- Email: [email protected]
Micellar effect on the photolysis of hydrogen peroxide
PII: S0043-1354(01)00023-9
Wat. Res. Vol. 35, No. 13, pp. 3276–3279, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE MICELLAR EFFECT ON THE PHOTOLYSIS OF HYDROGEN PEROXIDE YOUN-JOO AN*,y, SEUNG-WOO JEONGy and ELIZABETH R. CARRAWAYz Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA (First received 1 February 2000; accepted in revised form 1 December 2000) Abstract}Photolysis experiments were performed to quantify the effect of three anionic surfactants on the photolysis of hydrogen peroxide (H2O2) at the ambient laboratory temperature of 22 18C. H2O2 photolysis in water, methanol, and surfactant monomeric solution was also conducted to compare the photochemical reactivity of H2O2 in different media. Photolysis rates were highest for water, followed by micellar solutions, and lowest for methanol. The results show that the photochemical reactivity of H2O2 is less favorable in organic solvent than in water and surfactant micelles affect H2O2 photolysis. Retarded photolysis of H2O2 in micellar solutions implies that a fraction of H2O2 dissolved in water partitions into micellar pseudophase of surfactant. H2O2 partitioned into micelles has less photochemical reactivity and thus photolysis rate was retarded in the presence of micelles. Photolysis inhibitory level by micelles was shown to be dependent on the kinds of surfactants used in this study. In addition, the inhibitory effect by surfactant monomers was negligible due to the absence of micelles. # 2001 Elsevier Science Ltd. All rights reserved Key words}hydrogen peroxide, photolysis, surfactant, micelle, monomer, partition
INTRODUCTION
Hydrogen peroxide is widely used either alone or in combination with other treatment processes in environmental applications. H2O2 in water is a rather mild and ineffective oxidant without light or metal ion catalysis (Larson and Weber, 1994). It can also serve as a reducing agent as well as an oxidizing agent (Strukul, 1992). Decomposition of H2O2 in water depends on temperature, pH, and presence of impurities (e.g. metal ions, metal oxyhydroxides). The photolysis of H2O2 to yield reactive and nonselective hydroxyl radicals is an important indirect method of contaminant destruction. Enhanced degradation of organic pollutants by UV/ H2O2 system is well known. The addition of photocatalysts and thermal catalysts to H2O2 has also been shown to promote effective contaminant destruction under the irradiation of cool fluorescence light (Pignatello, 1992; Pignatello and Huang, 1993). A number of organic and inorganic compounds are reported to inhibit the photochemical decom*Author to whom all correspondence should be addressed. Tel.: +1-580-436-8547; fax: +1-580-436-8703; e-mail: [email protected] y Present address: Research Associateship Programs, U.S. Environmental Protection Agency, Ada, OK 74820, USA. z Present address: Department of Environmental Toxicology, Clemson University, Pendleton, SC 29670, USA.
position of H2O2 solutions. Mathews and Curtis (1914) investigated the effect of six substances on the photolysis of H2O2, including acetanilide, sulfuric acid, sodium chloride, sodium hydroxide, calcium hydroxide, and barium hydroxide. They showed that all the six compounds they studied acted as inhibitors for photochemical reaction. Anderson and Taylor (1923) reported 25 inhibitors for H2O2 photolysis, including acids, ethers, amides, alcohols, ketones, benzene and alkaloids. Photodegradation of contaminants in surfactant solution has been demonstrated as a post treatment method after surfactant enhanced aquifer remediation to recover and reuse the surfactant solution (Chu and Jafvert, 1994; Chu et al., 1998). However, photolysis combined with H2O2 will be a more general approach for environmental applications since direct photolysis is only susceptible for light absorbing compounds. To the best of our knowledge, the effect of surfactant on the photolysis of H2O2 has not been investigated. Previously, a patented process for preserving H2O2 used surfactants to inhibit the thermal decomposition of H2O2 (Schaidhauf, 1914). The motivation of the work presented here was to investigate the photolysis of H2O2 in surfactant solutions and provide fundamental information for future environmental applications. The objective of this study was to examine the effect of surfactant micelles on photolysis of H2O2. Surfactants used in this study were sodium dodecyl
3276
Micellar effect on H2O2 photolysis
sulfate (SDS), ammonium perfluorooctanoate (APFO), and lithium perfluorooctanesulfonate (LiFOS). H2O2 photolysis was conducted in micellar solutions of SDS, APFO and LiFOS. H2O2 was photolyzed in water, methanol and APFO monomeric solution to compare the photochemical reactivity of H2O2 in different media. The H2O2 photolysis in an APFO monomeric solution was compared with that in an APFO micellar solution. In addition, UV absorbance of surfactants was measured to quantify the background absorbance of surfactant alone.
