Developing an understanding of the reaction processes occurring at the surface-fluid interface at the atomic level of nanostructured materials in high-temperature aqueous environments is necessary for establishing general principles of behavior of nanomaterials operating in such extremes. In situ Co K-edge X-ray absorption spectroscopy (XAS) measurements were made on Fe3O4 nanoparticles in the presence of Co2+ ions in aqueous fluids to 500 degrees C and approximately 220 MPa. The results from analysis of the in situ EXAFS data, along with SEM-EDX spectra measured from reacted nanoparticles, indicate that adsorption of Co2+ ions on the surface of Fe3O4 nanoparticles is negligible at temperatures below 200 degrees C but becomes significant in the 250-500 degrees C temperature range. The low reaction temperature threshold of the Co2+ aqua ion with Fe3O4 nanoparticles is consistent with a relatively low value of the crystal field stabilization energy (CFSE) of Co2+ in octahedral site symmetry in spinels. Modeling of the pre-edge feature of the XANES and analysis of the extended X-ray absorption fine structure (EXAFS) shows that Co2+ adsorbs predominantly on octahedral sites of the surface of nanoparticles in aqueous fluids. Structural analyses using EXAFS and high resolution TEM show that the inverse spinel structure is preserved in the Co-incorporated surface atomic layers of the Fe3O4 nanoparticles. Our results suggest that the dissolved radioactive isotope Co-60 in the primary cooling loop of supercritical water-cooled nuclear reactors have a high likelihood of precipitating on the surfaces of spalled ferrite nanomaterial. (C) 2013 Elsevier B.V. All rights reserved.