Synergistic interfacial engineering of CNF@MN@MX composite membranes for highly efficient and recyclable photocatalytic detoxification of Cr(VI)

The efficient and sustainable removal of hexavalent chromium (Cr(VI)) from wastewater is a critical challenge in environmental engineering. Herein, an ultra-stable and recyclable cellulose nanofiber-based composite membrane (CNF@MN@MX) was fabricated via vacuum-assisted filtration by integrating cellulose nanofibers (CNFs), indium-based metal-organic frameworks (MIL-68(In)-NH₂, MN), and Ti₃C₂Tₓ MXene (MX). The CNFs provide a robust structural scaffold, MN imparts visible-light photoactivity, and MX establishes a highly conductive network that facilitates charge separation and transport. Comprehensive structural and photoelectrochemical characterizations confirmed strong interfacial coupling among the three components, leading to enhanced electron mobility and light-harvesting capacity. The optimized CNF@MN@MX membrane (mass ratio CNF:MN:MX = 1:1:1) exhibited a specific surface area of 22.65 m 2 /g, a maximum Cr(VI) adsorption capacity of 17.79 mg/g, and nearly complete Cr(VI) reduction under visible-light irradiation. Notably, the membrane retained over 70% removal efficiency after 20 regeneration cycles and maintained excellent performance in the presence of common anions. Density functional theory (DFT) calculations revealed that MX incorporation lowered the Gibbs free energy of Cr(VI) reduction from −1.19 to −1.72 eV, confirming the thermodynamic advantage of MX-facilitated electron transfer. Furthermore, the CNF@MN@MX membrane efficiently purified simulated industrial effluents, yielding residual Cr(VI) concentrations below the regulatory limit. This work provides a robust design strategy for constructing conductive, photoactive, and recyclable membrane systems for advanced heavy-metal wastewater treatment.