Efficient photocatalytic degradation of Rhodamine B dye using solar light-driven La˗Mn co-doped Fe2O3 nanoparticles

This work aims to develop a highly efficient solar light–induced photocatalyst based on La˗Mn co-doped Fe2O3 nanoparticles. Pure Fe2O3 and La˗Mn co-doped Fe2O3 nanoparticles were fabricated by a simple co-precipitation method. The photocatalysts were analyzed for their morphological, structural, and magnetic characteristics. Scanning electron microscopy analysis demonstrated the formation of semi-spherical nanoparticles along with small aggregations. The size of nanoparticles was measured using a transmission electron microscope and found in the range of 42–49 nm. The crystalline nature and geometry of synthesized nanoparticles were investigated using X-ray diffraction analysis. Due to the incorporation of La-Mn, the saturation magnetization and remanent magnetization of the nanoparticles decreased from 6.17 to 2.89 emu/g and 1.15 to 0.52 emu/g, respectively, while the coercivity was reduced from 756.72 to 756.67 Oe. The surface area of nanoparticles was increased from 77.93 to 87.45 m2/g as a result of La-Mn co-doping. The photocatalytic performance of the Fe2O3, La0.1Mn0.3Fe1.6O3, and La0.2Mn0.2Fe1.6O3 catalysts was assessed by their capability to degrade Rhodamine B (RhB) under solar light illumination. La0.2Mn0.2Fe1.6O3 displayed exceptional degradation performance, degrading RhB to 91.78% in 240 min, in comparison to La0.1Mn0.3Fe1.6O3 (71.09%) and pristine Fe2O3 (58.21%) under specified reaction conditions ((RhB) = 50 ppm; (catalyst) = 40 mg/L; pH = 7; T = 25 °C)). RhB degradation was affected by changing pH, catalytic dosage, dye concentration, and temperature. The degradation of RhB was found to be pseudo-1st order kinetics. The exceptional photocatalytic performance of La0.2Mn0.2Fe1.6O3 catalysts showed that the synthesized nanoparticles could be effectively utilized to remove organic pollutants from industrial wastewater.


