Sonochemical Degradation of Alkylbenzene Sulfonate Surfactants in

Ruiyang Xiao , Zongsu Wei , Dong Chen , and Linda K. Weavers. Environmental ... Limei Yang, Joe Z. Sostaric, James F. Rathman, and Linda K. Weavers...
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J. Phys. Chem. B 2006, 110, 18385-18391

18385

Sonochemical Degradation of Alkylbenzene Sulfonate Surfactants in Aqueous Mixtures Limei Yang,† James F. Rathman,‡ and Linda K. Weavers*,† Department of CiVil and EnVironmental Engineering and Geodetic Science and Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: April 14, 2006; In Final Form: July 26, 2006

The degradation of nonvolatile surfactants sodium 4-octylbenzene sulfonate (OBS) and dodecylbenzenesulfonate (DBS) and a nonvolatile nonsurfactant 4-ethylbenzene sulfonic acid (EBS), as single components and binary mixtures, were studied under 354 kHz ultrasound. In addition, the effects of pulsed ultrasound on degradation were also examined. Results show that in mixtures of the surfactant OBS and nonsurfactant EBS, the surfactant is selectively degraded. The reduced degradation of EBS was dependent on the mixed molar ratio of EBS/ OBS. The degradation of OBS was unaffected by the presence of EBS at a molar ratio of OBS/EBS g 1. Furthermore, OBS degradation was significantly enhanced under pulsed ultrasound. In OBS and DBS surfactant mixtures sonicated under pulsed ultrasound, surfactants strongly affected each other’s degradation rates due to competition for the reaction sites on the cavitation bubble surfaces. OBS exhibits a faster degradation rate than DBS at shorter pulse intervals due to its faster rate of transfer to the cavitation bubble interfaces. At longer pulse intervals, DBS, which is more surface active, degrades faster than OBS due to the increased amounts of DBS accumulation on the bubble surfaces.

Introduction Sonochemical processes mainly occur as a result of cavitations the growth and violent collapse of cavitation bubbles caused by alternate compression and rarefaction cycles of sound waves propagating through the liquid.1 The cavitation bubble collapse generates several thousands of degrees in transient localized temperature and hundreds of atmospheres in pressure,2,3 which dissociates water to produce reactive hydroxyl radicals.4,5 The degradation of organic compounds in aqueous sonochemical systems proceeds mainly by two reaction mechanisms. First, the organic compounds are oxidized by OH radicals at the bubble cavities and bubble surfaces and in the surrounding liquid. Second, organic compounds are thermolyzed directly inside cavitation bubbles and at bubble surfaces. The degradation depends to a large extent on the nature of the organic compounds.6 Thermal destruction processes inside the cavitation bubbles are not considered important for nonvolatile solutes because they do not partition significantly into bubbles.7 Thus, the pathway for degradation of nonvolatile compounds is by thermolysis at bubble surfaces and reaction with hydroxyl radicals either at bubble surfaces or in bulk solution.8-10 Because of the much higher concentration of hydroxyl radicals at the cavitation bubble surfaces than in the bulk solution, the degradation rates of nonvolatile compounds mainly depends on their ability to partition to bubble surfaces. In recent years, the application of ultrasound to the degradation of surfactants in single solutes has drawn much attention.8,9,11-15 Nonvolatile surfactants have been shown to adsorb at the bubble-solution interface.10 As a result, the sonochemical oxidation of surfactants is expected to be relatively fast as compared to that of nonsurfactants in aqueous mixtures. * To whom correspondence should be addressed. Phone: (614) 2924061; fax: (614) 292-3780; e-mail: [email protected]. † Department of Civil and Environmental Engineering and Geodetic Science. ‡ Department of Chemical and Biomolecular Engineering.

