Synthesis, Characterization, and Kinetic Insights of MnZnFe₂O₄/SrWO₄ Ferrite–Tungstate Nanocomposite for Rapid Photocatalytic Degradation of Imidacloprid

1. Introduction

Pesticide contamination in aquatic environments has become a major global concern due to the widespread and persistent use of neonicotinoids [1,2]. Among these, imidacloprid (IMI) is one of the most extensively applied systemic insecticides in agriculture, exhibiting high water solubility (0.61 g L⁻¹ at 20 °C) and chemical stability (half-life up to 190 days in water and 997 days in soil), which allow it to persist and accumulate in ecosystems. In surface water and ground water, IMI residues have been found in concentrations of 0.01-320 μg L^-1 especially where there is high agricultural runoff. Even at low levels, trace concentrations have very dangerous effects on aquatic life: in the case of the organism, the median of lethal concentration (LC 50) of 2.1 mg L^-1 in the case of Daphnia magna, and the levels of chronic toxicity were observed at concentrations of 1 μg L^-1 [3,4]. It also has low photodegradation rate (rate constant 0.004-0.012 h ^-1 in natural sunlight) which also adds to its persistence [5]. Although regulatory authorities in most jurisdictions such as in the European Union cannot renew the use of IMI and the U.S Environmental Protection Agency has recently prohibited the use of IMI, residual concentrations are still a significant issue within the environment, as they signal the need to find a lasting solution by removing it [6,7]. Among the remediation methods, the advanced oxidation processes (AOPs) based on heterogeneous photocatalysis have become the promising pathways in the mineralization of recalcitrant organic micro-pollutants including IMI. Photocatalysis has the potential to convert the contaminant to CO₂, H₂O and inorganic components as opposed to relocating the pollutant into a different phase as would be the case with conventional biological or adsorption techniques [8]. The use of semiconductors on a ceramic platform is of special interest in this regard because of its chemical stability, mechanical endurance, irradiation resistance, and the possibility of reuse in cyclic mode of operation, an imperative characteristic of effective water-treatment systems [9].

A group of ceramic photocatalysts, the so-called scheelite-type tungstates, with the general formula ABO₄ (A Ca, Sr, Ba; B Mo, W) are of great interest to researchers due to their wide band gaps, great chemical stability, and strong crystalline structures, making them applicable in severe aqueous conditions [10]. A good example is strontium tungstate (SrWO₄ ), that has tetragonal structure of scheelite, it has great photochemical stability, and has a large oxidative potential under ultraviolet light due to its wide band gap of about 4.1 eV [11]. However, the main disadvantage of SrWO 4 and other wide-gap tungstates is the low absorption of visible light and the rapid rate of photogenerated electron-hole pair recombination, which reduces quantum efficiency and prevents the effective operation in the sunlight [12]. In line with this, it is urgent to have strategies that will improve charge separation, expand the optical response and enable catalyst recovery. At the same time, spinel ferrites (Mn, Co, Ni, etc.) with the overall formula MₓZn₁₋ₓFe₂O₄, have been considered in photocatalytic reactions and Fenton-like oxidations. Specifically, manganese-zinc ferrite (MnₓZn₁₋ₓFe₂O₄) is both magnetic separable (high saturation magnetization, low coercivity) and tunable in their electronic structure through cation substitution to provide interfacial charge -transfer when coupled with other semiconductors [13]. As single components, ferrites act as photocatalysts, radical-generating oxidation catalysts; as a part of heterostructures, they may offer charge-sink properties, magnetically reconfigurable structures, and high photo stability [14].

Prior work on IMI degradation using heterogeneous catalysts has included both ferrite-based and non-ferrite systems. For instance, a mesoporous copper ferrite (CuFe₂O₄) catalyst in a heterogeneous Fenton-type system reported a pseudo-first order rate constant of approximately 1.0445 h⁻¹ for IMI (~10 mg L⁻¹) under optimal conditions [15]. Although this performance is high, full mineralisation was not demonstrated and catalyst recovery in repeated cycles remained limited. Another ferrite-system study reported a Z-scheme heterojunction of graphitic carbon nitride (g-C₃N₄) and slag-derived calcium ferrite, achieving ~2.5-fold faster IMI removal over 120 min versus pure g-C₃N₄ [16].On the non-ferrite side, ZnO nano catalysts have been reported to reach approximately 95 % IMI degradation within one hour under suitable UV conditions [6]. While the high removal rate is promising, such ZnO systems often require UV irradiation only, lack magnetic recovery, and are vulnerable to photocorrosion or catalyst sintering during reuse [17]. Another example includes an Ag-ZnO composite for IMI degradation, where performance improved over pure ZnO, yet visible-light efficiency remained low and long-term reusability was not reported [18]. More recently, an Ag₂O/CuO composite achieved ~92.3 % IMI degradation in 180 min under UV illumination (half-life ~3.7 h) but again lacked magnetic recovery and visible-light responsiveness [19,20]. Additionally, a composite of 10 % Co₃O₄–MoO₃ showed ~98 % removal of IMI under natural sunlight in ~3–4 h but the catalyst stability and recyclability were not fully documented [21].

A simple co-precipitation method along with hydrothermal treatment was used to synthesize MnZnFe₂O₄/SrWO₄ nanocomposite, which combined the desirable properties of ferrites and tungstate phases. The coexistence of both phases was confirmed by X-ray Diffraction, Scanning Electron Microscopy, Fourier-transform infrared spectroscopy, UV-Visible Spectroscopy and Photoluminiscence measurements that showed changed optical properties of composite in comparison to the constituents, which showed a sign of reduced charge-carrier recombination dynamics. The photocatalytic efficiency of the composite towards the degradation of imidacloprid was studied using UV illumination at different parameters like catalyst loading, pH of the solution, initial concentration of the pesticide and the duration of the irradiation.

The MnZnFe₂O₄/SrWO₄ photocatalyst demonstrated a significantly higher rate of the pesticide degradation in the presence of 0.2 g L^-1 catalyst load, 30 mg L^-1 imidacloprid concentration, and pH 9 conditions, which was about 87 percent in 30 minutes, a significantly higher rate than either of the two catalysts alone. This enhanced performance is attributed to the synergistic light-absorption of the two constituents and inhibition of recombination of charge-carriers as shown by strong photoluminescence quenching. Furthermore, after five consecutive catalytic cycles, the composite maintained more than 70% of its initial activity, demonstrating good operational stability and reusability. Overall, the results of this study highlight the relevance and originality of a simply synthesized MnZnFe₂O₄/SrWO₄ heterostructure composite, which acts as an efficient, reusable, and UV light responsive photocatalyst to remove imidacloprid, hence providing a practical solution to solar-inspired pesticide remediation.

