Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A Myunghee Lim a,b, Younggyu Son c,⇑, Jeehyeong Khim b,⇑ a
Future Environmental Research Center, Korea Institute of Toxicology, Jinju 660-844, Republic of Korea School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-701, Republic of Korea c Department of Environmental Engineering, Kumoh National Institute of Technology, Gumi 730-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 November 2013 Received in revised form 21 March 2014 Accepted 22 March 2014 Available online xxxx Keywords: Cavitation Hydrogen peroxide Phenol Bisphenol A Sonolysis
a b s t r a c t This report describes the effects of H2O2 concentration (0.01, 0.1, 1, and 10 mM) on the sonochemical degradation of phenol and bisphenol A (BPA) using an ultrasonic source of 35 kHz and 0.08 W/mL. The concentration of the target pollutants (phenol or BPA), total organic carbon (TOC), and H2O2 were monitored for each input concentration of H2O2. The effects of H2O2 on the sonochemical degradation of phenol was more significant than that of BPA because phenol has a high solubility and low octanol–water partition coefficient (Kow) value and is subsequently very likely to remain in the aqueous phase, giving it a greater probability of reacting with H2O2. The removal of TOC was also enhanced by the addition of H2O2. Some intermediates of BPA have a high Kow value and subsequently have a greater probability of pyrolyzing by the high temperatures and pressures inside of cavitation bubbles. Thus the removal efficiency of TOC in BPA was higher than that of phenol. The removal efficiencies of TOC were lower than the degradation efficiencies of phenol and BPA. This result is due to the fact that some intermediates cannot readily degrade during the sonochemical reaction. The H2O2 concentration decreased but was not completely consumed during the sonochemical degradation of pollutants. The initial H2O2 concentration and the physical/ chemical characteristics of pollutants were considered to be important factors in determining the formation rate of the H2O2. When high concentration of H2O2 was added to the solution, the formation rates were relatively low compared to when low concentrations of H2O2 were used. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The sonication of aqueous solutions causes cavitation events that involve the formation, growth, and collapse of bubbles [1]. The collapse of these cavitation bubbles results in localized high temperatures (5000 K) and pressures (1000 atm) [1–4]. These extreme conditions can induce the cleavage of dissolved oxygen and water molecules to produce radicals (O, OH, and H). The main degradation mechanisms of ultrasound are pyrolysis and radical reactions [5]. The degradation mechanism of organic compounds by ultrasound was determined by the properties of the target materials [2,4,5]. The surfaces of the cavitation bubbles are hydrophobic; therefore, hydrophobic compounds can enter the bubbles and be pyrolyzed directly by the high temperatures and pressures. On the other hand, hydrophilic compounds barely diffuse into the bubbles and are degraded indirectly by radicals in the bulk solution [2,5,6]. ⇑ Corresponding authors. Tel.: +82 54 478 7637 (Y. Son), +82 2 928 7656 (J. Khim). E-mail addresses: [email protected] (Y. Son), [email protected] (J. Khim).
A variety of parameters (combination of UV system [7–9], temperature [10–12], pH [8,9,13–15], ionic strength [13,14], initial concentration [10,12,13–16], saturating gas [8,10,15,17–22], frequency [12,15,17,19,21,22,24–26], additives [7,9,10,16,18–21,23,26–33], power [9,11,14,30], reactor type [18]) have been examined in sonochemical processes. It is believed that hydrogen peroxide is one of the most effective additives to enhance the sonochemical degradation of aqueous pollutants [14,18,19,27,31]. Under ultrasound irradiation hydrogen peroxide can dissociate into two hydroxyl radicals and acts as a secondary source of hydroxyl radicals [16,26,27,31]. Without adding hydrogen peroxide, hydrogen peroxide can be generated by the recombination of two hydroxyl radicals generated by cavitation in aqueous solution and linearly accumulated [16,20,22,28]. It is not clear whether generated hydrogen peroxide can be the secondary source of hydroxyl radicals as added hydrogen peroxide acted because the concentration of added hydrogen peroxide drastically decreases while generated hydrogen peroxide is linearly accumulated. It was found that the amount/concentration of added hydrogen peroxide concentration was an important parameter in the
http://dx.doi.org/10.1016/j.ultsonch.2014.03.021 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
ultrasound/H2O2 system (particularly when compared to the concentration of the target compounds). The presence of an optimum amount of H2O2 for the sonochemical degradation of the target compounds was verified, but it was difficult to determine the optimum concentration for each compound [2,4–7,13,14,16, 18–20,22,23,27–29,31,34–37] because the experimental conditions varied widely and a very limited range of H2O2 concentrations was used in most previous studies. In this paper, various concentrations of H2O2 were tested for the enhancement of the sonochemical degradation of phenol and BPA. The degradation rate constants of phenol and BPA were determined under a variety of experimental conditions. The total organic carbon (TOC) concentration was also measured to estimate the degree of mineralization. Furthermore, the formation rate of H2O2 was measured for each concentration of H2O2 in the presence and in the absence of the target pollutants. 2. Materials and methods 2.1. Chemicals Highly pure phenol (99%, Samchun, South Korea) and bisphenol A (99+% purity, Sigma–Aldrich, USA) were used in this investigation. The H2O2 solutions were diluted from a 34.5% stock solution of H2O2 (Junsei, Japan). Potassium biphthalate (Junsei, Japan), potassium iodide (Samchun, Korea), sodium hydroxide (Junsei, Japan), and ammonium molybdate (Junsei, Japan) were used to determine the H2O2 concentration. Methanol and acetic acid (HPLC grade, J. T. Baker, USA) were used to determine the phenol and BPA concentrations. The initial concentration of phenol and BPA was 0.044 mM. Four different concentrations of hydrogen peroxide (0.01, 0.1, 1, and 10 mM) were evaluated in this investigation. 2.2. Ultrasonic reactor The sonochemical reaction was carried out using a 1000 mL double-walled Pyrex reactor containing 500 mL of the solution containing phenol and BPA. Fig. 1 displays a schematic diagram of the reactor. The water was introduced to the wall of the reactor using a cooling water system and the solution temperature was maintained at 20 ± 2 °C. The mixing system was installed inside the reactor to homogenize the H2O2 and the solution containing phenol and BPA. The reactor was then fitted with a cup-horn type transducer using a Teflon tape. The Frexonic system (Mirae Ultrasonics Tech., Korea) operates at 35 kHz and consisted of a single piezoelectric transducer (PZT, Tamura corp.). The diameter of the transducer was 10 cm. The aqueous solution was aerated using an air diffuser for 30 min prior to the experiment. The delivered ultrasonic energy
