Ind. Eng. Chem. Res. 2004, 43, 5049-5056
5049
Effects of Surface Active Properties on the Cavitational Degradation of Surfactant Contaminants Gim-Yang Pee,†,‡ James F. Rathman,† and Linda K. Weavers*,‡ Department of Civil and Environmental Engineering and Geodetic Science and Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
The influence of surfactant properties on the sonochemical degradation of organic compounds was investigated in a 20-kHz probe reactor and a dual-frequency 16- and 20-kHz near-field acoustical processor (NAP). The initial sonochemical degradation rates of 4-octylbenzene sulfonic acid (OBS) and t-octylphenoxy polyethoxyethanol (OPE) in the NAP as a function of concentration resulted in complicated but similar trends to catalysis. At concentrations above the critical micelle concentration, the formation of micelles created additional weak spots in solution that acted as additional nucleation sites for the formation of cavitation bubbles. In comparing the degradation of surfactants to that of nonsurfactants, enhanced degradation of surfactants in the NAP was attributed to the accumulation of surfactants on cavitation bubble surfaces, resulting in localization of contaminants at the site of highest temperature and hydroxyl radical concentration, [•OH]. The lack of enhanced degradation of surfactants in the probe was attributed to the higher power intensity, resulting in rapid bubble growth cycles with insufficient time for surfactants to diffuse to and accumulate on cavitation bubble surfaces compared to the low-power-intensity reactor. Introduction Sonolysis results in the formation of cavitation bubbles when sound waves in the range of 20 kHz to several megahertz are applied to a liquid medium. Chemical reactions can occur through either thermolysis within the bubble cavity and at the interface or OH radical attack at the interface and in the bulk solution. Sonolysis has been found to be effective in the degradation of a variety of organic contaminants, such as halobenzenes,1,2 polychlorinated biphenyls,3,4 and p-nitrophenol (PNP).5,6 Physicochemical properties of organic compounds have been identified to be important in determining the site and rate of sonolytic reaction.7 For example, studies performed on volatile compound degradation by Drijvers et al.1 showed that diffusion of the contaminant to the interior of the cavity is an important parameter affecting the degradation rate constant of the contaminant. For hydrophobic nonvolatile compounds, degradation increases for uncharged, protonated species compared to charged, deprotonated species, indicating that reactants reaching the boundary layer are important.6 For surfactants, the rate of reaction is expected to be dependent on their accumulation at the interface. The amount of surfactant that accumulates at the liquidvapor interface, known as the surface excess (Γ), can be determined from measurements of the surface tension of the solution. In addition, surface tension alters bubble growth and collapse dynamics, potentially increasing or decreasing degradation rates.8,9 Previous studies conducted on surfactants have led to the proposal of various theories on the effect of surfactants on sonolysis. Alegria et al. showed that * To whom correspondence should be addressed. Tel.: (614) 292-4061. Fax: (614) 292-3780. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Civil and Environmental Engineering and Geodetic Science.
surfactants accumulate at the cavitation bubble interface.10 In addition, modeling has shown that the presence of surfactants allows more rapid growth of cavitation nuclei to active cavitation bubbles than occurs in the absence of surfactants.8 This growth process, enhanced by surfactants, is called rectified diffusion. These factors enable the degradation of surfactants to be faster than that of nonsurfactant contaminants following the same reaction pathway. However, reduced surface tension (i.e., higher surfactant concentration) affects the bubble dynamics of a cavitation bubble by partially countering the implosion energy; hence, lower collapse temperatures can be attained. Lower collapse temperatures are an undesirable effect as they lead to a reduced rate of free-radical formation and hence lower degradation rates.11 Destaillats et al.12 indicated that, at similar concentrations (below the critical micelle concentration, CMC), the degradation rate constants of t-octylphenoxy polyethoxyethanol and a nonsurfactant compound, t-octylphenol, were similar, suggesting that surfactant properties do not accelerate the degradation of surfactants. Above the CMC, the authors speculated that the reduction in the overall efficiency of the sonochemical process for surfactants was due to the isolation of free monomer from the interface as a result of micelle formation. Thus, it is unclear whether surface tension properties are beneficial or detrimental to the degradation of contaminants in sonolytic systems. The purpose of this study was to determine whether the use of sonochemistry is advantageous for the degradation of surfactants as compared to compounds that neither accumulate at the interface substantially nor reduce the surface tension of a solution. The destruction of anionic and nonionic surfactants was investigated to determine the applicability of sonolysis to treat surfactant-laden industrial wastewater. Of particular interest were mass-transfer limitations of the contaminants to reaction sites, degradation in the presence of surfactant micelles, and the applicability to
10.1021/ie0306022 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/03/2004
