J. Phys. Chem. C 2009, 113, 3735–3739
3735
Sonochemical Degradation of Alkylbenzene Sulfonates and Kinetics Analysis with a Langmuir Type Mechanism Ben Nanzai, Kenji Okitsu,* Norimichi Takenaka, and Hiroshi Bandow Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan ReceiVed: October 27, 2008; ReVised Manuscript ReceiVed: December 22, 2008
Sonochemical degradation of anion surfactants, such as p-octylbenzene sulfonate (LAS C8), p-nonylbenzene sulfonate (LAS C9), and p-dodecylbenzene sulfonate (LAS C12), was performed to obtain detailed information on the interfacial region of cavitation bubbles, because LASs are decomposed mainly in this region. In this study, a Langmuir type mechanism was applied, because heterogeneous reactions are induced by ultrasonic irradiation. The initial concentrations of LASs were varied from 15 to 2000 µM to analyze the reaction kinetics. In the case of low concentrations of LASs (15-40 µM), the order of degradation rate was LAS C8 > LAS C9 > LAS C12. This order is suggested to be due to diffusion-controlled degradation. Though the degradation rates increased with increasing initial LAS concentration, inflection points were observed around 40-100 µM for all LASs. The result obtained was reasonably well explained with a Langmuir type mechanism. For high concentrations of LASs (100-2000 µM), the same mechanism could not be applied to explain the results. The yields of H2O2 in the presence of LASs were also measured as a function of LAS concentration. It was found that the curve of H2O2 yield vs LAS concentration had an inflection point around 40 µM for all LASs. From these results, it is suggested that the physicochemical properties in the interfacial region of cavitation bubbles were different in the low and high LAS concentration regimes. 1. Introduction Ultrasonic irradiation in a liquid brings about the formation, growth, and collapse of microscale gas bubbles. During bubble collapse, a local reaction site of extremely high temperature (several thousand degrees) and high pressure (several hundred atmospheres) is created.1-4 OH and H radicals are formed from the pyrolysis of water vapor inside the collapsing bubble.5,6 Degradation of dissolved organic compounds proceeds via a reaction with these radicals and/or direct pyrolysis. Although there are a number of reports concerning sonochemical degradation of various types of organic compounds,7-14 the reaction kinetics of sonochemical degradation has not yet been proved even in a homogeneous solution system. Okitsu et al.15,16 reported that a Langmuir type kinetic model based on heterogeneous reaction systems can be applied to the analysis of sonochemical degradation of azo dyes, butyric acid, and benzoic acid. In sonochemistry, it is generally suggested that the inside and the gas/liquid interface of cavitation bubbles are the main reaction zones, and thus accumulation of organic compounds at the interface of the bubbles is important for their effective degradation.17 Linear alkylbenzene sulfonates (LASs) tend to accumulate at the gas/liquid interface of cavitation bubbles due to their surface active property, and do not evaporate into the cavitation bubble because they have no volatility. Consequently, if sonochemical degradation of LASs were investigated, detailed information on the cavitation bubble interface should be obtained. In addition, the behavior of LAS molecules toward the gas/liquid interface of cavitation bubbles would be clear. * Corresponding author. Telephone/fax: (+81)-72-254-9321. E-mail:
[email protected].
