5. Conclusions
The equilibria of Ni- and Co-complexes adsorption onto
Purolite S950 were investigated. Adsorption of Ni- and Co- complexed with citrate, malate and lactate ligands were generally low in comparison to metals complexed with sulfate or
hydrated metals from aqueous solutions. This was attributed
to the bulky organic ligands which promoted crowding effects
or steric hindrance to adsorption sites. The relative adsorption
capacities of the metals were found to be strongly dependent on
the size of the ligands. Lactate which was the smallest ligand
was found to have the highest adsorption capacity followed by
the larger malate and citrate complexes. Adsorption of the bulky
complexes, particularly under weakly acidic conditions where
the resins exhibited higher metal adsorption, was predicted by
the Freundlich models. This suggests metal uptake occurred
by multilayer adsorption. The strong affinity of the resin for
H+ under acidic conditions promoted competitive adsorption.
This hampered the adsorption of the metal complexes and
promoted monolayer adsorption. The relative stability of the
metal-organic acid complexes meant ion-exchange of these complexes with Purolite S950 is almost impossible. This adsorption behaviour highlights the challenges in recovering Ni and
Co from bioleaching solutions based on heterotrophic or fungi
organism.
The elution efficiencies of both Ni- and Co-complexes from
Purolite S950 resins were high (82–98%). It would appear the
adsorption process involves weak interactions, where the nature
of metal complexes and metal speciation has little effect on overall desorption efficiency from Purolite S950.