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Comparative Study of Sonocatalytic Enhancement for Removal of Bisphenol A and 17r-Ethinyl Estradiol Namguk Her,† Jong-Sung Park,† Jaekyung Yoon,‡ Jinsik Sohn,§ Sangho Lee,§ and Yeomin Yoon*,|| †
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Department of Chemistry and Environmental Sciences, Korea Army Academy at Young-Cheon, 135-1 Changhari, Kokyungmeon, Young-cheon, Gyeongbuk 770-849, South Korea ‡ New and Renewable Energy Research Division, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, Korea § School of Civil and Environmental Engineering, Kookmin University, Seoul 136-702, South Korea Department of Civil and Environmental Engineering, University of South Carolina, Columbia, South Carolina 29208, United States
bS Supporting Information ABSTRACT: A series of experiments were conducted to compare the effectiveness of various catalysts in the production of H2O2 and in the degradation of endocrine disrupting compounds (EDCs), including bisphenol A (BPA) and 17R-ethinyl estradiol (EE2), in water by sonocatalysis using ultrasonication at 28 kHz with a contact time of 60 min. The catalysts included a stainless steel wire mesh (SSWM), glass beads (GB), TiO2 powder, a Ti-wire mesh (Ti-WM), and an oxidized Ti-wire mesh (TiO2-WM). The most effective catalyst combination for the production of 215.7 μM of H2O2, was the use of SSWM with TiO2 as the cocatalyst. A significantly lower sonochemical reactivity was observed when there was no added catalyst (18.5 μM), SSWM (61.3 μM), and TiO2 alone (134.1 μM). For the given contact time and frequency, sonochemical reactivity was determined from the rate constants for H2O2 production. EDC sonodegradation occurred in the following order: SSWM þ TiO2 (powder) > GB > TiO2 (powder) > SSWM þ GB > TiO2-WM > Ti-WM > SSWM > no catalyst, although the catalyst type, surface area, and amount were varied. Our findings suggest that TiO2-WM can be continuously reused for the catalysis of H2O2 production and the degradation of EDC.
1. INTRODUCTION Recent reports of endocrine-disrupting compounds (EDCs) and pharmaceuticals present in wastewater effluents and surface waters used as drinking water have raised significant concerns by the public and by regulatory agencies.13 Some of the most important and commonly found EDCs include bisphenol A (BPA) and 17R-ethinyl estradiol (EE2). BPA, which is widely used as an important intermediate in the production of epoxy resins, polycarbonate plastics, polysulfone, and certain polyester resins, belongs to the well-known phenolic EDCs.4 Previous studies have shown that BPA leaches out of polycarbonate plastic materials and epoxy resins during autoclaving. This has recently become a matter of interest due to the effects of low doses of BPA on human health, especially in early postnatal exposures.5,6 The synthetic estrogen EE2 is the most active estrogen used for birth control medication, and its toxicity is 1050 times higher than that of 17β-estradiol (a natural sex hormone).7 In aquatic environments, EDCs accumulate in living organisms and negatively affect the reproductive system to exhibit effects such as male fish feminization, sexual disruption, and smoltification.8,9 Recent studies have shown that the sonochemical process is effective in the removal of contaminants from water, because this advanced oxidation process (AOP) enhances or promotes chemical reactions.1013 Previous studies have shown that the sonochemical process is an effective technique for removing EDCs, including bisphenol A and hormones.1420 These studies have described the treatability of EDCs in water by an AOP associated with sonochemistry, which generates hydroxyl radicals in water. r 2011 American Chemical Society
The relationship between the degradation of bisphenol A and the formation of H2O2 was taken into account, correlating this to the radicals that participate in the degradation process.16 The process of sonocatalytic degradation offers significant advantages over conventional remediation methods, in that it is facile and safe to perform and produces only a minimal amount of toxic byproducts.2123 Sonocatalysis using TiO2, H2O2, sand, steel beads, UV/ZnO, and Al2O3 represents a relatively new and efficient technique for degrading contaminants, in contrast to other AOPs such as ozone/H2O2, UV/H2O2, UV/TiO2, and Fenton/photo-Fenton treatments.2428 However, the development of a practical