The A values, in Table 2, suggest the order of adsorption
capacity of the nickel complexes, at both 0.01 and 0.1 M acid
concentrations is lactate > malate > citrate complexes. This order
is confirmed by the corresponding 1/n values. The sizes of the
ligands are in the following order; the lactate (C3H5O3
smallest followed by malate (C4H5O5
It is therefore likely that the size of the ligands played a role in
the order of the nickel complex adsorption, where the adsorption
capacity of the nickel complexes was influenced by the steric hindrance or the crowding effect associated with the comparative
bulkiness of the organic acid ligands in the nickel complexes. In
general, the adsorption of all the nickel-organic acid complexes
are all low in comparison to nickel from aqueous solutions and
nickel complexed with smaller anions like sulfate. Nickel citrate,
malate and lactate complexes, prepared using 0.01 M acids, have
low A values from 2.51 to 3.26 mg/g (see Table 2). In comparison, nickel adsorbed from an aqueous solution using an IRN77
cation-exchange resin (based on sulfonic acid functional group)
has an A value of 81.82 mg/g [21]. It would appear the bulkiness of the organic ligands hampers the uptake on the metal
complexes on the resin.
The adsorption mechanism of nickel ions and nickel complexed to smaller anions like sulfate are predicted by the
Langmuir model. Nickel sulfate adsorbed on Dowex M4195
(Qm = 94.51 mg/g and b = 2.54 × 10−3 L/g), Amberlite IRC748
(Qm = 125.03 mg/g and b = 1.22 × 10−3 L/g) and Ionac SR-5
(Qm = 79.26 mg/g and b = 5.15 × 10−3 L/g) have been shown to
fit the Langmuir model [13]. In comparison, the adsorption of
nickel complexed to the bulkier organic ligands, as shown in
Table 2, are predicted by the Freundlich model. In addition, the
monolayer adsorption of the smaller nickel sulfate complexes
and ions corresponds to higher adsorption capacities in comparison to adsorption of nickel-organic acid complexes onto the
Purolite S950 (see Table 2). These results suggest that the crowding effect imparted by the larger organic acid ligands on nickel
hampers the adsorption of the nickel-organic acid complexes
on the resins and promotes multilayer adsorption. Whereas the
smaller metal cation (Ni2+), the hydrated nickel ions or NiSO4
complex, which do not suffer from steric hindrance are adsorbed
as a monolayer. Similar behaviour has been observed in the
adsorption of nickel complexed to EDTA on Amberlite IRA-68.
Poorer adsorption of the nickel-EDTA complex was obtained
(Qm = 22 mg/g) because of the bulky EDTA ligand [22]. These
results reflect the challenges in recovering metals from metalorganic complexes.