3(s) 2Cl-(aq) + O3(g) + H2O f Cl2(g) + O2(g) + 2OH-(aq)

(1) (2)

N2O5(g) + NaCl(s) f ClNO2(g) + NaNO3(s)

(3)

ClNO2(g) + hν f Cl• + •NO2

(4)

Reaction 2 is enhanced upon irradiation in the presence of metal oxides (TiO2, Fe2O3), possibly under photocatalytic conditions (13). It has also been found that irradiation of titanium dioxide in the presence of chloride ions is able to chlorinate chloroform to tetrachloromethane (17). Furthermore, irradiation of other semiconductor compounds (Fe2O3, CdS) in the presence of various halide ions is able to lead to phenol halogenation (18), provided that the reduction potential of the halide is lower than the reduction potential of the semiconductor valence band. The described processes are possibly due to the oxidation of the halide ions to the corresponding radicals, which are responsible for the halogenation reactions. This paper reports results of chlorination studies in homogeneous aqueous solution, using phenol as a model molecule. Phenol chlorination was assessed in the presence of important environmental factors such as dissolved Fe(III) and hydrogen peroxide (19). These compounds have already been demonstrated to induce relevant phenol nitration in the presence of nitrite/nitrous acid (20, 21).

Experimental Section Reagents and Materials. Phenol (purity grade >99%), 2-chlorophenol (>99%), 3-chlorophenol (98%), 4-chlorophenol (>99%), catechol (>99%), hydroquinone (>99%), 1,4benzoquinone (98%), and Na2SO4 (>99.5%) were purchased from Aldrich, Fe(ClO4)3 × 9 H2O (>97%) and NaCl (>99.5%) from Fluka, H2O2 (35%), FeCl3 × 6 H2O (>99%), HClO4 (70%), H3PO4 (85%), NaH2PO4 × H2O (>99%), and dichloromethane (SupraSolv for organic trace analysis) from Merck, acetonitrile (Supergradient HPLC grade) from Scharlau, and N2 (99.998%) 10.1021/es0480567 CCC: $30.25

 2005 American Chemical Society Published on Web 05/26/2005

FIGURE 1. Absorption spectra of 6.6 × 10-4 M Fe(ClO4)3 in the presence of different amounts of chloride ions (the concentration of added chloride is reported on the Figures). pH values are 0.5 (A) and 2.0 (B), adjusted by addition of HClO4. The emission spectrum of the Philips TL 01 lamp is also reported (emission maximum at 313 nm). and He (99.995%) from SIAD. All reagents were used as received without further purification. Irradiation and Dark Experiments. Irradiation was carried out in cylindrical Pyrex glass cells (4.0 cm diameter, 2.3 cm height), containing a 5 mL aqueous solution. The radiation source was a 100 W Philips TL 01 lamp with emission maximum at 313 nm (22). The spectrum of the lamp and the spectra of Fe(III) at different concentration values of added chloride (pH 0.5 (A) and pH 2.0 (B)) are reported in Figure 1. The changes in the Fe(III) spectrum with increasing chloride are due to the formation of the species FeCl2+ and FeCl2+ (23). The difference in the Fe(III) spectrum between pH 0.5 and pH 2.0 in the absence of chloride is due to relevant absorption by FeOH2+ at the higher pH value. Total photon flux in the cells was 1.3 × 10-7 Ein s-1, actinometrically determined using the ferrioxalate method (24). Dark experiments were carried out in magnetically stirred vials, wrapped with aluminum foil. Analytical Determinations. After the scheduled reaction time, the solutions were HPLC analyzed with a Merck-Hitachi chromatograph equipped with a model 7125 manual Rheodyne injector (injection loop volume 54 µL), AS2000A Autosampler (injection volume up to 100 µL), L-6200 and L-6000 pumps, and L-4200 UV-vis detector (detection wavelength 224 nm). The manual injector was used to study the dark processes due to faster operation, while the autosampler was used for the photoinduced reactions (higher injection volume). The column used was a RP-C18 LichroCART (Merck, length 125 mm, diameter 4 mm), packed with

LiChrospher 100 RP-18 (5 µm diameter). The analytes were eluted with a 30:70 mixture of acetonitrile:phosphate buffer (H3PO4 + NaH2PO4, 0.050 M total phosphate, pH 2.8), flow rate 1.0 mL min-1. Under these conditions the retention times were (min) as follows: phenol (3.70), 2-chlorophenol (8.60), 3-chlorophenol (11.60), and 4-chlorophenol (10.70). The column dead time was 0.90 min. No formation of 3-chlorophenol from phenol was detected in the systems under study. Absorption spectra were obtained with a Varian Cary 100 Scan UV-vis spectrophotometer. An overview of the various transformation intermediates of phenol in the presence of FeCl3 upon irradiation was obtained by GC-MS analysis. After irradiation, aqueous samples were extracted with CH2Cl2, and the extract was then dried with anhydrous Na2SO4 and concentrated under a gentle stream of high-purity nitrogen. The concentrated extract was then injected into a Thermo-Quest Finnigan GCMS, equipped with a GC2000 gas chromatograph, a GCQ plus MS2000 ion trap mass analyzer and a 30 m capillary column (CP-Sil 8CB lowbleed/MS). The conditions employed were as follows: carrier flow 1 mL min-1 (He), injector temperature 300 °C, oven temperature 40 °C (3 min), then at 300 °C at 15 °C/min, stay at 300 °C for 20 min, adopting manual splitless injection (1 µL injected volume). Initial Rates and Relative Uncertainty. The time evolution data of phenol were fitted with the function [P]t ) [P]0 exp(-kdPt), where [P]t is the phenol concentration at time t, [P]0 is the initial phenol concentration, and kdP is the pseudofirst-order rate constant for phenol degradation. The data of each chlorophenol isomer were fitted with [ClP]t ) kfClP[P]0 (kdClP - kdP)-1 [exp(-kdPt) - exp(-kdClPt)], where [ClP]t is the concentration of the chlorophenol isomer at time t, [P]0 is the initial phenol concentration, kfClP is the pseudo-firstorder rate constant for the formation of the chlorinated isomer from phenol, and kdP and kdClP are the pseudo-firstorder rate constants for the degradation of phenol and the chlorophenol isomer, respectively. The initial degradation rate of phenol and the initial formation rates of chlorophenols were calculated as the slope of the tangent to the fitting curve at t ) 0. The initial disappearance rate of phenol and the initial formation rates of 2-chlorophenol and 4-chlorophenol will be reported, together with the associated standard errors (intraseries variability, (1σ, derived from the goodness of the fit to the experimental data). For further details see refs 20 and 21.