5050 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 1. Operating Parameters for the NAP and the Probe total sonication volume power volume power/volume power/area reactor (mL) (W) (mL) (W/mL) (W/cm2) NAP probe
2000 50
106.7 22.8
165 50
0.647 0.455
0.0847 18.2
treat a range of concentrations to elucidate whether surface tension reduction is beneficial or detrimental for degradation in sonolytic systems. Experimental Methods Materials. An anionic surfactant, octylbenzene sulfonic acid (OBS), and the nonionic surfactant t-octylphenoxy polyethoxyethanol (OPE) were chosen as model surfactants. The surfactants have similar hydrophobic tail groups but differing charges and abilities to decrease the surface tension of the solution. PNP and ethylbenzene sulfonate (EBS) were chosen as model nonsurfactant compounds. All compounds were obtained from Sigma-Aldrich with purities of >97% and were used as received. Water used was from a MilliQ water purification system (R ) 18.2 MΩ cm). Ultrasonic Reactors. Two ultrasonic reactors were used for sonochemical experiments: the near-field acoustic processor (NAP) ultrasonic system model NAP3606-HP-TC manufactured by Advanced Sonic Processing and the 20-kHz ultrasonic direct immersion probe model 550 manufactured by Fisher Scientific. The NAP has seven water-cooled magnetostrictive transducers fixed to each stainless steel parallel plate operating at 16 kHz on one plate and 20 kHz on the other. Cooling of the transducers was provided by a CFT-150 refrigerated recirculator manufactured by Neslab Instruments. The NAP system runs in a continuous circulation mode with a temperature-controlled glass vessel, doublediaphragm Teflon pump, and 1/2-in. PTFE Teflon tubing connecting the components. The total volume of solution in the circulating system was 2 L; however, the volume under active sonication was 165 mL. A direct immersion probe emitting ultrasound from a 1.20-cm2 tip was used to compare the efficiency of the NAP to the probe. This system was operated in batch mode with a sonication volume of 50 mL. The powers of the NAP and the probe were measured using calorimetry. The parameters of both systems are listed in Table 1. Experimental Conditions. For concentration effect studies, the pH of the solutions during sonication was maintained at pH 7.4 using a 25 mM phosphate buffer. For experiments comparing the sonochemical degradations of surfactants and nonsurfactants, a 33 mM phosphate buffer at pH 2.8 was used to ensure that the nonsurfactant compounds were neutraly charged (pKa,PNP ) 7.1, and pKa ,EBS ) 7.0). By definition, surfactants accumulate on bubble surfaces. The surface-active behavior of OPE and OBS is not dependent on pH, and so pH was expected to have no effect on reaction rate constants. As expected, experiments performed at pH 2.8 and pH 7.4 for OPE and OBS verified that pH had a negligible effect on the degradation of these surfactants.13 Samples from the NAP were obtained for analysis by withdrawing 5-mL samples from a tap on the external glass reactor at designated time points. For each sample, the sample container was rinsed with solution twice before sample collection. For experimental runs
using the probe, a new solution was sonicated for each time point to prevent systematic errors due to volume reduction of the sonicating solution. The samples were filtered with 0.45-µm Millex-FG filters before analysis. Control experiments indicated that the compounds of interest did not sorb onto the filters. Analysis. Quantification of the concentration in solution was performed using a Hewlett-Packard 1100 HPLC with a Hypersil ODS C-18 column. An eluent gradient of acetonitrile and phosphate buffer at pH 2.2 from 100% phosphate buffer to 55% phosphate buffer in 45 min was used for OBS. An isocratic eluent of 52% water and 48% acetonitrile was used for OPE. The surface tensions of the solutions were determined using a Sensadyne tensiometer (Chem-Dyne Research Corporation, model PC 500). Relative reaction rate constants of the four compounds with •OH were measured using Fenton’s reagent14 and the method of relative rate determinations.15 A 25-mL sample of a mixture containing 0.5 mM of PNP and 0.5 mM of either OPE, OBS, or EBS was allowed to undergo OH radical reaction. At specific times, 2.5 mL of 1 N NaOH was added to stop the production of OH radical by oxidizing Fe(II) to Fe(OH)3.16 Calorimetry. Calorimetry measures the amount of power dissipated as heat during cavitation and is calculated using eq 117
Pdiss )
|
dT dt
t)0msolventCp,solvent
|
dTv dt
+ (Awsxw)FvesselCp,vessel (1)
t)0
where m and Cp are the mass and heat capacity of the solvent, respectively; (dT/dt)t)0 is the initial slope of the temperature rise of the reaction mixture versus time of exposure to ultrasonic irradiation; Tv is the temperature of the vessel; Aws is the area of the wetted surface of the vessel; xw is the thickness of the vessel wall; and Fvessel is the density of the vessel material. For our experiments, 50 mL of water, OPE (at pH 7.4), or PNP (at pH 2.8) at various concentrations in phosphate buffer was sonicated without cooling. The slope for temperature rise per unit time was then obtained and used to calculate the power. Heat loss to the reactor was expected to be negligible over the short time of calorimetry measurements. In addition, we were using the same vessel for all calorimetry measurements, and thus, any heat loss was assumed to be constant for all experiments. This assumption might have slightly underestimated absolute power measurements but does not affect relative changes. Results The equilibrium amount of surfactant absorbed at an interface, the surface excess (Γ), is described by the classical Gibbs equation.18 The Gibbs equation is also valid for compounds that are only weakly surface-active and not considered to be conventional surfactants. The hydrophobicity of such solutes can lead to surface tension reduction at the interface.6 Surface excess is a means to calculate the equilibrium amount of reactant accessible for reaction at the interface. From the measurements of surface tension at various concentrations, the critical micelle concentration (CMC) was found to be 1.3 mM for OPE and 5.2 mM for OBS in phosphate buffer at 20 °C. Subsequently, the satu-
