Impact of Al and Mn doping on the catalytic activity of magnetite spinel for sulfamethoxazole degradation: kinetics and toxicity assessment†

B. Gokulakrishnan and G. Satishkumar*
Advanced Materials and Catalysis Lab, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore-632014, Tamilnadu, India. E-mail: satishkumar.g@vit.ac.in; satishgsamy@gmail.com; Tel: +91 416 2202736

First published on 21st July 2025

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Water impact Sulfamethoxazole, a persistent antibiotic in wastewater, poses environmental risks due to low biodegradability with a long half-life and potential to promote antibiotic resistance. This study highlights the significance of polarizing the Fe electrons in a commercially attractive magnetite catalyst to expedite the challenging Fe3+ reduction in the redox cycle. Aluminium in the Al-doped magnetite catalyst effectively polarized the Fe3+ in the spinel, degraded sulfamethoxazole with 60% TOC removal and detoxified the effluent within 30 minutes. The findings of this study expand the potential of economically viable magnetite as a Fenton-like catalyst for real wastewater treatment.

1. Introduction

Fenton oxidation is a promising advanced oxidation process widely recognized for its ability to generate highly reactive hydroxyl radicals (·OH) with a high redox potential of 2.8 V, making it highly effective for degrading a wide range of contaminants.^1,2 The core reaction involves Fe²⁺ and H₂O₂ (eqn (1)), and the oxidized Fe³⁺ reacts with H₂O₂ to regenerate Fe²⁺ through eqn (2).^1,3 However, conventional Fenton processes face challenges such as the generation of ferric sludge, excessive H₂O₂ consumption, challenges in separating reagents, and the need for stringent reaction conditions.^4–6 Solid catalysts like iron oxides have emerged as an alternative to traditional homogeneous processes, which drive the reaction at the solid–liquid interface.^3,6–8

Fe(II) + H2O2 → Fe(III) + ·OH + OH− k = 63 M−1 s−1	(1)

Fe(III) + H2O2 → Fe(II) + ·OOH + H+ k = 0.001–0.01 M−1 s−1	(2)

Despite the advantages, a major challenge in iron-based catalysts is that Fe(III) is the predominant form, and its reduction by H₂O₂ is significantly slower (eqn (1) and (2)).^1,4 Recently, natural minerals have gained attention as promising catalysts in catalytic wet peroxide oxidation (CWPO).^9,10 Among iron-oxide minerals, magnetite (Fe₃O₄), in particular, stands out as an effective iron oxide catalyst for heterogeneous Fenton oxidation due to the presence of both Fe(II) and Fe(III) at octahedral sites and its inverse spinel structure facilitates the Fe(II)/Fe(III) redox cycle within the structure and enhances catalytic performance.^2,11,12 In addition, it enables the incorporation of various transition metal cations, which significantly improves electron mobility and promotes fast regeneration of Fe(II) from the Fe(III) reduction reaction. Based on the similarity in ionic radii, magnetite can accommodate a various range of cationic substitutions, including divalent (e.g., Co, Ni, Zn, Cu, Mn), trivalent (e.g., Al, V, Cr), and tetravalent (e.g., Ti) metal cations while maintaining its spinel structure.^13–15 Amongst them, the introduction of redox-active metal ions, such as Mn²⁺/Mn³⁺, Cr³⁺/Cr²⁺ and Co²⁺/Co³⁺, can play a dual role in enhancing the catalytic performance of magnetite in Fenton reactions.^16–18 Redox-active metal ions can replace Fe ions partially in the octahedral sites of the spinel lattice, enhancing the Fenton reaction by aiding the challenging regeneration of Fe²⁺ by donating electrons to Fe³⁺.¹⁸ In addition, these redox-active metals can produce hydroxyl radicals by reacting directly with H₂O₂. Mn is often preferred over Co and Cr as a dopant for Fenton-like and photo-Fenton processes for its eco-friendliness.^{14–16,19,20}

