Enhanced Charge Separation and Visible-Light Utilization in La-Doped MIL-100(Fe) for Efficient Cr(VI) Photoreduction

Abstract

Lanthanum-doped MIL-100­(Fe) (MIL-100­(Fe_0.99/La_0.01)) was successfully synthesized through a facile and environmentally friendly room-temperature stirring method to enhance charge separation and visible-light utilization for Cr­(VI) photoreduction. Structural characterization using XRD, FT-IR, SEM-EDS, BET, and UV–Vis reflectance spectroscopy confirmed the formation of a highly crystalline MIL-100­(Fe) framework with uniform octahedral morphology, hierarchical micro–mesoporous structure, and effective La incorporation into the framework. The La³⁺ dopant modified the local electronic structure by introducing electron-trapping sites, suppressing electron–hole recombination, and extending visible-light absorption up to 700 nm. The La-doped sample had a narrower optical band gap (2.93 eV) than the pure MIL-100­(Fe), which means it could collect light better. Photocatalytic experiments under visible-light irradiation demonstrated that MIL-100­(Fe_0.99/La_0.01) achieved superior Cr­(VI) reduction efficiency (88.35%) within 35 min at pH = 5, outperforming undoped MIL-100­(Fe) (60.04%) under identical conditions. Enhanced electrostatic interaction between positively charged catalyst surfaces and negatively charged Cr­(VI) species (HCrO₄ ^–, Cr₂O₇ ^2–) facilitates interfacial charge transfer and is responsible for the best performance at mildly acidic pH. Active-species trapping experiments revealed that hydroxyl radicals (^•OH) were the dominant reactive species, with superoxide radicals (^•O^2–) and photogenerated holes (h⁺) contributing minor roles. Kinetic analysis followed a pseudo-first-order model (k ₁ = 0.088 min^–1, R ² = 0.9965), confirming efficient reaction dynamics. Improved light absorption, hierarchical porosity that facilitates mass transport, and effective charge separation through La-induced defect states all work together to produce the increased photocatalytic activity of La-doped MIL-100­(Fe). This study not only establishes a sustainable, low-energy synthesis route for rare-earth-modified MOFs but also offers useful information regarding the design of visible-light-responsive mixed-metal frameworks for environmental remediation. The findings highlight La-doped MIL-100­(Fe) as a promising photocatalyst for Cr­(VI) detoxification and broader applications in solar-driven wastewater treatment.

Introduction

In the context of rapid industrialization, large quantities of heavy metal ions have been released from various industrial activities such as leather tanning, cement production, electroplating, metallurgy, and mineral extraction. Among these, hexavalent chromium (Cr­(VI)) is a highly toxic contaminant. It can cause cellular damage, teratogenic effects, and carcinogenesis, thereby posing a serious threat to both human health and ecological systems. − Trivalent chromium (Cr­(III)), on the other hand, is significantly less hazardous and even functions as a necessary trace element in the metabolism of lipids and glucose. Thus, reducing Cr­(VI) to Cr­(III) is a viable strategy to lessen pollution caused by chromium.

Because it may use natural sunshine, the photocatalytic reduction of Cr­(VI) to Cr­(III) under visible light has become a promising treatment method among those that have been studied. Furthermore, the utilization of visible light in photocatalytic processes offers several distinct advantages over traditional ultraviolet-driven systems. First, visible light constitutes nearly 45% of the solar spectrum, whereas UV radiation accounts for less than 5%, meaning that visible-light-responsive catalysts can harvest solar energy more efficiently. , This improved solar utilization not only enhances photocatalytic performance under natural sunlight but also significantly reduces reliance on artificial UV sources, thereby lowering operational costs and energy consumption. Second, the use of visible light mitigates potential hazards associated with UV irradiation, such as equipment degradation and health risks to operators, ensuring safer and more sustainable operation. Finally, the ability to activate photocatalysts under ambient sunlight enhances their practical applicability in large-scale environmental remediation. Because of this, visible light-driven photocatalysis is a very appealing method for reducing Cr­(VI) and breaking down other persistent pollutants. − Furthermore, the photocatalytic process can degrade pollutants into innocuous substances like CO₂ and H₂O without the need for additional toxic reagents, thereby minimizing the risk of secondary pollution.