2. Experimental Details

2.1. Materials

Sigma-Aldrich provided all of the reagents, which were used exactly as supplied, Strontium nitrate (Sr(NO₃)₂) (99%), sodium tungstate dihydrate (Na₂WO₄·2H₂O) (99%), Potassium hydroxide (KOH) pellets (90%), manganese nitrate tetrahydrate (Mn(NO₃)₂·4H₂O) (98%), Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) (≥98%), Iron (III) Nitrate Nonahydrate (Fe(NO₃)₃·9H₂O) (98%), cetyltrimethylammonium bromide (CTAB, surfactant) (99%), ethanol C₂H₅OH (99.8%) and Confidor (17% imidacloprid) were bought from the agricultural crop company “Bayer”. Deionized water (2.0 MΩ/cm) was used throughout.

2.2. Synthesis of Strontium Tungstate (SrWO4)

Strontium tungstate (SrWO₄) nanoparticles synthesis was performed using a co-precipitation approach as shown in Figure 1.[22]. Initially, two separate 0.40 M solutions were prepared by dissolving 1.27 g of Sr(NO₃)₂ and 1.98 g of Na₂WO₄·2H₂O in 15 Ml of deionized water under moderate stirring at ambient temperature. The strontium solution was then added dropwise into the tungstate solution under vigorous stirring. The reaction mixture was heated to 70 °C and maintained under constant stirring for 3 hrs. while the pH was adjusted and stabilized at 8.0 using aqueous KOH to ensure enrolled nucleation This lead to the formation of fine white precipitate. After 3 hrs., the suspension was left to age for 12 hrs. at ambient temperature to allow the growth of the particles. The resulting precipitate was then centrifuged at 5000 rpm and washed repeatedly with deionized water until the supernatant is clear. The resultant solid was dried at 80 °C to eliminate surface moisture, followed by calcination at 200 °C for 2 hrs. in a muffle furnace, producing well-crystallized SrWO₄ nanoparticles.

2.3. Synthesis of Manganese Zinc Ferrite (MnZnFe₂O₄)

MnZnFe₂O₄ nanoparticles were synthesized via a CTAB assisted co-precipitation method. Stoichiometric amounts of metal nitrates i.e 2.5 mmol Mn(NO₃)₂ (0.717 g), 2.5 mmol (ZnNO₃)_2.6H₂O (0.743 g) and 10.0 mmol Fe(NO₃)_3.9H₂O (4.040 g), were separately dissolved in deionized water and diluted to 25 ml each. The solutions were combined under vigorous stirring and CTAB (0.1%) was used as a steric stabilizer. The mixture was heated to 80 °C, and precipitation was induced by dropwise addition of concentrated KOH solution to maintain pH. The suspension was aged at 80 °C for 3 h to promote crystallite growth. The product was isolated by centrifugation, washed repeatedly with deionized water and ethanol until neutral pH was achieved, and dried at 80 °C for 12 h. Finally, the powder was calcined at 200 °C for 2 h (5 °C/min ramp) to decompose organics and enhance crystallinity, yielding phase-pure MnZnFe₂O₄ nanoparticles

2.4. Synthesis of MnZnFe₂O₄/SrWO₄

The MnZnFe₂O₄/SrWO₄ nanocomposite was synthesized [23] by weighing MnZnFe₂O₄ (0.50 g) were dispersed in deionized water (30mL) containing PVP (1 wt%) and sonicated at (50 °C, 15 min). Separate 0.10 M solutions of Sr(NO₃)₂ (15mL) and Na₂WO₄. 2H₂O (15mL) were adjusted to pH 8.5. In the vigorous stirring at 80 ⁰C (600 rpm) the dropwise addition of the Sr²⁺ and WO4^2- solutions into the ferrite dispersion was done over a duration of 20 min with the pH under control at 8 by adding NH₄OH. The mixture was aged 30 min at 80 °C, transferred to a Teflon-lined autoclave, and heated at 140 °C for 6 h. The solid was recovered by centrifugation (8000 rpm, 10 min), washed sequentially with DI water and ethanol to a neutral supernatant, and dried at 80 °C overnight.

Upon completion, the product was collected by centrifugation at 8000 rpm for 10 minutes, then washed several times with deionized water and absolute ethanol until a neutral supernatant was obtained. Finally, the purified composite powder was dried at 80 °C overnight, yielding a fine, homogeneous MnZnFe₂O₄/SrWO₄ powder.

2.5. Photocatalytic Degradation of Imidacloprid (IMI)

The photocatalytic degradation mechanism of imidacloprid using the synthesized MnZnFe₂O₄/SrWO₄ nanocomposite is presented in Figure S1. Ultraviolet irradiation was supplied by a mercury lamp (emission wavelength: 365 nm; Konrad Benda Herolab) with a power rating of 8 W and an irradiance intensity in the range of 10–12 mW cm⁻² across an illuminated area of 42.75 cm². The light source was positioned 10 cm above the reaction system. Photocatalytic experiments were performed under UV exposure, while control studies in the absence of illumination were conducted to assess non-photocatalytic effects. Aqueous imidacloprid solutions were prepared using commercial Confidor within a concentration range of 10–50 mg L⁻¹. In a typical experiment, 100 mL of a 30 mg L⁻¹ solution was used and maintained at ambient temperature (25 °C). Before irradiation, the catalyst was introduced into the solution at concentrations ranging from 0.1 to 0.6 g L⁻¹ and stirred magnetically at approximately 400 rpm in the dark for 10 minutes to ensure adsorption–desorption equilibrium. Following this, the suspension was exposed to UV light while continuous stirring was maintained throughout the reaction. At specific time intervals, 3 mL aliquots were withdrawn and analyzed using a UV–visible spectrophotometer at 269 nm (Figure S2) to determine the residual imidacloprid concentration. The photocatalytic degradation efficiency was subsequently calculated using Equation (1).

$$\mathrm{Degradation}\mathrm{efficiency}\%=1-{\displaystyle \frac{C}{CO}}\times 100$$

(1)

where C denotes concentration at a particular time interval and C₀ represents initial concentration. This study examines the catalytic efficiency using a variety of experimental parameters, such as photodegradation time, catalytic concentration (0.1-0.6g/L), pesticide dosage (10-50 ppm) and pH (3,5,7,9 and 11). The pH of the solution can be changed using 0.1 KOH and HCl. The following formula is employed in the kinetic model to calculate the photocatalyst’s kinetic behavior and rate constant: ln (C/C₀) = k_app x t, where C₀ is the initial concentration and C is the concentration at a given time interval. The natural logarithm of C/C₀ is represented by ln (C/C₀) and the apparent rate constant (Kapp), which includes both the adsorption of Imidacloprid on the catalyst surface and the following photoreaction upon UV light irradiation, is the total degradation rate under pseudo-first-order kinetics. It measures how quickly the substance’s concentration drops over time, illustrating the combined effects of degradation, light intensity and the catalytic efficiency. The higher the value of (Kapp), the faster the reaction proceeds. It also provides information about the reaction order: t denotes the reaction time. The following formula can be used to determine the reaction’s half-life and rate constant: T_1/2= 0.693/ k_app,; where T_1/2 or half-life, denotes the amount of time needed for the concentration.

