Removal of sulfamethoxazole from aqueous solution by sono-ozonation in the presence of a magnetic catalyst

Separation and Purification Technology 117 (2013) 46–52

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Removal of sulfamethoxazole from aqueous solution by sono-ozonation in the presence of a magnetic catalyst Liwei Hou a, Hui Zhang a,⇑, Liguo Wang a, Lu Chen a, Yidong Xiong b, Xiaofei Xue a a b

Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079, China School of Physics and Technology, Wuhan University, Wuhan 430072, China

a r t i c l e

i n f o

Article history: Available online 21 May 2013 Keywords: Sulfamethoxazole Sono-catalytic ozonation Magnetic Acute toxicity Biodegradability

a b s t r a c t The degradation of sulfamethoxazole (SMX) was investigated under the sono-catalytic ozonation process over magnetic suspension. The effects of various operating conditions were investigated, including initial pH value, catalyst dosage, and power density. The results indicated that the degradation of SMX followed a pseudo-first-order kinetic model under different operating conditions. The removal rate increased with the increase of pH, but it decreased when the catalyst dosage or power density exceeded a certain value. The removal rate was inhibited in the presence of tert-butyl alcohol, isopropanol or NaF. After 20 min treatment, the COD removal reached 57.5%, the acute toxicity to Daphnia magna largely decreased, and the biodegradability (BOD5/COD ratio) increased from 0.02 to 0.41. COD was almost completely removed when the reaction time was extended to 120 min. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the occurrence of antibiotics residues in the aquatic environment has been recognized as one of the emerging issues in environmental chemistry and a human health risk where water is subsequently used to supply drinking water [1]. Although the concentration of the pollutants in the aquatic environment is low, its continuous input may constitute a potential risk for human and terrestrial organisms in the long-term period [2]. In this work, sulfamethoxazole (SMX), a commonly-used sulfonamide antibiotic, was chosen as the target contaminant due to its large global consumption in animal food industry [2]. SMX and its by-products have been identified along with other pharmaceuticals in the secondary wastewater effluents [2,3]. Although they have little or no direct effects on human health, risk for development of antibiotic resistant bacteria has been reported [4]. Therefore, it is important to develop efficient treatment technologies to control the concentration level of SMX in aquatic environments. The use of ozonation has attracted considerable interest for antibiotics treatment. It is considered as a promising alternative method to conventional processes due to its capability for oxidizing complex organic pollutants to simpler and more easily biodegradable compounds [5,6]. However, ozone treatment has the limitation with regard to the selectivity of organic compounds, high cost of the implementation and the poor mass transfer in

⇑ Corresponding author. Tel.: +86 27 68775837; fax: +86 27 68778893. E-mail address: [email protected] (H. Zhang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.05.014

aqueous solution [7,8]. As a consequence, ozone was usually combined with other technologies to improve its utilization, such as UV/O3, H2O2/O3, US/O3, and catalytic ozonation [8–13]. Among the stated ozone-based technologies, catalytic ozonation process is considered as an effective way to improve the degradation of recalcitrant pollutants [11,12]. Generally, catalytic ozonation includes homogeneous and heterogeneous catalytic ozonation [12]. Especially, heterogeneous catalytic ozonation presents a higher activity on the decomposition of ozone [13]. Supported and unsupported metal oxides are the catalysts most commonly used for the ozonation of organic compounds. However, one obstacle to the practical application of catalytic ozonation is the separation of these catalysts from the final wastewater effluent. To overcome this drawback, magnetite was used to simplify the recovery stage of the treatment process [14]. Recently, ultrasound has been used to combine with ozone for the non-biodegradable organic contaminants removal [8,10]. When ultrasound is introduced into the reaction system, the transient cavitation can result in turbulent flow conditions within the reactor. This will accelerate turbulence and reduce the liquid film thickness [15]. Accordingly, the mass transfer of ozone in aqueous phase is increased by ultrasonic irradiation. In the meanwhile, the cavitation effect could enhance the continuous cleaning of the catalyst surface and this effect could keep the catalyst away from poisoning during the reaction [16]. In addition, hydroxyl radical (OH) will be generated in the vapor phase of cavitation bubbles in the US/O3 process [8,17]. The internal loop-lift reactor has been regarded as a promising type of gas–liquid reactor for its good