sonocatalytic system has not yet been achieved successfully because many operational parameters must be considered.28 For example, the complexity involved in the separation of the fine catalyst powder from the treated water is a major limitation to the practical use of powdered catalysts in sonic degradation. Various supporting materials such as glass beads, stainless steel, silica, alumina, ceramics, zeolites, and magnesia are commonly used to immobilize powder catalysts for the sonodegradation of contaminants.24,27,2933 Among the various semiconductors employed as supporting materials, the anatase phase of TiO2 is the most preferable material for photocatalysis.27,28,32 Indeed, these previous studies Received: December 3, 2010 Revised: April 19, 2011 Accepted: April 21, 2011 Published: April 21, 2011 6638
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Industrial & Engineering Chemistry Research have indicated that TiO2 catalysts irradiated with ultrasonic sound waves create a synergistic effect that enhances the degradation of contaminants, as a result of the formation of highly reactive free radicals such as OH•, H•, and OH2•. The presence of TiO2 particles in these reactions contributes to an increase in bubble cavitation that promotes the transfer of the generated free radicals to the liquid bulk region as the bubble collapses.34 In other studies, the presence of a heterogeneous catalyst appears to increase the rate of formation of cavitation bubbles, by providing additional nucleation sites, which increase the pyrolysis of H2O molecules and the formation of OH•.35,36 Because highly reactive hydroxyl radicals play a major role in sonochemical reactions, it is important to enhance H2O2 production during sonocatalytic reactions to efficiently degrade organic contaminants such as EDCs. The generation of OH• from H2O2 has been reported for TiO2-coated glass bead sonocatalytic systems;11,27,28,37 however, there are no reports on the combined effects of various catalysts on H2O2 production and the sonocatalytic degradation of EDCs. In this study, the effect of a stainless steel wire mesh (SSWM), glass beads (GB), TiO2 powder, a Tiwire mesh (Ti-WM), and/or a thermally oxidized Ti-WM (TiO2WM) on the sonocatalytic activity at a frequency of 28 kHz was assessed by monitoring H2O2 production and degradation of BPA and EE2 in aqueous sonocatalytic environments.
2. MATERIALS AND METHODS 2.1. Materials. Commercially available stainless steel wire mesh (0.1 mm in diameter) with a pore size of 1 mm was purchased from Ducksoo Inc. (Seoul, South Korea), and a Ti wire mesh (99.6% purity, 0.10.2 mm in diameter) with pore sizes of 0.5 and 1 mm was purchased from Hyundai Titanium Inc. (Seoul, South Korea). Glass beads with a diameter of 0.1 mm were obtained from Goryeo-Ace Inc. (Seoul, South Korea). Anatase TiO2 powder was acquired from Sigma-Aldrich Chemical Co., Inc. (99%) BPA and EE2 were obtained from Sigma-Aldrich Chemical Co., Inc. The solvents (methanol and acetonitrile) were obtained from JT Baker Chemical Co., Inc. and were all of high-performance liquid chromatography (HPLC) grade. Table S1 (Supporting Information) lists the characteristics of BPA and EE2, which were obtained from the SRC PhysProp Database.38 All the sample solutions were prepared with ultrapure deionized (DI) water. 2.2. Sonocatalytic Experiments. A schematic diagram of the experimental setup and the Ti-WM thermal oxidation process is shown in Figure 1. Ultrasonic irradiation of aqueous samples was conducted in a 3000 mL stainless steel bath-type reactor (15 cm length 10 cm width 20 cm height) (Ul-Tech) at a frequency of 28 kHz with an applied power of 200 ( 3 W and at a constant temperature of 20 ( 1 °C. The reactor bath, which was connected to a temperature control unit (Thermo Haake), contained 1000 mL of DI water. TiO2 films were prepared by oxidizing the Ti-WM with oxygen gas at 600 °C for 4 h. The thermal oxidation method used here was described previously.39,40 Experiments were conducted both in the absence and in the presence of the various catalysts (SSWM, GB, TiO2 powder, Ti-WM, and TiO2-WM) at 28 kHz and a pH of 6.8. The SSWM, Ti-WM, and TiO2-WM catalysts were placed horizontally in the center of the reactor. The GB and SSWM beads were placed randomly in solution. The
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Figure 1. Schematic of (a) ultrasound system and (b) Ti-WM thermal oxidation process.