The term ln(C/C₀) represents the natural logarithm of the ratio between the instantaneous concentration (C) and the initial concentration (C₀). The apparent rate constant (kₐₚₚ) describes the overall degradation rate following pseudo–first-order kinetics, incorporating both the adsorption of imidacloprid onto the catalyst surface and its subsequent photochemical transformation under UV irradiation. This parameter reflects the rate at which the pollutant concentration decreases over time, accounting for the combined influence of catalytic activity, light intensity, and reaction conditions. A higher value of kₐₚₚ indicates a more rapid degradation process. The variable t corresponds to the reaction time.

The half-life (T₁/₂) of the reaction, defined as the time required for the pollutant concentration to decrease to half of its initial value, can be determined using the relation:

T₁/₂ = 0.693/kₐₚₚ,

where kₐₚₚ is the apparent rate constant.

For kinetic evaluation based on the Langmuir–Hinshelwood (L–H) model, the initial linear segment of the ln(C/C₀) versus time plot was considered. The slope of this region provides the value of kₐₚₚ, and the initial reaction rate (r₀) was subsequently calculated using Equation (2).

r_o =K_app C_o ……………

(2)

The L-H model shown in section 5.6 used these r_o values_.

2.6. Characterization of Structure, Morphology and Optics

The concentration of imidacloprid in solution was monitored using a UV–visible spectrophotometer (Specord 200 Plus, Germany), operating over a wavelength range of 190–1100 nm. The maximum absorption peak (λₘₐₓ) for the commercial Confidor formulation (containing 17% imidacloprid, a neonicotinoid pesticide) was identified at 269 nm, as illustrated in Figure S2. At predetermined time intervals, 3 mL aliquots were withdrawn from the reaction mixture to record absorbance values.

The crystalline phases of the synthesized photocatalysts were characterized by X-ray diffraction (XRD) using an X’Pert3 MRD diffractometer equipped with a Cu Kα radiation source, with data collected over a 2θ range of 10°–80°. Surface morphology was examined through scanning electron microscopy (SEM), while elemental composition was determined via energy-dispersive X-ray spectroscopy (EDS), with the corresponding results summarized in Table S1.

3. Results and Discussion

3.1. Optical Property

The optical behavior of MnZnFe₂O₄, SrWO₄, and MnZnFe₂O₄/SrWO₄ nanostructures was investigated using UV–Vis absorption spectroscopy, and their corresponding band gaps were evaluated using the Tauc method. In Figure 2, the absorption spectra of all three samples are indicated in the wavelength range of 200-800nm. A unique absorption edge is observed in each sample, indicative of its inherent electronic structure and transition properties. The MnZnFe₂O₄ nanoparticles exhibited two distinct absorption peaks at 450 nm and 620 nm, which are assigned to O²⁻(2p) → Fe³⁺(3d) ligand-to-metal charge-transfer (LMCT) transitions and weaker d–d crystal-field transitions of Fe³⁺ and Mn²⁺ ions occupying octahedral and tetrahedral sites, respectively. The fact that the XRD pattern does not show Fe^{2 +} means that no intervalence (Fe^{2 +} /Fe³⁺) transition is involved in the absorption; rather, the high intensity of the visible absorption is due purely to permitted ligand-to-metal charge transfer (LMCT) transitions as well as to defect-related surface states. The following equation 3 can be used to determine the interaction between the incident photon energy and the excess energy available to create electron-hole pairs in a semiconductor.

(αhν)n = A(hν − E_g)………………

(3)

where α is the absorption coefficient; hν is the photon energy (eV), which is the sum of the incident light (ν) and the planks constant (h); Eg is the optical band gap;. A stands for constant and n represents the quantity of electron-hole pairs formed. The direct optical band gap (E₉ = 1.8 eV) obtained from the Tauc plot, as shown in Figure 2, corresponds to electronic excitation from the O 2p valence band (HOMO) to the Fe 3d conduction band (LUMO). The fact that the band gap is relatively narrow with a high absorbance intensity is suggestive of efficient π→ π* and n→ π* transitions aided by centrally coordinated Fe³⁺. This extensive visible absorption corresponds to a bathochromic (red) shift compared to standard bulk ferrites, which is a consequence of strain on the lattice, redistribution of cations and increasing the orbital overlap (because of the nanoscale effect), which lowers the excitation energy and increases the absorption edge into the visible spectral range. Therefore, MnZnFe₂O₄ is an optically sensitive Fe ³⁺ controlled semiconductor in which Fe ²⁺ does not play a role. By comparison, the SrWO 4 nanostructures exhibited a single and strong absorption edge at 301 nm, which is typical of a wide-band-gap scheelite-type oxide. The XRD findings were in line with tetragonal phase of high crystallinity and zero impurity reflection. The UV absorption is based on the O 2p → W 5d (n → π*) charge-transfer transition, i.e., it is an excitation of electrons in the oxygen non-bonding (HOMO) orbital to tungsten antibonding (LUMO) orbital. The Tauc plot gave a direct band gap of 3.5 eV, which corresponded to intrinsic SrWO₄ which the Tauc plot verified that the strong UV absorption activity and little activity in the visible light region. The big HOMO-LUMO gap restricts optical excitation to larger photon energies, and this is the reason why the material is transparent in the visible spectrum. SrWO₄ has a distinctly hypsochromic (blue) shift at its absorption edge than MnZnFe₂O₄ (both of which have a smaller band gap and no defect-induced states) indicating a highly ordered electronic structure with minimal sub band gap transitions.

The MnZnFe₂O₄/SrWO₄ composite is a hybrid optical material with a combination of ferrite and tungstate transitions. The UV-Vis spectrum shows two large absorption bands, one of which is 266nm, which is the O 2p → W 5d (n → π*) transition of the SrWO₄, and the other one is 450nm, the O 2p → Fe ³⁺ (3d) (π→π*) transition of the ferrite material. This Tauc analysis based on the indirect band gap (E g = 2.4 eV) is between that of the parent oxides, indicating orbital hybridization between Fe ³⁺(3d) and W⁶⁺(5d) states at the interface. This hybridization produces shallow electronic states at the conduction band minimum, essentially lowering the excitation energy and increasing absorption in the visible light. The emergence of the lower energy absorption edge relative to pure SrWO₄ is a bathochromic (red) shift, which means that interfacial electronic coupling and charge delocalization reduces the band gap as a whole. The redshift also establishes a good transfer of charges across Fe-O-W junction with electrons exited in the O 2p HOMO state in the ferrite material is able to transfer to the hybrid Fe/W 3d-5d LUMO state which facilitates prolonged light collection. This composite, therefore, becomes a combination of the high UV response of SrWO₄ and visible absorption of MnZnFe₂O₄ to create a photo active material of a wide spectrum.