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2. Experimental section

1.0

0.8 SMX+

Fraction /%

mixing with low shear stress and energy consumption. Moreover, it can also enhance the mass transfer rate between the gas–liquid phases [18]. Herein, a rectangular air-lift reactor was employed for the degradation of SMX by sono-catalytic ozonation process, which has rarely been reported in the antibiotic treatment [19]. The effects of operating conditions such as pH, ultrasonic power density, addition of catalyst and hydroxyl radical scavengers on SMX removal were studied in the process of US/Fe3O4/O3. The mineralization, biodegradability and acute toxicity of SMX were also studied during the sono-catalytic ozonation process.

SMX-

SMX

0.6

0.4

0.2

2.1. Materials Sulfamethoxazole (C10H11N3O3S, analytical reagent, P99.0%) was purchased from Fluka (Buchs, Switzerland) and used as received without further purification. The structure and the relevant data of SMX were shown in Table 1. The forms of SMX depend on the pH of the solution, and it was shown in Fig. 1. Fe3O4 was purchased from Prolabo Co. (Paris, France). tert-butyl alcohol (TBA), isopropanol and NaF were purchased from Shanghai Reagent Co., Ltd. (Shanghai, China). Both oxalic acid and potassium iodide (AR) were obtained from Shanghai Zhangyun Chemical Co., Ltd. (China). All the solutions were prepared with deionized water. Table 1 Chemical structures and relevant data for sulfamethoxazole. Compound

Sulfamethoxazole

Molecular formula Structure

C10H11N3O3S

O S

O

NH2

NH

O Formula weight (g/mol) kmax (nm) pKa1 pKa2

N

253.3 278 1.6 5.7

O O

NH3+

S NH

O

2

4

6

8

pH Fig. 1. pH-dependent speciation of SMX in the solution.

2.2. Procedure Semi-batch experiments were performed in the rectangular airlift reactor as we used in our previous study [18]. SMX was prepared freshly in buffer solution (0.05 mol/L H3PO4 and 0.1 mol/L of NaOH solution to reach pH 9.0 [20], and 0.05 mol/L Na2HPO4 and KH2PO4 for other pH values, pH 3.0, 5.0, and 7.0) before each run. The solution pH was measured with a pH-meter (Mettler Toledo FE20). Ozone was generated by a laboratory ozone generator (XFZ-5BI, China). Concentrations of ozone in the gas and liquid phases were detected by the iodometric method and indigo method, respectively [21,22]. Then, a predetermined amount of Fe3O4 was added into the reactor and stirred by a magnetic stirrer (Model 78-1, Hangzhou Instrument Motors Factory, China) to keep a complete mixing of the solution in the reactor. Sonication was performed with a KS-250 ultrasonic generator (250 W, 20 kHz, Ningbo Kesheng Instrument Co., China). The tip of the probe was 1.0 cm in diameter and was placed 1.5 cm below the surface of the reaction solution. The sonication was administered in a pulse mode of 2.5 s on and 2.1 s off. The acoustic power was determined calorimetrically [23]. All experiments were conducted at ambient temperature (22 ± 2 °C). Samples were taken by syringe at different time intervals, and then filtered through 0.22 lm polyvinylidene fluoride (PVDF) syringe filters (Millipore Co.) before analysis.

2.3. Analysis methods

O

Chemical speciation

O

NH2

S NH

N

Non-protonated form (SMX)

O O

NH2

S -

N O

0

N

Protonated form (SMX+)

O

0.0

N

Deprotonated form (SMX-)

The SMX concentration was determined by reversed phase high-performance liquid chromatography (HPLC). HPLC consisted of a LC-20AB pump (Shimadzu), Shim-Pack VP-ODS-C18 column (4.6 mm  250 mm, 5 lm) with a flow rate of 1.0 mL/min and a UV detector (Shimadzu UV-1600 spectrophotometer). The UV detector was set at the maximum absorption wavelength of SMX (278 nm) using a Shimadzu UV-1700 spectrophotometer to scan from 200 to 800 nm. The mobile phase was 46% acetonitrile-54% oxalic acid solution (0.01 mol/L) and the injection volume was 20 lL. The COD concentration was determined by colorimetric method at 440 nm with Hach spectrophotometer (Hach DR/2000, Loveland, Co.) based on the standard method [24]. The five-day biochemical oxygen demand (BOD5) was measured using Oxi-top BOD analyzer (WTW Oxitop IS6, Germany).