The doping of Al (0.53 Å), a redox-inactive metal, in magnetite spinel structure is not an appealing idea to accelerate the Fe(II)/Fe(III) redox cycle, as it cannot exhibit variable oxidation states like Mn, Co, and Cr, despite having similar ionic radii to Fe³⁺ (0.64 Å). At the same time, the significance of Fe–Al interaction in enhancing the catalytic activity of iron/iron oxide supported alumina catalysts was demonstrated earlier towards CWPO of various pollutants.^21–24 In that respect, recently, we substituted Al for Fe-ions in the magnetite lattice spinel and observed a long-lasting catalytic performance for the degradation of different types of pollutants in batch and continuous reactors.^25,26 The high catalytic performance is attributed to the Lewis acid property of Al which polarised the electrons of Fe³⁺ in Fe₃O₄ and improved the kinetics of the challenging Fe³⁺ reduction reaction with H₂O₂ to produce HOO· radicals. Interestingly, substituted Mn in Fe₃O₄ spinel structure, besides reducing Fe³⁺ by the oxidation of Mn²⁺ to Mn³⁺,²⁷ can also polarise the electrons around Fe³⁺ since Mn is more electronegative than Fe.¹⁹ Hence, it is essential to investigate Mn and Al substituted Fe₃O₄ to determine which dopant significantly improves the catalytic activity and stability of the solid Fenton magnetite catalyst for the CWPO of pollutants. Sulfamethoxazole, a widely used sulfonamide antibiotic in animal husbandry, aquaculture, and disease prevention, is taken as a model pollutant in this study.^28,29 Approximately 45–70% of SMX is excreted through urine due to partial metabolism in both human and animal bodies. About 25% of SMX remains unchanged, leading to contamination of sediments, soils, and wastewater.³⁰ This contributes to antibiotic resistance and creates significant environmental issues.

In this work, Fe₃O₄, Mn and Al substituted Fe₃O₄ nanoparticles have been prepared using a simple co-precipitation method and encapsulated in the carbon matrix. The graphitic carbon matrix is highly beneficial in promoting electron transfer from the spinel nanoparticles to the oxidant through the π electron system. It also effectively prevents the accumulation of magnetic spinel nanoparticles like Fe₃O₄ in the reaction medium. The synthesised catalysts have been analysed using various characterisation techniques such as XRD, UV-Raman, and XPS to determine the impact of Mn and Al substitution on the structural and electronic properties. The performance of all three catalysts was evaluated to degrade 10 ppm SMX antibiotic as a model pollutant. The impact of reaction parameters on reaction kinetics, including temperature, pH, H₂O₂ dosage, catalyst loading, and reaction time, was investigated for all the catalysts. Natural wastewater contains inorganic and organic species, and hence, we investigated the degradation of SMX to explore how various inorganic anions, organic acids (Cl⁻, NO₃⁻, CO₃²⁻, H₂PO₄⁻ and HA) and water matrices (drinking water and tap water) influence the catalytic performance.³¹ The active radical generated in all three catalysts, involved in SMX mineralization, were identified through scavenging studies. Finally, the toxic nature of SMX and treated effluents from all the catalysts at various time intervals was assessed using the activated sludge respiration inhibition (ASRI) test and an acute toxicity assessment using the bioindicator Artemia salina.

2. Experimental

2.1. Reagents

Details of chemicals and reagents used in this study are provided in the ESI,† page S2.

2.2. Preparation of catalytic materials

2.3. Catalytic studies

The catalytic activity of the synthesized Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C catalysts was evaluated through the CWPO of SMX. For a detailed description of the experimental procedure and conditions, please refer to the ESI,† page S2–S5.

2.4. Characterization methods

The prepared catalysts were characterized using various techniques, including X-ray diffraction (XRD) (Rietveld refinement), Raman spectroscopy, Brunauer–Emmett–Teller (BET), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectrometry (XPS), zeta potential and inductively coupled plasma mass spectroscopy (ICP-MS). Total organic carbon analysis (TOC) and UV-vis spectroscopy (UV) were used to evaluate the extent of SMX degradation in the effluent water. Detailed information on these characterization and analytical techniques can be found in the ESI,† page S5–S7.

