Degradation Pathways of Pentachlorophenol by Photo-Fenton

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Environ. Sci. Technol. 2001, 35, 1771-1778

Degradation Pathways of Pentachlorophenol by Photo-Fenton Systems in the Presence of Iron(III), Humic Acid, and Hydrogen Peroxide MASAMI FUKUSHIMA* AND KENJI TATSUMI National Institute of Advanced Industrial Science and Technology (AIST) 16-1 Onogawa, Tsukuba 305-8569, Japan

The degradation characteristics and pathways of pentachlorophenol (PCP) by the photo-Fenton systems were studied in H2O2 aqueous solutions, which contained Fe(III) only [H2O2/Fe(III) system] and Fe(III) + humic acid (HA) [H2O2/Fe(III)/HA system] at pH 5.0. Although 40% of the PCP was degraded after 5 h of irradiation in the H2O2/ Fe(III) system, more than 90% was degraded in the H2O2/ Fe(III)/HA system. This shows that at pH 5.0 the degradation of PCP is clearly enhanced by the presence of HA in the photo-Fenton system. In the H2O2/Fe(III) system, the production of octachlorodibenzo-p-dioxin (OCDD) was detected, and 2-hydroxy nonachlorodiphenyl ether was also identified as a precursor of OCDD. However, no OCDD production was observed in the H2O2/Fe(III)/HA system. This indicates that the presence of HA represses the production of OCDD during the degradation of PCP by the photo-Fenton system. Such an effect by HA can be attributed to a reaction sequence wherein reaction intermediates derived from PCP, such as PCP•, are incorporated into HA. This was verified by 13C NMR and pyrolysis-GC/MS studies.

Introduction Highly chlorinated phenol derivatives, such as pentachlorophenol (PCP), have been listed as a priority pollutant by the U.S. Environmental Protection Agency (1). Because of the intensity of this toxicity, attention has been focused on its presence in and removal from the environment. PCP is mainly used as a wood preservative, and this use leads to soil contamination (2-4). In addition, PCP is transformed into a more toxic dimer, such as polychlorinated dibenzo-pdioxins (PCDDs) and dibenzofurans, by oxidative processes such as enzymatic reactions (5-7). Therefore, the studies on fate of PCP via the oxidative transformation are of critical importance to remedial processes in the environment. On the other hand, the formation of hydroxyl radicals (HO•) via the reaction of Fe(II) with H2O2, i.e., the Fenton reaction, has been utilized for the degradation of organic pollutants. Photo-Fenton systems are especially well-known as advanced oxidation process (8, 9). In the aquatic environment, natural organic matter, such as humic acid (HA), plays an important role in photo-Fenton systems, including the photosensitized generation of H2O2 and the photoreduction of Fe(III) to Fe(II) (10, 11). In these systems, the reducing and * Corresponding author phone: +81-298-61-8328; fax: +81-29861-8326; e-mail: [email protected]. 10.1021/es001088j CCC: $20.00 Published on Web 03/23/2001

 2001 American Chemical Society

complexing abilities of HA to Fe(III) (12, 13) assist in the catalytic reactions of Fe(III)/Fe(II) redox cycles. It is also known that HA is ubiquitous in the soil environment (14). Because of interest in the photochemical transformation of pollutants on the soil surfaces (15-17), the photo-Fenton systems promise to be an important remedial process in soil environment. Therefore, studies on the photo-Fenton systems in the presence of HA may help in understanding one of the remedial processes in the environment. However, little has been reported on the degradation of pollutants by the photo-Fenton systems in the presence of HA, e.g., nitrobenzene (18) and aniline (19). Previously, we have reported that indirect photolysis of PCP in the aqueous solution containing Fe(III) and HA can be observed even in the absence of H2O2 (20). Although the paper suggested a little contribution of HO• to the degradation of PCP, the detailed degradation pathways of PCP were not studied. In the present study, the degradation characteristics and pathways of PCP by the photo-Fenton systems were studied in aqueous solutions of H2O2, which contained Fe(III) only [H2O2/Fe(III) system] and Fe(III) + HA [H2O2/Fe(III)/HA system]. The reaction products were identified and determined by GC/MS, and the intermediates derived from PCP, which were incorporated into HA, were identified by 13C NMR and pyrolysis-GC/MS (pyrGC/MS).

