Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water

The photodegradation of chlorine-based disinfectants NH2Cl, 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 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 8.5×104 M−1 s−1. NH2Cl was relatively stable in all irradiated solutions, with <0.3 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 OH could have implications for chlorinated byproducts or decay of unwanted chemical contaminants.