Results and Discussion Phenol Photochlorination upon Irradiation of Fe(III)/Cl-. The 313 nm irradiation of phenol solutions containing Fe(III) and chloride ions produces 2-chlorophenol (2ClP) and

FIGURE 2. Time evolution of 1.0 × 10-3 M phenol, 2ClP and 4ClP in the presence of 6.6 × 10-3 M FeCl3 at pH 2.0 (adjusted by addition of HClO4), irradiation at 313 nm. Note separate concentration scales. VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Initial Degradation Rate of Phenol (P) and Initial Formation Rates of 2ClP and 4ClP upon Irradiation of Fe(III)/Cl- at 313 nma no.

conditions

pH

P initial degradation rate (M s-1)

2ClP initial formation rate (M s-1)

4ClP initial formation rate (M s-1)

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

P + FeCl3 P + FeCl3 P + FeCl3 P + FeCl3 P + FeCl3 P + Fe(ClO4)3 + 1 × 10-3 M NaCl P + Fe(ClO4)3 + 3 × 10-3 M NaCl P + Fe(ClO4)3 + 1 × 10-2 M NaCl P + Fe(ClO4)3 + 3 × 10-2 M NaCl P + Fe(ClO4)3 + 0.1 M NaCl P + Fe(ClO4)3 + 0.3 M NaCl P + Fe(ClO4)3 + 1 M NaCl P + Fe(ClO4)3 + 1 × 10-3 M NaCl P + Fe(ClO4)3 + 3 × 10-3 M NaCl P + Fe(ClO4)3 + 1 × 10-2 M NaCl P + Fe(ClO4)3 + 3 × 10-2 M NaCl P + Fe(ClO4)3 + 0.1 M NaCl P + Fe(ClO4)3 + 0.3 M NaCl P + Fe(ClO4)3 + 1 M NaCl 2.6 × 10-4 M P + FeCl3 5.3 × 10-4 M P + FeCl3 8.0 × 10-4 M P + FeCl3 P + FeCl3 2.6 × 10-4 M P + FeCl3, N2 5.3 × 10-4 M P + FeCl3, N2 8.0 × 10-4 M P + FeCl3, N2 P + FeCl3, N2

0.5 1.0 1.5 2.0 2.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

(1.35 ( 0.03) × 10-8 (1.45 ( 0.03) × 10-8 (4.43 ( 0.17) × 10-8 (1.01 ( 0.09) × 10-7 (2.06 ( 0.33) × 10-7 (4.80 ( 0.06) × 10-9 (7.00 ( 0.08) × 10-9 (7.91 ( 0.26) × 10-9 (9.45 ( 0.33) × 10-9 (1.24 ( 0.04) × 10-8 (1.32 ( 0.05) × 10-8 (1.25 ( 0.05) × 10-8 (1.50 ( 0.16) × 10-7 (1.21 ( 0.11) × 10-7 (1.10 ( 0.10) × 10-7 (8.89 ( 0.66) × 10-8 (6.58 ( 0.27) × 10-8 (4.10 ( 0.30) × 10-8 (4.20 ( 0.27) × 10-8 (7.94 ( 0.90) × 10-8 (6.83 ( 0.64) × 10-8 (8.46 ( 0.50) × 10-8 (8.10 ( 0.45) × 10-8 (5.51 ( 0.87) × 10-8 (4.85 ( 0.39) × 10-8 (5.85 ( 0.36) × 10-8 (4.54 ( 0.07) × 10-8

(2.37 ( 0.42) × 10-10 (4.31 ( 0.33) × 10-10 (1.06 ( 0.11) × 10-9 (3.14 ( 0.23) × 10-9 (2.19 ( 0.08) × 10-9 (7.66 ( 0.23) × 10-11 (3.51 ( 0.42) × 10-10 (3.93 ( 0.38) × 10-10 (2.36 ( 0.27) × 10-10 (2.28 ( 0.26) × 10-10 (2.18 ( 0.47) × 10-10 (1.79 ( 0.22) × 10-10 (2.47 ( 0.01) × 10-10 (1.19 ( 0.06) × 10-9 (2.08 ( 0.26) × 10-9 (2.22 ( 0.15) × 10-9 (1.70 ( 0.23) × 10-9 (7.92 ( 0.69) × 10-10 (3.40 ( 0.76) × 10-10 (2.60 ( 0.35) × 10-9 (3.40 ( 0.09) × 10-9 (3.16 ( 0.23) × 10-9 (3.14 ( 0.08) × 10-9 (2.95 ( 0.30) × 10-9 (3.08 ( 0.07) × 10-9 (2.98 ( 0.39) × 10-9 (3.40 ( 0.43) × 10-9