MATERIALS AND METHODS
Materials APFO (Fluka, 98%), LiFOS (3 M, 25% aqueous solution), SDS (Aldrich, 98%), H2O2 (Aldrich, 35%), potassium permanganate (Aldrich, 0.1 0.005 N), and ferrous sulfate (Aldrich, 99.999+%) were used as received; physicochemical properties for surfactants used in this study are in Table 1. An approximately 0.1 M stock solution of H2O2 was prepared from the 35% reagent solution and standardized by titration with potassium permanganate. H2O2 concentrations were measured iodometrically (Allen et al., 1952). Each calibration curve of H2O2 in surfactant solutions was prepared in the same surfactant solutions. Absorbance measurement was conducted using a Hewlett Packard 8452A diode array spectrophotometer in 10-mm pathlength quarts cells (Starna). Ferrous sulfate was used to prepare the standard solutions for the potassium ferrioxalate actinometry. Water was purified by a Barnstead Nanopure system resulting in a resistivity >17.8 MO cm1.
3277
(ACE glass, 7891-30) for ultraviolet irradiation. The light source was a low-pressure mercury lamp (UVP Pen-Ray Lamp) with predominant UV intensity at 254 nm. The temperature of the tubes was kept almost constant (22 18C) by providing a stream of fresh air using a fan. The photon flux of the lamp was measured by using a potassium ferrioxalate actinometry (Hatchard and Parker, 1956). The incident radiation was determined to be consistently near 5.0 108 einstein L1 s1. RESULTS AND DISCUSSION
UV spectroscopic studies of APFO Aliphatic carboxylic acids and their anions are known to produce absorption spectra in the UV region as a result of n !p and p!p transitions (Mukerjee et al., 1990). APFO is a perfluoroalkyl carboxylate which has a carbonyl group (C=O) in the molecular structure. The carbonyl group is the chromophore that absorbs ultraviolet light, and it undergoes an n !p transition when it absorbs light. Most n !p transitions have small molar absorptivity, leading to the low absorption intensity (Klessinger and Michl, 1995). Absorbance spectra of APFO measured at the concentrations ranging from 0.01 to 0.1 M is shown in Fig. 1(a). The molar absorptivity of APFO determined experimentally was 0.75 M1 cm1 at 254 nm (Fig. 1(b)). This value was used to calculate the initial photolysis rate of H2O2 in a later section.
UV absorbance of surfactant Among the three anionic surfactants used in this study, only APFO absorbs the UV light since it has the chromophore, carbonyl group. The UV absorption spectra of APFO were measured as a function of APFO concentrations ranging from 0.01 to 0.1 M to determine the molar absorptivities of APFO. The molar absorptivity of APFO at 254 nm was used to quantify the background absorbance by APFO on the photolysis of H2O2. Photolysis of hydrogen peroxide H2O2 was photolyzed in water, 90% methanol, monomeric solution (10 mM APFO), and micellar solutions (50 mM APFO, 14 mM LiFOS, and 20 mM SDS). All micellar solution concentrations used were about 2–2.5 times critical micelle concentration (CMC). The initial H2O2 concentration used in this study was 10 mM. Photolysis experiments were conducted in triplicate using cylindrical quartz tubes (13 OD 10 L mm). The samples were mixed completely and then placed in merry-go-round reactor
Table 1. Selected physical and chemical properties of surfactants in this study Surfactant
APFO LiFOS SDS a
Molecular formula
MW
Water solubilitya
CMC (mM)
C7F15COONH4 C8F17SO3Li C12H25SO4Na
431.10 128.17 288.38
230 mM Complete Complete
26.4b 6.25b 8.15c
Data provided by supplier; temperature not specified. An (1999). c Mukerjee and Mysels (1970). b
Fig. 1. (a) Absorption spectrum of APFO as a function of APFO concentrations. Concentration increases from left to right. (b) Molar absorptivity of APFO with wavelength. The concentration of APFO used in this study is 0.01–0.1 M.
3278
Youn-Joo An et al.