was measured by a power meter (METEX; M-4660M) to be 40 ± 3 W. The input power was calculated using the calorimetric method and the calorimetric power of the reaction solution is shown in Eq. (1):
Power ¼
dT cp M dt
ð1Þ
where dT/dt is the rise in temperature of the reaction volume as a function of the sonication time, cp is the heat capacity of water (4.2 J/g K), and M is the mass of water (g) [11]. The calorimetric power was 21.0 W in this study. 2.3. Analytical procedure A 10 mL sample was taken every 10 min for 60 min of the sonochemical reaction for the analysis of the target compounds (phenol, BPA), TOC, and H2O2 concentration. H2O2 was added to the phenol and BPA solution twice during the course of the reaction (at the beginning of the reaction and at 30 min). The phenol and BPA concentrations were measured by HPLC (Varian Prostar HPLC system) and the TOC concentration was measured using a TOC analyzer (SEIVERS 5310C laboratory analyzer, GE, US). The H2O2 concentration was determined using the iodometric method [17]. An iodide ion (I) reacts with H2O2 to form a triiodide ion (I 3 ) which absorbs at 352 nm when using a UV spectrophotometer (SPECORD 40, Analyticjena). Sample aliquots (2.0 mL) from each experiment were mixed in a quartz cuvette containing 0.75 mL of 0.10 M potassium biphthalate and 0.75 mL of a solution containing 0.4 M potassium iodide, 0.06 M sodium hydroxide, and 104 M ammonium molybdate. The mixed solution (total volume: 3.5 mL) was allowed to stand for 2 min before the absorbance was measured. 3. Results and discussion 3.1. Effects of H2O2 on the sonochemical degradation of phenol and BPA When an aqueous solution is ultrasonically irradiated, hydroxyl radicals are produced by cavitation (Eq. (2)). The hydroxyl radical exhibits a high oxidation potential and can directly degrade organic pollutants [4–6,10]; however, hydroxyl radicals has a very short lifetime and tend to combine with one another to form H2O2 (Eq. (4)). The following reactions (Eqs. (1)–(11)) occur under sonolysis [7–9,12,16,22,24,27,36,37].
H2 O ! H þ OH
ð2Þ
H2 O2 ! 2HO
ð3Þ
2HO ! H2 O2
ð4Þ
Fig. 1. Schematic diagram of the ultrasonic reactor.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
ð6Þ
O þ H2 O ! 2 OH
H þ H2 O ! OH þ H2
ð7Þ
H þ O2 ! OOH þ O
ð8Þ
2 OOH ! H2 O2 þ O2
ð9Þ
H2 O2 þ OH ! H2 O þ
HO2
ð10Þ
HO2 þ H2 O2 ! H2 O þ OH þ O2
ð11Þ 1
The reported formation rates of H2O2 (0.02–9.7 lM min ) in previous research varied widely in terms of frequency, power, and reaction volume [12,24–26,30,36–45]. Table 1 shows the formation rate of H2O2 during the sonochemical reaction in the absence of target materials as well as the range of formation rates at low frequencies. In this investigation, 0.58 lM min1 of H2O2 was produced which is within the range reported in previous studies. These H2O2 formation rates were slightly lower than those in the presence of target materials, as shown by previous research as well as this investigation, which will be discussed later in this paper. Four different concentrations of H2O2 (0.01, 0.1, 1, and 10 mM) were used in this experiment and the H2O2 was added to the phenol and BPA solution twice (at the beginning and after 30 min) within the 60 min reaction time. Therefore, the total input concentration of H2O2 was 0.02, 0.2, 2, or 20 mM, respectively, within the 60 min reaction time. Bremner et al. calculated the stoichiometric value of H2O2 for the complete degradation of phenol [16]. Based on Eqs. (12) and (13), 14 mol and 37 mol of H2O2 were required to completely degrade phenol and BPA, respectively. In the present study, 0.616 and 1.584 mM of H2O2 were required for the complete oxidation of 0.044 mM phenol and BPA.
C6 H6 O þ 14H2 O2 ! 6CO2 þ 17H2 O
ð12Þ
C15 H16 O þ 37H2 O2 ! 15CO2 þ 45H2 O
ð13Þ
1.0
Removal efficiency of phenol (C/C0)
ð5Þ
0.9
0.8
0.7 H2O2 0 mM
0.6
H2O2 0.01 mM H2O2 0.1 mM
0.5
H2O2 1 mM H2O2 10 mM
0.4 0
10
20
30
40
50
60
50
60
Irradiation time (min)
(a) 1.0
Removal efficiency of BPA (C/C0)
O2 ! 2O
0.9
0.8
0.7 H2O2 0 mM H2O2 0.01 mM
0.6
H2O2 0.1 mM H2O2 1 mM
0.5
H2O2 10 mM 0.4 0
10
20
30
40
Irradiation time (min)
(b) However, the stoichiometric amount of H2O2 cannot be used as a parameter in the complete degradation of organic compounds. Although H2O2 has a high oxidation potential (1.8 V), it is not capable of degrading many pollutants. In most cases, H2O2 was used in conjunction with solid catalysts or other treatment technologies [2,4–7,13,14,16,18–20,23,26,29,31,34–36]. Fig. 2 displays the degradation of phenol and BPA under the ultrasound/H2O2 system in the presence of different concentrations of H2O2. The degradation of phenol and BPA increased in the presence of H2O2. The degradation ratio of phenol was 12% in the absence of H2O2 during 60 min of sonication while at 20 mM of H2O2 the phenol degraded up to 48% (Fig. 2(a)). For BPA, the degradation ratio was 30% under ultrasonic irradiation in the absence of H2O2 and increased to 47% in the presence of 20 mM H2O2. There were no significant effects at high concentrations of H2O2 since there was only a 10% increase in the phenol degradation at the
Fig. 2. Effects of H2O2 concentration on the sonochemical degradation of phenol (a) and BPA (b). Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM, d: ultrasonic irradiation, s: ultrasonic irradiation + 0.01 mM H2O2, j: ultrasonic irradiation + 0.1 mM H2O2, h: ultrasonic irradiation + 1 mM H2O2, N: ultrasonic irradiation + 10 mM H2O2).
