A simple and effective synthesis of magnetic γ-Fe₂O₃@SiO₂@TiO₂–Ag microspheres as a recyclable photocatalyst: dye degradation and antibacterial potential

Abstract

Purpose and methods

In this study, an effective technique for synthesizing γ-Fe₂O₃@SiO₂@TiO₂–Ag magnetically separable photocatalyst was introduced by combining co-precipitation, sol-gel, and photo-deposition methods. A series of analyses including FTIR, SEM, EDS, XRD, and VSM were applied to characterize the prepared materials and the investigations on photocatalytic activity of the prepared composites were accomplished.

Results

Compared to bare γ-Fe₂O₃@SiO₂@TiO₂, the Ag-doped composite was more active in terms of photocatalytic characteristics. By applying γ-Fe₂O₃@SiO₂@TiO₂-Ag, the decomposition rate of the Basic blue 41 reached to about 94% after 3 h of UV irradiation; this rate was 63% for pure γ-Fe₂O₃@SiO₂@TiO₂. The results indicated that the dye degradation kinetics followed first-order kinetic model. During the five cycles of separation, it was observed that the Ag-doped composite was greatly effective and stable in terms of recycling. Moreover, the results indicated that antibacterial activity of γ-Fe₂O₃@SiO₂@TiO₂-Ag was remarkably stronger than that of pure Fe₂O₃@SiO₂@TiO₂ particles.

Conclusion

It was concluded that by modifying magnetic TiO₂ by silver nanoparticles, charge separation was eased by catching photo-generated electrons, resulted in an enhanced photo- and biological activity.

Graphical abstract

Introduction

Dyes are amongst the most common organic industrial contaminants and pollutants [1, 2]. Dying textiles, paper, plastics, leather, food, and cosmetics leads to the formation of toxic species discharging as colored wastewater to the environment [3–5]. These colored effluents disturb photosynthesis in water plants by reducing light penetration [6, 7]. In order to accomplish decolorization and detoxification of dyes, varieties of techniques have been developed including adsorption, degradation, oxidation, and catalytic reduction [8–12]. The advanced oxidation processes (AOPs) such as Fenton reaction, photocatalysis, sonolysis, ozonation and their combination rank among the most important methods for controlling pollution [6, 13]. Herein, they are highly efficient, simple, well reproducible, and can be handled easily [14]. Due to the industrializing of societies, photocatalytic degradation of harmful substances and organic pollutants in wastewater via semiconductor nanomaterials has attracted so much attention. ZnO, ZnS, Cu₂O, SnO_2, and TiO₂ are amongst the oxide semiconductor photocatalysts, of which TiO₂ has been proven to have wide-bandgap capable of removal of different organic dyes and contaminants under the irradiation of ultraviolet (UV) [15, 16]. Unfortunately, it is still a challenge to efficiently separate and recover TiO₂ nanomaterials from treated water and this fact restricts their widespread application.

Since magnetic nano or microparticles (such as Fe₃O₄ and Fe₂O₃), are easily separable from solution by applying an external magnetic field, they have been used in recent years to produce new catalysts to overcome these issues [17, 18]. Moreover, it is probable that photo-dissolution can separate TiO₂ from the magnetic iron oxide core. Consequently, in addition to altering the properties of the magnetic iron oxides, the photocatalytic activity of titanium-based catalysts will be reduced. To resolve this problem, SiO₂ can be applied as a barrier layer to form Fe₂O₃@SiO₂ which will not allow the interaction between the magnetic core and the TiO₂ coating [18, 19]. On the one hand, these magnetic photo-catalysts (Fe₂O₃@SiO₂@TiO₂) are very active in terms of photocatalytic features; on the other hand, they are very efficient in terms of magnetic recovery. Besides, Ag nanoparticles are often applied to adjust TiO₂ because they can catch electrons, help electron-hole separation, and make electron excitation possible by generating a local electrical field. Since silver nanoparticles are quite cheap compared to other noble metals and show bactericidal and special oxygen adsorption behavior, they have been considered as a good option for TiO₂ modification [20, 21]. The important task of heterogeneous photocatalysts as a nano TiO₂ based magnetic composite is improving the performance of photocatalytic process which could be increased by silver modification. Moreover, it has been proved that modification of TiO₂ nanoparticle with noble metal ions or transition metal ions can improve the antibacterial features [22].

