Journal of Colloid and Interface Science 377 (2012) 184–190

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Magnetic a-Fe2O3/MCM-41 nanocomposites: Preparation, characterization,and catalytic activity for methylene blue degradation

Irina Ursachi a, Alexandru Stancu a, Aurelia Vasile b,⇑a Department of Physics ‘‘Alexandru Ioan Cuza’’ University of Iasi, 11 Carol I Bvd., 700506 Iasi, Romaniab Department of Chemistry, ‘‘Alexandru Ioan Cuza’’ University of Iasi, 11 Carol I Bvd., 700506 Iasi, Romania

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 February 2012Accepted 23 March 2012Available online 4 April 2012

Keywords:HematiteMCM-41Magnetic nanoparticlesMagnetic propertiesUltrasonic irradiationDegradationMethylene blueHydrogen peroxide

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.03.066

⇑ Corresponding author. Fax: +40 232 201313.E-mail address: aurelia@uaic.ro (A. Vasile).

Catalysts based on nanosized magnetic iron oxide stabilized inside the pore system of ordered mesopor-ous silica MCM-41 have been prepared. The obtained materials were characterized by powder X-ray dif-fraction analysis (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), andN2 adsorption–desorption isotherm. XRD analysis showed that the obtained materials consist from thepure hematite crystalline phase (a-Fe2O3) dispersed within ordered mesoporous silica MCM-41. Mag-netic measurements show that the obtained nanocomposites exhibit at room temperature weak ferro-magnetic behavior with slender hysteresis. The catalytic activity of the magnetic a-Fe2O3/MCM-41nanocomposites was evaluated by the degradation of methylene blue (MB) aqueous solution. For thispurpose, an ultrasound-assisted Fenton-like process was used. The effect of solution pH on degradationof MB was investigated. The results indicated that US–H2O2–a-Fe2O3/MCM-41 nanocomposite system iseffective for the degradation of MB, suggesting its great potential in removal of dyes from wastewater. Itwas found that the degradation rate of MB increases with decrease in the pH value of the solution.

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1. Introduction

Magnetic iron oxide nanoparticles have attracted increasingattention due to their wide range of potential applications in catal-ysis, magnetic recording, microwave absorption, biological sensors,in vitro/in vivo applications, and environmental depollution [1–5].Among iron oxides, hematite (a-Fe2O3) is a promising materialfor different catalytic applications [6,7]. Generally, magnetic ironoxide nanoparticles have a low specific surface area that causespoor catalytic performances. Moreover, obtaining and maintainingthese particles in nanosized form is difficult due to the natural ten-dency of aggregation. One way used to overcome these drawbacksis to disperse iron oxide nanoparticles within a porous matrix hav-ing a high specific surface area. Due to their uniform pores withsize in the range 1.5–20 nm [1,8], high specific surface area andtunable surface functional groups [1] in conjunction with highhydrothermal and chemical stability, ordered mesoporous silicamaterials such as MCM-41 and SBA-15 can be used as a nanoreac-tor for the synthesis and stabilization of magnetic iron oxide nano-particles [9].

Various methods have been reported in the literature for the syn-thesis of ordered mesoporous silica focused on reducing synthesistime without affecting the quality of the material [10–12]. Althoughultrasonic irradiation has the major advantage of a very pronounced

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reduction in the duration of synthesis, only a few examples of sono-chemical synthesis of mesoporous silica materials [13–15] have yetbeen reported. The sonochemical technique for the synthesis of or-dered mesoporous silica can reduce the preparation time from 48to 72 h to several hours. Moreover, ordered mesoporous silicaMCM-41 obtained by sonochemical technique is thermally morestable compared with the corresponding material obtained by theconventional method. This stability is attributed to the thicker porewalls of MCM-41 obtained by sonochemical synthesis [13].