highest concentration (20 mM) of H2O2 compared to the lowest concentration (0.02 mM). In previous studies the effects of H2O2 in the presence of various organic pollutants such as trichloroethylene, 1,3-dichloro-2-propanol, o-chlorophenol, trihalomethanes, oxalic acid, phenol, alachlor, cyanide, and p-chlorobenzoic acid were investigated [2,4–7,10, 13,14,16,18–20,23,27–29,31,34–37]. The effects of H2O2 were dependent on the properties of the target compounds [2,4]. In particular, the addition of H2O2 had a significant influence on the degradation of non-volatile and
Table 1 Formation rate of H2O2 during sonochemical reactions from previous investigations. H2O2 formation (lM/min)
Frequency (kHz)
Power (W)
Solution volume (mL)
Refs.
0.7 3.5 0.02–0.06 4.7 1.5–3.6 0.58
20 20 20 20 20 35
30 125 12.7–160.7 19 30–80 40
300 50 400 20 50 500
[25] [32] [36] [37] [45] This study
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
hydrophilic compounds. Alternatively, the degradation of volatile compounds was not affected by the presence of H2O2 [2,4,27]. Our results agreed well with these previous results. The sonochemical degradation rate of BPA was higher than phenol because BPA is more hydrophobic and volatile than phenol [46,47]. Therefore, BPA has a greater probability of getting close to the surface of a cavitation bubble and can easily be degraded by hydroxyl radicals. Alternatively, the effects of H2O2 on the sonochemical degradation of phenol were more significant than BPA. Phenol has high solubility and low log Kow and therefore has a greater probability of reacting with H2O2 in aqueous solutions. H2O2 has a low Kow (1.57 at 25 °C) compared to phenol and BPA (Table 2) and subsequently remained in the aqueous phase. Fig. 3 displays the TOC degradation of phenol and BPA in solution as a function of H2O2 concentration. The degradation of TOC corresponds to the mineralization of organic pollutants, and the degradation of TOC through inorganic carbon (such as CO2) was also quite significant. Therefore, some researchers used the reduction of TOC as the key factor in evaluating the effectiveness of these processes [9,40,48]. In this research, TOC degradation was considered to be an effect of H2O2. The TOC degradation of phenol increased by approximately 8.5% in the presence of 0.01 mM H2O2 compared to the absence of H2O2. The effects of H2O2 concentration in the degradation of TOC in the phenol solution were not significant. There was only a 5% increase in TOC degradation when using 10 mM H2O2 compared to 1 mM H2O2. The degradation ratio of phenol was nearly 50% using 20 mM H2O2, while the TOC degradation of the phenol solution was only 25% (Figs. 2a and 3). These results imply that some intermediates cannot completely degrade during the sonochemical reaction (in our investigation, the sonochemical reaction was carried out for 1 h) [13,20]. According to previous research, the representative intermediates of phenol during the sonolysis reaction were hydroquinone, benzoquinone, and catechol [21,23,24,34,49]. The log Kow value of the intermediates were 0.59 (hydroquinone), 0.2 (benzoquinone), and 0.88 (catechol). These values were smaller than the log Kow of phenol, which confirmed that the compounds preferred to remain in the aqueous solution. These compounds further degraded into more hydrophilic compounds such as formic and oxalic acids [20,34]. It was determined that the degradation rate of TOC was slower than that of phenol. For BPA, the TOC degradation continuously increased as the concentration of H2O2 increased. However, the enhancement of TOC degradation was not significant when 1 or 10 mM H2O2 was added. As seen in the phenol experiments, the BPA concentration decreased by 48% within 60 min of ultrasonic irradiation, while the TOC concentration remained at 65%. Some intermediates reported by previous researchers such as 3-hydroxybisphenol A, 2-(4-hydroxyphenol)-2-propanol, 4-(1-methylethenyl)phenol, 4-hydroxyacetophenone, benzoquinone, phenol, catechol, and hydroquinone were also present [27,40,49]. Among these intermediates, 3-hydroxybisphenol A and 4-(1-methylethenyl)phenol exhibited similar log Kow values (3.16 and 2.9, respectively). These
Mineralization (TOC removal) efficiency (%)
4
40 Phenol BPA 30
20
10
0 0 mM
0.01 mM
0.1 mM
1 mM
10 mM
The concentration of hydrogen peroxide added Fig. 3. Degradation of TOC content in phenol and BPA solutions as a function of H2O2 concentration. Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM).
compounds were relatively degradable while all other intermediates have low log Kow values below 1.5. Therefore, the degradation ratio of TOC in the BPA solution was higher than that of phenol and the impact of H2O2 on the TOC degradation was quite significant in the BPA solution. The impact of H2O2 decreased as the concentration of H2O2 increased. Although the degradation rates of TOC increased as concentration of H2O2 increased, it was not significant compared to the input concentrations of H2O2 in both solutions. This result was caused by H2O2 acting as a hydroxyl radical scavenger during the sonolysis reaction, as described by Eq. (9) [6,7,16,18,20,34–37]. This reaction may consume H2O2 as well as hydroxyl radicals and was subsequently a non-productive reaction [50]. Therefore, the determination of the optimum concentration of H2O2 was crucial. Table 3 represents the calculated molar ratios (target compounds versus H2O2) used in previous investigations. The molar ratios in previous studies varied by up to a factor of 200,000 [4,5,7,23,26,28,29,31,36]. The reported values were in the range of approximately 1:6.8–228.3 (target material: H2O2) and were also affected by the target materials (initial concentration and properties) as well as the experimental conditions (frequency and power) [18,28,35,38]. Therefore, it was difficult to directly compare the optimum concentration of H2O2 between previous investigations and this study (the optimum ratio was 1:4.5 in our study, which was based on 0.2 mM H2O2). Most previous studies used a very narrow range of H2O2 concentrations and a single input method. In our study, a wide range of H2O2 concentrations (0.01– 10 mM) was used and the H2O2 was added to the solution twice because high concentrations can act as a scavenger of hydroxyl radicals.
