Role of Glycerol Oxidation Pathways in the Reductive Acid Leaching Kinetics of Manganese Nodules Using Glycerol

Manganese nodules from ocean bed are potential resources of Cu, Ni, and Co for which land-based deposits are scarce in India. The present work describes a novel approach of using glycerol, a nontoxic biomass-derived reductant, for the reductive acid leaching of manganese nodules. Parameters such as acid concentration, time, temperature, and pulp density were optimized for leaching. The optimal leaching conditions were found to be 10% (w/v) pulp density and 10% (v/v) H2SO4 at 80 degrees C with 1% (v/v) glycerol yielding >95% of Ni and >98% Cu, Co, and Mn extraction within an hour. Kinetic analysis of the data based on the initial rate method showed that the leaching process was chemical reaction-controlled with an apparent activation energy of 55.47 kJ/mol. Various oxidation intermediates of glycerol formed during leaching were identified using mass spectrometry and Raman spectroscopy, and a probable oxidation pathway of glycerol during the leaching process has been elucidated based on the analysis. Glycerol was oxidized to glyceraldehyde, glyceric acid, tartronic acid, dihydroxyacetone, hydroxy pyruvic acid, glyoxalic acid, oxalic acid, and finally converted to CO2 during leaching. The fast reaction kinetics, near-complete dissolution of manganese, and other associated metals in the nodule can be attributed to the participation of all intermediate products of glycerol oxidation in redox reactions with MnO2, enhancing the overall reduction leaching efficiency.