4. Results and discussion
4.1. Adsorption of nickel organic complexes
Equilibrium adsorption isotherms of nickel-complexes, pre-
pared using 0.01 and 0.1 M acids, are shown in Figs. 2–4. The
dependence of nickel adsorption on the type of ligands and acid
concentrations (or solution pH) is examined in these data. The
equilibrium isotherms of the various nickel complexes were
fitted to the Langmuir and Freundlich models. The solid and
dashed lines in Figs. 2–4 are predicted by Freundlich and Lang-
muir models, respectively. Table 2 lists the estimated constants
from the fitted models and the corresponding measures of fit (R2
and χ2 values) of the adsorption data. Evidently, the adsorption
of the nickel complexes fit the Freundlich model. The R2 and χ2
values for the Freundlich model reported in Table 2 confirm this
fit. The R2 values for the Freundlich model varied from 0.96 to
0.99 compared to 0.92–0.96 for the Langmuir model. Similarly
the χ2 values for the Freundlich model, which varied from 0.08
to 0.43, are lower compared to Langmuir model which had χ2
values from 1.27 to 4.52. This suggests the adsorption of nickel
complexed to citrate, malate and lactate ligands, onto Purolite
S950, occurs by multilayer adsorption. The Freundlich param-
eter, A values, reported in Table 2, reflects the effect of the type
of ligands and solution pH (organic acid concentration) on the
adsorption capacity of the nickel complexes.