Elimination of torasemide from aqueous medium: influence of sorption and photocatalytic processes parameters supported by DFT analysis

The sorption and photocatalytic activity of pharmaceutical torasemide using an immobilized TiO₂ photocatalyst were investigate. The experimental design included optimization of reaction conditions such as pH and initial pharmaceutical concentration in aqueous solution using the response surface modeling approach, scavenger tests to gain insight into the photocatalysis mechanism, and application of the process to more complex water matrices. TiO₂ in the role of sorbent showed a low capacity for torasemide (12.23–29.83 μg/g within 24 h of contact), making this type of removal inefficient on its own. Investigating the sorption process influenced by different process parameters such as pH, temperature, ionic strength, and dosage of TiO₂ applied, the low tendency to this kind of material was affirmed by low K_d values (0.70 to 6.78 mL/g) obtained by linear isotherms. Photocatalysis proved to be the better choice for the removal of torasemide from water, with the best kinetics at pH 4 and concentration of 5 mg/L with half-time for degradation of 34.83 min. Computational DFT analysis identified the zwitterionic torasemide structure as predominant under neutral and acidic conditions. It also showed that negatively charged areas around nitrogen-containing fragments probably have the highest potential to promote the TiO₂ sorption at low pH conditions, where it is highest, through electrostatic attractions and N–H∙∙∙∙∙OTiO₂ hydrogen-bonding contacts.