Ferryl Ion in the Photo-Fenton Process at Acidic pH: Occurrence, Fate, and Implications

Area of Science:

• Environmental Chemistry
• Photocatalysis
• Oxidation Processes

Background:

• Fenton processes are crucial for oxidizing organic compounds using reactive species.
• Hydroxyl radical (HO•) generation in Fenton reactions is well-established under acidic conditions.
• The formation and role of ferryl ion (FeIV O2+) in these systems require further elucidation.

Purpose of the Study:

• To demonstrate the generation of ferryl ion (FeIV O2+) in UV/Fe(III) and UV/Fe(III)/H2O2 systems at pH 2.8.
• To elucidate the parallel formation mechanisms of FeIV O2+ via Fe(III) oxidation by HO• and O-O homolysis of [FeIII-OOH]2+.
• To develop an optimized kinetic model incorporating FeIV O2+ reactions for predicting contaminant decay.

Main Methods:

• Utilized methyl phenyl sulfoxide (PMSO) as a probe compound to detect reactive species.
• Employed laser flash photolysis to measure the rate constant for HO• and Fe3+ reaction.
• Determined rate constants and quantum yield for [FeIII-OOH]2+ O-O homolysis by fitting PMSO2 formation.
• Developed and optimized a kinetic model by integrating FeIV O2+ reactions into the photo-Fenton model.

Main Results:

• Demonstrated FeIV O2+ generation in UV/Fe(III) and UV/Fe(III)/H2O2 systems at pH 2.8.
• Quantified the rate constant for HO• + Fe3+ reaction (4.41 × 10^7 M^-1 s^-1) and O-O homolysis of [FeIII-OOH]2+ (1.4 × 10^-2 s^-1, quantum yield 0.3).
• Showed that FeIV O2+ formation predominantly involves HO• + Fe3+ in the absence of H2O2, while its contribution from [FeIII-OOH]2+ O-O homolysis depends on reactant ratios, scavenging, and irradiance.
• The optimized kinetic model accurately predicted Fe(II) formation and contaminant decay.

Conclusions:

• Elucidated the formation mechanisms of reactive oxidative species, particularly FeIV O2+, in photo-Fenton processes.
• Highlighted the significant role of FeIV O2+ in modulating the iron cycle and pollutant abatement.
• The developed kinetic model provides a robust tool for understanding and optimizing UV/Fe-based oxidation systems.