Introduction

Synthetic dyes are of the chemical compounds that have been consumed in different industries such as leather, pharmaceutical, textile, and paper1,2,3,4,5. Because of wide applications and large-scale production of these dyes, they are main part of industrial wastewaters6,7,8,9,10,11. Also, they have high toxicity and chemical stability to degradation by general treatment procedures12,13,14,15. Thus, common treatment manners are not effective for degradation of organic dyes in wastewater16,17. Lately, researchers have been focused to the photocatalysis as the effective and useful method for wastewater treatment owing to their simple operation and high efficiency18,19,20,21,22,23,24,25.

Utilization of magnetic nanocomposites (MNCs) is one of the useful method to dyes degradation in water and wastewater because they have some merits such as easy and fast separation by constant magnet, economic viability, and recoverability and reusability for several runs26,27,28,29,30,31,32. Various methods have been reported for preparation the magnetic nanocomposites33,34. Sol–gel method by using natural gel is the easy, green, cost-effective, non-toxic, and eco-friendly manner for synthesis the nanocomposites35,36,37,38,39,40,41.

Scientists have demonstrated an increasing interest with the production of magnetic nanoparticles using environmentally friendly synthesis methods. Minghui Li group conducted a study on the synthesis of MgFe2O4–ZnO heterojunction photocatalysts and examined their effectiveness in degrading Rhodamine B (RhB) dye42. In 2022, Makofane et al. reported the green synthesis of NiFe2O4 nanoparticles using plant extracts from Monsonia burkeana. The nanoparticles were synthesized in cubic spinel crystalline structures. The researchers also investigated how different calcination temperatures affected the photocatalytic activity of the nanoparticles43. In 2019, Jia et al. synthesized a Z-scheme MgFe2O4/Bi2MoO6 heterojunction photocatalyst using hydrothermal and ball-milling techniques. They then studied the photocatalytic activity of this catalyst for the removal of malachite green44.

The research work aimed to synthesis MgFe2O4@ZnAl2O4 MNCs using the green sol–gel method. The synthesized materials were subjected to comprehensive characterization employing various techniques, including XRD, TEM, FESEM, BET, DRS, VSM, EDX, and elemental mapping techniques. The research aimed to evaluate the photocatalytic efficacy of magnetic nanocomposites in degrading RB222 dye under visible light irradiation. To find out the optimized conditions, the influence of photocatalyst amount, initial dye concentration, irradiation time, exposure to both dark and visible light, and the reusability of the photocatalyst was investigated. The degrading performance of the dye was verified by the utilization of UV–Vis spectroscopy and total organic carbon (TOC) measurement. Figure 1 depicts the molecular configuration of the RB222 dye.

Figure 1
figure 1

Chemical structure of RB222 dye.

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Experimental

Materials

The TG obtained as a natural gel, was sourced from a bio shop. Fe(NO3)3·9H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, and Mg(NO3)2·6H2O, as the metal salts, were prepared from Dae-Jung and Merck company.

Characterization of MgFe2O4@ZnAl2O4 MNCs

The sample's X-ray diffraction pattern was obtained using an X’Pert-PRO advance X-Ray diffractometer with Cu Kα radiation. Magnetization was assessed using the Meghnatis Kavir Kashan apparatus, and optical properties were recorded through UV–Vis diffuse reflectance spectra with a Shimadzu UV-2550 instrument. The band gap (Eg) was calculated employing Tauc's theory45. The TEM image was recorded by using an EM 208S, and SEM (equipped with EDX and Mapping) were obtained with Tescan Mira3. Total organic carbon (TOC) analysis was investigated with Shimadzu TOC-5000, and Jena-Specord 205 spectrophotometer was used for recorded the UV–Vis absorption spectra of dyed solutions. Also, The Belsorp Mini II instrument was utilized to conduct BET analysis.

Synthesis of MgFe2O4@ZnAl2O4 MNCs

Taghavi fardood’s method was used for synthesis the magnesium ferrite nanoparticles (MgFe2O4)46. 0.2 g of tragacanth gum was added to 40 ml deionized water under magnetic stirring to obtain the transparent gel solution (70 °C for 1 h). In next step, 0.1 g of prepared MgFe2O4 NPs was added to TG solution and dispersed in it for 2 min under ultrasonic irradiation. Then, 1 mmol Zn(NO3)3·6H2O, and 2 mmol of Al(NO3)3·9H2O were added to the prior solution and dispersed for 2 min. After that, the final solution was stirred for 12 h at 70 °C to achieve dried resin. Obtained resin calcined for 4 h at 600 °C for synthesis MgFe2O4@ZnAl2O4 MNCs.

Photocatalytic dye degradation

Photocatalytic experiments were performed utilizing a batch-style photoreaction setup with a 90 W lamp (λ > 400 nm), and the light intensity was measured using a Lux meter. The intensity of sunlight was examined by a Lux meter. Degradation efficiency was investigated in 50 mL of dye solution by changing amount of MgFe2O4@ZnAl2O4 MNCs, initial dye concentration, and irradiation time. When optimum conditions were determined, dye degradation experiment was done in the presence and absence of visible light to confirm the synergistic effect between visible light and synthesized nanocomposites. The efficiency of RB222 dye degradation was computed through UV–Vis spectroscopy at the λmax = 611 nm. The following equation was used for evaluation the degradation efficiency. A0 and A are the initial and final absorbance of dye solution, respectively.

$$\%Degradation=\frac{{A}_{0}-A}{{A}_{0}}$$

Result and discussion

Characterization of MgFe2O4@ZnAl2O4 MNCs

X-ray diffraction (XRD), and EDX analysis were performed to verify the phase identification, crystallinity, and presence of impurity in synthesized MgFe2O4@ZnAl2O4 MNCs. Figure 2 displayed the XRD patterns of the sample. It is evident that all of the diffraction peaks correspond to the cubic spinel phase for MgFe2O4 (JCPDS card no. 89-4924) and ZnAl2O4 (JCPDS card no. 73-1961). The estimation of the crystallite size of the sample was conducted using the Scherrer formula47 and obtained 13 nm. The XRD pattern, devoid of impurities, confirms the successful synthesis of MgFe2O4@ZnAl2O4 MNCs through the green sol–gel method.

Figure 2
figure 2

XRD pattern of the MgFe2O4@ZnAl2O4 MNCs.

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EDX, and elemental mapping of nanocomposite confirm that sample are contain Mg, Fe, Zn, Al, and O elements with good spatial distribution (Figs. 3, 4).

Figure 3
figure 3

EDX pattern of MgFe2O4@ZnAl2O4 MNCs.

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Figure 4
figure 4

Element mapping images of MgFe2O4@ZnAl2O4 MNCs.

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As can be observe in FESEM and TEM image, magnetic nanocomposites are quite regular in shape and size. Moreover, the TEM image certified the synthesis of MgFe2O4@ZnAl2O4 MNCs with a uniform size of nearly 20–25 nm (Fig. 5a,b).

Figure 5
figure 5

(a) FESEM image and (b) TEM image of MgFe2O4@ZnAl2O4 MNCs.

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The UV–Vis–DRS spectra of the sample are given in Fig. 6. It can be seen that the bandgap of MgFe2O4@ZnAl2O4 MNCs is about 1.88 eV. It is apparent synthesized samples exhibit considerable photocatalytic activity when exposed to visible light irradiation.

Figure 6
figure 6

UV–Vis spectrum and Tauc plot of MgFe2O4@ZnAl2O4 MNCs.

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The magnetic nanocomposite active surface area was appraised with BET analysis. Based to the results, the specific surface area, pore diameter, and total pore volume of sample are 78.109 m2/g, 14.728 nm, and 0.2876 cm3/g, respectively. The isotherm for nitrogen adsorption–desorption, as depicted in Fig. 7, verifies the characteristic Type IV behavior with hysteresis loops of Type H1, in accordance with the IUPAC categorization of materials48.

Figure 7
figure 7

The N2 absorption/desorption isotherm of the MgFe2O4@ZnAl2O4 MNCs.

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To study the magnetic properties of synthesized MgFe2O4@ZnAl2O4 MNCs, sample was characterized with the VSM technique. According to the magnetic hysteresis (Fig. 8), Ms = 6.59 emu/g and Hc and Mr have equal values (Hc = Mr = 0) and nanocomposites have superparamagnetic feature and well magnetic feature in the adjacency of a magnet. The MgFe2O4 MNPs demonstrate a saturation magnetization (Ms) of 13.32 emu/g49. Despite a decrease in saturation magnetization, the synthesized nanocomposite retains significant magnetic properties, facilitating easy separation using a magnet. This reduction in Ms confirms the coating of ZnAl2O4 on MgFe2O4, as noted in the literature44,50. In another study, Taghavi Fardood et al. reported on the green synthesis of MgFe2O4@CeO2 nanocomposites and compared their saturation magnetization to MgFe2O4 NPs. The saturation magnetization (Ms) of MgFe2O4@CeO2 MNCs was found to be 3.58 emu/g, representing a significant reduction compared to the MgFe2O4 sample49.

Figure 8
figure 8

Magnetic hysteresis loop of MgFe2O4@ZnAl2O4 MNCs.

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Photocatalytic efficiency of MgFe2O4@ZnAl2O4 MNCs in degradation of RB222 dye

The photocatalytic activity of MgFe2O4@ZnAl2O4 MNCs was investigated by survey the degradation of RB222 dye in ambient condition and natural pH. Photocatalyst experiments was followed by changing different parameters to find the optimum condition and results are given in details. In order to survey the impact of catalyst amount on degradation performance, 0.01–0.025 g of catalyst for RB222 solution was examined. By using 0.02 g of sample, 96% of RB222 dye was removed in 10 min when the initial concentration of dye was 20 mg/l (Fig. 9a). The efficiency of degradation increased when the amount of catalyst was increased from 0.01 to 0.025 g. This could be due to increased production of hydroxyl radicals51. Subsequent experiments were carried out by using 0.02 g of nanocomposites as the best amount.

