  Nickel (Ni) is an economically important metal characterized by its
mechanical strength and anticorrosion properties. With the increasing
industrial demand for Ni and the depleting accessible Ni primary ores,
sustainable technologies are required for the recovery of this metal from
alternative resources. In this study, adsorption using chitosan was
investigated as a sustainable technique to recover Ni from sulfate (SO42-) and
calcium (Ca) rich secondary resources. The effects of pH, contact time, and the
presence of SO42- and Ca on Ni adsorption were investigated in batch
experiments. Chemical speciation modeling was performed to analyze how the
predominant Ni species present under different conditions may affect the
efficiency of the adsorption process. The comparison of the maximum Ni
adsorption capacities of chitosan in the absence (1.00 mmol/g) and presence
(1.49 mmol/g) of 500 mM SO42- and 10 mM Ca indicated the positive effect of
these ions on Ni adsorption. The predominance of the neutrally charged (NiSO4)0
species in Ni/SO42- system has contributed to the enhanced Ni adsorption on
chitosan as verified by X-ray photoelectron spectroscopy (XPS) analysis.
However, kinetic studies confirmed that the Ni adsorption rate decreased by 4.5
times when SO42- was present. The subsequent continuous Ni adsorption from a
real SO42-rich leachate in a column setup revealed that chitosan is selective
for Ni over Ca and Cr with selectivity quotients of 9.6 (K Ni/Ca) and 3.0 (K
Ni/Cr). Overall, this study indicated that Ni complexation with SO42- enhances
the Ni adsorption capacity of chitosan, but slows down the adsorption process.
