Nanoemulsion assembly toward vaterite mesoporous CaCO3 for high-efficient uranium extraction from seawater

Uranium extraction from seawater is particularly significant and regarded as an indispensable strategy for satisfying the increasing demand for nuclear fuel owing to the high uranium reserves (about 4.5 billion tons) in seawater, while remains great challenges due to the low concentration, the interference of various cations and the complexity of the marine environment. Thus, developing a highly efficient adsorbent with high adsorption capacity, excellent selectivity, low cost, and facile synthesis method is significant and urgently required. Inorganic materials show many advantages in adsorption such as low cost, fast response, high stability, etc, while conventionally, have poor capacity and selectivity especially in real seawater. Herein, mesoporous CaCO3 (mCaCO3) with vaterite phase is synthesized by a facile nanoemulsion strategy and "ready-to-use" for uranium adsorption without functionalization and post treatment. Surfactant Pluronic F127 not only assembles into reverse micelles to form mesopores, but also stabilizes the active vaterite phase. The obtained mCaCO3 with high surface area (48.2 m2/g), interconnected mesopores (11 nm), and unique vaterite phase achieves highly efficient uranium adsorption with a maximum adsorption capacity of 850 ± 20 mg-U/g in uranium-spiked seawater and 6.5 ± 0.5 mg-U/g in 700 L of natural seawater for one week, as well as excellent selectivity, matching the state-of-the-art U adsorbents. After adsorption, mCaCO3-U is dissolved with a simple acid elution to obtain concentrated uranyl solution for purification, avoiding the disposal of adsorbents. To the best of our knowledge, this is the first case to report mesoporous CaCO3 for uranium adsorption from seawater with such a good performance. The facile synthesis, abundant raw materials and eco-friendly adsorption-desorption processes endow the mCaCO3 as a promising candidate for large-scale uranium extraction from seawater.