In the present study, it was attempted to highlight the benefits of heterogeneous catalysis, magnetic separation, and improved catalytic and antibacterial activities of TiO₂ nanoparticles. This object was carried out by applying γ-Fe₂O₃@SiO₂@TiO₂–Ag nanocomposite to degrade highly toxic Basic blue 41 dye and prevent the growth of two different types of bacterial strains (Escherichia coli and Staphylococcus aureus). The effect of initial pH, dye concentration and catalyst dosage on the reaction rate in order to determine the optimum conditions for maximum degradation were also investigated. The superparamagnetic behavior of the particles in nanosize would make the immobilized catalyst on superparamagnetic nanomaterials to be easily separated from the media during a strong interaction with the applied external magnetic field, and it can be easily dispersed again.

Materials and methods

Reagents, chemicals and Bacteria

Basic blue 41 (BB41) was purchased from Ciba Company, Basel, Switzerland. Chemical reagents such as anhydrous iron chloride (FeCl₃), sodium acetate trihydrate (CH₃COONa.3H₂O), silver nitrate (AgNO₃), ethylene glycol (CH₂OH)₂, ammonium hydroxide 30% (NH₄OH), tetraethyl orthosilicate (TEOS), tetraisopropyl orthotitanate (TIPOT) and ethanol were supplied by the Merck Company, Darmstadt, Germany and were used without any further purification. In order to obtain the desired pH value for all the experiments, diluted NaOH and HCl solution was used (if needed). Using double-distilled water, the aqueous solutions were prepared.

In addition, two different types of bacterial strains (E. coli and S. aureus) were used to evaluate the antimicrobial activity of the synthesized materials. E. coli and S. aureus can be found frequently in animals and human intestine and in the upper respiratory tract and on the skin, respectively. These bacteria are considered as important pathogens to aquatic environments and aquatic organisms including fish and controlling them would be a significant practical application for the synthesized nanoparticles.

The selection of the involved parameters was performed through a single factor analysis. In this method, at first the pH of the samples was analyzed while other parameters were fixed. After finding the optimum pH, the amount of optimum dose of catalyst was surveyed. In the end, the optimum initial concentration of dye was found in predetermined pH and catalyst dosage.

Synthesis of γ-Fe₂O₃ nanoparticles

Through the solvothermal method, the magnetic γ-Fe₂O₃ nanoparticles were synthesized [23]. Briefly, ethylene glycol (60 mL) was used to dissolve 0.74 g of FeCl₃; then, to obtain a clear solution, sonication was applied for 20 min. At the next step, 2.16 g of sodium acetate was added to this solution and mixing continued for 60 min; afterward, a sealed teflon-lined stainless-steel autoclave was used to store the dark brown mixture and heating was applied in the air up to 160 °C for 12 h; then, the temperature was allowed to decrease to normal status without any interventions. Ultimately, a magnet was used to separate the resulting products; ethanol and deionized water were used to rinse them thoroughly and they were left to dry at 60 °C for 1 day.

Synthesis of γ-Fe₂O₃@SiO₂ core-shell nanocomposite

Using a modified Stöber technique, the interlayer of SiO₂ was synthesized [24]. Ultrasonic was applied to the mixture of ethanol (40 mL) for 60 min, deionized water (10 mL), and concentrated ammonia aqueous solution (25 wt.%, 1.2 mL) to disperse the as-prepared γ-Fe₂O₃ particles (0.1 g). Then, 0.35 mL of tetraethyl orthosilicate was added dropwise and the mixture was stirred for 2 h. Followed by gathering the resulting product, deionized water was used to rinse the sample. Ultimately, they were dried under vacuum at 50 °C for the forthcoming experiments.

Synthesis of γ-Fe₂O₃@SiO₂@TiO₂ core-shell structured microspheres

A layer of TiO₂ was applied through TIPOT hydrolysis in ethanol solution to overcoat the γ-Fe₂O₃@SiO₂ nanoparticles [25]. Similarly, sonication was applied for 15 min to scatter 0.1 g of γ-Fe₂O₃@SiO₂ nanoparticles in a mixture of 35 mL ethanol and 1 mL TIPOT. After adding 2 mL deionized water to the mixture, mechanical stirring was started until the formation of TiO₂ solution. The mixture was introduced to the reflux system at 100 °C for 4 h after making sure the homogeneity of dispersion. Ethanol was used to rinse the resulting products several times and then they were dried under the room temperature. Consequently, calcination was accomplished in the air at 450 °C for 2 h.

