Photodegradation of nitroaromatic compounds by the UV/H2O2 process using polychromatic irradiation sources

FERNANDO S. GARCÍA EINSCHLAG; LUCIANO CARLOS; ALBERTO L. CAPPARELLI; ESTHER OLIVEROS; ANDRÉ M. BRAUN

Karlsruhe, Alemania

Simposio; Updates in Photochemical Technologies; 2001

Various advanced oxidation processes (AOP) are nowadays available for achieving oxidative degradation of organic pollutants in aqueous media, and have been increasingly employed in recent years as alternative or complementary treatments to conventional methods of detoxification. Most AOP are based on the production of hydroxyl radicals as primary oxidizing species. Among them, the UV/H2O2 process is widely applied to the treatment of wastewaters containing a variety of biocidal or non-biodegradable organic contaminants. In this work, we have investigated the efficiency of the degradation of nitroaromatic compounds by the UV/H2O2 process, using 1-chloro-2,4-dinitrobencene, 2,4-dinitrophenol and 4-chloro-3,5-dinitrobenzoic acid, as model compounds. The optimal concentration of H2O2 leading to the fastest rates of substrate degradation and DOC (dissolved organic carbon) depletion could be evaluated, by using a simplified equation based on the main reactions involved in the first stages of the degradation process. Production of hydroxyl radicals by photolysis of H2O2 is the primary step of the UV/H2O2 process (reaction 1). H2O2   +  hn  ----  2 HO.                                                                     (1) The molar absorption coefficient of H2O2 (eH2O2) being only 18.7 L mol-1 cm-1 at 254 nm, absorption of incident radiation by the aromatic substrate competes with absorption by H2O2 even at relative low substrate concentrations. Although substrate photolysis was negligible under the experimental conditions used in this work, inner filter effects by the aromatic compound had to be taken into account for the evaluation of the rate of photons absorbed by H2O2. The efficiency of substrate degradation was strongly dependent on the H2O2 concentration. In all cases, the degradation rate increased up to a maximum value with increasing H2O2 concentration, then decreased when a larger amount of H2O2 was added. The existence of such a maximum results from the competition between the reactions of hydroxyl radicals with the organic substrate (reaction 2) and H2O2 (reaction 3). It should be noted that hydroperoxyl radicals (HO2.) produced by reaction 3 are much less reactive toward organic substrates than HO.. Substrate (s) + HO.  ----  Radical intermediates                                                (2)  H2O2  +  HO.  -----  HO2.   + H2O                                                                 (3) Assuming a simplified reaction scheme including only reactions 1, 2 and 3 and taking into account competitive light absorption by H2O2 and the substrate, the optimal concentration of H2O2 (denoted as [H2O2]op) leading to a maximum initial rate of substrate degradation could be derived (equation 4). The value of [H2O2]op depends on the particular substrate and its concentration: [H2O2]op = [S]0 {(kS.eS)/(kH2O2.eH2O2)}1/2                                                    (4) where [S]0 and eS are the initial concentration and the molar absorption coefficient of the substrate, respectively. This equation may be used in two different ways: a) If the rate constant of reaction 2 (kS) is known, the optimal H2O2 concentration may be calculated. b) If the optimal H2O2 concentration is determined experimentally by measuring the initial rate of substrate degradation, then kS can be estimated. In this communication, we show the predictive character of equation 4 for the degradation of several chloronitroaromatic compounds by the UV/H2O2 process.