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Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light
Chemosphere 47 (2002) 343–348 www.elsevier.com/locate/chemosphere
Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light Taha M. El-Morsi a, Moustafa M. Emara b, Hassan M.H. Abd El Bary b, Alaa S. Abd-El-Aziz a, Ken J. Friesen a,* a
Department of Chemistry, The University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, Canada R3B 2E9 b Department of Chemistry, Al-Azhar University, Nasr City 11884, Cairo, Egypt Received 28 July 2000; received in revised form 30 June 2001; accepted 6 September 2001
Abstract The homogeneous degradation of the polychlorinated n-alkane, 1,2,9,10-tetrachlorodecane (T4 C10 ), was studied in aqueous solutions of hydrogen peroxide, including Fenton and photo-Fenton reaction conditions. All solutions were adjusted to a pH of 2.8 and an ionic strength of 0.1 M NaClO4 prior to photolysis. T4 C10 (2 106 M) was substantially degraded by the H2 O2 /UV system (1:0 102 M H2 O2 ), with 60% disappearance in 20 min of irradiation in a photoreactor equipped with 300 nm lamps of light intensity 3:6 105 Ein L1 min1 (established by ferrioxalate actinometry). The reaction produced stoichiometric amounts of chloride ion indicating complete dechlorination of the chlorinated n-alkane. T4 C10 degraded very slowly under Fenton (Fe2þ /H2 O2 /dark) and Fenton-like (Fe3þ /H2 O2 /dark) conditions. However, when the same solutions were irradiated, T4 C10 degraded more rapidly than in the H2 O2 /UV system, with 61% disappearance in 10 min of exposure. The rapid degradation is related to the enhanced degradation of hydrogen peroxide to oxidizing OH radicals under photo-Fenton conditions. Degradation was inhibited in both the H2 O2 /UV and photo-Fenton systems by the addition of KI and tert-butyl alcohol due to OH scavenging. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; UV; Tetrachlorodecane; Hydrogen peroxide; Photo-Fenton
1. Introduction Chlorinated paraffins are complex mixtures of polychloro-n-alkanes (PCAs) used industrially as high-temperature lubricants and metal-cutting fluids, plasticizers, and flame-retardant additives in sealants, adhesives and paints. PCAs with carbon chain lengths ranging from C10 to C13 , also known as short-chain chloroparaffins, are of particular concern due to their toxicity and potential for release into the environment (Environment
*
Corresponding author. Tel.: +1-204-786-9043; fax: +1-204775-2114. E-mail address: [email protected] (K.J. Friesen).
Canada, 1993). From a recent review (Tomy et al., 1998) it is evident that short-chain PCAs are widespread in the environment. Although PCAs have been detected in waters receiving industrial effluents, their occurrence in water, sediments and aquatic organisms of remote Arctic lakes (Tomy et al., 2000) suggests that these are relatively persistent chemicals. Advanced oxidation technologies (AOT) have been developed for the detoxification of water contaminated with persistent organic chemicals. The method utilizing the strong oxidizing potential of the hydroxyl radical (E0 ¼ þ1:8 V at neutral pH to þ2.7 V in acidic solutions (Buxton et al., 1988) has proven effective for the degradation of a wide variety of aqueous contaminants (Halmann, 1996), including saturated halocarbons such
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as chloroform (Hsiao et al., 1983; Ahmed and Ollis, 1984), dichloroethane (Ollis et al., 1984) and 1-bromododecane (Pelizzetti et al., 1990). The strategies for generating the reactive OH radicals for these oxidation methods include UV photolysis of ozone or hydrogen peroxide, Fenton or photo-Fenton type reactions, and TiO2 photocatalysis. We have recently demonstrated that the PCA, 1,10dichlorodecane, is rapidly photooxidized by the TiO2 photocatalyst system (El-Morsi et al., 2000) with the reaction involving adsorbed dichlorodecane and surface OH radicals generated by exposing aqueous TiO2 suspensions to 300 nm UV light. In this study we are investigating the effectiveness of aqueous hydrogen peroxide systems, including Fenton and photo-Fenton conditions, for the degradation of another member of the PCA group, 1,2,9,10-tetrachlorodecane. 2. Experimental 2.1. Chemicals Chlorine gas and 1,9-decadiene, used in the synthesis of tetrachlorodecane, were supplied by Aldrich Chemical. Reagent grade ferric perchlorate, Fe(ClO4 )3 9H2 O, and ferrous perchlorate, Fe(ClO4 )2 6H2 O, radical scavengers KI and tert-butyl alcohol, and NaClO4 were used as received. Hydrogen peroxide solutions were prepared by diluting 30 wt.% H2 O2 (BDH) with Milli-Q purity water to the desired concentration. 2,9-Dimethyl1,10-phenanthroline (DMP) from Sigma Chemical, 0.01 M CuSO4 5H2 O and phosphate buffer (prepared with K2 HPO4 and NaH2 PO4 and adjusted to pH 7 with 0.05 M H2 SO4 and 1.0 M NaOH) were used in the spectrophotometric determination of H2 O2 . DMP reagent was dissolved in 100 mL 95% ethanol, placed in a 100 mL flask wrapped with aluminum foil and refrigerated. All organic solvents used were HPLC grade. 2.2. Synthesis of 1,2,9,10-tetrachlorodecane (T4 C10 ) 1,2,9,10-Tetrachlorodecane was synthesized by chlorine addition to 1,9-decadiene using a variation of the procedure reported by Tomy (1998). Approximately 10 mL of 0.05 M NaOH was layered over 40 mL of dichloromethane in a round bottom flask. Chlorine gas was gently bubbled through the DCM layer for several minutes before introduction of 1,9-decadiene into the lower DCM layer by pipet. After a brief reaction period, the layers were separated and the DCM layer was analyzed by GC/MS. The total ion chromatogram produced one dominant peak with a retention time of 13.7 min and an EI mass spectrum confirming the identity of the product as tetrachlorodecane. The very small amounts of higher chlorinated decane isomers (with chlorine
