Reductive dissolution of colloidal ferrites by methyl viologen radicals

Abstract

The reductive dissolution of aqueous dispersions of colloidal magnetite (Fe₃O₄) and a cobalt ferrite (Co_0.6Fe_2.4O₄) by methyl viologen radicals (MV˙⁺) has been studied in the pH range 2–9 using a radiolytic method to generate MV˙⁺. Nickel-containing ferrites (Ni_0.7Fe_2.3O₄ and Ni_0.19 Co_0.56Fe_2.25O₄) have also been investigated briefly.

Dissolution is effected by reduction of Fe^III followed by its release from the lattice as Fe²⁺_{aq ˙} G(Fe²⁺_aq) was independent of dose for > 75 % reduction of Fe^III but decreased from ca. 0.45 µmol J^–1 at pH 2 to ca. 0.1 µmol J^–1 at pH 7 for magnetite and cobalt ferrite. the data suggest that the rate-determining step is the release of reduced Fe^III and that at the higher pH the particles behave as microelectrodes which can be discharged by MV^˙+ accepting an electron. At a pH of 7 magnetite dissolved stoichiometrically, but reduced Fe^III appeared to be the only iron species to dissolve form cobalt ferrite. The dissolving magnetite particles became very irregular in shape, whereas the coablt ferrite particles scarcely changed in shape or size until > 75 % Fe^III had been dissolved. These features are consistent with complete disruption of the magnetite lattice and with leaching of reduced Fe^III from cobalt ferrite.

At pH 7 neither Fe^IIEDTA[E₇= 0.12 V for (FeEDTA)^III/II] nor (CH₃)₂COH[E₇=–1.8 V for (CH₃)₂CO, H⁺/(CH₃)₂ĊOH] caused dissolution, although they were effective at pH 2. The relative efficiency of MV^˙+(E₇=–0.44 V for MV²⁺/ MV^˙+) at pH 7 suggests a better matching of donor and acceptor levels for the electron transfer process.