Under appropriate conditions, surfactants preferentially accumulate and react at cavitation bubble surfaces. Surfactants in the bulk solution may also react with OH radicals diffusing from bubble surfaces. Strong competition for OH radicals occurs with nonsurfactants when both surfactants and nonsurfactants are present in solution. EPR and spin-trapping studies show that surfactant chemical properties, namely, chemical structure, play an important role in the accumulation and equilibrium of surfactants at bubble surfaces.9 Results indicate that the radical scavenging efficiency decreases with increasing n-alkyl chain length at plateau concentrations, suggesting that surfactants that are more surface active require more time to accumulate and equilibrate at bubble surfaces.9 Degradation of surfactants under pulsed ultrasound shows that DBS, which is more surface active than OBS, degrades faster during longer pulse intervals due to increased amounts of DBS accumulation on bubble surfaces under pulsing.12 In contrast, pulsed ultrasound as compared to continuous wave (CW) ultrasound did not alter the degradation rate of a nonsurfactant, EBS. When sonochemical treatment of aqueous waste streams is applied, mixtures of compounds are expected. Thus, an understanding of how the presence of other compounds affects degradation is necessary. Previous researchers have investigated the degradation of mixtures of volatile and nonvolatile compounds16-18 and mixtures of volatile compounds.19,7 In some cases,7,16,17 the compounds affected each other’s degradation rate; in other cases,18,19 there was no effect on rates. The results in all the studies stated previously varied depending on physicochemical characteristics of compounds, ultrasonic parameters, and reaction mechanisms. Clearly, the systems are complicated, and there is little systematic understanding as to why compounds affect one another’s degradation in some cases but not in other cases. In this study, a nonvolatile nonsurfactant was sonicated in the absence and presence of a nonvolatile surfactant. We expected to observe selective degradation of surfactants because surfactants preferentially accumulate at the cavitation bubble

10.1021/jp062327d CCC: $33.50 © 2006 American Chemical Society Published on Web 08/24/2006

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interfaces. To investigate the competitive degradation of surfactants at the cavitation bubble surfaces, binary mixtures of the surfactants with the same headgroup but different n-alkyl chain lengths were studied to examine how the surface activity affected the accumulation of surfactants at bubble surfaces. Reduced degradation rates as compared to individual surfactants were expected due to competition for the reaction sites. Experiments of pulsed ultrasound were conducted to take advantage of increased surfactant accumulation on the bubble surfaces and examine how the enhanced surfactant accumulation would affect the degradation of the compounds in the mixtures. Experimental Procedures Material and Reagents. Two anionic surfactants, sodium 4-octylbenzene sulfonate (OBS, Aldrich, 97%) and sodium dodecylbenzenesulfonate (DBS, Wako, 99%), were used. 4-Ethylbenzene sulfonic acid (EBS, Aldrich, 95%) was used as a model nonsurfactant. All compounds were used as received without further purification. Water used to dissolve compounds was from a Milli-Q water purification system operating at R ) 18.2 MΩ cm. (Millipore, Billerica, MA). The initial concentrations of OBS in this study were 0.1, 0.2, 0.5, 1, 2, and 5 mM. The initial concentrations of EBS were 0.1, 0.5, and 1 mM, and the DBS concentrations were 0.1, 0.2, 0.5, and 1 mM. Initial solutions containing either surfactants or nonsurfactant were adjusted to pH 2.8 using a 33 mM phosphate buffer to ensure that the nonsurfactant EBS (pKa ) 7.0) was in the protonated form. Our previous work showed that the pH had a negligible effect on the degradation of the surfactants.8 Sonochemical Experiments. Sonication experiments were conducted in an USW 51-52 ultrasonic reactor and an ultrasonic transducer (ELAC Nautik, Inc., Kiel, Germany) operating at 354 kHz. A SM-1020 Function/Pulse generator (Signametrics Corporation, Seattle, WA) produced pulse signals that were then amplified by an AG 1021 linear amplifier (T & C Power Conversion, Inc., Rochester, NY). The amplifier was able to create electrical output in either CW or pulsed wave forms. The pulse signals received by the transducer were detected by a 100 MHz 54501A digitizing oscilloscope (Hewlett-Packard). Power transmitted to the solution was 33.0 ( 0.8 W, measured calorimetrically resulting in a power intensity of 1.41 W/cm2. The temperature of the aqueous solution was controlled at 23 ( 3 °C by a 1006S Isotemp cooling system (Fisher Scientific, Pittsburgh, PA). The sonication volume was 500 mL, and 1.5 mL samples were taken from the reactor at specific time intervals during sonication. The total sampling volume did not exceed 5% of the total sonication volume. All samples were filtered before analysis with 0.2 µm PTFE Millipore filters (Sigma-Aldrich). The compounds did not adsorb onto the filters as indicated by control experiments. Analysis. The concentrations of surfactants and the nonsurfactant were determined by a Hewlett-Packard 1100 high performance liquid chromatograph (HPLC).12 The surface tensions of the surfactants were measured by a Sensadyne Tensiometer PC 500 (Chem-Dyne Research Corporation, Mesa, AZ). To compare the initial degradation rates of compounds in mixtures with single solutes, the ratio of kmixC0/ksingleC0 was calculated, where C0 is the initial concentration of the compound, kmix is the pseudo-first-order rate constant of the compound in the mixture, and ksingle is the pseudo-first-order rate constant of the same compound as a single solute. To compare the initial degradation rates of compounds under pulsed ultrasound to continuous wave (CW) ultrasound, the

Figure 1. Plots of the concentration of EBS and OBS in binary mixtures of 1 mM OBS and 1 mM EBS as a function of sonication time under pulsed ultrasound with a pulse length of 100 ms and pulse interval of 5 s (error bars represent 95% confidence intervals).

enhanced degradation degrading under pulsed conditions over CW at the same initial concentration was defined as12

pulse enhancement (%) )

kpulsedC0 - kCWC0 × 100 (1) kCWC0

where C0 is the initial concentration of the compound, kpulsed is the pseudo-first-order rate constant of the compound under pulse mode, and kCW is the pseudo-first-order rate constant of the same compound under the same concentration and conditions but with CW mode. In the pulsed ultrasound mode, T represents the length of a pulse of ultrasound (pulse ON time) and T0 represents the length of the interval between each pulse of ultrasound (pulse OFF time). Rate constants from pulsed experiments were determined from the sonication time (i.e., pulse ON time). Results and Discussion Decomposition Products of Surfactants OBS and DBS. In this study, the decomposition products of EBS, OBS, and DBS were not investigated. However, based on the work of other researchers, two main reaction pathways have been suggested for the decomposition of nonvolatile surfactants in sonochemical systems. One is the OH• radical attack on the hydrophobic n-alkyl chain and the other is the OH• radical addition to the aromatic ring.13,15 It has been shown that continuous attack of the OH• radicals on the aromatic ring can lead to ring opening and complete mineralization after prolonged sonication.20 The OH• radical attack on the n-alkyl chain of DBS leads to the formation of volatile organic solutes that are subsequently pyrolyzed within the cavitation bubbles and water soluble organic products, such as oxalic and formic acid, that remain in bulk solution.13 Therefore, in this study, we expected that the decomposition products of EBS, OBS, and DBS were not likely surface active and would not affect the results in a significant way. Sonochemical Degradation of Binary Mixtures of OBS and EBS. Degradation at Various Mixed Molar Ratios. First, we investigated the degradation of the surfactant, OBS, and the nonsurfactant, EBS, individually and in binary mixtures under various mixing ratios. Figure 1 indicates that OBS is not greatly affected by the presence of EBS but that EBS is significantly slowed due to the presence of OBS. To explore the effects of surfactant concentration and sonication mode on the degradation of binary mixtures of EBS at a fixed concentration (1 mM) and OBS with various

Degradation of Alkylbenzene Sulfonate Surfactants

J. Phys. Chem. B, Vol. 110, No. 37, 2006 18387 TABLE 1: Pulse Enhancement (%) of EBS, OBS, and DBS in Mixtures under CW and Pulsed Conditions pulse enhancements (%) condition

EBS

OBS

DBS

T ) 100 ms, T0 ) 5 s 0.1 mM OBS and 1 mM EBS 23 ( 32 20 ( 8 0.5 mM OBS and 1 mM EBS 13 ( 19 24 ( 12 1 mM OBS and 1 mM EBS 17 ( 23 69 ( 13 2 mM OBS and 1 mM EBS 13 ( 14 132 ( 18 5 mM OBS and 1 mM EBS 10 ( 17 150 ( 21 T ) 100 ms, T0 ) 100 ms 0.1 mM OBS and 0.1 mM EBS 16 ( 21 25 ( 3 0.5 mM OBS and 0.5 mM EBS -5 ( 14 60 ( 3 1 mM OBS and 1 mM EBS -33 ( 35 69 ( 4 T ) 100 ms, T0 ) 100 ms 0.1 mM OBS and 0.1 mM DBS 0.2 mM OBS and 0.2 mM DBS 0.5 mM OBS and 0.5 mM DBS 1 mM OBS and 1 mM DBS

Figure 2. (A) Initial degradation rates of EBS and OBS in binary mixtures of 1 mM EBS and various concentrations of OBS sonicated under CW and pulsed ultrasound with a pulse length of 100 ms and pulse interval of 5 s (error bars represent 95% confidence intervals). (B) Initial degradation rates of OBS in single solutes at various concentrations sonicated under CW and pulsed ultrasound with a pulse length of 100 ms and pulse interval of 5 s (error bars represent 95% confidence intervals).