3. Results and discussion

3.1. Catalyst characterization

The XRD pattern of Fe₃O₄@C (Fig. 1) closely aligns with the standard JCPDS card of Fe₃O₄ (01-075-0033), confirming pure magnetite formation. The spinel structure of magnetite is represented as Fe_{(tetrahedral)}²⁺ [Fe_(octahedral)²⁺ Fe_(octahedral)³⁺] O₄, where Fe²⁺ ions occupy the tetrahedral sites, while both Fe²⁺ and Fe³⁺ reside in octahedral positions. Doping of manganese and aluminium into the magnetite spinel lattice preferentially replaces Fe²⁺/Fe³⁺ with Mn²⁺/Mn³⁺, and Fe³⁺ with Al³⁺ at the octahedral site.¹⁵ The successful incorporation of Mn and Al into the magnetite spinel structure was confirmed by lattice distortion. After substituting aluminium and manganese into the Fe₃O₄@C spinel lattice, the diffracted patterns are well indexed to Al-doped and Mn-doped Fe₃O₄@C, respectively, consistent with previously reported data.^19,26 No other peaks for Al and Mn were observed in the diffraction pattern, confirming the absence of aluminium and manganese oxide phases. This indicates the successful incorporation of these metal ions in the Fe₃O₄ spinel lattice. A noticeable peak shift towards higher 2θ values for the metal doped Fe₃O₄@C reflects lattice changes due to the smaller ionic radii of Al³⁺ (0.53 Å) and Mn²⁺/Mn³⁺ (0.67 Å/0.58 Å) compared to Fe³⁺ (0.65 Å) and Fe²⁺ (0.77 Å). Fe(FeAl)₂O₄@C exhibits a more noticeable peak shift due to the smaller ionic radius of Al³⁺, whereas Mn-doping results in a smaller shift because of the closer ionic sizes and mixed oxidation states.^32,33 The crystallite size was determined using the Scherrer equation, and interplanar spacing (d-spacing) and lattice parameter (a) values are summarized in Table 1. The incorporation of Mn and Al into the Fe₃O₄ spinel lattice results in a reduction of the lattice parameter (a) from 8.4 Å to 8.3 Å and 8.2 Å, respectively (Table 1). This reduction arises from the partial substitution of Fe ions in magnetite with Mn and Al ions. The decrease in the lattice parameter is further confirmed by the shift of the (311) and (440) peak positions to higher 2θ values in the XRD patterns, confirming the successful incorporation of Mn and Al into the Fe₃O₄ spinel structure. The nitrogen adsorption and desorption studies conducted on the three catalysts, Fe₃O₄@C, Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C, revealed that the carbon coating significantly enhanced the surface area of each of the catalysts, resulting in values of 251 m² g⁻¹ for Fe₃O₄@C, 291 m² g⁻¹ for Fe(FeAl)₂O₄@C, and 279 m² g⁻¹ for Fe(FeMn)₂O₄@C, as summarized in Table 1.

