Solar-Light-Driven Photodegradation of Eosin Y Dye Using Green-Synthesized Ag/S/ Modified Fe3O4 Nanocomposites with Enhanced Photocatalytic Activity

INTRODUCTION

The ongoing release of untreated industrial waste into bodies of water has become a global problem that threatens both the health of people and the balance of ecosystems [1,2]. Some of the most dangerous chemicals in such effluents include synthetic dyes, which the textile, paper, and pharmaceutical industries extensively use [3,4]. EosinY, a derivative of xanthines, and Benzidine, a powerful derivative of aromatic amines, are specifically mentioned as important polluters of the environment due to their high stability, low biodegradability, and potential to accumulate in the food chain [5,6]. Not only do these dyes raise the chemical oxygen demand (COD) of water bodies, but they also inhibit the penetration of sunlight, which is critical in the survival of aquatic organisms through photosynthesis [7,8]. Since they have the potential of being mutagenic and carcinogenic toxins, eliminating these recalcitrant molecules in wastewater has become a high-priority activity in the environmental engineering discipline [9,10]. The traditional remediation processes, including the biological treatment, coagulation, and adsorption, usually do not provide enough tools to completely mineralise these dyes or to produce secondary waste [11]. Therefore, the Advanced Oxidation Processes (AOPs), especially heterogeneous photocatalysis, have been identified as an extremely efficient and sustainable substitute, capable of producing powerful radicals to break down the organic pollutants into harmless final products such as CO₂ and H₂O [12,13]. Within photocatalytic materials, magnetite (Fe₃O₄) nanoparticles have become eminent due to their high surface area, low toxicity and their magnetic property that enables rapid recovery under the influence of an external magnetic field [14,15]. Nevertheless, the practical use of pure Fe₃O₄ is often hampered by the tendency of photogenerated charge carriers to recombine rapidly and the limited activity of photogenerated charge carriers in the visible light spectrum [16,17]. To overcome these problems, researchers have extensively studied how non-metal dopants hybridize with noble metals. The addition of silver (Ag) nanoparticles makes it possible to form Schottky barriers and harness the Surface Plasmon Resonance (SPR) effect, used to greatly enhance the separation of electrons and holes and the absorption of visible light [18,19]. In addition, the band structure of the catalyst can be modified with the addition of Sulfur (S) and surface stability, preventing particle aggregation and creating more active sites, which can be obtained by the addition of EDTA as a modifying agent [20,21].  In line with the principles of green chemistry, synthesis of these hybrid nanocomposites has changed towards using plant-based extracts, which are environmentally-friendly reducing and capping agents, thereby avoiding the use of toxic chemicals [22] The use of this plant-mediated technique not only provides a production process that is environmentally friendly but also improves the catalytic performance of the materials that are produced as a result of the synergistic action of natural phytochemicals [23,24]. The study is about the solar-light-driven photodegradation of Eosin Y using a unique green-synthesized Ag/S-modified Fe₃O₄ nanocomposite. These enhancements act as a starting point for the present study. This study aims to take advantage of high sun radiation during sunny days to develop an innovative, economical, and recyclable solution for efficient treatment of industrial dye contamination.

MATERIALS AND METHODS

Collection of Plant

Brassica napus leaves were harvested in the Abu Ghraib region of Baghdad, Iraq, in April and May 2025. The plant material is analysed and categorized at the Desert Studies Centre of Anbar University. After removing impurities, the leaves were dried in the shade at room temperature, then it was powdered, packed in opaque containers with airtight lids and stored until later study.

Chemicals of Study

Fe (NO₂)₃·6H₂O and 25% NaOH were purchased from Fluka and used in the experiment. Throughout the investigation, deionized water (DI) was consistently used. Use of silver nitrate (AgNO₂) and Sodium sulphide (Na₂S₂), with 98% analytical purity. Chemicals were obtained from the Sigma-Aldrich company.

Extraction of Crude Extract from Brassica napus

Brassica napus leaves were used to obtain the plant extract. The leaves have been dried, powdered and washed with an electric mixer. All the particulates that were dispersed were removed by sieving. Add 10 g of the powder to a beaker, then add 200 mL of (DI). A magnetic stirrer was used to stir the mixture continuously. Bring to 80°C, stir for 30 mins to obtain the brown extract, and cool. Filtration of the solution ensued. The plant extract was stored in a tumbler bottle in the refrigerator until it was required for use [25].