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The toxicity of the SMX and its degradation products were tested using 24 h born Daphnia magna at different dilutions [25]. These tests were evaluated with the same procedure as Organization for Economic Cooperation and Development (OECD) Guideline 202 [26]. Daphnia magna organisms were cultivated in the laboratory for more than three offsprings. They were put into the solution for culturing in an incubator for 24 h. The incubator was set at 20 °C in a 16 h light – 8 h dark cycle. Foods were not given during the acute toxicity test. Toxicity experiments were carried out with 5 Daphnia magna in each test beaker with 50 mL of reaction solution after different dilution rates. Results were evaluated on the basis of immobilization percentage obtained by counting the number of survived organisms after 24 h. The catalyst structure was characterized by X-ray diffractometer (XRD), using a D5005 diffractometer from Bruker equipped with a CuKa radiation (k = 1.5406 Å). Signals were recorded for 2h between 20° and 80° with a recording step of 0.05° each 3 s. Phase identification was made by comparison with Joint Committee on Powder Diffraction Standards (JCPDS) database (Fig. 2). The total dissolved iron (Fe2+, Fe3+) leached from Fe3O4 was determined by using the 1,10-phenanthroline method [27]. 3. Results and discussion 3.1. SMX decay in different oxidation systems To investigate the roles of ozone, ultrasound and catalyst in SMX removal, different sets of experiments were conducted. The experiments were carried out at 85.7 W/L of ultrasound power density, while the initial SMX concentration was 50 mg/L, the Fe3O4 dosage was 0.3 g/L, pH was 7.0 and gas ozone concentration was 15.8 mg/L. It can be seen that the degradation rate follows apparent pseudo-first order kinetics. The SMX removal was negligible in the presence of ultrasound alone (data not shown) due to the limited production of OH in the bulk solution at low frequency of ultrasound [28]. In addition, the reaction occurred mainly at the interface of cavitation bubbles or in the bulk solution via a radical reaction rather than inside of the bubbles via a pyrolytic reaction due to the nonvolatile and hydrophilic nature of the SMX [29]. In the meanwhile, slight adsorption of SMX on the catalyst was observed (data not shown). Even though the catalyst was combined with ultrasound, no significant difference on the removal efficiency of SMX was observed (data not shown).

Fig. 3 illustrates that only 0.18/min rate constant was obtained by ozonation while the rate constant increased to 0.22/min when ozonation was combined with ultrasonic irradiation. The decomposition of ozone would produce thermolytically free radicals in the vapor phase of a cavitation bubble [30]:

O3 þÞÞÞ ! O2 ðgÞ þ Oð3 PÞðgÞ

ð1Þ

Oð3 PÞ þ H2 O ! 2 OHðgÞ

ð2Þ

where the symbol ‘‘)))’’ represents ultrasonic irradiation. The generated free radicals might diffuse into the bulk solution to oxidize the SMX. Furthermore, in the presence of ultrasound, an energy gradient in the direction of propagation of the acoustic wave would be formed. It could lead to the reduction of liquid film thickness of gas bubbles containing ozone [19,31]. Moreover, the ultrasonic breakup of these gas bubbles due to ultrasonic irradiation would lead to the larger specific surface area between the gas and liquid phases [19,31,32]. Compared to US/O3 system, the Fe3O4/O3 process was favorable for the degradation of organics. When a metal oxide is introduced into water, it tends to strongly adsorb H2O molecules. The adsorbed H2O dissociates into OH and H+, forming surface hydroxyl groups with the surface metal and oxygen sites, respectively [33,34]. It was reported that the surface hydroxyl groups of the solid metal oxide were one of the active sites in heterogeneous catalytic ozonation [7,33]. In the meanwhile, ozone shows strong affinity to Lewis acid sites on the surface of metal oxide, enhancing the redox process involved in ozone decomposition [35,36]. As discussed above, it is very likely that ozone adsorption/ decomposition occurs on the active Lewis sites of the catalyst. Therefore, the rate of ozone decomposition was enhanced in the catalyst suspensions and the removal rate was 0.25/min (Fig. 3), paralleling the COD removal in the system (as shown in Section 3.6). A far higher removal rate of SMX (0.36/min) was achieved in the sono-catalytic ozonation process. It was the fact that the catalyst surface could keep from deactivation with ultrasonic irradiation and more available active sites could be obtained in the presence of ultrasonic irradiation. In meanwhile, ultrasound may cause the fragmentation of catalyst and increase the surface area as well as the active sites [17]. Moreover, the mass transfer between the reactants and the catalyst could be enhanced during the ultrasonic irradiation. Therefore, a global increase in the process of US/Fe3O4/O3 could be achieved.

0.4

1.0 4000

0.3 0.25

k (/min)

0.8 3000 0.6 2000

Fresh Fe3O4

0.22 0.2

0.18

0.1

Ct /C0

Intensity /a.u.

0.36

0.0

0.4

O3

US/O3 Fe 3 O4 /O3 US/Fe 3 O4/O3

O3

1000

O3 /US

0.2

O3 /Fe3 O4

Reference Fe3O4 0 20

O3 /Fe 3 O4 /US

0.0 30

40

50

2θ /

60

70

80

o

Fig. 2. XRD pattern of Fe3O4 particles. Reference X-ray lines belonging to Fe3O4 (JCPDS card number 89-2355) were given at the bottom of the figure.

0

5

10

15

20

t (min) Fig. 3. Comparative performance for different oxidation systems on SMX removal ([SMX] = 50 mg/L, [O3]g = 15.8 mg/L, Q = 30 L/h, P = 85.7 W/L, [Fe3O4] = 0.3 g/L, and pH = 7.0). The inset gives the removal rate of SMX in different oxidation systems.

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3.2. Effect of pH on SMX removal

 FeOH2þ ! FeOH þ Hþ

ð4Þ

The pH value is usually considered as one of the most important factors for sono-catalytic ozonation treatment. Herein, the degradation of SMX was investigated at fixed pH of 3.0, 5.0, 7.0, and 9.0, respectively. It is clearly shown in Fig. 4 that the SMX removal rate increased with increasing pH significantly. However, when the pH value exceeded 7.0, the enhancement effect on the SMX removal was limited. SMX has two pKa values of 1.6 and 5.7, resulting in protonated, non-protonated and deprotonated forms at different pH values [37]. Considering the structure of the SMX, the possible reaction centers that are more susceptible to ozone electrophilic attack can be proposed. The amino group seems to be the dominant sites where ozone attack can be expected. However, the pH of the medium has an important role on the mechanism of reaction [38]. When the pH was 3.0, the non-protonated form was the predominant form (Fig. 1 and Table 1), which was less susceptible to ozone attack than that of the deprotonated form [38]. Therefore, the reaction rate was low under the acidic condition (0.08/min, pH 3.0). When the pH increased to 5.0, the deprotonated component of SMX increased accordingly, leading to the higher reaction rate (0.12/min). Further increasing pH to 7.0, all the SMX molecules were present in a completely deprotonated form, which led to a higher reactivity towards ozone (0.25/min) [38]. However, the dissociation of the hydrogen presented in the –NH– group, when the pH increased to 9.0, promoted a slight increase of the SMX reactivity [38,39]. Hence, the enhancement of the reaction rate was limited when the pH value changed from 7.0 to 9.0. On the other hand, the decomposition rate of ozone depended on the pH. The direct attack on organic compounds by ozone occurred under acidic conditions [14]. At a high pH value, ozone was partially decomposed to non-selective OH (Eq. (3)), which could easily attack the molecule of SMX [14].