Introduction
From the past few years, it was found that the economies of the developed countries had shifted from agriculture to industry due to a huge increase in the world's population (Zheng et al. 2020). While industrialization has made life easier for humans, it has also had a negative impact on the environment by producing toxic and hazardous chemical species (Payra et al. 2019;Usman et al. 2021b). Most of these industries discharge their toxic and stable chemical species into the wastewater, which ultimately damages aquatic life and human health (Lv et al. 2020). The wastewater contaminants are mostly organic dyes that are stable and toxic due to their complex and aromatic structures and are mostly waste-products of the tanneries, paint, and textile industries (Ritika et al. 2019). Due to this stable nature, these organic pollutants are considered non-biodegradable (Luque et al. 2022). These pollutants cause serious environmental problems and human health issues, including skin and lung destruction (Ashar et al. 2020;Das and Mahalingam 2020). Many types of dyes, which are the most common organic pollutants, are produced in different industries and discharged into water bodies without proper treatment and are the major sources of water pollution (Ahmed et al. 2020;Xu et al. 2018).
Rhodamine B (RhB), one of the most extensively used organic dyes in industry, is exceedingly toxic and hazardous to both humans and the environment (Lin and Hsiao 2022). RhB is a water-soluble synthetic dye that is commonly used as a colorant in the leather, textile, and cosmetics industries (Tanveer et al. 2022). When RhB-containing wastes are not completely cleaned, they constitute a significant environmental risk due to their harmful effects on flora and human health . The RhB dye is carcinogenic, and both internal and external exposure causes eye and skin irritation, as well as impairment to the respiratory, reproductive, and brain systems in humans (Pei et al. 2022). RhB must therefore be eliminated before it may be released into water supplies.
These water pollutants threaten human and aquatic life when discharged untreated into water bodies. These contaminants must be eradicated from the waste material before it is discharged into the water bodies. These contaminants are most commonly removed from wastewater by employing different methods. These methods include some electrochemical and physio-chemical techniques such as oxidation, chemical precipitation, adsorption, and microwave-assisted catalysis (Mostafa et al. 2020;Tang et al. 2020;Usman et al. 2021a). These methods are very important and advantageous for removing organic pollutants from water, but there are various disadvantages and drawbacks associated with these methods ). Due to environmental issues caused by the above-mentioned methods, there was a need to develop an effective and eco-friendly technique to treat the harmful organic pollutants (Bibi et al. 2017).
The photocatalysis provided the required properties, including being non-toxic, cost-effective, and commercially simple to use Wu et al. 2020). In this technique, a photocatalyst is used to degrade the harmful organic pollutants in the presence of sunlight (Demirci et al. 2018;Raza et al. 2020). The active sites available on the surface of the catalyst are responsible for absorbing the sunlight and hence for the creation of active species to degrade harmful organic compounds (Gao et al. 2020;Kumar et al. 2020). For the last few years, metal oxide nanoparticles have been most commonly used as photocatalysts for the removal of toxic organic compounds from sewage water (Duan et al. 2020). These metallic nanoparticles have been fabricated by the use of different methods, i.e., hydrothermal, sol-gel, co-precipitation, and microwave-assisted techniques (Almeida et al. 2020;Yousuf et al. 2019;Zare et al. 2019). Although these fabrication methods are very effective and efficient for the production of nanoparticles, there are still some drawbacks to them, including high cost, requiring more energy, and producing harmful by-products . It is required to choose a comparatively more effective, cheaper, and least harmful technique for the fabrication of nanoparticles .
The solar light-sensitive metal oxide photocatalysts are very important for the decomposition of water pollutants. Recently, iron oxides (Fe 2 O 3 ) have been extensively used for photocatalysis. Because of its environmentally friendly potential, Fe 2 O 3 is an effective and advantageous photocatalyst to oxidize H 2 O. It absorbs a longer range of solar light, making it a more effective photocatalyst (Guo et al. 2017). The photocatalytic activity of Fe 2 O 3 nanoparticles has been found to be increased when treated with some other metals, more preferentially the transition metals. The Mn being cheaper, easily available, and with environmentally friendly properties, is the best transition metal used for doping of Fe 2 O 3 nanoparticles.
It is observed that the doping of nanoparticles with Mn imparts various tremendous characteristics to them. The enhancement of the photocatalytic performance of the Fe 2 O 3 nanoparticles was also observed by doping with Mn. This doping has enhanced the photo-induced charges, which in turn increased the capacity of the Fe 2 O 3 nanoparticles to transfer the charges (Wang et al. 2019). The nanoparticles having Mn doping also undergo agglomeration due to their higher surface energy. The photocatalytic effect was found to be further enhanced by the use of rare-earth metal ions in addition to Mn. The rare-earth metals are responsible for the inhibition of photo-induced electron-hole recombination (Rashmi et al. 2017). The rare earth metals can also increase the number of active sites responsible for degrading hazardous organic dyes by forming heterojunctions in nanoparticles, which eventually limits electron-hole recombination (Harish et al. 2013). The rare-earth metals, in combination with Mn, are also responsible for changing the intrinsic properties of the nanoparticles due to the unique 4f-3d coupling of electrons. La is more preferable to other rare-earth elements due to its distinct properties such as larger size (1.6061 Å), simple electronic spectra, reactivity, paramagnetic nature, higher resistivity (615 Ωm), and lower melting point (920 °C). La in combination with Mn has enhanced physical, chemical, and catalytic properties of the nanoparticles (Baig et al. 2020;Peng et al. 2016).
Because of these remarkable properties of cations of the lanthanide series, in the present work, we have chosen La in combination with Mn for the co-doping of Fe 2 O 3 nanoparticles to enhance the photocatalytic performance of these nanoparticles.

Chemicals
The chemicals required for the fabrication of La˗Mn codoped Fe 2 O 3 nanoparticles are listed here. Fe(NO 3 ) 3 . 9H 2 O (Merck, 98%), Mn(NO 3 ) 2 .4H 2 O (Merck 98%), La(NO 3 ) 3 , (Merck 98%), deionized water, and sodium hydroxide (NaOH, Sigma, 98%) were utilized as precursors and reagents for this research work. The RhB, an organic dye utilized during this work, was procured from Almadina chemicals. All these precursors and chemicals which were utilized in this research work were in highly purified form and were used as received from the supplier. All of the chemicals and precursors that were used in this research were of analytical grade and used as received.