Experimental Section Materials. HA was extracted from a peat soil from the Bibai Damp Plane in Hokkaido according to protocols of International Humic Substances Society (21). PCP was purchased from Nacalai Tesque (99.0% purity). Tetrachlorohydroquinone (TeCHQ) and tetrachlorocatechol (TeCC) were obtained from Tokyo Chemical Industry (98% purity) and Aldrich (98% purity), respectively. Other chlorophenols were used as standard materials for pyr-GC/MS: 2,3,5-trichlorophenol (TrCP), 2,3,5,6-tetrachlorophenol (TeCP) (Tokyo Chemical Industry, 98% purity), and 2,3,4,5-TeCP (Kanto Chemicals, 10 ppm in cyclohexane). The 2-chloro-4-phenyl phenol (98% purity, Tokyo Chemical Industry) was used as a model compound in the pyr-GC/MS study, and other model compounds were also chemically synthesized (see Appendix) (22). The 13C-labeled PCP (Cambridge Isotope Laboratories, 99% purity) and 14C-labeled PCP (American Radiolabeled Chemicals Inc., 15 and 0.1 mCi mL-1 in toluene, respectively) were used for the 13C NMR studies and for the measurement of the amount of 14CO2 generated, respectively. A standard sample of octachlorodibenzo-p-dioxin (OCDD) (50 µg mL-1 in toluene) was purchased from GL Sciences. Irradiation of the Test Solutions. A 220-mL aliquot of a test solution, which contained 50 µM PCP, 100 µM Fe(III), and 50 mg L-1 HA, was placed in a quartz cuvette (60 × 60 × 100 mm), and the pH was adjusted to 5.0 by the addition of H2SO4 or NaOH solutions. After bubbling O2 gas for 30 min, a 2.2-mL aliquot of 0.1 M aqueous H2O2 was added. The test solution was then irradiated with a 500-W xenon short arc lamp (Ushio Denki), which had first been passed through a glass filter (λ > 370 nm). The temperature of the test solution was maintained at 25.0 ( 0.3 °C, and O2 gas was flushed into the solution during the irradiation. After being irradiated, a 600-µL aliquot of the test solution was extracted with 600 µL of ethyl acetate, which contained 25 µM p-nitrotoluene as an internal standard. A 20-µL aliquot of the extract was then injected into a Jasco PU-980 type HPLC system (Japan Spectroscopic Co., Ltd.). The mobile phase consisted of a mixture of 0.8% aqueous H3PO4 and methanol (15/85 ) v/v). A 5C18-MS Cosmosil packed column (4.6 mm i.d. × 250 mm, VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Nacalai Tesque Inc.) was used as a solid phase, and column temperature was maintained at 50 °C. An SPD-6 UV-vis detector (Shimadzu Co., Ltd.) was used for detection with a detection wavelength of 220 nm. Recovery of the extraction at 50 µM PCP was 98 ( 5% (n ) 3). After the test solution (1.2 mL) was centrifuged, the Cl- ions were measured by ion chromatography. All of the irradiation runs were conducted in duplicate. Determination of Reaction Products. Reaction products, such as TeCHQ, TeCC, and OCDD, were identified and determined by GC/MS after extraction of the test solution (200 mL) with n-hexane according to the method reported by Morimoto et al. (23). On the extraction, a 500-µL aliquot of the hexane solution of anthracene (0.5 mM) was added to the test solution as an internal standard. A 1-µL aliquot of the extract was introduced into an HP5971/HP5890 series II (Hewlett-Packard) GC/MS system. A Quadrex methyl silicon capillary column (0.25 mm i.d. × 25 m) was employed in the separation. The temperature ramp was as follows: 65 °C for 1.5 min, 65-120 °C at 35 °C min-1, 120-300 °C at 7 °C min-1, and a 300 °C hold for 5 min. The recoveries of extractions (n ) 3) for 1 µM TeCHQ, TeCC, and OCDD were 85 ( 10%, 92 ( 7%, and 104 ( 10%, respectively. 