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Hydroxyl radical formation in batch and continuous flow ultrasonic systems
Ultrasonics Sonochemistry 22 (2015) 600–606
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Hydroxyl radical formation in batch and continuous flow ultrasonic systems Hrvoje Juretic a,b,c, Melissa Montalbo-Lomboy a, J. (Hans) van Leeuwen a,c,d, William J. Cooper e, David Grewell a,⇑ a
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50011-3080, USA Department of Energy, Power Engineering and Environment, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lucica 5, HR-10000 Zagreb, Croatia c Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50011-3232, USA d Department of Food Science and Human Nutrition, Iowa State University, Ames, IA 50011-3080, USA e Department of Civil and Environmental Engineering, Urban Water Research Center, University of California, Irvine, Irvine, CA 92697-2175, USA b
a r t i c l e
i n f o
Article history: Received 7 May 2013 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 9 July 2014 Keywords: Ultrasonication Batch Continuous flow Donut horn Hydroxyl radical Terephthalate dosimetry
a b s t r a c t The creation of free radicals by ultrasonic cavitation is the main mechanism that leads to chemical degradation of target pollutants and the process is considered an alternative advanced oxidation technology. The goal of this study was to compare the effects of batch and continuous flow ultrasonic systems on the formation of hydroxyl radicals. Ultrasonic batch experiments were conducted in two reactors (small and large) using a standard 20 kHz catenoidal titanium horn at varying amplitudes and sonication times. The effect of saturating gas was also investigated by introducing helium and air at 1 L min 1 into the larger 100 mL reactor. In the continuous flow system, the experiments were conducted with a 20 kHz, 3.3 kW ultrasonic systems using a titanium ‘‘donut’’ horn at varying volumetric flow rates and amplitudes. Formation of hydroxyl radicals was determined using terephthalic acid dosimetry measurements. At the same energy densities, higher hydroxyl radical concentrations were formed in the batch system than in the continuous flow system. Sonication time appeared to be the main factor that influenced the results in batch and continuous flow systems. The two gases (helium and air) did not increase the hydroxyl radical formation at any amplitude or sonication time tested. Ó 2014 Published by Elsevier B.V.
1. Introduction Advanced oxidation processes involve the production of hydroxyl radicals (OH) and are used for the oxidation of pollutants in water and wastewater [1]. Hydroxyl radicals react with pollutants by addition to the double bonds, hydrogen abstraction, or, electron transfer [2,4]. These highly reactive OH typically react with organic compounds with reaction rate constants in the order of 108–1010 M 1 s 1 [2,3]. Hydroxyl radicals are effective primarily because they are strong oxidants, they are non-selective in nature, and are capable of converting recalcitrant compounds into biodegradable forms [4]. Hydroxyl radicals can be produced using various methods, such as electrolysis, ozonation, ultraviolet radiation, microwave, ultrasonication, or as a combination of two of these processes. ⇑ Corresponding author. Address: 100 Davidson Hall, Iowa State University, Ames, IA 50011-3080, USA. E-mail address: [email protected] (D. Grewell). http://dx.doi.org/10.1016/j.ultsonch.2014.07.003 1350-4177/Ó 2014 Published by Elsevier B.V.
Here, we investigated ultrasonication as a promising alternative technology when considering advanced oxidation processes (AOPs). Ultrasound is sound waves at frequencies beyond the normal human hearing range (15–20 kHz) [5,6]. During ultrasonication, hydroxyl radicals are generated as a result of thermal decomposition of water [7]. As sound waves pass through a liquid, during the expansion half-cycle of the waves, microbubbles will be formed if sufficiently large negative pressure is applied to overcome the tensile strength of the liquid. Most liquids are contaminated by the presence of gas molecules and particulate matter creating weak sites serving as cavitation nuclei, which lower the tensile strength. The tension generated by ultrasound waves is responsible for the formation of cavitation bubbles at these sites. Once produced, these small bubbles will expand and contract in accordance with the energy absorbed from alternating rarefaction and compression cycles of the acoustic waves. The fate of these bubbles in the acoustic region depends on many factors, including the intensity of the applied ultrasound. For low-intensity ultrasound, the size of the bubble oscillates about some equilibrium size
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for many acoustic cycles and these stable bubbles may grow by a process called rectified diffusion [8]. As the bubble grows, it may be transformed into a transient bubble (normally produced by applying high-intensity ultrasound) that collapses violently, giving rise to very intense localized heating (5000 K) [9] and pressure spikes (1000 atm) [10] that lead to the pyrolysis of water, producing OH [7]. During thermal decomposition of water, other reactive species can also form, e.g., oxidizing hydrogen peroxide (H2O2) and oxygen radical (O) and the reducing hydrogen atom (H) [4]. There are a growing number of studies in water and wastewater treatment involving AOPs [1,2,11–14]. Several of these studies have shown the potential of ultrasonication in water and wastewater treatment [15–17] for enhanced pollution degradation. Hoffmann et al. [17] showed the successful degradation of several chemical contaminants, such as chlorinated hydrocarbons, pesticides, and phenols. They also found that the chemical degradation by ultrasonication followed three pathways: oxidation by OH, pyrolytic decomposition, and supercritical water oxidation. Méndez-Arriaga et al. [18] utilized ultrasound to treat water contaminated with ibuprofen, a pollutant that is typically not removed in municipal treatment systems [19]. Ultrasonics destroyed 98% of ibuprofen (21 mg L 1 initial ibuprofen concentration) after 30 min of treatment at a frequency of 300 kHz and 80 W of applied power. In addition, Pengphol et al. [20] has effectively combined ozone and ultrasonication treatment of chlorpyrifos, an organophosphate pesticide sometimes found in shrimp. Their results showed a reduction of chlorpyrifos toxicity to brine shrimp after this combined treatment. The formation of hydroxyl radicals during batch ultrasonication has been explored extensively. However, as with other technologies, the scale-up of the process can be challenging. Therefore, the goal of this study was to compare the OH generation efficiency in batch and continuous flow ultrasonic systems, where the continuous flow ultrasonic system corresponds to a scaled-up design. It should be noted that the continuous flow system contained the same ‘‘donut’’ horn technology typically used in a commercial scale ultrasonic system used to treat waste activated sludge for enhanced anaerobic digestion [21].
