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 (FeIIINTA) 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 FeIIINTA (1 mM) at neutral pH immediately lead to the formation of intermediates (presumably peroxocomplexes of FeIIINTA) which absorb light in the region 350–600 nm where FeIIINTA 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 FeIIINTA 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 FeIIINTA 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 FeIIINTA 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.

► The decomposition rate of H2O2 by FeIIINTA complexes follows first-order kinetics.
► The decomposition rate of H2O2 by FeIIINTA is at a maximum at pH ≈ 8.
► The presence hydroxyl radical scavengers slows down the rate of decomposition of H2O2.
► The degradation of NTA during the reaction decreases the activity of FeIIINTA at neutral pH.