IMAGING THE IN SITU REDUCTION OF CHROMIUM(VI) ON MAGNETITE SURFACES USING ELECTROCHEMICAL AFM

Chromium (VI) is a highly toxic and readily mobile metal contaminant. In the presence of reductants, such as Fe(II), and mineral catalysts, Cr(VI) may be reduced to a less toxic and relatively insoluble form, Cr(III). In this study, we investigate the interaction between Cr(VI) and the surfaces of Fe(II)-bearing minerals. We have developed a new approach to study the effect of variable redox conditions on surface-mediated redox reactions via electrochemical atomic force microscopy (EC-AFM). Magnetite (Fe²⁺Fe³⁺₂O₄), which has been previously shown to reduce Cr(VI), was chosen as a electrode and substrate. In these AFM experiments, the redox potential of the solution is controlled by polarizing the magnetite at specific electrochemical potentials while immersed in a Cr-containing electrolyte. Deposition on the surface is imaged in situas a function of potential and time. Analyses of surface coverage and volumetric measurements reveal that more precipitation occurs over time at very negative (–1.00 V at pH 3 and –1.25 V at pH 11) and very positive (+1.00 V) electrochemical potentials. At pH 11, particle growth is primarily vertical, while growth at pH 3 is predominantly lateral with a tendency to form more and smaller adsorbate particles.

Batch experiments were also performed on the same samples used in the AFM. Here, changes in the Cr concentration in solution were monitored using inductively-coupled mass spectrometry (ICP-MS). At pH 3, more Cr is removed from solution (~35% after 45 min., starting at 2 µM Cr(VI)) at high positive as well as negative potentials. XPS analyses of the magnetite surface suggest that only Cr(III) phases are present and that Cr and/or Fe-Cr oxide phases are more stable at very reducing conditions (–0.75 V), while Cr/Fe-Cr (oxy)hydroxide phases are present under more moderately reducing conditions (–0.25 V). These oxyhydroxide phases are very soluble below pH 5, which explains why there is little decrease in Cr from solution at intermediate potentials. Although some precipitation is observed at pH 11, less than 7% of the total Cr was removed from solution after 45 minutes. Fe(II)/Fe(III) ratios calculated from XPS analyses (0.33), however, are lower than the expected ratio of magnetite. This oxidized layer, possibly maghemite (γ-Fe₂O₃) or FeOOH, may inhibit the reduction of Cr(VI) on the surface.