2. Materials and methods Typically, the transient characteristics of hydroxyl radicals require an in-situ scavenger compound to quantify them [22]. In
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this study, terephthalic acid (TA) was used to scavenge hydroxyl radicals and the fluorescence properties of the product, 2-hydroxyterephthalate (2-HTA) used to measure the yield of 2-HTA, representing an estimate of steady-state concentration of OH radicals. 2.1. Materials and equipment Synthesized standard 2-hydroxyterephthalate (2-HTA) solution and 2 mM terephthalic acid (TA) solution were prepared using the method described by Mason et al. [22]. All chemicals used were reagent grade and obtained from Sigma Aldrich. The synthesis of 2-HTA yielded 2.975 g, and its purity of 95.7% was determined by high performance liquid chromatography (HPLC); its melting point was 304 °C as determined by differential scanning calorimetry (DSC). 2.2. Ultrasonic batch experiments The ultrasonic batch experiments were conducted in two reactors, 50 mL polypropylene conical centrifuge tubes (small batch) and an aluminum alloy chamber (large batch) (see Fig. 1). The small and large batch studies used working volumes of 35 mL and 100 mL, respectively. Aqueous TA solution (2 mM) was prepared and sonicated using a Branson 2000 Series bench-scale ultrasonic unit (Branson Ultrasonics, Danbury, CT) at varying amplitudes (9.6, 14.4, 19.2, 28.8, 38.4 and 48 lmpeak-to-peak(pp)) and sonication times (1, 1.5, 2, 3, 4, 5, 8, 10 min). The ultrasonic system was capable of operating at a maximum power output of 2.2 kW and a constant frequency of 20 kHz. The horn was a standard, 20 kHz catenoidal titanium horn with a flat 13 mm diameter face (gain = 8). The same horn was used in both batch experiments. Ultrasonication was assumed to be 70% efficient. This conservative assumption was based on another study where the same ultrasonic ‘‘donut’’ horn was used [23]. 2.3. Ultrasonic experiments with gas addition To determine the effects of dissolved gases on the formation of hydroxyl radicals during ultrasonication, solutions of TA were saturated with helium or air. One hundred milliliters (100 mL) of aqueous TA solution (2 mM) was saturated with air or helium for 10 min before the sample was sonicated. To insure that solutions were saturated during sonication, gases were continuously introduced into the reactor at flow rate of 1 L min 1.
Fig. 1. Ultrasonic large batch reactor experimental set-up.
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Fig. 2. Schematics of ultrasonic continuous system experimental set-up (T – tank with 20 L aqueous solution of terephthalic acid; P – pump; V – valve; PI – pressure indicator; R – reactor).
2.4. Ultrasonic continuous flow experiments Ultrasonic continuous flow experiments were conducted using a Branson 2000 Series bench-scale ultrasonic unit operating at 3.3 kW and 20 kHz. Aqueous TA solution (2 mM) was held in a large feed tank and pumped to the ultrasonic chamber where the Branson Ultrasonics ‘‘donut’’ horn [23,24] was installed (see Fig. 2). In order to continuously stir the feed tank (T), valves were installed after the pump (P) to recirculate a portion of the samples back to the feed tank. Flow rates were varied by adjusting the flow valve (V1) and recirculation valve (V2). Experiments were conducted at varying volumetric flow rates (0.3–3.3 L min 1) and amplitudes (3, 6, 9, 12 lmpp). It is important to note that in the batch systems, the catenoidal horn vibrated vertically, while in the continuous flow systems, the donut horn vibrated radially. The energy density was calculated assuming an ultrasonication of 70% efficiency, which is conservative compared to other studies [25].
and as a function of amplitude and volumetric flow rates in continuous flow systems. In both small and large batch systems the OH formation was logarithmically proportional to amplitude. This result matches those reported by Villeneuve et al. [26], in which OH formation was proportional to the logarithm of the acoustic power applied in focused ultrasound. Also, at longer sonication times, higher OH formation was observed at all amplitudes.
2.5. Analytical method Terephthalate dosimetry is based on the fact that terephthalic acid (TA) reacts rapidly with OH radicals via hydroxylation of the aromatic ring, preferentially forming highly fluorescent 2-hydroxyterephthalate ion (2-HTA) that is readily measured by a spectrofluorometer and can be correlated with the concentration of OH radicals. Immediately after sonication, the samples were analyzed for fluorescence using a Varian Cary Eclipse fluorescence spectrophotometer at excitation and emission wavelengths of 315 nm and 425 nm, respectively. The fluorescence measurements were then compared to the previously synthesized standard 2-HTA solution to subtract any initial fluorescence prior to calculation.
Fig. 3. Synthesized standard 2-hydroxyterephthalate solution as a function of fluorescence intensity.
3. Results and discussion Fig. 3 is the calibration curve for 2-HTA concentrations. Because the synthesized 2-HTA was obtained at 95.7% purity, the correction of the standard calibration curve was made accordingly. This calibration curve was used through the balance of the study to correlate the fluorometric intensity to the apparent hydroxyl radical formation represented by the 2-HTA concentration. 3.1. Effects of varying ultrasonic amplitude and time Figs. 4–6 show the formation of hydroxyl radicals as a function of amplitude and sonication time in small and large batch systems
Fig. 4. Formation of hydroxyl radicals as a function of amplitude and sonication time in 35 mL small batch systems.
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Fig. 5. Formation of hydroxyl radicals as a function of amplitude and sonication time in 100 mL small batch systems.
Fig. 6. Formation of hydroxyl radicals as a function of amplitude and volumetric flow rate in continuous systems.
It is important to note that despite identical amplitudes and sonication times in both batch systems, OH formation was up to 80% higher in the 35 mL, small batch system than in the 100 mL, large batch system. In a study by Nikitenko et al. [27], the sonochemical efficiency (g) was defined as directly proportional to the sonochemical yield (G) and volume (v) of the sonicated liquid divided by the surface of the horn (S). In the current study, the surface of the horn used was the same in both batch systems (small and large); the only factor to affect the sonochemical efficiency was the volume of the sonicated liquid, so that as the volume of the sonicated liquid increased, the sonochemical yield was decreased. The small batch systems had OH formation reaching approximately 1400 nmol/L, while the large batch systems only reached approx. 800 nmol/L. Being said this, it is important to note that the Nikitenko et al. [27]’s sonochemical efficiency (g) only included the geometry of the horn, sonochemistry yield and volume of the reactor. If we are to account the energy dissipated into the liquid, the large batch system will give better efficiency than the small batch system. Further discussions are found in Section 3.3. Unlike the batch system, for which all results, showed a logarithmic trend, the continuous flow system exhibited different trends. As seen in Fig. 6, with increasing amplitude, the OH formation at flow rates of 2.1 and 3.3 L min 1 followed a polynomial trend, while at flow rates between 0.3 and 1 L min 1, a logarithmic trend line was observed. It is interesting to note that the highest OH formation at all flow rates tested was inversely proportional
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to the amplitude. That is, as the amplitude and flow rates decreased, the OH formation increased. At relatively lower amplitudes, such as found in the continuous flow system, the cavitation field was larger and more effective in producing free radicals. Moreover, it took longer for cavitation bubbles to implode in the continuous flow system than in the batch system, which may have created a ‘‘vapor barrier’’ effect in the liquid. However, because of the relatively lower amplitude and unique horn geometry, which focused cavitation to the center of the horn, this effect may be insignificant. In this context, ‘‘vapor barrier’’ refers to the collection of bubbles that tend to coalesce near the top of the liquid and horn tip during sonication. In Fig. 6, it was shown that at flowrates, 0.3 L min 1 and 1 L min 1, the 2-HTA formation dropped after amplitudes >6 lm. This could indicate that as 2-HTA is produced, it also reacts with another OH radical. That is to say that both generation and degradation of 2-HTA occurs simultaneously. This is particularly observed at longer residence times and higher amplitudes (Fig. 6). While in the small and large batch systems in Figs. 4 and 5, because the trend of the 2-HTA formation is increasing as amplitude increases, it is believed that 2-HTA degradation may be negligible in comparison to those observed in the continuous flow systems (Fig. 6). Furthermore, as the substitution on the ‘‘benzene ring’’ increases, the reaction rate of additional substitution decreases [28–30]. Also, similar studies of photolysis of natural organic matter were found where results in the formation of the 2-HTA accounts for the initial slope of the curve, thus the effect of the degradation was minimized [29–33]. In Fig. 7, the ultrasonic energy densities of both batch and continuous flow ultrasonic systems are presented as a function of amplitude. The energy density of the small batch system ranged from 7.2 to 240 J mL 1, the large batch system ranged from 2.8 to 100.8 J mL 1, which was approximately one-half the energy density of the continuous flow system as highlighted in the box in Fig. 7. The figure shows that at lower amplitudes, the continuous flow system was able to dissipate higher ultrasonic energy to the solution compared to the batch system. Because of the unique geometry of the horn in the continuous flow system, the surface area was significantly larger than the catenoidal horn of the batch systems. Also, the donut horn focused the ultrasonic energy at the center of the horn thus increasing its efficiency. The lower energy density found in the batch systems may have been caused by the fact that ‘‘the sonochemical reactions occurred mainly in the active cavitating zones of the reactors and not in the whole sonicated volume’’ [27]. Because identical horns, amplitudes, and sonication times were used in both batch systems, the energy dissipated into
Fig. 7. Ultrasonic energy densities as a function of amplitude in batch and continuous systems.
