Oxidation of Cr(III) to Cr(VI) and Production of Mn(II) by Synthetic Manganese(IV) Oxide

Abstract

The heterogeneous oxidation of Cr(III) to Cr(VI), a toxic inorganic anion, by a synthetic birnessite (δ-MnO2) was investigated in batch reactions using a combination of analytical techniques including UV–Vis spectrophotometry, microwave plasma–atomic emission spectrometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), to evaluate both the solution speciation of Cr(III)/Cr(VI) and the surface of the reacted δ-MnO2. The formation of dissolved Mn(II) was determined during the batch reactions to evaluate the extent and stoichiometry of the Cr(III) oxidation reaction. A stoichiometric 3:2 Mn(II):Cr(VI) molar relationship was observed in the reaction products. The reductive dissolution of the δ-MnO2 by Cr(III) resulted in a surface alteration from the conversion of Mn(IV) oxide to reduced Mn(II) and Mn(III) hydroxides. The results of this investigation show that naturally occurring Cr(III) will readily oxidize to Cr(VI) when it comes in contact with MnO2, forming a highly mobile and toxic groundwater contaminant.

Chitin as a Sorbent Superior to Other Biopolymers: Features and Applications in Environmental Research, Energy Conversion, and Understanding Evolution of Animals

“…Thus, it will respond to redox or photoredox processes by changes of partition. Accordingly, some reduced (Eu 2+ ) or oxidized (HCrO 4 − ) forms of an element can be removed/extracted from the chitin layer by some slow flow of solvent, sometimes facilitated by light (reduction of Eu, U) or presence of a natural catalyst such as MnO 2 that promotes air oxidations of Ce, Cr [24], Cl − , phenols [25], alkyl benzenes, etc., with both solid Mn phases [26] and Mn 2+ , although weakly adsorbing to chitin [22]). Then, two electrodes placed on either side of the percolated chitin pack will record different potentials due to the concentration drop (scheme Figure 4, practical setup Figure 5 Here, sensor technology and energy conversion-meant to induce fuel cell conversion of organic compounds containing CH and NH bonds while not requiring Pt group metals for CH bond activation and storing some light energy by making Eu 2+ and H 2 (in a secondary step)-do merge.…”

Adsorption and Oxidative Dissolution of Silver Nanoparticles in Soil and Soil Mineral Suspensions

“…aniline [40], aliphatic amine [41], and triazine [42] and inorganic ions such as Cr(III) [43,44] and As(III) [45]. Preliminary investigations in this study suggested that MnO2 also oxidizes AgNPs [37] and thus is an important environmental pathway whereby AgNPs are transformed to the more toxic Ag + ion.…”

“…The Fe oxides included magnetite (Fe3O4), hematite (α-Fe2O3, hereafter Fe2O3), and goethite, an Fe oxyhydroxide (α-FeOOH, hereafter FeOOH). The Mn oxide was a synthetic birnessite (δ-MnO2, hereafter MnO2) characterized and studied previously [37,44]. Table 2.1…”

“…The MnO2 material has been studied previously in the Manning lab [37,44] showing direct evidence of oxidation capacity. The removal of AgNP from solution by MnO2 and subsequent oxidation to Ag + proceed via the following homogeneous redox reaction:…”

“…MnO2(s) + 2Ag 0 (s) + 4H + (aq) = Mn 2+ (aq) + 2Ag + (aq) + 2H2O [3.1] Equation 3.1 implies that oxidative dissolution of AgNPs will yield aqueous Ag + . In addition, reductive dissolution of MnO2 will form aqueous Mn 2+ [44]. Both Ag + and Mn 2+ cations will be subject to adsorption on the negatively charged MnO2 surface [38,39,52].…”