Thesis Title: «Photochemical degradation of the antibiotic meropenem in aqueous solutions»
Friday 4 October 2019, at: 10:30, Venue: Hall Κ2.A3
- Assistant Professor Nikolaos Xekoukoulotakis (advisor)
- Assistant Professor Paraskevi Panagiotopoulou
- Dr Konstantina Tyrovola
The present thesis describes the photochemical degradation of the antibiotic meropenem, which belongs to the category of carbapenem. The degradation took place in aqueous solutions with the use of ultraviolent (UV) radiation in the wavelength of 254 nm (UV-C radiation), with or without the presence of the chemical oxidizing reagent hydrogen peroxide (H2O2).
The experiments took place in a lab-scale batch reactor with a capacity of 450 mL, in ambient temperature (23-25 °C), with the solution being in contact with the atmospheric air. Samples were collected for the determination of the residual concentration of meropenem.
The laboratory work started with experiments of continuous mixing in the dark for the investigation of the hydrolysis of meropenem in different pH values between 6 and 9. From these experiments, it was noted that the hydrolysis of the antibiotic was negligible, since the experimental time was only some minutes, while the hydrolysis was starting after one hour. Moreover, experiments for the calculation of the absorbance spectra of the antibiotic in different pH values (pH = 6, 7 and 8) were conducted and the results showed that the molecular absorption coefficient of meropenem is relatively high. Specifically, ε254nm for pH = 6.0 was 3.4×103 L·mol–1·cm–1, for pH = 7.0 was 3.2×103 L·mol-1·cm–1 and for pH=8.0 was 2.9×103 L·mol–1·cm–1. Additionally, the effect of the initial concentration of meropenem in the reaction rate was tested by experiments with different initial concentrations: 1 μmol/L-50 μmol/L. The results showed that the rate of the reaction of the direct photolysis of the substance does not depend on the initial concentrations that were investigated, indicating that the photochemical degradation of meropenem follows first-order kinetics. From the rate constant of the first-order kinetics, the quantum yield of the direct photolysis of the antibiotic meropenem was calculated. The effect of the pH of the solution in the direct photolysis of meropenem was also studied, with results showing that the pH does not significantly affect the direct photolysis of the substances at the range of 4,5,6,7,8. Furthermore, the effect of the aqueous matrix (fresh synthetic urine and synthetic wastewater) was tested. It was found that the degradation of the antibiotic was not significantly affected by the aqueous matrix. From the direct photolysis experiments that were conducted, it was clear that it was an efficient process, as the concentration of meropenem was reduced by about 95% in only few minutes.
Additional irradiation experiments with the presence of the oxidizing agent hydrogen peroxide (Η2Ο2) were held in different initial concentrations: 0,1 mmol/L - 2,0 mmol/L. From these experiments, it can be noted that the photochemical degradation of meropenem during its indirect photolysis was better and faster, in comparison with the direct photolysis, and the optimum oxidant concentration was 1,25 mmol/L. It was also found that the concentration of meropenem decreased exponentially over time. It appears that the photochemical degradation of meropenem followed pseudo-first order kinetics. Furthermore, it was found that the different pH of the solutions do not significantly affect the indirect photolysis of the substance, unlike the aqueous matrix which reduced the rate of the reaction because their various organic and inorganic constituents consumed part of the photogenerated hydroxyl radicals.
Finally, electric energy per order, EEO, was calculated for experiments of different pH and aqueous matrix in direct and indirect photolysis, and for experiments of different initial concentration of hydrogen peroxide. It was found that in direct photolysis EEO was relatively high, while in indirect was relatively low.