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the solution was the same regardless of the volume. The difference in energy densities observed between the small and large batch systems was mainly caused by their difference in total volume and not by the absolute ultrasonic energy dissipated to the liquid. 3.2. Effects of gas addition Fig. 8(a–e) details the effect of gas addition on the formation of OH as a function of time and amplitude. Fig. 8 shows that for all amplitudes tested, the experiments without sparging resulted in the highest hydroxyl radical formation, which is contrary to the expected results. Hydroxyl radical formation in samples sparged with compressed air was higher than in those sparged with helium. Surprisingly, the addition of air or helium actually reduced the OH production. During sonication, the gas sparged into the liquid may have created a ‘‘vapor barrier’’ near the head space of the liquid and near the horn tip, thus reducing cavitation. Also, the bubbles caused by dissolved gas were not vaporous cavities; although they can implode during sonication, they do not produce hydroxyl radicals by thermal degradation of water molecules. Because only air and helium were tested in this study, these results do not imply the effect of other sparging gases. There are studies which successfully
remediated pollutant using ultrasound with saturation of other gases such as Argon and Oxygen [34,35]. Hydroxyl radical formation in helium saturated solution was 2–4 times lower than in air saturated solution. This result is comparable to results published by Hua and Hoffmann [36], who also reported that helium saturated solutions showed the lowest hydrogen peroxide concentration among the 4 gases tested (Kr, Ar, He, O2). In the same study [36], the authors determined that the ‘‘observed production rate in the presence of helium is slower than would be predicted solely by its low water solubility’’. It was also observed that as amplitude and energy supplied increased, the hydroxyl radical formation generally increased in all experiments. This indicates that the influence of amplitude and sonication time is independent of gas sparging. Table 1 reflects the reaction rates of 2-HTA formation at varying amplitudes and under different gas sparging routines. Zero-order kinetics was observed in all reactions at varying amplitudes. In almost all experiments conducted with and without gas sparging, the reaction rate constants were directly proportional to the amplitude. That is, the observed rates were proportional to amplitude independent of gas sparging, with the exception of the reaction rate at 38.4 lm(p–p) amplitude without gas sparging, which was
Fig. 8. (a–e) Effects of gas addition to the hydroxyl radical formation as a function of sonication time at varying amplitude (a) 9.6 lm (b) 19.2 lm (c) 28.8 lm (d) 38.4 lm (e) 48 lm. x-no sparging; – with 1 L min 1 air; N – with 1 L min 1 helium. No sparging indicates that the liquid was not saturated with any gas before or during sonication.
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Table 1 Reaction rates of 2-hydroxyterephthalate formation at varying amplitude and gas sparging. Amplitude (lm)
9.6 19.2 28.8 38.4 48
Reaction rate constant (nM/min) No sparging
With 1 L min
91.4 118.0 155.7 151.7 168.4
62.6 86.1 113.3 124.1 133.8
1
air
With 1 L min
1
He
23.1 26.1 31.9 42.9 52.5
slightly lower than the reaction rate observed at 28.8 lm(p–p) amplitude. Without sparging, the observed reaction rates ranged from 91.4 to 168.4 nM min 1 followed by air sparging ranging from 62.6 to 133.8 nM min 1. The lowest reaction rates were observed in helium saturated environments at 23.1–52.5 nM min 1, which is very close to results reported by Hua and Hoffmann [36]. In the same study [36], the reported reaction rate of 0.0699 lM min 1 in oxygen saturated solution at 20.2 kHz matched the results for air sparged solutions seen in this study. 3.3. Batch vs. continuous flow systems Fig. 9 compares hydroxyl radical formation in batch and continuous flow systems at varying ultrasonic energy densities. To best represent batch ultrasonic systems, this figure only shows the results for small batch systems (35 mL) because it resulted in higher OH formation than the large batch systems (100 mL). It is important to note that the batch system exhibited significantly higher hydroxyl radical formation than the continuous flow systems, even at similar energy densities. The relatively lower hydroxyl radical formation in the continuous flow system may be attributed to the significantly lower residence time of the liquid in the ultrasonic chamber. Namely, the contact times (residence time) of the liquid with the horn in the continuous flow systems were in the order of seconds compared to minutes in the batch systems. Yim et al. [37] compared sonochemical degradation of chlorinated hydrocarbons in batch and continuous flow flow systems and obtained comparable results in both systems after 120 min of ultrasonication. Here, this indicates that residence time is a more important factor than energy density in the formation of hydroxyl radical, because both systems (batch and continuous flow) exhibited the same energy densities and frequencies, yet the batch systems yielded higher OH levels. To conclusively compare batch and continuous flow systems in terms of OH formation,
Fig. 10. Hydroxyl radicals to energy ratio as in batch and continuous flow systems as a function of ultrasonic amplitude.
it is recommended that in future studies both systems should be exposed to the same ultrasonic residence times. It is suggested to utilize lower volumetric flow rates for continuous flow systems or shorter sonication times for batch systems to match residence times. Fig. 10 presents the hydroxyl radical formation and energy ratio as a function of ultrasonic amplitude in small and large batch systems, as well as the continuous flow systems. This figure depicts the efficiency of the different systems (batch vs. continuous flow) in terms of producing hydroxyl radical (nmol) per unit energy (kJ). It has been observed that as the amplitude increases, the hydroxyl radical formation decreases, similar to those discussed in Figs. 7 and 9. Furthermore, it can be seen that the small batch systems ranged from 4.2 to 21.5 nmol/kJ, large batch systems were from 5.6 to 57 nmol/kJ while the continuous flow were only from 0.6 to 9.5 nmol/kJ (encircled). When efficiency is solely based on hydroxyl formation per unit energy utilized, the most efficient will be the large batch systems. Similar to the results in Fig. 9, the continuous flow systems provided the lowest 2-HTA to energy ratio. The lower efficiency from the continuous flow systems may be attributed to the significantly lower residence times in the continuous flow systems compared to the batch systems. However, it is important to note that these results were obtained based only on the conditions tested in this study. Despite the lower hydroxyl radical formation in the continuous flow system, there is still potential for scale-up for the continuous flow system. Currently, the Branson ‘‘donut’’ shaped horn or the Sonix™ system has been successfully employed as the only ultrasonic system that is used in large scale liquid processes such as wastewater treatment plants [21,38]. The same ultrasonic horn in the continuous flow system used in this study has been tested in full-scale wastewater treatment plants across Europe, USA and Australia [38]. To utilize this available technology to scale-up the advanced oxidation process via ultrasonication, optimization study of the flowrates and amplitudes in the continuous flow system should be conducted. Also, multiple donut horn design can also be used similar to those found in Kruger and Fiona [21] and Roxburgh et al. [38]. 4. Conclusions
Fig. 9. Formation of hydroxyl radicals in batch and continuous systems as a function of ultrasonic energy densities.