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The ICP-MS analysis revealed the amount of (Fe), (Al) and (Mn) in the synthesized catalysts, as listed in Table 1. Detailed theoretical calculations for Fe, Al, and Mn composition in the synthesised catalysts are provided in the ESI,† page S7 and S8. The measured weight percentages of Al and Mn and the Fe/Mn and Fe/Al mole ratios closely matched the theoretical values. The FE-SEM images of Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C are shown in Fig. 2(a, e and i), which reveals particle sizes ranging from 5 μm to 30 μm without any distinct morphology. HR-TEM analysis confirmed that Fe₃O₄ nanoparticles exhibit a spherical morphology, uniformly dispersed within the carbon matrix (Fig. 2b and c), with an average particle size of 20 nm. Mn doping did not cause any significant change in the morphology of the magnetite spinel, as the nanoparticles retained their spherical shape, uniform size and were well-dispersed within the carbon matrix (Fig. 2f and g). However, Al doping led to nanoparticles with a distorted cubic morphology and an average particle size of 32 nm, evenly distributed within the carbon matrix (Fig. 2j and k). High-magnification TEM images provided insights into the structural characteristics and interplanar spacings of all the synthesized catalysts. The interplanar spacings of 0.25 nm and 0.29 nm, corresponding to the (311) and (220) planes, were identified for F₃O₄@C and Fe(FeMn)₂O₄@C catalysts (Fig. 2d and h). Fe(FeAl)₂O₄@C displayed lattice spacings of 0.25 and 0.24 nm corresponding to the (311) and (222) planes (Fig. 2l). The selected area electron diffraction (SAED) analysis of all three catalysts revealed spotty diffraction patterns, indicating high crystallinity, which was consistent with the XRD results (Fig. S1†). The identified planes from the SAED patterns (111), (311), (400), (511) and (440) are well aligned with the obtained HR-TEM and XRD results, demonstrating that all three catalysts were synthesized in a single phase. The uniform interplanar spacings across different dopants suggest that the doping process preserves the structural integrity of the Fe₃O₄ lattice. Additionally, the FE-SEM elemental mapping shown in Fig. 3 confirmed the presence of Fe, O, and C in the Fe₃O₄@C catalyst, as well as the successful incorporation of Mn and Al into the Fe₃O₄ lattice in Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C, respectively. The homogeneous distribution of these elements highlights the uniform dispersion of all the synthesised nanoparticles in the carbon matrix. Fig. 4 presents the Raman spectra of undoped Fe₃O₄, Fe(FeMn)₂O₄ and Fe(FeAl)₂O₄ nanoparticles, each revealing characteristic vibrational modes of the spinel structure. Three prominent peaks at 290, 490, and 685 cm⁻¹ correspond to the E_g, T_2g, and A_1g phonon modes, which are linked to oxygen vibrations at octahedral and tetrahedral sites, respectively.³⁴ These modes confirm the presence of Fe₃O₄'s cubic lattice, with Fe²⁺ (0.77 Å) and Fe³⁺ (0.65 Å) ions participating in distinct vibrational motions. Doping with Mn causes a slight shift in the A_1g peak from 685 to 698 cm⁻¹, suggesting refined changes in the Fe–O bond environment due to the incorporation of Mn ions.³⁵ The shift is relatively small due to the small ionic radii of Mn²⁺/Mn³⁺ (0.67 Å/0.58 Å).³⁶ Meanwhile, Al doping results in a more significant shift of the A_1g mode to 706 cm⁻¹, along with stronger changes in the peak intensity. This larger shift reflects greater lattice distortion, likely caused by the smaller ionic radius of Al³⁺ (0.53 Å) compared to Fe³⁺ (0.64 Å). The pronounced changes suggest that Al doping introduces substantial strain, altering bond lengths and angles in the Fe₃O₄ structure. To further analyse the surface composition and chemical states of Fe₃O₄@C, Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C, XPS analysis was performed for all the prepared materials (Fig. 5). The survey scan spectrum (Fig. 5a) confirms the presence of key elements, with Fe 2p, O 1s, and C 1s peaks present in the undoped Fe₃O₄@C. The spectra for Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C show Al 2p and Mn 2p peaks, respectively, verifying the successful incorporation of these dopants into the Fe₃O₄ spinel structure. Further insights were obtained from the high-resolution Fe 2p spectrum of Fe₃O₄@C (Fig. 5b), which exhibits two broad peaks centered at 711 eV and 724 eV, corresponding to the Fe 2p_3/2 and Fe 2p_1/2 states.^37,38 Deconvolution of these peaks indicates the presence of both Fe²⁺ and Fe³⁺ oxidation states. Specifically, Fe²⁺ peaks appeared at 710.7 eV and 722.8 eV, while Fe³⁺ peaks were located at 712.3 eV and 724.6 eV. The distribution of oxidation states in Fe₃O₄@C was found to be 40.3% Fe²⁺ and 59.7% Fe³⁺, indicating a mixed-valence structure. Similarly, Fe(FeAl)₂O₄@C showed all characteristic Fe²⁺ and Fe³⁺ peaks in the Fe2p_3/2 and Fe 2p_1/2 spectra, with Fe²⁺ observed at 710.5 eV and 724.1 eV, and Fe³⁺ at 712.0 eV and 725.7 eV (Fig. 5g).^25,26 Here, the iron species distribution was 43% Fe²⁺ and 57% Fe³⁺. Notably, the addition of Al into the spinel lattice of Fe₃O₄ significantly influences the electronic properties of Fe, as indicated by a 1.1 eV increase in Fe³⁺ binding energy relative to Fe₃O₄@C. This shift results from the Lewis acid nature of Al, which polarizes neighbouring Fe atoms in the spinel lattice and enhances their electropositive character. The strong electron-withdrawing nature of Al within the spinel structure pulls electron density away from Fe sites, resulting in a unique and stronger Al–Fe interaction, reflected by the increased binding energy.^23,25,26 In Fig. 5h, the peaks in the Al2p spectrum at 74.7 and 73.8 eV correspond to Al–O–Al and Al–O–Fe linkages in Fe(FeAl)₂O₄@C.²⁶ Fe(FeMn)₂O₄@C exhibited the same characteristic Fe²⁺ and Fe³⁺ peaks as Fe₃O₄@C in the Fe 2p_3/2 and Fe 2p_1/2 spectra, specifically at 710.7 eV and 723.6 eV for Fe²⁺, and 711.9 eV and 725.8 eV for Fe³⁺ (Fig. 5d). Like Al, Mn incorporation in the spinel lattice polarizes the neighbouring iron sites, increasing their positive charge and shifting Fe³⁺ binding energy by 1.1 eV due to Mn's higher electronegativity (1.55) compared to Fe.¹⁹ This shift confirms the successful doping of Mn and underscores its role in enhancing Fe's electropositive character. Fig. 5e shows peak fitting of the Mn 2p spectrum, revealing the two oxidation states of manganese Mn(II) at 640.5 eV and 652.2 eV, and Mn(III) at 642.1 eV and 653.3 eV in Fe(FeMn)₂O₄@C.¹⁹ The high-resolution C 1s XPS spectrum (Fig. 5c, f and i) shows peaks at 284.5 eV and 285.4 eV, indicating CC and C–C bonds, confirming the presence of non-oxidized carbon on the surface.³⁹ The additional peak at 286.1 eV corresponds to hydroxyl carbon and that at 288.9 eV corresponds to carboxylate carbon in Fe₃O₄@C and Fe(FeMn)₂O₄@C, respectively. The zeta potential analysis highlights distinct surface charge behaviours of the synthesized catalysts across various pH levels (Fig. 6). Fe₃O₄@C demonstrates highly negative zeta potential from pH 2 to pH 7. This pronounced negativity can be attributed to the presence of oxygen containing functional groups, such as hydroxyl carbon (C–OH) and carboxylate carbon (O–CO), on the surface of the carbon matrix.⁴⁰ The increasing pH further intensifies the negative surface charge of Fe₃O₄@C due to the deprotonation of these functional groups. XPS analysis supports these observations, showing that Fe₃O₄@C contains approximately 15% hydroxy carbon (C–OH) (Fig. 5c). The higher abundance of C–OH groups significantly enhanced the negative surface charge, making Fe₃O₄@C more negative at increasing pH levels. In contrast, Fe(FeMn)₂O₄@C contains carboxylate carbon (O–CO) in lower quantities (6%) on the carbon surface, which is shown by a peak at a binding energy of 288.9 eV (Fig. 5f). Meanwhile, Fe(FeAl)₂O₄@C does not have any functional groups as there are no detectable peaks corresponding to hydroxyl or carboxylate carbon (Fig. 5i). Consequently, these materials display a positive surface charge up to pH 3 and turn negative at higher pH values.