Table 2 Comparison of the chemical and physical properties of phenol and BPA. Compounds
Molar mass (g/mol)
Solubility at 25 °C
Log Kow
Phenol
Molecular structure
94.11
91,700
1.46
BPA
228.29
120–300
3.4
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 3 The molar ratios of target materials and H2O2 during the ultrasound/H2O2 process. Target compounds (conc. (mM))
Molar ratio (target compound: H2O2)
Applied process
Refs.
TCE (3.34), CP (0.362, 0.724), DCP (0.286) TCE (3.34) 4-CP (1.6) 2-CP (0.778) C. I. reactive red (0.102) 2-CP (0.778) 2-CP (0.778) Phenol (2.5) p-CP (0.395) Alachlor (0.371) NP (0.03) Phenol (0.662) Phenol (2.5) CHCl3 (0.084), CHBrCl2 (0.061), CHBr2Cl (0.048), CHBr3 (0.040), CHI3 (0.025) p-CP (1.556) p-NP (0.2) Rhodamine B (0.0569) Oxalic acid (2.3) 2-CP, 3-CP, 4-CP (0.12) Cyanide (0.769) BPA (0.0044) BPA and phenol (0.044)
1:0.299 1:276.2 1:0.299 1:0.18 1:1.81 1:0.018 1:0.38 1:17.25 1:7.56 1:1.7 1:16.9 1:6.82 1:14 1:10.1 1:121.5 1:0.027 1:2.70 1:33 1:3333 1:91.4 1:28 1:8.75 1:588.24 1:30.2 1:10 1:102,197 1:17.39 34.78 1:22.6 255.5 1:0.01, 0.26 1:22.8 1:22830 1:0.45 1:454.5
Fenton-like Fenton-like Fenton Fenton H2O2 Fenton H2O2 Fenton H2O2 H2O2 Fenton Fenton-like Fenton-like Fenton Fenton-like H2O2 Fenton-like H2O2 H2O2 H2O2 H2O2 H2O2
[2] [4] [5] [6] [7] [13] [14] [16] [18] [19] [20] [23] [26] [27] [28] [29] [31] [34] [35] [36] [38] This study
3.2. Formation rate of H2O2 under the sonochemical reaction Some previous studies investigated the effects of H2O2 concentration on sonochemical reactions [2,4–7,13,14,16,18–20,22–28, 31,35–37,47] and observed the formation of H2O2 during the sonolysis of water [12,25,30,32,29,37,44,49]. However, there have been no experiments that monitored H2O2 under the various input concentrations of H2O2 in the presence and in the absence of target materials. In this investigation, the concentration H2O2 was monitored and the formation rates were calculated (Fig. 4). The formation rates were obtained by the input concentration subtracted from the measured concentration under each experimental condition. The formation rates of H2O2 were affected by the H2O2 concentration and pollutant types. The formation rate of H2O2 decreased as the input concentration of H2O2 increased. The results that were obtained at an initial concentration of 1 and 10 mM H2O2 were not shown because the formation rates of H2O2 become negative and
the concentration of H2O2 decreased during the sonication time. This result confirms that the decay rates of H2O2 were slightly faster than the formation rates at high concentrations of H2O2. The negative values were higher in the presence of 10 mM H2O2 compared to 1 mM in both the phenol and BPA solutions. The H2O2 concentration slightly decreased but was maintained throughout the sonochemical reaction because H2O2 dissociated to hydroxyl radicals and reacted with other radicals (Eqs. (10) and (11)) in addition to producing some radical reactions (Eqs. (6) and (8)). H2O2 remained in both the phenol and BPA solutions but the degradation rates of the compounds were not significant in the presence of high concentrations of H2O2. Additionally, the hydroxyl radical scavenger effect increases at the high concentration of H2O2 [18,34,35,37]. These results confirm that high concentrations of H2O2 were not effective in the degradation of pollutants and it is therefore important to evaluate the optimum concentration of H2O2 with respect to the experimental conditions and the properties of the pollutants. 4. Conclusion
Water + H2O2 0 mM Water + H2O2 0.01 mM Water + H2O2 0.1 mM
Net hydrogen peroxide generation (mM)
0.04
Phenol + H2O2 0 mM Phenol + H2O2 0.01 mM Phenol + H2O2 0.1 mM
0.03
BPA + H2O2 0 mM BPA + H2O2 0.01 mM BPA + H2O2 0.1 mM
0.02
0.01
0.00 0
10
20
30
40
50
60
Irradiation time (min) Fig. 4. Formation rate of H2O2 during the sonochemical degradation of phenol and BPA. Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM).
The degradation of phenol and BPA was enhanced by the addition of H2O2 during sonochemical processes. For the sonochemical degradation of phenol and BPA, the degradation ratio increased from 10% to 50% and 30% to 50% upon the addition of H2O2 respectively. The effects of H2O2 on the sonochemical degradation of phenol were more effective than on BPA because of the hydrophobicity of BPA. There was no significant effect on the sonochemical degradation of phenol using 0.1, 1, or 10 mM H2O2. The degradation of TOC was enhanced by the addition of H2O2 and the effects of H2O2 were quite significant in the BPA solution compared to phenol because some intermediates of BPA have high Kow values and a greater probability to react with the hydroxyl radicals. The degradation ratios of TOC were lower than the degradation ratios of phenol and BPA because some intermediates cannot readily degrade during the sonochemical reaction. High concentrations of H2O2 can act as a radical scavenger (especially hydroxyl radicals). Therefore, the determination of an optimum H2O2 concentration is crucial. Additional research into the optimum H2O2 concentration under various experimental conditions (frequency, power) and in the presence of various target compounds (concentration, volatility) is required.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
Acknowledgements This work was supported by research fund, Kumoh National Institute of Technology. M. L. also acknowledges the research project for Environmental Risk Assessment of Manufactured Nanomaterials (KK-1303-03) funded by the Korea Institute of Toxicology (KIT, Korea), and a fellowship from the National Research Foundation of Korea Grant funded by the Korean Government (Ministry of Education, Science and Technology) (NRF-2011-357-D00143).
