Photocatalytic degradation and mineralization of tramadol pharmaceutical in aqueous TiO2 suspensions: Evaluation of kinetics, mechanisms and ecotoxicity

• The photocatalytic degradation and mineralization of tramadol was studied.
• Ηydroxylation, oxidation and dealkylation were found to be the main transformation pathways.
• HO are the main reactive species during the photocatalytic process.
• Near additive toxicity effects were observed between N-desmethyl and N-oxide tramadol.

Ιn the present study the transformation and mineralization of a common aquatic pollutant, Tramadol (TRA) was investigated for the first time by means of TiO2 photocatalysis. The degradation kinetics for both ΤRA and total organic carbon (TOC) followed apparent first-order model with rate constants of kapp = 15.3 × 10−2 min−1 and kapp = 9.7 × 10−3 min−1 and half lives (t1/2) of 4.5 min and 71.4 min, respectively. The transformation products (TPs) of TRA, were identified by high resolution accurate mass liquid chromatography (HR-LC–MS) suggesting that hydroxylation, oxidation and dealkylation are the main transformation pathways. The reactions were found to occur mainly at the surface of the photocatalyst via surface-bound HO radicals rather than by free diffusion into the homogeneous phase. Τhe potential risk of TRA and its TPs to aqueous organisms was investigated using Microtox bioassay before and during the process. The acute toxicity increased in the first stages and then decreased rapidly to very low values within 120 min of the photocatalytic treatment. The increase in the toxicity is associated with near additive or low synergistic effects between two TPs generated during the process i.e. N-desmethyl and N-oxide tramadol.

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