3.2. Functional Group Analysis

The FTIR spectra in Figure 3 clearly reflect the vibrational features of both the MnZnFe₂O₄, SrWO₄ and MnZnFe₂O₄/SrWO₄. Figure 3(b) demonstrates the FTIR spectra of nanocomposites with wave numbers between 400-950 cm^-1. The significant portion of the spectra is the vibrational area between 400 to 1000 cm^-1, which can be linked to the molecular vibration of the metal-oxygen bond in the photocatalysts. For the ferrite sample, two characteristic absorption bands are observed in the lower frequency region. The strong band at 560 cm⁻¹ corresponds to the stretching vibration of metal–oxygen bonds at the tetrahedral sites, while the band near 430 cm⁻¹ arises from the octahedral site vibrations of the spinel lattice. These two features are typical of Mn–Zn spinel ferrites and are consistent with earlier reports [24] The SrWO₄ phase is confirmed by the presence of a pronounced absorption peak at 820 cm⁻¹, which corresponds to the asymmetric stretching vibration of the W–O bonds in the WO₄²⁻ tetrahedral units. This peak position agrees well with previously reported FTIR data for scheelite-type SrWO₄ [12,25] indicating that the internal tetrahedral geometry of the tungstate group remains intact in both the pure SrWO₄ and the composite material. Broad O–H stretching and bending modes observed at 3400 cm⁻¹ and 1630 cm⁻¹, respectively, originate from adsorbed water on the particle surfaces, a common feature in oxide nanomaterials. Importantly, no additional absorption bands associated with impurity phases such as Fe₂O₃, FeOOH, or WO₃ are detected, indicating that the synthesized materials are phase-pure. The combination of ferrite (A–O/B–O) and tungstate (W–O) vibrational modes in the composite further supports the successful integration of both structures without chemical degradation.

3.3. XRD Analysis

The crystal structure and phase purity of Mn₀.₅Zn₀.₅Fe₂O₄, SrWO₄, and the SrWO₄/MnZnFe₂O₄ composite were examined by powder XRD (Cu Kα, λ ≈ 1.5406 Å). The diffraction patterns are shown in Figure 4. The diffraction pattern of Figure 4 (a) shows that SrWO₄ powder is completely different and can be indexed to a tetragonal scheelite structure (space group I4₁/a). Distinct peaks are observed at 2θ values typical of SrWO₄, which we assign to the (101), (004), (200), (211), (204), (220), (116), (312), (224) and (136) planes, in agreement with the scheelite-type earlier reports on nanocrystalline SrWO₄ with JCPDS 96-591-0264 (marked in red colored rectangle shape [12]. No additional peaks from SrO, WO₃, or other tungstate phases are detected, confirming that the co-precipitation route followed by mild calcination at 200 °C yields phase-pure SrWO₄ with good crystallinity. Similar diffraction behavior has been reported by Fathima et al. and by Chen et al. [26], who also observed intense (112) and (211) reflections for chemically synthesized SrWO₄. The XRD pattern of the MnZnFe₂O₄ sample exhibits a series of well-defined peaks at around 2θ ≈ 30°, 35°, 43°, 53°, 57°, 62° and 74°. These reflections can be indexed to the (220), (311), (400), (422), (511), (440) and (533) planes of a cubic spinel ferrite with space group Fd3̅m. The peak positions and their relative intensities agree well with the Mn₀.₅Zn₀.₅Fe₂O₄ reference pattern (JCPDS 96-230-0585) reported in the literature [27,28] This confirms that the main phase formed in our synthesis is monocrystalline MnZnFe₂O₄. Besides the spinel peaks, a set of much weaker reflections is also visible in the MnZnFe₂O₄ pattern at 2θ ≈ 24°, 33°, 35.5°, 40.8°, 49.4°, 54.0°, 57.5°, 62.4°, 63.9° and 71.9°. These can be indexed to the (012), (104), (110), (113), (024), (116), (018), (214) and (300) planes of rhombohedral α-Fe₂O₃ (hematite) [28]. Their very low intensity relative to the spinel (311) peak indicates that hematite is present only as a minor secondary phase in the as-prepared MnZnFe₂O₄ powder, most likely due to partial oxidation of iron during ferrite synthesis and calcination. MnZnFe₂O₄ prepared via sol–gel, co-precipitation, and combustion synthesis has repeatedly exhibited similar XRD signatures in the literature [29], confirming the reliability of the structural assignment.

XRD analysis of the MnZnFe₂O₄/SrWO₄ composite has shown that the diffractogram is a superposition of the respective phases’ patterns and that the appearance of new reflections of new phases that would indicate the formation of mixed or substituted phases is absent. This finding validates that the two constituents retain their unique crystallographic arrangements. The spinel ferrite typical peaks of the (220), (311), (400), (422), (511) and (440) planes are easily recognizable to indicate that the MnZnFe₂O₄ phase retains a cubic crystal structure after composite synthesis. At the same time, the archetypal scheelite reflections that can be indexed as (112), (004), (200), (204), (220), (116), (312) and (224) of SrWO₄ are also observed, which supports the fact that both materials coexist in the final specimen. The 2 theta values of these reflections are more or less similar to those reported in each of the individual pure phases, which indicates that there is no new bulk mixed phase present and it would be expected that the interaction between MnZnFe₂O₄ and SrWO₄ is a mostly interfacial interaction. The results have supported the development of a heterostructured composite in which the ferrite and tungstate phases retain their independent crystal structures. Similar behavior has been reported for ferrite-oxide composites, where the crystalline structures of the individual phases are retained during composite formation [23].

The minor hematite-related peaks, which could be seen in the pure MnZnFe2O4 sample, are not detectable in the composite structure. In particular, the diagnostic α-Fe₂O₃ reflections near 2θ ≈ 24° and 33° are absent. This disappearance can be attributed to surface encapsulation of the ferrite nanoparticles by the SrWO₄ shell, which suppresses the diffraction contribution of any surface-bound hematite, and secondly, the reduced Fe oxidation during the composite’s hydrothermal step, where the presence of WO₄²⁻ species offers a stabilizing chemical environment that discourages the formation of Fe–O–Fe clusters associated with hematite. Similar observations have been reported for ferrite–tungstate and ferrite–molybdate composites, where the secondary oxide layer inhibits surface oxidation [30,31]

The average crystallite sizes were estimated using the Debby Scherer equation 4.

$$D={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(4)

where,

K= Scherer constant (0.9)

λ= Wavelength of X-rays used (1.5406 A^o for Cu Kα)

β= Full width at half maximum (FWHM) value

θ= Angle of diffraction (2θ)

SrWO₄ exhibits a crystalline size of 54.8 nm, MnZnFe₂O₄ has 34.3 nm and MnZnFe₂O₄/SrWO₄ has 16.8 nm size calculated by using equation (2), while the percent crystallinity of MnZnFe₂O₄, SrWO₄ and MnZnFe₂O₄/SrWO₄ is 99.2%, 96.2% and 90% respectively. The percent crystallinity was calculated using equation (5). It represents how the atomic arrangement of material is ordered with respect to the ratio of crystalline to amorphous phase.

$$\mathrm{Percentage}\mathrm{of}\mathrm{Crystalinity}\left(\%\right)={\displaystyle \frac{\mathrm{Ac}}{\mathrm{Ac}+\mathrm{A}\mathsf{\alpha }}}\times 100$$

