Photocatalytic degradation of metformin and amoxicillin in synthetic hospital wastewater: effect of classical parameters

In this study, photocatalytic degradation of synthetic hospital wastewater containing two pharmaceutical contaminants, namely amoxicillin trihydrate (a commonly used antibiotic) and metformin HCl (a widely used diabetic medicine), was carried out using a laboratory-scale photoreactor (200 mL), TiO₂ as photocatalyst, and 125 W low-pressure mercury vapour lamp emitting UV rays at 365 nm. The response surface methodology based on face-centred central composite design was used to optimize the independent variables, namely the initial concentrations of contaminants (10–50 mg/L), TiO₂ dosage (250–1250 mg/L), initial pH (3–11), and reaction time (30–150 min). Results showed that both amoxicillin and metformin removals were influenced by all the four variables individually and also by the interaction between these variables. Response surface and overlaid contour plots were used to evaluate the optimum conditions. It was found that the maximum removal of amoxicillin (90%) and metformin (98%) could occur when the pH is 7.6, TiO₂ dosage is 563 mg/L, and reaction time is 150 min for an initial concentration of the contaminants at 10 mg/L. Further experiments were conducted to evaluate the characteristics of photocatalytic degradation. Low adsorption of contaminants on TiO₂ surface and negligible degradation of contaminants using acetonitrile as solvent suggest that hydroxyl radical attack could be the predominant pathway in the removal process. The COD and TOC analyses of the samples confirmed the mineralization of the compounds to more than 60%.