Probing the photolytic–photocatalytic degradation mechanism of DEET in the presence of natural or synthetic humic macromolecules using molecular-scavenging techniques and EPR spectroscopy

• DEET photodegradation is enhanced in the presence of DOM depending on structure.
• 3DOM*, HO and 1O2 are the major reactive species involved in DEET photolysis.
• EPR showed that DOM affect the generation of HO and hydroxy-DEET-radicals.
• FA was found more beneficial for the photocatalytic degradation of DEET than HALP.
• FA contribute to photocatalytic degradation through a sensitization mechanism.

The present work provides for the first time a detailed investigation of the photolytic and photocatalytic degradation mechanisms of DEET in the presence of different types of dissolved organic matter (DOM) e.g., a lignite HA (LHA) mainly of aliphatic character, a synthetic humic acid like polycondensate (HALP) with aromatic-ring rich properties and a soil fulvic acid (FA). The data are based on catalytic, molecular-scavenging and electron paramagnetic resonance (EPR) experiments. Photodegradation rates were significantly increased in the presence of different humic and fulvic acids and were highly dependent upon DOM composition, whereas they reached a threshold at approximately 1.0 mg L−1 of DOM. Scavenging experiments indicated that excited state DOM* plays a key role in the photochemical transformation in the presence of the studied humic and fulvic acids. FA and HALP showed an increasing trend in the photocatalytic degradation kinetics, while retardation was observed in the presence of LHA. EPR spin-trapping measurements in the presence of TiO2, showed that DOM type affect the generation of HO and the hydroxy-DEET-radicals which are formed in the initial steps of photodegradation. Fulvic acid was found to have the less significant effect on HO formation, while it contributed significantly to the photocatalytic degradation through a sensitization/charge injection mechanism with subsequent formation of O2− and 1O2.