number > 4) evident in the chromatogram indicated that free radical substitution reactions were minimal. 2.3. Light irradiation Photolytic experiments were performed using a Rayonet Photochemical Reactor equipped with a carousel and 16 UV lamps emitting 300 nm light with an intensity of 3:6 105 Ein L1 min1 as established by ferrioxalate actinometry (Hatchard and Parker, 1956). Aqueous solutions of T4 C10 (2 106 M) were prepared by pipeting from a stock solution (394 ng lL1 in 1:1 CH3 CN/H2 O) into Milli-Q water that exhibited a resistivity greater than 18 MX cm1 . The T4 C10 solutions were adjusted to pH 2.8 with perchloric acid (HClO4 ) in order to optimize the production of OH radicals (De Laat and Gallard, 1999; McGinnis et al., 2000). All aqueous solutions of T4 C10 contained <0.07% CH3 CN, which was determined to have a negligible effect on the rates of degradation of the substrate. Photo-Fenton reactions were initiated by adding 0.1 M NaClO4 , 1:0 103 M Fe(ClO4 )3 or Fe(ClO4 )2 , and 1:0 102 M H2 O2 in rapid succession with constant stirring to maintain oxygen saturation. Solutions were simultaneously irradiated in the photoreactor which had been thermally equilibrated for 10 min. Samples were typically removed after irradiation times of 10, 20, 30, 40, 50, 60, 90 and 120 min and then extracted with 60 mL of hexane/dichloromethane (5.6:1, v/v), with an extraction efficiency of 74:1% 1:5%. Control experiments were performed to determine the extent of thermal degradation (under dark conditions) and direct photolysis (in the absence of H2 O2 ). 2.4. Analyses Analyses of tetrachlorodecane were performed with a Hewlett–Packard (HP) 5890 Series II GC equipped with a 30-m 0:25-mm 0:25-lm PTE-5 column (Supelco) and coupled to a HP 5989A MS Engine. Column head pressure was maintained at 14 psi with an injection port temperature of 200 °C. The column was temperature programmed as follows: 90 °C (1 min), 10 °C min1 to 180 °C, 7 °C min1 to 210 °C (5 min). All experiments were performed in duplicate with the reproducibility in concentrations typically better than 2%. The concentration of hydrogen peroxide was determined spectrophotometrically by the DMP method with absorbance measurements at 454 nm (Kosaka et al., 1998). Spectrophotometric measurements were performed using a Hewlett–Packard 8450A diode array spectrophotometer with a quartz cell of path length 1.0 cm. Chloride ion (Cl ) was measured potentiometrically using an Orion 96-17B chloride selective electrode calibrated with NaCl standards. In order to reach a detectable concentration of Cl 1 L solutions were
T.M. El-Morsi et al. / Chemosphere 47 (2002) 343–348
concentrated by rotary evaporation, adjusted to a volume of 5 mL, followed by addition of 2 mL of Orion ISA reagent prior to measurement.
3. Results and discussion
Aqueous solutions of T4 C10 (2 106 M) did not absorb UV light, hence direct photolysis of T4 C10 was negligible as expected. However, T4 C10 degradation was sensitized by the presence of hydrogen peroxide due to the favorable absorption spectrum of H2 O2 . The direct photolysis of H2 O2 , requiring k < 360 nm (Pignatello et al., 1999), generates OH radicals according to: hm
H2 O2 ! 2 OH
ð1Þ
Although degradation products were not determined in this study, hydroxyl radicals are known to react with saturated organic chemicals by H-abstraction from alkyl or hydroxy groups (Walling, 1975) or by an electron transfer process (Bossmann et al., 1998):
OH þ RH ! R þ H2 O OH þ RH ! ½RH
þ
þ OH
absorbed, leading to a decrease in the rate of OH production. Increasing the concentration of H2 O2 to 1.5 M reduced the rate of degradation of T4 C10 . This is attributed to the self-scavenging of OH by H2 O2 (Stefan et al., 1996; Kang et al., 1999):
3.1. Effect of H2 O2 concentration
ð2Þ
ð3Þ
The rate of degradation of T4 C10 by reaction with OH radicals was dependent on the concentration of H2 O2 as illustrated in Fig. 1. At a pH of 2.8 and in 0.1 M NaClO4 the largest degradation rate was observed with a H2 O2 concentration of 1:0 102 M with 60% disappearance of the parent tetrachlorodecane in the first 20 min of photolysis. The rate decreased at lower H2 O2 concentrations as a smaller fraction of incident light was
6
Fig. 1. Degradation of 1,2,9,10-tetrachlorodecane (2 10 M) in 0.1 M NaClO4 and pH 2.8 using 300 nm UV light with H2 O2 concentrations of (a) 1:0 103 M, (b) 5:5 103 M and (c) 1:0 102 M.
