Effects of fulvic acid on Fe(II) oxidation by hydrogen peroxide

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 HO2./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.