concentrations, initial degradation rates of EBS and OBS in mixtures were determined and plotted as a function of OBS concentration under CW and pulsed ultrasound as shown in Figure 2A. At any OBS concentration studied, the degradation rates of EBS were similar under both CW and pulsed ultrasound. However, the degradation of EBS was significantly higher in the presence of lower OBS concentrations (i.e., 0, 0.1, and 0.5 mM) as compared to the presence of higher OBS concentrations (i.e., 1, 2, and 5 mM). Pulse enhancements calculated from eq 1 were not present for EBS as shown in Table 1. In contrast, the degradation rates of OBS in mixtures were significantly faster under pulsed ultrasound than under CW at all the concentrations studied. The enhancements in the rate under pulsing became more prominent as the OBS concentration increased. In addition, the degradation of OBS in mixtures with EBS was dependent on its initial concentration. Under CW, the degradation rates of OBS in the mixture were similar at concentrations above 0.1 mM. Under pulsing, the degradation rates of OBS increased with concentration until it reached a plateau at 2 mM. Although pulsed ultrasound allows for OBS to adsorb on the bubble surfaces, possibly reducing the amount of OH• diffusing to bulk solution where EBS reactions occur, similar degradation rates of EBS between CW and pulsed ultrasound were observed. The lack of difference in the degradation of EBS under CW and pulsed ultrasound indicated that EBS, a compound that is not surface active, does not accumulate on bubble surfaces but

19 ( 11 3(7 36 ( 13 47 ( 10

18 ( 11 38 ( 8 -21 ( 12 -57 ( 10

T ) 100 ms, T0 ) 5 s 0.1 mM OBS and 0.1 mM DBS 1(8 0.2 mM OBS and 0.2 mM DBS -2 ( 8 0.5 mM OBS and 0.5 mM DBS 24 ( 11 1 mM OBS and 1 mM DBS 20 ( 10

3(5 38 ( 10 74 ( 17 200 ( 13

reacts with OH• mainly in bulk solution.12,21,22 The longer bubble lifetime under pulsed ultrasound does not enhance the competitive ability of EBS to react with OH• at the cavitation bubble surfaces. In addition, it does not appear that enhanced accumulation of OBS with pulsing reduces the OH• available in bulk solution. The faster degradation of OBS under pulsed ultrasound suggests that pulsed ultrasound allows sufficient time for OBS to accumulate and possibly equilibrate with the bubble surfaces. It is more beneficial for OBS at higher concentrations because the adsorption mechanism switches from diffusion controlled to mixed kinetic diffusion controlled as the bulk concentration increases.23,24 As the mechanisms switched to mixed kinetic diffusion controlled, more time is required for OBS to reach and equilibrate with bubble surfaces. The persistence of cavitation bubbles in pulsed ultrasound depends on the bubble dissolution time, which is inversely proportional to the surface tension of the solution.25-27 It has been demonstrated by Epstein and Plesset that 10 µm spherical air bubbles completely dissolve in 6 s in gas-saturated water.26 Chan et al. determined that the lifetime of a 20 µm bubble in a saline solution could be increased from 6 s to 5-10 min by the addition of the surfactant sodium laurate (C12H23NaO2).27 Therefore, we expect that increased degradation rates of OBS at higher concentrations under pulsed ultrasound are due to increased surface excess and reduced surface tension, resulting in increased adsorption of OBS during pulse intervals and slower bubble dissolution rates. The faster degradation of OBS under pulsing as compared to CW in mixtures under all conditions further verified that increased surface excess and reduced surface tension play a dominant role in the pulse enhancements.12 Thus, it appears that the main pathway for OBS degradation is either through thermolysis or reaction with OH• at the bubble surfaces because a longer bubble lifetime facilitates surfactant accumulation on bubble surfaces at higher concentrations. The trend of increased OBS degradation in mixtures at higher concentrations under pulse conditions is consistent with our previous work12 showing increased degradation of single solute surfactants at higher bulk concentrations and pulsed intervals suggesting that EBS does not affect OBS degradation in mixtures. The significantly higher degradation of EBS at lower concentrations of OBS may be

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Figure 3. Initial degradation rate ratio of kmixC0/ksingleC0 for EBS and OBS in binary mixtures of 1 mM EBS and various concentrations of OBS sonicated under CW and pulsed ultrasound with a pulse length of 100 ms and a pulse interval of 5 s (error bars represent 95% confidence intervals).

