Mechanism and Kinetics of the OH-Radical Intervention during Fenton Oxidation in the Presence of a Significant Amount of Radical Scavenger (Cl^-)

Abstract

Orange II, the 4-(2-hydroxy-1-naphthylazo)benzenesulfonic acid Na-salt, was taken as a model for the oxidation of organic compounds in photoassisted Fenton process in the presence of Cl--anion in solution. The HO• radicals seem to originate due to the photolysis of Fe(OH)2+ complexes in solution, whereas the Cl2 -• radical was a product of the photolysis of the FeCl2+ complexes. The rate constants for the reaction of HO • and Cl2 -• radicals with Or II were determined by laser kinetic spectroscopy: k OH • +OrII = 6.0 × 109 (M s)-1 and k Cl2 • +OrII = 3.7 × 107 (M s)-1. A significant decrease in the rate of decoloration was observed for Orange II upon addition of Cl- (10 mM), but further addition of Cl- only marginally affected the latter reaction rate. Chlorinated hydrocarbons are observed as the products of Orange II oxidation in photoassisted and dark Fenton processes in the presence of Cl- anions. Light irradiation was observed to decrease the amount of chlorinated organic products (AOX) in solution as compared to dark Fenton processes. The results obtained are shown to be in agreement with the radical reaction theory for the particular case of the Fenton reaction. Kinetic modeling of the Orange II decoloration was carried out taking into consideration the rate constants found for the reaction k HO • +OrII and k Cl2 • +OrII.

Enhancement of Fenton oxidation for removing organic matter from hypersaline solution by accelerating ferric system with hydroxylamine hydrochloride and benzoquinone

“…At a high concentration of chloride ions, however, the oxidation of aniline ceased. Chloride ions slow down the oxidation reactions through the complexation of Fe 2+ /Fe 3+ with Cl − (Reactions (4)- (7)) and/or by acting as scavengers of hydroxyl radicals (Reactions (8)- (10)) (Bacardit et al, 2007;De Laat et al, 2004;Kiwi et al, 2000;Machulek et al, 2007). It can be seen from Reactions (8) and (10) that the scavenging effect of chloride ions was pH-dependent, which can be abated by maintaining the pH at an optimal level of 3 during the Fenton process (De Laat et al, 2004;Grebel et al, 2010).…”

Interference of inorganic ions on phenol degradation by the Fenton reaction

“…Among these new technologies, advanced oxidation processes (AOP) are potentially the most promising due to their high efficiency and versatility. Thus, a variety of classes of compounds can be completely mineralized by existing AOP, including compounds refractory to conventional biological treatment processes (Kiwi et al, 2000;Pérez et al, 2002;Pignatello et al, 2006).…”

Influence of the operating conditions on highly oxidative radicals generation in Fenton's systems

“…The chemistry of the Fenton's process is based upon the reaction of hydrogen peroxide (E 0 = 1.80 and 0.87 V at pH 0 and 14 respectively) with a proper catalyst, leading to the generation of a pool of radicals [1], capable of non-selectively oxidizing a wide range of biorefractory organic pollutants such as chlorinated aliphatics, halogenated phenols, PAHs and PCBs. The radical produced in the Fenton's initiation Reaction (I) is the hydroxyl radical, whose formation can be achieved by adding an homogeneous catalyst, such as a transition metal salt [3,4], by generating the metal catalyst electrochemically by means of sacrificial steel electrodes or by using the metals naturally occurring in the environment as heterogeneous catalyst [5][6][7][8][9][10][11][12] The hydroxyl radicals generated through Reaction (I) react with hydrogen peroxide and begin a series of propagation reactions [2] …”