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

Abstract

The decomposition rate of H(2)O(2) by iron(III)-nitrilotriacetate complexes (Fe(III)NTA) 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; [H(2)O(2)](0): 1 mM-4 M) and concentrations of HO· radical scavengers: 0-53 mM. Spectrophotometric analyses revealed that reactions of H(2)O(2) with Fe(III)NTA (1 mM) at neutral pH immediately lead to the formation of intermediates (presumably peroxocomplexes of Fe(III)NTA) which absorb light in the region 350-600 nm where Fe(III)NTA and H(2)O(2) do not absorb. Kinetic experiments showed that the decomposition rates of H(2)O(2) were first-order with respect to H(2)O(2) and that the apparent first-order rate constants were found to be proportional to the total concentration of Fe(III)NTA complexes, were at a maximum at pH 7.95 ± 0.10 and depend on the [NTA](T,0)/[Fe(III)](T,0) and [H(2)O(2)](0)/[Fe(III)](T,0) molar ratios. The addition of increasing concentrations of tert-butanol or sodium bicarbonate significantly decreased the decomposition rate of H(2)O(2), suggesting the involvement of HO· radicals in the decomposition of H(2)O(2). The decomposition of H(2)O(2) by Fe(III)NTA 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(III)NTA 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 H(2)O(2).