MIL-101(Cr)–cobalt ferrite magnetic nanocomposite: synthesis, characterization and applications for the sonocatalytic degradation of organic dye pollutants

Abstract

In this study, for the first time, a novel magnetically recyclable MIL-101(Cr)/CoFe₂O₄ nanocomposite was prepared via a facile solvothermal method. The morphology, structural, magnetic and optical properties of the nanocomposite were characterized via field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometer (VSM), UV-visible spectroscopy (UV-visible) and BET surface area analysis. Furthermore, the sonocatalytic activity of the MIL-101(Cr)-based magnetic nanocomposite was explored for the degradation of organic dye pollutants such as Rhodamine B (RhB) and methyl orange (MO) under ultrasound irradiation in the presence of H₂O₂. Under optimized conditions, the degradation efficiency reached 96% for RhB and 88% for MO. The sonocatalytic activity of MIL-101(Cr)/CoFe₂O₄ was almost 12 and 4 times higher than that of the raw MIL-101(Cr) and pure CoFe₂O₄, respectively. The improved sonocatalytic performance of the as-prepared binary nanocomposite can be attributed to the relatively high specific surface area of MIL-101(Cr) and magnetic property of CoFe₂O₄, as well as the fast generation and separation of charge carriers (electrons and holes) in MIL-101(Cr) and CoFe₂O₄. In addition, the trapping tests demonstrated that ·OH radicals are the main active species in the dye degradation process. Moreover, the most influencing factors on the sonocatalytic activity such as the H₂O₂ amount, initial dye concentration and catalyst dosage were investigated. Finally, the nanocomposite was magnetically separated and reused without any observable change in its structure and performance even after four consecutive runs.

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1. Introduction

Nowadays, considerable attention is being paid to the veritable color contamination issues. Dyes are widely utilized in various industries including the paint, fashion, printing, coloring, materials and other production industries, which can significantly cause water contamination when untreated wastewater is discharged into the environment due to their non-biodegradable, exceptionally poisonous, cancer-causing and mutagenic properties.^1–4 Thus, currently, various strategies including advanced chemical oxidation (AOP), ion exchange, organic treatment and adsorption have been developed and utilized for the remediation of dyes in wastewater.^1,5–11 Among the AOP techniques, sonolysis has been widely used to degrade organic pollutants. The chemical effect of ultrasound (US) comes from acoustic cavitation, which leads to the formation of bubbles that develop and collapse in the liquid. The collapse of bubbles in solution results in the generation of spots with high localized temperature and pressure. Under these extraordinary conditions, water molecules are broken, producing highly reactive hydroxyl radicals, which attack and oxidize the organic dyes in water.^12,13 However, the US alone is not efficient for the degradation of target organic pollutants because of the long time and high amount of energy required for considerable degradation. Thus, to overcome this issue, sonocatalysts (catalysts active under ultrasonic irradiation) can be utilized in aqueous solutions for the degradation of organic pollutants. The presence of the insoluble sonocatalysts in the reactive solution accelerates the generation of ·OH under ultrasonic irradiation.^14,15 In previous reports, different semiconductors such as Cu₂S,¹ ZnO–biosilica,¹⁶ KNbO_3,¹⁷ LuFeO₃,¹⁸ Fe₃O₄–graphene/ZnO@SiO₂ (ref. 19) and La-doped ZnO²⁰ have been studied for their sonocatalytic activity and utilized as sonocatalysts. However, their separation and reuse are still a challenge. Therefore, it is necessary to develop new magnetic sonocatalysts with high catalytic activity that can be easily recycled.

Nano/micrometer-scale spinel-type ferrites (MFe₂O₄; M = Ni, Fe, Co, and Cu) have exhibited extraordinary potential for numerous applications such as the removal of pollutants, medical diagnostics and drug delivery.²¹ Among them, cobalt ferrite (CoFe₂O₄) has been widely studied as a magnetic material and photocatalyst because of its magnetism, high thermal stability, electronic conductivity, and noteworthy catalytic capability.^22,23 These properties made it a suitable candidate for us to investigate its capability as a sonocatalyst. The probable sonocatalytic degradation mechanism is based on both light and the “hot spots” derived from the ultrasonic cavitation effect. Firstly, it is well known that the sonoluminescence caused by the ultrasonic cavitation effect can generally generate lights with a wide wavelength range.^24,25 The generated light with an appropriate wavelength can excite the applied nanoparticles in the solution, acting as a sonocatalyst to perform pollutant degradation. Secondly, as is known, the “hot spots” produced by the ultrasonic cavitation effect in aqueous medium can achieve very high temperatures (more than 1000 °C). These high temperatures are sufficient to break down some H₂O molecules on the surface of the catalyst and produce hydroxyl radicals (·OH). These heat energies of the “hot spots” can also excite the prepared nanoparticles similar to light.^24,25 Under the excitation of the generated light and heat, electrons migrate from the valence band (VB) to the conduction band (CB) of the nanoparticles, resulting in the formation of e⁻/h⁺ pairs in the inner or on the surface of the nanoparticles. However, CoFe₂O₄ has serious drawbacks including the easy recombination of e⁻/h⁺ pairs and high agglomeration in solution, which decrease its catalytic activity. Thus, it is conceivable to couple CoFe₂O₄ nanoparticles with other semiconductors such as porous materials with a high surface area, overcoming the abovementioned problems and upgrading their catalytic performance.