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Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A Myunghee Lim a,b, Younggyu Son c,⇑, Jeehyeong Khim b,⇑ a
Future Environmental Research Center, Korea Institute of Toxicology, Jinju 660-844, Republic of Korea School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-701, Republic of Korea c Department of Environmental Engineering, Kumoh National Institute of Technology, Gumi 730-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 November 2013 Received in revised form 21 March 2014 Accepted 22 March 2014 Available online xxxx Keywords: Cavitation Hydrogen peroxide Phenol Bisphenol A Sonolysis
a b s t r a c t This report describes the effects of H2O2 concentration (0.01, 0.1, 1, and 10 mM) on the sonochemical degradation of phenol and bisphenol A (BPA) using an ultrasonic source of 35 kHz and 0.08 W/mL. The concentration of the target pollutants (phenol or BPA), total organic carbon (TOC), and H2O2 were monitored for each input concentration of H2O2. The effects of H2O2 on the sonochemical degradation of phenol was more significant than that of BPA because phenol has a high solubility and low octanol–water partition coefficient (Kow) value and is subsequently very likely to remain in the aqueous phase, giving it a greater probability of reacting with H2O2. The removal of TOC was also enhanced by the addition of H2O2. Some intermediates of BPA have a high Kow value and subsequently have a greater probability of pyrolyzing by the high temperatures and pressures inside of cavitation bubbles. Thus the removal efficiency of TOC in BPA was higher than that of phenol. The removal efficiencies of TOC were lower than the degradation efficiencies of phenol and BPA. This result is due to the fact that some intermediates cannot readily degrade during the sonochemical reaction. The H2O2 concentration decreased but was not completely consumed during the sonochemical degradation of pollutants. The initial H2O2 concentration and the physical/ chemical characteristics of pollutants were considered to be important factors in determining the formation rate of the H2O2. When high concentration of H2O2 was added to the solution, the formation rates were relatively low compared to when low concentrations of H2O2 were used. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The sonication of aqueous solutions causes cavitation events that involve the formation, growth, and collapse of bubbles [1]. The collapse of these cavitation bubbles results in localized high temperatures (5000 K) and pressures (1000 atm) [1–4]. These extreme conditions can induce the cleavage of dissolved oxygen and water molecules to produce radicals (O, OH, and H). The main degradation mechanisms of ultrasound are pyrolysis and radical reactions [5]. The degradation mechanism of organic compounds by ultrasound was determined by the properties of the target materials [2,4,5]. The surfaces of the cavitation bubbles are hydrophobic; therefore, hydrophobic compounds can enter the bubbles and be pyrolyzed directly by the high temperatures and pressures. On the other hand, hydrophilic compounds barely diffuse into the bubbles and are degraded indirectly by radicals in the bulk solution [2,5,6]. ⇑ Corresponding authors. Tel.: +82 54 478 7637 (Y. Son), +82 2 928 7656 (J. Khim). E-mail addresses: [email protected] (Y. Son), [email protected] (J. Khim).
A variety of parameters (combination of UV system [7–9], temperature [10–12], pH [8,9,13–15], ionic strength [13,14], initial concentration [10,12,13–16], saturating gas [8,10,15,17–22], frequency [12,15,17,19,21,22,24–26], additives [7,9,10,16,18–21,23,26–33], power [9,11,14,30], reactor type [18]) have been examined in sonochemical processes. It is believed that hydrogen peroxide is one of the most effective additives to enhance the sonochemical degradation of aqueous pollutants [14,18,19,27,31]. Under ultrasound irradiation hydrogen peroxide can dissociate into two hydroxyl radicals and acts as a secondary source of hydroxyl radicals [16,26,27,31]. Without adding hydrogen peroxide, hydrogen peroxide can be generated by the recombination of two hydroxyl radicals generated by cavitation in aqueous solution and linearly accumulated [16,20,22,28]. It is not clear whether generated hydrogen peroxide can be the secondary source of hydroxyl radicals as added hydrogen peroxide acted because the concentration of added hydrogen peroxide drastically decreases while generated hydrogen peroxide is linearly accumulated. It was found that the amount/concentration of added hydrogen peroxide concentration was an important parameter in the
http://dx.doi.org/10.1016/j.ultsonch.2014.03.021 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
2
M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
ultrasound/H2O2 system (particularly when compared to the concentration of the target compounds). The presence of an optimum amount of H2O2 for the sonochemical degradation of the target compounds was verified, but it was difficult to determine the optimum concentration for each compound [2,4–7,13,14,16, 18–20,22,23,27–29,31,34–37] because the experimental conditions varied widely and a very limited range of H2O2 concentrations was used in most previous studies. In this paper, various concentrations of H2O2 were tested for the enhancement of the sonochemical degradation of phenol and BPA. The degradation rate constants of phenol and BPA were determined under a variety of experimental conditions. The total organic carbon (TOC) concentration was also measured to estimate the degree of mineralization. Furthermore, the formation rate of H2O2 was measured for each concentration of H2O2 in the presence and in the absence of the target pollutants. 2. Materials and methods 2.1. Chemicals Highly pure phenol (99%, Samchun, South Korea) and bisphenol A (99+% purity, Sigma–Aldrich, USA) were used in this investigation. The H2O2 solutions were diluted from a 34.5% stock solution of H2O2 (Junsei, Japan). Potassium biphthalate (Junsei, Japan), potassium iodide (Samchun, Korea), sodium hydroxide (Junsei, Japan), and ammonium molybdate (Junsei, Japan) were used to determine the H2O2 concentration. Methanol and acetic acid (HPLC grade, J. T. Baker, USA) were used to determine the phenol and BPA concentrations. The initial concentration of phenol and BPA was 0.044 mM. Four different concentrations of hydrogen peroxide (0.01, 0.1, 1, and 10 mM) were evaluated in this investigation. 2.2. Ultrasonic reactor The sonochemical reaction was carried out using a 1000 mL double-walled Pyrex reactor containing 500 mL of the solution containing phenol and BPA. Fig. 1 displays a schematic diagram of the reactor. The water was introduced to the wall of the reactor using a cooling water system and the solution temperature was maintained at 20 ± 2 °C. The mixing system was installed inside the reactor to homogenize the H2O2 and the solution containing phenol and BPA. The reactor was then fitted with a cup-horn type transducer using a Teflon tape. The Frexonic system (Mirae Ultrasonics Tech., Korea) operates at 35 kHz and consisted of a single piezoelectric transducer (PZT, Tamura corp.). The diameter of the transducer was 10 cm. The aqueous solution was aerated using an air diffuser for 30 min prior to the experiment. The delivered ultrasonic energy