power density applied to the solution was 0.2 W mL1. Sample aliquots of 1.5 mL were withdrawn at set intervals of 10 min over the total irradiation time of 60 min. Most of the values reported are an average of triplicate experimental results. Aqueous KI solution was used to determine the concentration of hydrogen peroxide formed during aqueous sonolysis.41 Briefly, a sample of 1.5 mL was collected, and 0.75 mL of 0.1 M potassium hydrogen phthalate was then added to the sample. An iodide reagent (0.75 mL) (0.4 M potassium iodide, 0.06 M NaOH, and 1.0 104 M ammonium molybdate) was then added at t = 0 min. The solution mixtures were allowed to stand for 2 min, after which the absorbance at 350 nm was measured using an ultraviolet spectrophotometer (Hewlett-Packard, Germany). 2.3. Analysis Using HPLC-MS/MS and HPLC-Fluorescence Spectroscopy. The concentration of BPA was determined using HPLC-mass spectrometry (MS/MS) on the Agilent Technologies 6410 LC/MS system (Santa Clara, CA, U.S.A.) equipped with an electrospray ionization (ESI) apparatus, employing a C18 reverse phase column (150 mm 4.6 mm, 5 μm) (Agilent Technologies). The concentration of EE2 was determined by HPLC-fluorescence spectroscopy using the Agilent Technologies 1200 Series instrument. HPLC-fluorescence detection was carried out using a fluorescence detector at an excitation wavelength of 280 nm and an emission wavelength of 310 nm to identify EE2. The mobile-phase solvent profile for both HPLC methods was 30% DI water and 70% methanol for 10 min at a constant flow rate of 1 mL min1 with a sample injection volume of 10 μL. Chromatographically separated samples for HPLCMS/MS were analyzed under the following conditions: ESI negative ionization mode; drying gas flow, 10 mL min1; nebulizer pressure, 345 kPa; drying gas temperature, 350 °C; fragmentor voltage, 130 V; collision energy, 15 V; precursor ion, 227 m/z; product ion, 212 and 133 m/z. Each eluted compound’s concentration was determined against an external calibration curve with five different concentrations ranging from 0.01 to 1 μM prepared in DI water. Calibration standards of 0.05 μM were run between approximately every 10 samples. The method detection limits were 0.13 nM for BPA by HPLC-MS/MS and 1.29 nM for EE2 by HPLC-fluorescence. 6639
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Figure 2. Comparison of (a) H2O2 production and (b) zero-order rate constants in response to the addition of SSWM and GB. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; SSWM pore size = 1 mm; SSWM size =15 cm (L) 10 cm (W); GB size = 0.1 mm; GB amount = 25 g (b, no addition; O, SSWM; 1, GB; Δ, SSWM þ GB).
2.4. Luminol Test. The sonochemiluminescence (SCL) image was captured with a digital camera. The luminol fluorescence test was performed to observe the sonochemical reaction zone in the presence and absence of various catalysts (SSWM, GB, Ti-WM, and TiO2-WM). Luminescence photographs were taken in the absence of BPA and EE2. In addition, luminescence was checked in the presence of BPA and EE2. The aqueous luminol solution was prepared by mixing 100 mM NaOH and 2 mM luminol (3aminophthalyl hydrazide). Luminol reacts with HO• radicals generated in the cavitation bubbles to produce aminophthalate anions that exhibit blue fluorescence when intense ultrasound propagates through the luminol solution;42 that is, the luminol exhibits SCL. Therefore, it can be interpreted that the light emission zone is the sonochemical reaction zone where the reaction of luminal with HO• occurs.