In recent years, increasing interest has been paid to the use ofultrasound in the degradation of hazardous organic pollutants[16–18]. The phenomenon of acoustic cavitation provides unique lo-cal conditions of extremely high temperature and pressure, whichinduce the formation of hydroxyl radicals (HO�) and hydrogen atoms[17]. The non-volatile, hydrophilic hazardous pollutants can be de-graded by HO� at the bubble surface or in the solution bulk. However,ultrasonic processes are limited by low degradation rate due to thehigh stability of non-volatile dyes [19]. Thus, the coupling of ultra-sonic irradiation with other oxidation processes (photocatalysis,ozonization or Fenton-type processes) should enhance the degrada-tion rate of organic pollutants. The coupling of ultrasound withphotocatalysts or ultrasound with Fenton-like processes has at-tracted attention of many researchers in recent years. In these cases,promising results were obtained in the degradation of hazardous or-ganic pollutants such as phenol [20] and methylene blue [17].

In the present paper, we report the preparation of a catalystbased on magnetic iron oxide nanoparticles dispersed inside pores

I. Ursachi et al. / Journal of Colloid and Interface Science 377 (2012) 184–190 185

of amino functionalized mesoporous silica MCM-41 and its appli-cation to the degradation of methylene blue (MB) by means of acoupled heterogeneous Fenton-like process with ultrasonic irradi-ation. Amino functionalized MCM-41 used as catalyst support wasobtained by ultrasound-assisted synthesis using cetyltrimethylam-monium bromide (CTAB) as structure-directing agent, followed bypost-synthesis functionalization with 3-aminopropyltriethoxysi-lane (APTES). The magnetic iron oxide precursor (iron nitrate)was loaded into the amino functionalized mesoporous support bythe wet impregnation technique. The choice of MB is motivatedby its positive charge in solution, which excludes volatility andgas-phase pyrolysis. Moreover, the blue color allows easy spectro-photometric monitoring of the system during MB degradationexperiments. The effect of solution pH on the degradation of MBwas investigated as well.

2. Materials and methods

2.1. Materials

Tetraethyl orthosilicate (TEOS, 98%) and sodium hydroxide(NaOH) were purchased from Merck; cetryltrimethylammoniumbromide (CTAB, 99%) and methylene blue (MB, C16H18N3Cl�3H2O)were purchased from Sigma. Iron nitrate nonahidrate (Fe(-NO3)3�9H2O, 98%), 3-aminopropyltriethoxysilane (APTES, 99%),and aqueous ammonia (30% NH3) were purchased from Aldrich;ethanol (EtOH, 99%) and hydrogen peroxide (30%, w/w) were pur-chased from S.C. Chemical Company S.A. (Romania). All chemicalswere used as received without further purification. Deionizedwater used throughout the experiments was prepared with anELGA purelab water system.

2.2. Mesoporous silica MCM-41 preparation

Pure ordered mesoporous silica MCM-41 was prepared by ultra-sound-assisted synthesis and conventional method [21,22]. Bothsynthetic methods, which have been detailed in [15], were per-formed using an initial reaction mixture with the following molarcomposition: 1TEOS:0.3CTAB:58EtOH:11NH3:144H2O. The prod-uct obtained via ultrasonic irradiation was denoted U, while theproduct obtained by conventional method was denoted M.

Modification of pure mesoporous silica samples with aminofunctional groups was carried out by post-synthesis procedure asfollows: 0.5 g MCM-41 prepared as above was dispersed in the eth-anol-diluted APTES under vigorous stirring at room temperaturefor 4 h. The mixture was then carefully heated to evaporate the sol-vent. The resulting products were denoted pU and pM.

2.3. Preparation of nanosized magnetic iron oxide/MCM-41nanocomposites

In order to disperse iron oxide inside pores of amino functional-ized mesoporous silica MCM-41 matrix, the samples pU and pM,respectively, were impregnated with an aqueous solution containing

Table 1Description of the synthesized samples.

