Photoinduced reduction of chromium(vi) by iron aminopolycarboxylate complex (FeNTA)

Abstract

Chromium(VI) reduction photoinduced by iron(III) nitrilotriacetate (FeNTA) was investigated under monochromatic excitation. 313 nm was used as the irradiation wavelength in order to minimize the absorption of the light by chromium(VI): 91% of the photons were absorbed by FeNTA at pH = 7.0. Quantum yields of FeNTA and chromium(VI) disappearance and iron(II) formation were measured at pH 2.0, 4.0 and 7.0. In all the cases, chromium(VI) reduction follows first order kinetics with maximum efficiency at pH 2.0. The observed rate constant is proportional to FeNTA concentrations up to a maximum of 6.0 × 10⁻⁴ mol L⁻¹. The effect of oxygen was also investigated. If there is no large pH effect and no effect of oxygen on the quantum yields of chromium(VI) and FeNTA disappearance, the pH strongly influences the nature of the reduced chromium species. At pH 2.0, only chromium(III) was detected, whereas at pH 4.0 and 7.0 no chromium(III) resulting from chromium(VI) reduction was observed. Chromium(V) is supposed to be formed and stabilized by the chelating groups of NTA or NTA photoproducts.