Green Synthesis of Iron Oxide Nanoparticles (Fe₃O₄)

This experiment was carried out with some slightly modifications. Dissolve 2.2 g of Fe (NO₃)₂.6H₂O in 200 mL of deionized water in a beaker, using a magnetic stir bar. Then carefully add 20 mL of the prepared plant extract. Then, use a burette to add the extract dropwise while stirring vigorously at 20°C. Thereafter, adjust the pH by adding 1 M NaOH and stirring well while increasing the temperature to 80°C. This process will result in a brown precipitate, which is described below. Allow the precipitate to reach room temperature, then centrifuge at 1200 rpm to remove any remaining fibres.

Filter through Whatman No. 1 paper and wash three times with deionized water, followed by a wash with 100% ethanol to remove contaminants; air dry overnight in an oven at 60°C for 2 hr.[25].

Preparation of Ag/S-Fe₃O₄ Nanohybrids

The nanohybrids are synthesized using an impregnation technique with slight modifications to obtain Ag, and S nanoclusters for the nanohybrid production. Add 5 M AgNO₃, Na₂S₂, and Fe₃O₄ nanoparticles (green synthesis) in 50 mL DI water. Stir the aggregate in at 80°C darkly. Two hr of synthesis are required at the temperature. The pellet containing particles of Ag, and S-Fe₃O₄ nanohybrids was washed three times with DI water to remove these nanohybrids from the pellet by centrifuging at 1200 rpm for 15 min at 20°C. Ag/S-Fe₃O₄ were purified, oven-dried at 60°C overnight and then powdered in a mortar and pestle [26].

Preparation of Eosin Y and Dye Solution

A certain amount of Eosin Y and dye was separately dissolved in 100 mL of DI to prepare a 1000 ppm stock solution. Then a 20-ppm working solution was made by taking 2 mL of the stock solution and diluting it to 100 mL with DI.

Spectrophotometric Determination and Calibration

For quantitative monitoring of the photodegradation process, a set of standard solutions of Eosin Y dye were prepared, ranging from 5 to 25 ppm. The UV-Vis absorption spectra showed a clear maximum absorption wavelength, λ_max, which was 516 nm, as shown in Fig. 4. Thus, the same wavelength was used throughout the remainder of the construction of the standard calibration curve shown in Fig. 5.

Calibration Curve and Quantitative Analysis of Eosin Y dye:

The analytical data showed a strong linear relationship between absorbance and concentration with a correlation coefficient (R) of 0.9995, which was satisfactory and in good agreement with the Beer-Lambert law. The resulting linear regression equation is:

Abs = 0.0597 · C + 0.0010

MATERIALS AND METHODS

Photocatalytic Degradation Procedures

The photocatalytic activity of the synthesized nanohybrids (Fe₃O₄, Ag/Fe₃O₄, and S/Fe₃O₄) was evaluated by the degradation of Eosin Y under natural solar light. The effect of medium acidity/alkalinity was investigated at three different levels: pH 3, pH 7 and pH 8, which optimized the catalytic performance.

The reaction Setup and Optimization of pH

In each experimental run, 0.1 g of the photocatalyst was added to 100 mL of the dye solution (20 ppm). The following procedure was carefully followed:

1- pH Adjustment: The pH of the dye solutions was adjusted to the desired values of 3, 7 and 8 with small quantities of the solutions of HCl and NaOH. The pH was adjusted by adding HCl (0.1 N) or NaOH (0.1 N), and it was measured by a pH meter.

2- Dark Adsorption Phase: The suspension was stirred in the dark for 30 min on a magnetic stirrer to attain adsorption-desorption equilibrium. Thus, the subsequent drop in absorbance was solely attributed to photocatalytic degradation and not just physical adsorption.

3- Solar Irradiance: The stabilized suspensions were then subjected to the direct sunlight at the highest level of radiation (12:00 PM – 3:00 PM) at 49 to 52°C. A magnetic stirrer was used continuously during the irradiation time to obtain a uniform suspension of the catalyst particles and to increase the interaction between the catalyst particles and the incident solar photons.

