Ultrasound-Assisted Catalytic Degradation of Methyl Orange with Fe

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Ultrasound-Assisted Catalytic Degradation of Methyl Orange with Fe3O4/Polyaniline in Near Neutral Solution Yang Wang, Ligang Gai, Wanyong Ma, Haihui Jiang, Xiangqian Peng, and Lichun Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504242k • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Industrial & Engineering Chemistry Research

Ultrasound-Assisted Catalytic Degradation of Methyl Orange with Fe3O4/Polyaniline in Near Neutral Solution Yang Wang,† Ligang Gai,*† Wanyong Ma,† Haihui Jiang,† Xiangqian Peng,† Lichun Zhao† †

Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry &

Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, People’s Republic of China

*Corresponding author. Tel: +86 531 89631208; Fax: +86 531 89631207; E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

An ultrasound-assisted advanced oxidation process (AOP) has been demonstrated for sonocatalytic degradation of methyl orange (MO) with Fe3O4/polyaniline (Fe3O4/PANI) microspheres in near neutral solution (pH ~6). The Fe3O4/PANI microspheres were characterized with XRD, SEM, TEM, FT-IR, XPS, and zeta-potential measurements, and further tested in the role of adsorption and sonocatalytic decolorization of MO in solution. The isotherms and kinetics of MO adsorption with Fe3O4/PANI follow the Langmuir model and the pseudo-second-order model, respectively. The kinetics of sonocatalytic decolorization of MO with Fe3O4/PANI conforms to a combinational model involving the pseudo-second-order adsorption model and the pseudo-first-order degradation model, since Fe3O4/PANI has a high capacity to adsorb MO in solution. The percentage of room-temperature sonocatalytic degradation of MO with Fe3O4/PANI is about 4.8, 8.8, and 5.7 times of that with Fe3O4, dedoped Fe3O4/PANI, and ultrasonication

alone,

respectively.

The

eco-friendly

Fe3O4/PANI

featured

with

superparamagnetism and excellent reusability offers a promising sonocatalyst for rapid decolorization and enhanced degradation of azodyes in effluents.

Keywords: Methyl orange; Fe3O4/PANI; Ultrasound-assisted degradation; Sonocatalysis


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1. INTRODUCTION The development of textile and dyeing industries produces plenty of daily necessities, and causes a series of environmental issues at the same time. It was estimated that around 15% of dye was lost during dyeing process and eventually discharged into environment,1 exerting heavy pressure on environmental protection and sustainability.2-5 The dyeing effluents are colored even in a very low concentration. This necessitates to degrade the dyes in effluents, at least to decolorize them before effluent disposal to environment. So far, many strategies have been designed to clear the dyeing effluents, such as adsorption,5-9 flocculation,10,11 extraction,12 ultrafiltration/nanofiltration,13 bioremediation,14 electrochemical reduction,15 and advanced oxidation processes (AOPs).1-4,16-31 An advanced oxidation process has been defined as a process that involves formation and subsequent reactions related to highly reactive free radicals, such as ·OH, ·O2‒, ·O2H, which are capable of oxidizing dyes to render decolorized effluents.2,19 The decolorization of dyes in an AOP process has two meanings, i.e., partial degradation to form aromatic intermediates and deep mineralization to generate oxyacids, CO2, and H2O.2,21,24 The reported AOP techniques comprise acoustic cavitation,3,16,20,28,29 sonocatalysis,2,19,26,30 photocatalysis,27 sonophotocatalysis,1,19,27 γ-irradiation,17 microwave degradation,18 and Fenton’s chemistry.4,21-25 To improve decolorization rate of the dye, combinational techniques are universally adopted involving two or three kinds of AOPs.1-4,18-30 The documented references related to AOPs are focused on increasing the concentration of radicals by adding chemical oxidants (e.g. O3, H2O2, K2Cr2O7),2,19,20,29 Fenton’s reagent,4,21-25 radical-producer (e.g. CCl4),3,4,28,29 and catalyst (e.g. TiO2, Fe3O4),1,2,18,25,26,30 and by using highfrequency ultrasonic atomizer.3 However, the addition of chemical reagent increases the cost and


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possibly causes secondary pollution associated with the chemicals.29 When nonmagnetic catalysts ( e.g. TiO2) are employed, their recovery is a troublesome issue. In a sonocatalytic process, dye degradation is promoted in acidic solution, based on the results that: (i) acid environment improves hydrophobic enrichment of dye molecules;2,3,32,33 and (ii) positively charged catalyst surface facilitates adsorption of negatively charged dye ions, resulting in enhancement in degradation rate of the dye.26 Considering the disposal of acidic solution does not meet the environmental protection, it is of great significance to make dye decolorization in neutral solution. Recently, polyaniline (PANI), a semiconducting polymer wealthy in amino groups, has been demonstrated to show a high capacity to separate sulfonated azodyes from aqueous solution.6,8,9 Since the nonmagnetic separation has limitations in adsorbent recovery, we report here on an advanced strategy in terms of ultrasound-assisted catalytic degradation of biorefractory azodyes with

superparamagnetic Fe3O4/PANI microspheres in near neutral

solution (pH ~6). This advanced strategy has four features: (i) positively charged Fe3O4/PANI in solution with pH 6 favors adsorption of negatively charged azodye molecules upon the catalyst, resulting in enhancement in degradation rate of the dye;26 (ii) sonogenerated electrons (ecb‒) and holes (hvb+) upon the semiconducting PANI shell contribute to produce highly reactive radicals capable of decomposing

dye

molecules;26,27 (iii)

superparamagnetic

Fe3O4/PANI facilitates

fast

recovery/redispersion of the catalyst by simply switching on/off an external magnet;34 and (iv) disposal of the decolorized and near neutral effluents meets the demand of environmental protection. In view of methyl orange (MO) being frequently investigated in dyeing effluent treatment,1,6,17,18 we performed our strategy by taking MO degradation as an example. As


