Less-precious nitrogen-rich covalent organic frameworks capable of effective rare earth recovery from water

The development of stable and inexpensive adsorbents for La(III) recovery from wastewater is of great importance for rare earth industries. In this work, two novel N-rich covalent organic frameworks (COFs) synthesized via a one-step solvothermal method were termed as COF-PA-CC and COF-ML-CC, and employed as the adsorbents for removal of rare earth (RE) ions from aqueous solution. The structures and properties of COFs were characterized with X-ray diffractometer, total reflection infrared spectrometer, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption instrument and thermogravimetric analyzer. The results revealed that COF-PA-CC composed of nanowires with the average length of 500 nm, whereas COF-ML-CC displayed the spherical morphology with the diameter of 100 ∼ 300 nm. Specific surface areas of COF-PA-CC and COF-ML-CC were calculated to be 82.13 and 191.08 m2/g, respectively. The adsorption process of La(III) was well-fitted with pseudo-second-order kinetic model and Langmuir isotherm model, exhibiting the high adsorption capacities of 150.88 and 168.19 mg/g for COF-PA-CC and COF-ML-CC after 120 min. Moreover, the influences of pH, temperature, different RE ions on adsorption performances were comprehensively tested, yielding an optimal adsorption condition (pH = 5.5, 65 °C and 120 min) and a better affinity for some light RE ions. Importantly, the fabricated COFs possess the excellent reusability after five adsorption-regeneration cycles, remaining 97.84 % and 95.51 % of initial capacities for COF-PA-CC and COF-ML-CC, respectively. The adsorption mechanism revealed that the complexation interaction between La(III) and the unoccupied N sites of N-rich COFs was recommended to interpret the remarkable adsorption performance of La(III) on the basis of the characterization analysis after the adsorption with elemental mapping, EDS and XPS. Consequently, these low-cost and robust COFs with abundant active N sites supplied a promising strategy for the recovery of RE ions from wastewater.