A number of previous studies on the sonolysis of LASs have been reported. Yim et al.18 reported accumulation of LAS C12 at the cavitation bubble interface. Ashokkumar et al.19 reported rates of LAS C12 degradation, and discussed mainly the reaction pathways and the relationship between the degradation rate and the intensity of sonoluminescence. Manousaki et al.20 investigated the effects of various parameters such as bulk temperature, initial solute concentration, ultrasonic power, frequency, and salt addition on the rate of LAS C12 degradation. In addition, the spin-trapping studies by Sostaric and Riesz21,22 showed that anion surfactants with shorter alkyl chain lengths were more efficiently accumulated at the cavitation bubble interface under 354 kHz continuous sonication. Yang et al.23 verified using pulse sonication that LAS C8 can diffuse more easily than LAS C12. Both Sostaric and Yang et al. reported that the degradation rates of anion surfactants were affected by their alkyl chain lengths. The reports described above were concerned only with events occurring in or at the gas/liquid interface of cavitation bubbles. In the present study, “the interfacial region of cavitation bubbles” is defined as the surrounding zone of the gas/liquid interface as shown in Figure 1. To investigate reaction kinetics in this region for the first time, sonochemical degradation of three kinds of LASs at various concentrations was performed, and a Langmuir type kinetic model was applied. From the data analysis, it was expected that the adsorption equilibrium of LAS molecules in the interfacial region of cavitation bubbles should become clear. In addition, changes in the physicochemical properties of bubbles induced by accumulation of LASs should also be clarified. Although there are many reports on the sonochemical degradation of LASs, the reaction kinetics model is still unclear. To analyze the kinetics correctly, the concentration of LASs
10.1021/jp809509g CCC: $40.75 2009 American Chemical Society Published on Web 02/10/2009
3736 J. Phys. Chem. C, Vol. 113, No. 9, 2009
Figure 1. Schematic for the definition of “interfacial region of hot cavitation bubbles” where molecules are degraded by direct pyrolysis or OH radical reaction.
should be as low as possible, because it is important to avoid the effects of degradation products. In this study, we have investigated sonochemical degradation of LASs over a wide concentration range. In particular, the sonochemical degradation of very low concentrations of LASs (below 100 µM) was intensively investigated to determine the reaction kinetics.
Nanzai et al.
Figure 2. Sonochemical degradation of LASs in aqueous solution under Ar atmosphere. Initial concentration of LASs: 40 µM. Each value is the average of five sonication trials, and the error bars indicate the standard deviation.
2. Experimental Section Sodium p-octylbenzene sulfonate (LAS C8; 99.9%), sodium p-nonylbenzene sulfonate (LAS C9; 99.9%), and sodium pdodecylbenzene sulfonate (LAS C12; 99.9%) were supplied by Nacalai Tesque Inc. All of the LAS sample solutions, with concentrations from 15 to 2000 µM, were prepared with Milli-Q water. Ultrasonic irradiation was carried out using a 65 mm diameter oscillator (Kaijo type 4611; manufacturing no. 37G4) and an ultrasonic generator (Kaijo type TA-4021; lot no. 19G9; frequency 200 kHz) operated at 200 W. The diameter of the reaction vessel was 55 mm, and the base of the vessel was 1 mm thick and fixed at 4 mm from the oscillator. Details of the irradiation setup and the characteristics of the reaction vessel are described elsewhere.24 A 60 mL Ar-saturated aqueous solution containing each LAS compound was sonicated in a water bath maintained at 20 °C by a cold water circulation system (TAITEC CP-150R). The concentration of each dissolved compound was determined using a high-performance liquid chromatograph (HPLC; Shimadzu LC-20AT, SPD-20AV) with UV detection at 223 nm, using a C18 column and a mobile phase of acetonitrile/50 mM NaClO4 aqueous solution (60:40 v/v) flowing at 0.3 mL min-1. The initial degradation rate was determined by the following procedure: the concentration of LASs during sonication was plotted as a function of sonication time, and the time profile was fitted to a logarithmic equation. The logarithmic equation obtained was differentiated with respect to sonication time, and time t ) 0 was substituted in the differential equation to obtain the initial degradation rate.16 The rate was determined as the average value from several experimental runs. Surface tension was measured at ca. 20 °C using a laboratorymade surface tension meter employing the Wilhelmy plate method. The Wilhelmy plate consisted of a thin platinum plate (Kyowa Interface Science Co., Ltd.), attached to an electronic balance (A&D GH-200) via a thin metal wire. The force on the plate due to wetting was measured and used to calculate the surface tension. Eastoe and Dalton25 reported that, in the case of surface-active solutes, a certain time was needed to achieve
Figure 3. Relationship between LAS concentration and degradation rate. Each value is the average of three to six sonication trials, and the error bars indicate the standard deviation.