345
OH þ H2 O2 ! HO2 þ H2 O
ð4Þ
Although the hydroperoxy radical (HO2 ) is formed in this process, its reactivity with organic compounds is low (Nadtochenko and Kiwi, 1998). Therefore, in all subsequent experiments an initial H2 O2 concentration of 1:0 102 M was used. Although H2 O2 concentrations do not decline significantly over the reaction period, as will be discussed later, we observed significant degradation of T4 C10 under these conditions. This indicates that, although steady-state OH concentrations are low, they are sufficient for reaction with 106 M T4 C10 . However, even under these conditions the reaction was incomplete over the 60 min irradiation period with T4 C10 disappearance reaching a plateau at 82% in 40 min. 3.2. Effect of OH scavengers The rate of degradation of T4 C10 by H2 O2 was inhibited by the presence of 0.01 M iodide (I ) ion as illustrated in Fig. 2. Iodide decreases the rate of degradation of T4 C10 by competitive reaction with hydroxyl radicals (Rabani et al., 1998; Kang et al., 1999)
OH þ I ! OH þ I
ð5Þ
and/or direct reaction with H2 O2 preventing the formation of OH radicals (Ruppert et al., 1994)
Fig. 2. Degradation of 1,2,9,10-tetrachlorodecane (2 106 M) with 1:0 102 M H2 O2 in 0.1 M NaClO4 and pH 2.8 using 300 nm UV light (a) without added hydroxyl radical scavengers, (b) in the presence of 0.01 M KI and (c) in the presence of 0.10 M tert-butyl alcohol.
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H2 O2 þ 2I þ 2Hþ ! I2 þ 2H2 O
ð6Þ
The degradation of T4 C10 was also inhibited by the presence of 0.10 M tert-butyl alcohol, a well-known OH radical scavenger (Buxton et al., 1988; Hislop and Bolton, 1999), further indicating that the degradation of tetrachlorodecane proceeds by reaction with OH free radicals. 3.3. Thermal and photochemically enhanced Fenton reactions Fenton’s reagent, a mixture of ferrous (Fe2þ ) ion and hydrogen peroxide which produces OH radicals by the reaction Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð7Þ
has also been used extensively for the chemical oxidation of chlorinated aromatic hydrocarbons (Barbeni et al., 1987; Sedlak and Andren, 1991). Modified Fenton conditions, incorporating a mixture of ferric (Fe3þ ) ion and hydrogen peroxide, avoid the problems of high local concentrations of Fe2þ at the time of mixing of reagents (Zepp et al., 1992). Under these conditions Fe2þ is produced in situ from Fe3þ in aqueous H2 O2 solutions, Fe3þ þ H2 O2 ! Fe2þ þ HO2 þ Hþ
ð8Þ
with Fe2þ then reacting with excess H2 O2 to generate OH radicals (Eq. (7)). Tetrachlorodecane degraded very slowly under modified Fenton dark conditions (Fig. 3) even though H2 O2 concentrations decreased significantly over the 50min reaction period (Fig. 4). Therefore, the decrease in H2 O2 concentration does not reflect increased production of OH radicals, but is rather attributed to the catalytic effect of Fe3þ on the decomposition of H2 O2 to H2 O and O2 (Murphy et al., 1989) for which a detailed mechanism is presented by Pignatello (1992). There is some evidence that Fe3þ and H2 O2 react to produce several FeIII -hydroperoxy complexes (De Laat and Gallard, 1999), FeIII (HO2 )2þ and FeIII (OH)(HO2 )þ , which decompose to produce the hydroperoxy radical (HO2 ) and Fe2þ . However, the general lack of reactivity of T4 C10 under Fenton-like conditions indicates that, if HO2 is formed, it is relatively non-reactive with T4 C10 . Similarly, the slow degradation of T4 C10 under Fenton conditions (Fig. 3) is due to the scavenging of OH radicals by the high concentration of Fe2þ (Eq. (9)) relative to T4 C10 2þ
Fe
þ OH ! Fe
3þ
þ OH
Fig. 3. Degradation of 1,2,9,10-tetrachlorodecane (2 106 M) with 1:0 102 M H2 O2 (a) under modified Fenton conditions (1:0 103 M Fe(ClO4 )3 /dark), (b) under Fenton conditions (1:0 103 M Fe(ClO4 )2 /dark), (c) using 300 nm UV light, (d) under photo-Fenton conditions (1:0 103 M Fe(ClO4 )2 /UV), and (e) under modified photo-Fenton conditions (1:0 103 M Fe(ClO4 )3 /UV).
