Construction of novel nitrogen-rich covalent organic frameworks for highly efficient La(III) adsorption

Two novel N-rich covalent organic frameworks (COFs), named COF-TZ-TP and COF-TA-TP, were prepared via a one-step solvothermal method and employed as the adsorbent to remove rare earth (RE) ions from solution. The as-prepared materials were characterized with XRD, FESEM, ATR-IR, BET, TGA and XPS analysis. The results show the rapid adsorption kinetics of La(III) with capacities of 165.6 and 89.8 mg/g for COF-TZ-TP and COF-TA-TP after 3 h. Moreover, Langmuir isotherm model and pseudo-second-order kinetic model are well fitted to describe the La(III) adsorption process. The analysis of the adsorption thermodynamic shows that the adsorption process is a spontaneous exothermic process with ΔS0 of 57.22 and 84.09 J/(mol ∙ K) for COF-TZ-TP and COF-TA-TP, respectively. In addition, the fabricated COFs exhibit the better adsorption results towards the light RE ions compared with heavy ones and the excellent reusability after five adsorption-regeneration cycles. The adsorption mechanism reveals that the splendid adsorption performance of RE for our N-rich COFs is mainly attributed to the surface coordination interaction between the N element and La(III) according to the characterization analysis for COF-TZ-TP before and after the adsorption process with elemental mapping, EDS and XPS. Therefore, these COFs with abundant active N sites offer the highly efficient adsorption capacity of La(III), and provide the promising strategy for the recovery of light RE from waste rare earth devices.