3. RESULTS AND DISCUSSION 3.1. Effect of Stainless Wire Mesh and Inert Glass Beads. To investigate the effect of solid catalysts on ultrasonic reactivity, SSWM (area, 150 cm2; pore size, 1 mm), GB (diameter, 0.1 mm; amount, 25 g), and SSWM (150 cm2) with GB (25 g) were added to aqueous samples at a pH of 6.8 and investigated by using ultrasound with a frequency of 28 kHz. In our previous study, a relatively high sonocatalytic reactivity, based on H2O2 production, was obtained in the presence of 25 g of GB among various doses at a frequency of 28 kHz.43 As shown in Figure 2, the most effective conditions (based on H2O2 production of 159.8 μM)
Figure 3. Comparison of sonodegradation for (a) BPA and (b) EE2 in response to the addition of SSWM and GB. (c) Comparison of pseudofirst-order rate constants. Sonication conditions: Co = 1 μM; 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; SSWM pore size = 1 mm; SSWM size = 15 cm (L) 10 cm (W); GB size = 0.1 mm; GB amount = 25 g (b, no addition; O, SSWM; 1, GB; Δ, SSWM þ GB).
were found to be the addition of GB with a contact time of 60 min. A significantly lower sonochemical reactivity was observed when there was no catalyst present (18.5 μM), SSWM (61.3 μM), and SSWMþGB (100.6 μM). It was assumed that the presence of inert solid glass-beads would increase sonochemical reactivity, because cavitation increases in the presence of solid catalysts due to the collapse and rebound of cavitation bubbles near the solid boundary.44 These results suggest that the combined effect of SSWM and GB is less effective in enhancing H2O2 production than GB alone. It is well-known that acoustic cavitation generates many transient hot spots, with local high temperatures and high pressures for short periods of time; this causes the sonolysis of H2O2 molecules, which in turn results in the production of radicals such as H•, HO•, and HOO•.45 6640
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Figure 4. Photographs of sonochemiluminescence in the addition of SSWM and GB: (a) no addition, (b) SSWM, (c) GB, and (d) SSWM þ GB. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; SSWM pore size = 1 mm; SSWM size = 15 cm (L) 10 cm (W); GB size = 0.1 mm; GB amount = 25 g.
A previous study reported that cavitation bubble collapse at solid surfaces adopted four different bubble geometries, namely, toroidal, spherical, symmetric, and asymmetric (via a microjet effect).44 In our study, at the low frequency of 28 kHz, the time available during the growth phase of the cavitation bubbles is expected to be greater in the presence of GB, compared to treatments with no catalyst, SSWM, and SSWM þ GB treatments. Thus, an increase in H2O2 production should occur in response to increased amounts of GB. To directly determine the effect of SSWM and GB addition on sonodegradation, we performed ultrasonic irradiation of two EDC compounds including BPA and EE2 in aqueous solution under the same experimental conditions as outlined in Figure 2. The ultrasound-assisted degradation of the target compounds in the presence of different catalysts is shown in Figure 3. The best sonochemical degradation of the target compounds in aqueous solution occurs with the addition of GB. The percentage degradation of the compounds (BPA/EE2) follows the order: GB (>99%/>99%) > SSWM þ GB (97.3%/97.4%) > SSWM (76.6%/82.5%) > no catalyst (53.9%/60.5%) at a contact time of 60 min. This occurs because the presence of GBs increases the number of free radicals in the system, as shown in Figure 2. Thus, we conclude that the degradation of both BPA and EE2 is primarily caused by HO• radical attack. As previously reported, the mechanism for sonochemical degradation of organic contaminants involves the production of free radicals and their subsequent attack on the pollutant species.46,47 Statistical analysis of the experimental data revealed that BPA and EE2 degradation follows pseudo-first-order kinetics. The degradation rate constants are shown in Figure 3c. It was observed that in the presence of the catalysts, the degradation rate of EE2 (0.0150.062 min1) was slightly faster than that of BPA (0.0120.098 min1). In general, the ultrasonic degradation of organic compounds is a function of their physicochemical properties, such as the octanolwater partition coefficient (KOW), water solubility (SW), vapor pressure (VP), and Henry’s law constant (KH). These parameter values, listed in Table S1 (Supporting Information) for BPA and EE2, indicate their relatively strong hydrophobicity (log Kow = 3.3 for BPA and 3.7 for EE2) and very low volatility. This result reflects the
Figure 5. Comparison of (a) H2O2 production and (b) zero-order rate constants in response to the addition of SSWM and TiO2. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; SSWM pore size = 1 mm; SSWM size = 15 cm (L) 10 cm (W); TiO2 (powder) amount = 100 mg (b, no addition; O, SSWM; 1, TiO2; Δ, SSWM þ TiO2).
