Visible-light-driven malachite green degradation by optimized mil-125(ti) immobilized in alginate hydrogel for continuous-flow remediation

2.1. Chemicals and materials

All chemicals were of analytical grade and used without further purification. Titanium(IV) butoxide (C₁₆H₃₆O₄Ti), terephthalic acid (C₈H₆O₄), N,N-dimethylformamide (DMF, C₃H₇NO), methanol (CH₃OH), absolute ethanol (C₂H₅OH), malachite green (MG, C₂₃H₂₅ClN₂), trichloromethane (CHCl₃), ethylenediaminetetraacetic acid (EDTA, C₁₀H₁₄N₂O₈), and potassium bromate (KBrO₃) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). A stock solution of MG (500 mg/L) was prepared by dissolving a precisely weighed amount of malachite green (MG) powder in ultrapure water, and working solutions were obtained through appropriate dilution. Sodium alginate (SA), SDS, and NaHCO₃ were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultra-pure water (18.25 MΩ·cm) was produced using a Milli-Q system (Millipore, Bedford, MA, USA) and used throughout the experiments.

2.2. Preparation of photocatalysts

2.2.2. Fabrication of MIL-125(Ti)/SA immobilized gel beads

As illustrated in Figure S1, MIL-125(Ti)-150°C powder (0-2 wt%) was dispersed in 20 mL ultrapure water and ultrasonicated for 30 min. Sodium alginate (0.3 g) was added and stirred overnight to obtain a uniform solution, followed by the addition of SDS (60 mg) and NaHCO₃ (0.3 g). The foamy pre-gel was extruded through a syringe (0.5 mm needle, 30 mL/h) into a coagulation bath containing 10% CaCl₂ and 10% acetic acid. The beads were cured overnight, washed thoroughly, and freeze-dried for 36 hrs. The composites were denoted as x%-MIL-125(Ti)/SA (x=0, 0.5, 1, 1.5, 2). Control beads without SDS/NaHCO₃ were prepared under identical conditions and denoted 0.5%-Control.

2.3. Characterizations of photocatalysts

The morphological features of the synthesized photocatalysts were examined using a Quantum FEG 650 scanning electron microscope (SEM). Fourier transform infrared (FT-IR) spectra were recorded on an IFS120HR spectrometer (FEI, Hillsboro, OR, USA) to identify functional groups. Crystalline structures were analyzed by X-ray diffraction (XRD) using a Rigaku D/Max-2400 diffractometer (Shimadzu, Japan) with Cu Kα radiation. The specific surface area and porosity were determined from N₂ adsorption-desorption isotherms measured at 77 K using an ASAP2020 analyzer (Thermo Fisher, Arbor, MI, USA), with the Brunauer-Emmett-Teller (BET) method applied for surface area calculation. Surface chemical composition and elemental states were analyzed by X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher ESCALAB 250 XI spectrometer (Waltham, MA, USA).

2.4. Photocatalytic activity evaluation

2.5. Recyclability test and radical trapping experiments

The stability and reusability of the photocatalysts were evaluated through consecutive recycling experiments. After each photocatalytic run, the used catalyst was recovered by centrifugation, washed thoroughly three times with absolute ethanol and deionized water to remove adsorbed MG and degradation intermediates, and dried overnight under vacuum at 60°C [23]. The regenerated catalyst was then reused in a fresh MG degradation cycle under identical reaction conditions.

To identify the primary reactive species responsible for MG degradation, radical trapping experiments were performed by introducing specific scavengers into the reaction system at a concentration of 10 mM prior to irradiation [15].The scavengers used were: EDTA for holes (h⁺), chloroform (CHCl₃) for superoxide radicals (•O₂⁻), methanol (CH₃OH) for hydroxyl radicals (•OH), and potassium bromate (KBrO₃) for electrons (e⁻). The degradation efficiency in the presence of each scavenger was monitored and compared to the control experiment conducted without any scavenger.