Sample Synthesis methods Sample desc

U Sonochemical Pure MCM-4pU Sonochemical Amino-functM Conventional Pure MCM-4pM Conventional Amino-functpUF Wet impregnation Iron oxides npMF Wet impregnation Iron oxides n

a Molar ratio used at functionalization.b Molar ratio used at impregnation.

an appropriate amount of Fe(NO3)3�9H2O to achieve the designedconcentration (see Table 1). Impregnation was carried out at roomtemperature (20 ± 2 �C) for 4 h under vigorous stirring. Wetimpregnation technique was followed by solvent evaporation.The dried powders were heated at 550 �C (heating rate of 2 �C/min) and then calcined at 550 �C for 6 h; the obtained sampleswere denoted pUF and pMF, respectively.

2.4. Characterization

The powder X-ray diffraction (XRD) patterns were recorded on aShimadzu diffractometer operating at intensity of 40 kV and 30 mAand using Ni-filtered Cu Ka (k = 1.542 ÅA

0

) radiation. The diffracto-grams were recorded in the 2h range of 1.5–8� and 20–80� witha 2h step size of 0.02� and a step time of 12 s. Scanning electronmicroscopy (SEM) investigations were performed using a Vega Tes-can electron microscope. Nitrogen adsorption–desorption iso-therms were recorded on a Quantachrome Nova 2200 Instrument& Pore Size Surface Area Analyzer at 77 K on samples degassed at473 K for 2 h under high vacuum. The Brunauer–Emmett–Teller(BET) specific surface area was calculated from the linear part ofthe BET plot. Pore size distribution was calculated using Barrett–Joyner–Halenda (BJH) method. Magnetic measurements were per-formed on a vibrating sample magnetometer (VSM) PrincetonInstruments Micromag™ 3600 at room temperature under an ap-plied field of 10,000 Oe. Contributions of holder and silica matrixwere subtracted from the recorded magnetic data.

2.5. Degradation of methylene blue aqueous solution

An ultrasonic processor SONICS VIBRA Cell™ Model CV 33 witha 1.13 cm diameter titanium horn operating at 20 kHz was used forultrasonic MB degradation experiments. Sonication was applied inpulsed mode (4 s on/1 s off). During experiments, the temperaturewas maintained at 20 ± 2 �C. A specific quantity of magnetic ironoxide/MCM-41 nanocomposite (0.1 g) was dispersed into 100 mLof 0.03 mM methylene blue aqueous solution, and the pH valuewas fixed to pH 3 using NaOH. The degradation of MB was initiatedby addition of 0.4 mL H2O2 (30% w/w) and immediately turning onthe ultrasonic irradiation. At given time intervals, 5 mL solutionwas withdrawn from the reaction medium and immediately cen-trifuged at 4000 rpm for 15 min to remove the suspended catalystparticles. The absorbance of the liquid sample was measured at665 nm (which corresponds to the maximum absorbance of theMB) with a Shimadzu UV-2401PC Spectrophotometer to determinethe MB concentration. For comparison, the MB degradation exper-iment was also done without ultrasounds or hydrogen peroxide.

3. Results and discussion

The synthetic methods, description of samples, functionaliza-tion level, and Fe loading level are presented in Table 1.

ription APTES:Sia Fe:Sib

1 – –ionalized MCM-41 0.1:0.9 –1 – –ionalized MCM-41 0.1:0.9 –anoparticles/pU 0.1:0.9 0.02anoparticles/pM 0.1:0.9 0.02

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3.1. Structural and textural analysis

It is well known that the X-ray diffraction pattern of orderedmesoporous silica MCM-41 presents a small number of reflectionsalways located at small angles of diffraction. Typically, only threediffraction peaks are well resolved and correspond to (100),(110), and (200) reflections. The fourth diffraction peak is usuallyof very low intensity and corresponds to (210) reflection.

The small-angle XRD results of the pure and amino functional-ized mesoporous silica samples are shown in Fig. 1.