physicochemical properties associated with the EDC compounds’ hydrophobicity at the location that HO• attacks a constituent aromatic ring. A previous study showed that ultrasonic treatment of BPA solution is associated with hydrogen peroxide, nitrite, and nitrate ion production.18,19 A separate study showed that the hydrophobicity of the compounds significantly affected their accumulation at the gasliquid interface of the bubbles and that it was the most important factor for the sonochemical degradation of aromatic compounds.12 The sonochemical reaction probably occurs in the interfacial region where moderate temperature (∼2000 K) and pressure are produced on cavity implosion. Therefore, the accumulated BPA and EE2 at the gasliquid interface are effectively degraded by the reaction of HO• and/or thermal degradation during cavity collapse. SCL identified the sonochemical reaction zone and reactivity in the presence of each catalyst. Figure 4 shows the photographs of SCL emission resulting from the presence of different catalysts. It is recognized that the SCL zone follows the order GB > SSWM þ GB > SSWM > no catalyst, at a contact time of 60 min, which is consistent with the H2O2 production rates. SSWM and/or GB, when added to the sonicated liquid, directly result in an increase in HO•, which is associated with an increase in the number of active collapsing bubbles resulting from sonochemical reaction. This observation indicates that the addition of these catalysts may enhance the sonochemical reaction rate. Figure S1 (Supporting Information) summarizes the SCL emission photographs of control experiments in the absence and presence of BPA and EE2, and inter presence of GB (0.1 mm and 25 g). The results show somewhat similar sonochemical reaction zones in both conditions. To confirm the more accurate sonochemical reaction zone, it would be necessary that the spatial variation of the H2O2 production rate is analyzed in the absence and presence of EDCs. 6641
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Figure 6. Degradation process during sonochemical reaction with TiO2.
3.2. Effect of TiO2 Powder. TiO2 powder was tested to compare the combined effect of SSWM, TiO2, and SSWM þ TiO2 on H2O2 production during 60 min of sonocatalytic reactivity in the presence of a constant amount of TiO2 in a solution of 100 mg L1. As shown in Figure 5, the production of H2O2 using sonocatalysis in conjunction with the addition of SSWM þ TiO2 (215.7 μM) was more effective than using no catalyst at all (18.5 μM) and was more effective than either catalyst by itself was (SSWM 61.3 μM, TiO2 134.1 μM). Sonochemical reactivity, based on the rate constants for H2O2 production, varied depending on reaction stages in the presence of TiO2 powder. The initial stage between 0 and 10 min showed significantly higher rate constants (0.415.7 μM1 min1) than the second stage between 10 and 60 min (0.31.18 μM1 min1). These results suggest that the combined effect of SSWM and TiO2 is a practical way to enhance H2O2 production. The degradation process by sonochemical reaction with TiO2 powder is outlined in Figure 6, and the detailed mechanisms of this process have been discussed previously.48,49 It is well-known that conduction band electrons (e) and valence band holes (hþ) are generated when aqueous TiO2 suspensions are irradiated with light of energy greater than its band gap energy. The photogenerated electrons can reduce organic contaminants or react with electron acceptors such as O2 adsorbed on the Ti(III) surface or dissolved in water, reducing the oxygen molecule to form the superoxide radical anion O2•.48 The photogenerated holes can then oxidize organic molecules to form Rþ or oxidize OH or H2O to form OH• radicals. Sonochemical reaction with TiO2 results in increased cavitation due to the heterogeneous nucleation of