Irreversibility of sorption of cobalt to goethite (α-FeOOH) and disparities in dissolution of aged synthetic Co-goethite

The behaviours of incorporated cobalt in goethite during desorption and dissolution is fundamental to our understanding of the best extraction techniques for maximum cobalt recovery. Here, Surface Complexation Modelling, desorption and dissolution of aged synthetic Co-goethite was investigated to demonstrate the transition between surface complexion, surface precipitation and structural incorporation in goethite with different amounts of cobalt found in oxic laterites. Surface Complexation Modelling of cobalt sorption on goethite reveals existence of both surface complexation and polynuclear complexes. Surface complexation starts at a low surface loading (≥ 0.004 wt% Co) to form Fe(OH)2Co, and as uptake (0.03 to 0.2 wt% Co) increases, two surfaces complexes, Fe(OH)2Co and (> FeOH)2Co, are formed. Additional increases in concentration (0.45 to 1.7 wt%) leads to the formation of poly-nuclear complexes (e.g., (> FeOH)3Co2) co-existing with other complexes. Structural incorporation would be manifest if Co sorption was irreversible. We find that after four weeks of sorption about 55 to 70% of cobalt is reversible and the retention capacity increases with decreasing concentration. Samples formed at low loading show strong evidence of structural incorporation relative to those formed at high loading. In particular, the sample with 0.004 wt% Co needs a 20% dissolution of goethite before 64% of its incorporated cobalt is released while samples with 1.74 wt% Co requires a dissolution of 7.8% of goethite before liberating 66% of adsorbed cobalt. All the grades of Co-goethite dissolves incongruently with iron, however, the mode of formation should be considered when determining the leaching conditions for maximum recovery. In addition, the mixed Co-goethite grades show strong deviation from congruent dissolution and have a negative effect on the amount of cobalt and iron released during extraction.