was measured by a power meter (METEX; M-4660M) to be 40 ± 3 W. The input power was calculated using the calorimetric method and the calorimetric power of the reaction solution is shown in Eq. (1):
Power ¼
dT cp M dt
ð1Þ
where dT/dt is the rise in temperature of the reaction volume as a function of the sonication time, cp is the heat capacity of water (4.2 J/g K), and M is the mass of water (g) [11]. The calorimetric power was 21.0 W in this study. 2.3. Analytical procedure A 10 mL sample was taken every 10 min for 60 min of the sonochemical reaction for the analysis of the target compounds (phenol, BPA), TOC, and H2O2 concentration. H2O2 was added to the phenol and BPA solution twice during the course of the reaction (at the beginning of the reaction and at 30 min). The phenol and BPA concentrations were measured by HPLC (Varian Prostar HPLC system) and the TOC concentration was measured using a TOC analyzer (SEIVERS 5310C laboratory analyzer, GE, US). The H2O2 concentration was determined using the iodometric method [17]. An iodide ion (I) reacts with H2O2 to form a triiodide ion (I 3 ) which absorbs at 352 nm when using a UV spectrophotometer (SPECORD 40, Analyticjena). Sample aliquots (2.0 mL) from each experiment were mixed in a quartz cuvette containing 0.75 mL of 0.10 M potassium biphthalate and 0.75 mL of a solution containing 0.4 M potassium iodide, 0.06 M sodium hydroxide, and 104 M ammonium molybdate. The mixed solution (total volume: 3.5 mL) was allowed to stand for 2 min before the absorbance was measured. 3. Results and discussion 3.1. Effects of H2O2 on the sonochemical degradation of phenol and BPA When an aqueous solution is ultrasonically irradiated, hydroxyl radicals are produced by cavitation (Eq. (2)). The hydroxyl radical exhibits a high oxidation potential and can directly degrade organic pollutants [4–6,10]; however, hydroxyl radicals has a very short lifetime and tend to combine with one another to form H2O2 (Eq. (4)). The following reactions (Eqs. (1)–(11)) occur under sonolysis [7–9,12,16,22,24,27,36,37].
H2 O ! H þ OH
ð2Þ
H2 O2 ! 2HO
ð3Þ
2HO ! H2 O2
ð4Þ
Fig. 1. Schematic diagram of the ultrasonic reactor.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
ð6Þ
O þ H2 O ! 2 OH
H þ H2 O ! OH þ H2
ð7Þ
H þ O2 ! OOH þ O
ð8Þ
2 OOH ! H2 O2 þ O2
ð9Þ
H2 O2 þ OH ! H2 O þ
HO2
ð10Þ
HO2 þ H2 O2 ! H2 O þ OH þ O2
ð11Þ 1
The reported formation rates of H2O2 (0.02–9.7 lM min ) in previous research varied widely in terms of frequency, power, and reaction volume [12,24–26,30,36–45]. Table 1 shows the formation rate of H2O2 during the sonochemical reaction in the absence of target materials as well as the range of formation rates at low frequencies. In this investigation, 0.58 lM min1 of H2O2 was produced which is within the range reported in previous studies. These H2O2 formation rates were slightly lower than those in the presence of target materials, as shown by previous research as well as this investigation, which will be discussed later in this paper. Four different concentrations of H2O2 (0.01, 0.1, 1, and 10 mM) were used in this experiment and the H2O2 was added to the phenol and BPA solution twice (at the beginning and after 30 min) within the 60 min reaction time. Therefore, the total input concentration of H2O2 was 0.02, 0.2, 2, or 20 mM, respectively, within the 60 min reaction time. Bremner et al. calculated the stoichiometric value of H2O2 for the complete degradation of phenol [16]. Based on Eqs. (12) and (13), 14 mol and 37 mol of H2O2 were required to completely degrade phenol and BPA, respectively. In the present study, 0.616 and 1.584 mM of H2O2 were required for the complete oxidation of 0.044 mM phenol and BPA.
C6 H6 O þ 14H2 O2 ! 6CO2 þ 17H2 O
ð12Þ
C15 H16 O þ 37H2 O2 ! 15CO2 þ 45H2 O
ð13Þ
1.0
Removal efficiency of phenol (C/C0)
ð5Þ
0.9
0.8
0.7 H2O2 0 mM
0.6
H2O2 0.01 mM H2O2 0.1 mM
0.5
H2O2 1 mM H2O2 10 mM
0.4 0
10
20
30
40
50
60
50
60
Irradiation time (min)
(a) 1.0
Removal efficiency of BPA (C/C0)
O2 ! 2O
0.9
0.8
0.7 H2O2 0 mM H2O2 0.01 mM
0.6
H2O2 0.1 mM H2O2 1 mM
0.5
H2O2 10 mM 0.4 0
10
20
30
40
Irradiation time (min)
(b) However, the stoichiometric amount of H2O2 cannot be used as a parameter in the complete degradation of organic compounds. Although H2O2 has a high oxidation potential (1.8 V), it is not capable of degrading many pollutants. In most cases, H2O2 was used in conjunction with solid catalysts or other treatment technologies [2,4–7,13,14,16,18–20,23,26,29,31,34–36]. Fig. 2 displays the degradation of phenol and BPA under the ultrasound/H2O2 system in the presence of different concentrations of H2O2. The degradation of phenol and BPA increased in the presence of H2O2. The degradation ratio of phenol was 12% in the absence of H2O2 during 60 min of sonication while at 20 mM of H2O2 the phenol degraded up to 48% (Fig. 2(a)). For BPA, the degradation ratio was 30% under ultrasonic irradiation in the absence of H2O2 and increased to 47% in the presence of 20 mM H2O2. There were no significant effects at high concentrations of H2O2 since there was only a 10% increase in the phenol degradation at the
Fig. 2. Effects of H2O2 concentration on the sonochemical degradation of phenol (a) and BPA (b). Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM, d: ultrasonic irradiation, s: ultrasonic irradiation + 0.01 mM H2O2, j: ultrasonic irradiation + 0.1 mM H2O2, h: ultrasonic irradiation + 1 mM H2O2, N: ultrasonic irradiation + 10 mM H2O2).