14CO 14CO was 2 Generation. Experiments in which 2 monitored were carried out on 10-mL scale in a 25-mL flat bottom glass tube. A 30-µL aliquot of a 14C-labeled PCP toluene solution (11.1 mM) was placed into the glass tube, and toluene was evaporated by a stream of nitrogen gas. Subsequently, a 10-mL aliquot of aqueous solution ([HA] ) 50 mg L-1, [Fe(III)] ) 100 µM, pH 5.0) was pipetted into the tube, and the mixture was stirred in the presence of bubbling O2 gas for 15 min. After the addition of 100 µL of a 0.1 M H2O2 aqueous solution, the tube was fitted with a stopper, which contained a polyethylene center well containing a plug of glass wool impregnated with 400 µL of 2 M aqueous NaOH as a 14CO2 trap. After 2 h of irradiation, 0.5 mL of 1 M H2SO4 was injected via a syringe needle from the top of stopper to drive off 14CO2 from the test solution. After stirring for 2 h, 1 mL of the test solution was pipetted into the glass vial containing the mixture of methanol (1 mL) and scintillation cocktail (9 mL). The glass wool and aqueous washings (600 µL) from the center well was transferred to the vial containing the mixture of methanol and scintillation cocktail. The radioactivities (dpm) in the vials were measured by an LS 6000 series liquid scintillation counter (Beckman Instruments, Inc.). To evaluate the amount of decrease in the concentrations of PCP (∆[PCP]), a test solution that contained 33.3 µM nonlabeled PCP was irradiated, and the [PCP] after irradiation was measured by HPLC. 13C NMR. In the H O /Fe(III)/HA system, the 13C-labeled 2 2 PCP was used for preparation of the powdered HA. After 5 h of irradiation, 10 mL of 2-propanol was added to the test solution. The fraction of HA was then concentrated by ultrafiltration through a Millipore YM1 ultrafiltration cellulose membrane (nominal cutoff 1000 Da). The concentrated fraction was acidified to below pH 1 via the addition of H2SO4 and stirred for 4 h. This mixture was then centrifuged (10 000 rpm, 10 min), and the precipitate was dialyzed against water with a Spectra/Por cellulose ester membrane (nominal cutoff 500 Da). This was freeze-dried, and the resulting powder (11 mg) was then dissolved in 500 µL of DMSO-d6. The 13C NMR inverse-gated spectrum was measured by means of a JEOL Lambda FT-NMR spectrometer (Nippon Denshi) with a resonance frequency for 13C of 125 MHz. The spectra were recorded using a pulse angle of 45° and a 2.0-s pulse delay, with 80 000 accumulated scans being obtained. Pyr-GC/MS. The powdered HA, which was irradiated with nonlabeled PCP, was prepared by the same method as the 13C NMR study. A JPH-3 type Curie-point pyrolyzer (Japan Analytical Industry) was connected with the GC/MS system, 1772

