Recovery of Co(ii), Ni(ii) and Zn(ii) using magnetic nanoparticles (MNPs) at circumneutral pH

Growing demand for metals, particularly those with irreplaceable utility within renewable energy technology dictates an urgent demand for the development of new innovative approaches for their extraction from primary and secondary sources. In this study, magnetic nanoparticles (MNP) were investigated for their ability to remove cobalt (Co²⁺), nickel (Ni²⁺), and zinc (Zn²⁺) ions from neutral pH aqueous solutions under anoxic conditions. A MNP suspension (1 g L^-1 or 5 g L^-1) was exposed to varying concentrations of Co(ii), Ni(ii), and Zn(ii) (10-1000 mg L^-1) in both single and mixed systems for 48 hours at pH 7.0 ± 0.1. Results show that MNPs can remove these ions to low concentrations (K _d values: Zn: 0.07 L g^-1; Co: 0.02 L g^-1; and Ni: 0.01 L g^-1 in single metal systems). Transmission Electron Microscopy (TEM) analysis confirmed relatively homogenous surface coverage of MNPs by each metal, while X-ray Absorption Spectroscopy (XAS) measurements determined sorption via the formation of coordinate bonds between the sorbed metals and surface oxygen atoms (Fe-O). Overall, our results show that MNPs can serve as an effective and reusable sorbent for Zn, Ni and Co ions from circumneutral pH waters.