Photocatalytic degradation kinetics, mechanism and ecotoxicity assessment of tramadol metabolites in aqueous TiO₂ suspensions

This study investigated for the first time the photocatalytic degradation of three well-known transformation products (TPs) of pharmaceutical Tramadol, N-desmethyl-(N-DES), N,N-bidesmethyl (N,N-Bi-DES) and N-oxide-tramadol (N-OX-TRA) in two different aquatic matrices, ultrapure water and secondary treated wastewater, with high (10 mg L⁻¹) and low (50 μg L⁻¹) initial concentrations, respectively. Total disappearance of the parent compounds was attained in all experiments. For initial concentration of 10 mg L⁻¹, the target compounds were degraded within 30–40 min and a mineralization degree of more than 80% was achieved after 240 min of irradiation, while the contained organic nitrogen was released mainly as NH₄⁺ for N-DES, N,N-Bi-DES and NO₃⁻ for N-OX-TRA. The degradation rates of all the studied compounds were considerably decreased in the wastewater due to the presence of inorganic and organic constituents typically found in effluents and environmental matrices which may act as scavengers of the HO^•. The effect of pH (4, 6.7, 10) in the degradation rates was studied and for N-DES-TRA and N,N-Bi-DES-TRA, the optimum pH value was 6.7. In contrast, N-OX-TRA showed an increasing trend in the photocatalytic degradation kinetic in alkaline solutions (pH 10). The major transformation products were identified by high resolution accurate mass spectrometry coupled with liquid chromatography (HR-LC–MS). Scavenging experiments indicated for all studied compounds the important role of HO^• in the photocatalytic degradation pathways that included mainly hydroxylation and further oxidation of the parent compounds. In addition, Microtox bioassay (Vibrio fischeri) was employed for evaluating the ecotoxicity of photocatalytically treated solutions. Results clearly demonstrate the progressive decrease of the toxicity and the efficiency of the photocatalytic process in the detoxification of the irradiated solutions. - Highlights: • Photocatalytic degradation of N-desmethyl, N,N-bidesmethyl and N-oxide-tramadol was studied. • Degradation of all studied compounds was strongly inhibited in wastewater. • Hydroxylation and oxidation are the main degradation pathways. • HO^• are the main reactive radicals during the photocatalytic process. • A significant abatement of the overall toxicity was accomplished.