Unraveling the respective roles of functional groups in PANI/TiO₂ for targeted pollutant removal: a molecular-level insight

Organic-inorganic composite materials, such as PANI/TiO₂, have shown great potential in water purification. However, the specific mechanism of the interface functional groups in the adsorption of pollutants at the molecular level has not been fully elucidated. This paper proposes a molecular group-directed masking strategy to systematically analyze the adsorption contributions of each functional site in the PANI/TiO₂ platform. We designed a series of TiO₂ with specific surface groups (-OH, -NO₂, -COOH) and structurally clear aniline oligomers as model probes to selectively mask or separate the key functional groups of the composite material. Comparative studies on the adsorption of five representative pollutants – Methylene blue (MB), acid red G (ARG), hexavalent chromium (HCrO₄⁻), pentavalent antimony (SbO₂⁺), and phosphate (H₂PO₄⁻) - revealed different site-specific adsorption mechanisms. The hydroxyl groups on the TiO₂ surface significantly enhanced the adsorption of MB through hydrogen bonds, while the Ti sites dominated the adsorption of phosphate and Sb(V), and the spatial steric hindrance effect of the organic surface groups selectively inhibited the binding of Sb(V). For the PANI component, the terminal amino groups were crucial for the removal of ARG, and the quinone imine nitrogen served as the main coordination site for Sb(V). The adsorption of MB was mainly controlled by π-π interactions and hydrophobic interactions, while the N-H groups in PANI participated in the redox reaction of Cr(VI) to Cr(III). By quantitatively correlating the functional groups with the specific adsorption mechanisms of pollutants, this study surpasses empirical performance evaluations and establishes a site-resolved, group-directed framework for understanding the interfacial cooperativity in composite adsorbents.