The pure mesoporous silica sample prepared by conventionalsynthesis (Fig. 1a, M sample) exhibits three diffraction peaks corre-sponding to (100), (110), and (200) reflections assigned to thehexagonal unit cell. The XRD pattern can be indexed to a highly or-dered hexagonal lattice typical of MCM-41 (space group p6mm).The pure mesoporous silica sample obtained by ultrasound-as-sisted synthesis (Fig. 1b, U sample) exhibits a well resolved diffrac-tion peak corresponding to (100) reflection, while diffractionpeaks corresponding to (110) and (200) reflections have verylow intensity and could not be resolved. Pinavaia et al. [23] havedemonstrated that such XRD patterns are obtained for sampleswith crystallographic structure analogous to MCM-41 material,which means that the synthesized samples may be consideredmaterial of MCM-41 type.

The XRD patterns of MCM-41 samples functionalized by post-synthesis treatment show a single diffraction peak correspondingto (100) reflection (Fig. 1c, pM sample; Fig. 1d, pU sample). Forboth samples, a significant decrease in the intensity of (100) dif-fraction peak and the reduction to detection limit of (110) and(200) reflections present in the parent sample was noticed. Thisindicates a distortion of the long-range ordering of the mesoporousstructure during functionalization process.

The small-angle XRD results of the pure and amino functional-ized mesoporous silica showed that the obtained samples by bothmethods exhibit ordered hexagonal lattice that is typical for MCM-41.

The wide-angle XRD and small-angle XRD results of the ironoxide/MCM-41 nanocomposites (pMF and pUF samples) are shownin Fig. 2. Presence of diffraction peaks corresponding to (104),(110), (113), (202), (024), (116), (018), and (214) reflectionsin wide-angle patterns clearly indicate the formation of hematitephase (a-Fe2O3) (space group R3c (167), JCPDS No. 72-0469). Thebroad peak at around 2h = 22� is assigned to amorphous silica.The small-angle XRD patterns of impregnated samples (Fig. 2, in-set) show that the diffraction peaks were shifted toward higher

Fig. 1. Small-angle XRD patterns of pure mesoporous silica and amino function-alized MCM-41 samples.

values of diffraction angle and (100) peak intensity decreased. Thisresult is due to functionalization agent removal by calcination.Also, the decrease in peak intensity could be attributed to a de-crease in the electron density contrast between the silica walland mesopore due to location of iron oxide particles inside mesop-ores of silica matrix [24].

Textural properties of prepared samples were determined fromnitrogen adsorption–desorption isotherms. The main structuraland textural characteristics of the prepared samples are presentedin Table 2. Nitrogen adsorption isotherms and corresponding poresize distribution of M and U samples are shown in Fig. 3. Both iso-therms are of type IV with a H1-type hysteresis loop [25] which ischaracteristic of mesoporous materials. A sharp inflection in therelative pressure (p/p0) between 0.2 and 0.3 corresponds to capil-lary condensation inside uniform mesopores. The sharpness of thisstep demonstrates the narrow pore size distribution (see Fig. 3B).For pure MCM-41 prepared by ultrasound-assisted synthesis (sam-ple U), the position of adsorption–desorption curve is slightlyshifted to lower values of p/p0 indicating decrease in the mesoporesize (see Table 2). The obtained data show that ultrasound-assistedsynthesis favors the formation of mesoporous structures with low-er values of interplanar spacing, pore diameter, and volume, com-pared with samples obtained by conventional synthesis.

The N2 sorption results reveal that modification with aminofunctional groups leads to a decrease in the porosity of samplescompared with that of the corresponding pure silica samples(see Table 2). The decrease in textural characteristics is a conse-quence of the grafting of organic functional groups onto poreswalls of the MCM-41 materials. Nevertheless, although the bind-ing of amino functional groups induce a certain decrease in silicaporosity, a significant volume of pores still remains unobstructed,as can be deduced from the textural properties listed in Table 2.The high specific surface area and average pore diameter of theiron oxide/MCM-41 nanocomposites indicate that the mesopor-ous structure is still maintained after loading of iron oxide spe-cies. Furthermore, the textural characteristics of obtainednanocomposites suggest that organic phase removal (functionali-zation agent) by calcination gives rise to an increase in the poros-ity of the nanocomposite compared with the correspondingamino functionalized MCM-41 samples. A decrease in the totalpore volume and specific surface area is registered for ironoxide/MCM-41 samples compared with the pure mesoporous sil-ica. This result could be attributed to the deposition of iron oxidenanoparticles into the mesopores of the silica matrix [24].