ð9Þ
Photo-Fenton reactions using either Fenton (Fe2þ / H2 O2 ) or modified Fenton (Fe3þ /H2 O2 ) conditions and UV light often accelerate the decomposition of organic chemicals (Pignatello, 1992; Safarzadeh-Amiri et al.,
Fig. 4. Degradation of hydrogen peroxide (1:0 102 M) (a) exposed to 300 nm UV light, (b) under modified Fenton conditions (1:0 103 M Fe(ClO4 )3 /dark), and (c) under modified photo-Fenton conditions (1:0 103 M Fe(ClO4 )3 /UV).
1997; Nadtochenko and Kiwi, 1998; Fukushima et al., 2000). With ferric ion in acidic solutions the dominant Fe(OH)2þ complex (Pignatello, 1992) photolyzes according to (Hislop and Bolton, 1999) hm
FeðOHÞ2þ ! Fe2þ þ OH
ð10Þ
This enhances the production of OH radicals and promotes cycling of Fe3þ to Fe2þ for reaction with H2 O2 (Eq. (7)). The degradation of 1,2,9,10-tetrachlorodecane in perchlorate solutions is significantly accelerated under modified photo-Fenton conditions (Fig. 3), with 61% disappearance of T4 C10 in the first 10 min of irradiation.
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The rapid degradation of tetrachlorodecane is attributed to the enhanced degradation of hydrogen peroxide and hence production of oxidizing OH radicals (Fig. 4). This supports recent work (Bossmann et al., 1998; De Laat and Gallard, 1999) which suggested that iron(IV) complexes may be the main precursors leading to the production of OH radicals. The degradation of T4 C10 under modified photo-Fenton conditions was also inhibited by the presence of tert-butyl alcohol (0.1 M), again supporting a free radical reaction. Under photoFenton conditions (Fe2þ /H2 O2 /UV) T4 C10 degradation rates were similarly enhanced in the early stages of the reaction (10 min) but slowed considerably as the reaction proceeded (Fig. 3). When 2:5 104 M FeSO4 is used in the photoFenton reaction, the extent of degradation of T4 C10 is significantly decreased. This may be due to the lower Fe2þ concentrations in this experiment and/or to the scavenging of OH radicals by sulfate.
OH þ
Hþ SO2 4 ! H2 O
þ
SO2 4
ð11Þ
3.4. Photolysis of H2 O2 The presence of Fe3þ has a dramatic effect on the rate of disappearance of hydrogen peroxide as illustrated in Fig. 4. In the absence of Fe3þ the photodegradation of H2 O2 (1:0 102 M) is very slow with the 300 nm light source used due to the recombination of OH radicals. However, this system is capable of degrading 106 M T4 C10 (Fig. 1) without significant decreases in H2 O2 concentrations. The thermal degradation of H2 O2 under Fenton-like conditions (Fe3þ /dark) resulted in 50% disappearance in 30 min. However, the lack of reactivity of T4 C10 under these conditions indicates that this decomposition does not involve significant production of OH radicals as discussed previously. Under modified photo-Fenton conditions (Fe3þ /UV) Fe3þ catalyzes the degradation of H2 O2 to such an extent that it is 85% depleted after 5 min of irradiation. The resultant increase in production of OH radicals (Eq. (10)) produced a similar dramatic increase in the rate of destruction of T4 C10 with 80% disappearance after 20 min of irradiation. 3.5. Dechlorination of T4 C10 Although degradation products were not the emphasis in this study, the extent of dechlorination of 1,2,9,10-tetrachlorodecane was determined during photolysis in 1:0 102 M H2 O2 at a pH of 2.8. A near stoichiometric release of Cl was observed with the ratio of [Cl ]/[T4 C10 ]0 reaching 4 after 80 min of irradiation (Fig. 5). An initial release of Cl accompanies the degradation of the parent T4 C10 with the [Cl ]/[T4 C10 ]0
347
Fig. 5. Dechlorination of 1,2,9,10-tetrachlorodecane (2 106 M) during photolysis using 300 nm UV light in aqueous H2 O2 solutions (1:0 102 M) containing 0.1 M NaClO4 and a pH of 2.8.
ratio approaching 2 in the 60 min in which the major degradation of T4 C10 is observed. The results are consistent with a pathway that includes formation of chlorinated organic intermediates that degrade more slowly than the parent tetrachlorodecane. Further Cl release, with the [Cl ]/[T4 C10 ]0 reaching 4, is observed after longer irradiation times (80 min) as the intermediates are degraded. Work on the identification of degradation products is continuing in order to understand the reaction pathway for the degradation of chlorinated n-alkanes with H2 O2 or with photo-Fenton conditions.
Acknowledgements The authors thank the Canadian Chlorine Coordinating Committee (C4) and the Canadian Chlorine Producers Association (CCPA) for financial support.
