Estudio de la degradación fotocatalítica de contaminantes emergentes: desarrollo de composites fotoactivos y aplicación en un reactor de spouted bed

Abstract

Photocatalysis is currently being considered for water decontamination due to its ability to degrade organic pollutants to CO2, water and mineral acids. In TiO2 photocatalysis, UV radiation is needed to create hole-electron pairs which can be transferred to water to form oxidizing species. However, this process generally suffers low apparent quantum yield. Hence, the first goal of this work was to investigate the parameters that deactivate the catalyst in real waters such as tap, river or wastewater. In general, the removal efficiency of clofibric acid (CFA) decreased with inorganic salts, especially with sulfates and carbonates, and also in environmental waters. A general kinetic model was developed to describe the CFA photodegradation depending on the type and concentration of substances present in water. High correlation was observed between experimental CFA concentrations and those predicted by the model. After that, several strategies were tried to improve quantum yields by adding electron scavengers and powdered activated carbon (PAC). Among the e--scavengers tested, only NaIO4 showed significant effect on the degradation of the analytes (a mixture of 5 pharmaceuticals: sulfamethoxazole, carbamazepine, CFA, diclofenac and ibuprofen), especially when the assays were performed in river water. Regarding the addition of PAC, different composites prepared from titania nanoparticles and PAC were tested. In any case, depending on each pharmaceutical tested and on its competitive affinity with the others, the combined treatment of adsorption and photocatalytic degradation improved the overall removal efficiency of drugs in water. One of the main drawbacks of TiO2 photocatalysis its nanometric size that usually leads to catalyst washing out from systems. Hence, a new procedure to immobilize TiO2 nanoparticles on a larger matrix such as calcium alginate was investigated to obtain millimetric-size particles with photocatalytic activity. Two natural thickeners, xanthan gum (XG) and locust bean gum (LBG), were also added to improve the mechanical stability of the photoactive beads. Nevertheless, the presence of thickening compounds affected negatively to their photocatalytic efficiency, while the most effective ones were alginate-TiO2 (TA) beads. Furthermore, a new type of photocatalytic material based on TiO2, calcium alginate and 1.5% of PAC was tested. It was confirmed that these so formed composites (TA/PAC) combined adsorption with photolysis and, thus, were more efficient at micropollutants removal. Finally, a new spouted bed photocatalytic reactor (SBPR) was designed at bench scale. After assessing the effect of light adsorption, inlet air flow and photocatalyst load, continuous-flow experiments were performed for the mixture of pharmaceuticals. Different operational conditions were set in order to evaluate the influence of influent flow rate and pollutant concentration. On the whole, it was noticed that the SBPR worked efficiently for high inlet loadings of contaminants and that it was able to treat satisfactorily micropollutant concentrations of up to 10 mg L-1.