Fig. 2. Wide-angle XRD and small-angle XRD (inset) patterns of magnetic ironoxide/MCM-41 nanocomposites: (a) pMF sample and (b) pUF sample.

Table 2The main structural and textural characteristics of the prepared samples.

Sample d100a (nm) a0

b (nm) Wtc (nm) Dp

d (nm) Vpe (cm3/g) SBET

f (m2/g)

U 3.38 3.9 1.74 2.16 1.086 1699M 3.48 4.01 1.77 2.24 1.129 1601pU 3.41 3.93 2.93 1.00 0.449 232pM 3.38 3.9 2.92 0.98 0.131 106pUF 3.39 3.91 1.92 1.99 0.788 1456pMF 3.49 4.03 2.17 1.86 0.555 1042

a d-Spacing of (100) reflection.b Unit cell constant, a0 = (2d100)/

p3.

c The pore wall thickness estimated from the difference (a0 � Dp).d Average pore diameter, estimated using the desorption branch of the isotherm and the Barrett–Joyner–Halenda (BJH) method.e Total pore volume, taken from the volume of N2 adsorbed at p/p0 = 0.95.f BET surface area, calculated from the linear part of the BET plot. See experimental section for designations of samples.

Fig. 3. (A) Nitrogen adsorption–desorption isotherms and (B) pore size distribution of pure mesoporous silica samples.

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3.2. Scanning electron microscopy analysis

Representative SEM micrographs of amino functionalizedMCM-41 and magnetic a-Fe2O3/MCM-41 nanocomposites areshown in Fig. 4. SEM images revealed that all samples clearly pres-ent spherical morphology that is preserved after functionalization(Fig. 4a and b) and after impregnation and calcination (Fig. 4c andd). Particle surface morphology is smooth and does not detect theodd structures on the surface of all samples.

3.3. Magnetic properties

To investigate the magnetic properties of the synthesized sam-ples, measurements were carried out in an applied magnetic fieldat room temperature. The field dependence of isothermal magneti-zation for a-Fe2O3/MCM-41 nanocomposite measured at roomtemperature is shown in Fig. 5. The magnetization curves recordedon synthesized samples show weak ferromagnetic behavior. Themeasured coercive force (Hc) and saturation magnetization (Ms)values were less than 39 Oe and 0.35 emu g�1, respectively. Thecalculated saturation magnetization values (listed in Table 3) aresmaller than that of commercial hematite (0.6 emu/g) [26]. A de-crease in Ms was observed in nanoparticles, and it was attributedto the surface contribution: spin canting, stoichiometry deviation,surface disorder and cation distribution [27]. In our case, this phe-nomenon can be caused by the presence of the mesoporous matrix.

The hysteresis loop of the samples indicates similar types ofmagnetic behavior. On the basis of the criteria given by Dunlopet al. [28], the value of Mr/MS should be larger than 0.5 for sin-gle-domain (SD) particles, between 0.1 and 0.5 for pseudosingle-domain (PSD) particles and lower than 0.1 for multidomain (MD)

particles. As seen from Table 3, the iron oxide nanoparticles dis-persed in MCM-41 show an MD-type behavior (Mr/MS < 0.1).

3.4. Degradation of MB on a-Fe2O3/MCM-41 nanocomposites

Our experiments showed that the MB degradation is insignifi-cant when only US–H2O2 system was used (see Fig. 6B). Futher-more, MB degradation experiments in the absence of ultrasoundsusing only H2O2–a-Fe2O3/MCM-41 nanocomposite systemsshowed that MB adsorption occurs on the surface of catalyst thatchanges its color to green (see Fig. 6C). The ultrasonic degradationof MB was investigated in the US–H2O2–a-Fe2O3/MCM-41 nano-composite systems at pH 2–5.

