Both cobalt (Co) and nickel (Ni) substitute for Fe in goethite (α-FeOOH) as well as adsorb on goethite. Co and Ni-rich goethite are the dominant ore mineral in oxide-type laterite deposits. A quantitative understanding of the aqueous solubility of Ni- and Co-rich goethite would help in modelling the formation of laterites and developing methods for metal extraction. In this contribution, we determined the aqueous solubility of substituted nickel and cobalt goethite in both the binary and ternary systems as a function of pH < 1 to 5 and varying concentrations found in Ni laterites. We found the dependency of the solubility product (K sp ) of goethite to the type of incorporated ion, concentrations, and pH. The solubility of Ni substituted goethite increases with increasing pH and increasing Ni substitution. A similar trend is observed for Co, although its substitution in goethite appears to increase the stability of goethite. Ksp increases with increasing Ni and Co substitution in bi-metal (Ni and Co) goethite, and are generally low compared to those of single metal substituted goethite with the same concentration. These findings have some implication for substitution mechanism and Ni and Co extraction from laterites and soils. The Ni(OH) 2 -Co(OH) 2 -FeOOH solid solution simulation with PHREEQC is inconsistent with our experimental data and reveals “nonideality” of mixing systems with different chemical and thermodynamic properties as end members, but attained equilibrium with goethite at lower pH.
- Ni laterite