2.6. Statistical analysis

All experiments were performed in triplicate (n=3) to ensure statistical reliability and to account for experimental variability, and data are presented as mean values ± standard deviation. Statistical significance of differences among experimental groups was assessed using one-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test, performed with SPSS version 22.0 statistical software (IBM SPSS Statistics, Chicago, IL, USA). A p-value of less than 0.05 (p < 0.05) was considered statistically significant.

3.1. Structural and morphological characterization

The morphological and structural properties of the synthesized MIL-125(Ti)-150°C were first investigated. SEM imaging (Figure 1a) revealed a characteristic tetragonal cake-like morphology with smooth surfaces and minimal structural defects, consistent with previous reports [24]. The particles exhibited a uniform size distribution, with diameters ranging from 600 to 800 nm and thicknesses of approximately 200 nm. This well-defined and uniform morphology is advantageous for providing a high specific surface area and enhanced light-harvesting efficiency [25]. Transmission electron microscope (TEM) analysis (Figures 1b and c) further confirmed the plate-like structure with sharp edges and consistent thickness, indicating high crystallinity achieved during synthesis. This uniform architecture is beneficial for facilitating the separation and transport of photogenerated charge carriers while providing abundant accessible active sites, thereby augmenting the overall photocatalytic activity. Complementary energy dispersive spectroscopy (EDS) (Figure 1d) quantitatively identified the elemental composition as C (34.4 wt%), O (27.4 wt%), and Ti (38.2 wt%), which aligns well with the theoretical stoichiometry of MIL-125(Ti) (Ti₈O₈(OH)₄[(O₂C)C₆H₄(CO₂)]₃) [24], thereby confirming successful synthesis of the target material.

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Figure 1.

Morphological and compositional analysis of MIL-125(Ti) synthesized at 150°C. (a) SEM image showing the uniform tetragonal cake-like morphology. (b, c) TEM images at different magnifications illustrating the plate-like structure and high crystallinity. (d) EDS spectrum confirming the elemental composition (C, O, Ti) consistent with MIL-125(Ti) stoichiometry.

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X-ray photoelectron spectroscopy (XPS) analysis was conducted to elucidate the surface electronic structure and oxidation states of the synthesized material. The full survey spectrum of MIL-125(Ti)-150°C (Figure 2a) exhibits prominent core-level peaks corresponding to C 1s (284 eV), Ti 2p (458 eV), and O 1s (531 eV), confirming the elemental composition. High-resolution analysis of the Ti 2p region (Figure 2b) reveals a characteristic spin-orbit doublet with peaks centered at binding energies of 457.9 eV (Ti 2p₃/₂) and 463.7 eV (Ti 2p₁/₂). The absence of satellite features and the precise binding energy positions confirm the predominant presence of Ti⁴⁺ species, consistent with the expected tetravalent state in the MIL-125(Ti) framework, thereby attesting to the successful synthesis and high chemical purity of the material. Deconvolution of the high-resolution C 1s spectrum (Figure 2c) yields three distinct components at 284.0 eV (C=C), 285.4 eV (C–C), and 287.9 eV (C=O), reflecting the diverse chemical environments of carbon atoms within the organic linker. Similarly, the O 1s spectrum (Figure 2d) is resolved into three contributions at 528.6 eV (Ti–O), 529.8 eV (C=O), and 532.0 eV (–OH), attributable to oxygen in metal-oxo bonds, carboxylate groups, and hydroxyl species, respectively. These XPS results provide compelling evidence for the formation of the MIL-125(Ti) structure with its characteristic coordination environments.

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Figure 2.

Comprehensive physicochemical characterization of MIL-125(Ti)-150°C. (a-d) X-ray photoelectron spectroscopy (XPS) spectra showing ((a) full survey, (b) Ti 2p, (c) C 1s, and (d) O 1s regions). (e) XRD pattern confirming the crystalline framework. (f) FT-IR spectrum indicating functional groups of organic linkers and Ti-O bonds. (g) UV-Vis DRS spectrum demonstrating visible-light absorption (h) N₂ adsorption-desorption isotherm with inset pore size distribution curve showing microporous structure.

