Comparative OH radical oxidation using UV-Cl2 and UV-H2O2 processes

Hydroxyl radicals ((OH)-O-center dot) 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-Cl-2(pH 7) < UV-H2O2 < UV-Cl-2 (pH 6) < UV-Cl-2 (pH 5) However water., 2 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-Cl-2 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-Cl-2 was the most efficient oxidation process, while at >0.15mM initial oxidant, UV-H2O2 was most efficient.