Figure 9
figure 9

The effect of (a) photocatalyst dosage, (b) the initial dye concentration, (c) light irradiation and (d) time on the removal of RB222 dye (%) at room temperature.

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Pollution concentration is one of the important factors that effected the efficiency in photodegradation experiments49,52. Hence, degradation of RB222 dye was considered in different concentration (10–40 mg/l of RB222) and constant dosage of MgFe2O4@ZnAl2O4 MNCs (0.02 g). Figure 9b, exhibits the variation in degradation efficiency by increasing the concentration. Reduce in degradation performance with increasing concentration can be attributed to the further number of dye molecules whereas photocatalysis (as oxidant agent) dosage is constant. Also, dye molecules functioned as the barrier in solution to attain the light to the surface of photocatalyst and decrease the degradation efficiency53.

Photocatalytic activity of MgFe2O4@ZnAl2O4 MNCs for degradation of RB222 dye was considered with monitoring the performance in three states; photolysis (visible light irradiation without MgFe2O4@ZnAl2O4 MNCs), adsorption (magnetic nanocomposites under dark), and photocatalysis (MgFe2O4@ZnAl2O4 MNCs under visible light irradiation). In the photolysis state, we don't have degradation for selected dye. In adsorption condition, we have dye removal of 18% for RB222 dye. As shown in Fig. 9c, in photocatalysis state 96% of RB222 dye were degraded at 10 min.

To find the best time of dye degradation, the experiment was followed by using the optimum dosage of catalyst and dye concentration at different irradiation time, and their results are given in Fig. 9d. The lambda max of RB222 is located at 611 nm, and it is clear that degradation performance is certified by a decrease in absorbance maximum values when irradiation time increased. It is noticeable that 96% of RB222 were degraded in 10 min. Additionally, the removal of TOC from the RB222 dye was evaluated both before and after the degradation test under identical conditions. The findings revealed a 59% reduction in the TOC content of the RB222 dye. This unique efficiency in short time confirm that synthesized nanocomposites are useful photocatalysis in ambient conditions and visible-light region for degradation of RB222 dye.

In photocatalytic reactions, crucial species influencing activity include singlet oxygen (1O2), hydroxy free radicals (.OH), and superoxide radicals (O2·−)54,55,56. Radical scavenging studies using KI, NaN3, and p-benzoquinone (BQ) were conducted to elucidate the involved reactive species, as depicted in Fig. 10. In the absence of scavengers, RB222 dye removal in 10 min reached 96%. However, with KI and NaN3, removal decreased to 47% and 36% respectively. Conversely, BQ had minimal impact on the selected dye. These results highlight the significant roles of 1O2 and OH· in the photocatalytic degradation of MgFe2O4@ZnAl2O4 MNCs. NaN3 exhibited superior efficacy over KI, while BQ had a negligible effect on RB222 dye degradation. Thus, photodegradation predominantly occurs via hydroxyl radical and singlet oxygen mechanisms.

Figure 10
figure 10

Impact of scavenging agents on RB222 dye degradation.

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In order to survey the stability and recyclability features of the MgFe2O4@ZnAl2O4 MNCs, cyclic experiments were performed. The photocatalytic experiments were executed in optimized condition under visible light. Then, magnetic nanocomposites were separated by using the external magnet and washed with deionized water, and next run was carried out with the fresh dye solution. As shown in Fig. 11, the photocatalytic efficiency of the sample reduced slightly after five cycles. However, the degradation performance was remaining up to 96% and 92% in the final cycle for RB222 dye. The results showed that the synthesized MgFe2O4@ZnAl2O4 MNCs had excellent potential to be used as useful and efficient reusable photocatalyst.

Figure 11
figure 11

Recyclability of MgFe2O4@ZnAl2O4 MNCs. Reaction conditions: RB222 = 20 mg/l, catalyst = 0.02 g, 10 min.

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Conclusions

In conclusion, MgFe2O4@ZnAl2O4 magnetic nanocomposites were successfully synthesized with the easy, green, and cost-effective sol–gel method. Base on the XRD pattern, magnetic nanocomposites were produced in very pure spinel structure within crystallite size 13 nm. Also, SEM and TEM images revealed that morphology of nanocomposites is spherical with average width of 20–25 nm. The nanocomposites exhibited a saturation magnetization (Ms) of 6.59 emu/g, confirming their superparamagnetic nature due to the absence of Hc and Mr values. BET analysis revealed a surface area of 78.109 m2/g, while DRS analysis determined a band gap of 1.88 eV. Photocatalytic, photolysis, and adsorption performance evaluations demonstrated superior photodegradation activity, indicating a synergistic effect between MgFe2O4@ZnAl2O4 MNCs and visible light irradiation for organic dye degradation. Specifically, 96% degradation of RB222 dye was achieved within 10 min, with a corresponding TOC removal rate of around 59%. The MgFe2O4@ZnAl2O4 MNCs was used as useful photocatalyst for degradation of RB222 dye. Photocatalytic activity under the visible light irradiation, low catalyst dosage, high concentration of pollutants, short irradiation time, and reusability the nanocomposites are the merits of this work and shown these photocatalysis have the promising potential for wastewater treatment.