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FIGURE 1. Degradation characteristics of PCP at pH 5.0. and the powdered HA (1 mg) was pyrolyzed at 500 °C for 4.0 s with 0.5 mg of the methylation reagent, tetramethylammonium hydroxide (TMAH). The temperature ramp for the GC/MS was as follows: 50 °C for 1 min, 50-300 °C at 5.0 °C min-1, and a 300 °C hold for 4 min. Chlorine Analysis. To check the organic chlorine species in the test solutions, adsorptive organic halogen (AOX) was determined according to our previous study (20). In this experiment, the powdered HA was prepared by the same method as the 13C NMR study using nonlabeled PCP.

Results and Discussion Removal Characteristics of PCP. Figure 1 shows the removal characteristics of PCP in the H2O2, H2O2/HA, H2O2/Fe(III), and H2O2/Fe(III)/HA systems at pH 5.0. It is known that PCP does not absorb light above 370 nm (24); therefore, no decrease in PCP as a result of irradiation was observed in the presence of H2O2 only (Figure 1, 0). However, in the H2O2/ HA system, approximately 15% of the PCP was removed after 300 min of irradiation. It has been reported that the photosensitization of HA gives rise to the production of active oxygen species, such as 1O2, HO•, and H2O2, and that these species are related to the degradation of pollutant (25). According to Ononye et al. (26), the photolysis of p-quinone, which may be contained in HA, gives rise to the semiquinone radical and HO•; therefore, the loss of PCP in the H2O2/HA system can be attributed to the photosensitization of HA. Although 40% of the PCP was removed in the H2O2/Fe(III) system after 300 min of irradiation, more than 90% was removed in the H2O2/Fe(III)/HA system. This indicates that the presence of HA results in an enhancement in PCP degradation. In addition, when 2.6 mM 2-propanol was added as a radical scavenger into the H2O2/Fe(III) and H2O2/Fe(III)/HA systems, the percentages of PCP removed after 300 min of irradiation were reduced to 20% and 25%, respectively (Figure 1). This suggests that the degradation of PCP is mainly due to HO• in both systems. In the absence of H2O2, the removal of PCP has been reported to be slightly reduced in the presence of 2-propanol (20), which suggests a little contribution of HO• to the PCP degradation. However, the enhancement in PCP degradation was not observed even in the presence of Fe(III) and HA (20). The results in the present study indicate that the reaction mechanisms for the indirect photolysis (i.e., in the absence of H2O2) are different from those for the photo-Fenton system. The typical reaction scheme in the Fenton system involves a Haber-Weiss mechanism:

Fe(II) + H2O2 f Fe(III) + HO• + OH-

(1)

In the absence of chelator such as HA, the possible redox reactions of iron species can be written as (27)

FIGURE 2. GC/MS chromatograms of the hexane extract of the reaction mixtures in the H2O2/Fe(III) and H2O2/Fe(III)/HA systems at pH 5.0 as a result of irradiation.

Fe3+ + H2O2 f Fe2+ + HO2• + H+

(2)

Fe3+ + HO2/O2•- f Fe2+ + O2

(3)

light

Fe(OH)2+ 98 Fe2+ + HO• H+

(4)

Fe2+ + HO2/O2•- 98 Fe3+ + H2O2

(5)

Fe2+ + HO• f Fe3+ + OH-

(6)

These reactions suggest that the Fe(III)-catalyzed photoFenton system appears to proceed, even in the H2O2/Fe(III) system. However, the formation of colloidal iron(III) hydroxide is extensive at pH 5.0, and this may prevent the reduction of Fe(III) as shown in eqs 2-4. In the presence of HA, the majority of the iron species are complexed with HA at pH 5.0 (28). Therefore, the iron species can be stabilized in the presence of HA, and this results in an improvement in the efficiency of the degradation of PCP at pH 5.0. Moreover, in the H2O2/Fe(III) and H2O2/Fe(III)/HA systems under dark conditions (Figure 1, 2 and [), no degradation of PCP could be observed. These results show that the degradation of PCP is enhanced by the light irradiation. Identification and Determination of Reaction Products. To identify and determine the reaction products, the hexane extracts from the reaction mixtures were examined by GC/ MS. The PCP, which was used in the present work, contained 1% impurities. Therefore, we first checked the impurities in the PCP by means of a GC/MS analysis of the hexane extract from an unirradiated aqueous solution of PCP. As shown in the chromatograms before irradiation (Figure 2), 2,3,5,6-TeCP was found to be the major impurity (1.0%). In both systems, TeCC (13.4 min) and TeCHQ (13.6 min) could be detected as reaction products after 5 h of irradiation (Figure 2). The concentrations of TeCHQ and TeCC increased during the irradiation period (Figure 3). In addition, dechlorination was observed during the degradation of PCP in both systems, as

FIGURE 3. Production of TeCHQ and TeCC and the release of chloride ions at pH 5.0 as a result of irradiation. shown in Figure 3. If TeCHQ and TeCC would be main byproducts, the [Cl-]/∆[PCP] ratio, which represented the amounts of chlorine released from PCP, might be approximately 1. However, the ratios during the irradiation VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Results in Light Irradiation of Test Solutions Containing 14C-Labeled PCPa [14C] mass balance (µM)d [14C]

systems Fe(III)/H2O2 Fe(III)/HA/H2O2

soln (µM)

[14C]

170 ( 4 140 ( 1

trap (µM)

6∆[PCP]