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Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light Taha M. El-Morsi a, Moustafa M. Emara b, Hassan M.H. Abd El Bary b, Alaa S. Abd-El-Aziz a, Ken J. Friesen a,* a
Department of Chemistry, The University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, Canada R3B 2E9 b Department of Chemistry, Al-Azhar University, Nasr City 11884, Cairo, Egypt Received 28 July 2000; received in revised form 30 June 2001; accepted 6 September 2001
Abstract The homogeneous degradation of the polychlorinated n-alkane, 1,2,9,10-tetrachlorodecane (T4 C10 ), was studied in aqueous solutions of hydrogen peroxide, including Fenton and photo-Fenton reaction conditions. All solutions were adjusted to a pH of 2.8 and an ionic strength of 0.1 M NaClO4 prior to photolysis. T4 C10 (2 106 M) was substantially degraded by the H2 O2 /UV system (1:0 102 M H2 O2 ), with 60% disappearance in 20 min of irradiation in a photoreactor equipped with 300 nm lamps of light intensity 3:6 105 Ein L1 min1 (established by ferrioxalate actinometry). The reaction produced stoichiometric amounts of chloride ion indicating complete dechlorination of the chlorinated n-alkane. T4 C10 degraded very slowly under Fenton (Fe2þ /H2 O2 /dark) and Fenton-like (Fe3þ /H2 O2 /dark) conditions. However, when the same solutions were irradiated, T4 C10 degraded more rapidly than in the H2 O2 /UV system, with 61% disappearance in 10 min of exposure. The rapid degradation is related to the enhanced degradation of hydrogen peroxide to oxidizing OH radicals under photo-Fenton conditions. Degradation was inhibited in both the H2 O2 /UV and photo-Fenton systems by the addition of KI and tert-butyl alcohol due to OH scavenging. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; UV; Tetrachlorodecane; Hydrogen peroxide; Photo-Fenton
1. Introduction Chlorinated paraffins are complex mixtures of polychloro-n-alkanes (PCAs) used industrially as high-temperature lubricants and metal-cutting fluids, plasticizers, and flame-retardant additives in sealants, adhesives and paints. PCAs with carbon chain lengths ranging from C10 to C13 , also known as short-chain chloroparaffins, are of particular concern due to their toxicity and potential for release into the environment (Environment
*
Corresponding author. Tel.: +1-204-786-9043; fax: +1-204775-2114. E-mail address: [email protected] (K.J. Friesen).
Canada, 1993). From a recent review (Tomy et al., 1998) it is evident that short-chain PCAs are widespread in the environment. Although PCAs have been detected in waters receiving industrial effluents, their occurrence in water, sediments and aquatic organisms of remote Arctic lakes (Tomy et al., 2000) suggests that these are relatively persistent chemicals. Advanced oxidation technologies (AOT) have been developed for the detoxification of water contaminated with persistent organic chemicals. The method utilizing the strong oxidizing potential of the hydroxyl radical (E0 ¼ þ1:8 V at neutral pH to þ2.7 V in acidic solutions (Buxton et al., 1988) has proven effective for the degradation of a wide variety of aqueous contaminants (Halmann, 1996), including saturated halocarbons such
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 1 ) 0 0 3 0 5 - 8
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as chloroform (Hsiao et al., 1983; Ahmed and Ollis, 1984), dichloroethane (Ollis et al., 1984) and 1-bromododecane (Pelizzetti et al., 1990). The strategies for generating the reactive OH radicals for these oxidation methods include UV photolysis of ozone or hydrogen peroxide, Fenton or photo-Fenton type reactions, and TiO2 photocatalysis. We have recently demonstrated that the PCA, 1,10dichlorodecane, is rapidly photooxidized by the TiO2 photocatalyst system (El-Morsi et al., 2000) with the reaction involving adsorbed dichlorodecane and surface OH radicals generated by exposing aqueous TiO2 suspensions to 300 nm UV light. In this study we are investigating the effectiveness of aqueous hydrogen peroxide systems, including Fenton and photo-Fenton conditions, for the degradation of another member of the PCA group, 1,2,9,10-tetrachlorodecane. 2. Experimental 2.1. Chemicals Chlorine gas and 1,9-decadiene, used in the synthesis of tetrachlorodecane, were supplied by Aldrich Chemical. Reagent grade ferric perchlorate, Fe(ClO4 )3 9H2 O, and ferrous perchlorate, Fe(ClO4 )2 6H2 O, radical scavengers KI and tert-butyl alcohol, and NaClO4 were used as received. Hydrogen peroxide solutions were prepared by diluting 30 wt.% H2 O2 (BDH) with Milli-Q purity water to the desired concentration. 2,9-Dimethyl1,10-phenanthroline (DMP) from Sigma Chemical, 0.01 M CuSO4 5H2 O and phosphate buffer (prepared with K2 HPO4 and NaH2 PO4 and adjusted to pH 7 with 0.05 M H2 SO4 and 1.0 M NaOH) were used in the spectrophotometric determination of H2 O2 . DMP reagent was dissolved in 100 mL 95% ethanol, placed in a 100 mL flask wrapped with aluminum foil and refrigerated. All organic solvents used were HPLC grade. 2.2. Synthesis of 1,2,9,10-tetrachlorodecane (T4 C10 ) 1,2,9,10-Tetrachlorodecane was synthesized by chlorine addition to 1,9-decadiene using a variation of the procedure reported by Tomy (1998). Approximately 10 mL of 0.05 M NaOH was layered over 40 mL of dichloromethane in a round bottom flask. Chlorine gas was gently bubbled through the DCM layer for several minutes before introduction of 1,9-decadiene into the lower DCM layer by pipet. After a brief reaction period, the layers were separated and the DCM layer was analyzed by GC/MS. The total ion chromatogram produced one dominant peak with a retention time of 13.7 min and an EI mass spectrum confirming the identity of the product as tetrachlorodecane. The very small amounts of higher chlorinated decane isomers (with chlorine
