Effects of Fulvic Acid on Fe(II) Oxidation by Hydrogen Peroxide

Abstract

Iron redox cycling can catalyze the oxidation of humic substances and increase the rate of oxygen consumption in surface waters rich in iron and organic carbon. This study examines the role of Fenton's reaction [oxidation of Fe(II) by hydrogen peroxide] in this catalytic cycle. A number of competing processes were observed in model systems containing dissolved Fe, hydrogen peroxide, and Suwannee River fulvic acid. First, the effective rate constant of Fenton's reaction increased with increasing fulvic acid concentration, indicating the formation of Fe(II)-fulvate complexes that react more rapidly with hydrogen peroxide than Fe(II)-aquo complexes. This effect was significant at pH 5 but negligible at pH 3. A second effect was scavenging of the HO • radical produced in Fenton's reaction by fulvic acid, forming an organic radical. The organic radical reduced oxygen to HO 2 • /O 2 •-, which then regenerated hydrogen peroxide by reaction with Fe(II). Finally, Fe(III) was reduced by a dark reaction with fulvic acid, characterized by an initially fast reduction followed by slower processes. The behavior of Fe(II) and hydrogen peroxide over time in the presence of fulvic acid and oxygen could be described by a kinetic model taking all of these reactions into account. The net result was an iron redox cycle in which hydrogen peroxide as well as oxygen were consumed (even though direct oxidation of Fe(II) by oxygen was not significant), and the oxidation of fulvic acid was accelerated.

Mecanismo e aplicações da reação de fenton assistida por compostos fenólicos redutores de ferro

“…Apesar do uso de cobre ou cobalto nesses sistemas apresentar grande eficiência, a toxicidade desses metais pode causar outros problemas ambientais. 47,48 , seqüestrando os radicais livres ou estimulando a produção destes para degradação de xenobióticos 10,15,[47][48][49] , ou ainda incorporando os produtos de oxidação de xenobióticos em suas estruturas poliméricas 10,15 . A Tabela 2 compila alguns trabalhos que avaliaram a participação de substâncias húmicas na degradação de xenobióticos por reações do tipo Fenton.…”

Degradation of Oxytetracycline by the Iron Wire/H2O2 System

“…The pH value was detected by pH meter (pHS-3E, Shanghai, China). The concentration of aqueous iron ions were determined by the ferrozine method [32].…”

Towards accounting for dissolved iron speciation in global ocean models

“…We disregard here complexation of Fe(II) by organic ligands. Ferrous iron complexes have been suggested to be possibly responsible for the long residence time of Fe(II) in the SOIREE iron fertilization experiment (Croot et al, 2001), but have only been demonstrated in riverine or coastal waters with high fulvic acid concentrations Sulzberger, 1996 andRose andWaite, 2003). However, such Fe(II) specific ligands may be difficult to identify using current techniques when they are at low abundance (e.g., Croot et al, 2007Croot et al, , 2008.…”