Photoinduced Decomposition of Trihalomethanes in Aqueous Solution Effects of H₂O₂ on the Decomposition Rate and Characteristics of the Reaction with OH Radical

Abstract

Photochemical reactions of trihalomethanes in aqueous H₂O₂ solution were studied at room temperature under the irradiation with a Vycor-filtered light from a low pressure mercury lamp. CHCl₃ did not decompose in air-saturated aqueous polution, but was converted to CO₂and HCl by the addition of H₂O₂ due to the reaction with OH radical formed by the photodecomposition of H₂O₂ at 254 nm. Photoirradiation of an Ar-saturated aqueous solution of H₂O₂ without CHCl₃ led to the formation of O₂. The quantum yield of the O₂ formation (Φ_O2)decreased and that of the CHCl₃ decomposition (Φ_-CHCl3) increased with inc reasing the concentration of CHCl₃. The sum of Φ_O2 and Φ_-CHCl3 was constant, indicating that two OH radicals are required for the decomposition of a CHCl₃ molecule in Ar-saturated aqueous solution. The simultaneous formation of CO and C_O2 occurred upon irradiation of several brominecontaining trihalomethanes (CHBr_nCl_(3-n) in the presence of H₂O₂. The yield ratio of CO to C_O2 was proportional to that of the numbers of quanta at 254 nm absorbed by trihalomethanes and H₂O₂. The bromine-containing trihalomethanes gave C_O2 exclusively by the reaction with OH radical as generated by Fenton's reagent or γ-radiolysis of N₂O-_O2-saturated aqueous solution.