The temporal changes in the spectral features of the solutionswere studied by monitoring the absorbance of UV–vis spectra at665 nm. As shown in Fig. 7A, MB was degraded slowly under ultra-sonic irradiation when US–H2O2 system was used. The very smalldecrease in the absorbance at 665 nm indicates that the ultrasonicirradiation produce only a weak increase in MB degradation due togeneration of additional HO free radicals by H2O2 decomposition.In the catalytic systems US–H2O2–pMF and US–H2O2–pUF, it canbe observed a clear decrease in the absorbance at 665 nm(Fig. 7B and C) and no new peaks were observed after sonocatalyticdegradation. As shown in Fig. 7B and C, ultrasonic irradiation for30 min caused a significant discoloring of the solution (decreasein the absorbance at 665 nm) and after 60 min the blue color al-most disappeared.

The temporal changes in the spectral features of the catalyticsystems US–H2O2–pMF and US–H2O2–pUF are similar. However,some slight differences can be observed between the two systemsstudied. In the case of US–H2O2–pMF catalytic system, after

Fig. 4. SEM images of prepared samples: (a) pM; (b) pU; (c) pMF, and (d) pUF.

Fig. 5. Room-temperature magnetization curves of a-Fe2O3/MCM-41nanocomposites.

Table 3Magnetic properties of iron oxide/MCM-41 samples.

Sample HC (Oe) Mr (emu/g) MS (emu/g) Mr/MS

pMF 19 0.05 0.94 0.05pUF 39 0.009 0.35 0.02

Fig. 6. Photographs of MB aqueous solution before (A) and after degradationexperiment using: (B) US–H2O2 system, and (C) H2O2–a-Fe2O3/MCM-41 nanocom-posite system.

188 I. Ursachi et al. / Journal of Colloid and Interface Science 377 (2012) 184–190

30 min, one can observe a significant decrease in the absorbance atwavelength of about 200 nm (attributed of H2O2 [29]) compared

with US–H2O2–pUF catalytic system. The significant decrease inthe absorbance at about 200 nm indicates that the consumptionof H2O2 in the US–H2O2–pMF system is higher than for US–H2O2–pUF. Also, it can be seen from Fig. 7 that the two majorabsorbance peaks of MB became weaker in intensity as the ultra-sonic irradiation treatment time increased. After 60 min, thesetwo peaks almost disappeared, indicating that the benzene ringand heteropolyaromatic linkage of MB were almost destroyed [30].

Experimental results on the degradation of MB were presentedalso in the form of a plot of the percentage of degradation of MB

Fig. 7. Changes of UV–vis spectra during the degradation of MB in different systems: (A) US–H2O2, (B) US–H2O2–pMF, and (C) US–H2O2–pUF; inset photographs of solution atthe end of the experiment.

Fig. 8. Removal efficiency of MB by using different systems: US–H2O2, US–H2O2–pMF and US–H2O2–pUF. The initial concentration of MB was 0.03 mM; pH 3; the USwas applied in pulsed mode (4 s on/1 s off); the reaction temperature was 20 ± 2 �C.

I. Ursachi et al. / Journal of Colloid and Interface Science 377 (2012) 184–190 189

versus irradiation time (Fig. 8). In the simple system US–H2O2, thedegradation rate of MB can be considered insignificant. The de-crease in MB concentration was only 17% of the initial concentra-tion after 270 min ultrasonic irradiation. The slow degradation ofMB in the US–H2O2 system indicates a very limited ultrasonic deg-radation rate of MB. In the catalytic systems US–H2O2–pMF andUS–H2O2–pUF, the degradation rate of MB is increased signifi-cantly. As shown in Fig. 8, the pMF and pUF samples exhibit a highcatalytic activity and removed about 94% and 96%, respectively, of

the initial quantity of MB within 60 min. If it is not necessary to re-move the MB total content from wastewater, then 60 min timemay be considered sufficient. After 270 min, the decrease in MBconcentration was about 99% of the initial concentration valuefor both systems. The almost complete degradation of the MB bythe US–H2O2–pMF and US–H2O2–pUF systems suggested that a-Fe2O3/MCM-41 nanocomposites behaved as a Fenton-like hetero-geneous catalyst with high catalytic activity.