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The crystalline structure of MIL-125(Ti)-150°C was confirmed by X-ray diffraction analysis (XRD) analysis (Figure 2e). The observed diffraction peaks at 6.7°, 9.7°, 11.6°, 14.9°, 15.2°, 16.5°, 17.9°, 18.8°, and 19.5° are well-matched with literature values, corresponding to the (101), (200), (221), (310), (103), (222), (312), (213), and (400) crystallographic planes, respectively, indicating a well-defined crystalline framework [26]. The sharp and intense peaks further signify high crystallinity and structural stability. FT-IR spectroscopy (Figure 2f) revealed characteristic functional groups: a broad peak at 3378 cm⁻¹ attributed to O–H stretching vibrations, peaks at 1669 cm⁻¹ and 1590 cm⁻¹ assigned to C=O and aromatic C=C stretching vibrations, respectively, a peak at 1385 cm⁻¹ indicative of asymmetric OCO stretching, and a peak at 630 cm⁻¹ corresponding to Ti–O stretching vibrations [26-28]. These spectral features collectively confirm the successful formation of MIL-125(Ti) with intact organic linkers and metal-oxo bonds. UV-Vis diffuse reflectance spectroscopy (DRS) analysis (Figure 2g) demonstrated strong absorption in the ultraviolet region, yielding a calculated band gap energy of 3.26 eV for MIL-125(Ti)-150°C. Furthermore, the N₂ adsorption-desorption isotherm (Figure 2h) exhibited a characteristic type I isotherm, with a pore size distribution centered around 2 nm, indicating a microporous structure. This favorable porosity is anticipated to enhance both the adsorption capacity and catalytic performance of the material.

3.2. Photocatalytic performance in batch mode

The photocatalytic performance of the synthesized MIL-125(Ti) samples was evaluated for MG degradation under visible light irradiation. As illustrated in Figure 3(a), the sample synthesized at 150°C demonstrated superior activity compared to those prepared at 120°C and 200°C. Kinetic analysis revealed that the apparent rate constant for MIL-125(Ti)-150°C was approximately two orders of magnitude higher, underscoring its exceptional intrinsic photocatalytic activity. This was visually corroborated by the distinct decolorization of the MG solution (Figure 3b) and the progressive attenuation of its characteristic absorption peak (Figure S3). Ultimately, the 150°C catalyst achieved a final degradation efficiency nearly 3.2 times higher than the 120°C variant, establishing a clear synthesis-temperature-performance relationship (Figure 3c).

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Figure 3.

Photocatalytic degradation of malachite green (MG) using MIL-125(Ti)-150°C. (a) Photodegradation kinetics under visible light irradiation. (b) Visual decolorization of MG solution (from blue-green to colorless). (c) Comparative analysis of dark adsorption and photocatalytic performance. (d) Effect of initial MG concentration (20–80 μg/mL). (e) Effect of catalyst dosage (0.05–0.25 mg/mL). The different small letters represent significant differences between treatments by ANOVA (p < 0.05). (f) Effect of solution pH (2–10). All degradation efficiencies are mean values of triplicate experiments with error bars representing standard deviations. The different small letters represent significant differences between treatments by ANOVA (p < 0.05).

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3.3. Optimization of reaction parameters

The effects of key operational parameters on the photodegradation were systematically investigated. As illustrated in Figure 3(d), increasing the MG concentration from 20 to 80 μg/mL resulted in a progressive decline in degradation efficiency, underscoring the inhibitory effect of high pollutant loading. This trend is attributed to the saturation of active sites on the catalyst surface and reduced light penetration through the more intensely colored solution [23].

Variation in catalyst dosage also significantly influenced the process (Figure 3e). The degradation efficiency and corresponding rate constants (k) increased with catalyst loading up to an optimum of 0.2 mg/mL, beyond which performance plateaued or slightly decreased. The initial improvement is due to greater availability of active sites [29], whereas the decline at higher loadings (>0.2 mg/mL) is likely caused by increased light scattering and reduced photon absorption in the suspension. Thus, 0.2 mg/mL was selected as the optimal dosage, offering a balance between efficiency and practical applicability.

