Sonolytic degradation of endocrine disrupting chemical 4-cumylphenol in water

Ultrasonics Sonochemistry 18 (2011) 943–950

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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch

Sonolytic degradation of endocrine disrupting chemical 4-cumylphenol in water Mahdi Chiha a, Oualid Hamdaoui a,⇑, Stéphane Baup b, Nicolas Gondrexon b a b

Laboratory of Environmental Engineering, Department of Process Engineering, Faculty of Engineering, University of Annaba, P.O. Box 12, 23000 Annaba, Algeria LEPMI, 1130 Rue de la Piscine, BP 75, 38402 Saint Martin d’Hères Cedex, France

a r t i c l e

i n f o

Article history: Received 28 May 2010 Received in revised form 16 December 2010 Accepted 27 December 2010 Available online 30 December 2010 Keywords: Endocrine disrupting chemical 4-Cumylphenol Ultrasound Advanced oxidation process

a b s t r a c t The sonolytic degradation of endocrine disrupting compound 4-cumylphenol (4-CyP) in aqueous solution was investigated. The influence of operating parameters for sonication process such as 4-CyP initial concentration, frequency, power, pH, temperature and saturating gas was examined. The extent of degradation was inversely proportional to the initial substrate concentration. The rate of 4-CyP degradation was frequency dependent. The degradation rate increased proportionally with increasing ultrasonic power from 20 to 100 W and temperature in the range of 20–50 °C. The most favorable degradation pH was acidic media. Destruction in the presence of saturating gas follows the order: argon > air > nitrogen. The 4-CyP degradation was inhibited in the presence of nitrogen gas owing to the free radical scavenging effect in vapor phase within the bubbles of cavitation. The ultrasonic degradation of 4-CyP was clearly promoted in the presence of bromide anions and the promoting effect on degradation increased with increasing bromide concentration. At low 4-CyP concentration (0.05 mg L1), bicarbonate ion drastically enhanced the rate of 4-CyP degradation. Experiments conducted using pure and natural water demonstrated that the sonolytic treatment was more efficient in the natural water compared to pure water. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Several environmental contaminants, classified as endocrine disrupting chemicals (EDCs), can interfere with the function of the endocrine system in living organisms. Chemical exposure to EDCs has been linked to neurological and reproductive effects on fish and wildlife, and may also affect human fertility [1–7]. These findings have raised public concern over their environmental and human health effects. Sewage treatment plants effluent outfalls constitute significant sources of EDCs to the receiving surface, coastal waters and regional environments, thus increasing the risk of exposure. Among the increasing list of substances classified as EDCs, an important attention has been paid to a selected group of endocrine-disrupting phenols, alkylphenols (APs), including bisphenol-A (BPA), due to their wide presence in household and industrial processes [8,9]. BPA was the most studied endocrine disrupting AP, but 4-cumylphenol (4-CyP) is 12 times as estrogenically active as BPA [10]. 4CyP, also known as 4-(a,a-dimethylbenzyl) phenol, is widely used as a material for polycarbonate plastics, surfactants, fungicides and preservatives. 4-CyP was chosen because this alkylphenol has been detected as a pollutant in municipal wastewaters, coastal waters

⇑ Corresponding author. E-mail addresses: (O. Hamdaoui).

[email protected],

[email protected]

1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.12.014

and rivers [11–14]. Therefore, 4-CyP should be removed from wastewater and water sources. Ultrasonic irradiation has received considerable interest as an advanced oxidation process. The passage of ultrasound through a liquid induces physical and chemical processes, largely through acoustic cavitation. Under the influence of an acoustic field, bubbles are generated from existing gas nuclei in liquids. These bubbles oscillate in a nonlinear manner, and under specific experimental conditions violently collapse to generate high temperatures and pressures [15,16]. The implosion of cavitation bubbles is sufficiently violent to generate localized temperatures and pressures on the order of 5000 K and hundreds of atmospheres, respectively [17,18]. The extremely high temperatures and pressures formed in collapsing cavitation bubbles in aqueous solutions lead to the thermal dissociation of water vapor into reactive hydroxyl radicals and hydrogen atoms, and with other species present (O2 and H2O), various other radicals may form (reactions (1)–(5)) [19,20].

H2 O ! H þ  OH

ð1Þ

O2 ! 2O

ð2Þ





H þ O2 ! OOH

ð3Þ

O þ H2 O ! 2 OH

ð4Þ

H þ O2 !  OH þ O

ð5Þ

In the absence of any solute, these primary radicals of sonolysis mostly recombine to form hydrogen peroxide that is released in the medium (reactions 6 and 7).