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expected, the degradation level of MO is dramatically enhanced with Fe3O4/PANI, as compared to that with Fe3O4, dedoped Fe3O4/PANI, and ultrasonication alone. 2. EXPERIMENTAL 2.1 Preparation of Fe3O4/PANI catalyst. Fe3O4 and Fe3O4/PANI were prepared by modifying the synthetic procedures in previous reports.34,35 Detailed procedures are provided in Supporting Information (S1). PANI was also prepared for comparison (S2, Supporting Information). To obtain the dedoped Fe3O4/PANI, the as-prepared Fe3O4/PANI sample was subjected to dedoping treatment in 3 mol L‒1 ammonia water for 6 h at room temperature, and then collected by filtration, washed with distilled water and ethanol, and finally dried in a vacuum oven at 40 °C. 2.2 Characterization. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. Detailed characterization procedures were provided in S3 (Supporting Information). Iron content of Fe3O4/PANI was measured on a PerkinElmer Optima 7000DV inductively coupled plasma–optical emission spectrometer (ICP–OES). Before the ICP-OES measurement, the sample was agitated in 2 M HCl aqueous solution for 12 h. After separation, the supernate was sampled for examination. Zeta potential measurements were conducted on a Malvern Zetasizer Nano-ZS90 apparatus, after quantitative addition of sample powders into pre-prepared solutions with pH ranging from 2 to 12. Zeta potential measurements on MO solutions with different pH were also performed for comparison. 2.3 Adsorption of MO with Fe3O4/PANI. MO was chosen as the typical azodye for investigation. The structural formula and characteristics of MO are given in Table S1


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(Supporting Information). Aqueous solutions with dye concentration ranging from 10 to 100 mg L‒1 were prepared by the isochoric method. Before the adsorption experiments, calibration curve based on the Beer–Lambert law was established for use to quantify the dye concentration. The time-dependent adsorption capacity was measured at 25 °C with time range of 5‒540 min, using an aliquot of 30 mL of dye solution (50 mg L‒1) containing 10 mg of adsorbent. To plot the adsorption isotherms, 10 mg of adsorbent was dispersed into an aliquot of 30 mL of dye solution with concentration ranging from 10 to 100 mg L‒1. The dispersions were oscillated in a constanttemperature water-bathing vibrator (HZ-9613Y, Jiangsu, China) for 12 h, operating with a reciprocating vibration rate of 100 rpm at 15, 25, 40, and 55 °C, respectively. The supernate was sampled and analyzed by recording the absorption value at 464 nm corresponding to the characteristic absorption peak of MO, using an ultraviolet/visible (UV/Vis) spectrometer (TU‒1901, Beijing, China). 2.4 Sonocatalytic degradation of MO with Fe3O4/PANI. In a typical sonocatalytic degradation process, 10 mg of Fe3O4/PANI microspheres were added into a conical flask containing 30 mL of dye solution (pH ~6), followed by applying ultrasonic treatment in a digital display ultrasonic bath (100 W, 40 kHz; KQ-100DB, Shanghai, China) for 3 h. The effect of dye concentration on degradation rate was checked by varying dye concentration from 10 to 50 mg L‒1. The temperature-dependent degradation was investigated by fixing dye concentration at 50 mg L‒1 at 25, 35, 45, and 55 °C, respectively. The variation in temperature of the ultrasonic bath was controlled to be ±1 °C by virtue of circulating water. The concentration of dye in solution was measured at different time intervals, using the UV/Vis spectrometer (TU‒1901, Beijing, China). The total organic content (TOC) of MO solution at different time intervals was measured on a Shimadzu CPH CN200 TOC-L analyzer. Also, the products of MO degradation were examined


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with the gas chromatography/mass spectroscopy (GC/MS) technique, operating on an Agilent 7890A/7000B GC/MS analyzer. 3. RESULTS AND DISCUSSION 3.1 Characterization of the catalyst 3.1.1 Structure, morphology, and magnetic properties. Figure 1 shows the XRD patterns of the samples. The diffraction peaks marked in Figure 1a can be indexed to (111), (220), (311), (400), (422), (511), (440), and (533) planes of face-centered cubic Fe3O4 (JCPDS no. 88-0866). It should be pointed out that the phase of magnetic component of Fe3O4/PANI (Figure 1b) has not been altered during the ultrasound-assisted coating process, since there is no observable shift in peak position between Figure 1a and b. Compared with that for bare Fe3O4 (Figure 1a), the slight decrease in peak intensity for the composite (Figure 1b) is due to the amorphous polymer coating.

Figure 1. XRD patterns of the samples: (a) Fe3O4; (b) Fe3O4/PANI.


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The morphology and size of the samples are revealed by the SEM and TEM images. The bare Fe3O4 sample consists of microspheres with average diameter of 360 nm (Figure 2a), and the microspheres are constructed by self assembly of primary nanospheres with size ranging from 20 to 50 nm. After polymer coating, the diameter of most of the composite microspheres increases to ca. 470 nm (Figure 2b and c), rendering a well-defined core–shell structure with shell thickness in the range of 40–90 nm (Figure 2c).