(2.37 ( 0.44) × 10-10 (7.21 ( 0.84) × 10-10 (1.34 ( 0.17) × 10-9 (3.14 ( 0.30) × 10-9 (2.49 ( 0.27) × 10-9 (4.01 ( 0.35) × 10-11 (2.56 ( 0.40) × 10-10 (2.45 ( 0.34) × 10-10 (2.36 ( 0.31) × 10-10 (2.52 ( 0.17) × 10-10 (2.17 ( 0.25) × 10-10 (2.94 ( 0.19) × 10-10 (2.03 ( 0.05) × 10-10 (9.80 ( 0.15) × 10-10 (1.92 ( 0.35) × 10-9 (2.38 ( 0.33) × 10-9 (2.28 ( 0.19) × 10-9 (1.00 ( 0.20) × 10-9 (7.36 ( 0.81) × 10-10 (2.63 ( 0.26) × 10-9 (3.22 ( 0.12) × 10-9 (3.14 ( 0.30) × 10-9 (3.15 ( 0.28) × 10-9 (2.99 ( 0.36) × 10-9 (3.12 ( 0.15) × 10-9 (2.96 ( 0.38) × 10-9 (3.40 ( 0.44) × 10-9

a Initial conditions (unless otherwise reported): 1.0 × 10-3 M P, 6.6 × 10-3 M FeCl , or Fe(ClO ) , pH adjusted with HClO . Error ) standard 3 4 3 4 deviation (σ), obtained from the goodness of the fit to the experimental data, representing intraseries variance.

4-chlorophenol (4ClP) as chloroderivatives. No 3ClP was detected in the studied systems. Chlorophenol detection limits under the adopted conditions were around 2-5 × 10-8 M. Figure 2 shows as an example the time evolution of 1.0 × 10-3 M phenol, 2ClP, and 4ClP in the presence of 6.6 × 10-3 M FeCl3 at pH 2.0, adjusted by addition of HClO4. The time evolution curves of 2ClP and 4ClP show a broad maximum after about 1 h of irradiation, which is most likely a consequence of the formation/transformation budget. Chlorophenol transformation starts to prevail over formation after 1 h, possibly due to the decrease in phenol concentration that increases the steady-state [•OH], mainly controlled by [Phenol], which enhances the degradation processes. The fraction of transformed phenol accounted for by chlorophenols is typically between 1% and 10% in the runs carried out in these systems (see Table 1). An overview of the various transformation intermediates of phenol in the system shown in Figure 2 was obtained by GC-MS analysis of the dichloromethane extracts. The main identified compounds were 1,4-benzoquinone, 2- and 4-ClP, and various dihydroxybiphenyls and phenoxyphenols. A C5 ketone and a C5 alcohol were also tentatively identified, which may represent ring-opening phenol derivatives. Catechol and hydroquinone, poorly extracted in dichloromethane, were not detectable in HPLC-UV runs due to the strong interference by Fe(III). Their formation from phenol upon Fe(III) photolysis is however quite likely as confirmed by the detection of 1,4-benzoquinone, an oxidation product of hydroquinone (25). Based on the literature stability constants (23), corrected for the ionic strength, the prevailing Fe(III) species under the conditions adopted in Figure 2 are Fe3+ (79%), FeCl2+ (13%), and FeOH2+ (8%). The chlorocomplexes FeCl2+ and FeCl3 are present in negligible concentration values under such conditions. The photolysis of Fe3+ and FeOH2+ yields hydroxyl radicals, with FeOH2+ being the most photoactive 5068

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compound (25-29).

FeOH2+ + hν f Fe2+ + •OH

[Φ5310 nm ) 0.195] (5)

Fe3+ + hν + H2O f Fe2+ + •OH + H+ [Φ6310 nm < 0.05] (6) Indeed, FeOH2+ photochemistry is thought to play a relevant role in natural waters (25-27) and in the atmospheric aqueous phase (28, 29). In the presence of chloride ions, hydroxyl radicals are involved into an equilibrium reaction yielding ClOH-• as a product (reaction 7). At pH < 7 ClOH-• evolves into Cl• (reaction 8 (30, 31)). An alternative pathway that can be postulated for the generation of Cl• involves photolysis of the FeCl2+ complex, in analogy with the photochemical behavior of FeOH2+ and FeSO4+ (28):

Cl- + •OH a ClOH-• ClOH-• + H+ a Cl• + H2O

[Keq,7 ) 0.70 M-1]

(7)

[Keq,8 ) 1.6 × 107 M-1] (8)

FeCl2+ + hν f Fe2+ + Cl•

(9)

The radical Cl•, produced by either pathway, can react with chloride to yield Cl2•- in an equilibrium reaction (3135):

Cl• + Cl- a Cl2•-

[Keq,10 ) 1.9 × 105 M-1]

(10)

The forward reaction of the equilibrium 10 is very fast (rate constant 2.1 × 1010 M-1 s-1 (31)). In the present work the adopted chloride concentration was in the interval 1.0 × 10-3-1.0 M, and reaction 10 indicates that [Cl2•-] . [Cl•] under the conditions we adopted. The radical species Cl2•-

is an oxidant that can be involved in electron-transfer, hydrogen abstraction, or addition reactions with organic compounds (34). Phenol (H-Ph-OH) undergoes a moderately fast reaction with Cl2•- (rate constant 2.5 × 108 M-1 s-1 (34, 36)), most likely involving a one-electron oxidation step forming chloride and phenol radical cation (H-Ph+•-OH (34)). The latter undergoes rapid deprotonation to yield the radical phenoxyl (H-Ph-O• (37, 38)). Interestingly, a hydrogen abstraction by Cl2•- on phenol would yield phenoxyl as well, thus one-electron oxidation and hydrogen abstraction are equivalent from the point of view of the products. Phenoxyl can be tentatively hypothesized to react with Cl2•-, yielding chlorophenols (Cl-Ph-OH (39, 40)). However, this point will be the object of further discussion.

