Nickel binding with magnetite nanoparticles†

Abstract

Nickel is generally found in trace amounts in the environment and can be beneficial to living organisms, but it is also an environmental contaminant of high concern, primarily due to anthropogenic releases. Fe oxides play a significant role in the behavior and fate of Ni in the environment, as they can interact with metal cations. However, the interactions between magnetite (Fe₃O₄) and Ni are not well described, and in particular the effect of magnetite stoichiometry (Fe(II)/Fe(III) = R) is not well considered. Ni sorption experiments were performed on stoichiometric (R0.5) and oxidized (R0.1) magnetite as a function of Ni concentration and pH under anaerobic conditions. Samples were analyzed by transmission electron microscopy, X-ray absorption spectroscopy (XAS) and magnetic circular dichroism at the Ni L_2,3-edges and XAS at the Ni K-edge. At high Ni concentrations, Ni precipitates as Ni(OH)₂ on the magnetite surface, but also as distinct sheet-like particles. At low Ni concentrations, high energy resolution fluorescence detection (HERFD) XAS analyses at the Ni K-edge revealed Ni incorporation into R0.5 magnetite and surface adsorption of Ni onto R0.1 magnetite. The present results were compared with those previously published for Co, which revealed an unexpected distinct behavior of Ni and Co. This element-specific binding mechanism highlights the unique properties of magnetite compared to other naturally occurring iron oxides (e.g. goethite, hematite), for which Ni and Co binding mechanisms are similar. Taken together, these results will help not only to predict the behavior and fate of Ni under environmental conditions in the presence of magnetite but also to synthesize magnetite nanoparticles doped by the addition of Ni with interesting magnetic properties.