Researchers have recently focused a lot of interest in metal–organic frameworks (MOFs) because of their exceptional qualities, which make them appropriate for a variety of uses such adsorption, catalysis, drug delivery, gas storage, and separation. − Among the common synthesis methods, hydrothermal synthesis remains the most prevalent, as it helps suppress the formation of amorphous phases. However, this method is energy-intensive and time-consuming, highlighting the urgent need for more efficient and environmentally friendly synthesis strategies to facilitate the industrial-scale application of MOFs. , In particular, MOFs-based materials have demonstrated remarkable potential in photocatalytic processes, benefiting from adjustable metal nodes and organic linkers that enable precise control over light absorption, charge transfer, and redox activity. Recent studies have reported that rational design strategies, such as metal doping, mixed-metal frameworks, and defect engineering, can significantly enhance the visible-light response and photocatalytic efficiency of MOFs for pollutant removal. − For example, advanced MOFs systems with tailored electronic structures exhibit improved charge separation and catalytic performance in wastewater treatment applications, underscoring the versatility of MOFs as visible-light-driven photocatalysts. Notably, more recent reports have further demonstrated that structural modulation and compositional optimization of MOFs can effectively promote interfacial charge transfer and suppress electron–hole recombination, thereby leading to enhanced activity and stability under visible-light irradiation. ,

MIL-100 and its derivatives are well-recognized representatives of highly stable MOFs, exhibiting remarkable properties such as very large surface areas (>2000 m²·g^–1), high thermal stability, and excellent solvent resistance. Structurally, MIL-100 is constructed from [M₃O­(X)­(H₂O)₂]⁶⁺ clusters (M = Fe, V, Al, Sc, Cr or doped metal combinations; X = OH^–, Cl^–, or F^–), coordinated with benzene-1,3,5-tricarboxylate (BTC^3–) ligands to form a stable three-dimensional network. , The Fe sites in MIL-100­(Fe) have been reported to show high catalytic and gas adsorption activity. Recent studies have indicated that doping rare earth elements (REEs) into MOFs frameworks can significantly improve their physicochemical properties, including porosity, active site density, and interactions with target molecules. , Among REEs, lanthanum (La), with its electronic configuration [Xe] 5d¹6s², has the ability to act as an electron trap due to the availability of empty 4f and 5d orbitals, thereby reducing charge recombination and enhancing photocatalytic performance. − Therefore, La-doping of photocatalysts has attracted considerable research interest.

In this work, we aimed to establish a synthesis platform for lanthanum-doped MIL-100­(Fe) materials using a simple, cost-effective, and environmentally friendly procedure. The conventional method is the hydrothermal approach, but this technique generally requires high temperature and pressure, leading to significant energy consumption and potential degradation of thermally sensitive organic linkers. Therefore, recent studies have highlighted that MOFs in general, and MIL-100­(Fe) in particular, can also be synthesized under ambient conditions. − This strategy relies on the self-assembly of Fe ions and organic linkers in solvents (e.g., ethanol, methanol, or water–organic mixtures) without external heating. Such an approach not only minimizes energy consumption but also preserves the structural integrity of heat-sensitive organic linkers, while yielding materials with uniform nano- to microscale particle sizes, high surface areas, and well-defined porous structures. Furthermore, the crystallization rate can be effectively tuned by adjusting parameters such as pH, the molar ratio of metal ions to linkers, and solvent composition, making room-temperature synthesis a highly promising strategy for large-scale and industrial production of MOFs.

X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier-transform infrared (FT-IR) spectroscopy were among the sophisticated characterization methods used to examine the produced photocatalysts. Moreover, ultraviolet–visible (UV–vis) spectroscopy was used to determine the residual concentration of Cr­(VI) in aqueous solutions after the photocatalytic treatment.

Experimental Section

Chemicals and Instrumentation

Iron­(II) sulfate heptahydrate (FeSO₄.7H₂O; 99 wt %), Lanthanum­(III) nitrate hexahydrate (La­(NO₃)₃.6H₂O; 98 wt %), Trimesic acid (C₆H₃-(COOH)₃; 98 wt %) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH; 96 wt %), Hydrochloric acid (HCl; 37 wt %), Potassium dichromate (K₂Cr₂O₇; 99.8 wt %) were purchased from Xilong Scientific. Ethanol (CH₃CH₂OH; 99.7 wt %) was purchased from Guangdong Guangshua Sci-Tech.

XRD, SEM, EDS, BET, FT-IR, and UV–vis DRS measurements were carried out using a Siemens D5005 diffractometer (Cu Ka radiation, l = 1.54056 Å), a Hitachi S4800 scanning electron microscope, an ISIS 300 energy-dispersive X-ray spectrometer, a Gemini VII 2390 surface analyzer, a NICOLET iS50FT-IR spectrometer, and a V-750 UV–visible spectrophotometer. Cr­(VI) concentrations were determined using an Agilent 8453 UV–visible spectroscopy instrument.

Synthesis of MIL-100­(Fe)

The MIL-100­(Fe) was synthesized via a green synthesis at room temperature (Figure ), referring to previously studied procedures. MIL-100­(Fe) was synthesized through a simple room-temperature stirring method. , MIL-100­(Fe) was synthesized through a simple room-temperature stirring method. Specifically, 1.9122 g of trimesic acid (H₃BTC, 9.12 mmol) was dissolved in 30 mL of 1.0 M NaOH solution under continuous magnetic stirring to facilitate deprotonation. The metal precursor solution was made by simultaneously dissolving 3.8087 g of ferrous sulfate heptahydrate (FeSO₄·7H₂O, 13.7 mmol) in deionized water. The Fe²⁺ solution was then continuously stirred while the deprotonated H₃BTC solution was gradually added. For a whole day, the resultant mixture was agitated at room temperature. After centrifuging the precipitate and washing it with ethanol and distilled water, it was dried to produce the orange-brown powder that is typical of MIL-100 (Fe).