A Fenton-type catalyst (Fe2+ + H2O2) makes possible the degra-dation of non-volatile hydrophilic organic dyes molecules viahighly reactive free radicals HO� attack. The degradation efficiencyis dependent on the formation rate of these radicals. Our resultssuggest that hematite nanoparticles dispersed in mesoporous silicaMCM-41 pores produce an increase in the decomposition rate ofH2O2 with formation of hydroxyl radicals. Furthermore, the exis-tence of heterogeneous catalyst in the system facilitates the pro-cess of cavitation. Also, the fast degradation of MB suggests astrong synergistic effect between the ultrasonic irradiation andcatalytic activity of a-Fe2O3/MCM-41 nanocomposites. Magnetica-Fe2O3/MCM-41 nanocomposite used as heterogeneous catalysthas the advantage of an easy recovery from reaction mixture bymagnetic separation or centrifugation.

3.5. Influence of pH

The sonocatalytic degradation of MB was investigated at differ-ent values of pH. Fig. 9 shows that the catalytic systems US–H2O2–pMF and US–H2O2–pUF give rise to different removal efficiency. Atthis initial pH value, the catalytic system US–H2O2–pUF exhibits alower catalytic activity within 180 min sonication compared with

Fig. 9. Effect of pH value on the removal efficiency of methylene blue: US–H2O2–pMF system (A) and US–H2O2–pUF system (B).

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US–H2O2–pMF. However, within 300 min sonication, the degrada-tion percentage of MB achieved 94% in both cases. When the initialpH value is adjusted at 3 and 2, respectively, the limiting efficiencyis clearly enhanced and slowly increases with the decreasing pH.

As at pH 2 and 3 the performance of the two studied systems israther similar, the MB degradation can be performed at pH 3 for60–90 min when approximately 95–97% of the initial amount ofMB is removed.

It is known that in homogenous systems pH of 3–4 is the opti-mal value for Fenton reactions and decreased catalyst efficiencywith increase in the pH value is closely related to the stability ofFe (at high pH values (pH > 5)) occurs precipitation of ferrous com-pounds) [18].

Based on the obtained results, it seems that in the heteroge-neous Fenton ultrasound-assisted MB degradation technique theused pH values lead to good efficiency of the system. However, de-tailed studies are needed to determine the optimal pH value wherestability and activity of the catalyst are the highest.

4. Conclusion

Magnetic a-Fe2O3/MCM-41 nanocomposites were stabilizedwithin the pore system of the MCM-41 mesoporous silica synthe-sized by conventional and ultrasounds-assisted methods, and thiscatalyst was tested toward the MB degradation by coupling ultra-sounds with Fenton-like process at room temperature.

It was found that the obtained a-Fe2O3/MCM-41 nanocompos-ites exhibit high activity in the degradation of MB by couplingultrasound with Fenton-like processes. Ultrasonic irradiation ofa-Fe2O3 stabilized inside channel of the MCM-41 may promotevery reactive hydroxyl radicals formation that initiate a series ofchemical reactions that promote the degradation of MB. Magnetica-Fe2O3/MCM-41 nanocomposites obtained by ultrasound-as-sisted synthesis showed catalytic activity similar to that of thenanocomposites synthesized by conventional method but the firstare synthesized in a very short time, which could be favorable inpractical applications. Magnetic a-Fe2O3/MCM-41 nanocompositecatalysts have the advantage of an easy catalyst recovery of thematerial by magnetic separation or centrifugation.

The results have indicated that MB could be degraded at roomtemperature faster by coupling ultrasonic irradiation with a Fen-ton-like process using a heterogeneous catalyst based on a-Fe2O3/MCM-41 nanocomposites.

Acknowledgments

This work was financially supported by Romanian CNCS-UEFI-SCDI project IDEI-EXOTIC No. 185/25.10.2011.

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