H-Ph-OH + Cl2•- f H-Ph+•-OH + 2ClH-Ph+•-OH a H-Ph-O• + H+

(11)

[pKa,12 ) -2.0] (12)

H-Ph-O• + Cl2•- f Cl-Ph-OH + Cl-

(13)

The actual presence of phenoxyl in the systems under study is confirmed by the GC-MS detection of dihydroxybiphenyls and phenoxyphenols, which form upon reaction between phenoxyl and phenol (40). Effect of pH. The generation of Cl2•- requires the preliminary formation of Cl• radicals. Cl• may be produced upon chloride oxidation by hydroxyl (the latter generated upon photolysis of Fe3+ and FeOH2+ (reactions 5-8)) or directly upon photolysis of FeCl2+ (reaction 9). A possible way to assess the contribution of the various pathways to phenol photochlorination is to study the effects that pH and chloride concentration have on the initial formation rates of 2ClP and 4ClP. The effect of pH was studied in the interval 0.52.0. The results at pH 2.5 are also reported for completeness, but they are hardly possible to discuss because under such conditions, at the adopted Fe(III) concentration values, formation of polynuclear iron species and iron colloids would take place (20, 25-28). The higher pH values are however more significant from an environmental point of view, thus pH 2.0 is the most reasonable compromise between experimental convenience and environmental significance. The majority of the experiments reported in this paper were in fact carried out at pH 2.0. Under such conditions one can ensure the presence of the photoactive compound FeOH2+ without formation of polynuclear and colloidal species (20, 25-28). Under typical environmental conditions, FeOH2+ is the prevailing Fe(III) hydroxocomplex in the pH interval 3-5, which is highly significant to aerosol chemistry (29). Figure 3 shows that chlorophenol formation rate increases with increasing pH (see also Table 1, entries no. 1-4). The pH trend of the rates is compatible with a pathway dominated by the photolysis of FeOH2+, because the concentration of this species can be expected to increase with increasing pH in the interval 0.5-2.0. It is interesting to observe that, while Fe3+ is about an order of magnitude more concentrated than FeOH2+ at pH 2.0, FeOH2+ absorbs a relevantly higher fraction of radiation under such conditions. Actually, the molar absorbivity of FeOH2+ at 310 nm is about 40 times higher than that of Fe3+ (28). This fact can be clearly seen in Figure 1A/B, by comparison of the Fe(III) spectra at pH 0.5 and 2.0 in the absence of chloride. At pH 0.5 and no chloride the absorption of the solution is mainly accounted for by Fe3+. At the emission maximum of the lamp (313 nm) the absorbance of 6.6 × 10-4 M Fe(III) is around 0.05. At pH 2.0 both Fe3+ and FeOH2+ are present, with [Fe3+] prevailing over [FeOH2+]. The absorbance of 6.6 × 10-4 M Fe(III) is now much higher (0.35), and the difference with pH 0.5 is due to the very relevant contribution of FeOH2+ absorption. Higher