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5051 Table 2. Surface Excesses, Diffusivities, Fenton’s Reactivities, and First-Order Kinetic Rate Constants for OPE, OBS, EBS, and PNP in the NAP and Probe at 0.1 mM
OPE OBS EBS PNP
Γ at 0.1 mM (molecule cm-2)
Diw/DPNPw
Fenton’s reactivity (ki,fen/kPNP,fen)
kNAP × 10-3 (min-1)
kProbe × 10-3 (min-1)
1.6 × 1014 1.0 × 1013 -
0.37 0.60 0.85 1.0
0.9 2.8 1.2 1.0
26 ( 1 19.0 ( 0.4 11.2 ( 0.2 9.7 ( 0.2
5.3 ( 0.2 6.0 ( 0.2 7.0 ( 0.2 4.9 ( 0.1
rated surface excesses were calculated to be 2.8 × 1014 molecules cm-2 and 1.4 × 1014 molecules cm-2 for OPE and OBS, respectively. Γ values for OPE and OBS at 0.1 mM were calculated to be 1.6 × 1014 and 1.0 × 1013 molecules cm-2, respectively, in phosphate buffer at 20 °C and pH 2.8. The surface tensions of PNP and EBS were measured at values up to 0.5 mM using tensiometery, and no significant changes in surface tension were observed. This suggests that PNP and EBS are not particularly surface-active. From nucleation through the point of unsustainable growth prior to an implosive cavitational collapse, cavitation bubbles grow isothermally because of the long time period of bubble growth compared to the extremely rapid cavitational collapse.19 Thus, bubbles are at ambient temperature during the vast majority of their lifetime. Therefore, although the collapse temperature was on the order of thousands of Kelvin, surface excess values at ambient temperature were used to quantify the system. Sonolysis of reactants such as PNP has been shown to follow apparent pseudo-first-order kinetics.20 For reactions where thermolysis occurs in parallel with hydroxylation, the overall rate expression can be described by21
-
d[C] ) kpyr[C] + kOH[C][•OH] dt
(2)
where [C] is the concentration of reactant, kpyr is the first-order rate constant for reaction undergoing thermolysis, and kOH is the second-order rate constant for hydroxylation. The OH radical concentration is assumed to be constant as cavitation bubbles are continually producing •OH; hence, the equation is simplified to
-
d[C] ) kpyr[C] + k′OH[C] ) k[C] dt
Figure 1. NAP and probe initial rates as a function of [OPE] at reaction temperature 20 °C.
(3)
where k′OH ) kOH[•OH] and k ) kpyr + k′OH. The first-order degradation results for OBS, OPE, EBS, and PNP obtained using the probe and the NAP are reported in Table 2. Experiments for OBS and OPE were repeated three times, and the rate constants were within 4% of each other. The pH after sonication was found to change by less than 0.1. The initial rate represents the number of moles of reactant that undergo reaction per unit time initially. Figure 1 shows the variation in the initial rates with OPE concentration in the probe and the NAP. Figure 2 shows the initial rates for OBS for various concentrations in the NAP. The plots for the NAP reactor have similar trends, resulting from the same factors influencing the degradation at the various concentrations. Discussion Comparison of Degradation Behaviors of Surfactants and Nonsurfactants. From Table 2, the degradation rates of OPE, OBS, EBS, and PNP for the
Figure 2. NAP initial rates as a function of [OBS] at reaction temperature 20 °C.
probe were found to be in the decreasing order EBS > OBS > OPE > PNP. The rate constants of the compounds differed by at most a factor of 1.4. However, the NAP degradation rate constants were found to be significantly different from each other in decreasing order OPE > OBS > EBS > PNP. The rate constants of OPE and OBS were approximately factors of 3 and 2 times greater, respectively, than those of nonsurfactants in the NAP. Table 2 demonstrates the enhanced degradation observed for surfactants compared to nonsurfactant organic compounds in a low-power-intensity dualfrequency 16- and 20-kHz reactor. It shows that the degradation of OPE, OBS, EBS, and PNP in a highpower-intensity 20-kHz probe reactor is not dependent on surface excess. The surface excess of OPE is greater than that of OBS at 0.1 mM; thus, the degradation of OPE is expected to be greater than that of OBS if the equilibrium parameter, surface excess, controls degradation. In addition, EBS and PNP were not observed to accumulate at interfaces as determined by surface tension measurements. Thus, if surface excess controls degradation, the degradations of EBS and PNP are
5052 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