due to less OH• consumption by OBS on the bubble surfaces allowing more OH• to diffuse to bulk solution. Figure 2B shows the initial degradation rates of OBS in single solutes at various concentrations sonicated under CW and pulsed ultrasound with a pulse length of 100 ms and pulse interval of 5 s. A comparison of OBS and EBS in mixtures as compared to single solutes calculated as kmixC0/ksingleC0 is shown in Figure 3. EBS values of kmixC0/ksingleC0 < 1 indicate that rate constants of EBS were significantly reduced in the presence of OBS at all the concentrations studied as compared to single EBS. Furthermore, the values of kmixC0/ksingleC0 of EBS in the mixtures were dependent on the initial concentration of OBS. Increasing the OBS concentration to 1 mM or higher resulted in a dramatically lower kmixC0/ksingleC0 value for EBS. Interestingly, a similar trend of kmixC0/ksingleC0 for EBS under pulsing and CW was found at all the concentrations studied, indicating that the degradation of EBS in mixtures is strongly dependent on the OBS concentration rather than sonication mode (i.e., CW or pulsing). Figure 3 shows that the degradation of EBS was affected in the presence of OBS at all the concentrations studied. However, the degradation of OBS was unaffected in the presence of EBS at OBS concentrations of 1 mM and higher. The results are consistent with that of Pe´trier et al., showing that chlorobenzene hindered the degradation of 4-chlorophenol.17 A significant reduction in OBS degradation in the mixture as compared to OBS alone at lower OBS concentrations was observed. Similar values of kmixC0/ksingleC0 for OBS under both CW and pulsed ultrasound at all the concentrations studied were also observed. Results indicate that selective degradation of OBS occurs in binary mixtures with EBS and that the selectivity is dependent on the OBS concentration when the EBS concentration was fixed at 1 mM. Selective degradation of surfactant is defined here as a relatively large reduction in nonsurfactant degradation in the presence of surfactant as compared to in its absence (i.e., kmixC0/ksingleC0 < 1) while at the same time observing negligible change in the degradation of the surfactant (i.e., kmixC0/ksingleC0 ≈ 1). OBS migrates to and eventually adsorbs on gas/solution interfaces of cavitation bubbles, allowing it to undergo degradation through high temperature thermolysis and OH radical reaction. EBS, which is not surface active,12 tends to remain in bulk solution where a small amount of the OH radical has diffused from cavitation bubbles. Dukhin et al. compared the adsorption time of two surface active compounds based on their diffusion coefficients. The characteristic adsorption time for

Yang et al. sodium octyl sulfate was 0.04 ms and approximately 2 orders of magnitude longer for sodium dodecyl sulfate (9 ms).28 It has been shown that the time required to reach the equilibrium surface excess (Γeq) depends on the structure of the surfactants and may vary drastically, ranging from milliseconds to hundreds of seconds for anionic surfactants.29 Fainerman et al. showed that 2 mM SDS (C12H25OSO3-Na+) reached equilibrium after more than 3 ms. Furthermore, approximately 700 µs passed before the surface excess concentration of SDS reached 50% Γeq.30 Thus, a fraction of OBS also stayed in bulk solution to react with OH radicals dependent on the sonolysis conditions (i.e., if time is long enough for adsorption to occur and equilibrium to be reached). It should be noted that although Figure 3 does not show a benefit of pulsing for selective degradation of OBS, the degradation rate of EBS is much slower in the presence of OBS. Results indicate that pulsing significantly enhances the degradation rates of OBS in mixtures and in single solute systems and that the enhancement depends on the initial concentration of OBS. Reduced degradation of EBS when more surfactant is added may be due to competition of OBS and EBS for OH radicals. Drijvers et al.7 observed an inhibition in TCE sonolysis in the presence of a second volatile cosolute, chlorobenzene, and attributed it to lower reaction rates with radicals formed during sonolysis. We expect that the amount of OBS reacting with OH• at the cavitation bubble surfaces is dependent on the bubble surface coverage by OBS molecules (i.e., surface excess); more surface coverage results in more OH• consumption at the surface. The amount of OBS reacting with OH• in the bulk solution is dependent on the initial bulk OBS concentration. Therefore, with more OBS present in bulk solution, more bulk OH• will be consumed. Accordingly, there will be less OH• left for EBS to react. Thus, the degradation of EBS which only reacts with OH• in the bulk solution will be significantly reduced in mixtures as compared to when it is a single solute as shown in Figure 3. Reduced OBS degradation in mixtures as compared to as a single solute at lower concentrations may result from a lack of significant OBS accumulation on bubble surfaces at low concentrations. Under this condition, OBS and EBS degradation in the mixture are dependent on OH• rate constants and not preferential accumulation on bubble surfaces. Thus, it appears that the ability of OBS to hinder EBS degradation is dependent on both the ratio of OBS/EBS in the mixture and the OBS concentration. Degradation of OBS and EBS Binary Mixtures at Equal Mixed Molar Ratio. To further examine the selective degradation of OBS in a binary mixture with EBS, mixed molar ratios of 1:1 (equal concentrations) for OBS and EBS were sonicated at CW and pulsed ultrasound at various concentrations. Pulsed ultrasound with a pulse length of 100 ms and a pulse interval of 100 ms was studied based on our previous results of single surfactant degradation, showing that OBS has sufficient time to reach bubble surfaces under this pulse condition and concentration range.12 Figure 4 shows no change in the results at 1 mM OBS and EBS between pulse intervals of 100 ms and 5 s. Similar to higher OBS concentrations in Figure 2, Figure 4 and Table 1 show selective degradation of OBS over EBS with enhanced degradation of OBS due to pulsing and no effect of pulsing on EBS. As with changing molar ratios, the results of mixtures at a molar ratio of 1:1 imply that the kinetics of the degradation of OBS is concentration dependent under pulsed ultrasound. However, the kinetics of EBS degradation in

