The ever-increasing demand for Rare Earth Elements (REEs) due to their increased use in various high-tech and futuristic applications has stimulated the development of new sustainable approaches for efficient REE separation and recovery. Herein, we report on the development of polyethylenimine (PEI) cross-linked cellulose nanocrystal (CNC) materials for use as high performance sustainable adsorbents for REEs. This cross-linking reaction occurs in aqueous solution and traditional toxic cross-linkers are not involved. Importantly, PEI acts not only as an emerging cross-linker but also as coordination sites for REE binding. The adsorbents were qualitatively and quantitatively characterized by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), elemental analysis, zeta-potential, thermogravimetric analysis (TGA), and conductometric–potentiometric titration. In a single-component system, the adsorption behavior showed heterogeneous adsorption capacities of 0.611, 0.670, and 0.719 mmol g⁻¹ for La(III), Eu(III), and Er(III), respectively. The similarity of chemical properties makes REEs difficult to separate from each other, but the materials displayed preferential adsorption for Er(III) compared to La(III) and Eu(III) in a ternary REE solution. The FT-IR, XPS and EDS mapping results revealed the importance of the primary and secondary amino functional groups as the principal REE binding sites. Overall, this study demonstrates a facile route for separation, recovery, and enrichment of REEs from aqueous solution.

The development of an environmentally benign process for the hydrolysis of cellulose into reducing sugars can be one of the key technologies for making full use of cellulosic biomass in the future. Here, a biomass char sulfonic acid (BC-SO₃H)-catalyzed hydrolysis of cellulose in water was achieved under microwave irradiation. The BC-SO₃H catalysts prepared cheaply from natural bamboo, cotton and starch, showed a much higher turnover number (TON, 1.33-1.73) for this reaction compared to a dilute H₂SO₄ solution (TON, 0.02), which was likely due to their strong affinity to β-1,4-glycosidic bonds of cellulose. In addition, microwave irradiation played key roles in activating cellulose molecules and strengthening particle collision, which can lead to a remarkable acceleration effect on this heterogeneously catalytic process.