1.

Schematic illustration of the synthesis of MIL-100­(Fe).

Synthesis of La-Doped MIL-100­(Fe)

La-doped MIL-100­(Fe) samples, specifically MIL-100­(Fe_0.99/La_0.01), were synthesized following the same room-temperature stirring method as described above, with the addition of lanthanum nitrate hexahydrate (La­(NO₃)₃·6H₂O) as the doping agent. The La³⁺ source was introduced into the Fe precursor solution to achieve a target molar ratio of Fe²⁺: La³⁺ = 99:1, while maintaining the total metal ion concentration constant. The subsequent steps, including the preparation of the organic linker solution, dropwise mixing, stirring conditions, centrifugation, washing, and drying, were identical to those used for the pristine MIL-100­(Fe) synthesis. The resulting La-doped MIL-100­(Fe) powders were then subjected to further characterization and performance evaluation.

Photocatalytic Experiments

The photocatalytic activity of La-doped MIL-100­(Fe), specifically MIL-100­(Fe_0.99/La_0.01), was assessed through the visible-light-driven reduction of Cr­(VI), using a 250 W lamp as the light source.

To investigate the influence of pH on photocatalytic performance, experiments were conducted at four different pH values (3, 5, 7 and 9), adjusted using HCl and NaOH solutions. A typical experiment involved dispersing the photocatalyst at a dosage of 0.3 g·L^–1 in an aqueous Cr­(VI) solution (initial concentration: 20 ppm). To achieve adsorption–desorption equilibrium, the suspension was agitated in the dark. Subsequently, 1 mL·L^–1 of 3% H₂O₂ was added to initiate the photocatalytic reaction under visible-light irradiation.

At selected time intervals, aliquots of the reaction solution were collected, and the catalyst was removed by centrifugation. The residual concentration of Cr­(VI) was determined using the diphenylcarbazide (DPC) method, in which Cr­(VI) ions react with 1,5-diphenylcarbazide in acidic medium to form a purple Cr–DPC complex that can be quantified by UV–Vis spectroscopy.

Comparative experiments were carried out to assess the Cr­(VI) reduction performance of pristine MIL-100­(Fe) and La-doped MIL-100­(Fe_0.99/La_0.01) after the ideal pH condition was determined. Additionally, the roles of reactive radicals and the kinetics of the photocatalytic reaction were examined.

Results and Discussion

Characterization of MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01)

X-Ray Diffraction (XRD) Patterns

The X-ray diffraction (XRD) patterns of the samples are shown in Figure . The diffraction profiles clearly confirm the formation of the MIL-100­(Fe) phase, as evidenced by the characteristic low-angle reflections at 2θ ≈ 4.25° and 10.91°, which can be indexed to the (311) and (020) crystallographic planes, respectively (CCDC 640536). , These reflections are widely recognized as the structural fingerprints of MIL-100­(Fe), indicating the successful formation and preservation of its porous framework under mild synthesis conditions. No additional sharp diffraction peaks attributable to iron oxide phases or other crystalline impurities are observed over the entire scanned 2θ range, demonstrating the high phase purity of the as-prepared material.

2.

XRD spectra of MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01) materials.

For the La-doped sample, MIL-100­(Fe_0.99/La_0.01), the characteristic diffraction peaks of MIL-100­(Fe) are well retained, indicating that La incorporation does not alter the crystal structure or phase formation of the parent framework. Importantly, apart from the intrinsic reflections of MIL-100­(Fe), no additional diffraction peaks corresponding to La-containing phases or other crystalline impurities are detected, confirming that the obtained material is single-phase within the detection limits of XRD. This result suggests that La is incorporated at low concentrations without inducing structural distortion or secondary phase formation. Based on the coordination chemistry of rare-earth elements and previously reported La-modified MOF systems, La³⁺ is expected to interact with the deprotonated carboxylate groups (−COO^–) of H₃BTC through La–O coordination bonds. Due to its larger ionic radius and higher coordination number compared to Fe³⁺, La³⁺ is unlikely to fully replace Fe³⁺ in the secondary building units of MIL-100­(Fe). Instead, La³⁺ is more plausibly incorporated via partial coordination with carboxylate ligands, forming La–O–C interactions or acting as a secondary coordination center within the framework or at defect sites. −

Overall, the XRD results verify the successful synthesis of both pristine and La-doped MIL-100­(Fe) with well-preserved framework structures and high phase purity, highlighting the structural robustness of the MIL-100 topology and its tolerance toward heterometal incorporation under environmentally benign synthesis conditions.

