The Fenton reaction is limited by a narrow acidic pH range, the slow reduction of Fe(III), and susceptibility of the nonselective
hydroxyl radical (HO•) to scavenging by water constituents. Here, we employed the biodegradable
chelating agent picolinic acid (
PICA) to address these concerns. Compared to the classical Fenton reaction at pH 3.0,
PICA greatly accelerated the degradation of
atrazine,
sulfamethazine, and various substituted
phenols at pH 5.0 in a reaction with autocatalytic characteristics. Although HO• served as the principal
oxidant, a high-spin, end-on hydroperoxo intermediate, tentatively identified as
PICA-FeIII-OOH, also exhibited reactivity toward several test compounds.
Chloride release from the oxidation of
2,4,6-trichlorophenol and the positive slope of the Hammett correlation for a series of halogenated
phenols were consistent with
PICA-FeIII-OOH reacting as a nucleophilic
oxidant. Compared to HO•,
PICA-FeIII-OOH is less sensitive to potential scavengers in environmental water samples. Kinetic analysis reveals that
PICA facilitates Fe(III)/Fe(II) transformation by accelerating Fe(III) reduction by H2O2. Autocatalysis is ascribed to the buildup of Fe(II) from the reduction of Fe(III) by H2O2 as well as
PICA oxidation products.
PICA assistance in the Fenton reaction may be beneficial to wastewater treatment because it favors
iron cycling, extends the pH range, and balances oxidation universality with selectivity.