Synthesis of La˗Mn co-doped Fe 2 O 3 nanoparticles
For the preparation of La˗Mn co-doped Fe 2 O 3 nanoparticles, different molar concentrations of solutions were made, such as Fe(NO 3 ) 3 . 9H 2 O (0.16 M), Mn(NO 3 ) 2 .4H 2 O (0.03 M, 0.02 M) and La(NO 3 ) 3 (0.01 M, 0.02 M) were made by the addition of specific quantities of these salts to water separately. Briefly, 16.6 g of ferric nitrate was added in 250 ml of deionized water and stirred for 15 min at 60 °C to get a homogeneous solution. 0.02 M solution of manganese nitrate was added to the above solution at the same temperature with constant stirring for 15 min. Following that, a 0.02-M solution of lanthanum nitrate was added dropwise while being continuously stirred. At the start of this experiment, the pH of the solution was found to be 4, and the color was orange-yellow. The pH of the solution was then increased from 4 to 11 by adding 1 M NaOH solution and maintained at 11. Stirring was continued for 4 h at 80 °C. The solution was then cooled, and the precipitates separated. The washing of these precipitates was carried out by deionized water many times till the pH became neutral. The precipitates were then dried in an oven at 150-200 °C and annealed at 450 °C for 4 h in a furnace to get the final product. For comparison, pristine Fe 2 O 3 nanoparticles were also synthesized under the same conditions in the absence of La and Mn. The schematic representation for the synthesis of La˗Mn co-doped Fe 2 O 3 nanoparticles is presented in Fig. S1.

Characterization
The morphology and particle size of pure Fe 2 O 3 and La˗Mn co-doped Fe 2 O 3 nanoparticles were examined by using a scanning electron microscope (SEM, Hitachi SX-650). Similarly, the transmission electron microscope (TEM, JEOL JEM-2100Plus) is used for the visualization and determination of morphological properties of nanoparticles. The fourier transform infrared spectrophotometer (FTIR, was used for taking the spectra of the fabricated doped and undoped nanoparticles in the 400-3000 cm −1 wavelength range. The X-ray diffraction (XRD) analysis of the prepared nanoparticles was performed to determine the nanoparticles' purity and crystallinity. The dynamic light scattering (DLS) analysis was performed using the NANO ZS Malvern Zetasizer for the measurement of zeta potential and particle size distribution. The magnetic properties of as fabricated particles were determined by the use of a magnetic field of − 15,000 to + 15,000 Oe strength. The N 2 adsorption-desorption on the surface of nanoparticles and subsequently plotting a BET isotherm were utilized to measure the surface area. This analysis was performed by using the micrometrics physisorption analyzer (ASAP 2020). The UV spectrophotometer (Perkin Elmer, USA) was used for the measurement of photocatalytic performance of the as-prepared nanoparticles in the region of 200-700 nm. All the graphs were made using OriginPro 2017 software.

Photocatalytic activity
The photocatalytic efficiency of pure Fe 2 O 3 and La˗Mn codoped Fe 2 O 3 nanoparticles was assessed by the degradation of RhB dye under solar light illumination. In a conventional degradation procedure, 40 mg/L of catalyst was mixed with 100 mL of RhB (50 ppm) in ultrapure water. To achieve the adsorption-desorption equilibrium, the resulting mixture was put in the dark and continuously agitated for 30 min. Following that, the mixture was illuminated with solar light, and 1 mL samples were taken at fixed time intervals and centrifuged at 5000 rpm to remove the residual catalyst particles. A UV-Vis spectrophotometer was used to check the concentration of RhB dye (Eq. 1).
The photocatalytic effect of the La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles was further studied at different experimental conditions. The kinetics experiments of RhB degradation were also performed using the Langmuir-Hinshelwood model and obeyed the pseudo-first-order kinetics as assessed by Eq. 2.
The slope of the straight line obtained by graphing ln(C/C o ) versus time is used to determine the value of k. (1)