Photolysis of hydrogen peroxide Figure 2 shows the concentration of H2O2 remaining as a function of irradiation time in water, methanol and surfactant solutions. Photochemical decomposition of H2O2 was found to follow first order kinetics in all solutions and the first order reaction constants are given in Table 2. Photolysis rates were highest for water, followed by micellar solutions (50 mM APFO>14 mM LiFOS>20 mM SDS), and lowest for methanol. A comparison of H2O2 photolysis in water and methanol indicated that the photochemical reactivity of H2O2 was less favorable in organic solvent than in water. This result is consistent with a previous observation that alcohols inhibit H2O2 photolysis (Anderson and Taylor, 1923). This study observed that H2O2 photolysis in micellar solutions was slower than in water, and faster than in methanol. Above the CMC, the surfactant monomers dissolved in water form micelles. Micelles have less polar region compared to the aqueous phase. Although H2O2 is miscible with water, it can be also dissolved in organic solvents (Dong and Guo, 1994). Retarded H2O2 photolysis in the presence of micelles implies that a fraction of H2O2 dissolved in water partitioned into micellar pseudophase of surfactant. H2O2 partition into micelles has less photochemical reactivity since micellar environment is more like organic solvent than water. Therefore, overall photolysis rates in the presence of micelles were retarded. In addition, photolysis inhibitory level by micelles was shown to be dependent on the kinds of surfactants used in this study. As expected, APFO monomers did not alter the H2O2 photolysis rate since no micelles exist below the CMC. Photolysis rate of H2O2 in APFO monomeric solution was very similar to that in water. Slight decrease of photolysis rate in the presence of APFO monomers was due to absorbance of APFO alone.
Fig. 2. Photolysis of H2O2 at 254 nm in water, 10 mM APFO, 50 mM APFO, 14 mM LiFOS, 20 mM SDS, and methanol (from bottom to top). Io =5.0 108 einstein L1 s1. The initial concentration of H2O2 is 10 mM. The lines show the fitting with the first order reaction rates.
Table 2. Photolysis rate constant of H2O2 in water, methanol and surfactant solutions, calculated by pseudofirst order kinetics Solutions
Water 90% methanol 10 mM APFO (monomeric) 50 mM APFO (micellar) 14 mM LiFOS (micellar) 20 mM SDS (micellar) a
Rate constant (min1)a
R2
0.0121 0.0006 0.0040 0.0002 0.0119 0.0005 0.0072 0.0003 0.0054 0.0002 0.0045 0.0001
0.984 0.977 0.994 0.993 0.992 0.998
Values are s.
To consider any effects of the absorbance of APFO alone in H2O2 photolysis, photolysis rates at t ¼ 0 (initial rates) in water and APFO solutions were calculated using general equation of photolysis, as shown in Table 3. These calculations predicted that initial rates would be decreased by factors of 1.01 and 1.04 in 10 and 50 mM APFO solutions, respectively, compared to water. However, initial rates calculated by the actual rate constants in Table 2 were decreased by factors of 1.02 and 1.68 in 10 and 50 mM APFO solutions, respectively. The comparison showed that APFO monomers had negligible effects on the H2O2 photolysis (1.02 1.01) and APFO micelles retarded the H2O2 photolysis more than what would be expected from APFO background absorbance alone (1.68>1.04). Therefore, the rate decrease of H2O2 in 50 mM APFO solution is largely due to the micellar effects. Change of pH affects the formation of hydroxyl radicals which leads to the change of photolysis rate (Beltran et al., 1996). Sample pHs in surfactant solutions were measured the and resulting solution pHs were all around neutral, so pH effect was found to be negligible in this study. CONCLUSIONS
The influence of surfactant micelles on the photolysis of hydrogen peroxide was investigated. Photolysis in water, methanol, and surfactant monomeric solution was also conducted for comparison. Photolysis rates were highest for water, followed by micellar solutions, and lowest for methanol. Photolysis in monomeric solution was shown to be very similar to that in water. The conclusions of this study are as follows: 1. Photolysis of H2O2 was slower in methanol than in water, indicating that the photochemical reactivity of H2O2 is less favorable in organic solvent than in water. 2. Photolysis of H2O2 was retarded in micellar solutions. It implies that a fraction of H2O2 dissolved in water partitions into micellar pseudophase of surfactant. H2O2 partitioned into micelles has less photochemical reactivity and thus photolysis rate was retarded in the presence of micelles.