Solution pH exerted a notable influence on degradation performance (Figure 3f). High efficiency (>95%) was maintained over a broad pH range from 4 to 8, with the maximum rate constant observed at pH 4. Performance decreased under strongly acidic (pH 2) or alkaline (pH 10) conditions. Since the natural pH of the malachite green (MG) solution was approximately 4.7—within the optimal range—no pH adjustment was made in subsequent experiments.

3.4. Catalyst stability and recyclability

The operational stability of MIL-125(Ti)-150°C was confirmed through four consecutive photocatalytic cycles (Figure 4a). Although the degradation efficiency experienced a discernible decline with each cycle, the catalyst maintained significant activity, underscoring its potential for reuse. The primary reduction in performance was attributed to diminished adsorption capacity during the dark phase, as evidenced by the trends in Figure 4(b). This phenomenon is likely due to the irreversible adsorption of MG molecules or reaction intermediates within the catalyst’s pores, which conventional washing fails to remove. Such fouling of active sites is a common challenge in photocatalysis and is identified as the key factor limiting long-term recyclability in this system [30].

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Figure 4.

Cyclic stability and reactive species identification of MIL-125(Ti)-150°C. (a) MG degradation efficiency across four consecutive photocatalytic cycles with error bars (mean ± SD, n = 3). (b) Variation in C/C₀ over repeated cycles. (c) Effects of individual radical scavengers on degradation rate. (d) Comparison of MG removal efficiency under different scavenger conditions. Error bars indicate experimental reproducibility. The different small letters represent significant differences between treatments by ANOVA (p < 0.05).

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3.5. Mechanistic investigation of photocatalytic activity

Radical trapping experiments were performed to decipher the photocatalytic mechanism of MG degradation over MIL-125(Ti)-150°C. The results (Figures 4c and d) clearly indicate that while hydroxyl radicals (•OH) play a negligible role, photogenerated holes (h⁺), superoxide radicals (•O₂⁻), and electrons (e⁻) are all pivotal active species. The significant, yet partial, suppression of degradation by their respective scavengers, and the near-complete quenching of activity upon their simultaneous addition, point to a synergistic mechanism where these species act in concert to mineralize the dye. This mechanistic insight is consistent with the outstanding performance of our catalyst, which, as detailed in Figure S4 and Table S1, demonstrates a superior reaction rate constant compared to benchmark materials in the literature, underscoring its competitive advantage.

3.6. Characterization of the immobilized hydrogel beads

To bridge the gap between powder catalysis and practical application, the optimized MIL-125(Ti) was encapsulated within a macroporous alginate hydrogel matrix. Cross-sectional SEM analysis (Figures 5a and b) revealed that the resulting composite beads possessed a uniform spherical morphology and a highly porous three-dimensional internal structure. This interconnected macroporous network, formed utilizing gas bubbles within the pre-gel solution as pore templates, resulted from the cross-linking of sodium alginate (SA) by Ca²⁺ upon contact with CaCl₂ and CH₃COOH, concurrent with the interconnection of individual pores by CO₂ gas generated from the reaction between NaHCO₃ and CH₃COOH [31]. The interconnected porous network facilitates enhanced adsorption and diffusion of pollutants while providing abundant active sites for photocatalytic reactions, thereby significantly boosting photocatalytic efficiency. Furthermore, pore connectivity promotes the transport of photogenerated charge carriers and rapid migration of reactants, collectively enhancing the overall catalytic performance.

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Figure 5.

Characterization of 0.5%-MIL-125(Ti)/SA macroporous hydrogel beads. (a, b) Cross-sectional SEM images (scale bars = 500 μm and 100 μm, respectively) showing interconnected macroporous network structure. The yellow box in (a) highlights the surface feature shown at higher magnification in (b). (c) FT-IR spectrum confirming the coexistence of MIL-125(Ti) and sodium alginate functional groups. The macroporous architecture promotes pollutant diffusion and light penetration within the beads.