SEM analysis
The shape and particle size of undoped Fe 2 O 3 and La˗Mn co-doped Fe 2 O 3 nanoparticles were examined by using SEM analysis, as shown in Fig. 1a. The SEM micrographs of Fe 2 O 3 nanoparticles have shown that the fabricated nanoparticles appear as semi-spherical shaped with small aggregations in clusters of different sizes (Su et al. 2016). The morphology of the doped nanoparticles was not significantly affected due to this bimetallic doping. Because the size of the dopant metal ion Mn +2 is not significantly different from that of the parent Fe +3 ions, its doping has little effect on the size of doped nanoparticles. The La +3 ion is much larger (1.6061 Å) than the Mn 2+ (0.83 Å) and Fe 3+ (0.63 Å) ions found in parent nanoparticles. These larger La ions induce lattice imperfections and also lattice strain. For this reason, the particle size of as-prepared doped nanoparticles was found to be reduced by increasing the molar concentration of La ions (Ravinder et al. 2022). Figure 1b shows the micrograph of La 0.1 Mn 0.3 Fe 1.6 O 3 , and a reduction in the size of doped particles was observed in comparison to the undoped Fe 2 O 3 nanoparticles. Similarly, La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles in Fig. 1c have shown a reduction in the size of the particles that take place by increasing the dopant La metal ions. Due to this reduction in the size of the fabricated doped nanoparticles, the active surface of the particles is increased, so the catalytic performance of these particles is also enhanced ).

TEM analysis
TEM analysis was performed to visualize and determine the morphological properties of nanoparticles as presented in Fig. 2. The doping of Fe 2 O 3 nanoparticles has resulted in aggregation of the particles, which in turn produces interparticle mesopores, resulting in enhancement of the surface area. It was observed from TEM micrographs of Fe 2 O 3 given in Fig. 2a that the average size as calculated by Nanomeasure software was found to be 48.137 nm, while the doping of Fe 2 O 3 with La and Mn reduces the size of these particles. The similar type of trend for the synthesis of metal-doped Fe 2 O 3 nanoparticles has been reported by other research groups (Haider et al. 2022;Ramprasath et al. 2022). Figure 2b shows the micrograph of La 0.1 Mn 0.3 Fe 1.6 O 3 nanoparticles; the size of these particles was found to be 46.923 nm. Similarly, Fig. 2c shows the micrograph of La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles, and their size was estimated to be 42.861 nm. The morphology of all the samples was found to be semi-spherical shaped, which is an indication of the fact that the morphology of particles is not significantly changed, but only the size of the particle is affected (Alotaibi et al. 2022 nanoparticles via a hydrothermal method and found that the doping of metal has no obvious influence on the shape of nanoparticles, which further proved our TEM analysis results (Cao et al. 2022).

FTIR analysis
The FTIR spectra of the prepared nanoparticles (undoped and doped) were taken to determine structural parameters. The spectra of nanoparticles have shown two absorption peaks in the 400-3000 cm −1 region for metal-oxygen stretching vibrations. Of these two prominent absorption peaks, one is observed in the low frequency region (400-500 cm −1 ) for octahedral M-O stretching vibrations (Lassoued 2021). Similarly, the peak in a higher frequency region (500-600 cm −1 ) appeared because of tetrahedral stretching vibrations ). The FTIR spectra for all three types of nanoparticles were taken and are represented in Fig. 3.
The spectra for Fe 2 O 3 , La 0.1 Mn 0.3 Fe 1.6 O 3 , and La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles have shown that there is a small shift of the absorption peak to the higher frequency region by the increase in the concentration of dopant metal La. It is because of the enhancement of large-sized La metal ions' concentration that decrease in particle size is observed, causing a decrease in bond length, which in turn causes the increase in the absorption frequency of the stretching bands (Dasan et al. 2017). Due to this fact, change in the position of the absorption band takes place, as shown in Fig. 3.