3.2. Catalytic performance of Fe₃O₄@C, Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C towards CWPO of SMX

The performance of the synthesized catalyst was evaluated by its ability to convert H₂O₂ into active radicals and to achieve high mineralization of SMX. H₂O₂ was decomposed by two distinct pathways: a selective pathway of generating ·OH, which is highly reactive and crucial for effective degradation of organic pollutants, and a non-selective pathway that produces water and oxygen without contributing significantly to pollutant degradation. A notable reduction in TOC confirms SMX mineralization via the selective ·OH pathway, while lower TOC removal suggested the dominance of the non-selective route. The pseudo-first-order rate constant was determined from the H₂O₂ conversion under the given experimental conditions. The balance between these pathways, influenced by the catalyst's structure and reaction conditions, underscores the need for optimization to enhance pollutant removal.

3.3. Proposed mechanism for the higher catalytic activity of Fe(FeAl)₂O₄@C compared to Fe(FeMn)₂O₄@C

In the Fe(FeMn)₂O₄@C spinel structure, both Fe and Mn can generate HO· by reacting with H₂O₂ as both are redox-active metals. Among them, the Mn²⁺/H₂O₂ reaction is kinetically slower than the Fe²⁺/H₂O₂ reaction.⁴⁴ Consequently, the catalytic decomposition of H₂O₂ is initiated by the kinetically favourable oxidation reaction involving the transition of Fe²⁺ to Fe³⁺ within the Fe(FeMn)₂O₄@C spinel structure, leading to the formation of HO· (Scheme 1a, step 1). This oxidation of Fe²⁺ to Fe³⁺ creates a charge imbalance in the spinel structure. It is important to note that Mn in the spinel structure polarized the Fe electrons and created additional electropositive character on Fe^3+(δ+), which is evident in the XPS analysis shift to a higher binding energy of 1.1 eV for Fe³⁺. Subsequently, the more electropositive Fe^3+(δ+) competes with H₂O₂ in oxidizing the nearby Mn²⁺. If Mn²⁺ is oxidized by reacting with H₂O₂ to produce HO·, both Mn and Fe contribute to a significant increase in HO· production, along with an enhanced rate of H₂O₂ conversion and TOC removal. However, experimental results showed that the k_obs for H₂O₂ conversion over Fe(FeMn)₂O₄@C was moderate (0.037 min⁻¹), indicating that this pathway is not predominant under the studied conditions. Thus, Mn²⁺ is oxidized by neighbouring more electropositive Fe^3+(δ+) as shown in Scheme 1a, step 2. Further, the formed Mn³⁺ ions are reduced back to Mn²⁺ by reacting with H₂O₂, producing HOO· (Scheme 1a, step 3). This process, as illustrated in Scheme 1a, repeats through steps 3 and 4, thereby sustaining the catalytic cycle. On the other hand, the redox inactive Al in the Fe(FeAl)₂O₄@C catalyst effectively decomposed H₂O₂ with a higher k_obs of 0.11 min⁻¹ compared to Fe(FeMn)₂O₄@C. In this case, initially, the kinetically favourable oxidation of Fe²⁺ to Fe³⁺ occurs by reacting with the oxidant H₂O₂, leading to the generation of HO· and creating a charge imbalance in the spinel structure (as shown in Scheme 1b, step 1). The presence of Al in the spinel structure polarizes the electrons around Fe due to its Lewis acid properties, resulting in a more electropositive character for Fe^3+(δ+). This shift is evident in the XPS analysis, which shows a binding energy increase of 1.1 eV for Fe³⁺. To restore charge neutrality, the more electropositive Fe^3+(δ+) is rapidly reduced to Fe²⁺ through a reaction with H₂O₂, producing HOO· and it is further oxidized by an adjacent Fe^3+(δ+) (Scheme 1b, step 2). The catalytic cycle proceeds through steps 1 and 2, as shown in Scheme 1b. It is worth mentioning that both Al and Mn equally polarized the Fe³⁺ electrons. However, under the given reaction conditions, the polarization effect of redox-active Mn is partially neutralized by its redox activity towards H₂O₂. Since both H₂O₂ and Fe compete for the oxidation of Mn²⁺, the reduction rate of Fe³⁺ is slowed down (as shown in Scheme 1a step 2). On the other hand, the redox-inactive metal Al effectively polarized Fe³⁺ in the spinel structure, which enhanced the reduction reaction of Fe³⁺, leading to a three-fold increase in the kinetics of H₂O₂ decomposition compared to redox-active metal Mn.