Metal organic frameworks (MOFs) are hybrid crystalline porous materials with structures consisting of a regular array of positively charged metal ions or metallic clusters associated with bi- or multipodal organic linkers.²⁶ Currently, MOFs have experienced rapid development due to their versatile applications such as catalysis, storage and separation. Recent research has shown that these semiconducting materials are absolutely effective as catalysts for the photocatalytic degradation of organic pollutants.²⁷ Among the different MOFs, MIL-101(Cr) is one of the foremost investigated and exceptionally interesting materials due to its mesoporous structure, huge surface area and great hydrothermal stability, and thus has been widely explored for the adsorption of pollutants, catalysis, photocatalysis and drug delivery.^28,29 Furthermore, these properties make MIL-101(Cr) a great candidate for sonocatalytic activity, and coupling it with narrow band-gap magnetic semiconductors (e.g. CoFe₂O₄) can be an effective way for enhancing its catalytic activity and its recyclability employing an appropriate external magnet.

In the present study, a new magnetic binary composite, namely MIL-101(Cr)/CoFe₂O₄, was prepared via a hydrothermal method and its sonocatalytic activity was studied for the degradation of Rhodamine B (RhB) and methyl orange (MO) in water as organic pollutants under US irradiation in the presence of H₂O₂ as a hydroxyl radical (·OH) source. Besides, the effects of various parameters such as initial dye concentration, sonocatalyst dosage, scavengers and H₂O₂ amount were evaluated. Moreover, the sonocatalytic activity of MIL-101(Cr)/CoFe₂O₄ was compared with that of pure CoFe₂O₄, and MIL-101(Cr) under the same conditions. To the best of our knowledge and based on the literature review, there is no report on the synthesis and application of the MIL-101(Cr)/CoFe₂O₄ nanocomposite as a sonocatalyst for the sonodegradation of organic dyes, particularly in the presence of H₂O₂ as an environmental-friendly oxidizing agent.

2. Experimental

2.1. Materials

Chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 98%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 98.5%), terephthalic acid (TPA, 98%), sodium hydroxide (NaOH), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 98%), dimethyl formamide (DMF, 99%), ethanol (C₂H₅OH, 96%), disodium ethylenediaminetetraacetate (Na₂EDTA, 99%), silver nitrate (AgNO₃, 98%), 1,4-benzoquinone (BQ, 98%), t-butyl alcohol (t-BuOH, 99%), Rhodamine B (RhB, C₂₈H₃₁ClN₂O₃, 98%), methyl orange (MO, C₁₄H₁₄N₃NaO₃S, 98%) and other chemicals were obtained from Merck Company and used as received.

2.2. Synthesis of CoFe₂O₄ nanoparticles

The CoFe₂O₄ nanoparticles were prepared via a hydrothermal method. Fe(NO₃)₃·9H₂O (1.71 g) and Co(NO₃)₂·6H₂O (0.62 g) were added to deionized water (25 mL) and the solution was stirred at room temperature for 1 h. Then, 1 mol L⁻¹ NaOH aqueous solution was added to the suspension to adjust the pH to 11 while stirring for 1 h. The solution was then transferred to a teflon-lined autoclave bomb and kept in an oven for 12 h at 180 °C. Finally, the black product was magnetically collected and was washed with absolute ethanol and deionized water.

2.3. Synthesis of MIL-101(Cr)/CoFe₂O₄

The binary magnetic MIL-101(Cr)/CoFe₂O₄ nanocomposite was synthesized via a hydrothermal method. Briefly, 0.16 g of synthesized CoFe₂O₄ nanoparticles was added to 14 mL deionized water and sonicated for 1 h. Then, 0.5 g of TPA and 1.2 g of Cr(NO₃)₃·9H₂O were added to CoFe₂O₄ suspension and it was rigorously agitated for 1 h at room temperature. Afterwards, the resultant suspension was kept for 12 h at 200 °C under autogenous pressure after it was transferred to a 50 mL teflon-lined stainless steel autoclave. The obtained product was then removed from the oven and left to cool to room temperature. To obtain an extra-pure sample, the resultant mixture was consecutively flushed several times with DMF and ethanol solvents to remove any possible impurities from the unreacted TPA on its surface. The ICP-AES results indicated that the loading amount of MIL-101(Cr) in the as-fabricated MIL-101(Cr)/CoFe₂O₄ nanocomposite was approximately 64.80 wt%.