Figure 2. SEM images of Fe3O4 (a) and Fe3O4/PANI (b); TEM image of Fe3O4/PANI (c); and room-temperature magnetization curves of the samples (d). Figure 2d shows the room-temperature magnetization curves of the samples. The magnetic saturation value (Ms), remnant magnetization (Mr), and coercivity (Hc) of the samples are given in Table 1. Compared with those for Fe3O4, the decrease in Ms and increase in Mr for the


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composite are due to the nonmagnetic polymer coating, which negatively contributes to magnetization readings in the standard practice of normalizing magnetization by sample mass.34 For Fe3O4/PANI, the Ms per magnetite mass based on the ICP‒OES result is 72.6 emu g–1, close to that of 71.3 emu g–1 for Fe3O4. This result confirms that the observed decrease in Ms (Table 1) is due to the polymer coating rather than the degradation of magnetic cores, in concert with the XRD result (Figure 1). It is worth noting that the composite remains superparamagnetism because the Hc value (38 Oe) is smaller than the maximum theoretical value of 50 Oe for superparamagnetic particles.35 The observed superparamagnetism is attributed to the primary Fe3O4 nanoparticles with size in the range of 20‒50 nm.36 It is well accepted that superparamagnetism enables magnetic particles to be separated from and rapidly redispersed into solution by simply switching on/off the magnetic field.37 Table 1. Room-temperature magnetic properties of the samples. Magnetic saturation value (Ms) (emu g‒1)

Remnant magnetization (Mr) (emu g‒1)

Coercivity (Hc) (Oe)

Fe3O4

71.3

2.04

38

Fe3O4/PANI

52.7

2.44

38

Sample

In previous report,34 the electrostatic attraction and hydrogen bonding are suggested to be responsible for the formation of core–shell structured Fe3O4/PANI without surfactant. In the present case, a SDS-assisted in situ polymerization is proposed for the formation of core–shell structure.38 Here, using the zeta-potential plot of Fe3O4 recorded as a function of solution pH (Figure 3), the formation mechanism of the core–shell structure can be clarified. In an acidic reaction medium, positively charged Fe3O4 (Figure 3) facilitates adsorption of SDS with SO42‒


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through electrostatic attraction. Thus, enrichment of aniline molecules upon Fe3O4/SDS occurs through interactions in terms of van der Waals forces. In addition, the shell thickness can be tuned by varying the solution pH, dosage of reactants, reaction time, and reaction temperature. The weight percent of the polymer in the composite sample is ca. 27.4% determined by the ICP– OES technique. On the basis of the above analysis, we conclude that the as-obtained Fe3O4/PANI sample is featured with core‒shell structure, good dispersibility, and superparamagnetism. These features are beneficial for the magnetic composite to be used as a sonocatalyst in dyeing effluent treatment.

Figure 3. Zeta potential plots of the samples: Fe3O4 (hollow square); Fe3O4/PANI (hollow sphere); dedoped Fe3O4/PANI (hollow triangle); PANI (inverted hollow triangle); and MO (hollow rhombus). 3.1.2 Surface physics and chemistry. Zeta potential measurements are widely adopted to evaluate dispersibility and stability of magnetic particles in aqueous solution.39,40 The surfacecharge characteristics of solid particles can be deduced from their zeta potential plots. In fact, the


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isoelectric points for PANI and PANI-containing composites are highly dependent on doping agent and doping concentration.40 In this research, we find that the testing procedure has a notable effect on the isoelectric point values for PANI and PANI-coated composite, due to the doping/dedoping mechanism related to semiconducting polymers. According to reference,41 the sol of particles were firstly diluted to a designated volume, followed by addition of HNO3/NaOH to tune the solution pH. Since it is a slow process for tuning the solution pH, the doping/dedoping effect is magnified by post addition of acid/alkali during the tuning process. In our experiments, quantitative sample powders were added into pre-prepared solutions with designated pH, followed by direct zeta potential measurements. For bare Fe3O4, the isoelectric point (i.p.) is located at pH 5.1 (Figure 3), whereas the isoelectric points for Fe3O4/PANI and PANI overlap at pH 6.4. This result indicates completely covered PANI shell upon Fe3O4 cores, in accordance with the TEM result (Figure 2c). For the measured MO aqueous solutions with concentration in the range of 10‒50 mg L‒1, the solution pH approaches to 6. At this pH, the surface of the as-prepared (i.e. doped) Fe3O4/PANI is positively charged, a situation that facilitates adsorption of negatively charged MO (i.p. 3.3) upon the composite through electrostatic attraction. However, the surface of Fe3O4 and dedoped Fe3O4/PANI (i.p. 4.4) is negatively charged at pH 6, making a negative contribution to MO adsorption. In addition, the dedoped Fe3O4/PANI is deprived of semiconducting properties,8,9 which play an important role in sonocatalytic degradation of the dye (discussion below). To further reveal the surface characteristics of the samples, FT-IR and XPS spectra were collected as shown in Figures S1 and S2, respectively. Detailed discussion on the FT-IR and XPS spectra of the samples is provided in S4 and S5 (Supporting Information).