Micellar effect on H2O2 photolysis
3279
Table 3. Photolysis rate of H2O2 at t ¼ 0 (initial rate) in water and APFO solutions calculated by first order reaction rate and photolysis rate Solutions Water
10 mM APFO
50 mM APFO
First order reaction rate r ¼ kC ro=kCoa
ro ro,water/ro
0.000121 1.00
0.000119 1.02
0.000072 1.68
Photolysis rate
Fsl
0.3543
0.3654
0.4078
r =Io f Fslfcl
fcl
1
0.9620
0.8352
Fsl ¼ 1 10ðesþecCÞl ec C fcl ¼ es þ ec C
ro =ðIo fÞ ¼ Fsl fcl
0.3543
0.3515
0.3406
ro;water =ro
1.00
1.01
1.04
b
c
d
a
ro is initial rate, k values from Table 2, Co =0.01 M of H2O2. General expression for photolysis rate (Zepp, 1978). Io is the incident light intensity at given wavelength (einstein L1 s1), Fsl is the fraction of light absorbed by the system at a wavelength l, fcl is the fraction of light absorbed by the photoreactant at a wavelength l, f is the quantum yield, es is the absorption coefficient of the solvent, ec is the molar absorptivity of the photoreactant at given wavelength (M1 cm1), and l is the cell pathlength (cm). c Io =5.0 108 einstein L1 s1. d Assumed to be unaffected by APFO micelles. b
3. The inhibitory effect by surfactant monomers on the photolysis of H2O2 was negligible due to the absence of micelles below the critical micelle concentration.
Acknowledgements}We gratefully acknowledge two reviewers who provide constructive comments for improving the manuscript. Support of this work was provided by the Office of the Vice President for Research and Associate Provost for Graduate Studies in Texas A&M University.
REFERENCES
Allen A. O., Hochanadel C. J., Ghormley J. A. and Davis T. W. (1952) Decomposition of water and aqueous solutions under mixed fast neutron and gamma radiation. J. Phys. Chem. 56, 575–586. An Y.-J. (1999) Solubilization and photochemical degradation of polycyclic aromatic hydrocarbons in perfluorinated surfactant solutions. Ph.D. Dissertation, Texas A&M University, USA. Anderson W. T. and Taylor H. S. (1923) The inhibition of the photochemical decomposition of hydrogen peroxide solutions. J. Am. Chem. Soc. 45, 650–662. Beltran F. J., Ovejero G. and Rivas J. (1996) Oxidation of polynuclear aromatic hydrocarbons in water. 3. UV radiation combined with hydrogen peroxide. Ind. Engng. Chem. Res. 35, 883–890. Chu W. and Jafvert C. T. (1994) Photodechlorination of polychlorobenzene congeners in surfactant micelle solutions. Environ. Sci. Technol. 28, 2415–2422. Chu W., Jafvert C. T., Diehl C. A., Marley K. and Larson R. A. (1998) Phototransformations of polychlorobiphenyls in Brij 58 micellar solutions. Environ. Sci. Technol. 32, 1989–1993.
Dong S. and Guo Y. (1994) Organic phase enzyme electrode operated in water-free solvents. Anal. Chem. 66, 3895–3899. Hatchard C. G. and Parker C. A. (1956) A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer. Roy. Soc. London Proc. 235, 518–536. Klessinger M. and Michl J. (1995) Excited States and Photochemistry of Organic Molecules. VCH Publishers Inc., New York. Larson R. A. and Weber E. J. (1994) Reaction Mechanisms in Environmental Organic Chemistry. Lewis Publishers, Boca Raton, FL. Mathews J. H. and Curtis H. A. (1914) The photochemical decomposition of hydrogen peroxide. J. Phys. Chem. 18, 166–178. Mukerjee P., Gumkowski M. J., Chan C. C. and Sharma R. (1990) Determination of critical micellization concentration of perfluorocarboxylates using ultraviolet spectroscopy: Some unusual counterion effects. J. Phys. Chem. 94, 8832–8835. Mukerjee P. and Mysels K. J. (1970) Critical Micelle Concentration of Aqueous Surfactant Systems. US Department of Commerce, Washington, DC. Pignatello J. J. (1992) Dark and photoassisted Fe3+catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ. Sci. Technol. 26, 944–951. Pignatello J. J. and Huang L. Q. (1993) Degradation of polychlorinated dibenzo-p-dioxin and dibenzofuran contaminants in 2,4,5-T by photoassisted iron-catalyzed hydrogen peroxide. Water Res. 27, 1731–1736. Schaidhauf A. (1914) Stable Hydrogen Peroxide and Method of Making the Same. US Patent 1109791. Strukul G. (1992) Catalytic Oxidations with Hydrogen Peroxide as Oxidant. Kluwer Academic Publishers, Dordrecht, Netherlands. Zepp R. G. (1978) Quantum yields for reactions of pollutants in dilute aquoeus solution. Environ. Sci. Technol. 12, 327–329.
