RESUMEN
The values of the bimolecular rate constants for the reactions of 2,2'-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid) radical cation with epicatechin (((2.4 ± 0.2)) s(-1) M(-1)), and epigallocatechingallate ((29 ± 5) s(-1) M(-1)) were obtained by spectrophotometric measurements. We propose a correlation between the Gibbs energy ΔG(o) for the one-electron charge-transfer reactions from several antioxidants to radical species and the rate constants of the corresponding bimolecular reactions. This correlation can be used to predict rate constants of reactions of known ΔG(o) values.
Asunto(s)
Electrones , Ácido Gálico/análogos & derivados , Benzotiazoles/química , Catequina/química , Ácido Gálico/química , Cinética , Oxidación-Reducción , Ácidos Sulfónicos/química , TermodinámicaRESUMEN
The effect of Hg(2+) on the fluorescence intensity of three fulvic acids (Pahokee Peat, Pony Lake and Suwannee River) was studied. The fluorescence intensity decreased in the presence of added Hg(2+), while the fluorescence lifetimes were independent of the concentration of Hg(2+) in solution. These results are indicative of ground-states association between the fulvic acids and Hg(2+) with formation of stable non-fluorescent complexes (static quenching process). The analysis of the excitation-emission matrices with the Singular Value Decomposition (SVD) and Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) methods provided additional valuable information regarding the binding properties between Hg(2+) ions and specific fluorescence components of the fulvic acids. The three fulvic acids were shown to contain the same three groups of fluorophores characterized by excitation/emission pairs in the following regions: (320-330 nm/425-450 nm), (370-375 nm/465-500 nm), (290-295 nm/370-395 nm). These pairs are almost not affected by the change of pH from 2.0 to 7.0. Ryan-Weber and modified Stern-Volmer methods were used to analyze the static fluorescence quenching of the individual components. Similar conditional stability constants of Hg(2+) binding for the three components were found by both methods. The obtained log K values are in the range of 4.4 to 5.4.
Asunto(s)
Benzopiranos/química , Fluorescencia , Mercurio/química , Sitios de UniónRESUMEN
The laser flash photolysis technique (λ(exc)=355 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(-) radicals generated from quenching of the triplet states of 1,4-naphthoquinone (NQ) by formic acid. Kinetic simulations of the experimental signals support the proposed reaction mechanism. This system is of potential interest in the development of UV-A photoinduced photolytic procedures for the treatment of Hg(II) contaminated waters. The successful replacement of NQ with a commercial fulvic acid, as a model compound of dissolved organic matter, showed that the method is applicable to organic matter-containing waters without the addition of quinones.
Asunto(s)
Formiatos/química , Mercurio/análisis , Quinonas/química , Contaminantes Químicos del Agua/análisis , Anaerobiosis , Restauración y Remediación Ambiental/métodos , Cinética , Mercurio/química , Fotólisis , Contaminantes Químicos del Agua/química , Purificación del Agua/métodosRESUMEN
The laser flash photolysis technique (λ(exc) = 266 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(·-) radicals in the temperature range 291.7-308.0 K. For this purpose, the CO(2)(·-) radicals were generated by scavenging of sulfate radicals by formic acid. The absorbance of the reduced radical of methyl viologen, a competitive scavenger of CO(2)(·-), was monitored at 390 nm. Moreover, theoretical calculations, including solvent effects, were also performed within the framework of the density functional theory for various chemical species of Hg(I) and Hg(II) to aid in the modeling of the reaction of reduction of HgCl(2) by CO(2)(·-).