bubbles, which leads to the induction of hot spots in solution and the formation of HO• on the TiO2 surface during the sonochemical reaction. Sonochemical reactions depend on the average bubble temperature and the number of active bubbles, whereas sonoluminescence intensity depends on the maximum bubble temperature of the collapsing bubbles and the number of active bubbles.50 The combined effect of SSWM þ TiO2 sonocatalysis can be explained by several reasons. The reasons include (i) the somewhat positive character of the semiconductor surface at pH < ∼7,51 at which the attractive forces between water molecules and the positively charged solid surfaces enhance sonochemical reactivity; (ii) the fact that the mass transfer processes and chemical reactivity at those catalyst surfaces are enhanced by the turbulent flow conditions;11 and (iii) the advantages of SSWM þ TiO2 attributed to an increased number of active reaction sites.11 3.3. Effect of Ti-WM and TiO2-WM. As previously described, the use of TiO2 powder has a practical limitation in that the fine catalyst powder needs to be separated from water. We compared SSWM, Ti-WM, and TiO2-WM (thermally oxidized with O2 gas in the absence of TiO2 powder) systems on the basis of the
Figure 7. Comparison of (a) H2O2 production and (b) zero-order rate constants in response to the addition of SSWM, Ti-WM, and TiO2-WM. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; WM pore size = 0.5 mm; WM size = 15 cm (L) 10 cm (W) (b, no addition; O, SSWM; 1, Ti-WM; Δ, TiO2-WM).
amount of H2O2 they produced. As shown in Figure 7, at 28 kHz and for a reaction time of 60 min, the H2O2 production amount and rate constants of the systems decreased in the following order: TiO2-WM (76.5 μM/4.0 μM1 min1) > Ti-WM (67.8 μM/3.6 μM1 min1) > SSWM (53.4 μM/2.8 μM1 min1) > no catalyst present (18.5 μM/1.0 μM1 min1). These findings suggest that the presence of oxidized Ti-WM improves H2O2 production. Although TiO2 has a large bandgap (3.2 eV), the main drawback of its use is the charge carrier recombination that occurs within nanoseconds. These results also indicate that the TiO2-WM coupled semiconductor system improves the lifetime of charge carriers by preventing this particular limitation. One proposed strategy to enhance the rate of sonochemical reactivity is to use an adsorbent as the sonocatalyst support, to increase the concentration of organic contaminants around the catalysts.28 Unfortunately, TiO2 has poor affinity for adsorption of organic compounds. Furthermore, contaminant organic compounds are usually present at very low concentration, and therefore the sonodegradation rates are typically very low. The observed increase in H2O2 production for the TiO2-WM system suggests that high degradation rates can be achieved even for trace target organic contaminants. The sonocatalytic degradation of BPA and EE2 in aqueous solution is illustrated in Figure 8. After 60 min of sonication, 5489% of BPA and 5991% of EE2 were degraded depending on the addition of different catalyst types, including SSWM, Ti þ WM, and TiO2 þ WM. The results show that sonocatalysis varied in its effectiveness in the degradation of BPA and EE2 as 6642
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Figure 9. Photographs of sonochemiluminescence in the presence of SSWM, Ti-WM, and TiO2-WM: (a) no addition, (b) SSWM, (c) TiWM, and (d) TiO2-WM. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; WM pore size = 0.5 mm; WM size = 15 cm (L) 10 cm (W).
Figure 8. Comparison of sonodegradation for (a) BPA and (b) EE2 in response to the addition of SSWM, Ti-WM, and TiO2-WM. (c) Comparison of pseudo-first-order rate constants. Sonication conditions: Co = 1 μM; 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; WM pore size = 0.5 mm; WM size = 15 cm (L) 10 cm (W) (b, no addition; O, SSWM; 1, Ti-WM; Δ, TiO2-WM).