FIGURE 3. Initial formation rates of 2ClP and 4ClP in the presence of 1.0 × 10-3 M phenol and 6.6 × 10-3 M FeCl3 as a function of pH (adjusted with HClO4), irradiation at 313 nm. The concentration values of the Fe(III) species Fe3+, FeCl2+, and FeOH2+ (relative to the ones at pH 2.0, where [Fe3+] ≈ 5.2 × 10-3 M, [FeCl2+] ≈ 9 × 10-4 M, and [FeOH2+] ≈ 5 × 10-4 M) are also reported as a function of pH. 4.0E-9 is a compact form for 4.0 × 10-9. radiation absorption by FeOH2+ at pH 2.0, combined with higher quantum yield for hydroxyl photoproduction when compared with Fe3+ (see reactions 5 and 6), indicate that most hydroxyl at pH 2.0 would be generated by the photolysis of FeOH2+. The importance of FeOH2+ photolysis in hydroxyl production as well as the absolute rate of hydroxyl photogeneration is then expected to decrease with decreasing pH, as [FeOH2+] gets lower. Coherently, for instance, the initial degradation rate of phenol decreases with decreasing pH in the interval under discussion (Table 1, entries no. 1-4). Upon consideration of the degradation intermediates, it can be expected that phenol transformation in our system takes place upon reaction with •OH and with Cl2•-, the formation of the latter being enhanced by high hydroxyl levels (reactions 7, 8, and 10). Further examination of the pH effect on chlorophenol formation rate can be carried out when considering the concentration values of Fe3+, FeOH2+, and FeCl2+ in the systems under study. These values can be calculated from the thermodynamic stability constants of the complexes, namely KFeCl ) a(FeCl2+) a(Fe3+)-1 a(Cl-)-1 ) 30.2 and KFeOH ) a(FeOH2+) a(Fe3+)-1 a(OH-)-1 ) 3.47 × 1011 (23) (a ) activity). It is also KFeCl2 ) a(FeCl2+) a(FeCl2+)-1 a(Cl-)-1 ) 4.47 and KFeCl3 ) a(FeCl3) a(FeCl2+)-1 a(Cl-)-1 ) 0.10 (23). To obtain the concentration values of the various species, the thermodynamic constants were corrected for the ionic strength of the solution according to the Guntelberg equation. Equilibrium calculations yielded [Fe3+] ≈ 5.2 × 10-3 M, [FeOH2+] ≈ 5 × 10-4 M, and [FeCl2+] ≈ 9 × 10-4 M in the presence of 6.6 × 10-3 M FeCl3 at pH 2.0. [FeCl2+] and [FeCl3] were below 10-5 M. Iterative calculations were necessary to adjust the equilibrium concentration values of the species in solution. The pH trend of the main species as resulting from calculations is reported in Figure 3. The comparison with the experimental data suggests that chlorophenol formation at pH 2.0 is dominated by the photolysis of the hydroxocomplex FeOH2+ (reactions 5, 7, 8, and 10). At lower pH the concentration of FeOH2+ decreases, and its role becomes less important. Chlorophenol formation rate gets lower as a consequence. The residual chlorophenol formation at pH 0.5 can hardly be accounted for by FeOH2+ photolysis. Under such conditions the residual formation of chlorophenols may be initiated by the photolysis of either Fe3+ (reactions 6-8 and 10) or FeCl2+ (reactions 9 and 10). Effect of Addition of NaCl. To assess the relative contribution of the photolysis of Fe3+ and FeCl2+ to chloVOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Initial formation rates of 2ClP and 4ClP in the presence of 1.0 × 10-3 M phenol and 6.6 × 10-3 M Fe(ClO4)3 as a function of the concentration of added Cl- (NaCl). pH 0.5, adjusted with HClO4, irradiation at 313 nm. The relative fractions r of Fe3+, FeCl2+, FeCl2+, and FeCl3 are also reported. 4.0E-10 is a compact form for 4.0 × 10-10. rophenol generation at pH 0.5, it is possible to study the initial formation rates of 2ClP and 4ClP upon irradiation of phenol and Fe(ClO4)3, as a function of the concentration of added chloride (NaCl). Actually, [FeCl2+] can be expected to increase and [Fe3+] to decrease with increasing chloride. Figure 4 shows the initial formation rates of 2ClP and 4ClP at pH 0.5 as a function of added chloride (see also Table 1, entries no. 6-12), together with the relative fractions of Fe3+, FeCl2+, FeCl2+, and FeCl3. The calculated fractions of the Fe(III) species should be regarded as rough estimates due to the high value of the ionic strength even at low chloride (it is [HClO4] ≈ 0.3 M at pH 0.5). The fraction of FeOH2+ is not reported: it is extremely low at pH 0.5, and the role of FeOH2+ photochemistry in chlorophenol formation at this pH value is negligible, as already discussed (see Figure 3). Above 0.1 M added chloride a relevant amount of FeCl2+ is present, together with FeCl2+. It was not possible to completely differentiate the photochemical behavior of the two species, thus they will be discussed together. As reported in Figure 4, the initial formation rates of chlorophenols as a function of the concentration of added chloride show an increase from 10-3 to 10-2 M NaCl. This can be due to the corresponding increase in [FeCl2+] or to the competition between 1.0 × 10-3 M phenol and chloride for the reaction with hydroxyl produced upon Fe3+ photolysis (the rate constant of the forward reaction between •OH and Cl- is 4.3 × 109 M-1 s-1 (30, 31), the one between phenol and hydroxyl is 1.4 × 1010 M-1 s-1 (30)). In the hypothesis that the photolysis of FeCl2+/FeCl2+ controls chlorophenol formation rate, the latter cannot be expected to increase up to 1.0 M added chloride because absorption by Fe(III) chlorocomplexes will reach saturation around 0.03 M added chloride (solution absorbance ≈ 1.5). This datum can be derived from Figure 1A, considering that the absorbance values are to be multiplied by about 10 since Fe(III) in the irradiation experiments (6.6 × 10-3 M) was 10 times more concentrated than in the solutions, the spectra of which are reported in Figure 1A (6.6 × 10-4 M Fe(III)). Coherently with the absorption data, chlorophenol formation rate keeps roughly constant above 0.03 M chloride (Log10([NaCladded]) = -1.5). Considerations concerning competition for light absorption give further evidence in favor of a relevant role played by FeCl2+/FeCl2+ photolysis at high [NaCladded]. Figure 1A shows that, with increasing chloride concentration, the light absorption by FeCl2+/FeCl2+ causes a relevant spectral interference on Fe3+ absorption. This observation is even more relevant in that the Fe(III) concentration adopted for 5070