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On the basis of characterization of the surface physics and chemistry of the samples, we conclude that: (i) the surface of Fe3O4/PANI is positively charged in solution with pH ~6, making a positive contribution to MO adsorption; (ii) the surface of Fe3O4/PANI sample is doped with Cl and S(VI) species (Figures S1 and S2); (iii) most of the dopants can be removed from the PANI shell after dedoping treatment (Table S3); and (iv) C‒Cl covalent bond is formed during the ultrasound-assisted interfacial polymerization process (Figure S2c). The formation of C‒Cl bond is probably completed through a radical reaction mechanism, by analogy to facile formation of Cl· radicals in an AOP process using CCl4 as the radical producer.3,4,28,29 In view of the negatively charged MO and the positively charged semiconducting shell of Fe3O4/PANI in near neutral solution, it is expected that the as-obtained Fe3O4/PANI will show enhanced sonocatalysis performance in degradation of the dye. 3.2 Adsorption of MO upon Fe3O4/PANI. Since MO is inevitably adsorbed upon the positively charged Fe3O4/PANI surface during the sonocatalytic degradation process, we first investigated the adsorption behaviors with respect to MO upon Fe3O4/PANI without ultrasonication. Detailed results and discussion are provided in S6 (Supporting Information). On the basis of investigation on the adsorption behaviors of MO upon Fe3O4/PANI, six major results are obtained as follows: (i) the adsorption behaviors of MO upon Fe3O4/PANI in the temperature range of 15–55 °C follow the Langmuir model (Figure S3a and b); (ii) the maximum adsorption capacity (Qm) of MO upon Fe3O4/PANI increases as the temperature increases (Table S4), since the thermal motion of MO molecules is intensified by increasing temperature; (iii) the kinetics of adsorption of MO upon Fe3O4/PANI conforms to the pseudo-second-order model, rather than the pseudo-first-order model (Figures S3d and S5); (iv) the rate constant of adsorption (k2) sluggishly increases from 3.91×10‒4 to 4.83×10‒4 g mg‒1 min‒1 as the temperature


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increases from 25 to 55 ºC; (v) a relatively lower equilibrium adsorption capacity produces a relatively higher adsorption heat, indicating an energetically heterogeneous surface of Fe3O4/PANI;42 and (vi) chemisorption plays a crucial role in solutions with MO concentration smaller than 50 mg L‒1, whereas electrostatic attraction-induced physisorption dominates the adsorption in solutions with MO concentration higher than 70 mg L‒1, and a balance between the physi- and chemisorption exists to contribute to the adsorption in solutions with MO concentration in the range of 50‒70 mg L‒1 (Figure S3e and f). 3.3 Sonocatalytic degradation of MO with Fe3O4/PANI 3.3.1 Kinetics. The decolorization kinetics of MO with and without Fe3O4, Fe3O4/PANI, and dedoped Fe3O4/PANI was analyzed with pseudo-first-order (equation 1) and Langmuir– Hinshelwood (L‒H) kinetic models (equation 2):21,26

ln

Ct = −kt C0

(1)

where Ct, C0, k and t are the dye concentration at time t (min), initial dye concentration, rate constant, and sonolysis time, respectively;21

r=

k 0C 1+ K 0C

(2)

where r is the degradation rate of dye, C is the dye concentration, k0 is a rate coefficient, and K0 is a combination of adsorption and kinetic terms. It has been demonstrated that k0 follows an Arrhenius relationship with temperature T, where K0 is almost independent of temperature.26 The parameters of k0 and K0 can be evaluated by changing equation (2) to (3):


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K 1 1 = + 0 (3) r k0C k 0 Thus, the plot of the reciprocal of degradation rate (1/r) against the reciprocal of dye concentration (1/C) would be linear, with the slope and intercept corresponding to 1/k0 and K0/k0, respectively.


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Figure 4. Sonocatalytic decolorization of MO with Fe3O4/PANI: (a) Temperature-dependent kinetic plots (MO: 50 mg L‒1); (b) Plots of t/Qt versus t corresponding to region I in a; (c) Plots of ln(Ct/C0) versus t corresponding to region II in a; (d) Concentration-dependent kinetic plots for MO degradation at 25 °C; (e) Plots of t/Qt versus t corresponding to region I in d; (f) Plots of ln(Ct/C0) versus t corresponding to region II in d. Insets in e and f are the plots of rate constant versus dye concentration corresponding to region I and II, respectively. Figure 4a shows the time-dependent sonocatalytic decolorization plots of Ct/C0 versus t at different temperatures. Compared with the plots in Figure S3c, the decolorization rate derived from the plots in Figure 4a is greatly improved, due to the enhanced mass transfer caused by ultrasound.4,43 The kinetics of sonocatalytic degradation of MO with Fe3O4/PANI is examined by the pseudo-first-order (equation 1)21 and L‒H (equation 3) models,26 respectively. However, the resulting plots do not match well with any of the two models due to their low correlation coefficients (Figure S6a and b). In view of the high capacity of Fe3O4/PANI for MO adsorption (128.7 mg g–1, 25 °C, Table S4) due to oppositely charged MO and Fe3O4/PANI in near neutral solution (Figure 3) as well as the enhanced mass transfer caused by ultrasound,4,43 we partition the decolorization process of MO with Fe3O4/PANI into two stages: the adsorption-dominated stage (region I) and the sonocatalysis-controlled degradation (region II) (Figure 4a). The partition of the two regions for each plot is based on the optimum correlation coefficients acquired by linear fitting of the two portions before and after the boundary (dotted green line in Figure 4a) according to equations (1), (3) and (4), respectively. The equation (4) is presented as follows:44

t 1 t = + 2 Qt k 2Qe Qe

(4)