(5)

where,

Ac= area under crystalline peak

Aα= area under the amorphous peak

3.4. SEM and EDX Analysis

Scanning electron microscopy (SEM) was used to study the morphology and particle size distribution of the synthesized materials, revealing that each of the three samples, i.e., MnZnFe2O4, SrWO4, and the MnZnFe₂O₄/SrWO₄ composite, has a different morphology and particle size distribution. The SEM image of MnZnFe2O4 (Figure 5a) shows that the particles are agglomerated clusters of approximately spherical nanoparticles. The size distribution of the MnZnFe₂ O₄ material is between 25 nm - 40 nm with most particles being between 30 nm-35 nm as represented in the corresponding graph (Fig. d). Such a small distribution indicates that the synthesis procedure successfully generated comparatively homogeneous particles. Nevertheless, the existence of agglomeration as seen in the SEM images implies that the nanoparticles are prone to sticking to each other due to the existence of strong magnetic dipole-dipole forces, an observation that is in accord with other past experiments involving Mn-based ferrites, whereby a similar morphology of particles and moderate agglomeration was observed in MnFe₂O₄ prepared through the co-precipitation and solvothermal reactions. This kind of clustering has been highly reported in Mn-Zn ferrites due to their high saturation magnetization and surface energy [32] Even though agglomerated, individual particle boundaries can be identified, which means that primary crystal growth was not significantly hindered in the course of synthesis [33]. Though agglomerated, the individual particle boundaries remain visible, suggesting that primary crystallite growth was not significantly hindered during synthesis Figure 5b shows that the pure SrWO₄ sample has a clear faceted morphology with grain sizes of between 35 nm to 60 nm with an average size of 50 nm. Such polyhedral characteristics are typical of the tungstates of a scheelite type and crystallize with smooth edges as a result of the anisotropic rate of growth of WO₄^2- tetrahedrons [12] The relatively homogeneous grain boundaries and well-defined edges are an indicator of good crystallinity and is in line with the strong diffraction peaks in XRD. The nucleation density in SrWO₄ seems to be higher than in the ferrite and this is why the size distribution is somewhat wider when compared to MnZnFe₂O₄ [9]. A significant change in the morphology is observed in the composite MnZnFe ₂O₄/SrWO₄ (Figure 5c). The composite particle size distribution (Figure 5f) is significantly smaller, with a range of 5 to 30nm, with the most frequent occurrence in the range of 15 to 20nm. This smaller size and more homogeneous distribution indicate that the size of the SrWO4 shell is crucial for regulating the particle size of the MnZnFe₂O₄ heterostructure [34]. The reduction in particle size suggests that SrWO4 controls the formation of composites by inhibiting ferrite grain growth and enhancing nucleation rate. The fine structure is probably due to WO4-2 complexes on ferrite particles inhibiting their fusion and limiting their further growth. The same grain-refining effects are also witnessed in ferrite-oxide hybrid systems in which tungstate and molybdate phases are used to alter the growth kinetics. The smaller particle size in the composite means there are no large, phase-segregated particles representing ferrite or tungstate. The absence of discrete isolated domains implies that the composite cannot be a simple physical mixture or can be aggregated in separate domains [35]. The reduction at 28 nm (MnZnFe₂O₄) and 49 nm (SrWO₄) to 17 nm (MnZnFe₂O₄ /SrWO₄) supports this explanation. Surface area is usually increased by finer particles, which in turn increases photodegradation.

Figure 6 (a, b, c) shows the EDX spectra of the synthesized materials that prove the presence of all the expected elements. In the case of MnZnFe₂O₄, the formation of the spinel ferrite is confirmed by the formation of distinct peaks of Mn, Zn, Fe and O without any impurity phases. A small carbon signal is visible, which can be attributed to residual surfactant from the low-temperature synthesis with CTAB; this is consistently observed and has no negative effect on phase purity. The SrWO₄ sample exhibits strong Sr, W, and O peaks with an almost perfect Sr:W ratio, indicating that the scheelite phase has formed successfully. In the MnZnFe₂O₄/SrWO₄ composite, the simultaneous presence of Mn, Zn, Fe, Sr, W, and O confirms the coexistence of both phases. No extra elemental peaks were observed, demonstrating that the composite is chemically pure and that the synthesis process did not introduce any unwanted species. The XRD study nearly supports the EDS analysis of the photocatalysts.

3.5. Photoluminescence (PL) Analysis

Photoluminescence (PL) studies of synthesized nanomaterials MnZnFe₂O_4, SrWO₄, and MnZnFe2O4/SrWO4 are shown in Figure 7 to investigate the charge carrier recombination behavior. Data show a sharp peak for MnZnFe2O4 and SrWO4 at 362nm, attributed to the intrinsic emission from the electronic transition of ionized oxygen vacancies, cationic disorder in the spinel structure of MnZnFe2O4, and the scheelite-type structure of SrWO₄. These charge transfer transition state of Fe⁺³/Mn⁺² → O^-2 and O^-2 → W⁺⁶ promote radiative recombination which lead to poor photocatalytic performance. However, the composite of MnZnFe₂O₄/SrWO₄ peak slightly shifted to 364nm with dramatically quenching PL intensity, indicating efficient interfacial charge transfer and suppressed photo carrier recombination.

4. Photocatalytic Degradation Mechanism of Imidacloprid Using MnZnFe₂O₄/SrWO₄ Catalyst

Light absorption, charge separation, and surface redox reactions all play a role in the synergistic heterojunction-driven imidacloprid photocatalytic degradation over the MnZnFe₂O₄/SrWO₄ composite. MnZnFe₂O₄ and SrWO₄ both generate electron–hole pairs when exposed to UV light. At the MnZnFe₂O₄/SrWO₄ junction, a built-in interfacial electric field is established due to the favorable band alignment between the two semiconductors. This field prevents charge carriers from recombination and encourages directional migration of charge carriers [6]. Photogenerated electrons preferentially accumulate in the MnZnFe₂O₄ rich region’s conduction band, where they react with dissolved oxygen to produce superoxide radicals (•O₂), which can further transform into H₂O₂ and ultimately produce hydroxyl radicals (•OH), which are highly reactive. Photogenerated holes move toward the SrWO₄ rich region at the same time and directly oxidize surface-adsorbed water molecules or hydroxide ions to produce additional •OH radicals [21]. The predominant oxidative agents that are responsible for attacking the imidacloprid molecules that are adsorbed on the surface of the catalyst are these reactive oxygen species, particularly •OH. The radicals attack imidacloprid’s electron-rich sites first, like the nitroimine group, heterocyclic rings, and chloropyridinyl moiety. This causes stepwise bond cleavage, ring opening, and the formation of intermediate fragments that are gradually mineralized into benign end products like CO₂, H₂O, and inorganic ions (like NO₃/NH₄⁺ and Cl^-) shown in Figure 9. Overall, the heterojunction architecture’s effective interfacial charge separation, extended carrier lifetime, and increased generation of reactive oxygen species all contribute to MnZnFe₂O₄/SrWO₄ composite a promising photocatalyst for environmental remediation, especially in pesticide-contaminated water. To verify the photodegradation process FTIR spectroscopy was employed in Figure 8.

