Dual-scenario photocatalytic degradation of polystyrene nanoplastics: Microstructure-dependent performance of rutile TiO2 in aqueous and solid-state laboratory systems

Nanoplastics are emerging contaminants of increasing environmental concern, and yet effective remediation strategies remain limited, particularly under non-aqueous conditions. In this work, rutile TiO 2 photocatalysts with tailored microstructures were synthesized via a controlled acid-hydrothermal route and evaluated for the degradation of polystyrene nanoplastics (PS-NPs) in both aqueous and solid-phase laboratory systems. By tuning the synthesis conditions, distinct microstructures were obtained, and their structural, optical, and surface properties were systematically characterized. Among the synthesized materials, a raspberry-like structured sample with enhanced surface hydroxylation exhibited the highest degradation efficiency in aqueous medium (~13% within 12 h under UV irradiation). In such condition, the photocatalytic degradation was primarily monitored by turbidimetry and supported by total organic carbon and gas chromatography–mass spectrometry analyses, which supported polymer transformation through the formation of soluble and volatile degradation products. Furthermore, to simulate the photocatalytic removal of airborne PS-NPs deposited on urban photocatalytic surfaces (e.g., walls, cement, and building facades), a solid-phase configuration was developed in which PS-NPs were directly deposited onto TiO 2 -coated substrates, enabling continuous interfacial contact during irradiation. Under these conditions, a degradation efficiency of 68% was achieved after 18 h of irradiation, as confirmed by FT-IR analysis, contact angle measurements, and morphological observations. The high degradation efficiency observed in the solid-phase configuration was attributed to interfacial oxidation processes driven by surface-bound reactive oxygen species and catalyst-polymer contact. These findings demonstrate that microstructural engineering of TiO 2 represents an effective strategy for PS-NPs degradation and highlight solid-phase photocatalysis as a promising approach for the remediation of surface-deposited airborne nanoplastics.