Effect of some parameters on the rate of the catalysed decomposition of hydrogen peroxide by iron(III)-nitrilotriacetate in water

The decomposition rate of H₂O₂ by iron(III)-nitrilotriacetate complexes (Fe^IIINTA) has been investigated over a large range of experimental conditions: 3 < pH < 11, Fe(III)]_T,0: 0.05-1 mM; NTA]_T,0/Fe(III)]_T,0 molar ratios : 1-250; H₂O₂]₀: 1 mM-4 M) and concentrations of HO radical scavengers: 0-53 mM. Spectrophotometric analyses revealed that reactions of H₂O₂ with Fe^IIINTA (1 mM) at neutral pH immediately lead to the formation of intermediates (presumably peroxocomplexes of Fe^IIINTA) which absorb light in the region 350-600 nm where Fe^IIINTA and H₂O₂ do not absorb. Kinetic experiments showed that the decomposition rates of H₂O₂ were first-order with respect to H₂O₂ and that the apparent first-order rate constants were found to be proportional to the total concentration of Fe^IIINTA complexes, were at a maximum at pH 7.95 ± 0.10 and depend on the NTA]_T,0/Fe(III)]_T,0 and H₂O₂]₀/Fe(III)]_T,0 molar ratios. The addition of increasing concentrations of tert-butanol or sodium bicarbonate significantly decreased the decomposition rate of H₂O₂, suggesting the involvement of HO radicals in the decomposition of H₂O₂. The decomposition of H₂O₂ by Fe^IIINTA at neutral pH was accompanied by a production of dioxygen and by the oxidation of NTA. The degradation of the organic ligand during the course of the reaction led to a progressive decomplexation of Fe^IIINTA followed by a subsequent precipitation of iron(III) oxyhydroxides and by a significant decrease in the catalytic activity of Fe(III) species for the decomposition of H₂O₂.