Degradation of Alkylbenzene Sulfonate Surfactants

Figure 4. Initial degradation rates of EBS and OBS in binary mixtures with molar ratio 1:1 at various concentrations under pulsed ultrasound with a pulse length of 100 ms and a pulse interval of 100 ms (error bars represent 95% confidence intervals).

Figure 5. Initial degradation rate ratio of kmixC0/ksingleC0 for EBS and OBS in binary mixtures with molar ratio 1:1 at various concentrations sonicated under CW and pulsed ultrasound with a pulse length of 100 ms and a pulse interval of 100 ms (error bars represent 95% confidence intervals).

mixtures is neither concentration nor sonication mode dependent. As described previously, an increase in OBS degradation with concentration under pulsed ultrasound may be due to increased surfactant concentrations reaching the bubble interface and a decrease in transfer rate to the bubble surfaces with increased concentration. The constant degradation rates of EBS between CW and pulsed ultrasound further indicate that EBS does not accumulate on bubble surfaces. The initial degradation rate ratio of kmixC0/ksingleC0 for OBS and EBS is shown in Figure 5. When OBS was present at the same concentration as EBS, the degradation of EBS was much slower than EBS alone as shown by the lower kmixC0/ksingleC0 values. The slight decrease in EBS kmixC0/ksingleC0 at 1 mM as compared to 0.1 mM under pulsed ultrasound may be due to increased OBS degradation at 1 mM, consequently reducing EBS degradation in the mixture. Increased accumulation of OBS at bubble surfaces and reaction with OH• may decrease the amount of OH• migrating to the bulk solution when bubbles approach saturation. However, differences in degradation rates between mixtures and individual compounds were not significant for OBS as shown by kmixC0/ksingleC0 values ranging from 0.83 to 1.01. Results indicate selective degradation of OBS in mixed solutions at molar ratios of 1:1 with EBS. Thus, it appears that the selective degradation is neither governed predominantly by sonication mode nor concentration at the molar ratio of 1:1. Degradation of OBS and DBS Surfactant Binary Mixtures. Pulsed ultrasound enhances the degradation of single

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Figure 6. Initial degradation rates of OBS and DBS in binary mixtures with molar ratio 1:1 at various concentrations under CW and pulsed ultrasound with a pulse length of 100 ms and pulse intervals of 100 ms and 5 s (error bars represent 95% confidence intervals).

surfactants depending on pulse length, pulse interval, and initial concentration and surface activity of surfactants.12 With binary surfactant mixtures, a series of experiments was conducted at mixtures of OBS and DBS with a molar ratio 1:1 under CW and pulsed ultrasound. We expected that we could alter the selective degradation of the surfactants depending on the pulse interval used. Comparison of the Degradation Rates of OBS with DBS in Mixtures. Figure 6 shows that the initial degradation rates of OBS and DBS in equal molar mixtures were dependent on surfactant concentration and pulse interval, T0. At low concentrations, the initial degradation rates of both OBS and DBS in mixtures were similar under CW and pulsing at both intervals. The similar degradation rates of both surfactants at lower concentrations indicated that adsorption to the bubble surfaces was diffusion controlled, and it took less time for surfactants to accumulate and approach equilibrium with the bubble surfaces.31 It is possible that both OBS and DBS reached the bubble surfaces at lower concentrations. In addition, the bubble surfaces were probably far away from the saturation, and the competition of surfactants to accumulate to bubble surfaces may not be a factor. Thus, when bubbles collapse, both OBS and DBS are present at the cavitation bubble surfaces and undergo degradation. At the 100 ms pulse interval, the initial degradation rates of OBS increased significantly when the concentrations increased. However, the degradation rates of DBS decreased significantly with increased concentration. Results suggest that the surfactants strongly affect each other’s degradation rates due to competition for the reaction sites on the cavitation bubble surfaces. OBS, which is less surface active than DBS, exhibits a faster degradation rate than DBS due to its faster transfer rate to the cavitation bubble interface.9 A pulse interval of 100 ms may provide sufficient time for OBS to accumulate and approach equilibrium with bubble surfaces. Although DBS is more surface active, it takes longer to accumulate and equilibrate with bubble surfaces. Therefore, the surfaces are dominated by the adsorption of OBS at this shorter pulse interval, resulting in increased degradation with increasing concentration. At a pulse interval of 5 s, the initial degradation rate of DBS shows a nearly linear increase when the concentration increased. We expected that OBS degradation rates would accordingly decrease as DBS competes effectively for the bubble surfaces. However, the degradation rate of OBS also increased when the concentration increased to 0.5 mM and then remained constant at 1 mM. In addition, the degradation rates of OBS and DBS