FT-IR Spectra

The FT-IR spectra of MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01) are presented in Figure . The O–H stretching vibration of adsorbed water molecules on the materials surface is responsible for the wide absorption band seen at about 3205 cm^–1. A distinct peak at 1624 cm^–1 corresponds to the CO stretching vibration, indicating the presence of carbonyl groups within the structure. Carboxylate (COO−) groups symmetric and asymmetric stretching vibrations are clearly identified by sharp peaks at 1450 cm^–1 and 1377 cm^–1, respectively. The C–H bending vibration of aromatic benzene rings is attributed to a band at 708 cm^–1, indicating the addition of aromatic ligands. Importantly, a characteristic peak at 456 cm^–1 is associated with Fe–O and La–O bonding vibrations, confirming the coordination interaction between Fe²⁺/La³⁺ ions and the carboxylate oxygen atoms of the organic linker. These results offer compelling proof of the effective integration of La³⁺ ions into the MIL-100­(Fe) framework and the structural stability of the resulting MOFs as a result of metal–ligand coordination.

3.

FT-IR spectra of NH₂-TPA, MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01).

SEM Images

The morphology of the material is shown in Figure as observed from the SEM images, the material displays a three-dimensional network composed of well-defined octahedral crystals, which is consistent with previous studies on MIL-100­(Fe) materials. − Upon the introduction of lanthanum (Figure c,d), the characteristic octahedral shape of the framework remains largely intact. The crystal facets appear sharper and more distinct in the doped samples, indicating that the incorporation of La ions does not interfere with crystal growth or the structural integrity of the parent MOFs. Notably, no separate bulk impurities or uncontrolled agglomerations of La species were detected on the polyhedral surfaces, suggesting a uniform distribution of the dopant within the MIL-100­(Fe) architecture.

4.

SEM images and particle size distribution of MIL-100­(Fe) (a,b) and MIL-100­(Fe_0.99/La_0.01) (c,d).

EDS Spectrum

Figure shows the EDS (Energy Dispersive X-ray Spectroscopy) spectrum of the synthesized MIL-100­(Fe_0.99/La_0.01) material. The spectrum confirms the presence of key elements including carbon (C), oxygen (O), iron (Fe), and lanthanum (La). The characteristic La peaks are clearly observed at approximately 4.6 and 5.0 keV, corresponding to the L_α and L_β emission lines of lanthanum, respectively. These peaks provide direct evidence that La³⁺ ions have been successfully incorporated into the MOFs structure. The Fe peaks confirm the iron-based framework of MIL-100, while the C and O signals are attributed to the organic ligand and framework oxygen atoms. Overall, the EDS results confirm the successful incorporation of La into the MIL-100­(Fe) framework, forming the La-substituted MOF MIL-100­(Fe_0.99/La_0.01).

5.

EDS spectrum of MIL-100­(Fe_0.99/La_0.01).

UV–vis-DRS Spectra

The Kubelka–Munk equation and the Tauc plot were used to calculate the bandgap energies of MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01) photocatalysts.

$${(\alpha h\nu )}^2=A({h\nu -E}_{\mathrm{g}})$$

where α is the absorption coefficient, h is Planck’s constant, n is the photon’s frequency, A is a proportionality constant and E _g is the bandgap energy.

UV–Vis diffuse reflectance spectroscopy was used to assess the optical absorption characteristics of the as-synthesized photocatalysts (Figure a). La-doped MIL-100­(Fe/La) showed much greater absorption in the UV–Vis region as compared to pure MIL-100­(Fe), while retaining a high absorption intensity in the visible light range (400–700 nm). This finding demonstrates that La incorporation enhances the light-harvesting capacity of MIL-100­(Fe), allowing for more efficient utilization of the solar spectrum and offering clear advantages for photocatalytic applications under natural sunlight.

6.

UV–vis-DRS spectra (a) and Tauc plots (b) of MIL-100­(Fe/La) and MIL-100­(Fe_0.99/La_0.01).

Tauc plots obtained from the Kubelka–Munk function were used to further determine the optical band gaps (Eg) (Figure b). The bandgap of MIL-100­(Fe/La) was calculated to be 2.93 eV by extending the linear part of the (αhν)² versus h ₀ curve. These enhanced optical properties are consistent with recent reports on La-modified photocatalysts, in which La³⁺ incorporation was shown to not only promote light absorption but also suppress electron–hole recombination. − As a result, the enhanced charge separation efficiency greatly increases MIL-100­(Fe/La) photocatalytic activity, especially in the reduction of Cr­(VI) under visible light irradiation.

BET

The N₂ adsorption–desorption isotherm of MIL-100­(Fe/La) exhibits a combined type I and type IV (Figure ), indicating the coexistence of microporous and mesoporous domains within the framework. The BET surface area was determined to be 1079.8 m² g^–1, with a total pore volume of 0.20 cm³ g^–1. The BJH pore size distribution analysis revealed mesopores with diameters of 4.9–5.5 nm, whereas DFT modeling identified micropores of approximately 0.97 nm. These results confirm that MIL-100­(Fe/La) possesses a hierarchical micro–mesoporous structure.

