The reaction of [Fe(pic)3] with hydrogen peroxide: a UV-visible and EPR spectroscopic study (Hpic = picolinic acid)

Picolinic acid promotes organic pollutants removal in Fe(III)/periodate process: Mechanism and relationship between removal efficiency and pollutant structure

The application of Fe-catalyzed periodate (PI) processes is often limited by both the narrow applicable pH range and weak reaction between Fe(III) and oxidant. Here, the biodegradable picolinic acid (PICA) was used as one kind of chelating ligands (CLs) to enhance the removal of organic pollutants (OPs) at initial pH 3.0-8.0, which displayed superior properties than the other CLs in Fe(III)/PI process. The dominant reactive species produced in the Fe(III)-PICA/PI process turned out to be high-valent iron-oxo (FeIV=O) species and hydroxyl radical (•OH) by quenching, sulfoxide probe transformation, and 18O isotope-labeling tests. The relative contribution of FeIV=O and •OH was dependent on OPs ionization potential (IP) and energy gap (ΔE). The degradation of OPs was also directly associated with their structure, the apparent rate constants correlated well with the highest occupied molecular orbital energy (EHOMO), IP, and ΔE, and among them ΔE had a greater effect. Furthermore, Fe(III)-PICA complexes displayed excellent long-term effectiveness for OPs removal in actual water matrixes, along with the non-toxic conversion of PI, indicating a broad application perspective of Fe(III)-PICA/PI process. This study provides an efficient method to improve the performance of Fe(III)/PI process and reveals the mechanism and relationship between removal efficiency and pollutant structure.

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.

Iron-Catalyzed Oxidation of 1-Phenylethanol and Glycerol With Hydrogen Peroxide in Water Medium: Effect of the Nitrogen Ligand on Catalytic Activity and Selectivity

The iron(II) complexes [Fe(bpy)₃](OTf)₂ (bpy = 2,2'-bipyridine; OTf = CF₃SO₃) (1) and [Fe(bpydeg)₃](OTf)₂ (bpydeg = N⁴,N⁴-bis(2-(2-methoxyethoxy)ethyl) [2,2'-bipyridine]-4,4'-dicarboxamide) (2), the latter being a newly synthesized ligand, were employed as catalyst precursors for the oxidation of 1-phenylethanol with hydrogen peroxide in water, using either microwave or conventional heating. With the same oxidant and medium the oxidation of glycerol was also explored in the presence of 1 and 2, as well as of two similar iron(II) complexes bearing tridentate ligands, i.e., [Fe(terpy)₂](OTf)₂ (terpy = 2, 6-di(2-pyridyl)pyridine) (3) and [Fe(bpa)₂](OTf)₂ (bpa = bis(2-pyridinylmethyl)amine) (4): in most reactions the major product formed was formic acid, although with careful tuning of the experimental conditions significant amounts of dihydroxyacetone were obtained. Addition of heterocyclic amino acids (e.g., picolinic acid) increased the reaction yields of most catalytic reactions. The effect of such additives on the evolution of the catalyst precursors was studied by spectroscopic (NMR, UV-visible) and ESI-MS techniques.