Sampling and Instrumental Analysis

Time Intervals: Aliquots of 5 mL were collected periodically every 30 minutes (0, 30, 60, 90, 120, 150, and 180 min). Centrifugation: The samples were centrifuged immediately after each reaction to separate the solid photocatalysts and end the reaction. Absorbance measurements were used to measure residual concentration, which was obtained with a UV-Vis spectrophotometer. Absorbance values at t=0 (A₀) and at the different times of irradiation (Aₜ) were measured at the λmax for Eosin dye.

The efficiency and Rate of Reactions/ Calculated

The degradation efficiency (%R) was calculated using the following expression:

The pseudo-first-order kinetic model was used to appraise the reaction rates.

The rate constant k was computed from the slope of the linear plot of the data ln(A₀/Aₜ) vs irradiation time t [27].

Characterization

The main goal of these measurements was to characterize the shape, morphology and structure of the synthesized nanomaterials. The change in colour of the reaction mixture was visually observed, and this change in colour indicates the formation of nanoparticles. FTIR spectroscopy was performed to identify functional groups, which were present in Fe₃O₄-NPs and nanohybrids, in the range 400-4000 cm⁻¹ [28]. X-ray diffraction (XRD; Panalytical X'Pert Pro) with Cu-Kα radiation (λ = 1.540 Å, 40 keV, 15 mA) was performed on the as-synthesized samples by performing scans in the 2θ angular range from 10° to 80°. The synthesized Fe₃O₄-NPs were characterized with regard to size and shape by using transmission electron microscopy (TEM) operated at 200 kV. Moreover, the field-emission scanning electron microscopy (FESEM; JEOL-JSM-7600F) was used to investigate the morphology and surface topography of the biosynthesized nanoparticles and nanocomposites [29].

RESULTS AND DISCUSSION

FTIR Spectral Analysis

The (FTIR) spectra of magnetite (Fe₃O₄) nanoparticles and their nanohybrids (Ag/Fe₃O₄ and S/Fe₃O₄). A wide absorption band between 3475 and 3415 cm⁻¹ is due to O–H stretching vibrations. This shows that phenolic compounds and flavonoids are present and are acting as reducing and stabilizing agents. This is in line with recent progress in the synthesis of nanoparticles using plants [30]. The peaks at 16351616 cm⁻¹ are either because of the stretching of C=O bonds in amides or vibration of C=C bonds in aromatic rings. This implies that the proteins and polyphenols are sticking to the surface of the nanoparticles, forming a stable layer that covers the nanoparticles. When these molecules stick to the surface of the nanoparticle, they seem to form this layer, which is called a bio-capping layer [31]. The band at a position of 1558 cm⁻¹ is designated to the N-H bending (amide II), thus showing that nitrogen-based biomolecules are used to stabilize nanoparticles. The peaks between 1382 and 1338 cm⁻¹ are due to C-N stretching or O-H bending, and the peaks between 1109 and 1067 cm⁻¹ are because of C-O stretching vibrations. This implies that alcohols and polysaccharides exist and assist in maintaining the stability of the colloids. Extra bands in the 999--850 cm -1 range indicate that the structure has altered due to the addition of silver and its reaction with the Fe₃O₄ matrix. This is in line with recent research on metal–magnetite nanocomposites [32]. The strong absorption bands seen in the low wavenumber range (628–470 cm⁻¹) are typical of Fe–O stretching vibrations, which show that crystalline magnetite has formed. Notably, slight changes in the positions and intensities of the peaks in the nanohybrids compared to pure Fe₃O₄ show that surface functionalization and interfacial interactions have worked, especially in the S/Fe₃O₄ system. This study follows the recent reports, which put a lot of emphasis on the role of surface engineering in enhancing the performance of nanoparticles. These results prove the efficiency of green synthesis using Brassica napus to produce stable and functional nanostructures with promising applications in nanotechnology.