7.

N₂ adsorption–desorption isotherms of MIL-100­(Fe_0.99/La_0.01).

The coexistence of micropores and mesopores is advantageous for photocatalytic applications. Micropores provide abundant adsorption sites for Cr­(VI) species, thereby enhancing their preconcentration near the active sites. Meanwhile, mesopores facilitate the diffusion of reactive radicals (^•OH, ^•O₂ ^–) and improve light penetration within the catalyst particles. This hierarchical pore structure facilitates effective mass transport and charge separation, which enhances photocatalytic reduction performance toward Cr­(VI).

As shown in the Table , the specific surface area of MIL-100­(Fe/La) is comparable and sits well within the typical range reported for pristine MIL-100­(Fe) synthesized via various methods. This confirms that the incorporation of Lanthanum via a green synthesis route preserves a high specific surface area, which effectively facilitates the photocatalytic reduction of Cr­(VI).

1. Comparison of Textural Properties between MIL-100­(Fe_0.99/La_0.01) and Previously Reported MIL-100­(Fe) Frameworks.

catalyst	SBET (m2/g)	synthesis method	ref
MIL-100(Fe)	1164.5	hydrothermal
MIL-100(Fe)	790.5	hydrothermal
MIL-100(Fe)	1316	hydrothermal
MIL-100(Fe/La)	1079.8	stirring at room temparature	This work

pH_pzc

Based on the ΔpH – pH_(initial) curve (Figure ), the point of zero charge (pH_pzc) of MIL-100­(Fe/La) was discovered to be 5.21, which is the pH at which the material’s net surface charge is zero. The protonation of surface hydroxyl and carboxyl groups coupled to Fe³⁺ and La³⁺ centers causes the surface to become positively charged when the pH of the solution falls below this threshold (pH < 5.21). In contrast, at pH values above pH_pzc, deprotonation predominates, leading to a negatively charged surface that favors the electrostatic adsorption of cationic species. Compared to the reported values for MIL-100­(Fe), which typically lie in the range of 3.5–4.5, − the incorporation of La³⁺ evidently shifts the pH_pzc toward a higher value.

8.

pH_pzc of MIL-100­(Fe_0.99/La_0.01).

Photocatalytic Study

Figure a shows the UV–vis absorption spectra of Cr­(VI)–DPC complex solutions at different concentrations ranging from 4 to 20 ppm. The spectra exhibit a characteristic absorption peak centered around 550 nm, which corresponds to the Cr­(VI)–diphenylcarbazide (DPC) complex. As the Cr­(VI) concentration increases, the absorbance intensity also increases proportionally, indicating a direct correlation between Cr­(VI) concentration and absorbance at this wavelength. Based on absorbance measurements at λ_max = 550 nm, a calibration curve for the quantitative assessment of Cr­(VI) was constructed using these results.

9.

(a) Adsorption spectra of Cr-DPC complex solutions with varying initial Cr­(VI) and (b) a linear correlation between the concentration of Cr­(VI) and the absorbance at 550 nm.

The linear working range of the method was determined by constructing a calibration curve representing the correlation between Cr­(VI) concentration and the absorbance of the Cr–DPC complex formed between Cr­(VI) and diphenylcarbazide. Standard Cr­(VI) solutions were prepared in the concentration range of 1–20 ppm. With a correlation coefficient of R ² = 0.9974 the calibration curve demonstrated strong linearity. Therefore, the linear dynamic range of the method was established as 1–20 ppm, with the corresponding regression equation: A = 0.0138­[Cr­(VI)] + 0.0018 (R ² = 0.9974) (Figure b).

The findings shown in Figure show that pH has a significant impact on MIL-100 (Fe_0.99/La_0.01) photocatalytic activity for Cr­(VI) reduction. The material exhibits the highest photocatalytic efficiency (88.35%) under mildly acidic conditions at pH = 5. When the pH increases, the catalytic performance declines significantly, reaching only 58.96% at pH = 7 and dropping further to 34.29% at pH = 9. Likewise, a decrease in pH to 3 also results in lower performance (62.1%). As a result, pH = 5 was chosen as the ideal setting for further investigations, such as kinetic analyses and evaluations of catalyst reusability.

10.

Effect of pH on the photocatalytic performance of MIL-100­(Fe_0.99/La_0.01) (initial Cr­(VI) concentration: 20 ppm; catalyst dosage: 0.3 g L; H₂O₂: 3% (1 mL L^–1), visible light irradiation).

