Efficient removal of uranium (VI) from water by a hyper-cross-linked polymer adsorbent modified with polyethylenimine via phosphoramidate linkers

Practical adsorbents with high efficiency are essential to effectively treating wastewater. Herein, a novel porous uranium adsorbent (PA-HCP) having a considerable amount of amine and phosphoryl groups was designed and synthesized by grafting polyethyleneimine (PEI) on a hyper-cross-linked fluorene-9-bisphenol skeleton via phosphoramidate linkers. Furthermore, it was used to treat uranium contamination in the environment. PA-HCP exhibited a large specific surface area (up to 124 m2/g) and a pore diameter of 2.5 nm. Batch uranium adsorptions on PA-HCP were investigated methodically. PA-HCP demonstrated a uranium sorption capacity of >300 mg/g in the pH range of 4-10 (C0 = 60 mg/L, T = 298.15 K), with its maximum capacity reaching 573.51 mg/g at pH = 7. The uranium sorption process obeyed the pseudo-second-order model and fitted well with the Langmuir isothermal. In the thermodynamic experiments, uranium sorption on PA-HCP was revealed to be an endothermic, spontaneous process. Even in the presence of competing metal ions, PA-HCP exhibited excellent sorption selectivity for uranium. Additionally, excellent recyclability can be achieved after six cycles. Based on FT-IR and XPS measurements, both the PO and -NH2 (and/or -NH-) groups on PA-HCP contributed to efficient uranium adsorption as a result of the strong coordination between these groups and uranium. Furthermore, the high hydrophilicity of the grafted PEI improved the dispersion of the adsorbents in water and facilitated uranium sorption. These findings suggest that PA-HCP can be used as an efficient and economical sorbent to remove U(VI) from wastewater.