expected to be lower than those of OPE and OBS. Another possibility controlling degradation in the reactors is reaction with •OH; however, the degradation trends observed in the probe and NAP are different in each system, with neither correlating with •OH as measured by relative reaction with Fenton’s reagent (Table 2). The lifetime of a bubble in a cavitating solution is expected to be on the order of milliseconds to seconds depending on the power intensity of the ultrasound.22 Results indicate that the time required for equilibrium adsorption of the surfactants studied here is significantly longer than the available bubble lifetime. This is consistent with dynamic surface tension results and published studies of similar surfactants.23 Therefore, it is unlikely that enough time exists for surfactants or other contaminants to accumulate at the bubble interface (or inside the bubble for volatile contaminants) to reach equilibrium, particularly in the probe system. Several researchers have investigated the degradation of compounds that partition to the cavitation bubble surface or interior. Many of these studies have assumed that the bubble is in equilibrium with the surrounding water, thus, employing Henry’s law constants (H) for volatile compounds,24-27 octanol-water partitioning constants or enrichment factors for hydrophobic compounds,6,27-29 and surface excesses for nonvolatile compounds.30-32 Modeling studies have verified this assumption in certain cases.25 On the contrary, in another modeling study, De Visscher et al.24 found no correlation between degradation rates and increasing H. A follow-up study reported the correlation of degradation with the aqueous-phase diffusivity of the compound.1 In addition, Weavers et al.21 reported a larger reduction in the degradation of pentachlorophenol with increasing concentration at low ultrasound frequency than at a higher frequency. This difference was attributed to a longer time for diffusion to the cavitation bubble at low frequency than at high frequency. However, to our knowledge, no studies have reported changing trends in degradation rate constants of a series of compounds in different ultrasonic reactors at either the same or different frequencies. The change in trends observed in this study implies that neither equilibrium parameters nor simple diffusion are adequate measures for predicting relative degradation rate constants. Although equilibrium cannot be reached within the short lifetime of a cavitating bubble, the difference in surface excess will contribute to a different flux of molecules moving toward the interface. In the bulk liquid, cavitation bubbles generate considerable turbulence such that convection and turbulence are expected to control transport; thus, the transport of all compounds in this region is expected to be approximately equal. In general, the dynamic surface excess is determined by the transport of material from the bulk phase to the surface by convection and diffusion, convection and diffusion within the interfacial region, and generation or consumption by chemical reaction.33 In the interfacial regime, we assume diffusion between the bulk phase and bubble surface to be the primary mode of transport. Fick’s first law for a one-dimensional binary system molar flux of solute i (Ji, mol m-2 s-1) provides a simple description of the system34
Ji ) -Diw
dCi dz
(4)
where Ci is the molar concentration of the reactant, Diw
is the diffusion coefficient of solute i in water, and distance z is the direction of one-dimensional diffusion (in this case the thickness of the interface). Integrating this equation across the width of the interfacial region35 gives
Ji ) -
Diw (Ceq,i - C0,i) z
(5)
where Ceq is the equivalent concentration of solute on the surface (i.e., proportional to surface excess for surfactant) and C0,i is the concentration of solute in bulk solution. If Ceq,i is related to the surface excess through a, the effective mass-transfer interfacial area per unit volume, the flux can be written as
Ji ) -
Diw (Keq,iΓia - C0,i) z
(6)
Ceq,i ∝ Γia
(7)
where
and Keq,i is the equilibrium relationship at the interface, which equals Ceq,i/Cs,i, where Cs,i is the saturated concentration at the gas-liquid interface. For two solutions containing 0.1 mM of solute each, the values of C0,i are assumed to be equal. The diffusion coefficient (Diw) of the solute i can be estimated using36
Diw )
13.16 × 10-9 µw1.14Vi0.589
(8)
where µw is the solvent viscosity and Vi is the molar volume of the solute, i, at the solute normal boiling point. The ratios Diw/DPNPw were calculated as 0.85, 0.603, and 0.368 for EBS, OBS, and OPE, respectively. From this ratio, we expect the diffusivity to be highest for PNP followed by EBS, OBS and OPE. From the viewpoint of diffusivity, PNP is expected to have the highest degradation rate constant because it has the largest diffusivity. However, the degradation rate constant of OPE, which diffuses the slowest, was higher in the NAP. From the calculated surface excesses at 0.1 mM for OPE and OBS, OPE will experience a larger flux (i.e., driving force × diffusivity) to the interface of the bubble. Its surface excess was an order of magnitude greater than that for OBS, whereas the diffusivities were estimated to be of the same magnitude for the two compounds. Assuming an average bubble size of 10 µm and Keq ) 1, the driving force at 0.1 mM was 1.5 mM for OPE but negligible for OBS. This OPE flux resulted in a larger concentration of OPE at the interface for reaction during collapse of the bubble. This effect of flux can also be seen in the sonolysis of phenol and phenolate. Although the second-order reaction rate constant for the hydroxylation of phenolate is higher than that for phenol,37 the sonolysis of phenol is considerably faster.38 The two compounds are assumed to have the same diffusion coefficient. Hence, a plausible explanation for a higher phenol degradation rate is that the flux of phenol to the cavitation bubble surface is greater than that of phenolate. At equilibrium, the concentration of phenol at the interface is greater than that of phenolate because of its higher hydrophobicity.
