Porous Covalent Organic Polymers Comprising a Phosphite Skeleton for Aqueous Nd(III) Capture

In order to meet the ever-increasing industrial demand for rare-earth elements (REEs), it is desirable to separate and recycle them at low concentrations from various sources including industrial and urban wastes. Here, we introduced phosphorus binding sites on the hydrophobic surface of a robust and high-surface area porous polymer backbone for environmentally benign and selective recovery of REEs via adsorption. For this purpose, two porous covalent organic polymer (COP) materials incorporated with in-built phosphite functionality (P-COP-1 and P-COP-2) were synthesized and applied for the adsorptive separation of Nd(III) ions from aqueous solution. A strategy to develop a series of P-COPs via a simple Friedel-Crafts reaction was introduced, and their application to the selective adsorption of REEs was explored for the first time. The newly synthesized P-COPs were amorphous and/or weakly crystalline and showed excellent chemical stability and large specific surface area with sufficient mesoporosity for enhanced diffusion of REE ions. P-COP-1 exhibited an exceptionally high Nd(III) adsorption capacity of 321.0 mg/g, corresponding to the stoichiometric ratio of P/Nd(III) = 1:0.7 and high selectivity of >86% over other competing transition and alkaline earth metal ions, whereas P-COP-2 gave a Nd(III) adsorption capacity of 175.6 mg/g at 25 °C and pH 5. Moreover, P-COP-1 showed a distribution coefficient value of 5.45 × 10⁵ mL/g, which is superior to other benchmark adsorbent materials reported so far. Finally, the P-COPs were reusable for a minimum of 10 cycles without deterioration in adsorption capacities.