Figure 8. FTIR spectrum of the product produced by imidacloprid photodegradation compared to the original imidacloprid.

Figure 8. FTIR spectrum of the product produced by imidacloprid photodegradation compared to the original imidacloprid.

Figure 9. Schematic diagram illustrates the proposed photocatalytic degradation mechanism of imidacloprid using MnZnFe₂O₄/SrWO₄photocatalyst.

Figure 9. Schematic diagram illustrates the proposed photocatalytic degradation mechanism of imidacloprid using MnZnFe₂O₄/SrWO₄photocatalyst.

The O-H stretching band at 3402 cm^-1 slightly overlapped the N-H stretching vibration of the imidazoline ring at 3338 cm^-1 in the pristine IMI spectra. Additionally, absorption peaks at 1558 cm^-1, 1236-1277 cm^-1, and 1439 cm^-1 represent the pyridine ring’s C=N, NO₂ and C=C stretching vibrations. Furthermore, the presence of peak at 2996 cm^-1 for tetrahedral carbon and aryl Cl at 1108 cm^-1 depict the existence of pyridine ring. Additionally, stretching peak at 1664 cm^-1 for C=N of imidazolidine. Following flourishing of IMI spectra observe peaks at 3423 cm^-1 for H-O-H stretching and bending vibration due to aqueous medium while N-H stretching at 1620 cm^-1indicate detection of byproduct [36].

The following equations (3-8) provides a complete picture of the green photocatalytic degradation of the imidacloprid pollutant using MnZnFe₂O₄/SrWO₄

• Light sensitization:

Light + MnZnFe₂O₄/SrWO₄ → e^- +h⁺………………. (3)

2.

Redox Process

a)

Reduction Site (electron warehouse: MnZnFe₂O₄)

e^- _CB + O₂ → •O₂⁻………………. (4)

•O₂⁻ ↔ •HO₂ → H₂O₂ ………………. (5)

H₂O₂ + e^- → •OH + OH^- ………………. (6)

Zn⁺²+ e^-→Zn⁰

Photo-Fenton reaction

Fe⁺² + H₂O₂ → Fe⁺³ + •OH + OH^-

Mn⁺² + H₂O₂ → Mn⁺³ + •OH + OH^-

• b) Oxidation site (holes factory: SrWO₄)

h⁺ _VB + H₂O/ OH^- → •OH + h⁺ ………………. (7)

3.

Removal of Pollutant

C₉H₁₀ClN₅O₂ + •OH → CO₂ + H₂O + NO₂^- + Cl^- NH₄⁺ ………………. (8)

5. Imidacloprid Degradation Study

Imidacloprid photocatalytic degradation was investigated by optimizing time, catalytic concentration, pH, dosage of pollutant and comparative evaluation of catalytic efficiency parameters.

5.1. Photocatalytic Efficiency Comparison

Figure 9 shows the comparative degradation efficiencies of Imidacloprid in the absence of any catalyst and in the presence of SrWO 4, MnZnFe₂O₄ and MnZnFe₂O₄/SrWO₄ nanocomposite under dark for 24 hrs. and a 30 minute under ultraviolet (UV). The photocatalytic efficiency of different catalyst exhibits different rate of efficiency based on their composition, ability to absorbed light. In this factor comparison of catalyst was carried out using 30 ppm solution containing Imidacloprid with 20 mg of photocatalyst under 9 pH. In the absence of a catalyst, the system only degrades by a fraction of about 7% in darkness (increasing to 18% during UV light, however, this confirms the very low role of direct photolysis in Imidacloprid removal). Pure SrWO₄ exhibits low activity with dark and UV removal of about 20 and 37 percent respectively. This low performance is due to its wide band gap (approximately 3.5 eV) that limits the absorption of photons and promotes quick electron-hole recombination, despite its good crystallinity.

MnZnFe₂O₄ is better active (approximately 47 percent in the dark and approximately 75 percent under UV light). Its narrower band gap (~1.8 -1 eV) leads to greater effectiveness with respect to light absorption and generation of charge-carriers. However, partial nanoparticle agglomeration (~28 nm) is observed as a result of SEM, which limits agglomeration access to surface-active sites and hence the overall catalytic efficiency. The highest degradation rate is obtained with MnZnFe₂O₄/SrWO₄ nanocomposite, reaching the values of about 52 and 87, respectively, in darkness and under UV radiation, respectively. Moderate dark remediation is explained by the fact that its surface area is increased due to a decrease in particle size (~17 nm). The radical improvement of the heterojunction structure is supported by the strong improvement when the structure works under UV light. X-ray diffraction ascertains coexistence of phases, and no contaminant is formed, and the optimum separation band gap (2.4 eV) and strong Fe-O-W interfacial connectivity promotes effective separation of charges. The repressed recombination, which is shown by the photoluminescence quenching, increases the carrier lifetime and enhances the generation of reactive oxygen species.

The optimization of band structure, the transfer of charge caused by heterojunction, and the refinement of nanoscale morphology are all directly correlated in the degradation trend of MnZnFe₂O₄/SrWO₄ > MnZnFe₂O₄ > SrWO₄ > no catalyst. Therefore, the high performance of the composite can be explained because of the synergistic combination of structural integrity, electronic properties optimization, and improved surface reactivity making the composite the best photocatalyst in degrading Imidacloprid under UV irradiation.

Figure 9. Comparative percentage degradation efficiency of MnZnFe₂O₄, SrWO₄, and MnZnFe₂O₄/SrWO₄ catalysts.

Figure 9. Comparative percentage degradation efficiency of MnZnFe₂O₄, SrWO₄, and MnZnFe₂O₄/SrWO₄ catalysts.

5.2. Time Exposure’s Impact on Imidacloprid’s Photodegradation Efficiency

The influence of irradiation time on the photocatalytic degradation of IMI was studied by dispersing 0.2 g L^-1 MnZnFe₂O₄/SrWO₄ of imidacloprid (30 mg L^-1), subsequently followed by the continuous stirring for 50 min at pH 9. Figure 10(a) showed that under UV light exposure degradation shown up to an extent of 87% within half an hour. It is noteworthy that the degradation pattern increase significantly in first 30 minutes due to formation of reactive oxygen species and charge separation, after that a slight or statistically insignificant degradation was observed due to either the total breakdown of IMI molecules or the restricted availability of active sites [37]. All of the data is an average of three independent replicates (n = 3) with error bars showing the standard deviation that can be attributed to normal experimental variability.

