PhD Thesis Defence Presentations - Efstathios Avramiotis

In the present thesis, advanced oxidation processes were tested for the degradation of pharmaceuticals, like antibiotics and antihypertensive drugs, in water matrices. The main process that was studied is persulfate activation by biochars for the degradation of antibiotics, where untreated and pre-conditioned biochars were used. In addition to persulfate salts, experiments were performed with hydrogen peroxide and sodium percarbonate. Special attention was given on the effect of pyrolysis temperature on the activity of the biochar.

Biochars were studied in detail regarding their physicochemical properties using a series of characterization techniques such as nitrogen adsorption – desorption at liquid nitrogen temperature (BET), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR) and potentiometric mass titration (PMT) in combination with H⁺ consumption at the biochar surface.

Moreover, a critical bibliographic review of the photocatalytic degradation of organic micropollutants using complex titania – carbonaceous material catalysts was carried out. Based on its findings, a series of photocatalytic degradation experiments of an antihypertensive compound were executed, using stimulated sunlight and a titania – biochar catalyst.

In the forementioned processes, several parameters that determine their performance were studied. More specifically, the influence of the concentration of reactants (i.e. oxidant, catalyst, pollutant) was explored. A certain focus was given on the effect of the water matrix and the initial pH of the solution. The latter was also examined in conjunction with the pollutant type that was used each time and, thus, the occurring electrostatic interactions. Finally, it was attempted to determine the oxidative species that contribute to the degradation process, as well as the role of adsorption on the catalyst surface.

It is known that the degradation of organic micropollutants does not lead necessarily to complete mineralization since a series of stable transformation products are likely to be generated. Thus, a set of experiments were realized for their detection and relative quantification, where persulfate activation by biochar was compared to that by FeCl₂ or heating. Finally, the ecotoxicity that these transformation products exhibit was estimated for a series of indicative aquatic organisms.

Based on the results obtained, several conclusions can be reached:

1. Carbon/titania composites have been shown to be capable of photocatalytically degrading a range of emerging pollutants. In particular, the photocatalytic degradation of losartan using biochar/titania composite was quite efficient, showing improved performance compared to pure titania;

2. Biochars have the potential to activate persulfates to degrade antibiotics. Higher pyrolysis temperatures increase process efficiency and favor non-radical degradation pathways. Moreover, biochar pre-treatment with KOH was shown to increase efficiency;

3. Transformation products accompanying the degradation of title pollutant with greater or lesser stability and/or ecotoxicity than the parent compound were detected.