Both the catalyst’s surface charge (zeta potential) and the speciation of Cr­(VI) in aqueous solution can be used to explain how pH affects the photocatalytic reduction of Cr­(VI). Changes in pH affect the photocatalyst’s surface charge properties as well as the distribution of Cr­(VI) species, including CrO₄ ^2–, HCrO₄ ^–, and Cr₂O₇ ^2–, which affects the adsorption affinity and redox reactivity. Within the acidic range (pH = 2–6), Cr­(VI) primarily exists as HCrO₄ ^– and Cr₂O₇ ^2–, which are more susceptible to reduction. The abundance of H⁺ ions in acidic media promotes electron transfer, thereby facilitating the conversion of Cr­(VI) to Cr­(III). In contrast, Cr­(III) tends to precipitate as Cr­(OH)₃, whereas Cr­(VI) primarily occurs as CrO₄ ^2– at neutral and slightly alkaline circumstances (pH > 7). This precipitate’s buildup on the catalyst surface can obstruct active sites and prevent reactants from coming into touch with the photocatalyst, which lowers catalytic efficiency because of a less favorable redox potential. One noteworthy discovery is that the MIL-100­(Fe_0.99/La_0.01) catalyst produced the maximum reduction efficiency of Cr­(VI) to Cr­(III) at pH = 5, which is marginally below the point of zero charge (pH_pzc = 5.21, Figure ). Proton adsorption causes the catalyst surface to become positively charged at pH values lower than the pH_pzc. As a result, the predominant Cr­(VI) species at pH = 5 are electrostatically drawn to and enriched on the catalyst surface by HCrO₄ ^– and Cr₂O₇ ^2–, which facilitates charge transfer and increases the reduction of Cr­(VI) to Cr­(III).

The photocatalytic activities of MIL-100­(Fe) and MIL-100­(Fe_0.99/La_0.01) were evaluated via visible-light-driven photoreduction of Cr­(VI) to Cr­(III) in the presence of 3% H₂O₂ (1 mL L^–1) at pH = 5, using a catalyst dosage of 0.3 g·L^–1 (Figure ). Control experiments were conducted under identical conditions, including: (i) light irradiation without catalyst, (ii) light irradiation with catalyst only, and (iii) light irradiation with H₂O₂ only. After 30 min of irradiation, no noticeable change in Cr­(VI) concentration was observed in the absence of catalyst (i), while a moderate reduction of approximately 38% occurred in the presence of the catalyst alone (ii), and only a slight decrease (∼28%) was detected with H₂O₂ alone (iii). In contrast, the combined MIL-100­(Fe_0.99/La_0.01)/H₂O₂ system exhibited a pronounced enhancement in photocatalytic activity, characterized by a rapid decrease in Cr­(VI) concentration within the first 15 min of irradiation. After 35 min of visible-light exposure, the Cr­(VI) removal efficiency reached 88.35%, which is substantially higher than that obtained with pristine MIL-100­(Fe) under the same conditions (60.04%). These results demonstrate that La incorporation, in conjunction with H₂O₂, effectively enhances the photocatalytic performance of MIL-100­(Fe) toward Cr­(VI) reduction under visible-light irradiation. The improved activity is attributed to a synergistic effect rather than a single-factor contribution. Further electrochemical investigations are expected to provide deeper insight into the charge-transfer behavior of the La-doped system.

11.

Cr­(VI) photoreduction of different materials over time (catalyst dosage: 0.3 g L; H₂O₂: 3% (1 mL L-1), visible light irradiation; initial Cr­(VI) concentration: 20 ppm, pH = 5).

The effects of different radical scavengers on the visible-light-driven photocatalytic reduction of Cr­(VI) over MIL-100­(Fe_0.99/La_0.01) are presented in Figure . To elucidate the relative contributions of the reactive species involved in the reduction process, control experiments were conducted using 1,4-benzoquinone (BQ) as a scavenger for superoxide radicals (^•O₂ ^–), isopropanol (IPA) as a hydroxyl radical (^•OH) scavenger, and ammonium oxalate (AO) as a quencher for photogenerated holes (h⁺). In the absence of any scavenger, the catalyst exhibits a high Cr­(VI) reduction efficiency of 88.35% under visible-light irradiation. Upon the addition of IPA, the reduction efficiency markedly decreases to 48.05%, indicating that ^•OH radicals play an important role in the photocatalytic reduction process under the present reaction conditions. It should be emphasized that this observation does not imply that the reaction is exclusively governed by ^•OH species. The presence of BQ results in a moderate decrease in efficiency to 73.66%, suggesting that ^•O₂ ^– radicals also participate in the reduction pathway, although with a comparatively smaller contribution. Meanwhile, the addition of AO causes only a slight decrease in efficiency (82.06%), implying a limited direct involvement of photogenerated holes. This behavior is consistent with recent studies on (photo)­electrocatalytic molecular oxygen activation, demonstrating that the nature and effectiveness of reactive species strongly depend on catalyst-specific O₂ activation pathways. , In such systems, ^•OH and ^•O₂ ^– are frequently detected as reactive intermediates, with their relative importance varying with catalyst structure and reaction environment.

12.

Photocatalytic reduction of Cr­(VI) by MIL-100 (Fe_0.99/La_0.01) via active species trapping (initial Cr­(VI) concentration: 20 ppm, pH = 5; catalyst dosage: 0.3 g L; H₂O₂: 3% (1 mL L^–1), visible light irradiation).