Transmission Electron Microscopy (TEM) Analysis

The quasi-spherical form of synthesized green Fe₃O₄-NPs is densely clustered. Much of this aggregation could be due to the high surface energy of magnetite nanoparticles and the strong magnetic dipole-dipole interactions. This partial aggregation may be due to the fact that the phytochemicals used for reduction and capping (phenolics, flavonoids, proteins, etc.) are not as effective as synthetic surfactants in stabilizing the micrograph [33]. The result of agglomeration was obtained, but the basic particles were also homogeneous and small, showing that the green synthesis process controlled the nucleation and growth.

Fig. 7 shows the particle size distribution of the synthesized nanogels in the form of a histogram, with a mean particle size of 7.5 nm and an SD of 1.09 nm, indicating a fairly narrow size range. As expected from a well-controlled, reproducible synthesis, the red Gaussian fitted curve shows a regular particle size distribution. Optimized synthesis parameters (pH, temperature and extract concentration) influence reduction kinetics and particle stabilization, leading to excellent uniformity [34]. The small size (<10 nm) of the Fe₃O₄ nanoparticles indicates that these may be superparamagnetic, which is advantageous for medical uses, including targeted drug delivery, hyperthermia therapy and MRI. Just like the green synthesis of magnetite nanoparticles, this method could be employed for the synthesis of functional nanomaterials because of the structural features, small size, quasi-spherical shape, and acceptable dispersion [35]. Overall, TEM and particle size studies indicate that the Fe₃O₄ nanoparticles were successfully synthesized using the green synthesis method, yielding homogeneous, ultrasmall nanoparticles with minimal aggregation, which are typical of magnetic nanomaterials.

Field Emission Scanning Electron Microscopy (FESEM) Analysis

The structural changes post-hybridization with Ag, and S are important to understand the particle size (mean=10.7 nm, SD=±2.8 nm) of the Fe₃O₄ nanoparticles green synthesized using the extract of Brassica napus. The small mean size and moderate standard deviation reflect the efficiency of the green synthesis approach in nucleation and regulation of growth. Data of dispersion imply that the reduction mediated by phytochemicals is unpredictable. The bioactive compounds phenolics, flavonoids and proteins present in Brassica napus plant extracts, although having a reducing and capping action, have a less stabilizing effect than synthetic surfactants, resulting in partial aggregation and mild polydispersity [36]. Fig. 8 shows the Fe₃O₄ NP size, and the results for nanohybrids show a systematic increase in particle size for Ag-prepared (16.7 nm (SD ± 4.3 nm)) and S-prepared (19.18 nm (SD ± 4.46 nm)). This increase is due to the fact that secondary components often influence nucleation and crystal development, allowing for effective surface modification and hybrid formation.

Fig. 9 shows that the coated Ag/Fe₃O₄ nanoparticles form a core/shell structure on Fe₃O₄. The Ag⁺ ions used in the synthesis could have increased the number of nucleation centres and the heterogeneous growth, and thus the expansion of the particles and larger size dispersion [33]. Silver atoms may also reduce surface energy differences, improving partial particle coalescence [37]. Furthermore, atoms of silver could decrease the differences in surface energy and promote partial coalescence of adjacent particles [37]. The S/Fe₃O₄ nanohybrid in Fig. 10 indicated a slight increase in the average size of 19.18 nm and SD of 0.97 nm, which clearly indicated that the incorporation of sulphur had a relatively larger effect on the particle assembly. In SEM micrographs, the process increases particle size, forms particle clusters, and bridges particles through strong bonding and adsorption between Fe and S [38]. This is due to the surface chemistry, the deposition of the secondary phase, the organic functionalization, and the interaction between the particles. These factors resulted in small nanoparticles with moderate aggregation (10.7 nm), larger particles surface-decorated with Ag (16.7 nm), and larger and more clustered particles with S (19.18 nm) [39]. All nanoscale systems (< 20 nm) possess superparamagnetic properties, a very high surface area ideal for catalysis, antibacterial activity, and biological uses. The literature suggests that green-made iron oxide nanohybrids develop and change their aggregation state in response to changes in surface energetics or reaction pathways [40].