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where Qt and Qe are the MO uptake at time t and at the adsorption equilibrium per weight of adsorbent (mg g‒1), respectively; and k2 is the pseudo-second-order rate constant (g mg–1 min–1). Figure 4b and c show the plots of t/Qt versus t and the plots of ln(Ct/C0) versus t corresponding to region I and region II in Figure 4a, respectively. After linear fitting, the plots in Figures 4b and c exhibit high correlation coefficients (R2 > 0.99), whereas the plots of 1/r versus 1/C based on the L–H model for MO degradation at region II give much lower correlation coefficients (Figure S6c). These results indicate that the kinetics of sonocatalytic degradation of MO with Fe3O4/PANI conforms to a pseudo-second-order adsorption model at region I and a pseudo-firstorder degradation model at region II. For MO degradation with Fe3O4 or dedoped Fe3O4/PANI in the whole decolorization process, the kinetics follows the pseudo-first-order model as exhibited by Figure S7b, rather than the L‒H model (Figure S7c). No partition is performed on the plots in Figure S7a. The difference in decolorization kinetics between Fe3O4/PANI, Fe3O4, and dedoped Fe3O4/PANI is attributed to the large discrepancy in capacity to adsorb MO in near neutral solution (Tables S4 and 5). The kinetics of the concentration-dependent decolorization at room temperature was also investigated, as shown in Figure 4d‒f. By analogy to the temperature-dependent decolorization process, the plots in Figure 4d are partitioned into two regions to simulate the kinetic model, since the individual models based on equations (1) and (3) cannot be used to well explain the whole decolorization process (Figure S8). In accordance with the result mentioned before, a combinational model involving equations (1) and (4) matches well with the whole decolorization process, offering high correlation coefficients (R2 > 0.99) for the linearly fitted plots in Figure 4e and f. The rate constant of the adsorption-controlled region decreases from 1.59×10‒2 to


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1.13×10‒3 g mg‒1 min‒1 as the MO concentration increases from 20 to 50 mg L‒1 (Figure 4e, inset), as a result of competitive adsorption of MO molecules upon the adsorbent. The degradation rate corresponding to region II decreases from 4.70×10‒2 to 5.72×10‒3 min‒1 as the MO concentration increases from 20 to 50 mg L‒1 (Figure 4f, inset). This result indicates that low concentration of solution is preferred for sonocatalytic degradation of MO with Fe3O4/PANI, in accordance with the result for sonocatalytic degradation of Rhodamine B with TiO2.26

Figure 5. Adsorption and sonocatalytic decolorization of MO with YP-D at room temperature: (a) Adsorption isotherm; (b) Sonocatalytic decolorization kinetic plot of Ct/C0 versus t; (c) Plot of t/Qt versus t corresponding to region I in b; (d) Plot of ln(Ct/C0) versus t corresponding to region II in b.


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It is known that activated carbons with high specific surface area have high capacities for dye adsorption.5 To consolidate the above suggestion that a combinational model should be employed to analyze the kinetics of sonocatalytic degradation of dyes with catalyst possessing a high capacity for dye adsorption, the decolorization behavior of MO with activated carbon (YPD, 1500 m2 g–1, Keleli Trade Co., Shanghai, China) was examined. Figure 5 shows the adsorption isotherm (Figure 5a) and sonocatalytic decolorizaiton behavior of MO with YP-D (Figure 5b to d). The isotherm and kinetics of MO adsorption upon YP-D follow the Langmuir model (Figure S9a) and the pseudo-second-order model (Figure S9b), respectively. The maximum adsorption capacity (Qm) for MO adsorption upon YP-D is calculated to be 917.4 mg g–1 according to equation S1 (S6, Supporting Information). In the presence of ultrasound, a rapid decolorization rate is observed within 10 min (Figure 5b). Similar to the situation for MO decolorization with Fe3O4/PANI, the sonocatalytic decolorization kinetics of MO with YP-D does not simply follow the pseudo-first-order model (Figure S9c)21 or the L‒H model (Figure S9d).26 By partition of the plot in Figure 5b into two regions, the plots at the two portions match well with the pseudo-second-order adsorption model and the pseudo-first-order degradation model, respectively (Figure 5c and d). This result consolidates the feasibility to partition the sonocatalytic decolorization kinetic plot of MO with catalyst which has a high capacity to adsorb the dye.

3.3.2 Rate constant of decolorization and apparent activation energy. According to equation (4) and Figure 4b, the rate constants of decolorization corresponding to region I are calculated to be 1.13×10‒3 (25 °C), 1.25×10‒3 (35 °C), 3.08×10‒3 (45 °C), and 4.69×10‒3 g mg‒1 min‒1 (55 °C), higher than the corresponding rate constant of simple adsorption (Figure S3c) at 25 °C (3.91×10‒4 g mg‒1 min‒1) and 55 °C (4.83×10‒4 g mg‒1 min‒1). This result is attributed to the