Wat. Res. Vol. 35, No. 13, pp. 3276–3279, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE MICELLAR EFFECT ON THE PHOTOLYSIS OF HYDROGEN PEROXIDE YOUN-JOO AN*,y, SEUNG-WOO JEONGy and ELIZABETH R. CARRAWAYz Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA (First received 1 February 2000; accepted in revised form 1 December 2000) Abstract}Photolysis experiments were performed to quantify the effect of three anionic surfactants on the photolysis of hydrogen peroxide (H2O2) at the ambient laboratory temperature of 22 18C. H2O2 photolysis in water, methanol, and surfactant monomeric solution was also conducted to compare the photochemical reactivity of H2O2 in different media. Photolysis rates were highest for water, followed by micellar solutions, and lowest for methanol. The results show that the photochemical reactivity of H2O2 is less favorable in organic solvent than in water and surfactant micelles affect H2O2 photolysis. Retarded photolysis of H2O2 in micellar solutions implies that a fraction of H2O2 dissolved in water partitions into micellar pseudophase of surfactant. H2O2 partitioned into micelles has less photochemical reactivity and thus photolysis rate was retarded in the presence of micelles. Photolysis inhibitory level by micelles was shown to be dependent on the kinds of surfactants used in this study. In addition, the inhibitory effect by surfactant monomers was negligible due to the absence of micelles. # 2001 Elsevier Science Ltd. All rights reserved Key words}hydrogen peroxide, photolysis, surfactant, micelle, monomer, partition
INTRODUCTION
Hydrogen peroxide is widely used either alone or in combination with other treatment processes in environmental applications. H2O2 in water is a rather mild and ineffective oxidant without light or metal ion catalysis (Larson and Weber, 1994). It can also serve as a reducing agent as well as an oxidizing agent (Strukul, 1992). Decomposition of H2O2 in water depends on temperature, pH, and presence of impurities (e.g. metal ions, metal oxyhydroxides). The photolysis of H2O2 to yield reactive and nonselective hydroxyl radicals is an important indirect method of contaminant destruction. Enhanced degradation of organic pollutants by UV/ H2O2 system is well known. The addition of photocatalysts and thermal catalysts to H2O2 has also been shown to promote effective contaminant destruction under the irradiation of cool fluorescence light (Pignatello, 1992; Pignatello and Huang, 1993). A number of organic and inorganic compounds are reported to inhibit the photochemical decom*Author to whom all correspondence should be addressed. Tel.: +1-580-436-8547; fax: +1-580-436-8703; e-mail: [email protected] y Present address: Research Associateship Programs, U.S. Environmental Protection Agency, Ada, OK 74820, USA. z Present address: Department of Environmental Toxicology, Clemson University, Pendleton, SC 29670, USA.
position of H2O2 solutions. Mathews and Curtis (1914) investigated the effect of six substances on the photolysis of H2O2, including acetanilide, sulfuric acid, sodium chloride, sodium hydroxide, calcium hydroxide, and barium hydroxide. They showed that all the six compounds they studied acted as inhibitors for photochemical reaction. Anderson and Taylor (1923) reported 25 inhibitors for H2O2 photolysis, including acids, ethers, amides, alcohols, ketones, benzene and alkaloids. Photodegradation of contaminants in surfactant solution has been demonstrated as a post treatment method after surfactant enhanced aquifer remediation to recover and reuse the surfactant solution (Chu and Jafvert, 1994; Chu et al., 1998). However, photolysis combined with H2O2 will be a more general approach for environmental applications since direct photolysis is only susceptible for light absorbing compounds. To the best of our knowledge, the effect of surfactant on the photolysis of H2O2 has not been investigated. Previously, a patented process for preserving H2O2 used surfactants to inhibit the thermal decomposition of H2O2 (Schaidhauf, 1914). The motivation of the work presented here was to investigate the photolysis of H2O2 in surfactant solutions and provide fundamental information for future environmental applications. The objective of this study was to examine the effect of surfactant micelles on photolysis of H2O2. Surfactants used in this study were sodium dodecyl
3276
Micellar effect on H2O2 photolysis
sulfate (SDS), ammonium perfluorooctanoate (APFO), and lithium perfluorooctanesulfonate (LiFOS). H2O2 photolysis was conducted in micellar solutions of SDS, APFO and LiFOS. H2O2 was photolyzed in water, methanol and APFO monomeric solution to compare the photochemical reactivity of H2O2 in different media. The H2O2 photolysis in an APFO monomeric solution was compared with that in an APFO micellar solution. In addition, UV absorbance of surfactants was measured to quantify the background absorbance of surfactant alone.
