Green Synthesis of AgNPs-Modified TiO₂-Fe₃O₄ Magnetic Spheres for Aqueous Organic Pollutant Removal

1. Introduction

The contamination of water bodies by persistent organic compounds represents one of the most significant environmental challenges today, due to their high chemical stability, toxicity and resistance to conventional treatment processes [1,2]. Dyes, pharmaceuticals, pesticides and other emerging contaminants can remain in aqueous solutions for long periods, negatively affecting aquatic ecosystems and human health. In this context, the development of advanced materials capable of efficiently removing and degrading these pollutants has gained increasing interest, particularly those based on advanced oxidation processes and sustainable technologies [3]. Heterogeneous photocatalysis has established itself as a promising alternative for water treatment, as it allows the degradation of organic pollutants through the generation of reactive oxygen species, mainly hydroxyl radicals (•OH), without the need to add external chemical reagents [3,4]. Among the photocatalytic semiconductors studied, titanium dioxide (TiO₂) stands out as the reference material due to its high chemical stability, low cost, non-toxicity and high oxidative power [4,5]. However, its limited absorption in the visible region of the electromagnetic spectrum and the rapid recombination of photoinduced electron-hole pairs restrict its photocatalytic efficiency, which has led to the modification of TiO₂ through doping, coupling with other materials and the design of hybrid systems [6]. An effective strategy for improving the performance of TiO₂ consists of coupling it with magnetite (Fe₃O₄), which acts as a redox co-catalyst capable of facilitating electron transfer and promoting the in-situ generation of highly oxidizing species through heterogeneous Fenton-type mechanisms [7,8]. The reversible alternation between the Fe²⁺/Fe³⁺ states promote the continuous production of hydroxyl radicals, which synergizes with the photocatalysis of TiO₂ and accelerates the degradation of organic pollutants. Additionally, the presence of Fe₃O₄ confers magnetic properties to the system, allowing for the efficient recovery of the material after use [9]. Doping with metallic silver nanoparticles (AgNPs) is another key strategy for enhancing the photocatalytic and functional activity of the system. The formation of a Schottky junction at the TiO₂-Ag interface promotes the trapping of photoexcited electrons, inhibiting electron-hole recombination and extending the photoactive response toward the visible region of the spectrum [10,11]. In addition, Ag NPs provide intrinsic antimicrobial activity, contributing to the inactivation of microorganisms present in aqueous solutions [12]. It is important to note that the use of silver in controlled concentrations and its immobilization in a solid matrix minimize the risks associated with its release into the environment [13]. In order to maximize the efficiency and safety of the system, the integration of adsorbent materials and biopolymers is essential. Activated carbon acts as a concentrator of organic pollutants, adsorbing them on its surface and promoting their proximity to active photocatalytic sites, thus increasing the probability of degradation [14]. Meanwhile, chitosan provides structural stability, biocompatibility and additional antimicrobial properties, while sodium alginate is used as an encapsulating matrix for the synthesis of semiconductor spheres [15,16]. The use of biopolymers allows for the creation of biodegradable, non-toxic and mechanically stable structures, preventing the direct dispersion of nanoparticles in the aqueous medium. This paper reports on the synthesis of TiO₂-Fe₃O₄ semiconductor spheres doped with AgNPs based on biopolymers, with the aim of removing organic contaminants from aqueous solutions. The innovation of the system lies in the integration of photocatalysis, adsorption, antimicrobial activity and magnetic recovery in a single reusable and environmentally safe material. The presence of magnetite allows for the rapid separation of spheres using an external magnetic field, facilitating their reuse and reducing the generation of secondary waste [9]. The objective of this work is to manufacture AgNPs-doped TiO₂-Fe₃O₄ composite contained within a biopolymeric alginate-chitosan matrix, as a low-cost and green alternative for water sanitation.

