Novel Polyethylenimine–Acrylamide/SiO₂ Hybrid Hydrogel Sorbent for Rare-Earth-Element Recycling from Aqueous Sources

Abstract

Recycling rare-earth elements (REEs) becomes increasingly important because of their supply vulnerability and increasing demands in industry, agriculture, and national security. Hybrid hydrogel sorbents are outstanding, because of their high stability and selectivity. Organic–inorganic hybrid hydrogels were synthesized by thermopolymerization of acrylamide onto PEI polymer chains with N,N′-methylene bis(acrylamide) as a cross-linker. The grafted network was evidenced by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The porous structure was observed by scanning electron microscopy (SEM). The degree of cross-linking, the degree of PEI grafting, and the SiO2 concentration were studied to optimize the adsorption of REEs. The pH value of the medium greatly affected REE adsorption capacity, where the almost-neutral conditions gave the strongest bonding of REEs to active sites. Moreover, kinetic studies showed that the rate-determining step of the adsorption process was chemical sorption, and that REE diffusion within micropores was the control step for, specifically, intraparticle diffusion. The adsorbents showed excellent selectivity and recyclability for REEs through five adsorption–desorption cycles in contact with synthetic acid mine drainage solution. A high separation toward REEs over fouling metals was achieved by using a citrate-based buffer eluent solution. This hybrid hydrogel shows promise for the recycling of REEs from aqueous solutions.