2.3. Photocatalytic Tests

The photocatalytic activity of the synthesized catalysts was measured in the phenol and methyl orange (MO) photodegradation reactions. Both processes were carried by using a discontinuous batch system, including a 400 mL pyrex reactor enveloped by an aluminum foil, filled with an aqueous suspension (250 mL) containing 25 ppm of phenol or MO and the photocatalyst (1 g/L). This system was illuminated through a UV-transparent Plexiglas® top window (threshold absorption at 250 nm) by a 300 W Osram Ultra-Vitalux lamp (Munich, Germany) with sun-like radiation spectrum and a main line in the UVA range at 365 nm. The intensity of the incident UV-Visible light on the solution was measured with a Delta OHM photoradiometer HD2102.1 (Caselle di Selvazzano, Padova, Italy), being ca. 120 W/m². The visible photocatalytic experiments were performed by using a polyester UV filter sheet (Edmund Optics, Barrington, NJ, USA) showing 99.9% of absorbance below 400 nm (0.15 W/m² for λ < 400 nm and 150 W/m² for λ > 400 nm). In order to favor the adsorption–desorption equilibrium, prior to the irradiation, the suspension was magnetically stirred for 10 min in the dark. Furthermore, a constant oxygen flow of 25 L/h used as the oxidant was passed through the suspension for improving the homogeneous dispersion of the photocatalyst in the solution. For this purpose, a bubbler tank was used as a source of natural oxygen. All photocatalytic tests started at the natural pH of pollutants solutions which was ca. 6, and the total reaction time was 120 min.

During the phenol and MO photoreactions, samples were collected at different times and analyzed by UV-Visible spectrophotometry, considering the main absorption band observed for these compounds and located at 270 and 465 nm, for phenol and MO, respectively. For these analyses, a Genesys 10UV Thermo Electron instrument (Waltham, MA, USA) was used. Taking into account the Lambert–Beer law which stated that the absorbance is proportional to the concentrations, the evolution of these pollutants concentration as a function of the reaction time was calculated from the calibration curve obtained from the UV-Vis analyzes. The pollutants photodegradation rate was also determined by using Equation (1):

where, v = photodegradation rate, k = Initial reaction constant, taken from the slope of the graph representing concentration vs. reaction time (s⁻¹), C₀ = initial concentration of the substrates (Phenol or MO) (mol/L), V = volume of Phenol or MO (L).

Photolysis tests of phenol and MO under UV-Visible light and in the absence of catalyst were carried out. Reproducibility of the measurements was ensured by double testing of selected samples.