Phase evolution matters: How Fe(II) reshapes coprecipitate surfaces for enhanced U(VI) adsorption

Iron (oxyhydr)oxides serve as critical substrates for uranium sequestration in environmental systems. However, the immobilization behavior of U(VI) by co-precipitates of Fe(II)/Fe(III) at moderate molar ratios remains unclear. This study investigates phase evolution during Fe(II)/Fe(III) co-precipitation and subsequent U(VI) immobilization mechanisms. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses revealed that co-precipitates with a low Fe(II)/Fe(III) molar ratio (1:50) retained as the phase of ferrihydrite for 48 h, whereas those with a high Fe(II)/Fe(III) molar ratio (1:10) transformed to goethite within 12 h. Acid dissolution experimental results are consistent with Fe(II) being incorporated throughout the bulk of the precipitate particles. The accelerated U(VI) immobilization kinetics in the Fe(II)/Fe(III) ratio of 1:10 system may be attributed to the enhanced surface reactivity of goethite as its dominant crystalline phase. Combined density functional theory (DFT) and spectroscopic analysis reveal a critical energy barrier that inhibits electron transfer from structurally incorporated Fe(II) to U(VI) at low Fe(II) density. Integrating these findings with literature evidence of U(VI) reduction in magnetite (Fe(II)/Fe(III) ≈ 1:2) supports the proposed concept of a reductive inert threshold for structural Fe(II), bracketed between ratios of 1:10 and 1:2. In contrast, supplementation with high concentrations of aqueous Fe(II) induced partial reduction, highlighting the critical role of Fe(II) speciation and availability. This finding reveals a non-reductive, adsorption-dominated immobilization pathway for uranium at the iron oxide interface, which improves the theoretical understanding and provides a new foundational basis for evaluating and predicting the environmental behavior of uranium.