Organic contaminants, such as pharmaceuticals and personal care products, are ubiquitous in drinking water sources, but are not removed by conventional drinking water treatment processes. Photolysis of chlorine (i.e., hypochlorous acid) produces a suite of reactive oxidants, including hydroxyl radical, chlorine radical, and ozone. Therefore, chlorine photolysis is an advanced oxidation process that could potentially be used in drinking water treatment applications to remove organic contaminants. This study investigates the production of reactive oxidants during chlorine photolysis as a function of both pH (6-9) and irradiation wavelength (254, 311, and 365 nm). These variables are critical to the formation of reactive oxidants due to the differences in the quantum yield, reaction mechanisms, and molar absorptivity of hypochlorous acid versus hypochlorite (pKa = 7.5). The production of hydroxyl and chlorine radicals was determined using nitrobenzene and benzoate as probe compounds, while ozone was quantified using cinnamic acid. The rate of hydroxyl radical production is higher when hypochlorous acid is the most important chlorine species (i.e., at low pH and at shorter wavelengths). In contrast, ozone generation is higher at higher pH values and when light in the UV-A region is used for irradiation due to the photochemical transformation mechanism of hypochlorite. The laboratory results were used to validate a comprehensive kinetic model that can be used to predict reactive species production as a function of pH and irradiance conditions (i.e., wavelength and light intensity).

Effect of pH and wavelength on reactive oxidant production during chlorine photolysis

255th National Meeting (2018)

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