X-ray diffraction (XRD) analysis

The XRD pattern of green synthesized Fe₃O₄ nanoparticles exhibits sharp peaks at 2θ 30, 35, 43, 57 and 62 degrees corresponding to crystallographic planes (220), (311), (400), (511) and (440) respectively. The product is found to be consistent with the JCPDS (No. 19-0629) data of cubic inverse spinel magnetite (Fe₃O₄) which indicates the successful synthesis of the product. Literature reports [31,41,42]. The importance of synthesis of Fe₃O₄ nanoparticles using the green technique with respect to the peak position and indexing.High diffraction peak at (311) plane indicates a good orientation and crystalline order of the nanoparticles.

The absence of secondary peaks of haematite (Fe₃O₄) or maghemite (γ-Fe₃O₄) confirms the phase-purity of magnetite and their stabilizing effect in green synthesis [43]. Also, the “broadening” of the diffraction peaks indicates nanoscale particles.Crystallite size was calculated using Debye Scherrer equation.

Average crystallite size ranges from 4.6 to 7.3 nm, with an average crystallite size of 6.26 nm. This value is lower than that of the green synthesized Fe₃O₄ nanoparticles, which are dependent on the synthesis conditions [44]. A comparison of this kind of XRD data to TEM and SEM results revealed a definite and scientifically sound trend. The XRD crystal size is ~6 nm, which is smaller than TEM (~10.7 nm) and SEM (~16-19 nm) measurements. This difference might be due to natural differences in methodology. TEM measures particle size (there may be many crystallite sizes), while XRD measures coherent crystalline domain size. SEM may observe larger agglomerated structures resulting from particle clustering. Capping chemicals present in plant extracts during the preparation of green nanoparticles can cause the particle sizes of TEM and SEM to differ [45].

Photocatalytic Activity:

Effect of Green-Synthesized Fe₃O₄ (SPIONs) on Eosin Y Degradation pH3

The UV–Vis absorbance data of Fe₃O₄ nanoparticles, which were synthesized using greenery, are summarized in Table 1 at pH 3 (A₀ = 1.221). The surface of Fe₃O₄ is positively charged at pH 3 (below its pH_pzc ≈ 6.8), thus increasing the interaction between the dye and the catalyst due to the electrostatic attraction between the EY molecule (anionic at pH 3, pKa = 3.8) and the surface. The Fe²⁺/Fe³⁺ cycling under solar photons further promotes the production of hydroxyl radicals (•OH) for mineralization [46]. The ln(A₀/Aₜ) plot (Fig. S1a) presents an almost linear plot, with k_app = 0.01438 min⁻¹ and an R² = 0.7018 value.

Effect of Ag/Fe₃O₄ Nanohybrid on Eosin Y Degradation pH3

The Ag/Fe₃O₄ nanohybrid (A₀ = 1.541, Table 2) achieved 98.19% removal at 180 min, with k_app = 0.02139 min⁻¹ (R² = 0.9061), the highest goodness-of-fit among all pH 3 systems. The superiority of kinetic linearity is explained by the synergistic effect of the localized surface plasmon resonance (LSPR) of the Ag nanoparticles, which capture photons from the visible range of the solar spectrum (≈400-450 nm) and put hot electrons into the conduction band of Fe₃O₄. At the Ag–Fe₃O₄ interface, the Schottky junction is formed to inhibit the recombination of electron–hole pairs, which increases the lifetime of the photogenerated holes (h⁺) and superoxide radicals (•O₂⁻) [47]. The Ag–Fe₃O₄ interface, a metal–semiconductor junction, prevents charge recombination and promotes efficient charge transfer. The initial removal at 30 min (86.89%) is slightly lower than Table 1 (87.63%), possibly due to increased initial absorbance (A₀ = 1.541 vs 1.221), indicating dye loading is momentarily clogging active sites [48].

Effect of S/Fe₃O₄ Nanohybrid on Eosin Y Degradation pH3

S/Fe₃O₄ (A₀ = 1.193, Table 3) exhibited the highest initial removal rate at pH 3, reaching 94.72% within 30 min and 98.83% at 180 min (k_app = 0.01896 min⁻¹). By doping Fe₃O₄ with sulphur, mid-gap energy states will be added, which will reduce the optical band gap of Fe₃O₄ and therefore increase the absorption of visible-light photons. Furthermore, the active species produced at the sulphur-functionalised surface, in addition to •OH, [49], gives rise to a dual-radical degradation pathway, making for the quick initial reaction rates. The A₀/Aₜ and A₀/Aₜ (Ln) values are high and constant, even at the first time point (2.94 at 30 min), indicating that the catalytic activity of S/Fe₃O₄ is approaching its highest value after the initial exposure to the sunlight.