This study explored the effects of batch and continuous flow ultrasonic systems on hydroxyl radical formation using terephthalic acid dosimetry. For batch systems, amplitude and sonication time was directly proportional to the formation of OH. For continuous flow systems, amplitude (less than 6 lm(p–p)) and
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volumetric flow rate were inversely related to OH formation. The comparison of batch and continuous flow systems showed that residence time or sonication time had more impact on hydroxyl radical formation than energy densities. Using various amplitudes and sonication times, the effects of gas sparging on OH formation was also examined. Both helium and air sparging resulted in lower levels of hydroxyl radical formation compared to samples sonicated without gas sparging independent of amplitude and sonication time. Based on the parameters tested on this study, it is concluded that helium and air are not optimal gas choices to enhance hydroxyl radical formation. The effects of the horn geometry is also as important as the energy density and type of system (batch or continuous flow) as studied here. It is recommended that further investigation on this is conducted in the future. Acknowledgements The authors would like to thank Branson Ultrasonics for providing the ultrasonic system used in this study. H. Juretic thanks the Fulbright Program for a doctoral fellowship to study at Iowa State University and the University of California, Irvine. W.J. Cooper thanks NSF CBET Grant 1034555 for partial support. References [1] C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769–776. [2] A. Matilainen, M. Sillanpaa, Removal of natural organic matter from drinking water by advanced oxidation processes, Chemosphere 80 (2010) 351–365. [3] S. Parsons, Advanced Oxidation Processes for Water and Wastewater Treatment, IWA publishing, London, 2004. [4] M. Mohajerani, M. Mehrvar, F. Ein-Mozaffari, Recent achievements in combination of ultrasonolysis and other advanced oxidation processes for wastewater treatment, Int. J. Chem. React. Eng. 8 (2010) (2010) 1–76. [5] S.K. Khanal, M. Montalbo, J.(H.) van Leeuwen, G. Srinivasan, D. Grewell, Ultrasound enhanced glucose release from corn in ethanol plants, Biotechnol. Bioeng. 98 (2007) 978–985. [6] M. Montalbo-Lomboy, L. Johnson, S.K. Khanal, J. van Leeuwen, D. Grewell, Sonication of sugary-2 corn: a potential pretreatment to enhance sugar release, Bioresour. Technol. 101 (2010) 351–358. [7] P. Riesz, T. Kondo, Free radical formation induced by ultrasound and its biological implications, Free Radical Biol. Med. 13 (1992) 247–270. [8] D.Y. Hsieh, M.S. Plesset, Theory of rectified diffusion of mass into gas bubbles, J. Acoust. Soc. Am. 33 (1961) 206–215. [9] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399. [10] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara, M.M. Mdleleni, M. Wong, Acoustic cavitation and its chemical consequences, Philos. Trans. A. Math. Phys. Eng. Sci. 357 (1999) (1999) 335–353. [11] N.N. Mahamuni, Y. Adewuyi, Advanced oxidation processes involving ultrasound for wastewater treatment: a review with emphasis on cost estimation, Ultrason. Sonochem. 17 (2010) 990–1003. [12] B.A. Wols, C.H.M. Hofman-Caris, Review of photochemical reaction constants of organic micropollutants required for UV advanced oxidation processes in water, Water Res. 46 (2012) 2815–2827. [13] K. Ayoub, E.D. van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: A review, J. Hazard. Mater. 178 (2010) 10–28. [14] S. Esplugas, D. Bila, L.G.T. Krause, M. Dezotti, Ozonation and advanced oxidation technologies to remove endocrine disrupting chemicals (EDCs) and pharmaceuticals and personal care products (PPCPs) in water effluents, J. Hazard. Mater. 149 (2007) 631–642.
[15] V.V. Goncharuk, V.V. Malyarenko, V.A. Yaremenko, Use of ultrasound in water treatment, J. Water Chem. Technol. 30 (2008) 137–150. [16] T.J. Mason, E. Joyce, S.S. Phull, J.P. Lorimer, Potential uses of ultrasound in the biological decontamination of water, Ultrason. Sonochem. 10 (2003) 319–323. [17] M.R. Hoffmann, I. Hua, R. Höchemer, Application of ultrasonic irradiation for the degradation of chemical contaminants in water, Ultrason. Sonochem. 3 (1996) S163–S172. [18] F. Méndez-Arriaga, R.A. Torres-Palma, C. Petrier, S. Esplugas, J. Gimenez, C. Pulgarin, Ultrasonic treatment of water contaminated with ibuprofen, Water Res. 42 (2008) 4243–4248. [19] K. Fent, A.A. Weston, D. Caminada, Ecotoxicology of human pharmaceuticals, Aquat. Toxicol. 76 (2006) 122–159. [20] S. Pengphol, J. Uthaibutra, O.A. Arquero, N. Nomura, K. Whangchai, Oxidative degradation and detoxification of Chlorpyrifos by ultrasonic and ozone treatments, J. Agric. Sci. 4 (2012) 164–172. [21] R. Kruger, F. Hogan, Using SonixTM to enhance anaerobic digestion: an overview from different trials and installations, in: Proceedings of the Water Environment Federation, Residuals and Biosolids Management, Covington, KY, Water Environment Federation, Alexandria, VA, 2005, pp. 945–955. [22] T.J. Mason, J.P. Lorimer, D.M. Bates, Y. Zhao, Dosimetry in sonochemistry: the use of aqueous Terephthalate ion as a fluorescence monitor, Ultrason. Sonochem. 1 (1994) S91–S95. [23] M. Montalbo-Lomboy, S.K. Khanal, J.(H.) van Leeuwen, D.R. Raman, D. Grewell, Simultaneous saccharification and fermentation and economic evaluation of ultrasonic and jet cooking pretreatment of corn slurry, Biotechnol. Prog. 27 (2011) 1561–1569. [24] M. Montalbo-Lomboy, S.K. Khanal, J.(H.) van Leeuwen, D.R. Raman, L. Dunn Jr., D. Grewell, Ultrasonic pretreatment of corn slurry for saccharification: a comparison of batch and continuous flow systems, Ultrason. Sonochem. 17 (2010) 939–946. [25] J.M. Löning, C. Horst, U. Hoffmann, Investigations on the energy conversion in sonochemical processes, Ultrason. Sonochem. 