Deciphering the Temperature-Governed Dynamic Evolution of Eu Surface Complexes on Magnetite

Abstract: This study elucidates the synergistic control of temperature and pH on the adsorption/desorption kinetics of Eu on magnetite by integrating batch adsorption, stirred-flow kinetics, and surface complexation modeling. The results reveal that pH is the dominant factor governing adsorption kinetics, while the effect of temperature exhibits strong pH dependence.Under acidic conditions, adsorption is dominated by outer-sphere complexation with low adsorption capacity and activation energy, rendering temperature effects negligible. Under neutral conditions, however, the adsorption mechanism shifts to inner-sphere complexation, where elevated temperature significantly accelerates adsorption kinetics by promoting Eu 3+ hydrolysis and enhancing the reactivity and mobility of surface hydroxyl groups. Crucially, the dynamic evolution of surface complexes reveals that temperature acts as a molecular switch controlling the adsorption pathway. Below 40 °C, adsorption follows a stepwise from outer-sphere to inner-sphere.Above 40 °C, this pathway shifts to direct inner-sphere complexation, bypassing the outer-sphere intermediate entirely. This temperature-triggered pathway shift indicates that in the hightemperature environment of a nuclear waste repository, stable inner-sphere complexes form rapidly during the early adsorption stage, significantly enhancing radionuclide retention and accelerating the onset of slow-desorption kinetics.