equilibrium at the surface of a solution. In our measurement, the surface tension gradually decreased with time and it took several tens of minutes to achieve equilibrium. While long time measurement led to poor reproducibility, short time measurement before establishment of equilibrium led to good reproducibility. Thus the apparent surface tension was determined within 3 s measurement time, which was much longer than the lifetime of a cavitation bubble: it has been reported that a cavitation bubble has a lifetime of several hundred microseconds.26 3. Results and Discussion The degradation of LASs was clearly observed during sonication. The concentration changes in LASs are shown in Figure 2, revealing that the concentration of each of the LASs decreased with sonication time. The relationships between the initial degradation rate and the initial concentration of each of the LASs (15-250 µM) are shown in Figure 3. For concentrations less than 40 µM, the degradation rates were in the order LAS C8 > LAS C9 > LAS C12. These results are almost the same as previous reports on sonolysis of LAS C8 and LAS C12.23,27 Furthermore, the degradation rates of LASs increase with increasing initial concentrations of LASs. Two straight lines were able to be drawn with an inflection point at 40-100 µM for all LASs.
Sonochemical Degradation of LASs
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3737
Figure 4. Relationship between LAS concentration and yield of H2O2 formed during the sonochemical degradation of LAS.
It has been reported that the following reactions occur5,6,28,29 when an aqueous solution is irradiated under Ar atmosphere. )))
H2O 98 OH + H
(pyrolysis)
(1)
the other hand, for initial concentration of >40 µM, the rate of reaction (eq 5) becomes a maximum value because most of the OH radicals formed are consumed by reaction with LAS molecules. Second, it is suggested that the physicochemical properties in the interfacial region of cavitation bubbles are changed. Even if the bulk concentration of LASs were not high, the LAS concentration in the interfacial region of cavitation bubbles would be high because of the surface activity of LASs. Accordingly, it has been reported that the accumulated molecules induce interactions such as electrostatic repulsion due to their charge, and this repulsion encumbers further accumulation.23 In previous reports15,16 a heterogeneous kinetics model based on a Langmuir type mechanism was proposed. This model was used to explain the local reaction zone in the interfacial region of cavitation bubbles, where organic solutes were rapidly decomposed because a high temperature and an extremely high concentration of OH radicals exist in this region.18,30 To discuss the reaction kinetics in the interfacial region of cavitation bubbles, the sonochemical degradation of LASs is analyzed by the Langmuir type mechanism. The kinetic model used in this study makes the following assumptions. The rate of adsorption, r1, of LAS molecule from the bulk solution to the interfacial region of cavitation bubbles is proportional to the concentration of the bulk solution of LAS and to (1 - θ), where θ corresponds to the occupied ratio of LAS molecules in the interfacial region of cavitation bubbles. The rate of desorption, r-1, is proportional to θ. Therefore, the rates of r1 and r-1 are expressed by use of the rate constants k1 and k-1 for absorption and desorption, respectively, as follows:
2OH f H2O2
(2)
2H f H2
(3)
r1 ) k1[LAS](1 - θ)
(7)
OH + H f H2O
(4)
r-1 ) k-1θ
(8)
where the symbol “)))” represents ultrasonic irradiation. In the presence of LASs, reaction with OH radicals occurs.
