Visible-light driven synergistic adsorption-photocatalysis by S-scheme CoTiO3@Cu-BDC-NH2 MOF photocatalyst for tetracycline and chromium removal

This study focuses on the fabrication of an effective heterojunction composite, CoTiO 3 /Cu-BDC-NH 2 MOF (CT@CuMOF) prepared for TC and Cr(VI) removal from water through a synergistic adsorptive-photocatalytic degradation process. CT@CuMOF composite exhibited maximum removal of 21.4 mg/g for TC and 42.1 mg/g for Cr(VI) under dark equilibrium, and visible light irradiation led to 72 % TC and 80 % Cr(VI) degradation within 30 min. VB-XPS results confirmed the alignment of band edges, supporting an S-scheme charge transfer mechanism between the p-type CT and n-type CuMOF. TEM results reveled a well-formed p-n heterojunction, while EIS and photocurrent density measurements demonstrated enhanced charge carrier mobility and separation. Kinetic results followed pseudo-second order for TC and Cr(VI), with Langmuir adsorption isotherm showing maximum capacities of 25.36 mg/g for TC and 45.94 mg/g for Cr(VI). ESR and radical scavenger studies confirmed hydroxyl (•OH) and superoxide (•O 2 − ) radicals as key species in TC degradation, while superoxide played dominant roles in Cr(VI) reduction. CT@CuMOF composite exhibited favorable thermodynamic conditions (ΔG° <0), endothermic processes (ΔH° >0) and excellent reusability (80 % retention after five cycles). This research highlights the novelty of CT@CuMOF composite as a sustainable material for water purification.