Defect-Engineered Black TiO₂ as a Rapid and Sustainable Adsorbent for Water Remediation

Rapid removal of chemically diverse organic pollutants remains a major challenge in aqueous decontamination. In this study, atmosphere-controlled defect engineering was used to activate anatase TiO₂ as a rapid adsorbent operating on the minute scale, exhibiting low charge selectivity under the investigated conditions. A reduced black TiO₂ (B–TiO₂), produced by inert annealing, achieved ≈100% removal of cationic methylene blue within ~6 min and ≈91% uptake of anionic methyl orange within ~3 min, whereas pristine and air-annealed TiO₂ showed only marginal adsorption under identical conditions. Correlative structural and surface-sensitive analyses indicated that this behaviour was associated with a chemically activated near-surface region enriched in reduced titanium contributions, defect-associated or non-lattice oxygen environments and a locally perturbed anatase framework, together with finely dispersed carbon-related motifs integrated within the oxide matrix. Adsorption kinetics were described, within experimental resolution, by pseudo-second-order fitting, while intraparticle diffusion analysis supported sequential regimes initiated by rapid interfacial attachment. Equilibrium analysis yielded apparent maximum capacities of 6.116 mg g⁻¹ for methylene blue and 2.950 mg g⁻¹ for methyl orange, reflecting adsorption governed by surface heterogeneity for cationic species and an apparent saturation-type response for anionic uptake. Overall, controlled surface non-stoichiometry emerges as a viable strategy to enhance adsorption kinetics in TiO₂, providing a transferable design framework for developing oxide-based adsorbents for sustainable water-treatment applications.