5.3. Impact of pH on Imidacloprid Photodegradation

The impact of pH on the photocatalytic degradation of imidacloprid (30 mg/L) using 0.2 g/L MnZnFe₂O₄/SrWO₄ under UV light is depicted in Figure 10(b). The pH of solution was adjusted to 3,5,7,9 and 11 using 0.1M HCl and NaOH. The electrostatic interaction between the catalyst surface, charged radicals and pollutant molecules can be effected by pH variations, which can change the effectiveness of photocatalytic degradation. Under the influence of UV light data observed maximum degradation % at pH 5, 9, and 11 are higher than degradation efficiencies at pH 3 and 7 during the first irradiation stage (15 min) suggesting that slightly acidic to alkaline environments are favorable to the initial stage of reaction. With the increase in the duration of irradiation (30-60 min), a noticeable difference is formed. Optimal degradation activity is at pH of 9, where about 87% removal was obtained in 30 min. This fact proves that pH 9 is the best condition of photocatalytic degradation. This increased performance at pH 9 can be attributed by increased concentration of OH radicals. The OH radicals are able to target the electron-deficient sites of Imidacloprid. At this point -NO₂ electron withdrawing group present in IMI associate with imidazolidine ring induced partial positively charge on the adjacent –C=N, rendering it more susceptible to nucleophilic attack by OH^- ionic group to fasten the process of hydrolysis [38]. Consequently, at 11 pH show minimum degradation efficiency owing to excessive generation of OH^-, which in turn start repealing and does not allow the reactive radicals to interact effectively with the pollutant particles [19]. Under acidic conditions (pH 3) the efficiency of degradation is relatively low, probably because of reduced production of hydroxyl radical and possible recombination of charge carriers in the proton rich medium

5.4. Imidacloprid Dosage Effect on the Photodegradation of Imidacloprid

The impact of commercially available Confidor (Imidacloprid) concentration governed by mass transfer and concentration gradient was evaluated from 10-50 ppm treated with 0.1 g MnZnFe₂O₄/SrWO₄ catalyst under 9 pH in Figure 10 (c). The rate of degradation shows a definite dependence on the concentration of pollutants and reaction time. The IMI sequestration efficiency for MnZnFe₂O₄/SrWO₄ catalyst increased with increasing initial dosage up to 40 ppm, followed by the gradual decrease, in contrast data observed optimum at 40 ppm with removal rate of 80%. This efficient performance is driven by accessibility of surface active site. However after the optimal point, at high concentration pollutant loading increases in the solution lead to diffusion limitation and more competition for the scarce active site, which in turn restrict the generation of hydroxyl ion and radical play a pivotal role in photodegradation [20,39]. In addition saturation of solution are responsible for the deactivation of sites due to slow diffusion of IMI molecules from catalytic surface [40,41]. Based on the final degradation efficiency, the performance of different Imidacloprid dosages (10-50 ppm) are in the following order: 40 ppm (80%) > 30 ppm (76%) > 20 ppm (68%) > 10 ppm (57%) > 50 ppm (56%) On the other hand, as the concentration is increased further to 50 ppm, the degradation efficiency is low as compared to that at 40 ppm. With that great amount of pollutants loaded, the amount of Imidacloprid molecules exceeds the active sites available on the surface of the catalyst. Since the reactive oxygen species (•OH, •O₂^-) generation is regulated by a constant load of catalysts, light intensity and the time taken to irradiate the sample, the generation of radicals is constant. Taking this into consideration, too much concentration of pollutants leads to inadequate availability of radicals to degrade fully. Also, an increase in concentrations can facilitate the presence and concentration of intermediate species on the catalyst surface, thus inhibiting active sites and lowering photocatalytic performance [42].

5.5. Initial Catalytic Concentration for Imidacloprid Degradation

The degradation efficiency of IMI is shown in Figure 10 (d) which was evaluated over the range of 0.1g/L-0.6g/L MnZnFe₂O₄/SrWO₄ catalytic composite added in 10 ppm solution of Imidacloprid under UV light. The degradation efficiency progressively increases from 0.1-0.5 g/L. This increase is explained by the fact that the availability of active surface sites is increased and leads to a more efficient adsorption of imidacloprid molecules and an increase in the formation of photogenerated electrons and holes. This means that if the active size increases, the more there would be the generation of ROS (reactive oxygen species; ∙OH and ∙O₂^-) and thus the oxidative degradation of the pollutant will be more efficient. However, a significant decrease is observed when the amount of MnZnFe₂O₄/SrWO₄ further increase to 0.6 g/L. The following is the order of catalysts photocatalytic efficacy at various dosages:

0.5 g/L (94%) > 0.6 g/L (76%) > 0.4 g/L (75%) > 0.3 g/L (64%) > 0.2 g/L (52%) > 0.1 g/L (43%)

Removal efficiency of pollutant primarily influenced by presence of active sites on the catalyst surface. However, beyond a certain threshold, decrease in degradation attributed to nanoparticles agglomeration and impending penetration of photon flux (e^- and h⁺ ), reduced light penetration due to increased turbidity [43,44]. Furthermore, active sites become saturated, reaching equilibrium lead to the desorption of organic pollutant

5.6. Kinetic Analysis of Reaction

IMI degradation by photocatalysis is observed over an irradiation period of 30 min -1hr by plotting linear regression line ln (C/C_o) vs t, thus system followed first order of reaction kinetic in Figure 11 (a, b, c). The degradation rate constant of 10, 20, 30, 40, and 50 ppm were 0.015 min^-1, 0.032 min^-1, 0.037 min^-1, 0.04 min^-1 and 0.02 min^-1, so, 40 ppm found the efficient results with the half-life of 17.3 min. Furthermore, removal of pollutant in the presence of 0.1 g, 0.2 g, 0.3 g, 0.4 g, 0.5 g, 0.6 g, catalyst reveal the fitting rate constant (K) were 0.014 min^-1, 0.026 min^-1, 0.026 min^-1, 0.038 min^-1, 0.065 min^-1, 0.037 min^-1. Under the light exposure value of K increases to 0.065 min^-1 corresponding to the half-life of 10.66 min which reduced to 18.7 min as the rate of reaction accelerated to 0.037 min^-1. This pattern of fitting rate constant indicate overuse of MnZnFe₂O₄/SrWO₄ nanocomposite are responsible for inducing light shielding effect which lead to decrease in removal efficiency of IMI. As the pH of solution increases from 3, 5, 7, 9 and 11 in Figure 11 (c), show linear relationship by applying Fitting kinetics (K) 0.016 min^-1, 0.024 min^-1, 0.024 min^-1, 0.047 min^-1, and 0.028 min^-1. The Study reveal role of H⁺ and OH^- in photocatalytic degradation mechanism. It is obvious to select 9 pH with the quite low half-life of 14.7 min for the followed up experiment in basic condition due to enhancement of OH^- which involve in nucleophilic attack on imine (C=N) or (N-NO₂) of imidazolidine group. Pseudo first order also confirmed by using linear plot of Langmuir–Hinshelwood (L-H) kinetic model (1/r⁰ vs 1/C^o) along with to analyze heterogeneous surface of MnZnFe₂O₄/SrWO₄ catalyst under UV light in equation 9. This kinetic model reveal the whole mechanistic description consist of pollutant adsorption on the active sites attain by stirring the reactant solution for 10 min in the dark, subsequently followed the interaction of IMI and photo charge carrier of catalyst and finally desorption of degraded pesticide.

$${\displaystyle \frac{1}{r_0}}={\displaystyle \frac{1}{\mathrm{ҡ}}}+\left({\displaystyle \frac{1}{\mathrm{ҡ}K}}\right){\displaystyle \frac{1}{C_0}}$$

(9)

The linear plot yields an apparent rate constant of (K_app) of 14.7 min with an excellent correlation coefficient (R²=0.99) in Figure 12. However, intrinsic Surface reaction constant ҡ (0.06 mg/L min^-1) and moderate adsorption affinity value of K (5.8 L^-1 mg). This relative high Langmuir adsorption and favorable surface reaction demonstrate efficient catalytic adsorption and surface reactivity confirming its stability for degradation.

