Comparative Study of Arsenate Removal by MIL‐88B (Fe) and Bimetallic MIL‐88B (Fe/Ni) MOFs Obtained via Microwave‐Assisted Synthesis

ABSTRACT Arsenate remediation remains a critical environmental challenge, motivating the development of efficient and scalable adsorbents. Here, we report a comparative study of monometallic Fe‐MIL‐88B and its bimetallic analogue Fe/Ni‐MIL‐88B synthesized via microwave‐assisted methods, yielding crystalline frameworks with octahedral morphologies (~200 nm) and uniform metal distribution. Comprehensive characterization (XRD, FTIR, SEM, BET) confirms the integrity and porosity of both MOFs after synthesis. Adsorption experiments reveal rapid arsenate uptake for both materials, with equilibrium reached within 30 min. Kinetics follow a pseudo‐second‐order model, indicating an adsorption process dominated by chemisorption with additional contributions from physisorption, and equilibrium isotherms fit the Langmuir model, consistent with monolayer adsorption on homogeneous sites. Thermodynamic analysis at 298 K shows spontaneous adsorption for both materials: Fe‐MIL‐88B with Δ G ° ≈ −20.6 kJ/mol, Δ H ° ≈ −10.0 kJ/mol, and Δ S ° ≈ +35.6 J/mol·K, and Fe/Ni‐MIL‐88B with Δ G ° ≈ −24.5 kJ/mol, Δ H ° ≈ −11.9 kJ/mol, and Δ S ° ≈ +42.5 J/mol·K, indicating more favorable thermodynamics and a stronger entropic drive in the bimetallic framework. The monometallic Fe‐MIL‐88B exhibits a maximum arsenate capacity of 167.5 mg/g, while the bimetallic Fe/Ni‐MIL‐88B shows a superior capacity of 207.2 mg/g, demonstrating a clear synergistic enhancement from Ni 2+ incorporation. Spectroscopic analyses (FTIR and XPS) corroborate the formation of Fe–O–As coordination bonds, supporting a chemically bonded adsorption mechanism, in addition to electrostatic interactions induced by Ni 2+ sites. The presence of Ni 2+ also improves structural stability against hydrolysis and maintains porosity after multiple adsorption–desorption cycles, highlighting the practical viability of the Fe/Ni‐MIL‐88B system for arsenate removal from water. Overall, the study underscores the advantages of integrating secondary metals into MIL‐88B MOFs to boost adsorption performance through synergistic interactions, enhanced site diversity, and robust thermodynamic and structural features.