Heterogeneous photocatalytic degradation of the pharmaceutical agent salbutamol in aqueous titanium dioxide suspensions

The present paper deals with the photocatalytic transformation of salbutamol [2-(tert-butylamino)-1-(4-hydroxyl-3-hydroxymethylphenyl) ethanol] under simulated solar irradiation using titanium dioxide as a photocatalyst. The investigation has involved a kinetic study of the drug decomposition, the identification of intermediate compounds, the assessment of mineralization, as well as toxicity evaluation. The variation of TiO2 concentration and pH on the reaction rate was investigated. The use of the response surface methodology allowed to fit the optimal values of the parameters leading to the degradation of the micro-pollutant. High resolution mass spectrometry was employed in assessing the temporal course of the photocatalyzed process. According to our findings, a tentative degradation pathway is proposed for the photocatalytic degradation of salbutamol based on the formation of both oxidative and reductive compounds. 2-(Methylamino)-1-(4-hydroxyl-3-methylphenyl)ethanol, 2-(tertbutylamino)- 1-(4-hydroxylphenyl)ethanol, 2-(tert-butylamino)-1-(3,4-dihydroxyphenyl)ethanol, 2-(tert-butylamino)-1-(4-hydroxyl-3-methylphenyl) ethanol, and 2-(tert-butylamino)-1-(3,4-dihydroxyphenyl)ethanone are initially formed, which are then transformed into 2-(tert-butylamino)- acetic acid and hydroquinone. All these intermediates are easily degraded themselves and until 2 h of irradiation 2-(tert butylamino)-acetic acid is the only compound recognized to endure. Photomineralization of the substrate was rather a quick process; within 2 h of irradiation nitrogen is completely mineralized, where the amino moiety is mainly transformed into NH4+ and in a lesser extent into NO3- ions (ratio 2:1). Carbon complete mineralization is achieved within 3 h of irradiation. Finally, Microtox bioassay (Vibrio fischeri) was employed in evaluating the ecotoxicity of solutions treated by photocatalysis.