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M. Chiha et al. / Ultrasonics Sonochemistry 18 (2011) 943–950

2 OH ! H2 O2

ð6Þ

2 OOH ! H2 O2 þ O2

ð7Þ

However, when aqueous sonolysis is conducted in the presence of organic solutes, a number of chemical processes can occur, depending on the physical and chemical nature of the solute [21]. According to ‘‘hot spot’’ theory, sonochemical reactions can occur in three regions [20]: (i) the region inside the bubble cavity where the volatile and hydrophobic molecules are degraded via pyrolytic reactions, (ii) bubble–liquid interfacial region where the hydroxyl radical reactions are predominant and (iii) the liquid bulk region where the free radicals that migrate from the bubble–liquid interface into the liquid, create secondary sonochemical reactions. To the best of our knowledge, data on the sonochemical degradation of 4-CyP have not been reported previously. Additionally, the influence of inorganic species such as bicarbonate and bromide ions on the sonochemical degradation of organic compounds remains controversial. It is also of considerable practical interest to examine the sonochemical treatment in complex matrix such as natural water. Therefore, in this study, the application of high frequency ultrasound for the destruction of 4-CyP in aqueous systems was examined. The effect of operational conditions for sonication process such as substrate initial concentration, frequency, power, pH, temperature and saturating gas was studied. The influence of mineral anionic species such as bicarbonate and bromide on the sonolytic degradation of 4-CyP was also investigated. Additionally, sonochemical destruction of 4-CyP in pure and natural water was compared.

2. Materials and methods 2.1. Chemicals 4-CyP for synthesis (Sigma) containing more than 99% of pure compound was used for the preparation of solutions. Structural formula of 4-CyP is shown in Fig. 1. Acetonitrile supplied by Acros organics was HPLC-UV quality. Pure water, obtained with activated carbon and ion exchanger resins from Fisher Bioblock Scientific (Illkirch, France), was used throughout the study for the preparation of aqueous solutions, and as a component of the mobile phase in analysis by high performance liquid chromatography (HPLC). Sodium bicarbonate, potassium iodide, ammonium heptamolybdate, tert-butyl alcohol and potassium bromide were commercial products of the purest grade available (analytical grade).

2.3. Analyses 4-CyP concentration was determined by using HPLC (Waters model 515) equipped with a Supelcosil LC-18 column (ID = 4.6 mm, length = 250 mm) and UV detector (Waters model 486). Sample injections were achieved with a Rheodyne injection system equipped with a 20 lL sample loop for 4-CyP concentrations above 1 mg L1 and a 200 lL loop for the lowest concentrations. The used eluent was a mixture of acetonitrile and pure water (60:40 v/v) at a flow rate of 1 mL min–1. Hydrogen peroxide concentrations were determined using the iodometric method [23]. Sample aliquots taken from the reactor were added in the quartz cell of the spectrophotometer containing potassium iodide (0.1 M) and ammonium heptamolybdate (0.01 M). The mixed solutions were allowed to stand for 5 min before absorbance was measured. All experiments were carried out at least three times. 3. Results and discussion 3.1. Effect of ultrasound frequency Application of ultrasonic irradiations to 4-CyP solutions with initial concentration of 5 mg L1 was carried out at frequencies of 300 or 600 kHz and 80 W. Fig. 2 illustrates the sonolytic degradation of 4-CyP at two frequencies (300 and 600 kHz). The best sonochemical destruction rate of 4-CP in aqueous solution is observed to occur at 300 kHz. It is known that ultrasound frequency affect cavitation by modifying bubble number, bubble size, cavitation threshold and the temperatures reached during the collapse. With an increase in acoustic frequency, the bubble oscillation time per acoustic cycle decreases. Additionally, the lifetime of the acoustic cavitation bubbles also decrease with an increase in acoustic frequency. A decrease in both of these factors will lead to lesser release of OH radical in this frequency range [24]. 3.2. Effect of initial 4-CyP concentration The effect of 4-CyP initial concentration was studied in the range of 0.05–30 mg L1 and the results were depicted in Fig. 3.

