AN ELECTROCHEMICAL STUDY OF CHROMIUM OXIDATION BY MANGANESE OXIDES

Natural production of Cr(VI), a highly toxic contaminant, has been linked to redox reactions involving Mn oxide minerals. Previous studies have investigated Cr(III) oxidation by Mn oxides, but relatively few have attempted to evaluate and compare how the composition and structure of the Mn oxide phase influences its capacity to produce Cr(VI). In this study we evaluate the energetics, kinetics, and mechanisms of Cr(III) oxidation by four Mn oxides (pyrolusite, low crystalline Na-birnessite, high crystalline Na-birnessite, and hexagonal H⁺-birnessite) using electrochemical amperometric techniques and Pt-, Au-, and powder-microelectrodes (PMEs), which directly measure the kinetic and thermodynamic properties of Cr(III) oxidation in-situ. We will quantify the electron-accepting (oxidizing) capacity of the Mn oxide phases using new mediated electrochemical methods (Sander et al., 2015)

A pyrolusite PME shows two redox peaks from cyclic voltammograms (pH 4): an anodic peak centered at +1.1 V (vs. Ag/AgCl) and a broad cathodic peak centered at -0.24 V. For comparison, low crystalline Na-birnessite (LCB) shows two broad, possibly bimodal peaks: an anodic peak centered at +0.83 V and a cathodic peak centered at +0.36 V. Differences in the voltammetric characteristics of these two phases suggest that their redox interactions with Cr(III) will also be different. The bimodal peaks for LCB suggest the presence of mixed valence states, Mn(II/III), on the mineral surface that have been previously implicated to enhance the capacity for Mn oxides to oxidize Cr(III) (Landrot et al., 2012). The oxidizing capacities of the Mn oxide phases will allow for more accurate assessment of the reactivity of different Mn oxide phases toward Cr(III), aiding in the development of fundamental models for the behavior of Cr in the natural environment.