Photo-deposition of ag on γ-Fe₂O₃@SiO₂@TiO₂ substrate

After dissolving the appropriate amount of AgNO₃ in 40 mL deionized water and adding 0.1 g γ-Fe₂O₃@SiO₂@TiO₂, 0.5 mM solution was obtained. Stirring was performed for 20 min in the ultrasound bath until the magnetic nanostructures started to scatter in the solvent. 0.4 mL methanol was added to the mixture after transferring it to the quartz tube. Nitrogen gas was applied for 30 min to degas the fluid for eliminating oxygen from the mixture. Under ultraviolet irradiation, stirring was done for 12 h; Subsequently, Ag ions started to reduce to metallic silver and they were settled down on TiO₂ surface. Ultimately, an external magnetic field was used to separate the solid. Ethanol and deionized water were applied for rinsing and they were dried overnight at 60 °C.

Characterization

The functional group of the material was studied using the Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer Spectrophotometer Spectrum One) in the range of 4000-450 cm⁻¹. The samples were examined in terms of morphological structure using a scanning electron microscopy (SEM) by LEO 1455VP scanning microscope. To detect the crystal structure, the XRD model Siemens D-5000 diffractometer with Cu Kα radiation (1.5406 Å) was applied. The magnetic field was determined in a vibrating sample magnetometer (VSM 7400 Lake Shore) at ambient temperature.

Photocatalytic performance

A UV-C lamp (Philips 9 W) was applied as the irradiation source to degrade the photocatalytic dye in a photoreactor. The tube-like UV lamp was inserted in the bulk volume of samples in a 600 mL beaker which was covered by aluminum foil. The temperature of the bulk volume was controlled by circulating fresh cold water (15 °C) around the beaker which prevented rising the bulk temperature. To conduct the experiments, various amounts of catalyst were dissolved in 500 mL of a dye solution (20 mg L⁻¹). To have a suitable dispersion, the solution was stirred before the start of the photocatalytic reaction in dark for 1 h and the organic molecules and the catalyst surface reached an adsorption-desorption equilibrium. Then, to have comparable data, all the photocatalytic performances were carried out at the same place with identical UV intensity. At regular intervals, the specimens were withdrawn from the wastewater to verify dye concentration and a magnetic field was applied externally to separate the catalyst from solution. An amount of 2 mL of initial sample was pipetted out each time and after measurement, it was not returned to the solution.

The UV–vis spectrophotometer (Perkin-Elmer Lambda 25) was applied to specify BB41 dye absorbance at its maximum wavelength (λ_max = 605 nm) and a pH-meter (S47-K seven Multi, Mettler Toledo, Columbus, OH, USA) was used to specify the pH of the solutions. The ultrasonic bath (Parsonic 15 s, Pars Nahand Engineering, Iran) functioning at the frequency of 30 kHz and power of 200 W was used for particle dispersion. A centrifuge (Model Universal 320R, Hettich, Tuttlingen, Germany) was used to speed up the phase separation. It was attempted to observe how the photocatalytic dye degradation is affected by the catalyst quantity (0, 0.01, 0.02, 0.03 and 0.04 g), initial dye concentration (15, 20, 30 and 40 mg L⁻¹) and pH (2, 5, 8 and 10) through contacting 500 mL of dye solution at room temperature for 180 min. The presented data are the average of three times repeated experiments with the standard deviation of less than 5%.

Antibacterial experiment

Agar plate disk diffusion method was accomplished to measure the inhibition zone according to the standard method [26–28]. Before this experiment, all glassware and samples were sterilized by autoclaving at 120 °C for 30 min. Sterile nutrient agar culture media was poured into the separated sterile petri dishes. 100 μL of each bacteria suspension was transmitted to plates and dissipated uniformly to inoculation occur and 3 mg of nanostructured materials were straightly placed onto the inoculated agar surface. Then plates incubated at 37 °C for 24 h under different dark and visible light conditions and finally, the inhibition zones were measured [29].