2. Materials and Methods

2.1. Spheres Manufacturing Process

The sphere manufacturing process begins with the selection and precise dosing of raw materials, including TiO₂ in the anatase phase (0.10 g), Fe₃O₄ (0.06 g), powdered activated carbon (0.10 g), silver nanoparticles (AgNPs, 0.010 g), microencapsulated carbamide peroxide, sodium alginate (0.75 g), chitosan (0.10 g), distilled water (25 mL), calcium chloride (CaCl₂ at 2% w/v), glacial acetic acid to prepare a 1% v/v solution, and glutaraldehyde diluted to 0.05% w/v. In parallel, auxiliary solutions are prepared: an alginate solution (25 mL of distilled water heated to 45 °C), a chitosan coating solution (100 mL of distilled water with 2 mL of acetic acid and 0.1 g of chitosan), a glutaraldehyde reinforcement solution (100 mL of distilled water with 0.2 mL of glutaraldehyde), and a CaCl₂ gelation solution (100 mL of distilled water with 2 g of CaCl₂). Subsequently, the powders of TiO₂ (3 μm, 99.9%, Sigma-Aldrich, St. Louis, MO, USA), Fe₃O₄ (5 μm, 99.9%, Sigma-Aldrich, St. Louis, MO, USA), activated carbon (<100 nm, 99.9%, Bulk Superfoods, CDMX, México), AgNPs (<100 nm, 99.9%, SkySpring Nanomaterials, Inc. Houston, TX, USA), and microencapsulated carbamide peroxide are thoroughly mixed in a mortar until a homogeneous blend is obtained. This powder mixture is then incorporated into the previously heated and homogenized alginate solution (using 0.6 g of sodium alginate), forming a uniform suspension, to which 0.03 mL of alkyl polyglucoside is added. The resulting suspension is loaded into a needle-free insulin syringe and dropped into the CaCl₂ solution, where the droplets instantly gel to form spheres, which are kept in the solution for 15 minutes to ensure complete crosslinking. The spheres are then recovered using a magnet (due to the presence of magnetite) and dried on absorbent paper for 30 minutes to 2 hours at room temperature (~25 °C). Once dried, the spheres are coated by immersion in the chitosan solution for 2–3 minutes, followed by drying for approximately 20 minutes, and subsequently treated with the glutaraldehyde solution for 2–3 minutes to reinforce the matrix, followed by a final drying step under the same conditions. After this process, the obtained spheres are ready for use in aqueous water sanitation tests.

Figure 1 Shows the flow diagram of the methodology used to prepare spheres containing AgNPs-doped TiO₂-Fe₃O₄. The figure presents the sequential stages involved in the synthesis process, including precursor preparation, incorporation of Ag nanoparticles into the TiO₂-Fe₃O₄ matrix, sphere formation, and post-treatment steps leading to the final composite material. Key processing conditions at each stage are summarized to provide a clear overview of the fabrication route.

2.2. Powder Characterization

Once the powders were homogenized and before encapsulating them with chitosan, the particle size distribution was determined by laser diffraction using a Mastersizer 2000 system (Malvern Panalytical, Almelo, The Netherlands). The morphological characteristics of the powders were examined by scanning electron microscopy (SEM) using a JEOL 6300 microscope (Akishima, Tokyo, Japan). The crystalline structure was analyzed by X-ray diffraction (XRD) employing a Bruker D8 Advance diffractometer (Billerica, MA, USA). Data were collected over a 2θ range of 20-80°, with a step size of 0.016° and a counting time of 10 s per step. The resulting diffraction patterns were analyzed using X’Pert HighScore Plus software (version 2.2b).

2.3. Absorbance Measurements

Optical absorbance measurements were performed using a deuterium lamp as the UV-Vis light source. All optical analyses were carried out with a compact CCD spectrometer (Thorlabs CCS200, New Jersey, USA) operating in the 200-1000 nm wavelength range. Absorbance was determined by first recording the transmission spectrum of distilled water in a standard quartz cuvette with an optical path length of 10 mm, which served as the reference. Subsequently, the transmission spectra of AgNPs-doped TiO₂-Fe₃O₄ suspensions in distilled water, contained in identical quartz cuvettes, were acquired. Optical absorbance was then calculated using the spectrometer software based on the reference and sample measurements. The photocatalytic activity of AgNPs-doped TiO₂-Fe₃O₄ was evaluated via the degradation of organic contaminants in aqueous solution under natural solar irradiation for 5 h. The experiments were conducted in a glass beaker containing 200 mL of distilled water, 0.01 g of the AgNPs-doped TiO₂-Fe₃O₄ photocatalyst and one drop of methylene blue as a model pollutant. The suspension was magnetically stirred at 400 rpm throughout the experiment and was exposed to light for 5 hours with the help of a solar simulator. Aliquots were collected at 60 min intervals to monitor the degradation kinetics and evaluate the water decontamination efficiency.

