N-Doped TiO2: An Efficient Catalyst for the Photocatalytic Treatment of Water and Wastewater under Visible Light Irradiation

Abstract

As a consequence of the rapid growth of population in urban areas, water use and reuse has become a major concern, leading to an urgent imperative to develop effective and affordable technologies for wastewater treatment. Traditional methods for wastewater treatment are usually based on physical and biological processes but unfortunately, some organic pollutants, classified as bio-recalcitrant, are not biodegradable. In this way heterogeneous photocatalysis may become an interesting water treatment technology to remove organic pollutants not treatable by conventional technique. Photocatalysis, also called the "green" technology, represents one of the main challenges in the field of treatment and decontamination of water and air because it is able to work at ambient temperature and atmospheric pressure. Heterogeneous photocatalysis is a catalytic process that uses the energy associated to a light source to activate a catalyst with semiconducting proprieties. The most common used photocatalyst is TiO 2 . It is able to oxidize a wide range of toxic organic compounds into harmless compounds such as CO 2 and H 2 O. Due to the value of TiO 2 band-gap energy, about 3.2 eV, it is effective only under irradiation of UV light. This is a technological limitation when aiming at implementation of large scale sustainable "green" technologies with renewable energy sources such as solar light. The main research objectives for the application of TiO 2 as a photocatalyst is the increase the photocatalytic performances of TiO 2 through the doping of its crystalline structure with non-metal ions (nitrogen) that reduce the band-gap making possible the fruitful absorption of the visible light.The results obtained from our research activity evidenced that the doping of TiO 2 with nitrogen (N-TiO 2 ) has led to an enhanced photocatalytic activity in presence of visible light irradiation. The optimized formulation of N-TiO 2 has shown very effective in the removal of organic dyes, such as methylene blue and methyl orange [1], antibiotics such as spyramicin [2] and in the inactivation of E. coli [3]. It is also important to evidence that this optimized N-TiO 2 photocatalyst resulted also able to reduce the total chemical oxygen demand (COD) of a highly polluted wastewater such as tannery wastewater [4].A successful implementation of photocatalytic processes at large scales, however, calls for a global interdisciplinary view over the interplay between all the important parameters that affect the capture and utilization of photons in a photoreactor [4]. Moreover, one of the most important drawbacks of photocatalytic process is that photocatalysts are often used in slurry reactors. The limitation of slurry process is that the photocatalyst in powder form must be separated from the purified water after the treatment, and the cost of this separation stage may even invalidate economically this technique. With the aim to overcome this technical limitation, two different research strategies have been developed.The first research strategy was foc...