number > 4) evident in the chromatogram indicated that free radical substitution reactions were minimal. 2.3. Light irradiation Photolytic experiments were performed using a Rayonet Photochemical Reactor equipped with a carousel and 16 UV lamps emitting 300 nm light with an intensity of 3:6 105 Ein L1 min1 as established by ferrioxalate actinometry (Hatchard and Parker, 1956). Aqueous solutions of T4 C10 (2 106 M) were prepared by pipeting from a stock solution (394 ng lL1 in 1:1 CH3 CN/H2 O) into Milli-Q water that exhibited a resistivity greater than 18 MX cm1 . The T4 C10 solutions were adjusted to pH 2.8 with perchloric acid (HClO4 ) in order to optimize the production of OH radicals (De Laat and Gallard, 1999; McGinnis et al., 2000). All aqueous solutions of T4 C10 contained <0.07% CH3 CN, which was determined to have a negligible effect on the rates of degradation of the substrate. Photo-Fenton reactions were initiated by adding 0.1 M NaClO4 , 1:0 103 M Fe(ClO4 )3 or Fe(ClO4 )2 , and 1:0 102 M H2 O2 in rapid succession with constant stirring to maintain oxygen saturation. Solutions were simultaneously irradiated in the photoreactor which had been thermally equilibrated for 10 min. Samples were typically removed after irradiation times of 10, 20, 30, 40, 50, 60, 90 and 120 min and then extracted with 60 mL of hexane/dichloromethane (5.6:1, v/v), with an extraction efficiency of 74:1% 1:5%. Control experiments were performed to determine the extent of thermal degradation (under dark conditions) and direct photolysis (in the absence of H2 O2 ). 2.4. Analyses Analyses of tetrachlorodecane were performed with a Hewlett–Packard (HP) 5890 Series II GC equipped with a 30-m 0:25-mm 0:25-lm PTE-5 column (Supelco) and coupled to a HP 5989A MS Engine. Column head pressure was maintained at 14 psi with an injection port temperature of 200 °C. The column was temperature programmed as follows: 90 °C (1 min), 10 °C min1 to 180 °C, 7 °C min1 to 210 °C (5 min). All experiments were performed in duplicate with the reproducibility in concentrations typically better than 2%. The concentration of hydrogen peroxide was determined spectrophotometrically by the DMP method with absorbance measurements at 454 nm (Kosaka et al., 1998). Spectrophotometric measurements were performed using a Hewlett–Packard 8450A diode array spectrophotometer with a quartz cell of path length 1.0 cm. Chloride ion (Cl ) was measured potentiometrically using an Orion 96-17B chloride selective electrode calibrated with NaCl standards. In order to reach a detectable concentration of Cl 1 L solutions were
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concentrated by rotary evaporation, adjusted to a volume of 5 mL, followed by addition of 2 mL of Orion ISA reagent prior to measurement.
3. Results and discussion
Aqueous solutions of T4 C10 (2 106 M) did not absorb UV light, hence direct photolysis of T4 C10 was negligible as expected. However, T4 C10 degradation was sensitized by the presence of hydrogen peroxide due to the favorable absorption spectrum of H2 O2 . The direct photolysis of H2 O2 , requiring k < 360 nm (Pignatello et al., 1999), generates OH radicals according to: hm
H2 O2 ! 2 OH
ð1Þ
Although degradation products were not determined in this study, hydroxyl radicals are known to react with saturated organic chemicals by H-abstraction from alkyl or hydroxy groups (Walling, 1975) or by an electron transfer process (Bossmann et al., 1998):
OH þ RH ! R þ H2 O OH þ RH ! ½RH
þ
þ OH
absorbed, leading to a decrease in the rate of OH production. Increasing the concentration of H2 O2 to 1.5 M reduced the rate of degradation of T4 C10 . This is attributed to the self-scavenging of OH by H2 O2 (Stefan et al., 1996; Kang et al., 1999):
3.1. Effect of H2 O2 concentration
ð2Þ
ð3Þ
The rate of degradation of T4 C10 by reaction with OH radicals was dependent on the concentration of H2 O2 as illustrated in Fig. 1. At a pH of 2.8 and in 0.1 M NaClO4 the largest degradation rate was observed with a H2 O2 concentration of 1:0 102 M with 60% disappearance of the parent tetrachlorodecane in the first 20 min of photolysis. The rate decreased at lower H2 O2 concentrations as a smaller fraction of incident light was
6
Fig. 1. Degradation of 1,2,9,10-tetrachlorodecane (2 10 M) in 0.1 M NaClO4 and pH 2.8 using 300 nm UV light with H2 O2 concentrations of (a) 1:0 103 M, (b) 5:5 103 M and (c) 1:0 102 M.