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Figure 7. Initial degradation rate ratio of kmixC0/ksingleC0 for OBS and DBS in binary mixtures with molar ratio 1:1 at various concentrations sonicated under CW and pulsed ultrasound with pulse length of 100 ms and pulse interval of 5 s (error bars represent 95% confidence intervals).

in mixtures were comparable at concentrations from 0.1 to 0.5 mM at a pulse interval of 5 s. However, a lower OBS degradation rate as compared to DBS was observed at a concentration of 1 mM. Competitive adsorption has been studied at the air-water interface.32,33 DBS accumulates on the interfaces more slowly than OBS.9 To allow DBS to compete with OBS for adsorption at the air-solution interfaces of the cavitation bubbles, longer pulse intervals provide longer times for DBS to adsorb on the bubble surfaces. Although OBS adsorbs on the bubble surfaces more rapidly than DBS, DBS competes for the bubble surfaces with OBS with increased adsorption time, forcing some adsorbed OBS back to bulk solution. Thus, lower amounts of OBS are present at bubble surfaces with higher concentrations of DBS due to a greater surface excess of DBS as observed by a lower degradation rate of OBS than DBS at a higher DBS concentration and a longer pulse interval. Comparison of the Degradation of Single Surfactants with Surfactants in Mixtures. Figure 7 shows the degradation rate ratio kmixC0/ksingleC0 for both OBS and DBS in mixtures as compared with single solutes under various sonication modes. Slower degradation rates were observed for both surfactants as compared to their degradation in single solutes as observed by kmixC0/ksingleC0 < 1, indicating that surfactants affect each other’s degradation. Because both compounds tend to accumulate at bubble surfaces, there is competition for reaction sites. In addition, the adsorption of surfactants to bubble surfaces in mixtures at concentrations below CMCs might be slower than in single surfactant solutions.34 Figure 8 shows the degradation rates of single OBS and DBS and the calculated total surfactant degradation rates in mixtures with a pulse length of 100 ms and an interval of 5 s. The same total initial concentrations of single and mixed solutes were used to compare the initial degradation rates of total surfactant and the mixture of one-half OBS and one-half DBS. Regardless of concentration, there was no significant difference between the total initial degradation rates of mixtures and the total initial degradation rates of single compounds of OBS. However, faster total surfactant degradation as compared to DBS alone at a concentration of 0.2 mM and slower total surfactant degradation as compared to DBS alone at a concentration of 1 mM were observed. As the surface excess of DBS is larger than OBS, we expect that more DBS will undergo degradation than OBS, assuming that both OBS and DBS adsorb and approach equilibrium at

Yang et al.

Figure 8. Initial rates of single surfactants and total initial rates in binary mixtures of OBS and DBS with molar ratio 1:1 at various concentrations under pulsed ultrasound with pulse length of 100 ms and pulse interval of 5 s (error bars represent 95% confidence intervals).