1

300 kHz

0.8

600 kHz

2.2. Ultrasonic reactor

0

0.6 C/C

Experiments were conducted in a cylindrical water-jacketed glass reactor in order to control the temperature. Ultrasonic waves (300 kHz) were emitted from the bottom of the reactor through a piezoelectric disc (diameter 4 cm) fixed on a Pyrex plate (diameter 5 cm). The temperature of the solution was monitored using a thermocouple immersed in the reacting medium. The temperature inside the reactor was maintained at 20 ± 1 °C by circulating cooling water through a jacket surrounding the cell. Acoustic power dissipated in the reactor was estimated using standard calorimetric method [22]. The reactor was periodically sampled for analysis.

0.4

0.2

0 0

CH3 OH CH3 Fig. 1. Structural formula of 4-CyP.

5

10

15

20

25

30

35

40

Time (min) Fig. 2. Effect of ultrasonic frequency on the degradation of 4-CyP (power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5); temperature: 20 ± 1 °C).

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1

0.8

-1

C/C

0

0.6

0.7

0.6

-1

0.8

4-CyP initial degradation rate (mg L min )

0.05 mg/L 0.1 mg/L 0.5 mg/L 1 mg/L 5 mg/L 10 mg/L 20 mg/L 30 mg/L

0.4

0.2

0.5

0.4 Experimental

0.3

Predicted 0.2

0.1 0

0 0

5

10

15

20

25

30

35

40

0

5

10

Time (min)

25

30

35

Fig. 4. Experimental and predicted initial degradation rate as a function of the 4CyP initial concentration (frequency: 300 kHz; power: 80 W; volume: 300 mL; pH: natural (6.5); temperature: 20 ± 1 °C).

140

0 mg/L 10 mg/L 20 mg/L 30 mg/L

120

100

2

80

2

The extent of degradation is found to be inversely proportional to the initial concentration of 4-CyP. The variation in the concentration of 4-CyP with time showed an exponential decrease resembling a pseudo-first order kinetics generally observed for the sonochemical destruction of pollutants. The initial sonolytic degradation rates of 4-CyP at different concentrations are shown in Fig. 4. The curve shows that the higher the substrate concentration was, the higher the initial decomposition rate was. However, a linear relationship was not observed, as expected, for a first-order kinetic law. Similar results have been obtained for the sonolytic degradation of BPA [25]. Besides the degradation process, the ultrasonic treatment resulted in the concurrent formation of hydrogen peroxide. Fig. 5 illustrates the formation rate of H2O2 for three initial concentrations of 4-CyP and its respective value without compound. A higher concentration of H2O2 is reached for the lower concentration of substrate. At low 4-CyP concentrations, the probability of reaction with OH radical decreases and recombination of hydroxyl radicals occurs. At high 4-CyP concentration, the formation rate of H2O2 is low due to low recombination of OH radicals that react with 4CyP. This tendency is in good agreement with the results shown in Fig. 4. An increase in solute concentration would increase the probability of hydroxyl radical attack on pollutant molecules. Because at the interface of the bubbles the OH radical concentration is very high and the hydroxyl radical recombination would be the dominant process, an increase of substrate concentration would increase the fraction of OH that reacts with the substrate, and the degradation rate would be increased accordingly. 4-CyP exhibits low fugacity and therefore it cannot be degraded by pyrolysis inside the cavitation bubble because of its low Henry’s law constant (8.8  108 atm m3 mol1). Additionally, due to its low solubility in water (43.3 mg L1) and relative high octanol/ water partition coefficient (log KOW = 4.1), 4-CyP may be eliminated through reaction with hydroxyl radical at the interface of cavitation bubble. Thus, the OH radicals generated by ultrasound is the main responsible for the degradation of 4-CyP. In a recent work, Okitsu et al. [26–28] proposed a heterogeneous reaction model to describe the sonochemical degradation of azo dyes, butyric acid, benzoic acid and alkylbenzene sulfonates in water based on the Langmuir mechanism. They considered an

20

-1

H O (µM)

Fig. 3. Ultrasonic degradation of 4-CyP for various initial substrate concentrations (frequency: 300 kHz; power: 80 W; volume: 300 mL; pH: natural (6.5); temperature: 20 ± 1 °C).