2.4. Characterization methods

FT-IR spectra were recorded on a spectrophotometer (Shimadzu FT-IR 8400S, Japan) using KBr pellets in the wavelength range of 400–4000 cm⁻¹. XRD patterns of the powder samples were recorded using an X-ray diffractometer (XPertPro Panalytical, Netherland) using Ni-filtered CuKα radiation (λ = 1.5406 Å). UV-vis spectra were measured in the spectral range of 200–800 nm on a spectrophotometer (Cary 100 Conc, Varian Company, USA). A homogeneous suspension in water was formed by sonication for 40 min for the UV–vis studies of the samples. The magnetic measurements were performed utilizing a vibrating sample magnetometer (VSM, Magnetic Danesh Pajoh Kashan Co., Iran) at a maximum magnetic field of 10 kOe. The Brunauer–Emmett–Teller surface area measurement (PHS-1020, China) was performed utilizing N₂ at 77 K as the adsorbing gas. FE-SEM images were obtained using a scanning electron microscopy system (MIRA3 TESCAN, Czech Republic) coupled with a link energy-dispersive X-ray (EDX). TEM analysis was performed using a transmission electron microscope (Philips CM120, The Netherlands) at the acceleration voltage of 120 kV. In addition, an ultrasonic bath (Sonic 6MX; England) with 100 W output acoustic power and frequency of 37 kHz was utilized to investigate all the sonodecomposition reactions of dyes. Inductively coupled plasma atomic emission spectroscopy (ICP-AES, model OEC-730, USA) was utilized to identify the concentrations of Cr, Fe and Co in the filtrate.

2.5. Sonodecomposition tests

To examine the sonocatalytic decomposition of RhB as an organic cationic dye and MO as an organic anionic dye over the desired catalysts in aqueous solution, an ultrasonic device was utilized. Typically, a specific amount of magnetic MIL-101(Cr)/CoFe₂O₄ catalyst (0.5 g L⁻¹) was transferred to the dye aqueous solution (25 mg L⁻¹, 50 mL), and subsequently the resulting solution was stirred for 30 min in the dark. This blending step allows adsorption–desorption equilibrium to be achieved between the catalyst surface and organic dye. At that point, the solution was subjected to US irradiation in the presence of H₂O₂ (60 mmol L⁻¹). After the required interval, the reaction process was halted and 2 mL of the aqueous solution was withdrawn, followed by the separation of the MIL-101(Cr)/CoFe₂O₄ nanocomposite from the supernatant solution utilizing an external magnetic field, but in some cases it was centrifuged for 5 min at 5000 rpm for better separation. Finally, the residual solution of the organic dye was analyzed using a UV-vis spectrophotometer at a wavelength of 557 and 467 nm to evaluate the concentration of residual RhB and MO dyes, respectively. Also, the effects of various parameters, such as initial concentration of RhB (5, 15, 25, and 35 mg L⁻¹), sonocatalyst amount (0, 0.25, 0.5, and 0.75 g L⁻¹), and concentration of H₂O₂ (0, 10, 20, 30, 40, 50, 60, and 70 mmol L⁻¹), on the sonocatalytic activity of MIL-101(Cr)/CoFe₂O₄ were investigated. The percentage degradation efficiency (DE%) was also determined using eqn (1) as follows:

DE% = [(C₀ − C_t)/C₀] × 100% (1)

where, C₀ and C_t represent the concentration (mg L⁻¹) of decomposed dye at the initial concentration (after adsorption equilibrium) and the specified reaction time interval in min, respectively.

3. Results and discussion

3.1. Characterization of the MIL-101(Cr)/CoFe₂O₄ nanocomposite

Fig. 1 demonstrates the X-ray diffraction (XRD) patterns of CoFe₂O₄, MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄ to characterize their phase and crystallization properties. As shown in Fig. 1(a), the main peaks identified for raw CoFe₂O₄ were located at 2θ values of 30.13°, 35.5°, 43.08°, 53.44°, 57.07° and 62.59°, which crystallized in the cubic structure with JCPDS card (01-1121). The average crystalline size of the CoFe₂O₄ nanoparticles was measured to around 12 nm utilizing the Scherrer equation.³⁰ The XRD pattern of MIL-101(Cr) is shown in Fig. 1(b). The diffraction peaks corresponding to reflection planes are indexed, which are consistent with the peak positions reported for the standard MIL-101(Cr) structure, demonstrating that the synthesized material has the MIL-101(Cr) phase with high crystallinity.² The XRD pattern of the nanocomposite in Fig. 1(c) exhibits diffraction peaks corresponding to both cubic structure CoFe₂O₄ and MIL-101(Cr), and no other impurity peaks can be observed. These results reveal that the sample is composed of spinel CoFe₂O₄ and MIL-101(Cr), and the MIL-101(Cr)/CoFe₂O₄ nanocomposite was successfully synthesized.

Fig. 1 XRD patterns of the as-fabricated (a) raw CoFe₂O₄, (b) raw MIL-101(Cr) and (c) MIL-101(Cr)/CoFe₂O₄.

