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 containi ng dissolved Fe, hydrogen peroxide, and Suwannee River fulvic acid. Fi rst, the effective rate constant of Fenton's reaction increased with i ncreasing fulvic acid concentration, indicating the formation of Fe(II )-fulvate complexes that react more rapidly with hydrogen peroxide tha n Fe(II)-aquo complexes. This effect was significant at pH 5 but negli gible at pH 3. A second effect was scavenging of the HO. radical produ ced in Fenton's reaction by fulvic acid, forming an organic radical. T he organic radical reduced oxygen to HO2./O-2(.-), which then regenera ted hydrogen peroxide by reaction with Fe(II), Finally, Fe(III) was re duced by a dark reaction with fulvic acid, characterized by an initial ly fast reduction followed by slower processes. The behavior of Fe(II) and hydrogen peroxide over time in the presence of fulvic acid and ox ygen could be described by a kinetic model taking all of these reactio ns into account. The net result was an iron redox cycle in which hydro gen peroxide as well as oxygen were consumed (even though direct oxida tion of Fe(II) by oxygen was not significant), and the oxidation of fu lvic acid was accelerated.