Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: Removal of trace pollutants in water using nanomaterials

This doctoral research focused on new photocatalytic nanomaterial synteshes based on titanium dioxide (titania, TiO2), for wastewater treatment using visible light, avoiding the UV light use, which is costly and hazardous to health. Various doping strategies were employed: silver (Ag), strontium (Sr), iron (Fe), and biochar (BC) were tested as doping agents. Among these options, iron doping through the solid-state method proved to be the preferred approach due to its superior performance, simplicity of synthesis, and by-products absence. The optimum material (titania loaded with 1wt% of Fe) was characterized using SEM, XRD, UV-Vis-DRS, BET, and AAS techniques. The characterization confirmed the effectiveness of the synthesis method and the positiv effect of doping. The synthesized TiO2 nanoparticles had an approximate size of 40 nm, a band gap of 2.52 eV, making them sensitive to visible light, as well as a mesoporous structure with a surface area of about 9 m2/g. Iron effectively replaced titanium in the crystalline lattice, comprising mainly anatase and rutile phases, while a fraction of iron (approximately 5% of the total) formed a distinct Fe2O3 phase on the titanium dioxide surface. The photocatalyst's performance was evaluated through experiments with various model contaminants, including methylene blue, rhodamine B, methyl orange, and paracetamol. The results showed performance comparable to the results obtained by other researchers working under similar conditions albeit the doping methods were far more complex to adopt. For instance, a kinetic constant of 0.522 h–1 was obtained, in line with the findings of N. Abbass et al. (2016), who reported a kinetic constant of 0.971 h–1, and C. Afonso et al. (2022), who reported a K value of 0.202 h-1. Photocatalytic experiments were carried out employing an Osram 13 W LED lamp in batch tests by varying operating conditions, including pH, contaminant concentration, catalyst concentration and irradiance, to determine optimal working conditions. In addition, photocatalyst behavior was studied in more complex matrices, particularly in mixtures of multiple organic contaminants and in the presence of inorganic ions such as chloride, nitrate, iodide, sulfate, sodium, potassium, and iron. It has been observed that pH influences contaminant adsorption with repercussions on photocatalysis as well, high contaminant concentrations disfavor photocatalysis, photocatalytic efficiency increases as irradiance increases, there is an optimum of catalyst concetration to be used, the presence of iron and nitrate ions, enhance photocatalytic performance. In the second part of the study, the enhancment of photocatalytic process was investigated using ultrasound and hydrogen peroxide as coadjuvant agents to assess any synergistic effects. Ultrasound was found to promote contaminant removal through the production of hydroxyl radicals, but no synergistic effect with the photocatalyst was observed. However, the addition of hydrogen peroxide showed an additional effect not present when the two techniques were used individually. This additional effect was due to the Fenton reaction involving surface iron. Furthermore, in the view of a scale up of the process, the immobilization of the photocatalyst on polystyrene pellets with dimensions of 1-3 mm was achieved through a thermal method. Optimization of the photocatalyst load and pellet dimensions ensured efficient utilization. The thermal method was chosen for its simplicity and lack of effluent production. It is important to note that the photocatalytic performance of the material was reduced due to surface passivation, albeit providing an economical alternative to more industrially demanding centrifugation processes (aimed at separating the catalyst from the treated solution). Immobilization on polystyrene pellets demonstrated the reusability of the material, as the photocatalyst was tested for up to 10 cycles without a significant decrease in performance. In addition, a laboratory-scale continuous treatment system was implemented, the results of which matched the expectations of batch testing, providing a practical approach to wastewater treatment. Specifically, by employing 5 Osram lamp 13 W LEDs and a 42 mL reactor, setting a flow rate of 8.7 mL/h (hydraulic residence time of 4.5 h) and 718 mg of supported catalyst, about 35 % rhodamine B was removed, working under steady-state conditions for the monitoring time (9 days).