Results and discussion

FTIR analysis

The FTIR spectra of the samples were determined to verify the composition and structure of the nanocomposites (Fig. 1). Owing to the specific peaks of γ-Fe₂O_3, FT-IR spectra of γ-Fe₂O₃ showed the strong absorption bands at 587, 888, and 1087 cm⁻¹. It was shown that the strong peak at 587 cm⁻¹ was due to the Fe–O functional group vibration. After applying a silica layer to coat the magnetic nanoparticles, an absorption peak at 800 cm⁻¹ was presented as symmetric Si–O–Si vibration and wide bands from 1080 to 1100 cm⁻¹ represented the asymmetric Si–O–Si stretching vibration for γ-Fe₂O₃@SiO₂ nanoparticles [30]. The absorption band at approximately 1630 cm⁻¹ and the absorption band at 3375–3432 cm⁻¹ for all samples was because of the Si–O–H stretching and remnant OH vibrations on the surface of silica-coated magnetic nanoparticles.

Fig. 1.

FTIR spectra of prepared materials

Then, γ-Fe₂O₃@SiO₂ was modified with TIPOT in order to obtain γ-Fe₂O₃@SiO₂@TiO₂ and an absorption peak from 940 to 960 cm⁻¹ corresponded to the vibration of Si–O–Ti was observed [23]. The broadband at 500-900 cm⁻¹ was added to the spectrum assigned to Ti–O bond stretching and O–Ti–O bending vibrations indicating the successful formation of γ-Fe₂O₃@SiO₂@TiO₂ composite [31].

SEM and EDS analysis

Figure 2 shows the samples’ morphologies observed by SEM. Based on SEM images, it was found that the γ-Fe₂O₃ and γ-Fe₂O₃@SiO₂ particles almost had mediocre mono size and shape (Fig. 2a, b). An increase was seen in particles roughness and size after modification of γ-Fe₂O₃@SiO₂ composite surface with TIPOT and followed by Ag (Fig. 2c, d). As shown in SEM images, owing to calcination at high temperature, the nanocatalysts were aggregated.

Fig. 2.

SEM images of prepared materials: a γ-Fe₂O₃, b γ-Fe₂O₃@SiO₂, c γ-Fe₂O₃@SiO₂@TiO₂ and d γ-Fe₂O₃@SiO₂@TiO₂-Ag

Energy-dispersive X-ray spectrometry (EDS) is typically applied as an analytical technique to examine the specimen in terms of the elemental analysis or chemical properties. The individual elements are detected by EDS using X-rays emitted from the sample. Energy peaks correspond to the various elements in the sample. It was observed that the as-prepared nanocomposites were thoroughly composed of Fe, O, Si, Ti, and Ag (Fig. 3), thus, verifying the deposition of TiO₂–Ag nanocrystals on the γ-Fe₂O₃@SiO₂ nanoparticles surface.

Fig. 3.

EDS curve of the γ-Fe₂O₃@SiO₂@TiO₂-Ag composite

XRD analysis

To examine the as-prepared samples in terms of phase and purity, XRD analysis was applied. As shown in Fig. 4a, a normal XRD pattern of the resulting γ-Fe₂O₃ specimen and all of the diffraction peaks could be simply categorized to a face-centered cubic structure of magnetite according to the JCPDS card No. 39-1346. The XRD pattern of γ-Fe₂O₃@SiO₂ micro-spheres and pure γ-Fe₂O₃ were quite similar (Fig. 4b); however, a broad peak positioned at 2ϴ = 22° matched SiO₂ indicated that the coated SiO₂ shell is formless. The new diffraction peaks of γ-Fe₂O₃@SiO₂@TiO₂ could be categorized to TiO₂ anatase phase from JCPDS card No. 21-1272 (Fig. 4c) compared to Fig. 4a and b. TiO₂ peak broadening indicates that small nano-crystalline grains made the outer shells. Due to the small Ag content, the absence of Ag diffraction peaks was seen simultaneously [30].

Fig. 4.