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5053
For OBS and OPE, surface excess accounts for the higher degradation rate constant for OPE. For the nonsurfactant models, PNP and EBS, surface tension measurements indicated that the surface excesses of these compounds were negligible at concentrations used in this study. Although the diffusivity, Diw, is larger for the two nonsurfactant models, accumulation at the surface does not occur. Thus, diffusion and the resulting mean-weighted travel distance are appropriate in determining the number of molecules that can interact with the reactive bubble.21 As observed in Table 2, EBS degradation is a factor of 1.4 larger than PNP degradation in the probe but only 1.2 larger in the NAP. This result is consistent with the longer bubble lifetime in the NAP, allowing more PNP molecules to interact with the reactive bubble than EBS because of its higher diffusivity. For non-surface-accumulating compounds, diffusion and reactivity control degradation. Thus, as the lifetime of a bubble changes, the observed degradation rates and trends might also change as a result of changes in the controlling mechanisms that influence the process. In the probe sonicator, degradation rate constants generally decreased with increasing surface excess of the compounds. With the exception of PNP, the slowestreacting contaminant with •OH as determined by Fenton’s reagent (Table 2), degradation rate constants increased with increasing diffusivity. However, in the NAP sonication unit, degradation appeared to decrease with increasing diffusivity and to increase with increasing surface excess. These contrasting results in the two reactors can be explained by the difference in power intensity (I) between the NAP and the probe. The intensity in the probe was 18.2 W cm-2, whereas that of the NAP was 0.0847 W cm-2, corresponding to acoustic pressures of 2.4 and 0.16 bar, respectively. Assuming that the bubbles in the probe and NAP can be well-understood by analysis of a single representative bubble,39 the work of Louisnard and Gomez22 investigating the growth of bubbles under single, low-frequency conditions indicate a growth time of a bubble at the probe driving pressure to be in the millisecond range, whereas the growth time in the NAP would be orders of magnitude higher. Furthermore, the time required to reach equilibrium surface tension (i.e., surface accumulation) is in the range of 10-100 s for these surfactants.23 Therefore, with substantially faster net bubble growth in the probe unit, surfactant compounds have insufficient time to reach the bubble interface to a large degree, as observed by comparing the rate constants in the probe to those in the NAP. In the probe, the diffusivity term in the flux equation plays a dominant role because the short lifetime of the bubble limits surface excess at the interface (i.e., the system remains far from equilibrium). The trend observed in the probe is similar to that seen in a higher-frequency 354-kHz reactor at a moderate power intensity.13 Concentration Effects. Initial rates of degradation of the surfactant compounds as a function of concentration in the NAP and probe verify the accumulation of surfactants on cavitation bubbles in the NAP but not in the probe reactor. For the NAP system, Figures 1 and 2 are divided into three regions dominated by different factors. At low concentrations (0.05-0.5 mM for OPE, 0.05-1.5 mM for OBS), the initial rate of reactant conversion increased linearly with concentration. This increase is attributed to a substantial increase in surface
excess in this region, indicating the rapid transport of surfactant molecules toward the interface for reaction. In the second region (0.5-1.3 mM for OPE, 1.5-5.2 mM for OBS), a maximum initial rate was attained, followed by a slight decrease in the rate. In this concentration range, the surface excess was constant. However, the observed steep decrease in surface tension (55-39 dyne cm-1 for OPE and 60-47 dyne cm-1 for OBS) would greatly reduce the Blake radius as the concentration increased. The Blake radius is the threshold bubble radius; bubbles smaller than this radius will not grow into active cavitation bubbles for a particular acoustic pressure.40 A decrease in surface tension results in a smaller Blake radius, thus increasing the number of nuclei able to grow into active cavitation bubbles. Thus, more surfaces were formed, and flux toward the interface was increased as a result of an increase in the bulk concentration. Thermolysis and hydroxylation are parallel reactions that occur simultaneously. For volatile organics, the amount of reactant that enters the cavity affects the collapse temperature and subsequently the amount of OH radical produced. Similarly, for surfactants, the number of molecules that accumulate at the interface and participate in thermolysis reactions can reduce the production of OH radicals, hence reducing the overall reaction rate. As the concentration at the interface increases, the number of surfactant molecules undergoing thermolysis reactions increases.41 Although thermolysis will also result in the reduction of surfactant molecules, it inhibits OH radical production to a greater extent by reducing the bubble collapse temperature, hence acting to reduce the reaction rate (i.e., the decrease in k′OH in greater than the increase in kpyr in eq 3). Judging from the NAP results in Figures 1 and 2, in the first portion of the second region, the reduction in Blake radius is the dominating factor. In the second portion, initial rates decrease because thermolysis results in the reduction of OH radical production because