Radiation chemistry of colloidal haematite and magnetite in water. Reductive dissolution by (CH3)2ĊOH radicals and FeIIEDTA

Abstract

The radiation-induced dissolution of colloidal α-Fe₂O₃ and Fe₃O₄ has been investigated in aqueous solution at ca. pH 2. Reductive dissolution is achieved with (CH₃)₂ĊOH radicals and Fe^IIEDTA by electron transfer to Fe^III in the crystal lattice followed by dissolution of the product Fe^II. (CH₃)₂ĊOH dissolves α-Fe₂O₃ at the diffusion-controlled rate under the conditions used, but the fraction of the radicals that reach the particles is limited by competing radical–radical reactions. The rate of dissolution of Fe^III in Fe₃O₄ is diffusion-controlled at low particle concentrations, but becomes slower at high particle concentrations. It is suggested that Fe^II in the original Fe₃O₄ lattice does not dissolve at the same rate as Fe^II resulting from reduction of Fe^III.

Fe^IIEDTA transfers an electron to Fe^III in both colloidal oxides at a rate some 10³-fold slower than diffusion-controlled, but a high rate of radiation-induced dissolution is achieved by reducing the product Fe^IIIEDTA with (CH₃)₂ĊOH. When excess EDTA is present to complex the dissolved Fe²⁺, the rate of dissolution increases in an autocatalytic manner as the irradiation proceeds. This increase is more marked for Fe₃O₄ and is also observed in the thermal dissolution of this oxide by Fe^IIEDTA because of the contribution from Fe^II in the lattice. It is also found that Fe^II is released more slowly than reduced Fe^III from the Fe₃O₄ lattice during thermal dissolution by Fe^IIEDTA.

The radiation chemistry of colloidal α-Fe₂O₃ solutions containing Fe³⁺ and Fe²⁺ has been used to probe the location of these ions. Results show that the ions are concentrated in the electrical double layer of the untreated sol prepared from FeCl₃ in dilute HCl, but not when the colloid is washed and redispersed in dilute HClO₄.