345
OH þ H2 O2 ! HO2 þ H2 O
ð4Þ
Although the hydroperoxy radical (HO2 ) is formed in this process, its reactivity with organic compounds is low (Nadtochenko and Kiwi, 1998). Therefore, in all subsequent experiments an initial H2 O2 concentration of 1:0 102 M was used. Although H2 O2 concentrations do not decline significantly over the reaction period, as will be discussed later, we observed significant degradation of T4 C10 under these conditions. This indicates that, although steady-state OH concentrations are low, they are sufficient for reaction with 106 M T4 C10 . However, even under these conditions the reaction was incomplete over the 60 min irradiation period with T4 C10 disappearance reaching a plateau at 82% in 40 min. 3.2. Effect of OH scavengers The rate of degradation of T4 C10 by H2 O2 was inhibited by the presence of 0.01 M iodide (I ) ion as illustrated in Fig. 2. Iodide decreases the rate of degradation of T4 C10 by competitive reaction with hydroxyl radicals (Rabani et al., 1998; Kang et al., 1999)
OH þ I ! OH þ I
ð5Þ
and/or direct reaction with H2 O2 preventing the formation of OH radicals (Ruppert et al., 1994)
Fig. 2. Degradation of 1,2,9,10-tetrachlorodecane (2 106 M) with 1:0 102 M H2 O2 in 0.1 M NaClO4 and pH 2.8 using 300 nm UV light (a) without added hydroxyl radical scavengers, (b) in the presence of 0.01 M KI and (c) in the presence of 0.10 M tert-butyl alcohol.
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H2 O2 þ 2I þ 2Hþ ! I2 þ 2H2 O
ð6Þ
The degradation of T4 C10 was also inhibited by the presence of 0.10 M tert-butyl alcohol, a well-known OH radical scavenger (Buxton et al., 1988; Hislop and Bolton, 1999), further indicating that the degradation of tetrachlorodecane proceeds by reaction with OH free radicals. 3.3. Thermal and photochemically enhanced Fenton reactions Fenton’s reagent, a mixture of ferrous (Fe2þ ) ion and hydrogen peroxide which produces OH radicals by the reaction Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð7Þ
has also been used extensively for the chemical oxidation of chlorinated aromatic hydrocarbons (Barbeni et al., 1987; Sedlak and Andren, 1991). Modified Fenton conditions, incorporating a mixture of ferric (Fe3þ ) ion and hydrogen peroxide, avoid the problems of high local concentrations of Fe2þ at the time of mixing of reagents (Zepp et al., 1992). Under these conditions Fe2þ is produced in situ from Fe3þ in aqueous H2 O2 solutions, Fe3þ þ H2 O2 ! Fe2þ þ HO2 þ Hþ
ð8Þ
with Fe2þ then reacting with excess H2 O2 to generate OH radicals (Eq. (7)). Tetrachlorodecane degraded very slowly under modified Fenton dark conditions (Fig. 3) even though H2 O2 concentrations decreased significantly over the 50min reaction period (Fig. 4). Therefore, the decrease in H2 O2 concentration does not reflect increased production of OH radicals, but is rather attributed to the catalytic effect of Fe3þ on the decomposition of H2 O2 to H2 O and O2 (Murphy et al., 1989) for which a detailed mechanism is presented by Pignatello (1992). There is some evidence that Fe3þ and H2 O2 react to produce several FeIII -hydroperoxy complexes (De Laat and Gallard, 1999), FeIII (HO2 )2þ and FeIII (OH)(HO2 )þ , which decompose to produce the hydroperoxy radical (HO2 ) and Fe2þ . However, the general lack of reactivity of T4 C10 under Fenton-like conditions indicates that, if HO2 is formed, it is relatively non-reactive with T4 C10 . Similarly, the slow degradation of T4 C10 under Fenton conditions (Fig. 3) is due to the scavenging of OH radicals by the high concentration of Fe2þ (Eq. (9)) relative to T4 C10 2þ
Fe
þ OH ! Fe
3þ
þ OH
Fig. 3. Degradation of 1,2,9,10-tetrachlorodecane (2 106 M) with 1:0 102 M H2 O2 (a) under modified Fenton conditions (1:0 103 M Fe(ClO4 )3 /dark), (b) under Fenton conditions (1:0 103 M Fe(ClO4 )2 /dark), (c) using 300 nm UV light, (d) under photo-Fenton conditions (1:0 103 M Fe(ClO4 )2 /UV), and (e) under modified photo-Fenton conditions (1:0 103 M Fe(ClO4 )3 /UV).