The Fourier transform infrared spectra (FTIR) of the as-prepared catalysts, namely CoFe₂O₄, MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄, are displayed in Fig. 2. Fig. 2(a) shows the FT-IR spectrum of the CoFe₂O₄ nanoparticles with two peaks in the range of 400 to 600 cm⁻¹, corresponding to the Fe(III)–O and Co(II)–O bonds of spinel-type oxide.³¹ In addition, the –OH group of adsorbed water resulted in the appearance of bands at about 2800–3700 cm⁻¹ and 1400–1700 cm⁻¹ related to the typical stretching vibrations.³² As shown in Fig. 2(b), the characteristic absorption peaks of MIL-101(Cr) can be observed the region of 1400 to 1600 cm⁻¹, which mainly originate from the carboxylate group vibrations and are identical to that reported in the literature.² The two sharp peaks at 1600 and 1404 cm⁻¹ can be attributed to the asymmetric and symmetric vibrations of the carboxyl groups, respectively, confirming the presence of the dicarboxylate linker inside the sample. After the formation of the MIL-101(Cr)/CoFe₂O₄ nanocomposite, as exhibited Fig. 2(c), the characteristic peaks related to MIL-101(Cr) are nearly the same and another band at 590 cm⁻¹ is assigned to CoFe₂O₄. Combined, these results confirm the successful synthesis of the MIL-101(Cr)/CoFe₂O₄ nanocomposite.

Fig. 2 FT-IR spectra of the as-fabricated (a) raw CoFe₂O₄, (b) raw MIL-101(Cr) and (c) MIL-101(Cr)/CoFe₂O₄.

Energy Dispersive X-ray (EDX) was performed to characterize the composition of the as-prepared nanocomposite. The chemical purity of the raw CoFe₂O₄ (Fig. S1(a)†) and raw MIL-101(Cr) (Fig. S1(b)†) was confirmed by EDX analysis. As shown in Fig. S1(c),† the EDX elemental spectrum of the nanocomposite shows elemental peaks corresponding to both MIL-101(Cr) (C, O and Cr) and CoFe₂O₄ (Co, Fe and O), and no other impurity peaks can be observed, demonstrating that the composite sample is composed of MIL-101(Cr) and CoFe₂O₄. The distribution of elements in the nanocomposite was examined utilizing SEM mapping analysis, and the results are shown in Fig. S1(d).† The corresponding elemental mappings show that Co, Fe, C, O and Cr elements are consistently distributed over the nanocomposite, confirming the homogeneity of the sample.

To survey the structure, morphology and size of the as-prepared samples, field emission scanning electron microscopy (FE-SEM) analysis was employed. The FE-SEM images of the raw CoFe₂O₄, raw MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄ nanocomposite are shown in Fig. 3. According to Fig. 3(a), the obtained CoFe₂O₄ nanoparticles are loosely aggregated with a sphere-like morphology. Moreover, it can be seen in Fig. 3(b) that the raw MIL-101(Cr) has an octahedral shape with crystals sizes in the submicrometer range (0.2–0.7 μm). Based on the SEM images of MIL-101(Cr)/CoFe₂O₄ (Fig. 3(c) and (d)), the different morphology of the binary MIL-101(Cr)/CoFe₂O₄ sample compared to that of the raw MIL-101 (Cr) indicates a different formation mechanism due to the presence of CoFe₂O₄ nanoparticles. However, although there was a change in morphology, the XRD, FT-IR, and EDX results strongly confirm that the MIL-101 (Cr) framework was present in the structure of as-synthesized nanocomposite.

Fig. 3 FE-SEM images of the as-fabricated (a) CoFe₂O₄, (b) MIL-101(Cr) and (c and d) MIL-101(Cr)/CoFe₂O₄ nanocomposite.

The morphology and microstructure of the as-prepared MIL-101(Cr)/CoFe₂O₄ nanocomposite were further confirmed by TEM analysis. Fig. 4 presents the TEM images of the MIL-101(Cr)/CoFe₂O₄ nanocomposite. It is clear that the MIL-101(Cr) crystals and CoFe₂O₄ nanoparticles are both present in the composite. The nanoparticle size distribution is narrow, ranging from 15 to 40 nm. These results are consistent with the average particle size calculated in the XRD and FE-SEM analysis and are interesting and significant.

Fig. 4 TEM images of the as-fabricated MIL-101(Cr)/CoFe₂O₄ nanocomposite.

The UV-visible absorption spectra of CoFe₂O₄ and MIL-101(Cr)/CoFe₂O₄ nanocomposite are presented in Fig. S2.† As can be seen in Fig. S2(a),† the raw CoFe₂O₄ shows a broad absorption band with absorption edges at ca. 520 nm.³¹ Compared with the spectrum of the raw CoFe₂O₄ sample, the spectral features of the MIL-101(Cr)/CoFe₂O₄ nanocomposite are shifted to the visible light region (Fig. S2(b)).† Additionally, the much stronger absorption band of MIL-101(Cr)/CoFe₂O₄ than that of raw CoFe₂O₄ indicates that the MIL-101(Cr)/CoFe₂O₄ nanocomposite has potential as an efficient sonocatalyst under visible light irradiation.