3. Results

3.1. Structure X-Ray Diffraction

Figure 2 shows the X-ray diffraction (XRD) pattern corresponding to the mixture of TiO₂ + Fe₃O₄ powders. The diffractogram identifies the characteristic peaks of the anatase phase of TiO₂, which can be indexed to the crystallographic planes (101), (103), (004), (200), (211), (204), (116) and (220). The most intense peak, located at 2θ ~25.3°, corresponds to the (101) plane, with an interplanar spacing d ~3.52 Å, consistent with the tetragonal crystal structure of anatase (JCPDS card No. 21-1272). Additionally, the pattern exhibits peaks corresponding to the inverse spinel cubic crystal structure of magnetite (Fe₃O₄), identified at 2θ ~30. 1° (220), 35.5° (311), 43.2° (400/422), 57.0° (511) and 62.6° (440), in good agreement with JCPDS card No. 19-0629. The presence of these peaks confirms the coexistence of the ferrimagnetic phase within the system. The average crystallite size was estimated from the broadening of the most intense peak of each phase using the Scherrer equation, obtaining values of 20 nm for anatase and 25 nm for magnetite, indicating a nanocrystalline microstructure favorable for functional applications. The coexistence of TiO₂ in the anatase phase and magnetite is particularly attractive for photocatalysis applications, since anatase provides high photocatalytic activity under UV-visible irradiation, while magnetite can promote efficient charge separation and enable magnetic recovery of the material, facilitating its reuse in environmental treatment processes. Silver cannot be observed in the diffraction pattern because its content in the TiO₂ + Fe₃O₄ mixture is below the detection limits of the diffraction equipment.

3.2. Particle Size Distribution

Figure 3 shows the particle size distribution and cumulative fraction of the AgNPs-doped TiO₂-Fe₃O₄ powders before manufacturing the spheres. The particle size is mainly within the submicrometric range (0.55-0.92 µm). The cumulative curve indicates that ~75% of the particles lie between 0.55 and 0.72 µm, confirming the predominance of fine particles. About 25% of the particles are smaller than 0.60 µm, indicating an ultrafine fraction, while ~25% exceed 0.72 µm, corresponding to the coarse tail. The continuous unimodal distribution suggests homogeneous milling without bimodality. The prevalence of fine submicrometric particles is advantageous for photocatalysis, as it increases specific surface area and dispersion stability in aqueous media, thereby enhancing photocatalytic performance.

3.3. Powders Morphology

Scanning electron microscopy (SEM) micrograph (Figure 4, scale = 5 µm) reveals that the synthesized powders consist of irregular submicrometric aggregates with a highly textured, cauliflower-like surface. These entities correspond to compact secondary particles formed by the coalescence of much smaller primary nanometric domains, which are not individually resolved at this magnification but are evidenced by the granular surface and diffuse boundaries of the aggregates. This hierarchical architecture is consistent with the particle size distribution (Figure 3), where the dominant fraction lies within the 0.55-0.92 µm range. Therefore, the measured particle size reflects the dimensions of the secondary aggregates observed in SEM rather than individual crystallites.

Morphologically, three structural levels can be distinguished: (i) primary nanoparticles forming the nanogranular surface; (ii) submicrometric aggregates (~0.5-0.9 µm) representing the predominant entities in both SEM and granulometry and (iii) larger agglomerates (>1 µm) arising from the association of several aggregates. The prevalence of isolated aggregates within the submicrometric interval supports the cumulative fraction (~75% between 0.55 and 0.72 µm), whereas the tail toward larger sizes is explained by the clustered regions visible in the micrograph. Such limited agglomeration is typical for powders containing Fe₃O₄, whose magnetic interactions promote interparticle association during drying without destroying aggregate integrity.

