Adsorption of nickel ions by oleate-modified magnetic iron oxide nanoparticles

In this work, magnetic nanoparticles of iron oxide (MNPs) were synthesized, and then the surface was recovered with an oleate double layer in order to investigate the ability of this material to adsorb nickel ions. First, the solution chemistry of oleate ions was investigated in order to determine the critical micellar concentration (CMC) value and the arrangements of ions above the CMC. Then, the synthesized oleate-modified MNP was characterized (TEM, DLS, XRD, FTIR, zeta potential, magnetometry). Finally, adsorption experiments were carried out as a function of pH and as a function of nickel concentration in 0.1 g L^-1 suspensions of oleate-modified MNP. The results show that CMC of oleate ranges from 1 to 2.5∙10^-3 mol L^-1. Above CMC, arrangement of oleate ions as droplets, vesicles, or micelles depends on pH and influences the average size and solution absorbance. Potentiometric titrations allowed determining a pKa value of 7.8 for sodium oleate. The high stability in aqueous suspensions and characterization of oleate-modified MNP confirm that oleate ions are arranged as a bilayer coating at the surface of MNP. Retention of nickel was found to be highly dependent on pH, with a maximum adsorption (90%) beginning from pH = 7.5. The sorption isotherms were well fitted with the Langmuir model and the maximum nickel adsorption capacities were found to be 44 and 80 mg g^-1 for pH = 6.8 and 7.2, respectively. The efficient removal of nickel combined with the magnetic properties of the NMP make the oleate-modified MNP an interesting water purification tool.