MATERIALS AND METHODS
Materials APFO (Fluka, 98%), LiFOS (3 M, 25% aqueous solution), SDS (Aldrich, 98%), H2O2 (Aldrich, 35%), potassium permanganate (Aldrich, 0.1 0.005 N), and ferrous sulfate (Aldrich, 99.999+%) were used as received; physicochemical properties for surfactants used in this study are in Table 1. An approximately 0.1 M stock solution of H2O2 was prepared from the 35% reagent solution and standardized by titration with potassium permanganate. H2O2 concentrations were measured iodometrically (Allen et al., 1952). Each calibration curve of H2O2 in surfactant solutions was prepared in the same surfactant solutions. Absorbance measurement was conducted using a Hewlett Packard 8452A diode array spectrophotometer in 10-mm pathlength quarts cells (Starna). Ferrous sulfate was used to prepare the standard solutions for the potassium ferrioxalate actinometry. Water was purified by a Barnstead Nanopure system resulting in a resistivity >17.8 MO cm1.
3277
(ACE glass, 7891-30) for ultraviolet irradiation. The light source was a low-pressure mercury lamp (UVP Pen-Ray Lamp) with predominant UV intensity at 254 nm. The temperature of the tubes was kept almost constant (22 18C) by providing a stream of fresh air using a fan. The photon flux of the lamp was measured by using a potassium ferrioxalate actinometry (Hatchard and Parker, 1956). The incident radiation was determined to be consistently near 5.0 108 einstein L1 s1. RESULTS AND DISCUSSION
UV spectroscopic studies of APFO Aliphatic carboxylic acids and their anions are known to produce absorption spectra in the UV region as a result of n !p and p!p transitions (Mukerjee et al., 1990). APFO is a perfluoroalkyl carboxylate which has a carbonyl group (C=O) in the molecular structure. The carbonyl group is the chromophore that absorbs ultraviolet light, and it undergoes an n !p transition when it absorbs light. Most n !p transitions have small molar absorptivity, leading to the low absorption intensity (Klessinger and Michl, 1995). Absorbance spectra of APFO measured at the concentrations ranging from 0.01 to 0.1 M is shown in Fig. 1(a). The molar absorptivity of APFO determined experimentally was 0.75 M1 cm1 at 254 nm (Fig. 1(b)). This value was used to calculate the initial photolysis rate of H2O2 in a later section.
UV absorbance of surfactant Among the three anionic surfactants used in this study, only APFO absorbs the UV light since it has the chromophore, carbonyl group. The UV absorption spectra of APFO were measured as a function of APFO concentrations ranging from 0.01 to 0.1 M to determine the molar absorptivities of APFO. The molar absorptivity of APFO at 254 nm was used to quantify the background absorbance by APFO on the photolysis of H2O2. Photolysis of hydrogen peroxide H2O2 was photolyzed in water, 90% methanol, monomeric solution (10 mM APFO), and micellar solutions (50 mM APFO, 14 mM LiFOS, and 20 mM SDS). All micellar solution concentrations used were about 2–2.5 times critical micelle concentration (CMC). The initial H2O2 concentration used in this study was 10 mM. Photolysis experiments were conducted in triplicate using cylindrical quartz tubes (13 OD 10 L mm). The samples were mixed completely and then placed in merry-go-round reactor
Table 1. Selected physical and chemical properties of surfactants in this study Surfactant
APFO LiFOS SDS a
Molecular formula
MW
Water solubilitya
CMC (mM)
C7F15COONH4 C8F17SO3Li C12H25SO4Na
431.10 128.17 288.38
230 mM Complete Complete
26.4b 6.25b 8.15c
Data provided by supplier; temperature not specified. An (1999). c Mukerjee and Mysels (1970). b
Fig. 1. (a) Absorption spectrum of APFO as a function of APFO concentrations. Concentration increases from left to right. (b) Molar absorptivity of APFO with wavelength. The concentration of APFO used in this study is 0.01–0.1 M.
3278
Youn-Joo An et al.