The absence of faceted crystals and the predominance of rounded to irregularly spherical aggregates indicate a formation mechanism governed by multiple nucleation events followed by isotropic growth and controlled aggregation. This mechanism simultaneously accounts for the quasi-spherical morphology, the relatively narrow particle size distribution, and the nanogranular surface texture expected for AgNPs-doped TiO₂-Fe₃O₄ spheres. Consequently, the SEM observations validate the granulometric results: the particle size analysis quantifies the same submicrometric aggregates visualized morphologically, while the observed aggregation state explains the distribution profile. This distribution suggests effective interfacial contact between the phases, which is favorable for enhancing synergistic effects in catalytic and photocatalytic applications by increasing the active surface area and facilitating charge transfer processes.

3.5. Mappings

Figure 5 illustrates the elemental mapping of the TiO₂ + Fe₃O₄ powder mixture with AgNPs, where the characteristic signals of Ti, Fe, and O are clearly identified, confirming that the intrinsic chemical structure of both TiO₂ and Fe₃O₄ phases remains intact after the modification process. Additionally, the Ag signal is distinctly observed in the modified samples, indicating its successful incorporation into the system; this analysis therefore unequivocally confirms the presence of silver within the material. Although the Ag content is relatively low, its uniform spatial distribution across the analyzed region suggests that the synthesis method promotes effective dispersion of the modifier, reducing the possibility of segregated phases and enabling a homogeneous modification of the TiO₂/Fe₃O₄ surface. Moreover, the absence of extraneous elemental signals implies that no significant impurities were introduced during synthesis, supporting the chemical purity of the composite. The elemental maps further reveal a strong spatial correlation between Ti and O, as well as Fe and O, consistent with the expected stoichiometry of TiO₂ and Fe₃O₄, respectively. In contrast, the Ag signal appears finely and evenly distributed rather than concentrated in specific regions, which is advantageous for enhancing surface-related properties such as photocatalytic activity. Complementary EDS spectra support these observations by showing the characteristic peaks of Ti, Fe, and O along with the detectable presence of Ag, thereby confirming the formation of a TiO₂/Fe₃O₄-Ag composite system. Overall, these results demonstrate that the synthesis approach is effective in incorporating silver nanoparticles into the TiO₂/Fe₃O₄ matrix while preserving structural integrity and ensuring a uniform distribution of the modifier.

3.4. Irradiation System

Figure 6 shows the experimental system in operation inside a sunlight simulator, designed to reproduce the spectrum and intensity of solar radiation in a controlled manner. During the process, the spheres are kept in homogeneous suspension by continuous magnetic agitation, which promotes effective and constant contact between the contaminants present in the aqueous solution and the active catalytic surface of the spheres. Irradiation with artificial light similar to sunlight activates the photocatalytic properties of AgNPs-doped TiO₂-Fe₃O₄ spheres, promoting the generation of reactive species (such as hydroxyl and superoxide radicals). These species play a key role in the degradation and mineralization of organic compounds and dyes, significantly accelerating oxidation reactions and improving the efficiency of the removal process.

Figure 7 shows representative images of water subjected to photocatalytic treatment for 5 hours of exposure to radiation from the solar simulator. After this period, the liquid still shows slight traces of coloration and turbidity; however, there is a clear visual reduction compared to the initial untreated sample. This reduction indicates the progressive advancement of the purification process, associated with the partial degradation of organic contaminants and the decrease in species responsible for optical absorption in the aqueous medium. The same figure includes an image corresponding to a sample treated for 15 hours of irradiation, in which water initially contaminated with dirt, soap, stagnant water and methylene blue shows a notable improvement in its transparency and clarity. The marked reduction in coloration and turbidity suggests more advanced photodegradation of dyes and organic compounds, as well as possible partial mineralization of these compounds. These qualitative results demonstrate the high efficiency of the photocatalytic system, particularly under prolonged irradiation times, and confirm the material’s ability to promote the removal of complex contaminants in aqueous solutions through photoinduced processes.