15

4-CyP initial concentration (mg L )

60

40

20

0 0

5

10

15

20

25

30

Time (min) Fig. 5. Formation of hydrogen peroxide for various 4-CyP initial concentrations (frequency: 300 kHz; power: 80 W; volume: 300 mL; pH: natural (6.5); temperature: 20 ± 1 °C).

effective reaction site in the vicinity of the gas–liquid interface where the organic substrate is attacked by the increased concentration of radicals. In this work, the heterogeneous reaction equation developed by Okitsu et al. [26] is used to model the kinetics of 4-CyP degradation. The reaction rate was given by the following equation:

r¼

kKC 4CyP 1 þ KC 4CyP

ð8Þ

where r is the initial degradation rate (mg L1 min1), k is the pseudo-rate constant (mg L1 min1), K is the equilibrium constant (L mg1) and C4-CyP (mg L1) is the 4-CyP initial concentration. The sonolytic degradation data was analyzed by non-linear curve fitting analysis method, using MicrocalTM OriginÒ software,

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to fit the kinetic model. The value of the model parameters are 0.063 ± 0.004 L mg1 and 1.047 ± 0.028 mg L1 min1 for the equilibrium constant (K) and the pseudo-rate constant (k), respectively. In order to check the validity of the Okitsu et al.’s model, it is interesting and essential to reconstitute the predicted curve using the initial concentration values and Okitsu et al.’s parameters. Fig. 4 shows the superposition of experimental results (points) and the theoretical calculated points (line). These results show that the heterogeneous kinetic model describes adequately the sonochemical degradation of 4-CyP. These results indicate that the sonochemical degradation of 4-CyP occurs mainly at the interfacial region of cavitation bubbles by hydroxyl radical attack. 3.3. Effect of ultrasonic power The sonolytic degradation of aqueous 4-CyP solution (5 mg L1) was investigated at different ultrasonic powers (20–100 W). As shown in Fig. 6, it can be observed that the destruction rate increased by increasing ultrasonic power. This result is expected because it is known that the number of active cavitation bubbles increases with an increase in the acoustic power leading to an increase in the amount OH radical generated [29,30], which might be responsible for the observed enhancement in the degradation rates. Additionally, an increase in ultrasound power means an increase in the acoustic amplitude. The collapse time, the temperature and the pressure on collapse are all dependent on the acoustic amplitude; the cavitation bubble collapse will be more violent at higher acoustic amplitudes [31]. An increase in power will thus result in greater sonochemical effects in the collapsing bubble. It is important to note that degradation rates increase with ultrasonic power in the range of 20–100 W, because a continual increase in power does not necessarily imply continual increases in sonochemical destruction. It is very common that at very high powers, the degradation rate may be decelerated. 3.4. Effect of initial pH The effect of solution pH on the sonolytic degradation of 4-CyP was studied by varying the initial pH under constant process parameters. The initial pH values were adjusted with dilute

sulfuric acid and sodium hydroxide solutions. The sonochemical degradation of 4-CyP at acidic and alkaline initial pH together with a control representing the data at the initial natural pH of the solution is shown in Fig. 7. It may be observed from this figure that the decrease in 4-CyP concentration was faster under acidic media. The degradation rate decreases by increasing solution pH. This can be attributed to the dependency of 4-CyP ionization on the pH value. The ionic fraction of 4-CyP ion (uions) can be calculated from:

uions ¼

1

ð9Þ

1 þ 10ðpKapHÞ

Obviously, uions increases as the pH value increased. The dissociation constant of 4-CyP (pKa) is 10.1. Hence, at pH 2 and 6.5, the compound exists mainly in molecular form, whereas at pH 10 it is partially in ionic form (uions = 0.44) due to deprotonation of the phenolic group. Ionic form of 4-CyP is much more hydrophilic and soluble than the neutral state and thus is less likely to approach the interfacial region of cavitation bubbles. Thus, at pH 10, the degradation is mainly carried out in the bulk of the solution where there is a lower concentration of OH radical. The faster decomposition of 4-CyP in acidic condition must be due to enriched hydrophobicity of the molecule by protonation of the phenolic group. This behavior leads to the more likely diffusion of the 4-CyP molecule at lower pH to the bubble–liquid interface, where the concentration of hydroxyl radicals is maximum. 3.5. Effect of temperature Sonolytic degradation of 4-CyP was conducted under different temperatures ranging from 20 to 50 °C for an initial substrate concentration of 5 mg L1. With an increase in the temperature, the degradation rate was also increased (Fig. 8). The temperature of the bulk phase affects the viscosity, gas solubility, vapor pressure and surface tension. The increase of aqueous temperature certainly increases the number of cavitation bubbles on sonolysis and thus the rate of production of radicals though results in a lowering of the cavitation threshold due to the rise in vapor pressure associated with heating of the liquid. In fact, there is always a possibility that the higher concentration of chemical species is present in the cavitating bubble due to higher vapor pressure at higher operating temperature and this generates much higher amounts of free