Photolysis of hydrogen peroxide Figure 2 shows the concentration of H2O2 remaining as a function of irradiation time in water, methanol and surfactant solutions. Photochemical decomposition of H2O2 was found to follow first order kinetics in all solutions and the first order reaction constants are given in Table 2. Photolysis rates were highest for water, followed by micellar solutions (50 mM APFO>14 mM LiFOS>20 mM SDS), and lowest for methanol. A comparison of H2O2 photolysis in water and methanol indicated that the photochemical reactivity of H2O2 was less favorable in organic solvent than in water. This result is consistent with a previous observation that alcohols inhibit H2O2 photolysis (Anderson and Taylor, 1923). This study observed that H2O2 photolysis in micellar solutions was slower than in water, and faster than in methanol. Above the CMC, the surfactant monomers dissolved in water form micelles. Micelles have less polar region compared to the aqueous phase. Although H2O2 is miscible with water, it can be also dissolved in organic solvents (Dong and Guo, 1994). Retarded H2O2 photolysis in the presence of micelles implies that a fraction of H2O2 dissolved in water partitioned into micellar pseudophase of surfactant. H2O2 partition into micelles has less photochemical reactivity since micellar environment is more like organic solvent than water. Therefore, overall photolysis rates in the presence of micelles were retarded. In addition, photolysis inhibitory level by micelles was shown to be dependent on the kinds of surfactants used in this study. As expected, APFO monomers did not alter the H2O2 photolysis rate since no micelles exist below the CMC. Photolysis rate of H2O2 in APFO monomeric solution was very similar to that in water. Slight decrease of photolysis rate in the presence of APFO monomers was due to absorbance of APFO alone.
Fig. 2. Photolysis of H2O2 at 254 nm in water, 10 mM APFO, 50 mM APFO, 14 mM LiFOS, 20 mM SDS, and methanol (from bottom to top). Io =5.0 108 einstein L1 s1. The initial concentration of H2O2 is 10 mM. The lines show the fitting with the first order reaction rates.
Table 2. Photolysis rate constant of H2O2 in water, methanol and surfactant solutions, calculated by pseudofirst order kinetics Solutions
Water 90% methanol 10 mM APFO (monomeric) 50 mM APFO (micellar) 14 mM LiFOS (micellar) 20 mM SDS (micellar) a
Rate constant (min1)a
R2
0.0121 0.0006 0.0040 0.0002 0.0119 0.0005 0.0072 0.0003 0.0054 0.0002 0.0045 0.0001
0.984 0.977 0.994 0.993 0.992 0.998
Values are s.
To consider any effects of the absorbance of APFO alone in H2O2 photolysis, photolysis rates at t ¼ 0 (initial rates) in water and APFO solutions were calculated using general equation of photolysis, as shown in Table 3. These calculations predicted that initial rates would be decreased by factors of 1.01 and 1.04 in 10 and 50 mM APFO solutions, respectively, compared to water. However, initial rates calculated by the actual rate constants in Table 2 were decreased by factors of 1.02 and 1.68 in 10 and 50 mM APFO solutions, respectively. The comparison showed that APFO monomers had negligible effects on the H2O2 photolysis (1.02 1.01) and APFO micelles retarded the H2O2 photolysis more than what would be expected from APFO background absorbance alone (1.68>1.04). Therefore, the rate decrease of H2O2 in 50 mM APFO solution is largely due to the micellar effects. Change of pH affects the formation of hydroxyl radicals which leads to the change of photolysis rate (Beltran et al., 1996). Sample pHs in surfactant solutions were measured the and resulting solution pHs were all around neutral, so pH effect was found to be negligible in this study. CONCLUSIONS
The influence of surfactant micelles on the photolysis of hydrogen peroxide was investigated. Photolysis in water, methanol, and surfactant monomeric solution was also conducted for comparison. Photolysis rates were highest for water, followed by micellar solutions, and lowest for methanol. Photolysis in monomeric solution was shown to be very similar to that in water. The conclusions of this study are as follows: 1. Photolysis of H2O2 was slower in methanol than in water, indicating that the photochemical reactivity of H2O2 is less favorable in organic solvent than in water. 2. Photolysis of H2O2 was retarded in micellar solutions. It implies that a fraction of H2O2 dissolved in water partitions into micellar pseudophase of surfactant. H2O2 partitioned into micelles has less photochemical reactivity and thus photolysis rate was retarded in the presence of micelles.