Overall, these scavenger results support the proposed photocatalytic mechanism, in which multiple reactive species cooperatively contribute to Cr­(VI) reduction. Photogenerated electrons are considered to be primarily responsible for the direct reduction of Cr­(VI) to Cr­(III), while ^•OH and ^•O₂ ^– radicals participate as important auxiliary reactive species. Such a synergistic mechanism highlights the system-dependent nature of active species involvement in Fe-based MOFs photocatalysts rather than a single-species-controlled pathway.

Based on the experimental results and physicochemical characterizations, a plausible photocatalytic mechanism for the reduction of Cr­(VI) to Cr­(III) over MIL-100­(Fe/La) under visible-light irradiation is proposed and schematically illustrated in Scheme X and Figure . The photocatalytic reaction follows a heterogeneous surface-mediated mechanism, in which the redox transformations predominantly occur on the catalyst surface. Upon visible-light excitation, the MIL-100­(Fe_0,99/La_0,01) framework generates photogenerated electrons (e^–) in the conduction band (CB) and holes (h⁺) in the valence band (VB). The incorporation of La³⁺ into the MIL-100­(Fe) framework is suggested to promote charge separation through electronic interactions with Fe–O clusters, thereby suppressing electron–hole recombination and prolonging the lifetime of charge carriers, as commonly reported for rare-earth-modified MOF systems. ,, In parallel, the intrinsic Fe³⁺/Fe²⁺ redox couple within the MIL-100 framework facilitates interfacial electron transfer and photoassisted redox processes.

13.

Schematic illustration of the proposed photocatalytic mechanism for Cr­(VI) reduction over MIL-100­(Fe_0,99/La_0,01) under visible-light irradiation.

Photogenerated electrons play a primary role in the direct reduction of adsorbed Cr­(VI) species to Cr­(III), particularly under mildly acidic conditions (pH = 5), where Cr­(VI) predominantly exists as HCrO₄ ^– and Cr₂O₇ ^2–. , Meanwhile, a fraction of electrons can react with dissolved oxygen to generate superoxide radicals ^•O₂ ^–, which further participate in the reduction of Cr­(VI) as auxiliary reactive species.

Simultaneously, photogenerated holes oxidize surface-adsorbed H₂O or OH^– to form hydroxyl radicals (^•OH). In the presence of H₂O₂, the Fe³⁺/Fe²⁺ redox cycling promotes photoassisted Fenton-like reactions, resulting in enhanced ^•OH generation. − Radical scavenging experiments confirm that ^•OH radicals play an important role in the photocatalytic process under the applied reaction conditions. Nevertheless, ^•OH radicals do not exclusively govern the reduction pathway; instead, they contribute synergistically by assisting charge separation and participating in indirect redox reactions.

Overall, the photocatalytic reduction of Cr­(VI) over MIL-100­(Fe_0,99/La_0,01) proceeds through a cooperative mechanism in which photogenerated electrons act as the primary reducing species, while ^•OH and ^•O₂ ^– radicals function as important auxiliary reactive species. The synergistic involvement of Fe and La centers accounts for the enhanced photocatalytic activity and stability of the La-modified MIL-100­(Fe) catalyst under visible-light irradiation, in good agreement with the scavenger experiments and the proposed reaction scheme.

The durability and reusability of the MIL-100­(Fe_0.99/La_0.01) photocatalyst were systematically evaluated over five consecutive photocatalytic cycles, as shown in Figure . After each reaction cycle, the catalyst was recovered, thoroughly washed with deionized water and ethanol to remove residual species, and subsequently dried prior to reuse. Notably, no significant decline in photocatalytic activity is observed throughout the five cycles, demonstrating the excellent reusability of the catalyst. This stable performance indicates the high structural robustness and chemical stability of the MIL-100­(Fe_0.99/La_0.01) framework under repeated visible-light irradiation and reaction conditions.

14.

Reusability of MIL-100­(Fe_0,99/La_0,01) in the photocatalytic reduction of Cr­(VI).

The photocatalytic reduction of Cr­(VI) over MIL-100­(Fe_0.99/La_0.01) during the first 20 min was studied using Pseudo-first-order (1.1), Pseudo-second-order (1.2), Elovich (1.3), Bangham (1.4), and Intrapartical diffusion (1.5) kinetic models. With a rate constant of k ₁ = 0.088 min^–1 (R ² = 0.9965), the computed results show that the photocatalytic reaction adheres to the Pseudo-first-order model (Figure and Table ).