20 ( 3 51 ( 7

(µM)b

56 ( 2 170 ( 4

%

CO2c

44 ( 11 34 ( 5

measd

calcd

190 ( 10 190 ( 8

200 200

a n ) 3, pH 5.0, [PCP] ) 33.3 µM, [Fe(III)] ) 100 µM, [H O ] ) 1 mM, [HA] ) 50 mg L-1, irradiation time ) 2 h. b Determined by HPLC using 0 2 2 nonlabeled PCP; ∆[PCP] ) [PCP]0 - [PCP]t ) 2h. c Average and standard deviation of the calculated values from eq 8. d The measured values represent [14C]soln + [14C]trap, and the calculated values represent [PCP]0 × 6.

period were in the range of 2-3 in both systems. Moreover, the percentages of the sum of the [TeCHQ] and [TeCC] to the ∆[PCP] after 5 h of irradiation were 2% for the H2O2/Fe(III) system and 9% for the H2O2/Fe(III)/HA system, respectively. These results suggest the presence of further byproducts. In both systems, the large amounts of remaining species derived from PCP, except for TeCHQ and TeCC, may be accounted for as follows: (i) mineralization to CO2, (ii) formation of dimers between reaction intermediates, and (iii) binding to HA. CO2 Generation. To verify mineralization to CO2 in both systems at pH 5.0, irradiation studies were carried out using 14C-labeled PCP. The total concentration of 14C ([14C] total), which originated from the 14C-labeled PCP, can be calculated from the initial concentration of PCP ([PCP]0). The concentration of 14C in the solution after irradiation ([14C]soln) is calculated as

[14C]soln ) (dpm after irradiation)/ (dpm before irradiation) × [14C]total (7) Similarly, the concentrations of 14C in the 14CO2 trap after irradiation ([14C]trap) can be evaluated. The mass balance of 14C before and after irradiation was verified by comparing [14C]soln + [14C]trap with [14C]total (Table 1). The percentages of 14CO mineralized (%14CO ) are evaluated as 2 2

% 14CO2 )

[14C]total - [14C]soln ∆[PCP] × 6

× 100 or )

[14C]trap

× ∆[PCP] × 6 100 (8)

As shown in Table 1, the % 14CO2 values were 44% for the H2O2/Fe(III) system and 34% for the H2O2/Fe(III)/HA system, respectively. Production of OCDD and Its Repression by the presence of HA. As shown in Figure 2, in the H2O2/Fe(III) system, peaks at 27.9, 28.5, and 28.8 min were observed after 5 h of irradiation. These peaks were identified from a mass spectral library as 2-hydroxynonachlorodiphenyl ether (2H-NCDE), 4-hydroxynonachlorodiphenyl ether (4H-NCDE), and OCDD, respectively (29). These compounds could be produced by the dimerization of the reaction intermediates derived from PCP. However, these peaks could not be observed in the H2O2/Fe(III)/HA system (Figure 2). Figure 4 shows the time-dependent variations of the OCDD concentrations. It is known that the optimum pH of the photo-Fenton system is at approximately 3.0 (30). At this pH, PCP was completely degraded within 5 h of irradiation in both systems. Therefore, the production of OCDD was examined at pH 3.0 as the significant controls (Figure 4, 0 and O). At pH 3.0, no OCDD production was observed in either system. At pH 5.0, the production of OCDD increased with irradiation time in the H2O2/Fe(III) system. However, no OCDD production as well as 2H-NCDE and 4H-NCDE was observed in the H2O2/Fe(III)/HA system. Moreover, as shown in Figure 1 (O), approximately 15% of the PCP was degraded in the presence of HA only after 5 h of irradiation. Although we examined to determine OCDD in this case, this 1774

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FIGURE 4. Time-dependent variations of the OCDD production at pH 5.0 and pH 3.0 as a result of irradiation. production could not be observed. Moreover, in the absence of H2O2, the formation of OCDD and 2H-NCDE can be observed at pH 3.0 and pH 5.0 in the presence of Fe(III) only (20). However, these formations were also prevented by the presence of HA. These results suggest that the formation of dimeric compounds, such as OCDD, 2H-NCDE, and 4HNCDE, can be repressed by the presence of HA in the photoFenton system. The mechanism of formation of OCDD has been discussed in terms of the H2O2/Fe(III) system. In the photo-Fenton systems, radical species, which are produced by the HO• attack of PCP, may contribute to the production of dimeric compounds. Since the pKa value of PCP is 4.7 (24), 67% of the PCP exists as the dissociated species (PCP-) at pH 5.0. It has been reported that PCP- is oxidized to the pentachlorophenoxy radical by an attack on PCP- by HO• (31):