highest concentration (20 mM) of H2O2 compared to the lowest concentration (0.02 mM). In previous studies the effects of H2O2 in the presence of various organic pollutants such as trichloroethylene, 1,3-dichloro-2-propanol, o-chlorophenol, trihalomethanes, oxalic acid, phenol, alachlor, cyanide, and p-chlorobenzoic acid were investigated [2,4–7,10, 13,14,16,18–20,23,27–29,31,34–37]. The effects of H2O2 were dependent on the properties of the target compounds [2,4]. In particular, the addition of H2O2 had a significant influence on the degradation of non-volatile and
Table 1 Formation rate of H2O2 during sonochemical reactions from previous investigations. H2O2 formation (lM/min)
Frequency (kHz)
Power (W)
Solution volume (mL)
Refs.
0.7 3.5 0.02–0.06 4.7 1.5–3.6 0.58
20 20 20 20 20 35
30 125 12.7–160.7 19 30–80 40
300 50 400 20 50 500
[25] [32] [36] [37] [45] This study
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
hydrophilic compounds. Alternatively, the degradation of volatile compounds was not affected by the presence of H2O2 [2,4,27]. Our results agreed well with these previous results. The sonochemical degradation rate of BPA was higher than phenol because BPA is more hydrophobic and volatile than phenol [46,47]. Therefore, BPA has a greater probability of getting close to the surface of a cavitation bubble and can easily be degraded by hydroxyl radicals. Alternatively, the effects of H2O2 on the sonochemical degradation of phenol were more significant than BPA. Phenol has high solubility and low log Kow and therefore has a greater probability of reacting with H2O2 in aqueous solutions. H2O2 has a low Kow (1.57 at 25 °C) compared to phenol and BPA (Table 2) and subsequently remained in the aqueous phase. Fig. 3 displays the TOC degradation of phenol and BPA in solution as a function of H2O2 concentration. The degradation of TOC corresponds to the mineralization of organic pollutants, and the degradation of TOC through inorganic carbon (such as CO2) was also quite significant. Therefore, some researchers used the reduction of TOC as the key factor in evaluating the effectiveness of these processes [9,40,48]. In this research, TOC degradation was considered to be an effect of H2O2. The TOC degradation of phenol increased by approximately 8.5% in the presence of 0.01 mM H2O2 compared to the absence of H2O2. The effects of H2O2 concentration in the degradation of TOC in the phenol solution were not significant. There was only a 5% increase in TOC degradation when using 10 mM H2O2 compared to 1 mM H2O2. The degradation ratio of phenol was nearly 50% using 20 mM H2O2, while the TOC degradation of the phenol solution was only 25% (Figs. 2a and 3). These results imply that some intermediates cannot completely degrade during the sonochemical reaction (in our investigation, the sonochemical reaction was carried out for 1 h) [13,20]. According to previous research, the representative intermediates of phenol during the sonolysis reaction were hydroquinone, benzoquinone, and catechol [21,23,24,34,49]. The log Kow value of the intermediates were 0.59 (hydroquinone), 0.2 (benzoquinone), and 0.88 (catechol). These values were smaller than the log Kow of phenol, which confirmed that the compounds preferred to remain in the aqueous solution. These compounds further degraded into more hydrophilic compounds such as formic and oxalic acids [20,34]. It was determined that the degradation rate of TOC was slower than that of phenol. For BPA, the TOC degradation continuously increased as the concentration of H2O2 increased. However, the enhancement of TOC degradation was not significant when 1 or 10 mM H2O2 was added. As seen in the phenol experiments, the BPA concentration decreased by 48% within 60 min of ultrasonic irradiation, while the TOC concentration remained at 65%. Some intermediates reported by previous researchers such as 3-hydroxybisphenol A, 2-(4-hydroxyphenol)-2-propanol, 4-(1-methylethenyl)phenol, 4-hydroxyacetophenone, benzoquinone, phenol, catechol, and hydroquinone were also present [27,40,49]. Among these intermediates, 3-hydroxybisphenol A and 4-(1-methylethenyl)phenol exhibited similar log Kow values (3.16 and 2.9, respectively). These
Mineralization (TOC removal) efficiency (%)
4
40 Phenol BPA 30
20
10
0 0 mM
0.01 mM
0.1 mM
1 mM
10 mM
The concentration of hydrogen peroxide added Fig. 3. Degradation of TOC content in phenol and BPA solutions as a function of H2O2 concentration. Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM).
compounds were relatively degradable while all other intermediates have low log Kow values below 1.5. Therefore, the degradation ratio of TOC in the BPA solution was higher than that of phenol and the impact of H2O2 on the TOC degradation was quite significant in the BPA solution. The impact of H2O2 decreased as the concentration of H2O2 increased. Although the degradation rates of TOC increased as concentration of H2O2 increased, it was not significant compared to the input concentrations of H2O2 in both solutions. This result was caused by H2O2 acting as a hydroxyl radical scavenger during the sonolysis reaction, as described by Eq. (9) [6,7,16,18,20,34–37]. This reaction may consume H2O2 as well as hydroxyl radicals and was subsequently a non-productive reaction [50]. Therefore, the determination of the optimum concentration of H2O2 was crucial. Table 3 represents the calculated molar ratios (target compounds versus H2O2) used in previous investigations. The molar ratios in previous studies varied by up to a factor of 200,000 [4,5,7,23,26,28,29,31,36]. The reported values were in the range of approximately 1:6.8–228.3 (target material: H2O2) and were also affected by the target materials (initial concentration and properties) as well as the experimental conditions (frequency and power) [18,28,35,38]. Therefore, it was difficult to directly compare the optimum concentration of H2O2 between previous investigations and this study (the optimum ratio was 1:4.5 in our study, which was based on 0.2 mM H2O2). Most previous studies used a very narrow range of H2O2 concentrations and a single input method. In our study, a wide range of H2O2 concentrations (0.01– 10 mM) was used and the H2O2 was added to the solution twice because high concentrations can act as a scavenger of hydroxyl radicals.