Pseudo-First-Order Kinetic Analysis pH 3

The pseudo-first-order kinetic plots [Ln(A₀/Aₜ) vs. irradiation time] Table 5 presents the apparent rate constants kₐₚₚ and the corresponding R² values from the linear regression graphs. See Fig. S1a, b, c.

Photocatalysis at Neutral pH 7

Effect of green-Synthesised Fe₃O₄ SPIONs at pH 7 on Eosin Y Degradation

The Fe₃O₄ surface is close to its point of zero charge (pH_pzc) at neutral pH (7.0), which lessens the electrostatic interaction between dye and catalyst and inhibits the homogeneous Fenton cycle. Even with these considerations, all three systems achieved high removal efficiencies (98.47–99.10%) at 180 min [50], which suggests that •O₂⁻ and surface-bound •OH-mediated heterogeneous photocatalytic pathways are still active under solar irradiation at elevated temperatures (49–52°C). The kinetic parameters for all the systems at pH 7 are listed in Table 8.

Effect of Ag/Fe₃O₄ Nanohybrid at pH 7 on Eosin Y Degradation

The Ag/Fe₃O₄ nanohybrid recorded the highest overall performance at pH 7: 94.38% removal within 30 min and 99.10% at 180 min (k_app = 0.01977 min⁻¹; Table 6). The pH-independent LSPR mechanism of Ag NPs is a process that keeps pumping hot electrons into the conduction band of Fe₃O₄, no matter what the pH of the bulk solution is. The Schottky barrier precludes back-transferring, which assures efficient charge separation during the irradiation period. The photoactivation effect of LSPR increases the activity of Ag/Fe₃O₄ SPIONs at neutral pH as compared to bare SPIONs [51].

Effect of S/Fe₃O₄ Nanohybrid at pH 7 on Eosin dye

S/Fe₃O₄ showed the best linear kinetic curve at pH 7 with R² = 0.8660 and the removal efficiency of 98.86%. The high and stable k_app = 0.02045 min⁻¹ is indicative of the stable bandgap engineered photocatalysis, where the oxygen vacancies created by sulphur do not deactivate the catalyst and continue to generate •OH and •O₂⁻ throughout the irradiation window [52].

Pseudo-First-Order Kinetic Analysis and Comparative Performance

Fig. S2 shows the pseudo-first-order kinetic plots for all catalytic systems. The linear behavior of Ln(A₀/Aₜ) with irradiation time for all the datasets validates the validity of the L–H pseudo-first-order model during the solar irradiation period of 180 min, which is consistent with the previous works of Fe₃O₄-based photocatalysts [53].

At neutral pH, the LSPR-driven hot-electron injection has the maximum kinetic efficiency (kₐₚₚ = 0.01977min⁻¹) and final Ln(A₀/Aₜ) = 4.71. No reduction was observed after irradiation of the S/Fe₃O₄ sample, which exhibited the most linear (R² = 0.8660) catalytic activity. The study reports that the green synthesis of surface modification of Fe₃O₄ can overcome the limitations of neutral pH photocatalytic reactions and perform better than the solar dye-removal system [54,55].

Significance of Mildly Alkaline (pH 8) Conditions

Effect of green-Synthesised Fe₃O₄ SPIONs at pH 8 on Eosin Y

The data for the Fe₃O₄ SPIONs are tabulated for A₀ = 1.595 at pH 8 in Table 9. The system achieves 99.18% removal at 180 min, its highest value across all three pH conditions studied (pH 3: 96.23%; pH 7: 98.47%; pH 8: 99.18%), with kₐₚₚ = 0.02463 min⁻¹ (R² = 0.9052) [56].

