Hydroxyl radicals (•OH) are a direct photolysis product of both aqueous free chlorine and hydrogen peroxide under UV irradiation. The photooxidation rates of an OH-radical probe compound, nitrobenzene, were compared in aqueous solutions of free chlorine, and H2O2, exposed to monochromatic UV irradiation (254 nm). Acidic (< pH 6) free chlorine solutions had the highest observed rates of nitrobenzene decay. In deionized water, with initial oxidant concentrations < 0.15 mM, the first order nitrobenzene decay rates increased in the following order of treatments: UV-Cl2(pH 7) < UV-H2O2 < UV-Cl2 (pH 6) < UV-Cl2 (pH 5). However, water quality had a significant impact; background photon and radical scavengers led to reduced rates of nitrobenzene decay in natural waters. Analysis of the necessary energy input required to achieve 90% nitrobenzene degradation showed UV-Cl2 at pH 5 to be the most efficient oxidation process whereas at neutral pH, process efficiency was a function of the initial concentration of the oxidant; at 0.05 or 0.06 mM initial oxidant, UV-Cl2 was the most efficient oxidation process, while at >0.15 mM initial oxidant, UV-H2O2 was most efficient.
Comparative OH radical oxidation using UV-Cl2 and UV-H2O2 processes
Michael J. Watts, Erik J. Rosenfeldt, Karl G. Linden; Comparative OH radical oxidation using UV-Cl2 and UV-H2O2 processes. Journal of Water Supply: Research and Technology-Aqua 1 December 2007; 56 (8): 469–477. doi: https://doi.org/10.2166/aqua.2007.028
Download citation file: