pH-responsive MIL-101(Fe)/NiFe-LDH composite membranes for high-flux removal of cationic and anionic dyes in wastewater treatment

Synthetic dyes, known for their toxicity, mutagenicity, and carcinogenicity, pose severe threats to aquatic ecosystems and human health. Membrane filtration offers an energy-efficient solution, but achieving simultaneously high selectivity, permeability, and stability for both cationic and anionic dyes remain challenging. Here, we design a hierarchical composite membrane by integrating MIL-101(Fe), a high-porosity metal-organic framework (MOF), with NiFe-layered double hydroxide (LDH) nanosheets. The MIL-101(Fe) matrix provides a large surface area (∼19.2 m²/g) and tailored pore architecture for size-selective adsorption. At the same time, the NiFe-LDH enhances electrostatic interactions via its pH-tunable surface charge (isoelectric point = 3.16). This synergy enables exceptional removal efficiencies for methyl green (MG, 88 %) and methyl orange (MO, 91 %) at ultrahigh water flux (>100 L·m⁻²·h⁻¹·bar⁻¹), outperforming conventional membranes (Table S1). The membrane's pH-responsive selectivity, driven by zeta potential reversal, allows targeted removal of cationic (e.g., MG at pH > 3.16) and anionic (e.g., MO at pH < 3.16) dyes. Density functional theory (DFT) simulations reveal that MG adsorption occurs via π-π stacking with MIL-101's benzene linkers and electrostatic attraction to LDH hydroxyl groups. In contrast, MO uptake involves anion exchange with LDH interlayers. With excellent recyclability (>80 % efficiency after 5 cycles) and antifouling properties (flux recovery >90 %), this work advances the design of multifunctional membranes for scalable wastewater treatment.