XRD analysis
The XRD analysis of the undoped Fe 2 O 3 and La˗Mn codoped Fe 2 O 3 nanoparticles was performed for the determination of purity and crystallinity of the nanoparticles. The analysis of these nanoparticles has confirmed the hexagonal geometry of the synthesized nanoparticles. The XRD results are shown in Fig. 4. According to these results, there are sharp and intense diffraction peaks at 2θ values, 24.13° (012) All the values of the peaks were compared with a standard pattern of Fe 2 O 3 having hexagonal geometry (Wang et al. 2021). The XRD results were found to be greatly comparable to the standard reference (JCPDS card no. 33-0664), which confirmed the synthesis of Fe 2 O 3 (Ma and Chen 2018). There were no other irrelevant peaks observed in this result, which is confirmation of the purity of as-synthesized nanoparticles (Ge et al. 2022). It was also found that the decrease in intensity of the peaks was observed for the La and Mn-doped nanoparticles. The peaks were also shifted due to this doping, and the shifting was increased by increasing the La ion concentration (Baig et al. 2020).
The cr ystal size of the synthesized Fe 2 O 3 , La 0.1 Mn 0.3 Fe 1.6 O 3 , and La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles was calculated by using the Debye Scherrer equation and measured to be 59.143, 58.237, and 51.948 nm, respectively. The decrease in size of the doped nanoparticles is explained by the fact that the insertion of large-sized La 3+ ions (1.6061 Å) into the small-sized Fe 3+ ions (0.63 Å) caused an increase in constant lattice value, which in turn decreases the crystallite size (Ahmad et al. 2016).

Dynamic light scattering analysis
The DLS technique is utilized to measure the zeta potential and particle size distribution. The results plotted in Fig. 5 have shown a regular reduction in the average size of  Fig. 5a. Similarly, the size of the particles is reduced after bimetallic doping in a composition of La 0.1 Mn 0.3 Fe 1.6 O 3 . In this case, the average size is 67.42 nm, as shown in Fig. 5b. The average size of nanoparticles is further decreased to 57.28 nm when the concentration of dopant metal La is increased in La 0.2 Mn 0.2 Fe 1.6 O 3, as presented in Fig. 5c.

Magnetic properties
The magnetic properties of the nanoparticles were measured at standard conditions of temperature and pressure with a field strength of − 15,000 to + 15,000 Oe. The magnetic hysteresis (M-H) curves for Fe 2 O 3 nanoparticles and La˗Mn co-doped Fe 2 O 3 nanoparticles with different dopant concentrations are given in Fig. 6. These M-H curves are used for the measurement of various magnetic properties, i.e., Ms, Hc, Mr, and squareness ratio (Table S1).
These magnetic properties of as-synthesized undoped and doped nanoparticles are greatly influenced by various factors such as surface charge, cation stoichiometry, cation distribution, size of particles, and the method of fabrication used (Sharifi et al. 2012). The results presented in Table S1 show that the value of Ms is decreased for the doped nanoparticles compared to undoped particles. Fe 2 O 3 nanoparticles have a value of 6.17 emu/g, while La 0.1 Mn 0.3 Fe 1.6 O 3 particles have a value of 3.73 emu/g and La 0.2 Mn 0.2 Fe 1.6 O 3 particles have a value of 2.89 emu/g. Similarly, the Mr values are also decreased by increasing the concentration of the La 3+ ion. Fe 2 O 3 nanoparticles have a Mr of 1.15 emu/g, which was decreased to 0.68 emu/g for La 0.1 Mn 0.3 Fe 1.6 O 3 particles and was further reduced by increasing the dopant ions concentration up to 0.52 emu/g for La 0.2 Mn 0.2 Fe 1.6 O 3 . The values for coercivity and squareness ratio are also affected in a similar way, but there is very little change for these parameters.
The decrease in the values of magnetic parameters is considered due to the insertion of La +3 ion into the Fe 2 O 3 nanoparticle sublayers, which decreases the crystallinity and uniform morphology of the undoped Fe 2 O 3 nanoparticle