9 (2002) 169–179. [26] L. Villeneuve, L. Alberti, J.-P. Steghens, J.M. Lancelin, J.L. Mestas, Assay of hydroxyl radicals generated by focused ultrasound, Ultrason. Sonochem. 16 (2009) 339–344. [27] S.I. Nikitenko, C. Le Naour, P. Moisy, Comparative study of sonochemical reactors with different geometry using thermal and chemical probes, Ultrason. Sonochem. 14 (2007) 330–336. [28] S. Ayatollahi, D. Kalnina, W. Song, M. Turks, W.J. Cooper, Radiation chemistry of salicylic and methyl substituted salicylic acids: models for the radiation chemistry of pharmaceutical compounds, Radiat. Phys. Chem. 92 (2013) 93– 98. [29] B. Razavi, S.B. Abdelmelek, W. Song, W.J. Cooper, Photochemical fate of Atorvastatin (Lipitor) in simulated natural waters, Water Res. 45 (2011) 625– 631. [30] H. Santoke, W. Song, B.M. Peake, W.J. Cooper, Advanced oxidation treatment and photochemical fate of selected antidepressant pharmaceuticals in solutions of Suwannee river humic acid, J. Hazard. Mater. 217–218 (2012) 382–390. [31] S.E. Page, W.A. Arnold, K. McNeill, Terephthalate as a probe for photochemically generated hydroxyl radical, J. Environ. Monitor. 12 (2010) 1658–1665. [32] H. Xu, W.J. Cooper, J.Y. Jung, W. Song, Photosensitized degradation of Amoxicillin in natural organic matter isolate solutions, Water Res. 45 (2011) 632–638. [33] L. Wang, X.H. Haomin, W.J. Cooper, W. Song, Photochemical fate of betablockers in NOM enriched waters, Sci. Total Environ. 426 (2012) 289–295. [34] D.K. Kim, K.E. O’Shea, W.J. Cooper, Mechanistic considerations for the degradation of methyl tert-butyl ether (MTBE) by sonolysis: effect of argon vs. oxygen saturated solutions, Ultrason. Sonochem. 19 (2012) 959–968. [35] D.K. Kim, K.E. O’Shea, W.J. Cooper, Oxidative degradation of alternative gasoline oxygenates in aqueous solution by ultrasonic irradiation: mechanistic study, Sci. Total Environ. 430 (2012) 246–259. [36] I. Hua, M.R. Hoffmann, Optimization of ultrasonic irradiation as an advanced oxidation technology, Environ. Sci. Technol. 31 (1997) 2237–2243. [37] B. Yim, H. Okuno, Y. Nagata, Y. Maeda, Sonochemical degradation of chlorinated hydrocarbons using a batch and continuous flow flow system, J. Hazard. Mater. B81 (2001) (2001) 253–263. [38] R. Roxburgh, R. Sieger, B. Johnson, B. Rabinowitz, S. Goodwin, G. Crawford, G. Daigger, Sludge minimization technologies – doing more to get less, in: Proceedings of the Water Environment Federation, Technical Exhibition and Conference (WEFTEC), Dallas, Texas, 2005, pp. 506–525.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Hydroxyl radical formation in batch and continuous flow ultrasonic systems Hrvoje Juretic a,b,c, Melissa Montalbo-Lomboy a, J. (Hans) van Leeuwen a,c,d, William J. Cooper e, David Grewell a,⇑ a
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50011-3080, USA Department of Energy, Power Engineering and Environment, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lucica 5, HR-10000 Zagreb, Croatia c Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50011-3232, USA d Department of Food Science and Human Nutrition, Iowa State University, Ames, IA 50011-3080, USA e Department of Civil and Environmental Engineering, Urban Water Research Center, University of California, Irvine, Irvine, CA 92697-2175, USA b
a r t i c l e
i n f o
Article history: Received 7 May 2013 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 9 July 2014 Keywords: Ultrasonication Batch Continuous flow Donut horn Hydroxyl radical Terephthalate dosimetry
a b s t r a c t The creation of free radicals by ultrasonic cavitation is the main mechanism that leads to chemical degradation of target pollutants and the process is considered an alternative advanced oxidation technology. The goal of this study was to compare the effects of batch and continuous flow ultrasonic systems on the formation of hydroxyl radicals. Ultrasonic batch experiments were conducted in two reactors (small and large) using a standard 20 kHz catenoidal titanium horn at varying amplitudes and sonication times. The effect of saturating gas was also investigated by introducing helium and air at 1 L min 1 into the larger 100 mL reactor. In the continuous flow system, the experiments were conducted with a 20 kHz, 3.3 kW ultrasonic systems using a titanium ‘‘donut’’ horn at varying volumetric flow rates and amplitudes. Formation of hydroxyl radicals was determined using terephthalic acid dosimetry measurements. At the same energy densities, higher hydroxyl radical concentrations were formed in the batch system than in the continuous flow system. Sonication time appeared to be the main factor that influenced the results in batch and continuous flow systems. The two gases (helium and air) did not increase the hydroxyl radical formation at any amplitude or sonication time tested. Ó 2014 Published by Elsevier B.V.
1. Introduction Advanced oxidation processes involve the production of hydroxyl radicals (OH) and are used for the oxidation of pollutants in water and wastewater [1]. Hydroxyl radicals react with pollutants by addition to the double bonds, hydrogen abstraction, or, electron transfer [2,4]. These highly reactive OH typically react with organic compounds with reaction rate constants in the order of 108–1010 M 1 s 1 [2,3]. Hydroxyl radicals are effective primarily because they are strong oxidants, they are non-selective in nature, and are capable of converting recalcitrant compounds into biodegradable forms [4]. Hydroxyl radicals can be produced using various methods, such as electrolysis, ozonation, ultraviolet radiation, microwave, ultrasonication, or as a combination of two of these processes. ⇑ Corresponding author. Address: 100 Davidson Hall, Iowa State University, Ames, IA 50011-3080, USA. E-mail address: [email protected] (D. Grewell). http://dx.doi.org/10.1016/j.ultsonch.2014.07.003 1350-4177/Ó 2014 Published by Elsevier B.V.