XRD patterns of as-prepared materials

VSM analysis

As Fig. 5 shows, to examine the magnetization treatment of the as-prepared nanoparticles and composites, samples’ magnetization curve was determined under the room temperature. The magnetization saturation (Ms) values of pure γ-Fe₂O₃, γ-Fe₂O₃@SiO₂, γ-Fe₂O₃@SiO₂@TiO_2, and γ-Fe₂O₃@SiO₂@TiO₂-Ag were 66.58, 39.16, 33.29, and 27.41 emu g⁻¹, respectively. In comparison with other samples, the γ-Fe₂O₃ nanospheres’ Ms. value was higher because of the coating and subsequent deposition of Ag. Compared to γ-Fe₂O₃@SiO₂@TiO₂, γ-Fe₂O₃@SiO₂@TiO₂-Ag nanocomposite had a slightly smaller Ms. value that could be related to the minor increase in the mass and size due to Ag nanoparticles deposition on the γ-Fe₂O₃@SiO₂@TiO₂ nanosphere surfaces. However, the core-shell structured γ-Fe₂O₃@SiO₂@TiO₂-Ag nanocomposite still has strong magnetization showing this nanocomposite is appropriate for the purpose of magnetic separation and recovery. After placing a magnet near the vial, γ-Fe₂O₃@SiO₂@TiO₂-Ag microspheres were quickly drawn towards the sides of the vial within 50 s, and a transparent solution was reached (Fig. 5, inset) confirming their magnetic nature.

Fig. 5.

Magnetization curve of magnetic particles at room temperature

Effects of operational parameters on dye degradation

The current research was conducted to investigate the effect of catalyst dosage, dye concentration, and pH on dye decolorization. Under similar conditions using γ-Fe₂O₃@SiO₂@TiO₂ and γ-Fe₂O₃@SiO₂@TiO₂–Ag composites, the photocatalytic dye degradation of BB41 was measured.

By applying the UV irradiation and oxygen bubbling the experiments were carried out. The quantity of dye degradation in the specific time of (R(t)) was calculated through the following equation [32]:

$$R\left(t\right)=\frac{C_0-C_t}{C_0}\times 100$$ 1

Effect of pH

It was shown that the pH of the wastewater influences the chemical structure of pollutants. Thus, various pH values of 2, 5, 8, and 10 were applied (Fig. 6). Furthermore, the surface charge characteristics of the γ-Fe₂O₃@SiO₂@TiO₂-Ag were influenced by pH. The catalyst surface charge is positive at acidic media, thus resists BB41 as a cationic dye. The γ-Fe₂O₃@SiO₂@TiO₂-Ag charge is negative at high pH values, consequently, attracts cationic dye (BB41) through the electrostatic attraction [33]. Nevertheless, some decolorization mechanisms such as hydroxyl radical degradation, hole direct oxidation and electron direct reduction in the conducting band are likely to happen at diverse pH values.

Fig. 6.

Effect of initial pH on the degradation of BB41. Experimental conditions: 20 mg L⁻¹ BB41 and 0.04 g L⁻¹ catalyst

When the solution pH is changed, it causes to increase protone (H⁺) or hydroxyle (OH⁻) groups in the liquid media based on the acidic or basic conditions, respectively. Therefore, these hydroxyl groups can produce hydroxyl radicals which act as a degradation source. Also, the protons can help the degradation process as below:

$$\begin{array}{l}{\mathrm{TiO}}_2{\mathrm{h\upsilon B}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{TiO}}_2+{\mathrm{H}}^{+}+{\mathrm{OH}}^{-}\\ {\mathrm{TiO}}_2\left({\mathrm{h\upsilon B}}^{+}\right)+{\mathrm{OH}}^{-}\to {\mathrm{TiO}}_2+\mathrm{OH}•\\ {\mathrm{TiO}}_2\left({\mathrm{eCB}}^{-}\right)+{\mathrm{O}}_2\to {\mathrm{TiO}}_2+\mathrm{O}2\\ {\mathrm{O}}_2•-+{\mathrm{H}}^{+}\to {\mathrm{HO}}_2•\\ \mathrm{Dye}+\mathrm{OH}•\to \text{degradation products}\end{array}$$

Effect of catalyst dosage

The effect of different catalyst dosages on dye decolorization is shown in Fig. 7. The results indicated a significant percentage of dye degradation by UV/catalyst compared to the UV in the isolate. Since powerful oxidants such as free radicals (hydroxyl radicals, superoxide, etc.) are generated in the aqueous phase, dye degradation can be carried out significantly by the UV/catalyst. In addition, increment of the catalyst dosage caused the degradation ability of the catalyst to reach to its maximum value degradating 99.2% of BB41 molecules. Applying over optimum dosage leads to a decrease in degradation of BB41 molecules due to the agglomeration of magnetic catalysts.

Fig. 7.

Effect of catalyst dosage on the degradation of BB41. Experimental conditions: 20 mg L⁻¹ BB41 concentration and initial pH = 10

Effect of initial dye concentration

Another important parameter affecting the dye degradation is initial dye concentration. Various concentrations of 15, 20, 30 and 40 mg L⁻¹ have been used in the present study to examine the effect of initial dye concentration. As shown in Fig. 8, since the intermediates generated during the degradation of dye molecules interfere with the process, decolorization percentage decreases as its concentration increases and this would be obvious when the concentration of intermediates is high.

