Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: A review

Highlights: ► Kd data of Pu, U, Np, Am and Tc onto Fe (hydr)oxides under different conditions. ► Molecular-level speciation of Pu, U, Np, and Am sorbed onto Fe (hydr)oxides. ► Effects of CO32+, Ca, PO43+, humic acid, dissolved/structural Fe2+ on U sorption. - Abstract: Pu, U, Np, Am and Tc are among the major risk drivers at nuclear waste management facilities throughout the world. Furthermore, uranium mining and milling operations have generated an enormous legacy of radioactively contaminated soils and groundwater. The sorption process of radionulcides onto ubiquitous Fe (hydr)oxides (FHOs; hematite, magnetite, goethite and ferrihydrite) is one of the most vital geochemical processes controlling the transport and fate of radionuclides and nuclear wastes in the subsurface zones. Meanwhile, understanding molecular-level chemical speciation of radionuclides onto FHOs is crucial to model their behavior in subsurface environments, and to develop new technologies for nuclear waste treatment and long-term remediation strategies for contaminated soils and groundwater. This review article aims (1) to provide risk or performance assessment modelers with macroscopic distribution coefficient (Kd) data of Pu, U, Np, Am and Tc onto FHOs under different conditions (pH, radionuclide concentration, solution ion strength, sorbent loading, partial pressure of CO2 (PCO2), equilibrium time) pertinent to environmental and engineered systems, and (2) to provide a microscopic or molecular-level understanding of the chemical speciation and sorption processes of these radionuclides to FHOs.