Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions

[Display omitted] •The precipitation rate of U(VI) by Fe(II) under O2-rich conditions is up to 96%.•16–20% of U is adsorbed on the surface of the solids as uranyl compounds.•52–56% of U is absorbed at the internal pores of Fe precipitates as uranyl phases.•27–29% of U is likely to be incorporated into the FeO(OH) structure.•U(VI) partially precipitates as U(V) that remains stable under O2-rich conditions. Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16–20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52–56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27–29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O2 and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H+ concentration ([H+]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O2 has potential applications in the separation and recycling of aqueous uranium.