Effects of iron oxide coatings on the mobility and retardation of U(VI) in water saturated media

Iron oxide coatings occur commonly in the environment and affect significantly the fate of heavy metals. However, U(VI) transport in water saturated iron oxide-coating porous media has been seldom investigated. In this study, the adsorption and retardation of U(VI) on iron oxide-coated sand were comparatively investigated over a wide range of experimental conditions by batch and column experiments combined with modelling. The presence of iron oxide coatings significantly enhanced U(VI) adsorption which could be described by a two-site surface complexation model. The breakthrough curves of U(VI) in iron oxide coated quartz sand column show a two-plateau profile, which could not be directly reproduced by the commonly used two-site kinetic model. More sites for U(VI) have to be considered, implying that the heterogeneity of surface sites might be more significant in the non-equilibrium transport system than that in the equilibrium adsorption system. In addition, the presence of salicylic acid (SA) facilitated the transport of U(VI) at acidic pH, due to the competition between surface complexation of U(VI) and the complexation of U(VI) with SA in aqueous phase. Overall, this study provides a comprehensive understanding of the migration behavior of U(VI) in water saturated iron oxide-coated sand system.