of the increased flux to the bubble surface. Although there was still an increase in the number of nucleus-forming bubbles, reaction via hydroxylation was reduced to a greater extent compared to the increase in the amount of surfactant undergoing reaction via thermolysis reactions. The third region is defined by the onset of micelle formation (1.3 mM for OPE, 5.2 mM for OBS). At concentrations only slightly higher than the CMC, both systems show a relatively constant initial rate, consistent with the fact that the surface tension and surface excess are constant in this region. At higher concentrations, as the number of micelles in solution becomes appreciably large, a significant increase in the initial rates is observed. Micelles effectively increase the number of sites in solution40 where nucleation and subsequent growth of cavitation bubbles occurs. The presence of micelles initiates more cavity formation, rather then sequestering surfactant molecules from the cavitation bubble interface, as has been suggested.12 This effect is observed with the increase in rate at concentrations above the CMC, as shown for the NAP in Figures 1 and 2. Although micelles were also present in the constant-rate portion, micelle aggregates might be too small or the number of micelles might still be insufficient to affect the reaction rates. Although the concentrations at which these regions occur are different for OPE and OBS, the consistency of the trend within
5054 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
these regions indeed suggests that surfactants accumulate on cavitation bubbles, thereby affecting degradation. In the probe there is a linear trend that does not appear to be affected appreciably by surfactant properties. Cavitation bubbles are similar to catalytic particles. The diffusion and adsorption of surfactant molecules at the bubble/solution interface are the same mechanisms by which reactants accumulate at a solid/solution interface in heterogeneous catalysis. An important difference is that cavitation bubbles grow and collapse over time, so desorption of products is fast. The reaction rate vs concentration plot of an adsorption-limited catalytic process35 is similar to the NAP results in Figures 1 and 2. In a catalytic reaction, each catalyst particle can be modeled as a reactor. The reaction is affected by factors such as mass transfer to the interface, adsorption at the interface, reaction rates, and desorption and mass transfer of the product to the bulk.35 In addition, the catalytic reaction is dependent on the number of sites available. For a reaction that is adsorption-limited (i.e., amount of reactant reaching the catalysis particle for reaction is limiting), mass transfer to the interface is dependent on the concentration of the reactant in the bulk phase. The NAP trend in Figures 1 and 2 is consistent with a system that is approaching equilibrium. The initial rates observed in the probe sonication unit show a linearly increasing trend with concentration in Figure 1. This is indicative of an adsorption-limited competitive reaction observed in catalysis systems. This competitive reaction is likely a result of OH radical recombining in the interfacial region rather than reacting with target compounds. This trend is similar to processes occurring in the first region in Figures 1 and 2 for the NAP and in catalytic processes where the surface is not saturated but adsorption to the interface is limited, suggesting that the cavitation bubble surfaces are not saturated. Thus, the system is not at equilibrium in the probe. It is also possible that this phenomenon might be due to localized heating that occurs near collapsing bubbles. The CMC of a solution is dependent on the temperature of the solution. The localized high temperature in the region surrounding cavitation bubbles produced from cavitational collapses42 causes micelle breakup, releasing free monomers in the vicinity near the bubble and forming a supersaturated free monomer solution.43 This process was not observed to be significant in the NAP system. The time that the solution spent in the external reactor with no sonication likely allowed any excess free monomer to re-form micelles. In the probe system, continuous sonication was employed, so that re-formation of micelles in the vicinity of bubble surfaces most likely did not occur. An increase in the free monomer concentration in bulk solution might have allowed more surfactant molecules to move to the interface by increasing the flux to the bubble surface. Therefore, at concentrations above or near the CMC, the rate continued to increase instead of leveling off with increasing surfactant concentration. The results on concentration effects in both the NAP and probe confirm the importance of flux in predicting degradation in different reactor systems. Calorimetry. As the surface tension of the solution decreases, the temperature reached inside the bubble upon collapse is expected to be lower.44 In addition,
Figure 3. Calorimetry for different concentrations of PNP at pH 2.8 and OPE at pH 7.4 in the probe without cooling. The probe system volume was 50 mL. The power was normalized by the power of the buffer solution, Power0.
