A solar-driven UV/Chlorine advanced oxidation process Po Yee Chan, Mohamed Gamal El-Din**, James R. Bolton* Department of Civil and Environmental Engineering, University of Alberta, 3-133 Markin/CNRL, Edmonton, AB, T6G 2W2 Canada article info Article history: Received 25 January 2012 Received in revised form 23 July 2012 Accepted 25 July 2012 Available online 10 August 2012 Keywords: Advanced oxidation Free chlorine Hydroxyl radicals Solar process abstract An overlap of the absorption spectrum of the hypochlorite ion (OCl  ) and the ultraviolet (UV) end of the solar emission spectrum implies that solar photons can probably initiate the UV/chlorine advanced oxidation process (AOP). The application of this solar process to water and wastewater treatment has been investigated in this study. At the bench-scale, the OCl  photolysis quantum yield at 303 nm (representative of the lower end of the solar UV region) and at concentrations from 0 to 4.23 mM was 0.87  0.01. Also the hydroxyl radical yield factor (for an OCl  concentration of 1.13 mM) was 0.70  0.02. Application of this process, at the bench-scale and under actual sunlight, led to methylene blue (MB) photobleaching and cyclohexanoic acid (CHA) photodegradation. For MB photobleaching, the OCl  concentration was the key factor causing an increase in the pseudo first-order rate constants. The MB photobleaching quantum yield was affected by the MB concen- tration, but not much by the OCl  concentration. For CHA photodegradation, an optimal OCl  concentration of 1.55 mM was obtained for a 0.23 mM CHA concentration, and a scavenger effect was observed when higher OCl  concentrations were applied. Quantum yields of 0.09  0.01 and 0.89  0.06 were found for CHA photodegradation and OCl  photolysis, respectively. In addition, based on the Air Mass 1.5 reference solar spectrum and experimental quantum yields, a theoretical calculation method was developed to estimate the initial rate for photoreactions under sunlight. The theoretical initial rates agreed well with the experimental rates for both MB photobleaching and CHA photodegradation. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Advanced oxidation processes in wastewater treatment Advanced Oxidation Processes (AOPs) have proven to be effective in degrading organic constituents in industrial wastewaters, including persistent organic pollutants (Ikehata et al., 2008). Most persistent organic pollutants are highly stable chemically and are not readily biodegradable; therefore, AOPs can serve as an alternative treatment to accelerate the biodegradability of these constituents (Gamal El-Din et al., 2011; Martin et al., 2010). In AOPs, a powerful oxidizing agent is generated [often this is the hydroxyl radical (  OH)], which oxidizes persistent organic compounds to smaller molecules. The oxidizing ability of the hydroxyl radical arises from its high standard electrode potential of þ2.7 V in acid solution, so it can react with most organic compounds through abstrac- tion or addition reactions at almost diffusion-controlled rates (Andreozzi et al., 1999; Bolton, 2010). The combination of AOPs with other treatment processes, such as pre-treatment or post-treatment, is widely used to improve the performance of AOPs and to increase the overall treatment efficiency (Gamal El-Din et al., 2011; Litter, 2005; * Corresponding author. Tel.: þ1 780 439 4709; fax: þ1 780 439 7792. ** Corresponding author. Tel.: þ1 780 492 5124. E-mail addresses: mgamalel-din@ualberta.ca (M. Gamal El-Din), jb3@ualberta.ca, jbolton@boltonuv.com (J.R. Bolton). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 5672 e5682 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.07.047