Micellar effect on H2O2 photolysis
3279
Table 3. Photolysis rate of H2O2 at t ¼ 0 (initial rate) in water and APFO solutions calculated by first order reaction rate and photolysis rate Solutions Water
10 mM APFO
50 mM APFO
First order reaction rate r ¼ kC ro=kCoa
ro ro,water/ro
0.000121 1.00
0.000119 1.02
0.000072 1.68
Photolysis rate
Fsl
0.3543
0.3654
0.4078
r =Io f Fslfcl
fcl
1
0.9620
0.8352
Fsl ¼ 1 10ðesþecCÞl ec C fcl ¼ es þ ec C
ro =ðIo fÞ ¼ Fsl fcl
0.3543
0.3515
0.3406
ro;water =ro
1.00
1.01
1.04
b
c
d
a
ro is initial rate, k values from Table 2, Co =0.01 M of H2O2. General expression for photolysis rate (Zepp, 1978). Io is the incident light intensity at given wavelength (einstein L1 s1), Fsl is the fraction of light absorbed by the system at a wavelength l, fcl is the fraction of light absorbed by the photoreactant at a wavelength l, f is the quantum yield, es is the absorption coefficient of the solvent, ec is the molar absorptivity of the photoreactant at given wavelength (M1 cm1), and l is the cell pathlength (cm). c Io =5.0 108 einstein L1 s1. d Assumed to be unaffected by APFO micelles. b
3. The inhibitory effect by surfactant monomers on the photolysis of H2O2 was negligible due to the absence of micelles below the critical micelle concentration.
Acknowledgements}We gratefully acknowledge two reviewers who provide constructive comments for improving the manuscript. Support of this work was provided by the Office of the Vice President for Research and Associate Provost for Graduate Studies in Texas A&M University.
REFERENCES
Allen A. O., Hochanadel C. J., Ghormley J. A. and Davis T. W. (1952) Decomposition of water and aqueous solutions under mixed fast neutron and gamma radiation. J. Phys. Chem. 56, 575–586. An Y.-J. (1999) Solubilization and photochemical degradation of polycyclic aromatic hydrocarbons in perfluorinated surfactant solutions. Ph.D. Dissertation, Texas A&M University, USA. Anderson W. T. and Taylor H. S. (1923) The inhibition of the photochemical decomposition of hydrogen peroxide solutions. J. Am. Chem. Soc. 45, 650–662. Beltran F. J., Ovejero G. and Rivas J. (1996) Oxidation of polynuclear aromatic hydrocarbons in water. 3. UV radiation combined with hydrogen peroxide. Ind. Engng. Chem. Res. 35, 883–890. Chu W. and Jafvert C. T. (1994) Photodechlorination of polychlorobenzene congeners in surfactant micelle solutions. Environ. Sci. Technol. 28, 2415–2422. Chu W., Jafvert C. T., Diehl C. A., Marley K. and Larson R. A. (1998) Phototransformations of polychlorobiphenyls in Brij 58 micellar solutions. Environ. Sci. Technol. 32, 1989–1993.
Dong S. and Guo Y. (1994) Organic phase enzyme electrode operated in water-free solvents. Anal. Chem. 66, 3895–3899. Hatchard C. G. and Parker C. A. (1956) A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer. Roy. Soc. London Proc. 235, 518–536. Klessinger M. and Michl J. (1995) Excited States and Photochemistry of Organic Molecules. VCH Publishers Inc., New York. Larson R. A. and Weber E. J. (1994) Reaction Mechanisms in Environmental Organic Chemistry. Lewis Publishers, Boca Raton, FL. Mathews J. H. and Curtis H. A. (1914) The photochemical decomposition of hydrogen peroxide. J. Phys. Chem. 18, 166–178. Mukerjee P., Gumkowski M. J., Chan C. C. and Sharma R. (1990) Determination of critical micellization concentration of perfluorocarboxylates using ultraviolet spectroscopy: Some unusual counterion effects. J. Phys. Chem. 94, 8832–8835. Mukerjee P. and Mysels K. J. (1970) Critical Micelle Concentration of Aqueous Surfactant Systems. US Department of Commerce, Washington, DC. Pignatello J. J. (1992) Dark and photoassisted Fe3+catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ. Sci. Technol. 26, 944–951. Pignatello J. J. and Huang L. Q. (1993) Degradation of polychlorinated dibenzo-p-dioxin and dibenzofuran contaminants in 2,4,5-T by photoassisted iron-catalyzed hydrogen peroxide. Water Res. 27, 1731–1736. Schaidhauf A. (1914) Stable Hydrogen Peroxide and Method of Making the Same. US Patent 1109791. Strukul G. (1992) Catalytic Oxidations with Hydrogen Peroxide as Oxidant. Kluwer Academic Publishers, Dordrecht, Netherlands. Zepp R. G. (1978) Quantum yields for reactions of pollutants in dilute aquoeus solution. Environ. Sci. Technol. 12, 327–329.