The addition of a chlorophenoxy radical to the ortho-position of chlorophenol yields a 2-phenoxyphenol radical (32), and the PCDDs are formed via the 2-chlorophenoxyphenol anion (33). As shown in Figure 2, 2H-NCDE was found in the present system. In the production of PCDDs during the enzymatic oxidation of chlorophenols (5), 2-phenoxyphenols have been identified in the reaction mixtures. Therefore, 2H-NCDE represents a reaction intermediate in the production of OCDD. The possible pathways of OCDD production in the H2O2/Fe(III) system are presented in Scheme 1. Coupling between either PCP• or PCP• and PCP results in the production of a 2H-NCDE radical (2H-NCDE•). However, the production of 2H-NCDE was detected as shown in Figure 2. It has been reported that, in the photo-Fenton systems, the phenol radical is reduced to phenol by Fe(II) (34); therefore, in the present

SCHEME 1

TABLE 2. Mass Balance of Chlorine from PCP before and after 5 h of Irradiationa chlorine (Cl) species (unit)

values

Before Irradiation [PCP]0 measd by HPLC (µM) Cl content calcd from [PCP]0 (mg L-1) AOX in the test solution (mg L-1)

50 ( 2 8.8 ( 0.3 8.9 ( 0.5

After 5 h of Irradiation [PCP] measd by HPLC (µM) Cl content calcd from [PCP] (mg L-1) AOX in the test solution (mg L-1) [Cl-] measd by ion chromatography (µM) Cl content calcd from [Cl-] (mg L-1) AOX in the HA fraction (mg L-1) total Cl content from degraded PCP (mg L-1)a

4.0 ( 0.5 0.7 ( 0.1 4.5 ( 0.2 128 ( 4 4.5 ( 0.2 2.2 ( 0.4 8.3 ( 0.4

Percentages to Total Cl Content from Degraded PCP org Cl in aqueous phase (%) 19 ( 7 inorg Cl in aqueous phase (Cl-) (%) 54 ( 1 org Cl in HA fraction (%) 27 ( 6 a At pH 5.0, [HA] ) 50 mg L-1, and [Fe(III)] ) 100 µM. b (AOX in the test solution after irradiation) - (Cl content calculated from [PCP] after irradiation) + (Cl content calculated from [Cl-] after irradiation).

system, 2H-NCDE• may be reduced to 2H-NCDE by Fe(II) species, and OCDD is then produced via 2H-NCDE- (as shown in Scheme 1). Moreover, since PCP is not dissociated at pH 3.0, PCP• is not formed by an attack by HO•. Therefore, OCDD formation may not be observed at pH 3.0. This suggests that PCP• is an initial intermediates to the OCDD production. Here one has a question why the presence of HA represses the production of OCDD in the photo-Fenton system. It is known that oxidation of phenol compounds by the Fenton reaction yields the coupling compounds (34). Thus, it can be expected that formation of the coupling compound, in which the reaction intermediates derived from PCP bind with HA, may inhibit the production of OCDD via the bimolecular process. The coupling between HA and the reaction intermediates was verified by chlorine analysis in HA, 13C NMR, and pyr-GC/MS, as described below. Chlorine Analysis in the HA Fraction after Irradiation. As described in the previous study (20), if the chlorinated species from PCP are able to bind to HA, then chlorine should be detectable in the HA fraction after irradiation. To detect the organic chlorine species in HA, AOX in the HA fraction was measured. The mass actions of the chlorinated species before and after irradiation are summarized in Table 2. The chlorine content (8.8 mg L-1), which was calculated from [PCP]0, was in good agreement with the AOX values for the

