Steric hindrance effect on adsorption of metal–organic complexes onto aminophosphonate chelating resin

Recovery of nickel and cobalt from organic acid complexes generated by bioleaching of nickel laterite ores with heterotrophic organism and their metabolites were conducted by ion-exchange. This study investigated specifically the adsorption of nickel and cobalt complexed with citrate, malate and lactate on chelating aminophosphonate Purolite S950 and the possible steric hindrance imposed by the larger bulky metal species on the resin loading capacity. Equilibrium adsorption tests of various concentrations of metal complexes were measured and analysed by fitting the experimental adsorption data to two empirical isotherm models: Langmuir and Freundlich. The distribution of nickel and cobalt speciation estimated from the stability and formation quotient constants of these metal complexes were used in examining the roles of steric hindrance in the adsorption of metal complexes on S950. Analysis of the mechanism of adsorption of these complexes suggests the size and the nature of the metal species have a significant influence on the metal uptake. In general the metal loading capacity of the resin increased as the size of the metal complexes decreased. The type and the number of anions which coordinated with the metal as ligands dictated the size of the complexes. An interesting outcome of this study, however, is the generally higher uptake of the weakly acidic lactate complex in comparison to the smaller hydrous nickel ion; thus suggesting the important roles of both physical and chemical properties of the metal species in their adsorption on resins. In general the order of the uptake of metal species on S950 is: lactate > metal ion > malate > citrate.