Surface-modified 3D-printed residue-containing inorganic polymers with CNTs/TiO₂ for water pollutant photodegradation

Supported photocatalysts are essential for overcoming the recovery and post-treatment challenges of water remediation. This study reports the development of an innovative, self-standing 3D-printed photocatalytic scaffold for the degradation of emerging contaminants (ECs) from water. Residue-containing inorganic polymers (IPs) were fabricated using 3D-printing by combining bauxite residue (red mud, RM) and metakaolin. The high iron content in the red mud served as an intrinsic catalyst for the in situ, catalyst-free growth of multi-walled carbon nanotubes (MWCNTs) via thermal chemical vapor deposition (TCVD). Subsequently, a conformal anatase TiO 2 thin film was deposited onto the CNT-decorated scaffolds via Atomic Layer Deposition (ALD), resulting in a hierarchical core-shell nanostructure. Characterization revealed a significant 3.7-fold increase in the specific surface area, rising from 55 to 202 m 2 g −1 and a complex bimodal pore architecture. Coupling CNTs and the conformal anatase TiO₂ shell enhances charge separation and electron transport, thereby boosting photocatalytic efficiency. The 3D-CNTs/TiO 2 composite demonstrated high photocatalytic activity under UV-A irradiation, achieving 92% removal of ciprofloxacin (CIP) from water within 60 min in a continuous-flow process (C 0 = 10 mg L −1 , pH 6.5). The degradation followed pseudo-first-order kinetics, with the highest efficiency observed under acidic conditions (pH 3), with roughly complete removal observed at pH 3 within 30 min of irradiation. This superior efficiency in acidic medium is attributed to enhanced electrostatic interactions between the composite and the pollutant. Scavenger assays identified holes and superoxide radicals as the dominant reactive species. Beyond its high stability and reusability over five cycles, the composite proved effective in degrading a complex mixture of four selected ECs. This work offers a sustainable strategy for upcycling industrial residues into robust, reusable photocatalytic materials water remediation.