Ion-selective uranium capture from seawater by UiO-66 metal-organic framework modified with amidoxime groups

Seawater is a valuable source of uranium (U) resources, and harnessing it effectively can play a crucial role in promoting nuclear energy. However, current polymer-based adsorbents typically exhibit slow adsorption rates and insufficient selectivity. In this work, the efficient uranyl ion (UO₂ ²⁺) adsorbent (UiO-66-AO) was obtained from UiO-66-NH₂ through a simple and mild approach post synthesis modification (PSM). During the PSM process, not only the octahedral morphology (particle size ∼200 nm) but also the crystal structure of UiO-66-NH₂ was well maintained. The integration of amidoxime groups (AO) improved the selectivity and adsorption performance towards UO₂ ²⁺. According to the Langmuir model, the maximum adsorption capacity (q _max) of UiO-66-AO was 413.2 mg g^-1. In the competitive ion solution containing vanadium(v), iron (Fe), magnesium (Mg), calcium (Ca), etc., UiO-66-AO exhibited a much higher UO₂ ²⁺ adsorption capacity of 12.3 mg g^-1 than UiO-66-NH₂ (0.9 mg g^-1). Furthermore, UiO-66-AO achieved equilibrium for UO₂ ²⁺ adsorption in natural seawater within 3 days, and the adsorption capacity was 3.0 mg g^-1. Finally, near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) demonstrated the chelation effect of AO on U, proving that the PSM successfully achieved performance enhancement. It is hoped that the simple and mild AO modification approach will provide new insights for the preparation of rapid, highly selective, and efficient UO₂ ²⁺ adsorbents.