Lignin-derived carbon quantum dot–sensitized TiO2 for enhanced visible-light photocatalytic simultaneous degradation of pharmaceuticals

Titanium dioxide (TiO 2 ) photocatalysis is a promising advanced oxidation method for water treatment, but its reliance on UV irradiation limits practical solar-driven applications. This study addresses this limitation by synthesizing lignin-based carbon quantum dots (L–CQDs) from kraft lignin and combining them with anatase TiO 2 to create a visible-light-active hybrid photocatalyst. L–CQDs/TiO₂ composites with 1–4 wt.% L–CQD loadings were prepared, thoroughly characterized, and evaluated for photocatalytic performance under simulated visible light (400–700 nm). The optimized 3 wt.% L–CQDs/TiO 2 hybrid achieved simultaneous degradation of sulfamethoxazole (SMX) and ibuprofen (IBU) with removal efficiencies of 98.6% and 79.7%, respectively, after 90 minutes – representing 118-fold and 5-fold increases in pseudo-first-order rate constants compared to bare TiO 2 . Radical scavenging experiments revealed a dual degradation mechanism: SMX was degraded mainly by superoxide radicals in the bulk solution, while IBU was degraded at the photocatalyst surface via photogenerated holes, with hydroxyl radicals playing a negligible role. Density functional theory calculations of frontier molecular orbital energies supported this mechanistic distinction. Matrix effects in tap water selectively suppressed IBU degradation due to competing hole consumption by CO₃²⁻ and Cl⁻, while SMX degradation remained unaffected. These findings demonstrate that L–CQDs act as a sustainable photosensitizer, shifting TiO 2 activity into the visible range through a charge-transfer mechanism rather than direct band gap modification, enabling efficient multi-pollutant removal under solar-relevant irradiation.