Evaluation of the efficiency of photodegradation of nitroaromatics applying the UV/H2O2 technique

Photolysis of nitroaromatic compounds in aqueous solution is a very slow and inefficient process. As already observed for a variety of organic pollutants, considerably faster degradation rates of nitrobenzene (NBE), 1-chloro-2,4-dinitrobenzene (CDNB), 2,4-dinitrophenol (DNP), and 4-nitrophenol (PNP) could be achieved, when the oxidative degradation of these compounds was initiated by hydroxyl radicals produced by UV-C photolysis of H2O2. Analysis of intermediate products formed during irradiation by HPLC and IC showed that cleavage of the aromatic ring should occur at an early stage of the oxidation process and that organic nitrogen was almost completely converted to nitrate. The optimal initial concentration of hydrogen peroxide ([H2O2](OPT)) leading to the fastest oxidation rate, which depends on the initial substrate concentration ([S](0)) could be evaluated using a simplified expression based on the main reactions involved in the first stages of the degradation process. Using only a minimum of kinetic and analytical information, this expression shows that the ratio R-OPT (= [H2O2](OPT)/[S](0)) is related to the bimolecular rate constants for the reactions of hydroxyl radicals with substrate (k(S)) and H2O2 (k(HP)) and to the corresponding molar absorption coefficients (epsilon(S), epsilon(HP)). Competition experiments between selected pairs of the substrates showed that their relative reactivity toward hydroxyl radicals could be correctly predicted using the same simplified approach. The results of our investigations as well as literature data support the general validity of the proposed procedure for optimizing oxidation rates of the UV/H2O2 process.