1

1

pH 2

0.8

0.8 pH 6.5 pH 10

0.6

C/C

0

C/C

0

0.6

0.4

0.4 20 W 40 W 60 W 80 W 100 W

0.2

0.2

0 0

5

10

15

20

25

30

35

40

Time (min) Fig. 6. Effect of ultrasonic power on the degradation of 4-CyP (frequency: 300 kHz; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5); temperature: 20 ± 1 °C).

0 0

5

10

15

20

25

30

35

40

Time (min) Fig. 7. Effect of pH on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; temperature: 20 ± 1 °C).

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1

1

0.8

0.8

20 °C 30 °C 40 °C 50 °C

0.6

C/C

C/C

0

0

0.6

Air Argon Nitrogen

0.4

0.4

0.2

0.2

0

0

0

5

10

15

20

25

30

35

40

0

5

Time (min)

10

15

20

25

30

35

40

Time (min)

Fig. 8. Effect of temperature on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5).

Fig. 9. Effect of the saturating gas on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5); temperature: 20 ± 1 °C).

radicals in the system leading to higher degradation rates. On the other hand, an increase in temperature increases the vapor pressure. Consequently, the cavitation bubbles are filled with water vapor. The increased vapor content increases the resistance to the inward motion of a bubble during the collapse resulting in the reduced intensity of the collapse. Combining all of the above, the effect of temperature on sonochemical degradation rate is complicated. In this work, an increase in destruction rate showed moderate change with respect to temperature, especially in the range of 30–50 °C (Fig. 8).

increasing the oxygen concentration. On the contrary, dissolved nitrogen present in aqueous solution might scavenge the free radicals [35] and inhibit the free radical oxidation of 4-CyP, as shown in reactions (10)–(15).

3.6. Effect of saturating gas Sonolytic destruction of organic pollutants in aqueous solution depends on the saturating gas. Gas injection during sonication enhances cavitation and selection of the right gas is very important, because it may change the reaction efficiency by promoting radical species not only from sonolysis of water but also from the gas involved [15,32]. In order to investigate the influence of the saturating gas on the degradation of 4-CyP, continuous injection of argon, air or nitrogen at equivalent flow rates during sonication and the 4-CyP concentration was monitored. Fig. 9 shows the time dependence of the 4-CyP degradation rate under argon, air and nitrogen. It can be seen from this figure that the order of degradation ratio is as follows: argon > air > nitrogen. Because 4-CyP is a non-volatile compound, the gaseous atmosphere is mainly composed of the saturating gas and water vapor. It is well known that high polytropic gas ratio (c) and low thermal conductivity (k) favor higher collapse temperatures [33]. In accordance, the values of c and k for Ar (c = 1.67, k = 179  104 W m1 K1), air (c = 1.4, k = 259  104 W m1 K1) and nitrogen (c = 1.4, k = 240  104 W m1 K1) suggest that maximum collapse temperature is expected in the presence of argon, and lower temperatures in the presence of air. Nevertheless, the degradation in the presence of air (Fig. 8) is not consistent with this rule. In the presence of air, cavitation bubble is able to promote radical species not only from sonolysis of water but also from oxygen that promotes parallel reactions (reactions 2 and 4) and OH radical species [34]. Thus, the rate of sonochemical degradation naturally increases with