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FIGURE 5. Initial formation rates of 2ClP and 4ClP in the presence of 1.0 × 10-3 M phenol and 6.6 × 10-3 M Fe(ClO4)3 as a function of the concentration of added Cl- (NaCl). pH 2.0, adjusted with HClO4, irradiation at 313 nm. The relative fractions r of Fe3+, FeOH2+, FeCl2+, FeCl2+, and FeCl3 are also reported. 3.0E-9 is a compact form for 3.0 × 10-9. the absorption spectra measurements was 10 times lower than the one used in irradiation experiments, to make spectra measurements possible. Furthermore, [Fe3+] decreases with increasing chloride (see Figure 4). In the presence of 6.6 × 10-3 M total Fe(III) and 0.30 M added Cl- (Log10([NaCladded]) = -0.5), FeCl2+/FeCl2+ would reduce the radiation intensity available for Fe3+ at about 1/10 the one in the absence of chloride: see Figures 1A and 4, and consider that the ratio of the absorptances (the fractions of radiation absorbed by the species) is equal to the ratio of the absorbances (20). Calculations also show that, in the presence of 0.30 M added chloride (Log10([NaCladded]) = -0.5), the rate of Fe3+ photolysis should be about 5 times lower than in the presence of 0.03 M added chloride (Log10([NaCladded]) = -1.5). In the reasonable hypothesis that chlorophenol formation rate as initiated by Fe3+ photolysis is directly proportional to the rate of Fe3+ photolysis, if Fe3+ plays a major role one would expect an equivalent 5 times decrease in the initial formation rates of 2ClP and 4ClP. However, the experimental data reported in Figure 4 show that chlorophenol formation rates keep about constant between 0.03 and 0.30 M added chloride (Log10([NaCladded]) = -1.5 f -0.5). Chlorophenol formation at high chloride levels should thus imply a relevant contribution by the photolysis of FeCl2+/FeCl2+, and the role of Fe3+ photolysis would be minor at high chloride concentration. Conversely, it can be important at low chloride and account for the maximum in chlorophenol formation rate around (3-10) × 10-3 M added chloride (Log10([NaCladded]) = -2.5 f -2.0), which would otherwise be difficult to account for. The trend of chlorophenol formation rate reported in Figure 4 can thus be accounted for as follows: at low chloride the photolysis of both Fe3+ and FeCl2+ can play a role in chlorophenol formation. Figure 1A indicates that at 0.01 M added chloride the radiation absorption by FeCl2+ is already quite relevant, despite its low concentration. Under such conditions, on the contrary, [FeCl2+] is probably too low for this species to have any importance. The initial increase in chlorophenol formation rate with increasing [NaCladded] can be due to the corresponding increase in [FeCl2+] and to the competition between phenol and chloride for reaction with hydroxyl, formed upon photolysis of Fe3+ (reaction 6). The decrease in chlorophenol formation rate after the maximum is likely caused by a lower contribution of Fe3+ photolysis, due to the combination of two factors: the decrease of [Fe3+] and the light extinction by the chlorocomplexes FeCl2+ and FeCl2+, which reduces the radiation intensity available for Fe3+. At high chloride the initial formation rate of chlorophenols is however still relevant, most likely due to the

FIGURE 6. Initial formation rates of 2ClP and 4ClP in the presence of 6.6 × 10-3 M FeCl3, at pH 2.0 by addition of HClO4, as a function of phenol concentration. Irradiation at 313 nm. 5.0E-9 is a compact form for 5.0 × 10-9.

SCHEME 1. Proposed Early Steps in the Transformation Pathways of Phenol in the Presence of Fe(III) + Cl- under 313 nm Irradiation.

contribution by the photolysis of FeCl2+/FeCl2+. Chlorophenol formation rate keeps roughly constant with increasing chloride concentration, despite the increasing [FeCl2+/ FeCl2+], because light absorption by the chlorocomplexes reaches saturation above 0.03 M added chloride. Accordingly, further increase in [FeCl2+/FeCl2+] would not change relevantly their rate of radiation absorption. The effect of the concentration of added chloride on chlorophenol formation rate was also studied at pH 2.0, that is under conditions that are nearer to the environmental ones. Under such conditions the formation of chlorophenols should be dominated by the photolysis of FeOH2+, as already discussed. The initial chlorophenol formation rates as a function of added chloride concentration are reported in Figure 5 (1.0 × 10-3 M phenol, 6.6 × 10-3 M Fe(ClO4)3, pH

FIGURE 7. Initial formation rates of 2ClP and 4ClP in the presence of 1.0 × 10-3 M phenol, 0.20 M H2O2, and 0.20 M NaCl in the dark, as a function of pH (adjusted by addition of HClO4). 4.0E-9 is a compact form for 4.0 × 10-9. 2.0; see also Table 1, entries no. 13-19). The initial formation rates of 2ClP and 4ClP have a maximum for [NaCladded] = 10-1.5 M and then decrease with increasing chloride. Figure 5 also reports the relative fractions of Fe3+, FeOH2+, FeCl2+, FeCl2+, and FeCl3 as a function of added chloride. These data have been calculated from the already reported stability constants, corrected for the ionic strength of the solution according to the Guntelberg equation. The points at the highest chloride levels (high ionic strength) are probably less correctly estimated, but the general trend is unmistakable. The decrease in chlorophenol initial formation rate after the maximum is most likely linked with the concentration decrease of FeOH2+, due to the higher proportion of the less photoactive Fe(III)-chloride complexes and to the increasing ionic strength. On the contrary, the spectral interference by FeCl2+/FeCl2+ on FeOH2+ at pH 2.0 is more limited than in the case of Fe3+ at pH 0.5 (see Figure 1A/B). Figure 5 shows that below 10-1.5 M added chloride the initial formation rates increase with increasing [NaCladded]. This is possibly due to the competition between phenol and Cl- for reaction with hydroxyl, formed upon photolysis of FeOH2+ (reaction 5). In summary, the photolysis of FeOH2+ plays a major role at pH 2.0, while it is negligible at pH 0.5. At the lower pH value the formation of chlorophenols is initiated by the photolysis of Fe3+ and of FeCl2+/FeCl2+, with the contribution of the chlorocomplexes prevailing at high chloride. At pH 2.0 the initial formation rate of chlorophenols is maximum at [NaCladded] = 10-1.5 M. The increase in chlorophenol formation before the maximum is most likely a consequence of the competition between phenol and Cl- for reaction with hydroxyl: higher [Cl-] means that chloride would compete more efficiently with phenol, with a higher generation rate of Cl•/Cl2•-. The decrease in chlorophenol formation after the maximum is linked with the decrease of [FeOH2+] at high chloride (see Figure 5). A comparison between Figures 4 and 5 indicates that both at pH 0.5 and 2.0 the chlorocomplexes FeCl2+/FeCl2+ are present in a relevant amount at sufficiently high chloride. At both pH values the photolysis of FeCl2+/FeCl2+ can be expected to contribute to chlorophenol formation. However, due to the much higher photoactivity of FeOH2+ when compared with Fe3+, the relative contribution of FeCl2+/ FeCl2+ photolysis to chlorophenol formation at pH 2.0 is much lower than at pH 0.5. Given these premises, under most environmental conditions the photolysis of FeCl2+/FeCl2+ can be expected to have lower importance than the one of FeOH2+. VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Initial Degradation Rate of Phenol (P) and Initial Formation Rates of 2ClP and 4ClP in the Presence of H2O2 and Cl- in Dark Acidic Solutionsa no.