3.4. Scavenging experiments

Quenching experiments were performed to determine the reactive oxygen species involved in the mineralization of SMX over Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C catalysts. In this study, 3 mM of scavengers such as isopropanol for ·OH, chloroform for ·O₂⁻, and dimethyl sulfoxide for ·OOH/·OH were used to assess the contribution of these radicals. In the absence of scavengers, all three catalysts achieved complete SMX removal within 5 minutes, demonstrating their high catalytic efficiency. When chloroform was introduced to scavenge ·O₂⁻, complete SMX removal was still observed (Fig. 8), indicating that superoxide radicals did not play a significant role in the mineralization process. However, in the presence of isopropanol, which targets ·OH radicals, the SMX removal efficiency dropped significantly.

The removal efficiencies declined to 14%, 20%, and 16% for Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C, respectively (Fig. 8). This pronounced reduction highlights the crucial involvement of ·OH radicals in the SMX mineralization process. Further, DMSO, which scavenges both ·OOH and ·OH radicals, caused a more pronounced reduction in SMX removal efficiency, reducing it to 4%, 9%, and 6% for Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C, respectively (Fig. 8). These results indicate that both ·OOH and ·OH radicals are involved in the reaction and ·OH is the dominant reactive oxygen species responsible for SMX mineralization across all three catalysts.

3.5. Effect of co-existing inorganic anions, organic acids and water matrices

The presence of various anions in natural water environments poses considerable challenges in wastewater treatment, as these anions can interfere with free radical-induced reactions. To explore their effects on the catalytic degradation of SMX, a study was conducted by introducing 10 mmol of specific inorganic salts and organic acids into the reaction system.⁴⁵ These included NaCl, NaNO₃, Na₂CO₃, K₂HPO₄, and humic acid. With the introduction of Cl⁻, the SMX removal efficiencies were 87%, 94%, and 96% for Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C, respectively (Fig. 9). This indicates only a slight inhibitory effect of Cl⁻ on the degradation process, likely due to the interaction between Cl⁻ and ·OH, generating ClOH·⁻. Under acidic conditions, ClOH·⁻ may further react with protons to form less reactive Cl· species, contributing to the minor reduction in SMX degradation efficiency.³¹ Similarly, NO₃⁻ could form nitrate radicals upon reacting with hydroxyl radicals, which might slightly reduce the removal efficiency.³¹ The presence of CO₃²⁻ led to a significant decrease in degradation efficiency, with removal rates dropping to 71%, 81%, and 84% for all three catalysts (Fig. 9). This reduction occurs as CO₃²⁻ reacts with hydroxyl radicals to produce carbonate radicals, thereby diminishing the concentration of hydroxyl radicals and lowering the possibility of their interaction with SMX.³¹ Phosphate anions (HPO₄³⁻) showed a more pronounced inhibitory effect on SMX degradation across all three catalyst systems, with efficiency reductions of 60%, 72%, and 74% over Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C, respectively (Fig. 9). This significant inhibition is likely due to the formation of phosphate radicals which react with organic contaminants at a slower rate compared to hydroxyl radicals.⁴⁵ Similarly, humic acid substantially inhibited SMX removal, reducing efficiency by 79%, 87%, and 90% for Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C, respectively (Fig. 9). The competition between humic acid and hydroxyl radicals likely accounts for this decline in removal efficiency.⁴⁶ Further experiments conducted in different water matrices, such as drinking water and tap water, demonstrated complete removal of SMX under optimized conditions, as shown in Fig. 9. Despite significant interference in SMX degradation caused by carbonate anions, phosphate anions, and humic acid, the Fe(FeAl)₂O₄@C/H₂O₂ system outperformed the other two catalysts.

