Kinetics of hydrogen peroxide decomposition with complexed and “free” iron catalysts

The hydrogen peroxide decomposition kinetics were investigated for both “free” iron catalyst Fe(II) and Fe(III)] and complexed iron catalyst Fe(II) and Fe(III)] complexed with DTPA, EDTA, EGTA, and NTA as ligands (L). A kinetic model for free iron catalyst was derived assuming the formation of a reversible complex (Fe–HO₂), followed by an irreversible decomposition and using the pseudo‐steady‐state hypothesis (PSSH). This resulted in a first‐order rate at low H₂O₂ concentrations and a zero order rate at high H₂O₂ concentrations. The rate constants were determined using the method of initial rates of hydrogen peroxide decomposition. Complexed iron catalysts extend the region of significant activity to pH 2–10 vs. 2–4 for Fenton's reagent (free iron catalyst). A rate expression for Fe(III) complexes was derived using a mechanism similar to that of free iron, except that a L–Fe–HO₂ complex was reversibly formed, and subsequently decayed irreversibly into products. The pH plays a major role in the decomposition rate and was incorporated into the rate law by considering the metal complex specie, that is, EDTA–Fe–H, EDTA–Fe–(H₂O), EDTA–Fe–(OH), or EDTA–Fe–(OH)₂, as a separate complex with its unique kinetic coefficients. A model was then developed to describe the decomposition of H₂O₂ from pH 2–10 (initial rates = 1 × 10⁻⁴ to 1 × 10⁻⁷ M/s). In the neutral pH range (pH 6–9), the complexed iron catalyzed reactions still exhibited significant rates of reaction. At low pH, the Fe(II) was mostly uncomplexed and in the free form. The rate constants for the Fe(III)–L complexes are strongly dependent on the stability constant, K_ML, for the Fe(III)–L complex. The rates of reaction were in descending order NTA > EGTA > EDTA > DTPA, which are consistent with the respective log K_MLs for the Fe(III) complexes. Because the method of initial rates was used, the mechanism does not include the subsequent reactions, which may occur. For the complexed iron systems, the peroxide also attacks the chelating agent and by‐product‐complexing reactions occur. Accordingly, the model is valid only in the initial stages of reaction for the complexed system. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 24–35, 2000