Fig. 8.

Effect of initial dye concentration on the degradation of BB41. Experimental conditions: initial pH = 10 and 0.04 g L⁻¹ catalyst

Figure 9 shows the concentration of degraded dye caused by γ-Fe₂O₃@SiO₂@TiO₂ and γ-Fe₂O₃@SiO₂@TiO₂–Ag composites under UV irradiation. In accordance with calculations, BB41 degradation by γ-Fe₂O₃@SiO₂@TiO₂ catalyst after 180 min was 63%, while it was about 94% for γ-Fe₂O₃@SiO₂@TiO₂-Ag. Since Ag particles deposit selectively on the electron trapping sites in photo-deposition process, the resulting clusters could be used as new electron sinks to catch the photo-induced electron and use them for degradation of pollutants. Consequently, γ-Fe₂O₃@SiO₂@TiO₂-Ag was chosen for further studies.

Fig. 9.

Comparison of catalysts performance on the degradation of BB41. Experimental conditions: 20 mg L⁻¹ BB41 concentration initial pH = 10 and 0.04 g L⁻¹ catalyst

Dye degradation mechanism

The UV-Vis spectra at various intervals were measured to examine BB41 absorption during the photocatalytic degradation by γ-Fe₂O₃@SiO₂@TiO₂-Ag at 200 nm ≤ λ ≤ 800 nm (Fig. 10a). Dye absorbance at the visible area of spectrum decreased during the decolorization process because of the azo band (-N=N-) degradation as the most reactive sites of dye molecule [34]. After applying UV irradiation, a photon with enough energy for TiO₂ bandgap excitations will generate electron and hole in the conduction band (CB) and valence band (VB) of TiO₂, respectively. Since a large amount of photo-excited electrons can be accumulated by Ag clusters, these clusters can play an important role as a very active site with high potential to decompose the pollutants. It was shown that the oxidation state of Ag is zero. Since Ag nanoparticles were attached on the substrate through photo-deposition technique, previous studies have revealed that metals settled down on TiO₂ surface nanostructures by UV deposition are mostly metallic species having zero oxidation state (M₀) [35, 36]. The metal particles provide a sink for photo-generated electrons and decrease their recombination rate with holes that boosts the photocatalytic activity. The photocatalytic activity of TiO₂ semiconductor is further boosted by the metallic state (M₀) through building a Schottky junction bneverneverneetween the metal and the semiconductor.

Fig. 10.

a The UV–Vis spectral changes of BB41 solution under photocatalytic degradation by γ-Fe₂O₃@SiO₂@TiO₂-Ag at different time intervals and b Mechanism of catalyst performance

Dye degradation kinetics

Various decolorization kinetic models (Table 1) at several dye concentrations were examined (Table 2). The data fitting was done to the zero-order, first-order and second-order models [40]. Table 2 shows the decolorization kinetic constants for several dye concentrations. The results indicated that the BB41 decolorization kinetics followed the first-order kinetic model since dye degradation is dependent on γ-Fe₂O₃@SiO₂@TiO₂-Ag dosage.

Table 1.

Different kinetics models of decolorization (C0 = initial dye concentration (mg L⁻¹), C = dye concentration at time t (mg L⁻¹), k0 = zero-order rate constant (mg min⁻¹ L⁻¹), k1 = first-order rate constant (L min⁻¹) and k2 = second-order rate constant (L mg⁻¹ min⁻¹))

Kinetics	Equation	Ref.
Zero-order	C – C0 = −k0 t	[37]
First-order	C/C0 = exp. (−k1 t)	[38]
Second-order	(1/C) - (1/C0) = (k2 t)	[39]

Table 2.