the bubble surfaces during a pulse interval of 5 s. Nevertheless, the reactivity of DBS with OH• is approximately 2.5 times lower than OBS;12 thus, the total degradation rates in mixtures are expected to be slower than single OBS but faster than single DBS at the same total concentrations. However, similar degradation rates between the single OBS and the mixed solutes were found. Therefore, there is a balance between surface excess and reactivity with OH•. In addition, the amounts of OBS and DBS that adsorb and approach equilibrium on the bubble surfaces are unequal depending on their concentrations, surface activities, and orientation on the bubble surfaces.35 Faster total degradation in mixtures than DBS alone at a concentration of 0.2 mM might be due to unsaturated bubble surfaces resulting in possible similar surface excesses of OBS and DBS. It appears that a faster reactivity of OBS with OH• plays a dominant role at lower concentrations, giving a total faster degradation in mixtures of OBS and DBS as compared to DBS alone. Slower total degradation in mixtures than single DBS at a concentration of 1 mM might be due to saturated bubble surfaces resulting in lower total surface excess as compared to single DBS at the same concentration. It was found that when the chain length of the two surfactants is unequal, the molecular packing at the gas/solution interface of cavitation bubbles is looser.35 Therefore, it is also possible that a smaller surface excess of OBS and DBS in mixtures than single DBS occurs due to the looser packing on the surfaces when both compounds are present. It seems that surface excess plays an important role in the degradation of mixtures at higher concentrations. In addition, surface tension may play a role in the degradation of mixtures because the adsorption of the more surface active solute to the surface causes a larger reduction in surface tension.36 Figure 9 shows that the surface tension in mixtures at total concentrations of 0.4 and 1 mM were significantly lower than that of single OBS but higher than that of DBS alone. Lower surface tension may lower the cavitation threshold, facilitate the generation of additional cavitation bubbles,1,37 and also retard bubble dissolution.26,27 Therefore, a lower surface tension of DBS alone at 1 mM than the mixtures of 0.5 mM OBS and DBS also may be responsible for the slower total surfactant degradation rates than single DBS at total concentrations of 1 mM. In summary, all these effects including reactivity of surfactants with OH•, surface excess, and surface tension may play a

Degradation of Alkylbenzene Sulfonate Surfactants

Figure 9. Surface tension as a function of initial total surfactant concentration below CMCs of OBS alone, DBS alone, and mixtures of OBS/DBS (error bars represent 95% confidence intervals).

combined role in the degradation of binary mixtures of surfactants at a molar ratio 1:1 by pulsed ultrasound. However, the dominant factors appear to be the concentration of the surfactants and the pulse interval. Conclusion In binary mixtures of a surfactant, OBS, and a nonsurfactant, EBS, the presence of OBS strongly hinders the degradation of EBS due to the preferential accumulation of OBS on the cavitation bubbles surfaces. In contrast, the degradation of OBS was unaffected by EBS, suggesting that surfactants can be selectively degraded. The selectivity is dependent on the mixed molar ratio rather than sonication mode (CW and pulsing). Although the selectivity is not altered, the initial degradation rates of OBS in mixtures can be significantly enhanced by pulsed ultrasound. In binary mixtures of the surfactants OBS and DBS, there exists a competition for reaction sites on cavitation bubble surfaces. Reactivity of surfactants with OH•, surface excess, and surface tension may play a combined role in the degradation of binary mixtures of surfactants by pulsed ultrasound, but dominant factors appear to be the concentration of the surfactants and the pulse interval. Results provide information applicable to the sonochemical degradation of complex wastewaters containing surfactants. Acknowledgment. The authors are grateful for financial support provided by the Office of Naval Research (ONR). References and Notes (1) Leighton, T. G. The Acoustic Bubble; Academic Press: London, 1994. (2) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. Sonochemical hot spot. J. Am. Chem. Soc. 1986, 108, 5641-5642. (3) Suslick, K. S. Sonochemistry. Science 1990, 247, 1439-1445. (4) Suslick, K. S. The chemical effects of ultrasound. Sci. Am. 1989, 260 (2), 80-86. (5) Suslick, K. S., Ed. Ultrasound: Its Chemical, Physical, and Biological Effects; VCH: New York, 1988. (6) Weavers, L. K. Sonolytic ozonation for the remediation of hazardous pollutants. In AdVances in Sonochemistry; Mason, T. J., Tiehm, A., Eds.; Elsevier Science: Amsterdam, 2001. (7) Drijvers, D.; van Langenhove, H.; Kim, L. N. T.; Bray, L. Sonolysis of an aqueous mixture of trichloroethylene and chlorobenzene, Ultrason. Sonochem. 1999, 6, 115-121. (8) Weavers, L. K.; Pee, G. Y.; Frim, J. A.; Yang, L.; Rathman, J. F. Ultrasonic destruction of surfactants: application to industrial wastewaters. Water EnViron. Res. 2005, 77 (3), 259-265. (9) Sostaric, J. Z.; Riesz, P. Sonochemistry of surfactants in aqueous solutions: an EPR spin-trapping study. J. Am. Chem. Soc. 2001, 123 (44), 11010-11019.

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