BET analysis
When BET analysis was performed for Fe 2 O 3 nanoparticles, it was found that a type IV isotherm was formed, as shown in Fig. 7a (Shikha et al. 2017). The surface area of Fe 2 O 3 nanoparticles was found to be 77.93 m 2 /g. Similarly, the surface area of the La 0.1 Mn 0.3 Fe 1.6 O 3 and also of La 0.2 Mn 0.2 Fe 1.6 O 3 was also measured. It was observed that the surface area of La 0.1 Mn 0.3 Fe 1.6 O 3 was found to be 80.23 m 2 /g, as shown in Fig. 7b. Further analysis of the La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles has shown that by changing the concentrations of the dopant metals, the surface properties of the La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles were also changed to 87.45 m 2 /g, as given in Fig. 7c.
From the above results, it is concluded that the surface area of the La˗Mn co-doped Fe 2 O 3 nanoparticles is increased by increasing the dopant concentrations (Keerthana et al. 2022). Due to this fact, the interaction between gas molecules and the nanomaterials is increased, which in turn also increases the adsorption-desorption (Zhou et al. 2016). This increased interaction between gas and nanoparticles will enhance the gaseous diffusion into the inner layers of the material having active sites ). The increased surface area is beneficial for degrading the harmful organic compound.

Photocatalytic activity
The UV spectrophotometer is used for the measurement of photocatalytic performance of the as-prepared Fe 2 O 3 nanoparticles and La˗Mn co-doped Fe 2 O 3 nanoparticles. This experiment was performed by calculating the degradation of RhB dye under sunlight. The dye solution was prepared and kept in the dark to avoid any disturbance before applying the catalyst. The dye solution and the catalyst solution were then mixed in the dark and kept for some time to attain the equilibrium. This mixture was irradiated by keeping it under sunlight.
The degradation rate of dye was monitored by taking samples from time to time and measuring the concentration by using a UV spectrophotometer. The decrease in dye concentration was detected with the increase in time, and ultimately, the dye was degraded after 240 min. It is due to the fact that the nanoparticles acted as catalysts to degrade the dye molecules. These results of RhB degradation have shown 58.21% degradation by using Fe 2 O 3 as represented in Fig. 8a. After the doping of Fe 2 O 3 with Mn and La in the composition of La 0.1 Mn 0.3 Fe 1.6 O 3, the degradation of dye was found to be increased by 71.09%, which is much higher than undoped nanoparticles, as represented in Fig. 8b. The results have shown that with the increase in concentration of dopant metal La, the percentage degradation of the RhB was also increased, as shown in Fig. 8c. It is evident that when the La concentration is increased to La 0.2 Mn 0.2 Fe 1.6 O 3 , dye degradation was also increased up to 91.78%. To highlight the importance of the present study, the photocatalytic degradation of RhB over several Fe 2 O 3 -based catalysts was also compared with the catalysts previously described (Table S2).
Based on the above discussion, it is concluded that the degradation rate is increased by doping and is further increased by increasing the composition of large-sized La +3 cations due to the reduction in size and increase in surface area of the doped nanoparticles (Keerthana et al. 2022). The photocatalytic activity of La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles is further studied under different experimental conditions.

Effect of pH
As it is evident from the above discussion, degradation of RhB is best carried out by La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles. Further studies were also carried out to explore the effects of the other factors on degradation process. Amongst these factors, the pH of the solution is considered as the most important and noticeable dynamic factor to affect the degradation (Wang et al. 2022b). This factor is responsible for the development of charge on the catalyst surface, either positive or negative (Ali et al. 2022). Due to this reason of charge development, the adsorption rate of dye on the catalyst surface is also changed. In addition to the charge development, the size and aggregation of the catalyst are also affected by the change in pH, which in turn influences the degradation of dye . In this experiment, a fixed amount of catalyst (40 mg/L) and that of dye (50 ppm) are used at These dye solutions having different pH values were then exposed to sunlight, and degradation was calculated. To determine the effect of pH on degradation, the zeta potential was used for the measurement of the surface charge of the catalyst, as shown in Fig. 9.
The point of zero potential (also known as IEP) was measured from zeta potential values. The point of zero potential for La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles was found to be 8.46, as shown in Fig. 9. The catalyst surface attains a positive charge when the solution's pH was decreased below 8.46, and the surface became negative by increasing the pH above 8.46. As RhB is a cationic dye, which means its attachment and also degradation are favored at a pH value above 8.46 because above this point, the catalytic surface becomes negative. This negative surface attracts the cationic dye; hence, degradation of the dye is increased. Similarly, when the pH of the solution is decreased, a positive charge on the catalytic surface will be developed. This positive charge ultimately repels the cationic dye; hence, the degradation of the dye is decreased with the decrease in pH of the solution (Ali et al. 2022). Due to this reason, the minimum degradation value of 43.3% was observed at pH 4. An increase in degradation of dye was observed with the enhancement of pH, and a maximum degradation of 89.34% was observed at pH 10 as represented in Fig. 10 and also in Table 1.
The kinetics of degradation of RhB dye has shown a pseudo-first-order reaction as observed for different initial concentrations of dye solution. It is evident from the above results that the rate constant is increased from 0.0025 to 0.0092 min −1 by varying the pH from 4 to 10.

