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

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.