To precisely reveal the extraordinary structure of the as-prepared catalyst, BET analysis was performed. Fig. 5(a) and (c) show the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves for MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄, respectively. The N₂ adsorption–desorption isotherms of the MIL-101(Cr)/CoFe₂O₄ nanocomposite in Fig. 5(c) demonstrate a type (IV) curve with H3 hysteresis loop. The BET specific surface area values for the raw MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄ were measured to be around 2452 and 3353 m² g⁻¹ with the total pore volume of 0.532 and 3.0515 cm³ g⁻¹, respectively. Also, the pore sizes of the raw MIL-101(Cr) and MIL-101(Cr)/CoFe₂O₄ are 1.30 and 1.21 nm, which were computed utilizing the Barrett–Joyner–Halenda (BJH) method from the adsorption and desorption branches of the N₂ isotherms (Fig. 5(b) and (d)), respectively. Thus, based on these results, it can be concluded that adding CoFe₂O₄ during the synthesis process had a great effect on the structure and size of MIL-101(Cr) in the nanocomposite rather than the microstructure of pure MIL-101(Cr). Also confirming the above, the SEM images show that the size of MIL-101(Cr) in the MIL-101(Cr)/CoFe₂O₄ nanocomposite is in the nano-scale rather than that on the micro-scale of pure MIL-101(Cr), resulting in a significant increase in the pore volume and surface area of the nanocomposite, which can enhance the available active sites for its sonocatalytic performance.

Fig. 5 BET analysis and BJH plots of the as-fabricated (a and b) raw MIL-101(Cr), and (c and d) MIL-101(Cr)/CoFe₂O₄.

Fig. S3† shows the magnetic hysteresis loops of the raw CoFe₂O₄ and MIL-101(Cr)/CoFe₂O₄, which were studied at room temperature with an applied field of −10 000 ≤ H ≤ 10 000 Oe. As shown in Fig. S3,† the ferromagnetic properties of both CoFe₂O₄ and MIL-101(Cr)/CoFe₂O₄ were obviously confirmed. As anticipated, the MIL-101(Cr)/CoFe₂O₄ nanocomposite had a lower saturation magnetization (Ms) (22 emu g⁻¹) than that of the raw CoFe₂O₄ (62.78 emu g⁻¹) due to the presence of the nonmagnetic MIL-101(Cr). However, compared with the non-magnetic MIL-101(Cr) catalyst, the magnetic MIL-101(Cr)/CoFe₂O₄ catalyst could be separated from the heterogeneous reaction system using an external magnetic field, as shown in the inset of Fig. S3.†

3.2. Sonocatalytic activity of MIL-101(Cr)/CoFe₂O₄ nanocomposite

The sonocatalytic performance of the MIL-101(Cr)/CoFe₂O₄ nanocomposite was investigated for the degradation of the cationic RhB dye and anionic MO dye under US irradiation. Fig. 6(a) shows the change in the UV-vis absorbance spectrum and color of RhB as a function of irradiation time. Firstly, the adsorption percentage of RhB by the MIL-101(Cr)/CoFe₂O₄ nanocomposite at 30 min in the dark under adsorption–desorption equilibrium was only 11%, which is related to the fact that the cationic charges on the MIL-101(Cr)/CoFe₂O₄ nanocomposite repel the cationic charged RhB dye molecules.³³ In the presence of the sonocatalyst, H₂O₂ and US irradiation, the intensity of the characteristic absorption peak of RhB at 557 nm diminished continuously with time and it roughly disappeared after 140 min. The sonocatalytic activity of the MIL-101(Cr)/CoFe₂O₄ nanocomposite for the degradation of the anionic MO dye was considered to confirm the generality of this sonocatalytic system. The adsorption percentage of MO by the MIL-101(Cr)/CoFe₂O₄ nanocomposite within 30 min of adsorption–desorption equilibrium was 39%, which can attributed to the adsorption of the anionic molecules of MO with the cationic charged nanocomposite. As shown in Fig. 6(b), the degradation efficiency was 88% for MO in the MIL-101(Cr)/CoFe₂O₄/H₂O₂/US system within 140 min. It was discovered that the as-prepared nanocomposite acted differently in the sonocatalytic degradation of the RhB and MO dyes due to their distinct structures, compositions, size and electric charge.³⁰

Fig. 6 UV-vis absorption spectra of the sonodegradation process of (a) RhB and (b) MO dye over the MIL-101(Cr)/CoFe₂O₄ nanocomposite as a function of irradiation time, (c) sonocatalytic degradation of RhB and (d) plot of ln(C₀/C) against the irradiation time under different conditions. (Experimental conditions: catalyst dosage = 0.5 g L⁻¹, RhB dye = 25 mg L⁻¹ (50 mL), pH = 7 (about 6.5), H₂O₂ = 60 mmol L⁻¹, temperature = 25 °C and reaction time = 140 min.)

