Spinel Manganese Oxide as a Selective Adsorbent for Cobalt Recovery from Metal-Rich Waste Streams

Abstract

A spinel phase of MnO2 (“λMnO2”) was synthesized and characterized as a sorbent for cobalt (Co) and 5 other metals common to Co-rich waste streams (Ba, Ca, Mg, Ni, and Sr). Sorption was characterized in both single and multi-metal solutions to understand potential applications for metal recovery under realistic conditions. The particles had a maximum Co uptake of 3.74 × 10−5 mol/g with 10 g/L particle loading and demonstrated significant selectiveness toward Co (separation factors between 2.5 and 190). Selectivity could be in part attributed to a redox exchange between Co and Mn, which is supported by an increase of Mn at a nearly 1:1 mol ratio to Co removed in metal solutions with initial concentrations above 0.0005 M. The sorption behavior toward Co was better described using the Freundlich isotherm (R2 = 0.973) than that of the Langmuir isotherm, indicating sorption site heterogeneity. The rate of sorption was generally well described with a first-order kinetic model (R2 for Co between 0.73 and 0.99). To study the regenerability of the λMnO2 sorbent, the particles were cycled between metal and nitric acid solutions. While Co sorption remained nearly constant over the 7 cycles tested, the uptake of other metals increased with each cycle, reducing the Co selectivity. Additionally, less than 70% of Co was recovered after each cycle. This, in combination with data indicating Mn/Co redox exchange, indicates that the λMnO2 particles are converted to a Co-enriched MnO2 spinel, which does not retain Co selectivity following the initial exchange. The irreversibility of the Co−Mn exchange under the conditions studied suggests that λMnO2 is not suitable for use as a regenerable Co-selective sorbent without potentially intensive metallurgical or hydrothermal regeneration. Importantly, because Co-enriched MnO2 spinel materials are used in electrodes and capacitors, λMnO2 may offer a pathway to generate valuable Co-enriched MnO2 spinel materials from complex metal waste streams.