Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water Michael J. Watts, Karl G. Linden à Department of Civil and Environmental Engineering, Duke University, Box 90287 Hudson Hall, Durham, NC 27708-0287, USA article info Article history: Received 1 December 2006 Received in revised form 19 March 2007 Accepted 20 March 2007 Available online 11 May 2007 Keywords: Ultraviolet Chlorine Monochloramine Quantum yield Hydroxyl radical Disinfection abstract The photodegradation of chlorine-based disinfectants NH 2 Cl, HOCl, and OCl  under UV irradiation from low- (LP) and medium-pressure (MP) Hg lamps was studied. The quantum yields of aqueous chlorine and chloramine under 254 nm (LP UV) irradiation were greater than 1.2 mol Es 1 for free chlorine in the pH range of 4–10 and 0.4 mol Es 1 for monochloramine at pH 9. Quantum yields for MP (200–350 nm) ranged from 1.2 to 1.7 mol Es 1 at neutral and basic pH to 3.7 mol Es 1 at pH 4 for free chlorine, and 0.8 mol Es 1 for monochloramine. Degradation of free chlorine was enhanced under acidic water conditions, but water quality negatively impacted the MP Hg lamp degradation of free chlorine, compared to the LP UV source. The production of hydroxyl radical via chlorine photolysis was assessed along with the rate of reaction between d OH and HOCl using radical scavengers (parachlorobenzoic acid and nitrobenzene) in chlorinated solutions at pH 4. The quantum yield of OH radical production from HOCl at 254 nm was found to be 1.4 mol Es 1 , while the reaction of HOCl with OH radical was measured as 8.5  10 4 M 1 s 1 . NH 2 Cl was relatively stable in all irradiated solutions, with o0.3 mg L 1 increase in nitrate following a UV dose of 1000 mJ cm 2 . For water treatment plants, no significant changes in chlorine concentration would be expected under typical pH levels and UV doses; however, the formation of d OH could have implications for chlorinated byproducts or decay of unwanted chemical contaminants. & 2007 Elsevier Ltd. All rights reserved. 1. Introduction Chlorine addition during primary stages of drinking water treatment can be a useful tool for plant operators to help control taste and odor, color, and bacterial growth in filtration beds. The required prechlorination dose is a function of the raw water chlorine demand and the desired chlorine residual. However, the detection of known or suspected carcinogenic disinfection-by-products (DBPs) in distribution systems, from treatment of raw waters containing even moderate levels of natural organic matter, has led to closer inspection and careful monitoring of chlorine dosages and residuals. Re- cently, the use of ultraviolet light (UV) for disinfection, in addition to pre- or post-chlorination, is gaining interest due to the effectiveness of UV for inactivation of Cryptosporidium and providing part of a multiple-barrier disinfection system (Craik et al., 2001; Kashinkunti et al., 2004; Zimmer et al., 2003). A multiple-barrier disinfection scenario incorporating chlorine and UV-based disinfectants could be sequenced using chlorine as a preoxidant, UV as a primary disinfectant, and additional chlorine for residual disinfection. Thus, chlorine could be present before, during, and/or after UV disinfection. The concentration of the free chlorine species, HOCl (hypochlorous acid) and OCl  (hypochlorite), in aqueous solution is pH dependent (pk a 7.5). Free chlorine in the form ARTICLE IN PRESS 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.03.032 à Corresponding author. Tel.: +1 919 660 5200; fax: +1 919 660 5219. E-mail address: kglinden@duke.edu (K.G. Linden). WATER RESEARCH 41 (2007) 2871– 2878