Phosphorylated hyper-crosslinked calix[4]arene for selective uranyl extraction from seawater

Seawater contains abundant uranium, which could provide a long–term supply for developing the nuclear power industry. Developing an efficient uranium–selective adsorbent is therefore an urgent requirement. In this work, we used a simple Friedel–Crafts alkylation reaction and a subsequent phosphorylation reaction to prepare porous phosphorylated hyper-crosslinked calix[4]arene (C4HP) for potential use in the recovery of uranium from seawater. The effect of the initial pH, adsorbent composition, sorbent dose and contact time on the sorption of uranyl ions was investigated. Because of the introduction of the phosphate groups, C4HP shows an excellent adsorption capacity of 336.4 mg·g−1 for uranium, which is much higher than that of hyper-crosslinked calix[4]arene (C4H) without phosphorylation. Meanwhile, C4HP functioned as a promising adsorbent for uranyl extraction with fast capture kinetics (the adsorption equilibrium can be reached in 150 min). After 6 cycles, C4HP maintained its original adsorption capacity of 76.2 % (255.4 mg·g−1). Furthermore, C4HP possesses excellent adsorption selectivity for uranium in artificial seawater, with a KdU of ~4.1 × 104 mL·g−1, much higher than that of other metals. This makes it useful for broad applications and capturing uranium from natural seawater. The effective extraction of uranyl was primarily attributed to the chelation of uranyl ions with phosphate groups.