where [LAS] is the initial concentration of LAS in the bulk solution. When a pseudosteady state is reached
LASs + OH f degradation
k1[LAS](1 - θ) ) k-1θ
(5)
In addition, LASs are accumulated in the interfacial region of cavitation bubbles, and thus a direct pyrolysis of LASs proceeds: )))
LASs 98 degradation
(pyrolysis)
(6)
Thus LAS molecules are decomposed both by reaction with OH radicals and by pyrolysis as per eqs 5 and 6. To confirm the decomposition pathway, the relationship between the yield of H2O2 and the initial concentrations of LASs was investigated as shown in Figure 4. The yield of H2O2 decreases almost linearly with increasing LAS concentration and passes the inflection point less than 40 µM. Then the yield continues to decrease above 40 µM. This tendency is in good agreement with the results shown in Figure 3. On the basis of these results, it is suggested that the reaction mechanism for LAS concentrations less than 40 µM should be different from that for concentrations larger than 40 µM for the following reasons. First, the decomposition pathway should depend on the initial concentration of LAS, because the yield of OH radicals is limited. For initial concentrations of LAS C9 > LAS C12, and they have significantly different magnitudes (the value for LAS C8 and LAS C9 are 15 and 2.1 times, respectively, larger than that for LAS C12). It can be considered that these differences are derived from the accumulation of molecules. It is implied by eq 10 that K is positively correlated with the occupied ratio θ. The θ value can be expressed as
θ)
Figure 5. Reciprocal plot based on the Langmuir type mechanism at (a) low concentrations of LASs (15-40 µM), and (b) high concentrations of LASs (100-2000 µM).
In this model, the adsorption equilibrium constant of LAS toward the interfacial region, K ) k1/k-1, is defined such that LAS molecules are concentrated or nonconcentrated in the interfacial region of cavitation bubbles. At the moment of bubble collapse, it is assumed that LAS molecules existing in the interfacial region of cavitation bubbles are decomposed with the pseudorate constant k. The following equation is obtained by inverting both sides of eq 11:
1 1 1 1 ) + r kK [LAS] k
net number of accumulated molecules (13) maximum number of accumulable molecules
As mentioned above, LASs with shorter alkyl chain are more easily accumulated in the interfacial region of cavitation bubbles.21-23 Thus, “net number of accumulated molecules” should follow the relative order LAS C8 > LAS C9 > LAS C12. To compare the “maximum number of accumulable molecules” of LASs, the surface tension of each LAS was measured. The results are shown in Figure 6. The surface tension decreased with increasing initial concentration of LASs, and the value was smaller in the case of LASs with longer alkyl chains. It follows that the relative order of “maximum number of accumulable molecules” is LAS C12 > LAS C9 > LAS C8. On the basis of eq 13, it was found that the K value is affected by both “net number of accumulated molecules” and “maximum number of accumulable molecules”. Table 1 shows that the order of k values is LAS C12 > LAS C9 > LAS C8. This result is different from the case of degradation of butyric acid and benzoic acid, for which the degradation rates were positively correlated with the k values.16 This suggests that different phenomena occurred in the case of LASs, and it is difficult to fully interpret the pseudorate constants k derived from the Langmuir type mechanism in this study. Further experiments will be needed to clarify the details.
(12)
On the basis of eq 12, a graph of 1/r versus 1/[LAS] was plotted as shown in Figure 5. For [LAS] < 40 µM (Figure 5a), a linear relationship is observed. On the other hand, for [LAS] > 100 µM (Figure 5b), the data points were not able to be fitted to the straight line. The following two reasons are proposed for the case [LAS] > 100 µM. First, it is probable that the reaction pathway was dramatically changed between the low and high concentration regimes of LASs. Second, a dense accumulation of LASs induces changes in physicochemical properties in the interfacial region of cavitation bubbles, and in addition, an electric repulsion among molecules might affect the accumulation of LASs. Moreover, because a concentration gradient should exist in the interfacial region of cavitation bubbles, the rate of
Figure 6. Surface tension as a function of LAS concentration.
