Physicochemical and structural factors in the sulfuric acid leaching of nickel- and copper-bearing synthetic birnessites

Abstract

A large number of nickel-and copper-doped samples of birnessite (0.7 nm phase), a layered-structure manganese mineral, were synthesized by dehydration of respective buserites (1 nm phase). The samples were characterized in terms of chemical composition, specific surface area, phase constituents, crystallinity, strain, morphological features, and structural complexity, in order to study the influence of the physicochemical characteristics of the samples on the leachability of doped elements and manganese in sulfuric acid. In contrast to manganese, the leaching behavior of doped nickel and copper is found to be more sensitive to the structural characteristics of the host birnessite phase. The leachability of the doped elements does not show any correlation with the specific surface area of the samples. Significant parameters affecting leachability are the interlayer spacing of the parent buserite phase used in the synthesis and the microcrystalline dimension and strain in the ͗001͘ crystallographic direction of the birnessite phase. In addition, leachability is also controlled by the crystal field stabilization energy (CFSE) of the doped metal ion. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies on leach residues indicated the transformation of birnessite phase into other minerals such as nsutite (␥-MnO 2 ). A significant fraction of the doped nickel and copper (20 to 40 pct) remains unleached, even after prolonged leaching up to 6 days, and this is attributed to the compact structure of the newly formed phases during leaching.

Electrically Rearranged Birnessite-Type MnO2by Repetitive Potential Steps and Its Pseudocapacitive Properties

“…According to the sorption experiments of transition metals onto birnessite conducted by Kumar et al, Ni 2+ (0.069 nm in radius) ions in the interlayer are impossible to substitute Mn 4+ ions (0.053 nm) in the MnO 2 layer owing to their ionic radii, whereas the substitution with Mn 2+ (0.083 nm) can take place to gain additional stabilization energy; i.e., the crystal field stabilization energy (CFSE) = 0 and 122.2 kJ mol −1 for Mn 2+ and Ni 2+ , respectively. 36 Although Mn 2+ is unlikely formed directly at less negative potential than 0 V, Mn 3+ ions generated at the cathodic step can be disproportionated into Mn 2+ and Mn 4+ ions. 37 The dissolved Mn 2+ ions near the MnO 2 layers are oxidized to be redeposited on the anodic polarization.…”

Fabrication of a Hybrid Capacitor Composed of Vertically-Aligned Multilayered Manganese Oxide Film

“…Newly appeared peaks at 12.2 and 24.7° in all the patterns demonstrate that the layered structure itself remains, where Na + ions are accommodated as the chargecompensating ions. A decrease in diffraction intensity is attributable to an orientation change of the MnO 2 layers and/or an effect of Ni-doping where the basal (or 00l-type) reflections in birnessite are significantly broadened by doping (21).…”

“…Electron transfer from/to Ni 2+ ions is not involved in the examined potential region, which was in fact confirmed experimentally. According to the sorption experiments of transition metals onto birnessite, Mn 4+ ions (ionic radius 0.053 nm) in MnO 2 layers are not substituted by Ni 2+ (0.069 nm) ions in the interlayer because of their ionic radius, whereas the substitution of Mn 2+ (0.083 nm) can take place to acquire crystal field stabilization energy (CFSE); i.e., CFSE = 0 and 122.2 kJ mol -1 for Mn 2+ and Ni 2+ , respectively (21). Although Mn 2+ is unlikely formed directly at less negative potential than 0 V, Mn 3+ ions generated electrochemically can disproportionate to Mn 2+ and Mn 4+ ions (22).…”