The recovery of the spheres after the treatment process was successful, as shown in Figure 8. Thanks to the incorporation of magnetite (Fe₃O₄) in their composition, the spheres exhibit magnetic behavior that allows for their rapid and efficient separation from the aqueous medium through the application of an external magnetic field. This feature significantly facilitates the recovery and reuse of the material in successive treatment cycles, contributing to the operational viability and sustainability of the process, without any significant loss of photocatalytic efficiency.

3.5. Photocatalytic Activity

The UV-Vis absorbance spectra recorded during solar irradiation in the presence of AgNPs-doped TiO₂-Fe₃O₄ spheres shows in Figure 9, exhibits a pronounced band in the 600–690 nm region, attributed to the chromophores’ structure of the organic contaminant in solution. As the exposure time increases, a systematic decrease in the maximum absorbance intensity is observed, indicating progressive photocatalytic degradation and disruption of the conjugated molecular system responsible for coloration. The absence of significant peak shifts suggests that the dominant process is the reduction of dye concentration rather than the accumulation of strongly absorbing intermediates, while the gradual flattening of the visible region at longer irradiation times evidences near-complete decolorization. Minor variations in the short-wavelength region (< 500 nm) may be associated with transient aromatic by-products or light scattering from suspended catalyst particles, which also diminish with continued irradiation. In general, the spectral evolution confirms efficient solar-driven photocatalytic activity of the AgNPs-doped TiO₂-Fe₃O₄ compound, attributable to synergistic effects between plasmonic Ag nanoparticles, the TiO₂ semiconductor matrix, and the Fe₃O₄ component that enhances interfacial charge transfer and facilitates catalyst recovery.

3.6. Absorbance Rate

The decrease in absorbance with irradiation time (Figure 10) follows an exponential tendency (A = 1.491·e^(−0.112·t); R² = 0.8791), indicating that the photodegradation of the contaminant over AgNPs-doped TiO₂-Fe₃O₄ spheres obeys pseudo-first-order kinetics consistent with the Langmuir-Hinshelwood model at low dye concentrations [17]. The apparent rate constant (kapp = 0.112 h^-1) and the ~46% decolorization after 5 h of solar exposure fall within the range reported for Ag-modified TiO₂ photocatalysts operating under natural or simulated solar light, confirming efficient visible-light utilization and suppressed electron-hole recombination due to the plasmonic effect of Ag nanoparticles. Moreover, the sustained decay without kinetic plateau suggests stable availability of active sites, while the Fe₃O₄ phase likely facilitates interfacial electron transfer and enables magnetic recovery of the catalyst, contributing to the overall photocatalytic performance. Similar kinetic behavior and synergistic mechanisms in Ag/TiO₂ and TiO₂-Fe₃O₄ heterostructures for solar-driven dye degradation have been widely documented in recent literature [18,19,20].

3.7. Photocatalytic Antibacterial Activity

Table 1 presents the microbiological results obtained for contaminated water after 5h of solar irradiation in the presence of AgNPs-doped TiO₂-Fe₃O₄. A marked reduction in bacterial indicators was observed, with total coliforms decreasing by 87% and fecal coliforms by more than 93% relative to the untreated control. These results confirm the strong antibacterial efficiency of the composite under solar-driven photocatalytic conditions and its effectiveness in improving microbiological water quality. The high inactivation efficiency is associated with the synergistic interaction among Ag, TiO₂ and Fe₃O₄, whose crystalline phases were confirmed by XRD. As noted, the diffraction patterns showed the characteristic reflections of anatase TiO₂ (tetragonal structure) and magnetite Fe₃O₄ (cubic inverse spinel), indicating that the synthesis preserved the photocatalytically active anatase phase and the magnetic phase without structural degradation. As mentioned, anatase provides high surface reactivity and efficient charge generation, while Fe₃O₄ promotes interfacial electron transfer, reducing electron-hole recombination. Ag nanoparticles act as electron sinks and enhance visible-light absorption, while released Ag+ ions contribute intrinsic antimicrobial effects, consistent with recent reports on Ag-TiO₂ antibacterial photocatalysts and magnetically recoverable TiO₂-Fe₃O₄ disinfection systems. [21,22]