follows (BPA/EE2%): TiO2 þ WM (89/91%) > Ti þ WM (81/ 84%) > SSWM (77/82%) > no catalyst present (54/59%). Figure 8c shows the respective degradation rate constants. We observed that the degradation rate of EE2 (0.0150.038 min1) was slightly faster than that of BPA (0.0120.035 min1). As described previously, the low values of KH for BPA and EE2 imply that insignificant volatilization occurs within the cavity during sonolysis. Therefore, the pyrolysis reaction mechanism inside the cavity is unimportant. The pKa values (acid ionization constant) of the compounds are less than 10, as shown in Table S1 (Supporting Information). At low solution pH, they would exist in nonionic molecular form and exhibit greater larger hydrophobicity. Due to their low solubility associated with high hydrophobicity, both BPA and EE2 tend to diffuse into the cavity-liquid interface. The supercritical environment produced
Figure 10. Comparison of (a) H2O2 production and (b) zero-order rate constants depending on the reuse of TiO2-WM. Sonication conditions: 0.2 W mL1; 20 ( 1 °C; pH 6.8; 28 kHz; WM pore size = 3 mm; WM size = 15 cm (L) 10 cm (W) (b, 1st; O, 2nd; 1, 3rd; Δ, 4th).
in this interfacial region10 increases BPA and EE2 solubility. Therefore, the reaction presumably occurs within the interfacial region, where high temperature and pressure are produced 6643
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Industrial & Engineering Chemistry Research during cavity implosion. This favors thermal degradation or supercritical oxidation in the interfacial region during cavity collapse. In addition, our results show that oxidative degradation by the strong radicals in or near the interface is also very important in the presence of the catalysts. The sonocatalytic degradation of both BPA and EE2 increases linearly with increasing H2O2 production; the EDC degradation rates and H2O2 production rates show strong correlation with r2 > 0.96. Figure 9 shows the photographs of SCL emission; from these photographs, the cavitation field under the various sonocatalytic conditions can be observed. SCL confirmed both the sonochemical reaction zone and the reactivity in the presence of each catalyst. Luminol mapping relies on luminol emission resulting from the capture of HO• radical, produced by sonolysis during cavitation collapse. Thus, luminescence is a good indicator of where chemically active cavitation bubbles occur in a reactor. As expected on the basis of the H2O2 production shown in Figure 7, the SCL results visually support that the sonocatalytic reactivity is the highest in the presence of TiO2 and WM together, indicating the largest population of active bubbles (Figure 9). Most catalysts used in chemical investigations and by the chemical industry are reclaimed and recycled by a suitable and simple treatment. In aqueous solution, TiO2 powder catalyst settles and is easily separated from the treated solution. Unlike TiO2 powder, no treatment is necessary for the reuse of the TiO2-WM. Figure 10 shows the relationship between the degrees of H2O2 production and the number of times TiO2-WM was used over a 30 min contact time. Very similar H2O2 production amounts/rate constants (31.234.4 μM/1.051.11 μM1 min1) were observed over four replicates, suggesting that the reused TiO2-WM can be used continuously to promote H2O2 production. However, further research is necessary to enhance H2O2 production by modifying the Ti-WM oxidation conditions.
4. CONCLUSIONS In this study, several catalysts, including stainless steel wire mesh, glass beads, TiO2 powder, Ti-wire mesh, and oxidized Tiwire mesh were tested under ultrasonic irradiation for their effects on H2O2 production and on the degradation of BPA and EE2 (EDCs used in this study). The results clearly indicate the importance of the material used to construct the support system in determining H2O2 production and EDC sonodegradation rates. Sonochemical reactivity and the EDC degradation rates followed the decreasing order of catalyst systems: GB > TiO2 (powder) > SSWM þ GB > TiO2-WM > TI-WM > SSWM > no catalyst. Although SSWM alone had an insignificant effect on H2O2 production and EDC degradation in response to ultrasonic irradiation, the cocatalytic effect of SSWM and TiO2 powder together increased the sonochemical reactivity. The SCL photographs shown here confirm that the cavitation fields produced by a 28 kHz sonicator are significantly different depending on the presence of various catalysts. While a more comprehensive assessment is necessary, these results provide relevant information that can be applied to the optimization of sonochemical reactors for practical applications in water and wastewater treatment. ’ ASSOCIATED CONTENT
bS
Supporting Information. Table of physicochemical properties. Sonochemiluminescence photographs. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
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