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

• Published in: Chemical geology 2016-07, Vol.429, p.60-74
• Main Authors: Walker, Sarah M., Marcano, Maria C., Bender, Will M., Becker, Udo
• Format: Article
• Language: English
• Subjects: Atomic force microscopy; Catalysts; Chromium; Crystallography; Hexavalent chromium; Magnetite; Phases; Redox; Toxic; X-ray photoelectron spectroscopy
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2016.02.025

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)2O4, 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 (−500mV at pH3, −750mV at pH7, and −1000mV at pH11) and very positive (+1000mV at pH3 and +500mV at pH11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH7, while less coverage is observed at pH11 (