 FeOH ! FeO þ Hþ

ð5Þ

2O3 þ OH !  OH þ O2 þ 2O2

ð3Þ

In addition, the surface activity of metal oxides could be affected by pH. There exists proton transference on the surface of metal oxides at the different pH values [40]. When the pH value was increased, the species of  FeOHþ 2 decreased and the species of FeO were formed (Eqs. (4) and (5)). The catalytic activity of Fe3O4 reached the maximum when the pH was 7.0, because the FeO had a high affinity for molecular ozone and was easy for the surface OH-ozone combination [40]. Thus, the removal rate of SMX increased with the increase of pH when at pH < 7.0 and the increase became insignificant when at pH > 7.0.

3.3. Effect of power density on SMX removal The removal of SMX by sono-catalytic ozonation with Fe3O4 at different ultrasound power densities was investigated. As observed in Fig. 5, the removal rate of SMX increased from 0.26 to 0.32/min when the power density increased from 36.9 to 85.7 W/L. However, the removal rate decreased to 0.30/min with further increase in power density to 101.3 W/L. With the increase in the power density, the cavitation effect would be accelerated. This would enhance mass transfer from gas phase to liquid phase, and that of the SMX between the liquid phase and the catalyst surface. In addition, the cleaning effect of ultrasonic irradiation and the generation of free radicals from ozone decomposition would also be improved with the increase of cavitation effect. When ultrasound power density further increased to 101.3 W/L, an insufficient collapse of the cavitation bubbles and an acoustic screen could be generated, and the scattering effect would come forth which consumed a part of ultrasonic power and translated it into heat [41,42]. Therefore, the generation of OH would decrease to some extent. Although degassing effect should be taken into account in the presence of ultrasonic irradiation, it was not pronounced in the sparged system [43,44]. Hence, the removal rate of SMX decreased at 101.3 W/L of power density. 3.4. Effect of Fe3O4 dosage on the removal of SMX The effect of the catalyst dosage on the SMX removal rate is shown in Fig. 6. The removal rate of SMX slightly increased when Fe3O4 dosage increased from 0.3 to 0.6 g/L (corresponding to 0.24 and 0.35/min, respectively). When the catalyst dosage exceeded 0.6 g/L, the removal rates decreased accordingly (0.30 and 0.26/ min at 1.0 and 1.5 g/L, respectively). Logically, the increase in Fe3O4 dosage provided more Fe(II)/Fe(III) active sites, which enhanced the degradation rate accordingly. In the meanwhile, the formation of gas nuclei in the catalyst porosity was increased with an increasing catalyst dosage and the cavity formation rate was increased. However, the degradation rate decreased when the Fe3O4 dosage exceeded 0.6 g/L. This is because the overdose of the catalyst would inhibit the collisions between OH and organic compounds. Similar studies have been reported on the ozonation of

0.31

0.3

k (/min)

0.3

0.8

0.12

0.1

0.08

0.6

0.6 0.0

pH = 3.0 pH = 5.0 pH = 7.0 pH = 9.0

0.4

0.2

0.1

0.4 0.0

0.2

pH= 3.0 pH= 5.0 pH= 7.0 pH= 9.0

0.0 0

0.2 0.0 5

0.30

0.26

0.2

Ct /C0

Ct/C0

0.8

0.32

1.0

0.25

k (/min)

1.0

10

15

20

t (min) Fig. 4. The effect of pH on SMX removal ([SMX] = 50 mg/L, [O3]g = 14.7 mg/L, Q = 30 L/h, P = 85.7 W/L, and [Fe3O4] = 0.3 g/L). The inset shows the removal rate of SMX at different pH values (pH = 3.0, 5.0, 7.0, and 9.0).

36.9 W/L

85.9 W/L

101.3 W/L

P=36.9 W/L P=85.7 W/L P=101.3 W/L

0

5

10

15

20

t (min) Fig. 5. The effect of power density on the removal of SMX ([SMX] = 50 mg/L, [O3]g = 15.4 mg/L, Q = 30 L/h, [Fe3O4] = 0.3 g/L, and pH = 7.0). The inset shows the reaction rate of SMX at different power densities.