ð9Þ
Photo-Fenton reactions using either Fenton (Fe2þ / H2 O2 ) or modified Fenton (Fe3þ /H2 O2 ) conditions and UV light often accelerate the decomposition of organic chemicals (Pignatello, 1992; Safarzadeh-Amiri et al.,
Fig. 4. Degradation of hydrogen peroxide (1:0 102 M) (a) exposed to 300 nm UV light, (b) under modified Fenton conditions (1:0 103 M Fe(ClO4 )3 /dark), and (c) under modified photo-Fenton conditions (1:0 103 M Fe(ClO4 )3 /UV).
1997; Nadtochenko and Kiwi, 1998; Fukushima et al., 2000). With ferric ion in acidic solutions the dominant Fe(OH)2þ complex (Pignatello, 1992) photolyzes according to (Hislop and Bolton, 1999) hm
FeðOHÞ2þ ! Fe2þ þ OH
ð10Þ
This enhances the production of OH radicals and promotes cycling of Fe3þ to Fe2þ for reaction with H2 O2 (Eq. (7)). The degradation of 1,2,9,10-tetrachlorodecane in perchlorate solutions is significantly accelerated under modified photo-Fenton conditions (Fig. 3), with 61% disappearance of T4 C10 in the first 10 min of irradiation.
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The rapid degradation of tetrachlorodecane is attributed to the enhanced degradation of hydrogen peroxide and hence production of oxidizing OH radicals (Fig. 4). This supports recent work (Bossmann et al., 1998; De Laat and Gallard, 1999) which suggested that iron(IV) complexes may be the main precursors leading to the production of OH radicals. The degradation of T4 C10 under modified photo-Fenton conditions was also inhibited by the presence of tert-butyl alcohol (0.1 M), again supporting a free radical reaction. Under photoFenton conditions (Fe2þ /H2 O2 /UV) T4 C10 degradation rates were similarly enhanced in the early stages of the reaction (10 min) but slowed considerably as the reaction proceeded (Fig. 3). When 2:5 104 M FeSO4 is used in the photoFenton reaction, the extent of degradation of T4 C10 is significantly decreased. This may be due to the lower Fe2þ concentrations in this experiment and/or to the scavenging of OH radicals by sulfate.
OH þ
Hþ SO2 4 ! H2 O
þ
SO2 4
ð11Þ
3.4. Photolysis of H2 O2 The presence of Fe3þ has a dramatic effect on the rate of disappearance of hydrogen peroxide as illustrated in Fig. 4. In the absence of Fe3þ the photodegradation of H2 O2 (1:0 102 M) is very slow with the 300 nm light source used due to the recombination of OH radicals. However, this system is capable of degrading 106 M T4 C10 (Fig. 1) without significant decreases in H2 O2 concentrations. The thermal degradation of H2 O2 under Fenton-like conditions (Fe3þ /dark) resulted in 50% disappearance in 30 min. However, the lack of reactivity of T4 C10 under these conditions indicates that this decomposition does not involve significant production of OH radicals as discussed previously. Under modified photo-Fenton conditions (Fe3þ /UV) Fe3þ catalyzes the degradation of H2 O2 to such an extent that it is 85% depleted after 5 min of irradiation. The resultant increase in production of OH radicals (Eq. (10)) produced a similar dramatic increase in the rate of destruction of T4 C10 with 80% disappearance after 20 min of irradiation. 3.5. Dechlorination of T4 C10 Although degradation products were not the emphasis in this study, the extent of dechlorination of 1,2,9,10-tetrachlorodecane was determined during photolysis in 1:0 102 M H2 O2 at a pH of 2.8. A near stoichiometric release of Cl was observed with the ratio of [Cl ]/[T4 C10 ]0 reaching 4 after 80 min of irradiation (Fig. 5). An initial release of Cl accompanies the degradation of the parent T4 C10 with the [Cl ]/[T4 C10 ]0
347
Fig. 5. Dechlorination of 1,2,9,10-tetrachlorodecane (2 106 M) during photolysis using 300 nm UV light in aqueous H2 O2 solutions (1:0 102 M) containing 0.1 M NaClO4 and a pH of 2.8.
ratio approaching 2 in the 60 min in which the major degradation of T4 C10 is observed. The results are consistent with a pathway that includes formation of chlorinated organic intermediates that degrade more slowly than the parent tetrachlorodecane. Further Cl release, with the [Cl ]/[T4 C10 ]0 reaching 4, is observed after longer irradiation times (80 min) as the intermediates are degraded. Work on the identification of degradation products is continuing in order to understand the reaction pathway for the degradation of chlorinated n-alkanes with H2 O2 or with photo-Fenton conditions.
Acknowledgements The authors thank the Canadian Chlorine Coordinating Committee (C4) and the Canadian Chlorine Producers Association (CCPA) for financial support.
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