Under solar irradiation, these structural and electronic features favor the generation of reactive oxygen species (ROS), such as •OH and O₂•⁻^-, which oxidatively damage bacterial membranes and intracellular components. The greater reduction observed for fecal coliforms suggests higher susceptibility of enteric bacteria to oxidative stress and silver toxicity. Overall, achieving >90% fecal coliform removal within 5h demonstrates that the structurally confirmed AgNPs-doped TiO₂-Fe₃O₄ compound is an effective photocatalyst for solar-assisted water disinfection, combining strong antibacterial activity with magnetic recoverability.

4. Discussion

The XRD results confirmed the successful coexistence of anatase TiO₂ and magnetite Fe₃O₄ without the formation of secondary phases, preserving both the photocatalytic active and magnetic functionalities. The nanocrystalline nature of the material (20–25 nm) is favorable for enhancing surface reactivity and charge carrier generation. Although Ag was not detected by XRD, its presence is inferred as highly dispersed nanoparticles, consistent with its role as a surface modifier.

Particle size analysis and SEM observations revealed a homogeneous unimodal distribution in the submicrometric range, composed of hierarchical aggregates formed by primary nanoparticles. This morphology provides a high specific surface area and promotes effective interfacial contact between phases, which is essential for improving photocatalytic performance. Limited agglomeration, likely influenced by magnetic interactions, does not hinder active site accessibility.

The photocatalytic tests demonstrated a progressive degradation of contaminants under simulated solar irradiation, with significant decolorization and reduction of turbidity over time. UV-Vis spectra confirmed a decrease in absorbance without peak shifts, indicating direct degradation rather than accumulation of intermediates. The process followed pseudo-first-order kinetics, consistent with the Langmuir-Hinshelwood model, with a rate constant comparable to similar Ag-modified TiO₂ systems under solar light.

The enhanced photocatalytic activity is attributed to the synergistic interaction among TiO₂, Fe₃O₄, and AgNPs, where TiO₂ acts as the photoactive matrix, Fe₃O₄ facilitates charge transfer, and Ag nanoparticles improve visible-light absorption and suppress electron-hole recombination. Additionally, the incorporation of Fe₃O₄ enables efficient magnetic recovery, improving the reusability and sustainability of the system.

Finally, the material exhibited strong antibacterial activity, achieving over 90% removal of fecal coliforms after 5 h of irradiation. This effect is associated with the combined action of reactive oxygen species and Ag⁺ ions, highlighting the multifunctional capability of the composite for simultaneous pollutant degradation and water disinfection.

5. Conclusions

• Multifunctional spheres containing AgNPs-doped TiO₂-Fe₃O₄ were successfully synthesized through ionic gelation and biopolymer coating, integrating photocatalytic, adsorptive, antimicrobial and magnetic recovery functionalities in a single reusable material with structural stability and environmental safety.
• Structural and morphological analyses confirmed the coexistence of nanocrystalline anatase TiO₂ and magnetite phases with finely dispersed Ag nanoparticles and a predominantly submicrometric particle size distribution, providing high surface area and favorable interfacial charge transfer for photocatalytic processes.
• Photocatalytic experiments under simulated solar irradiation demonstrated effective degradation of methylene blue following pseudo-first-order kinetics (kapp ~0.112 h^-1), evidencing enhanced visible-light activity and reduced electron-hole recombination due to the synergistic interaction of AgNPs, TiO₂ and Fe₃O₄.
• The spheres exhibited strong antibacterial performance, achieving reductions of 87% in total coliforms and >93% in fecal coliforms after 5 h of irradiation, confirming their dual capability for organic contaminant removal and solar-driven water disinfection.
• The incorporation of magnetite enabled rapid magnetic recovery and reuse of the spheres without functional loss, supporting the feasibility of AgNPs-doped TiO₂-Fe₃O₄ spheres as an efficient, green and low-cost and sustainable platform for solar water treatment applications.