Effects of chloride ions on the iron(III)-catalyzed decomposition of hydrogen peroxide and on the efficiency of the Fenton-like oxidation process

Abstract

The effects of chloride concentration on the rates of decomposition of H 2O 2 by ferric ion and on the rate of oxidation of an organic solute in homogeneous aqueous solution have been investigated. Experiments were carried out in a batch reactor, in the dark, at pH ≤ 3, 25.0 ± 0.5 °C and at controlled ionic strength (≤1 M). The concentrations of chloride ranged from 0 to 1 M ([Fe(III)] 0 = 0.2 or 1 mM, [H 2O 2] 0 = 1, 10 or 50 mM). The spectrophotometric study shows that chloride ions compete with hydrogen peroxide for the complexation of Fe(III) and that H 2O 2 does not form complexes with iron(III)-chlorocomplexes. The kinetic study showed that the rates of decomposition of H 2O 2 decreased in the presence of chloride. The measured rates were accurately predicted by a kinetic model which incorporates the formation of iron(II) and iron(III)-chlorocomplexes and reactions involving Cl 2 − radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(III) complexed with chloride. The kinetic model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H 2O 2 in the presence of chloride. Computer simulations indicate that Cl 2 − which represents the predominant radical contributes to the oxidation of atrazine.