The Fenton Reaction in Water Assisted by Picolinic Acid: Accelerated Iron Cycling and Co-generation of a Selective Fe-Based Oxidant

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–Fe^III–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–Fe^III–OOH reacting as a nucleophilic oxidant. Compared to HO^•, PICA–Fe^III–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.