Biomedical subjects

Luciano Carlos

Publications and source records attributed to Luciano Carlos.

3 recordsLinked to original sources

PubMed2003

Competition kinetics using the UV/H2O2 process: a structure reactivity correlation for the rate constants of hydroxyl radicals toward nitroaromatic compounds.

The rate constants for hydroxyl radical reaction toward a set of nitroaromatic substrates kS, have been measured at 25 degrees C using competition experiments in the UV/H2O2 process. For a given pair of substrates S1 and S2, the relative reactivity beta (defined as kS1/kS2) was calculated from the slope of the corresponding double logarithmic plot, i.e., of ln[S1] vs. ln[S2]. This method is more accurate and remained linear for larger conversions in comparison with the plots of ln[S1] and ln[S2] against time. The rate constants measured ranged from 0.33 to 8.6 x 10(9) M(-1)s(-1). A quantitative structure-reactivity relationship was found using the Hammett equation. Assuming sigma values to be additive, a value of -0.60 was obtained for the reaction constant rho. This value agrees with the high reactivity and the electrophilic nature of HO* radical.

Environmental Pollutants↗

PubMed2002

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 (kS) and H2O2 (kHP) and to the corresponding molar absorption coefficients (epsilonS, epsilonHP). 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.

2,4-Dinitrophenol↗

PubMed2002

Degradation of nitroaromatic compounds by the UV-H2O2 process using polychromatic radiation sources.

The UV-H2O2 process, a standard advanced oxidation process (AOP) for water treatment, has been applied to the degradation of a series of nitroaromatic compounds (nitrobenzene, 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, 3-nitrophenol, 4-nitrophenol and 4-chloro-3,5-dinitrobenzoic acid) using polychromatic radiation sources. The optimal concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest degradation rate of a given substrate (S) was determined experimentally and estimated using a simplified kinetic model based on the main reactions involved in the first stages of the oxidation. We have shown that, under conditions of monochromatic irradiation, the ratio ROPT (= [H2O2]OPT/[S]0) is given by a simple mathematical expression containing only a few parameters, whereas, under conditions of polychromatic irradiation, ROPT is expressed by a complex mathematical equation (involving the spectral distribution of the lamp emission and the absorption spectra of H2O2 and the substrate). Two numerical analysis procedures are proposed for obtaining the bimolecular rate constants for the reaction of hydroxyl radicals with a substrate (kS) from this equation. The rate constants, kS, determined for the substrates investigated in this work are in agreement with the expected reactivity trend, taking into account the effects of substituents on the distribution of electron density in the aromatic ring. The methods proposed in this work offer a double advantage: i) a standard AOP may be used to evaluate the rate constants of reaction of substrates with hydroxyl radicals under polychromatic as well as under monochromatic irradiation, ii) optimal amounts of additive may be obtained using only a few parameters as predictive tools.

Journal Article↗