Here, we investigated ultrasonication as a promising alternative technology when considering advanced oxidation processes (AOPs). Ultrasound is sound waves at frequencies beyond the normal human hearing range (15–20 kHz) [5,6]. During ultrasonication, hydroxyl radicals are generated as a result of thermal decomposition of water [7]. As sound waves pass through a liquid, during the expansion half-cycle of the waves, microbubbles will be formed if sufficiently large negative pressure is applied to overcome the tensile strength of the liquid. Most liquids are contaminated by the presence of gas molecules and particulate matter creating weak sites serving as cavitation nuclei, which lower the tensile strength. The tension generated by ultrasound waves is responsible for the formation of cavitation bubbles at these sites. Once produced, these small bubbles will expand and contract in accordance with the energy absorbed from alternating rarefaction and compression cycles of the acoustic waves. The fate of these bubbles in the acoustic region depends on many factors, including the intensity of the applied ultrasound. For low-intensity ultrasound, the size of the bubble oscillates about some equilibrium size
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for many acoustic cycles and these stable bubbles may grow by a process called rectified diffusion [8]. As the bubble grows, it may be transformed into a transient bubble (normally produced by applying high-intensity ultrasound) that collapses violently, giving rise to very intense localized heating (5000 K) [9] and pressure spikes (1000 atm) [10] that lead to the pyrolysis of water, producing OH [7]. During thermal decomposition of water, other reactive species can also form, e.g., oxidizing hydrogen peroxide (H2O2) and oxygen radical (O) and the reducing hydrogen atom (H) [4]. There are a growing number of studies in water and wastewater treatment involving AOPs [1,2,11–14]. Several of these studies have shown the potential of ultrasonication in water and wastewater treatment [15–17] for enhanced pollution degradation. Hoffmann et al. [17] showed the successful degradation of several chemical contaminants, such as chlorinated hydrocarbons, pesticides, and phenols. They also found that the chemical degradation by ultrasonication followed three pathways: oxidation by OH, pyrolytic decomposition, and supercritical water oxidation. Méndez-Arriaga et al. [18] utilized ultrasound to treat water contaminated with ibuprofen, a pollutant that is typically not removed in municipal treatment systems [19]. Ultrasonics destroyed 98% of ibuprofen (21 mg L 1 initial ibuprofen concentration) after 30 min of treatment at a frequency of 300 kHz and 80 W of applied power. In addition, Pengphol et al. [20] has effectively combined ozone and ultrasonication treatment of chlorpyrifos, an organophosphate pesticide sometimes found in shrimp. Their results showed a reduction of chlorpyrifos toxicity to brine shrimp after this combined treatment. The formation of hydroxyl radicals during batch ultrasonication has been explored extensively. However, as with other technologies, the scale-up of the process can be challenging. Therefore, the goal of this study was to compare the OH generation efficiency in batch and continuous flow ultrasonic systems, where the continuous flow ultrasonic system corresponds to a scaled-up design. It should be noted that the continuous flow system contained the same ‘‘donut’’ horn technology typically used in a commercial scale ultrasonic system used to treat waste activated sludge for enhanced anaerobic digestion [21].
2. Materials and methods Typically, the transient characteristics of hydroxyl radicals require an in-situ scavenger compound to quantify them [22]. In
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this study, terephthalic acid (TA) was used to scavenge hydroxyl radicals and the fluorescence properties of the product, 2-hydroxyterephthalate (2-HTA) used to measure the yield of 2-HTA, representing an estimate of steady-state concentration of OH radicals. 2.1. Materials and equipment Synthesized standard 2-hydroxyterephthalate (2-HTA) solution and 2 mM terephthalic acid (TA) solution were prepared using the method described by Mason et al. [22]. All chemicals used were reagent grade and obtained from Sigma Aldrich. The synthesis of 2-HTA yielded 2.975 g, and its purity of 95.7% was determined by high performance liquid chromatography (HPLC); its melting point was 304 °C as determined by differential scanning calorimetry (DSC). 2.2. Ultrasonic batch experiments The ultrasonic batch experiments were conducted in two reactors, 50 mL polypropylene conical centrifuge tubes (small batch) and an aluminum alloy chamber (large batch) (see Fig. 1). The small and large batch studies used working volumes of 35 mL and 100 mL, respectively. Aqueous TA solution (2 mM) was prepared and sonicated using a Branson 2000 Series bench-scale ultrasonic unit (Branson Ultrasonics, Danbury, CT) at varying amplitudes (9.6, 14.4, 19.2, 28.8, 38.4 and 48 lmpeak-to-peak(pp)) and sonication times (1, 1.5, 2, 3, 4, 5, 8, 10 min). The ultrasonic system was capable of operating at a maximum power output of 2.2 kW and a constant frequency of 20 kHz. The horn was a standard, 20 kHz catenoidal titanium horn with a flat 13 mm diameter face (gain = 8). The same horn was used in both batch experiments. Ultrasonication was assumed to be 70% efficient. This conservative assumption was based on another study where the same ultrasonic ‘‘donut’’ horn was used [23]. 2.3. Ultrasonic experiments with gas addition To determine the effects of dissolved gases on the formation of hydroxyl radicals during ultrasonication, solutions of TA were saturated with helium or air. One hundred milliliters (100 mL) of aqueous TA solution (2 mM) was saturated with air or helium for 10 min before the sample was sonicated. To insure that solutions were saturated during sonication, gases were continuously introduced into the reactor at flow rate of 1 L min 1.
Fig. 1. Ultrasonic large batch reactor experimental set-up.
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Fig. 2. Schematics of ultrasonic continuous system experimental set-up (T – tank with 20 L aqueous solution of terephthalic acid; P – pump; V – valve; PI – pressure indicator; R – reactor).
2.4. Ultrasonic continuous flow experiments Ultrasonic continuous flow experiments were conducted using a Branson 2000 Series bench-scale ultrasonic unit operating at 3.3 kW and 20 kHz. Aqueous TA solution (2 mM) was held in a large feed tank and pumped to the ultrasonic chamber where the Branson Ultrasonics ‘‘donut’’ horn [23,24] was installed (see Fig. 2). In order to continuously stir the feed tank (T), valves were installed after the pump (P) to recirculate a portion of the samples back to the feed tank. Flow rates were varied by adjusting the flow valve (V1) and recirculation valve (V2). Experiments were conducted at varying volumetric flow rates (0.3–3.3 L min 1) and amplitudes (3, 6, 9, 12 lmpp). It is important to note that in the batch systems, the catenoidal horn vibrated vertically, while in the continuous flow systems, the donut horn vibrated radially. The energy density was calculated assuming an ultrasonication of 70% efficiency, which is conservative compared to other studies [25].
and as a function of amplitude and volumetric flow rates in continuous flow systems. In both small and large batch systems the OH formation was logarithmically proportional to amplitude. This result matches those reported by Villeneuve et al. [26], in which OH formation was proportional to the logarithm of the acoustic power applied in focused ultrasound. Also, at longer sonication times, higher OH formation was observed at all amplitudes.
2.5. Analytical method Terephthalate dosimetry is based on the fact that terephthalic acid (TA) reacts rapidly with OH radicals via hydroxylation of the aromatic ring, preferentially forming highly fluorescent 2-hydroxyterephthalate ion (2-HTA) that is readily measured by a spectrofluorometer and can be correlated with the concentration of OH radicals. Immediately after sonication, the samples were analyzed for fluorescence using a Varian Cary Eclipse fluorescence spectrophotometer at excitation and emission wavelengths of 315 nm and 425 nm, respectively. The fluorescence measurements were then compared to the previously synthesized standard 2-HTA solution to subtract any initial fluorescence prior to calculation.
Fig. 3. Synthesized standard 2-hydroxyterephthalate solution as a function of fluorescence intensity.
3. Results and discussion Fig. 3 is the calibration curve for 2-HTA concentrations. Because the synthesized 2-HTA was obtained at 95.7% purity, the correction of the standard calibration curve was made accordingly. This calibration curve was used through the balance of the study to correlate the fluorometric intensity to the apparent hydroxyl radical formation represented by the 2-HTA concentration. 3.1. Effects of varying ultrasonic amplitude and time Figs. 4–6 show the formation of hydroxyl radicals as a function of amplitude and sonication time in small and large batch systems
Fig. 4. Formation of hydroxyl radicals as a function of amplitude and sonication time in 35 mL small batch systems.
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Fig. 5. Formation of hydroxyl radicals as a function of amplitude and sonication time in 100 mL small batch systems.
