Chemical Kinetic and Mechanistic Investigation of Cobalt and Manganese Recovery from Aqueous Solutions by Ozone...

Cobalt (Co) and manganese (Mn) are designated as critical elements by the U.S. Department of the Interior, yet the mechanisms governing their recovery from aqueous systems remain poorly understood. Ozone-induced oxidative precipitation offers a promising, chemical-minimizing approach for recovering these elements from aqueous solutions. Building on our previously published, patent-pending, work, this study investigated the effect of gas flow rate, stirring rate, ion concentration, and temperature on Co-Mn precipitation performance using ozone. Model solutions replicating typical Co and Mn concentrations in aqueous secondary sources were prepared for controlled laboratory experiments. The influence of individual parameters was first evaluated, followed by a statistically designed parametric study. Results showed maximum Co recovery at a medium gas flow rate (1100 cc/min) and moderate temperature (50 °C), whereas Mn recovery was optimized at an elevated temperature (80 °C) and a high flow rate (2000 cc/min). Ozone mass-transfer limitations were described using the two-film theory, and measured volumetric mass transfer coefficient (K_La) values were consistent with the observed precipitation trends. Quantitatively, K_La increased from 0.0032 s^-1 at 20 °C to 0.0057 s^-1 at 40 °C, then decreased at higher temperatures due to reduced ozone solubility and stability. Similarly, increasing gas flow from 200 cc/min to 1400 cc/min raised K_La to 0.0147 s^-1, with a slight decline at 2000 cc/min caused by incomplete ozone generation and accelerated decomposition. Kinetic modeling during the first 30 s identified that the pseudohomogeneous model best described the system, highlighting both ozone mass transfer and oxidant availability in solution as rate-limiting factors. Activation energies were calculated: 8.6 kJ/mol (Co) and -15.3 kJ/mol (Mn) in pure solutions, and 11.4 kJ/mol (Co) and -12.3 kJ/mol (Mn) in mixed solution, indicating a diffusion-controlled process. Notably, Co recovery in the mixed solution increased with temperature as Mn oxides (MnO₂, MnOOH) formed nucleation sites that promoted Co adsorption and coprecipitation, a mechanism supported by SEM-EDS, XRD, FT-IR, XPS, and Zeta Potential analyses confirming the formation of Mn oxide phases facilitating surface-mediated Co capture. Overall, these findings provide mechanistic and kinetic insight into ozone-based Co-Mn recovery and establish a basis for process design and optimization in critical metal recovery applications.