Fe-Doped TiO2/Hard Carbon-Coated PLA 3D-Printed Structures for Synergistic Adsorption–Photocatalytic Removal of Methylene Blue and Tetracycline in Water

This study presents a sustainable waste-to-resource approach for water treatment through the development of an immobilized adsorption–photocatalytic system based on Fe-doped TiO2 and biomass-derived hard carbon. Hard carbon was synthesized from durian peel via a hydrothermal–carbonization process, transforming agricultural waste into a high-value adsorbent with a high specific surface area (279 m2 g−1) and hierarchical pore structure. Fe-doped TiO2 nanoparticles were prepared using a solution combustion method with titanium isopropoxide (TTIP) and glycine, resulting in porous anatase-dominant structures. The Fe-doped TiO2/hard carbon composite was subsequently coated onto 3D-printed polylactic acid (PLA) scaffolds to form an immobilized catalytic system. Structural and physicochemical characterization (X-ray diffraction (XRD), Rietveld refinement, scanning electron microscopy/transmission electron microscopy (SEM/TEM), Brunauer–Emmett–Teller (BET), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and photoluminescence (PL)) confirmed that Fe doping effectively reduced the bandgap (from 3.02 to 2.89 eV) and introduced defect states that enhance charge separation. The 3 mol% Fe-doped TiO2 exhibited the most optimal structural and textural properties, including the highest surface area (123 m2 g−1) among TiO2-based samples. Photocatalytic performance was evaluated using methylene blue (MB) as a model organic pollutant. The Fe-doped TiO2/hard carbon-coated PLA structures demonstrated enhanced removal efficiency compared to pristine TiO2, attributed to a synergistic mechanism combining pollutant adsorption and photocatalytic degradation. The hard carbon component facilitates pollutant pre-concentration, while Fe doping improves light absorption and suppresses charge carrier recombination. The 3 mol% Fe-doped TiO2/hard carbon system exhibited the best performance, consistent with its optimized structural, optical, and surface properties. Importantly, the use of biomass-derived carbon and 3D-printed biodegradable PLA substrates highlights the environmental and practical advantages of the developed system, including resource recovery, catalyst stability, and ease of reuse. This work demonstrates a promising pathway for integrating waste valorization with advanced water treatment technologies, contributing to sustainable and circular approaches for environmental remediation.