What are the effects of environmental factors on Co speciation at the magnetite surface?

Abstract

Magnetite nanoparticles are abundant in the environment and are notably used for environmental applications due to their unique magnetic, adsorption and redox properties. The stoichiometry of magnetite (Fe(II)/Fe(III)) is dependent on environmental factors (pH, presence of organic ligands, redox conditions), which largely affects not only magnetite nanoparticles' physicochemical properties, such as redox reaction, but also adsorption of contaminants. However, the joint effects of environmental factors and magnetite stoichiometry on magnetite–metal cation interaction are elusive. This study focuses on Co as an important contaminant and because Co-doped magnetite nanoparticles are of high interest for nanotechnology applications. Magnetite nanoparticles (∼10 nm) with different stoichiometries were synthesized by coprecipitation and partially oxidized using H₂O₂ to obtain the desired stoichiometry. Batch studies were carried out with different [Co], using three different stoichiometries (0.1, 0.3 and 0.5) at different pH values and in the presence or the absence of natural organic matter (OM) or atmospheric O_2(g). Experimental and modeling results and XAS and XMCD analyses revealed that interactions between Co and magnetite varied, with oligomer formation and surface precipitation favored at higher stoichiometries. It was also highlighted that the formation of different species of Co depended on environmental conditions with the adsorption of monomers favored at low pH and in the presence of organic matter, and the partial oxidation of Co(II) to Co(III) under aerobic conditions. These results will help to predict the behavior and fate of Co in the environment and to understand the impact of environmental factors for an appropriate use of magnetite nanoparticles for environmental applications, or to synthesize Co-modified magnetite nanoparticles using water as solvent for high technology applications.