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Fourier transform infrared spectroscopy (FT-IR) spectroscopy (Figure 5c) confirmed the successful incorporation of MIL-125(Ti) into the alginate matrix. The spectrum of the composite bead displayed characteristic alginate vibrations alongside features attributable to the Ti–O–C or O–Ti–O bonds of the MIL-125(Ti) framework. Importantly, the chemical integrity of the photocatalyst was preserved within the beads, demonstrating its stability under the acidic preparation conditions.

3.7. Continuous-flow photodegradation in a fixed-bed reactor

The performance of the immobilized photocatalyst was further evaluated under continuous-flow conditions using a fixed-bed reactor. As shown in Figure 6(a), the degradation efficiency was highly dependent on the MIL-125(Ti) loading within the alginate beads. While the 0%-MIL-125(Ti)/SA beads (blank control) showed only 34.3% removal, primarily through adsorption, the incorporation of the photocatalyst significantly enhanced performance. The optimal loading was found to be 0.5%, achieving a peak removal efficiency of 83.4% within 90 min. However, a further increase in the photocatalyst loading beyond this optimum led to a noticeable decline in efficiency (Figure 6b), which is attributed to reduced light penetration and potential pore blocking within the hydrogel matrix at higher loadings [32]. Kinetic analysis confirmed that the degradation process for all loadings followed pseudo-first-order kinetics (Figure 6c). The 0.5%-MIL-125(Ti)/SA composite exhibited the highest apparent rate constant (k = 0.01661 min⁻¹), which was substantially greater—by approximately 5-fold and 2.5-fold, respectively—than that of the blank beads and the higher-loading (1.5% and 2%) composites, unequivocally establishing 0.5% as the optimal loading.

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Figure 6.

Continuous-flow photodegradation of MG using MIL-125(Ti)/SA beads in a fixed-bed reactor. (a) Degradation curves for varying MIL-125(Ti) loadings (0–2%). (b) Corresponding degradation rate. The different small letters represent significant differences between treatments by ANOVA (p <0.05) and (c) pseudo-first-order kinetic fittings. (d) Comparison of adsorption and photocatalytic performance of gel beads with and without SDS/NaHCO₃ additives under dark and light conditions. (e) Pseudo-first-order kinetic plots confirming enhanced activity with macroporous structure. (f) Effect of flow rate (12–60 mL/h) on MG removal efficiency under visible light irradiation.

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The engineered macroporous structure, created using SDS and NaHCO₃, was identified as a critical factor governing performance. As shown in Figure 6(d), the macroporous beads demonstrated significantly superior adsorption in the dark (∼27% removal in 30 min) compared to the non-porous control beads, which showed negligible uptake. Under visible light irradiation, this performance gap widened dramatically, with the macroporous beads achieving a removal rate approximately 4.7 times higher than that of the control beads (Figure 6d). This enhanced performance is directly linked to the improved diffusion of pollutants and photons within the interconnected porous network. The vital role of the macropores was further quantified through kinetic analysis, which revealed that the degradation rate constant for the macroporous beads was nearly an order of magnitude higher than that of their non-porous counterparts (Figure 6e).

Finally, the influence of hydraulic retention time, controlled by the flow rate, on the system’s performance was investigated (Figure 6e). As expected, the degradation efficiency was inversely related to the flow rate. The highest efficiency (>94%) was observed at the lowest flow rate of 12 mL/h, which provides the longest contact time. An increase in flow rate to 30 mL/h resulted in a moderate decrease in efficiency, while a further increase to 60 mL/h led to a significant drop to 71.1%, due to insufficient contact time for complete adsorption and photocatalytic degradation. A flow rate of 30 mL/h was selected as the optimal compromise, balancing a high processing throughput with satisfactory degradation efficiency (Figure 6f).