Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM

Abstract

Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, 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 surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III) 2 O 4 , as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (-500 mV at pH 3,-750 mV at pH 7, and-1000 mV at pH 11) and very positive (+1000 mV at pH 3 and +500 mV at pH 11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH 7, while less coverage is observed at pH 11 (< 8%) and pH 3 (< 2%). Particle growth at pH 3 is predominantly lateral in nature with a tendency to form a higher number of smaller adsorbate particles. At pH 11, growth is primarily vertical (perpendicular to the surface), and smaller particles tend to aggregate into larger clusters on the surface with increasingly negative redox polarization. These

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

“…To minimize the 60 Hz noise from the power line, all cables were co-axially shielded and all electrical grounds were common to the bipotentiostat (CHI760D, CH Instrument, Texas, USA) that was used for controlling the electrochemical conditions. A Bruker EC-AFM cell [37] was used for PeakForce SECM imaging. In this cell, a Pt wire loop along the internal perimeter of the cell was used as a counter electrode and a AgCl-coated Ag wire was used as a pseudo-reference electrode (AgQRE).…”

Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe3O4 and α-FeOOH

“…This result revealed the presence of precipitated Cr 2 O 3 species on the α-FeOOH surface at pH 3. 55 It was noticeable that a high CrO(OH) content in Cr@Fe 3 O 4 might proceed through Cr 3+ migration on the Fe 3 O 4 surface.…”

Effects of Hydroxyl and Carboxyl Functional Groups on Calcite Surface Wettability Using Atomic Force Microscopy and Density Functional Theory

“…21 Surface Coverage and Volumetric Analyses of AFM Images. Image analyses were performed for the two samples shown in Figures 1 and 2 using an in-house developed MATLAB script (described in Walker et al 37 ) to quantify adsorbates observed on the AFM images. This script's binary images (Figure 5a−c) show the adsorbate distribution patterns and surface coverages on the calcite surface by using white dots on a black background.…”