4.2. Adsorption of cobalt organic complexes
Equilibrium adsorption isotherms of cobalt complexes are
shown in Figs. 5–7. Table 3 lists the corresponding fitted model
parameters and the measures of fit, R2 and χ2. Further examina-tion of the results in Table 3 and Figs. 5–7 suggest the adsorption
behaviours observed for Co-complexes are consistent with that
obtained for the corresponding Ni-complexes. Higher adsorption capacities were obtained under the weakly acidic conditions
and the relative order of adsorption for the various complexes is
lactate, malate and citrate. Similarly comparison of the adsorption capacities of Co-organic acid complexes were shown to be
generally lower in comparison to their smaller counter parts,
cobalt ion and cobalt sulfate [21]. The adsorption capacities of
cobalt ions from aqueous solution on IRN77 resin (reflected by
measured Freundlich parameters A, 75.63 mg/g) [21] is higher
in comparison to cobalt organic complexes in Table 3. The effect
of the size on ligands complexed to cobalt on the relative affinity of Co-complexes on the resin is similar to that observed for
nickel, that is lactate > malate > citrate.
These results suggest the factors which influences the Ni and Co-complex adsorption are similar. This also lends further
support to the proposed effect of the bulkiness of the organic ligands or crowding effects that promote multilayer adsorption and
steric hindrance, which hampers Co-complexes adsorption. At
the more acidic conditions, although the Co-complexes adsorption occur to some extent, the higher affinity of the resin to H+ under these condition [24–26] promotes lower adsorption of the
Co-complexes.
The Co-complexes adsorptions are shown to have a better
agreement with the Langmuir, although the difference in the
extent of fit between the two models is not significant, particularly at the lower acid concentrations. The R2 (0.95–0.99) for
the Langmuir model and the χ2 (0.12–0.45) are comparable with
the R2 values of 0.94–0.98 and χ2 0.64–1.02 for the fitted Freundlich model. This would suggest that adsorption occurs by
multilayer and monolayer adsorption. It is apparent that the fit of
the Co-complexes adsorption to the Freundlich model is higher
at the more acidic conditions. The dependence of the mechanism
for cobalt adsorption to the Langmuir mechanism under acidic
conditions also corresponds with the greater competition of the
complexes with hydronium ions under this condition [24–26].
The stronger affinity of the resins towards the smaller cation (H+)
under acidic conditions would obviously reduce the crowding
effects associated with the larger Co-complexes. This, as shown
by the results in Table 3, promotes monolayer adsorption (see Fig. 8. Monolayer adsorption of cobalt complexes as a result of site competition between hydrogen ions and metal complexes (Co-L: cobalt complex; H+:
hydrogen ion).
Fig. 8). With increasing pH, Co-complexes were preferentially
adsorbed. This appears to lead to steric hindrance that, as shown
in Table 3, promoted multilayer adsorption. The overall outcome
of these effects is the low Ni- and Co-organic acid complexes
adsorption at the various pH.