To provide an overview on the potential and workability of each component in the decomposition reaction of RhB dye, several systems including US alone, H₂O₂/US, MIL-101(Cr)/CoFe₂O₄/H₂O₂, MIL-101(Cr)/H₂O₂/US, MIL-101(Cr)/CoFe₂O₄/US, CoFe₂O₄/H₂O₂/US, and MIL-101(Cr)/CoFe₂O₄/H₂O₂/US were utilized for the degradation of the dyes and the results are presented in Fig. 6(c). Briefly, the results from the executed tests revealed that decomposition efficiency followed the order of US alone (0%) < H₂O₂/US (12%) < MIL-101(Cr)/CoFe₂O₄/H₂O₂ ∼ MIL-101(Cr)/H₂O₂/US (25%) < MIL-101(Cr)/CoFe₂O₄/US (31%) < CoFe₂O₄/H₂O₂/US (52%) < MIL-101(Cr)/CoFe₂O₄/H₂O₂/US (96%). According to the analysis of the results, MIL-101(Cr)/CoFe₂O₄/H₂O₂/US was recognized as the best system for the decomposition of RhB, which can be related to the obstruction of electron–hole recombination by the linkage between CoFe₂O₄ and MIL-101(Cr).

The kinetics of the RhB dye degradation was investigated using the rate constant (k) for a much better comparison of the sonocatalytic efficiency of the samples. Therefore, to assess the rate of the reaction in the sonodegradation experiments, the pseudo-first-order equation was used¹ as follows: ln(C₀/C) = kt, where C₀ and C are the dye concentrations before and after irradiation, respectively, k is the pseudo-first-order rate constant and t is the reaction time. As shown in Fig. 6(d), the rate constant, k, in the different systems increased in the order of US alone (0 min⁻¹) < H₂O₂/US (0.0009 min⁻¹) < MIL-101(Cr)/CoFe₂O₄/H₂O₂ ∼ MIL-101(Cr)/H₂O₂/US (0.0018 min⁻¹) < MIL-101(Cr)/CoFe₂O₄/US (0.0026 min⁻¹) < CoFe₂O₄/H₂O₂/US (0.0055 min⁻¹) < MIL-101(Cr)/CoFe₂O₄/H₂O₂/US (0.0215 min⁻¹). Among them, the MIL-101(Cr)/CoFe₂O₄/H₂O₂/US system exhibited the best catalytic performance (96% within 140) with a rate constant of 0.0215 min⁻¹, which was enhanced by more than 12 and 4 times (k = 0.0215 min⁻¹) compared to that of the raw MIL-101(Cr) (k = 0.0018 min⁻¹) and CoFe₂O₄ (k = 0.0055 min⁻¹), respectively. The raw MIL-101(Cr) and CoFe₂O₄ showed poor sonocatalytic activities of only 25% and 52% degradation of RhB within 140 min of US, respectively, which resulted from the quick recombination of photoexcited charges. Thus, when MIL-101(Cr) was composited with CoFe₂O₄ nanoparticles, then the sonodegradation rate increased prominently.

3.2.1. Effect of H₂O₂ amount, sonocatalyst dosage and initial concentration of RhB. To explore the influence of H₂O₂ concentration on the degradation of RhB, a series of tests were applied with different amounts of H₂O₂, while the initial concentration of dye and other parameters were kept constant, and the results are displayed in Fig. S4(a).† It can be seen that only 31% of RhB was removed without H₂O₂. When the concentration of H₂O₂ increased from 10 to 60 mmol L⁻¹, the degradation efficiency was enhanced correspondingly from 38% to 96%. This improvement in the degradation rate can be attributed to the increase in hydroxyl radicals (·OH) produced via H₂O₂ decomposition. However, the further addition of H₂O₂ to 70 mmol L⁻¹ led to a decline in the degradation proficiency. Therefore, excess H₂O₂ molecules act as scavengers of ·OH to create perhydroxyl (·OOH) radicals with a lower oxidation potential.¹

To optimize the effect of sonocatalyst dosage on the degradation process of RhB dye, a series of experiments was performed with different concentrations of MIL-101(Cr)/CoFe₂O₄ nanocomposite (0, 0.25, 0.5 and 0.75 g L⁻¹, Fig. S4(b)),† while the other parameters were kept constant. Apparently, with an increase in the catalyst concentration from 0 to 0.5 g L⁻¹, an increase in the degradation rate of RhB in aqueous solution occurred (from 12% to 96%), which is reasonable due to the increase in the number of active sites resulting from a higher amount of catalyst. Clearly, it generated more ·OH radicals, facilitating the further and faster degradation of the dye molecules.³⁴ However, a higher concentration of MIL-101(Cr)/CoFe₂O₄ (0.5 to 0.75 g L⁻¹) lead to a decline in the degradation efficiency because of the aggregation of the nanoparticles and decrease in the number of active sites, as well as the scattering and blockage of the transmission of ultrasound waves within the solution. Subsequently, the optimal value of 0.5 g L⁻¹ for MIL-101(Cr)/CoFe₂O₄ was considered for the other tests.³⁵

