Characterizations of FMMWCNTs@GLYMO@WO3 (III)
The characterization of the ferromagnetic photocatalyst, FMMWCNTs@GLYMO@WO3 (III), was effectively carried out using a combination of different microscopic and spectroscopic techniques such as FT-IR, XRD, TEM, FESEM, EDS, EDS-mapping, VSM, TGA, and DRS. Figure 1 shows the FT-IR spectra of (a) MWCNTs, (b) FMMWCNTs (I), (c) FMMWCNTs@GLYMO (II), and (d) FMMWCNTs@GLYMO@WO3 (III). As depicted in Fig. 1a, the wide absorption peak at approximately 3345 cm-1 is indicative of the hydroxyl groups (ν OH) present in the untreated MWCNTs. Vibrational frequencies associated with C = C bonds within graphite sheets are observed around 1638 cm-1. Furthermore, the distinctive band within the 600 − 400 cm-1 range confirms the stretching vibration mode of the Fe-O bond, as illustrated in Fig. 1b. The confirmation of the attachment of (3-glycidyloxypropyl)trimethoxysilane onto the FMMWCNTs (I) surface is evidenced by the presence of characteristic bands at 2925 and 2859 cm-1, corresponding to the stretching vibrations of alkyl chains (C-H). Additionally, the distinct stretching vibration associated with the Si-O-Si bond is evident at 830 cm-1 (as shown in Fig. 1c). The presence of WO3 attached to the FMMWCNTs@GLYMO (II) framework is identified by the stretching vibration of the W-O band, observed at approximately 550 cm-1 (depicted in Fig. 1d).
The XRD method was utilized to assess the structure of the crystallographic of FMMWCNTs@GLYMO@WO3 (III) (Fig. 2). The pattern of XRD displays reflection peaks at 2θ = 32º, 48º, 50º, 56º and 76.9º which correspond to (2 2 0), (2 2 2), (4 0 0), (4 2 2) and (6 2 2), respectively, demonstrating the existence of the cubic structure of Fe3O4 (Ghasemzadeh et al. 2018). In addition, the presence of diffraction peaks at 2θ = 33.3º, 34.1º, and 61.7º, which correspond to the crystallographic faces (2 0 0), (2 0 1), and (4 0 0), respectively can be attributed to the WO3 tetragonal structure (Ghasemzadeh et al. 2017). In addition, the profile of XRD indicates that there is a diffraction peak at 2θ = 23.1º and 42.0, which corresponds to the (0 0 2) and (0 1 0) planes of the hexagonal graphite structure (Ghasemzadeh et al. 2019). Furthermore, the mean size of the crystalline structure of FMMWCNTs@GLYMO@WO3 (III) was measured to be 24.8 nm, calculated using the following equation of Debye-Scherrer.
$$D=\frac{k{\lambda }}{\beta \text{cos}\theta }$$
1
D represents the size of the crystalline nanoparticles, k is Scherrer’s constant (0.98), λ represents the wavelength (1.54), and full width at half maximum (FWHM) is denoted by β.
TEM was performed to examine the structure and average size of NPs that are immobilized on the surface of FMMWCNTs@GLYMO@WO3 (III) and the 5th recycled FMMWCNTs@GLYMO@WO3 (III) from the degradation of MTZ in aqueous media (Fig. 3). TEM images of the fresh FMMWCNTs@GLYMO@WO3 (III) (Figs. 3a and 3b) confirm that the nano photocatalyst is decorated successfully with well-dispersed ferromagnetic nanoparticles. In addition, no aggregation of nanoparticles is detected on the surfaces of the MWCNTs. TEM images also show that the ferromagnetic nanoparticles on the surface of FMMWCNTs (I) are spherical in shape, with an average size of approximately 20–33 nm. Likewise, the structure of the MWCNTs has remained predominantly intact, showing no signs of damage during the process of functionalization.
The FESEM method was utilized to illustrate the surface morphology of FMMWCNTs@GLYMO@WO3 (III), as depicted in Fig. 4. The corresponding FESEM images reveal a rod-like structure. Moreover, the presence of WO3 NPs adhered to the ferromagnetic photocatalyst is distinctly visible as bright spots in Fig. 4.
The energy dispersive spectrum (EDS) of FMMWCNTs@GLYMO@WO3 (III) exhibited the presence of all the anticipated elements, such as C, O, Si, Fe, and W (Fig. 5). There are no additional peaks observed in the ferromagnetic photocatalyst's structure, suggesting the absence of any impurities.
The EDS-mapping images of FMMWCNTs@GLYMO@WO3 (III) (Fig. 6) show the dispersion of carbon, oxygen, silicon, iron, and tungsten throughout the surface of the ferromagnetic photocatalyst.
The magnetic properties of FMMWCNTs (I) and FMMWCNTs@GLYMO@WO3 (III) were examined using VSM analysis at ambient temperature (Fig. 7). According to the analysis using the VSM technique, it was observed that the magnetic hysteresis curves exhibited the existence of a hysteresis loop, indicating that the nanoparticles possess ferromagnetic properties. In addition, the saturation magnetization values (Ms) of the FMMWCNTs (I) and FMMWCNTs@GLYMO@WO3 (III) are 17.25 and 7.3 emu/g, respectively. The decrease in Ms of FMMWCNTs@GLYMO@WO3 (III) is because of the existence of non-magnetic substances on the surface of FMMWCNTs (I) (Ghasemzadeh et al. 2019).