Fig. 6. Formation of hydroxyl radicals as a function of amplitude and volumetric flow rate in continuous systems.
It is important to note that despite identical amplitudes and sonication times in both batch systems, OH formation was up to 80% higher in the 35 mL, small batch system than in the 100 mL, large batch system. In a study by Nikitenko et al. [27], the sonochemical efficiency (g) was defined as directly proportional to the sonochemical yield (G) and volume (v) of the sonicated liquid divided by the surface of the horn (S). In the current study, the surface of the horn used was the same in both batch systems (small and large); the only factor to affect the sonochemical efficiency was the volume of the sonicated liquid, so that as the volume of the sonicated liquid increased, the sonochemical yield was decreased. The small batch systems had OH formation reaching approximately 1400 nmol/L, while the large batch systems only reached approx. 800 nmol/L. Being said this, it is important to note that the Nikitenko et al. [27]’s sonochemical efficiency (g) only included the geometry of the horn, sonochemistry yield and volume of the reactor. If we are to account the energy dissipated into the liquid, the large batch system will give better efficiency than the small batch system. Further discussions are found in Section 3.3. Unlike the batch system, for which all results, showed a logarithmic trend, the continuous flow system exhibited different trends. As seen in Fig. 6, with increasing amplitude, the OH formation at flow rates of 2.1 and 3.3 L min 1 followed a polynomial trend, while at flow rates between 0.3 and 1 L min 1, a logarithmic trend line was observed. It is interesting to note that the highest OH formation at all flow rates tested was inversely proportional
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to the amplitude. That is, as the amplitude and flow rates decreased, the OH formation increased. At relatively lower amplitudes, such as found in the continuous flow system, the cavitation field was larger and more effective in producing free radicals. Moreover, it took longer for cavitation bubbles to implode in the continuous flow system than in the batch system, which may have created a ‘‘vapor barrier’’ effect in the liquid. However, because of the relatively lower amplitude and unique horn geometry, which focused cavitation to the center of the horn, this effect may be insignificant. In this context, ‘‘vapor barrier’’ refers to the collection of bubbles that tend to coalesce near the top of the liquid and horn tip during sonication. In Fig. 6, it was shown that at flowrates, 0.3 L min 1 and 1 L min 1, the 2-HTA formation dropped after amplitudes >6 lm. This could indicate that as 2-HTA is produced, it also reacts with another OH radical. That is to say that both generation and degradation of 2-HTA occurs simultaneously. This is particularly observed at longer residence times and higher amplitudes (Fig. 6). While in the small and large batch systems in Figs. 4 and 5, because the trend of the 2-HTA formation is increasing as amplitude increases, it is believed that 2-HTA degradation may be negligible in comparison to those observed in the continuous flow systems (Fig. 6). Furthermore, as the substitution on the ‘‘benzene ring’’ increases, the reaction rate of additional substitution decreases [28–30]. Also, similar studies of photolysis of natural organic matter were found where results in the formation of the 2-HTA accounts for the initial slope of the curve, thus the effect of the degradation was minimized [29–33]. In Fig. 7, the ultrasonic energy densities of both batch and continuous flow ultrasonic systems are presented as a function of amplitude. The energy density of the small batch system ranged from 7.2 to 240 J mL 1, the large batch system ranged from 2.8 to 100.8 J mL 1, which was approximately one-half the energy density of the continuous flow system as highlighted in the box in Fig. 7. The figure shows that at lower amplitudes, the continuous flow system was able to dissipate higher ultrasonic energy to the solution compared to the batch system. Because of the unique geometry of the horn in the continuous flow system, the surface area was significantly larger than the catenoidal horn of the batch systems. Also, the donut horn focused the ultrasonic energy at the center of the horn thus increasing its efficiency. The lower energy density found in the batch systems may have been caused by the fact that ‘‘the sonochemical reactions occurred mainly in the active cavitating zones of the reactors and not in the whole sonicated volume’’ [27]. Because identical horns, amplitudes, and sonication times were used in both batch systems, the energy dissipated into
Fig. 7. Ultrasonic energy densities as a function of amplitude in batch and continuous systems.
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the solution was the same regardless of the volume. The difference in energy densities observed between the small and large batch systems was mainly caused by their difference in total volume and not by the absolute ultrasonic energy dissipated to the liquid. 3.2. Effects of gas addition Fig. 8(a–e) details the effect of gas addition on the formation of OH as a function of time and amplitude. Fig. 8 shows that for all amplitudes tested, the experiments without sparging resulted in the highest hydroxyl radical formation, which is contrary to the expected results. Hydroxyl radical formation in samples sparged with compressed air was higher than in those sparged with helium. Surprisingly, the addition of air or helium actually reduced the OH production. During sonication, the gas sparged into the liquid may have created a ‘‘vapor barrier’’ near the head space of the liquid and near the horn tip, thus reducing cavitation. Also, the bubbles caused by dissolved gas were not vaporous cavities; although they can implode during sonication, they do not produce hydroxyl radicals by thermal degradation of water molecules. Because only air and helium were tested in this study, these results do not imply the effect of other sparging gases. There are studies which successfully
remediated pollutant using ultrasound with saturation of other gases such as Argon and Oxygen [34,35]. Hydroxyl radical formation in helium saturated solution was 2–4 times lower than in air saturated solution. This result is comparable to results published by Hua and Hoffmann [36], who also reported that helium saturated solutions showed the lowest hydrogen peroxide concentration among the 4 gases tested (Kr, Ar, He, O2). In the same study [36], the authors determined that the ‘‘observed production rate in the presence of helium is slower than would be predicted solely by its low water solubility’’. It was also observed that as amplitude and energy supplied increased, the hydroxyl radical formation generally increased in all experiments. This indicates that the influence of amplitude and sonication time is independent of gas sparging. Table 1 reflects the reaction rates of 2-HTA formation at varying amplitudes and under different gas sparging routines. Zero-order kinetics was observed in all reactions at varying amplitudes. In almost all experiments conducted with and without gas sparging, the reaction rate constants were directly proportional to the amplitude. That is, the observed rates were proportional to amplitude independent of gas sparging, with the exception of the reaction rate at 38.4 lm(p–p) amplitude without gas sparging, which was
Fig. 8. (a–e) Effects of gas addition to the hydroxyl radical formation as a function of sonication time at varying amplitude (a) 9.6 lm (b) 19.2 lm (c) 28.8 lm (d) 38.4 lm (e) 48 lm. x-no sparging; – with 1 L min 1 air; N – with 1 L min 1 helium. No sparging indicates that the liquid was not saturated with any gas before or during sonication.
