Development of unconventional photocatalytic reactors and processes for the abatement of harmful N-containing pollutants

The development of effective methods for the abatement of some harmful pollutants from waste waters and from hydric resources is a challenging task. Nitrogen containing compounds, such as inorganic ammonia, nitrites and nitrates, and some organic N-containing compounds (dyes, pesticides, drugs, etc.), are harmful contaminants for drinking water, inducing acute and/or chronic diseases, especially affecting infants and children. Furthermore, when released in waste waters they contribute to eutrophication, or possibly contaminate ground water. This is particularly relevant in agriculturally intensive zones and in the case of some relevant industrial processes involving e.g. nitration reactions. In the present work, we developed innovative photocatalytic processes for the abatement of N-containing compounds, focusing on selectivity towards innocuous N2, to be applied for the treatment of waste waters to meet legislative specifications. The photocatalytic performance of the samples, to be correlated with nanomaterials properties, has been checked for the photoreduction of nitrate ions and the photooxidation of ammonia and organic N-containing compounds by means of two semi-batch photoreactors, specifically designed and optimized for this application. A first design was based on external irradiation through a quartz window, a second prototype on an immersed UV lamp. The optimization of reactor design was the main scope of this investigation, to achieve uniform and effective photoirradiation. The advantages of continuous flow or high pressure were considered. The photocatalysts have been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise in one step single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. In the present investigation TiO2 and SrTiO3 have been prepared by means of a home made apparatus. Different pyrolysis conditions have been set in order to optimize materials properties. Noble metals have been in case added to the samples by post synthesis deposition-precipitation (Au), impregnation and photodeposition (Pt, Pd) and/or directly during the FP synthesis (Pt, Pd, Ag). The role of the metal nanoparticles was the enhancement of the lifetime of the photoproduced charges by electron trapping and/or the enhancement of the materials absorption in the visible region, which is one of the most practically relevant goals of this research. The same materials formulations have been prepared starting from commercial nanostructured TiO2 supports for comparison purposes. ACKNOWLEDGMENTS The financial support of Fondazione Cariplo through the measure “Ricerca sull’inquinamento dell’acqua e per una corretta gestione della risorsa idrica”, grant no. 2015-0186, is gratefully acknowledged.