Electrochemically-Controlled Magnetite Dissolution

The electrochemically-controlled dissolution of magnetite (Fe3O4) within a natural sample was investigated in acidic solutions. The magnetite was electrically conductive and occurred in an igneous rock consisting primarily of a nonconductive fayalite (Fe2SiO4) matrix. The techniques of voltammetry, electrogenerated fluorescence imaging, scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) were used to characterize aspects of the magnetite dissolution process. Voltammetric experiments showed that magnetite dissolution was pH and potential dependent. Important steps in the dissolution process included electron-transfer (to reduce Fe3+ to Fe2+) within the magnetite and diffusion of hydrogen ion to the surface. Electrogenerated fluorescence imaging showed that there was a very high degree of interconnectedness between magnetite grains in the sample and that microscopic magnetite veins which laced the sample were also interconnected SECM was used to detect directly Fe2+, the electroactive product of the dissolution reaction, as it diffused away from the surface AFM was used to determine dissolution rates by measuring the physical retreat of magnetite surfaces at various potentials The rates measured with AFM compared well with rates measured for the same process with voltammetry and SECM Mathematical modelling was done to compare experimental measurements of current and retreat with theoretically expected values. Two-dimensional digital simulation of diffusion-controlled dissolution provided visualization of diffusion layer geometry and spatially-resolved retreat characteristics. Closed equations for diffusion-controlled current were used to model the sample surface as a combination of planar and band electrodes. The currents calculated in the case of diffusion-limited dissolution were fitted to observed current transients to estimate the relative proportions of large grains and veins in the sample The analytical techniques described above were used to examine magnetite dissolution in the presence of chemical reducing agents. The agents used were sulfite and a vanadium (ID)/vanadyl mixture. Sulfite anions appeared to adsorb and inhibit magnetite dissolution. Vanadium species appeared to accelerate dissolution without further interaction