Innovative Photoreactors to remove N-containing pollutants from water

The photooxidation of ammonia and photoreduction of nitrates have been investigated in semibatch mode, developing a suitable process to achieve the desired selectivity to innocuous N2. Proper photoreactor prototypes have been tested for both applications, together with process design. 1. Scope The development of efficient processes in photocatalysis is a challenging task. In particular, N-containing pollutants1, such as inorganic ammonia, nitrites and nitrates, and some organic N-containing compounds (dyes, pesticides, drugs, etc.)2, 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-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. 2. Results and discussion Different photocatalysts have been tested and compared. TiO2 has been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. This method is suitable for continuous one-step synthesis of oxides3. The apparatus includes a nozzle/burner, which is co-fed with oxygen and with an organic solution of the oxide precursors. The organic solvent acts as fuel for the flame, whereas oxygen is the comburent and the dispersing agent. The TiO2 samples was prepared from a solution of Titanium(IV)-isopropylate (Aldrich, pur. 97 %) in xylene and propionic acid. The burner was fed with a flow rate of 2.2 mL/min of the solution and 5 L/min of O2. The pressure drop across the nozzle was set to 1.5 bar. Several metals such as Pd and Ag were added as co-catalysts with a 0.1-0.5 wt% concentration by wet impregnation from a solution of nitrate precursors. Photocatalytic reduction of NO3− and oxidation of NH3/NH4+ in water were carried out in a specifically designed Pyrex reactor, with a top quartz window (Figure 1). The catalysts were suspended in an aqueous solution of NH4Cl (0.2M) or NaNO3 (0.006M) and the suspension was stirred using a magnetic stirrer. The reaction suspension was thoroughly degassed and then exposed to He. The reactor was operated in semi-batch mode: the solution containing the pollutant to be photoconverted was added at the beginning of the reaction in batch mode, whereas a gas stream continuously flowed through the reactor. The gas was composed by He during the conditioning-outgassing phase preliminary to every measurement. He was fed in continuous mode also during the nitrate photoreduction tests, whereas it was substituted by synthetic air (80 vol% He + 20 vol% O2) during the ammonia photooxidation experiments. A trap for ammonia, possibly stripped from the reactor, was placed downstream. Comparison with commercial nanostructured TiO2 supports revealed several differences. The TiO2 sample prepared by FP exhibited higher activity with respect to the commercial sample, achieving ca. 20 % conversion after 5 h, without indication of decay or deactivation. By contrast, the commercial sample was less active and most of all its activity was rapidly ruled out. Some induction period was observed, due to the need of sample conditioning upon irradiation. The most interesting point is that no trace of nitrites or nitrates was observed, confirming a full selectivity to the desired product, i.e. molecular N2. The photoreduction of nitrates was also tested over the same samples. Very low conversion (<5%) was achieved with pure TiO2 catalysts, whereas Pd doping improved conversion by ca. one order of magnitude under the same experimental conditions. The key problem remained process selectivity: 100% to the undesired NH3 for the undoped photocatalyst, max 30% for the Pd-doped sample prepared by FP. Figure 1. Scheme of the photocatalytic plant set up 3. Conclusions An innovative photoreactor has been set up allowing to achieve the photoreduction of nitrates and the photooxidation of ammonia in semibatch mode. Its configuration, to be further optimised as for geometry and mixing, is suitable for scale up, easily implements immobilised photocatalysts and is ready to use with direct sunlight. These first results on both reactions let us conclude that a two step process can be designed to cope with the insufficient selectivity to N2 during the nitrate photoreduction step. The ammonia produced is removed during a second, fully selective step of photooxidation. The flame pyrolysis procedure is a viable technique for the preparation of either bare or metal-doped semiconductors in nanosized form, to be used for the photocatalytic abatement of inorganic N-containing pollutants in waste or drinking waters. Even if a photoreactor with external irradiation is less efficient than one base on immersion lamps, it is much more suitable for scale up and most of all for the direct application with solar light. Sufficiently high ammonia conversion was achieved during photooxidation in semibatch confirguration, with 100 % selectivity to N2. The financial support of Fondazione Cariplo (Italy) through the measure “Ricerca sull’inquinamento dell’acqua e per una corretta gestione della risorsa idrica”, project “DEN - Innovative technologies for the abatement of N-containing pollutants in water”, grant no. 2015-0186, is gratefully acknowledged. References 1. Compagnoni, M.; Ramis, G.; Freyria, F. S.; Armandi, M.; Bonelli, B.; Rossetti, I. Photocatalytic Processes for the Abatement of N-Containing Pollutants from Waste Water. Part 1: Inorganic Pollutants. J. Nanosci. Nanotechnol. 2017, in press. 2. Freyria, F. S.; Armandi, M.; Compagnoni, M.; Ramis, G.; Rossetti, I.; Bonelli, B. Catalytic and Photocatalytic Processes for the Abatement of N-Containing Pollutants From Wastewater . Part 2 : Organic Pollutants. J. Nanosci. Nanotechnol., 2017, in press. 3. Compagnoni, M.; Lasso, J.; Di Michele, A.; Rossetti, I. Flame Pyrolysis Prepared Catalysts for the Steam Reforming of Ethanol. Catal. Sci. Technol. 2016, 6247-6256.