Effect of catalytic dosage
The degradation rate was also significantly affected by the change in catalytic dosage. The catalytic dosage is also an important factor for the degradation of dyes because it also causes a change in surface area. The effect of the photocatalyst La 0.2 Mn 0.2 Fe 1.6 O 3 on photodegradation of RhB was observed using the different amounts of 10, 20, 30, 40, and 50 mg/L while keeping other factors constant at the dye concentration of 50 ppm, temperature of 25 °C, the pH at 7, as shown in Fig. 11 and Table 2. These experimental results have shown that the dye degradation is increased as the catalytic dosage is enhanced by 10 to 50 mg/L. The minimum degradation of dye was found at a lower catalytic dosage of 10 mg/L, which was 60.49% and went on increasing by an increase of catalytic dosage up to 50 mg/L, which shows the maximum photodegradation efficiency of the photocatalyst La 0.2 Mn 0.2 Fe 1.6 O 3 as 90.07% at 25 °C, 50 ppm dye concentration and pH of the solution was maintained at 7. This increase in photodegradation is due to an increase in surface area, which in turn increases the active sites (Cheng . For this reason, the rate of degradation is also increased by increasing catalytic amount. There will be an increased absorption of photons with the increase of surface area, which further increases the interfacial interaction among catalytic surface and dye molecules (Ahmed et al. 2021c).
The kinetics of degradation of RhB dye has shown a pseudo-first-order reaction as studied under different catalytic dosages. The results have shown that the value of the rate constant is increased by 0.0037 to 0.0092 min −1 as the catalytic dosage is increased by 10 to 50 mg/L. The values of the regression coefficient also increase from 0.9830 to 0.9961, which is in good agreement with the degradation percentage.

Effect of dye concentration
The rate of the degradation of organic dyes is also greatly affected by the change in dye's concentration. The experiments have shown an increase in the rate of reaction by increasing the concentration of dye. The effect of the photocatalyst La 0.2 Mn 0.2 Fe 1.6 O 3 on photodegradation of RhB was observed by using the various dye concentrations, i.e., 10,20,30,40,50, and 60 ppm while keeping other factors constant at the catalytic dosage of 40 mg/L, temperature of 25 °C, and pH at 7, as shown in Fig. 12 and Table 3.
The results have shown an increase in the photocatalytic performance of La 0.2 Mn 0.2 Fe 1.6 O 3 from 46.19 to 89.82% as the dye concentration was increased from 10 to 50 ppm while keeping other factors constant, such as 25 °C temperature, 40 mg/L catalytic dosage, and pH 7. The results   have also shown that by further increasing the dye amount, the rate of degradation is reduced. It is because the active sites are available to a particular limit. But when the amount of dye is further increased, there are not sufficient active sites available for the attachment of molecules (Ahmed et al. 2020). Therefore, the decrease in degradation of dye is observed, as presented in Fig. 12.
The kinetics of degradation of RhB dye has shown a pseudo-first-order reaction as studied under different amounts of dye. These results have shown that rate constant is increased from 0.0023 to 0.0093 min −1 with the increase of dye concentration by 10 to 50 ppm. Similarly, the regression coefficient is increased from 0.9664 to 0.9974 min −1 .