conditions

pH

P initial degradation rate (M s-1)

2ClP initial formation rate (M s-1)

4ClP initial formation rate (M s-1)

1 2 3 4 5 6 7

P + H2O2 + NaCl P + H2O2 + NaCl P + H2O2 + NaCl P + H2O2 + NaCl P + H2O2 + NaCl P + H2O2 + NaCl P + H2O2 + NaCl

0.3 0.4 0.8 1.1 1.5 2.0 3.0

(1.10 ( 0.01) × 10-8 (5.58 ( 0.04) × 10-9 (5.73 ( 0.03) × 10-9 (7.45 ( 0.03) × 10-9 (1.38 ( 0.031) × 10-9 (2.95 ( 0.34) × 10-10 (1.98 ( 1.23) × 10-10

(2.83 ( 0.27) × 10-9 (2.03 ( 0.26) × 10-9 (9.00 ( 1.90) × 10-10 (3.17 ( 0.21) × 10-10 (8.67 ( 0.71) × 10-11 (4.83 ( 1.11) × 10-11 Lb

(3.42 ( 0.27) × 10-9 (2.37 ( 0.21) × 10-9 (1.22 ( 0.05) × 10-9 (5.83 ( 0.11) × 10-10 (1.63 ( 0.39) × 10-10 (8.17 ( 1.15) × 10-11 Lb

a Initial conditions: 1.0 × 10-3 M P, 0.20 M H O , 0.20 M NaCl, pH adjusted with HClO . Error ) standard deviation (σ), obtained from the goodness 2 2 4 of the fit to the experimental data, representing intraseries variance. b Chlorophenol concentration was too low to be detected.

Effect of Phenol and Oxygen. Figure 6 shows the initial formation rates of 2ClP and 4ClP as a function of phenol concentration in the presence of 6.6 × 10-3 M FeCl3, at pH 2.0 adjusted with HClO4, upon 313 nm irradiation (see also Table 1, entries no. 20-23). Chlorophenol initial formation rate reaches a plateau above 2.6 × 10-4 M phenol, possibly indicating that phenol under such conditions is the main scavenger of chlorinating reactive species in solution (probably Cl2•-). The effect of oxygen on chlorophenol formation rate was also studied. The results obtained in aerated solution (see Figure 6 and Table 1, entries no. 20-23) were compared with the ones obtained in the absence of oxygen (Table 1, entries no. 24-27). In the latter case the cells, before irradiation, were purged for 20 min with a gentle stream of high-purity nitrogen. The initial formation rates of both 2ClP and 4ClP are around 3 × 10-9 M s-1 at all phenol concentration values, in aerated solution as well as under nitrogen atmosphere. The comparison indicates that oxygen has practically no effect on chlorophenol formation. This result can give insight into phenol photochlorination pathways. Various alternative hypotheses can actually be developed, in analogy with the ones advanced for phenol photonitration (41-46). Let H-Ph-OH be phenol, HO-Ph-OH catechol or hydroquinone, and Cl-Ph-OH either 2ClP or 4ClP. Chlorophenol formation might be hydroxyl-mediated, that is initiated by a hydroxyl radical attack on the aromatic ring of phenol to yield the dihydroxycyclohexadienyl radical (HO(H)-Ph•OH). In the presence of oxygen this radical should evolve into dihydroxybenzenes (43-45, 47), but an alternative pathway might be the reaction with Cl2•- to give 2ClP + 4ClP upon water elimination.

H-Ph-OH + •OH f HO(H)-Ph•-OH •

(14)

HO(H)-Ph -OH + O2 f HO-Ph-OH + HO2

•

In the framework of reactions 14-16 chlorophenol formation should be inhibited by oxygen (43-45, 48), which is not the case. Furthermore, since ortho- and para HO(H)-Ph•-OH radicals would preferentially form (catechol and hydroquinone are the main phenol hydroxyderivatives upon reaction with hydroxyl (43-45, 47)), relevant amounts of 3ClP should be detected in the system. The absence of 3ClP thus constitutes additional evidence against a hydroxylmediated pathway. As an alternative, chlorination might take place upon radical addition of Cl2•- to phenol aromatic ring, followed by hydrogen abstraction to yield chlorophenols. In these cases, however, molecular oxygen is usually the most efficient hydrogen abstractor (43-45, 47), and phenol chlorination 9

H-Ph-OH + Cl2•-f Cl(H)-Ph•-OH + Cl-

(17)

Cl(H)-Ph•-OH + O2 f Cl-Ph-OH + HO2•

(18)