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enhancement of mass transfer caused by high-pressure shock waves and acoustic vortex microstreaming due to ultrasonication.4,43 According to equation (1) and Figure 4c, the rate constants of decolorization corresponding to region II are calculated to be 5.72×10‒3 (25 °C), 5.96×10‒3 (35 °C), 8.54×10‒3 (45 °C), and 9.84×10‒3 min‒1 (55 °C), respectively. The apparent activation energy (Ea) for decolorization can be calculated with the Arrhenius equation:45

ln k = −

Ea + ln A (5) RT

where A is the Arrhenius factor, and k is the rate constant of degradation. As mentioned before, the kinetics of MO decolorization with Fe3O4/PANI conforms to the pseudo-second-order adsorption model followed by the pseudo-first-order degradation model. Therefore, the rate constants obtained at region II are applied as the k in equation (5), producing an Ea value of 14.87±0.82 kJ mol‒1 for MO degradation at region II. For MO degradation with ultrasonication alone, the Ea is calculated to be 16.38±0.12 kJ mol‒1 through the data of temperature-dependent MO degradation plots (Figure S10b). As a result, the apparent activation energy for MO degradation with Fe3O4/PANI is reduced by 0.57–2.45 kJ mol‒1, in comparison with that for MO degradation with ultrasonication alone. Compared with the case using TiO2 as the sonocatalyst,45 the relatively lower reduction in Ea is attributed to that the contribution from MO degradation at region I to the rate constant has not been taken into account. In fact, although adsorption of MO upon the sonocatalyst dominates the stage of region I, sonocatalytic degradation of MO inevitably occurs at this stage.


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Table 2. Contributions from ultrasonication alone, adsorption, and catalyst to the total decolorization of MO solutions. Temperature (°C)a

Decolorization

Concentration (mg L‒1)b

25

35

45

55

20

30

40

50

50c

Pus

5.9%

6.1%

7.0%

7.6%

4.9%

5.1%

5.4%

5.7%

5.7%

Pad

46.6%

47.0%

49.8%

50.9%

55.7%

55.5% 54.1% 46.6%

73.0%

Pc

33.4%

35.3%

40.2%

38.7%

39.3%

39.0% 35.3% 33.4%

14.3%

Ptotal

85.9%

88.4%

97.0%

97.2%

99.9%

99.6% 94.8% 85.7%

93.0%

a. Degradation of MO solution with concentration of 50 mg L‒1. b. Degradation of MO solutions at 25 °C. c. Degradation of MO with YP-D (0.056 mg mL–1).

3.3.3 Promoted sonocatalytic degradation of MO with Fe3O4/PANI. To determine the role of Fe3O4/PANI in a sonocatalytic process, three factors are considered as contributions to the total decolorization of MO solution, including ultrasonic degradation of MO without catalyst, MO adsorption upon the catalyst, and sonocatalytic degradation of MO promoted by the catalyst. In the case of MO degradation with ultrasonication alone (S9, Supporting Information), the percentage of MO degradation slowly increases from 5.9% to 7.6% as the temperature increases from 25 to 55 °C (Table 2 and Figure S10a). Also, the degradation level of MO sluggishly increases from 4.9% to 5.7% as the MO concentration increases from 20 to 50 mg L–1 (Table 2 and Figure S11a). The slight increase in MO degradation with ultrasonication alone as the temperature or MO concentration increases is in accordance with previous reports.30,45 To determine the percentage of contribution from surface adsorption, the recovered catalyst was eluted in 3 mol L–1 ammonia water for 6 h with constant stirring, and the eluent was sampled for UV/Vis measurement. The absence of elemental S in the energy dispersive spectrometry (EDS)


20

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pattern with respect to the recovered catalyst after eluting treatment evidences complete removal of the adsorbed MO upon the catalyst (Figure S12). Therefore, the promoted degradation of MO with the catalyst can be determined according to equation (6): Ptotal = Pus + Pad + Pc (6) where Ptotal is the total percentage of decolorized MO; Pus is the percentage of MO degraded by ultrasonication alone; Pad is the percentage of MO adsorbed upon the catalyst; and Pc is the percentage of MO degradation promoted by the catalyst. The corresponding values are provided in Table 2. It is found that the percentage of room-temperature (25 °C) sonocatalytic degradation of MO with Fe3O4/PANI is about 4.8, 8.8, and 5.7 times of that with Fe3O4, dedoped Fe3O4/PANI, and ultrasonication alone (Table 2 and Table S6), respectively. This result is attributed to the large discrepancy in MO adsorption upon the catalysts under otherwise identical conditions. Also, relatively higher temperature (45 °C) and relatively lower MO concentration (20 mg L‒1) facilitate great enhancement of MO degradation in the Fe3O4/PANI-based sonocatalytic process. In the case of MO decolorization with YP-D, the percentage of MO decolorization is enhanced by 14.3% corresponding to the promotion by the catalyst (Table 2). The enhancement in dye decolorization level with the help of solid catalyst in an AOP process is due to that the addition of solid catalyst particles increases the number of cavitation events,2,46 resulting in enhancement in degradation level. The relatively higher contribution from Fe3O4/PANI (33.4%) to the total MO decolorization as compared with that (14.3%) from YP-D is attributed to the semiconducting characteristic of PANI shell which can generate highly active electrons and holes under ultrasonic treatment (discussion below).