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Table 1 Reaction rates of 2-hydroxyterephthalate formation at varying amplitude and gas sparging. Amplitude (lm)
9.6 19.2 28.8 38.4 48
Reaction rate constant (nM/min) No sparging
With 1 L min
91.4 118.0 155.7 151.7 168.4
62.6 86.1 113.3 124.1 133.8
1
air
With 1 L min
1
He
23.1 26.1 31.9 42.9 52.5
slightly lower than the reaction rate observed at 28.8 lm(p–p) amplitude. Without sparging, the observed reaction rates ranged from 91.4 to 168.4 nM min 1 followed by air sparging ranging from 62.6 to 133.8 nM min 1. The lowest reaction rates were observed in helium saturated environments at 23.1–52.5 nM min 1, which is very close to results reported by Hua and Hoffmann [36]. In the same study [36], the reported reaction rate of 0.0699 lM min 1 in oxygen saturated solution at 20.2 kHz matched the results for air sparged solutions seen in this study. 3.3. Batch vs. continuous flow systems Fig. 9 compares hydroxyl radical formation in batch and continuous flow systems at varying ultrasonic energy densities. To best represent batch ultrasonic systems, this figure only shows the results for small batch systems (35 mL) because it resulted in higher OH formation than the large batch systems (100 mL). It is important to note that the batch system exhibited significantly higher hydroxyl radical formation than the continuous flow systems, even at similar energy densities. The relatively lower hydroxyl radical formation in the continuous flow system may be attributed to the significantly lower residence time of the liquid in the ultrasonic chamber. Namely, the contact times (residence time) of the liquid with the horn in the continuous flow systems were in the order of seconds compared to minutes in the batch systems. Yim et al. [37] compared sonochemical degradation of chlorinated hydrocarbons in batch and continuous flow flow systems and obtained comparable results in both systems after 120 min of ultrasonication. Here, this indicates that residence time is a more important factor than energy density in the formation of hydroxyl radical, because both systems (batch and continuous flow) exhibited the same energy densities and frequencies, yet the batch systems yielded higher OH levels. To conclusively compare batch and continuous flow systems in terms of OH formation,
Fig. 10. Hydroxyl radicals to energy ratio as in batch and continuous flow systems as a function of ultrasonic amplitude.
it is recommended that in future studies both systems should be exposed to the same ultrasonic residence times. It is suggested to utilize lower volumetric flow rates for continuous flow systems or shorter sonication times for batch systems to match residence times. Fig. 10 presents the hydroxyl radical formation and energy ratio as a function of ultrasonic amplitude in small and large batch systems, as well as the continuous flow systems. This figure depicts the efficiency of the different systems (batch vs. continuous flow) in terms of producing hydroxyl radical (nmol) per unit energy (kJ). It has been observed that as the amplitude increases, the hydroxyl radical formation decreases, similar to those discussed in Figs. 7 and 9. Furthermore, it can be seen that the small batch systems ranged from 4.2 to 21.5 nmol/kJ, large batch systems were from 5.6 to 57 nmol/kJ while the continuous flow were only from 0.6 to 9.5 nmol/kJ (encircled). When efficiency is solely based on hydroxyl formation per unit energy utilized, the most efficient will be the large batch systems. Similar to the results in Fig. 9, the continuous flow systems provided the lowest 2-HTA to energy ratio. The lower efficiency from the continuous flow systems may be attributed to the significantly lower residence times in the continuous flow systems compared to the batch systems. However, it is important to note that these results were obtained based only on the conditions tested in this study. Despite the lower hydroxyl radical formation in the continuous flow system, there is still potential for scale-up for the continuous flow system. Currently, the Branson ‘‘donut’’ shaped horn or the Sonix™ system has been successfully employed as the only ultrasonic system that is used in large scale liquid processes such as wastewater treatment plants [21,38]. The same ultrasonic horn in the continuous flow system used in this study has been tested in full-scale wastewater treatment plants across Europe, USA and Australia [38]. To utilize this available technology to scale-up the advanced oxidation process via ultrasonication, optimization study of the flowrates and amplitudes in the continuous flow system should be conducted. Also, multiple donut horn design can also be used similar to those found in Kruger and Fiona [21] and Roxburgh et al. [38]. 4. Conclusions
Fig. 9. Formation of hydroxyl radicals in batch and continuous systems as a function of ultrasonic energy densities.
This study explored the effects of batch and continuous flow ultrasonic systems on hydroxyl radical formation using terephthalic acid dosimetry. For batch systems, amplitude and sonication time was directly proportional to the formation of OH. For continuous flow systems, amplitude (less than 6 lm(p–p)) and
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volumetric flow rate were inversely related to OH formation. The comparison of batch and continuous flow systems showed that residence time or sonication time had more impact on hydroxyl radical formation than energy densities. Using various amplitudes and sonication times, the effects of gas sparging on OH formation was also examined. Both helium and air sparging resulted in lower levels of hydroxyl radical formation compared to samples sonicated without gas sparging independent of amplitude and sonication time. Based on the parameters tested on this study, it is concluded that helium and air are not optimal gas choices to enhance hydroxyl radical formation. The effects of the horn geometry is also as important as the energy density and type of system (batch or continuous flow) as studied here. It is recommended that further investigation on this is conducted in the future. Acknowledgements The authors would like to thank Branson Ultrasonics for providing the ultrasonic system used in this study. H. Juretic thanks the Fulbright Program for a doctoral fellowship to study at Iowa State University and the University of California, Irvine. W.J. Cooper thanks NSF CBET Grant 1034555 for partial support. References [1] C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769–776. [2] A. Matilainen, M. Sillanpaa, Removal of natural organic matter from drinking water by advanced oxidation processes, Chemosphere 80 (2010) 351–365. [3] S. Parsons, Advanced Oxidation Processes for Water and Wastewater Treatment, IWA publishing, London, 2004. [4] M. Mohajerani, M. Mehrvar, F. Ein-Mozaffari, Recent achievements in combination of ultrasonolysis and other advanced oxidation processes for wastewater treatment, Int. J. Chem. React. Eng. 8 (2010) (2010) 1–76. [5] S.K. Khanal, M. Montalbo, J.(H.) van Leeuwen, G. Srinivasan, D. Grewell, Ultrasound enhanced glucose release from corn in ethanol plants, Biotechnol. Bioeng. 98 (2007) 978–985. [6] M. Montalbo-Lomboy, L. Johnson, S.K. Khanal, J. van Leeuwen, D. Grewell, Sonication of sugary-2 corn: a potential pretreatment to enhance sugar release, Bioresour. Technol. 101 (2010) 351–358. [7] P. Riesz, T. Kondo, Free radical formation induced by ultrasound and its biological implications, Free Radical Biol. Med. 13 (1992) 247–270. [8] D.Y. Hsieh, M.S. Plesset, Theory of rectified diffusion of mass into gas bubbles, J. Acoust. Soc. Am. 33 (1961) 206–215. [9] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399. [10] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara, M.M. Mdleleni, M. Wong, Acoustic cavitation and its chemical consequences, Philos. Trans. A. Math. Phys. Eng. Sci. 357 (1999) (1999) 335–353. [11] N.N. Mahamuni, Y. Adewuyi, Advanced oxidation processes involving ultrasound for wastewater treatment: a review with emphasis on cost estimation, Ultrason. Sonochem. 17 (2010) 990–1003. [12] B.A. Wols, C.H.M. Hofman-Caris, Review of photochemical reaction constants of organic micropollutants required for UV advanced oxidation processes in water, Water Res. 46 (2012) 2815–2827. [13] K. Ayoub, E.D. van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: A review, J. Hazard. Mater. 178 (2010) 10–28. [14] S. Esplugas, D. Bila, L.G.T. Krause, M. Dezotti, Ozonation and advanced oxidation technologies to remove endocrine disrupting chemicals (EDCs) and pharmaceuticals and personal care products (PPCPs) in water effluents, J. Hazard. Mater. 149 (2007) 631–642.
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