The effect of initial dye concentration as a critical parameter on its sonocatalytic degradation efficiency was investigated. Accordingly, different initial concentrations of RhB ranging from 5 to 35 mg L⁻¹ were tested under US irradiation. As shown in Fig. S4(c),† more time was required to achieve a reasonable degradation efficiency at a higher concentration of RhB dye. However, the number of generated ·OH radicals required for attacking the dye molecules remained constant at a constant reaction time, amount of H₂O₂ and catalyst dosage, and increasing the initial dye concentration resulted in a decrease in the degradation efficiency.³

The pH of the initial solution is a significant parameter that affects the degradation of dye molecules. The influence of the initial pH of the RhB and MO dye solutions on their sonocatalytic degradation is shown in Fig. S4(d).† To explore the effect of pH on the degradation efficiency, the pH of the dye solutions was adjusted over a wide range, ranging from acidic, neutral/near-neutral to basic. For these trial runs, the dose of the catalyst was 0.5 g L⁻¹, the initial dye concentration was set to 25 mg L⁻¹ (50 mL), and the amount of H₂O₂ was 60 mmol L⁻¹. Approximately 51%, 91%, 96%, 100% and 53% of RhB and 44%, 95%, 88%, 80% and 40% of MO was degraded after 140 min of reaction at the initial pH 3, 5, 7, 9 and 11, respectively. Based on the results of the effect of initial pH on the degradation of the dyes, the degradation efficiency reached the highest level when the initial RhB and MO solutions had a neutral pH. Thus, the acceptable performance of MIL-101(Cr)/CoFe₂O₄ as an efficient heterogeneous catalyst in a tunable neutral solution of organic dyes are a noteworthy advantage for increasing its efficiency and reducing the cost of the process. This may be related to the function of the solid-acid catalyst of MIL-101(Cr), which will be introduced in detail below. On the other hand, the degradation dramatically decreased at pH 3 and 11, which is reasonable because MOFs suffer from a lack of stability in alkaline and acidic conditions compared to a neutral environment, hindering the catalytic process.³⁶

3.2.2. The proposed sonocatalytic degradation mechanism of organic dye. The sonocatalytic degradation of organic contaminants over a sonocatalyst is based on both sonoluminescence and hot spots, which originate from the ultrasonic-cavitation effect.^24,25 Sonoluminescence seems can produce abundant visible light, which excites the prepared MIL-101(Cr)/CoFe₂O₄ nanocomposite, and subsequently it acts as a sonocatalyst. Moreover, the formation of hot spots results in the generation of electrons and holes on the surface of the MIL-101(Cr)/CoFe₂O₄ sonocatalyst.^17,31,37 Actually, the electrons in the full valence band of MIL-101(Cr) and CoFe₂O₄ are excited to the empty conduction band separately under ultrasound irradiation, leaving an equal number of holes with a positive charge in the corresponding positions. The presence of CoFe₂O₄ favors the sonocatalytic activity under ultrasound irradiation due to its narrow band gap. Moreover, the difference in the band structures of CoFe₂O₄ and MIL-101(Cr) facilitates the transfer of sono-induced electrons from the CB of CoFe₂O₄ to the CB of MIL-101(Cr) and holes migrate from the VB of MIL-101(Cr) to the VB of CoFe₂O₄. These processes productively prevent the recombination of sonogenerated electron–hole pairs and significantly improve the sonocatalytic activity. The sono-excited electrons on the surface of MIL-101(Cr) may be captured by H₂O₂ and O₂ in the solution to yield ·OH and O₂˙⁻ radicals, while the holes are scavenged by the adsorbed water to form ·OH radicals. Finally these active species can oxidize the dye molecules adsorbed on the active sites of MIL-101(Cr) and CoFe₂O₄ through electrostatic attraction, resulting in the degradation of the dye and production of CO₂, H₂O, etc. as safe materials^{17,30,31,38,39} (eqn (2)–(9)). Thus, based on this process, a schematic representation of the mechanism is illustrated in Fig. 7.

US irradiation → heat–light (2)

MIL-101(Cr)/CoFe₂O₄ + heat or light → (MIL-101(Cr)/CoFe₂O₄)* (3)

(CoFe₂O₄)* → CoFe₂O₄ (h_VB⁺) + CoFe₂O₄ (e_CB⁻) (4)

(MIL-101(Cr))* → MIL-101(Cr) (h_VB⁺) + MIL-101(Cr) (e_CB⁻) (5)

CoFe₂O₄ (e_CB⁻) + MIL-101(Cr) → CoFe₂O₄ + MIL-101(Cr) (e_CB⁻) (6)

MIL-101(Cr) (e_CB⁻) + H₂O₂ and O₂ → MIL-101(Cr) + OH⁻ + ·OH and O₂˙⁻ (7)

CoFe₂O₄ (h_VB⁺) + H₂O/OH⁻ → CoFe₂O₄ + ·OH (8)

RhB + ·OH and O₂˙⁻ → CO₂ + H₂O + etc. (9)

Fig. 7 Conceivable mechanism for the sonodegradation process of dye contaminants using the MIL-101(Cr)/CoFe₂O₄/H₂O₂/US system in aqueous solution.

