Heterogeneous photocatalytic processes for the abatement of N-containing pollutants from wastewater

Abstract

Since the last four decades, the viability of photocatalytic degradation of organic compounds in water streams has been demonstrated. Nitrogen containing compounds, such as dyes, pesticides, drugs, etc. as well as inorganic ammonia (1), nitrites and nitrates (2), are harmful contaminants for drinking water and ground water (3). These compounds are particularly relevant in agriculturally intensive zones and in the case of some industrial processes involving e.g. nitration reactions. Therefore, the development of effective methods for the abatement of these harmful pollutants from waste waters and from hydric resources is a challenging task. Different configurations for solar TiO2 photocatalytic reactors have been used (4), however pilot and demonstration plants are still countable. This work represents the photocatalytic processes for the abatement of N-containing compounds focusing mainly on the photoreduction of nitrate ions and the photooxidation of ammonia in semibatch mode, aiming at maximum selectivity towards N2 (5). Innovative photoreactors were specifically designed and optimized for this application. Different photocatalysts have been prepared with two different methods and their photocatalytic performance has been compared with commercial nanostructured TiO2 (P25). TiO2 has been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, and mesoporous TiO2 was prepared by sol-gel method. Pd has been in case added TiO2 by post synthesis impregnation. The physical/chemical properties of the photocatalysts were studied by means of XRD, BET and UV-Vis spectroscopy. The results of both photooxidaton and photoreduction of photocatalysts imply that the flame pyrolysis procedure is a viable technique for the preparation of nanosized TiO2 for this application, leading to ca. double conversion with respect to commercial samples. Furthermore, the addition of a small amount of Pd (0.1 wt%), likely enhanced the lifetime of the photoproduced charges by electron trapping and resulted in higher conversion in almost all photocatalysts. However, the best performance was obtained by Pd doped on TiO2 (FP) especially in the photooxidation process with conversion up to 31% after 5h and selectivity up to 100%. The specific configuration of designed photoreactor, enhanced the uniform mixture of catalyst and light distribution and opens possibility for further optimization for scale up in order to be used with direct sunlight. Key words: Photoreduction, Photooxidation, N-containing compounds, Waste water treatment, TiO2, Flame pyrolysis Acknowledgements: Fondazione Cariplo (grant 2015-0186 \u201cDEN - Innovative technologies for the abatement of N-containing pollutants in water\u201d) is gratefully acknowledged. References: (1) Lee J., Park H., Choi W., 2002, Environ. Sci. Technol., 36, 5462. (2) Nolan B.T., Puckett L.J., Ma L.W., Green C.T., Bayless E., Malone R.W., 2010, J. Environ. Qual., 39, 1051. (3) Burow K.R., Nolan B.T., Rupert M.G., Dubrovsky N.M., 2010, Environ. Sci. Technol., 44, 4988. (4) Bukowski J., Somers G., Bryanton J., 2001, J. Occup. Environ. Med., 43, 377. (5) Compagnoni M., Ramis G., Freyria F.S., Armandi M., Bonelli B., Rossetti I, 2017, J. Nanosci. Nanotech, 17, 3632