If chlorophenol formation passes through phenoxyl as an intermediate (reactions 11-13), no effect of oxygen can be expected because phenoxyl reacts with O2 at a very low rate (49). Reactions 11-13 can thus account for the lack of an oxygen effect on chlorophenol formation. Furthermore, the actual presence of phenoxyl radicals in the systems under study is confirmed by the GC-MS detection of dihydroxybiphenyls and phenoxyphenols, which form upon reaction between phenoxyl and phenol (40). Proposed phenol transformation pathways in the studied systems, leading to the detected intermediates (2ClP, 4ClP, benzoquinone, dihydroxybiphenyls, and phenoxyphenols), are reported in Scheme 1. The formation of hydroquinone has been postulated as an intermediate step to account for the occurrence of 1,4-benzoquinone (25). Since hydroquinone is likely to form upon reaction between phenol and hydroxyl, and since this process also yields catechol (4345), catechol formation has been postulated as well. Phenol Chlorination in the Dark in the Presence of H2O2/ Cl-. Some runs were carried out using hydrogen peroxide under irradiation as a hydroxyl source. H2O2 is a key intermediate in aqueous atmospheric chemistry (50, 51), and its photolysis quantum yield in aqueous solution approaches 0.5 (52):

H2O2 + hν f 2•OH

(19)

(15)

HO(H)-Ph•-OH + Cl2•-f Cl-Ph-OH + H2O + Cl(16)

5072

via this pathway should be inhibited by the absence of oxygen, which again is not the case.

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 13, 2005

However, only trace levels of chlorophenols were detected upon irradiation of hydrogen peroxide and chloride at pH 6. Chlorophenol formation under such conditions can be attributed to reactions 19, 7, 8, and 10-13. Various reasons can account for the low chlorophenol formation rate. First of all the molar absorbivity of H2O2 around 313 nm is rather low (below 1 M-1 s-1 (50, 52)). Furthermore, the presence of hydrogen peroxide and the reactions induced by its photolysis can result in the depletion of the reactive species for phenol photochlorination (Cl2•- and the phenoxyl radical, H-PhO•; H-Ph-OH ) phenol) (30, 49, 53):

H2O2 + •OH f HO2• + H2O

[k20 ) 2.7 × 107 M-1 s-1] (20)

H2O2 + Cl2•- f HO2• + H+ + 2Cl-

[k21 ) 1.4 × 105 M-1 s-1] (21)

HO2• + Cl2•-f O2 + H+ + 2Cl-

[k22 ) 3 × 109 M-1 s-1] (22)

HO2• + H-Ph-O• f H-Ph-OH + O2

[k23 ) 2 × 109 M-1 s-1] (23)

Relevant chlorophenol formation was on the contrary detected in the dark in the presence of hydrogen peroxide and chloride at acidic pH. Figure 7 reports the initial formation rates of 2ClP and 4ClP, in the presence of 1.0 × 10-3 M phenol, 0.20 M H2O2, and 0.20 M NaCl, as a function of pH in the dark (see also Table 2, entries no. 1-7). Differently from the Fe(III)/chloride system under irradiation, in the present case the fraction of transformed phenol accounted for by chlorophenols is always above 10%. Chlorophenol formation is enhanced at low pH, and the initial formation rates are proportional to [H+] (the fitting equations are of the form: rate ) k × 10-pH). Hydrogen peroxide reacts with chloride in acidic solution to yield hypochlorous acid, HClO (54, 55). The latter is however partly consumed upon reaction with H2O2 (56, 57):

H2O2 + Cl- + H+ f HClO + H2O

(24)

H2O2 + HClO f H2O + O2 + H+ + Cl-

(25)

The species HClO gives then rise to chlorination reactions (58-60), and in acidic solution the chlorination rate is directly proportional to [H+] (60). Different reactive species have been hypothesized to take part to chlorination (Cl+, Cl2O, H2ClO+, Cl2), and all require H+ for their formation (61):

HClO + H+ a Cl+ + H2O

(26)

2HClO + H+ f Cl2O + H+ + H2O

(27)

HClO + H+ a H2ClO+

(28)

HClO + H+ + Cl- a Cl2 + H2O

(29)

Chlorination of anisole by HClO has indicated that Cl2 and Cl+ are unlikely to be involved in the process (61). Actually, Cl+ is very unstable and therefore unlikely to be involved in electrophilic chlorination reactions at all. Due to the similarity between anisole and phenol, Cl2O and H2ClO+ are the possible chlorinating agents also in our case (61). Environmental Significance. The processes studied in this paper may be relevant to the chemistry of marine aerosol, naturally rich in chloride. Field studies carried out on the Antarctic coast have shown that the aerosol in the region is rich in hydrogen peroxide during summer (62-64) and has often very acidic pH values due to the presence of sulfuric acid of biogenic origin, as revealed from anion exchange processes in the snow (63-65). The occurrence of Fe(III) species has also been observed (66), which is a common feature in particulate and aerosols (28, 29, 67). Under such conditions, chlorination processes in the presence of Fe(III) and chloride under irradiation and of hydrogen peroxide and chloride at acidic pH are a likely possibility. Finally, it is interesting to point out an analogy between nitrite and chloride in their interaction with dissolved Fe(III) under irradiation and hydrogen peroxide in the dark. Hydroxyl formed upon Fe(III) photolysis can in fact oxidize nitrite to nitrogen dioxide, which causes phenol nitration (20), and chloride to Cl•/Cl2•-, finally resulting in phenol chlorination. Moreover, hydrogen peroxide can react with nitrous acid in

acidic solution to yield the nitrating agent HOONO (21, 68, 69) and with chloride in acidic solution to give HClO, involved in chlorination reactions.

Acknowledgments Financial support by PNRA - Progetto Antartide, MIUR (FIRB 2001, contract no. RBAU01HLFX), Italian Interuniversity Consortium “Chemistry for the Environment” (INCA), and Universita` di Torino - Ricerca Locale is gratefully acknowledged. Thanks are due to Mirco Lucchiari for technical assistance with the GC-MS measures.

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Received for review December 9, 2004. Revised manuscript received April 20, 2005. Accepted April 21, 2005. ES0480567

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