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3.3.4 Proposed mechanism for sonocatalytic degradation of MO with Fe3O4/PANI. As demonstrated in literature,2,27 an ultrasound-assisted AOP process produces cavitation where collapse of bubbles occurs in an extremely small time interval. This generates localized hot spots, highly reactive free radicals, and sonoluminescence, and causes enhancement of mass transfer.2,27 The degradation of organic pollutants usually involves pyrolysis of volatile pollutant molecules entrapped inside the bubble and attack of free radicals to the pollutants.2,47 The nonvolatile nature of MO suggests that pyrolytic reactions within the collapsing bubbles can be negligible and the degradation exclusively occurs through the attack of free radicals in the liquid phase.21 The formation mechanisms of ·OH, ·O2‒, and ·HO2‒ radicals in solution with simple ultrasonication has been well documented in literature,23,26,27 involving pyrolysis of H2O by virtue of cavitation to form ·OH and ·H and subsequent combination with O2 dissolved in water. In a sonocatalytic process, the presence of the solid catalyst particles provides additional nuclei for the formation of cavities and hence the number of cavitional events is enhanced, resulting in increase in decolorization rate.2 On the other hand, sonoluminescence caused by ultrasonic cavitation generates lights with wide wavelength range.27 Under excitation of the lights and/or hot spots, some electrons in the valence band (VB) of semiconducting PANI shell can be transferred to the conduction band (CB), leading to formation of sonochemically generated electrons and holes. Similar to the photocatalytic process,48 the resulting electrons and holes react with H2O and O2 adsorbed upon Fe3O4/PANI, producing ·OH, ·O2‒, and ·HO2‒ radicals. These reactive oxygen species have high ability to oxidize organic pollutants in solution, contributing positively to the enhanced degradation level of the dye.


22

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Fe3O4/PANI;

intermediates

methyl orange (MO) molecule;

bubble

MO adsorption upon Fe3O4/PANI

ultrasonication

sonocatalytic degradation of MO

(b)

(c)

(a)

H2O OH

O2

O2-

MO solution bu

H2O

bb

le

OH

Scheme 1. Proposed sonocatalytic degradation mechanism of MO upon Fe3O4/PANI. As demonstrated before, the kinetics of sonocatalytic decolorization of MO with Fe3O4/PANI does not simply follow the pseudo-first-order21 and L‒H models,26 since Fe3O4/PANI has a high capacity to adsorb MO in solution. Therefore, an adsorption-promoted sonocatalytic degradation mechanism is proposed as illustrated in Scheme 1. At the initial stage, MO molecules are captured by Fe3O4/PANI due to electrostatic attraction and ultrasonication, resulting in fast decolorization of MO solution (Scheme 1a). Secondly, the captured MO molecules react with the sonogenerated free radicals from/around the semiconducting PANI shell (Scheme 1b), producing intermediates without color. Further reactions between the intermediates and the radicals result in mineralization of the dye (Scheme 1c). With increasing the time of ultrasonic treatment, the amount of MO molecules upon the catalyst rapidly decreases, because the adherence of MO upon the catalyst can shorten the path for radicals/cavities to decompose the dye. Thirdly, the


23


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addition of solid catalyst particles increases the number of cavitation events and hence increases the degradation rate.2

Figure 6. (a, b) Plots of TOCt/TOC0 versus t for MO solutions with ultrasonication alone (a. CMO = 20 mg L–1; b. CMO = 30 mg L–1); (c, d) Plots of TOCt/TOC0 versus t for sonocatalytic degradation of MO with Fe3O4/PANI (c. CMO = 20 mg L–1; d. CMO = 30 mg L–1); (e, f) Plots of TOCt/TOC0 versus t corresponding to TOC removal of MO by adsorption (e. CMO = 20 mg L–1; f. CMO = 30 mg L–1). As mentioned before, the decolorization of dye solution in an AOP process involves partial degradation of the dye to form aromatic intermediates and deep mineralization to generate oxyacids, CO2, and H2O.2,21,24 To consolidate the adsorption-promoted decolorization mechanism and determine the degradation products of MO in the Fe3O4/PANI-based sonocatalytic decolorization process, the total carbon content (TOC) and the components of MO solution at different time intervals were analyzed with the TOC and GC/MS techniques. Figure 6 shows the plots of TOCt/TOC0 versus t collected at room temperature, where TOCt is the TOC at time t and TOC0 is the initial TOC. For MO degradation with ultrasonication alone, the percentage of TOC for MO solutions with concentration of 20 and 30 mg L–1 gradually


24

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decreases to 96.5% and 95.8% within 180 min (Figure 6a and b), respectively. The decreased TOC percentages of 3.5% and 4.2% are separately lower than the decolorization percentages of 4.9% and 5.1% for MO solutions with concentration of 20 and 30 mg L–1 (Table 2). This results indicates incomplete mineralization of MO with ultrasonication alone in the present case. A noteworthy point is that the shape of the plots for MO degradation with Fe3O4/PANI (Figure 6c and d) is much different from that of Figure 6a and b. In Figure 6c and d, a dramatic decrease in TOC percentage occurs within 30 min, followed by a small increase at 60 min and then a slow decrease up to 180 min, presenting ultimate TOC percentages at 57% and 57.4% for MO solutions with concentration of 20 and 30 mg L–1, respectively. Likewise, the decreased TOC percentages (43% and 42.6%) are much lower than the decolorization percentages (99.9% and 99.6%) for MO decolorization with Fe3O4/PANI (Table 2), indicating incomplete mineralization of MO. The dramatic decrease in TOC percentage within 30 min is mainly due to the rapid adsorption of MO upon Fe3O4/PANI. As the time increases from 30 to 60 min, more captured MO molecules react with the sonogenerated free radicals from/around the semiconducting PANI shell, producing intermediates in solution. The newly-formed intermediates are responsible for the slow increase in TOC percentage. The decrease in TOC percentage with prolonging the time from 60 to 180 min is attributed to deep mineralization of the intermediates. It should be pointed out that the time at which the points of inflection in the TOC plots occur (Figure 6c and d) approaches to that at which the cut-off points in the decolorization kinetic plots have been optimized (Figure 4d). Moreover, the point of inflection delays to appear in the TOC plot as the MO concentration increases (Figure S13), in accordance with the situation for the cut-off points in the decolorization kinetic plots (Figure 4d). These results consolidate the adsorption-promoted decolorization mechanism and confirm the feasibility to partition the decolorization kinetic plots


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into two regions as shown in Figure 4a and d. On the basis of the above analysis, TOC removal of MO by adsorption can be deduced to be 33.4% and 31.9% for MO solutions with concentration of 20 and 30 mg L–1, respectively (Figure 6e and f).