3.6. Toxicity assessment (sludge inhibition test and acute toxicity)

Toxicity assessment of treated effluent is critical when complete mineralization is not achieved. Intermediates produced from the partial oxidation of SMX reportedly exhibit greater biological toxicity than the parent pollutant.⁴⁷ To evaluate the toxicity of treated effluent obtained from Fe₃O₄@C, Fe(FeMn)₂O₄@C, and Fe(FeAl)₂O₄@C catalysts, we conducted an activated sludge inhibition test alongside a widely recognized acute toxicity assessment. As shown in Fig. S5,† the initial SMX demonstrated moderate biotoxicity, resulting in a 52% inhibition of oxygen uptake at 0 minutes. During the initial 15 minutes of the reaction, the inhibition of oxygen uptake increased, peaking at 64%, 70%, and 68% for Fe₃O₄@C, Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C, respectively (Fig. S5†). This trend suggests the formation and accumulation of highly toxic intermediates in the early stages of the reaction. However, as the reaction progressed, the inhibition percentage gradually decreased, indicating further mineralization of the toxic intermediates into less harmful molecules. After 60 minutes of reaction time, the inhibition percentages were reduced to 22% and 18% for Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C, respectively, (Fig. S5†) which were lower than the initial SMX inhibition. These results highlight that higher mineralization efficiency correlates with reduced toxicity, underscoring the effectiveness of these systems in mitigating the environmental impact of SMX degradation. In contrast, the inhibition percentage for Fe₃O₄@C remained higher than the initial SMX even after 60 minutes of reaction. This finding indicates the persistence of toxic intermediates, suggesting that the Fe₃O₄@C system is less effective in degrading the harmful by-products compared to Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C.

The above toxicity study outcomes were further validated using established acute toxicity assessment methods, which measured the mortality rate of aquatic species Artemia salina, which serves as a bioindicator to evaluate the toxicity of the treated effluent.⁴⁸ The bioindicator's mortality rate was assessed after a 24 hour incubation period. Effluent samples collected at 15, 30, 45, and 60 minutes of reaction time for Fe₃O₄@C, Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C catalysts were analyzed for toxicity, with their corresponding TOC values provided in Table S1.† The negative control (saline water) showed no mortality, as presented in Table S1,† whereas the positive control (10 ppm SMX) resulted in significant mortality even at lower doses. Consistent with the results of the oxygen inhibition method, the reaction effluent collected after the first 15 minutes demonstrated significant toxicity, with a mortality rate exceeding 60% for all three catalysts, even at higher dilutions. This suggests that the intermediate in the solution was more toxic than the initial SMX antibiotic. As the reaction progressed and mineralization increased, the mortality rate decreased. After 60 minutes of reaction, Fe(FeMn)₂O₄@C and Fe(FeAl)₂O₄@C achieved 48% and 60% mineralization, respectively, rendering the effluent non-toxic to Artemia salina, with mortality rates of 40% and 30% at 100% v/v concentrated effluent (Table S1†). In contrast, the effluent collected after 60 minutes using the Fe₃O₄@C catalyst with only18% mineralization remained more toxic, exhibiting a high mortality rate of above 50% across all dilution levels. During the initial phase of the reaction, toxic intermediates are produced from SMX, including benzene rings with hydroxyl and amine groups, as well as isoxazole rings.⁴⁹ These intermediates lead to the generation of toxic effluents (Table S1†). As the reaction continues, these intermediates further oxidized, primarily resulting in the formation of non-toxic malic acid and oxalic acid.⁵⁰ It is important to note that the Fe(FeAl)₂O₄@C catalyst effectively detoxifies the effluent within 30 minutes, resulting in a 30% mortality rate. In contrast, the effluent treated with the Fe(FeMn)₂O₄@C catalyst remains more toxic, showing a 60% mortality rate after the same reaction time (Table S1†). This difference highlights the crucial role of the redox-inactive Al substituted in the spinel structure of Fe₃O₄, which enhances the kinetics of the Fe²⁺/Fe³⁺ redox cycle and leads to the rapid degradation of SMX.

3.7. Recycle study

The Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C catalysts demonstrated excellent catalytic activity and stability over three consecutive recycles (Fig. 10). Both catalysts completely removed SMX throughout the recycle study, with only a minimal reduction in mineralization efficiency. Specifically, the mineralization efficiency decreased from 60% to 50% for the Fe(FeAl)₂O₄@C catalyst (Fig. 10b), and from 50% to 41% for the Fe(FeMn)₂O₄@C catalyst after three cycles (Fig. 10a). In the initial cycles, the Fe(FeAl)₂O₄@C catalyst exhibited higher leaching of Fe 2 ppm (1%) and Al 1.7 ppm (6%). However, by the third cycle, metal leaching arrested significantly with Fe concentration falling to 0.2 ppm (0.09%) and Al to 0.2 ppm (0.7%). Similarly, the Fe(FeMn)₂O₄@C catalyst showed elevated leaching of Fe 2.4 ppm (1.1%) and Mn 2 ppm (3.6%) in the initial run, but leaching declined in the subsequent cycles, stabilizing at 0.7 ppm (0.3%) for Fe and 0.6 ppm (1.2%) for Mn after the 3rd recycle. The XRD patterns of the spent Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C catalysts (Fig. S6†) were nearly identical to those of the fresh catalysts, showing a complete match with all the characteristic peaks. This clearly demonstrates that the catalysts remained unchanged after three consecutive cycles. The FE-SEM analysis of the spent Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C catalysts, after 3-cycles, shown in Fig. S7,† demonstrates that the catalyst particles size remained almost unchanged when comparing the fresh and spent catalysts. Additionally, the FE-SEM elemental mapping of the spent Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C catalysts (Fig. S8†) revealed a uniform distribution of Fe, Al and Mn within the carbon matrix, similar to the fresh catalyst. In the recycle study, although SMX mineralization % marginally declined, still, the effluent obtained was non-toxic to Artemia salina, as the mortality rate remained low (Table S1†). The cost analysis of the synthesized catalysts shows that the cost per kilogram for Fe₃O₄@C, Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C is 3998 (46.5 USD), 4250 (51 USD), and 4257 (49.6 USD), respectively. These costs, based on a straightforward synthesis method, are comparable to the reported Fe₃O₄.⁵¹ Detailed calculations are provided in the ESI† (page S8).