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L. Hou et al. / Separation and Purification Technology 117 (2013) 46–52

0.4 0.35

0.8

0.2

0.25 0.2

0.6 0.1

0.4

0.4 0.0

0.3 g/L

0.6 g/L

1.0 g/L

1.5 g/L

0.0 0

10

15

0

20

5

3.5. The role of different active radicals on SMX removal

10

15

20

3.6. Mineralization, biodegradability and acute toxicity tests To investigate the mineralization of SMX in the US/Fe3O4/O3 system, experiments were carried out when the SMX concentration was 50 mg/L, pH was 7.0, the gas flow rate was 30 L/h, gaseous ozone concentration was 12.3 mg/L, ultrasonic power density was 85.7 W/L and the Fe3O4 addition was 0.3 g/L. As can be seen in Fig. 8 and 57.5% COD was removed after 20 min reaction compared with almost complete removal of SMX. After the reaction time was extended to 120 min, a nearly complete COD removal was achieved. At the beginning of the process, the COD removal rate was found to be relatively high, but it dropped slowly with the extension of the reaction time. It is the fact that intermediates, such as carboxyl acids, are more difficult to be oxidized compared to their parent compound [47]. The biodegradability and the toxicity of the effluent in the US/ Fe3O4/O3 process were also evaluated in this study. The BOD5/ COD ratio of the initial solution was 0.02, indicating that SMX was hardly biodegraded. After 20 min treatment when SMX was nearly completely removed, the value of BOD5/COD was increased to 0.41. It showed that the effluent after 20 min treatment was much more easily biodegradable than the influent. To determine the potential environmental impact of SMX and its by-products, the variation of acute toxicity during US/Fe3O4/ O3 process before and after the reaction was examined by 24-h

1.0

Ct/C0 1.0 COD/COD0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60

80

100

COD/COD0

To further determine whether the radical reaction played a major role in the degradation of SMX in the US/Fe3O4/O3 system, some radical inhibitors were employed when the SMX concentration was 50 mg/L, gaseous ozone concentration was 14.1 mg/L, ultrasonic power density was 85.7 W/L, Fe3O4 addition was 0.3 g/L, the gas flow rate was 30 L/h and the pH was 7.0. It is generally accepted that the ozone oxidation pathways include the direct oxidation by molecular ozone and the indirect radical oxidation by hydroxyl radicals, which could be formed both in the solution and on the solid surface [17,19]. To go inside into the oxidation mechanism, two different radical scavengers (TBA, isopropanol) and a reactive surface hydroxyl group indicator (NaF) [45] were employed to investigate the role of the reactive species in this sono-catalytic ozonation process. Their respective quenching and replacing effects allowed us to verify the roles of the total hydroxyl radicals (OH), the hydroxyl radicals in solution (OHfree) and the hydroxyl radicals on the catalyst surface (OHads), respectively. As shown in Fig. 7, the degradation rates were suppressed to a certain extent upon TBA (0.1 mol/L), isopropanol (0.1 mol/L) and NaF (0.1 mol/L) addition, suggesting that both direct molecular ozone reaction and indirect radical reaction were involved in the process. The removal rate of SMX was decreased from 0.31 to 0.21/min when TBA was applied to the reaction system. However, when 0.1 mol/L isopropanol was added into the reaction solution, the removal rate slightly decreased to 0.27/min. In addition, NaF was used as an indicator of reactive surface hydroxyl groups [45,46]. When 0.1 mol/L NaF was added into the reaction solution, the hydroxyl groups on the surface of Fe3O4 were almost replaced by fluoride, and ozone decomposition was greatly inhibited. Consequently, the surface radicals (OHads) generated on the catalyst surface was suppressed, and the reaction rate dropped from 0.31 to 0.24/min accordingly. This result further confirmed that the ozone decomposition mainly occurred on the catalyst surface. Furthermore, the amount of iron dissolved in the solution was also determined at the end of the experiment. The total concentration of leached iron ions was 0.24 mg/L, indicating that Fe3O4 catalyst was relatively stable.