Vinodgopal et al.45 observed reduced sonoluminescence with increasing surfactant concentration and correlated it with reduced bubble collapse temperatures due to the formation of degradation products. Within a given reactor (i.e., probe or NAP), assuming that there are the same number of cavitational collapses per unit time, the temperature released to the bulk from the collapsing bubble also will be reduced with increasing surfactant concentration. To test whether the time available for adsorption of surfactant in the probe system is sufficient for the accumulation of surfactants on cavitation bubble surfaces, we investigated the energy input into solution in the probe system as measured by calorimetry. Calorimetry was used as a method to determine the total heat energy input into the solution. This heat energy results from cavitational collapse, sound waves, and heating of the transducer. If surfactants are interacting with cavitation bubbles to a large degree, the surfactants would be expected to lower the collapse temperature of cavitation bubbles,24 thus altering the heat energy (other heat energy terms are not expected to change). It is assumed that, if surfactant accumulation does not occur on a bubble surface (as we suspected occurs in the probe), the calorimetry results in the presence of PNP and OPE in the probe as a function of concentration would be similar (i.e., neither interacts with the cavitation bubble to reduce the collapse temperature). Figure 3 shows the trend of power as measured by calorimetry in the probe system in the presence of varying concentrations of either OPE or PNP. According to Figure 3, less reduction in heating is observed in solution as the surfactant concentration increases, opposite to the trend observed for PNP. One possible explanation for this result is that the number of collapses per unit time increased as a result of rectified diffusion allowing faster bubble growth for solutions containing surfactant.8 Also, as the surface tension decreases, the total number of nucleation sites increases because of a reduction in the Blake radius. This effect of surface tension is consistent with region I in Figures 1 and 2. Hence, a higher power input into sonication was observed as surface tension decreased compared to PNP. The decrease in power input in the absence of OPE indicates that the collapse temperature of the bubble decreases, likely because of thermolysis of molecules adsorbed at the interface of the cavity (i.e., surface
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5055
excess). This thermolysis reduces the temperature of the solution as evidenced by the degradation of volatile organic compounds and surfactants.24,44,45 Thus, it appears that the effects of thermolysis and decreased Blake radius balance to minimize the net heat effect measured by calorimetry. The Blake radius merely increases the number of active sites; thus, a reduction in the temperature of a collapsing bubble occurs similar to that observed by Vinodgopal et al.45 For PNP, the increase in concentration resulted in a larger decrease in the power input into solution than for OPE, indicating that a larger number of molecules participate in the thermolysis reactions, reducing the temperature of cavitational collapse. Unlike surfactant solutions, there was no enhancement caused by surface tension reduction, so a reduction in power for PNP solutions as concentration increased was observed. It does appear that the probe reactor allows for some accumulation of compounds on the bubble surface or interior, although the extent of accumulation is expected to be much less than in the NAP. Conclusions The degradation of surfactants using sonolysis is advantageous because of surfactant localization at the interface as compared to nonvolatile nonsurfactant solutes, but it is strongly dependent on the reactor conditions, particularly the lifetime of a cavitation bubble. As predicted in modeling results8 and verified in this study, properties of surfactants are beneficial to compound degradation. Adequate predictions of the accumulation of compounds on a cavitation bubble surface cannot be realized on the basis of equilibrium or simple diffusivity parameters alone. Instead, the flux (i.e., driving force × diffusivity) must also be considered to determine whether a compound will accumulate on a bubble surface. It appears that both the NAP and the probe reactors allow for some compound accumulation on the cavitation bubble, but there appears to be a much greater degree of accumulation in the NAP reactor than the probe reactor. This accumulation allows the reactants to come into contact with the greatest temperature and OH radical concentrations. Acknowledgment Financial support provided by the Naval Facilities Engineering Service Center, the Office of Naval Research, and the Water Environmental Research Foundation (WERF) is gratefully acknowledged. Literature Cited (1) Drijvers, D.; van Langenhove, H.; Herrygers, V. Sonolysis of fluoro-, chloro-, bromo- and iodobenzene: A comparative study. Ultrason. Sonochem. 2000, 7, 87-95. (2) Kruus, P.; Burk, R. C.; Entezari, M. H.; Otson, R. Sonication of aqueous solutions of chlorobenzene. Ultrason. Sonochem. 1997, 4, 229-233. (3) Lu, Y.; Weavers, L. K. Sonochemical Desorption and Destruction of 4-Chlorobiphenyl from Synthetic Sediments. Environ. Sci. Technol. 2002, 36, 232-237. (4) Zhang, G.; Hua, I. Cavitation Chemistry of Polychlorinated Biphenyls: Decomposition Mechanisms and Rates. Environ. Sci. Technol. 2000, 34, 1529-1534. (5) Weavers, L. K.; Ling, F. H.; Hoffmann, M. R. Aromatic Compound Degradation in Water Using a Combination of Sonolysis and Ozonolysis. Environ. Sci. Technol. 1998, 32, 2727-2733.
