ORIGINAL PAPER Visible light N-TiO 2 -induced photodegradation of Congo red: characterization, kinetics and mechanistic study H. S. Wahab 1 • H. M. Hadi 1 Received: 3 June 2016 / Revised: 12 March 2017 / Accepted: 8 May 2017 Ó Islamic Azad University (IAU) 2017 Abstract In this study, the photocatalytic degradation of Congo red has been investigated in N-doped TiO 2 (N- TiO 2 ) aqueous suspensions under visible light irradiation. Visible light-active N-TiO 2 was successfully prepared at three different weight contents (2.5, 5, and 7%) employing sol–gel method. It was able to harvest the visible irradia- tion with wavelength suitable for activation. X-ray diffraction, scanning electron microscopy, energy-disper- sive X-ray spectrometer, diffused reflectance UV–Vis spectroscopy, nitrogen adsorption Brunauer–Emmert– Teller, Raman spectroscopy, photoluminescence and X-ray photoelectron spectrometer were used to characterize the doped catalyst. The samples had a relatively large specific Brunauer–Emmert–Teller surface areas of about 42 m 2 g -1 with average X-ray diffraction crystalline size of 52 nm and showed visible light photocatalytic activity at about 408 nm. The impacts of several operating parameters on the Congo red photodegradation process were examined. Langmuir–Hinshelwood model exhibited pseudo-first- order degradation kinetics. N-TiO 2 -assisted plausible pho- todegradation mechanism has been suggested based on the qualitatively detected intermediate compounds. Keywords Advanced oxidation processes  Congo red  Langmuir–Hinshelwood  N-TiO 2  Photocatalysis Introduction The impacts of azo dyes on the environment are a great concern because of the potentially cancer-causing and allergenic properties of these materials especially when dyes suffer from anaerobic discolorations (Bruschweiler et al. 2014). In addition, these dyes possibly preclude both sunlight penetration and dissolved oxygen, which are necessary for aquatic life (Zainal et al. 2005). Thus, these colored effluents ought to be treated before discharging them to various water bodies. Advanced oxidation pro- cesses (AOPs) which relies mainly on hydroxyl radical (  OH) formation using excitation sources are the most widely used approach that is employed for dye degradation studies (Calze et al. 1987). Heterogeneous photocatalysis through combination of TiO 2 and visible light or ultraviolet is one of the promising advanced oxidation processes for the degradation of water-soluble organic pollutants that exist in wastewater (Khataeea et al. 2009). However, anatase TiO 2 material cannot efficiently utilize visible light (k [ 380 nm) of the solar energy because of its compara- bly large band gap. To overcome this problem, many reports showed that nitrogen is the best option among the nonmetal doping types due to its small ionization energy, comparable atomic size and atomic radius with oxygen, formation of meta stable center, thermal stability, low recombination centers and can absorb visible light (Jinlong et al. 2010; Xu et al. 2013). The mechanism of photocat- alytic activity of N-TiO 2 is explained by assumptions of oxygen vacancy formation when two nitrogen atoms introduced substitutionally or interstitially into lattice of Editorial responsibility: M. Abbaspour. Electronic supplementary material The online version of this article (doi:10.1007/s13762-017-1361-8) contains supplementary material, which is available to authorized users. & H. S. Wahab hswahab@yahoo.com 1 Department of Chemistry, College of Science, Al-Nahrain University, P.O. Box 64090, Al-Jadriya, Baghdad, Iraq 123 Int. J. Environ. Sci. Technol. DOI 10.1007/s13762-017-1361-8