Table 3. GC/MS results. Intermediates

tR (min)

MW

Main fragment ions (m/z)

Aniline (1)

5.33

93

93/92/91, 77, 65, 51, 39

N,N-dimethylbenzene-1,4diamine (2)

5.71

136

136/135, 121, 107, 93, 77, 66, 53, 41

4-Diazenyl-N,Ndimethylbenzenamine (3)

5.83

149

148/147, 136, 121, 107, 93, 77, 66, 53, 41

Oct-1-en-3-ol (4)

10.05

128

128/127, 110, 99, 85, 72, 57, 43, 29

The intermediates of MO degradation with Fe3O4/PANI were examined by the GC/MS technique, using the supernate of MO solution with initial concentration of 30 mg L–1 after ultrasonic treatment for 60 min. The GC retention time (tR), molecular weight (MW), and main fragments are summarized in Table 3. The identified intermediate 3 indicates dissociation of the C–N bond close to the side of benzenesulfonic group, due to the attack of reactive oxygencontaining radicals on the benzene ring with electron-attracting group.17 The intermediates of 1 and 2 arise from further degradation of 3. The intermediate 4 is due to recombination of the radical fragments derived from opening the benzene ring by oxidation. From the GC/MS results, we can conclude that the part of 4-diazenyl-N,N-dimethylbenzenamine in MO molecule is more difficult to be degraded in a sonocatalysis process as compared with the part of benzenesulfonic group, since the electron-donating groups (N,N-dimethyl, amino, and imine groups ) can stabilize the benzene ring.


26

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3.3.5 Reusability of the catalyst. To examine the reusability of Fe3O4/PANI, the catalyst was recovered after every degradation run, dispersed into 30 mL of 3 mol L–1 ammonia water, and agitated for 6 h. Then, the solid was magnetically separated from solution. The eluent was sampled and analyzed by the UV/Vis spectrometer to quantify the adsorbed MO upon Fe3O4/PANI. The dedoped catalyst was washed with water, followed by re-doping treatment in 0.1 M HCl for 3 h to resume the original semiconducting state. The regenerated catalyst was washed with water and ethanol, and finally dried in a vacuum oven for further use. Five consecutive sonocatalysis cycles were performed to test the reusability of the catalyst. Figure 7 shows the percentage of decolorization of MO solutions after five consecutive sonocatalysis cycles. The percentage of total decolorization (Ptotal) of MO is 97.9% and the percentage of MO degradation promoted by the catalyst (Pc) is 37.3% in the fifth cycle, indicating excellent regenerability and reusability of the catalyst.

Figure 7. Cycling performance of Fe3O4/PANI for sonocatalytic degradation of MO solution (20 mg L‒1) at 25 °C.

4. Conclusions


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In summary, an ultrasound-assisted advanced oxidation process has been developed that involves sonocatalytic degradation of MO in near neutral solutions (pH ~6) with Fe3O4/PANI, which has been demonstrated to have high capacity for MO adsorption upon the surface. The high adsorption capacity is attributed to the oppositely charged MO and Fe3O4/PANI, contrary to the situations for MO with Fe3O4 or dedoped Fe3O4/PANI. The significance of this paper includes: (1) the decolorization behavior of dye with Fe3O4/PANI possessing a high capacity to adsorb the dye does not simply follow the known pseudo-first-order degradation model and the Langmuir– Hinshelwood model, and a combinational model is suggested by partitioning the decolorization kinetic plot; (2) room-temperature sonocatalytic degradation of MO is greatly enhanced with Fe3O4/PANI, being about 4.8, 8.8, and 5.7 times of that with Fe3O4, dedoped Fe3O4/PANI, and ultrasonication alone, respectively; (3) the part of 4-diazenyl-N,N-dimethylbenzenamine in MO molecule is more difficult to be degraded in a sonocatalysis process as compared with the part of benzenesulfonic group, since the electron-donating groups (N,N-dimethyl, amino, and imine groups ) can stabilize the benzene ring; and (4) Fe3O4/PANI exhibits excellent regenerability and reusability, presenting a promising sonocatalyst for rapid decolorization and enhanced degradation of azodyes.

ASSOCIATED CONTENT Supporting Information. Synthetic procedures of Fe3O4, Fe3O4/PANI and PANI, FT-IR spectra, XPS spectra, adsorption isotherms and kinetic plots, additional sonocatalytic decolorization kinetic plots, EDS pattern of the recycled catalyst, and additional TOC result. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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*Corresponding Author. Fax: +86 531 89631207. Phone: +86 531 89631208. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by NSFC (no. 51272143).

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Table of Contents Graphic Fe3O4/PANI;

intermediates

methyl orange (MO) molecule;

bubble

MO adsorption upon Fe3O4/PANI

ultrasonication

sonocatalytic degradation of MO

(b)

(c)

(a)

H2O OH

O2

O2-

MO solution bu

H2O

bb

le

OH


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Ultrasound-Assisted Catalytic Degradation of Methyl Orange with Fe

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