FIGURE 5. 13C NMR spectra of DMSO-d6 solution in the presence of HA (2%): (a) PCP, (b) TeCHQ, (c) TeCC, and (d) HA after the reaction with 13C-labeled PCP. test solution before irradiation (8.9 mg L-1). This indicates that all of the PCP can be detected as AOX species. In addition, the summation of AOX and Cl- in the test solutions after irradiation (9.0 mg L-1) was in good agreement with the AOX value beforehand (8.9 mg L-1). The percentages to the total chlorine content from the degraded PCP after irradiation (Table 2) indicate that 27% of the chlorine could be found in the HA fraction. This suggests that the reaction intermediates from PCP are incorporated into the HA by various interactions. 13C NMR Study. To observe the nature of the binding between HA and the reaction intermediates from PCP, 13Clabeled PCP was irradiated in the Fe(III)/HA/H2O2 system, and the 13C NMR spectrum of the HA fraction was then measured. Figure 5 shows a comparison of the 13C NMR spectra of PCP, TeCHQ, TeCC, and the HA fraction after irradiation, respectively. Peaks corresponding to PCP, TeCHQ, and TeCC were not observed in the spectrum of HA (Figure 5d). This indicates that unreacted PCP, TeCHQ, and TeCC may not be adsorbed to HA via hydrophobic interactions. In addition, no signals could be observed in 2% HA DMSO-d6 solution under the same accumulation conditions. This indicates that the origin of many peaks in Figure 5d is from 13C-labeled PCP. It is possible that a large variety of C-C and C-O binding can occur in the present system. Hatcher et al. (35) reported that, in the enzymatic oxidation of 13C-labeled 2,4-dichlorophenol with HA, the peaks from ester and ether C-O linkages appeared in the 139-141 and 143-151 ppm ranges, respectively. However, in the photo-Fenton system, the formation of an ester linkage may be difficult because of the complexation between carboxylic groups in HA and Fe(III). Therefore, the spectral region at 143-153 ppm in Figure 5d may indicate the presence of ether linkages. According to VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Pyr-GC/MS chromatograms of HA: (a) chromatogram after irradiation in the absence of PCP (reaction blank) and (b) chromatogram after irradiation in the presence of PCP.

FIGURE 7. Pyr-GC/MS chromatograms of the model compounds: (a) 2-chloro-4-phenyl phenol and (b) 2-methoxy-4-(pentachlorophenoxy) benzaldehyde. the substitute effect of benzene (36), C-O and C-C bindings bring about the decrease and/or increase in the chemical shift for other carbons on aromatic rings in the range of 1-14 ppm. Hence, the peaks in the 128-133 ppm regions in Figure 5d can be attributed to the shifts as the result of C-C and C-O binding. The peaks at 161-171 ppm in Figure 5d may correspond to carbonyl carbons, such as quinones, and/or carbons on the dissociated phenolic hydroxyl groups (36). 1776

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Pyr-GC/MS Study. To identify reaction intermediates from PCP that were incorporated into HA, pyr-GC/MS chromatograms were studied. Figure 6a shows the reaction blank in which the test solution was irradiated in the absence of PCP. As shown in Figure 6b, the methylated forms of 2,3,5-TrCP (14.8 min), 2,3,4,5-TeCP (20.1 min), 2,3,5,6-TeCP (20.3 min), and PCP (24.8 min) were detected as new peaks. It has been reported that dimers, such as 1-(3,4-dimethoxyphenyl)-2(4-propylphenoxy)-1-ethanol, are degraded to monomers

FIGURE 8. Possible degradation pathways of PCP by the photo-Fenton systems. during the pyrolysis (37); therefore, the appearance of chlorophenols in Figure 6b can be attributed to the degradation of coupling compounds with HA during the pyrolysis. The 13C NMR study suggests the occurrence of C-C and ether C-O binding between HA and chlorophenols. Therefore, the degradation characteristics during the pyrolysis were studied by using the model compounds, which involved the C-C and C-O binding. Figure 7 shows the pyr-GC/MS chromatograms of the model compounds. The degradation of 2-chloro-4-phenylphenol, which involves C-C binding, could not be observed. However, the ether C-O linkage, 3-methoxy-4-(pentachlorophenoxy) benzenealdehyde, was degraded during the pyrolysis. This type of degradation could also be observed in the case of another synthesized ether, 2,6-dimethoxy(pentachlorophenoxy) phenyl ether [3,4,5trimethoxybenzene (14.2 min), pentachloroanisol (24.7 min), and the parent (40.4 min)]. These results indicate that ether linkages represent the main mode of binding between HA and the reaction intermediates derived from PCP. Possible Degradation Pathways. On the basis of the results described above, possible degradation pathways of PCP are proposed, as shown in Figure 8. Initially, PCP is oxidized to PCP• by an attack by HO•. In the H2O2/Fe(III) system, OCDD is produced via 2H-NCDE-. On the other hand, the addition of HO• to phenolic sites in HA yields hydroxycyclohexadienyl radicals (HCHD•) (34, 38). The radical coupling between PCP• and HCHD• gives rise to ether linkages between HA and PCP•. Moreover, an attack of HO• on PCP can yield the semiquinone radicals of TeCHQ and TeCC (31). These radicals may also couple with HCHD• to form the coupling compounds. The summation of the percentages of TeCHQ and TeCC to the ∆[PCP] after 5 h of irradiation were 2% for the H2O2/