(6) Tauber, A.; Schuchmann, H.-P.; von Sonntag, C. Sonolysis of aqueous 4-nitrophenol at low and high pH. Ultrason. Sonochem. 2000, 7, 45-52. (7) Henglein, A.; Kornmann, C. Scavenging of OH Radicals Produced in the Sonolysis of Water. Int. J. Radiat. Biol. 1985, 48, 251-258. (8) Fyrillas, M. M.; Szeri, A. J. Dissolution or Growth of Soluble Spherical Oscillating Bubbles: The Effect of Surfactants. J. Fluid Mech. 1995, 289, 295-314. (9) Fyrillas, M. M.; Szeri, A. J. Surfactant dynamics and rectified diffusion of microbubbles. J. Fluid Mech. 1996, 311, 361378. (10) Alegria, A.; Lion, Y.; Kondo, T.; Riesz, P. Sonolysis of Aqueous Surfactant Solutions. Probing the Interfacial Region of Cavitation Bubbles by Spin Trapping. J. Phys. Chem. 1989, 93, 4908-4913. (11) Colussi, A. J.; Weavers, L. K.; Hoffmann, M. R. Chemical Bubble Dynamics and Quantitative Sonochemistry. J. Phys. Chem. A 1998, 102, 6927-6934. (12) Destaillats, H.; Hung, H. M.; Hoffmann, M. R. Degradation of Alkylphenol Ethoxylate Surfactants in Water with Ultrasonic Irradiation. Environ. Sci. Technol. 2000, 34, 311-317. (13) 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., in press. (14) Haag, W. R.; Yao, C. C. D. Rate Constants for Reaction of Hydroxyl Radicals with Several Drinking Water Contaminants. Environ. Sci. Technol. 1992, 26, 1005-1013. (15) Pilling, M. J.; Seakins, P. W. Reaction Kinetics; Oxford University Press: New York, 1995. (16) Neyens, E.; Baeyens, J. A review of classic Fenton’s peroxidation as an advanced oxidation technique. J. Hazard. Mater. 2003, B98, 33-50. (17) Thompson, L. H.; Doraiswamy, L. K. Sonochemistry: Science and engineering. Ind. Eng. Chem. Res. 1999, 38, 12151249. (18) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1997. (19) Leighton, T. G. Bubble Population Phenomena in Acoustic Cavitation. Ultrason. Sonochem. 1995, 2, S123-S136. (20) Kontronarou, A.; Mills, G.; Hoffmann, M. R. Ultrasonic Irradiation of p-Nitrophenol in Aqueous Solution. J. Phys. Chem. 1991, 95, 3630-3638. (21) Weavers, L. K.; Malmstadt, N.; Hoffmann, M. R. Kinetics and Mechanism of Pentachlorophenol Degradation by Sonication, Ozonation, and Sonolytic Ozonation. Environ. Sci. Technol. 2000, 34, 1280-1285. (22) Louisnard, O.; Gomez, F. Growth by rectified diffusion of strongly acoustically forced gas bubbles in nearly saturated liquids. Phys. Rev. E 2003, 67, 036610. (23) Chang, C.-H.; Franses, E. I. Adsorption dynamics of surfactants at the air/water interface: A critical review of mathematical models, data, and mechanisms. Colloids Surf. A 1995, 100, 1-45. (24) De Visscher, A.; Van Eenoo, P.; Drijvers, D.; van Langenhove, H. Kinetic Model for the Sonochemical Degradation of Monocyclic Aromatic Compounds in Aqueous Solution. J. Phys. Chem. 1996, 100, 11636-11642. (25) Colussi, A. J.; Hung, H. M.; Hoffmann, M. R. Sonochemical Degradation Rates of Volatile Solutes. J. Phys. Chem. A 1999, 103, 2696-2699. (26) Hung, H. M.; Hoffmann, M. R. Kinetics and Mechanism of the Sonolytic Degradation of Chlorinated Hydrocarbons: Frequency Effects. J. Phys. Chem. A 1999, 103, 2734-2739. (27) Weavers, L. K. In Advances in Sonochemistry; Mason, T. J., Tiehm, A., Eds.; Elsevier Science: Amsterdam, 2001; Vol. 6. (28) Adewuyi, Y. G. Sonochemistry: Environmental Science and Engineering. Ind. Eng. Chem. Res. 2001, 40, 4681-4715. (29) Seymour, J. D.; Gupta, R. B. Oxidation of Aqueous Pollutants Using Ultrasound: Salt-Induced Enhancement. Ind. Eng. Chem. Res. 1997, 36, 3453-3457. (30) Ashokkumar, M.; Vinodgopal, K.; Grieser, F. Sonoluminescence Quenching in Aqueous Solutions Containing Weak Organic Acids and Bases and Its Relevance to Sonochemistry. J. Phys. Chem. B 2000, 194, 6447-6451. (31) Ashokkumar, M.; Hall, R.; Mulvaney, P.; Grieser, F. Sonoluminescence from Aqueous Alcohol and Surfactant Solutions. J. Phys. Chem. B 1997, 101, 10845-10850.
5056 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 (32) Ashokkumar, M.; Crum, L. A.; Frensley, C. A.; Grieser, F.; Matula, T. J.; McNamara, W. B. I.; Suslick, K. S. Effect of Solutes on Single-Bubble Sonoluminescence in Water. J. Phys. Chem. A 2000, 104, 8462-8465. (33) Miller, C. A.; Neogi, P. Interfacial Phenomena; Marcel Dekker: New York, 1985; Vol. 17. (34) Hines, A. L.; Maddox, R. N. Mass Transfer Fundamentals and Applications; Prentice Hall: Upper Saddle River, NJ, 1985. (35) Folger, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1992. (36) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997. (37) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (•OH/•O-) in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17, 513-817. (38) Serpone, N.; Terzian, R.; Colarusso, P.; Minero, C.; Pelizzetti, E.; Hidaka, H. Sonochemical Oxidation of Phenol and Three of Its Intermediate Products in Aqueous Media: Catechol, Hydroquinone, and Benzoquinone. Kinetic and Mechanistic Aspects. Res. Chem. Intermediates 1992, 18, 183-202. (39) Storey, B. D.; Szeri, A. J. A reduced model of cavitation physics for use in sonochemistry. Proc. R. Soc. London A 2001, 457, 1685-1700.
(40) Leighton, T. G. The Acoustic Bubble; Academic Press: London, 1994. (41) Sostaric, J. Z.; Riesz, P. Sonochemistry of Surfactants in Aqueous Solutions: An EPR Spin-Trapping Study. J. Am. Chem. Soc. 2001, 123, 11010-11019. (42) Storey, B. D.; Szeri, A. J. Water vapour, sonoluminescence and sonochemistry. Proc. R. Soc. London A 2000, 456, 1685-1709. (43) Qi, Y.; Weavers, L. K.; Zakin, J. L. Enhancing HeatTransfer Ability of Drag Reducing Surfactant Solutions with Ultrasonic Energy. J. Non-Newtonian Fluid Mech. 2003, 116, 7193. (44) Tauber, A.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C. Sonolysis of tert-Butyl Alcohol in Aqueous Solution. J. Chem. Soc., Perkin Trans. 2 1999, 1129-1135. (45) Vinodgopal, K.; Ashokkumar, M.; Grieser, F. Sonochemical Degradation of a Polydisperse Nonylphenol Ethoxylate in Aqueous Solution. J. Phys. Chem. B 2001, 105, 3338-3342.
Received for review July 17, 2003 Revised manuscript received April 22, 2004 Accepted April 29, 2004 IE0306022