Mildly alkaline (pH 8) conditions lead to the Fe₃O₄ surface becoming negatively charged due to the presence of a large number of hydroxyl groups (Fe–OH and Fe–O⁻). These groups, on the surface, are oxidized by solar photons to produce ferryl radicals (Fe–O•), and the conduction band reduces O₂ to •O₂⁻. These conditions are especially desirable for Fe₃O₄ and S/Fe₃O₄, which have the highest removal efficiency at pH 8 (Table 10).[57].

Effect of Ag/Fe₃O₄ Nanohybrid at pH 8 on Eosin Y dye

The Ag/Fe₃O₄ nanohybrid (Table 10, A₀ = 1.051) achieved 97.91% removal at 180 min with kₐₚₚ = 0.01750 min⁻¹ (R² = 0.8115) — the lowest kₐₚₚ and R² among the pH 8 systems. There are, however, two context points to consider [58].

The highest k_app value was obtained for Green Fe₃O₄ SPIONs at pH 8 (k_app = 0.02463 min⁻¹, R² = 0.9052), which was the highest value obtained among the three pH values. The Ag/Fe₃O₄ nanohybrid (Table 8, A₀ = 1.051) exhibited a relatively low kapp = 0.01750 min⁻¹ at pH 8, which is believed to be due to partial passivation of the hydroxyl sites on the Fe₃O₄ by Ag NPs, thereby reducing the beneficial alkaline effect on the generation of •O₂⁻.[59].

Effect of S/Fe₃O₄ Nanohybrid at pH 8 on Eosin Y dye

Table 11 indicates that the highest kₐₚₚ value (0.02508 min⁻¹), R² value (0.9491), and removal efficiency (99.21%) were obtained at pH 8 for S/Fe₃O₄ with an A₀ value of 1.642, confirming that this system is the best under alkaline conditions [60].

The reaction may be pH-dependent, with "acid" activation dominating at pH 3, "band-gap narrowing" effects at pH 7 and "alkaline" activation at pH 8. Concurrently, SO₄²⁻ and OH are generated, which in turn results in a dual-radical mechanism that isn't achievable at lower pH. The trend in the 𝑘_app pH 3 (0.01896) < pH 7 (0.02045) < pH 8 (0.02508 min⁻¹) supports this interpretation. The kinetics are broken down into three steps: medium initiation (0-30 min), fast degradation (30-120 min), and the step of refinement (120-180 min) [61].

Pseudo-First-Order Kinetic Analysis and Comparative Performance/ pH 8

 pseudo-first-order kinetic plots of all systems at pH 8. Table 12 reports the values of kₐₚₚ and R², which are corrected values obtained directly from these regression lines. All of the values are self-consistent; that is, the slope of each regression line in Fig. S3a, b, c is the same as the corresponding kₐₚₚ value in Table 12 [62,63].

Comparative Removal Efficiency

The photocatalytic removal efficiency (%R) after 180 min is compared for the three noncatalytic systems at pH 3, 7 and 8 in a grouped bar chart, as shown in Fig. 12. The numbers in Table 13 are the values for each system, and the numbers in the column labelled "incremental change" (Δ) show how much these values change from pH 3 to pH 8.

CONCLUSION

The solar-driven photodegradation rate of Eosin Y using green-synthesised nanohybrid composites of Ag/Fe₃O₄ and S/Fe₃O₄ is outstanding over a broad range of pH (3–8) and reaches 96.23–99.21% in 180 min under optimal solar irradiation (49–52°C) without any pH adjustment. Ag/Fe₃O₄ shows the highest rate constant (kₐₚₚ = 0.02139 min⁻¹) at pH 3 due to LSPR-driven charge separation; S/Fe₃O₄ achieves its overall maximum performance at pH 8, with kₐₚₚ = 0.02508 min⁻¹ and removal = 99.21% via base-activated dual SO₄•⁻/•OH radical generation. The results strongly validate the ability of surface modification of green-synthesised Fe₃O₄ to address the mechanistic constraints of each pH domain and suggest that these nanocatalysts are viable, economically competitive, recyclable, and sustainable candidates for actual solar-driven wastewater treatment applications.

ACKNOWLEDGEMENT

The authors would like to thank the Department of Chemistry in the College of Education for pure sciences, University of Anbar, for providing the required laboratory, facilities, and equipment for conducting this research.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.