5.7. Reusability of the Photocatalyst for Imidacloprid Degradation

For the economical removal of pesticide from the aqueous solution necessary to analyze the effectiveness and chemical stability of catalyst under the optimal condition. The reusability of catalyst was evaluated over five consecutive cycle seen in Figure 13, yielding 72% removal at 30 ppm of IMI dosage. FTIR spectra of MnZnFe₂O₄/SrWO₄ in figure confirm the sustainability after five recycle, which make it attractive and cost-effective for commercial use. At the completion of each cycle Catalyst recovered by centrifugation, thoroughly washed using distilled water and dried in oven for 1hr at 60 C^o. The observed reduction in photocatalytic activity preliminary concerned with the loss during recovery of catalyst, aggregation of nanoparticles which in turn restrict sequestration and progressive pour fouling by intermediates.[45].

5.8. Mineralization Study of Imidacloprid

The degree of mineralization of imidacloprid after the photocatalytic degradation of Imidacloprid using MnZnFe₂O₄/SrWO₄ nanocomposite was analyzed using total organic carbon (TOC) (30 0.2 g L -1 catalyst, pH 9). The percentage removal of the TOC was determined using the following equation 10:

$$\%\mathrm{TOC}\mathrm{removal}=\left({\displaystyle \frac{\mathrm{TOCo}-\mathrm{TOCt}}{\mathrm{TOCo}}}\right)\times 100\dots \dots \dots \dots \dots$$

(10)

TOC₀ is the TOC prior to UV irradiation, whereas TOC_t represents the TOC at a certain irradiation time. Figure 14 shows the removal of Imidacloprid (30 ppm/L) at pH 9 with 0.2 g/L of photocatalyst after 30 min in the UV light using Total Organic Carbon Analyzer (TOC) (Bk-TOC (1500), China). The initial batch of imidacloprid solution with 100mL at the time zero had 515 µg/L of total carbon. After 10 min UV exposure, the TOC reduced to 390 µg/L which was equivalent to mineralization level of 24%. Further irradiation reduced the TOC to 250 µg/L (51% removal) in 20 minutes.

There was a strong decrease in the first 30 minutes during which the TOC was 155 µg/L and this was almost the same as 70%. After 30 min, the decrease in TOC was slower, which means that most of the mineralization was already obtained. The values of the TOC at 40, 50 and 60 min were 148 µg/L (71 percent), 145 µg/L (72 percent) and 142 µg/L (73 percent), respectively. The TOC results indicate that MnZnFe₂O₄/SrWO₄ does not only demineralize the parent Imidacloprid molecules but also oxidizes organic carbon to final inorganic products. Whereas UV-Vis analysis demonstrated that 87% degradation in 30 min and TOC elimination was approximately 72% which indicate good but incomplete removal. This is due to the fact that Imidacloprid is effectively mineralized into a comparatively less hazardous chemical with little organic residue or intermediate material that may not be fully oxidized.

5.9. Comparison of Photocatalyst with Previously Reported Photocatalyst

Table 1 compares the photocatalytic degradation efficiency of the MnZnFe₂O₄/SrWO₄ photocatalyst to other previously reported photocatalysts when exposed to UV light. After 30 minutes, the MnZnFe₂O₄/SrWO₄ material (0.2 g/L) is able to photodegrade Imidacloprid (30 ppm) with an efficiency of 87 percent in pH 9. It has a rate constant of 0.047 min^-1.

It has been reported by Tariq et al. [20] that Ag₂O/CuO nanocomposites may break down imidacloprid (20 mg/L) by 92.3% after 180 minutes with a 0.3g/L catalyst and a rate constant of 0.0031 min^-1. The removal efficiency is also very high but the degradation rate constant is much lower and thus long irradiation is required. Recently Alqarni [46] achieved 98.1% photodegradation efficiency of Imidacloprid (10mg/L) at 90 min using 0.5 g/L of TiO₂/PHEMA ( poly(2-hydroxyethyl methacrylate) catalyst with a rate constant of 0.0377 min^-1 whereas the ZnO [6] photocatalyst has reached an efficiency of 95% at same pollutant concentration at 60 min with a rate constant of 0.038 min^-1. It has also been demonstrated that carbon-based systems, such as CDs-QDs/CQDs, can degrade imidacloprid (200 mg/L-1) at a rate of 0.021 mins-1 with an 85% degradation efficiency using a 0.4g/L catalyst. With a catalyst concentration of 1g/L-1, CQDsSH/CdS QDs were reported to be 92% efficient with imidacloprid (10 mg/L-1) in 90 minutes; however, the rate constant of degradation was not specified. Recently the DyMo@MnFe₂O₄ showed the 87% degradation in 60 min while in contrast in current research work the 0.2 g MnZnFe₂O₄/SrWO₄ nanocomposite loading demonstrate 87% degradation of IMI within significant UV light exposure of 30 min. Collectively, these findings highlight the advantages of UV light response, sustainability and practicality of proposed catalyst over the previous reported.

6. Conclusions and Future Perspectives

The current investigation shows that manganese-doped ferrite–tungstate nanocomposites exhibit excellent photocatalytic performance for the degradation of imidacloprid under UV irradiation. The optimized catalyst dosage (0.2 g L⁻¹) achieved up to 87% degradation of 30 mg L⁻¹ imidacloprid within 60 minutes at pH 9, following pseudo-first-order kinetics with a rate constant of 0.031 min⁻¹. The enhanced photocatalytic efficiency is attributed to the narrow band gap (~2.5 eV), increased surface area, and improved charge separation resulting from heterojunction formation and synergistic manganese doping, which collectively suppress electron–hole recombination.

Photoluminescence analysis further confirms reduced recombination rates and efficient generation of reactive oxygen species, contributing to improved photocatalytic activity. The nanocomposite also exhibited good stability and reusability, retaining approximately 72% of its activity after five successive cycles, along with facile magnetic recovery.

Overall, these findings highlight the potential of the developed nanocomposite as a promising and sustainable photocatalyst for wastewater treatment. Future research should focus on scalable, cost-effective synthesis; evaluation under natural sunlight; long-term stability in complex wastewater matrices; and extension to a broader range of organic pollutants to validate its practical environmental applicability.