Monitoring advanced oxidation processes’ efficiency via fluorescence loss in carbon quantum dots

Carbon quantum dots (CQDs) synthesized from ammonium citrate were investigated as fluorescence-based probes for assessing ultraviolet (UV) fluence and hydroxyl radicals (•OH) in advanced oxidation processes (AOPs). CQDs characterized through fluorescence spectroscopy showed two distinct emission bands at 440 nm and 520 nm, corresponding to the monomeric and agglomerated forms of citrazinic acid (CZA), respectively. Under low-pressure (LP) mercury lamp irradiation, CQDs exhibited fluorescence loss that was significantly enhanced in the presence of hydrogen peroxide. Dose–response analysis identified three distinct processes: the monomer (440 nm) reaction followed Langmuir-Hinshelwood (L-H) kinetics, while the other two processes are the agglomerate (520 nm), showed fast and slow reactions. Methanol scavenging confirmed •OH involvement in the monomer and slow agglomerate reactions, while the fast aggregate reaction proceeded via another mechanism. Comparative experiments with carbamazepine (CBZ) showed first-order degradation kinetics with linear dependence on H 2 O 2 concentration, contrasting with the L-H kinetics observed for CQDs. These findings demonstrate that CQDs cannot serve as dual actinometers for both UV and UV-based AOP monitoring.