Green approach for perfluorocarboxylic acids (PFCAs) removal with density functional theory (DFT) insights: Peanut-shell biomass-based carbon quantum dots (PCQDs) coupled with TiO₂ photocatalyst

Perfluoroalkyl carboxylic acids (PFCAs), widely used in industrial applications, have led to significant bioaccumulation in aquatic ecosystems. While regulatory efforts have focused on phasing out long-chain PFCAs, short-chain alternatives (C₃–C₆) have emerged as substitutes. However, these compounds exhibit similar environmental persistence and toxicity while displaying increased mobility in water systems, posing additional ecological risks. This study examines the photodegradation of perfluorooctanoic acid (PFOA) and short-chain PFCAs employing TiO₂ and peanut shell biomass-derived carbon quantum dot (PCQD)-doped TiO₂ photocatalysts under UVC and visible light irradiation. The role of scavengers in PFOA degradation under visible light was also examined. Structural and optical characterization confirmed the successful synthesis of pure carbon quantum dots, TiO₂, and PCQD/TiO₂ composites, with boosted optical properties owing to PCQD incorporation. The PCQD/TiO₂ composite achieved PFOA degradation efficiencies of 78.6 % under UVC and 55.0 % under visible light, outperforming pure TiO₂ (41.0 % and 24.0 %, respectively). Degradation efficiencies for short-chain PFCAs (C₃–C₆) also improved significantly. Additionally, experimental and density functional theory (DFT) analyses validated a defluorination pathway involving chain-shortening and H/F exchange, confirming the H/F exchange mechanism as the dominant degradation route. The composite photocatalyst demonstrated excellent reusability over three cycles, highlighting its potential for sustainable environmental remediation.