N2 þ  OH ! NO2 þ H

ð10Þ

N2 þ O ! NO þ N O2 þ N ! NO þ O

ð11Þ ð12Þ

NO þ O !  NO2

ð13Þ

NO þ  OH ! HNO2

ð14Þ



ð15Þ



NO2 þ OH ! HNO3

3.7. Effect of KBr and tert-butyl alcohol Many advanced oxidation processes for water and wastewater decontamination undergo a variable level of inhibition by water components, particularly the anions, because of the scavenging of reactive species. The effect on sonochemical water treatment processes may be quite different. Therefore, the influence of bromide ions on sonolytic degradation of 4-CyP was investigated. Effect of addition of KBr (300–5000 mg L1) on sonochemical destruction of 4-CyP is shown in Fig. 10. As can be seen from this figure, the degradation rate was considerably enhanced by the addition of bromide ion. The sonolytic destruction increased with increasing bromide concentration. The improvement of the degradation rate should involve the presence of the dibromine radical anion (Br 2 ) coming from the reaction of bromide ion with OH radical (reactions 16 and 17). Bromide at sufficiently high concentration could be able to reach the interfacial region of cavitation bubbles and therefore react with OH to yield radical anion Br 2 , which is reactive but less than OH radical toward organic and inorganic compounds [36]. It is possible to account for the enhancement of 4-CyP sonochemical degradation by bromide anion under the hypothesis that the radical Br 2 undergoes more limited radical– radical recombination (reaction (18), k18 = 2  109 M1 s1) compared to OH (reaction (6), k6 = 5.5  109 M1 s1) on the surface of the cavitation bubbles [37]. Although the OH radical is consumed in the presence of bromide, the total amount of radical increased at the interfacial region. In this way, the radicals Br 2

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1

0.8 0 mg/L KBr 300 mg/L KBr 5000 mg/L KBr

C/C

0

0.6

0.4

0.2

0 0

5

10

15

20

25

30

35

40

been reported by Tauber et al. [39]. The tert-butyl alcohol is able to scavenge OH radicals in the bubble and prevent the accumulation of OH radicals at the interface of the bubble. In the present work, the scavenging effect of tert-butyl alcohol on the degradation of 4-CyPP (5 mg L1) was investigated (Fig. 11). The degradation was effectively quenched, but not completely, by the addition of tert-butyl alcohol, suggesting that the main mechanism of 4-CyP destruction is chemical oxidation by hydroxyl radicals. This low degradation suggests that the degradation takes place at the interface of liquid–gas bubbles where it is oxidized by hydroxyl radicals formed within the cavitation bubbles as a result of the sonolysis of water. Another factor that also affects the rate of 4-CyP degradation is the formation of volatile products from the tert-butyl alcohol degradation that accumulate inside the bubble. Such volatile products decrease the temperature inside the bubble, which, in turn, slow the sonolytic reactions [40]. From these results, it can infer that 4-CP does not undergo direct pyrolysis in the sonolytic bubbles but rather becomes oxidized at the interface by reacting with OH radicals.

Time (min) Fig. 10. Effect of bromide concentration on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5); temperature: 20 ± 1 °C).

could be more available than OH to react with 4-CyP and induce its degradation. Accordingly, the anions could transform OH into less reactive species that could however be involved at a higher extent into substrate decomposition. Additionally, compared with OH, the non-recombined Br 2 radicals are likely able to migrate from the cavitation bubbles to react with 4-CyP molecules [38].

Br þ  OH ! Br þ OH

ð16Þ

Br þ Br ! Br 2

ð17Þ

   Br 2 þ Br2 ! Br3 þ Br

ð18Þ

If indeed the hydroxyl radical is a major participant in the sonolytic degradation of 4-CyP, we should be able to suppress this reaction by using a known OH radical scavenger in the solution. Extensive work on the sonochemistry of tert-butyl alcohol has

3.8. Effect of bicarbonate The negative effect of bicarbonate ions on several advanced oxidation processes is well known. The influence of bicarbonate ions on the sonochemical degradation of organic pollutants is controversial. In order to evaluate this, the sonolytic destruction of 4CyP (0.05–30 mg L1) was investigated in the presence of various concentrations of bicarbonate (30–1000 mg L1 of NaHCO3) at pH 8.3. Addition of bicarbonate ion had no significant effect on the degradation rate in the 4-CyP concentration range of 0.5– 30 mg L1. At low substrate concentration (0.05 mg L1), the rate of 4-CyP decomposition was drastically enhanced in the presence of bicarbonate (Fig. 12). The sonochemical degradation increases by increasing bicarbonate concentration and reached a maximum at 250 mg L1 of NaHCO3. The increase of bicarbonate concentration above this value retards the process, but the destruction rate was higher than that obtained in the absence of bicarbonate. The improvement of 4-CyP destruction is due to the formation of carbonate radical (CO 3 ) coming from the reaction of bicarbonate ion with OH radical (reaction (19), k19 = 8.5  106 M1 s1 [40]).

1

1 0 mg/L NaHCO

3

30 mg/L NaHCO

0.8

0.8

0 mg/L 300 mg/L 1000 mg/L 5000 mg/L

3

3

250 mg/L NaHCO

3

1000 mg/L NaHCO

0.6

3

C/C

C/C

0

0

0.6

70 mg/L NaHCO

0.4

0.4

0.2

0.2

0

0

0

5

10

15

20

25

30

35

40

Time (min) Fig. 11. Effect of tert-butyl alcohol concentration on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 5 mg L1; pH: natural (6.5); temperature: 20 ± 1 °C).

0

1

2

3

4

5

6

7

8

Time (min) Fig. 12. Effect of bicarbonate concentration on the degradation of 4-CyP (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 0.05 mg L1; pH: 8.3; temperature: 20 ± 1 °C).

M. Chiha et al. / Ultrasonics Sonochemistry 18 (2011) 943–950

1

0.8 Pure water Natural water

C/C

0

0.6

0.4

0.2

0 0

5

10

15

20

Time (min) Fig. 13. Sonochemical degradation of 4-CyP in pure and natural water (frequency: 300 kHz; power: 80 W; volume: 300 mL; 4-CyP initial concentration: 0.05 mg L1; pH: 7.6; temperature: 20 ± 1 °C).

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concentrations shows that the higher the substrate concentration was, the higher the initial decomposition rate was. The kinetic model based on a Langmuir mechanism was capable of describing adequately the sonochemical degradation of 4-CyP. The rate of 4CP degradation is frequency dependant. The degradation rate increased proportionally with increasing ultrasonic power from 20 to 100 W and temperature in the range of 20–50 °C. The most favorable condition for the degradation was observed in acidic media. The dependence of the 4-CyP degradation rate in the presence of various saturating gas shows the following order: argon > air > nitrogen. The degradation was inhibited in the presence of nitrogen gas owing to the free radical scavenging effect in vapor phase within the cavitation bubbles. The ultrasonic degradation of 4-CyP was clearly promoted in the presence of bromide ion and the promoting effect on degradation increased with increasing bromide concentration. The improvement of the degradation rate should involve the presence of the dibromine radical  anion (Br 2 ) coming from the reaction of bromide ion with OH radical. At low 4-CyP concentration (0.05 mg L1), bicarbonate ion considerably improved the degradation. Experiments conducted using pure and natural water demonstrated that the sonolytic treatment was more efficient in the natural water compared to pure water. Acknowledgement

CO 3

The radical is a strong one-electron oxidant (1.78 [41] and 1.59 V [42] vs. SHE at pH 7.0 and 12.5, respectively).

HCO3



þ OH !

CO 3

þ H2 O

ð19Þ

The financial support by the Ministry of Higher Education and Scientific Research of Algeria to the Project No. J0101120090018 is greatly acknowledged.



At low concentration of 4-CyP, the combination of OH radical (reaction (6), k6 = 5.5  109 M1 s1) is dominant, but in the presence of bicarbonate, the formation of CO 3 radical that is less reactive than OH radical, minimizing the combination of CO 3 that decays by reacting with itself according to reaction 20, 21 (k20,21 = 2  107 M1 s1). The substitution of OH with CO 3 could enhance degradation if the latter, although less reactive than OH, undergoes radical–radical recombination at a lesser extent than the hydroxyl radical. The combination of OH is known to be 275 times higher than that of the reaction of CO 3 with itself.  2 2 CO 3 þ CO3 $ C2 O6 ! CO4 þ CO2

CO 3 þ

H2 O CO 3 ! 2CO2

þ HO2 þ OH

ð20Þ ð21Þ

3.9. 4-CyP treatment in natural water Since 4-CyP presents an endocrine disrupting effect at low levels, and because, in several cases, 4-CyP has been found in natural waters at low concentrations, the study of the elimination of low 4CyP level in natural water was investigated. The ultrasonic degradation of 4-CyP was studied by dissolving the pollutant in a natural mineral water (0.05 mg L1). The main characteristics of the natural water are: pH = 7.6, Ca2+ = 486 mg L1, Na+ = 9.1 mg L1, 1 1 Cl = 10 mg L1, SO2 , HCO . The deg4 = 1187 mg L 3 = 403 mg L radation rate in the natural water is higher than in the pure water (Fig. 13). Similarly, Torres et al. [43] have showed that the degradation of bisphenol A by 300 kHz ultrasound in a natural mineral water was higher than in the deionised water. 4. Conclusion Removal of the endocrine disrupting chemical 4-CyP by 300 kHz ultrasonic irradiation was reached. The extent of degradation was inversely proportional to the initial concentration of 4-CyP. The initial sonolytic degradation rate of 4-CyP at different

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