The kinetics of BB41 photocatalytic degradation at various dye concentrations (R² = Correlation coefficient)

Dye concentration (mg/L)		Zero-order		First-order		Second-order
		K0	R2	K1	R2	K2	R2
BB41	0	0.0151	0.8090	0.0011	0.8328	0.0001	0.8482
	15	0.0401	0.9809	0.0033	0.9931	0.0003	0.9700
	20	0.0468	0.9328	0.0042	0.9965	0.0004	0.9836
	30	0.0775	0.8462	0.0092	0.9722	0.0013	0.9739
	40	0.1053	0.8084	0.0287	0.9810	0.0322	0.6751

Recycling

Since the as-prepared superparamagnetic photocatalyst has magnetic responsive features, it is possible to separate the catalyst quickly, recycle it from the reaction media, and reuse it in further reactions. As previously explained, an external magnet was used to collect the catalyst every 40 min; it was then rinsed using ethanol, water, and acetone, consecutively. The dried powder at 80 °C was used once more in the next catalytic cycle with new reactants; Fig. 11 shows the results. As the results show, the γ-Fe₂O₃@SiO₂@TiO₂–Ag catalyst was used once more for five successive cycles whereas the catalytic performance did not decrease considerably. Consequently, the γ-Fe₂O₃@SiO₂@TiO₂–Ag was applied as effective heterogeneous catalysts for dye degradation reaction under UV irradiation. It was observed that in comparison to γ-Fe₂O₃@SiO₂@TiO₂, the catalytic activity was greatly influenced by modifying the γ-Fe₂O₃@SiO₂@TiO₂ magnetic nanoparticle through Ag clusters.

Fig. 11.

Recycling experiment for γ-Fe₂O₃@SiO₂@TiO₂–Ag catalyst

Antibacterial activity

The antibacterial activity of the prepared materials was evaluated by measuring the inhibition zone emerged on agar plates under visible-light irradiation and in dark condition (Fig. 12). The first experiment evaluated under dark condition illustrated that the synthesized compounds did not have sufficient antibacterial activity, except silver doped nanocomposite. In contrast with the dark condition, prepared materials showed good performance in counteraction with both E. coli and S. aureus strains. The greatest inhibition zone belonged to γ-Fe₂O₃@SiO₂@TiO₂–Ag compound. As can be seen from Fig. 12c and d, the antibacterial ability of the silver doped γ-Fe₂O₃@SiO₂@TiO₂ was assessed in comparison with pure γ-Fe₂O₃@SiO₂@TiO₂, γ-Fe₂O₃ and γ-Fe₂O₃@SiO₂. The results showed that the improved magnetic nanocomposite by silver had attractive antibacterial activity, but the others showed no inhibition zone. The antibacterial activities of silver can be explained due to the following mechanisms: 1) the bacterial cell membrane is enriched with sulfur-containing proteins, which could be the preferential sites for Ag particle attachment due to sulfur-Ag affinity; therefore, silver nanoparticles can damage or change the structure of bacteria by attaching to the bacterial cell membrane [28, 41]. 2) Ag can form complexes with DNA and RNA which cause DNA condensation and loss of replication ability [42]. Such complexation can lead to silver binding with thiol groups in proteins and inactivate the respiratory enzyme activity [43]. 3) Free radicals formed on silver particles, especially reactive oxygen species (ROS) can represent antimicrobial activities [44].

Fig. 12.

Antibacterial activity by measuring the inhibition zones for E. coli and S. aureus under different light conditions: a, b in the dark and c, d visible light

Conclusion

In order to synthesize γ-Fe₂O₃@SiO₂@TiO₂–Ag magnetically separable photocatalyst, a combination of co-precipitation, sol-gel, and photo-deposition methods were introduced. Using XRD, SEM, EDS, FTIR and VSM techniques, as-prepared nanoparticles were successfully characterized. For photocatalytic degradation of BB41 dye, the Ag-doped nanocomposite was used as an environmentally friendly catalyst. Dye removal increased by increasing the catalyst dosage and decolorization percentage decreased as dye concentration increased and this was obvious when the concentration of intermediates was high. Furthermore, dye degradation was positively influenced by the pH of the wastewater and was based on first-order kinetic model. After the end of the photodegradation reaction, it was possible to separate the catalysts from the product solution using a simple magnetic separation process. According to the results, γ-Fe₂O₃@SiO₂@TiO₂–Ag photocatalyst can be used after recycling process and would be stable within five separation cycles which indicates the promising performance of the composites as a low-cost and environmentally friendly technique for photocatalytic treatment of wastewaters. Further antibacterial experiments demonstrated that the γ-Fe₂O₃@SiO₂@TiO₂–Ag had excellent antibacterial activity against two different types of bacterial strains under visible light condition.

Compliance with ethical standards

Conflict of interest

The authors have no conflicts of interest to declare.