Sonochemical Degradation of LASs 4. Conclusion At very low concentration of LASs, the order of degradation rate is LAS C8 > LAS C9 > LAS C12. In the relationship between the degradation rate and the initial concentration of LASs, the inflection points are observed at 40-100 µM. Inflection points are also observed in the yield of H2O2 in the same concentration range. Taking into account the Langmuir type mechanism, it is suggested that the reaction pathway and physicochemical properties in the interfacial region of cavitation bubbles are changed at 40-100 µM. In the sonochemical degradation of LASs, the kinetic model based on the Langmuir type mechanism is applicable only at very low concentration of LASs. The model does not explain the sonochemical degradation of LASs at high concentrations. Acknowledgment. This work was funded by the Sasakawa Scientific Research Grant from The Japan Science Society. References and Notes (1) Didenko, Y. T.; McNamara, W. B., III; Suslick, K. S. J. Am. Chem. Soc. 1999, 121, 5817. (2) McNamara, W. B., III; Didenko, Y. T.; Suslick, K. S. J. Phys. Chem. B 2003, 107, 7303. (3) Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2005, 127, 5326. (4) Okitsu, K.; Suzuki, T.; Takenaka, N.; Bandow, H.; Nishimura, R.; Maeda, Y. J. Phys. Chem. B 2006, 110, 20081. (5) Makino, K.; Mossoba, M. M.; Riesz, P. J. Am. Chem. Soc. 1982, 104, 3537. (6) Makino, K.; Mossoba, M. M.; Riesz, P. J. Phys. Chem. 1983, 87, 1369. (7) Drijvers, D.; Langenhove, H. V.; Herrygers, V. Ultrason. Sonochem. 2000, 7, 87. (8) Adewuyi, Y. G. Ind. Eng. Chem. Res. 2001, 40, 4681, and references cited therein.
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3739 (9) Hatanaka, S.; Mitome, H.; Yasui, K.; Hayashi, S. J. Am. Chem. Soc. 2002, 124, 10250. (10) Lu, Y.; Weavers, L. K. EnViron. Sci. Technol. 2002, 36, 232. (11) Ciawi, E.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2006, 110, 9779. (12) Kidak, R.; Ince, N. H. Ultrason. Sonochem. 2006, 13, 195. (13) Petrier, C.; Combet, E.; Mason, T. Ultrason. Sonochem. 2007, 14, 117. (14) Okitsu, K.; Kawasaki, K.; Nanzai, B.; Takenaka, N.; Bandow, H. Chemosphere 2008, 71, 36. (15) Okitsu, K.; Iwasaki, K.; Yobiko, Y.; Bandow, H.; Nishimura, R.; Maeda, Y. Ultrason. Sonochem. 2005, 12, 255. (16) Okitsu, K.; Nanzai, B.; Kawasaki, K.; Takenaka, N.; Bandow, H. Ultrason. Sonochem. 2009, 16, 155. (17) Nanzai, B.; Okitsu, K.; Takenaka, N.; Bandow, H.; Maeda, Y. Ultrason. Sonochem. 2008, 15, 478. (18) Yim, B.; Okuno, H.; Nagata, Y.; Nishimura, R.; Maeda, Y. Ultrason. Sonochem. 2002, 9, 209. (19) Ashokkumar, M.; Niblett, T.; Tantiongco, L.; Grieser, F. Aust. J. Chem. 2003, 56, 1045. (20) Manousaki, E.; Psillakis, E.; Kalogerakis, N.; Mantzavinos, D. Water Res. 2004, 38, 3751. (21) Sostaric, J. Z.; Riesz, P. J. Am. Chem. Soc. 2001, 123, 11010. (22) Sostaric, J. Z.; Riesz, P. J. Phys. Chem. B 2002, 106, 12537. (23) Yang, L.; Rathman, J. F.; Weavers, L. K. J. Phys. Chem. B 2005, 109, 16203. (24) Nanzai, B.; Okitsu, K.; Takenaka, N.; Bandow, H.; Tajima, N.; Maeda, Y. Ultrason. Sonochem. 2009, 16, 163. (25) Eastoe, J.; Dalton, J. S. AdV. Colloid Interface Sci. 2000, 85, 103. (26) Sunartio, D.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2007, 129, 6031. (27) Yang, L.; Rathman, J. F.; Weavers, L. K. J. Phys. Chem. B 2006, 110, 18385. (28) Henglein, A. Ultrasonics 1987, 25, 6. (29) Henglein, A. AdV. Sonochem. 1993, 3, 17. (30) Gutierrez, M.; Henglein, A.; Ibanez, F. J. Phys. Chem. 1991, 95, 6044.
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