Photocatalytic reactors and processes for the abatement of harmful N-containing pollutants from waste and drinking waters

The N-contamination problem has been effectively synthesized in the following Figure (on the left) and is addressed as one of the main concerns by the European Environment Agency (EEA). Indeed, environmental limits are defined by ‘planetary boundaries’ (right Figure, [Rockström et al., Nature, 461 (2009) 472]), where the inner blue-delimited zones represents estimated planetary safe boundaries. The nitrogen cycle has already disrupted the safe operating limits, well beyond climate change. Initial estimates suggest the ecosystem impacts of nitrogen pollution cost the EU-27 between €25 billion and €115 billion per year, and total costs (including those from N-related health and climate change impacts) could amount to €320 billion a year. “A Blueprint to Safeguard Europe's Water Resources” has been published in Nov. 2012, highlighting that diffuse and point-source pollution are still significant pressures on the water environment in about 38% and 22% of EU water bodies, respectively. Eutrophication due to excessive nutrient load remains a major threat to the good status of waters as nutrient enrichment is found in about 30% of water bodies in 17 Member States. In the same frame, environmental pollution with pharmaceutical residua is recognized as an emerging pressing problem. The Commission is going to present a report on the scale of this problem, in order to propose adequate legislation. The same document fosters the use of Best Available Techniques (BAT), or the development of new ones, to ensure the improvement of water quality. Inorganic nitrogen containing pollutants, such as inorganic ammonia, nitrites and nitrates, are responsible of acute and/or chronic diseases, especially for infants and children. Effective methods for their abatement from waste and drinking waters are still missing. Biological treatments are standardized processes allowing the abatement of biodegradable organics from waste waters to meet common regulations. However, the legislative limits for N containing compounds can hardly be met by this standardized treatment, imposing further processes for N removal (and additional costs). The revision of the whole capacity for waste water treatments of Lombardy evidenced that sufficient depuration efficiency for organic compounds characterizes large scale depuration plants, efficiency decreasing with plant size. In any case, however, the regional objective for the abatement of 75% of N-containing compounds has not been met in any plant, removal efficiency attesting on 60-66% for small and large scale installation, respectively [http://www.arpalombardia.it]. Ion exchange traditionally has been used to remove nitrates from drinking water sources, but this process leaves behind a highly concentrated brine solution that still requires treatment. Alternatively, reverse osmosis and electrodialysis may be used, which however are expensive processes due to their installation and management costs. In addition, several methods have been developed and applied to the treatment of ammonia-containing wastewater, including biological processes, air stripping, ion exchange, breakpoint chlorination, and chemical oxidation. Among these methods, biological processes are generally regarded to be the most efficient. However, these processes have disadvantages, including handling difficulties and large equipment requirements. Moreover, they are difficult to apply to treatment of wastewater that contains harmful co-existing species for bacteria. Therefore, the development of cheaper and easily scalable systems for their abatement is more than welcome and is at the basis of our strategy. Together with the review of existing tertiary treatments, 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 as innovative technologies to be implemented as tertiary treatment in depuration plants. Different photocatalysts have been compared and prepared by means of an innovative flame-based technique. TiO2 has been prepared in nanosized form by flame pyrolysis (FP). This method allows the one step synthesis of single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. Pd has been also added to TiO2 by post synthesis impregnation. The role of the metal nanoparticles was the enhancement of the lifetime of the photoproduced charges by electron trapping. The same materials formulations have been prepared starting from commercial nanostructured TiO2 supports for comparison purposes. A first experimental set up was tested for both the ammonia photooxidation and the nitrates photoreduction reactions, with the goal of maximum selectivity to N2. The reactor was irradiated externally from the topside, which was closed through a quartz window, transparent in the UV-Vis range of interest. The sketch of the reactor is reported in the following Figure. A semi-batch reactor configuration was adopted, with a gas stream constituted by 20 vol% O2 in He continuously bubbling in the ammonia-containing suspension of the photocatalyst during the photo-oxidation process, whilst only He was used during the nitrate reduction tests. In the former case one of the reactants was fed in batch mode (ammonia), oxygen continuously, and one of the products, molecular nitrogen, was continuously removed. The possible over-oxidation byproducts, nitrates and nitrites, were kept in the photoreactor, thus allowing their possible combined post treatment. Similar approach was adopted during the tests for the photoreduction of nitrates, where the reactant, sodium nitrate, was fed in batch mode, whereas the desired product (N2) was continuously withdrawn, leaving in the reactor the possible over-reduction product, i.e. ammonia/ammonium. The external lamp was placed on the top of the photoreactor in order to simulate a future application directly by sunlight. This configuration allows easier scale up with respect to reactor configurations based on immersion lamps sometimes found in the literature, which have the advantage of closer (and thus more efficient) irradiation of the reacting suspension, but are not easily feasible for scale up, especially if the final scope is the use of sunlight. A preliminary set of activity tests for both reactions have been carried out on bare titania photocatalysts. In particular, we compared the performance of a FP-prepared catalyst with the commercial P25 benchmark (see Figure below). The same catalysts were poorly active for the photoreduction of nitrates (conversion < 5%), leading to 100% selectivity to ammonia. When 0.1 wt% Pd was added conversion was improved to 15% after 6h, but still with significant selectivity to ammonia (up to 40%). Based on these results and given the full selectivity to N2 during the photooxidation of ammonia, a two step process may be conceived, with the photoreduction of nitrates (and nitrites) as a first stage, followed by the photooxidation of the originally present and newly formed ammonia. Acknowledgments 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. The valuable help of Paola Marzullo, Gaia Mascetti, Veronica Praglia and Alberto Riva is gratefully acknowledged.