$$\ln \left(\frac{C_0}{C_t}\right)=k_{1}t$$ 1

$$\frac{1}{C_t}-\frac{1}{C_0}=k_{2}t$$ 2

$$q_t=a+b\ln (t)$$ 3

$$\log \left(\log \left(\frac{C_0}{C_0-q_t}\right)\right)=\log \left(\frac{k_{0}m}{2.303\mathrm{V}}\right)+\alpha \log (t)$$ 4

$$q_t=k_i{t}^{0.5}+\mathrm{C}$$ 5

where t is the reaction time (min); C ₀ and C correspond to initial and remaining concentrations of Cr­(VI) (mg L^–1); k ₁ (min^–1) and k ₂ (L mg^–1 min^–1) correspond to rate constants of the pseudo first- and second-order models. q _t is the adsorption capacity at time t (mg g^–1); a and b are the Elovich constants related to the initial adsorption rate and the surface coverage, respectively. In the Bangham model, k ₀ and α are constants reflecting the pore diffusion rate and the adsorption intensity, while m and V denote the mass of catalyst (g) and the solution volume (mL), respectively. In the intraparticle diffusion model, k _i (mg g^–1 min^–0.5) represents the intraparticle diffusion rate constant, and C (mg g^–1) is the intercept associated with the boundary layer thickness.

15.

Pseudo-1st-order (a), Pseudo-2nd-order (b), Elovich (c), Bangham (d) and Intrapartical diffusion (e) kinetic model for the photocatalysis process of MIL-100­(Fe_0.99/La_0.01) (dosage: 0.3 g·L^–1).

2. Comparison of kinetic model parameters for the photocatalytic reduction of Cr­(VI) using MIL-100­(Fe_0.99/La_0.01) Under visible light irradiation.

model	R 2	parameters	value
pseudo-first order	0.9965	k 1 (min–1)	0.086
pseudo-second order	0.9409	k 2 (L.mg–1.min–1)	0.013
Elovich	0.9643	a (mg.g–1)	–7.497
		b (g.mg–1)	18.101
Bangham	0.9913	A	1.074
		log(k o.m/2.303 V)	–1.519
Intrapartical diffusion	0.9443	k i (mg.g–1.min-0.5)	2.224
		C (mg.g–1)	7.599

In this study, the MIL-100­(Fe/La) material shows several favorable characteristics, including a relatively energy-efficient synthesis approach based on room-temperature stirring, observable photocatalytic activity under visible-light irradiation, and a comparatively short reaction time. To facilitate an objective evaluation, Table presents a systematic comparison between the performance and synthesis conditions of MIL-100­(Fe/La) reported in this work and those of related materials described in previous studies.

3. Comparison of Photocatalytic Cr­(VI) Reduction Performance of MIL-100­(Fe₀.₉₉/La₀.₀₁) with Reported MOF-Based Photocatalysts.

catalyst	light source	pH	initial Cr(VI) (ppm)	time (min)	Cr(VI) reduction (%)	synthesis method	ref
MIL-53(Fe)	visible	2	10	120	82	hydrothermal
NH2-MIL-125(Ti)	visible	3	10	90	85	solvothermal
NH2-MIL-101(Fe)	visible	7	20	30	80	hydrothermal
La-doped MIL-88B(Fe)–NH2	UV	6	20	20	81	hydrothermal
MIL-100(Fe)	visible	5	20	35	60.04	room-temperature stirring	this work
MIL-100(Fe 0,99/La 0,01)	visible	5	20	35	88.35	room-temperature stirring	this work

Conclusion

In this study, MIL-100­(Fe) and La-doped MIL-100­(Fe_0,99/La_0,01) materials were successfully synthesized via a facile approach. XRD analysis confirms that both samples exhibit a well-preserved MIL-100 framework with high crystallinity and single-phase characteristics, indicating that La incorporation does not alter the crystal structure or induce secondary phase formation. Comprehensive characterizations using XRD, SEM–EDS, BET surface area analysis, FT-IR spectroscopy, and UV–vis diffuse reflectance spectroscopy collectively demonstrate the successful synthesis, structural integrity, porous nature, and enhanced visible-light absorption of the obtained materials.

The photocatalytic performance of the materials was evaluated through the visible-light-driven reduction of Cr­(VI) to Cr­(III). Notably, MIL-100­(Fe, La) exhibits a high Cr­(VI) removal efficiency of 88.35% at pH = 5 within only 30 min, which is significantly superior to that of the pristine MIL-100­(Fe). The excellent photocatalytic activity under visible light highlights the strong potential of the La-modified MIL-100­(Fe) material for practical wastewater treatment applications. Radical scavenging experiments were conducted to elucidate the role of reactive species involved in the photocatalytic process. The results suggest that photogenerated electrons are primarily responsible for the direct reduction of Cr­(VI) to Cr­(III), while ^•OH and ^•O₂ ^– radicals act as important auxiliary reactive species, contributing synergistically to the overall photocatalytic efficiency under the specific reaction conditions employed.

Furthermore, the photocatalytic kinetics were analyzed using several kinetic models, including pseudo-first-order, pseudo-second-order, Elovich, Bangham, and intraparticle diffusion models. Among them, the pseudo-first-order kinetic model provides the best fit to the experimental data, with a high correlation coefficient (R ² = 0.9965), indicating that the photocatalytic reduction process is predominantly governed by surface reaction kinetics.

The authors declare no competing financial interest.