Table 2 Comparison of the chemical and physical properties of phenol and BPA. Compounds
Molar mass (g/mol)
Solubility at 25 °C
Log Kow
Phenol
Molecular structure
94.11
91,700
1.46
BPA
228.29
120–300
3.4
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 3 The molar ratios of target materials and H2O2 during the ultrasound/H2O2 process. Target compounds (conc. (mM))
Molar ratio (target compound: H2O2)
Applied process
Refs.
TCE (3.34), CP (0.362, 0.724), DCP (0.286) TCE (3.34) 4-CP (1.6) 2-CP (0.778) C. I. reactive red (0.102) 2-CP (0.778) 2-CP (0.778) Phenol (2.5) p-CP (0.395) Alachlor (0.371) NP (0.03) Phenol (0.662) Phenol (2.5) CHCl3 (0.084), CHBrCl2 (0.061), CHBr2Cl (0.048), CHBr3 (0.040), CHI3 (0.025) p-CP (1.556) p-NP (0.2) Rhodamine B (0.0569) Oxalic acid (2.3) 2-CP, 3-CP, 4-CP (0.12) Cyanide (0.769) BPA (0.0044) BPA and phenol (0.044)
1:0.299 1:276.2 1:0.299 1:0.18 1:1.81 1:0.018 1:0.38 1:17.25 1:7.56 1:1.7 1:16.9 1:6.82 1:14 1:10.1 1:121.5 1:0.027 1:2.70 1:33 1:3333 1:91.4 1:28 1:8.75 1:588.24 1:30.2 1:10 1:102,197 1:17.39 34.78 1:22.6 255.5 1:0.01, 0.26 1:22.8 1:22830 1:0.45 1:454.5
Fenton-like Fenton-like Fenton Fenton H2O2 Fenton H2O2 Fenton H2O2 H2O2 Fenton Fenton-like Fenton-like Fenton Fenton-like H2O2 Fenton-like H2O2 H2O2 H2O2 H2O2 H2O2
[2] [4] [5] [6] [7] [13] [14] [16] [18] [19] [20] [23] [26] [27] [28] [29] [31] [34] [35] [36] [38] This study
3.2. Formation rate of H2O2 under the sonochemical reaction Some previous studies investigated the effects of H2O2 concentration on sonochemical reactions [2,4–7,13,14,16,18–20,22–28, 31,35–37,47] and observed the formation of H2O2 during the sonolysis of water [12,25,30,32,29,37,44,49]. However, there have been no experiments that monitored H2O2 under the various input concentrations of H2O2 in the presence and in the absence of target materials. In this investigation, the concentration H2O2 was monitored and the formation rates were calculated (Fig. 4). The formation rates were obtained by the input concentration subtracted from the measured concentration under each experimental condition. The formation rates of H2O2 were affected by the H2O2 concentration and pollutant types. The formation rate of H2O2 decreased as the input concentration of H2O2 increased. The results that were obtained at an initial concentration of 1 and 10 mM H2O2 were not shown because the formation rates of H2O2 become negative and
the concentration of H2O2 decreased during the sonication time. This result confirms that the decay rates of H2O2 were slightly faster than the formation rates at high concentrations of H2O2. The negative values were higher in the presence of 10 mM H2O2 compared to 1 mM in both the phenol and BPA solutions. The H2O2 concentration slightly decreased but was maintained throughout the sonochemical reaction because H2O2 dissociated to hydroxyl radicals and reacted with other radicals (Eqs. (10) and (11)) in addition to producing some radical reactions (Eqs. (6) and (8)). H2O2 remained in both the phenol and BPA solutions but the degradation rates of the compounds were not significant in the presence of high concentrations of H2O2. Additionally, the hydroxyl radical scavenger effect increases at the high concentration of H2O2 [18,34,35,37]. These results confirm that high concentrations of H2O2 were not effective in the degradation of pollutants and it is therefore important to evaluate the optimum concentration of H2O2 with respect to the experimental conditions and the properties of the pollutants. 4. Conclusion
Water + H2O2 0 mM Water + H2O2 0.01 mM Water + H2O2 0.1 mM
Net hydrogen peroxide generation (mM)
0.04
Phenol + H2O2 0 mM Phenol + H2O2 0.01 mM Phenol + H2O2 0.1 mM
0.03
BPA + H2O2 0 mM BPA + H2O2 0.01 mM BPA + H2O2 0.1 mM
0.02
0.01
0.00 0
10
20
30
40
50
60
Irradiation time (min) Fig. 4. Formation rate of H2O2 during the sonochemical degradation of phenol and BPA. Power = 40 ± 3 W, initial phenol and BPA concentration = 0.044 mM).
The degradation of phenol and BPA was enhanced by the addition of H2O2 during sonochemical processes. For the sonochemical degradation of phenol and BPA, the degradation ratio increased from 10% to 50% and 30% to 50% upon the addition of H2O2 respectively. The effects of H2O2 on the sonochemical degradation of phenol were more effective than on BPA because of the hydrophobicity of BPA. There was no significant effect on the sonochemical degradation of phenol using 0.1, 1, or 10 mM H2O2. The degradation of TOC was enhanced by the addition of H2O2 and the effects of H2O2 were quite significant in the BPA solution compared to phenol because some intermediates of BPA have high Kow values and a greater probability to react with the hydroxyl radicals. The degradation ratios of TOC were lower than the degradation ratios of phenol and BPA because some intermediates cannot readily degrade during the sonochemical reaction. High concentrations of H2O2 can act as a radical scavenger (especially hydroxyl radicals). Therefore, the determination of an optimum H2O2 concentration is crucial. Additional research into the optimum H2O2 concentration under various experimental conditions (frequency, power) and in the presence of various target compounds (concentration, volatility) is required.
Please cite this article in press as: M. Lim et al., The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.021
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M. Lim et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
Acknowledgements This work was supported by research fund, Kumoh National Institute of Technology. M. L. also acknowledges the research project for Environmental Risk Assessment of Manufactured Nanomaterials (KK-1303-03) funded by the Korea Institute of Toxicology (KIT, Korea), and a fellowship from the National Research Foundation of Korea Grant funded by the Korean Government (Ministry of Education, Science and Technology) (NRF-2011-357-D00143).
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