Isopropanol No scavengers

Fig. 7. The role of different active radicals on the degradation of SMX ([SMX] = 50 mg/L, [O3]g = 14.1 mg/L, Q = 30 L/h, P = 85.7 W/L, [Fe3O4] = 0.3 g/L, [TBA] = 0.1 mol/L, [Isopropanol] = 0.1 mol/L, [NaF] = 0.1 mol/L, and pH = 7.0).

Ct/C0

oxalic acid and reactive black 5 where an optimal catalyst dosage was observed [13,14]. Finally, considering the same final removal efficiency for SMX, 0.3 g/L was chosen as the most applicable dosage of the catalyst.

NaF

t (min)

t (min) Fig. 6. The effect of Fe3O4 dosage on the degradation of SMX ([SMX] = 50 mg/L, [O3]g = 13.6 mg/L, Q = 30 L/h, P = 85.7 W/L, and pH = 7.0). The inset gives the removal rate of SMX at different dosages of Fe3O4.

TBA

No scavengers 0.1 mol/L Isopropanol 0.1 mol/L NaF 0.1 mol/L TBA

0.0 5

0.21

0.1

0.0

0.2

0.3 g/L 0.6 g/L 1.0 g/L 1.5 g/L

0.2

0.31 0.27 0.24

0.26

Ct/C0

k (/min)

Ct/C0

0.6

0.3

030

0.3

0.8

1.0 k (/min)

1.0

120

t (min) Fig. 8. The variation of COD with reaction time ([SMX] = 50 mg/L, [O3]g = 12.3 mg/L, Q = 30 L/h, P = 85.7 W/L, [Fe3O4] = 0.3 g/L, and pH = 7.0).

L. Hou et al. / Separation and Purification Technology 117 (2013) 46–52

Before treatment After treatment

D. magna immobilization (/24 h)

1.0

[4] [5]

0.8

[6]

0.6 [7] [8]

0.4

[9]

0.2

[10]

0.0

0

1

2

3

4

5

[11]

Tested sample (/mL) [12] Fig. 9. The effect of SMX and its degradation products on immobilization of D. magna. Experimental conditions for SMX degradation ([SMX] = 0.5, 1.25, 2.5, 5.0 mg/L, [O3] = 13.2 mg/L, Q = 30 L/h, P = 85.7 W/L, [Fe3O4] = 0.3 g/L, and pH = 7.0).

immobilization test with Daphnia magna. Based on the protocol of the method [25,26], the volume (in mL) of tested sample was a measurement for toxicity. The larger value corresponds to the smaller toxicity [25]. As shown in Fig. 9, a maximum toxicity of 100% was observed in the initial solution, while only 45% immobilization of Daphnia magna was obtained after the treatment. It indicated that US/Fe3O4/O3 process could reduce the acute toxicity of SMX to a large degree. 4. Conclusion SMX can be effectively degraded and mineralized in the sonocatalytic ozonation process. The removal rate of SMX increased with the increase of pH value. There existed optimum values of Fe3O4 concentration and power density for the effective degradation of SMX. SMX could quickly react with ozone and the generated hydroxyl radicals. The presence of TBA, isopropanol or NaF inhibited SMX removal, indicating that both direct molecular ozone reaction and indirect radical reaction were involved in the US/ Fe3O4/O3 system. Further study showed that an improved biodegradability and a reduced toxicity of SMX solution after treatment. These findings suggested that US/Fe3O4/O3 process could be an optional method for enhancing the degradation of SMX and other refractory and toxic organic pollutants. Acknowledgements This study was supported by Natural Science Foundation of Hubei Province, China through ‘‘The Outstanding Youth Scholars Program’’ (Grant No. 2007ABB028) and Natural Science Foundation of China (Grant Nos. 20977069 and 21211130108). Liwei Hou would like to acknowledge the financial support by the Fundamental Research Funds for the Central Universities (Grant No. 20102050201000053). We appreciate the valuable comments of the anonymous reviewers.

[13]

[14]

[15]

[16]

[17]

[18]

[19] [20] [21] [22] [23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

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