The optical properties related to the absorbance of ferromagnetic photocatalysts were evaluated by UV-Vis diffuse reflectance spectroscopy (DRS). Figure 8 illustrates that the maximum absorption in the visible-light range was detected for FMMWCNTs@GLYMO@WO3 (III).
The profile of FMMWCNTs@GLYMO@WO3 (III) exhibits a three-step thermal decomposition, as depicted in the TGA thermogram (Fig. 9). Figure 9 reveals that the initial step, ranging from 50 to 170.34 ºC, corresponds to a weight loss of approximately 6.25%, ascribed to the evaporation of physically absorbed water. Subsequently, the (3-glycidyloxypropyl)trimethoxysilane (GLYMO) fragment attached to the FMMWCNTs (I) surface undergoes decomposition in the second and third stages, resulting in weight losses of 30% (200-287.56 ºC) and 10% (500-608.84 ºC), respectively. Based on TGA analysis, the estimated quantity of organic components anchored onto FMMWCNTs (I) is 0.23 mmol g-1.
Probing the performance of FMMWCNTs@GLYMO@WO 3 (III) in the degradation of MTZ and ACE in aqueous media
Metronidazole and acetaminophen were chosen as representative pollutants, and the photocatalytic efficiency of the ferromagnetic photocatalyst that was prepared was assessed for the degradation of these contaminants through the utilization of various factors under visible light. Before commencing the photocatalytic experiment, several parameters were fine-tuned, including the initial concentration of MTZ and ACE, the catalyst dosage, and the pH level, as illustrated in Figs. 10 and 11. The distinctive catalytic efficiency of FMMWCNTs@GLYMO@WO3 (III) in the degradation of MTZ and ACE was shown by investigating the standard reactions separately in the existence of Fe3O4 NPs, MWCNTs, and WO3. It is important to note that, even after prolonged reaction times, a substantial yield of MTZ and ACE decomposition was not achieved (Figs. 10a and 11a). In the initial stage, the mixture of reactions underwent magnetic stirring for 60 min under dark atmospheric conditions. This was done to establish an equilibrium state of adsorption-desorption between MTZ, ACE, and the photocatalyst. After 60 min, 3.5 mL of the reaction mixture was extracted from the suspension, after which the ferromagnetic photocatalyst was separated using a magnet rod. Subsequently, the obtained results were examined using a UV-vis spectrophotometer to assess the degradation of MTZ and ACE (Figs. 10b and 11b). As can be observed from these Figs, the most optimal outcomes were attained by employing a quantity of 0.6 g/L of ferromagnetic photocatalyst to a volume of 50 mL of each contaminant solution with a concentration of 10 mg/L at a pH level of 7. Taking into consideration the influence of pH, the pH range of MTZ and ACE solution was adjusted from pH 3 to 11, after which the efficiency of degradation was examined. The critical variables, including the dosage of ferromagnetic photocatalyst and the concentration of contaminants, were kept consistent at 0.6 g/L and 10 mg/L, respectively, during the pH testing. As depicted in Figs. 10c and 11c, the maximum ferromagnetic photocatalytic degradation of MTZ and ACE was achieved at a pH equal to 7 within 15 min, resulting in a degradation percentage of 93% and 97%, respectively. Subsequently, an investigation was conducted to analyze the impact of loading the ferromagnetic photocatalyst on the photocatalytic degradation of these two contaminants (Figs. 10d and 11d). By examining the degradation curve of MTZ and ACE, it is evident that the optimal outcomes were obtained when 0.6 g/L of the ferromagnetic photocatalyst was applied. Subsequently, the effect of MTZ and ACE concentration was examined to fix pH and ferromagnetic photocatalyst dosage for further studies of degradation. The variations in MTZ and ACE concentrations at 5, 10, and 20 g/L under visible light are shown in Figs. 10e and 11e. Evaluation of reaction rate kinetics is of significant importance in the processes involved in catalytic wastewater treatment. The Langmuir-Hinshelwood (L-H) kinetic model is employed to study the degradation kinetics of organic compounds, such as antibiotics, via photocatalysis. According to the Langmuir-Hinshelwood (L-H) model, the efficiency of the photocatalytic process adheres to the principles of pseudo-first-order kinetics and can be mathematically represented by the following equation.
$$ln\left(\frac{{C}_{0}}{{C}_{t}}\right)=kt$$
2
where k is the pseudo-first-order rate constant, C0 is the initial pollutant concentration, and Ct is the pollutant concentration at time t. The photocatalytic degradation kinetics of ACE were investigated in the presence of ferromagnetic photocatalyst, FMWCNTs (I) and WO3 under optimal conditions. Figure 12 illustrates that the estimated degradation rate constants are 0.0278, 0.0216, and 0.0198 min− 1, respectively, for FMWCNTs (I), WO3, and FMMWCNTs@GLYMO@WO3 (III).