Effect of temperature
The rate of photodegradation of organic pollutants is affected by the change in temperature because the degradation of dyes is either an endothermic or an exothermic reaction.
The temperature is one of the major factors, so the dye degradation is much affected by changing the temperature. The photodegradation of RhB is an endothermic reaction and is favored by an increase in temperature (Ahmed et al. 2021c).
As it was found that the temperature has a very small effect on the degradation of dye, there is still an optimum temperature at which degradation of dye is maximum. For RhB degradation, the experiment was performed at different temperatures (25, 35, 45, and 55 °C) by keeping other factors, catalytic dosage (40 mg/L), pH 7, and concentration of dye (50 ppm) constant. The experimental results shown in Fig. 13 and Table 4 reveal that dye was degraded up to 94.87% at 45 °C.

Degradation mechanism of dye
The results of dye degradation by using La 0.2 Mn 0.2 Fe 1.6 O 3 as a photocatalyst have shown that the dye is being converted into harmless components by the action of sunlight. The most probable mechanism to degrade the dyes is shown in Fig. S2. As the sunlight irradiates on the catalytic surface, it causes the excitation of electrons to produce an electron (e − ) hole (h + ) pair that further degrades the RhB dye. The mechanism shows that the excited electrons produced in the conduction band are responsible for the production of super oxide anion radical (O 2 •− ) which is further converted into hydroperoxyl radical ( • OOH) by the reaction with water (Al-Hakkani et al. 2022). In the presence of sunlight, • OOH radical rearranges itself to form H 2 O 2 . The hydrogen peroxide thus produced reacts with oxygen to produce • OH.
Similarly, in the valance band, a reaction between water molecules and holes also takes place and converts it into • OH. These hydroxyl radicals then react with RhB and

Reusability of catalyst
Since the catalyst's stability and durability are crucial factors to consider when considering large-scale industrial applications, four degradation tests were conducted under specific reaction conditions, as shown in Fig. S3. After each cyclic run, the La 0.2 Mn 0.2 Fe 1.6 O 3 catalyst was extracted from the aqueous solution using a magnet, thoroughly washed with deionized water, and dried at 70 °C for reuse in the next cycle. When the catalyst is used repeatedly, the degrading efficiency of RhB falls steadily from 91.78 to 85.08%. Photocatalyst aggregation, metal ion leaching, and catalytic site degradation as a result of photocorrosion all contribute to the ultimate loss of catalytic potential Zhu et al. 2022). The catalyst used in degradation tests might be recovered in the presence of a high magnetic field. As a result, the reusability experiment anticipates and supports the future use of the La 0.2 Mn 0.2 Fe 1.6 O 3 catalyst for photocatalytic wastewater treatment.

Conclusion
The Fe 2 O 3 and La˗Mn co-doped Fe 2 O 3 nanoparticles having a composition of La 0.1 Mn 0.3 Fe 1.6 O 3 and La 0.2 Mn 0.2 Fe 1.6 O 3 were prepared by the co-precipitation method. The SEM analysis revealed the formation of semi-spherical shaped nanoparticles, while the average size of nanoparticles was measured using TEM analysis and found in the range of 42-49 nm. The phase purity and crystalline nature of the nanoparticles were further confirmed by FTIR and XRD analysis. The saturation magnetization and remanent magnetization of the nanoparticles declined from 6.17 to 2.89 emu/g and 1.15 to 0.52 emu/g, respectively, caused by the addition of La-Mn. The coercivity decreased from 756.72 to 756.67 Oe. As a consequence of La-Mn co-doping, the surface area of La 0.2 Mn 0.2 Fe 1.6 O 3 nanoparticles was enhanced from 77.93 to 87.45 m 2 /g. The surface area of undoped and doped particles, which was determined by BET, was 77.93, 80.23, and 87.45 m 2 /g, respectively. The synthesized nanoparticles were effectively utilized as photocatalyst materials for the solar-light-driven degradation of RhB dye. It was observed from the results that La 0.2 Mn 0.2 Fe 1.6 O 3 showed maximum degradation performance of up to 91.78% for RhB, much higher than that of La 0.1 Mn 0.3 Fe 1.6 O 3 (71.09%) and pristine Fe 2 O 3 (58.21%). Degradation of RhB dye was increased from 43.30 to 89.34% with the increase of pH from 4 to 10 and increased by changing catalytic