3.2.3. The influence of the scavenger type. To investigate what type of species plays a crucial role in the sonodecomposition process of the dyes and confirm the mechanism of sonocatalysis, disodium ethylenediaminetetraacetate (EDTA), AgNO₃, 1,4-benzoquinone (BQ) and t-butyl alcohol (t-BuOH) were used as the scavengers of holes (h_VB⁺), electrons, O₂˙⁻ and ·OH radicals, respectively, with the same initial concentration of 10 mmol L⁻¹. As shown in Fig. 8, RhB sonodecomposition using the MIL-101(Cr)/CoFe₂O₄ nanocomposite was significantly suppressed after the addition of t-BuOH, demonstrating that ·OH is the most reactive species in the sonocatalytic process. When AgNO₃, BQ, and EDTA were added, the sonodecomposition activity of the MIL-101(Cr)/CoFe₂O₄ nanocomposite slightly diminished, which shows that electrons, O₂˙⁻ and h⁺ play a minor role. Thus, the most reactive species involved in the decomposition of RhB over the MIL-101(Cr)/CoFe₂O₄ nanocomposite are ·OH radicals, while electrons, O₂˙⁻ and h⁺ are also generated during the sonocatalytic reaction under ultrasound irradiation.^40,41

Fig. 8 Effects of different scavengers on the sonocatalytic degradation of RhB in the MIL-101(Cr)/CoFe₂O₄/H₂O₂/US system. (Experimental conditions: scavenger = 10 mmol L⁻¹, catalyst dosage = 0.5 g L⁻¹, RhB dye = 25 mg L⁻¹ (50 mL), pH = 7 (about 6.5), H₂O₂ = 60 mmol L⁻¹, temperature = 25 °C and reaction time = 140 min.)

3.2.4. Comparison with other reported sonocatalysts. To show the benefit of the present degradation strategy, the obtained results from sonodegradation of RhB over the MIL-101(Cr)/CoFe₂O₄ nanocomposite were compared with some reported catalysts in the literature.^3,18,42–50 The comparison between the results in this study with the catalysts in the literature in shown in Table S1† based on different parameters including initial dye concentration, degradation time, consumed catalyst amount and percentage, which illustrates the excellent degradation efficiency of the MIL-101(Cr)/CoFe₂O₄ nanocomposite. Especially, the degradation reaction in the presence of most detailed catalysts took longer times and required a higher catalyst to dye ratio.

3.2.5. Recyclability and stability tests. One of the most interesting advantages with the use of heterogeneous catalysts in wastewater treatment is their potential for reuse over several runs. Thus, the reusability of the sonocatalyst was investigated after its magnetic separation, washing completely with water and ethanol and then assessment of sonocatalysis for four cycles (Fig. 9(a)). The results showed that the degradation efficiency only diminished slightly even after four times of reuse, demonstrating that the MIL-101(Cr)/CoFe₂O₄ sonocatalyst has a great stability. The amounts of Cr, Co and Fe metals in the filtrate were also determined to be approximately 0.1% by ICP-AES investigation. This data proves that the leaching of Co²⁺, Cr³⁺ and Fe³⁺ ions from the catalyst was not significant. The nature of the recovered catalyst was also tested. As shown in Fig. 9(b)–(e), the XRD, FT-IR, FE-SEM and EDX analyses of the recycled MIL-101(Cr)/CoFe₂O₄ catalyst showed no change compared with that of the fresh sample. These findings confirm that the structure of the MIL-101(Cr)/CoFe₂O₄ nanocomposite is stable under the reaction conditions as a special advantage for its practical application.

Fig. 9 (a) Recyclability of MIL-101(Cr)/CoFe₂O₄ nanocomposite. (b) XRD, (c) FT-IR, (d) FE-SEM and (e) EDX results of MIL-101(Cr)/CoFe₂O₄ recovered after the fourth run.

4. Conclusion

In summary, a new binary magnetic MIL-101(Cr)/CoFe₂O₄ sonocatalyst for the degradation of dyes was fabricated via a hydrothermal method and completely characterized using various techniques including XRD, FT-IR, FE-SEM, EDX, TEM, UV-vis, BET and VSM. The results revealed that the as-prepared nanocomposite had good sonocatalytic activity toward the degradation of RhB (in the presence of H₂O₂ as an ·OH source) particularly compared to its individual counterparts. The performance of the sonocatalyst was additionally proven to be dependent on the amount of applied H₂O₂, initial dye concentration and the catalyst dosage. Additionally, the excellent superparamagnetism at room temperature and stability of the MIL-101(Cr)/CoFe₂O₄ nanocomposite guarantee its applicable magnetic separation. The high sonocatalytic performance of MIL-101(Cr)/CoFe₂O₄ was attributed to the combination of the adsorption and catalytic properties of MIL-101(Cr) and magnetic properties of CoFe₂O₄ together with the efficient separation of electron–hole pairs, making it a successful and efficient sonocatalyst for environmental applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors sincerely acknowledge all supports from Dr Firouzeh Siadatnasab for her scientific and laboratory assistance at Lorestan University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra04945j

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