4. Conclusion

In this study, a redox-inactive Al-doped magnetite catalyst was shown to be a more efficient Fenton-like catalyst than redox-active Mn-doped magnetite and un-doped magnetite catalysts. XPS analysis revealed the polarisation of Fe³⁺ electrons in both Fe(FeAl)₂O₄@C and Fe(FeMn)₂O₄@C, indicated by a shift of the Fe³⁺ peak to a higher binding energy of about 1.1 eV. The doped Al polarises Fe electrons in the magnetite spinel structure through its Lewis acid property, whereas the doped Mn polarises Fe electrons owing to its higher electronegativity than Fe. The Fe(FeAl)₂O₄@C catalyst decomposed H₂O₂ with a three times higher k_obs value of 0.11 min⁻¹ compared to Fe(FeMn)₂O₄@C, though both redox-active Fe and Mn being capable of decomposing H₂O₂ in the Fe(FeMn)₂O₄@C catalyst. The polarized Fe^3+(δ+) in Fe(FeAl)₂O₄@C, after the kinetically favourable oxidation of Fe²⁺ to Fe³⁺ by H₂O₂, expedites the kinetics of the challenging Fe³⁺ reduction reaction with H₂O₂ to regain charge neutrality in the spinel structure. Meanwhile, subsequent to kinetically favourable oxidation of Fe²⁺ to Fe³⁺ in Fe(FeMn)₂O₄@C, the polarization effect of Mn on Fe is partially neutralized by Mn's redox activity towards H₂O_2. The competition between H₂O₂ and neighbouring Fe^3+(δ+) for the oxidation of Mn²⁺ in the spinel structure slows down the reduction rate of Fe³⁺. Under the optimized conditions, the Fe(FeAl)₂O₄@C catalyst exhibited superior catalytic performance for the degradation of SMX with 60% TOC removal, compared to 50% and 18% TOC removal attained from Fe(FeMn)₂O₄@C and Fe₃O₄@C, respectively. Furthermore, Fe(FeAl)₂O₄@C selectively decomposed H₂O₂ to produce HO· and HOO· with good stoichiometric efficiency at pH levels of 3, 4 and 5. In contrast, the Fe(FeMn)₂O₄@C catalyst from pH 4 to 7 produced O₂ and H₂O by non-selective decomposition of H₂O₂ resulting in poor stoichiometric efficiency. The Fe(FeAl)₂O₄@C catalyst effectively detoxifies the effluent within 30 minutes. In contrast, the effluent treated with the Fe(FeMn)₂O₄@C catalyst remains more toxic, showing a 60% mortality rate in acute toxicity assessment after the same reaction time. The Fe(FeAl)₂O₄@C catalyst demonstrated excellent catalytic activity and stability over three consecutive recycles with an Fe leaching of 0.2 ppm (0.09%) and Al of 0.6 ppm (2%). This study highlights the importance of polarizing the Fe electrons in the magnetite spinel by doping with redox-inactive metal Al with Lewis acid character to expedite the challenging Fe³⁺ reduction in the redox cycle. A redox-inactive metal like Al is identified as a suitable dopant to effectively polarise the Fe electrons in the magnetite spinel compared to the redox-active metal Mn. The findings of this research expand the potential of magnetite as an economically viable Fenton-like catalyst for real wastewater treatment applications.

Data availability

The data supporting this article are provided in the manuscript and its ESI.†

Author contributions

B. Gokulakrishnan: data curation, formal analysis, writing original draft; G. Satishkumar: conceptualization, supervision and writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors acknowledge VIT Seed Grant (File No.: SG20230101) for providing the financial support for carrying out this research work. The authors are grateful to the Department of Science and Technology (DST), New Delhi, India for providing financial support to acquire X-ray Photoelectron Spectroscopy (XPS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) equipment through “Promotion of University Research and Scientific Excellence (PURSE)” under Grant No. SR/PURSE/2020/34 (TPN 56960) and carry out the work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ew00452g

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