Fe(III) system and 9% for the H2O2/Fe(III)/HA system, respectively. In the H2O2/Fe(III) system, the percentage of OCDD to the ∆[PCP] was approximately 1%. Moreover, mineralization to CO2 was clearly observed: 44% for the H2O2/ Fe(III) system and 34% for the H2O2/Fe(III)/HA system. From the chlorine analysis in the Fe(III)/HA/H2O2 system, 27% of chlorine species were found in the HA fraction. Therefore, it is estimated that in the percentages of unknown products to the ∆[PCP], approximately 50% can be attributed to the H2O2/Fe(III) system and approximately 30% can be attributed to the H2O2/Fe(III)/HA system. It is known that chlorohydroquinones and chlorocatechols are further oxidized to completion leading to the mineralization to Cl- and CO2 (39). Typically these reactions produce C6 dicarboxylic acids and C4 maleic acid derivatives (40). Although these compounds were not identified in the present study, the unknown products in both systems might be the ring cleavage products via TeCC and/or TeCHQ. It was found that the presence of HA was effective in repressing the production of dimeric compounds from the reaction intermediates during the degradation of PCP by the photo-Fenton systems at pH 5.0. The repressive effects of HA on the production of dimers, such as OCDD, can be attributed to the fact that the reaction intermediates from PCP are covalently incorporated into HA.

Acknowledgments We thank Dr. T. Samukawa (Radioisotope Laboratory in AIST) for his technical advises and useful comments on the experiment using 14C-labeled PCP. VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Appendix Synthesis of 3-Methoxy-4-(pentachlorophenoxy) Benzenealdehyde and 2,6-Dimethoxy(pentachlorophenoxy) Phenyl Ether. Equimolar amounts of hexachlorobenzene and vanillin or 2,6-dimethoxyphenol (9.4 mmol) were refluxed with K2CO3 (5 g) in N,N-dimethylformamide (40 mL) for 2 h. After cooling, water was added to the reaction mixture, followed by extraction with diethyl ether. The organic phase was dried over Na2SO4 and evaporated to dryness at reduced pressure. The residue was purified by recrystallization from a hexane-benzene mixture. 3-Methoxy-4-(pentachlorophenoxy) Benzenealdehyde. Elemental analysis (% C, % H, % Cl): C 41.99, H 1.76, Cl 44.26 (calculated), C 41.94, H 1.51, Cl 43.82 (observed). The mass spectrum (mass number and [fragments and relative intensity]): 404 [M + 4, 20.8], 402 [M + 2, 63.7], 400 [M, 100], 398 [M - 2, 62.7], 350 [M-Cl-CH3, 17.1], 300 [M-CHO-2Cl, 15.4], 265 [M-C6H3(OCH3)CHO, 6.9], 249 [M-C6H3O(OCH3)CHO, 3.9]. 2,6-Dimethoxy(pentachlorophenoxy) Phenyl Ether. Elemental analysis: C 41.78, H 2.25, Cl 44.04 (calculated), C 42.34, H 2.29, Cl 43.96 (observed). The mass spectrum: 406 [M + 4, 20.9], 404 [M + 2, 63.5], 402 [M, 100], 400 [M - 2, 63.7], 352 [M-Cl-CH3, 36.5], 265 [M-C6H3(OCH3)2, 2.4], 249 [M-C6H3O(OCH3)2, 3.4], 153 [M-C6Cl5, 20.8], 137 [M-C6Cl5O, 1.6].

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Received for review March 13, 2000. Revised manuscript received September 25, 2000. Accepted December 14, 2000. ES001088J